Polytetrafluoroethylene composite microporous membrane for air purification and its preparation method

Abstract

This application discloses a polytetrafluoroethylene (PTFE) composite microporous membrane for air purification and its preparation method. The PTFE composite microporous membrane comprises: a porous base membrane, including a fibrous material; and a PTFE membrane disposed on both sides of the porous base membrane. The thickness ratio of the porous base membrane to the PTFE membrane on either side is 1:(1-1.5), and the average pore size of the PTFE membrane is 280 nm to 450 nm. The PTFE composite microporous membrane of this application has good thickness uniformity, which can maximize the filtration effect of the PTFE composite microporous membrane and avoid the problem of short lifespan caused by a large ratio of transverse to longitudinal tensile strength.

Description

Technical Field

[0001] This application relates to the field of air purification technology, specifically to a polytetrafluoroethylene composite microporous membrane for air purification and its preparation method. Background Technology

[0002] Polytetrafluoroethylene (PTFE) composite microporous membranes are porous membrane products obtained by extruding PTFE rods or strips, calendering them into thin films, stretching and heat-setting them at temperatures below their melting point. PTFE composite microporous membranes are flexible and elastic microporous materials with high porosity and uniform pore size distribution. They possess air-permeable but water-impermeable properties and can be used as bactericidal filter membranes, ultra-clean filter membranes for various solvents, and have a wide range of applications.

[0003] Air purification plays a vital role in high-tech fields such as biology, pharmaceuticals, chemicals, electronics, environmental protection, and scientific experiments. Traditional air purification filters use microporous membranes made of polypropylene, polyester, and polytetrafluoroethylene. These filters have relatively low filtration efficiency during use, and the penetration of filtered particles affects their mechanical properties, resulting in a shorter lifespan. Summary of the Invention

[0004] This application provides a polytetrafluoroethylene composite microporous membrane for air purification and its preparation method. The polytetrafluoroethylene composite microporous membrane has good transverse tensile strength and longitudinal tensile strength, small thickness deviation, and can also improve the air permeability, filtration effect and service life of the polytetrafluoroethylene composite microporous membrane.

[0005] In a first aspect, embodiments of this application disclose a polytetrafluoroethylene composite microporous membrane for air purification, comprising:

[0006] Porous base membranes, including fibrous materials;

[0007] A polytetrafluoroethylene (PTFE) membrane is disposed on both sides of a porous base membrane, wherein the thickness ratio of the porous base membrane to the PTFE membrane on either side is 1:(1 to 1.5), and the average pore size of the PTFE membrane is 280 nm to 450 nm.

[0008] In some alternative embodiments, the fiber material includes at least one of polyester fiber material and polyamide material.

[0009] In some optional embodiments, the porosity of the porous base membrane is 40%-60%;

[0010] In some optional embodiments, the porosity of the polytetrafluoroethylene membrane is 80%-90%;

[0011] In some optional embodiments, the average thickness of the polytetrafluoroethylene film is 3–4.5 μm;

[0012] In some optional embodiments, the average pore size of the polytetrafluoroethylene membrane is 300 nm to 400 nm;

[0013] In some optional embodiments, the average thickness of the porous base film is 2–3 μm.

[0014] In some optional embodiments, the polytetrafluoroethylene composite microporous membrane has a weight-average molecular weight of 700,000 to 900,000 for polytetrafluoroethylene.

[0015] In some optional embodiments, the transverse tensile strength of the polytetrafluoroethylene composite microporous membrane is 130 MPa to 160 MPa.

[0016] In some optional embodiments, the longitudinal tensile strength of the above-mentioned polytetrafluoroethylene composite microporous membrane is 100 MPa to 150 MPa.

[0017] In some optional embodiments, the porosity of the polytetrafluoroethylene composite microporous membrane is 70%-85%;

[0018] In some optional embodiments, the ratio of the transverse tensile strength to the longitudinal tensile strength of the polytetrafluoroethylene composite microporous membrane is 1:(0.8 to 1).

[0019] Secondly, embodiments of this application provide a method for preparing a polytetrafluoroethylene composite microporous membrane according to the first aspect, comprising:

[0020] A porous base membrane and a polytetrafluoroethylene membrane are provided, wherein the porous base membrane comprises a fibrous material;

[0021] A polytetrafluoroethylene (PTFE) membrane is placed on both sides of a porous base membrane and subjected to hot pressing to obtain the PTFE composite microporous membrane.

[0022] In some optional embodiments, the temperature of the hot pressing process is controlled at 90–110°C, the linear pressure of the hot pressing process is 0.5–0.8 MPa, and the speed of the hot pressing process is 0.8–1.5 m / min.

[0023] In some optional embodiments, the method for preparing the polytetrafluoroethylene membrane includes:

[0024] The polytetrafluoroethylene dispersion resin was heat-treated at a temperature of 300℃~600℃ to obtain a melt.

[0025] The melt and additives are mixed and extruded at 40°C to 150°C to obtain a paste.

[0026] The paste is subjected to heat shrinking, cooling, extrusion stretching and heat setting at a temperature of 600℃~900℃ to obtain a polytetrafluoroethylene film, wherein the final cooling temperature is 60℃~100℃; the extrusion stretching includes longitudinal stretching and transverse stretching in sequence, and the ratio of longitudinal stretching to transverse stretching is (2~5):1.

[0027] In some optional embodiments, the longitudinal stretching ratio is 100 to 160 times, and the transverse stretching ratio is 20 to 50 times. High porosity, good filtration efficiency, and the embodiments of this application have at least the following beneficial effects:

[0028] This application embodiment uses a porous base membrane and a polytetrafluoroethylene (PTFE) membrane. The porous base membrane serves as a support material, possessing good stability, which is beneficial for providing good mechanical properties for the PTFE composite microporous membrane. The average pore size of the PTFE membrane is 280 nm to 450 nm, which can better balance the transverse and longitudinal tensile strengths of the PTFE membrane. The PTFE membrane is disposed on both sides of the porous base membrane, and the thickness ratio of the porous base membrane to the PTFE membrane on either side is 1:(1~1.5), which can better facilitate hot pressing, improve the bonding effect between layers, improve the thickness uniformity of the PTFE composite microporous membrane, further enhance the filtration effect of the PTFE composite microporous membrane, reduce the thickness deviation of the PTFE composite microporous membrane, and reduce the excessively large ratio of transverse and longitudinal tensile strengths, thus avoiding a short lifespan due to transverse and longitudinal tensile strength. Attached Figure Description

[0029] The features, advantages, and technical effects of exemplary embodiments of this application will now be described with reference to the accompanying drawings.

[0030] Figure 1 This is an electron microscope image of the polytetrafluoroethylene composite microporous membrane surface of Example 1 of this application. Detailed Implementation

[0031] The features and exemplary embodiments of various aspects of this application will now be described in detail. Numerous specific details are set forth in the following detailed description to provide a comprehensive understanding of this application. However, it will be apparent to those skilled in the art that this application can be implemented without requiring some of these specific details. The following description of embodiments is merely intended to provide a better understanding of this application by illustrating examples. In the accompanying drawings and the following description, at least some well-known structures and techniques are not shown to avoid unnecessarily obscuring the application; and, for clarity, the dimensions of some structures may be exaggerated. Furthermore, the features, structures, or characteristics described below can be combined in any suitable manner in one or more embodiments.

[0032] It should be noted that in this application, the terms "first," "second," etc., are used to distinguish similar objects and not to describe a specific order or sequence. Furthermore, the term "comprising," or any other variation thereof, is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus 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, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0033] Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application). Unless otherwise stated, the test temperature for all parameters mentioned in this application is 25°C and the test pressure is standard atmospheric pressure.

[0034] The list of items connected by the term "one or more of" can mean any combination of the listed items. The term "multiple" means two or more. Similarly, the list of items connected by "one or more of" can mean any combination of the listed items.

[0035] The embodiments of this application, by selecting and limiting the mass ratio of the adhesive and polyvinylidene fluoride polymer, produce a polytetrafluoroethylene composite microporous membrane with good bonding strength and good peel strength, while also improving the internal resistance and air permeability of the polytetrafluoroethylene composite microporous membrane.

[0036] The term "permeability" reflects the permeability of a polytetrafluoroethylene (PTFE) composite microporous membrane. Specifically, it measures the time required for a given volume of gas (typically 100 cc) to pass through a 1 square inch PTFE composite microporous membrane under a given pressure, for example, rapidly increasing the pressure to 10 kPa within 2 seconds.

[0037] like Figure 1 As shown in the embodiments of this application, a polytetrafluoroethylene composite microporous membrane for air purification is proposed, comprising:

[0038] Porous base membranes, including fibrous materials;

[0039] A polytetrafluoroethylene (PTFE) membrane is disposed on both sides of a porous base membrane, wherein the thickness ratio of the porous base membrane to the PTFE membrane on either side is 1:(1 to 1.5), and the average pore size of the PTFE membrane is 280 nm to 450 nm.

[0040] The membrane employs a porous base membrane and a polytetrafluoroethylene (PTFE) membrane. The porous base membrane, as a supporting material, possesses excellent stability, which is beneficial for providing good mechanical properties to the PTFE composite microporous membrane. The average pore size of the PTFE membrane is 280 nm to 450 nm, which can better balance the transverse and longitudinal tensile strengths of the PTFE membrane. The PTFE membrane is disposed on both sides of the porous base membrane, and the thickness ratio of the porous base membrane to the PTFE membrane on either side is 1:(1~1.5), which allows for better hot-pressing, improves the bonding effect between layers, enhances the thickness uniformity of the PTFE composite microporous membrane, and further improves the filtration effect of the PTFE composite microporous membrane, avoiding a short lifespan caused by an excessively high ratio of transverse and longitudinal tensile strengths.

[0041] Optionally, the thickness ratio of the porous base membrane and the polytetrafluoroethylene membrane on either side is any one of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, and 1:1.5.

[0042] In some optional embodiments, the fiber material includes at least one of polyester fiber material and polyamide material. Further, the average diameter of the fiber material is 0.3 to 0.8 micrometers. The melting point of the polyester fiber material is 130°C to 160°C.

[0043] In some optional embodiments, the porosity of the porous base membrane is 40%-60%; this is beneficial for increasing the airflow rate in the polytetrafluoroethylene composite microporous membrane and improving the filtration efficiency of the polytetrafluoroethylene composite microporous membrane.

[0044] In some optional embodiments, the porosity of the polytetrafluoroethylene membrane is 80%-90%, which is beneficial to increasing the airflow rate in the polytetrafluoroethylene composite microporous membrane and improving the filtration efficiency in the polytetrafluoroethylene composite microporous membrane.

[0045] In some optional embodiments, the average thickness of the polytetrafluoroethylene (PTFE) film is 3–4.5 μm. Optionally, the average thickness of the PTFE film can be any one of 3 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, and 4 μm.

[0046] In some optional embodiments, the average pore size of the polytetrafluoroethylene membrane is 300 nm to 400 nm; optionally, the average pore size of the polytetrafluoroethylene membrane can be any one of 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm, 360 nm, 370 nm, 380 nm, 390 nm, and 400 nm.

[0047] In some optional embodiments, the average thickness of the porous base film is 2–3 μm. The average thickness of the porous base film can be any one of 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, and 3 μm.

[0048] In some optional embodiments, the polytetrafluoroethylene composite microporous membrane has a weight-average molecular weight of 700,000 to 900,000, optionally 750,000 to 850,000, and further optionally 800,000.

[0049] In some optional embodiments, the transverse tensile strength of the polytetrafluoroethylene composite microporous membrane is 130 MPa to 160 MPa; optionally, it is 140 MPa to 155 MPa.

[0050] In some optional embodiments, the longitudinal tensile strength of the above-mentioned polytetrafluoroethylene composite microporous membrane is 100 MPa to 150 MPa; optionally, it is 110 MPa to 140 MPa.

[0051] In some optional embodiments, the porosity of the polytetrafluoroethylene composite microporous membrane is 70%-85%; optionally, it is 75% to 80%.

[0052] In some optional embodiments, the maximum thickness deviation of the polytetrafluoroethylene composite microporous membrane is 0.4 μm to 1.0 μm, and optionally 0.48 μm to 0.96 μm.

[0053] In some optional embodiments, the peel strength of the polytetrafluoroethylene composite microporous membrane is from 148 N / m to 170 N / m.

[0054] In some optional embodiments, the toluene diffusion rate of the polytetrafluoroethylene composite microporous membrane is 1.25 to 1.5 ug / min.

[0055] In some optional embodiments, the ratio of the transverse tensile strength to the longitudinal tensile strength of the polytetrafluoroethylene composite microporous membrane is 1:(0.8 to 1).

[0056] Secondly, embodiments of this application provide a method for preparing a polytetrafluoroethylene composite microporous membrane according to the first aspect, comprising:

[0057] A porous base membrane and a polytetrafluoroethylene membrane are provided, wherein the porous base membrane comprises a fibrous material;

[0058] A polytetrafluoroethylene (PTFE) membrane is disposed on both sides of a porous base membrane and subjected to hot pressing to obtain the PTFE composite microporous membrane. The thickness ratio of the porous base membrane to the PTFE membrane on either side is 1:(1 to 1.5). The average pore size of the PTFE membrane is 280 nm to 450 nm.

[0059] According to the embodiments of this application, a polytetrafluoroethylene (PTFE) membrane with a thickness ratio of 1:(1~1.5) is disposed on both sides of a porous base membrane, which can provide a good support effect for the PTFE composite microporous membrane, facilitate the control of hot pressing, improve the bonding effect between layers, and thus improve the stability of the PTFE composite microporous membrane. The average pore size of the PTFE membrane is 280nm to 450nm, which is conducive to the fine-tuning of the PTFE membrane performance during hot pressing, thereby better balancing the transverse tensile strength and longitudinal tensile strength of the PTFE membrane. Improving the thickness uniformity of the PTFE composite microporous membrane can further enhance the filtration effect of the PTFE composite microporous membrane and avoid the short lifespan caused by a large ratio of transverse and longitudinal tensile strength.

[0060] In some optional embodiments, the temperature of the hot pressing process is controlled at 90–110°C, the linear pressure of the hot pressing process is 0.5–0.8 MPa, and the speed of the hot pressing process is 0.8–1.5 m / min.

[0061] In some optional embodiments, the method for preparing the polytetrafluoroethylene membrane includes:

[0062] The polytetrafluoroethylene dispersion resin was heat-treated at a temperature of 300℃~600℃ to obtain a melt.

[0063] The melt and additives are mixed and extruded at 40°C to 150°C to obtain a paste.

[0064] The paste is subjected to heat shrinking, cooling, extrusion stretching and heat setting at a temperature of 600℃~900℃ to obtain a polytetrafluoroethylene film, wherein the final cooling temperature is 60℃~100℃; the extrusion stretching includes longitudinal stretching and transverse stretching in sequence, and the ratio of longitudinal stretching to transverse stretching is (2~5):1.

[0065] According to the embodiments of this application, by controlling the ratio of longitudinal stretching and transverse stretching, the average pore size of the polytetrafluoroethylene (PTFE) membrane can be further adjusted, which is beneficial to balancing the transverse tensile strength and longitudinal tensile strength of the PTFE membrane, reducing the ratio of transverse tensile strength to longitudinal tensile strength, and improving the service life of the membrane.

[0066] In some optional embodiments, the longitudinal stretching factor is 100 to 160 times, and the transverse stretching factor is 20 to 50 times. This allows for further adjustment of the average pore size, balancing the transverse and longitudinal tensile strengths of the PTFE membrane.

[0067] Example

[0068] The following embodiments describe the disclosure of this application in more detail. These embodiments are for illustrative purposes only, as various modifications and variations will be apparent to those skilled in the art within the scope of the disclosure of this application. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized by conventional methods and can be used directly without further processing, and the instruments used in the embodiments are commercially available.

[0069] Example 1

[0070] Preparation of polytetrafluoroethylene film: Polytetrafluoroethylene dispersion resin with a weight average molecular weight of 760,000 was heat-treated at a temperature of 510℃ to obtain a melt.

[0071] The molten material and the extrusion aid liquid paraffin and dibutyl phthalate were mixed at a mass ratio of 99:0.2:0.8 and extruded at 125°C to obtain a paste.

[0072] The paste was subjected to heat shrinking, cooling, extrusion stretching and heat setting at a temperature of 820°C to obtain a polytetrafluoroethylene film, wherein the final cooling temperature was 100°C; the longitudinal stretching ratio was 140 times and the transverse stretching ratio was 30 times.

[0073] Preparation of a polytetrafluoroethylene (PTFE) composite microporous membrane for air purification: A PTFE membrane is placed on both sides of a porous base membrane and subjected to hot pressing to obtain the PTFE composite microporous membrane. The porous base membrane has an average thickness of 3 μm and a porosity of 55%, the PTFE membrane has a porosity of 86%, an average thickness of 3.4 μm, and an average pore size of 310 nm. The hot pressing temperature is 108 °C, the linear pressure is 0.75 MPa, and the hot pressing speed is 0.9 m / min, so that the obtained PTFE composite microporous membrane has an average thickness of 9.5 μm and a porosity of 74.6%.

[0074] Example 2

[0075] The difference between this embodiment and Embodiment 1 is that the average pore size of the polytetrafluoroethylene membrane is 450 nm.

[0076] Example 3

[0077] The difference between this embodiment and Embodiment 1 is that the average pore size of the polytetrafluoroethylene membrane is 380 nm.

[0078] Example 4

[0079] The difference between this embodiment and Embodiment 1 is that the average thickness of the polytetrafluoroethylene film is 3 μm.

[0080] Example 5

[0081] The difference between this embodiment and Embodiment 1 is that the average thickness of the polytetrafluoroethylene film is 4.5 μm.

[0082] Example 6

[0083] The difference between this embodiment and Embodiment 1 is that the weight-average molecular weight of polytetrafluoroethylene is 850,000.

[0084] Example 7

[0085] The difference between this embodiment and Embodiment 1 is that the weight-average molecular weight of polytetrafluoroethylene is 700,000.

[0086] Example 8

[0087] The difference between this embodiment and Embodiment 1 is that the longitudinal stretching ratio is 90 times, the transverse stretching ratio is 45 times, the hot pressing temperature is controlled at 107°C, the hot pressing linear pressure is 0.6 MPa, and the hot pressing speed is 0.8 m / min.

[0088] Example 9

[0089] The difference between this embodiment and Embodiment 1 is that the longitudinal stretching ratio is 160 times, the transverse stretching ratio is 32 times, the hot pressing temperature is controlled at 107°C, the hot pressing linear pressure is 0.5 MPa, and the hot pressing speed is 0.8 m / min.

[0090] Comparative Example 1

[0091] The difference between this comparative example and Example 1 is that two layers of polytetrafluoroethylene film are sequentially placed on one side of the porous base film and then subjected to hot pressing.

[0092] Test methods

[0093] (1) Gas diffusion rate: 5 mL of toluene was added to the sample bottle, and the bottle mouth was sealed with the polytetrafluoroethylene composite microporous membrane prepared in the examples or comparative examples. The sealing diameter was 8 mm. The sample bottle was placed in a constant temperature diffusion cell. High-purity nitrogen was introduced into the diffusion cell at a temperature of 25℃ and a humidity of less than 80%. The flow rate of nitrogen was 0.8 L / min. The diffusion rate of toluene gas through the polytetrafluoroethylene composite microporous membrane was tested for a certain period of time. The test time was 1 month and 3 months. The results are shown in Table 1 below.

[0094] The toluene diffusion rate is (weight of the sample vial at the test endpoint - weight of the sample vial at the test start) / test time.

[0095] (3) Peel strength test: A 2.5cm × 30cm sample was cut along the longitudinal direction (MD) of the polytetrafluoroethylene composite microporous membrane using a membrane cutter. 3M (200MP) double-sided tape was applied smoothly to a peeling plate, and the entire sample was then adhered to the peeling plate with the coated side facing down. The sample area should not be touched by hand. The sample was then rolled back and forth twice using the pressure roller (2kg) provided with the equipment (4 times for individual rolling). Finally, the test was performed on a tensile testing machine at a speed of 50mm / min, and the average value was taken from three tests. The peel strength of Comparative Example 1 also measures the strength between the porous base membrane and the polytetrafluoroethylene microporous membrane.

[0096] (4) The test method for tensile strength shall be in accordance with GB / T 1040.3-2006.

[0097] (5) Maximum thickness deviation: Take a sample of polytetrafluoroethylene composite microporous membrane, measure multiple thicknesses, sort the measured thicknesses by size, take the first 5 larger thickness values ​​and the last 5 smaller thickness values; subtract the sum of the 5 smaller thickness values ​​from the sum of the 5 larger thickness values, and divide by 5 to obtain the maximum thickness deviation value.

[0098] The air permeability values ​​of the polytetrafluoroethylene composite microporous membranes of Examples 1-9 and the comparative examples are shown in Table 1.

[0099] Table 1

[0100]

[0101] Although this application has been described with reference to preferred embodiments, various modifications can be made thereto and components can be replaced with equivalents without departing from the scope of this application. In particular, the technical features mentioned in the various embodiments can be combined in any manner, provided there is no structural conflict. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for preparing a polytetrafluoroethylene composite microporous membrane, characterized in that, include: A porous base membrane and a polytetrafluoroethylene membrane are provided, wherein the porous base membrane comprises a fibrous material, the fibrous material being at least one of polyester fiber material and polyamide material, and the average thickness of the porous base membrane is 2-3 μm; A polytetrafluoroethylene (PTFE) membrane is placed on both sides of a porous base membrane and subjected to hot pressing. The temperature of the hot pressing is controlled at 107–108°C, the linear pressure of the hot pressing is 0.5–0.75 MPa, and the speed of the hot pressing is 0.8–0.9 m / min, in order to obtain the PTFE composite microporous membrane. The thickness ratio of the porous base membrane to the polytetrafluoroethylene (PTFE) membrane on either side is greater than or equal to 2 / 3 and less than 1; the average pore size of the PTFE membrane is 280 nm to 450 nm; the ratio of the transverse tensile strength to the longitudinal tensile strength of the PTFE composite microporous membrane is 1:(0.8 to 1); the toluene diffusion rate of the PTFE composite microporous membrane is 1.25 to 1.5 μg / min; and the maximum thickness deviation of the PTFE composite microporous membrane is 0.4 μm to 0.6 μm.

2. The preparation method according to claim 1, characterized in that, The polytetrafluoroethylene composite microporous membrane meets one or more of the following conditions: (1) The porosity of the porous base membrane is 40% to 60%; (2) The porosity of the polytetrafluoroethylene membrane is 80% to 90%; (3) The average thickness of the polytetrafluoroethylene film is 3 to 4.5 μm; (4) The average pore size of the polytetrafluoroethylene membrane is 300 nm to 400 nm.

3. The preparation method according to claim 1, characterized in that, In the polytetrafluoroethylene composite microporous membrane, the weight-average molecular weight of polytetrafluoroethylene is 700,000 to 900,000.

4. The preparation method according to claim 1, characterized in that, The polytetrafluoroethylene composite microporous membrane meets one or more of the following conditions: (1) The transverse tensile strength of the polytetrafluoroethylene composite microporous membrane is 130 MPa to 160 MPa. (2) The longitudinal tensile strength of the polytetrafluoroethylene composite microporous membrane is 100 MPa to 150 MPa. (3) The porosity of the polytetrafluoroethylene composite microporous membrane is 70% to 85%.

5. The preparation method according to claim 1, characterized in that, The method for preparing the polytetrafluoroethylene membrane includes: Polytetrafluoroethylene dispersion resin is heat-treated at a temperature of 300℃~600℃ to obtain a melt; the melt and additives are mixed and extruded at 40℃~150℃ to obtain a paste. The paste is subjected to heat shrinking, cooling, extrusion stretching and heat setting at a temperature of 600℃~900℃ to obtain a polytetrafluoroethylene film, wherein the final cooling temperature is 60℃~100℃; the extrusion stretching includes longitudinal stretching and transverse stretching in sequence, and the ratio of longitudinal stretching to transverse stretching is (2~5):

1.

6. The preparation method according to claim 5, characterized in that, The longitudinal stretching ratio is 100 to 160 times, and the transverse stretching ratio is 20 to 50 times.

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