Porous membranes of polytetrafluoroethylene and / or modified polytetrafluoroethylene having high strength and small pore size

Abstract

The present invention provides a porous membrane containing polytetrafluoroethylene and / or modified polytetrafluoroethylene, which has a small pore diameter, a thin film thickness, a high porosity, and a high strength, and a production method thereof. The porous membrane containing polytetrafluoroethylene and / or modified polytetrafluoroethylene has a bubble point of isopropyl alcohol of 600 kPa or more according to JIS K3832, and a tensile strength of 90 MPa or more according to JIS K6251.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of Japanese Patent Application No. JP 2020-82449, filed on May 8, 2020, and Japanese Patent Application No. JP 2021-29375, filed on February 26, 2021, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to porous membranes of polytetrafluoroethylene and / or modified polytetrafluoroethylene, the porous membranes having film thickness, small pore size, high porosity, tear resistance in the tensile direction and in the direction orthogonal to the tensile direction, and high strength; and a method for manufacturing the same. Background Technology

[0004] Polytetrafluoroethylene (PTFE), containing copolymers with trace amounts of monomers, is used in various fields due to its excellent heat resistance, chemical resistance, water repellency, weather resistance, and low dielectric constant. Numerous PTFE porous membranes with various properties and their manufacturing methods have been invented to facilitate the porosification of PTFE through stretching.

[0005] PTFE porous membranes have high permeability and high water repellency, and are therefore used in applications such as waterproof and permeable clothing, exhaust filters for regulating internal pressure in automotive parts, and waterproof and sound-permeable membranes for communication equipment.

[0006] Waterproof performance is expressed by values ​​from water pressure resistance tests. For example, membranes used in devices such as 100m waterproof mobile phones require a water pressure resistance of 1 MPa. However, membranes with a water pressure resistance of 1 MPa must have pore sizes of tens of nanometers or smaller.

[0007] Furthermore, because waterproof and acoustically permeable membranes must not attenuate or degrade signals, such as speech, they require small pore sizes, thin film thickness, and high porosity—that is, low surface density (i.e., the weight of the membrane per unit area)—to prevent signal attenuation and / or the increase of accompanying sound due to the inherent vibrations of the porous membrane itself. Surface density is determined by porosity and membrane thickness. For example, if the membrane thickness is 30 μm and the porosity is 70%, the surface density is approximately 20 g / m³. 2 In waterproof and sound-permeable applications, the surface density is 10 g / m³. 2 Or lower, preferably several g / m 2 High intensity is also required.

[0008] In dust control applications, PTFE porous membranes are used in filters for air purifiers or cleaners, bag filters for dust collection such as waste incinerators, and air filters for cleanrooms used in semiconductor manufacturing.

[0009] In addition, due to the purity of PTFE, that is, due to the small amount of eluent present, PTFE porous membranes have replaced traditional ultrafiltration membranes as the final filter in the production of ultrapure water.

[0010] In addition, due to the excellent chemical resistance of PTFE porous membranes, they are also used in applications such as filtration (including etching solutions of circuit boards in corrosive liquids and organic solvents) or semiconductor manufacturing, as well as applications such as collecting valuable substances in etching solutions.

[0011] In semiconductor manufacturing applications, the integration density of circuits has recently increased. Therefore, there is a need for PTFE porous membranes (with nanoscale pore sizes) capable of removing nanoscale particles from etching solutions, as the presence of these particles allows them to remain on the wiring of integrated circuits, leading to reduced manufacturing yields. Unfortunately, it is difficult to obtain PTFE porous membranes (with nanoscale pore sizes) that possess the film thickness and strength to withstand filtration pressures or operations without reducing permeability.

[0012] Generally, PTFE porous membranes can be used for their intended purpose, but many are used in combination with and integrated with a substrate. In this case, the base material is a nonwoven fabric, cloth, mesh, etc. The base material does not have functions such as filtration, filtration performance, water and dust resistance, but serves to maintain the porous membrane. When PTFE porous membranes are used for filtration, dust collection, and dust prevention, it is necessary to reduce the membrane thickness to effectively perform these functions. In particular, it is desirable to reduce the membrane thickness in composite materials containing a base material. For example, although PTFE porous membranes with a thickness of 30 μm to 50 μm are generally commercially available for liquid filtration applications, the thickness of the PTFE porous membrane is preferably thinner, more preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. The thickness of the PTFE porous membrane is preferably 30 μm or less, more preferably 20 μm or less, and particles cannot be effectively collected when the thickness of the air filter or bag filter does not exceed 10 μm. In this case, when the thickness of the PTFE porous membrane becomes thinner, the strength decreases, processing becomes difficult, and even when the PTFE porous membrane is combined with the base material, the desired purpose cannot be achieved due to insufficient strength.

[0013] Typically, PTFE porous membranes are manufactured using the following steps: 1.) Mixing PTFE with additives (hydrocarbon solvents, etc.); 2.) Increasing the ratio (RR) of the cylindrical cross-sectional area to the outlet cross-sectional area, and then applying shear (shear force) to the PTFE by extrusion molding to obtain sheet or bead extrusions during fibrillation; 3.) After properly rolling the resulting extrusions into sheets using a rolling mill (rolls), evaporating and removing the hydrocarbon solvent; 4.) Stretching the obtained sheet product in the extrusion direction (hereinafter also referred to as MD) and in a direction perpendicular to the extrusion direction (hereinafter also referred to as CD), and then obtaining a PTFE porous membrane by sintering at the melting point of PTFE or at a higher temperature (e.g., 342°C to 343°C or higher).

[0014] However, using such general methods, it is difficult to obtain PTFE porous membranes with small pore sizes. Furthermore, problems can arise during the manufacturing process or during tearing of the porous membrane under operating conditions. The cause of porous membrane tearing is believed to be in the rolling step, where rolls are used to adjust the thickness. When the thickness of the porous membrane is reduced during the rolling step to ensure permeability, tearing occurs during stretching. Additionally, even when adjusting the stretch ratio on the MD and CD sides, the resulting porous membrane tends to have higher tensile strength on the MD side and lower tensile strength on the CD side. A large tensile strength ratio on the MD to CD sides is considered one of the reasons for the formation of easily tearable porous membranes.

[0015] In Patent Document 1, a PTFE dispersion is cast onto an aluminum foil and dried to produce a microporous fluoropolymer membrane (containing PTFE as the main component). This microporous fluoropolymer membrane is laminated with a commercially available PTFE porous membrane having small pore sizes. The aluminum foil is then dissolved and removed using an acid or the like, and stretched at a low ratio. The PTFE porous membrane with small pore sizes is then integrated into a filter and used in semiconductor applications.

[0016] Furthermore, in Patent Document 2, a PTFE-coated film is formed by immersing a polyimide film in a PTFE dispersion, a PTFE film is obtained by repeating drying / sintering steps, the PTFE film is peeled off from the polyimide film, and the peeled PTFE film is sequentially stretched on CD and MD. The porous film obtained by this method does not attenuate or degrade the signal and can be used as a thin PTFE film (with a small surface density) when applying a waterproof and sound-permeable membrane.

[0017] In Patent Document 3, a stretched membrane with high filtration efficiency is produced by sequentially stretching and thermally fixing a partially sintered membrane in the extrusion direction (MD) and the direction perpendicular to the extrusion direction (CD) (wherein, during the manufacturing of the PTFE porous membrane, a temperature gradient is formed in the thickness direction of the membrane by heating one side of the membrane before stretching). The membrane has an asymmetric structure (wherein the average pore size in the thickness direction continuously decreases 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, etc.

[0018] However, the acid dissolution in the aluminum foil removal step of Patent Document 1 and the peeling of the PTFE membrane from the polyimide membrane in Patent Document 2 are not easy, with the PTFE membrane prone to rupture. Furthermore, Patent Document 3 also requires complex steps. While these conventionally known techniques are effective in limited applications, problems remain, such as increased membrane surface density or lack of membrane strength in other applications, making it difficult to obtain a PTFE porous membrane possessing all the properties of small pore size, film thickness, high porosity, and high strength.

[0019] Patent documents

[0020] Patent Document 1: WO 2013 / 084858

[0021] Patent Document 2: JP 6178034 B

[0022] Patent Document 3: JP 4850814 B

[0023] Patent Document 4: WO 2007 / 119829

[0024] Patent Document 5: JP 5054007 B

[0025] Patent Document 6: JP 2010-99889 A Summary of the Invention

[0026] The problem to be solved by the present invention

[0027] The problems of the present invention are solved by providing: novel porous membranes comprising polytetrafluoroethylene (PTFE) and / or PTFE, the porous membranes having a film thickness, small pore size, and high porosity, wherein the tensile strength difference between MD and CD is small, such that the ratio of their tensile strengths is close to 1, and the porous membranes have high strength; and / or methods for manufacturing porous membranes comprising PTFE and / or modified PTFE, wherein tearing of the porous membrane is prevented during the manufacturing process. The present invention provides porous membranes comprising PTFE and / or modified PTFE, the porous membranes having a film thickness and high strength.

[0028] Problem Solving Methods

[0029] The present invention provides a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein, according to JIS K3832, the bubble point of isopropanol (IPA) is 600 kPa or higher, according to JIS K6251, the tensile strength is 90 MPa or higher, and the ratio of tensile strength in the extrusion direction (MD) to tensile strength in the direction perpendicular to the extrusion direction (CD) is 0.5 to 2.0.

[0030] The present invention also provides porous membranes comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein the heat of fusion of the porous membrane at 360°C to 385°C (when the temperature increases to 400°C at a rate of 10°C / min and is determined using a differential scanning calorimeter) is 5.0 J / g or higher.

[0031] It should be noted that in this application, the heat of fusion is determined using a differential scanning calorimeter by subtracting a baseline over a certain temperature range. For example, the heat of fusion (J / g) is determined at 300°C to 360°C or 360°C to 385°C.

[0032] A preferred aspect of the invention is a porous membrane in which the temperature is first increased to 400°C at a rate of 10°C / min (first run), then cooled to 200°C at a rate of 10°C / min, and then the temperature is increased again to 400°C at a rate of 10°C / min (second run) to obtain a DSC curve, wherein the heat of fusion (J / g) (H4) of the porous membrane containing polytetrafluoroethylene and / or modified polytetrafluoroethylene, determined using the DSC curve at 290°C to 335°C during the second temperature increase (second run), is 20 J / g or lower, as determined using a differential scanning calorimeter.

[0033] A preferred aspect of the invention is a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein the degree of sintering (S) of the porous membrane represented by Equation 1 is 0.8 or higher.

[0034] Equation 1: Degree of sintering (S) = (H1 - H3) / (H1 - H4)

[0035] in:

[0036] H1 is the heat of fusion (J / g) of polytetrafluoroethylene (PTFE) and / or modified PTFE with no heating history at 300°C and above, used for the preparation of porous membranes. It was measured by DSC curves over a temperature range of 300°C to 360°C using a differential scanning calorimeter (DSC), where the sample temperature increased at a rate of 10°C / min.

[0037] H3 is the heat of fusion (J / g) of polytetrafluoroethylene and / or modified polytetrafluoroethylene porous membrane in the first melt (first run), which was measured by DSC curves using a differential scanning calorimeter over a temperature range of 300°C to 360°C, wherein the sample temperature increased at a rate of 10°C / min.

[0038] H4 is the heat of fusion (J / g) of polytetrafluoroethylene and / or modified polytetrafluoroethylene porous membrane in the second melt (second run), which is measured by DSC curves using a differential scanning calorimeter in a temperature range of 290°C to 335°C, wherein the sample temperature is increased to 400°C at a rate of 10°C / min (first melt), then the sample is cooled to 200°C at a rate of 10°C / min, and then the sample temperature is increased to 400°C at a rate of 10°C / min (second melt) to produce the DSC curve by which H4 is determined.

[0039] A preferred aspect of the invention is a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene with a porosity of 70% or higher.

[0040] A preferred aspect of the invention is a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene with a membrane thickness of 30 μm or less.

[0041] A preferred aspect of the invention is a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene obtained from polytetrafluoroethylene, having a standard specific gravity of 2.15 or lower and satisfying Equation 2.

[0042] Equation 2: H1-H2 ＞ 12

[0043] in:

[0044] H1 is as defined previously in this article.

[0045] H2 is the heat of fusion (J / g) of the polytetrafluoroethylene and / or modified polytetrafluoroethylene formed into the stretched film product, which has no heating history at 300°C and above.

[0046] H2 was measured using a differential scanning calorimeter (DSC) over a temperature range of 300°C to 360°C by DSC curves, with the sample temperature increasing to 400°C at a rate of 10°C / min.

[0047] The resulting stretched film product is obtained by mixing 100g of polytetrafluoroethylene and / or modified polytetrafluoroethylene with about 28.7ml of naphtha with a boiling point of about 150°C-180°C for about 3 minutes and then letting it stand at about 25°C for about 2 hours. Then, a beaded extrudate is formed from the mixture of fluoropolymer and naphtha using an extruder. The extrudate is formed at an extruder cylinder cross-sectional area to outlet cross-sectional area ratio (RR) of about 100 and a plunger extrusion rate of about 0.5m / min and a temperature of about 25°C, resulting in the formation of beaded extrudates. The beaded extrudates are then dried at about 25°C for about 1.5 hours and then further dried at about 150°C for about 2 hours. Subsequently, the dried beaded extrudates are stretched 25 times in the extrusion direction at a temperature of about 300°C and a stretching rate of about 100% / second, and then cooled to room temperature to obtain the formed and stretched film product.

[0048] A preferred aspect of the invention is a porous membrane, wherein the modified polytetrafluoroethylene is a copolymer comprising: tetrafluoroethylene; and at least one monomer selected from hexafluoropropylene, perfluoro(alkyl vinyl ether), fluoroalkyl ethylene, trichlorofluoroethylene, vinylidene fluoride, vinyl fluoride, and ethylene, or mixtures thereof. The at least one such monomer in the copolymer is from 0.005 mol% to 1 mol% of the total copolymer.

[0049] The present invention also provides a method for manufacturing a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, the method comprising: adding and mixing a hydrocarbon-based solvent with a boiling point of 150°C to 290°C into a specific polytetrafluoroethylene; extruding the mixture using an extruder at a RR of 35 to 120 to obtain a sheet or bead extrusion; rolling the extrusion together at least once in the extrusion direction (MD) and in a direction perpendicular to the extrusion direction (CD) to obtain a rolled product with a thickness of 400 μm or less; heating the rolled product to 150°C or higher to evaporate and remove the hydrocarbon-based solvent; and subsequently biaxially stretching the rolled product sequentially in the MD and CD to obtain a porous membrane; and then sintering the porous membrane at a temperature not lower than the melting point of polytetrafluoroethylene.

[0050] Additionally, a preferred aspect of the invention is a method for manufacturing a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein a rolled product is sequentially biaxially stretched five times or more on a MD and biaxially stretched five times or more on a CD, such that the strain rate expressed by Equation 3 is 20% / sec or more on the MD.

[0051] Equation 3: Strain rate (% / sec) = (Vex - Vin) / L × 100

[0052] in:

[0053] a) Under continuous tension:

[0054] Vex is the outlet velocity (mm / sec) of the vertical (extrusion direction) stretching device.

[0055] Vin is the inlet velocity (mm / sec) of the vertical (extrusion direction) stretching device.

[0056] and

[0057] L is the stretching distance (mm) (the distance between the two sets of rollers); and

[0058] b) In the case of discontinuous stretching:

[0059] (Vex-Vin) is the stretching rate (mm / sec) of the biaxial stretching device, and

[0060] It is the stretching distance (mm) (a value obtained by subtracting the size of the pre-stretched sheet rolled product from the size of the stretched sheet material).

[0061] Effects of the present invention

[0062] The PTFE porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene according to the present invention has a thin film thickness, small pore size, high porosity, and a small tensile strength difference between MD and CD, thereby imparting it with a tensile strength ratio close to 1 and high strength. Furthermore, using the manufacturing method of the present invention, tearing of the porous membrane due to stretching during the manufacturing process can be prevented.

[0063] This invention can be used in waterproof and sound-permeable applications in communication devices, automotive exhaust filters requiring high water resistance, dustproof applications such as dust bag filters and air filters, filtration applications such as etching solutions for circuit boards in corrosive liquids and organic solvents, semiconductor manufacturing applications, and applications such as collecting valuable substances in etching solutions. This invention also allows for the manufacture of PTFE porous membranes without the need for complex steps. Attached Figure Description

[0064] Figure 1 This is a schematic diagram of the rolling method on CD used in this invention.

[0065] Figure 2 These are schematic diagrams of continuous tensioning devices and discontinuous tensioning devices.

[0066] Figure 3 The DSC curves are determined using the differential scanning calorimeter of PTFE from Example 1.

[0067] Figure 4 The DSC curves are determined using a differential scanning calorimeter with the PTFE porous membrane from Example 1.

[0068] Figure 5 This is an electron micrograph of the surface of the PTFE porous membrane in Example 1 (magnification: 5000x).

[0069] Description of symbols used in the attached figures

[0070] 1 and 2: A set of rollers on the inlet side of the biaxial stretching machine

[0071] 3 and 4: A set of rollers on the exit side of the biaxial stretching machine

[0072] 5: Heating furnace

[0073] 6: Sheet-rolled products

[0074] 7: Longitudinal (extrusion direction) stretched film

[0075] 8: Fixed chuck of biaxial stretching machine

[0076] 9: Sheet-rolled products

[0077] 10: Biaxially stretched membrane (PTFE porous membrane) Detailed Implementation

[0078] According to JIS K3832 of the present invention, the bubble point of isopropanol (IPA) is 600 kPa or higher, preferably 700 kPa or higher, and more preferably 750 kPa or higher. A bubble point of 600 kPa or higher indicates that the pore size of the PTFE porous membrane is small enough to remove nanoscale particles. Generally, the maximum pore size of the PTFE porous membrane is calculated using the bubble point and Equation 4.

[0079] Equation 4:

[0080] The maximum pore size (nm) of PTFE porous membranes = 4 × T × cosθ / P × 10 9

[0081] in:

[0082] T: IPA surface tension (Pa·m)

[0083] θ: Contact angle between IPA and porous membrane (θ=0)

[0084] P: Bubble point pressure (Pa)

[0085] If the bubble point is 600 kPa, the maximum pore size of the PTFE porous membrane according to the present invention, calculated in Equation 4, is approximately 130 nm. However, because a large number of pores of 130 nm or smaller exist in the PTFE porous membrane, particles of tens of nanometers can be captured when filtering liquids. Generally speaking, when the bubble point is less than 400 kPa, removing nanoscale particles is difficult, and the water resistance deteriorates, which is not preferred.

[0086] Because the PTFE porous membrane according to the present invention has a bubble point of 600 kPa or higher, it is a porous membrane with small pore size and high strength, and will not leak or tear even under water pressure close to 100 m.

[0087] According to the present invention, based on JIS K6251, tensile strength is a value (MPa) obtained by dividing the tensile stress by the cross-sectional area, and is therefore unaffected by membrane thickness, wherein PTFE porous membranes with different membrane thicknesses can be compared at their own tensile strength values. The tensile strength of the PTFE porous membrane according to the present invention is preferably 90 MPa or higher, more preferably 100 MPa or higher. If the tensile strength is 90 MPa or higher, the PTFE porous membrane has sufficient strength and, in addition to increasing permeate flow, is preferably resistant to thinning of the PTFE porous membrane and to filtration pressures and operations of liquids or gases. If the tensile strength is less than 90 MPa, in addition to the difficulty in thinning the PTFE porous membrane, a thinned PTFE porous membrane is undesirable in the steps of bonding the PTFE porous membrane to the substrate or processing the PTFE porous membrane and the substrate into a pleated shape during the manufacture of the filter membrane, because it has insufficient strength and is prone to tearing.

[0088] Furthermore, although Patent Document 2 describes a tensile strength of 30 MPa or greater for the application of waterproof and sound-permeable membranes, the PTFE porous membrane according to the present invention has a tensile strength of 90 MPa or higher and can be a thinner membrane, thus allowing for further improvement in sound transmission properties. Additionally, welding with the waterproof and sound-permeable components described in the aforementioned patent is also possible.

[0089] The tensile strength of PTFE porous membranes is related to the sintering conditions of PTFE. If the degree of sintering (S) calculated by Equation 1 above is 0.8 or greater, a PTFE porous membrane with high bubble point and high tensile strength is obtained. In contrast, when the degree of sintering (S) is too high, the PTFE fibrillary structure breaks due to stretching, and the pore size of the PTFE porous membrane increases. Therefore, the degree of sintering (S) is preferably less than 0.98.

[0090] The degree of sintering (S) is generally understood by those skilled in the art; however, the specific degree of sintering (S) of the present invention allows the PTFE porous membrane to have both high tensile strength and small pore size.

[0091] The ratio of the tensile strength of the PTFE porous membrane in the extrusion direction (MD) to its tensile strength in the direction perpendicular to the extrusion direction (CD) is preferably in the range of 0.5 to 2.0. The tensile strength ratio is preferably 0.5 to 1.8, more preferably 0.6 to 1.5. Generally, the difference in tensile strength between the MD and CD directions of the PTFE porous membrane is preferably small, and the ratio is preferably close to 1, because the porous membrane is difficult to tear when an external force is applied.

[0092] The PTFE porous membrane according to the invention preferably has a heat of fusion of 5.0 J / g or higher at 360°C to 385°C, as determined using a differential scanning calorimeter (when the temperature increases to 400°C at a rate of 10°C / min). More preferably, it has a heat of fusion of 6.0 J / g or higher. If the heat of fusion of the PTFE porous membrane at 360°C to 385°C (when the temperature increases to 400°C at a rate of 10°C / min) is less than 5.0 J / g, a tensile strength of 90 MPa or higher cannot be obtained, resulting in a membrane with poor tensile strength.

[0093] In PTFE porous membranes, the thermal absorption peaks measured by differential scanning calorimetry in the temperature range of 300°C or higher are typically endothermic peaks at 300°C to 360°C (from unsintered crystals formed during PTFE polymerization) and at 327°C (from crystals obtained by melting unsintered PTFE crystals at their melting point temperature or higher, then cooling and recrystallizing them). In contrast, the PTFE porous membrane according to the invention exhibits an endothermic peak at 360°C to 385°C, rather than both of these. The endothermic peak at 360°C to 385°C is not observed in the PTFE itself used in the invention, in sheet or bead extrusions of PTFE, or in sheet-rolled products from which the extrusions are rolled (see [link to relevant documentation]). Figure 2 However, it was first observed in stretched films (PTFE porous films) obtained by stretching sheet-rolling products (see...). Figure 3 Furthermore, since the endothermic peak does not disappear even when sintering the PTFE porous membrane at 385°C, it is believed that this is a new PTFE crystal produced by fiberizing the PTFE. Because this new PTFE crystal is a very large and strong PTFE crystal that melts at about 375°C, the heat of fusion of this PTFE porous membrane at 360°C to 385°C is 5.0 J / g or higher, which is an indicator of a PTFE porous membrane with high tensile strength.

[0094] In the PTFE membrane according to the invention, the temperature is first increased to 400°C at a rate of 10°C / min (first run), then cooled to 200°C at a rate of 10°C / min, and then the temperature is increased again to 400°C at a rate of 10°C / min (second run) to obtain a DSC curve, wherein the heat of fusion (J / g) (H4) of the porous membrane containing polytetrafluoroethylene and / or modified polytetrafluoroethylene, determined using the DSC curve at 290°C to 335°C during the second temperature increase (second run), is 20 J / g or lower, preferably 18 J / g or lower, as determined using a differential scanning calorimeter.

[0095] It was found that as H4 decreases, the standard specific gravity (SSG) of PTFE used to manufacture the PTFE porous membrane according to the invention also decreases, resulting in high molecular weight PTFE. If H4 exceeds 20 J / g, this SSG is high, meaning the molecular weight of PTFE is low, which is not preferred because it is difficult to obtain the target PTFE porous membrane with small pore size and high strength according to the invention.

[0096] The porosity of the PTFE porous membrane according to the present invention refers to the ratio of the total volume of the pores to the volume of the PTFE porous membrane, and can be determined using the Archimedes method, the gravimetric porosity method, or the mercury porosity method. The porosity of the PTFE porous membrane according to the present invention can be determined by measuring the density of the PTFE porous membrane according to ASTM D792, which is 70% or higher, preferably 75% or higher, more preferably 80% or higher, and less than 100%. The porosity is preferably high to improve the liquid filtration performance and permeability of the PTFE porous membrane, where excellent properties can be obtained as a porous membrane for filtering liquids, such as etching solutions for circuit boards in corrosive liquids, organic solvents, or semiconductor manufacturing applications; porous membranes for gas filtration, such as gas filters and exhaust filters; or porous membranes as waterproof and translucent porous membranes. Furthermore, a higher porosity is preferred because the surface density (weight of the membrane per unit area) required to achieve waterproof and translucent properties is reduced.

[0097] The PTFE porous membrane according to the present invention has a membrane thickness of 30 μm or less, preferably 25 μm or less, and more preferably 20 μm or less. While PTFE porous membranes are preferably thinner, generally speaking, thinner membranes result in reduced strength, making them more prone to problems during production. Because the PTFE porous membrane according to the present invention has sufficient strength and can be a film of 30 μm or less, it is possible to produce membranes with a porosity of approximately 3 g / m³, in addition to having sufficient strength at a membrane thickness of 10 μm or less and a porosity of 85% or greater. 2Waterproof and sound-permeable membrane with surface density (membrane weight per unit area).

[0098] According to ASTM D4895, the PTFE used to manufacture the PTFE porous membrane according to the present invention preferably has a standard specific gravity (SSG) of 2.15 or lower. The SSG is preferably 2.14 or lower. This indicates that the SSG is related to the molecular weight of PTFE, such that as the SSG decreases, the molecular weight of PTFE increases. Generally, as the molecular weight of PTFE increases, the primary particles of PTFE are more likely to be fibrillated, making it possible to manufacture PTFE porous membranes with smaller pore sizes. Furthermore, the tensile strength also increases with the increase of the molecular weight of PTFE.

[0099] It should be noted that the PTFE forming the porous membrane can be modified PTFE (which is not melt-processable) (modified by a comonomer that can copolymerize with tetrafluoroethylene (TFE)) or a mixture of PTFE and modified PTFE, as long as the properties of the PTFE are not impaired. Exemplary modified PTFE includes copolymers of TFE (described in Patent Document 4) and trace amounts of monomers other than TFE, specific examples of which include copolymers of tetrafluoroethylene and 0.005 mol% to 1 mol%, preferably 0.01 mol% to 0.1 mol%, and more preferably 0.01 mol% to 0.05 mol% of at least one monomer selected from hexafluoropropylene, perfluoro(alkyl vinyl ether), trichlorochlorofluoroethylene, vinylidene fluoride, vinyl fluoride, and ethylene, wherein the copolymer is not melt-processable. The perfluoro(alkyl vinyl ether) is preferably a perfluoro(alkyl vinyl ether) having 1 to 6 carbon atoms, more preferably a perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), and perfluoro(butyl vinyl ether). Fluorinated alkyl ethylene is preferably fluoroalkyl ethylene having 1 to 8 carbon atoms, and more preferably perfluorobutyl ethylene.

[0100] However, despite their low molecular weight, some modified PTFEs can have low specific gravity (SSG). This is because SSG is determined by the amount of specific gravity obtained by temporarily increasing the temperature to the melting temperature or higher, followed by cooling and recrystallization. In other words, in the case of recrystallization, the specific gravity is lower due to the inhibition of recrystallization by monomers (comonomers) other than trace amounts of TFE, resulting in reduced crystallinity compared to polymers of TFE alone. Therefore, even if the SSG is 2.15 or less, the molecular weight can still be low. In such resins, the primary particles tend not to be fibrillated, making it impossible to produce porous membranes with small pore sizes.

[0101] Therefore, the PTFE used to manufacture the PTFE porous membrane according to the present invention is more preferably a PTFE with an SSG of 2.15 or less, no heating history at 300°C or higher, and satisfying Equation 2 above. Regarding PTFE with an SSG of 2.15 or less and no heating history at 300°C or higher, when the PTFE is stretched in the extrusion direction, the primary particles are prone to fibrillation due to shear (shear force), wherein some crystals of the primary particles break. When the PTFE is more likely to be fibrillated, a PTFE porous membrane with a smaller pore size can be prepared. In contrast, since the heat of fusion of the remaining PTFE primary particles without fibrillation can be determined using a differential scanning calorimeter, the degree of fibrillation of the PTFE can be determined by the difference in the heat of fusion of the crystals before and after fibrillation, making it possible to determine whether a PTFE porous membrane with a small pore size can be manufactured.

[0102] The H1-H2 represented by Equation 2 above is 12 or greater. The naphtha used in the H2 determination of Equation 2 is a hydrocarbon-based solvent consisting of at least one branched saturated hydrocarbon having 8 to 14 carbon atoms and a boiling point of 150°C to 180°C. Examples include Isopar G (purchased from Exxon Mobil Corporation) (carbon atoms: 9 to 12, boiling point: 160°C to 176°C), Supersol FP25 (purchased from Idemitsu Kosan Co., Ltd., etc.) (carbon atoms: 11 to 13, boiling point: 150°C or higher), etc., wherein Supersol FP25 (purchased from Idemitsu Kosan Co., Ltd., etc.) is preferred in terms of the ease with which the solvent is removed from the bead extruder under H2. Because the fibrillation of PTFE is affected by the type and amount of hydrocarbon solvent added, but more so by the amount added, it is preferable to add 28.7 mL of Supersol FP25 purchased from Idemitsu Kosan Co., Ltd. relative to 100 g PTFE.

[0103] In addition, H2 was measured using a molded product obtained by fixing both ends of a bead-shaped extruder with a length of 50 mm and stretching the extruder 25 times in the extrusion direction. The bead-shaped extruder can be extruded and molded using a PTFE porous membrane manufacturing apparatus or an extruder capable of molding extruders with a diameter of approximately 1 mm, while the bead-shaped molded product can be stretched using a stretching apparatus or a stretch tester.

[0104] In the stretching method of the present invention, Equation 2 above relates to the IPA bubble point of the PTFE porous membrane. If PTFE with a standard specific gravity of 2.15 or less satisfies Equation 2 above, the obtained PTFE porous membrane has an IPA bubble point of 400 kPa or higher and a pore size with excellent tensile strength. Furthermore, a porous membrane with a heat of fusion of 5.0 J / g or higher at 360°C to 385°C is used, which is determined using a differential scanning calorimeter (when the temperature increases to 400°C at a rate of 10°C / min).

[0105] In the manufacture of the PTFE porous membrane according to the present invention, the PTFE used, in addition to a dried or granulated / dried aqueous dispersion, can also be obtained as PTFE by obtaining an aqueous dispersion containing PTFE primary particles (which is obtained by emulsion polymerization of tetrafluoroethylene (TFE) in an aqueous medium in the presence of a polymerization initiator (potassium permanganate, oxalic acid), a fluorinated surfactant, a polymerization stabilizer (higher alkanes), succinic acid, and an ionic strength modifier (zinc chloride)). This PTFE has an SSG of 2.15 or less, and satisfies Equation 2. As mentioned above, the PTFE can be a modified PTFE (modified by a comonomer that can copolymerize with tetrafluoroethylene (TFE)) or a mixture of PTFE and modified PTFE, as long as it does not impair the properties of the PTFE.

[0106] The PTFE porous membrane according to the present invention can be obtained by adding and mixing a hydrocarbon-based solvent with a boiling point of 150°C to 290°C into the PTFE; extruding the mixture using an extruder at a RR of 35 or higher; rolling the extrudate on MD and CD; heating the extrudate to 150°C or higher to evaporate and remove the hydrocarbon-based solvent; and subsequently biaxially stretching the product sequentially in the extrusion direction (MD) and the direction perpendicular to the extrusion direction (CD) to obtain a porous membrane; and then sintering the porous membrane at a temperature not lower than the melting point of PTFE. It should be noted that when rolling the extrudate on MD and CD, the order of rolling on MD and rolling on CD can be any, but each sequence must be performed together at least once until the extrudate reaches a predetermined thickness.

[0107] In addition to the naphtha used to determine Equation 2 above, exemplary hydrocarbon solvents used to manufacture the PTFE porous membrane according to the invention include straight-chain saturated hydrocarbon solvents and / or branched-chain saturated hydrocarbon solvents of the type having a boiling point of 150°C to 290°C and having at least one type having 8 to 16 carbon atoms, wherein exemplary straight-chain saturated hydrocarbon solvents include Norpar 13 (carbon atoms: 12 to 14, boiling point: 222°C to 243°C) and Norpar 15 (carbon atoms: 9 to 16, boiling point: 255°C to 279°C), wherein exemplary branched-chain saturated hydrocarbon solvents include: Isopar G (carbon atoms: 9 to 12, boiling point: 160°C to 176°C), Isopar H (carbon atoms: 10 to 13, boiling point: 178°C to 188°C) and Isopar M (carbon atoms: 11 to 16, boiling point: 223°C to 254°C), each of which is available from Exxon Mobil Corporation; and Supersol FP25 (carbon atoms: 11 to 13, boiling point 150°C or higher, available from Idemitsu Kosan Co., Ltd.), etc., and Isomper M is preferred because it prevents the solvent from evaporating during rolling, can be easily removed by heating, and is odorless.

[0108] The manufacturing method is more specifically as follows.

[0109] Manufacturing method step 1. To facilitate extrusion molding, a hydrocarbon-based solvent (preferably Isopar M from Exxon Mobil Corporation) is added to PTFE in an amount of 20% by weight or less, preferably 18% by weight or less, and more preferably 16% by weight or less, and mixed for 3 to 5 minutes, and then allowed to stand at 20°C or higher for 12 hours or longer.

[0110] Manufacturing method step 2. (After obtaining a cylindrical preform at 25°C ± 1°C, if necessary), extrusion molding is performed using an extruder at an RR of 35 to 120, preferably 50 to 120, and more preferably 50 to 80, a molding temperature of 40 to 60°C, preferably 40 to 50°C, and a plunger extrusion rate of 10 mm / min to 60 mm / min, preferably 20 mm / min to 30 mm / min, to obtain a sheet extruder. Bead extruders can also be obtained instead of sheet extruders. When extruding in a bead shape, it may be preferable to set the molding temperature 5°C to 10°C higher than that for sheet extrusion. This is not particularly problematic even at the same rate. Note that sheet extruders and bead extruders are collectively described as sheet extruders below.

[0111] If the plunger extrusion rate is less than 10 mm / min, productivity deteriorates, which is not preferred. If the extrusion rate exceeds 60 mm / min, it becomes difficult to increase the extrusion pressure or obtain a uniform extrudate, which is also not preferred.

[0112] If RR is less than 35, the strength of the extrudate decreases, which is not preferred because the PTFE primary particles will not fibrillate without sufficient shear (shear force) on them.

[0113] 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 preferred.

[0114] Furthermore, if the molding temperature is below 40°C, the compatibility between the hydrocarbon solvent and PTFE is poor, resulting in deteriorated flowability, which is not preferred. If the molding temperature exceeds 60°C, the hydrocarbon solvent evaporates, which is also not preferred.

[0115] Manufacturing method step 3. Using two pairs of rollers, the sheet extrusion is rolled at least once in both directions MD and CD to obtain a sheet-rolled product with a predetermined thickness or less. At this point, in order to set the tensile strength ratio on MD and CD (after evaporation and removal of the hydrocarbon solvent) to 0.5 to 2.0, each of the rolling ratios in both directions of MD and CD is determined, while considering the relationship between the thickness and strength of the sheet extrusion. The tensile strength ratio of the obtained sheet-rolled product on MD and CD is preferably 0.5 to 2.0.

[0116] After the extruded sheet is cut into appropriate lengths, such as Figure 1 a) Rolling is performed on MD as shown. Figure 1 As shown in b), regarding rolling on the CD, the CD is rotated to the MD at 90°C and then unfolded, resulting in deformation of the CD. Rolling in both directions can be used in combination to roll the sheet extrusion to a thickness of 400 μm or less, preferably 300 μm or less, and more preferably 200 μm or less, to obtain a sheet-rolled product.

[0117] When the thickness of the sheet-rolled product is 400 μm or less, porous membranes with a thickness of 30 μm or less can eventually be readily obtained. Generally, the thickness of the porous membrane is adjusted by the rolling thickness and the draw ratio on the MD and CD. However, because the draw ratio also significantly affects the permeability, bubble point, and other properties of the porous membrane, those working in porous membrane manufacturing can easily understand that the draw ratio on the MD and CD cannot be changed solely for thickness adjustment. By setting the rolling thickness to 400 μm or less, membranes with a thickness of 30 μm or less can eventually be obtained without strictly limiting the draw conditions to achieve the desired membrane properties.

[0118] In this invention, although the order is not limited to this, the sheet extrusion must be rolled together on the MD and CD at least once. Preferably, during rolling on the MD, the sheet extrusion, extruded using an extruder at an RR of 35 to 120, is vertically clamped by two sets of rollers heated to 40°C or higher to reduce its thickness. Then, during rolling on the CD, two sets of rollers are used instead of stretching, the sheet is rotated at 90°C, unfolded from the CD and clamped, and deformed on the CD to reduce its thickness. Two sets of rollers are used to reduce the thickness on both the MD and CD.

[0119] Generally speaking, PTFE sheets containing hydrocarbon solvents (auxiliaries) tend to have high strength in the direction in which the thickness is reduced by applying external force, regardless of methods such as rolling or stretching. This is a phenomenon easily understood by those engaged in the manufacture of PTFE substrates or porous membranes. For example, when an extruded sheet is halved on the MD (Medium Depth) by two sets of rollers, if the length on the CD (Dielectric Center) is not deformed, the sheet deforms on the MD to set the length on the MD to approximately twice its original length, and thus double its strength. In contrast, when the length on the CD is stretched to twice its original length, the thickness becomes approximately half, and the strength also doubles. Therefore, by reducing the thickness on the MD only by rolling, a sheet with high MD tensile strength can be obtained, thus increasing the ratio of tensile strength on the MD to that on the CD.

[0120] Furthermore, even when a hydrocarbon solvent is included and the sheet is stretched on CD, a sheet with strong CD tensile strength can be prepared, wherein stretching on CD can also be broadly regarded as rolling on CD. In the aforementioned Patent Document 5, after the extruded sheet is stretched 3.7 times in a direction perpendicular to the extrusion direction (CD), the extruded sheet is heated to evaporate and remove the hydrocarbon solvent, and then sequentially biaxially stretched and sintered on MD and CD to produce a PTFE porous membrane with a small tensile strength difference between MD and CD. As an apparatus for performing such stretching on CD, in the case of continuous stretching, a tenter frame for stretching on CD is preferably used; however, as a simpler method, the extruded sheet can be rolled on MD, and the extruded sheet can be continuously rolled using rollers depending on the purpose of using an apparatus such as that reported in the aforementioned Patent Document 6.

[0121] In this invention, after evaporation and removal of the hydrocarbon solvent, the tensile strength ratio (strength ratio) of the sheet-rolled product on the MD and CD axes is 0.5 to 2.0, preferably 0.5 to 1.8, and more preferably 0.6 to 1.7, as described later in 4, in order to determine each rolling ratio in both the MD and CD directions. By adding this step, without adjusting the MD / CD stretch ratio in biaxial stretching on the MD and CD axes, as described later in 4, the difference between the tensile strengths of the obtained PTFE porous membrane on the MD and CD axes can be reduced, and the strength ratio can be made close to 1 to obtain a PTFE porous membrane with excellent strength. Therefore, in addition to obtaining the effect of preventing tearing of the PTFE porous membrane, the tensile strength on the MD and CD axes can be significantly improved.

[0122] Sheets rolled at this strength ratio are more easily biaxially stretched sequentially on the MD and CD, as described later in step 4 of the manufacturing method. (Compared to sheet-rolled products of the same thickness produced by rolling only on the MD), even under stretching rate and temperature conditions (where PTFE porous membranes typically cannot be produced in stretching due to sheet tearing), porous membranes can be produced without tearing, resulting in increased yield and improved productivity.

[0123] Furthermore, since the PTFE porous membrane according to the present invention has a smaller pore size compared to a porous membrane produced only by rolling on MD, a porous membrane with a high isopropanol bubble point according to JIS K3832 can be obtained.

[0124] It should be noted that if the tensile strength ratio on MD and CD is 3 or greater after evaporation and removal of the hydrocarbon solvent as described later in step 4 of the manufacturing method, the tensile ratio on MD and CD is typically adjusted so that the tensile strength of the obtained porous membrane on MD and CD is not significantly different. In this case, the porous membrane itself has poor tensile strength on MD and CD, and therefore this is not preferred.

[0125] Manufacturing method step 4. Evaporate and remove the hydrocarbon solvent from the sheet-rolled product at 150°C or higher, preferably 200°C or higher, for 5 minutes or longer, preferably 15 minutes or longer. Subsequently, using a stretching device, perform sequential biaxial stretching on MD and CD to obtain stretched material (wherein the molding temperature is 150°C to 320°C, preferably 300°C, and the strain rate expressed by Equation 3 is 20% / sec or higher, preferably 40% / sec or higher), and then obtain the PTFE porous membrane according to the invention by sintering (heat-fixing) at the melting point of PTFE or higher, preferably at 350°C to 400°C, more preferably at 370°C to 385°C for 10 seconds to 120 seconds.

[0126] By rolling sheet extrusions to 200 μm or less, hydrocarbon solvents in sheet-rolled products tend to be evaporated and removed, while PTFE porous membranes with a thickness of 30 μm or less tend to be molded.

[0127] When obtaining a stretched material, a higher strain rate is preferred. However, when the strain rate is high, a strain rate of 130% / sec or lower is preferred because large equipment is required to ensure the heating time.

[0128] The strain rate represented by Equation 3 of the present invention relates to the rate of deformation and is 20% / sec or higher, preferably 30% / sec or higher, and more preferably 60% / sec. As the strain rate increases, the bubble point increases; that is, a PTFE porous membrane with a small pore size can be obtained. Although the strain rate in the extrusion direction (MD) does not need to be equal to the strain rate in the direction perpendicular to the extrusion direction (CD), the strain rate in each direction can be determined according to the purpose. The strain rate is particularly effective during stretching in the MD. If the strain rate during stretching in the CD is lower than the strain rate in the MD, a PTFE porous membrane with the target pore size according to the present invention can be obtained.

[0129] In the stretching step used to obtain a PTFE porous membrane, a discontinuous stretching method is employed, which involves discontinuously stretching a sheet-rolled product (to batch type) using a biaxial stretching machine. In this invention, a PTFE porous membrane can be obtained by appropriately selecting a stretching method or stretching apparatus according to the target characteristics of the PTFE porous membrane.

[0130] The draw ratio on the MD and CD is 5 times or higher, preferably 7 times or higher, and more preferably 10 times or higher. Additionally, while it is not necessary to set the draw ratio on the MD and CD to be the same, the draw ratio in each direction can be determined according to the purpose. Depending on the thickness after rolling, the draw ratio in the extrusion direction is preferably 7 times or higher, since the thickness of the PTFE porous membrane is more likely to be 30 μm or less.

[0131] In the continuous stretching method, a longitudinal (extrusion direction) stretching device is first used to continuously stretch the sheet-rolled product in the same direction as the extrusion direction (MD) of the sheet-rolled product. This stretching device has multiple rollers (clamping rollers) capable of being heated and vertically clamped (extruded). When using multiple sets of rollers for continuous stretching in the extrusion direction (MD), the rate ratio is preferably set to the rotational speed of each set of rollers. For example, in Figure 1 In a), it is preferable to allow the pair of rollers on the exit side to rotate at a faster rate than the pair of rollers on the inlet side, as this allows for greater stretching (stretching at a high ratio of 10 times or more). While not limited to this, the roller diameter is typically around 200 mm.

[0132] In addition, a method of continuous stretching in the extrusion direction (MD) is appropriately used (using a device with a heating zone between each set of rollers, for example, having...). Figure 1 (The apparatus of the heating furnace shown in Figure a).

[0133] use Figure 1 a) The extrusion direction (MD) stretching device shown has two sets of rollers (clamping rollers) capable of clamping (extruding). If Vex in Equation 3 is 500 mm / sec, Vin is 100 mm / sec, and L is 1000 mm (that is, the distance between the two sets of rollers is 1000 mm), then the strain rate is 40% / sec (((500-100) / 1000)×100=40).

[0134] Next, a tenter frame that can be continuously stretched in a direction perpendicular to the extrusion direction (CD) is used to continuously clamp both sides of the sheet-like stretched material with a chuck (continuously stretched in the extrusion direction (MD)). The chuck is moved while heating, and the stretched material is continuously extended in a direction perpendicular to the extrusion direction (CD) to obtain a PTFE porous membrane.

[0135] In the discontinuous stretching method, the sheet-rolled product is cut into a predetermined shape and size. A biaxial stretching machine is used to fix the four corners or perimeter of the cut sheet-rolled product via a chuck, and the chuck is stretched sequentially on the MD and CD. Figure 2 b). Repeat this batch type to obtain PTFE porous membranes discontinuously.

[0136] In the discontinuous stretching method, (Vex-Vin) is defined in Equation 3 as the stretching rate (the rate at which the chuck is moved). L (stretch distance) is obtained by subtracting the size of the pre-stretched sheet rolled object from the size of the stretched sheet material. For example, when the stretching rate on MD is 400 mm / sec and L is 400 mm (that is, when the size of the PTFE sheet before stretching is 100 mm² and it is stretched to 500 mm², L is 400 mm), the strain rate is 100% / sec ((400 / (500-100))×100=100).

[0137] Example

[0138] While not strictly limited to the embodiments described herein, the invention will be further described in detail below using these embodiments.

[0139] Standard Specific Gravity (SSG)

[0140] The standard specific gravity of PTFE was determined according to ASTM D4895.

[0141] Bubble

[0142] The bubble point of isopropanol (IPA) was determined according to JIS K3832 using a Pololax 1000 purchased from MicrotracBEL Corp.

[0143] Tensile strength and permeability

[0144] Tensile strength was determined using porous membrane sample sheets (MD stretching direction: 50 mm, CD stretching direction: 10 mm) prepared from PTFE porous membranes obtained under the conditions indicated in Table 1, according to JIS K6251, using a Tensilon RTC1310A (available from Orientec Co., Ltd.) at 25°C, a chuck spacing of 22 mm, and a stretching rate of 200 mm / min. Permeability was determined using a Frazier type tester.

[0145] Porosity

[0146] The true density of PTFE (2.2 g / cm³) 3The density of the PTFE porous membrane according to the present invention, as determined according to ASTM D792, is used to determine the porosity of the PTFE porous membrane based on the following formula.

[0147] Porosity (%) = (1 - (PTFE porous membrane density / actual density of PTFE in the PTFE porous membrane)) × 100 (membrane thickness)

[0148] The measurement was performed using a dial thickness gauge available from Peacock.

[0149] Heat of fusion

[0150] 1. The heat of fusion (J / g) of H1 above was determined using a differential scanning calorimeter (Diamond DSC from PerkinElmer Co., Ltd.) by a DSC curve obtained by raising the temperature of 10 mg PTFE (without a heating history of 300 °C or higher) to 400 °C at a rate of 10 °C / min.

[0151] 2. The heat of fusion (J / g) at 300°C to 360°C was determined as described in 1 above, except that 10 mg of the sample mentioned below for H2 determination was used to determine the heat of fusion of H2.

[0152] Samples used for H2 determination

[0153] Add 28.7 ml of naphtha (Supersol FP25 from Idemitsu Kosan Co., Ltd.) with a boiling point of 150°C to 180°C to 100 g of PTFE without a heating history of 300°C or higher and mix for 3 minutes. Let stand at 25°C for 2 hours, then extrude using an extruder at a cylinder cross-sectional area / outlet cross-sectional area ratio (RR) of 100, a molding temperature of 25°C ± 1°C and a plunger extrusion rate of 0.5 m / min to form beaded extrudates. The beaded extrudates were then dried at 25°C ± 1°C for 1.5 hours and further dried at 150°C for 2 hours. After evaporation and removal of naphtha, they were cut to a length of 51 mm to fix both ends and stretched 25 times in the extrusion direction at a molding temperature of 300°C and a strain rate of 100% / sec (stretch rate 100% / sec) to obtain molded products, which were used as samples for H2 determination.

[0154] 3. The heat of fusion of the PTFE porous membrane was determined using a differential scanning calorimeter (DSC). For 10 mg of PTFE porous membrane obtained under the conditions indicated in Table 1, the temperature was first increased to 400 °C at a rate of 10 °C / min (first run), then cooled to 200 °C at a rate of 10 °C / min, and then increased to 400 °C again at a rate of 10 °C / min (second run) to obtain the DSC curve. When using the DSC curve, the temperature was first increased (first run) to determine the heat of fusion (J / g) at 300 °C to 360 °C as H3, and then the temperature was increased again (second run) to determine the heat of fusion (J / g) at 290 °C to 335 °C as H4.

[0155] Structure of PTFE porous membrane

[0156] After depositing a porous PTFE membrane by sputtering with a platinum-palladium alloy, the porous PTFE membrane was observed under an electron microscope (SU-8000 purchased from Hitachi High-Tech Corporation).

[0157] PTFE

[0158] Mix 60g of paraffin wax, 2300ml of deionized water, 12g of ammonium salt of fluoromonoether acid (formula: C3F7-O-CF(CF3)COOH), and 0.05g of ammonium salt of fluoropolyether acid (C3F7-O-[CF(CF3)CF2]). n 0.75 g of succinic acid, 0.026 g of oxalic acid, and 0.01 g of zinc chloride were placed in a 4-liter autoclave (made of stainless steel, SUS316) equipped with stirring blades and a jacket for temperature control. The autoclave was then heated to 80°C while the interior was purged three times with nitrogen to remove oxygen, followed by vacuum evacuation. The internal temperature was then maintained at 63°C while stirring with tetrafluoroethylene (TFE) at 111 rpm and an internal pressure of 2.75 MPa.

[0159] Next, add 510 ml of an aqueous solution containing 40 mg of potassium permanganate (KMnO4) dissolved in 2000 ml of water. At the end of the potassium permanganate injection, increase the internal temperature to 85°C, then supply TFE. Stop stirring when 740 g of TFE has been consumed. Release the gas in the autoclave to atmospheric pressure, evacuate, restore the pressure to atmospheric pressure with nitrogen, and remove the contents to complete the reaction.

[0160] The obtained PTFE dispersion had a solids content of 27% and an average primary particle size of 0.23 μm. The PTFE dispersion was dried at 190 °C for 11 hours to obtain fine PTFE powder. The standard specific gravity (SSG) and molten heat of the obtained fine PTFE powder (H1, H2, and H1-H2) are indicated in Table 1.

[0161] Examples 1 to 4

[0162] In addition to the amounts indicated in Table 2, fine PTFE powder, purchased from Exxon Mobil Corporation's Isopar M, was mixed for five minutes using a Turbula shaker from Willy A. Bachofen AG. The mixture was allowed to stand at 25°C for 24 hours and then placed in an 80mm diameter cylinder of a preforming machine. The upper portion of the cylinder was then covered with a lid, and the cylinder was subsequently compressed at a rate of 50mm / min at room temperature (approximately 15°C to 30°C) to obtain a cylindrical preform. The obtained preform was extruded and molded using an extruder at an RR of 36, a molding temperature of 50°C, and an extrusion rate of 20mm / min. The extruded material was then extruded using an extrusion die (thickness: 1mm × width: 140mm) to obtain a sheet extrusion. The obtained sheet extrusion was cut into 120mm lengths and rolled multiple times using two sets of rollers heated to 50°C in the extrusion direction (MD) and perpendicular to the extrusion direction (CD) until the rolled thickness indicated in Table 2 was achieved. Subsequently, the Isopar M was evaporated and removed at 200°C for 15 minutes to obtain a sheet-like rolled product, which was then cut into squares (90 mm square millimeters). The ratio of the tensile strength of the sheet-like rolled product in the extrusion direction (MD) to that in the direction perpendicular to the extrusion direction (CD) (MD / CD strength ratio) is indicated in Table 2.

[0163] Using a biaxial stretching apparatus (EX10-S5 type, purchased from Toyo Seiki Seisaku-sho, Ltd.), the periphery of a square (90 mm²) rolled product was secured by a chuck (size: 72 mm angle, excluding the chuck clamp of the biaxial stretching apparatus). The material was sequentially stretched 10 times on MD and CD at a molding temperature of 300°C at the stretching rate (chuck movement rate) and strain rate indicated in Table 2 to obtain stretched material (size: 720 mm angle, excluding the chuck clamp of the biaxial stretching apparatus) (batch type). Two plates heated to 370°C were held 5 mm above and below the stretched material for 10 seconds, then sintered. The chuck around the material was then removed to obtain a porous PTFE membrane.

[0164] The bubble point, tensile strength (in MD and CD), MD / CD strength ratio, porosity, membrane thickness, permeability, heat of fusion (H3 and H4), and degree of sintering of the obtained PTFE porous membrane are indicated in Table 2. The DSC curves of the PTFE porous membrane obtained in Example 1 are shown in Table 2. Figure 3 In the middle, the electron micrograph is shown in Figure 5 middle.

[0165] Comparative Example 1

[0166] An attempt was made to produce a PTFE porous membrane as in Example 1, except that rolling was performed only on the MD, without rolling on the CD. However, during stretching, the sheet-like rolled product, after evaporation and removal of the hydrocarbon solvent, tore, resulting in the inability to produce a porous membrane. The results are shown in Table 1.

[0167] Comparative Example 2

[0168] The PTFE porous membrane was produced as in Example 2, except that rolling was performed only on the MD, not on the CD. The MD / CD strength ratio of the sheet, the bubble point of the obtained PTFE porous membrane, its tensile strength (on both MD and CD), its MD / CD strength ratio, its porosity, its membrane thickness, its permeability, the heat of fusion (H3 and H4) of the PTFE porous membrane, and its degree of sintering are indicated in Table 1.

[0169] Comparative Example 3

[0170] PTFE porous membranes were prepared as in Comparative Example 2, except that a biaxial stretching apparatus was used, with a stretch ratio of 7.5 times on the MD and 10 times on the CD. The sheet was a sheet-rolled product with an MD / CD strength ratio of 6.5 and high tensile strength on the MD. Stretching was performed at a stretch ratio of 7.5 times on the MD and 10 times on the CD, resulting in PTFE porous membranes with an MD / CD strength ratio of 0.5 to 2.0 obtained by stretching the sheet-rolled product. The bubble point, tensile strength (on the MD and CD), MD / CD strength ratio, porosity, membrane thickness, permeability, heat of fusion (H3 and H4), and degree of sintering of the obtained PTFE porous membranes are indicated in Table 1.

[0171] Comparative Example 4

[0172] The porous membrane was produced as in Comparative Example 2, except that instead of rolling on CD, it was rolled on MD to obtain a thickness of 400 μm, and the strain rate was 144 mm / sec. The resulting membrane thickness was 22.8 μm.

[0173] Comparative Example 5

[0174] The porous membrane was produced as in Comparative Example 4, except that the strain rate was 1288 mm / sec. The resulting membrane had a thickness of 21.8 μm.

[0175] Table 1

[0176]

[0177] Table 2

[0178]

[0179]

[0180] Industrial applicability

[0181] The present invention provides: a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, having small pore size, film thickness, high porosity and high strength, and a small tensile strength difference between MD and CD; and a method thereof.

[0182] This invention is applicable to waterproof and sound-permeable applications in communication devices, automotive exhaust filters requiring high water resistance, dustproof applications such as dust bag filters and air filters, filtration applications such as etching solutions for circuit boards in corrosive liquids and organic solvents, semiconductor manufacturing applications, and applications such as collecting valuable substances in etching solutions.

Claims

1. A porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, wherein, according to JIS K3832, the bubble point of isopropanol is 600 kPa or higher, according to JIS K6251, the tensile strength in the extrusion direction (MD) is 90 MPa or higher, and the ratio of the tensile strength in the extrusion direction (MD) to that in the direction perpendicular to the extrusion direction (CD) is 0.6 to 1.

5. The degree of sintering (S) of the porous membrane, expressed by the following formula, is from 0.8 to less than 0.98: Degree of sintering (S) = (H1 - H3) / (H1 - H4), where: H1 is the heat of fusion of the polytetrafluoroethylene (PTFE) and / or modified PTFE used in the preparation of the porous membrane, without a heating history at 300°C and above, expressed in J / g. It is measured using a differential scanning calorimeter (DSC) from a differential scanning calorimeter (DSC) curve obtained in the temperature range of 300°C to 360°C, wherein the sample temperature is increased to 400°C at a rate of 10°C / min. H3 is the heat of fusion of polytetrafluoroethylene and / or modified polytetrafluoroethylene porous membrane in the first melt, which is measured by differential scanning calorimetry (DSC) curves in the temperature range of 300°C to 360°C, wherein the sample temperature increases to 400°C at a rate of 10°C / min. H4 is the heat of fusion of the polytetrafluoroethylene and / or modified polytetrafluoroethylene porous membrane in the second melt, which is measured by differential scanning calorimetry (DSC) in the temperature range of 290°C to 335°C, wherein the sample temperature is increased to 400°C at a rate of 10°C / min, then the sample is cooled to 200°C at a rate of 10°C / min, and then the sample temperature is increased to 400°C at a rate of 10°C / min to produce the differential scanning calorimetry curve by which H4 is determined.

2. The porous membrane according to claim 1, wherein the heat of fusion of the porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene, as determined by differential scanning calorimetry in the temperature range of 360°C to 385°C, is 5.0 J / g or higher when the temperature increases at a rate of 10°C / min.

3. The porous membrane according to claim 2, wherein the heat of fusion of the porous membrane is determined by the following process: i) The temperature is first increased to 400°C at a rate of 10°C / min. ii) Then cool to 200°C at a rate of 10°C / min, after which iii) The temperature is then increased again to 400°C at a rate of 10°C / min to obtain a differential scanning calorimetry (DSC) curve. Furthermore, the heat of fusion is determined by differential scanning calorimetry within a temperature range of 290°C to 335°C during the second temperature increase, and the heat of fusion of the porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene is 20 J / g or lower.

4. The porous membrane according to claim 1, wherein the degree of sintering (S) of the porous membrane, expressed by the following formula, is from 0.9 to less than 0.

98.

5. The porous membrane according to claim 1, wherein the porosity is 70% or higher.

6. The porous membrane according to claim 1, wherein the membrane thickness is 30 µm or less.

7. The porous membrane according to claim 1, wherein the polytetrafluoroethylene used to manufacture the porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene has a standard specific gravity of 2.15 or lower and satisfies formula H1-H2. > 12% polytetrafluoroethylene, wherein: H1 is the heat of fusion of the polytetrafluoroethylene (PTFE) and / or modified PTFE used to prepare the porous membrane, having no heating history at 300°C and above, measured using a differential scanning calorimeter (DSC) from a differential scanning calorimeter curve obtained in the temperature range of 300°C to 360°C, wherein the sample temperature is increased to 400°C at a rate of 10°C / min. H2 is the heat of fusion of the polytetrafluoroethylene (PTFE) and / or modified PTFE formed into a stretched film product, wherein the PTFE and / or modified PTFE has no heating history at 300°C and above, and H2 was measured using a differential scanning calorimeter within a temperature range of 300°C to 360°C by differential scanning calorimetry, wherein the sample temperature was increased to 400°C at a rate of 10°C / min. The resulting stretched film product is obtained by mixing 100g of polytetrafluoroethylene (PTFE) and / or modified PTFE with 28.7ml of naphtha with a boiling point of 150°C-180°C for 3 minutes and then allowing it to stand at 25°C for 2 hours. Then, a beaded extrudate is formed from the mixture of fluoropolymer and naphtha using an extruder via a plunger. The extrudate is formed at an extruder cylinder cross-sectional area to outlet cross-sectional area ratio (RR) of 100, a plunger extrusion rate of 0.5m / min, and a temperature of 25°C. The beaded extrudate is then dried at 25°C for 1.5 hours, followed by further drying at 150°C for 2 hours. Subsequently, the dried beaded extrudate is stretched 25 times in the extrusion direction at a temperature of 300°C and a stretching rate of 100% / second, and then cooled to room temperature to obtain the formed and stretched film product.

8. The porous membrane according to claim 1, wherein the modified polytetrafluoroethylene used to manufacture the porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene is a copolymer comprising: tetrafluoroethylene; and 0.005 mol% to 1 mol% of at least one monomer selected from hexafluoropropylene, perfluoro(alkyl vinyl ether), fluoroalkyl ethylene, trifluorochloroethylene, vinylidene fluoride, vinyl fluoride and ethylene.

9. A method for manufacturing a porous membrane comprising polytetrafluoroethylene and / or modified polytetrafluoroethylene according to any one of claims 1 to 8, the method comprising: A hydrocarbon-based solvent with a boiling point of 150°C to 290°C is added and mixed into polytetrafluoroethylene (PTFE) and / or modified PTFE; the mixture is extruded using an extruder at a RR of 35 to 120 to obtain sheet or bead extrusions; the extrusions are rolled together at least once in the extrusion direction (MD) and in a direction perpendicular to the extrusion direction (CD) to obtain a rolled product with a thickness of 400 µm or less; the rolled product is heated to 150°C or higher to evaporate and remove the hydrocarbon-based solvent; and the rolled product is then sequentially biaxially stretched in the MD and the CD to obtain a porous membrane; the porous membrane is then sintered at a temperature not lower than the melting point of PTFE.

10. The method for manufacturing a porous membrane according to claim 9, wherein the tensile strength ratio of the rolled product from the sheet or bead extrusion on the MD and the CD is 0.6 to 1.

5.

11. The method for manufacturing a porous membrane according to claim 9, wherein the rolled product is sequentially biaxially stretched five times or more on the MD and biaxially stretched five times or more on the CD, such that the strain rate expressed by the following formula is 20% / sec or more on the MD: strain rate / % / sec = ((Vex-Vin) / L) × 100, where: a) Under continuous tension: Vex is the velocity of the vertical stretching device outlet in the extrusion direction, measured in mm / sec. Vin is the inlet velocity of the vertical stretching device in the extrusion direction, and L is the distance between stretching sections, in mm; and b) In the case of discontinuous stretching: (Vex-Vin) is the stretching rate of the biaxial stretching device, and L is the stretching distance, which is a value obtained by subtracting the size of the pre-stretched sheet rolled product from the size of the stretched sheet material.

Images