Polyimide porous membrane and porous composite

Abstract

The polyimide porous film (1a) of the present application has a first porous layer (11) and a second porous layer (12). The second porous layer (12) is disposed on one or both sides of the first porous layer (11). The average pore diameter D 11 of the surface of the second porous layer (12) is smaller than the average pore diameter D 12 of the first porous layer (11). The polyimide porous film satisfies at least one of the following conditions (I) and (II). (I) The air permeability P1 of the polyimide porous film (1a) is 30 seconds / 100 cm 3 or less. (II) The proportion R of the pore diameter of 0.2 μm or less in the number-based pore size distribution of the surface of the second porous layer (12) is 80% or more, and the opening ratio R 3 of the surface of the second porous layer (12) is 1 to 60%. P O ​​

Description

Technical Field

[0001] This invention relates to polyimide porous membranes and porous composites. Background Technology

[0002] Previously, polyimide porous membranes were known.

[0003] For example, Patent Document 1 describes a three-layer porous polyimide membrane. This porous polyimide membrane has two surface layers (a) and (b), and a macroporous layer sandwiched between these surface layers (a) and (b). The macroporous layer has spacer walls bonded to the surface layers (a) and (b), and a plurality of macropores with an average pore size of 10 to 500 μm in the membrane plane direction, surrounded by the spacer walls and the surface layers (a) and (b). The thickness of the spacer walls of the macroporous layer is 0.1 to 50 μm, and at least one spacer wall has one or more pores that allow adjacent macropores to communicate with each other. 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 fine pores with an average pore size greater than 5 μm and less than 200 μm, and the other surface layer has a plurality of fine pores with an average pore size of 0.01 to 200 μm. The surface aperture ratio of at least one surface layer is 10% or more, and the surface aperture ratio of the other surface layer is 5% or more. The micropores and macropores of the surface layers (a) and (b) are interconnected. The total thickness of the porous polyimide membrane is 5~500 μm, the Gare de laitance of the porous polyimide membrane is less than 20 seconds / 100 cc, and the porosity of the porous polyimide membrane is 60~95%.

[0004] Patent Document 2 describes a polyimide porous membrane manufactured using a given reaction product as a raw material. Both surfaces of this polyimide porous membrane are substantially open. The polyimide porous membrane has a Gryllium value of 2000 seconds / 100cc or less and a porosity of 40% or more.

[0005] Patent document 3 describes a porous polyimide membrane. The porous polyimide membrane has a porosity of 45% by volume or more and 95% by volume or less. The average pore size of the porous polyimide membrane is 10 nm or more and 1000 nm or less.

[0006] Patent document 4 describes a method for increasing the air permeability of a polyimide porous body using a given method. The method for increasing the air permeability of a polyimide porous body is characterized by performing a dry etching process on the polyimide porous body obtained using a polyimide solution or a polyamic acid solution and by solvent displacement-induced phase separation or thermally induced phase separation.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. 2018 / 021356

[0010] Patent Document 2: Japanese Patent Application Publication No. 2015-71755

[0011] Patent Document 3: International Publication No. 2016 / 104309

[0012] Patent Document 4: Japanese Patent Application Publication No. 2011-1434 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] The technologies described in Patent Documents 1-4, from the viewpoint of balancing breathability and water resistance, still have room for further exploration. Therefore, the present invention provides a polyimide porous membrane that is advantageous from the viewpoint of balancing breathability and water resistance.

[0015] Problem Solving Methods

[0016] The present invention provides a polyimide porous membrane having 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 of the surface of the second porous layer is smaller than the average pore size of the first porous layer, and the polyimide porous membrane satisfies at least one of the conditions selected from (I) and (II) below.

[0017] (I) The air permeability of the polyimide porous membrane, as measured by Method B (Gree method) of air permeability test as specified in Japanese Industrial Standard (JIS) L1096:2010, is 30 seconds / 100cm. 3 Furthermore, the air permeability of the sample obtained by removing the second porous layer from the polyimide porous membrane, as measured by the air permeability test method B (Gree method), is 2 seconds / 100cm lower than the air permeability of the polyimide porous membrane. 3 above.

[0018] (II) The proportion of pores with a diameter of less than 0.2 μm in the pore size distribution of the surface of the second porous layer is more than 80%, and the surface opening ratio of the second porous layer is 1 to 60%.

[0019] The effects of the invention

[0020] The aforementioned polyimide porous membrane is advantageous from the perspective of balancing air permeability and water resistance. Attached Figure Description

[0021] [ Figure 1 ] Figure 1 This is a cross-sectional view schematically illustrating an example of the polyimide porous membrane of the present invention.

[0022] [ Figure 2 ] Figure 2 This is a cross-sectional view schematically illustrating another example of the polyimide porous membrane of the present invention.

[0023] [ Figure 3 ] Figure 3 This is a cross-sectional view schematically illustrating an example of the porous composite of the present invention.

[0024] [ Figure 4A ] Figure 4A This is a binarized image of a scanning electron microscope (SEM) photograph of the surface of the second porous layer of the polyimide porous membrane in Example 1.

[0025] [ Figure 4B ] Figure 4B This is a binarized image of the surface of the first porous layer of the polyimide porous membrane in Example 1, obtained by SEM imaging.

[0026] [ Figure 4C ] Figure 4C This is an SEM image of a cross-section along the thickness direction of the polyimide porous membrane of Example 1.

[0027] [ Figure 4D ] Figure 4D It is Figure 4C A magnified version of a portion of the SEM image.

[0028] [ Figure 5A ] Figure 5A This is a binarized image of the surface of the second porous layer of the polyimide porous membrane in Example 2, obtained by SEM imaging.

[0029] [ Figure 5B ] Figure 5B This is a binarized image of the surface of the first porous layer of the polyimide porous membrane in Example 2, obtained by SEM imaging.

[0030] [ Figure 6A ] Figure 6A This is a binarized image of the surface of the second porous layer of the polyimide porous membrane in Comparative Example 1, obtained by SEM imaging.

[0031] [ Figure 6B ] Figure 6B This is a binarized image of the surface of the first porous layer of the polyimide porous membrane in Comparative Example 1, obtained by SEM imaging.

[0032] [ Figure 7A ] Figure 7A This is a binarized image of the surface of the second porous layer of the polyimide porous membrane in Comparative Example 2, obtained by SEM imaging.

[0033] [ Figure 7B] Figure 7B This is a binarized image of the surface of the first porous layer of the polyimide porous membrane in Comparative Example 2, obtained by SEM imaging.

[0034] [ Figure 8A ] Figure 8A This is a binarized image of the surface of the second porous layer of the polyimide porous membrane in Comparative Example 3, obtained by SEM imaging.

[0035] [ Figure 8B ] Figure 8B This is a SEM image of the surface of the first porous layer of the polyimide porous membrane in Comparative Example 3.

[0036] [ Figure 9A ] Figure 9A This is a binarized SEM image of the surface of the second porous layer of the polyimide porous membrane in Comparative Example 4.

[0037] [ Figure 9B ] Figure 9B This is a SEM image of the surface of the first porous layer of the polyimide porous membrane in Comparative Example 4. Detailed Implementation

[0038] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is not limited to the following embodiments.

[0039] Figure 1 This is a cross-sectional view schematically illustrating an example of the polyimide porous membrane of the present invention. Figure 2 This is a cross-sectional view schematically illustrating another example of the polyimide porous membrane of the present invention. Figure 1 and Figure 2 As shown, the polyimide porous membrane 1a comprises a first porous layer 11 and a second porous layer 12. Figure 1 As shown, the second porous layer 12 is disposed, for example, on one side of the first porous layer 11. Figure 2 As shown, the second porous layer 12 can also be disposed on both sides of the first porous layer 11, for example. The average pore size D of the surface of the second porous layer 12 is... 12 The average pore size D of the first porous layer 11 is smaller than that of the first porous layer 11. 11 In this specification, the average pore size is the number-average pore size.

[0040] The polyimide porous membrane 1a satisfies at least one of the conditions selected from (I) and (II) below.

[0041] (I) The air permeability P1 of the polyimide porous membrane 1a, determined according to the air permeability test method B (Gree method) specified in JIS L1096:2010, is 30 seconds / 100cm. 3Furthermore, according to the air permeability test method B (Greley method), the air permeability P2 of the sample obtained by removing the second porous layer 12 from the polyimide porous membrane 1a is 2 seconds / 100cm lower than the air permeability P1 of the polyimide porous membrane 1a. 3 above.

[0042] (II) The proportion R of pores smaller than 0.2 μm in the pore size distribution of the surface of the second porous layer 12 P The aperture ratio R of the surface of the second porous layer 12 is over 80%, and the aperture ratio R of the surface of the second porous layer 12 is also over 80%. O The range is 1% to 60%.

[0043] In polymer porous membranes, air permeability and water resistance are inversely related: increasing air permeability tends to decrease water resistance, and vice versa. The air permeability and water resistance of polymer porous membranes are closely related to their porosity and pore size. Therefore, it is considered important to adjust the porosity and pore size of the polymer porous membrane to achieve both desired air permeability and water resistance. However, the inventors have found that, for polyimide porous membranes, it is sometimes difficult to achieve both desired air permeability and water resistance by adjusting the porosity and pore size. In particular, it is known that the higher hydrophilicity of polyimide is not advantageous in improving water resistance.

[0044] Based on this, the inventors considered that it might be possible to achieve both desired air permeability and desired water resistance by including a first porous layer with a larger average pore size and a second porous layer with a smaller average pore size in the polyimide porous membrane. Through repeated experiments and explorations, the inventors ultimately discovered that by including a second porous layer in the polyimide porous membrane that meets given conditions, it is easy to achieve both desired air permeability and desired water resistance.

[0045] By ensuring that the polyimide porous membrane 1a meets at least one of the conditions selected from (I) and (II), both the desired air permeability and the desired water resistance can be easily achieved. The polyimide porous membrane 1a may meet only the condition of (I), only the condition of (II), or both the conditions of (I) and (II). The water resistance pressure R1 of the polyimide porous membrane 1a is, for example, 0.15 MPa or higher. The water resistance pressure R1 may be determined, 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 in condition (I) may be a sample obtained by removing only one side of the second porous layer 12 disposed on one side of the pair of second porous layers 12 disposed on both sides of the first porous layer 11, or a sample obtained by removing both of the pair of second porous layers 12.

[0046] In the preparation of the sample under conditions (I), the method for removing the second porous layer 12 from the polyimide porous membrane 1a is not limited to a specific method. This method is, for example, dry etching. In this case, it is easy to improve the etching selectivity for the second porous layer 12 and to easily suppress damage or destruction of the first porous layer 11. Examples of dry etching include etching 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 can be vacuum plasma treatment or atmospheric pressure plasma treatment.

[0047] The water pressure resistance R1 can be above 0.20 MPa or above 0.25 MPa. For example, the water pressure resistance R1 can be below 0.50 MPa.

[0048] Regarding condition (I), the air permeability P1 can be, for example, 25 seconds / 100cm. 3 Below, 20 seconds / 100cm 3 Below, 19 seconds / 100cm 3 Below, 18 seconds / 100cm 3 Below or 17 seconds / 100cm 3 Below. Air permeability P1, for example, greater than 2 seconds / 100cm. 3 Air permeability P2, for example, is 0.01 seconds / 100cm. 3 The above can also be expressed as 0.05 seconds per 100cm. 3 or above 0.1 seconds / 100cm 3 That's all. For example, breathability P2 is 28 seconds / 100cm. 3 The following can also be 25 seconds / 100cm. 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 Below or 1.0 seconds / 100cm 3 the following.

[0049] As mentioned above, in polymer porous membranes, air permeability and water resistance are inversely related. Under conditions (I), air permeability P2 is 2 seconds / 100cm lower than air permeability P1. 3This demonstrates that the second porous layer 12 plays a crucial role in achieving the desired water resistance. Under conditions (I), the sample thickness need only be equal to the thickness of the second porous layer 12 removed, for example, 80% to 99% of the thickness of the polyimide porous membrane 1a.

[0050] Breathability P2 can be 6 seconds / 100cm lower than breathability P1. 3 Above, 8 seconds / 100cm 3 or more than 10 seconds / 100cm 3 That's all. For example, breathability P2 is 15 seconds / 100cm lower than breathability P1. 3 the following.

[0051] Under the condition (II) is met, the air permeability P1 is, for example, 30 seconds / 100cm. 3 The following can also be 25 seconds / 100cm. 3 Below, 20 seconds / 100cm 3 Below, 19 seconds / 100cm 3 Below, 18 seconds / 100cm 3 Below or 17 seconds / 100cm 3 Below. Under the condition of (II), the air permeability P1 is, for example, 1 second / 100cm. 3 The above can also be expressed as 2 seconds per 100cm. 3 Above, 5 seconds / 100cm 3 or more than 10 seconds / 100cm 3 above.

[0052] Regarding condition (II), proportion R P It can be above 82% or 85%. Ratio R P For example, it could be below 99%, or below 98% or 95%. Opening ratio R O It can be above 2%, above 3%, or above 4%, or below 50%, below 40%, below 30%, below 20%, or below 15%.

[0053] The thickness of the polyimide porous membrane 1a is not limited to a specific value. Its thickness is, for example, 1 to 30 μm. In this case, the polyimide porous membrane 1a is easy to handle and can be used in a variety of applications requiring the desired air permeability and water resistance.

[0054] The thickness of the polyimide porous membrane 1a can be greater than 2μm, greater than 3μm, greater than 4μm or greater than 5μm, and can be less than 25μm, less than 20μm, less than 18μm or less than 15μm.

[0055] The thickness of the second porous layer 12 is not limited to a specific value. For example, its thickness is less than 1 μm. As a result, the air permeability P1 of the polyimide porous membrane 1a can easily become lower within a desired range.

[0056] The thickness of the second porous layer 12 is, for example, 0.01 μm or more. This easily improves the water resistance of the polyimide porous membrane 1a. The thickness of the second porous layer 12 can be less than 0.9 μm, or it can be 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.

[0057] The average pore size D of the surface of the second porous layer 12 12 Not limited to a specific value. Average pore size D 12 For example, the pore size is 0.05~1μm. In this case, the polyimide porous membrane 1a more easily achieves a balance between the desired air permeability and the desired water resistance. Average pore size D 12 For example, calculations can be performed based on SEM images of the surface of the second porous layer 12 and according to the methods described in the embodiments.

[0058] Average aperture D 12 It can be above 0.06μm or above 0.08μm, or below 0.5μm, 0.4μm, 0.3μm or 0.2μm.

[0059] For the polyimide porous membrane 1a, as long as at least one of the conditions selected from (I) and (II) is satisfied, the maximum pore size D of the surface of the second porous layer 12 is [value missing]. max It is not limited to a specific value. Maximum aperture D max For example, below 5 μm. In this case, the water resistance of the polyimide porous membrane 1a is easily improved. Maximum pore size D max Preferably, it is 4μm or less, more preferably 3μm or less, further preferably 2μm or less, and especially preferably 1μm or less.

[0060] The average pore size D of the surface of the first porous layer 11 or the surface of the sample obtained by removing the second porous layer 12 from the polyimide porous membrane 1a is D. 11 Not limited to a specific value. In the above-described sample obtained by removing the second porous layer 12 from the polyimide porous membrane 1a, for example, the first porous layer 11 is formed as the surface of the sample. Average pore size D 11 For example, the pore size is 0.1~10 μm. In this case, the polyimide porous membrane 1a more easily achieves a balance between the desired air permeability and the desired water resistance. Average pore size D 11 For example, calculations can be performed based on SEM images of the first porous layer 11 or the surface of the sample described above, and according to the methods described in the embodiments.

[0061] Average aperture D 11 It can be above 0.2μm or above 0.3μm, or below 8μm, below 5μm, below 4μm, below 3μm, below 2μm or below 1μm.

[0062] The porosity p1 of the polyimide porous membrane 1a is not limited to a specific value. For example, the porosity p1 is 40-90%. In this case, the polyimide porous membrane 1a more easily achieves a balance between desired air permeability and desired water resistance. The porosity p1 can be calculated, for example, according to the method described in the examples. As will be stated later, when the polyimide porous membrane 1a together with a porous substrate such as a nonwoven fabric constitutes a porous composite, the porosity p1 can also be calculated based on the area ratio of the cross-section.

[0063] Porosity p1 can be above 40%, above 45%, above 50%, above 55%, above 60%, or above 65%, or below 85%, below 80%, or below 75%.

[0064] The method for manufacturing the polyimide porous membrane 1a is not limited to a specific method. The polyimide porous membrane 1a can be manufactured, 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 membrane is obtained by inducing phase separation through the introduction of a non-solvent such as water into a coating of a polymer solution or polymer precursor solution. In the DIPS method, a porous membrane is obtained by inducing phase separation through solvent evaporation within a coating of a polymer solution or polymer precursor solution.

[0065] An example of a method for manufacturing a polyimide porous membrane 1a will be described. A soluble polyimide solution is coated onto a given substrate to form a coating film. An example of a soluble polyimide is KPI-MX3000F supplied by Kawamura Industries, Ltd. An example of a solvent in the soluble polyimide solution is a polar solvent with a high boiling point (e.g., above 140°C), such as 1-methyl-2-pyrrolidone. An example of a substrate is a porous material containing a fluoropolymer. The substrate is preferably a given polytetrafluoroethylene (PTFE) porous membrane. For example, the air permeability of a PTFE porous membrane, measured according to the air permeability test method B (Gerley method) specified in JIS L1096:2010, is 30 seconds / 100 cm. 3 ~50 seconds / 100cm 3The PTFE porous membrane has, for example, a thickness of 50 to 100 μm. The average pore size of the surface of the PTFE porous membrane is, for example, 100 to 200 nm. In the manufacture of the polyimide porous membrane 1a, a second porous layer 12 can be formed by contacting the substrate with the coating of a soluble polyimide solution. By using such a substrate, it is easy to form a second porous layer 12 that satisfies at least one of the conditions selected from (I) and (II).

[0066] Next, the coating of the soluble polyimide solution is humidified under given conditions. This promotes phase separation within the coating of the soluble polyimide solution. In this humidification process, the coating of the soluble polyimide solution is, for example, placed in an environment with a temperature of 5–30°C and a relative humidity of 60–80% for a given period. The given period is, for example, 1 minute to 30 minutes.

[0067] Next, solvent extraction is performed. For example, the coating of soluble polyimide solution is immersed in water at 20°C to 40°C. The immersion time is, for example, 1 minute to 30 minutes. Solvents such as 1-methyl-2-pyrrolidone can then be extracted.

[0068] Next, the coating film after solvent extraction is dried. The polyimide porous membrane 1a is obtained by peeling the dried film off the substrate. Alternatively, by performing the above-described humidification, solvent extraction and drying processes while the coating film of soluble polyimide solution is sandwiched between a pair of substrates, a polyimide porous membrane 1a with a second porous layer disposed on both sides of the first porous layer can also be obtained.

[0069] like Figure 3 As shown, for example, a porous composite 3 comprising a polyimide porous membrane 1a can be provided. The porous composite 3 comprises a polyimide porous membrane 1a and a porous substrate 2. In the porous composite 3, the polyimide porous membrane 1a is disposed in at least a portion of the pores 2v of the porous substrate 2.

[0070] The polyimide porous membrane 1a can be disposed on a portion of the voids 2v of the porous substrate 2, or it can be disposed on 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 portion of the porous substrate 2, and the second porous layer 12 is disposed on another surface of the first porous layer 11 in the thickness direction.

[0071] When the polyimide porous membrane 1a is disposed on 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 average pore size can be calculated by etching away the second porous layer 12. When the polyimide porous membrane 1a is disposed entirely on the voids 2v of the porous substrate 2, the surface of the polyimide layer with smaller surface pore size can be included in the second porous layer, and the surface of the polyimide layer with larger surface pore size can be included in the first porous layer. In this case, the second porous layer 12 is removed by etching one surface, and the average pore size and porosity are calculated.

[0072] The porous substrate 2 is not limited to a specific porous substrate. For example, the porous substrate 2 is a nonwoven fabric. Examples of nonwoven fabrics include those primarily composed of resins 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, for example, approximately 50 nm to approximately 3000 nm. From the viewpoint of heat resistance, the nonwoven fabric preferably contains fibers such as PI, PPS, LCP, polyamide, and glass.

[0073] The thickness and weight per unit area of ​​the porous substrate 2 are not limited to specific values. For example, the thickness of the porous substrate 2 is 1 μm to 200 μm. For example, the weight per unit area of ​​the porous substrate 2 is 2 to 200 g / m³. 2 By making the thickness of the porous substrate 2 1 μm or more, the operation of the porous substrate 2 is facilitated during the manufacture of the porous composite 3. By making the thickness of the porous substrate 2 200 μm or less, the porous composite 3 easily achieves the desired air permeability. By making the weight per unit area of ​​the porous substrate 2 2 g / m², the porous composite 3 achieves the desired air permeability. 2 The above facilitates the operation of the porous substrate 2 during the manufacture of the porous composite 3. This is achieved by setting the weight per unit area of ​​the porous substrate 2 to 200 g / m². 2 The porous composite 3 is thus able to readily possess the desired air permeability.

[0074] An example of a method for manufacturing porous composite 3 will be described. A soluble polyimide solution is coated onto a given nonwoven fabric to form a coating film. An example of the soluble polyimide used is Neoprim (registered trademark) S100, a soluble polyimide varnish provided by Mitsubishi Gas Chemical Co., Ltd. Neoprim S100 is a solution in which soluble polyimide is dissolved in γ-butyrolactone and N,N-dimethylacetamide. An example of a solvent for adjusting the concentration of the soluble polyimide varnish is a polar solvent with a high boiling point (e.g., above 140°C), such as γ-butyrolactone. Alternatively, as an additive, an additive such as polyethylene glycol, which is soluble in solution but has low solubility for polyimide, can also be used. As the nonwoven fabric used, for example, PPS nonwoven fabric (HC) provided by Toray Industries, Inc. 100, thickness 100μm, weight per unit area 86g / m² 2 Next, solvent extraction is performed. For example, the nonwoven fabric coated with the above-mentioned polyimide solution is immersed in a water bath at 20°C to 40°C for solvent extraction. The immersion time is, for example, 1 minute to 30 minutes. This allows the polar solvent to be extracted. Then, the coating after solvent extraction is dried to create a porous composite containing a polyimide porous membrane in at least a portion of the pores of the nonwoven fabric.

[0075] Example

[0076] The present invention will now be described in more detail through embodiments. However, the present invention is not limited to the following embodiments.

[0077] <Example 1>

[0078] 10g of soluble polyimide (KPI-MX300F) manufactured by Kawamura Sangyo Co., Ltd. was dissolved in 70g of 1-methyl-2-pyrrolidone manufactured by Tokyo Chemical Industry Co., Ltd., to obtain a solution. Then, polyethylene glycol 200 manufactured by Fujifilm and Koichi Pure Chemical Industries Co., Ltd. was added to the solution in 20g increments and stirred until homogeneous. Thus, the PI solution of Example 1 was obtained.

[0079] A coating film was obtained by applying the PI solution of Example 1 to a substrate of PTFE porous membrane TEMISH (registered trademark) manufactured by Nitto Denko Co., Ltd., with a thickness of 50 μm using a coating applicator. The average pore size of the substrate surface was approximately 120 nm. This average pore size was calculated in the same manner as the average pore size of the surface of the second porous layer (skin layer) of the polyimide membrane described later. The air permeability of this PTFE porous membrane, measured according to the air permeability test method B (Greley method) specified in JIS L1096:2010, was 40 seconds / 100 cm. 3 The thickness of the PTFE porous membrane is 70 μm.

[0080] Next, the coating was left to stand for 10 minutes inside a constant temperature and humidity apparatus set to 10°C and 70% relative humidity to promote phase separation. Then, the coating was immersed in a water bath at 30°C for 10 minutes for solvent extraction. Next, the coating was fixed to a square component with one side length of 10 cm (viewed from top view) and dried at 80°C for 10 minutes. Subsequently, the coating was dried at 250°C for 2 minutes. Thus, a polyimide porous membrane was obtained on the substrate. The polyimide porous membrane was then peeled off from the substrate to obtain the polyimide porous membrane of Example 1.

[0081] <Example 2>

[0082] Except that the thickness of the coating film in the coating of the PI solution in Example 1 was adjusted to 25 μm, the polyimide porous membrane of Example 2 was obtained in the same manner as in Example 1.

[0083] <Example 3>

[0084] 15g of soluble polyimide varnish (Neoprim S100) manufactured by Mitsubishi Gas Chemical Co., Ltd., and 7.5g of γ-butyrolactone manufactured by Tokyo Chemical Industry Co., Ltd. were added and mixed until homogeneous. Subsequently, 7.5g of polyethylene glycol PEG400 manufactured by Tokyo Chemical Industry Co., Ltd. was added and mixed until homogeneous. This yielded the PI solution of Example 3. The PI solution of Example 3 was applied to a substrate obtained by hydrophilizing the surface of the PTFE porous membrane used in Example 1 through corona treatment, using a coating applicator to a thickness of 50μm, thus obtaining a coating film.

[0085] The coated substrate was immersed in a water bath at 20°C for 30 minutes to perform phase separation-based pore formation and solvent extraction.

[0086] Next, the coating was fixed to a square component with a length of 10 cm on one side (viewed from top view) and dried at 80°C for 10 minutes. Then, the coating was dried at 250°C for 2 minutes. This resulted in a polyimide porous membrane on the substrate. The polyimide porous membrane was then peeled off from the substrate to obtain the polyimide porous membrane of Example 3. The obtained polyimide porous membrane has a second porous layer (skin layer) formed on the vapor phase side and a first porous layer formed on the substrate side.

[0087] <Comparative Example 1>

[0088] The PI solution of Example 1 was applied to a substrate, specifically Lumirror (registered trademark), a polyethylene terephthalate (PET) film manufactured by Toray Industries, Inc., to obtain a coating film with a thickness of 50 μm. The thickness of the PET film was 100 μm.

[0089] Next, the coating was left to stand for 10 minutes inside a constant temperature and humidity apparatus set to 10°C and 70% relative humidity to promote phase separation, resulting in a polyimide (PI) film on the substrate. The obtained PI film was then peeled off from the substrate. The peeled PI film was immersed in a water bath at 30°C for 10 minutes for solvent extraction. The solvent-extracted PI film was then dried at 80°C for 10 minutes. Subsequently, the PI film was dried at 250°C for 2 minutes. Thus, the polyimide porous membrane of Comparative Example 1 was obtained.

[0090] <Comparative Example 2>

[0091] On a substrate consisting of an electrolytic copper foil with a thickness of 0.018 mm, the PI solution of Example 1 was applied to a thickness of 50 μm using a coater to obtain a coating film.

[0092] Next, the coating was allowed to stand for 10 minutes inside a constant temperature and humidity apparatus set to 10°C and 70% relative humidity to promote phase separation. Then, the coating was immersed in a water bath at 30°C for 10 minutes for solvent extraction, resulting in a PI film on the substrate. The obtained PI film was then peeled off from the substrate. Next, the peeled PI film was dried at 80°C for 10 minutes. Subsequently, the PI film was dried at 250°C for 2 minutes. Thus, the polyimide porous membrane of Comparative Example 2 was obtained.

[0093] <Comparative Example 3>

[0094] A PTFE porous membrane with the following permeability, average pore size and thickness on the surface of the substrate was used as the substrate, and the thickness of the coating in the application of the PI solution in Example 1 was adjusted to 127 μm. Otherwise, the polyimide porous membrane of Comparative Example 3 was obtained in the same manner as in Example 1.

[0095] Breathability: 40 seconds / 100cm 3

[0096] Average pore size of substrate surface: 50nm

[0097] Thickness: 30μm

[0098] <Comparative Example 4>

[0099] 2.43 g of 4,4'-diaminodiphenyl ether (manufactured by Tokyo Chemical Industry Co., Ltd.) and 30 g of N,N-dimethylacetamide (manufactured by the same company) were added to a 50 ml glass vial and dissolved. Then, 3.47 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (manufactured by the same company) was slowly added and dissolved. The mixture was stirred at room temperature for 48 hours to carry out polymerization, yielding a polyimide precursor solution. The solid content of the polyimide precursor solution was 16.7% by mass. 8.35 g of N,N-dimethylacetamide was added to 10 g of the polyimide precursor solution for dilution, followed by 0.117 g of 1,2-dimethylimidazole (0.2 molar equivalent). Then, 20.2 g of triethylene glycol dimethyl ether (manufactured by the same company) was added in small, repeated additions, and the mixture was stirred until homogeneous to allow the reaction to proceed. This yielded the coating solution of Comparative Example 4, containing both a good solvent and a non-solvent. On the surface of a PET film manufactured by Mitsubishi Chemical Corporation, which has a hydrophilic surface obtained through corona treatment, the coating solution of Comparative Example 4 was applied using a coater to achieve a coating thickness of 250 μm, resulting in a coating film. This coating film was heated at 80°C for 30 minutes to obtain a white porous film. The porous film was peeled off from the PET film and fixed to a square component with one side length of 10 cm when viewed from above. 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. Subsequently, the polyimide porous film of Comparative Example 4 was obtained by natural cooling.

[0100] (Breathability)

[0101] The air permeability of the polyimide porous membranes of each example and comparative example was determined according to the air permeability test method B (Gerley method) specified in JIS L1096:2010. The results are shown in Table 1.

[0102] Samples were prepared by removing (etching) the second porous layer (skin layer) from each polyimide porous membrane. Such samples were prepared by removing a portion approximately 1 μm thick from the surface of each polyimide porous membrane containing the second porous layer (skin layer). This portion was removed by dry etching, including vacuum plasma treatment. The air permeability of the sample after removing the second porous layer (skin layer) was determined according to Method B (Gerley method) as specified in JIS L1096:2010. The results are shown in Table 1.

[0103] (Water pressure resistant)

[0104] The water pressure resistance of the polyimide porous membranes of each example and comparative example was determined according to the water resistance test method B (high water pressure method) specified in JIS L1092:2009. The shape of the water inlet of the double-sided adhesive tape used to fix the polyimide porous membrane to the testing fixture was adjusted to be 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.

[0105] (Surface observation)

[0106] Platinum vapor-deposited films were formed on both sides of the polyimide porous membranes in each embodiment and comparative example to prepare samples for surface observation of each polyimide porous membrane. In the manufacturing process of the polyimide porous membrane, one surface of the polyimide porous membrane in contact with the substrate is provided with a second porous layer, and the other surface is formed by a first porous layer. These samples were observed using a Hitachi High-Technology Co., Ltd. scanning electron microscope (SEM) SU8220. The image data of the obtained SEM images were binarized to determine the pores, and the pore diameter (maximum diameter) of each pore was measured to establish a pore size distribution based on the number of pores on both sides of the polyimide porous membrane. Figure 4A , Figure 5A , Figure 6A , Figure 7A , Figure 8A and Figure 9A These are binarized SEM images of the second porous layer surfaces of Examples 1, 2, 1 (Comparative Example), 2 (Comparative Example), 3 (Comparative Example), and 4 (Comparative Example). Figure 4B , Figure 5B , Figure 6B , Figure 7B , Figure 8B and Figure 9B The images shown are SEM photographs or their binarized images of the first porous layer surfaces of Examples 1, 2, 1 (Comparative Example), 2 (Comparative Example), 3 (Comparative Example), and 4 (Comparative Example). Image processing of the SEM photographs was performed using the image processing software ImageJ. The results related to the pore size distribution based on the number of surfaces of the second and first porous layers of each polyimide porous membrane are shown in Table 1.

[0107] (Cross-section observation)

[0108] Using an IM4000 ion mill manufactured by Hitachi High-Tech Corporation, cross-sections along the thickness direction of each polyimide porous membrane were exposed. Platinum vapor-deposited films were then formed on the exposed cross-sections to create samples for cross-sectional observation of each polyimide porous membrane. These samples were then observed using a Hitachi High-Tech Corporation scanning electron microscope (SEM) SU8220. Figure 4C This is an SEM image of a cross-section along the thickness direction of the polyimide porous membrane of Example 1. Figure 4D It is Figure 4CA magnified SEM image of a portion of the SEM image (enclosed by a white dashed line). The thickness of each polyimide porous membrane and the thickness of the second porous layer were determined based on the obtained SEM images. The thickness of the second porous layer was measured as the distance from the surface to the pores, observed by SEM at a magnification of 20,000x. The results are shown in Table 1.

[0109] (Porosity)

[0110] The porosity of the polyimide porous membranes in each embodiment and comparative example was determined according to 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 portion of the polyimide porous membrane, S is the area of ​​the polyimide porous membrane in top view, and T is the thickness of the polyimide porous membrane.

[0111] Porosity (volume %) = 100 - 100(W / D) / (S×T) Equation (1)

[0112] As shown in Table 1, the polyimide porous membranes of each embodiment have a strength of 30 seconds / 100cm. 3 The membranes exhibit the following air permeability and water pressure resistance of 0.15 MPa or higher. On the other hand, the polyimide porous membranes of Comparative Examples 1 and 2 show an air permeability significantly exceeding 30 seconds / 100 cm². 3 It is difficult to conclude that these polyimide porous membranes possess the desired air permeability. Furthermore, the water pressure resistance of the polyimide porous membranes in Comparative Examples 3 and 4 is below 0.10 MPa, making it difficult to conclude that these polyimide porous membranes possess the desired water resistance. Based on the comparison between the examples and the comparative examples, it can be understood that when a polyimide porous membrane has a structure comprising a first porous layer with a large average pore size and a second porous layer with a small average pore size, and satisfies at least one of the conditions selected from (I) and (II) above, it is advantageous from the viewpoint of balancing air permeability and water resistance.

[0113]

[0114] A first aspect of the present invention provides a polyimide porous membrane having 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 of the surface of the second porous layer is smaller than the average pore size of the first porous layer, and the polyimide porous membrane satisfies at least one of the conditions selected from (I) and (II) below.

[0115] (I) The air permeability of the polyimide porous membrane, as measured by Method B (Gree method) of air permeability test as specified in Japanese Industrial Standard (JIS) L1096:2010, is 30 seconds / 100cm. 3Furthermore, the air permeability of the sample obtained by removing the second porous layer from the polyimide porous membrane, as measured by the air permeability test method B (Gree method), is 2 seconds / 100cm lower than the air permeability of the polyimide porous membrane. 3 above.

[0116] (II) The proportion of pores with a diameter of less than 0.2 μm in the pore size distribution of the surface of the second porous layer is more than 80%, and the surface opening ratio of the second porous layer is 1 to 60%.

[0117] A second aspect of the present invention provides a polyimide porous membrane, wherein, in the first aspect, the polyimide porous membrane has a thickness of 1 to 30 μm.

[0118] A third aspect of the present invention provides a polyimide porous membrane in which, in the first or second aspect, the second porous layer has a thickness of less than 1 μm.

[0119] A fourth aspect of the present invention provides a polyimide porous membrane, wherein, in any one of the first to third aspects, the average pore size of the surface of the second porous layer is 0.05 to 1 μm.

[0120] A fifth aspect of the present invention provides a polyimide porous membrane in which, in any one of the first to fourth aspects, the maximum pore size of the surface of the second porous layer is 5 μm or less.

[0121] The sixth aspect of the present invention provides a polyimide porous membrane, wherein, in any one of the first to fifth aspects, the average pore size of the surface of the first porous layer or the surface of the sample is 0.1 to 10 μm.

[0122] A seventh aspect of the present invention provides a polyimide porous membrane, wherein, in any one of the first to sixth aspects, the polyimide porous membrane has a porosity of 30 to 90%.

[0123] The eighth aspect of the present invention provides a polyimide porous membrane, wherein, in any one of the first to seventh aspects, the water pressure resistance of the polyimide porous membrane, as determined by the water resistance test method B (high water pressure method) specified in JIS L1092:2009, is 0.15 MPa or higher.

[0124] A ninth aspect of the present invention provides a porous composite comprising a polyimide porous membrane according to any one of the first to eighth aspects, and a porous substrate, wherein the polyimide porous membrane is disposed in at least a portion of the pores of the porous substrate.

[0125] A tenth aspect of the present invention provides a porous composite in which, in the ninth aspect, one surface of the first porous layer in the thickness direction contacts at least a portion of the porous substrate, and the second porous layer is disposed on another surface of the first porous layer in the thickness direction.

[0126] The eleventh aspect of the present invention provides a porous composite in which, in the ninth or tenth aspect, the porous substrate is a nonwoven fabric.

Claims

1. A polyimide porous membrane, comprising: First porous layer, and A second porous layer disposed on one or both sides of the first porous layer. The average pore size of the surface of the second porous layer is smaller than that of the first porous layer. The polyimide porous membrane satisfies at least one of the conditions selected from (I) and (II) below: (I) The air permeability of the polyimide porous membrane, as measured by Method B (Gree method) of air permeability test as specified in Japanese Industrial Standard (JIS) L1096:2010, is 30 seconds / 100cm. 3 Furthermore, the air permeability of the sample obtained by removing the second porous layer from the polyimide porous membrane, as measured by the air permeability test method B (Gree method), is 2 seconds / 100cm lower than the air permeability of the polyimide porous membrane. 3 above, (II) The proportion of pores with a diameter of less than 0.2 μm in the pore size distribution of the surface of the second porous layer is more than 80%, and the surface opening ratio of the second porous layer is 1 to 60%.

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

3. The polyimide porous membrane according to claim 1, wherein, The second porous layer has a thickness of less than 1 μm.

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

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

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

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

8. The polyimide porous membrane according to claim 1, wherein, The water pressure resistance of the polyimide porous membrane, as determined by the water resistance test method B (high water pressure method) specified in JIS L1092:2009, is above 0.15 MPa.

9. A porous composite material comprising: The polyimide porous membrane according to claim 1, and Porous substrate, The polyimide porous membrane is disposed in at least a portion 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. The second porous layer is disposed on another surface in the thickness direction of the first porous layer.

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

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