Porous membrane prepared by stretching heat-treated sheet comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene

Abstract

This invention provides: a porous membrane comprising polytetrafluoroethylene (PTFE) and / or modified PTFE, having small pore size, being resistant to breakage, and resistant to external forces such as puncture and the like; and a method for manufacturing the same. Further, a porous membrane comprising PTFE and / or modified PTFE is provided, wherein the isopropanol bubble point is 500 kPa or greater according to JIS K3832, the value obtained by dividing the maximum force until needle penetration by the specimen thickness is 200 mN / µm or greater (based on the needle strength test according to JIS Z1707), the percentage of pore openings in the surface image obtained by electron microscopy is 10% to 30%, and the fiber thickness is 250 nm or greater.

Description

[Technical Field]

[0001] This invention relates to a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, which has small pore size and high strength and is particularly resistant to tearing and breakage; and a method thereof for manufacturing the membrane. [Previous Technology]

[0002] Polytetrafluoroethylene (PTFE) is used in a wide variety of fields due to its excellent heat resistance, chemical resistance, water repellency, weather resistance, and low dielectric constant. Many PTFE porous membranes with different properties and their manufacturing methods have been invented to make PTFE porous by stretching easier.

[0003] PTFE porous membranes have high water resistance, and are therefore used in applications such as waterproof and permeable clothing, vent filters for adjusting the internal pressure of automotive parts, and waterproof sound transmitting membranes for communication equipment.

[0004] Waterproof performance is indicated by values ​​from a waterproof pressure test. For example, the membrane used in a 100m waterproof phone or similar requires a water pressure resistance of 1 MPa. However, a membrane with a water pressure resistance of 1 MPa must have pore sizes of tens of nanometers or smaller.

[0005] Furthermore, in the case of vent filters used to adjust the internal pressure of automotive parts, welding of the parts requiring internal pressure adjustment is necessary, and the membrane must have the strength to withstand the welding. In addition, a moving vehicle may encounter harsh conditions such as dust, dirt, gravel, and other foreign materials that come into contact with the membrane at high speed, thereby damaging the membrane and reducing its waterproof function. Therefore, the membrane may have a protective cover or cap to provide a structure that prevents direct contact between foreign materials and the membrane. However, if the focus is on vehicle safety, this may be insufficient, and the membrane itself also needs to have high strength.

[0006] Furthermore, for waterproof items, the external forces applied to the membrane by agitation during cleaning from a washing machine or similar device must be taken into account, and the item must also be resistant to breakage. Not to mention, membrane strength is also required in these areas.

[0007] In dustproof applications, PTFE porous membranes are used in filters for air purifiers or cleaners, bag filters for dust collection (such as those for waste incinerators), air filters for cleanrooms used in semiconductor manufacturing, and the like.

[0008] Furthermore, due to the pure nature of PTFE, that is, because there is almost no precipitate, PTFE porous membranes have been used instead of conventional ultrafiltration membranes as the final filter in the production of ultrapure water.

[0009] In addition, it has excellent chemical resistance, so it may also be used in filtration applications (including circuit board etching solutions in corrosive liquids, organic solvents), or semiconductor manufacturing applications, and similar applications such as collecting valuable materials from etching solutions, and similar applications.

[0010] In semiconductor manufacturing applications, the integration of circuits has recently increased. Therefore, there is a need for PTFE porous membranes (with nanoscale pore sizes) capable of removing nanoscale fine particles from etching solutions, as the presence of nanoscale fine particles in etching solutions allows these particles to remain on the wiring of integrated circuits, causing yield reductions during manufacturing. Such porous membranes must possess sufficient strength to withstand filtration pressures and filtration operations.

[0011] Furthermore, the application of PTFE porous membranes has attracted attention in the energy and battery / capacitor fields, particularly in hydrogen production, which is an alternative method for storing energy. Hydrogen is produced using the electrolysis of water and similar substances, and the porous membrane is used as a separator between the positive and negative electrodes, subjected to redox reactions in concentrated alkaline solutions and at high temperatures. The electrodes are not smooth and have some inhomogeneity, and the crystalline materials generated by the electrode reactions can also damage the membrane. In addition, the membrane is strongly fixed between the electrodes, so it can suffer damage similar to that caused by pressure from, for example, a needle-like external object. In these fields, high strength of PTFE porous membranes is also required.

[0012] Based on the foregoing, the PTFE porous membrane must not only have a small pore size, but also be resistant to tearing and even have sufficient strength to resist breakage when pressed by a needle-like object.

[0013] However, it is difficult to obtain PTFE porous membranes that simultaneously have nano-sized pores and high strength.

[0014] Typically, PTFE porous membranes are manufactured using the following steps: 1. Mixing PTFE and additives (hydrocarbon-based solvents or similar). 2. Increasing the ratio of cylindrical cross-sectional area to outlet cross-sectional area (RR), and then applying shear (shear force) to the PTFE by extrusion to obtain sheet or bead-like extrusions during fiberization. 3. After properly rolling the obtained extrusion into a sheet shape using a rolling mill (roll) or similar, evaporating and removing the hydrocarbon-based solvent. 4. Stretching the obtained sheet product at high temperature along the extrusion direction (hereinafter also referred to as MD) and in a direction orthogonal to the extrusion direction (hereinafter also referred to as CD), and then baking at a temperature above the melting point of PTFE (342 to 343°C) to obtain a PTFE porous membrane.

[0015] However, using this conventional method, it is difficult to obtain PTFE porous membranes with small pore size, high strength, and resistance to tearing. This is believed to be because the stretching conditions are below the melting point of PTFE (342 to 343°C), which leads to the formation of many fine fibers and prevents strength increase. Furthermore, the increased draw ratio and porosity when PTFE is stretched well below its melting point are also considered factors. Increasing the draw ratio has the advantage of increasing tensile strength because PTFE molecules have stronger orientation but are often less resistant to tearing and the like.

[0016] Therefore, many methods for manufacturing PTFE porous membranes have been proposed, which involve preheating rolled and dried sheets to above the melting point of PTFE, and then stretching the sheets. Although heating to above the melting temperature is performed, this can still be referred to as stretching under conditions of incomplete baking, or semi-baked stretching. This method is known to produce thick fibers and obtain membranes with small pore sizes.

[0017] Patent document 1 details a method for measuring the heat of fusion of PTFE crystals, and describes the preparation of a sheet using a resin having a heat of fusion of 32 J / g or higher and lower than 47.8 J / g. The sheet is then heated above its melting point, cooled, and subsequently stretched to obtain a porous membrane with a porosity of 30% or higher and a thickness of 50 µm or lower. The resin having a heat of fusion of 32 J / g or higher and lower than 47.8 J / g mainly includes commercially available low molecular weight resins or resins whose molecular weight has been reduced by decomposing commercially available resins using radiation or the like.

[0018] Furthermore, in Patent Document 2, a PTFE coated film is formed by immersing a polyimide film in a PTFE dispersion, and a PTFE film is obtained by repeating drying / baking steps. The PTFE film is then peeled off from the polyimide film, and the peeled PTFE film is stretched sequentially along CD and MD. As a porous film obtained by this method, a needle-punch resistant film is prepared, which has a characteristic value of 5 to 15 gf / µm (49 to 147 mN / µm) per unit thickness.

[0019] In Patent Document 3, a stretch membrane with high filtration efficiency is prepared by sequentially stretching and thermally fixing a partially sintered membrane along the extrusion direction (MD) and a direction orthogonal to the extrusion direction (CD) (in which, in the process of manufacturing a PTFE porous membrane, a temperature gradient is formed along the thickness direction of the membrane by heating one side of the membrane before stretching). The stretch membrane has an asymmetric structure (in which the average pore size along the thickness direction is continuously reduced, and the average pore size of the heated surface is 0.05 µm to 10 µm) and is used for fine filtration of gases, liquids, and the like.

[0020] However, in Patent Document 1, the resin used has a low molecular weight, and from a strength point of view, low molecular weight resins are generally inferior, as is the case with general plastic materials. Furthermore, although Patent Document 2 details the needle-punching strength, it cannot be said that this strength is sufficient. In Patent Document 3, the strength of an extremely thin portion of the heated surface may be strong, but the overall film is not strong enough.

[0021] While these prior art techniques are effective in limited applications, they have problems (such as insufficient membrane strength in other applications and the like) and are considered insufficient in providing porous membranes with small pore sizes that can be used under more demanding conditions. Previous Art Documents Patent Documents

[0022] Patent Document 1: Japanese Patent Publication No. 5008850

[0023] Patent Document 2: Japanese Unexamined Patent Application No. 2018-204006

[0024] Patent Document 3: Japanese Patent Publication No. 4850814

[0025] Patent Document 4: WO 2016 / 117565

[0026] Patent Document 5: WO 2007 / 119829

[0027] Patent Document 6: Japanese Patent Publication No. 5054007

[0028] Patent Document 7: Japanese Unexamined Patent Application No. 2018-16697

[0029] Patent Document 8: U.S. Patent No. 3,037,953 [Summary of the Invention]

[0030] The problem that this invention aims to solve

[0031] One object of the present invention is to provide: a novel porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, having small pore size, being resistant to breakage, and resisting external forces such as puncture and the like. Means for solving the problem

[0032] The present invention provides a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein the isopropanol bubble point is 500 kPa or greater according to JIS K3832, the value obtained by dividing the maximum force until needle penetration by the specimen thickness is 200 mN / µm or greater (based on the needle penetration strength test according to JIS Z1707), the percentage of pore openings in the surface image by electron microscopy is 10 to 30%, and the fiber thickness is 250 nm or greater.

[0033] One preferred embodiment of the present invention comprises a porous membrane of polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein the polytetrafluoroethylene and / or modified polytetrafluoroethylene has a heat of fusion of less than 32 J / g at 296 to 343°C. This is achieved using a differential scanning calorimeter, wherein the polytetrafluoroethylene and / or modified polytetrafluoroethylene resin is heated to 365°C at a rate of 10°C / min, cooled to 330°C at a rate of -10°C / min, cooled from 330°C to 305°C at a rate of -1°C / min, cooled from 305°C to 245°C at a rate of -10°C / min, and then heated to 365°C at a rate of 10°C / min, wherein the bubble point is 600 kPa or greater, and the value obtained by dividing the maximum force up to a needle puncture by the thickness of a specimen is 250 mN / µm or greater (based on a needle penetration test).

[0034] The present invention also provides a method for manufacturing a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene as claimed in claim 1 or 2, comprising: 1. obtaining a sheet or coating comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene that has not been heat-treated at 250°C or higher; 2. fixing and heat-treating the sheet or coating such that the ratio (ΔH / ΔH0) of the following crystalline heat of fusion (ΔH) to (ΔH0) is 1.0 to 2.0; (ΔH0 is a heat of fusion between 295 and 360°C, which is obtained by heating the sheet or coating containing polytetrafluoroethylene and / or modified polytetrafluoroethylene resin, which has not been heat-treated at 250°C or higher, at 360°C for 20 minutes, and then increasing the temperature to 380°C at a rate of 10°C / min after cooling at room temperature; and ΔH is a heat of fusion between 295 and 360°C, which is obtained by heat-treating the sheet or coating containing polytetrafluoroethylene and / or modified polytetrafluoroethylene, which has not been heat-treated at 250°C or higher, and then increasing the temperature to 380°C at a rate of 10°C / min); and 3. a step of stretching the heat-treated sheet or coating in one direction and then sequentially stretching it in a second direction orthogonal to the first direction.

[0035] In this document, a preferred embodiment of the present invention is a method for manufacturing a porous membrane, wherein the heat treatment step is a step of fixing and heat-treating a sheet or coating such that the ratio (ΔH / ΔH0) of the melting heat of the crystal (ΔH) to (ΔH0) is one to 1.2 to 1.8.

[0036] In addition, a preferred embodiment of the present invention is a method for manufacturing a porous membrane, wherein a heat-treated sheet is stretched along an extrusion direction and then sequentially stretched along an orthogonal direction.

[0037] Furthermore, a preferred embodiment of the present invention is a method for manufacturing a porous membrane, wherein the sheet that has not been heat-treated at 250°C or higher is obtained by rolling a sheet-like or bead-like extrusion, which is obtained by mixing polytetrafluoroethylene and / or modified polytetrafluoroethylene with a hydrocarbon-based solvent having a boiling point of 150 to 290°C and then extruding it using an extruder at a molding temperature of RR 35 to 120 and room temperature to 120°C.

[0038] A preferred embodiment of the present invention is also a method for manufacturing a porous membrane, wherein the coating film, which has not been heat-treated at 250°C or higher, is obtained by coating a dispersion of polytetrafluoroethylene and / or modified polytetrafluoroethylene having a solids fraction concentration of 5 to 75% by mass, comprising a surfactant, a film-forming agent, and a thickener, onto a plate having a heat resistance of 400°C or higher, such that the thickness after drying is 1 to 50 µm, and then drying at 100 to 150°C for 10 to 20 minutes. Effects of the present invention

[0039] The present invention can be used as a separator or part of a separator in fuel cells, capacitors, lithium battery packs, and the like, and can be used as part of a separator that physically separates various positive and negative electrodes. In addition, it can be used in waterproof sound transmission applications for communication devices that require high water resistance and high strength, such as ventilation filters for automobiles, circuit board etching solutions in corrosive liquids, filtration applications for organic solvents, or semiconductor manufacturing applications, and the like, such as applications for collecting valuable materials from etching solutions, and the like.

Implementation Method

[0041] Cross-reference to related applications

[0042] This application claims priority to Japanese Patent Application No. 2021-14316, filed on February 1, 2021, the disclosure of which is incorporated herein by reference in its entirety.

[0043] The PTFE forming the porous membrane of the present invention may alternatively be modified PTFE, which can be copolymerized with tetrafluoroethylene (TFE) and modified to less than 1 wt.% without impairing the properties of PTFE. Examples of modified PTFE may include copolymers of TFE as described in Patent Document 5 and trace amounts of monomers other than TFE. More specific examples include copolymers of tetrafluoroethylene with at least one type of monomer that can copolymerize with tetrafluoroethylene, selected from less than 1 wt.% of hexafluoropropylene, perfluoro(alkyl vinyl ether), fluoroalkyl vinyl, and chlorotrifluoroethylene, which are copolymers that do not have melt-formability.

[0044] The PTFE and / or modified PTFE used in this invention refers to PTFE with a high molecular weight, which has a heat of fusion of less than 32 J / g at 296 to 343°C. This is obtained by using a differential scanning calorimeter to heat to 365°C at a rate of 10°C / min, cool to 330°C at a rate of -10°C / min, cool from 330°C to 305°C at a rate of -1°C / min, cool from 305°C to 245°C at a rate of -10°C / min, and then heat to 365°C at a rate of 10°C / min. More preferably, this is because it allows for the production of high-strength PTFE stretched films, in other words, PTFE porous films with high needle-punch strength. The PTFE and / or modified PTFE of this application can achieve higher mechanical strength with increasing molecular weight, similar to general-purpose plastic materials.

[0045] The molecular weight of PTFE or modified PTFE is related to the standard specific gravity (SSG) according to ASTM D4895, and the SSG of the PTFE or modified PTFE of the present invention is 2.19 or less, preferably 2.18 or less, and even more preferably 2.16 or less, which is suitable for preparing porous membranes with high strength.

[0046] Examples of resins that can be used include: 660J, 650J and modified polytetrafluoroethylene with high molecular weight manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd.; F106 and F104 manufactured by Daikin Industries, Ltd.; CD123E and CD145E manufactured by AGC Inc.; and the like.

[0047] The porous membrane containing PTFE or modified PTFE of the present invention meets all of the following requirements: ▪ Isopropanol (IPA) bubble point is 500 kPa or greater according to JIS K3832, ▪ The value obtained by dividing the maximum force until needle penetration by the specimen thickness is 200 mN / µm or greater (based on the needle penetration strength test according to JIS Z1707), ▪ The percentage of pore openings is 10 to 30%, and ▪ The fiber thickness is 250 nm or greater.

[0048] The isopropanol (IPA) bubble point of the porous membrane of the present invention is 500 kPa or greater according to JIS K3832, and preferably 600 kPa or greater. A bubble point of 500 kPa or greater indicates that the pore size of the PTFE porous membrane is a small pore size capable of removing nanoscale particles.

[0049] Generally, the maximum pore size of the PTFE porous membrane is calculated using the bubble point and the following equation. Maximum pore size of the PTFE porous membrane (diameter: nm) = 4 × γ × cosθ / P × 10⁹ γ: IPA surface tension (Pa•m) θ: Contact angle between IPA and the porous membrane (θ = 0) P: Bubble point pressure (Pa)

[0050] If the bubble point is 500 kPa, then the maximum pore size calculated by the aforementioned equation is approximately 146 nm. However, PTFE porous membranes have a large number of pores of 146 nm or smaller, thus they can capture particles of tens of nanometers when filtering liquids. Generally speaking, when the bubble point is less than 400 kPa, it is difficult to remove nano-sized nanoparticles and the waterproof performance deteriorates, which is not ideal.

[0051] The PTFE porous membrane of the present invention has a bubble point of 500 kPa or greater, and therefore the pore size of the porous membrane is small. In addition, the PTFE porous membrane of the present invention has high strength, so it will not tear even under a water pressure of about 100 m in ventilation filters and waterproof sound transmission applications, and will not cause water leakage.

[0052] With the PTFE porous membrane of the present invention, the value obtained by dividing the maximum force until needle penetration by the specimen thickness is 200 mN / µm or greater (in the needle penetration strength test according to JIS Z1707). The needle penetration strength test according to JIS Z1707 is a physical property required for applications in vent filters and battery pack separators, and this test is more suitable as an indicator of the tear resistance and fracture resistance of porous membranes than physical property values ​​such as tensile strength and the like. Specifically, in the present invention, a semi-circular needle (as specified in JIS Z1707) with a diameter of 1.0 mm and a tip shape radius of 0.5 mm is prepared, and puncture is performed at a test rate of 50 ± 5 mm / min, and the value obtained by dividing the maximum force (mN) until needle penetration by the specimen thickness (µm) is measured (needle penetration strength). A thicker membrane thickness increases the needle penetration strength, therefore the present invention specifies the strength per unit thickness to provide a film that is not easily broken.

[0053] The needle penetration intensity of the present invention is 200 mN / µm or greater, preferably 250 mN / µm or greater, and even more preferably 300 mN / µm or greater.

[0054] Furthermore, according to the aforementioned Patent Document 2, the needle penetration intensity is described as being between 49 and 146 mN / µm, while the PTFE porous membrane of the present invention has a value of 200 mN / µm or greater.

[0055] The pore opening portion (surface porosity) of the PTFE porous membrane of the present invention is 10 to 30%. The surface porosity is a physical property value related to the air permeability of the porous membrane, and therefore is preferably higher. However, if the surface porosity exceeds 30%, the needle penetration strength decreases (which is not preferred), and if the surface porosity is less than 10%, the air permeability or liquid permeability (flow rate) decreases (too low) (which is not preferred).

[0056] The surface openings of the porous membrane of the present invention are formed by stretching to deform (break) the crystalline portion, which is generated by heat-treating the PTFE sheet or coating during the manufacturing process of the porous membrane. This is well known to those skilled in the art of PTFE porous membranes. In the recrystallization process by heat treatment, the heating temperature, heating time, and the degree of crystal growth by slow cooling (degree of recrystallization) affect the porosity. If the heat treatment is insufficient, it is difficult to obtain a porous membrane with small pore size, and if the heat treatment is excessive, deformation by stretching is not easy to occur, so the membrane will not become porous.

[0057] The fiber thickness of the PTFE porous membrane of the present invention is 250 nm or greater, preferably 300 nm or greater. If the fiber thickness is less than 250 nm, the strength of the PTFE porous membrane cannot be achieved (the strength is reduced), which is not preferred.

[0058] The fibers in the PTFE porous membrane of the present invention are the amorphous portion of PTFE (e.g., irregularly aligned portions of PTFE molecular chains, whereas they are regularly aligned in the crystalline portion), which is produced by the heat treatment in the manufacturing process of the aforementioned PTFE porous membrane. Furthermore, the PTFE molecular chains have a high degree of entanglement and are difficult to deform (break) under load (such as tension, needle punching, and shear forces during the like), thus exhibiting excellent mechanical strength (needle punching strength).

[0059] The fiber of the present invention is considered to be different from the fiber with poor mechanical strength (PTFE molecular chain) caused by the dispersion of molecular chains in PTFE particles, which is caused by stretching a sheet that has not undergone heat treatment at 250°C or higher.

[0060] Regarding the surface porosity and fiber thickness of the aforementioned PTFE porous membrane, a method can be used to observe the surface of the porous membrane with an electron microscope and directly measure its size or area from the image. However, in this invention, the imaging software described in Patent Document 4 is preferred. For example, image analysis software such as Image-Pro-Plus, manufactured by Media Cybernetics, Inc., is used to color-code the porous membrane and openings as black and white, and automatically calculate their respective ratios. This allows for the automatic calculation of the porous membrane ratio. This method is called binary processing. The electron microscope image used for binary processing can be an image captured at a magnification capable of resolving pore and fiber structures, and the magnification is not limited. However, electron microscope images with magnifications of 5000x to 20000x are suitable for porous membranes with small pore sizes, wherein the IPA bubble point of this invention is 500 kPa or greater.

[0061] The thickness of the PTFE porous membrane of the present invention is not particularly limited, but a polytetrafluoroethylene porous membrane with a thickness of 70 µm or less is a preferred embodiment of the present invention. The preferred range of membrane thickness is 50 µm or less, and even more preferably 20 µm or less.

[0062] Next, a method for manufacturing a PTFE porous membrane will be described.

[0063] In this invention, a heat treatment and stretching method is used, wherein a pre-stretched sheet is heated above its melting point and then stretched, as described in Patent Document 1. In the general method described above, the fibers are prepared by stretching below their melting point and then baking, resulting in fine fiber diameters and insufficient strength. Furthermore, high needle-punching strength may not be achieved, which is the object of this invention.

[0064] As described above, stretching causes crystal deformation and porosity, and in the heat treatment and stretching method used in this invention, the larger heat of fusion of the PTFE used for manufacturing is beneficial for stretching and porosity. This is because PTFE is known to have the property of partially recrystallizing upon cooling, even after being heated and melted into crystals, and more crystals are present as the heat of fusion of the crystals increases.

[0065] If the crystal content is low, the membrane will not become porous even by stretching. On the other hand, if the crystal content is high, not only is it difficult to obtain a membrane with small pore size, but also a porous membrane with low needle penetration strength will be generated. Regarding this point, Patent Document 1 also describes the need for a crystal melting heat of 32 J / g or higher but lower than 47 J / g, even after heating to above the melting point and then cooling.

[0066] However, the resin with the heat of melting has a low molecular weight, making it difficult to prepare a film with high needle penetration strength.

[0067] In this invention, a method for preparing a membrane with small pore size (by stretching) and high needle penetration strength has been discovered, which is achieved by setting the heat treatment conditions within a specific range. Furthermore, when using PTFE of less than 32 J / g, a membrane with higher needle penetration strength can be obtained.

[0068] The details of the method for manufacturing the PTFE porous membrane of the present invention are as follows.

[0069] The manufacturing method of the present invention has three main steps: 1. obtaining a sheet or coating containing PTFE and / or modified PTFE that has not been heat-treated at 250°C or higher; 2. a heat treatment step; and 3. a stretching step.

[0070] First, the step of obtaining a sheet or coating containing PTFE and / or modified PTFE that has not been heat-treated at 250°C or higher will be described, followed by the description of the heat treatment step.

[0071] There are no particular restrictions on the method of obtaining sheets or coatings containing PTFE and / or modified PTFE.

[0072] First, the methods commonly used in the art can be used to obtain PTFE and / or modified PTFE.

[0073] In addition, the method of using "sheet" is preferably based on the following method of general method for manufacturing PTFE porous membrane, wherein polytetrafluoroethylene and / or modified polytetrafluoroethylene powder and a hydrocarbon-based solvent with a boiling point of 150 to 290°C are added and mixed, and then extruded using an extruder at RR 35 to 120 to obtain a sheet-like or bead-like extrusion or similar, and then the extrusion is rolled to prepare a sheet-like rolled product, and then the hydrocarbon-based solvent is removed.

[0074] Examples of hydrocarbon-based solvents used in the manufacture of the PTFE porous membrane according to the present invention include solvents based on straight-chain saturated hydrocarbons and / or branched-chain saturated hydrocarbons with boiling points of 150 to 290°C, and having at least one type having 8 to 16 carbon atoms. Examples of solvents based on straight-chain saturated hydrocarbons include: naphtha (a hydrocarbon-based solvent containing at least one type of straight-chain saturated hydrocarbon having 8 to 14 carbon atoms, boiling point: 150 to 180°C); Norpar 13 (carbon atoms: 12 to 14, boiling point: 222 to 243°C); Norpar 15 (carbon atoms: 9 to 16, boiling point: 255 to 279°C); and the like. Examples of solvents based on branched saturated hydrocarbons include: Isoper G (9 to 12 carbon atoms, boiling point 160 to 176°C), Isoper H (10 to 13 carbon atoms, boiling point 178 to 188°C), and Isomper M (11 to 16 carbon atoms, boiling point 223 to 254°C), each manufactured by Exxon Mobil Corporation; Supersol FP25 (11 to 13 carbon atoms, boiling point 150°C or higher), manufactured by Idemitsu Kosan Co., Ltd.; and similar products. Isomper M is preferred because it prevents solvent evaporation during rolling, can be easily removed by heating, and is odorless.

[0075] From the viewpoint of ease of extrusion molding, a hydrocarbon-based solvent (preferably Isopar M manufactured by Exxon Mobil Corporation) is added to PTFE in an amount of 16% wt.% to 22% wt.%, and more preferably 18 wt% to 20 wt%, and mixed for 3 to 5 minutes, and then allowed to stand at 20°C or higher for 12 hours or longer. Afterward, the resin is introduced into a cylindrical pressurizing device and pressurized within the cylinder, and air contained in the resin powder and hydrocarbon-based solvent is expelled to obtain a cylindrical preform.

[0076] Next, using an extruder, the cylindrical preform is extruded at an RR of 35 to 120, preferably 50 to 120, and more preferably 50 to 80, a molding temperature of 40 to 60, and more preferably 40 to 50, and a ram extrusion speed of 10 to 60 mm / min, and more preferably 20 to 30 mm / min, to obtain sheet extrusions, bead-like extrusions, tubular extrusions, and the like. Tubular extrusions can be cut and opened in the longitudinal direction by a blade to obtain sheet shapes.

[0077] If the extrusion rate is less than 10 mm / min, the productivity will be poor, which is not ideal. If the extrusion rate exceeds 60 mm / min, it will be difficult to increase the extrusion pressure or obtain a uniform extruded product, which is also not ideal.

[0078] If RR is less than 35, the strength of the extruded material will be reduced, which is not better because the primary particles of PTFE will not fiberize without sufficient shear (shear force) on them.

[0079] Furthermore, as the RR increases, the extrusion pressure during extrusion molding also increases. If the RR exceeds 120, a large molding machine is required, which is not ideal.

[0080] Furthermore, if the molding temperature is below room temperature, the compatibility between the hydrocarbon-based solvent and PTFE is poor, and the flowability will also deteriorate, which is not ideal. If the molding temperature exceeds 120°C, the hydrocarbon-based solvent will evaporate too quickly, which is also not ideal.

[0081] Two sets of rollers are used to roll sheet-like extruders and the like along MD rollers to achieve a predetermined thickness. Rolling is performed such that the rolled thickness is 200 µm or less, preferably 100 µm or less, and even more preferably up to 50 µm, but 50 to 100 µm is a limitation using this method.

[0082] Furthermore, Patent Document 6 describes a method for adjusting the tensile strength ratio of a rolled sheet between the MD and CD directions, which involves not only rolling the extruded sheet along the MD direction but also pulling the sheet, still containing additives, along a direction orthogonal to the extrusion direction. In this invention, the rolling method is not limited, and a similar method can be used before heat treatment if necessary to reduce the strength difference between the MD and CD directions of the sheet. In other words, the extruded sheet is cut to an appropriate length and then rolled along the MD direction. Subsequent rolling along the CD direction involves rotating the sheet rolled along the MD direction by 90 degrees relative to the MD direction and then deforming it along the CD direction. Rolling along these two directions can be combined to roll sheet extrusions and the like to a thickness of 400 µm or less, preferably 300 µm or less, and more preferably 200 µm or less, thereby obtaining a sheet-like rolled product.

[0083] The hydrocarbon-based solvent in the sheet-like rolled product is evaporated and removed at 150°C or higher, preferably 200°C or higher, for 5 minutes or longer, and preferably 15 minutes or longer, to obtain a rolled sheet that has not been heat-treated at 250°C or higher.

[0084] Next, the sheet is heated at 360°C for 20 minutes, cooled to room temperature, and then heated to 380°C at a rate of 10°C / min. The heat of fusion between 295°C and 360°C is measured at this point and is denoted as ΔH0. Next, the heat of fusion is measured under the same conditions, except that only the heating temperature and heating time are changed. When this is set as ΔH, the heating temperature and heating time are determined such that the value of ΔH / ΔH0 is between 1.0 and 2.0. The heat treatment temperature must be higher than the melting point of PTFE. The value of ΔH / ΔH0 is preferably between 1.2 and 1.8, and even more preferably between 1.2 and 1.6.

[0085] The heat treatment is performed by fixing a sheet containing PTFE and / or modified PTFE so that its dimensions do not change.

[0086] This heat treatment step also applies to the heat treatment steps described below for coatings.

[0087] Please note that the rolled sheet obtained by the aforementioned method (which has not yet been heat-treated at 250°C or higher) can be continuously passed through a heating furnace to undergo heat treatment in the range of ΔH / ΔH0 from 1.0 to 2.0. Furthermore, after being cut to a predetermined area, heat treatment may also be performed in a high-temperature dryer. Although specific heat treatment conditions cannot be universally specified because combustion conditions vary depending on the type of resin, general PTFE, or modified PTFE, general PTFE or modified PTFE can be obtained by heating at a temperature of approximately 400°C, preferably 350°C to 400°C, and more preferably 350°C to 385°C for about 30 to 500 seconds to achieve a ΔH / ΔH0 value in the range of 1.0 to 2.0.

[0088] The method of using a "coating" will be described next. This method is based on the method of applying a dispersion liquid onto a plate according to Patent Document 7.

[0089] A preferred method involves adding a water-soluble polymer and an organic solvent, which serve as surfactants, film-forming agents, and thickeners, to a dispersion having a solids fraction concentration of 5 to 75% by mass, and more preferably 40 to 65% by mass, the dispersion comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene having an average particle size of 0.01 to 5.00 µm, more preferably 0.10 to 1.00 µm, and even more preferably 0.10 to 0.50 µm, then coating it onto a smooth plate of 1 mm or more having a heat resistance of 400°C or higher, and heating it to a temperature of 360°C or higher after removing the water content, cooling it to room temperature after decomposing and removing the additives, and then peeling it off from the smooth plate to prepare a coating film.

[0090] The dispersion of the water-soluble polymer and organic solvent added as surfactant, film-forming agent and thickener preferably has a viscosity of 1 to 600 cps, more preferably 100 to 600 cps, and even more preferably 200 to 500 cps, which is obtained by means of a type B viscometer (viscosity at 30 rpm using rotor No. 2).

[0091] Examples of surfactants added to the dispersion include: LEOCOL manufactured by Lion Specialty Chemicals Co., Ltd., TRITON and TERGITOL series manufactured by Dow Chemical Company, EMGULGEN manufactured by Kao Corporation, and other nonionic surfactants based on polyoxyethylene alkyl ethers and polyoxyethylene alkyl phenyl ethers; LIPAL manufactured by Lion Specialty Chemicals Co., Ltd., EMAL and PELEX manufactured by Kao Corporation, and other sulfosuccinates, sodium alkyl ether sulfonates, and anionic surfactants based on mono-long-chain alkyl sulfates; LEOARL manufactured by Lion Specialty Chemicals Co., Ltd., OROTAN manufactured by Dow Chemical Company, and other polymeric surfactants based on polycarboxylate esters and acrylates; and similar. Examples of film-forming agents include polyamides, polyamide-imides, acrylic polymers, acetates, and other polymeric film-forming agents with film-forming effects, higher alcohols, ethers, and polymeric surfactants, and the like. Examples of thickeners that may be added include water-soluble cellulose, solvent-dispersible thickeners, sodium alginate, casein, sodium caseinate, styrax, polyacrylic acid, acrylates, and the like.

[0092] In addition, a freshly polymerized dispersion can be used as the dispersion liquid, but the dispersion liquid is preferably a dispersion liquid that has been concentrated and stabilized by known techniques (such as the methods described in Patent Document 8 or similar). The concentration of the dispersion liquid is preferably 5 to 75% by mass, and the concentration of the PTFE resin is preferably increased by concentrating it to 40 to 70% by mass.

[0093] Furthermore, as described above, the resin with less than 32 J / g (medium / high molecular weight resin) described in Patent Document 1 is preferred as a resin system.

[0094] Next, the aforementioned surfactant, film-forming agent, water-soluble polymer as thickener, and organic solvent are added to the dispersion. The dispersion is then applied to a stainless steel plate, aluminum plate, polyimide film, or glass plate with a heat resistance of 400°C or higher, and then heated to approximately 100°C to dry the water content. In this invention, the coating method is not limited, but suitable methods include: spraying with a nozzle and drying the water content; immersing the plate in a dispersion containing the aforementioned surfactant, film-forming agent, water-soluble polymer as thickener, and organic solvent, removing the plate at a predetermined speed, and then drying; and the like. The coating thickness can be freely controlled by the viscosity of the dispersion, the number of sprays, the spray volume, the removal speed, and the like. In the coating step, coating can also be performed continuously on the polyimide film or aluminum plate, and the speed and length of the oven can be adjusted for production to maintain a long time in the hot air drying oven. In addition, the target coating can also be obtained by applying it to a glass or aluminum plate over a predetermined area and then heat-treating it in a high-temperature drying oven.

[0095] Subsequently, the additives are decomposed and removed at a temperature of 360°C or higher. When the heating temperature is low, the coating film will be colored due to the effect of carbonized additives, so heating is required until the coating film is completely white. The heating temperature will vary depending on the type of additive, but must be performed at a melting point of 400°C, preferably 350 to 400°C, and more preferably 350 to 385°C for the PTFE or modified PTFE used. In addition, the heating time must be determined by confirming the degree of decomposition and removal, but heating is preferably performed for at least 20 minutes. After heating, cooling is performed at room temperature, and then a porous membrane is prepared by a stretching step. Note that the coating film cooled at room temperature has already been heated above the melting point, so the heat treatment for heat treatment is no longer required.

[0096] Please note that in this case, the coating is considered to be fully baked rather than under heat treatment conditions. However, in this invention, any heat treatment is defined as heat treatment as long as the ratio of ΔH to ΔH0 obtained by measuring ΔH and calculating ΔH0 is 1.0 to 2.0.

[0097] A method is specifically described for preparing sheets by mixing PTFE resin with a hydrocarbon-based solvent, extruding, rolling, and drying, followed by heat treatment of the sheets; and a method is described for preparing thin porous membranes by coating a dispersion of PTFE resin and then heat-treating to simultaneously decompose and remove additives to prepare a coating film, for use in preparing heat-treated sheets or coatings in methods for manufacturing porous membranes comprising PTFE and / or modified PTFE according to the present invention. In the present invention, there are no limitations on the preparation of heat-treated sheets or coatings, but the use of both methods is suitable. Regarding the heating method for further performing heat treatment, high-temperature drying is described. However, the heating method is not limited to this method; methods using infrared heaters followed by contact heating to surfaces above their melting point (including heated rollers) can also be used in the present invention.

[0098] Finally, describe step 3, stretching.

[0099] In the stretching step, the heat-treated sheet obtained as described above is stretched in one direction in an atmosphere of 150 to 320°C, and preferably 300°C, and then sequentially stretched in a direction orthogonal to it to prepare a porous membrane.

[0100] In methods for stretching materials that have not yet undergone the 2. heat treatment step, it is common to determine to some extent which direction the stretching initially occurs, and to stretch along the direction of extrusion. However, the stretching and stretching method using heat treatment used in this invention can be performed in any direction. Specifically, porous membranes can be prepared without problems when using sheets prepared by coating with a dispersion or when using the method described in Patent Document 6 to prepare sheets with no strength difference along the MD and CD, regardless of the direction in which the stretching begins.

[0101] If heat fixation is required in the stretched porous membrane, baking for 10 to 120 seconds can be performed at a PTFE melting point of 400°C, preferably 350 to 400°C, and more preferably 350 to 385°C.

[0102] In the stretching step for obtaining a PTFE porous membrane, a discontinuous (batch-type) stretching method and a continuous stretching method are used to obtain the sheet-like rolled product from step 2. In this invention, a PTFE porous membrane can be obtained by appropriately selecting a stretching method or stretching apparatus according to the target properties of the PTFE porous membrane.

[0103] The sheet-shaped rolled product undergoing step 2 is less likely to have a stretch ratio along MD and CD that exceeds that of a sheet-shaped rolled product that has not been heat-treated in this manner. Therefore, the stretch ratio along MD and CD is limited to 5 to 7 times. Furthermore, although it is not necessary to set the stretch ratio along MD and CD to the same ratio, the stretch ratio in each direction can be determined according to the purpose.

[0104] The discontinuous (batch type) stretching method is a method of cutting sheet-like products that have been heat-treated at a temperature above the melting point, rolling them, and then stretching them sequentially using a biaxial stretching machine.

[0105] In the continuous stretching method, the sheet-like rolled product from step 2 is first continuously stretched in the same direction (MD) as the extrusion direction of the sheet-like rolled product from step 2 using a longitudinal (extrusion direction) stretching device. This longitudinal stretching device has multiple sets of rollers (rolls) capable of being heated and vertically pinning. When using multiple sets of rollers for continuous stretching along the extrusion direction (MD), the speed difference is preferably set to the rotational speed of each set of rollers. More specifically, the rotational speed of the second and subsequent roller sets is increased to be faster than the rotational speed of the first roller set, thus completing the longitudinal stretching. Therefore, the stretching ratio is the ratio of the rotational speeds. When using three or more roller sets, the longitudinal stretching (continuous stretching along the extrusion direction (MD)) is preferably performed by increasing the speed in the travel direction.

[0106] Although not limited to this, the diameter of the roll is usually about 200 mmΦ. In addition, a method of continuous stretching in the extrusion direction (MD) using a device with a heating furnace between each set of rolls can also be suitably used.

[0107] Next, using a tenter frame capable of continuous stretching along a direction orthogonal to the extrusion direction (CD), the sheet-like stretched material (continuously stretched along the extrusion direction (MD)) is continuously clamped on both sides with chucks. While heating, the chucks are moved, and the stretched material is continuously extended along a direction orthogonal to the extrusion direction (CD) to obtain a PTFE porous membrane. Example

[0108] While not strictly limited to these examples, the invention will be described in further detail below using these examples. Standard Specific Gravity (SSG)

[0109] The standard specific gravity of PTFE is determined according to ASTM D4895. Heat of fusion of crystals.

[0110] The melting heat system of the crystal was determined using a differential scanning calorimeter (a Diamond DSC manufactured by PerkinElmer Co., Ltd.). 1. PTFE or modified PTFE

[0111] The heat of fusion of crystals at 296 to 343°C is determined by heating 10 mg of PTFE and / or modified PTFE (not previously heated to 250°C or higher) to 365°C at a rate of 10°C / min, cooling it to 330°C at a rate of -10°C / min, cooling it from 330°C to 305°C at a rate of -1°C / min, cooling it from 305°C to 245°C at a rate of -10°C / min, and then heating it to 365°C at a rate of 10°C / min. 2. Sheets or coatings

[0112] ΔH0: The heat of fusion of crystals (J / g) between 295 and 360 °C is obtained from the DSC curve. It is obtained by heating a sheet or coating of PTFE or modified PTFE that has not been heated to 250 °C or higher at 360 °C for 20 minutes, and then heating 10 mg of the sheet or coating cooled at room temperature to 380 °C at a rate of 10 °C / min.

[0113] ΔH: Heat of fusion of crystals (J / g) between 295 and 360 °C, obtained by heating 10 mg of a sheet or coating containing polytetrafluoroethylene and / or modified polytetrafluoroethylene (not yet heat-treated at 250 °C or higher) to 380 °C at a rate of 10 °C / min, such that the ratio of heat of fusion of crystals (ΔH0) to (ΔH) (ΔH / ΔH0) is 1.0 to 2.0. IPA bubble point

[0114] Isopropyl alcohol (IPA) bubble point is determined according to JIS K3832 using Porolux 1000 manufactured by MicrotracBEL Corp. Tensile strength and air permeability (Gurley value) are also considered.

[0115] Using porous membrane samples prepared from PTFE porous membranes under the conditions indicated in Table 1, tensile strength was measured according to JIS K6251 using Tensilon RTC1310A manufactured by Orientec Co., Ltd. at 25°C, a clamping distance of 22 mm, and a stretching speed of 200 mm / min. Note that the MD 50 mm × CD 10 mm porous membrane sample is used for strength along the MD (extrusion direction), while the MD 50 mm × CD 10 mm porous membrane sample is used for strength along the CD (direction orthogonal to the extrusion direction). Furthermore, MD strength × CD strength is an indicator of the overall strength of the PTFE porous membrane, and as its value increases, the strength becomes superior.

[0116] Air permeability was measured using a Garley Densometer manufactured by Toyo Seiki Seisaku-sho, Ltd. The PTFE porous membrane's open-cell ratio (surface open-cell ratio) and fiber diameter were also measured.

[0117] After the PTFE porous membrane was deposited by sputtering with a platinum-palladium alloy, it was observed under an electron microscope (SU-8000 manufactured by Hitachi High-Tech Corporation).

[0118] In this example, the surface structure was observed at 10,000× and binarization was performed using Image-Pro-Plus image analysis software manufactured by Media Cybernetics to calculate the pore opening ratio of the porous membrane.

[0119] Fiber diameter was analyzed using the Fibermetric function of Phenom™ Pro Suite, a ProX staged scanning electron microscope software manufactured by Phenom World. Film thickness

[0120] Measurements were performed using a needle-plate thickness gauge manufactured by Peacock. Needle penetration test (needle penetration strength).

[0121] A semi-circular needle (as specified in JIS Z1707) with a diameter of 1.0 mm and a tip radius of 0.5 mm was fabricated. Puncture was performed at a test rate of 50 ± 5 mm / min, and the value obtained by dividing the maximum force (mN) until the needle penetrated the specimen by the specimen thickness (µm) was measured (needle penetration strength). The PTFE used in Example 1 was polymerized...

[0122] 60 g of paraffin wax, 2300 ml of deionized water, 12 g of ammonium salt of fluoromonoether acid (chemical formula: C3F7-O-CF(CF3)COOH), 0.05 g of ammonium salt of fluoropolyether acid (C3F7-O-[CF(CF3)CF2]n-CF(CF3)COOH), 0.75 g of succinic acid, 0.026 g of oxalic acid, and 0.01 g of zinc chloride were placed in a high-pressure reactor (with a capacity of 4 liters) made of stainless steel (SUS316) equipped with stirring blades and a temperature regulating jacket. The system was then heated to 80°C, and the internal pressure was purged three times with nitrogen to remove oxygen, thus performing a vacuum process. Afterwards, 1 g of perfluorobutylethylene was added, and the internal temperature was maintained at 63°C while stirring at 111 rpm. The internal pressure was maintained at 2.75 MPa using tetrafluoroethylene (TFE).

[0123] Next, 510 ml of an aqueous solution was pumped, in which 40 mg of potassium permanganate (KMnO4) was dissolved in 2000 ml of water. At the end of the potassium permanganate injection, the internal temperature was increased to 85°C, and then TFE was supplied. Stirring was stopped when 740 g of TFE was consumed. The gas in the autoclave was released to atmospheric pressure, a vacuum was created, and the pressure was restored to atmospheric pressure using nitrogen. The contents were then removed to complete the reaction. The obtained PTFE dispersion had a solids fraction of 28%, and the average particle size of the primary particles was 0.24 µm. Subsequently, the obtained dispersion was diluted with water to a solids fraction of 15%, and stirring continued at room temperature until the agglomerated secondary particles were separated by mechanical stirring.

[0124] The obtained agglomerated secondary particles (PTFE powder) were dried at 190°C for 11 hours to obtain fine PTFE powder. The standard specific gravity (SSG) of the obtained fine PTFE powder and the heat of fusion of the crystals specified in Patent Document 1 are shown in Table 1. Example 1

[0125] Using fine PTFE powder, Isopar M manufactured by Exxon Mobil Corporation, and the amounts indicated in Table 1, the mixture was mixed for five minutes using a Turbula shaker manufactured by Willy A. Bachofen AG, and allowed to stand at 25°C for 24 hours. The mixture was then placed in a cylinder of a preforming machine with a diameter of 80 mm Φ. The top of the cylinder was then covered with a lid, and the cylinder was compressed at room temperature (approximately 15 to 30°C) at a speed of 50 mm / min to obtain a cylindrical preform. The obtained preform was extruded and shaped using an extruder at an RR of 36, a molding temperature of 50°C, and an extrusion speed of 20 mm / min. The extruded material was then extruded using an extrusion die (thickness: 1 mm × width: 140 mm) to obtain a sheet extrusion. The obtained sheet extrusion was cut into 250 mm lengths and rolled multiple times using two sets of rollers heated to 50°C along the extrusion direction (MD) and in a direction perpendicular to the extrusion direction (CD) until the thickness after rolling shown in Table 1 was achieved. Then, the aforementioned Isopar M was evaporated at 200°C and removed for 15 minutes to obtain a sheet-like rolled product, which was then cut into squares (120 mm square).

[0126] The obtained sheet was fixed to a 1 mm thick aluminum plate (100 mm square) at its four corners, and then heat-treated using a high-temperature dryer at the temperatures and times shown in Table 1. After heating, cooling was performed at room temperature, and then ΔH and ΔH0 were measured. Table 1 shows the temperature and time of the heat treatment.

[0127] For stretching, a biaxial stretching device (EX10-S5 type, manufactured by Toyo Seiki Seisaku-sho, Ltd.) is used to fix the periphery of a square (90 mm square) rolled product by means of a chuck (size: 72 mm angle, excluding the chuck grip of the biaxial stretching device). The product is stretched twice along MD and CD sequentially at a molding temperature of 300°C and a stretching speed (the speed of chuck movement) of 4.32 m / min to obtain a stretched material (PTFE porous membrane) (batch type).

[0128] Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0129] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions. Examples 2 and 3

[0130] In Example 2, PTFE resin 650J manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. was used, and sheets were prepared under all the same conditions as in Example 1, except for the heating times shown in Table 1.

[0131] In Example 3, PTFE resin 660J manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. was used, and sheets were prepared under all the same conditions as in Example 1, except for the heating times shown in Table 1.

[0132] Table 1 shows the physical properties of the PTFE porous membranes obtained in Examples 2 and 3.

[0133] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions.

[0134] Please note that Figures 1 and 2 show photographs of the PTFE porous membrane surface obtained in Example 2 by electron microscopy at 10,000× and photographs obtained by binarizing the image. Example 4

[0135] Using PTFE resin 650J manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd. as the resin, a glass plate (10 cm × 10 cm) was immersed in a PTFE dispersion (PTFE specific gravity: 2.16, average particle size: 0.25 µm) with a viscosity of 418 cps and a solids fraction concentration of 65% by mass. Oil adhering to the glass plate was removed using a Kimwipe containing isopropanol. The PTFE dispersion contained a solvent dispersant thickener (decomposition temperature: below 380°C), an acrylic film-forming agent (decomposition temperature: below 380°C), and a nonionic surfactant (LEOCOL TDN90-80 manufactured by Lion Specialty Chemicals Co., Ltd., decomposition temperature: 300°C or lower). The plate was vertically removed at a removal rate of 10 mm / s, dried at 120°C for 15 minutes, and then heat-treated at 380°C for 60 minutes to obtain a 35% PTFE solution. A coating film with a thickness of µm was then applied. The coating was then peeled off the glass plate.

[0136] The peeled coating was stretched under the same conditions as in Example 1 to obtain a stretched material (PTFE porous membrane). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0137] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions. Example 5

[0138] Except for the stretch ratios shown in Table 1, stretching was performed under the same conditions as in Example 3 to obtain a stretched material (PTFE porous membrane). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0139] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions. Comparative Example 1

[0140] Using PTFE resin 650J manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd., sheet-like material was rolled and stretched twice along MD and CD sequentially at a molding temperature of 300°C and a stretching speed (speed of moving chuck) of 4.32 m / min without heat treatment to obtain a stretched material (PTFE porous membrane) (batch type). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0141] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions. Comparative Example 2

[0142] Using PTFE resin 650J manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd., sheet-like material was rolled and stretched 10 times sequentially along MD and CD at a molding temperature of 300°C and a stretching speed (speed of the moving chuck) of 4.32 m / min without heat treatment to obtain stretched material (batch type). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0143] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions.

[0144] Please note that Figures 3 and 4 show photographs of the porous membrane surface structure obtained in Comparative Example 2 by electron microscopy at 5000× and photographs obtained by binarizing the image. Comparative Example 3

[0145] Except for the heating time shown in Table 1, stretching was performed under the same conditions as in Example 1 to obtain a stretched material (PTFE porous membrane). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0146] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions. Comparative Example 4

[0147] Except for the heating time shown in Table 1, stretching was performed under the same conditions as in Example 3 to obtain a stretched material (PTFE porous membrane). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0148] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions. Comparative Example 5

[0149] Except for the heating time shown in Table 1, stretching was performed under the same conditions as in Example 3 to obtain a stretched material (PTFE porous membrane). Table 1 shows the physical properties of the obtained PTFE porous membrane.

[0150] In addition, Table 1 shows the heat of fusion and ΔH / ΔH0 of the resin under predetermined conditions.

[0151] The present invention can provide a porous membrane with small pore size, high needle penetration strength and high tensile strength.

[0152] Films with strengths far exceeding those of conventionally stretched films can be prepared by heat treatment followed by stretching. Table 1 PTFE porous membrane Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Manufacturing conditions Resin type - Polymer products 650J 660J 650J 660J 650J 650J Polymer products 660J 660J Modified PTFE PTFE PTFE PTFE PTFE PTFE PTFE Modified PTFE PTFE PTFE PTFE specific gravity (SSG) 2.14 2.16 2.17 2.16 2.17 2.16 2.16 2.14 2.17 2.17 Heat of fusion of PTFE powder J / g 16.4 23.3 26.6 23.3 26.6 23.3 23.3 16.4 26.6 26.6 Thickness after rolling µm 200 200 200 35 200 200 200 200 200 200 Heating temperature ℃ 360 360 360 380 360 N / A (Unbaked) N / A (Unbaked) 360 360 360 Heating time sec 210 240 240 3600 240 N / A (Unbaked) N / A (Unbaked) 150 150 180 ΔH J / g 17.8 23.2 24.8 24.9 24.8 65.5 66.9 54.7 58.8 44.2 ΔH0 J / g 12.1 16.2 17.9 16.2 17.9 16.2 16.2 12.1 17.9 17.9 ΔH / ΔH0 - 1.47 1.43 1.39 1.54 1.39 4.04 4.13 4.52 3.28 2.47 stretch ratio - Vertical: 2 / Horizontal: 2 Vertical: 3 / Horizontal: 2.5 Vertical: 2 / Horizontal: 2 Vertical: 10 / Horizontal: 10 Vertical: 2 / Horizontal: 2 Vertical: 2 / Horizontal: 2 Vertical: 2 / Horizontal: 2 physical properties IPA Bubble Point kPa 665.0 953.9 810.7 521.0 746.0 46.3 107.9 298.0 56.0 201.2 Tensile strength along MD MPa 74.8 55.1 48.8 42.0 61.3 12.7 29.2 26.7 8.9 26.6 Tensile strength along CD MPa 70.5 57.7 55.4 37.1 44.1 9.7 8.2 23.6 6.1 25.1 Intensity along MD × Intensity along CD MPa 2 5273.4 3179.3 2703.5 1558.2 2703.3 123.2 239.4 630.1 54.3 667.7 Surface porosity % 12.7 12.0 13.6 12.2 23.2 29.1 51.7 22.2 28.6 27.3 Fiber diameter nm 373 382 348 378 410 218 177 356 308 258 film thickness µm 56.6 66.8 60.4 13.0 53.9 195.0 15.4 190.0 169.0 91.5 Needle intensity mN / µm 318 214 210 262 200 twenty three 45 64 15 59 Breathability (Gore rating) s / 100 ml 768 3012 1037 295 303 9 1 210 twenty four 63 Industrial application

[0153] The present invention provides: a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, having small pore size, not easily broken, and resistant to external forces such as puncture and the like; and a method thereof for manufacturing the same.

[0154] The present invention can be used as a separator or part of a separator in fuel cells, capacitors, lithium battery packs and the like, and can also be used as part of a separator that physically separates various positive and negative electrodes. In addition, it can be used in waterproof sound transmission applications of communication equipment that require high water resistance and high strength, such as ventilation filters for automobiles, circuit board etching solutions in corrosive liquids, filtration applications of organic solvents, or semiconductor manufacturing applications, and the like, such as applications for collecting valuable materials from etching solutions, and the like. [Simplified Explanation of the Diagram]

[0040] Figure 1 is an electron micrograph of the PTFE porous membrane surface of Example 2 (magnification: 10,000x). Figure 2 is a binary image of the PTFE porous membrane surface of Example 2 (magnification: 10,000x). Figure 3 is an electron micrograph of the PTFE porous membrane surface of Comparative Example 2 (magnification: 5000x). Figure 4 is a binary image of the PTFE porous membrane surface of Comparative Example 2 (magnification: 5000x).

Claims

1. A porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein: The polytetrafluoroethylene (PTFE) and / or modified PTFE have a heat of fusion of less than 32 J / g at 296 to 343 °C. This was obtained using a differential scanning calorimeter by heating the PTFE and / or modified PTFE to 365 °C at a rate of 10 °C / min, cooling it to 330 °C at a rate of -10 °C / min, cooling it from 330 °C to 305 °C at a rate of -1 °C / min, cooling it from 305 °C to 245 °C at a rate of -10 °C / min, and then heating it back to 365 °C at a rate of 10 °C / min. The isopropanol bubble point is 600 kPa or greater according to JIS K3832. A value obtained by dividing the maximum force required to penetrate the specimen by the thickness of the specimen is 250 mN / µm or greater, based on a needle penetration strength test according to JIS Z1707. The percentage of the pore opening is 10% to 30%, and the fiber thickness is 250 nm or greater.

2. A method for manufacturing a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene as claimed in claim 1, comprising: (1) obtaining a sheet or coating comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene that has not been heat-treated at 250°C or higher; and (2) fixing and heat-treating the sheet or coating such that the ratio (ΔH / ΔH0) of the following crystalline melting heat (ΔH) to (ΔH0) is 1.0 to 2.

0. (where ΔH0 is a heat of fusion between 295 and 360°C, which is when the sheet or coating containing polytetrafluoroethylene and / or modified polytetrafluoroethylene resin that has not been heat-treated at 250°C or higher is heated at 360°C for 20 minutes, and then the sheet or coating obtained by cooling at room temperature is increased to 380°C at a rate of 10°C / min; and ΔH is a heat of fusion between 295 and 360°C, which is when the sheet or coating containing polytetrafluoroethylene and / or modified polytetrafluoroethylene that has not been heat-treated at 250°C or higher is heat-treated, and then the temperature is increased to 380°C at a rate of 10°C / min); and (3) a step of stretching the heat-treated sheet or coating in one direction and then sequentially stretching it in a second direction orthogonal to the first direction.

3. The method for manufacturing a porous membrane as claimed in claim 2, wherein the heat treatment step (2) is a step of fixing and heat treating the sheet or coating obtained in the aforementioned step (1) such that the ratio (ΔH / ΔH0) of the melting heat (ΔH) and (ΔH0) of the crystals is one of 1.2 to 1.

8.

4. The method of manufacturing a porous membrane as claimed in claim 2 or 3, wherein in the stretching step (3), the sheet heat-treated in the heat treatment step (2) is stretched along an extrusion direction and then sequentially stretched along an orthogonal direction.

5. The method of manufacturing a porous membrane as claimed in claim 2 or 3, wherein the sheet used in the aforementioned step (1) is a sheet obtained by rolling a sheet-like or bead-like extrusion, the extrusion being obtained by mixing polytetrafluoroethylene and / or modified polytetrafluoroethylene with a hydrocarbon-based solvent having a boiling point of 150 to 290°C and then extruding it using an extruder at a molding temperature of RR 35 to 120 and room temperature to 120°C.

6. The method of manufacturing a porous membrane as claimed in claim 2 or 3, wherein the coating used in the aforementioned step (1) is obtained by coating a dispersion of polytetrafluoroethylene and / or modified polytetrafluoroethylene having a solids fraction concentration of 5 to 75% by mass, comprising a surfactant, a film-forming agent, and a thickener, onto a plate having a heat resistance of 400°C or higher, such that the thickness after drying is 1 to 50 µm, and then drying at 100 to 150°C for 10 to 20 minutes.

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