High air permeability ptfm porous membrane and its preparation process

Abstract

The application relates to a high-air-permeability PTFE porous membrane and a preparation process thereof. The porous membrane comprises a membrane body, the membrane body comprises a first outer surface and a second outer surface, the cross section of the membrane body comprises long-strip-shaped main fibers and block-shaped main nodes, and the two ends of the main fibers are connected to the main nodes or adjacent main fibers; the thickness of the membrane body is 3-40 mu m; the main fibers comprise support fibers with a diameter not less than 0.6 mu m, the number proportion of the support fibers is 10-60%, and long-and-narrow strip-shaped holes are formed between adjacent main fibers; and the air permeability of the porous membrane is not less than 3*10 4 ml / min / cm 2 @7kPa. The long-and-narrow strip-shaped hole structure is introduced to ensure that the PTFE porous membrane has high air permeability; and the membrane thickness and the support fibers are controlled to ensure that the hole structure of the PTFE porous membrane has high mechanical strength. The application further discloses a preparation process of the aforementioned porous membrane.

Description

Technical Field

[0001] This application relates to the field of microporous membranes, and in particular to a high-permeability PTFE porous membrane and its preparation process. Background Technology

[0002] Polytetrafluoroethylene (PTFE) possesses a unique molecular structure that endows it with outstanding chemical stability, excellent resistance to high and low temperatures, good non-stick properties, lubricity, and superior electrical insulation and aging resistance. Based on these advantages, PTFE accounts for a significant proportion of the total fluoroplastics usage and is widely applied in the petroleum, chemical, and textile industries.

[0003] In addition, PTFE is also used to prepare microporous membrane products. PTFE microporous membranes are widely used in the fields of microelectronics and semiconductors, as well as in waterproofing and breathability applications for various electronic components, automobiles, and electrical appliances. Because PTFE has relatively low molecular forces, only a small stretching force is needed to stretch the ribbon-like crystalline structure into a fibrous structure. Therefore, the most common preparation method for PTFE microporous membranes is the stretching method.

[0004] For example, US patents US 3953566 and US 4187390 disclose methods for preparing PTFE microporous membranes. These methods involve mixing PTFE dispersion resin with a lubricant (such as kerosene) to obtain a film-forming liquid, extruding the paste-like film-forming liquid to remove the lubricant, and then performing unidirectional or bidirectional stretching at a certain temperature (below the melting point of PTFE) to obtain the desired PTFE microporous membrane.

[0005] For applications requiring high air permeability, such as various acoustic devices (headphones, headsets, etc.), the rapid and large-amplitude vibration of the internal diaphragm causes rapid and drastic changes in the internal air pressure. If the PTFE membrane's air permeability is insufficient, these pressure fluctuations will restrict the diaphragm's movement, affecting the audio curve and causing audio distortion. This is unacceptable for acoustic devices requiring low distortion. Therefore, PTFE membranes used in low-distortion acoustic devices must have high air permeability to ensure that the internal and external air pressures of the acoustic device can balance within a very short time to reduce changes in the audio curve.

[0006] For example, U.S. Patent Application Publication No. US4902423A (GORE & ASS application) discloses a thin, low-density porous polytetrafluoroethylene membrane having an open structure defined by small nodes interconnected with fibrils. The membrane is characterized by a porosity equal to or greater than 90%, a density of 0.2 g / cc or less, and an air permeability between 100 and 350 Fraser numbers. Most of the nodes are arranged in a series of generally aligned parallel rows on the membrane and are connected to the nodes by fibrils and fibril bundles, with the parallel rows of nodes interconnected by long fibrils.

[0007] The PTFE membrane in the aforementioned patent is a thin, low-density porous PTFE membrane. It is thin with a porosity of at least 90% and very large pores. The extremely high porosity and large pores mean that the solid portion (i.e., the fibrils and nodes) accounts for a very small proportion of the membrane. This indicates that the diameter of the fibril structure constituting the membrane is extremely small. The thin thickness and small proportion of the fibril structure result in extremely low gas resistance, thus exhibiting extremely high air permeability (i.e., breathability). In other words, to achieve high air permeability, the membrane in the aforementioned patent ideally has low thickness and high porosity; however, a membrane with low thickness and high porosity is expected to have lower mechanical strength, making it prone to deformation and even mechanical damage.

[0008] For many applications requiring outdoor use, in addition to breathability, mechanical strength is also a crucial requirement. This is because applications such as outdoor base stations, lighting fixtures, and battery packs for new energy vehicles require not only balancing internal and external air pressure through the PTFE membrane, but also ensuring that the PTFE membrane is not easily damaged by mechanical forces (such as from impacts from sand and gravel) to guarantee the normal operation and safety of electronic components.

[0009] For example, Chinese invention patent application CN101242889A (applied by Gore Enterprises Holding Co., Ltd.) discloses a porous PTFE material and articles made therefrom, whose bubble point-Grey value relationship is equal to or lower than the equation log(Grey value) = 5.13 × 10⁻⁶. -3 (Bubble point) - 1.26 is the limit line; the surface area is at least 20 meters. 2 / gram; a combination of at least 50% light transmittance and at least 50% porosity.

[0010] The PTFE membrane in the aforementioned patent exhibits a significantly reduced porosity (compared to a porosity of not less than 90%), implying a higher proportion of fibrous structure in the membrane matrix. These larger-diameter fibers contribute to the higher overall mechanical strength of the PTFE membrane. Furthermore, increasing the thickness of the PTFE membrane is also expected to improve its mechanical strength. However, while increasing the density or thickness of the PTFE membrane can lead to a predictable improvement in mechanical strength, it often results in a significant decrease in air permeability, which is also undesirable.

[0011] Clearly, PTFE membranes that combine high air permeability and high mechanical properties are urgently needed; however, for current PTFE membranes, there is a contradiction between good mechanical strength and high air permeability, making it difficult to achieve both simultaneously. Summary of the Invention

[0012] This application provides a high-permeability PTFE porous membrane and its preparation process. This application ensures that the PTFE porous membrane has a permeability of not less than 3 × 10⁻⁶ pores by introducing a narrow, elongated strip-shaped pore structure. 4 ml / min / cm 2 The air permeability is 7 kPa; and by controlling the membrane thickness to 3–40 μm and introducing 10–60% of supporting fibers with a diameter of not less than 0.6 μm, the PTFE porous membrane's pore structure can be ensured to have high mechanical strength. Furthermore, the PTFE porous membrane preparation process in this application, by controlling the molecular weight and swelling degree of the PTFE resin in the film-forming raw materials, and employing a special intermittent micro-stretching process during heat treatment after biaxial stretching, can produce the desired special structured PTFE porous membrane.

[0013] In a first aspect, this application provides a high-permeability PTFE porous membrane, employing the following technical solution:

[0014] A high-permeability PTFE porous membrane includes a membrane body, the membrane body including a first outer surface and a second outer surface, the cross-section of the membrane body including elongated main fibers and block-shaped main nodes, the two ends of the main fibers being connected to the main nodes or adjacent main fibers;

[0015] The thickness of the membrane body is 3–40 μm;

[0016] The main fiber includes supporting fibers with a diameter of not less than 0.6 μm, and the ratio of the number of supporting fibers to the number of main fibers is 10 to 60%, with narrow strip-shaped pores formed between adjacent main fibers;

[0017] The air permeability of the porous membrane is not less than 3×10⁻⁶. 4ml / min / cm 2 @7kPa.

[0018] Optionally, the thickness of the membrane body is 5–35 μm; further optionally, the thickness of the membrane body is 5–30 μm; even more optionally, the thickness of the membrane body is 5–20 μm.

[0019] For applications such as low-distortion acoustic devices, high air permeability of PTFE porous membranes is essential when employing the aforementioned technical solutions. Currently, the most common approach is to reduce membrane thickness and / or increase membrane porosity. However, both reducing membrane thickness and increasing membrane porosity lead to a decrease in mechanical strength, making the membrane pore structure susceptible to unpredictable deformation or even damage under external forces. This is unacceptable for applications requiring both high air permeability and high mechanical strength.

[0020] Contrary to the common belief that membranes with large thicknesses (such as greater than 3 μm, or even greater than 5 μm or 10 μm) should not be used to achieve high air permeability, the inventors of this application have discovered that the air permeability of PTFE porous membranes does not necessarily increase with increasing membrane thickness. In particular, when the thickness of the membrane substrate is 3–40 μm and the substrate fibers form elongated strip-shaped pores in the thickness direction, the PTFE porous membrane can still achieve an air permeability of not less than 3 × 10⁻⁶. 4 ml / min / cm 2 High air permeability of 7 kPa.

[0021] This may be because, when gas permeates the membrane matrix, it encounters significant resistance from the walls of the membrane pore structure (i.e., the surfaces of the fiber structures or nodes). In this application, the elongated main fibers enclose and form a narrow, strip-shaped pore structure. The long axis of this strip-shaped pore structure extends from the pore structure and does not contain any fiber structure, thus eliminating the resistance to gas flow. Although the gas is still inevitably subject to resistance from the main fibers, the low resistance in the long axis direction of the strip-shaped pores results in significantly lower gas resistance, allowing the membrane to maintain high permeability even with a relatively high thickness.

[0022] Aside from the unique strip-shaped pores, the main nodes in this application are not the continuous, elongated node structures found in conventional EPTFE membranes, but rather block-shaped nodes. Because of their significantly larger size compared to fibers, these nodes are a crucial component of the solid structure of the membrane and a significant source of gas flow resistance. Compared to elongated nodes that divide the membrane into multiple relatively independent regions (with poor gas flow between regions), the smaller block-shaped nodes not only inherently possess lower gas resistance but also significantly reduce the gas flow resistance between different regions within the membrane, greatly reducing the restriction on gas flow paths. This significantly reduces the effective flow path of gas through the membrane, allowing the membrane of this application to maintain high permeability even with a relatively high thickness.

[0023] In summary, by combining the low gas resistance of the strip-shaped pores and the low gas flow path restriction of the block-shaped nodes within the membrane matrix, the membrane of this application possesses both low gas resistance and a short gas flow path, thus enabling the PTFE porous membrane to maintain high gas permeability even with a relatively high thickness. This is a surprising result, contrary to the currently accepted understanding that gas flux decreases with increasing membrane thickness.

[0024] In addition to high gas flux, the PTFE porous membrane of this application also has high mechanical strength. This is because the membrane in this application has a thickness of not less than 3 μm, and there are supporting fibers accounting for 10-60% of the thickness of the membrane body with a diameter of not less than 0.6 μm. The large membrane thickness and the introduction of the unconventional "coarse fibers", combined with the blocky node structure that can play a good local reinforcement role, enable the PTFE porous membrane in this application to have good mechanical strength.

[0025] It is worth noting that improving the uniformity of fibers in the membrane matrix is ​​generally considered beneficial, as a uniform fiber structure often represents higher dimensional stability and higher pore uniformity. However, while the specific strip-shaped pore structure in this application brings high air permeability, its large major diameter results in weak self-supporting ability. The introduction of supporting fibers, accounting for no less than 10% and with a diameter of no less than 0.6 μm, can significantly improve the self-supporting ability of the strip-shaped pores. Combined with the blocky node structure with good local reinforcement effect, the strip-shaped pores are supported, thereby ensuring that the membrane has both high air permeability and high mechanical strength. Therefore, the strip-shaped pores can compensate for the air permeability loss caused by the supporting fibers and blocky nodes, while the supporting fibers and blocky nodes can compensate for the mechanical strength loss caused by the high air permeability of the strip-shaped pores, thus enabling the membrane to have both high air permeability and high mechanical strength.

[0026] Of course, the thickness of the membrane substrate should not exceed 40 μm, and the proportion of supporting fibers, which constitute the solid part, should not exceed 60%. This is because excessive thickness and a high proportion of supporting fibers often mean a significant increase in gas impediment. The high-permeability strip pores cannot compensate for the decrease in permeability caused by the increased thickness and supporting fibers, thus resulting in an undesirable decrease in the membrane's permeability. Furthermore, while further increasing the membrane thickness and the proportion of supporting fibers can indeed further improve the mechanical strength of the membrane substrate, the significance of further improving mechanical strength is greatly reduced once the application requirements are met.

[0027] Understandably, the air permeability should be no less than 3×10. 4 ml / min / cm 2 @7kPa means that under a gas pressure of 7kPa, the amount of gas that can pass through the PTFE porous membrane per square centimeter per minute is not less than 3×10⁻⁶. 4 ml. The testing method is as follows: [The text abruptly shifts to a different topic] ...with an effective air permeability area of ​​Scm... 2 A membrane sample was adhered to the surface of a stainless steel fixture with an opening (1.0 mm diameter). The other end of the fixture was connected to a pressure sensor, a gas flow sensor, a gas pressure regulating valve, and a gas source. The gas pressure regulating valve was adjusted to bring the pressure sensor reading to 7 kPa, and the gas flow sensor reading was taken as Q mL / min. Therefore, the air permeability rate of the waterproof and breathable membrane assembly is Q / S ml / min / cm². 2 @7kPa.

[0028] The various surface morphology parameters of porous membranes (such as thickness, fiber diameter, pore size, and pore area ratio) can be measured by characterizing the membrane structure using a scanning electron microscope (SEM), followed by measurement and calculation using computer software (such as Matlab, NIS-Elements, etc.) or manually. In actual measurement, the membrane surface (or cross-section) can be characterized first using an electron microscope to obtain the corresponding SEM image, and a certain area, such as 10000 μm, can be selected. 2 (100μm x 100μm), the specific area size depends on the actual situation. Then, use appropriate computer software or manual measurement to determine the morphological parameters such as the pore diameter and fiber diameter of all holes in the area, and then calculate to obtain the average pore diameter, average fiber diameter and other morphological parameters of the region. Of course, those skilled in the art can also obtain the above parameters through other measurement methods. The above measurement methods are for reference only.

[0029] Optionally, the main fiber includes breathable fibers with a fiber diameter of no more than 0.2 μm, the breathable fibers are distributed between adjacent supporting fibers, and the ratio of the number of breathable fibers to the number of main fibers is 5 to 25%.

[0030] By adopting the above technical solution, in addition to the supporting fibers, the main fibers in this application also include breathable fibers with a diameter of no more than 0.2 μm. The smaller diameter of the breathable fibers results in a relatively weaker obstruction force on gas flow. However, the combination of the supporting fibers and the breathable fibers distributed among them forms a highly intact mesh structure of coarse and fine fibers. The breathable fibers can effectively connect adjacent supporting fibers, allowing the force to be more evenly transmitted to the mechanically stronger supporting fibers and node structures when the membrane body is under pressure, thereby significantly reducing local stress concentration.

[0031] The fact that the proportion of breathable fibers is not less than 5% means that the membrane body has a fine fiber structure with sufficient conductive force, which greatly improves the overall pressure-bearing capacity of the three-dimensional network structure of the membrane body, so that the membrane body has higher mechanical strength. On the other hand, the proportion of breathable fibers is not greater than 25% means that a dense three-dimensional network structure with high gas flow resistance composed of fine fibers has not been formed in the membrane body, and the main pressure-bearing components in the membrane body are still the main body fibers with a larger diameter, so as to ensure that the introduction of breathable fibers can further improve the mechanical strength of the membrane body without having an excessive impact on the air permeability of the membrane body.

[0032] It is important to note that the mechanical strength of porous membranes increases significantly with the increase of the number of breathable fibers, while the air permeability is less affected. However, with a further increase in the number of breathable fibers, the rate of increase in mechanical strength slows down significantly, while the air permeability decreases significantly. This means that for porous membranes with strip-shaped pores and supporting fibers, the degree to which the number of breathable fibers affects air permeability and mechanical strength varies. This may be because the introduction of a small number of breathable fibers can significantly improve the interrelationship between supporting fibers, thereby increasing the mechanical strength of the porous membrane. However, the impact of a small number of breathable fibers on air permeability is relatively small for membranes with large-volume strip-shaped pores. Furthermore, since a small number of breathable fibers can already connect the supporting fibers into a whole, when there are excessive breathable fibers, the rate of increase in mechanical strength slows down significantly. The introduction of excessive breathable fibers means that the three-dimensional network structure of the membrane is too dense, significantly increasing the resistance force during gas flow, thus significantly reducing the air permeability of the porous membrane.

[0033] Optionally, the arrangement density of the supporting fibers in the thickness direction of the membrane body is 0.8 to 5 fibers / 10 μm;

[0034] The density of the breathable fibers in the thickness direction of the membrane body is 0.4 to 2.5 fibers / 10 μm;

[0035] The air permeability of the porous membrane is no greater than 25 × 10⁻⁶. 4 ml / min / cm 2@7kPa.

[0036] By adopting the above technical solution, the arrangement density of the supporting fibers and breathable fibers in the thickness direction of the membrane body largely characterizes the distance between the fibers, that is, the short diameter of the strip pores.

[0037] For support fibers with a diameter of not less than 0.6 μm, if the fiber density is less than 0.8 fibers / 10 μm, it means that the fiber spacing in the thickness direction of the membrane body is large. Even with breathable fibers connecting the support fibers and providing local reinforcement through block nodes, the membrane body often struggles to achieve optimal mechanical strength. If the fiber density is greater than 5 fibers / 10 μm, although the high-permeability strip pores compensate for the gas impediment caused by the support fibers, the introduction of excessive coarse fibers as solid components still makes it difficult for the membrane body to achieve optimal high permeability.

[0038] For breathable fibers with a diameter of no more than 0.2 μm, if the arrangement density of the breathable fibers is less than 0.4 fibers / 10 μm, their connection effect on the supporting fibers is weak, and it is often impossible to make the membrane body have the preferred high mechanical strength. If the arrangement density of the breathable fibers is greater than 2.5 fibers / 10 μm, the three-dimensional network structure formed by the combination of the supporting fibers and the breathable fibers is often too dense, making it difficult to obtain the preferred high air permeability.

[0039] Optionally, the areal density of the membrane substrate is 1.2–7 g / m³. 2 ;

[0040] The porosity S of the membrane matrix is ​​30-90%, and the porosity S is calculated by the following formula:

[0041] S = 100% - D 实 / D×100%;

[0042] In the above formula, D 实 D is the sum of the areas of all entities per unit width in the thickness section of the membrane body, where the entities include the main fibers and main nodes, and D is the membrane cross-sectional area per unit width in the thickness section of the membrane body.

[0043] By adopting the above technical solution, the porosity S refers to the area ratio of the solid portion per unit width (e.g., 10 μm, 20 μm, or 100 μm, which can be freely selected as needed) for the thickness cross-section of the membrane body. For ease of characterization, this application selects a thickness cross-section with a width of 20 μm for measurement. It should be noted that the porosity S is only used to characterize the thickness cross-section of the membrane body as a two-dimensional plane, and cannot directly characterize the porosity (e.g., porosity) of the entire membrane.

[0044] It should be noted that the porosity S in this application is based on the porous membrane having a proportion of 10-60% supporting fibers, blocky nodes, and strip-shaped pores with high air permeability. When gas flows through the membrane body, it actually continuously bypasses the solid fiber and node structures in the membrane thickness direction. During this process, the gas is subject to the resistance force of the solid surface. The introduction of blocky nodes combined with a porosity of not less than 30% ensures that the actual flow path of the gas is shorter when flowing through the membrane body; while the strip-shaped pores combined with a porosity of not more than 90% ensure that the gas is subject to a lower gas resistance force when flowing through the membrane body. Based on the high mechanical strength of the porous membrane, the lower gas resistance force and shorter flow path mean that the porous membrane has a high air permeability.

[0045] Optionally, the ratio of the porosity S to the thickness of the membrane body is 2 to 15% / μm.

[0046] By adopting the above technical solution, for porous membranes with high air permeability strip-shaped pores and supporting fibers, the ratio of porosity to thickness in the thickness direction largely characterizes the porosity per unit thickness of the porous membrane. For porous membranes with a thickness of 3–40 μm, if the ratio of porosity to thickness is less than 2% / μm, it often means that the porous membrane has a large thickness while having low porosity. Although this can give the porous membrane high mechanical strength, it also often means a large gas blocking force and a long flow path, thus resulting in low air permeability. On the other hand, if the ratio of porosity to thickness is greater than 15% / μm, it often indicates that the porous membrane has a small thickness while having high porosity. Although the air permeability of the porous membrane is large, its mechanical strength is often not guaranteed.

[0047] Optionally, both the first outer surface and the second outer surface include surface fibers and surface nodes, wherein the surface fibers are connected to the surface nodes or to adjacent surface fibers, and the gaps between adjacent surface fibers form pores.

[0048] The pores are radially distributed around the surface nodes, and the relative deviation between the transverse tensile strength and the longitudinal tensile strength of the membrane body is no more than 60%.

[0049] By adopting the above technical solution, the porous membrane of this application has elongated, highly permeable strip-shaped pores in its thickness cross-section. Furthermore, by observing the surface structure of the porous membrane, a radially distributed pore structure can be observed. This radial pore structure indicates that the fibers are stacked on top of each other in the membrane thickness direction. Although this structure implies a slight elongation of the gas flow path, the fiber structures support each other in the thickness direction. Through the mutual support of the three-dimensional network structure at different thicknesses, the mechanical strength of the porous membrane is significantly improved.

[0050] Currently, the common PTFE porous membranes with long nodes and fibrillary structures have a high degree of overlap in the thickness direction of their pore structure and weak mutual support between fibers, resulting in relatively poor mechanical strength. In addition, the long node structure divides the membrane body into multiple relatively independent regions, which greatly reduces the flow of gas between regions, thereby effectively increasing the gas flow path and thus having a low permeability.

[0051] To ensure the porous membrane possesses this mutually supporting structure in the thickness direction, the fiber / pore structure distribution orientation needs to be low, manifested as a relative deviation of no more than 60% between the transverse and longitudinal tensile strengths of the membrane matrix. It can be understood that the relative deviation refers to the ratio of the absolute deviation to the average value. That is, the transverse and longitudinal tensile strengths of the membrane matrix are tested separately to obtain their average values, and then the relative deviations of the transverse and longitudinal tensile strengths from the average values ​​are calculated. Finally, the membrane matrix is ​​calculated by comparing the relative deviations with the average values.

[0052] Optionally, the water contact angle of both the first and second outer surfaces is 110° to 150°, the soap water permeation time of the PTFE porous membrane is not less than 10 minutes, and the acoustic impedance of the porous membrane is 150 to 500 rayl.

[0053] By adopting the above technical solutions, for porous membranes that combine high mechanical strength and high air permeability, in order to further meet their waterproof requirements for outdoor use, the water contact angles on both sides of the porous membrane can be further preferably controlled to be 110°–150°, and the soap water penetration time can be controlled to be no less than 10 minutes. Furthermore, to further ensure low impact on the audio curve when used in acoustic equipment, the acoustic impedance can be further preferably controlled to be 150–500 rayl.

[0054] The requirement that the soap water penetration time is not less than 10 minutes means that the PTFE porous membrane is attached to the fixture, soaked in soap water with a concentration of 0.1 g / L, and shaken in a roller for at least 10 minutes, with no water entering the fixture.

[0055] Water contact angle test: Lay the membrane to be tested flat on a horizontal platform, add 4 μL of water to the surface of the membrane, and then use an image analyzer to measure the angle between the water droplet profile and the membrane plane. Repeat 3 times and calculate the average value of the water contact angle.

[0056] Optionally, the holes include longitudinal holes and transverse holes, wherein the angle between the longitudinal holes and the longitudinal stretching direction is not greater than 55°, the angle between the transverse holes and the transverse stretching direction is less than 35°, the number of longitudinal holes is greater than the number of transverse holes, the average major diameter of the longitudinal holes is greater than the average major diameter of the transverse holes, and the average minor diameter of the longitudinal holes is not less than the average minor diameter of the transverse holes.

[0057] By adopting the above technical solution, the radial pore structure on both sides of the porous membrane can be further divided into longitudinal pores and transverse pores. The number of longitudinal pores is greater than the number of transverse pores, the average major diameter of the longitudinal pores is greater than the average major diameter of the transverse pores, and the average minor diameter of the longitudinal pores is not less than the average minor diameter of the transverse pores.

[0058] The larger and more numerous longitudinal pores effectively ensure that the porous membrane has a pore structure that allows gas to flow through, thereby reducing the obstruction force during gas flow. While the smaller and relatively fewer transverse pores can support the larger longitudinal pores through their layering with the longitudinal pores in the thickness direction, this ensures the porous membrane has high mechanical strength. Furthermore, unlike support through purely solid long nodes, supporting the longitudinal pores with transverse pores significantly improves the mechanical strength of the porous membrane, but its impact on the gas flow path and the forces acting on the gas flow organization is relatively small. Therefore, the porous membrane possesses both high mechanical strength and high air permeability.

[0059] Furthermore, the fact that there are fewer transverse pores than longitudinal pores means that although the pore structure is radially distributed, it is not completely isotropic. The size and number of longitudinal pores are larger and more numerous than those of transverse pores. Therefore, the short axis of the longitudinal pores is not excessively stretched, so as to ensure that the porous membrane has high air permeability while the mechanical strength is not excessively reduced due to excessive stretching.

[0060] Optionally, the average major axis of the transverse holes is 25–65 μm;

[0061] The average minor diameter of the longitudinal holes is 2.5–7.5 μm.

[0062] By adopting the above technical solution, as mentioned earlier, the longitudinal hole has a larger average major diameter than the transverse hole, while the average minor diameter of the longitudinal hole is not less than that of the transverse hole. Based on this, the average major diameter of the transverse hole is controlled to be 25-65 μm and the average minor diameter of the longitudinal hole is 2.5-7.5 μm, which means that the longitudinal hole has an average major diameter of at least 25-65 μm and the transverse hole has an average minor diameter of no more than 2.5-7.5 μm.

[0063] The longitudinal pores have an average major diameter of at least 25–65 μm, which, combined with the average major diameter of the transverse pores, is controlled at 25–65 μm. This ensures that the surface of the porous membrane also has a distinctly elongated pore structure, which greatly reduces the obstruction force on gas flow. The transverse pores have an average minor diameter of no more than 2.5–7.5 μm, which, combined with the average minor diameter of the longitudinal pores, is 2.5–7.5 μm. This ensures that while the longitudinal pores have high air permeability, their mechanical strength is not prone to rapid decline due to excessively large transverse dimensions (excessive distance between fibers). The relatively smaller transverse pores can better support the large longitudinal pores, ensuring that the porous membrane still has high mechanical strength even with large longitudinal pores.

[0064] Optionally, the average cross-sectional area of ​​the main node is 20–80 μm. 2 The distribution density of the main nodes is 10–40 per 10,000 μm. 2 .

[0065] By adopting the above technical solution, as mentioned above, the block-shaped main node can play a good local reinforcement role to compensate for the decrease in mechanical strength caused by the strip-shaped hole. However, as an important part of the three-dimensional network structure, the main node not only plays a positive structural reinforcement role, but also plays a negative role in impeding gas flow and separating the membrane structure.

[0066] Therefore, the average cross-sectional area of ​​the main nodes should not exceed 80 μm. 2 The distribution density should not exceed 40 per 10000 μm. 2 To avoid the formation of localized dense solid structures by excessively large and numerous main nodes, which would generate significant gas-trapping forces and divide the membrane substrate into several regions with poor gas flow, thus elongating the gas flow path and reducing the gas flux of the porous membrane. Furthermore, the average cross-sectional area of ​​the main nodes should not be less than 20 μm². 2 The distribution density should not be less than 10 per 10,000 μm. 2 For the porous membrane with specific elongated strip-shaped pores in this application, based on a certain proportion of supporting fibers, the main nodes with a certain distribution density and volume can play a good reinforcing role, thereby significantly improving the mechanical strength of the porous membrane. This allows the porous membrane to possess both high mechanical strength and high air permeability.

[0067] Optionally, the cross-sectional area of ​​the main node is not less than 100 μm. 2 These are locally enhanced nodes, and the distribution density of these locally enhanced nodes is 0.5 to 4 per 10000 μm. 2 .

[0068] As mentioned above, by adopting the above technical solution, the large-volume main node has a very good local reinforcement effect on the porous membrane with strip-shaped pores in this application. However, the large-volume main node also often means a high gas blocking force. The inventors of this application have found that by controlling the cross-sectional area to be not less than 100 μm 2 The distribution density of locally enhanced nodes is 0.5–4 per 10,000 μm. 2 This allows the porous membrane to have further optimized mechanical strength, while the air permeability decreases only slightly.

[0069] This is likely because the combination of supporting fibers and relatively small main nodes forms a relatively uniform three-dimensional network structure. The introduction of locally reinforcing nodes can then provide effective reinforcement to specific local areas. Combined with the supporting fibers and relatively small main nodes, this creates a three-dimensional network structure that combines general reinforcement with further local reinforcement, thereby significantly improving the mechanical strength of the porous membrane. The distribution density is no greater than 4 nodes / 10000μm. 2 It can be ensured that the area ratio of the local reinforcement nodes does not exceed 4%. Therefore, although the impact on the air permeability of the porous membrane is inevitable, the main nodes are blocky and the area ratio is low, so the impact on the air permeability of the porous membrane is very limited. This results in the porous membrane of this application having higher mechanical strength and only a slight decrease in air permeability.

[0070] Optionally, the porosity of the membrane substrate is not less than 50%; the ratio of the porosity of the membrane substrate to the thickness of the membrane substrate is 10% / μm.

[0071] Secondly, this application provides a process for preparing a high-permeability PTFE porous membrane, using the following technical solution:

[0072] A process for preparing a high-permeability PTFE porous membrane includes the following steps:

[0073] S1. Mixing and swelling: High molecular weight PTFE resin and additive oil are mixed evenly for a first swelling, and then low molecular weight PTFE resin is added and mixed evenly for a second swelling to obtain a paste-like mixture. The low molecular weight resin is selected from PTFE resin with a molecular weight of 4 million to 6 million, and the high molecular weight resin is selected from PTFE resin with a molecular weight of 7 million to 9 million.

[0074] S2. Preforming: The paste mixture is preformed at 30-45℃ to obtain a preformed blank.

[0075] S3. Calendering: The preformed blank is extruded to form an oil-containing base strip with a thickness of 0.15 to 0.45 mm;

[0076] S4. Degreasing: Heat the oil-containing base strip to remove the auxiliary oil and obtain an oil-free base strip.

[0077] S5. Longitudinal stretching: The oil-free base tape is placed in an environment of 200-300℃ for longitudinal stretching, with a stretching ratio of 10-20 times, to obtain a uniaxial stretched film.

[0078] S6. Lateral stretching: The uniaxially stretched film is placed in an environment of 100-300℃ and stretched laterally by 15-40 times to obtain a biaxially stretched film.

[0079] S7. Heat treatment: Place the biaxially oriented film at a temperature of 330-390℃ for 1-10 minutes for heat setting. During heat setting, the biaxially oriented film is subjected to a micro-stretch with a stretch ratio of 1.1-1.5, and an intermittent stretching process is adopted. Specifically, the stretching force is removed after every 20 seconds of stretching and held for 10 seconds.

[0080] By adopting the above technical solutions, the common PTFE membrane manufacturing process currently involves using PTFE resin of a single molecular weight as the film-forming component. It is believed that the higher the molecular weight of the PTFE resin, the higher the mechanical strength of the PTFE membrane. This is because the intermolecular forces of PTFE resin are relatively small, requiring a very high molecular weight to achieve good mechanical strength. Furthermore, high molecular weight PTFE resin has relatively better fiber-forming properties, thus allowing for the stretching of more fiber structures during thickness measurement. However, the denser fiber structure inevitably leads to a decrease in the air permeability of the porous membrane.

[0081] However, the inventors of this application unexpectedly discovered that by using specific high and low molecular weight PTFE resins to make the film, compared with using a single high molecular weight PTFE resin to make the film, the PTFE film not only has higher mechanical strength, but also higher air permeability.

[0082] This may be because, for fiber structures of the same size, higher molecular weights do tend to have higher mechanical strength. However, in this application, the low molecular weight PTFE resin with a molecular weight of 4-6 million has poorer fiber-forming properties compared to high molecular weight PTFE resin. Therefore, during the subsequent stretching and fiber-forming process, the stretching force tends to pull the high molecular weight PTFE resin, which has weaker intermolecular forces, apart to form the main fiber structure. The low molecular weight PTFE resin, on the other hand, is more likely to form the required blocky node structure and relatively larger diameter support fibers. The larger size allows it to have greater mechanical strength than the smaller diameter main fibers, even with a lower molecular weight. As mentioned above, the large-diameter support fibers combined with the blocky node structure enable the porous membrane to have good mechanical strength.

[0083] Furthermore, even the low molecular weight PTFE resin used in this application has a molecular weight of not less than 4 million, while higher molecular weight PTFE resins are insoluble and infusible at high temperatures. Therefore, they often need to be swollen into a paste using auxiliary oils. Currently, it is generally understood that to ensure low intermolecular forces between PTFE molecular chains during stretching, facilitating subsequent fiber formation, the PTFE resin needs to be fully swollen. However, this application, by further controlling the degree of swelling of both high and low molecular weight PTFE resins, ensures the acquisition of the desired blocky nodes and supporting fiber structure. Specifically, the high molecular weight PTFE resin undergoes a longer primary and secondary swelling process, resulting in sufficient swelling, reduced intermolecular forces, and easier fiber formation during the stretching process. The low molecular weight PTFE resin undergoes only a shorter secondary swelling process, thus its swelling degree is lower than that of the high molecular weight PTFE resin, and its intermolecular forces are higher. Therefore, under the same tensile force, high molecular weight PTFE resin, which is already easier to form fibers, is more likely to form the main fiber structure due to its higher degree of swelling; while low molecular weight PTFE resin, which is relatively difficult to form fibers, is more likely to form larger diameter support fibers and blocky node structures due to its relatively lower degree of swelling and the difficulty in fully separating the molecular chains.

[0084] Furthermore, in the heat setting process following biaxial stretching, this application introduces a micro-stretching with a stretching ratio of 1.1 to 1.5 and employs a specific intermittent stretching process. This, combined with the high and low molecular weight PTFE resin systems and control of the swelling degree of the low molecular weight PTFE resin, ensures that the porous membrane obtains the required supporting fibers and blocky nodes. This is likely because the biaxially stretched membrane exhibits high ductility during heat setting at higher temperatures. Applying a small stretching ratio at this temperature yields relatively high strain, allowing some adjacent fiber structures to contact and even fuse, thus obtaining the desired supporting fibers. It is important to note that if the stretching ratio is too high or a continuous stretching process is used, it may lead to excessive fusion of the fiber structures, resulting in excessively large fiber structures that generate greater gas flow resistance.

[0085] In summary, this application controls the different swelling degrees of high and low molecular weight PTFE resins by using specific high and low molecular weight PTFE resins in conjunction with a swelling process, and further introduces an intermittent stretching process with a small stretching ratio in the heat setting process after biaxial stretching, to ensure that the desired porous membrane with blocky nodes and large-diameter supporting fibers is obtained, so that the porous membrane has both high air permeability and high mechanical strength.

[0086] Optionally, in step S1, the solid content of the paste mixture is 70% to 85%; the mass ratio of the low molecular weight PTFE resin to the high molecular weight PTFE resin is 1:1 to 1:2.

[0087] By adopting the above technical solution, the solid content in the paste mixture largely determines the swelling degree of each high and low molecular weight PTFE resin. To ensure that the high molecular weight PTFE resin is fully swollen, the solid content of the paste mixture should not exceed 85%. To ensure that the low molecular weight PTFE resin is not completely swollen, in addition to controlling the low molecular weight PTFE resin to undergo only a short period of secondary swelling, the solid content also needs to be controlled to be no less than 70%. The ratio of high and low molecular weight PTFE resins, combined with the small-scale intermittent stretching process during heat setting, determines the proportion of supporting fibers in the porous membrane to a certain extent.

[0088] Optionally, in step S1, the swelling time for one swelling is 6 to 20 hours, and the sum of the swelling times for the first and second swellings is 8 to 24 hours. The ambient temperature during the swelling process is maintained at 30 to 60°C.

[0089] By adopting the above technical solutions, swelling time and swelling temperature are important means to control the swelling degree of polymer PTFE resin. It is generally believed that the longer the swelling time and the higher the swelling temperature, the higher the swelling degree of PTFE resin. By separately controlling the time of the first and second swelling and the temperature during the swelling process, it is possible to ensure the high swelling degree of high molecular weight PTFE resin and the relatively low swelling degree of low molecular weight PTFE resin, so as to ensure that during subsequent stretching and fiber formation, each of them forms the main fiber structure, the supporting fiber structure, and the blocky node structure.

[0090] Optionally, step S3 involves two molding processes, including:

[0091] S31. First calendering: the preformed blank is extruded and calendered to form a sheet with a thickness of 2-5 mm. The temperature of the first calendering is 35-55℃.

[0092] S32. Secondary calendering: The sheet is further calendered into a base strip with a thickness of 0.15 to 0.45 mm. The temperature of the secondary calendering is 50 to 80°C and is not lower than the temperature of the primary calendering.

[0093] Optionally, in step S4, the degreasing temperature is 150-250℃, the heating rate is 15-20℃ / 10min, and heating needs to be stopped and held for 10min after every 50℃ increase.

[0094] Optionally, the auxiliary oil is at least one of petroleum ether, solvent oil, and aviation kerosene.

[0095] By adopting the above technical solution, this application employs a special intermittent heating and intermittent heat preservation process during the degreasing operation of the oil-containing substrate, resulting in a porous membrane with better overall performance. This may be because this application uses different conventional high and low molecular weight PTFE resins, and the swelling degrees of high and low molecular weight PTFE resins are not the same. This means that the difficulty and speed at which the auxiliary oil escapes from the molecular chains of high and low molecular weight PTFE resins are different, which may affect the interaction force between the PTFE molecular chains in the final oil-free substrate. In particular, for low molecular weight PTFE resins with controlled swelling, if a continuous heating process is used, it is likely to lead to uneven heating in various places. Under local high temperatures, the auxiliary oil may further swell the low molecular weight PTFE resin, reducing the interaction force between the molecular chains of the low molecular weight PTFE resin, making it easier to form a smaller diameter main fiber structure in the stretching and fiber forming step, rather than forming the required supporting fiber structure.

[0096] In summary, this application includes at least one of the following beneficial technical effects:

[0097] 1. This application ensures that the PTFE porous membrane has a pore size of not less than 3×10⁻⁶ by introducing a narrow, elongated strip-shaped pore structure. 4 ml / min / cm 2 @7kPa air permeability; and by controlling the membrane thickness to 3-40μm and introducing 10-60% of the supporting fibers with a diameter of not less than 0.6μm, the pore structure of the PTFE porous membrane can be ensured to have high mechanical strength.

[0098] 2. This application controls the different swelling degrees of high and low molecular weight PTFE resins by using specific high and low molecular weight PTFE resins in combination with a swelling process, and further introduces an intermittent stretching process with a small stretching ratio in the heat setting process after biaxial stretching, so as to ensure that the desired porous membrane with block nodes and large diameter supporting fibers is obtained, thereby making the porous membrane have both high air permeability and high mechanical strength. Attached Figure Description

[0099] Figure 1 This is a scanning electron microscope image of one side surface of the PTFE porous membrane prepared in Example 1 of this application, with a magnification of 200×.

[0100] Figure 2 This is a scanning electron microscope image of the other side surface of the PTFE porous membrane prepared in Example 1 of this application, with a magnification of 200×.

[0101] Figure 3 This is a scanning electron microscope image of the cross-section of the PTFE porous membrane prepared in Example 1 of this application, with a magnification of 1000×.

[0102] Figure 4 yes Figure 3 A further magnified image is shown to illustrate the structure of the main fibers and main nodes, with a magnification of 2000×. Detailed Implementation

[0103] The following is in conjunction with the appendix Figures 1-4 This application will be described in further detail.

[0104] Example 1

[0105] This application discloses a high-permeability PTFE porous membrane, the preparation process of which includes the following steps:

[0106] S1. Mixing and swelling: High molecular weight PTFE resin and auxiliary oil are mixed evenly for a first swelling, and then low molecular weight PTFE resin is added and mixed evenly for a second swelling to obtain a paste-like mixture. The auxiliary oil is aviation kerosene, the low molecular weight resin is PTFE resin with a molecular weight of 5 million, and the high molecular weight resin is PTFE resin with a molecular weight of 8 million. The mass ratio of low molecular weight PTFE resin to high molecular weight PTFE resin is 1:1.5, and the solid content of the paste-like mixture is 75%. Furthermore, in this embodiment, the first swelling time is 12 hours, and the sum of the first and second swelling times is 14 hours. The ambient temperature during the swelling process is maintained at 45°C.

[0107] S2. Preforming: The paste mixture is preformed at 40°C to obtain a cylindrical preform.

[0108] S3, Calendering, specifically...

[0109] S31. First calendering: The preformed blank is extruded and calendered to form a sheet with a thickness of 3mm. The temperature of the first calendering is 45℃.

[0110] S32, Secondary calendering, further calendering the sheet into a base strip with a thickness of 0.25mm, the secondary calendering temperature is 70℃.

[0111] S4. Degreasing: Heat the oil-containing base strip to remove the auxiliary oil and obtain an oil-free base strip. The degreasing temperature is 200℃, the heating rate is 17℃ / 10min, and the heating must be stopped and held for 10min after every 50℃ increase.

[0112] S5. Longitudinal stretching: The oil-free base tape is placed in an environment of 250°C and longitudinally stretched with a stretching ratio of 15 times to obtain a uniaxial stretched film.

[0113] S6. Lateral stretching: The uniaxial stretching film is placed in an environment of 200°C for lateral stretching, with a stretching ratio of 27 times, to obtain a biaxial stretching film.

[0114] S7. Heat treatment: Place the biaxially oriented film at 360°C for 5 minutes for heat setting. During heat setting, the biaxially oriented film is subjected to a micro-stretch with a stretch ratio of 1.2, and an intermittent stretching process is adopted. Specifically, the stretching force is removed after every 20 seconds of stretching and held for 10 seconds.

[0115] Examples 2-7

[0116] The main difference between Examples 2-7 and Example 1 is that the raw materials and proportions of the paste mixture are different, and the process parameters of each step are different, as detailed in Table 1.

[0117] In the paste mixture of Example 2, the mass ratio of low molecular weight PTFE resin to high molecular weight PTFE resin is 1:0.5.

[0118] In Example 7, the intermittent heating process was not used when degreasing the oil-containing substrate.

[0119] Comparative Example 1

[0120] In Comparative Example 1, the paste mixture did not use high or low molecular weight PTFE resins, but only high molecular weight PTFE resin with a molecular weight of 9 million. In addition, by controlling the transverse and longitudinal stretch ratio, the PTFE membrane prepared in this comparative example has a classic long node, fibrillary structure, rather than a blocky node structure.

[0121] Comparative Example 2

[0122] In Comparative Example 2, the heat treatment steps used higher heat treatment temperatures and times, and although a micro-stretching process was used, an intermittent stretching process was not employed.

[0123] Comparative Example 3

[0124] In Comparative Example 3, the segmented swelling process was not used. Instead, high and low molecular weight PTFE resins were blended with aviation kerosene to ensure that both high and low molecular weight PTFE resins had a high degree of swelling. In addition, the micro-stretching process was not used in the heat treatment stage in Comparative Example 3.

[0125] Table 1. Formulation and process parameters for each embodiment and comparative example.

[0126]

[0127]

[0128] The performance parameters of the porous membranes prepared in each embodiment and comparative example are detailed in Table 2:

[0129] Table 2 Performance parameters of porous membranes in each embodiment and comparative example.

[0130]

[0131]

[0132] in conclusion

[0133] By comparing the technical solutions and performance parameters of Example 1 and Comparative Example 1, it is easy to find that, compared with the film-forming process of using a single high molecular weight PTFE resin and full swelling, the use of blending high and low molecular weight PTFE resins and controlling the swelling degree of low molecular weight PTFE resin in this application can achieve unexpectedly high mechanical strength.

[0134] By comparing the technical solutions and performance parameters of Example 2 and Comparative Example 2, it is not difficult to find that even when both high and low molecular weight PTFE resins are blended, if the heat treatment temperature, heat treatment time and stretching ratio are too high and the intermittent stretching process is not used, the resulting porous membrane will have excessive supporting fibers. Although it has higher mechanical strength, it has obviously too low air permeability.

[0135] By comparing the technical solutions and performance parameters of Example 1 and Comparative Example 3, it is not difficult to find that even when both high and low molecular weight PTFE resins are blended, if the low molecular weight PTFE resin is highly swollen and a micro-stretching process and intermittent stretching process are not introduced during the heat treatment stage, the resulting porous membrane has obviously too low mechanical strength due to insufficient supporting fibers.

[0136] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high-permeability PTFE porous membrane, characterized in that: The membrane body includes a first outer surface and a second outer surface. The cross-section of the membrane body includes elongated main fibers and block-shaped main nodes. The two ends of the main fibers are connected to the main nodes or adjacent main fibers. The thickness of the membrane body is 3–40 μm; The main fiber includes supporting fibers with a diameter of not less than 0.6 μm, and the ratio of the number of supporting fibers to the number of main fibers is 10 to 60%, with narrow strip-shaped pores formed between adjacent main fibers; The air permeability of the porous membrane is not less than 3×10⁻⁶. 4 ml / min / cm 2 @7kPa.

2. The high-permeability PTFE porous membrane according to claim 1, characterized in that: The main fiber includes breathable fibers with a fiber diameter of no more than 0.2 μm. The breathable fibers are distributed between adjacent supporting fibers, and the ratio of the number of breathable fibers to the number of main fibers is 5 to 25%.

3. The high-permeability PTFE porous membrane according to claim 2, characterized in that: The arrangement density of the supporting fibers in the thickness direction of the membrane body is 0.8 to 5 fibers / 10 μm; The density of the breathable fibers in the thickness direction of the membrane body is 0.4 to 2.5 fibers / 10 μm; The air permeability of the porous membrane is no greater than 25 × 10⁻⁶. 4 ml / min / cm 2 @7kPa.

4. The high-permeability PTFE porous membrane according to claim 1, characterized in that: The areal density of the membrane bulk is 1.2–7 g / m³. 2 ; The porosity S of the membrane matrix is ​​30-90%, and the porosity S is calculated by the following formula: S=100%-D 实 / D×100%; In the above formula, D 实 D is the sum of the areas of all entities per unit width in the thickness section of the membrane body, where the entities include the main fibers and main nodes, and D is the membrane cross-sectional area per unit width in the thickness section of the membrane body.

5. The high-permeability PTFE porous membrane according to claim 4, characterized in that: The ratio of the porosity S to the thickness of the membrane body is 2 to 15% / μm.

6. The high-permeability PTFE porous membrane according to claim 1, characterized in that: Both the first outer surface and the second outer surface include surface fibers and surface nodes. The surface fibers are connected to the surface nodes or to adjacent surface fibers, and the gaps between adjacent surface fibers form pores. The pores are radially distributed around the surface nodes, and the relative deviation between the transverse tensile strength and the longitudinal tensile strength of the membrane body is no more than 60%.

7. The high-permeability PTFE porous membrane according to claim 1, characterized in that: The water contact angles of the first and second outer surfaces are both 110° to 150°, and the soap water permeation time of the PTFE porous membrane is not less than 10 minutes; the acoustic impedance of the porous membrane is 150 to 500 ray l.

8. The high-permeability PTFE porous membrane according to claim 6, characterized in that: The holes include longitudinal holes and transverse holes. The angle between the longitudinal holes and the longitudinal stretching direction is no greater than 55°, and the angle between the transverse holes and the transverse stretching direction is less than 35°. The number of longitudinal holes is greater than the number of transverse holes. The average major diameter of the longitudinal holes is greater than the average major diameter of the transverse holes. The average minor diameter of the longitudinal holes is not less than the average minor diameter of the transverse holes.

9. A high-permeability PTFE porous membrane according to claim 8, characterized in that: The average major diameter of the transverse holes is 25–65 μm; The average minor diameter of the longitudinal holes is 2.5–7.5 μm.

10. A high-permeability PTFE porous membrane according to claim 1, characterized in that: The average cross-sectional area of ​​the main node is 20–80 μm. 2 The distribution density of the main nodes is 10–40 per 10,000 μm. 2 .

11. The high-permeability PTFE porous membrane according to claim 1, characterized in that: The cross-sectional area of ​​the main node is not less than 100 μm. 2 These are locally enhanced nodes, and the distribution density of these locally enhanced nodes is 0.5 to 4 per 10000 μm. 2 .

12. The preparation process of the high-permeability PTFE porous membrane according to any one of claims 1 to 11, characterized in that: The process includes the following steps: S1. Mixing and swelling: High molecular weight PTFE resin and additive oil are mixed evenly for a first swelling, and then low molecular weight PTFE resin is added and mixed evenly for a second swelling to obtain a paste-like mixture. The low molecular weight resin is selected from PTFE resin with a molecular weight of 4 million to 6 million, and the high molecular weight resin is selected from PTFE resin with a molecular weight of 7 million to 9 million. S2. Preforming: The paste mixture is preformed at 30-45℃ to obtain a preformed blank. S3. Calendering: The preformed blank is extruded to form an oil-containing base strip with a thickness of 0.15 to 0.45 mm; S4. Degreasing: Heat the oil-containing base strip to remove the auxiliary oil and obtain an oil-free base strip. S5. Longitudinal stretching: The oil-free base tape is placed in an environment of 200-300℃ for longitudinal stretching, with a stretching ratio of 10-20 times, to obtain a uniaxial stretched film. S6. Lateral stretching: The uniaxially stretched film is placed in an environment of 100-300℃ and stretched laterally, with a stretching ratio of 15-40 times, to obtain a biaxially stretched film. S7. Heat treatment: Place the biaxially oriented film at a temperature of 330-390℃ for 1-10 minutes for heat setting. During heat setting, the biaxially oriented film is subjected to a micro-stretch with a stretch ratio of 1.1-1.5, and an intermittent stretching process is adopted. Specifically, the stretching force is removed after every 20 seconds of stretching and held for 10 seconds.

13. The preparation process of the high-permeability PTFE porous membrane as described in claim 12, characterized in that: In step S1, the solid content of the paste mixture is 70% to 85%; the mass ratio of the low molecular weight PTFE resin to the high molecular weight PTFE resin is 1:1 to 1:

2.

14. The preparation process of the high-permeability PTFE porous membrane as described in claim 12, characterized in that: In step S1, the swelling time for the first swelling is 6 to 20 hours, and the sum of the swelling times for the first and second swellings is 8 to 24 hours. The ambient temperature during the swelling process is maintained at 30 to 60°C.

15. The preparation process of the high-permeability PTFE porous membrane as described in claim 12, characterized in that: Step S3 involves two molding processes, including: S31. First calendering: the preformed blank is extruded and calendered to form a sheet with a thickness of 2-5 mm. The temperature of the first calendering is 35-55℃. S32. Secondary calendering: The sheet is further calendered into a base strip with a thickness of 0.15 to 0.45 mm. The temperature of the secondary calendering is 50 to 80°C and is not lower than the temperature of the primary calendering.

16. The preparation process of the high-permeability PTFE porous membrane as described in claim 12, characterized in that: In step S4, the degreasing temperature is 150-250℃, the heating rate is 15-20℃ / 10min, and heating must be stopped and held for 10min after every 50℃ increase.

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