A method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane

Abstract

This invention relates to a method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane, belonging to the field of polytetrafluoroethylene (PTFE) stretching membrane technology. The method includes steps such as mixing, curing, sieving, pressing, extrusion, calendering, degreasing, longitudinal stretching, transverse co-stretching and compounding, and heat setting. Specifically, the transverse co-stretching and compounding step involves stacking substrates of different molecular weights, then, using the same stretching equipment, transversely co-stretching the multilayer PTFE substrates while they are being forward-delivered, followed by compounding using a roller group. The above technical solution of this invention can effectively improve the uniformity of pore size and pore size distribution, and improve the effective interception rate and interception stability of the membrane. The PTFE film obtained by this invention has superior performance compared to existing similar products and can be used to manufacture pleated filter cartridges, as well as for the filtration and purification of gases and liquids in pharmaceutical processes.

Description

Technical Field

[0001] This invention relates to a method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane, belonging to the field of polytetrafluoroethylene stretch membrane technology. Background Technology

[0002] Polytetrafluoroethylene (PTFE) membranes are widely used in defense, aerospace, electronics, electrical, chemical, food, medical and textile industries.

[0003] Patent application CN200610145995.6 discloses a processing method for polytetrafluoroethylene (PTFE) nanoporous filter membranes. The method includes steps such as mixing, pressing, pushing, calendering, degreasing, longitudinal stretching, and transverse stretching of double-layer PTFE base tapes, and heat setting. During degreasing, two or more base tapes are stacked, followed by longitudinal and transverse stretching and heat setting to obtain a PTFE microporous membrane with a pore size less than 0.1 micrometers. While this patent solves the problems of large PTFE pore size distribution and the difficulty in preparing pores smaller than 0.1 micrometers, it presents challenges in controlling the membrane's thickness and pore size during production. Furthermore, the interlayer strength of the composite membrane is difficult to achieve the desired effect.

[0004] Patent application CN202010960028.5 discloses a method for preparing a multilayered PTFE microporous membrane. During degreasing, it involves hot-pressing two or more base layers with different molecular weights to obtain a composite base layer, which is then subjected to longitudinal and transverse stretching and heat setting. While this invention solves the problem of interlayer adhesion, it makes it difficult to control the membrane's thickness and pore size during production. Summary of the Invention

[0005] This invention aims to solve the above-mentioned problems by providing a method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane. The method of this invention allows for the control of the membrane's structure and pore size in the thickness direction, and further improves the interlayer adhesion, i.e., peel strength, of the membrane prepared by this method.

[0006] The technical solution of the present invention to solve the above problems is as follows:

[0007] A method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane includes the following steps:

[0008] S1. Mixing: Polytetrafluoroethylene dispersion resins with different molecular weights are mixed with extrusion aids and then mixed separately in a mixer to form polytetrafluoroethylene pastes with at least two orders of magnitude of molecular weight.

[0009] S2. Curing: Place the pastes with different molecular weights in an oven and cure them at 30~60℃ for 5~12 hours.

[0010] S3. Screening: The cooked materials with different molecular weights are screened separately to remove lumps caused by cooking.

[0011] S4. Pressing: The smelted materials with different molecular weights and sieved out of agglomerates are placed in a pressing machine for pre-pressing to obtain polytetrafluoroethylene blanks with different molecular weights.

[0012] S5. Extrusion: Using a pusher, polytetrafluoroethylene blanks with different molecular weights are extruded to obtain polytetrafluoroethylene extrudates with different molecular weights.

[0013] S6. Calendering: The extrusions of polytetrafluoroethylene with different molecular weights are calendered using a calendering machine to obtain polytetrafluoroethylene tapes with different molecular weights.

[0014] S7. Degreasing: The polytetrafluoroethylene belts with different molecular weights are heat-treated in a degreasing machine to remove the extrusion aid;

[0015] S8. Longitudinal stretching: Polytetrafluoroethylene tapes with different molecular weights are subjected to longitudinal stretching treatment; the longitudinal stretching temperature is 150~320℃.

[0016] S9. Transverse Co-stretching and Composite: Polytetrafluoroethylene (PTFE) tapes with different molecular weights that have undergone longitudinal stretching are stacked together. Then, using the same tensioning device, they are stretched transversely under the clamps at both ends and fed forward as the clamps move. During forward feeding, the multilayer PTFE tapes are transversely co-stretched. Then, they are composited using a pressure roller group to obtain a composite microporous membrane semi-finished product. In this step, the transverse stretching temperature is 120~260℃; the composite pressure is 0.01~0.80 MPa; and the composite temperature is 180~320℃.

[0017] S10. Heat setting: The composite microporous membrane semi-finished product is heat set to obtain the composite microporous membrane. The heat setting temperature is 300~500℃ and the heat setting time is 50~200 s.

[0018] In existing technologies, multilayer basebands are typically laminated or hot-pressed during degreasing and stretching, followed by simultaneous longitudinal and transverse stretching. The inventors discovered that this method results in a high degree of membrane symmetry. Furthermore, they found that higher symmetry is less conducive to controlling the membrane's structure and pore size along its thickness direction. This affects the membrane's interception efficiency and stability, an area where existing technologies have received little attention. Current technologies often focus only on permeability and porosity. However, permeability and porosity do not fully represent the membrane's function. For example, pore size, pore size uniformity, and pore size distribution uniformity are also crucial.

[0019] From the perspective of the impact of process technology on microstructure, the higher the symmetry of the membrane, the more through-pores it has; and the more through-pores there are, the easier it is for the intercepted target to escape. This is equivalent to stacking multiple layers of filter screens of the same specifications with aligned meshes. Obviously, if multiple layers of filter screens of the same specifications are stacked with staggered meshes, the effective interception rate and interception stability of the composite filter screen can be effectively improved.

[0020] The commonality with the examples above lies in enhancing asymmetry, but the methods of enhancing asymmetry differ. The asymmetry that this invention aims to enhance includes not only the structural asymmetry of each membrane layer itself, but also the asymmetry exhibited by the substrate layers during stretching, and the degree of mesh misalignment maximized during lamination. Based on these objectives, firstly, substrates with different molecular weights are prepared using polytetrafluoroethylene dispersion resin raw materials; then, longitudinal stretching is performed separately to enhance the structural asymmetry of the substrates, thereby improving subsequent control over the membrane thickness direction structure and pore size; finally, simultaneous transverse stretching and lamination are performed. Since the structural asymmetry has already been enhanced before simultaneous transverse stretching, and the simultaneous transverse stretching further enhances the asymmetry (due to the different molecular weights of the substrate raw materials in different layers, stretching under the same conditions produces different effects, i.e., the aforementioned asymmetry exhibited by the different substrate layers during stretching), the above-mentioned technical solution of this invention can effectively improve the uniformity of pore size and pore distribution, and improve the effective interception rate and interception stability of the membrane.

[0021] The non-stickiness and extremely high chemical stability of PTFE membrane material result in poor surface adhesion between its layers. Simultaneous lateral stretching and lamination primarily enhance the bonding strength between different layers, i.e., improve peel strength. While asynchronous lateral stretching can indeed further increase the asymmetry of the composite membrane, it significantly reduces peel strength. Therefore, this invention is actually a comprehensive solution that balances functional performance and peel strength.

[0022] As a preferred embodiment of the above technical solution, the molecular weight of the polytetrafluoroethylene dispersion resin is in the range of 10^4 to 10^8.

[0023] As a preferred embodiment of the above technical solution, the multilayer consists of three layers, with the bottom layer film using polytetrafluoroethylene dispersion resin having a molecular weight of 10^5 to 10^6, and the middle and top layers using polytetrafluoroethylene dispersion resin having a molecular weight of 10^6 to 10^7.

[0024] As a preferred embodiment of the above technical solution, the extrusion aid is kerosene. The amount of kerosene used in the polytetrafluoroethylene paste of the bottom film layer, w1, is 30-35% of the mass of the polytetrafluoroethylene dispersion resin; the amount of kerosene used in the polytetrafluoroethylene paste of the middle film layer, w2, is 27-33% of the mass of the polytetrafluoroethylene dispersion resin; and the amount of kerosene used in the polytetrafluoroethylene paste of the surface film layer, w3, is 25-30% of the mass of the polytetrafluoroethylene dispersion resin; and w1 > w2 > w3.

[0025] As a preferred embodiment of the above technical solution, in step S1, the mixing temperature is 2~25℃ and the mixing time is 5~50 min. More preferably, the mixing temperature is 8~20℃ and the mixing time is 20~40 min. The mixing temperature refers to the ambient temperature.

[0026] As a preferred embodiment of the above technical solution, in step S4, the pressing speed is 0.01~0.2 m / min.

[0027] As a preferred embodiment of the above technical solution, in step S5, the compression ratio is 20~200, the extrusion speed is 1~20 cm / min, and the extruder pressure is 0.1~1.0 MPa. More preferably, the compression ratio is 50~150, the extrusion speed is 5~15 cm / min, and the extruder pressure is 0.6~1.2 MPa.

[0028] As a preferred embodiment of the above technical solution, in step S6, the temperature of the pressure roller is set to 35~60℃, and the calendering speed is 5~25 m / min. More preferably, the temperature of the pressure roller is set to 50~60℃, and the calendering speed is 8~15 m / min.

[0029] As a preferred embodiment of the above technical solution, in step S8, the longitudinal stretching ratio is 2 to 20 times, and the stretching speed is 2 to 20 m / min. More preferably, the longitudinal stretching ratio is 3 to 10 times, and the stretching speed is 3 to 8 m / min.

[0030] In the above technical solution of the present invention, the vehicle speed is the rate at which the equipment delivers the workpiece (baseband or film) forward; the same applies below.

[0031] It should be further explained that longitudinal stretching is achieved through the speed difference between the front and rear delivery rollers, but the stretching speed is not the speed difference between the front and rear rollers, but the forward delivery rate of the rear roller, because the linear speed of the rear roller is greater than that of the front roller. The delivery rate of the rear roller is the rate at which the equipment delivers the workpiece (baseband or film) forward.

[0032] As a preferred embodiment of the above technical solution, in step S9, the transverse stretching ratio is 5 to 40 times, the transverse stretching rate is 100 to 1000% / min, the transverse stretching temperature is 50 to 260℃, the composite pressure is 0.03 to 0.60 MPa, and the composite speed is 2 to 15 m / min. More preferably, the transverse stretching ratio is 8 to 15 times, the transverse stretching rate is 300 to 800% / min, the transverse stretching temperature is 120 to 260℃, and the composite speed is 5 to 12 m / min.

[0033] In the above technical solution of the present invention, the unit of transverse stretching rate is % / min, which means the multiple by which the stretching occurs per unit time (min). Since the width of the baseband is usually fixed in the art, even if there is a difference, it will not be significant. Therefore, the unit % / min is clear to those skilled in the art, and is more intuitive and has more practical significance.

[0034] It should be further explained that lateral stretching is achieved through a spreading device, thus having two rate parameters: vehicle speed and stretching rate. The stretching rate is the rate at which the left and right clamps move away from each other as the workpiece is being delivered forward. Rows of left and right clamps hold the workpiece at its left and right ends respectively, and their mutual movement achieves lateral stretching of the workpiece.

[0035] As a preferred embodiment of the above technical solution, the shaping method is either contact shaping or non-contact shaping.

[0036] Another object of the present invention is to provide a multilayer substrate transversely co-stretched polytetrafluoroethylene composite microporous membrane prepared by the above method, wherein the microporous membrane has an average porosity of 50-60%, an air permeability of 3-6 m^3 / (m^2·h), and an average pore size of 0.04-0.06 μm.

[0037] In summary, the present invention has the following beneficial effects:

[0038] 1) The present invention provides a method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane. The method adopts a scheme of separate longitudinal stretching followed by simultaneous transverse stretching. While taking into account the peel strength, it significantly improves the asymmetry of the membrane, which is beneficial to the control of the structure and pore size in the thickness direction of the membrane, and effectively improves the effective interception rate and interception stability of the membrane.

[0039] 2) In the preparation process of the composite microporous membrane of the present invention, the base bands of different layers are prepared with raw materials of different molecular weights. The different molecular weights refer to different orders of magnitude of molecular weight. The simultaneous transverse stretching of multiple base bands can play a good complementary role in mechanical properties and also improve the average porosity.

[0040] 3) The polytetrafluoroethylene film prepared by this invention has an average porosity of 50-60%, an air permeability of 3-6 m^3 / (m^2·h), an average pore size of 0.04-0.06μm, and a peel strength of 30N. Its performance is superior to existing similar products. It can be used to make pleated filter cartridges and can also be used for the filtration and purification of gases and liquids in the pharmaceutical process.

[0041] 4) The technical solution of the present invention has different molecular weights of the base materials of different layers, which leads to different effects of stretching under the same conditions, improves the asymmetry, and improves the peel strength during subsequent composite, so that the mechanical properties and filtration performance of the composite membrane are guaranteed. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the principle of the present invention;

[0043] Figure 2 This is a SEM image of the upper surface of the membrane in Embodiment 1 of the present invention;

[0044] Figure 3 This is a SEM image of the underside of the membrane in Embodiment 1 of the present invention;

[0045] Figure 4 This is a SEM image of the membrane cross-section in Embodiment 1 of the present invention. Detailed Implementation

[0046] The present invention will be further described below with reference to specific embodiments.

[0047] In this embodiment of the invention, the average porosity is tested using mercury porosimetry; a 9500 AutoPore IV fully automatic mercury porosimeter from Micron Instruments, Inc. is used to test the porosity and directly read the data.

[0048] The air permeability test method refers to GB / T36138-2018 "Polytetrafluoroethylene flat sheet microfiltration membrane for sterilization" and uses YG(B)461E fully automatic fabric air permeability tester with a pressure difference of 200Pa to directly read the data.

[0049] Before operating the instrument, it should be leveled by rotating the adjustable feet until the instrument is horizontal. Check that the sample set ring is tightened and that its airtight ring is tightly fitted to the two mating surfaces. Check that the pressure head can be controlled to move up and down freely. Check that the connection between the suction hose and the flow cylinder and the suction fan is tight. Check that the door cover and lock of the flow cylinder can be closed tightly. Check that the printer is correctly connected. Calibrate. Check for air leaks in the instrument. Cut the sample to the specified size; select the sample set ring and install it on the instrument; select the nozzle; connect the instrument to the power supply; set the parameters; start the instrument; the instrument will automatically press the sample and start the test. When the set pressure difference is reached, the instrument will automatically release the sample, automatically calculate the test result, and stop; the test result can be observed on the instrument's air permeability / pressure difference display screen; after the test, turn off the power, clean the instrument and all accessories, and remove the set ring. During the test, 10 circular pieces of membrane material with an area of ​​100 cm² were cut, and the test pressure difference was 200 Pa. The YG(B)461E fully automatic fabric air permeability tester was used to measure the air permeability 10 times, and the average air permeability was taken.

[0050] Peel strength was tested using a universal tensile testing machine, and the data was read directly.

[0051] Average pore size was measured using a PMI CFP-1500AE pore size analyzer with a surface tension of 15.9 mN / m and an effective sample area diameter of 2 cm.

[0052] Example 1

[0053] This embodiment uses a three-layer PTFE composite membrane as an example. The extrusion aid is kerosene. The amount of extrusion aid in the paste corresponding to the bottom substrate is 33% of the mass of the polytetrafluoroethylene dispersion resin. The molecular weight of the PTFE resin is 10. 5 ~10 6 The intermediate layer baseband corresponds to an extrusion aid dosage of 30% of the mass of the polytetrafluoroethylene (PTFE) dispersion resin, and the PTFE resin has a molecular weight of 10. 6 ~10 7 The amount of extrusion aid corresponding to the upper base layer paste is 28% of the mass of the polytetrafluoroethylene (PTFE) dispersion resin, and the molecular weight of the PTFE resin is 10. 6 ~10 7 .

[0054] A method for preparing a PTFE microporous membrane with a multilayer structure, such as... Figure 1 As shown, it includes the following steps:

[0055] S1. Mixing:

[0056] Two polyvinyl fluoride dispersion resins with different molecular weights (10) were used. 5 ~10 6 106 ~10 7 After being mixed with extrusion aids in different proportions (see above), the mixtures were placed in a mixer and mixed at high speed at a temperature of 15°C for 35 minutes to obtain three different PTFE pastes (denoted as bottom layer paste, middle layer paste, and top layer paste).

[0057] S2, ripening:

[0058] Three different pastes were placed in an oven for curing at 50°C for 15 hours.

[0059] S3, Screening:

[0060] The three different pastes were sieved separately to remove any lumps that formed after cooking.

[0061] S4, Pressed blank:

[0062] Three different matured pastes were placed in a prepressing machine for pre-pressing to obtain three different blanks; the pressing speed was 0.18 m / min for all three.

[0063] S5, Extrusion:

[0064] Three different blanks were processed by a press and extruded into sheets to obtain bottom sheet, middle sheet and top sheet. In this step, the compression ratio of the bottom sheet was 80, the middle sheet was 95 and the top sheet was 110. The extrusion speed was 10 cm / min and the press pressure was 0.9 MPa.

[0065] S6, Calendering:

[0066] The bottom layer, middle layer, and top layer are calendered separately using a calender to produce the bottom base strip, middle layer base strip, and top layer base strip; in this step, the temperature of the pressure rollers is 55℃ and the calendering speed is 12 m / min.

[0067] S7, Degreasing:

[0068] The bottom layer, middle layer, and top layer base strips were heat-treated in a degreasing machine to remove the extrusion aid and fully expand them. The degreasing temperature for each layer was 150 ℃.

[0069] S8, Longitudinal tension:

[0070] The three types of degreased base strips were subjected to longitudinal stretching treatment. The longitudinal stretching ratios of the bottom base strip, the middle base strip, and the top base strip were 3, 5, and 7 times, respectively. The stretching temperature was 210℃ and the stretching speed was 5 m / min. In this step, the longitudinal stretching was achieved by the speed difference between the front and rear rollers. The speed is not the speed difference, but refers to the linear speed of the rear roller, that is, the linear speed of the roller with the faster rotation speed.

[0071] S9, Multilayer baseband transverse co-stretching and composite:

[0072] Three types of longitudinally stretched base tapes are stacked together, and then, using the same tensioning device, they are stretched laterally under the clamps at both ends. As the clamps move, the tapes are fed forward, and simultaneously, the multilayer polytetrafluoroethylene (PTFE) tapes are co-stretched laterally. Then, they are composited using a pressure roller group to obtain a composite microporous membrane semi-finished product. In this step, the initial width of each base tape is approximately 20 cm, the lateral stretching ratio is 10 times, the lateral stretching speed is 400% / min, and the lateral stretching temperature is 160℃; the composite pressure is 0.06 MPa, the composite speed is 8 m / min, and the composite temperature is 260℃.

[0073] S10, Heat setting:

[0074] The setting time is 180 seconds, and the setting temperature is 450℃.

[0075] Figures 2-4 The image shows a multilayer PTFE microporous membrane prepared according to the above method. Measurements show that it has an air permeability of 4 m³ / (m²·h), an average pore size of 0.05 μm, a porosity of 55%, a peel strength of 30 N, and an effective width of approximately 1600 mm.

[0076] Examples 2-5

[0077] The only difference from Example 1 is the parameters, as shown in the table below.

[0078]

[0079] Comparative Example 1

[0080] A method for preparing a PTFE microporous membrane with a multilayer structure, such as... Figure 1 As shown, it includes the following steps:

[0081] S1. Mixing:

[0082] Two polyvinyl fluoride dispersion resins with different molecular weights (10) were used. 5 ~10 6 10 6 ~10 7After being mixed with extrusion aids in different proportions (see above), the mixtures were placed in a mixer and mixed at high speed at a temperature of 15°C for 35 minutes to obtain three different PTFE pastes (denoted as bottom layer paste, middle layer paste, and top layer paste).

[0083] S2, ripening:

[0084] Three different pastes were placed in an oven for curing at 50°C for 15 hours.

[0085] S3, Screening:

[0086] The three different pastes were sieved separately to remove any lumps that formed after cooking.

[0087] S4, Pressed blank:

[0088] Three different matured pastes were placed in a prepressing machine for pre-pressing to obtain three different blanks; the pressing speed was 0.18 m / min for all three.

[0089] S5, Extrusion:

[0090] Three different blanks were processed by a press and extruded into sheets to obtain bottom sheet, middle sheet and top sheet. In this step, the compression ratio of the bottom sheet was 80, the middle sheet was 95 and the top sheet was 110. The extrusion speed was 10 cm / min and the press pressure was 0.9 MPa.

[0091] S6, Calendering:

[0092] The bottom layer, middle layer, and top layer are calendered separately using a calender to produce the bottom base strip, middle layer base strip, and top layer base strip; in this step, the temperature of the pressure rollers is 55℃ and the calendering speed is 12 m / min.

[0093] S7, Degreasing:

[0094] The bottom base strip, the middle base strip, and the top base strip are stacked together and heat-treated together in a degreasing machine to remove the extrusion aid and fully expand. The degreasing temperature is 150℃.

[0095] S8, longitudinal stretching and transverse stretching

[0096] The degreased substrate was longitudinally co-stretched and then transversely co-stretched to obtain a composite microporous membrane semi-finished product.

[0097] In this step, the initial width of the baseband is about 20cm, the longitudinal stretching ratio is 5 times, the longitudinal stretching temperature is 210℃, and the longitudinal stretching speed is 5 m / min; the transverse stretching speed is 400% / min, and the transverse stretching temperature is 160℃.

[0098] S10, Heat setting:

[0099] The setting time is 180 seconds, and the setting temperature is 450℃.

[0100] The multilayer PTFE microporous membrane prepared according to the above method was tested and found to have the following properties: air permeability 35 m^3 / (m^2·h), average pore size 0.1 μm, porosity 80%, peel strength 50 N, and effective width up to about 1600 mm.

[0101] The table below shows the performance data for Examples 1-5 and the comparative examples.

[0102]

Claims

1. A method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane, comprising the following steps: S1. Mixing: Polytetrafluoroethylene dispersion resins with different molecular weights are mixed with extrusion aids and then mixed separately in a mixer to form polytetrafluoroethylene pastes with at least two orders of magnitude of molecular weight. S2. Curing: Place the pastes with different molecular weights in an oven and cure them at 30~60℃ for 5~12 hours. S3. Screening: The cooked materials with different molecular weights are screened separately to remove lumps caused by cooking. S4. Pressing: The smelted materials with different molecular weights and sieved out of agglomerates are placed in a pressing machine for pre-pressing to obtain polytetrafluoroethylene blanks with different molecular weights. S5. Extrusion: Using a pusher, polytetrafluoroethylene blanks with different molecular weights are extruded to obtain polytetrafluoroethylene extrudates with different molecular weights. S6. Calendering: The extrusions of polytetrafluoroethylene with different molecular weights are calendered using a calendering machine to obtain polytetrafluoroethylene tapes with different molecular weights. S7. Degreasing: The polytetrafluoroethylene belts with different molecular weights are heat-treated in a degreasing machine to remove the extrusion aid; S8. Longitudinal stretching: Polytetrafluoroethylene tapes with different molecular weights are subjected to longitudinal stretching treatment; the longitudinal stretching temperature is 150~320℃. S9. Transverse Co-stretching and Composite: Polytetrafluoroethylene (PTFE) tapes with different molecular weights that have undergone longitudinal stretching are stacked together. Then, using the same tensioning equipment, they are stretched in the transverse direction under the clamps at both ends and fed forward as the clamps move. During forward feeding, the multilayer PTFE tapes are transversely co-stretched. Then, they are composited using a pressure roller group to obtain a composite microporous membrane semi-finished product. The transverse stretching temperature is 120~260℃; the composite pressure is 0.01~0.80 MPa; and the composite temperature is 180~320℃. S10. Heat setting: The composite microporous membrane semi-finished product is heat set to obtain the composite microporous membrane. The heat setting temperature is 300~500℃ and the heat setting time is 50~200s. The multilayer consists of three layers. The bottom layer membrane uses polytetrafluoroethylene dispersion resin with a molecular weight of 10^5 to 10^6, while the middle and top layers use polytetrafluoroethylene dispersion resin with a molecular weight of 10^6 to 10^7. The extrusion aid is kerosene. The amount of kerosene used in the polytetrafluoroethylene paste of the bottom film layer, w1, is 30-35% of the mass of the polytetrafluoroethylene dispersion resin; the amount of kerosene used in the polytetrafluoroethylene paste of the middle film layer, w2, is 27-33% of the mass of the polytetrafluoroethylene dispersion resin; and the amount of kerosene used in the polytetrafluoroethylene paste of the surface film layer, w3, is 25-30% of the mass of the polytetrafluoroethylene dispersion resin; and w1 > w2 > w3.

2. The method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane according to claim 1, characterized in that: In step S1, the mixing temperature is 2~25℃; the mixing time is 5~50 min.

3. The method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane according to claim 1, characterized in that: In step S4, the pressing speed is 0.01~0.2 m / min.

4. The method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane according to claim 1, characterized in that: In step S5, the compression ratio is 20~200, the extrusion speed is 1~20 cm / min, and the extruder pressure is 0.1~1.0 MPa.

5. The method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane according to claim 1, characterized in that: In step S6, the temperature of the pressure roller is set to 35~60℃, and the calendering speed is 5~25 m / min.

6. The method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane according to claim 1, characterized in that: In step S8, the longitudinal stretching ratio is 2 to 20 times, and the stretching speed is 2 to 20 m / min.

7. The method for preparing a multilayer substrate transversely co-stretched PTFE composite microporous membrane according to claim 1, characterized in that: In step S9, the transverse stretching ratio is 5 to 40 times, the transverse stretching rate is 100 to 1000% / min, the transverse stretching temperature is 50 to 260℃, the composite pressure is 0.03 to 0.60 MPa, and the composite speed is 2 to 15 m / min.

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