Polyimide porous film and porous composite

Abstract

A polyimide porous film 1a comprises a first porous layer 11 and a second porous layer 12. The second porous layer 12 is disposed on one surface or both surfaces of the first porous layer 11. The average pore diameter D11 on the surface of the second porous layer 12 is smaller than the average pore diameter D12 in the first porous layer 11. At least one condition selected from the group consisting of the condition (I) and the condition (II) described below is satisfied. (I) The air permeability P1 of the polyimide porous film 1a is 30 sec / 100 cm3 or less, and the air permeability P2 of a sample that is obtained by removing the second porous layer 12 is lower than the air permeability P1 of the polyimide porous film 1a by 2 sec / 100 cm3 or more. (II) The ratio RP of the pore diameters of 0.2 μm or less in the number-based pore size distribution on the surface of the second porous layer 12 is 80% or more, and the aperture ratio RO on the surface of the second porous layer 12 is 1% to 60%.

Description

Polyimide porous membranes and porous composites

[0001] The present invention relates to a polyimide porous membrane and a porous composite.

[0002] Polyimide porous membranes have been known in the past.

[0003] For example, Patent Document 1 describes a porous polyimide membrane with a three-layer structure. This porous polyimide membrane has two surface layers (a) and (b) and a macrovoid layer sandwiched between these surface layers (a) and (b). The macrovoid layer has partition walls bonded to the surface layers (a) and (b), and a plurality of macrovoids surrounded by the partition walls and the surface layers (a) and (b), with an average pore size in the membrane plane direction of 10 to 500 μm. The partition walls of the macrovoid layer have a thickness of 0.1 to 50 μm, and at least one partition wall has one or more holes connecting adjacent macrovoids. The thickness of each of the surface layers (a) and (b) is 0.1 to 50 μm. At least one surface layer has a plurality of pores with an average pore size of more than 5 μm and not more than 200 μm, and the other surface layer has a plurality of pores with an average pore size of 0.01 to 200 μm. At least one of the surface layers has a surface porosity of 10% or more, and the other surface layer has a surface porosity of 5% or more. The pores in the surface layers (a) and (b) communicate with macrovoids. The total thickness of the porous polyimide membrane is 5 to 500 μm, the Gurley value of the porous polyimide membrane is 20 seconds / 100 cc or less, and the porosity of the porous polyimide membrane is 60 to 95%.

[0004] Patent Document 2 describes a polyimide porous membrane produced from a predetermined reaction product. Both surfaces of the polyimide porous membrane are substantially open. The polyimide porous membrane has a Gurley value of 2000 seconds / 100 cc or less and a porosity of 40% or more.

[0005] Patent Document 3 describes a porous polyimide film. The porosity of this porous polyimide film is 45% by volume or more and 95% by volume or less. The average pore diameter of this porous polyimide film is 10 nm or more and 1000 nm or less.

[0006] Patent Document 4 describes a polyimide porous body obtained by a method for improving the air permeability of a predetermined polyimide porous body. The method for improving the air permeability of a polyimide porous body is characterized by subjecting a polyimide porous body obtained by solvent-substituted phase separation or thermally induced phase separation using a polyimide solution or a polymic acid solution to a dry etching treatment.

[0007] International Publication No. 2018 / 021356 Japanese Patent Application Laid-Open No. 2015-71755 International Publication No. 2016 / 104309 Japanese Patent Application Laid-Open No. 2011-1434

[0008] The techniques described in Patent Documents 1 to 4 need to be reconsidered from the viewpoint of achieving both breathability and water resistance. Therefore, the present invention provides a polyimide porous membrane that is advantageous from the viewpoint of achieving both breathability and water resistance.

[0009] The present invention provides a polyimide porous film comprising: a first porous layer; and a second porous layer disposed on one or both sides of the first porous layer, wherein the average pore size at the surface of the second porous layer is smaller than the average pore size in the first porous layer, and wherein at least one condition selected from the group consisting of the following conditions (I) and (II) is satisfied: (I) The polyimide porous film has an air permeability of 30 sec / 100 cm as measured according to the Air Permeability Measurement Method B (Gurley method) specified in Japanese Industrial Standards (JIS) L1096:2010. 3 and the air permeability of a sample obtained by removing the second porous layer from the polyimide porous membrane, measured according to the Air Permeability Measurement Method B (Gurley method), is 2 seconds / 100 cm or less than the air permeability of the polyimide porous membrane. 3 (II) The proportion of pores with diameters of 0.2 μm or less in the number-based pore diameter distribution on the surface of the second porous layer is 80% or more, and the opening ratio on the surface of the second porous layer is 1 to 60%.

[0010] The above-mentioned polyimide porous film is advantageous from the viewpoint of achieving both breathability and water resistance.

[0011] FIG. 1 is a cross-sectional view schematically showing an example of a polyimide porous membrane according to the present invention. FIG. 2 is a cross-sectional view schematically showing another example of a polyimide porous membrane according to the present invention. FIG. 3 is a cross-sectional view schematically showing an example of a porous composite according to the present invention. FIG. 4A is a binarized image of a scanning electron microscope (SEM) photograph of the surface of the second porous layer of the polyimide porous membrane according to Example 1. FIG. 4B is a binarized image of an SEM photograph of the surface of the first porous layer of the polyimide porous membrane according to Example 1. FIG. 4C is an SEM photograph of a cross section along the thickness direction of the polyimide porous membrane according to Example 1. FIG. 4D is an SEM photograph enlarging a portion of the SEM photograph of FIG. 4C. FIG. 5A is a binarized image of an SEM photograph of the surface of the second porous layer of the polyimide porous membrane according to Example 2. FIG. 5B is a binarized image of an SEM photograph of the surface of the first porous layer of the polyimide porous membrane according to Example 2. FIG. 6A is a binarized image of an SEM photograph of the surface of the second porous layer of the polyimide porous membrane according to Comparative Example 1. FIG. 6B is a binarized image of an SEM photograph of the surface of the first porous layer of the polyimide porous membrane according to Comparative Example 1. FIG. 7A is a binarized image of an SEM photograph of the surface of the second porous layer of the polyimide porous membrane according to Comparative Example 2. FIG. 7B is a binarized image of an SEM photograph of the surface of the first porous layer of the polyimide porous membrane according to Comparative Example 2. FIG. 8A is a binarized image of an SEM photograph of the surface of the second porous layer of the polyimide porous membrane according to Comparative Example 3. FIG. 8B is a SEM photograph of the surface of the first porous layer of the polyimide porous membrane according to Comparative Example 3. FIG. 9A is a binarized image of an SEM photograph of the surface of the second porous layer of the polyimide porous membrane according to Comparative Example 4. FIG. 9B is a SEM photograph of the surface of the first porous layer of the polyimide porous membrane according to Comparative Example 4.

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings, but the present invention is not limited to the following embodiments.

[0013] Fig. 1 is a cross-sectional view schematically showing one example of a polyimide porous membrane according to the present invention. Fig. 2 is a cross-sectional view schematically showing another example of a polyimide porous membrane according to the present invention. As shown in Figs. 1 and 2, a polyimide porous membrane 1a includes a first porous layer 11 and a second porous layer 12. As shown in Fig. 1, the second porous layer 12 is disposed, for example, on one side of the first porous layer 11. As shown in Fig. 2, the second porous layer 12 may be disposed, for example, on both sides of the first porous layer 11. The average pore diameter D at the surface of the second porous layer 12 is 12 is the average pore diameter D in the first porous layer 11 11 In this specification, the average pore size is the number average pore size.

[0014] The polyimide porous membrane 1a satisfies at least one condition selected from the group consisting of the following conditions (I) and (II): (I) The polyimide porous membrane 1a has an air permeability P1 of 30 sec / 100 cm, as measured according to the air permeability measurement method B (Gurley method) specified in JIS L1096:2010. 3 The air permeability P2 of a sample obtained by removing the second porous layer 12 from the polyimide porous membrane 1a, measured according to Air Permeability Measurement Method B (Gurley method), is 2 seconds / 100 cm or less than the air permeability P1 of the polyimide porous membrane 1a. 3 (II) The proportion R of pore sizes of 0.2 μm or less in the number-based pore size distribution on the surface of the second porous layer 12 P is 80% or more, and the opening ratio R O is 1 to 60%.

[0015] In a polymer porous membrane, breathability and water resistance are in a trade-off relationship; increasing breathability tends to decrease water resistance, while increasing water resistance tends to decrease breathability. The breathability and water resistance of a polymer porous membrane are closely related to the porosity and pore size of the polymer porous membrane, and it is considered important to adjust the porosity and pore size of the polymer porous membrane so that the desired breathability and desired water resistance can be achieved simultaneously. On the other hand, according to the studies of the present inventors, it has been found that in a polyimide porous membrane, it is sometimes difficult to achieve the desired breathability and desired water resistance by adjusting the porosity and pore size. In particular, it has been found that the relatively high hydrophilicity of polyimide is not advantageous for improving water resistance.

[0016] Therefore, the present inventors thought that a polyimide porous membrane comprising a first porous layer having a relatively large average pore size and a second porous layer having a relatively small average pore size might be able to achieve both the desired breathability and the desired water resistance. After extensive trial and error, the present inventors finally found that a polyimide porous membrane comprising a second porous layer that satisfies certain conditions can easily achieve both the desired breathability and the desired water resistance.

[0017] In the polyimide porous membrane 1a, satisfying at least one condition selected from the group consisting of condition (I) and condition (II) makes it easy to achieve both the desired breathability and the desired water resistance. The polyimide porous membrane 1a may satisfy only condition (I), only condition (II), or both conditions (I) and (II). The water pressure resistance R1 of the polyimide porous membrane 1a is, for example, 0.15 MPa or more. The water pressure resistance R1 is measured, for example, according to the water resistance test method B (high water pressure method) specified in JIS L1092:2009. When the second porous layer 12 is disposed on both sides of the first porous layer 11, the sample under condition (I) may be obtained by removing only one of the pair of second porous layers 12 disposed on both sides of the first porous layer 11, or may be obtained by removing both of the pair of second porous layers 12.

[0018] The method for removing the second porous layer 12 from the polyimide porous film 1a in the preparation of the sample under condition (I) is not limited to a specific method. For example, dry etching is used. In this case, etching selectivity for the second porous layer 12 is likely to be high, and destruction or damage to the first porous layer 11 is likely to be suppressed. Examples of dry etching include etching including one selected from sputtering, plasma treatment, and corona discharge treatment, as well as dry blasting. From the viewpoint of etching effect, dry etching preferably includes plasma treatment. The plasma treatment may be vacuum plasma treatment or atmospheric pressure plasma treatment.

[0019] The water pressure resistance R1 may be 0.20 MPa or more, or 0.25 MPa or more. The water pressure resistance R1 is, for example, 0.50 MPa or less.

[0020] Regarding the condition (I), the air permeability P1 is, for example, 25 seconds / 100 cm 3 Below, 20 seconds / 100cm 3 Below, 19 seconds / 100cm 3 Below, 18 seconds / 100cm 3 or less, or 17 seconds / 100 cm 3 The air permeability P1 may be, for example, 2 seconds / 100 cm 3 The air permeability P2 is, for example, 0.01 seconds / 100 cm 3 or more, and 0.05 seconds / 100 cm 3 or more than 0.1 seconds / 100 cm 3 The air permeability P2 may be, for example, 28 seconds / 100 cm 3 less than 25 seconds / 100 cm 3 Below, 20 seconds / 100cm 3 Below, 15 seconds / 100cm 3 Below, 10 seconds / 100cm 3 Below, 5 seconds / 100cm 3 Below, 1.5 seconds / 100cm 3 Below, 1.2 seconds / 100cm 3 Less than or equal to 1.0 seconds / 100 cm 3 It may be the following:

[0021] As described above, in a porous polymer membrane, breathability and water resistance are in a trade-off relationship. Under the condition (I), the air permeability P2 is 2 seconds / 100 cm lower than the air permeability P1. 3 The fact that the water resistance is lower than this indicates that the second porous layer 12 plays an important role in realizing the desired water resistance. Under the condition (I), the thickness of the sample may be such that the second porous layer 12 can be removed, and is, for example, 80% to 99% of the thickness of the polyimide porous film 1a.

[0022] The air permeability P2 is 6 seconds / 100 cm 3 Above, 8 seconds / 100cm 3 or more, or 10 seconds / 100 cm 3 The air permeability P2 may be lower than the air permeability P1 by, for example, 15 seconds / 100 cm 3 Below is low.

[0023] When the condition (II) is satisfied, the air permeability P1 is, for example, 30 seconds / 100 cm 3 less than 25 seconds / 100 cm 3 Below, 20 seconds / 100cm 3 Below, 19 seconds / 100cm 3 Below, 18 seconds / 100cm 3 or less, or 17 seconds / 100 cm 3 When the condition (II) is satisfied, the air permeability P1 is, for example, 1 second / 100 cm 3 or more, 2 seconds / 100 cm 3 Above, 5 seconds / 100cm 3 or more, or 10 seconds / 100 cm 3 It may be more than that.

[0024] Regarding the condition (II), the ratio R P The ratio R may be 82% or more or 85% or more. P is, for example, 99% or less, and may be 98% or less or 95% or less. O may be 2% or more, 3% or more, or 4% or more, and may be 50% or less, 40% or less, 30% or less, 20% or less, or 15% or less.

[0025] The thickness of the polyimide porous film 1a is not limited to a specific value. The thickness is, for example, 1 to 30 μm. In this case, the polyimide porous film 1a is easy to handle and can be used in various applications requiring desired breathability and water resistance.

[0026] The thickness of the polyimide porous film 1a may be 2 μm or more, 3 μm or more, 4 μm or more, or 5 μm or more, and may be 25 μm or less, 20 μm or less, 18 μm or less, or 15 μm or less.

[0027] The thickness of the second porous layer 12 is not limited to a specific value. For example, the thickness is 1 μm or less. This makes it easy to keep the air permeability P1 of the polyimide porous film 1a low within a desired range.

[0028] The thickness of the second porous layer 12 is, for example, 0.01 μm or more. This tends to increase the water resistance of the polyimide porous film 1a. The thickness of the second porous layer 12 may be 0.9 μm or less, 0.05 μm or more, 0.1 μm or more, 0.2 μm or more, 0.3 μm or more, 0.4 μm or more, or 0.5 μm or more.

[0029] Average pore diameter D on the surface of the second porous layer 12 12 is not limited to a specific value. 12 In this case, the polyimide porous film 1a is more likely to have both the desired breathability and the desired water resistance. 12 can be calculated, for example, based on an SEM photograph of the surface of the second porous layer 12 according to the method described in the Examples.

[0030] Average pore diameter D 12 may be 0.06 μm or more, or 0.08 μm or more, or may be 0.5 μm or less, 0.4 μm or less, 0.3 μm or less, or 0.2 μm or less.

[0031] In the polyimide porous membrane 1a, as long as at least one condition selected from the group consisting of the condition (I) and the condition (II) is satisfied, the maximum pore diameter D on the surface of the second porous layer 12 max is not limited to a specific value. maxIn this case, the water resistance of the polyimide porous film 1a tends to be high. max is preferably 4 μm or less, more preferably 3 μm or less, even more preferably 2 μm or less, and particularly preferably 1 μm or less.

[0032] The average pore diameter D on the surface of the first porous layer 11 or on the surface of the above sample obtained by removing the second porous layer 12 from the polyimide porous film 1a 11 is not limited to a specific value. In the above sample obtained by removing the second porous layer 12 from the polyimide porous film 1a, for example, the first porous layer 11 forms the surface of the sample. 11 In this case, the polyimide porous film 1a is more likely to have both the desired breathability and the desired water resistance. 11 can be calculated, for example, according to the method described in the Examples based on an SEM photograph of the surface of the first porous layer 11 or the above sample.

[0033] Average pore diameter D 11 may be 0.2 μm or more, or 0.3 μm or more, or may be 8 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, or 1 μm or less.

[0034] The porosity p1 of the polyimide porous membrane 1a is not limited to a specific value. The porosity p1 is, for example, 40 to 90%. In this case, the polyimide porous membrane 1a is more likely to achieve both the desired breathability and the desired water resistance. The porosity p1 can be calculated, for example, according to the method described in the Examples. As described below, when the polyimide porous membrane 1a constitutes a porous composite together with a porous substrate such as a nonwoven fabric, the porosity p1 may be calculated based on the area ratio of the cross section.

[0035] The porosity p1 may be 40% or more, 45% or more, 50% or more, 55% or more, 60% or more, or 65% or more, and may be 85% or less, 80% or less, or 75% or less.

[0036] The method for producing the polyimide porous film 1a is not limited to a specific method. The polyimide porous film 1a can be produced, for example, by a phase separation method. Examples of phase separation methods include non-solvent-induced phase separation (NIPS) and drying-induced phase separation (DIPS). In the NIPS method, a porous film is obtained by inducing phase separation in a coating of a polymer solution or a polymer precursor solution by incorporating a non-solvent such as water. In the DIPS method, a porous film is obtained by inducing phase separation in a coating of a polymer solution or a polymer precursor solution by solvent evaporation.

[0037] An example of a method for producing a polyimide porous membrane 1a will be described. A soluble polyimide solution is applied to a predetermined substrate to form a coating. An example of a soluble polyimide is KPI-MX3000F provided by Kawamura Sangyo Co., Ltd. An example of a solvent in the soluble polyimide solution is a polar solvent with a relatively high boiling point (e.g., 140°C or higher), such as 1-methyl-2-pyrrolidone. An example of a substrate is a porous material containing a fluororesin. The substrate is preferably a predetermined polytetrafluoroethylene (PTFE) porous membrane. For example, the air permeability of a PTFE porous membrane measured according to the Air Permeability Measurement Method B (Gurley method) specified in JIS L1096:2010 is 30 seconds / 100 cm. 3 ~50 seconds / 100cm 3 The PTFE porous membrane has a thickness of, for example, 50 to 100 μm. The average pore size on the surface of the PTFE porous membrane is, for example, 100 to 200 nm. In the production of the polyimide porous membrane 1a, the second porous layer 12 can be formed in contact with the contact surface of the coating of the soluble polyimide solution with the substrate. By using such a substrate, the second porous layer 12 is likely to be formed so as to satisfy at least one condition selected from the group consisting of condition (I) and condition (II).

[0038] Next, the coating film of the soluble polyimide solution is subjected to a humidification treatment under predetermined conditions. This promotes phase separation in the coating film of the soluble polyimide solution. In this humidification treatment, the coating film of the soluble polyimide solution is placed in an environment, for example, at a temperature of 5 to 30°C and a relative humidity of 60 to 80%, for a predetermined period of time. The predetermined period is, for example, 1 to 30 minutes.

[0039] Next, a solvent extraction treatment is carried out. For example, the coating film of the soluble polyimide solution is immersed in water at 20°C to 40°C. The immersion time is, for example, 1 minute to 30 minutes. This extracts the solvent, such as 1-methyl-2-pyrrolidone.

[0040] Next, the coating film after the solvent extraction treatment is subjected to a drying treatment. The film obtained after the drying treatment is peeled off from the substrate to obtain a polyimide porous film 1a. Alternatively, by sandwiching a coating film of a soluble polyimide solution between a pair of substrates and applying the above-mentioned humidification treatment, solvent extraction treatment, and drying treatment, a polyimide porous film 1a in which second porous layers are arranged on both sides of a first porous layer can also be obtained.

[0041] As shown in Fig. 3, for example, a porous composite 3 including a polyimide porous film 1a can be provided. The porous composite 3 includes a polyimide porous film 1a and a porous substrate 2. In the porous composite 3, the polyimide porous film 1a is disposed in at least a portion of the voids 2v of the porous substrate 2.

[0042] The polyimide porous membrane 1a may be disposed in a part of the voids 2v of the porous substrate 2, or may be disposed in the entire voids 2v of the porous substrate 2. For example, one surface of the first porous layer 11 in the thickness direction is in contact with at least a part of the porous substrate 2, and the second porous layer 12 is disposed on the other surface of the first porous layer 11 in the thickness direction.

[0043] When the polyimide porous membrane 1a is disposed in a portion of the voids 2v of the porous substrate 2, the second porous layer 12 has a surface exposed from the voids 2v of the polyimide porous membrane 1a. In this case, the second porous layer 12 can be removed by etching to calculate the average pore size. When the polyimide porous membrane 1a is disposed in the entire voids 2v of the porous substrate 2, the surface of the polyimide layer having a small pore size on the surface can be included in the second porous layer, and the surface of the polyimide layer having a large pore size on the surface can be included in the first porous layer. In this case, one surface can be etched to remove the second porous layer 12, and the average pore size and porosity can be calculated.

[0044] The porous substrate 2 is not limited to a specific porous substrate. The porous substrate 2 is, for example, a nonwoven fabric. Examples of nonwoven fabrics include nonwoven fabrics whose main component is a resin such as polyethylene terephthalate (PET), liquid crystal polyester (LCP), polyimide (PI), polyphenylene sulfide (PPS), polyacrylonitrile, and polyamide. Other examples of nonwoven fabrics include glass fibers and other silica fibers. The fiber diameter of the fibers contained in the nonwoven fabric is not limited to a specific value and is, for example, about 50 nm to about 3000 nm. From the viewpoint of heat resistance, the nonwoven fabric preferably contains fibers such as PI, PPS, LCP, polyamide, and glass.

[0045] The thickness and basis weight of the porous substrate 2 are not limited to specific values. The thickness of the porous substrate 2 is, for example, 1 μm to 200 μm. The basis weight of the porous substrate 2 is, for example, 2 to 200 g / m 2 When the thickness of the porous substrate 2 is 1 μm or more, the porous substrate 2 can be easily handled when producing the porous composite 3. When the thickness of the porous substrate 2 is 200 μm or less, the porous composite 3 can easily have the desired breathability. When the basis weight of the porous substrate 2 is 2 g / m 2 When the porous substrate 2 has a basis weight of 200 g / m or more, the porous substrate 2 can be easily handled when producing the porous composite 3. 2 When the above condition is satisfied, the porous composite 3 is likely to have the desired air permeability.

[0046] An example of a method for producing the porous composite 3 will be described. A soluble polyimide solution is applied to a predetermined nonwoven fabric to form a coating film. An example of the soluble polyimide used is the soluble polyimide varnish Neoprim (registered trademark) S100 provided by Mitsubishi Gas Chemical Company, Inc. Neoprim S100 is a solution in which a soluble polyimide is dissolved in γ-butyllactone and N,N-dimethylacetamide. An example of a solvent for adjusting the concentration of the soluble polyimide varnish is a polar solvent with a relatively high boiling point (e.g., a boiling point of 140°C or higher), such as γ-butyllactone. It is also possible to use an additive such as polyethylene glycol, which is soluble in the solution but has low solubility in polyimide. The nonwoven fabric used may be, for example, a PPS nonwoven fabric (HC#100, thickness 100 μm, basis weight 86 g / m) provided by Toray Industries, Inc. 2 ) are listed. Next, a solvent extraction treatment is carried out. For example, the nonwoven fabric coated with the polyimide solution is immersed in a water bath at 20°C to 40°C to carry out the solvent extraction treatment. The immersion time is, for example, 1 minute to 30 minutes. This allows the polar solvent to be extracted. Next, the coating film after the solvent extraction treatment is dried to produce a porous composite containing a polyimide porous film in at least some of the pores of the nonwoven fabric.

[0047] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0048] Example 1: 10 g of soluble polyimide (KPI-MX300F) manufactured by Kawamura Sangyo Co., Ltd. was dissolved in 70 g of 1-methyl-2-pyrrolidone manufactured by Tokyo Chemical Industry Co., Ltd. to obtain a solution. 20 g of polyethylene glycol 200 manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. was then added to the solution and stirred uniformly. In this way, a PI solution according to Example 1 was obtained.

[0049] The PI solution according to Example 1 was applied to a substrate, a PTFE porous membrane TEMISH (registered trademark) manufactured by Nitto Denko Corporation, with an applicator to a thickness of 50 μm to obtain a coating film. The average pore diameter on the surface of the substrate was about 120 nm. This average pore diameter was calculated in the same manner as the average pore diameter on the surface of the second porous layer (skin layer) of the polyimide membrane described below. The air permeability of this PTFE porous membrane, measured according to the air permeability measurement method B (Gurley method) specified in JIS L1096:2010, was 40 sec / 100 cm 3 The thickness of the PTFE porous film was 70 μm.

[0050] Next, the coating film was left to stand for 10 minutes inside a thermo-hygrostat adjusted to a temperature of 10°C and a relative humidity of 70% to promote phase separation. Next, the coating film was immersed in a 30°C water bath for 10 minutes to perform solvent extraction. Next, the coating film was fixed to a square member having a side length of 10 cm in a plan view and dried at 80°C for 10 minutes. Thereafter, the coating film was dried at 250°C for 2 minutes. This resulted in a polyimide porous film on the substrate. Next, the polyimide porous film was peeled off from the substrate to obtain the polyimide porous film according to Example 1.

[0051] Example 2 A polyimide porous film according to Example 2 was obtained in the same manner as in Example 1, except that the thickness of the coating film in the coating of the PI solution according to Example 1 was adjusted to 25 μm.

[0052] Example 3 7.5 g of γ-butyl lactone manufactured by Tokyo Chemical Industry Co., Ltd. was added to 15 g of soluble polyimide varnish (Neoprim S100) manufactured by Mitsubishi Gas Chemical Company, Inc. and mixed uniformly. Then, 7.5 g of polyethylene glycol PEG400 manufactured by Tokyo Chemical Industry Co., Ltd. was added and mixed uniformly. In this manner, a PI solution according to Example 3 was obtained. The PI solution according to Example 3 was applied to a substrate obtained by hydrophilizing the surface of the PTFE porous membrane used in Example 1 by corona treatment using an applicator to a thickness of 50 μm, thereby obtaining a coating film.

[0053] After coating, the substrate was immersed in a water bath at 20° C. for 30 minutes to cause porosity due to phase separation and solvent extraction.

[0054] Next, the coating film was fixed to a square member having a side length of 10 cm in plan view and dried at 80°C for 10 minutes. Thereafter, the coating film was dried at 250°C for 2 minutes. As a result, a polyimide porous film was obtained on the substrate. Next, the polyimide porous film was peeled off from the substrate to obtain the polyimide porous film according to Example 3. The obtained polyimide porous film had a second porous layer (skin layer) formed on the gas phase side and a first porous layer formed on the substrate side.

[0055] Comparative Example 1 The PI solution of Example 1 was applied to a substrate, a polyethylene terephthalate (PET) film Lumirror (registered trademark) manufactured by Toray Industries, Inc., to a thickness of 50 μm using an applicator to obtain a coating film. The thickness of the PET film was 100 μm.

[0056] Next, the coating film was placed in a thermo-hygrostat adjusted to a temperature of 10°C and a relative humidity of 70% for 10 minutes to promote phase separation, yielding a polyimide (PI) film on the substrate. The resulting PI film was peeled off from the substrate. The peeled PI film was immersed in a 30°C water bath for 10 minutes to perform solvent extraction. Next, the PI film after solvent extraction was dried at 80°C for 10 minutes. Thereafter, the PI film was dried at 250°C for 2 minutes. This resulted in a polyimide porous film according to Comparative Example 1.

[0057] Comparative Example 2 The PI solution of Example 1 was applied to a substrate, which was an electrolytic copper foil having a thickness of 0.018 mm, using an applicator to a thickness of 50 μm to obtain a coating film.

[0058] Next, the coating film was placed in a thermo-hygrostat adjusted to a temperature of 10°C and a relative humidity of 70% for 10 minutes to promote phase separation. The coating film was then immersed in a 30°C water bath for 10 minutes to perform solvent extraction, resulting in a PI film on the substrate. The resulting PI film was peeled off from the substrate. The PI film peeled off from the substrate was then dried at 80°C for 10 minutes. The PI film was then dried at 250°C for 2 minutes. This resulted in a polyimide porous film according to Comparative Example 2.

[0059] <Comparative Example 3> A PTFE porous membrane according to Comparative Example 3 was obtained in the same manner as in Example 1, except that a PTFE porous membrane having the following air permeability, average pore diameter on the surface of the substrate, and thickness was used as the substrate, and the thickness of the coating film in the coating of the PI solution according to Example 1 was adjusted to 127 μm. Air permeability: 40 sec / 100 cm 3 Average pore diameter on the surface of the substrate: 50 nm Thickness: 30 μm

[0060] Comparative Example 4: 2.43 g of 4,4'-diaminodiphenyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) and 30 g of N,N-dimethylacetamide (manufactured by Tokyo Chemical Industry Co., Ltd.) were placed in a 50 ml glass vial and dissolved. Then, 3.47 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (manufactured by Tokyo Chemical Industry Co., Ltd.) was gradually added and dissolved, and the mixture was stirred at room temperature for 48 hours to polymerize, yielding a polyimide precursor solution. The solids content of the polyimide precursor solution was 16.7% by mass. 10 g of the polyimide precursor solution was diluted with 8.35 g of N,N-dimethylacetamide, and 0.117 g (0.2 molar equivalents) of 1,2-dimethylimidazole was added. 20.2 g of triethylene glycol dimethyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) was added in small portions and allowed to react with stirring until homogeneous. This yielded a coating solution according to Comparative Example 4, containing a good solvent and a non-solvent. On the surface of a PET film manufactured by Mitsubishi Chemical Corporation, which had a surface hydrophilized by corona treatment, the coating solution according to Comparative Example 4 was applied using an applicator to a coating thickness of 250 μm to obtain a coating film. This coating film was heated at 80 ° C for 30 minutes, and a porous film with a white appearance was obtained. The porous film was peeled off from the PET film, and fixed to a square member having a side length of 10 cm in plan view. In this state, the temperature around the porous film was increased from 130 ° C to 320 ° C at a rate of 10 ° C / min, and the temperature around the porous film was maintained at 320 ° C for 10 minutes. Thereafter, a polyimide porous film according to Comparative Example 4 was obtained by natural cooling.

[0061] (Air Permeability) The air permeability of the polyimide porous membranes according to each Example and Comparative Example was measured according to the air permeability measurement method B (Gurley method) specified in JIS L1096: 2010. The results are shown in Table 1.

[0062] The second porous layer (skin layer) of each polyimide porous membrane was removed (etched) to prepare a sample from which the second porous layer (skin layer) had been removed. A portion of approximately 1 μm in thickness was removed from the surface of each polyimide porous membrane, including the second porous layer (skin layer). This portion was removed by dry etching including vacuum plasma treatment. The air permeability of the sample from which the second porous layer (skin layer) had been removed was measured according to the air permeability measurement method B (Gurley method) specified in JIS L1096:2010. The results are shown in Table 1.

[0063] (Water Pressure Resistance) The water pressure resistance of the polyimide porous membranes according to each Example and Comparative Example was measured according to the water resistance test method B (high water pressure method) specified in JIS L1092:2009. The shape of the water passage hole in the double-sided adhesive tape used to fix the polyimide porous membrane to the measuring jig was adjusted to a circle with a diameter of 1.6 mm when viewed from a direction perpendicular to the main surface of the double-sided adhesive tape. The results are shown in Table 1.

[0064] (Surface Observation) Platinum vapor-deposited films were formed on both sides of the polyimide porous membrane according to each Example and Comparative Example to prepare samples for surface observation of each polyimide porous membrane. A second porous layer was disposed on one surface of the polyimide porous membrane, originating from the surface that contacted the substrate during the production of the polyimide porous membrane, and the other surface was formed by the first porous layer. SEM observation of these samples was performed using a scanning electron microscope (SEM) SU8220 manufactured by Hitachi High-Tech Corporation. The image data of the obtained SEM photographs was subjected to binarization processing to identify holes, and the pore diameter (maximum diameter) of each hole was measured to create a number-based pore size distribution on both sides of the polyimide porous membrane. Figures 4A, 5A, 6A, 7A, 8A, and 9A are binarized images of SEM photographs of the surface of the second porous layer according to Example 1, Example 2, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4, respectively. 4B, 5B, 6B, 7B, 8B, and 9B are SEM photographs or binarized images thereof of the first porous layer surfaces according to Example 1, Example 2, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4, respectively. Image processing software ImageJ was used for image processing of the SEM photographs. Table 1 shows the results relating to the number-based pore size distribution on the second porous layer surface and the first porous layer surface of each polyimide porous membrane.

[0065] (Cross-Section Observation) A cross section along the thickness direction of each polyimide porous membrane was exposed using an ion milling device IM4000 manufactured by Hitachi High-Tech Corporation, and a platinum vapor-deposited film was formed on the exposed cross section to prepare a sample for cross-sectional observation of each polyimide porous membrane. SEM observation of these samples was performed using a scanning electron microscope (SEM) SU8220 manufactured by Hitachi High-Tech Corporation. FIG. 4C is an SEM photograph of a cross section along the thickness direction of the polyimide porous membrane according to Example 1. FIG. 4D is an SEM photograph enlarging a portion (enclosed by the white dashed line) of the SEM photograph of FIG. 4C. The thickness of each polyimide porous membrane and the thickness of the second porous layer were determined from the obtained SEM photograph. The thickness of the second porous layer was measured as the distance from the surface to the pores observed at a magnification of 20,000 times by SEM observation. The results are shown in Table 1.

[0066] (Porosity) The porosity of the polyimide porous membranes according to each Example and Comparative Example was calculated using the following formula (1). The results are shown in Table 1. In formula (1), W is the mass of the polyimide porous membrane, D is the density of the solid part of the polyimide porous membrane, S is the area of ​​the polyimide porous membrane in a plan view, and T is the thickness of the polyimide porous membrane. Porosity (volume %)=100-100(W / D) / (S×T) Formula (1)

[0067] As shown in Table 1, the polyimide porous membranes according to the respective examples had a thermal conductivity of 30 seconds / 100 cm 3 On the other hand, the polyimide porous films according to Comparative Examples 1 and 2 had an air permeability of 30 sec / 100 cm or less and a water pressure resistance of 0.15 MPa or more. 3 The water pressure resistance of the polyimide porous membranes according to Comparative Examples 3 and 4 was 0.10 MPa or less, and it was difficult to say that these polyimide porous membranes had the desired breathability. Comparison between the Examples and the Comparative Examples revealed that a polyimide porous membrane having a structure including a first porous layer having a large average pore size and a second porous layer having a small average pore size and satisfying at least one condition selected from the group consisting of (I) and (II) above is advantageous from the viewpoint of achieving both breathability and water resistance.

[0068]

[0069] A first aspect of the present invention provides a polyimide porous film comprising: a first porous layer; and a second porous layer disposed on one or both sides of the first porous layer, wherein the average pore size at the surface of the second porous layer is smaller than the average pore size in the first porous layer, and at least one condition selected from the group consisting of the following conditions (I) and (II) is satisfied: (I) The polyimide porous film has an air permeability of 30 sec / 100 cm as measured according to the Air Permeability Measurement Method B (Gurley method) specified in Japanese Industrial Standards (JIS) L1096:2010. 3and the air permeability of a sample obtained by removing the second porous layer from the polyimide porous membrane, measured according to the Air Permeability Measurement Method B (Gurley method), is 2 seconds / 100 cm or less than the air permeability of the polyimide porous membrane. 3 (II) The proportion of pores with diameters of 0.2 μm or less in the number-based pore diameter distribution on the surface of the second porous layer is 80% or more, and the opening ratio on the surface of the second porous layer is 1 to 60%.

[0070] A second aspect of the present invention provides the polyimide porous film according to the first aspect, wherein the polyimide porous film has a thickness of 1 to 30 μm.

[0071] A third aspect of the present invention provides the polyimide porous film according to the first or second aspect, wherein the second porous layer has a thickness of 1 μm or less.

[0072] A fourth aspect of the present invention provides the polyimide porous film according to any one of the first to third aspects, wherein the average pore size on the surface of the second porous layer is 0.05 to 1 μm.

[0073] A fifth aspect of the present invention provides the polyimide porous film according to any one of the first to fourth aspects, wherein the maximum pore size on the surface of the second porous layer is 5 μm or less.

[0074] A sixth aspect of the present invention provides the polyimide porous film according to any one of the first to fifth aspects, wherein an average pore size on the surface of the first porous layer or on the surface of the sample is 0.1 to 10 μm.

[0075] A seventh aspect of the present invention provides the polyimide porous film according to any one of the first to sixth aspects, wherein the polyimide porous film has a porosity of 30 to 90%.

[0076] An eighth aspect of the present invention provides the polyimide porous membrane according to any one of the first to seventh aspects, wherein the polyimide porous membrane has a water pressure resistance of 0.15 MPa or more, measured in accordance with water resistance test method B (high water pressure method) specified in JIS L1092:2009.

[0077] A ninth aspect of the present invention provides a porous composite comprising: a polyimide porous film according to any one of the first to eighth aspects; and a porous substrate, wherein the polyimide porous film is disposed in at least a part of pores of the porous substrate.

[0078] A tenth aspect of the present invention provides the porous composite of the ninth aspect, wherein one surface of the first porous layer in the thickness direction is in contact with at least a part of the porous substrate, and the second porous layer is disposed on the other surface of the first porous layer in the thickness direction.

[0079] An eleventh aspect of the present invention provides the porous composite of the ninth or tenth aspect, wherein the porous substrate is a nonwoven fabric.

Claims

1. A polyimide porous film comprising: a first porous layer; and a second porous layer disposed on one or both sides of the first porous layer, wherein the average pore size at the surface of the second porous layer is smaller than the average pore size in the first porous layer, and wherein at least one condition selected from the group consisting of the following conditions (I) and (II) is satisfied: (I) The polyimide porous film has an air permeability of 30 sec / 100 cm as measured according to the Air Permeability Measurement Method B (Gurley method) specified in Japanese Industrial Standards (JIS) L1096:2010. 3 and the air permeability of a sample obtained by removing the second porous layer from the polyimide porous membrane, measured according to the Air Permeability Measurement Method B (Gurley method), is 2 seconds / 100 cm or less than the air permeability of the polyimide porous membrane. 3 (II) The proportion of pores with diameters of 0.2 μm or less in the number-based pore diameter distribution on the surface of the second porous layer is 80% or more, and the opening ratio on the surface of the second porous layer is 1 to 60%.

2. The polyimide porous film according to claim 1, wherein the polyimide porous film has a thickness of 1 to 30 μm.

3. The polyimide porous film according to claim 1, wherein the second porous layer has a thickness of 1 μm or less.

4. The polyimide porous film according to claim 1, wherein the average pore size on the surface of the second porous layer is 0.05 to 1 μm.

5. The polyimide porous film according to claim 1, wherein the maximum pore size on the surface of the second porous layer is 5 μm or less.

6. The polyimide porous film according to claim 1, wherein the average pore size on the surface of the first porous layer or on the surface of the sample is 0.1 to 10 μm.

7. The polyimide porous film according to claim 1, wherein the polyimide porous film has a porosity of 30 to 90%.

8. The polyimide porous film according to claim 1, wherein the water pressure resistance of the polyimide porous film is 0.15 MPa or more, as measured in accordance with the water resistance test method B (high water pressure method) specified in JIS L1092:2009.

9. A porous composite comprising the polyimide porous film according to claim 1 and a porous substrate, wherein the polyimide porous film is disposed in at least some of the pores of the porous substrate.

10. The porous composite according to claim 9, wherein one surface of the first porous layer in the thickness direction is in contact with at least a portion of the porous substrate, and the second porous layer is disposed on the other surface of the first porous layer in the thickness direction.

11. The porous composite according to claim 9 or 10, wherein the porous substrate is a nonwoven fabric.

Images