Polytetrafluoroethylene microporous membrane and preparation method thereof

Abstract

The invention provides a polytetrafluoroethylene microporous membrane and a preparation method of the polytetrafluoroethylene microporous membrane. The polytetrafluoroethylene microporous membrane has a membrane thickness fluctuation range of-1 [mu] m to + 1 [mu] m, the porosity of the polytetrafluoroethylene microporous membrane is 70%-90%, the longitudinal tensile strength of the polytetrafluoroethylene microporous membrane is 60-80 MPa, and the ratio of the longitudinal tensile strength to the transverse tensile strength of the polytetrafluoroethylene microporous membrane is 1.1-1.5. The thickness fluctuation range of the film is narrow, the thickness uniformity is good, the longitudinal strength and the transverse strength of the polytetrafluoroethylene microporous film are high, the ratio of the longitudinal tensile strength to the transverse tensile strength is larger than 1, and the polytetrafluoroethylene microporous film material has anisotropy. The longitudinal tensile strength and the transverse tensile strength are greatly improved.

Description

[0001] Technology

[0002] This application relates to the field of polytetrafluoroethylene (PTFE) microporous membrane technology, and more particularly to a PTFE microporous membrane and a method for preparing the PTFE microporous membrane. Background Technology

[0003] Homogeneous proton exchange membranes swell under repeated wet-dry cycles, leading to a decrease in their mechanical strength and eventual failure. To improve the mechanical stability of proton exchange membranes, expanded polytetrafluoroethylene (ePTFE) membranes are commonly used industrially as a reinforcing framework to mechanically reinforce perfluorosulfonic acid resins. In the field of flow batteries, ePTFE membranes can serve as a reinforcing framework, particularly suitable for flow battery ion exchange membranes requiring high mechanical strength, ultra-thinness, or corrosion resistance, such as zinc-based and high-power-density flow batteries.

[0004] As a reinforcing framework, the strength of the ePTFE membrane plays a crucial role in the mechanical stability, lifespan, and conductivity of the proton exchange membrane. If the ePTFE strength is poor, it is prone to deformation under humid heat and mechanical stress, causing the conductive channel structure to collapse or become unevenly distributed, thus reducing proton conductivity. Furthermore, insufficient ePTFE strength also leads to a decrease in the overall mechanical strength of the composite membrane, making it susceptible to physical damage such as membrane perforation and cracking, affecting battery efficiency. Current ePTFE membrane fabrication processes produce ePTFE membranes with poor thickness uniformity and low strength; existing ePTFE membranes cannot simultaneously meet the performance requirements of high porosity, high mechanical strength, and uniform membrane thickness. Summary of the Invention

[0005] The first aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane, wherein the membrane thickness fluctuates within the range of -1 μm to +1 μm, the porosity of the PTFE microporous membrane is 70% to 90%, the longitudinal tensile strength of the PTFE microporous membrane is 60 MPa to 80 MPa, and the ratio of the longitudinal tensile strength to the transverse tensile strength of the PTFE microporous membrane ranges from 1.1 to 1.5.

[0006] In some optional embodiments of the first aspect of this application, the thickness of the polytetrafluoroethylene microporous membrane is 3 μm to 15 μm.

[0007] In some optional embodiments of the first aspect of this application, the average pore size of the polytetrafluoroethylene microporous membrane is 0.1 μm to 0.2 μm.

[0008] A second aspect of this application provides a method for preparing a polytetrafluoroethylene microporous membrane, comprising:

[0009] S1: After thoroughly mixing polytetrafluoroethylene dispersion resin and additives, a curing treatment is carried out to obtain a cured polytetrafluoroethylene resin containing additives.

[0010] S2: After curing, polytetrafluoroethylene resin containing additives is pre-compressed into a cylindrical preform, and the cylindrical preform is extruded by an extruder with a slit die to obtain sheet or strip material.

[0011] S3: Rolling sheet-like strips to produce base strips of a predetermined thickness containing additives;

[0012] S4: Remove the additives from the baseband at the first temperature T1 and stretch the baseband longitudinally;

[0013] S5: The longitudinally stretched base strip is heat-treated at a temperature of T2-10℃ to T2-30℃, where T2 is the melting point of polytetrafluoroethylene resin (approximately 330-340℃).

[0014] S6: The substrate that has been longitudinally stretched and heat-treated is stretched laterally, then sintered and cured, cooled and wound up to obtain a polytetrafluoroethylene microporous membrane.

[0015] In some optional embodiments of the second aspect of this application, in step S2, the extrusion compression ratio of the extruder is 100 to 140, and the extrusion compression ratio is the ratio of the cross-sectional area of ​​the barrel S1 to the exit area of ​​the die head S2.

[0016] In some optional embodiments of the second aspect of this application, the thickness of the sheet material is 0.5 mm to 2 mm, and the width is 100 mm to 250 mm.

[0017] In some optional embodiments of the second aspect of this application, in step S4, longitudinal stretching is performed using a roller differential stretching method, with a stretching temperature of 200-300°C and a stretching ratio of 4-10 times.

[0018] In some optional embodiments of the second aspect of this application, the first temperature T1 ranges from 200°C to 250°C.

[0019] In some optional embodiments of the second aspect of this application, the heat treatment time in step S5 is 5 min to 15 min.

[0020] In some optional embodiments of the second aspect of this application, during the heat treatment process, the two sides of the baseband in the lateral direction are fixed so that the width of the baseband remains unchanged.

[0021] In some optional embodiments of the second aspect of this application, in step S6, the transverse stretching adopts a chain clamp track-type expansion stretching method. During the transverse stretching process, the base strip passes through the preheating section and the stretching section in the oven in sequence. The temperature of the preheating section is 150℃~300℃, the temperature of the stretching section is 200℃~300℃, and the stretching ratio is 10~25 times.

[0022] In some optional embodiments of the second aspect of this application, after transverse stretching, the sintering and curing step is also carried out in an oven, with the sintering and curing temperature range being 300°C to 400°C.

[0023] The third aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane, which is prepared using the method for preparing PTFE microporous membranes as provided in the second aspect of this application.

[0024] Beneficial effects:

[0025] The first aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane with a narrow thickness fluctuation range (the thickness fluctuation range can be understood as the difference between the thickness of the PTFE microporous membrane at various points and the average thickness of the entire PTFE microporous membrane), good thickness uniformity, and high longitudinal and transverse strength. The ratio of longitudinal tensile strength to transverse tensile strength is greater than 1, and the PTFE microporous membrane material exhibits anisotropy. Both longitudinal and transverse tensile strengths have been significantly improved.

[0026] This application provides a method for preparing a polytetrafluoroethylene (PTFE) microporous membrane. It proposes to use a slit die to extrude a sheet-like base strip. Compared with the compression ratio of a conventional round die, the compression ratio of the slit die is increased, thereby improving the strength and thickness uniformity of the base strip. After removing the additives, the base strip is stretched longitudinally and then sintered and shaped by heat treatment below the melting point to form a porous skeleton along the longitudinal direction. This helps to improve the effective stretching degree of transverse stretching, thereby improving the orientation degree. Combined with high-temperature sintering and shaping after transverse stretching, the longitudinal and transverse tensile strength of the PTFE microporous membrane is improved, while the membrane thickness uniformity is further improved.

[0027] The third aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane with excellent thickness uniformity and improved tensile strength in both the transverse and longitudinal directions. The PTFE microporous membrane significantly improves the mechanical strength of proton exchange membranes in reinforced fuel cells and ion exchange membranes in flow batteries, enhancing their durability and stability. Simultaneously, the highly ordered microporous structure in the longitudinal (thickness direction) (such as a vertically arranged fiber-node network) can directionally guide the transport of protons, gas, or water molecules, reducing the tortuosity of the mass transfer path and thus lowering internal resistance. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the extrusion barrel and slit die head structure. Detailed Implementation

[0029] The present application will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

[0030] The first aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane, wherein the membrane thickness fluctuates within the range of -1 μm to +1 μm, the porosity of the PTFE microporous membrane is 70% to 90%, the longitudinal tensile strength of the PTFE microporous membrane is 60 MPa to 80 MPa, and the ratio of the longitudinal tensile strength to the transverse tensile strength of the PTFE microporous membrane ranges from 1.1 to 1.5.

[0031] The first aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane with a narrow thickness fluctuation range, excellent thickness uniformity, and high longitudinal and transverse strength. The ratio of longitudinal tensile strength to transverse tensile strength is greater than 1, and the PTFE microporous membrane material exhibits anisotropy. Both longitudinal and transverse tensile strengths have been significantly improved.

[0032] Membrane thickness fluctuation range: The difference between the thickness of the polytetrafluoroethylene (PTFE) microporous membrane at various points and the average thickness of the entire PTFE microporous membrane. Measuring the membrane thickness fluctuation range can be done in some cases by taking multiple PTFE microporous membranes, with at least 5 points measured on each membrane.

[0033] In some optional embodiments of the first aspect of this application, the thickness of the polytetrafluoroethylene microporous membrane is 3 μm to 15 μm.

[0034] In some optional embodiments of the first aspect of this application, the average pore size of the polytetrafluoroethylene microporous membrane is 0.1 μm to 0.2 μm.

[0035] A second aspect of this application provides a method for preparing a polytetrafluoroethylene microporous membrane, comprising:

[0036] S1: After thoroughly mixing polytetrafluoroethylene dispersion resin and additives, a curing treatment is carried out to obtain a cured polytetrafluoroethylene resin containing additives.

[0037] S2: After curing, polytetrafluoroethylene resin containing additives is pre-compressed into a cylindrical preform, and the cylindrical preform is extruded by an extruder with a slit die to obtain sheet or strip material.

[0038] S3: Rolling sheet-like strips to produce base strips of a predetermined thickness containing additives;

[0039] S4: Remove the additives from the baseband at the first temperature T1 and stretch the baseband longitudinally;

[0040] S5: Heat treatment is performed on the longitudinally stretched base strip at a temperature of T2-10℃ to T2-30℃, where T2 is the melting point of polytetrafluoroethylene resin (the melting point of polytetrafluoroethylene resin is about 330 to 340℃).

[0041] S6: The substrate that has been longitudinally stretched and heat-treated is stretched laterally, then sintered and cured, cooled and wound up to obtain a polytetrafluoroethylene microporous membrane.

[0042] In some optional embodiments of the second aspect of this application, the curing treatment in step S1 includes allowing the material obtained after thoroughly mixing the polytetrafluoroethylene dispersion resin and the additives to stand at 40°C to 50°C for approximately 12 to 15 hours, so that the polytetrafluoroethylene dispersion resin and the additives can fully react and form a stable structure. This process can improve subsequent processing performance and prevent the precipitation of additives.

[0043] In some optional embodiments of the second aspect of this application, in step S2, the extrusion compression ratio of the extruder is 100 to 140, and the extrusion compression ratio is the ratio of the cross-sectional area of ​​the barrel to the exit area of ​​the die head. Figure 1 This is a schematic diagram of the extrusion barrel and slit die head structure. (Example) Figure 1 As shown, the extrusion barrel of the extruder is cylindrical, meaning its cross-section is circular, and its cross-sectional area is S1. The slit-type die head has a rectangular slit-shaped die head outlet, with an outlet area of ​​S2. Therefore, the extrusion compression ratio is the ratio of the barrel cross-sectional area to the die head outlet area, which is the ratio of S1 to S2.

[0044] In some optional embodiments of the second aspect of this application, the thickness of the sheet material is 0.5 mm to 2 mm, and the width is 100 mm to 250 mm.

[0045] In some optional embodiments of the second aspect of this application, in step S4, longitudinal stretching is performed using a roller differential stretching method, with a stretching temperature of 200-300°C and a stretching ratio of 4-10 times.

[0046] In some optional embodiments of the second aspect of this application, the first temperature T1 ranges from 200°C to 250°C.

[0047] In some optional embodiments of the second aspect of this application, the heat treatment time in step S5 is 5 min to 15 min.

[0048] In some optional embodiments of the second aspect of this application, during the heat treatment process, the two sides of the baseband in the lateral direction are fixed so that the width of the baseband remains unchanged.

[0049] In some optional embodiments of the second aspect of this application, in step S6, the transverse stretching adopts a chain clamp track-type expansion stretching method. During the transverse stretching process, the base strip passes through the preheating section and the stretching section in the oven in sequence. The temperature of the preheating section is 150℃~300℃, the temperature of the stretching section is 200℃~300℃, and the stretching ratio is 10~25 times.

[0050] In some optional embodiments of the second aspect of this application, after transverse stretching, the sintering and curing step is also carried out in an oven, with the sintering and curing temperature range being 300°C to 400°C.

[0051] This application provides a method for preparing a polytetrafluoroethylene (PTFE) microporous membrane. It proposes using a slit-type die to extrude a sheet-like substrate. Compared to the conventional round die with a compression ratio of 60-80, the slit-type die increases the compression ratio to 100-140, improving the strength and thickness uniformity of the substrate. After removing additives, the substrate is stretched longitudinally and then sintered and shaped to form a porous framework along the longitudinal direction through heat treatment below its melting point. This helps to improve the effective stretching degree of transverse stretching, thereby improving the orientation degree. Combined with high-temperature sintering after transverse stretching, the longitudinal and transverse tensile strength of the PTFE microporous membrane is improved, while the membrane thickness uniformity is further enhanced.

[0052] The third aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane, which is prepared using the method for preparing PTFE microporous membranes as provided in the second aspect of this application.

[0053] The third aspect of this application provides a polytetrafluoroethylene (PTFE) microporous membrane with excellent thickness uniformity and improved tensile strength in both the transverse and longitudinal directions. The PTFE microporous membrane significantly improves the mechanical strength of proton exchange membranes in reinforced fuel cells and ion exchange membranes in flow batteries, enhancing their durability and stability. Simultaneously, the highly ordered microporous structure in the longitudinal (thickness direction) (such as a vertically arranged fiber-node network) can directionally guide the transport of protons, gas, or water molecules, reducing the tortuosity of the mass transfer path and thus lowering internal resistance. [Specific Implementation Examples]

[0055] The following specific embodiments and comparative examples illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0056]

Example 1

[0057] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are thoroughly mixed at a mass ratio of 4:1 and then subjected to a curing treatment. Specifically, the mixture is allowed to stand at 40–50°C for approximately 10 hours to allow the resin and additive to fully react and form a stable structure. This process improves subsequent processing performance, prevents additive precipitation, and yields cured PTFE resin containing the additive.

[0058] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 100, and the thickness of the oil-containing base strip after extrusion is 0.9mm and the width is 200mm.

[0059] S3: Rolling sheet-like strips to produce a base strip with an additive thickness of 300μm;

[0060] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 220℃ and a stretching ratio of 4 times.

[0061] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 30°C below the melting point of PTFE resin (approximately 340-350°C) for 12 minutes.

[0062] S6: The longitudinally stretched substrate after heat treatment is subjected to transverse stretching (preheating section temperature 150℃, stretching section temperature 200℃, stretching ratio 15 times), followed by high-temperature sintering and curing (temperature 300℃). After cooling, it is wound up to obtain the highly longitudinally anisotropic polytetrafluoroethylene microporous membrane. Both transverse stretching and high-temperature sintering and curing are performed in an oven. The following examples and comparative examples also involve transverse stretching and high-temperature sintering and curing performed in an oven, and will not be described again.

[0063] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 8.8 μm, a thickness fluctuation of ±1 μm, a porosity of 75%, a longitudinal tensile strength of 60.4 MPa, a transverse tensile strength of 51.7 MPa, and an average pore size of 0.11 μm.

[0064] The aging process steps in the following examples and comparative examples are the same as those in Example 1, and will not be repeated here.

[0065]

Example 2

[0066] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 12 hours.

[0067] S2: The PTFE resin containing additives after curing is made into a cylindrical preform by a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 110, and the thickness of the oil-containing base strip after extrusion is 1mm and the width is 210mm.

[0068] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0069] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 220℃ and a stretching ratio of 4 times.

[0070] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 20°C below the melting point of PTFE resin for 10 minutes.

[0071] S6: The longitudinally stretched base strip after heat treatment is stretched laterally (preheating section temperature 160℃, stretching section temperature 210℃, stretching ratio 15 times), then sintered and cured at high temperature (temperature 310℃), and after cooling, it is wound up to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0072] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 9.4 μm, a thickness fluctuation of ±0.8 μm, a porosity of 78%, a longitudinal tensile strength of 62.0 MPa, a transverse tensile strength of 47.2 MPa, and an average pore size of 0.11 μm.

[0073]

Example 3

[0074] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 12 hours.

[0075] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 120, and the thickness of the oil-containing base strip after extrusion is 1.1mm and the width is 208mm.

[0076] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0077] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 230℃ and a stretching ratio of 5.5 times.

[0078] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 20°C below the melting point of PTFE resin for 10 minutes.

[0079] S6: The longitudinally stretched base strip after heat treatment is stretched laterally (preheating section temperature 180℃, stretching section temperature 240℃, stretching ratio 17 times), then sintered and cured at high temperature (temperature 335℃), and after cooling, it is wound up to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0080] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 12.5 μm, a thickness fluctuation of ±0.7 μm, a porosity of 80%, a longitudinal tensile strength of 66.3 MPa, a transverse tensile strength of 55.7 MPa, and an average pore size of 0.13 μm.

[0081]

Example 4

[0082] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 15 hours.

[0083] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 120, and the thickness of the oil-containing base strip after extrusion is 1.05mm and the width is 213mm.

[0084] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0085] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 230℃ and a stretching ratio of 5.5 times.

[0086] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 25°C below the melting point of PTFE resin for 12 minutes.

[0087] S6: The longitudinally stretched base strip after heat treatment is stretched laterally (preheating section temperature 170℃, stretching section temperature 220℃, stretching ratio 16 times), then sintered and cured at high temperature (temperature 329℃), and after cooling, it is wound up to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0088] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 13.8 μm, a thickness fluctuation of ±0.9 μm, a porosity of 79%, a longitudinal tensile strength of 65.4 MPa, a transverse tensile strength of 51.8 MPa, and an average pore size of 0.12 μm.

[0089]

Example 5

[0090] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 15 hours.

[0091] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 130, and the thickness of the oil-containing base strip after extrusion is 1.10mm and the width is 200mm.

[0092] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0093] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 240℃ and a stretching ratio of 6 times.

[0094] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 10°C below the melting point of PTFE resin for 10 minutes.

[0095] S6: The longitudinally stretched base strip after heat treatment is stretched laterally (preheating section temperature 180℃, stretching section temperature 250℃, stretching ratio 18 times), then sintered and cured at high temperature (temperature 390℃), and after cooling, it is wound up to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0096] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 13.8 μm, a thickness fluctuation of ±0.9 μm, a porosity of 79%, a longitudinal tensile strength of 67.1 MPa, a transverse tensile strength of 50.8 MPa, and an average pore size of 0.13 μm.

[0097]

Example 6

[0098] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 15 hours.

[0099] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 140, and the thickness of the oil-containing base strip after extrusion is 1.14mm and the width is 210mm.

[0100] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0101] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 240℃ and a stretching ratio of 6 times.

[0102] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 10°C below the melting point of PTFE resin for 10 minutes.

[0103] S6: The longitudinally stretched base strip after heat treatment is stretched laterally (preheating section temperature 180℃, stretching section temperature 250℃, stretching ratio 25 times), then sintered and cured at high temperature (temperature 390℃), and after cooling, it is wound up to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0104] Comparative Example 1

[0105] (1) Mix polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) at a mass ratio of 4:1 and then cure for 15 hours.

[0106] (2) The PTFE resin containing additives after curing is made into a cylindrical preform by a pre-compression molding machine, and then extruded into a sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 140, and the thickness of the oil-containing base strip after extrusion is 1.14 mm and the width is 210 mm.

[0107] (3) The sheet strip is rolled into a 300μm base strip containing additives;

[0108] (4) Remove the additives in the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 240℃ and a stretching ratio of 6 times.

[0109] (5) The longitudinally stretched base strip is stretched laterally (preheating section temperature 180℃, stretching section temperature 250℃, stretching ratio 25 times), then sintered and cured at high temperature (temperature 390℃), and then wound up after cooling to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0110] The polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 6.8 μm, a thickness fluctuation of ±1.5 μm, a porosity of 80%, a longitudinal tensile strength of 54.5 MPa, a transverse tensile strength of 45.9 MPa, and an average pore size of 0.15 μm.

[0111] Comparative Example 2

[0112] (1) Mix polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) at a mass ratio of 4:1 and then cure for 15 hours.

[0113] (2) The PTFE resin containing additives after curing is made into a cylindrical preform by a pre-compression molding machine, and then extruded into a sheet strip by a pusher. The pusher uses a slit die head, the pusher compression ratio is 130, and the thickness of the oil-containing base strip after extrusion is 1.10 mm and the width is 200 mm.

[0114] (3) The sheet strip is rolled into a 300μm base strip containing additives;

[0115] (4) Remove the additives in the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 240℃ and a stretching ratio of 6 times.

[0116] (6) The longitudinally stretched base strip is stretched laterally (preheating section temperature 180℃, stretching section temperature 250℃, stretching ratio 18 times), then sintered and cured at high temperature (temperature 390℃), and then wound up after cooling to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0117] The polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 11.4 μm, a thickness fluctuation of ±1.9 μm, a porosity of 82%, a longitudinal tensile strength of 48.4 MPa, a transverse tensile strength of 44.0 MPa, and an average pore size of 0.13 μm.

[0118] Comparative Example 3

[0119] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 15 hours.

[0120] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a conventional round die head, the pusher compression ratio is 80, and the thickness of the oil-containing base strip after extrusion is 1.25mm and the width is 200mm.

[0121] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0122] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 240℃ and a stretching ratio of 6 times.

[0123] S5: The longitudinally stretched base strip is then transversely stretched (preheating section temperature 180℃, stretching section temperature 250℃, stretching ratio 18 times), and then sintered and cured at high temperature (temperature 390℃). After cooling, it is wound up to obtain the highly longitudinally anisotropic polytetrafluoroethylene microporous membrane.

[0124] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 11.2 μm, a thickness fluctuation of ±1.8 μm, a porosity of 81%, a longitudinal tensile strength of 37.5 MPa, a transverse tensile strength of 39.1 MPa, and an average pore size of 0.11 μm.

[0125] Comparative Example 4

[0126] S1: Polytetrafluoroethylene (PTFE) dispersion resin and additive (isohexadecane) are mixed thoroughly at a mass ratio of 4:1 and then cured for 15 hours.

[0127] S2: The matured PTFE resin containing additives is made into a cylindrical preform using a pre-compression molding machine, and then extruded into sheet strip by a pusher. The pusher uses a conventional round die head, the pusher compression ratio is 80, and the thickness of the oil-containing base strip after extrusion is 1.25mm and the width is 200mm.

[0128] S3: Rolling sheet-like strips to produce 300μm base strips containing additives;

[0129] S4: Remove the additives from the baseband at 200℃ and stretch the baseband longitudinally at a stretching temperature of 240℃ and a stretching ratio of 6 times.

[0130] S5: The longitudinally stretched base strip is subjected to high-temperature heat treatment at a temperature 10°C below the melting point of PTFE resin for 10 minutes.

[0131] S6: The longitudinally stretched base strip after heat treatment is stretched laterally (preheating section temperature 180℃, stretching section temperature 250℃, stretching ratio 18 times), then sintered and cured at high temperature (temperature 390℃), and after cooling, it is wound up to obtain the high longitudinal anisotropy polytetrafluoroethylene microporous membrane.

[0132] The highly longitudinally anisotropic polytetrafluoroethylene microporous membrane prepared in this embodiment has a thickness of 12.8 μm, a thickness fluctuation of ±2.4 μm, a porosity of 76%, a longitudinal tensile strength of 42.5 MPa, a transverse tensile strength of 30.3 MPa, and an average pore size of 0.10 μm.

[0133] II. Performance Testing of Polytetrafluoroethylene Microporous Membranes

[0134] 2.1 Membrane thickness detection

[0135] The film thickness was measured using a thickness gauge, in accordance with the provisions of GB / T 6672—2001. The resolution of the thickness gauge should not exceed 0.1 μm, and the measurement points should be set according to... Figure 1 As shown, at least 3 points are tested at equal intervals along the horizontal direction as a group, and another group is tested every 200mm along the vertical direction, for a total of 5 groups.

[0136] 2.2 Porosity Detection

[0137] The tests were conducted according to GB / T 6673-2001 and GB / T 6672-2001. Three membrane pieces were cut longitudinally at 150 mm intervals, with each sample measuring 100 mm × 100 mm. The length, width, and thickness of the membrane and the mass of the sample were measured, and then the areal density and porosity were calculated according to formulas (1) and (2), respectively.

[0138]

[0139] Where: ρ1—area density of the diaphragm, g / cm³ 2 ;

[0140] m—mass of the diaphragm, in grams;

[0141] L—Length of the septum (cm);

[0142] b—width of the diaphragm, in cm.

[0143]

[0144] Where: ρ1—area density of the diaphragm, g / cm³ 2 ;

[0145] P—Porosity of the membrane, %;

[0146] d—Thickness of the diaphragm, μm;

[0147] ρ0 — density of the raw material, g / cm³ 3 .

[0148] 2.3 Longitudinal tensile strength test

[0149] The test was conducted according to GB / T 1040.3-2006. Taking the type II sample as an example, the diaphragm sample was cut into strips 150 mm long and 15 mm wide, with a clamping distance of (100±5) mm and a tensile rate of (250±10) mm / min. The breaking strength and elongation at break of the membrane sample strip were measured at room temperature using a universal electronic tensile testing machine.

[0150] 2.4 Transverse tensile strength test

[0151] The test was conducted according to GB / T 1040.3-2006. Taking the type II sample as an example, the diaphragm sample was cut into strips 150 mm long and 15 mm wide, with a clamping distance of (100±5) mm and a tensile rate of (250±10) mm / min. The breaking strength and elongation at break of the membrane sample strip were measured at room temperature using a universal electronic tensile testing machine.

[0152] 2.5 Average pore size detection

[0153] The aperture is tested using a bubble point aperture tester, and the aperture is calculated using equation (3):

[0154]

[0155] In equation (3), D(μm) is the pore diameter, γ(mN / m) is the liquid surface tension, θ is the liquid contact angle, and p(Psi) is the gas pressure.

[0156] Table 1. Summary of preparation conditions and performance of polytetrafluoroethylene (PTFE) microporous membranes for each comparative example and embodiment.

[0157]

[0158]

[0159] The thickness fluctuation values ​​in Table 1, i.e. the numbers after ±, reflect the thickness of the polytetrafluoroethylene microporous membrane.

[0160] Analysis of the results in Table 1 shows that:

[0161] A comparative analysis was conducted between Comparative Example 1 and Example 6. Both Comparative Example 1 and Example 6 used a slit die to extrude sheet-like strips. However, Example 6 involved an additional longitudinal stretching followed by high-temperature treatment. Therefore, the material in Example 6 exhibited stronger longitudinal and transverse strengths than that in Comparative Example 1. The porosity and pore size of Example 6 were also greater than those of Comparative Example 1. Furthermore, the uniformity of the polytetrafluoroethylene microporous membrane prepared in Example 6 was lower than that of the membrane prepared in Example 6. The membrane thickness of Comparative Example 1 was greater than that of Example 6. After heat treatment, the membrane thickness uniformity increased by 60%, the longitudinal tensile strength increased by 34.3%, and the transverse tensile strength increased by 23.7%.

[0162] Comparative analysis of Comparative Example 2 and the Example showed that the film thickness uniformity was improved by 52.6% after heat treatment, the longitudinal tensile strength was improved by 38.6%, and the transverse tensile strength was improved by 15.5%.

[0163] Comparative Examples 1 and 2 show that a higher extrusion compression ratio can reduce film thickness and film thickness fluctuation, while a higher extrusion ratio can also improve material uniformity and reduce thickness.

[0164] Comparative Example 3 did not change the extrusion die at all, nor did it undergo high-temperature treatment before longitudinal stretching. The compression ratio was smaller, the material strength was lower, the film thickness was thicker, and the film thickness fluctuation range was larger.

[0165] Analysis of Comparative Examples 2 and 3 shows that the use of a slit die in the extrusion process increases the extrusion compression ratio, which is beneficial for enhancing the strength of the membrane.

[0166] Comparative Example 4, with no changes to the extrusion die, underwent high-temperature treatment before longitudinal stretching. The results show that the film thickness also increased, exhibiting the largest thickness fluctuation range, without improved film uniformity, and with lower strength and porosity. Simply performing high-temperature heat treatment after longitudinal stretching cannot comprehensively improve the issues of film thickness, thickness uniformity, strength, and permeability.

[0167] Analysis of Comparative Examples 1, 4 and 6 shows that changing the extrusion die to a slit die and performing high-temperature treatment after longitudinal stretching can comprehensively improve the problems of film thickness, thickness uniformity, strength and air permeability.

[0168] A comparison of Examples 1 and 2, as well as Examples 5 and 6, reveals that increasing the compression ratio can improve material strength and reduce film thickness.

[0169] Overall, the film thickness of Examples 1 to 6 fluctuates within the range of -1μm to +1μm, the film has good uniformity, and the longitudinal tensile strength and transverse tensile strength are both above 45MPa. The material has high mechanical strength and porosity greater than or equal to 75%.

[0170] This application proposes using a slit die to extrude sheet-like strip material. Compared to the conventional round die with a compression ratio of 60-80, the slit die increases the compression ratio to 100-140, improving the strength and thickness uniformity of the base strip. After removing the additives, the base strip is stretched longitudinally and then sintered and shaped to form a porous skeleton along the longitudinal direction through heat treatment below the melting point. This helps to improve the effective stretching degree of transverse stretching, thereby improving the orientation degree. Combined with high-temperature sintering and shaping after transverse stretching, the longitudinal and transverse tensile strength of the polytetrafluoroethylene microporous membrane is improved, while the membrane thickness uniformity is further improved.

[0171] It should be noted that, in this document, "comprising," "including," or any other variation thereof is intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0172] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0173] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the principles of this application should be included within the scope of protection of this application.

Claims

1. A polytetrafluoroethylene microporous membrane, characterized in that, The thickness of the polytetrafluoroethylene (PTFE) microporous membrane fluctuates from -1 μm to +1 μm, the porosity of the PTFE microporous membrane is 70% to 90%, the longitudinal tensile strength of the PTFE microporous membrane is 60 MPa to 80 MPa, and the ratio of the longitudinal tensile strength to the transverse tensile strength of the PTFE microporous membrane ranges from 1.1 to 1.

5.

2. The polytetrafluoroethylene microporous membrane according to claim 1, characterized in that, The thickness of the polytetrafluoroethylene microporous membrane is 3μm to 15μm.

3. The polytetrafluoroethylene microporous membrane according to claim 1, characterized in that, The average pore size of the polytetrafluoroethylene microporous membrane is 0.1 μm to 0.2 μm.

4. A method for preparing a polytetrafluoroethylene microporous membrane, characterized in that, include: S1: After thoroughly mixing polytetrafluoroethylene dispersion resin and additives, a curing treatment is carried out to obtain a cured polytetrafluoroethylene resin containing additives. S2: After curing, polytetrafluoroethylene resin containing additives is pre-pressed into a cylindrical preform, and the cylindrical preform is extruded by an extruder with a slit die to obtain a sheet or strip. S3: The sheet-like strip is rolled into a base strip containing additives of a predetermined thickness; S4: Remove the additives from the baseband at the first temperature T1 and stretch the baseband longitudinally; S5: The longitudinally stretched base strip is subjected to heat treatment at a temperature of T2-10℃ to T2-30℃, where T2 is the melting point of the polytetrafluoroethylene resin. S6: The base strip, which has been longitudinally stretched and heat-treated, is then stretched laterally, sintered and cured, cooled and wound up to obtain the polytetrafluoroethylene microporous membrane.

5. The method for preparing the polytetrafluoroethylene microporous membrane according to claim 4, characterized in that, In step S2, the extrusion compression ratio of the extruder is 100 to 140, and the extrusion compression ratio is the ratio of the cross-sectional area of ​​the barrel to the exit area of ​​the die head. Preferably, the thickness of the sheet material is 0.5mm to 2mm, and the width is 100mm to 250mm.

6. The method for preparing the polytetrafluoroethylene microporous membrane according to claim 4, characterized in that, In step S4, the longitudinal stretching is performed using a roller differential stretching method, with a stretching temperature of 200-300℃ and a stretching ratio of 4-10 times. Preferably, the first temperature T1 is in the range of 200℃~250℃.

7. The method for preparing the polytetrafluoroethylene microporous membrane according to claim 4, characterized in that, In step S5, the heat treatment time is 5 min to 15 min; Preferably, during the heat treatment process, the two sides of the baseband in the lateral direction are fixed so that the width of the baseband remains unchanged.

8. The method for preparing the polytetrafluoroethylene microporous membrane according to claim 4, characterized in that, In step S6, the transverse stretching adopts a chain clamp track-type expansion stretching method. During the transverse stretching process, the base strip passes through the preheating section and the stretching section in the oven in sequence. The temperature of the preheating section is 150℃~300℃, the temperature of the stretching section is 200℃~300℃, and the stretching ratio is 10~25 times.

9. The method for preparing a polytetrafluoroethylene microporous membrane according to claim 8, characterized in that, After transverse stretching, the sintering and curing step is also carried out in the oven, and the sintering and curing temperature range is 300℃~400℃.

10. A polytetrafluoroethylene microporous membrane, characterized in that, The polytetrafluoroethylene microporous membrane is prepared by the method for preparing polytetrafluoroethylene microporous membrane as described in any one of claims 4 to 9.

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