Composite porous body and method for producing same

Abstract

A composite porous body comprising a substrate having a first surface, a carbon nanotube layer covering at least a part of the first surface, and a polyvinyl alcohol layer covering at least a part of the carbon nanotube layer, wherein the substrate includes a first layer comprising a polytetrafluoroethylene porous body having the first surface, the polyvinyl alcohol layer comprises a non-porous body of polyvinyl alcohol, and the polyvinyl alcohol layer has an average pore diameter of 1-5 nm.

Description

Composite porous body and method for manufacturing the same

[0001] This disclosure relates to a composite porous body and a method for producing the same.

[0002] Polyvinyl alcohol (hereinafter also referred to as "PVA") porous membranes are used in various separation membranes such as filtration filters.

[0003] Patent Document 1 discloses a filtration filter in which a PVA porous membrane is arranged on a filter made of a porous polytetrafluoroethylene (hereinafter also referred to as "PTFE") material. The PVA porous membrane used in Patent Document 1 has physical pores that can be confirmed when observed at 100,000x magnification using a scanning electron microscope (SEM) (see Figures 10 and 11 of Patent Document 1, etc.). The diameter of these physical pores contributes to the separation performance of the filtration filter in Patent Document 1.

[0004] Japanese Patent Publication No. 2013-34973

[0005] The composite porous body of the present disclosure comprises a substrate having a first surface, a carbon nanotube layer covering at least a portion of the first surface, and a polyvinyl alcohol layer covering at least a portion of the carbon nanotube layer, wherein the substrate includes a first layer made of a polytetrafluoroethylene porous material having the first surface, the polyvinyl alcohol layer is made of a non-porous polyvinyl alcohol material, and the average pore size of the polyvinyl alcohol layer is 1 nm or more and 5 nm or less.

[0006] Figure 1 is an overview view of the composite porous body according to Embodiment 1. Figure 2 is a cross-sectional view of the composite porous body according to Embodiment 1. Figure 3 is a schematic diagram of the test apparatus used for the liquid permeability test. Figure 4 is an overview view of the composite porous body according to Embodiment 2. Figure 5 is a flowchart showing the manufacturing process of the composite porous body according to Embodiment 3.

[0007] [Problems this disclosure aims to solve] In the fields of solvent separation membranes and water treatment membranes, there is a desire to further reduce the size of substances that can be removed from the fluid being filtered.

[0008] Therefore, the present disclosure aims to provide a composite porous body having high separation performance.

[0009] [Effects of this disclosure] According to this disclosure, it is possible to provide a composite porous body having high separation performance.

[0010] [Description of Embodiments of the Disclosure] Embodiments of the Disclosure will be described first by listing them. (1) The composite porous body of the Disclosure comprises a substrate having a first surface, a carbon nanotube layer covering at least a portion of the first surface, and a polyvinyl alcohol layer covering at least a portion of the carbon nanotube layer, wherein the substrate includes a first layer made of a polytetrafluoroethylene porous body having the first surface, the polyvinyl alcohol layer is made of a non-porous polyvinyl alcohol, and the average pore size of the polyvinyl alcohol layer is 1 nm or more and 5 nm or less.

[0011] The composite porous material of this disclosure includes a polyvinyl alcohol layer (hereinafter also referred to as the "PVA layer") made of a non-porous polyvinyl alcohol (hereinafter also referred to as the "PVA non-porous material"). The non-porous polyvinyl alcohol (non-porous membrane) is mainly composed of PVA and does not have any physical pores that can be confirmed when observed at 100,000x magnification using a SEM. The PVA non-porous material separates target substances by utilizing the gaps in the polymer network. The composite porous material of this disclosure includes a PVA layer with an average pore diameter of 1 nm to 5 nm. As a result, the composite porous material of this disclosure can have high separation performance. The higher the separation performance, the smaller the substances that can be separated.

[0012] (2) In (1) above, the polyvinyl alcohol may be crosslinked. This suppresses the elution of polyvinyl alcohol and improves the durability of the film.

[0013] (3) In (1) or (2) above, the average thickness of the polyvinyl alcohol layer may be 10 nm or more and 500 nm or less. If the average thickness of the polyvinyl alcohol layer is 10 nm or more, the polyvinyl alcohol layer can be formed uniformly, and the composite porous body can separate smaller substances. If the average thickness of the polyvinyl alcohol layer is 500 nm or less, the flux of the composite porous body is improved.

[0014] (4) In any of (1) to (3) above, the carbon nanotube layer may contain polyvinyl alcohol. Polyvinyl alcohol adheres well to polytetrafluoroethylene porous materials and non-porous polyvinyl alcohol materials. Therefore, a carbon nanotube layer containing polyvinyl alcohol (hereinafter also referred to as the "CNT layer") has excellent adhesion to the substrate and the polyvinyl alcohol layer.

[0015] (5) In any of (1) to (4) above, the average thickness of the carbon nanotube layer may be 0.1 μm or more and 20 μm or less. The CNT layer has excellent chemical resistance and durability, and its pore size is suitable for holding the PVA layer. When the average thickness of the CNT layer is 0.1 μm or more, the CNT layer and the polyvinyl alcohol layer on the CNT layer can be formed uniformly, and the composite porous body can separate smaller substances. When the average thickness of the CNT layer is 20 μm or less, the flux of the composite porous body is improved.

[0016] (6) In any of (1) to (5) above, the average aperture diameter of the carbon nanotube layer may be 20 nm or more and 200 nm or less, and the aperture ratio of the carbon nanotube layer may be 0.5% or more and 20% or less. When the average aperture diameter of the CNT layer is 20 nm or more and the aperture ratio of the CNT layer is 0.5% or more, the permeability is improved. When the average aperture diameter of the CNT layer is 200 nm or less and the aperture ratio of the CNT layer is 20% or less, a polyvinyl alcohol layer can be uniformly formed on the CNT layer, and the composite porous body can separate smaller substances.

[0017] (7) In any of (1) to (6) above, polyvinyl alcohol may be present on at least a portion of the first surface. This improves the adhesion between the substrate and the CNT layer because the polyvinyl alcohol present on the first surface adheres closely to the CNT layer.

[0018] (8) In any of (1) to (7) above, the average pore size on the first surface may be 20 nm or more and 2000 nm or less. When the average pore size on the first surface of the substrate is 20 nm or more, the composite porous body including the substrate has excellent liquid permeability. A composite porous body with high liquid permeability can shorten the filtration time. When the average pore size on the first surface of the substrate is 2000 nm or less, the carbon nanotubes constituting the carbon nanotube layer are more likely to get caught on the first surface, and the carbon nanotube layer is more likely to be properly formed on the first surface. When the average pore size on the first surface of the substrate is 2000 nm or less, the composite porous body has excellent strength. The average pore size on the first surface of the substrate is larger than the average pore size of the carbon nanotube layer.

[0019] (9) In any of (1) to (8) above, the shape of the substrate may be a sheet. This makes it possible to obtain a sheet-shaped composite porous body. The sheet-shaped composite porous body is easy to process into various shapes and can be applied to various types of filtration devices. Because the composite porous body has excellent flexibility, it is not easily damaged even when subjected to processing such as bending.

[0020] (10) In (9) above, the average thickness of the substrate may be 1 μm or more and 1000 μm or less. When the average thickness of the substrate is 1 μm or more, the composite porous body has excellent strength. When the average thickness of the substrate is 1000 μm or less, the composite porous body has excellent flexibility. Also, the filtration time by the composite porous body does not become too long.

[0021] (11) In any of (1) to (8) above, the shape of the base material is a tube, and the first surface may be the outer surface of the tube.

[0022] The overall shape of the composite porous body, which comprises a tubular substrate, is tubular. The tubular substrate also includes a hollow fiber membrane. In a tubular composite porous body, fluids containing impurities flow to the outside of the tubular composite porous body. Fluids that have permeated the composite porous body flow to the inside of the tubular composite porous body. The above-mentioned composite porous body can constitute a fluid flow path within itself. Multiple of these composite porous bodies can be bundled together to form a module for a purification device.

[0023] (12) In (11) above, the average thickness of the substrate may be 50 μm or more and 1000 μm or less. When the average thickness of the substrate is 50 μm or more, the composite porous body has excellent strength. When the average thickness of the substrate is 1000 μm or less, the composite porous body has excellent flexibility. In addition, the filtration time by the composite porous body does not become too long.

[0024] (13) In any of (1) to (12) above, the substrate comprises the first layer and a second layer adjacent to the first layer, wherein the average pore size of the second layer may be larger than the average pore size of the first layer. The permeability of a substrate having a second layer with a larger average pore size is superior to that of a substrate having the same thickness as the substrate and consisting only of the first layer. Therefore, by having the substrate composed of a first layer and a second layer, the filtration time of the composite porous body does not tend to increase even if the substrate is made thicker.

[0025] (14) A method for producing a composite porous body according to the present disclosure is a method for producing a composite porous body according to any one of (1) to (13) above, comprising the steps of: preparing a substrate including the first layer made of the polytetrafluoroethylene porous body having the first surface; preparing a first liquid containing carbon nanotubes; preparing a second liquid containing polyvinyl alcohol; applying the first liquid to at least a part of the first surface to form the carbon nanotube layer; and applying the second liquid to at least a part of the surface of the carbon nanotube layer to form the polyvinyl alcohol layer made of a non-porous polyvinyl alcohol to obtain the composite porous body, wherein the average pore size of the polyvinyl alcohol layer is 1 nm or more and 5 nm or less.

[0026] According to this disclosure, it is possible to provide a composite porous body having high separation performance.

[0027] (15) In (14) above, the step of obtaining the composite porous body may include either or both of the following steps: immersing the composite porous body in a crosslinking solution to crosslink the polyvinyl alcohol in the polyvinyl alcohol layer; and adding a crosslinking agent to the second solution, then applying the second solution to at least a portion of the surface of the carbon nanotube layer to form the polyvinyl alcohol layer and crosslink the polyvinyl alcohol.

[0028] According to this, the elution of polyvinyl alcohol is suppressed, and the durability of the film is improved.

[0029] [Details of Embodiments of the Disclosure] Specific examples of the composite porous body and method for manufacturing the same of the Disclosure will be described below with reference to the drawings. In the drawings of the Disclosure, the same reference numerals represent the same part or a corresponding part. In addition, dimensional relationships such as length, width, thickness, and depth have been appropriately modified for clarity and simplification of the drawings and do not necessarily represent actual dimensional relationships.

[0030] In this disclosure, the notation "A to B" means A or greater and B or less. If no unit is specified for A, and only a unit is specified for B, then the unit for A and the unit for B are the same.

[0031] In this disclosure, when compounds and the like are represented by chemical formulas, unless otherwise specified, the atomic ratios should include all conventionally known atomic ratios and should not necessarily be limited to those within the stoichiometric range.

[0032] In this disclosure, if one or more numerical values ​​are listed as the lower and upper limits of a numerical range, any combination of any one numerical value listed as the lower limit and any one numerical value listed as the upper limit shall also be disclosed.

[0033] In this disclosure, “equipment,” “includes,” “possesses,” and variations thereof are open-ended terms. Open-ended terms may or may not include additional elements in addition to the essential elements. The statement “consists of” is a closed term. However, even a configuration expressed in closed terms may include additional elements that are usually incidental or irrelevant to the subject technology.

[0034] In this disclosure, it has been confirmed that, as long as the measurements are taken from the same sample, there is almost no variation even if the measurement location is arbitrarily selected.

[0035] [Embodiment 1: Composite Porous Body (1)] A composite porous body according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 1") will be described with reference to Figures 1 and 2. As shown in Figures 1 and 2, the composite porous body 1 according to Embodiment 1 is in the shape of a sheet. The composite porous body 1 according to Embodiment 1 comprises a substrate 2 having a first surface 21, a carbon nanotube layer 3 covering at least a part of the first surface 21, and a polyvinyl alcohol layer 4 covering at least a part of the carbon nanotube layer 3. The substrate 2 includes a first layer 2A made of a polytetrafluoroethylene porous body having a first surface 21. The polyvinyl alcohol layer 4 is made of a non-porous polyvinyl alcohol. The average pore size of the polyvinyl alcohol layer 4 is 1 nm or more and 5 nm or less.

[0036] <Substrate> In Embodiment 1, the substrate 2 includes a first layer 2A made of a polytetrafluoroethylene porous material having a first surface 21. As shown in Figure 2, the substrate 2 is in the shape of a sheet. As shown in Figure 2, the substrate 2 has a plurality of pores 2h. Since Figure 2 is a cross-sectional view, each pore 2h of the substrate 2 appears to be independent, but each pore 2h is connected to other pores 2h, forming countless channels from the first surface 21 to the second surface 22. Countless channels are formed in the substrate 2, extending from the first surface 21 to the second surface 22.

[0037] PTFE is excellent in heat resistance and chemical resistance. Therefore, a composite porous body including a polytetrafluoroethylene porous body can have excellent heat resistance and chemical resistance. The first layer 2A may contain components other than polytetrafluoroethylene as long as the effects of the present disclosure are not impaired. Examples of such components include perfluoroalkoxyalkane (PFA) and tetrafluoroethylene-hexafluoropropylene copolymer (FEP).

[0038] Polyvinyl alcohol may be present on at least a part of the first surface 21 of the base material 2. The surface of the base material 2 includes the inner peripheral surface of the pores 2h of the base material 2. PVA may enter the pores 2h of the base material 2. The presence of PVA can be confirmed by, for example, XPS. PVA may disappear by heat treatment in the process of manufacturing the composite porous body 1. Therefore, PVA may not be detected by XPS.

[0039] The thickness of the base material 2 is the length between the first surface 21 and the second surface 22. The average thickness of the base material 2 may be 1 μm or more and 1000 μm or less, may be 10 μm or more and 900 μm or less, or may be 20 μm or more and 500 μm or less.

[0040] The average thickness of the base material 2 is a value obtained by subtracting the average thickness of the carbon nanotube layer 3 and the average thickness of the polyvinyl alcohol layer 4 from the average value of the thicknesses at three different locations of the composite porous body 1. The thickness of the composite porous body 1 is measured by a micrometer. The measurement methods for the average thickness of the carbon nanotube layer 3 and the average thickness of the polyvinyl alcohol layer 4 are as described below.

[0041] The average pore diameter on the first surface 21 of the base material 2 may be 20 nm or more and 2000 nm or less, may be 50 nm or more and 450 nm or less, or may be 200 nm or more and 350 nm or less.

[0042] Multiple pores 2h are formed on the first surface 21. When a carbon nanotube layer 3 is formed on the first surface 21, it is difficult to measure the average pore diameter of the pores 2h on the first surface 21. In this disclosure, the average pore diameter of the first surface 21 is determined from an SEM image of a cross-section along the thickness direction of the substrate 2. In this disclosure, the thickness direction of the substrate 2 means the direction from the first surface 21 to the second surface 22. The magnification of the SEM image is 50,000x. The size of the SEM image is 1 μm in the thickness direction and 10 μm in the width direction. The SEM image is binarized and each pore 2h in the SEM image is extracted. The equivalent circle diameter of each pore 2h in the SEM image is determined, and the arithmetic mean of the equivalent circle diameters of all pores 2h is calculated. The equivalent circle diameter is the diameter of a perfect circle with the same size as the area of ​​the pore 2h. This arithmetic mean of the equivalent circle diameters is considered to be the average pore diameter on the first surface 21 of the substrate 2.

[0043] The average minor diameter of the pores 2h on the first surface 21 may be between 4 nm and 400 nm. The first surface 21 having pores 2h with an average minor diameter of 4 nm or more improves the liquid permeability of the composite porous body 1. If the average minor diameter of the pores 2h on the first surface 21 is 400 nm or less, the carbon nanotubes 6 constituting the carbon nanotube layer 3 are more likely to get caught on the first surface 21, and the carbon nanotube layer 3 is more likely to be properly formed on the first surface 21. The average minor diameter of the pores 2h on the first surface 21 may be, for example, between 10 nm and 300 nm.

[0044] In this disclosure, the average minor axis of the voids 2h on the first surface 21 is determined from an SEM image of a cross-section along the thickness direction of the substrate 2. The magnification of the SEM image is 50,000x. The size of the SEM image is 1 μm in the thickness direction and 10 μm in the width direction. The SEM image is binarized and each void 2h in the SEM image is extracted. The minor axis of the ellipse approximating each void 2h in the SEM image is determined. The average of the minor axes of all the ellipses in the SEM image is the average minor axis of the voids 2h.

[0045] The substrate 2 may comprise multiple layers, as shown in Figure 2. The substrate 2 may comprise a first layer 2A and a second layer 2B. In Figure 2, the boundary between the first layer 2A and the second layer 2B is schematically shown by a dashed line. The first layer 2A includes a first surface 21. The second layer 2B is adjacent to the main surface of the first layer 2A opposite to the first surface 21. In Embodiment 1, the second layer 2B includes a second surface 22. The average pore diameter of the second layer 2B is larger than the average pore diameter of the first layer 2A. The liquid permeability of the substrate 2 comprising the second layer 2B with a larger average pore diameter is superior to that of the substrate 2 having the same thickness as the substrate 2 and consisting only of the first layer 2A. Therefore, by having the substrate 2 comprised of a first layer 2A and a second layer 2B, the filtration time of the composite porous body 1 does not tend to increase even if the substrate 2 is made thicker. The average pore size of the second layer 2B may be, for example, 2 to 2000 times, or 10 to 1000 times, the average pore size of the first layer 2A. The average pore size of the first layer 2A is larger than the average pore size of the carbon nanotube layer 3. If the substrate 2 consists of three or more layers, the average pore size may be larger for layers further from the first surface 21. Unlike this example, the substrate 2 may consist only of the first layer 2A. In this case, the first layer 2A includes the first surface 21 and the second surface 22.

[0046] The average pore diameter of the first layer 2A and the average pore diameter of the second layer 2B are determined from SEM images of the cross-section along the thickness direction of the substrate 2. The first layer 2A and the second layer 2B are heat-fused together during the fabrication of the substrate 2. Therefore, the boundary between the first layer 2A and the second layer 2B can be confirmed in the SEM image. In the SEM image, the arithmetic mean of the equivalent circular diameters of each void 2h in the region including the first surface 21 across the boundary is the average pore diameter of the first layer 2A. Similarly, in the SEM image, the arithmetic mean of the equivalent circular diameters of each void 2h in the region adjacent to the first layer 2A across the boundary is the average pore diameter of the second layer 2B.

[0047] The second layer 2B may be made of a porous polytetrafluoroethylene material. The second layer 2B may be made of a nonwoven fabric made of polyethylene terephthalate, polypropylene, polyethylene, polytetrafluoroethylene, or polyphenylene sulfide.

[0048] <Carbon Nanotube Layer> In Embodiment 1, the carbon nanotube layer 3 includes a plurality of carbon nanotubes 6. The carbon nanotube layer 3 may cover at least a portion of the first surface 21 of the substrate 2, or it may cover the entire first surface 21 of the substrate 2. The carbon nanotube layer 3 is positioned directly above the first surface 21 of the substrate 2.

[0049] As the carbon nanotube 6, you can use a single-walled carbon nanotube, which is a cylindrical structure with only one layer of carbon (graphene); a double-walled carbon nanotube or multi-walled carbon nanotube, which is a cylindrical structure with multiple layers of carbon stacked on top of each other; or a cup-stack type nanotube, which has a structure in which graphene is stacked in the shape of a paper cup with a hole in the bottom. The presence of carbon nanotube 6 in the carbon nanotube layer 3 can be confirmed by observation with an electron microscope.

[0050] The length of the carbon nanotube 6 is not particularly limited. For example, the length of the carbon nanotube 6 may be between 0.1 μm and 50 μm, or between 0.5 μm and 30 μm. The length of the carbon nanotube can be measured by observation using atomic force microscopy or electron microscopy.

[0051] The diameter of the carbon nanotube 6 is not particularly limited. For example, the diameter of the carbon nanotube 6 may be between 1 nm and 100 nm, or between 3 nm and 80 nm. The diameter of the carbon nanotube refers to the average outer diameter of a single carbon nanotube. The average outer diameter of a single carbon nanotube can be measured and calculated by directly observing the projection image of the carbon nanotube using a transmission electron microscope.

[0052] The carbon nanotube layer 3 may contain carbon nanotubes 6 and polyvinyl alcohol 5. The carbon nanotube layer 3 may consist of carbon nanotubes and polyvinyl alcohol. Insofar as the effects of this disclosure are not impaired, the carbon nanotube layer 3 may contain other components in addition to carbon nanotubes and polyvinyl alcohol. Other components include one or both of a binder component and a dispersant. Examples of binder components include acrylic resins and silicone resins. Examples of dispersants include compounds that disperse carbon nanotubes in a medium, such as sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, carboxymethylcellulose, and polycarboxylates.

[0053] The average thickness of the carbon nanotube layer 3 may be 0.1 μm or more and 20 μm or less, 2 μm or more and 8 μm or less, or 4 μm or more and 6 μm or less.

[0054] The average thickness of the carbon nanotube layer 3 is determined by SEM-EDX or FIB (Focused Ion Beam)-SEM. The thickness of the carbon nanotube layer 3 is the distance from the interface between the substrate 2 and the carbon nanotube layer 3 to the interface between the carbon nanotube layer 3 and the polyvinyl alcohol layer 4. The average thickness of the carbon nanotube layer 3 is the average of the thicknesses of three different carbon nanotube layer 3 locations. The image magnification is set appropriately according to the thickness of the carbon nanotube layer 3. For example, the image magnification can be set to ×10000, ×20000, or ×50000.

[0055] The average aperture diameter of the carbon nanotube layer 3 is 20 nm or more and 200 nm or less, and the aperture ratio of the carbon nanotube layer 3 may be 0.5% or more and 20% or less. The average aperture diameter of the carbon nanotube layer 3 is 20 nm or more and 200 nm or less, and may be 40 nm or more and 150 nm or less, or 80 nm or more and 120 nm or less. The aperture ratio of the carbon nanotube layer 3 may be 0.5% or more and 20% or less, or 5% or more and 18% or less, or 10% or more and 16% or less. The above ranges of average aperture diameter and aperture ratio of the carbon nanotube layer 3 can be combined as appropriate.

[0056] In this disclosure, the average aperture diameter and aperture ratio of the carbon nanotube layer 3 are determined from an SEM image of the cross-section along the thickness direction of the substrate 2. The magnification of the SEM image is 50,000x. In the SEM image, a rectangular measurement field of view of 0.1 μm × 2 μm is set within the region sandwiched between the interface between the carbon nanotube layer 3 and the polyvinyl alcohol layer 4 and a virtual line at a distance of 0.1 μm from the interface to the carbon nanotube layer 3 side.

[0057] The SEM image is binarized, and each aperture is extracted from the SEM image. The equivalent circle diameter of each aperture in the SEM image is determined, and the arithmetic mean of the equivalent circle diameters of all apertures is calculated. The equivalent circle diameter is the diameter of a perfect circle with the same size as the area of ​​the aperture. In this disclosure, this arithmetic mean corresponds to the average aperture diameter of the carbon nanotube layer 3.

[0058] In the image after binarization, the percentage of the total aperture area relative to the total area of ​​the measurement field (aperture ratio) is determined. In this disclosure, this aperture ratio corresponds to the aperture ratio of the carbon nanotube layer 3.

[0059] When a polyvinyl alcohol layer made of a non-porous polyvinyl alcohol material is placed directly on a substrate, the adhesion between the substrate and the polyvinyl alcohol layer is insufficient. Furthermore, because the pore size of the substrate and the pore size of the PVA layer differ significantly, it is not possible to form a thin polyvinyl alcohol layer directly on the substrate. In the composite porous body of Embodiment 1, the presence of a carbon nanotube layer 3 between the substrate 2 and the polyvinyl alcohol layer 4 improves the adhesion between the substrate 2, the carbon nanotube layer 3, and the polyvinyl alcohol layer 4, and allows for the formation of a thin polyvinyl alcohol layer 4.

[0060] The carbon nanotube layer 3 has excellent durability, chemical resistance, and heat resistance. Therefore, a composite porous body comprising the carbon nanotube layer 3 can also have excellent durability, chemical resistance, and heat resistance.

[0061] <Polyvinyl Alcohol Layer> In Embodiment 1, the polyvinyl alcohol layer 4 is made of a non-porous polyvinyl alcohol material. The polyvinyl alcohol layer 4 may cover at least a portion of the carbon nanotube layer 3, or it may cover the entire carbon nanotube layer 3. The polyvinyl alcohol layer 4 is placed directly on top of the carbon nanotube layer 3.

[0062] The polyvinyl alcohol layer 4 consists of a non-porous polyvinyl alcohol. The polyvinyl alcohol layer 4 may also contain other components in addition to the non-porous polyvinyl alcohol, provided that the effects of the present disclosure are not impaired. Examples of other components include glutaraldehyde, silica, titania, and epoxy. Furthermore, the polyvinyl alcohol may have functional groups. Examples of functional groups include carboxyl groups, sulfonic acid groups, amino groups, ethylene oxide groups, and acetoacetyl groups.

[0063] The average pore size of the polyvinyl alcohol layer 4 is 1 nm or more and 5 nm or less, and may be 1.0 nm or more and 5.0 nm or less, 1.2 nm or more and 3 nm or less, or 1.6 nm or more and 1.7 nm or less.

[0064] Because the average pore size of the polyvinyl alcohol layer 4 is very small, it is difficult to determine from an SEM image of the cross-section of the polyvinyl alcohol layer 4. In this disclosure, the average pore size of the polyvinyl alcohol layer 4 is determined by a liquid permeability test. Figure 3 is a schematic diagram of the test apparatus 7 used for the liquid permeability test. The test apparatus 7 comprises a beaker 70, a pressurized cylindrical chamber 71, and a holder 72. The composite porous body 1 is sandwiched between the lower end opening 71U of the chamber 71 and the upper end opening 72D of the holder 72. The polyvinyl alcohol layer 4 of the composite porous body 1 faces the chamber 71.

[0065] The procedure for determining the average pore size of the polyvinyl alcohol layer 4 is as follows. First, a test solution is prepared containing polyethylene glycol particles (hereinafter also referred to as "PEG particles"), vitamin B12, or gold nanoparticles having the average particle size listed in Table 1. Here, the average particle size refers to the Stokes diameter for PEG particles, the Stokes diameter for vitamin B12 as described in the literature (e.g., Sungil Jeon et al., RSC Adv., 2018, 8, 19879-19882), and the manufacturer's stated value for gold nanoparticles. The average particle sizes of the test solutions to be prepared are as shown in Table 1. As described below, the average particle size (Stokes diameter) of PEG particles is calculated based on the average molecular weight of the PEG particles. Average molecular weight of PEG particles in the test solution, concentration of vitamin B12 (g / cm³) ３ ), or the concentration of gold nanoparticles (g / cm³) ３ ) is known.

[0066]

[0067] The particle size (Stokes diameter) of PEG particles is calculated based on the average molecular weight using the following formula: r ｓ [nm]=(0.262×M ｗ ０．５ -0.3) × 0.2 In the above formula, M ｗ is the molecular weight, r ｓ This is the Stokes diameter.

[0068] Next, a liquid permeability test is performed. A test liquid is put into the chamber 71. By pressurizing the inside of the chamber 71, the filtrate that has passed through the composite porous body 1 is stored in the beaker 70. The pressure is 0.6 MPa or more. The concentration of particles contained in the filtrate is measured.

[0069] When the particles contained in the test liquid are PEG particles, the test liquid uses a solution with a concentration of 3000 ppm based on the mass of PEG. In this case, the concentration of particles contained in the filtrate (hereinafter also referred to as "filtrate concentration") is calculated by the following formula based on the amount of carbon [mg / L] obtained by measuring the total organic carbon of the test liquid and the filtrate. Filtrate concentration [%] = test liquid concentration [%] × filtrate carbon amount [mg / L] / test liquid carbon amount [mg / L]

[0070] When the particles contained in the test liquid are vitamin B12 or gold nanoparticles, the concentration of particles contained in the filtrate is calculated by the following formula based on the absorbance at a wavelength of 550 nm by ultraviolet-visible near-infrared spectroscopy of the test liquid and the filtrate. Filtrate concentration [%] = test liquid concentration [%] × filtrate absorbance ５５０ / test liquid absorbance ５５０

[0071] The liquid permeability test is performed in the above procedure in order from the test liquid with a small average particle size of the particles. Among the average particle sizes of the particles of the test liquid in which the concentration of the particles in the filtrate is 10% or less of the concentration of the particles in the test liquid, the smallest average particle size of the particles is regarded as the average pore diameter of the polyvinyl alcohol layer 4. For example, in the test liquid with an average particle size of 1 nm of PEG particles, the concentration of particles in the filtrate is 50% of the concentration of particles in the test liquid. In the test liquid with an average particle size of 1.2 nm of PEG particles, the concentration of particles in the filtrate is 10% of the concentration of particles in the test liquid. In the test liquid with an average particle size of 1.7 nm of vitamin B₁₂, when the concentration of particles in the filtrate is 5% of the concentration of particles in the test liquid, the average pore diameter of the polyvinyl alcohol layer 4 is 1.2 nm.

[0072] In the composite porous body of Embodiment 1, the average pore diameter of the polyvinyl alcohol layer 4 is smaller than the average pore diameter of the carbon nanotube layer 3 and the average pore diameter of the base material 2. In the composite porous body of Embodiment 1, the average pore diameter of the carbon nanotube layer 3 is smaller than the average pore diameter of the base material.

[0073] In a non-porous polyvinyl alcohol material, the polyvinyl alcohol may be crosslinked. Examples of crosslinking agents include glutaraldehyde, silica, and alkoxysilane, and titania (TiO2). ２ ) and titanium alkoxides, titanium chelates, or epoxides may also be used.

[0074] If the rate of change in flux before and after immersion of the composite porous material in 80°C hot water for 20 minutes is within 20%, it can be confirmed that the polyvinyl alcohol is cross-linked in the non-porous polyvinyl alcohol material.

[0075] The rate of change in flux is expressed as {(T2-T1) / T1} × 100, based on the flux T1 before immersion and the flux T2 after immersion. The flux of the composite porous material is measured by the following filtration test. A sample of the composite porous material is placed in the sample holder of a test apparatus consisting of a chamber and a sample holder. The test liquid is added to the chamber and pressurized to 0.6 MPa. The amount passing through the inspection surface per unit time is calculated from the weight change of the filtrate permeating from the sample holder, and the flux is obtained. The test liquid is water.

[0076] The average thickness of the polyvinyl alcohol layer 4 may be 10 nm to 500 nm, 50 nm to 400 nm, or 100 nm to 200 nm.

[0077] The average thickness of the polyvinyl alcohol layer 4 is determined by SEM-EDX or FIB-SEM. The distance from the interface between the carbon nanotube layer 3 and the polyvinyl alcohol layer 4 to the surface of the polyvinyl alcohol layer 4 is the thickness of the polyvinyl alcohol layer 4. The average thickness of the polyvinyl alcohol layer 4 is the average of the thicknesses of five different points on the polyvinyl alcohol layer 4. The image magnification is set appropriately according to the thickness of the polyvinyl alcohol layer 4. For example, the image magnification may be ×10000, ×20000, or ×50000.

[0078] [Embodiment 2: Composite Porous Body (2)] A composite porous body according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 2") will be described with reference to Figure 4. As shown in Figure 4, the composite porous body 1 according to Embodiment 2 is tubular in shape. The composite porous body 1 according to Embodiment 2 can have the same configuration as the composite porous body 1 of Embodiment 1, except that it is tubular in shape.

[0079] The base material 2 of the composite porous body 1 is tubular in shape. The first surface 21 of the tubular base material 2 constitutes the outer surface of the base material 2. The polyvinyl alcohol layer 4 constitutes the outer surface of the tubular composite porous body 1.

[0080] [Embodiment 3: Method for Manufacturing a Composite Porous Body] A method for manufacturing a composite porous body according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 3") will be described. The method for manufacturing a composite porous body of Embodiment 3 is a method for manufacturing a composite porous body of Embodiment 1 or Embodiment 2. As shown in Figure 5, the method for manufacturing a composite porous body of Embodiment 3 comprises the steps of: preparing a substrate including a first layer made of a polytetrafluoroethylene porous body having a first surface (hereinafter also referred to as "substrate preparation step S1"); preparing a first liquid containing carbon nanotubes (hereinafter also referred to as "first liquid preparation step S2"); preparing a second liquid containing polyvinyl alcohol (hereinafter also referred to as "second liquid preparation step S3"); applying the first liquid to at least a part of the first surface to form a carbon nanotube layer (hereinafter also referred to as "CNT layer formation step S4"); and applying the second liquid to at least a part of the surface of the carbon nanotube layer to form a polyvinyl alcohol layer made of a non-porous polyvinyl alcohol to obtain a composite porous body (hereinafter referred to as "step S5 for obtaining a composite porous body"). The average pore size of the polyvinyl alcohol layer is between 1 nm and 5 nm. The substrate preparation step S1, the first liquid preparation step S2, and the second liquid preparation step S3 may be performed in any order or simultaneously. The second liquid preparation step S3 and the CNT layer deposition step S4 may be performed in any order or simultaneously.

[0081] <Substrate Preparation Step S1> In the substrate preparation step S1, a substrate 2 is prepared, which includes a first layer 2A made of a polytetrafluoroethylene porous material. The substrate 2 is the same as the substrate 2 described in Embodiment 1.

[0082] The method for producing the base material 2 is not particularly limited. For example, the first layer, which is made of a porous polytetrafluoroethylene body, may be produced by the manufacturing method disclosed in Japanese Patent Application Publication No. 2010-94579. When a thin film made of PTFE is stretched, the thin film becomes porous. As a result, a first layer 2A made of a porous PTFE body is obtained. The average pore size of the base material 2 changes depending on the stretching conditions. The first layer 2A may be used as the base material 2 as is. Alternatively, the first layer 2A may be bonded to the second layer 2B to produce the base material 2. Details of the second layer 2B are as described in Embodiment 1.

[0083] <First Liquid Preparation Step S2> In the first liquid preparation step S2, the first liquid containing carbon nanotubes is prepared. A carbon nanotube dispersion may be prepared as the first liquid.

[0084] In a carbon nanotube dispersion, carbon nanotubes are dispersed in a dispersion medium. The dispersion medium may be at least one selected from the group consisting of water, ethanol, isopropyl alcohol, acetone, dimethyl sulfoxide, butyl acetate, and methyl ethyl ketone. The dispersion medium may further contain a surfactant. The carbon nanotube content in the carbon nanotube dispersion medium may be 0.01% by mass or more and 15% by mass or less.

[0085] The carbon nanotube content of the first liquid may be 0.01% by mass or more and 5% by mass or less. If the carbon nanotube content is 0.01% by mass or more, the amount of carbon nanotubes in the carbon nanotube mixture is sufficient, so carbon nanotubes are easily deposited on the first surface 21. If the carbon nanotube content is 5% by mass or less, the carbon nanotubes can be uniformly coated onto the first surface 21 without impairing their dispersibility.

[0086] The first solution may contain polyvinyl alcohol as a binder along with the carbon nanotube dispersion. The polyvinyl alcohol content of the first solution may be 0.001% by mass or more and 1% by mass or less. When polyvinyl alcohol is crosslinked after coating, the crosslinking network becomes denser and the pore size of the film decreases.

[0087] <Second Liquid Preparation Step S3> In the second liquid preparation step S3, a second liquid containing polyvinyl alcohol is prepared. As the second liquid, for example, a polyvinyl alcohol dispersion can be prepared using at least one selected from the group consisting of water, ethanol, isopropyl alcohol, acetone, dimethyl sulfoxide, butyl acetate, and methyl ethyl ketone as the dispersion medium. The polyvinyl alcohol content of the second liquid may be 0.1% by mass or more and 5% by mass or less. If the polyvinyl alcohol content is 0.1% by mass or more, the amount of polyvinyl alcohol in the polyvinyl alcohol mixture is sufficient, so the polyvinyl alcohol is easily deposited on the carbon nanotube layer 3. If the polyvinyl alcohol content is 5% by mass or less, the dispersibility of the polyvinyl alcohol is not impaired, and the second liquid can be uniformly coated onto the carbon nanotube layer 3. When polyvinyl alcohol is crosslinked after coating, the crosslinking network becomes denser and the pore size of the film becomes smaller. The higher the degree of polymerization of polyvinyl alcohol, the denser the crosslinking network becomes and the smaller the pore size of the film becomes.

[0088] <CNT layer deposition process S4> In the CNT layer deposition process S4, the first liquid is applied to at least a portion of the first surface 21 of the substrate 2 to form a carbon nanotube layer 3. For example, the first liquid may be applied to the first surface 21 using a bar coater, applicator, dip coater, spin coater, spray coater, or die coater. Alternatively, the first liquid may be applied to another substrate, and the first surface 21 may be placed on top of it with the first surface 21 facing downwards to transfer the first liquid onto the first surface 21. Alternatively, the first liquid may be dropped onto the first surface 21 and spread uniformly to coat it. The smaller the diameter and the shorter the length of the carbon nanotubes, the denser the film structure becomes, and the aperture diameter and aperture ratio decrease.

[0089] If the first liquid contains polyvinyl alcohol, the CNT layer formation step S4 may include either or both of the following steps: immersing the substrate 2 on which the carbon nanotube layer 3 has been formed in the crosslinking solution to crosslink the polyvinyl alcohol in the CNT layer; and adding a crosslinking agent to the first liquid, then applying the first liquid to at least a portion of the first surface 21 of the substrate 2 to form the carbon nanotube layer 3 and crosslink the polyvinyl alcohol (hereinafter also referred to as "crosslinking step S41").

[0090] The crosslinking solution can be prepared by adding a crosslinking agent to a solvent such as water, ethanol, or isopropyl alcohol. Examples of crosslinking agents include glutaraldehyde, silica, alkoxysilane, titania (TiO2). ２ ) and titanium alkoxides, titanium chelates, or epoxides may also be used. The crosslinking reaction may also be carried out by adding a reaction catalyst such as sulfuric acid, nitric acid, or hydrochloric acid, or by heating.

[0091] <Step S5 for obtaining a composite porous body> In step S5 for obtaining a composite porous body, the second liquid is applied to at least a portion of the surface of the carbon nanotube layer 3 to form a polyvinyl alcohol layer 4 made of a non-porous polyvinyl alcohol, thereby obtaining a composite porous body 1. The method of applying the second liquid is not particularly limited. For example, the second liquid may be applied to the surface of the carbon nanotube layer 3 using a bar coater, applicator, dip coater, spin coater, spray coater, or die coater. Alternatively, the second liquid may be applied to another substrate, and the first surface 21 may be placed on top of it with the first surface 21 facing downwards to transfer the second liquid onto the first surface 21. Alternatively, the second liquid may be dropped onto the first surface 21 and spread uniformly to coat it.

[0092] Step S5 for obtaining a composite porous body may include either or both of the following steps: immersing the composite porous body in a crosslinking solution to crosslink the polyvinyl alcohol in the polyvinyl alcohol layer, and adding a crosslinking agent to the second solution, then applying the second solution to at least a portion of the surface of the carbon nanotube layer to form a polyvinyl alcohol layer and crosslink the polyvinyl alcohol (hereinafter also referred to as "crosslinking step S51"). Crosslinking step S51 may further include immersing the composite porous body in a crosslinking solution to crosslink the polyvinyl alcohol in the carbon nanotube layer.

[0093] The crosslinking solution can be prepared by adding a crosslinking agent to a solvent such as water, ethanol, or isopropyl alcohol. Examples of crosslinking agents include glutaraldehyde, silica, alkoxysilane, titania (TiO2). ２ ) and titanium alkoxides, titanium chelates, or epoxides may also be used. The crosslinking reaction may also be carried out by adding a reaction catalyst such as sulfuric acid, nitric acid, or hydrochloric acid, or by heating.

[0094] When a crosslinking agent is added to the second solution, the crosslinking agent content in the second solution may be 0.1% by mass or more and 5% by mass or less. When polyvinyl alcohol is crosslinked after application, the crosslinking network becomes denser and the pore size of the film becomes smaller. The higher the degree of polymerization of polyvinyl alcohol, the denser the crosslinking network becomes and the smaller the pore size of the film becomes.

[0095] This embodiment will be described in more detail by reference to examples. However, this embodiment is not limited by these examples.

[0096] [Preparation of Composite Porous Materials] Composite porous materials were prepared for each sample using the following procedure.

[0097] <Substrate Preparation Process S1> In substrate preparation process S1, a substrate made of a polytetrafluoroethylene porous material was prepared. The shape of the substrate, average thickness, average pore diameter of the first surface, and presence or absence of PVA on the first surface are shown in Table 2.

[0098]

[0099] <First Liquid Preparation Step S2> In the first liquid preparation step S2, a dispersion of carbon nanotubes was prepared as the first liquid containing carbon nanotubes, by dispersing carbon nanotubes in water. The carbon nanotube content of the first liquid is as shown in Table 2. Furthermore, for samples where "Yes" is indicated in the "PVA Addition" column of "First Liquid" in Table 3, 0.01% by mass of polyvinyl alcohol was added to the first liquid.

[0100] <Second Liquid Preparation Step S3> In the second liquid preparation step S3, a dispersion of polyvinyl alcohol was prepared as the second liquid containing polyvinyl alcohol by dispersing polyvinyl alcohol in water. The polyvinyl alcohol content of the second liquid is 1% by mass. The degree of polymerization of the polyvinyl alcohol used in each sample is shown in Table 3.

[0101] <CNT layer deposition process S4> In the CNT layer deposition process S4, the first liquid was applied to the first surface of the substrate using an applicator to deposit a carbon nanotube layer. For samples where "Yes" is indicated in the "Post-deposition crosslinking" column of "CNT layer deposition process S4" in Table 3, the substrate 2 on which the carbon nanotube layer 3 was deposited was immersed in a crosslinking solution made by adding glutaraldehyde to water to crosslink the polyvinyl alcohol in the CNT layer. The glutaraldehyde content of the crosslinking solution was 3% by mass.

[0102] <Step S5 for obtaining a composite porous body> In step S5 for obtaining a composite porous body, the second liquid was applied to the surface of the carbon nanotube layer using an applicator to form a PVA layer and obtain a composite porous body.

[0103] In the samples where "Yes" is indicated in the "Immersion in Crosslinking Solution" column of "Step S5 for Obtaining Composite Porous Body" in Table 3, the composite porous body was immersed in a crosslinking solution containing glutaraldehyde added to water to crosslink the PVA in the PVA layer. The glutaraldehyde content of the crosslinking solution was 3% by mass.

[0104] In the samples where "Yes" was indicated in the "Addition of crosslinking agent to the second liquid" column of "Step S5 for obtaining a composite porous body" in Table 3, glutaraldehyde (sample 9) or ethylene glycol diglycidyl ether (sample 4) was added to the second liquid, and then the second liquid was applied to at least a portion of the surface of the carbon nanotube layer to form a polyvinyl alcohol layer and crosslink the polyvinyl alcohol. The crosslinking agent content of the second liquid was 1 to 2% by mass.

[0105] In sample 1-1, a polyvinyl alcohol layer was not formed.

[0106]

[0107] [Measurement of Composite Porous Materials] For each sample, the average thickness, average aperture diameter, and aperture ratio of the carbon nanotube layer, and the average thickness and average pore diameter of the polyvinyl alcohol layer were measured using the method described in Embodiment 1. The results are shown in Tables 4 and 5. The average pore diameter of the polyvinyl alcohol layer corresponds to the average pore diameter of the composite porous material. The smaller the average pore diameter of the polyvinyl alcohol layer, the better the composite porous material can separate smaller substances, indicating higher separation performance. In sample 1-1, since no polyvinyl alcohol layer was formed, the average pore diameter of the composite porous material was measured.

[0108]

[0109]

[0110] [Fluid Measurement] The flux of each composite porous material was measured using the following filtration test. A sample of each composite porous material was placed in the sample holder of a test apparatus consisting of a chamber and a sample holder. The test liquid was added to the chamber and pressurized to 0.6 MPa. The amount passing through the inspection surface per unit time was calculated from the weight change of the filtrate permeating from the sample holder, and the flux was obtained. The test liquid was water. The results are shown in Table 5. The flux of the composite porous materials of samples 1 to 10 was 0.59 L / (m). ２ The flux was above (h·bar), confirming that the composite porous materials of samples 1 to 10 had sufficient flux.

[0111] [Durability Test] Using the composite porous body of Sample 1, the average pore size of the polyvinyl alcohol layer was measured using the method described in Embodiment 1, with only the pressure condition changed to 6 MPa. The results are shown in the "Durability Test" column of Table 6. The composite porous body of Sample 1 was found to have pressure resistance, and it was confirmed that it did not deform or dissolve even under high pressure conditions, maintaining the same average pore size of the polyvinyl alcohol layer as under low pressure conditions.

[0112] [Heat Resistance Test] After heating the composite porous body of sample 1 at 150°C for 15 minutes, the average pore size of the polyvinyl alcohol layer was measured using the method described in Embodiment 1. The results are shown in the "Heat Resistance Test" column of Table 6. The composite porous body of sample 1 was found to have heat resistance, and it was confirmed that it did not deform or melt even under high temperature conditions, maintaining the same average pore size of the polyvinyl alcohol layer as under low temperature conditions.

[0113] [Chemical Resistance Test] Using the composite porous body of Sample 1, the average pore size of the polyvinyl alcohol layer was measured using the method described in Embodiment 1, but with the test solution changed to ethanol, N,N-dimethylformamide, dimethyl sulfoxide, or N-methylpyrrolidone. The results are shown in the "Chemical Resistance Test" column of Table 6. The composite porous body of Sample 1 was found to have chemical resistance, and it was confirmed that it did not deform or dissolve even in organic solvents, maintaining the same average pore size of the polyvinyl alcohol layer as when the solvent was water.

[0114] [Pressure Dehydration Test] Using the composite porous material of sample 1, a filtration test, which is performed to measure flux, was carried out under pressure conditions of 6 MPa with a 5% by mass aqueous solution of 2-propanol as the test solution. As a result, the flux was 0.093 L / (m ２ The concentration was (h·bar), and the concentration of 2-propanol in the filtrate was 42.8% of the concentration of 2-propanol in the test solution. It was confirmed that the composite porous material of sample 1 is capable of separating not only solute and solvent, and dispersed phase and solvent, but also solvents from each other, i.e., liquids from each other.

[0115]

[0116] [Discussion] Samples 1 to 10 are examples. Sample 1-1 is a comparative example. It was confirmed that the composite porous materials of Samples 1 to 10 can separate smaller particles and have higher separation performance compared to the composite porous material of Sample 1-1.

[0117] While embodiments and examples of this disclosure have been described above, it is intended from the outset that the configurations of each of the embodiments and examples described above may be combined or modified in various ways as appropriate. The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the embodiments and examples described above, and all modifications within the scope of the claims are intended to be included in the meaning of equivalences.

[0118] 1. Composite porous body, 2. Substrate, 2h. Cavities, 2A. First layer, 2B. Second layer, 3. Carbon nanotube layer, 4. Polyvinyl alcohol layer, 5. Polyvinyl alcohol, 6. Carbon nanotube, 7. Test apparatus, 21. First surface, 22. Second surface, 30. Surface, 70. Beaker, 71U. Lower end opening, 71. Chamber, 72D. Upper end opening, 72. Holder.

Claims

1. A composite porous body comprising: a substrate having a first surface; a carbon nanotube layer covering at least a portion of the first surface; and a polyvinyl alcohol layer covering at least a portion of the carbon nanotube layer, wherein the substrate includes a first layer made of a polytetrafluoroethylene porous material having the first surface; the polyvinyl alcohol layer is made of a non-porous polyvinyl alcohol material; and the average pore size of the polyvinyl alcohol layer is 1 nm or more and 5 nm or less.

2. The composite porous body according to claim 1, wherein the polyvinyl alcohol is crosslinked.

3. The composite porous body according to claim 1 or claim 2, wherein the average thickness of the polyvinyl alcohol layer is 10 nm or more and 500 nm or less.

4. The composite porous body according to any one of claims 1 to 3, wherein the carbon nanotube layer contains polyvinyl alcohol.

5. The composite porous body according to any one of claims 1 to 4, wherein the average thickness of the carbon nanotube layer is 0.1 μm or more and 20 μm or less.

6. The composite porous body according to any one of claims 1 to 5, wherein the average aperture diameter of the carbon nanotube layer is 20 nm or more and 200 nm or less, and the aperture ratio of the carbon nanotube layer is 0.5% or more and 20% or less.

7. The composite porous body according to any one of claims 1 to 6, wherein polyvinyl alcohol is present in at least a portion of the first surface.

8. The composite porous body according to any one of claims 1 to 7, wherein the average pore diameter on the first surface is 20 nm or more and 2000 nm or less.

9. The composite porous body according to any one of claims 1 to 8, wherein the shape of the substrate is a sheet.

10. The composite porous body according to claim 9, wherein the average thickness of the substrate is 1 μm or more and 1000 μm or less.

11. The composite porous body according to any one of claims 1 to 8, wherein the shape of the substrate is a tube, and the first surface is the outer surface of the tube.

12. The composite porous body according to claim 11, wherein the average thickness of the substrate is 50 μm or more and 1000 μm or less.

13. The composite porous body according to any one of claims 1 to 12, wherein the substrate comprises the first layer and a second layer adjacent to the first layer, and the average pore diameter of the second layer is greater than the average pore diameter of the first layer.

14. A method for producing a composite porous body according to any one of claims 1 to 13, comprising the steps of: preparing a substrate including the layer made of the polytetrafluoroethylene porous body having the first surface; preparing a first liquid containing carbon nanotubes; preparing a second liquid containing polyvinyl alcohol; applying the first liquid to at least a portion of the first surface to form the carbon nanotube layer; and applying the second liquid to at least a portion of the surface of the carbon nanotube layer to form the polyvinyl alcohol layer made of a non-porous polyvinyl alcohol to obtain the composite porous body, wherein the average pore size of the polyvinyl alcohol layer is 1 nm or more and 5 nm or less.

15. The method for producing a composite porous body according to claim 14, wherein the step of obtaining the composite porous body includes either or both of the steps of: immersing the composite porous body in a crosslinking solution to crosslink the polyvinyl alcohol in the polyvinyl alcohol layer; and adding a crosslinking agent to the second solution, then applying the second solution to at least a portion of the surface of the carbon nanotube layer to form the polyvinyl alcohol layer and crosslink the polyvinyl alcohol.

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