An asymmetric polytetrafluoroethylene porous membrane, its preparation method and use

Abstract

The application discloses an asymmetric polytetrafluoroethylene porous membrane and a preparation method and application thereof, the porous membrane comprises a main body, one side of the main body is a first outer surface, and the other side is a second outer surface, the main body comprises a pretreatment layer and a separation layer, the average pore size of the pretreatment layer is greater than that of the separation layer; the pretreatment layer comprises support nodes and support fibers, the support nodes are in a strip shape, the separation layer comprises separation nodes and separation fibers, and the separation nodes are in a point shape; the IPA bubble point of the porous membrane is 5-100 KPa, the grammage is 5-25 g / m 2 , and the thickness is 5-100 mu m; the porous membrane is integrally formed, has high retention efficiency, low pressure loss, good mechanical strength, is not easy to separate between layers, has a long service life, and is particularly suitable for application in filtration of stripping liquid and etching liquid in a semiconductor process and use as a waterproof and breathable membrane in electronic equipment and medical equipment; and the application further provides a preparation method of the porous membrane, and the method is simple to operate and easy to prepare.

Description

Technical Field

[0001] This invention relates to the field of membrane materials technology, and more specifically to an asymmetric polytetrafluoroethylene porous membrane, its preparation method, and its applications. Background Technology

[0002] Polytetrafluoroethylene (PTFE) is renowned for its chemical inertness and solvent resistance, earning it the nickname "King of Plastics." Due to its extremely low surface tension and strong hydrophobicity, PTFE microfiltration membranes produced by the stretching-extrusion method are not easily clogged and are extremely easy to clean; even clogging can be removed by backflushing with compressed air. This makes them highly advantageous in the food, pharmaceutical, and biopharmaceutical industries. Furthermore, due to their strong hydrophobicity, PTFE microporous membranes show great promise for applications in membrane contactors such as membrane distillation and membrane extraction. Currently, commercially available PTFE porous membranes are mainly produced using the extrusion-stretching method, and the product forms include sheet membranes and hollow fiber membranes. For example, polytetrafluoroethylene (PTFE) resin powder is thoroughly mixed with liquid extrusion aid to form a paste, which is then extruded to obtain PTFE strips. These strips are then calendered into raw sheets, which are heat-treated to further improve the crystallinity of PTFE. After biaxial stretching (longitudinal and transverse stretching), a porous membrane with a "node-fiber" structure is obtained. Finally, the microporous structure is fixed by heat setting to obtain a biaxially stretched PTFE membrane.

[0003] For example, in the prior art, Chinese patent application number CN200780011828.5, entitled "Manufacturing method of polytetrafluoroethylene porous membrane, filter material and filter unit" (applied by Nitto Denko Corporation), discloses a polytetrafluoroethylene porous membrane. The average pore size of this porous membrane is 1-5 μm. It is a symmetrical membrane with several dot-like nodes on its outer surface (the nodes are relatively disordered). Adjacent nodes are connected by fibers (often referred to by those skilled in the art as a "spider web structure"). This node-fiber structure ensures that the porous membrane has high capture efficiency and low pressure loss. However, in practical applications, it has been found that the strength of this type of porous membrane is relatively low. When dealing with some special situations (such as sudden power on / off, filtering fluids with high viscosity, etc.), the membrane pores are prone to collapse or shrinkage after being subjected to high pressure, thus failing to function properly and greatly reducing the service life of the porous membrane. At the same time, its water pressure resistance is also low.

[0004] Therefore, some researchers have developed asymmetric polytetrafluoroethylene (PTFE) composite membranes. For example, US Patent No. 7306841B2 discloses a nodally polymerized PTFE material, comprising multiple aggregates connected by relatively long fibrils; each aggregate is formed by PTFE nodes, which are connected by relatively short fibrils; by selecting ideal aggregates, the PTFE material can be guaranteed to have satisfactory performance, making the product usable in biomedical materials, filtration structures, and fabrics; however, since this membrane is a composite material, its mechanical strength is relatively low, especially when subjected to external forces, the layers are prone to separation, especially during pleating (flat membranes are often folded to increase membrane area), which poses a risk of delamination, thus affecting the service life of the material and limiting its application in filtration, waterproofing, and breathability.

[0005] In summary, the aforementioned problems have, to some extent, limited the development of polytetrafluoroethylene porous membranes (especially polytetrafluoroethylene macroporous membranes). Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide an asymmetric polytetrafluoroethylene (PTFE) porous membrane, its preparation method, and its applications. This PTFE porous membrane has an asymmetric structure and is integrally molded, exhibiting not only high retention efficiency and low pressure loss but also good mechanical strength. The layers are not easily separated, and the membrane has a long service life. It is particularly suitable for filtering stripping and etching solutions in semiconductor manufacturing processes, as well as for use as a waterproof and breathable membrane in electronic and medical devices.

[0007] To achieve the above objectives, the present invention provides the following technical solution: an asymmetric polytetrafluoroethylene porous membrane, comprising a main body, one side of which is a first outer surface and the other side of which is a second outer surface, the porous membrane being integrally formed; the main body having non-directional tortuous pathways; the main body comprising a pretreatment layer and a separation layer, one side of which is a first outer surface and the other side of which is a second outer surface; the average pore size of the pretreatment layer is larger than the average pore size of the separation layer;

[0008] The pretreatment layer includes support nodes and support fibers for forming a porous structure. The support nodes are elongated structures, and the length of the support nodes in a first direction is greater than the length in a second direction. The first direction is parallel to the thickness direction of the porous membrane, and the second direction is perpendicular to the thickness direction of the porous membrane. Adjacent support nodes are connected by support fibers.

[0009] The separation layer includes separation nodes and separation fibers for forming a porous structure. The separation nodes are point-like structures, and adjacent separation nodes are connected by separation fibers.

[0010] The porous membrane has an IPA bubble point of 5-100 kPa and a basis weight of 5-25 g / m³. 2 The thickness is 5-100 μm. Preferably, the IPA bubble point of the porous membrane is 10-80 kPa, and the basis weight is 8-22 g / m³. 2 The thickness is 10-90μm.

[0011] In the main structure of the polytetrafluoroethylene (PTFE) porous membrane provided by this invention, it can be clearly seen that there are two distinct node-fiber structures in different regions of the main cross-section. In one region, the membrane pores are relatively large (macropore region), which we call the pretreatment layer. In the pretreatment layer, the nodes are elongated structures, relatively thick and long, and the length of the node is greater than its width. We call these supporting nodes (the presence of supporting nodes ensures the mechanical strength of the membrane, which is quite different from the spider web structure of the PTFE porous membrane). Adjacent supporting nodes are connected by supporting fibers. In the other region, the membrane pores are relatively small, which we call the separation layer. In the separation layer, the nodes... The structure resembles a dot and is called a separation node (compared to the support nodes, the separation nodes are relatively smaller in width and shorter in length). Adjacent separation nodes are connected by separation fibers. As is well known, the filtration process is mainly completed in the thickness direction of the membrane. The filter medium (fluid) passes through the membrane thickness from one surface and exits from the other surface. In this invention, in the cross-sectional direction of the membrane, there are strip-shaped node structures in the pretreatment layer (macroporous layer) and dot-shaped node structures in the separation layer (microporous layer). Since the pretreatment layer is a macroporous layer and also a pressure-bearing layer, and since filtration mainly occurs in the thickness direction, the support nodes are relatively thick in the thickness direction to facilitate pressure resistance. The fluid passes through multiple... In porous membranes, the pores are less prone to change (collapse or shrinkage), thus ensuring stable and efficient filtration. In the direction perpendicular to the thickness direction (the second direction), the length of the supporting nodes is relatively small (the length of the supporting nodes in the first direction is greater than that in the second direction; the first direction is parallel to the thickness direction of the porous membrane, and the second direction is perpendicular to the thickness direction of the porous membrane). This results in less resistance to the fluid passing through the porous membrane, i.e., less pressure loss and higher flow velocity. When the fluid passes through the separation layer, the separation nodes within the separation layer have a point-like structure. These point-like separation nodes are relatively disordered, and adjacent separation nodes are connected by separation fibers. The membrane as a whole is a non-directional tortuous pathway. This refers to a randomly oriented groove structure and / or a discretely distributed pore structure, where each non-directional tortuous path is interconnected. During filtration, the liquid flows within the tortuous pore structure, trapping impurities through sieving and adsorption, thus facilitating thorough impurity retention. Furthermore, within the separation layer, the interlacing of upper and lower separation fibers further tortuous the flow path, ensuring sufficient impurity retention. The relatively small diameter of the point-like separation nodes results in lower overall membrane pressure loss even with a large number of nodes. As a filter membrane, this porous membrane exhibits lower pressure loss and faster flow rate at the same filtration precision; at the same flow rate, it has higher retention efficiency and higher mechanical strength.

[0012] Bubble point is an important performance characteristic of polymer filter membranes. The pressure at which continuous bubbling begins in the middle of the filter membrane is recorded as the bubble point. The testing methods for bubble point are well-known in the art. For example, the procedures for these tests are explained in detail in ASTM F316-70 and ANS / ASTM F316-70 (re-approved in 1976), which are incorporated herein by reference. The size of the bubble point reflects the overall pore size of the membrane. The IPA bubble point of the membrane in this invention is 5-100 kPa, indicating that the overall pore size of this porous membrane is relatively large, belonging to the micrometer scale, and can also be called a macroporous membrane. Compared to mesoporous membranes (porous membranes with an overall pore size of several hundred nanometers) and microporous membranes (porous membranes with an overall pore size of tens of nanometers or even smaller), there are significant differences in various properties, making them incomparable. The combined effect of basis weight and thickness reflects the overall density of the membrane, and its value is closely related to the nodes and fiber structure of each layer in the cross-section. The structure (which is the result of the combined action of the fibers at each node) is crucial. If the basis weight is too high when the membrane thickness remains essentially constant, the membrane will become too dense, affecting the overall flow rate and pressure loss, and significantly impacting its air permeability and sound transmission. Conversely, if the basis weight is too low, the membrane will become too porous, greatly reducing its tensile strength and pressure resistance, rendering it impractical. This invention, through a combination of thickness, basis weight, and bubble point, along with the ideal node structure of the pretreatment and separation layers, ensures that the porous membrane possesses high retention efficiency, low pressure loss, and good mechanical strength.

[0013] Furthermore, when used as a breathable membrane, it combines water resistance and breathability; of course, this porous membrane can also be used as a waterproof and sound-permeable membrane. Due to the very low surface tension of polytetrafluoroethylene (PTFE) and its strong hydrophobicity, water itself is difficult to penetrate. The presence of dotted separation nodes within the separation layer, with the separation fibers of the upper and lower layers interlacing, ensures that the membrane pores in this area are not too large (this can be observed through bubble point analysis), further increasing the difficulty of water penetration and thus further improving waterproofness. In addition, compared to porous membranes with a pure "spider web" structure, the porous membrane of this invention has good strength (not easily deformed, resistant to water pressure) and good waterproofness under certain pressure, i.e., excellent waterproof and pressure-resistant performance; one membrane with multiple uses, high economic efficiency. Therefore, this porous membrane is particularly suitable for filtering stripping and etching solutions in semiconductor manufacturing processes, and for use as a waterproof and breathable membrane in electronic and medical devices.

[0014] The polytetrafluoroethylene porous membrane structure of the present invention is integrally formed and asymmetrical. Integral formation means that the entire membrane is composed of the same material and is formed directly during the membrane preparation process. In the transition from the membrane thickness direction, only the membrane structure changes. In contrast, composite membranes have multi-layered structures. They are formed by applying a dense layer, which serves as a retention layer, onto a porous layer or porous membrane, often a microporous support layer or support membrane, in a separate process. The materials constituting the support layer and the retention layer in a composite membrane are often different. Integral porous membranes have higher mechanical strength than composite membranes and almost no risk of delamination.

[0015] The membrane thickness can be determined by characterizing the membrane structure using a scanning electron microscope, followed by calculation using computer software (such as Matlab, NIS-Elements, etc.) or manual measurement. When the membrane thickness is too small, its mechanical strength will be low, and the filtration time will be too short, resulting in ineffective filtration. Conversely, when the membrane thickness is too large, the filtration time will be too long, leading to excessive time costs. The membrane of this invention has a thickness of 5-100 μm, ensuring that it not only possesses high mechanical strength but also achieves effective filtration with high efficiency, short filtration time, and low time costs.

[0016] As a further improvement of the present invention, the first water contact angle of the porous membrane is 110-150°, the crystallinity is 25%-65%, and the porosity is 65%-95%.

[0017] The first water contact angle refers to the contact angle formed when 10-100 microliters of water droplets are uniformly applied to the material surface and tested with a contact angle tester. A first water contact angle greater than 90° indicates that the material has excellent hydrophobicity, and the larger the value, the stronger the hydrophobicity. The size of the water contact angle is related to the membrane material and the pore size. After testing, the first water contact angle of this porous membrane is 110-150°, which indicates that the porous membrane has strong hydrophobicity, water is particularly difficult to penetrate, and it has excellent waterproof effect, so it can be used as a waterproof, breathable and / or sound-permeable membrane.

[0018] In this invention, the crystallinity of the porous membrane was measured by DSC. The test results showed that the porous membrane has a suitable crystallinity (20%-65%), which means that most of the molecular chains of the fibers inside the membrane body have been arranged into an ordered and compact structure, and the intermolecular forces are relatively suitable. This improves the tensile strength and tensile strength at break of the porous membrane to a certain extent, and further ensures the mechanical strength. At the same time, it can also ensure the heat resistance of the porous membrane, and can still ensure the stability of the membrane pores at higher temperatures, thus having a wide range of applications.

[0019] Commonly used porosity testing methods include mercury intrusion porosimetry, density method, and wet / dry membrane weighing method; of course, those skilled in the art can also obtain the above parameters through other measurement methods, which are for reference only. After testing, the porosity of this PTFE porous membrane is 65%-95%, which makes the filter membrane have a high dirt holding capacity, can retain more impurity particles, and has a long service life, making it particularly suitable for filtering stripping fluids and / or etching solutions; in addition, the combination of suitable node-fiber structure and high porosity in different regions of the membrane ensures that the porous membrane has a large flow rate (and a slow flow rate decay) and high mechanical strength, thus meeting the needs of practical applications.

[0020] As a further improvement of the present invention, the average diameter of the separation nodes in SEM is 0.5-15 μm; the average density of the separation nodes is 5-54 per 4000 μm. 2 .

[0021] The diameter and number of separation nodes (reflected by density) significantly affect the membrane's pressure loss and mechanical strength. Excessive node length and / or number of nodes greatly reduces fluid velocity as it passes through the separation layer (small pores + relatively dense structure), leading to a substantial increase in membrane pressure loss. Conversely, insufficient node length and / or number of nodes result in lower overall membrane mechanical strength. Furthermore, fewer separation fibers hinder effective impurity retention, especially when the fluid contains a high concentration of impurities. It is prone to leakage; therefore, in this invention, when the porous membrane is used as a filter membrane, the appropriate size (reflected by the diameter) and appropriate number (reflected by the density) of the separation nodes can ensure both high retention efficiency and low pressure loss, while also ensuring that the separation layer (pore area) of the membrane has good mechanical strength. During long-term filtration, the membrane pores will not deform, and it can efficiently retain various impurities for a long time. When the porous membrane is used as a waterproof and breathable membrane, the appropriate size and appropriate number of separation nodes, under the combined effect of the tortuous path, can achieve both high waterproofness and high air permeability.

[0022] The average diameter and average density of the separation nodes, as well as the average length and average width of the subsequent separation fibers, can all be characterized by scanning electron microscopy to characterize the membrane structure, and then measured and calculated using computer software (such as Matlab, NIS-Elements, etc.) or manually.

[0023] During membrane fabrication, in the direction perpendicular to the membrane thickness (if the membrane is a flat sheet, this direction is planar; if the membrane is a hollow fiber membrane, this direction is perpendicular to the radius), its characteristics, such as the average diameter and density of separation nodes, and the average length and width of separation fibers, are roughly uniform and consistent. Therefore, the density and diameter of separation nodes and the length and width of separation fibers in a partial area on the corresponding plane can reflect the overall density and diameter of separation nodes and the length and width of separation fibers on that plane. In actual measurement, the main structure of the membrane can be characterized using an electron microscope to obtain the corresponding SEM image. Since the density and diameter of separation nodes and the length and width of separation fibers within the membrane separation layer are roughly uniform, a certain area, such as 1000 μm, can be selected. 2 (40μm x 25μm), 4000μm 2 (40μm x 100μm) or 10000μm 2 (100μm x 100μm), the specific area size depends on the actual situation. Then, use appropriate computer software or manual measurement to determine the density and diameter of the separation nodes on this area, as well as the length and width of the separation fibers. Perform several tests and take the average value to obtain the average density and average diameter of the separation nodes in the separation layer, and the average length and average width of the separation fibers. 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.

[0024] As a further improvement of the present invention, the average SEM length of the separated fibers is 3-50 μm and the average SEM width is 0.2-3 μm.

[0025] The porous membrane of this invention can achieve multiple uses and has high economic value. When used as a filter membrane, the separation layer effectively traps impurities. The separation fibers have a suitable length, and the upper and lower separation fibers interweave to form a tortuous path, thus ensuring sufficient capture of impurities. The width of the separation fibers ensures that they will not break during filtration, enabling stable trapping over a long period. Fiber breakage will not occur, which would enlarge the membrane pores and affect trapping. At the same time, the separation fibers are not too thick, ensuring that the porous membrane has low pressure loss and fast flow rate. When used as a waterproof and breathable membrane, the width and length of the separation fibers ensure that the porous membrane is not easily permeated by water, exhibiting excellent waterproof performance. At the same time, the porous membrane also has good air permeability, allowing for rapid air permeation and maintaining the balance of air pressure inside and outside the membrane.

[0026] As a further improvement of the present invention, the thickness of the separation layer is 4-30 μm; the thickness of the separation layer accounts for 3%-35% of the thickness of the membrane body; and the aspect ratio of the separation fiber is 5-45.

[0027] In this invention, the separation layer has a certain thickness, neither too thin nor too thick (relatively thinner than the pretreatment layer), ensuring high retention efficiency while also exhibiting low pressure loss and a relatively fast flow rate. As a waterproof and breathable membrane, it ensures excellent waterproof performance while maintaining high breathability. The separation fibers have a suitable aspect ratio (length:width), resulting in an ideal node-fiber structure within the separation layer, forming ideal pore size and number. This ensures the membrane effectively retains impurity particles while maintaining a fast flow rate for long-term, efficient filtration. Furthermore, it ensures good pressure resistance in the separation zone, preventing pore collapse under pressure and maintaining a relatively stable flow rate. Simultaneously, it provides long-term, efficient waterproofing, making it difficult for water to penetrate the porous membrane and ensure the normal operation of internal components.

[0028] As a further improvement of the present invention, the average SEM length of the support node in the first direction is 6-50 μm, and the average SEM length in the second direction is 0.5-10 μm.

[0029] The length direction (first direction) of the support node is basically parallel to the thickness direction, allowing the support node to act as a skeleton support in the thickness direction, thus ensuring that the porous membrane still has high tensile strength even when the membrane pores are large. The support node also has a certain length in the direction perpendicular to the thickness (second direction), which can further improve the tensile strength of the porous membrane. In addition, it was found that the support node has a suitable length in the second direction. This length has a small resistance to the fluid when it passes through the porous membrane, that is, it will not produce a large pressure loss. Therefore, the support node of the present invention with a suitable length (length in the first direction) and a suitable width (length in the second direction) ensures that the porous membrane not only has high tensile strength and high pressure resistance, but also has low pressure loss and high energy conversion efficiency, meeting various industrial needs and having a wide range of applications.

[0030] As a further improvement of the present invention, the ratio of the average SEM length of the support node in the first direction to the average SEM length in the second direction is 3-30; the ratio of the average SEM length of the support node in the first direction to the average SEM diameter of the separation node is 5-40.

[0031] When the ratio of the length of the support node in the first direction to its length in the second direction is too large, it is detrimental to achieving high tensile strength in the membrane; conversely, when the ratio is too small, it is detrimental to achieving low pressure loss. Therefore, to ensure both high tensile strength and low pressure loss, the ratio of the length of the node in the first direction to its length in the second direction must be within a reasonable range. In this invention, the ratio of the length of the node in the first direction to its length in the second direction is 3-30, thereby achieving both high tensile strength and low pressure loss. Furthermore, by controlling the ratio of the length of the support node in the first direction to the diameter of the separation node, this invention ensures that suitable nodes are present in regions with different membrane pore sizes, and that both the support and separation nodes play their respective ideal roles. This ensures that the porous membrane not only has high retention efficiency and low pressure loss but also good mechanical strength, is not easily separated between layers, and has a long service life. It is particularly suitable for use in filtering stripping and etching solutions in semiconductor manufacturing processes, as well as for use as a waterproof and breathable membrane in electronic and medical devices.

[0032] As a further improvement of the present invention, the average SEM distance between adjacent support nodes along the second direction is 3-25 μm; and the projections of two adjacent support nodes along the film thickness direction partially overlap, and the length of the overlapping area is not less than 10% of the film thickness.

[0033] The second direction is perpendicular to the membrane thickness direction. Since the nodes are all elongated structures, the distance between adjacent nodes in this invention refers to the distance between the center points of adjacent nodes in the second direction. The center point of a node is the point where the major axis (the line segment with the maximum length of the node in the first direction) and the minor axis (the line segment with the maximum length of the node in the second direction) intersect. Support nodes refer to relatively dense blocky regions with low porosity. If the distance between adjacent support nodes is too small, the overall porosity of the membrane will be too low, and the pressure loss of the membrane will be large. If the distance between adjacent support nodes is too large, the tensile strength of the membrane will be too low, and the mechanical properties will be poor. Along the length of the second direction, the average SEM distance between adjacent support nodes in this invention is 3-25 μm, which enables the porous membrane to have both low pressure loss and high tensile strength, resulting in a wide range of applications. It is particularly suitable for filtering stripping and etching solutions in semiconductor manufacturing processes, and for use as a breathable membrane in electronic and medical devices. Furthermore, we found that the projections of two adjacent support nodes along the membrane thickness direction partially overlap, and the length of the overlapping area is not less than 10% of the membrane thickness. That is, the distribution of support nodes is basically in an alternating (staggered) arrangement, which makes the filter pores S-shaped. With the spacing between nodes remaining unchanged, the staggered arrangement makes the filter channel more tortuous, increases the travel distance of the filter medium in the filter membrane, makes impurities easier to intercept and capture, and improves the filtration efficiency; at the same time, it can also further improve the waterproof performance.

[0034] As a further improvement of the present invention, the main body also has a transition layer, which is located between the pretreatment layer and the separation layer; the transition layer has transition nodes and transition fibers for forming a porous structure, and adjacent transition nodes are connected by transition fibers; the transition nodes have a square structure.

[0035] The average SEM length of the transition node in the first direction is 1.2-10 μm; the ratio of the average SEM length of the transition node in the first direction to the average SEM length in the second direction is 1.5-5.

[0036] In some porous membranes, we found an additional region besides the pretreatment layer and the separation layer, located between them. This region is called the transition layer. One side of the transition layer is connected to one side of the pretreatment layer by continuous fibers, and the other side is connected to one side of the separation layer by continuous fibers. Within the transition layer, the corresponding nodes (which we call transition nodes) are neither elongated nor dotted structures, but rather square structures (also called short strip structures), meaning their length (average SEM length in the first direction) is equal to their width. The ratio of length (average SEM length in the second direction) is neither too large nor too small. The transition node has a suitable length (average SEM length in the first direction is 1.2-10 μm) and a suitable aspect ratio (ratio of average SEM length in the first direction to average SEM length in the second direction is 1.5-5). This further ensures that the membrane as a whole has high mechanical strength, while also improving the retention efficiency and ensuring efficient retention of impurities. At the same time, it has little impact on the fluid flow rate, and the fluid can still pass through the porous membrane in a short time.

[0037] As a further improvement of the present invention, the SEM average width of the transition fiber is 0.2-5 μm; the thickness of the transition layer is 2-18 μm, and the thickness of the transition layer is 1%-20% of the film thickness.

[0038] The transition fibers have a suitable thickness, ensuring that they will not break during long-term filtration, thus guaranteeing the stability of the membrane pores and the overall mechanical strength of the porous membrane. The transition layer is relatively thin, and through its interaction with transition fibers of a certain thickness, it can play a preliminary interception role while simultaneously hindering the passage of fluid, ensuring that the porous membrane still has a low pressure loss. When used as a waterproof and breathable membrane, it can further ensure that the membrane has high waterproof performance while also having high breathability.

[0039] As a further improvement of the present invention, the first outer surface includes strip-shaped nodes and strip-shaped fibers for forming a porous structure, and adjacent strip-shaped nodes are connected by the strip-shaped fibers; the extension directions of the strip-shaped nodes are substantially consistent; the SEM average width of the strip-shaped nodes is 1-6 μm; the SEM average width of the strip-shaped fibers is 0.4-3.2 μm; and the average spacing between adjacent strip-shaped fibers is 0.5-5 μm.

[0040] On the first outer surface of the membrane, there are strip-shaped nodes and fibers, which we call strip nodes and strip fibers. Compared with strip fibers, strip nodes are wider and longer. The extension direction of several strip nodes is basically consistent, and the strip nodes are arranged in parallel with each other. Adjacent strip nodes are connected by several strip fibers. This node-fiber structure makes the first outer surface have large pores, thereby ensuring that the fluid passes through the first outer surface quickly, thus giving the membrane a fast flow rate. The presence of relatively thick strip nodes (i.e., strip nodes with appropriate thickness) ensures that the first outer surface has good pressure resistance and increases the mechanical strength of the membrane.

[0041] In this invention, the first outer surface of the membrane has strip-shaped fibers of suitable width, and adjacent strip-shaped fibers have a suitable average spacing (this spacing is an important factor affecting the pore size of the membrane on the first outer surface). This not only gives the membrane high tensile strength and good mechanical properties, but also a suitable pore size and low pressure loss; it also has a large dirt-holding capacity, capable of retaining various impurities for extended periods, making it particularly suitable for filtering stripping solutions and etching solutions in semiconductor manufacturing processes. Preferably, the first outer surface is more suitable for use as a liquid inlet surface.

[0042] In this invention, the average width of the strip nodes, the average width of the strip fibers, and the average spacing between adjacent strip fibers can all be obtained by characterizing the morphology of the first outer surface of the membrane using a scanning electron microscope, then selecting a certain area and measuring it using computer software (such as Matlab, NIS-Elements, etc.) or manually. The average value is then taken to calculate the corresponding values. Of course, it is understood that those skilled in the art can also obtain the above parameters through other measurement methods.

[0043] As a further improvement of the present invention, the second outer surface includes surface nodes and surface fibers, a plurality of the surface fibers are radially distributed along the circumference of the surface nodes, and adjacent surface nodes are connected by the surface fibers; the SEM average particle size of the surface nodes is 1-15 μm; the SEM average width of the surface fibers is 0.1-2.5 μm.

[0044] On another outer surface (where the nodes and fibers are relatively fine, and the membrane pores are relatively small), which we define as the second outer surface, surface nodes resembling dots and strip-shaped surface fibers are distributed. Several surface nodes are distributed relatively irregularly (no longer basically parallel), and adjacent dot-like nodes are connected by surface fibers. Several surface fibers are distributed radially along the circumference of the surface nodes. This structure of surface nodes and surface fibers, together with the appropriate particle size of the surface nodes, gives the second outer surface suitable pressure resistance and filtration accuracy. If the particle size of the surface nodes is too small, it will reduce the pressure resistance of the second outer surface, further reducing the overall mechanical strength of the membrane. If the particle size is too large, it will prolong the time required for the fluid to pass through the membrane, increase the filtration time, increase the pressure loss, and affect economic efficiency. The surface fibers have a suitable width (thickness), which further ensures that the porous membrane has a high flow rate (low pressure loss). At the same time, the surface fibers will not break when the porous membrane filters fluid for a long time, which can ensure the retention efficiency for a long time. It also facilitates various processing of the porous membrane, such as folding the membrane, and defects are not easily generated during the folding process. It also improves the heat resistance of the porous membrane. In addition, this structure of the second outer surface further ensures that the porous membrane has excellent waterproof and high air permeability.

[0045] In this invention, the average particle size of the surface nodes and the average width of the surface fibers can be obtained by characterizing the morphology of the second outer surface of the membrane using a scanning electron microscope, then selecting a certain area and measuring it using computer software (such as Matlab, NIS-Elements, etc.) or manually. The average value is then taken to calculate the corresponding value. Of course, it is understood that those skilled in the art can also obtain the above parameters through other measurement methods.

[0046] As a further improvement of the present invention, the porous membrane has a tensile strength of 5-30 MPa and an elongation at break of 70%-180%; at 0.03 MPa and a temperature of 20°C, the time required for 50 ml of water to pass through the porous membrane with a diameter of 47 mm is 0.3-7 s; the porous membrane has a retention efficiency of more than 90% for impurity particles with a particle size of 1-15 μm; and the porous membrane has a water pressure resistance of 20-200 kPa.

[0047] When air flows through the porous membrane at a velocity of 5.3 cm / s, the pressure loss is 10-110 mmH2O.

[0048] The tensile strength and elongation at break are important indicators for evaluating the mechanical strength of filter membranes. Under certain conditions, the greater the tensile strength of the membrane, the better its mechanical strength. Tensile strength refers to the membrane's ability to withstand parallel tensile forces. During testing under certain conditions, the membrane sample is subjected to a tensile load until it breaks. Based on the maximum tensile load at the point of failure and the change in the membrane sample's dimensions (length), the tensile strength and elongation at break can be calculated. Both tensile strength and elongation at break can be measured using a universal tensile testing machine. The testing method for tensile strength is well-known in the field; for example, ASTM D790 or ISO 178 details the procedure for tensile strength testing. In this invention, the PTFE porous membrane exhibits a tensile strength of 5-30 MPa and an elongation at break of 70%-180%, indicating that the porous membrane of this invention has high tensile strength and elongation at break, good mechanical properties, high industrial practical value, and fully meets market demands.

[0049] Flow rate tests on the porous membrane showed that, under conditions of 0.03 MPa pressure and 20°C, 50 ml of water required only 0.3-7 seconds to pass through a 47 mm diameter filter membrane. This demonstrates that the polytetrafluoroethylene (PTFE) porous membrane exhibits a fast flow rate, short filtration time, and low time cost. Furthermore, this invention's porous membrane is suitable for filtering stripping and etching solutions in semiconductor manufacturing processes. In addition, retention efficiency tests showed that the PTFE porous membrane of this invention achieves a retention efficiency of over 90%, preferably over 95%, for impurity particles with a diameter of 1-15 μm. This demonstrates its highly efficient retention of impurity particles, ensuring fluid purity. Depending on the pore size, impurities of various particle sizes can be removed, making it suitable for a wide range of applications.

[0050] Water pressure resistance tests were conducted on the polytetrafluoroethylene porous membrane, which showed a water pressure resistance of 20-200 kPa, indicating good water pressure resistance. Combined with the material of polytetrafluoroethylene, its corresponding bubble point and porosity, the porous membrane exhibits good waterproof pressure resistance and breathability, making it suitable for use as a waterproof and breathable membrane.

[0051] Pressure loss was measured on the porous membrane at a flow rate of 5.3 cm / s (membrane surface area is 100 cm²). 2 When the pressure loss is only 10-110 mmH2O (for example, a 10cm*10cm membrane), it shows that the porous membrane of the present invention has particularly low pressure loss, high energy utilization, and low filtration cost.

[0052] Furthermore, the present invention also provides a method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0053] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained. The polytetrafluoroethylene dispersion resin includes at least one polytetrafluoroethylene dispersion resin with a number average molecular weight of 1 million to 12 million. The lubricant is at least one of lubricating oil, palm oil, naphthenic oil, white oil, aviation kerosene, defatted kerosene, and paraffin wax.

[0054] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0055] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0056] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0057] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 180-280℃, a stretching ratio of 1-15 times, and a stretching rate of 5-25% / s to obtain the first strip matrix.

[0058] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with a heat carrier, and the second outer surface is brought into contact with a cold carrier to perform the first heat setting. The heat setting time is 0.5-6 minutes. The temperature of the heat carrier is 350-400℃, and the temperature of the cold carrier is 10-40℃.

[0059] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 200-290℃ (below the melting point) for transverse pre-stretching, with a transverse stretching ratio of 2-20 times, to obtain the second strip matrix; wherein the first outer surface is close to the heat source and the second outer surface is far away from the heat source;

[0060] H. Lateral stretching: The first strip-shaped substrate, after lateral pre-stretching, is placed in an environment with a temperature of 300-350℃ for lateral stretching, and the lateral stretching ratio is 3-15 times to obtain the second strip-shaped substrate; wherein the first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0061] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 345-380℃ for a second heat setting, and the heat setting time is 3-20 minutes to obtain a polytetrafluoroethylene porous membrane.

[0062] As a further improvement of the present invention, the strip-shaped substrate formed in step C is rolled again, so that the thickness of the rolled strip-shaped substrate is 20%-80% of the thickness of the strip-shaped substrate before rolling; during the second rolling, the first outer surface is close to the heat source, the second outer surface is far away from the heat source, and the temperature of the heat source is 60-80°C.

[0063] As a further improvement of the present invention, the thickness of the gap between the heat carrier and the cold carrier in step F is 50%-90% of the thickness of the first strip matrix before the first heat setting.

[0064] The stretching rate during transverse pre-stretching in step G is 1% / s-20% / s;

[0065] The stretching rate during transverse stretching in step H is 5% / s-30% / s.

[0066] In preparing polytetrafluoroethylene (PTFE) porous membranes, the raw material PTFE dispersion resin and lubricant are first mixed and stirred. The PTFE dispersion resin includes at least one type of PTFE dispersion resin with a number average molecular weight of 1 million to 12 million; that is, the PTFE dispersion resin used can be a single type with a number average molecular weight of 1 million to 12 million. Alternatively, multiple types of PTFE dispersion resin can be used (two or more types), such as a mixed dispersion resin composed of one type of PTFE dispersion resin with a number average molecular weight of 1 million and another type with a number average molecular weight of 12 million. By selecting a suitable PTFE dispersion resin with a suitable number average molecular weight and combining it with appropriate stretching processes (stretching temperature and stretching ratio, etc.), sufficient mechanical strength of the PTFE porous membrane can be obtained. The lubricant is at least one of lubricating oil, palm oil, naphthenic oil, white oil, aviation kerosene, degreased kerosene, and paraffin wax. The lubricant can be only one type. The material can be a mixture of the above-mentioned materials. By selecting a suitable lubricant, it is easier to process the polytetrafluoroethylene (PTFE) dispersion resin and obtain a porous membrane with the desired membrane structure. Preferably, in this invention, 15-35 parts by weight of lubricant are required for every 100 parts by weight of PTFE dispersion resin. Preferably, in this invention, the PTFE dispersion resin and lubricant are mixed and stirred at a temperature of 5-25°C to ensure that the PTFE dispersion resin does not become fibrous too early, which is beneficial for subsequent stretching treatments and thus obtaining a porous membrane with an ideal node-fiber structure. After the PTFE dispersion resin and lubricant are mixed evenly, they are placed in a heating container such as an oven for curing to obtain a paste. The curing temperature is preferably 25-50°C and the curing time is 8-48 hours. The purpose of curing is to ensure that the PTFE dispersion resin and lubricant are mixed more evenly and that the lubricant is evenly dispersed in the PTFE dispersion resin, which is convenient for subsequent preform extrusion.

[0067] Next, the preform is prepared by pre-pressing the above-mentioned paste into a cylindrical preform. Then, extrusion is performed by placing the preform into an extruder for extrusion, forming a flat strip matrix. One outer surface is the first outer surface, and the other outer surface is the second outer surface. Preferably, the formed strip matrix is ​​calendered again, so that the thickness of the calendered strip matrix is ​​20%-80% of the thickness of the strip matrix before calendering. During the second calendering, the first outer surface is close to the heat source, and the second outer surface is away from the heat source. The heat source temperature is 60-80°C. This re-calendering process is beneficial to the resin... The particles generate certain forces with each other, which facilitates subsequent fusion into fibers. After appropriate stretching, an ideal nodule-fiber structure is formed, and the membrane pores become more uniform with high porosity. At the same time, due to the different heating conditions of the two outer surfaces, the crystallinity of the two outer surfaces and the surrounding areas of the membrane differs. By combining this with subsequent transverse and longitudinal stretching processes, different nodule-fiber structures appear on the cross-section of the membrane body. The strip matrix is ​​then dried to allow the lubricant to evaporate; the drying temperature is 100-250℃.

[0068] The dried strip substrate is then subjected to longitudinal stretching on a film stretching machine. The longitudinal stretching temperature is 180-280℃, the longitudinal stretching ratio is 1-15 times, and the longitudinal stretching rate is 5-25% / s, resulting in the first strip substrate. In this invention, the stretching rate (including the longitudinal stretching rate and the transverse stretching rate) is specifically achieved by the distance between the rollers and the speed difference between the rollers. During the longitudinal stretching process, nodes and fibers begin to split. Since the activation energy of polytetrafluoroethylene resin for fiber formation is very low, a higher stretching temperature is required to easily form longer fibers and larger nodes, resulting in a relatively larger pore size. Therefore, a longitudinal stretching temperature of 180-280℃ is suitable. If the longitudinal stretching temperature exceeds 280℃, the uniformity of the membrane pores and fibers after stretching will be very poor. If the longitudinal stretching temperature is below 180℃, the fiber density and length will be insufficient, leading to a decrease in the final porosity of the membrane, an increase in pressure loss, and adverse effects on the mechanical properties and flow rate of the membrane.

[0069] After longitudinal stretching, the first outer surface of the first strip matrix is ​​brought into contact with a heat carrier, and the second outer surface is brought into contact with a cold carrier, thus performing the first heat setting for 0.5-6 minutes. The temperature of the heat carrier is 350-400℃, and the temperature of the cold carrier is 10-40℃. The carrier can be a roller or other appropriate tool that meets the requirements of this invention. The first heat setting plays a role in the initial structural locking, making the nodes less prone to breakage during subsequent transverse stretching (not completely eliminating nodes). The nodes in the two regions will form an ideal shape during the subsequent transverse stretching process, and at the same time, the fibers will disperse, resulting in a large number of fibers connecting the nodes. In the region close to the heat carrier, the nodes partially fuse in the thickness direction and basically do not recover after leaving the heat source, remaining in a partially fused state. This is because the molten PTFE has a high viscosity, and during partial melting, the nodes in the thickness direction... After fusion under pressure, it is difficult to rebound to its previous state. Due to the temperature gradient, the area near the cold carrier does not melt, and the film has a certain degree of resilience. That is, after being compressed and detached from the gap, it becomes a dispersed nodule. Therefore, it will cause strip-shaped support nodes on one side and point-shaped separation nodes on the other side. The temperature (heat carrier) of the first heat setting needs to be above the melting point, and the setting time should be 0.5-6 minutes to achieve a suitable setting effect. If the setting temperature is too low or the time is too short, it will not achieve the setting effect. If the setting temperature is too high or the time is too long, it will affect the overall crystallization performance of the film and will not produce the ideal fiber structure and membrane pore size. Preferably, the thickness of the gap between the heat carrier and the cold carrier is 50%-90% of the thickness of the first strip matrix before the first heat setting, which further reduces the resilience and forms an ideal temperature gradient in the film thickness direction, thereby forming corresponding nodule-fiber structures in different regions.

[0070] After the first heat setting, the first strip-shaped substrate is placed in an environment with a temperature of 200-290℃ (below the melting point) for transverse pre-stretching, with a transverse stretching ratio of 2-20 times, to obtain the second strip-shaped substrate. The first outer surface is close to the heat source, and the second outer surface is far from the heat source. An important condition for transverse pre-stretching is temperature control, which must not exceed the melting point. By pre-stretching at a temperature below the melting point, the nodes in the region far from the heat source are easily broken and dispersed into smaller nodes, resulting in a large number of smaller nodes. When the temperature is too high, it is impossible to form a large number of point-like structures, which in turn leads to the failure to form a porous membrane with an ideal structure (the formed porous membrane has larger pores and a lower bubble point). The nodes in the region close to the heat source mainly undergo deformation, becoming elliptical strips. However, the stretching temperature cannot be too low either, because the crystallinity in this region is relatively low. If the stretching temperature is too low, the nodes in this region will have difficulty deforming, making it difficult to obtain strip-shaped support nodes and a membrane with high porosity.

[0071] Next, the first strip-shaped substrate, after being transversely pre-stretched, is placed in an environment with a temperature of 300-350℃ (near the melting point, which needs to be higher than the transverse pre-stretching temperature, but not too high) for transverse stretching. The transverse stretching ratio is 3-15 times, resulting in a second strip-shaped substrate. The first outer surface is close to the heat source, and the second outer surface is far from the heat source. During transverse stretching, setting a higher temperature promotes the fibers to be pulled out from the nodes, increasing the number of fibers and thus improving the porosity of the membrane. This also helps to improve water resistance and the efficient retention of impurities. On the side away from the heat source, the nodes will be further reduced to separate nodes with a dot-like structure. The region near the heat source will change from an elliptical elongated shape to a relatively slender elongated shape. If the transverse stretching temperature is too high, the temperature gradient in the film thickness direction will be basically the same, thus preventing the formation of an asymmetric structure and ultimately resulting in a symmetric structure. Preferably, the stretching rate during transverse pre-stretching is 1% / s-20% / s, and the stretching rate during transverse stretching is 5% / s-30% / s. Both transverse pre-stretching and transverse stretching have suitable stretching rates, which facilitates the production of porous membranes with ideal structures and pore sizes. At the same time, it can ensure that the membrane pores are relatively uniform, and the thickness of each layer node and fiber is more uniform, thereby further improving the membrane's retention efficiency and reducing the membrane's pressure loss.

[0072] Finally, a second heat setting is performed. The second strip substrate is placed in an environment with a temperature of 345-380℃ for a second heat setting time of 5-20 minutes to obtain a polytetrafluoroethylene (PTFE) porous membrane. The membrane is then fully set to obtain a product with good dimensional stability and high strength. Through this preparation process, a polytetrafluoroethylene porous membrane with an ideal membrane structure can be obtained. This PTFE porous membrane is an asymmetric membrane and is integrally formed. It not only has high retention efficiency and fast flow rate, but also low pressure loss and good mechanical strength. It has a wide range of applications and is particularly suitable for filtering stripping and etching solutions in semiconductor manufacturing processes, as well as for use as a waterproof and breathable membrane in electronic and medical devices. Of course, it can also be used as a waterproof and sound-permeable membrane.

[0073] As a further improvement of the present invention, the polytetrafluoroethylene porous membrane is used for filtering stripping and etching solutions in semiconductor manufacturing processes, and as a waterproof and breathable membrane in electronic and medical devices.

[0074] The composition of stripping and etching solutions in semiconductor manufacturing processes varies depending on the process, but generally, both are mixtures of various chemical substances. Stripping solutions are typically organic solvents, such as ethanol and acetone, along with surfactants and additives; they are generally organic solutions. Etching solutions are typically strong acid solutions, such as nitric acid and hydrofluoric acid. Both stripping and etching solutions are recycled. They often contain various impurities (often numerous), such as photoresist. The polytetrafluoroethylene porous membrane of this invention is used for filtering stripping and etching solutions in semiconductor manufacturing processes, especially in panel manufacturing (e.g., LCD panels); and as a breathable membrane in electronic and medical devices, such as a waterproof and breathable membrane in mobile phones. This membrane can also be used as a waterproof and sound-permeable membrane.

[0075] The beneficial effects of this invention are as follows: The polytetrafluoroethylene porous membrane provided by this invention includes a main body, one side of which is a first outer surface and the other side is a second outer surface. The porous membrane is integrally formed. The main body has non-directional tortuous pathways. The main body includes a pretreatment layer and a separation layer. One side of the pretreatment layer is the first outer surface, and one side of the separation layer is the second outer surface. The average pore size of the pretreatment layer is larger than that of the separation layer. The pretreatment layer includes support nodes and support fibers for forming a porous structure. The support nodes are elongated structures, which helps to improve the mechanical strength of the membrane. The separation layer includes separation nodes and separation fibers for forming a porous structure. The separation nodes are dotted structures, which helps to improve the retention efficiency and water resistance of the membrane. Meanwhile, the porous membrane has an IPA bubble point of 5-100 kPa and a basis weight of 5-25 g / m³. 2 With a thickness of 5-100 μm, the porous membrane not only possesses high retention efficiency (efficiently retaining impurity particles with a diameter of 1-15 μm), but also exhibits low pressure loss, good mechanical strength, and minimal layer-to-layer separation, resulting in a long service life. It is particularly suitable for filtering stripping and etching solutions in semiconductor manufacturing processes, as well as for use as a waterproof and breathable membrane in electronic and medical devices. Furthermore, this invention provides a method for preparing this porous membrane, which is convenient, rapid, effective, simple to operate, energy-saving, and suitable for large-scale promotion. Attached Figure Description

[0076] Figure 1 The image shows a scanning electron microscope (SEM) image of the overall cross-section of the polytetrafluoroethylene porous membrane prepared in Example 1, with a magnification of 2000×.

[0077] Figure 2 The image shows a scanning electron microscope (SEM) image of the overall cross-section of the polytetrafluoroethylene porous membrane prepared in Example 2, with a magnification of 1000×.

[0078] Figure 3 The image shows a scanning electron microscope (SEM) image of the first outer surface of the polytetrafluoroethylene porous membrane prepared in Example 2, with a magnification of 500×.

[0079] Figure 4 The image shown is a scanning electron microscope (SEM) image of the second outer surface of the polytetrafluoroethylene porous membrane prepared in Example 2, with a magnification of 500×. Detailed Implementation

[0080] To more clearly illustrate the overall concept of this application, a detailed description is provided below by way of embodiments. Numerous specific details are set forth in the following description to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be implemented without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described.

[0081] Unless otherwise specified, the raw materials and equipment used in the preparation of the PTFE porous membranes in the following embodiments are commercially available. The structural morphology of the PTFE porous membranes was characterized using a Hitachi S-5500 scanning electron microscope.

[0082] Example 1: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0083] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained. The polytetrafluoroethylene dispersion resin has a number average molecular weight of 3 million. The lubricant is paraffin wax.

[0084] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0085] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0086] D. Drying: Dry the strip-shaped substrate to allow the lubricant to evaporate;

[0087] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 190℃, a stretching ratio of 2 times, and a stretching rate of 20% / s to obtain the first strip matrix.

[0088] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 1 minute; wherein the temperature of the heat carrier is 350°C and the temperature of the cold carrier is 20°C; and the thickness of the gap between the heat carrier and the cold carrier is 85% of the thickness of the first strip substrate before the first heat setting.

[0089] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 210℃ for transverse pre-stretching. The transverse stretching ratio is 4 times and the stretching rate is 18% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0090] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 310℃ for lateral stretching. The lateral stretching ratio is 4 times and the stretching rate is 25% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0091] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 350℃ for a second heat setting time of 4 minutes to obtain a polytetrafluoroethylene porous membrane.

[0092] Example 2: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0093] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained; the polytetrafluoroethylene dispersion resin is a polytetrafluoroethylene dispersion resin with a number average molecular weight of 5 million; the lubricant is degreased kerosene.

[0094] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0095] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0096] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0097] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 210℃, a stretching ratio of 4 times, and a stretching rate of 18% / s to obtain the first strip matrix.

[0098] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 1.5 minutes; wherein the temperature of the heat carrier is 355°C and the temperature of the cold carrier is 20°C; and the thickness of the gap between the heat carrier and the cold carrier is 80% of the thickness of the first strip substrate before the first heat setting.

[0099] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 230℃ for transverse pre-stretching. The transverse stretching ratio is 6 times and the stretching rate is 16% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0100] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 315℃ for lateral stretching. The lateral stretching ratio is 6 times and the stretching rate is 20% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0101] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 355℃ for a second heat setting time of 6 minutes to obtain a polytetrafluoroethylene porous membrane.

[0102] Example 3: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0103] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained. The polytetrafluoroethylene dispersion resin is a polytetrafluoroethylene dispersion resin with a number average molecular weight of 7 million. The lubricant is aviation kerosene.

[0104] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0105] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0106] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0107] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 230℃, a stretching ratio of 6 times, and a stretching rate of 15% / s to obtain the first strip matrix.

[0108] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 2 minutes; the temperature of the heat carrier is 360°C, and the temperature of the cold carrier is 25°C; and the thickness of the gap between the heat carrier and the cold carrier is 75% of the thickness of the first strip substrate before the first heat setting.

[0109] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 250℃ for transverse pre-stretching. The transverse stretching ratio is 8 times and the stretching rate is 15% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0110] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 320℃ for lateral stretching. The lateral stretching ratio is 8 times and the stretching rate is 16% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0111] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 360℃ for a second heat setting time of 8 minutes to obtain a polytetrafluoroethylene porous membrane.

[0112] Example 4: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0113] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained; the polytetrafluoroethylene dispersion resin is a polytetrafluoroethylene dispersion resin with a number average molecular weight of 9 million; the lubricant is white oil.

[0114] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0115] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0116] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0117] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 250°C, a stretching ratio of 10 times, and a stretching rate of 12% / s to obtain the first strip matrix.

[0118] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 3 minutes; wherein the temperature of the heat carrier is 370°C and the temperature of the cold carrier is 30°C; and the thickness of the gap between the heat carrier and the cold carrier is 70% of the thickness of the first strip substrate before the first heat setting.

[0119] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 260℃ for transverse pre-stretching. The transverse stretching ratio is 12 times and the stretching rate is 10% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0120] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 330℃ for lateral stretching. The lateral stretching ratio is 10 times and the stretching rate is 12% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0121] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 370℃ for a second heat setting time of 12 minutes to obtain a polytetrafluoroethylene porous membrane.

[0122] Example 5: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0123] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained; the polytetrafluoroethylene dispersion resin is a polytetrafluoroethylene dispersion resin with a number average molecular weight of 11 million; the lubricant is naphthenic oil.

[0124] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0125] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0126] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0127] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 270℃, a stretching ratio of 14 times, and a stretching rate of 10% / s to obtain the first strip matrix.

[0128] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 5 minutes; wherein the temperature of the heat carrier is 380°C and the temperature of the cold carrier is 35°C; and the thickness of the gap between the heat carrier and the cold carrier is 60% of the thickness of the first strip substrate before the first heat setting.

[0129] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 280℃ for transverse pre-stretching. The transverse stretching ratio is 16 times and the stretching rate is 5% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0130] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 340℃ for lateral stretching. The lateral stretching ratio is 12 times and the stretching rate is 8% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0131] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 380℃ for a second heat setting time of 16 minutes to obtain a polytetrafluoroethylene porous membrane.

[0132] Example 6: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0133] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained. The polytetrafluoroethylene dispersion resin includes polytetrafluoroethylene dispersion resins with a number average molecular weight of 3 million and 9 million (in a mass ratio of 1:1). The lubricant is palm oil.

[0134] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0135] C. Extrusion: The blank obtained in step B is extruded to form a flat strip-shaped substrate; one outer surface of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface; the strip-shaped substrate formed in step C is rolled again, so that the thickness of the rolled strip-shaped substrate is 40% of the thickness of the strip-shaped substrate before rolling; during the second rolling, the first outer surface is close to the heat source, and the second outer surface is away from the heat source, and the heat source temperature is 60℃. D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0136] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 200℃, a stretching ratio of 4 times, and a stretching rate of 16% / s to obtain the first strip matrix.

[0137] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 1.5 minutes; the temperature of the heat carrier is 355°C, and the temperature of the cold carrier is 20°C; the thickness of the gap between the heat carrier and the cold carrier is 80% of the thickness of the first strip substrate before the first heat setting.

[0138] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 220℃ for transverse pre-stretching. The transverse stretching ratio is 6 times and the stretching rate is 12% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0139] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 315℃ for lateral stretching. The lateral stretching ratio is 5 times and the stretching rate is 20% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0140] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 355°C for a second heat setting time of 6 minutes to obtain a polytetrafluoroethylene porous membrane.

[0141] Example 7: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0142] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained. The polytetrafluoroethylene dispersion resin includes polytetrafluoroethylene dispersion resins with a number average molecular weight of 5 million and 7 million (the mass ratio is 1:1). The lubricant is a lubricating oil.

[0143] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0144] C. Extrusion: The blank obtained in step B is extruded to form a flat strip-shaped substrate; one outer surface of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface; the strip-shaped substrate formed in step C is rolled again, so that the thickness of the rolled strip-shaped substrate is 60% of the thickness of the strip-shaped substrate before rolling; during the second rolling, the first outer surface is close to the heat source, and the second outer surface is away from the heat source, and the heat source temperature is 80℃. D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0145] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 260℃, a stretching ratio of 14 times, and a stretching rate of 8% / s to obtain the first strip matrix.

[0146] F. First heat setting: The first outer surface of the first strip substrate is brought into contact with the heat carrier, and the second outer surface is brought into contact with the cold carrier, thereby performing the first heat setting for 5 minutes; wherein the temperature of the heat carrier is 380°C and the temperature of the cold carrier is 35°C; wherein the thickness of the gap between the heat carrier and the cold carrier is 60% of the thickness of the first strip substrate before the first heat setting.

[0147] G. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 260℃ for transverse pre-stretching. The transverse stretching ratio is 16 times and the stretching rate is 4% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0148] H. Lateral stretching: The first strip matrix after lateral pre-stretching is placed in an environment with a temperature of 340℃ for lateral stretching. The lateral stretching ratio is 12 times and the stretching rate is 10% / s to obtain the second strip matrix. The first outer surface is close to the heat source and the second outer surface is far away from the heat source.

[0149] I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 380°C for a second heat setting for 16 minutes to obtain a polytetrafluoroethylene porous membrane.

[0150] Comparative Example 1: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0151] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained; the polytetrafluoroethylene dispersion resin is a polytetrafluoroethylene dispersion resin with a number average molecular weight of 5 million; the lubricant is degreased kerosene.

[0152] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0153] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0154] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0155] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 210℃, a stretching ratio of 4 times, and a stretching rate of 18% / s to obtain the first strip matrix.

[0156] F. First heat setting: The first strip substrate is subjected to the first heat setting at an ambient temperature of 390℃ for 7 minutes.

[0157] G. Lateral stretching: The first strip matrix after the first heat setting is placed in an environment with a temperature of 330℃ and subjected to lateral stretching. The lateral stretching ratio is 30 times and the stretching rate is 20% / s to obtain the second strip matrix.

[0158] H. Second heat setting: The second strip substrate is placed in an environment with a temperature of 385℃ for a second heat setting time of 15 minutes to obtain a polytetrafluoroethylene porous membrane.

[0159] Comparative Example 2: A method for preparing an asymmetric polytetrafluoroethylene porous membrane, comprising the following steps:

[0160] A. Mixing: The polytetrafluoroethylene dispersion resin and the lubricant are mixed and stirred until a paste is obtained; the polytetrafluoroethylene dispersion resin is a polytetrafluoroethylene dispersion resin with a number average molecular weight of 5 million; the lubricant is degreased kerosene.

[0161] B. Blank making: The above paste is pre-pressed into a cylindrical blank;

[0162] C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface;

[0163] D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate;

[0164] E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 210℃, a stretching ratio of 4 times, and a stretching rate of 18% / s to obtain the first strip matrix.

[0165] F. First heat setting: The first strip substrate is subjected to the first heat setting at an ambient temperature of 400℃ for 8 minutes.

[0166] G. Lateral stretching: The first strip matrix after the first heat setting is placed under an ambient temperature of 360℃ for lateral stretching. The lateral stretching ratio is 35 times and the stretching rate is 20% / s to obtain the second strip matrix.

[0167] H. Second heat setting: The second strip substrate is placed in an environment with a temperature of 390℃ for a second heat setting time of 20 minutes to obtain a polytetrafluoroethylene porous membrane.

[0168] 1. Structural Characterization

[0169] The morphology of the PTFE porous membranes obtained in each embodiment and comparative example was characterized using a scanning electron microscope to obtain the required data.

[0170] Table 1: Structural characteristics of the separation layer

[0171]

[0172] Table 2: Characteristics of Support Layer and Transition Layer

[0173]

[0174] As shown in Tables 1 and 2, the porous membranes prepared in Examples 1-7 all have ideal membrane structures and are all integrally formed asymmetric membranes. The porous membranes prepared in Examples 1-5 mainly include a pretreatment layer and a separation layer. The porous membranes prepared in Examples 6 and 7 mainly include a pretreatment layer, a transition layer, and a separation layer. However, the porous membranes prepared in Comparative Examples 1 and 2 are symmetric membranes with strip-shaped cross-sections on their main body, which is not the membrane structure required by this invention. Therefore, their performance is relatively coarse.

[0175] Table 3: Surface Structure Characteristics

[0176]

[0177] The porous membranes prepared in Examples 1-7 have ideal knot-fiber structures on both outer surfaces.

[0178] Table 4: Overall Characteristics of the Membrane

[0179] Example 1 45 20 30 45 70 Example 2 28 17 40 40 73 Example 3 18 15 50 37 76 Example 4 13 13 60 35 80 Example 5 9 10 80 32 84 Example 6 26 16 45 42 75 Example 7 8 11 85 34 85 Comparative Example 1 22 10 40 28 62 Comparative Example 2 20 6 40 24 57

[0180] Table 5: Membrane Properties

[0181] The examples show the retention efficiencies of 1 μm impurities in Example 1; the retention efficiencies of 3 μm impurities in Examples 2 and 6, and Comparative Examples 1 and 2; the retention efficiencies of 5 μm impurities in Example 3; the retention efficiencies of 10 μm impurities in Example 4; and the retention efficiencies of 15 μm impurities in Examples 5 and 7.

[0182]

[0183] As shown in the table above, the PTFE porous membrane prepared by this invention has high tensile strength and elongation at break, exhibiting excellent mechanical properties that meet various industrial needs and have a wide range of applications. It also boasts high flow rate, fast filtration speed, low pressure loss, and efficient retention of various impurities, making it particularly suitable for filtering stripping and etching solutions in semiconductor manufacturing processes. Furthermore, this PTFE porous membrane exhibits high air permeability and water pressure resistance, making it suitable for use as a waterproof and breathable membrane in electronic and medical devices. Of course, this membrane can also be used as a waterproof and sound-permeable membrane.

[0184] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. An asymmetric polytetrafluoroethylene porous membrane, comprising a body, wherein one side of the body is a first outer surface and the other side is a second outer surface, characterized in that, The porous membrane is integrally formed; the main body has non-directional tortuous pathways; The main body includes a pretreatment layer and a separation layer. One side of the pretreatment layer is a first outer surface, and one side of the separation layer is a second outer surface. The average pore size of the pretreatment layer is larger than the average pore size of the separation layer. The pretreatment layer includes support nodes and support fibers for forming a porous structure. The support nodes are elongated structures, and the length of the support nodes in the first direction is greater than the length in the second direction. The first direction is parallel to the thickness direction of the porous membrane, and the second direction is perpendicular to the thickness direction of the porous membrane; adjacent support nodes are connected by support fibers. The separation layer includes separation nodes and separation fibers for forming a porous structure. The separation nodes are point-like structures, and adjacent separation nodes are connected by separation fibers. The porous membrane has an IPA bubble point of 5-100 kPa and a basis weight of 5-25 g / m³. 2 Thickness ranges from 5 to 100 μm; The first outer surface includes strip-shaped nodes and strip-shaped fibers for forming a porous structure, and adjacent strip-shaped nodes are connected by the strip-shaped fibers; the extension directions of the strip-shaped nodes are substantially the same; The second outer surface includes surface nodes and surface fibers. A plurality of the surface fibers are radially distributed along the circumference of the surface nodes, and adjacent surface nodes are connected by the surface fibers.

2. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The porous membrane has a first water contact angle of 110-150°, a crystallinity of 30%-65%, and a porosity of 65%-95%.

3. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The average diameter of the separation nodes in SEM is 0.5-15 μm; the average density of the separation nodes is 5-54 per 4000 μm².

4. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The average length of the separated fibers measured by SEM is 3-50 μm, and the average width measured by SEM is 0.2-3 μm.

5. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The thickness of the separation layer is 4-30 μm; the thickness of the separation layer accounts for 3%-35% of the thickness of the membrane body; the aspect ratio of the separation fiber is 5-45.

6. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The average SEM length of the support node in the first direction is 6-50 μm, and the average SEM length in the second direction is 0.5-10 μm.

7. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The ratio of the average SEM length of the support node in the first direction to the average SEM length in the second direction is 3-30. The ratio of the average SEM length of the support node in the first direction to the average SEM diameter of the separation node is 5-40.

8. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, Along the second direction, the average SEM distance between adjacent support nodes is 3-25 μm; and the projections of two adjacent support nodes along the film thickness direction partially overlap, and the length of the overlapping area is not less than 10% of the film thickness.

9. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The main body also includes a transition layer located between the pretreatment layer and the separation layer; the transition layer contains transition nodes and transition fibers for forming a porous structure, and adjacent transition nodes are connected by transition fibers; the transition nodes have a square structure; the average SEM length of the transition nodes in the first direction is 1.2-10 μm; The ratio of the average SEM length of the transition node in the first direction to the average SEM length in the second direction is 1.5-5.

10. An asymmetric polytetrafluoroethylene porous membrane according to claim 9, characterized in that, The average SEM width of the transition fiber is 0.2-5 μm; the thickness of the transition layer is 2-18 μm, and the thickness of the transition layer is 1%-20% of the film thickness.

11. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The average SEM width of the strip nodes is 1-6 μm; the average SEM width of the strip fibers is 0.4-3.2 μm; and the average spacing between adjacent strip fibers is 0.5-5 μm.

12. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The average SEM particle size of the surface nodes is 1-15 μm; the average SEM width of the surface fibers is 0.1-2.5 μm.

13. The asymmetric polytetrafluoroethylene porous membrane according to claim 1, characterized in that, The porous membrane has a tensile strength of 5-30 MPa and an elongation at break of 70%-180%. At 0.03 MPa and 20°C, it takes 0.3-7 seconds for 50 ml of water to pass through a porous membrane with a diameter of 47 mm. The porous membrane has a retention efficiency of over 90% for impurity particles with a diameter of 1-15 μm. The porous membrane has a water pressure resistance of 20-160 kPa; When air flows through the porous membrane at a velocity of 5.3 cm / s, the pressure loss is 10-110 mmH2O.

14. A method for preparing an asymmetric polytetrafluoroethylene porous membrane according to any one of claims 1-13, characterized in that, Includes the following steps: A. Mixing: Mix and stir the polytetrafluoroethylene dispersion resin and lubricant until a paste is obtained; The polytetrafluoroethylene dispersion resin includes at least one polytetrafluoroethylene dispersion resin with a number average molecular weight of 1 million to 12 million; the lubricant is at least one selected from lubricating oil, palm oil, naphthenic oil, white oil, aviation kerosene, degreased kerosene, and paraffin wax. B. Blank making: The above paste is pre-pressed into a cylindrical blank; C. Extrusion: The preform obtained in step B is extruded to form a flat strip-shaped substrate; one of the outer surfaces of the strip-shaped substrate is the first outer surface, and the other outer surface is the second outer surface; D. Drying: The strip-shaped substrate is dried to allow the lubricant to evaporate; E. Longitudinal stretching: The dried strip matrix is ​​stretched longitudinally at a temperature of 180-280℃, a stretching ratio of 1-15 times, and a stretching rate of 5-25% / s to obtain the first strip matrix. F. First heat setting: The first outer surface of the first strip substrate is brought into contact with a heat carrier, and the second outer surface is brought into contact with a cold carrier to perform the first heat setting. The heat setting time is 0.5-6 minutes. The temperature of the heat carrier is 350-400℃, and the temperature of the cold carrier is 10-40℃. Transverse pre-stretching: The first strip matrix is ​​placed in an environment with a temperature of 200-290℃ for transverse pre-stretching, with a transverse stretching ratio of 2-20 times, to obtain the second strip matrix; wherein the first outer surface is close to the heat source and the second outer surface is far away from the heat source. Lateral stretching: The first strip-shaped substrate, after lateral pre-stretching, is placed in an environment with a temperature of 300-350℃ for lateral stretching, and the lateral stretching ratio is 3-15 times to obtain the second strip-shaped substrate; wherein the first outer surface is close to the heat source and the second outer surface is far away from the heat source. I. Second heat setting: The second strip substrate is placed in an environment with a temperature of 345-380℃ for a second heat setting, and the heat setting time is 3-20 minutes to obtain a polytetrafluoroethylene porous membrane.

15. The method for preparing an asymmetric polytetrafluoroethylene porous membrane according to claim 14, characterized in that, The strip-shaped substrate formed in step C is rolled again, so that the thickness of the rolled strip-shaped substrate is 20%-80% of the thickness of the strip-shaped substrate before rolling; during the second rolling, the first outer surface is close to the heat source and the second outer surface is far away from the heat source, and the temperature of the heat source is 60-80℃.

16. The method for preparing an asymmetric polytetrafluoroethylene porous membrane according to claim 14, characterized in that: In step F, the thickness of the gap between the heat carrier and the cold carrier is 50%-90% of the thickness of the first strip substrate before the first heat setting. The stretching rate during transverse pre-stretching in step G is 1% / s-20% / s; The stretching rate during the transverse stretching described in step H is 5% / s-30% / s.

17. The use of an asymmetric polytetrafluoroethylene porous membrane according to any one of claims 1-13, characterized in that: The porous membrane is used for filtering stripping and etching solutions in semiconductor manufacturing processes, and as a waterproof and breathable membrane in electronic and medical devices.

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