High-strength, small-pore polytetrafluoroethylene porous membrane
Patent Information
- Authority / Receiving Office
- TW · TW
- Patent Type
- Patents
- Current Assignee / Owner
- CHEMOURS MITSUI FLUOROPRODUCTS CO LTD
- Filing Date
- 2020-09-25
- Publication Date
- 2026-08-01
AI Technical Summary
Existing methods for producing polytetrafluoroethylene (PTFE) porous membranes struggle to achieve a combination of small pore diameter, thin film thickness, high porosity, and high strength, leading to issues such as high surface density, insufficient membrane strength, and complexity in manufacturing processes.
A PTFE porous membrane with a bubble point of 400 kPa or higher and tensile strength of 50 MPa or higher is produced through a method involving specific extrusion, drying, and successive biaxial stretching, using controlled strain rates and sintering conditions to enhance fibrillation and crystallization, resulting in a membrane with a heat of crystallization of 5.0 J/g or more at 360-385°C and a sintering degree of 0.9 or higher.
The resulting PTFE porous membrane exhibits small pore diameter, thin film thickness, high porosity, and high strength, suitable for applications requiring high water resistance, sound transmission, and filtration of nano-sized particles, with improved production efficiency and reduced complexity.
Smart Images
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Abstract
Description
[Technical Field] This invention relates to a polytetrafluoroethylene porous membrane with small pore size, thin film thickness, high porosity and high strength, and a method for manufacturing the same. [Previous Technology] Polytetrafluoroethylene (PTFE) has been used in various fields due to its excellent heat resistance, chemical resistance, water repellency, weather resistance, and low dielectric constant. Since PTFE can be easily made porous by stretching, various PTFE porous membranes with different properties and their manufacturing methods have been invented to date. PTFE porous membranes, due to their high breathability and water repellency, are used in waterproof and breathable clothing, as breathable filters for internal pressure regulation in automotive parts, and as waterproof and sound-transmitting membranes in communication devices. Waterproof performance is expressed through water pressure resistance tests. For example, membranes used in 100m waterproof mobile phones are required to withstand 1MPa of water pressure, but a membrane with 1MPa water pressure resistance must have a pore size of less than tens of nanometers. Furthermore, waterproof and sound-transmitting membranes require small pore sizes (maximum pore size), thin membrane thickness, and high porosity to prevent signal attenuation or deterioration of sound transmitted through the membrane, and to prevent signal attenuation or additional noise caused by the inherent vibration of the porous membrane itself. This means they require low areal density (membrane weight per unit area). Areal density is calculated from porosity and membrane thickness; for example, a membrane thickness of 30μm and a porosity of 70% results in an areal density of approximately 20g / m². For waterproof and sound-permeable applications, the surface density is required to be below 10 g / m2, preferably a few g / m2, and high strength is also required. For dustproof applications, it is used in air purifiers or vacuum cleaners as filter sheets, dust collection bag filters for waste incinerators, and cleanroom air filters for semiconductor manufacturing. Furthermore, due to the purity of PTFE, meaning it contains virtually no leaching substances, it can be used as a final filter for ultrapure water production, replacing traditional ultrafiltration membranes. Furthermore, due to its excellent chemical resistance, it is also used for filtering corrosive liquids, organic solvents, or etching solutions for circuit boards used in semiconductor manufacturing, as well as for recovering valuable substances from etching solutions. In semiconductor manufacturing, the integration density of circuits has increased in recent years. If nano-sized microparticles are present in the etching solution, these microparticles remain on the wiring of the integrated circuit, causing a decrease in manufacturing yield. Therefore, there is a demand for PTFE porous membranes with nano-sized pores that can remove nano-sized microparticles from the etching solution. However, it is difficult to obtain PTFE porous membranes with nano-sized pores that do not reduce permeability, have a thinner membrane thickness, and can withstand filtration pressure or filtration operation. Generally, PTFE porous membranes are manufactured using the following steps: 1. Mixing PTFE with a hydrocarbon solvent. 2. Increasing the ratio of cylinder cross-sectional area to outlet cross-sectional area (RR), while applying shear (shear force) to the PTFE through extrusion molding to obtain sheet-like or bead-like extrudates. 3. Calendering the obtained extrudate into sheets using rollers or other suitable methods, and then drying the hydrocarbon solvent. 4. Extending the obtained sheets longitudinally and laterally at high temperatures, and then firing them at a temperature above the melting point of PTFE (above 347°C) to obtain a PTFE porous membrane. However, it is difficult to obtain a PTFE porous membrane with small pore size using these conventional methods. Therefore, in Patent Document 1, a PTFE dispersion is cast onto an aluminum foil and dried to produce a non-porous fluoropolymer membrane with PTFE as the main component. After the fluoropolymer membrane is laminated with a commercially available PTFE porous membrane with small pore size, the aluminum foil is dissolved and removed with an acid or the like, and then stretched at a low magnification to form a PTFE porous membrane with small pore size, which is then used in semiconductor applications through an integrated filter. Furthermore, in Patent Document 2, a polyimide membrane is immersed in a PTFE dispersion to form a PTFE coated membrane. After repeating the drying and firing steps to obtain a PTFE membrane, the PTFE membrane is peeled off from the polyimide membrane. The peeled PTFE membrane extends sequentially in the TD direction (transverse) and MD direction (longitudinal) to form a thin PTFE membrane with no signal attenuation or deterioration and low surface density, which is used for waterproof sound transmission membrane applications. In Patent Document 3, during the manufacturing process of a PTFE porous membrane, a semi-sintered membrane is formed by heating one side of the membrane before stretching to create a temperature gradient in the thickness direction. By successively stretching and thermally fixing in the length and width directions, an extended membrane with a continuously decreasing average pore size in the thickness direction is produced. The average pore size of the heated surface is 0.05μm to 10μm, and it has an asymmetric structure, resulting in a highly efficient membrane suitable for precision filtration of gases and liquids. However, the aluminum foil removal step in Patent Document 1, which utilizes acid dissolution, and the self-polyimide film peeling of the PTFE membrane in Patent Document 2, are not easy and can lead to PTFE membrane rupture. Furthermore, Patent Document 3 also requires complex steps. While these prior art techniques are effective for their specific applications, they suffer from problems such as increased membrane areal density or insufficient membrane strength in other applications, making it difficult to obtain a PTFE porous membrane with all the characteristics of small pore size, thin film thickness, high porosity, and high strength. [Prior Art Documents] [Patent Documents] [Patent Document 1] International Publication No. 2013 / 084858 [Patent Document 2] Japanese Patent No. 6178034 [Patent Document 3] Japanese Patent No. 4850814 [Patent Document 4] International Publication No. 2007 / 119829 [Summary of the Invention] [The problem the invention aims to solve] The present invention addresses the problem of providing a novel polytetrafluoroethylene (PTFE) porous membrane with small pore size, thin film thickness, high porosity, and high strength, and provides a method for manufacturing the same. [Means for solving the problem] The present invention provides a polytetrafluoroethylene porous membrane, which has a bubble point of 400 kPa or higher based on JIS K3832 using isopropanol (IPA) and a tensile strength of 50 MPa or higher based on JIS K6251. This invention also provides a polytetrafluoroethylene (PTFE) porous membrane, the heat of fusion for crystallization of which is 5.0 J / g or higher at 360-385°C when heated to 400°C at a rate of 10°C / min using a differential scanning calorimeter. Furthermore, the heat of fusion for crystallization in this application is measured using a differential scanning calorimeter by plotting a baseline within a certain temperature range. For example, in this project, the heat of fusion for crystallization (J / g) is measured at 300-360°C or 360-385°C. The preferred polytetrafluoroethylene porous membrane of the present invention has a crystallization melting heat (J / g) (H4) of less than 20 J / g at 300~360°C during the second heating (2nd, RUN) determined by DSC curve. The DSC curve is obtained by using a differential scanning calorimeter to perform a first heating (1st, RUN) at a rate of 10°C / min to 400°C, cooling to 200°C at a rate of 10°C / min, and then performing a second heating (2nd, RUN) at a rate of 10°C / min to 400°C. The preferred polytetrafluoroethylene porous membrane of the present invention has a firing degree (S) of 0.9 or higher as shown in the following formula [number 1]: [number 1] Firing degree (S) = (H1-H3) / (H1-H4) H1: Heat of melting of polytetrafluoroethylene at 300~360°C without heating process above 300°C when heated to 400°C at a rate of 10°C / min using a differential scanning calorimeter H3: Heat of melting of polytetrafluoroethylene porous membrane at 300~360°C in the first heating (1st, RUN) mentioned above (J / g) H4: As mentioned above. The preferred embodiment of the present invention has a polytetrafluoroethylene porous membrane system with a porosity of 70% or more. The preferred embodiment of the present invention is a polytetrafluoroethylene porous membrane with a membrane thickness of less than 30 μm. The preferred polytetrafluoroethylene porous membrane of the present invention is obtained from polytetrafluoroethylene with a standard specific gravity of 2.15 or less and satisfying the following formula [number 2]: [number 2] H1-H2≧12 H1: As mentioned above, H2: For 100g of polytetrafluoroethylene without a heating process above 300°C, add 28.7ml of naphtha with a boiling point of 150~180°C and mix for 3 minutes. After standing at 25°C for 2 hours, use an extruder to extrude the granular extrudate at a forming temperature of 25±1°C and a plunger (ram) extrusion speed of 0.5m / min with the cylinder cross-sectional area / exit cross-sectional area ratio (RR) set to 100. Dry the granules at 25±1°C for 1.5 hours and then at 150°C for 2 hours. Extend the granules 24 times in the extrusion direction at a forming temperature of 300°C and a speed of 100% / sec. Use a differential scanning calorimeter to measure the heat of melting at 300~360°C when the temperature is increased to 400°C at a rate of 10°C / min. The present invention also provides a method for manufacturing a polytetrafluoroethylene porous membrane, which involves adding a hydrocarbon solvent with a boiling point of 150~290°C to the aforementioned specific PTFE and mixing them, extruding the mixture using an extruder at RR35~120, calendering it to a thickness of less than 800μm, evaporating and removing the hydrocarbon solvent by heating at a temperature above 150°C, and then biaxially extending the porous membrane successively in the extrusion direction (longitudinal) and in a direction perpendicular to the extrusion direction (transverse), and firing it at a temperature above the melting point of polytetrafluoroethylene to obtain the polytetrafluoroethylene porous membrane. Furthermore, the preferred method for manufacturing the polytetrafluoroethylene porous membrane of the present invention involves setting the strain rate in the extrusion direction, expressed by the following formula [number 3], to be 20% / sec or higher, and performing successive biaxial stretching at a rate of 5 times or higher in the extrusion direction (longitudinal) and at least 5 times higher in the direction perpendicular to the extrusion direction (transverse). [number 3] Strain rate (% / sec) = (Vex-Vin) / L×100 a) Continuous stretching: Vex: Exit speed of the longitudinal stretching device (mm / sec) Vin: Inlet speed of the longitudinal stretching device (mm / sec) L: Stretching distance (distance between two sets of rollers) (mm) b) Discontinuous stretching: (Vex-Vin): Stretching speed of the biaxial stretching device (mm / sec) L: Stretching distance (the value obtained by subtracting the size of the sheet-like calender before stretching from the size of the sheet-like calender before stretching) (mm). [Effects of the Invention] The PTFE porous membrane of this invention features small pore size, thin film thickness, high porosity, and high strength (tensile strength). This invention can be used for waterproofing and sound transmission in communication equipment, dustproofing applications such as automotive ventilation filters requiring high water resistance, dust collection bag filters, or air filters, filtration of corrosive liquids, organic solvents, or etching solutions for circuit boards in semiconductor manufacturing, and recovery of valuable materials from etching solutions. Furthermore, this invention allows for the manufacture of PTFE porous membranes without complex steps.
Implementation Method
Claims
1. A polytetrafluoroethylene (PTFE) porous membrane, wherein the bubble point of the PTFE porous membrane using isopropanol is above 400 kPa according to JIS K3832, and the tensile strength is above 50 MPa according to JIS K6251, wherein the heat of fusion for crystallization of the PTFE porous membrane at 360-385°C when heated to 400°C at a rate of 10°C / min using a differential scanning calorimeter is above 5.0 J / g, wherein the PTFE used in the manufacture of the PTFE porous membrane has a standard specific gravity (SSG) of 2.15 or less according to ASTM D4895, and wherein the porosity of the PTFE porous membrane is determined by measuring the density of the PTFE porous membrane based on ASTM D792, and is above 70% but below 100%.
2. For the polytetrafluoroethylene porous membrane of claim 1, wherein the heat of fusion (J / g) (H4) of the polytetrafluoroethylene porous membrane at 300~360°C during the second heating (2nd, RUN) determined by DSC curve is less than 20J / g. The DSC curve is obtained by using a differential scanning calorimeter to perform a first heating (1st, RUN) at a rate of 10°C / min to 400°C, a cooling to 200°C at a rate of 10°C / min, and a second heating (2nd, RUN) at a rate of 10°C / min to 400°C.
3. The polytetrafluoroethylene porous membrane of claim 1, wherein the degree of sintering (S) of the porous membrane as shown in Formula 1 below is 0.9 or above, [Formula 1] Degree of sintering (S) = (H1-H3) / (H1-H4) H1: Heat of fusion of polytetrafluoroethylene at 300~360°C without heating above 300°C when heated to 400°C at a rate of 10°C / min using a differential scanning calorimeter H3: Heat of fusion of polytetrafluoroethylene porous membrane at 300~360°C in the first heating (1st, RUN) mentioned above (J / g) H4: Heat of fusion of polytetrafluoroethylene porous membrane at 300~360°C in the second heating (2nd, RUN) mentioned above (J / g).
4. The polytetrafluoroethylene porous membrane as requested in item 1, wherein the thickness of the porous membrane is less than 30 μm.
5. The polytetrafluoroethylene porous membrane as requested in item 1, wherein the polytetrafluoroethylene porous membrane is manufactured in accordance with the following formula 2: [Formula 2] H1-H2≧12 H1: Heat of melting of polytetrafluoroethylene at 300~360°C without heating above 300°C when heated to 400°C at a rate of 10°C / min using a differential scanning calorimeter (J / g). H2: For 100g of polytetrafluoroethylene without a heating process above 300°C, add 28.7ml of naphtha with a boiling point of 150~180°C and mix for 3 minutes. After standing at 25°C for 2 hours, use an extruder to extrude the granular extrudate at a forming temperature of 25±1°C and a plunger (ram) extrusion speed of 0.5m / min with the cylinder cross-sectional area / exit cross-sectional area ratio (RR) set to 100. Dry the granules at 25±1°C for 1.5 hours and then at 150°C for 2 hours. Extend the granules 24 times in the extrusion direction at a forming temperature of 300°C and a speed of 100% / sec. Use a differential scanning calorimeter to measure the heat of melting at 300~360°C when the temperature is increased to 400°C at a rate of 10°C / min.
6. A method for manufacturing a polytetrafluoroethylene porous membrane, comprising adding a hydrocarbon solvent with a boiling point of 150 to 290°C to the polytetrafluoroethylene of the polytetrafluoroethylene porous membrane as claimed in claim 5, mixing the mixture, extruding it using an extruder at RR35 to 120, calendering it to a thickness of less than 800 μm, evaporating and removing the hydrocarbon solvent by heating at a temperature above 150°C, and then biaxially extending the porous membrane successively in the extrusion direction and in a direction perpendicular to the extrusion direction, and firing it at a temperature above the melting point of polytetrafluoroethylene to obtain the polytetrafluoroethylene porous membrane.
7. The method for manufacturing a polytetrafluoroethylene porous membrane as claimed in claim 6, wherein the strain rate in the extrusion direction, expressed by the following formula 3, is set to 20% / sec or more, and the biaxial stretching is performed successively at a rate of 5 times or more in the extrusion direction and at a rate of 5 times or more in the direction perpendicular to the extrusion direction, [Formula 3] Strain rate (% / sec) = (Vex-Vin) / L×100 a) Continuous stretching case Vex: Exit speed of the longitudinal stretching device (mm / sec) Vin: Inlet speed of the longitudinal stretching device (mm / sec) L: Stretching distance (distance between 2 sets of rollers) (mm) b) Non-continuous stretching case (Vex-Vin): Stretching speed of the biaxial stretching device (mm / sec) L: Stretching distance (the value obtained by subtracting the size of the sheet-like calender before stretching from the size of the sheet-like calender before stretching) (mm).