A ptfc porous membrane with a low degree of orientation pore structure and a preparation process thereof

Abstract

The application relates to a PTFE porous membrane with a low-orientation pore structure and a preparation process thereof, the porous membrane comprising a membrane body, the surface of the membrane body being provided with surface fibers and block-shaped surface nodes, strip-shaped holes being formed between the adjacent surface fibers and being distributed in a radial manner, and the fiber structures being cross-laminated in the thickness direction of the membrane body to form connected flow paths; the thickness of the porous body is 3-40 mu m; reinforced fibers and distributed fibers, the reinforced fibers accounting for 10-40% of all the surface fibers; the air permeation rate of the air permeation membrane is not less than 2*10 4 The porous membrane in the application can have a high air permeation rate due to the strip-shaped pore structure distributed in a radial manner, the block-shaped surface nodes and the reinforced fibers; the high tortuosity caused by the radial pore structure and the three-dimensional network structure formed by the thick and thin fibers ensure that the porous membrane has high air permeation rate and waterproof effect.

Description

Technical Field

[0001] This application relates to the field of microporous membranes, and in particular to a PTFE porous membrane with a low orientation pore structure and its preparation process. Background Technology

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

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

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

[0005] Currently, a major application area for PTFE microporous membranes is as waterproof porous membranes for outdoor equipment. This requires the PTFE membrane to have high air permeability and good waterproof performance. For example, in the use of battery packs in new energy vehicles, communication towers, and lighting fixtures, the internal electronic components generate a large amount of heat, causing a rapid increase in internal air pressure. To prevent this high pressure from affecting the operation of electronic components and creating unnecessary safety risks, a porous membrane is needed to quickly expel the high-pressure gas. Similarly, in acoustic devices such as headphones and headsets, the rapid and large-amplitude vibration of the internal diaphragm causes rapid and drastic changes in internal air pressure. If the PTFE membrane's air permeability is insufficient, the changing internal air pressure will restrict the diaphragm's movement, affecting the audio curve and causing audio distortion. This is unacceptable for acoustic devices requiring low distortion. Building upon the high breathability of porous membranes, applications such as battery packs, communication towers, lighting fixtures, and other outdoor-use equipment, as well as acoustic devices like headphones and headsets requiring outdoor use, all demand higher waterproofing capabilities from the porous membranes to prevent external liquids from penetrating the membranes and damaging the equipment. However, obtaining a PTFE membrane that combines high waterproofing with high breathability is not easy.

[0006] For example, the patent application CN112717729A submitted by Hangzhou Takizawa Filter Material Co., Ltd. discloses a PTFE porous membrane and its preparation method and uses. The membrane cross-section parallel to the membrane thickness direction contains primary nodes, which are granular structures. Several primary nodes are stacked to form nodal nodes, and adjacent nodes are connected by a first fiber. Primary nodes on the same nodal node are connected by a second fiber. On the outer surface of the membrane, the nodes extend in the same direction, the average width of the nodes is 1.3-4.3 μm, and the length of the nodes is at least 50 μm.

[0007] The PTFE porous membrane disclosed in the above patent has a relatively obvious fibril-long node structure. Its surface is composed of long nodes that are roughly parallel and at least 50 μm in length, as well as fibers that connect adjacent long nodes. The pore structure between adjacent fibers is the pore structure of the surface, and this pore structure exhibits a high degree of orientation.

[0008] The company's patent application, CN112717728A, also describes a PTFE macroporous membrane with long nodes and fibrils on its surface, whose surface pore structure also exhibits a high degree of orientation. This highly oriented PTFE porous membrane has a high flow rate, fast filtration speed, and good air permeability. This is because the highly oriented pore structure results in a high degree of overlap of the pore structure in the thickness direction. When the liquid flows through these continuously stacked but highly overlapping pores in the thickness direction, the actual flow path length is relatively close to the membrane thickness, thus resulting in both high water permeability and often high air permeability. Therefore, the excessively high water permeability of highly oriented macroporous PTFE membranes makes them unsuitable for breathable applications requiring waterproofing. If external liquids penetrate and enter the equipment, it could potentially damage the equipment.

[0009] To apply the aforementioned PTFE membrane to applications requiring both waterproofing and breathability, its waterproofing capability needs to be improved to ensure a low risk of water leakage. Common practices include increasing the density and thickness of the PTFE membrane. Utilizing the small pore size of the PTFE membrane and its inherent hydrophobic properties, this enhances its waterproofing performance. However, increasing the density and thickness of the PTFE membrane predictably leads to a decrease in its breathability, which is unacceptable for applications demanding high breathability.

[0010] Clearly, obtaining a PTFE membrane that combines high air permeability and high waterproof performance is a problem that urgently needs to be solved but is difficult to solve. Summary of the Invention

[0011] This application provides a PTFE porous membrane with a low-orientation pore structure and its preparation process. The PTFE membrane of this application has radially distributed, elongated pores on its surface. By reducing the air resistance of the pore structure along its long axis, it has a large air permeability. The stacking of the pore structure in the thickness direction brings high waterproof capability. The surface fibers include reinforcing fibers with a large diameter accounting for 10-40%, and distribution fibers with a smaller diameter distributed among the reinforcing fibers. The combination of coarse and fine fibers forms a hydrophobic network structure with high water pressure resistance, which can further improve the waterproof performance of the porous membrane. The block-shaped surface nodes have less air resistance than the long node structure and do not hinder the gas from automatically adjusting to the optimal flow path in the membrane, thus ensuring that the porous membrane has a large air permeability.

[0012] In a first aspect, this application provides a PTFE porous membrane with a low orientation degree pore structure, employing the following technical solution:

[0013] A PTFE porous membrane with a low orientation degree pore structure includes a membrane body, the membrane body including a first outer surface and a second outer surface, both the first outer surface and the second outer surface including surface fibers and block-shaped surface nodes, the surface fibers being connected to the surface nodes or to adjacent surface fibers, and the gaps between adjacent surface fibers forming elongated pores;

[0014] The pores are radially distributed around the surface nodes, and the fiber structures are intersected and stacked in the thickness direction of the membrane body to form interconnected flow paths;

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

[0016] The average diameter of the surface fibers is Z, and the surface fibers have reinforcing fibers with a diameter not less than 1.2 times that of Z and distribution fibers with a diameter not greater than 0.6 times that of Z. The reinforcing fibers account for 10 to 40% of all surface fibers.

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

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

[0019] Optionally, the air permeability of the porous membrane is not less than 5 × 10⁻⁶. 4 ml / min / cm 2 @7kPa; Further optionally, the air permeability of the porous membrane is not less than 8×10 4ml / min / cm 2 @7kPa; the air permeability of the porous membrane is not less than 10×10 4 ml / min / cm 2 @7kpa.

[0020] By adopting the above technical solutions, for applications requiring outdoor waterproofing and rapid pressure balance, such as battery packs for new energy vehicles, base stations, lighting fixtures, and low-distortion acoustic equipment, PTFE porous membranes must possess high air permeability to ensure rapid pressure balance. In addition, the PTFE porous membrane also needs good waterproof performance to prevent external liquid water from entering the equipment and damaging electronic components in outdoor applications. However, high air permeability and high waterproof performance are difficult to achieve simultaneously. This is because, to obtain high air permeability, whether reducing membrane thickness or increasing membrane porosity, a decrease in waterproof performance is expected, and vice versa.

[0021] For example, commonly used long-node-fiber structure PTFE membranes often struggle to achieve both high air permeability and high waterproof performance. For instance, the long-node-fiber structure PTFE membrane disclosed in application publication number CN112717728A has a pore structure with a significantly high degree of orientation. Furthermore, because the PTFE porous membrane is fabricated using a stretching method, its thickness variation is small, resulting in a basically symmetrical structure. Therefore, the highly oriented pore structure exists not only on the surface but also within the membrane. That is, in the thickness direction of the membrane body, the major axis of the pore structure extends in approximately the same direction. This means that the pore structure has a large degree of overlap in the thickness direction. To ensure high air permeability, the membrane body needs to have a large-sized pore structure and / or a small membrane thickness, both of which lead to a reduction in the waterproof capability of the porous membrane.

[0022] Based on the above understanding, the inventors of this application unexpectedly discovered that, contrary to the commonly held belief that membranes with large thicknesses (such as greater than 3 μm, or even greater than 5 μm or 10 μm) should not be used to obtain high air permeability, the air permeability of PTFE porous membranes does not necessarily increase with increasing membrane thickness. In particular, when the thickness of the membrane body is 3–40 μm and the surface has radially distributed elongated pores, and the membrane surface forms a highly intact three-dimensional network structure with strong self-supporting ability due to the reinforcement and distribution fibers and the blocky surface nodes, the PTFE porous membrane can still achieve an air permeability of not less than 2 × 10⁻⁶. 4 ml / min / cm 2 With a high air permeability of 7 kPa, the porous membrane of this application also has good waterproof effect, thus enabling the porous membrane of this application to have both high waterproofness and high air permeability, which are generally considered difficult to achieve simultaneously.

[0023] When gas flows inside a porous membrane, it is mainly resisted by the surface of the solid portion of the membrane (such as fiber structures and node structures). Therefore, it is generally believed that the higher the porosity of the membrane and the less solid portion it has, the lower the resistance to gas flow; and the smaller the membrane thickness, the lower the resistance to gas flow from the solid portion. Thus, porous membranes with high porosity and low thickness naturally have higher gas permeability. However, in this application, by introducing elongated pore structures combined with blocky surface node structures (rather than elongated node structures), the porous membrane can still have a permeability of not less than 2 × 10⁻⁶ μm even with a membrane thickness of 3–40 μm. 4 ml / min / cm 2 High breathability of 7kPa.

[0024] This may be because the elongated holes in this application mean that there is no solid structure that forms gas flow resistance in the long axis direction of the hole structure. Although the gas is still inevitably subject to the resistance of the fiber structure on both sides and the node structure, the low resistance in the long axis direction of the elongated holes makes the gas subject to significantly lower gas resistance.

[0025] More importantly, nodes are also a crucial component of the membrane's solid structure and a significant source of gas flow resistance. The node structure in this application is not a conventional long node structure, but rather a blocky node structure. This blocky structure means that some long nodes are transformed into fiber structures. The porous structure formed between these fibers obviously has lower gas resistance compared to a purely solid long node structure. Fewer solid structures also often mean relatively higher porosity, ensuring low gas resistance. Furthermore, long node structures often extend beyond the surface into the membrane (as clearly shown in the figures of application publication number CN112717728A), thus dividing the membrane structure into multiple relatively independent regions. When gas flows within the membrane, it is difficult to flow between these independent regions, significantly restricting the gas flow path and making it difficult to optimize the gas flow path automatically.

[0026] Compared to long-node structures, the block-shaped nodes in this application, while also extending into the membrane, do not separate the various regions within the membrane. Therefore, when gas flows within the membrane, it is only subject to resistance at the node surfaces, not excessive flow restriction by the nodes. This significantly reduces the mutual flow resistance between different regions within the membrane body and greatly reduces the limitation on the gas flow path. Gas flow through the membrane body tends to form a low-resistance flow path, further increasing permeability. Therefore, the combination of the long-strip pore structure and the block-shaped node structure not only inherently possesses low gas resistance but also optimizes the gas flow path, enabling the porous membrane of this application to maintain a thickness of 3–40 μm while still possessing a permeability of at least 2 × 10⁻⁶. 4 ml / min / cm 2 High breathability of 7kPa.

[0027] While ensuring that the porous membrane has high air permeability, the surface of the porous membrane in this application can be observed to have a distinct radial pore distribution. Radial means that the long axis of the pores points to various angles (low orientation degree) rather than roughly pointing to the same direction (high orientation degree).

[0028] The radial pores indicate that the fiber structure in each plane along the thickness direction is also arranged with a low orientation. Under pressure, the fibers in each layer support each other, and combined with the surface node structure that provides local strength reinforcement, this significantly reduces the possibility of excessive deformation and water leakage under water pressure. Furthermore, in the three-dimensional network structure formed by the coarse and fine fiber structures and blocky nodes, the pore structure between adjacent coarse fibers creates channels for gas flow. The fine fiber structures located between the coarse fibers, due to their smaller size, offer less resistance to gas flow. However, for a porous membrane with an inherently elongated pore structure, the fine fiber structures between the coarse fibers, through their hydrophobicity, greatly enhance the repulsion force against liquid water. Therefore, the combination of coarse and fine fibers creates both size exclusion and hydrophobic exclusion effects against liquid water, while only creating a size exclusion effect against gas. In this case, gas can relatively easily pass through the pore structure between the fibers, while liquid water cannot easily pass through the three-dimensional network structure with its relatively complete hydrophobic repulsion force. In addition, the overlapping of radial pores in the thickness direction means that the overlap of the pore structure in the thickness direction is low, resulting in a relatively higher degree of tortuosity. This increase in tortuosity has little impact on gas flow under the action of block nodes, while the larger size of the barrier and the more complete hydrophobic three-dimensional network structure mean a significantly improved waterproof capability.

[0029] It is important to note that the radial pores mean that the fiber structures forming the pores are also interwoven and mutually supportive in the thickness direction. Combined with the specific blocky surface nodes for local reinforcement and the strengthening of the three-dimensional network structure by reinforcing fibers comprising 10-40%, the porous membrane of this application still possesses high mechanical strength even without large-sized, highly reinforcing long node structures. High mechanical strength means that the porous membrane is less prone to pore structure collapse under high air pressure. Therefore, even though the PTFE membrane with radial pore structure in this application has a longer actual flow path, it still exhibits lower air resistance and higher permeability.

[0030] For PTFE membranes with a common long-fiber-primary-node structure, the highly oriented pore structure results in a high degree of overlap of the pores in the thickness direction. When the liquid flows through these continuously stacked but highly overlapping pores in the thickness direction, the actual flow path length is very close to the membrane thickness. Therefore, while the membrane has high air permeability, it also tends to have high water permeability, leading to poor waterproofing. Consequently, the excessively high water permeability of PTFE membranes with highly oriented pore structures makes them unsuitable for breathable applications requiring waterproofing. If external liquids penetrate and enter the equipment, it can easily damage the equipment.

[0031] In summary, the PTFE membrane in this application, due to its radially distributed elongated pore structure, combined with blocky surface nodes and reinforcing fibers, enables the porous membrane to have a gas flow path with lower air resistance and higher resistance to pore collapse, thus exhibiting high air permeability. Furthermore, the high tortuosity caused by the low overlap of the radially distributed elongated pores in the thickness direction, and the hydrophobic but not air-repellent three-dimensional network structure formed by the combination of coarse and fine fibers, ensure that the porous membrane still has good waterproof performance even with high air permeability.

[0032] Understandably, the air permeability should be no less than 2×10. 4 ml / min / cm 2 @7kPa means that under a gas pressure of 7kPa, the amount of gas that can pass through each square centimeter of PTFE porous membrane per minute is not less than 2×10⁻⁶. 4 ml. The testing method is as follows: [The text abruptly shifts to a different topic] ...with an effective air permeability area of ​​Scm... 2 A membrane sample was adhered to the surface of a stainless steel fixture with an opening (1.0 mm diameter). The other end of the fixture was connected to a pressure sensor, a gas flow sensor, a gas pressure regulating valve, and a gas source. The gas pressure regulating valve was adjusted to bring the pressure sensor reading to 7 kPa, and the gas flow sensor reading was taken as Q mL / min. Therefore, the air permeability rate of the waterproof porous membrane module is Q / S ml / min / cm². 2 @7kPa.

[0033] The various surface morphology parameters of porous membranes (such as thickness, fiber diameter, pore size, and pore area ratio) can be measured by characterizing the membrane structure using a scanning electron microscope (SEM), followed by measurement and calculation using computer software (such as Matlab, NIS-Elements, etc.) or manually. In actual measurement, the membrane surface (or cross-section) can be characterized first using an electron microscope to obtain the corresponding SEM image, and a certain area, such as 10000 μm, can be selected. 2(100μm x 100μm), the specific area size depends on the actual situation. Then, use appropriate computer software or manual measurement to determine the morphological parameters such as the pore diameter and fiber diameter of all holes in the area, and then calculate to obtain the average pore diameter, average fiber diameter and other morphological parameters of the region. Of course, those skilled in the art can also obtain the above parameters through other measurement methods. The above measurement methods are for reference only.

[0034] Optionally, the relative deviation between the width tensile strength and the membrane tensile strength of the membrane body is not greater than 60%; the soap water permeation time of the PTFE porous membrane is not less than 10 minutes.

[0035] By adopting the above technical solution, the commonly used long-node-fiber structure PTFE membrane, due to the high orientation of its fibers and pore structure, also exhibits a significant orientation in its mechanical properties, with a large difference between its tensile strength in the width direction and its tensile strength in the membrane direction. This application, however, reduces the orientation of the pores and fibers in the porous membrane by controlling the pore structure to be radially distributed. Furthermore, when the relative deviation between the width tensile strength and the membrane tensile strength is no greater than 60%, the waterproof effect of the PTFE porous membrane is further improved, ensuring high air permeability while maintaining a soap water penetration time of no less than 10 minutes.

[0036] This may be because when the relative deviation between the width tensile strength and the membrane tensile strength of the porous membrane is no more than 60%, it indicates that the orientation of the pore structure and fiber structure of the porous membrane is low, and it has a better cross-support effect in the thickness direction. This ensures that even if the porous membrane has a high porosity, it still has good anti-pore collapse performance with the help of the block node structure and reinforcing fibers. As a result, the porous membrane of this application still has very good air permeability under high gas pressure, so as to ensure the high air permeability of the porous membrane.

[0037] If the relative deviation between the width tensile strength and the membrane tensile strength of a porous membrane is greater than 60%, it indicates that the fiber and pore structures have a high degree of orientation, and the cross-support effect of the fiber structure in the thickness direction is poor. Even with local reinforcement by block nodes and structural reinforcement by reinforcing fibers, the three-dimensional network structure may still experience some pore collapse under high air pressure, leading to a decrease in air permeability. Furthermore, a high degree of pore orientation also means a decrease in the tortuosity of the internal flow channels of the porous membrane, resulting in more water channels in the formed hydrophobic three-dimensional network structure, making it more prone to water leakage.

[0038] It is understood that relative deviation refers to the ratio of absolute deviation to average value. In this application, the relative deviation between width tensile strength and membrane tensile strength refers to the calculation of the average value of the two values ​​by testing the tensile strength in the width direction and the tensile strength in the membrane direction (membrane length direction) of the porous membrane separately using a universal tensile tester, and further calculating the relative deviation between the width tensile strength, the membrane tensile strength and the average value of the two values.

[0039] A soap water penetration time of at least 10 minutes means that when a PTFE porous membrane is attached to a fixture, immersed in soap water with a concentration of 0.1 g / L, and shaken in a roller for at least 10 minutes, no water enters the fixture. Soap water wets and penetrates porous membranes more easily than ordinary deionized water; therefore, the soap water penetration time can be used to characterize the waterproof performance of porous membranes.

[0040] Optionally, the distribution fibers are dispersed among the reinforcing fibers, and the distribution fibers account for 10-60% of all surface fibers.

[0041] By adopting the above technical solution, the reinforcing fibers in this application, through their strong supporting capacity and the mutual cross-support of fibers in the thickness direction, can reduce the collapse of pores in the porous membrane under high air pressure, ensuring high air permeability. However, to ensure high air permeability of the porous membrane, the pore size of the porous membrane should not be too small. Therefore, the hydrophobic three-dimensional network structure formed by the reinforcing fibers alone often has high porosity and many hydrophobic weak points, making it difficult to ensure good waterproof performance. In contrast, the smaller diameter distributed fiber structure is dispersed among the reinforcing fibers and together with the reinforcing fibers forms a hydrophobic three-dimensional network structure with higher hydrophobic integrity, thereby ensuring high waterproof performance. Furthermore, although the three-dimensional hydrophobic network formed by the reinforcing fibers and distributed fibers has fewer hydrophobic weak points, because the fiber diameter of the distributed fibers is small, the hydrophobicity does not have a barrier effect on gas. Therefore, although the three-dimensional hydrophobic network significantly improves the waterproof performance, the increase in gas resistance is relatively small.

[0042] If the proportion of distributed fibers is low (e.g., below 10%), it indicates the presence of numerous medium-sized fiber structures (diameter smaller than reinforcing fibers but larger than distributed fibers) in the surface fibers. While these medium-sized fiber structures possess good mechanical strength and hydrophobic effects, their larger size often translates to greater gas resistance, thus affecting the air permeability of the porous membrane. Conversely, if the proportion of distributed fibers is high (e.g., below 60%), it indicates the presence of numerous small-sized distributed fibers in the surface fibers. The excessively small size of these distributed fibers means that while the resulting three-dimensional network structure is relatively dense, its mechanical strength is poor. Under significant gas pressure, the collapse of the pore structure due to pressure will lead to a decrease in air permeability, and damage to the pore structure in some areas can easily result in water seepage. Therefore, by further controlling the proportion of distributed fibers, the porous membrane can achieve even better air permeability and waterproof performance.

[0043] Optionally, the aspect ratio of the reinforcing fiber is 60 to 150, and the aspect ratio of the distribution fiber is 450 to 650.

[0044] By adopting the above technical solution, the reinforcing fiber has a significant impact on the pressure resistance of the three-dimensional network structure of the porous membrane. With the reinforcing fiber accounting for 10-40% and having block node reinforcement, further controlling the aspect ratio of the reinforcing fiber to 60-150 can ensure that the porous membrane has high air permeability and high pressure resistance.

[0045] If the aspect ratio of the reinforcing fibers exceeds 150, although a longer aspect ratio often means larger long-axis dimensions of the pores formed by the reinforcing fibers and smaller fiber diameters, and although porous membranes have lower gas flow resistance, excessively large long-axis dimensions of the pores mean poor self-supporting performance of the resulting three-dimensional network structure. Even with mutual support between fibers in the thickness direction and reinforcement by blocky nodes, it is still difficult to ensure good pressure resistance of the porous membrane. Furthermore, even if the large long-axis pore structure achieves low overlap in thickness through radial pore distribution, the tortuosity of the flow channels may still be low, leading to a decrease in the waterproofing effect of the porous membrane. The collapse of the pore structure under pressure will affect its air permeability, and localized pore structure damage will also increase the possibility of water leakage.

[0046] If the aspect ratio of the reinforcing fiber is less than 60, it often means that the long axis dimension of the pores formed by the reinforcing fiber is small, while the fiber diameter is large. Although this can ensure that the porous membrane has good pressure resistance and waterproof performance, the small long axis dimension of the pores and the large fiber size, combined with the low overlap of the fiber and pore structure in the thickness direction caused by the radial distribution of the pores, will lead to excessively high gas resistance of the porous membrane, thus resulting in low air permeability of the porous membrane.

[0047] Distributed fibers between reinforcing fibers can form a more complete hydrophobic three-dimensional network together with the reinforcing fibers and other surface fibers, while only slightly increasing gas resistance, thereby improving the waterproofness of the porous membrane while ensuring high air permeability. However, based on the aspect ratio of the reinforcing fibers being 60-150, if the aspect ratio of the distributed fibers exceeds 650, it indicates that the diameter of the distributed fibers is too fine. Although this results in lower gas resistance and higher air permeability, for the already small-diameter distributed fibers, even further reduction in diameter will have limited improvement in air permeability. Furthermore, excessively fine distributed fibers lack sufficient hydrophobicity, making it difficult to ensure the hydrophobic integrity of the formed three-dimensional hydrophobic network, allowing water to easily leak through weak points. Conversely, if the aspect ratio of the distributed fibers is below 450, it indicates that the diameter of the distributed fibers dispersed between the reinforcing fibers is too large. The larger-diameter distributed fibers, together with the larger-sized other surface fibers, form a denser three-dimensional network structure. Furthermore, due to the specific radially distributed pore structure in this application, the pore structures are stacked in the thickness direction, leading to excessively high gas resistance in the porous membrane.

[0048] Optionally, the distribution density of the surface nodes is 10–30 per 10,000 μm. 2 The surface nodes have an area of ​​not less than 150 μm. 2 Node aggregates with an area not exceeding 50 μm 2 The number of the node clusters is less than the number of the scattered nodes.

[0049] By adopting the above technical solution, while the long-node-fiber structure with a significant long diameter, which is currently more common, provides good reinforcement for the three-dimensional network structure of porous membranes, the large solid portion poses a high resistance to gas flow, and the restriction of gas flow paths caused by dividing the membrane structure into multiple regions leads to a decrease in the gas flux of the porous membrane. In contrast, the porous membrane of this application has a special node aggregate and dispersed node structure. Although both can produce a reinforcing effect on the three-dimensional network structure, the size is larger, with an area of ​​not less than 150 μm. 2 The node aggregates obviously have a greater advantage compared to those with an area not exceeding 50 μm. 2 While dispersed nodes offer stronger reinforcement, node aggregates, while providing better reinforcement, often result in greater gas resistance. Therefore, this application controls the number of node aggregates to be less than the number of dispersed nodes. By combining a small number of large, highly reinforcing node aggregates with a large number of small, less resistive dispersed nodes, the three-dimensional network structure is reinforced. This, along with the intersecting fiber structure in the thickness direction, ensures that the porous membrane is less prone to pore structure collapse and damage under high pressure.

[0050] In this application, the distribution density of surface nodes is controlled to be 10–30 per 10,000 μm. 2 Ensure that the node density in the three-dimensional network structure of the porous membrane is not too low (not less than 10 nodes / 10000μm). 2 By combining the local reinforcement effect of the node structure with the mutual intersection and support of the fiber structure in the thickness direction, the porous membrane is ensured to be less prone to collapse and damage of the pore structure under high gas pressure, thus reducing the possibility of decreased air permeability and water seepage. Of course, the distribution density of the node structure in the three-dimensional network structure of the porous membrane should not be too high (not exceeding 30 per 10000 μm). 2 To avoid excessive gas resistance caused by large-sized node structures that are part of the solid structure, the distribution density of surface nodes is controlled, and the proportion of large node aggregates is kept small. Combined with a radially distributed fiber structure, the porous membrane can achieve high pressure resistance while still maintaining high air permeability.

[0051] Optionally, the distribution density of the node aggregates is 0.2 to 2 per 10000 μm. 2 The distribution density of the dispersed nodes is 5–25 per 10,000 μm. 2 .

[0052] By adopting the above technical solution, the area is controlled to be no less than 150μm. 2 The distribution density of node aggregates is not less than 0.2 per 10000 μm. 2 This ensures that the node clusters provide good reinforcement to the regional three-dimensional network structure; while controlling the distribution density of the node clusters to no more than 2 per 10000 μm 2 This ensures that large-volume nodal aggregates do not have an excessive impact on gas drag. The control area is no larger than 50 μm. 2 The distribution density of the scattered nodes is not less than 5 per 10000 μm 2 This ensures that distribution nodes, which already have relatively poor reinforcement effects, can achieve better reinforcement through a relatively large number of nodes; while the control area is no more than 50μm. 2 The distribution density of the scattered nodes is no greater than 25 per 10000 μm. 2 It can reduce the possibility of excessive gas resistance caused by dispersed nodes that are larger than the fiber structure, while ensuring good reinforcement effect.

[0053] In summary, the number of node aggregates with good reinforcement effect but high air resistance should be controlled to a small amount, while the number of dispersed nodes with slightly poorer reinforcement effect but low air resistance should be controlled to a relatively large amount. The combination of the two forms a good reinforcement effect, and the synergistic radial distribution of the fiber structure provides mutual support in the thickness direction, which can ensure that the porous membrane has good pressure resistance and high air permeability.

[0054] Optionally, the surface fibers include longitudinal fibers and transverse fibers, wherein the angle between the longitudinal fibers and the longitudinal stretching direction is no greater than 55°, and the angle between the transverse fibers and the transverse stretching direction is less than 35°, and the average SEM measurement length of the longitudinal fibers is greater than the average SEM measurement length of the transverse fibers; the air permeability of the porous membrane is no greater than 25 × 10⁻⁶. 4 ml / min / cm 2 @7kpa.

[0055] By adopting the above technical solution, since the porous membrane in this application has pores distributed radially in a fibrous manner, and the pore structure is formed by the surrounding fiber structure, the fiber structure, similar to the pore structure, is also distributed radially in a fibrous manner. The radially distributed fiber structure can be divided into transverse fibers and longitudinal fibers. In the thickness direction, the transverse and longitudinal fibers intersect and support each other, thereby improving the pressure resistance of the porous membrane. In addition, longitudinal fibers refer to surface fibers with an angle of no more than 55° to the longitudinal stretching direction, which refers to the direction of membrane flow during production (membrane length direction); while transverse fibers refer to surface fibers with an angle of less than 35° to the transverse stretching direction, which refers to the direction perpendicular to the membrane flow direction during production (membrane width direction).

[0056] In this application, by controlling the length of the longitudinal fibers to be greater than the length of the transverse fibers, the longer longitudinal fibers can form a pore structure with a larger long-diameter, ensuring high air permeability. While the shorter transverse fibers form a pore structure with a shorter long-diameter, they can provide good support for the longitudinal fibers in the thickness direction, thereby improving the pressure resistance of the pore structure formed by the longitudinal fibers, which have weaker self-supporting capacity due to their larger long-diameter. Furthermore, although the pore structure formed by the transverse fibers has a shorter long-diameter, it still has significantly greater air permeability compared to the currently common long-node structures (pure solid structures). Therefore, the transverse fibers not only have the effect of forming pores to increase air permeability but also have the effect of forming longitudinal fibers with poor self-supporting performance to improve the pressure resistance of the porous membrane, thus enabling the porous membrane to possess both high air permeability and high pressure resistance.

[0057] Optionally, the average spacing between adjacent longitudinal fibers is 2.5 to 7.5 μm; the average spacing between adjacent transverse fibers is 0.5 to 3 μm.

[0058] By adopting the above technical solution, the spacing between adjacent longitudinal fibers reflects the short axis length of the hole structure formed by the longitudinal fibers to a certain extent, and the spacing between transverse fibers reflects the short axis length of the hole structure formed by the transverse fibers to a certain extent. The short axis length of the hole structure has an important impact on the gas resistance and waterproof performance of the hole structure.

[0059] When the average spacing between longitudinal fibers is less than 2.5 μm and / or the average spacing between transverse fibers is less than 0.5 μm, it indicates that the spacing between transverse and longitudinal fibers is small. Adjacent transverse and longitudinal fibers form a slender pore structure with high density. Although it can form a more complete hydrophobic three-dimensional network structure and thus have a better waterproof effect, the excessive density also means greater gas resistance, resulting in low air permeability. When the average spacing between longitudinal fibers is greater than 7.5 μm and / or the average spacing between transverse fibers is greater than 3 μm, it indicates that the spacing between the longitudinal and transverse fibers is relatively large. Although the long axis of the resulting pore structure is large, the short axis is also large. For the pore structure formed by longitudinal fibers, this means lower air resistance and greater air permeability. However, the integrity of the hydrophobic network formed is poor. Although water is still affected by the hydrophobic forces of the fibers and nodes on both sides of the pore, water seepage is still likely to occur under certain water pressure. For the pore structure formed by transverse fibers, this means that the transverse fibers themselves have poor support capacity and are difficult to form good support for the adjacent longitudinal fibers in the thickness direction. This leads to a decrease in the pressure resistance of the porous membrane and a greater possibility of pore collapse and damage under greater pressure.

[0060] Optionally, the ratio of the number of longitudinal fibers to the number of transverse fibers is 1.5 to 6; the ratio of the average length of the longitudinal fibers measured by SEM to the average length of the transverse fibers measured by SEM is 1.5 to 4.5.

[0061] By adopting the above technical solution, if the ratio of longitudinal fibers to transverse fibers is greater than 6, it indicates that the proportion of longitudinal fibers in the surface fibers is relatively high. Since longitudinal fibers have a certain orientation, a higher proportion of longitudinal fibers means a higher degree of orientation of the surface fibers. This results in insufficient tortuosity of the flow path, making it easy for water to seep in. Furthermore, the insufficient number of transverse fibers makes it difficult to provide good support for the relatively poor self-supporting properties of the longitudinal fibers, causing the porous membrane to be easily compressed, leading to pore collapse or damage, resulting in a decrease in air permeability and water seepage. If the ratio of longitudinal fibers to transverse fibers is less than 1.5, it indicates that the proportion of longer longitudinal fibers in the surface fibers is not high, while the proportion of shorter transverse fibers is not low. Although the higher proportion of transverse fibers can provide good support for the longitudinal fibers, the pore structure formed by the transverse fibers has a shorter long axis and poorer air permeability. Although the pore structure formed by the longitudinal fibers has a larger long axis and thus greater air permeability, the lower proportion of longitudinal fibers makes it difficult to ensure that the porous membrane has a high air permeability.

[0062] Based on a longitudinal to transverse fiber ratio of 1.5–6, the length ratio of the two fibers is further controlled to 1.5–4.5. On the one hand, the longitudinal fibers have sufficient length to ensure the air permeability of the porous membrane; on the other hand, while supplementing the air permeability, the transverse fibers are ensured not to be too long and affect their support for the longitudinal fibers. When the length ratio is 1.5–4.5, the longer longitudinal fibers form a porous structure with higher air permeability, while the shorter transverse fibers form a porous structure with a certain amount of supplementary air permeability, and their self-supporting properties are strong, which can well support the longitudinal fibers with weaker self-supporting properties.

[0063] Optionally, the gap between adjacent longitudinal fibers is a longitudinal hole, and the gap between adjacent transverse fibers is a transverse hole. The SEM measured major diameter of the longitudinal hole is larger than that of the transverse hole. The SEM measured major diameter of the longitudinal hole is 60-130 μm, and the average major diameter of the transverse hole is 25-65 μm.

[0064] By adopting the above technical solution, the pores formed by the longitudinal fibers are called longitudinal pores, and the pores formed by the transverse fibers are called transverse pores, with the major axis of the longitudinal pores being larger than that of the transverse pores. The longitudinal pores, with a major axis of 60–130 μm, ensure good air permeability of the porous membrane, while the transverse pores, with a major axis of 25–65 μm, not only further compensate for the lack of air permeability, ensuring good air permeability, but also provide support for the longitudinal fibers, which have poorer self-supporting properties, through their stronger self-supporting ability. This ensures that even with high air permeability, the porous membrane still maintains good pressure resistance, thereby reducing the possibility of water leakage due to pore structure damage when subjected to high water pressure.

[0065] Optionally, the water contact angle of both the first and second outer surfaces is 110–150°, and the areal density of the membrane substrate is 1.2–7.5 g / m³. 2 .

[0066] By adopting the above technical solution, the water contact angle reflects the water resistance performance of the porous membrane's hydrophobic three-dimensional network structure to a certain extent. In this application, a highly complete hydrophobic network structure is formed by combining a radially distributed pore structure with reinforcing fibers, distribution fibers, and blocky node structures, resulting in a hydrophobic but not air-repellent porous membrane structure. A water contact angle of not less than 110° on the porous membrane surface ensures good waterproofing, while controlling the water contact angle to not exceed 150° avoids the problem of reduced air permeability caused by an overly dense hydrophobic three-dimensional network structure.

[0067] The areal density of the membrane substrate reflects, to some extent, the compactness of the hydrophobic three-dimensional network structure of the porous membrane. Higher compactness results in better waterproofing but lower air permeability, and vice versa. This application controls the membrane substrate to achieve both good air permeability and waterproofing performance through special fiber and node structures.

[0068] Secondly, this application provides a process for preparing a PTFE porous membrane with a low orientation degree pore structure, using the following technical solution:

[0069] A process for preparing a PTFE porous membrane with a low orientation degree pore structure includes the following steps:

[0070] S1. Mixing and swelling: The film-forming resin and auxiliary oil are mixed evenly and swelled at a temperature of 30-60°C for 8-24 hours to obtain a paste-like mixture. The film-forming resin is obtained by mixing low-crystallinity PTFE resin with a crystallinity of 90-93% and high-crystallinity PTFE resin with a crystallinity of not less than 94% in a mass ratio of 1:1 to 1:4.

[0071] S2. Pre-film formation: The paste mixture is extruded to form an oil-containing base tape with a thickness of 0.15–0.45 mm;

[0072] S3. Degreasing: Heat the oil-containing base strip to remove the auxiliary oil and obtain an oil-free base strip.

[0073] S4. Longitudinal stretching: The oil-free base tape is placed in an environment of 200-300℃ for longitudinal stretching, with a stretching ratio of 8-20 times, to obtain a uniaxial stretched film.

[0074] S5. Heat treatment: The uniaxially stretched film is heat-treated at a temperature of 300-360℃ for 0.5-2 minutes to obtain the unwound film.

[0075] S6. Lateral stretching: The unwound film is stretched laterally to obtain a biaxially stretched film. The lateral stretching ratio is 1.2 to 2 times that of the longitudinal stretching ratio, and the stretching temperature is 20 to 60°C lower than the longitudinal stretching temperature.

[0076] S7. Heat setting: Place the biaxially oriented film at a temperature of 330-390℃ for 1-10 minutes to heat set.

[0077] By adopting the above technical solutions, it is generally believed that film-forming resins with low crystallinity should not be used to ensure film-forming performance and membrane strength. This is because film-forming resins with low crystallinity often have poor mechanical strength, making it difficult to ensure the mechanical strength of the resulting porous membrane. However, the inventors of this application unexpectedly discovered that, compared to using a single high-crystallinity PTFE resin as the film-forming resin, using a mixture of high- and low-crystallinity PTFE resins as the film-forming resin, and controlling the crystallinity of the low-crystallinity PTFE resin to be 90-93% and the crystallinity of the high-crystallinity PTFE resin to be no less than 94%, with a mass ratio of 1:1 to 1:4, combined with a special subsequent biaxial stretching process, can produce a porous membrane with high strength, as well as high air permeability and good waterproof effect.

[0078] This may be because, compared to highly crystalline PTFE resin, low-crystalline PTFE resin has poorer fiber-forming properties and is more prone to forming blocky nodes and coarser reinforcing fibers under tensile stress. For the porous membrane with its unique radial pores and fiber distribution, the reinforcing effect of the reinforcing fibers and blocky nodes is superior to the effect of the increased fiber strength brought by highly crystalline PTFE resin on the strength of the porous membrane. By controlling the crystallinity of the low-crystalline PTFE resin to be no too low (e.g., below 90%) and its proportion to be no too high (no more than 50 wt%), it can be ensured that even the low-crystalline PTFE resin still has a certain degree of fiber-forming ability, thus preventing the formation of excessive or coarse fiber structures and blocky nodes. Therefore, the proportion of reinforcing fibers and blocky node structures is not too high, ensuring that the resulting porous membrane has both waterproof and mechanical properties while maintaining high air permeability. Furthermore, by controlling the crystallinity of the low-crystallinity PTFE resin to be neither too high (e.g., above 93%) nor too low (e.g., below 25wt%), the low-crystallinity PTFE resin, while possessing a certain degree of fiber-forming ability, does not exhibit excessive fiber-forming ability. This prevents the low-crystallinity PTFE resin from generating a fine fiber structure during biaxial stretching, ensuring that the surface of the porous membrane has the required blocky nodes and a reinforcing fiber structure accounting for 10-40% of the total structure. Combined with the radially distributed pore and fiber structure, this ensures that the porous membrane, while having high air permeability, also possesses good pressure resistance and waterproof performance.

[0079] Furthermore, in addition to using a mixture of high and low crystallinity PTFE resins as the film-forming resin, a specific biaxial stretching process is required to ensure the acquisition of the desired porous membrane with high waterproofness and high air permeability. Specifically, longitudinal stretching must first be performed to form the longitudinal fibers in the porous membrane; after longitudinal stretching, heat treatment is carried out to remove stress concentration and molecular chain deentanglement caused during the longitudinal stretching process; subsequently, a low-temperature transverse stretching process is used to stretch the transverse fibers and separate the micro-coalesced fibers formed during heat treatment; resulting in a porous membrane with a radially distributed pore structure.

[0080] It is important to note that this application does not employ the currently considered superior high-temperature hot stretching process, but rather a unique high-temperature longitudinal stretching + high-temperature thermal unwinding + low-temperature transverse stretching biaxial stretching process. It is generally believed that for stretch-forming films, higher temperatures facilitate softening, faster stress transfer within the film, and easier unwinding of molecular chains, naturally making it easier to draw into fibers and form the desired fiber structure. However, this application includes a certain amount of low-crystallinity PTFE resin with a crystallinity of 90-93%. This low-crystallinity PTFE resin inherently possesses good toughness, softens more easily at higher temperatures, and has a lower elastic modulus at high temperatures. This means that for the specific high- and low-crystallinity PTFE mixed resin system used in this application as the film-forming resin, the film already has high porosity due to longitudinal stretching, and the high temperature will cause a rapid decrease in the film's elastic modulus. Therefore, a relatively small tensile stress will lead to a large strain in the high-porosity, low-elastic-modulus film, and the presence of the low-crystallinity PTFE resin makes this strain increase far greater than the strain change caused by the decrease in tensile strength. Since tensile stress is primarily used to cause deformation of the membrane rather than to stretch it into fibers, a counterintuitive phenomenon occurs in the special film-forming system of this application: low-temperature transverse stretching (transverse stretching temperature 20–60°C lower than longitudinal stretching temperature) facilitates fiber formation. At lower temperatures, the membrane's elastic modulus is higher, and the tensile stress is mainly used for stretching into fibers, thus making fiber formation easier. Furthermore, since the temperature is not excessively low (only 20–60°C lower than the longitudinal stretching temperature), stress transmission and molecular chain de-entanglement capabilities remain strong. Therefore, the membrane still exhibits relatively good fiber-forming ability during low-temperature transverse stretching.

[0081] Of course, a decrease in transverse stretching temperature inevitably leads to slower stress transmission and difficulty in untangling molecular chains. To address this issue, this application employs high-temperature heat treatment after longitudinal stretching and before transverse stretching. High-temperature heat treatment of the membrane without applying external stretching force eliminates stress concentration within the membrane during longitudinal stretching (thus stress is more easily and quickly transmitted during low-temperature transverse stretching) and promotes the untangling of molecular chains within the membrane. However, the heat treatment time and temperature should not be too long or too high to avoid excessive melting and fusion of the fiber structure during longitudinal stretching (two or more fibers fusing into one fiber), forming excessively large local fiber structures that affect the permeability of the resulting porous membrane. Furthermore, excessive heat treatment can increase the membrane strength before low-temperature transverse stretching, making it difficult for the stronger fiber structure to transmit stress. This not only makes it difficult to stretch into fibers during low-temperature transverse stretching, but excessive stress concentration can also lead to fiber breakage and membrane pore structure damage during low-temperature transverse stretching.

[0082] It is important to note that controlling the longitudinal stretching ratio to 8–20 times ensures a sufficient number of fibers are formed during longitudinal stretching (sufficient stretching ensures the porous membrane has the required fiber distribution). However, heat treatment after longitudinal stretching inevitably leads to slight fiber tufting, which can cause stress concentration during low-temperature transverse stretching. By controlling the transverse stretching ratio to 1.2–2 times the longitudinal stretching ratio, the slightly tufted fibers from heat treatment can be separated (the low-crystallinity PTFE resin is integral and difficult to separate; only slightly tufted fibers generated during short-term heat treatment can be separated), and long node structures can be stretched into block nodes and fiber structures, resulting in a porous membrane with a significant radial pore structure.

[0083] In summary, this application, by selecting a mixed film-forming resin system of high and low crystallinity PTFE resins and combining it with a special high-temperature longitudinal stretching + high-temperature thermal dewinding + low-temperature transverse stretching biaxial stretching process, can ensure that the prepared porous membrane has the required radially distributed pore structure and a proportion of 10-40% reinforcing fibers and blocky surface nodes, thereby enabling the porous membrane to have both high air permeability and high waterproof performance.

[0084] It is understood that the high and low crystallinity PTFE resins in this application can be obtained by directly purchasing resins with the required crystallinity, or by purchasing resins with a certain crystallinity and then adjusting the crystallinity of the resins through heat treatment or other methods to obtain resins with the required crystallinity.

[0085] Optionally, in step S1, the solid content of the paste mixture is 70% to 85%; the auxiliary oil is at least one of petroleum ether, solvent oil and aviation kerosene.

[0086] Optionally, in step S4, the longitudinal stretching is performed in 4 to 20 steps;

[0087] In step S7, the biaxially oriented film is subjected to a micro-stretching of 1.1 to 1.5 during heat setting, and an intermittent stretching process is adopted. Specifically, the stretching force is removed after every 20 seconds of stretching and held for 10 seconds. After heat setting, the finished PTFE film is obtained.

[0088] By adopting the above technical solution, the membrane has high density during longitudinal stretching. If it is stretched rapidly, the tensile stress may not be able to be transmitted in time, which will lead to a decrease in the uniformity of the pore structure obtained by stretching, and even cause fiber breakage defects in some areas.

[0089] Furthermore, in the heat setting process following biaxial stretching, this application introduces a micro-stretching with a stretch ratio of 1.1 to 1.5 and employs a specific intermittent stretching process. Combined with a high- and low-crystallinity PTFE resin system and a special biaxial stretching process, this further ensures that the porous membrane obtains the required supporting fibers and blocky nodes. This is likely because the biaxially stretched membrane exhibits higher ductility during heat setting at higher temperatures. Applying a small stretch ratio at this temperature yields relatively higher strain, causing some adjacent fiber structures to contact or even fuse, resulting in partial fiber fusion and thus obtaining the desired coarser reinforcing fibers. It is important to note that if the stretch ratio is too high or a continuous stretching process is used during heat setting, it may lead to excessive fusion of the fiber structures, resulting in oversized fiber structures. This would create greater air resistance and affect air permeability. Furthermore, the oversized pore structures formed after fiber fusion can become weak points in the hydrophobic three-dimensional network structure, leading to a decrease in waterproof performance.

[0090] Optionally, step S2 specifically includes the following process steps:

[0091] S21. Preforming: The paste mixture is preformed at 30-45℃ to obtain a cylindrical preformed blank.

[0092] S22. First calendering: The preformed blank is extruded and calendered to form a sheet with a thickness of 2-5 mm. The temperature of the first calendering is 35-55℃.

[0093] S23. Secondary calendering: The sheet is further calendered into a base strip with a thickness of 0.15 to 0.45 mm. The temperature of the secondary calendering is 50 to 80°C and is not lower than the temperature of the primary calendering.

[0094] Optionally, in step S6, when the heat treatment temperature is lowered to the transverse stretching temperature, the cooling rate is 3-15℃ / 10min, and the temperature needs to be held for 10min for every 50℃ decrease.

[0095] By adopting the above technical solution, this application employs a special low-temperature transverse stretching process, which is more prone to stress concentration and difficulty in untangling molecular chains. To ensure that the film does not develop defects due to stress concentration during low-temperature transverse stretching, nor does it become difficult to form fibers due to excessive molecular chain entanglement, local stress concentration generated during longitudinal stretching should be eliminated as much as possible during heat treatment, and the molecular weight should be properly untangled. By controlling the cooling rate after heat treatment to be not too fast (not exceeding 15℃ / 10min), and by using a special cooling + heat preservation process, the concentrated stress in the longitudinally stretched film can have sufficient time to relax, and the entangled molecular weight can also have sufficient time to untangle.

[0096] Of course, the cooling rate after heat treatment should not be too slow (below 3℃ / 10min). This is because a special high and low crystallinity PTFE mixed resin system is used in this application, and the cooling rate has a certain impact on the crystallinity of the film-forming resin. A slow crystallization rate may cause the crystallinity of the low crystallinity PTFE resin, which is not already very low, to increase, thereby causing its fiber-forming performance to increase rapidly, making it difficult to produce the required reinforcing fibers and blocky node structures.

[0097] In summary, this application includes at least one of the following beneficial technical effects:

[0098] 1. The porous membrane of this application, by introducing a radially distributed elongated pore structure, combined with blocky surface nodes and reinforcing fibers, enables the porous membrane to have a gas flow path with lower gas resistance and higher resistance to pore collapse, and enables porous membranes with a thickness of 3-40 μm to have a strength of not less than 2 × 10⁻⁶. 4 ml / min / cm 2 The porous membrane boasts a high air permeability of 7 kPa. Furthermore, the large-diameter reinforcing fibers and the smaller-diameter distributed fibers form a highly intact hydrophobic three-dimensional network structure, giving the porous membrane excellent hydrophobic properties. In addition, the radially distributed fiber structure, with its cross-support in the thickness direction and the regional reinforcement effect of the block nodes and reinforcing fibers, enables the porous membrane to withstand high water pressure. Therefore, the porous membrane not only has high air permeability but also excellent waterproof performance.

[0099] 2. Based on the introduction of a radially distributed elongated pore structure, by further controlling the relative deviation between the width tensile strength and the membrane tensile strength of the porous membrane to be no more than 60%, the porous membrane can be further made to have more selective waterproofing on the basis of high air permeability, and the soap water penetration time is no less than 10 minutes.

[0100] 3. By further controlling the aspect ratio of reinforcing fibers, the aspect ratio of distributed fibers, the proportion of distributed fibers, and the density of surface nodes, the porous membrane can achieve both better air permeability and high waterproof effect.

[0101] 4. The preparation process of this application introduces low-crystallinity PTFE resin into the film-forming resin in a counterintuitive way to form a high-low crystallinity PTFE mixed resin. Combined with a special high-temperature longitudinal stretching + high-temperature thermal dewinding + low-temperature transverse stretching biaxial stretching process, it can ensure that the prepared porous membrane has the required radially distributed pore structure and 10-40% of reinforcing fibers and blocky surface nodes, so that the porous membrane has both high air permeability and high waterproof performance. Attached Figure Description

[0102] Figure 1 This is a scanning electron microscope image of one side surface of the porous membrane prepared in Example 1 of this application, with a magnification of 100×.

[0103] Figure 2 yes Figure 1 A further magnified image, used to show the structure of the main fibers and main nodes, with a magnification of 500×.

[0104] Figure 3 yes Figure 2 A further magnified view is provided to show the structure of the main fibers and main nodes, with a magnification of 1000×.

[0105] Figure 4 This is a scanning electron microscope image of one side surface of the porous membrane prepared in Example 4 of this application, with a magnification of 100×.

[0106] Figure 5 yes Figure 4 A further magnified image, used to show the structure of the main fibers and main nodes, with a magnification of 500×.

[0107] Figure 6 yes Figure 5 A further magnified view is provided to show the structure of the main fibers and main nodes, with a magnification of 1000×. Detailed Implementation

[0108] The following is in conjunction with the appendix Figures 1-6 This application will be described in further detail.

[0109] This application discloses a PTFE porous membrane with a low orientation degree pore structure and its preparation process.

[0110] Example 1

[0111] This embodiment discloses a PTFE porous membrane with a low orientation degree pore structure, the preparation process of which includes the following steps:

[0112] S1. Mixing and swelling: The film-forming resin and auxiliary oil are mixed evenly and swelled at 45°C for 16 hours to obtain a paste-like mixture with a solid content of 75wt%. The film-forming resin is obtained by mixing low-crystallinity PTFE resin with a crystallinity of 91.4% and high-crystallinity PTFE resin with a crystallinity of 95.3% at a mass ratio of 1:2, while the auxiliary oil is aviation kerosene.

[0113] S2. Pre-film formation, specifically including the following process steps:

[0114] S21. Pre-forming: Take the swollen paste mixture from step S1 and pre-form it at 35°C into a pre-formed blank for the installation assembly.

[0115] S22. First-stage calendering: The preformed blank is calendered into a sheet with a thickness of 2.5mm. The calendering temperature is 45℃.

[0116] S23. Secondary calendering: The sheet is further calendered into a base strip with a thickness of 0.25mm. The temperature of the secondary calendering is 65℃.

[0117] S3. Degreasing: Heat the oil-containing base tape to 205°C to remove the auxiliary oil and obtain an oil-free base tape.

[0118] S4. Longitudinal stretching: The oil-free base tape is placed in an environment of 250°C for longitudinal stretching. The stretching ratio is 12 times. The stretching is carried out in 6 steps, and each stretching is 2 times to obtain a uniaxial stretched film. Of course, it can be understood that when stretching is divided into multiple steps, the stretching ratio of each stretching does not have to be the same.

[0119] S5. Heat treatment: Place the uniaxially stretched film at 330℃ for 1 minute to obtain the unwound film.

[0120] S6. Lateral stretching: The unwrapped film is stretched laterally to obtain a biaxially stretched film. The lateral stretching ratio is 1.6 times the longitudinal stretching ratio, i.e., the lateral stretching ratio is 19.2 times. The stretching temperature is 35°C lower than the longitudinal stretching temperature, i.e., the lateral stretching temperature is 215°C. Furthermore, when the unwrapped film is cooled from 330°C to 215°C, the cooling rate is 7°C / 10min, and a holding time of 10min is required for every 50°C drop.

[0121] S7. Heat setting: Place the biaxially oriented membrane at 350℃ for 4 minutes for heat setting. During heat setting, perform micro-stretching of the biaxially oriented membrane with a stretch ratio of 1.25 and use an intermittent stretching process. Specifically, the stretching force needs to be removed every 20 seconds and held for 10 seconds. After heat setting, the finished PTFE porous membrane is obtained.

[0122] Examples 2-7

[0123] The main differences between Examples 2-7 and Example 1 are that the film-forming system is different and the film-forming process is adjusted.

[0124] The main difference between Example 2 and Example 1 is that by controlling the biaxial stretching process, a longitudinal stretching process with a very large stretching ratio and a transverse stretching process with a small stretching ratio (1.2 times the longitudinal stretching ratio) are adopted. In addition, no heat preservation operation is performed during the cooling process after heat treatment, which increases the difficulty of fiber formation during transverse stretching. This results in a porous membrane with a relatively large orientation degree. Other process parameters are detailed in Table 1.

[0125] The main difference between Example 3 and Example 1 is that a relatively high longitudinal stretching ratio and a relatively high transverse stretching ratio are used to make the porous membrane obtained by stretching have a distributed fiber structure with a high proportion and a small diameter. Other process parameters are detailed in Table 1.

[0126] The main difference between Example 4 and Example 1 is that both the high-crystallinity PTFE resin and the low-crystallinity PTFE resin in Example 4 have high crystallinity, and the high-crystallinity PTFE resin accounts for a larger proportion of the film-forming resin, giving it good fiber-forming properties. Combined with the use of a higher longitudinal stretching ratio and a higher transverse stretching ratio during biaxial stretching, the resulting porous membrane not only has a higher proportion of distributed fibers, but also, although there are many block nodes, the overall size is small, and the number of node aggregates is small. Other process parameters are detailed in Table 1.

[0127] The main difference between Example 5 and Example 1 is that by controlling the low-crystallinity PTFE resin in the film-forming resin to have a relatively low crystallinity and a high proportion, while the high-crystallinity PTFE resin also has a relatively low crystallinity and a relatively low proportion; and by controlling the use of a higher longitudinal stretching ratio and a lower transverse stretching ratio and a higher transverse stretching temperature during biaxial stretching, the difficulty of fiber formation during transverse stretching is increased, so that although the number of block nodes in the resulting porous membrane is small, the number of larger node aggregates is large. Other process parameters are detailed in Table 1.

[0128] The main difference between Example 6 and Example 1 is that by controlling the biaxial stretching, a lower longitudinal stretching ratio and a higher transverse stretching ratio are used to make the difference in the number and length of longitudinal and transverse fibers in the resulting porous membrane smaller. Other process parameters are detailed in Table 1.

[0129] The main difference between Example 7 and Example 1 is that, by controlling the low-crystallinity PTFE resin in the film-forming resin to have a relatively low crystallinity and a high proportion, while the high-crystallinity PTFE resin also has a relatively low crystallinity and a relatively low proportion; and by controlling the use of a lower ratio of longitudinal stretching and a lower ratio of transverse stretching during biaxial stretching, combined with a higher heat setting temperature, heat setting time, and heat setting stretching ratio during heat setting, the fibers are encouraged to twinnize, thereby resulting in a porous membrane with a higher proportion of reinforcing fibers and a lower proportion of distributed fibers. Other process parameters are detailed in Table 1.

[0130] Comparative Example

[0131] Comparative Example 1

[0132] The main difference between Comparative Example 1 and the various embodiments is that the film-forming resin contains only highly crystalline PTFE resin, and the commonly used biaxial stretching process is employed. In this process, no heat treatment is performed after longitudinal stretching, and a higher stretching temperature is used during transverse stretching. Furthermore, no micro-stretching is performed during heat setting.

[0133] Comparative Example 2

[0134] The main difference between Comparative Example 2 and the various embodiments is that, although the film-forming resin contains both high-crystallinity PTFE resin and low-crystallinity PTFE resin, the proportion of high-crystallinity PTFE resin is relatively low (the mass ratio of low-to-high-crystallinity PTFE resin is 1:0.5); in addition, the commonly used biaxial stretching process is adopted, in which no heat treatment is performed after longitudinal stretching, and a higher stretching temperature is used during transverse stretching.

[0135] Comparative Example 3

[0136] The main difference between Comparative Example 3 and the various embodiments is that, although the film-forming resin contains both high-crystallinity PTFE resin and low-crystallinity PTFE resin, a higher longitudinal stretching ratio and a lower transverse stretching ratio are used during biaxial stretching (the stretching ratio is only 0.5 times the longitudinal stretching ratio), and a higher stretching temperature is used during transverse stretching, while no heat treatment is performed after longitudinal stretching.

[0137] Table 1 Film-forming systems and film-forming process parameters for each example and comparative example

[0138]

[0139]

[0140] Performance testing method 1: Water resistance

[0141] 1.1 Soap water penetration time

[0142] The PTFE porous membranes prepared in each embodiment or comparative example were used as samples. These samples were attached to a fixture and immersed in soapy water with a concentration of 0.1 g / L. The membranes were then placed in a drum and shaken for a certain period. The water ingress within the fixture was observed. If no water entered the fixture after the shaking time, the soapy water penetration time of the sample was considered to be longer than that time. If no water entered the fixture after shaking for 10 minutes, the soapy water penetration time of the sample was considered to be longer than 10 minutes. It should be noted that for samples with a soapy water penetration time longer than 10 minutes, further testing for a higher soapy water penetration time was not conducted. For samples with a soapy water penetration time shorter than 10 minutes, further testing for soapy water penetration at even lower shaking times was required. The shaking time was reduced by 1 minute at a time, i.e., the water ingress within the fixture was tested sequentially at shaking times of 9 minutes, 8 minutes, and so on, up to 5 minutes. If the soapy water penetration time of the sample was shorter than 5 minutes, the waterproofing effect of the sample was considered insufficient, and further testing for soapy water penetration at even lower shaking times was not conducted.

[0143] 1.2 Water pressure resistance test

[0144] With appropriate adjustments made to Method B (high water pressure method) of the water resistance test method specified in JIS L1092, the component is mounted on the surface of a tool with a 1mm diameter water outlet hole, and the annular adhesive portion of the component is pressed together using a clamp. The component is held under the corresponding water pressure for 30 minutes, and then the front and sides of the component are observed for leakage. If there is no leakage, the component is deemed to have passed the water pressure resistance test under the corresponding water pressure for 30 minutes.

[0145] II. Relative Deviation of Strength

[0146] The tensile strength of the porous membrane in the width direction and the tensile strength in the length direction were tested using a universal tensile tester (each test was performed 3 times and the average value was taken). The average value of the two values ​​was calculated, and the relative deviations of the width tensile strength, the membrane length tensile strength and the average value of the two values ​​were further calculated.

[0147] The morphological and performance parameters of each embodiment and comparative example are detailed in Table 2.

[0148] Table 2. Morphological and performance parameters of each embodiment and comparative example.

[0149]

[0150]

[0151] Summarize

[0152] By comparing the technical solutions and performance parameters of Examples 1-7 and Comparative Examples 1-3, the embodiments in this application, by controlling the pore structure with a low degree of orientation (radially distributed pore structure) in conjunction with reinforcing fibers and distribution fibers of a certain proportion, form a hydrophobic three-dimensional network structure with high integrity and high pressure resistance, thereby giving the porous membrane good waterproof performance; and by reinforcing the three-dimensional network structure with good pressure resistance through block nodes, the possibility of pore collapse of the porous membrane under high air pressure is reduced. Combined with the low air resistance of the elongated pores and block nodes, the porous membrane has high air permeability on the basis of good waterproof performance.

[0153] As with Comparative Example 1, the porous membrane prepared with only highly crystalline PTFE resin and a high biaxial stretching ratio will produce a three-dimensional network structure with high density and small fiber diameter. The proportion of reinforcing fibers in the three-dimensional network structure is low and there are almost no node structures. Therefore, the three-dimensional network structure of this porous membrane has poor pressure resistance. Such a membrane structure, with similar thickness (as with Example 6), is more prone to pore structure collapse or damage under higher air pressure, and therefore has not only low air permeability but also low water pressure resistance.

[0154] For example, although the porous membrane prepared in Comparative Example 2 uses high- and low-crystallinity PTFE resins as film-forming resins, the low-crystallinity PTFE resin accounts for a relatively high proportion of the film-forming resin. Furthermore, it employs a common biaxial stretching process without heat treatment. The high temperature during transverse stretching makes fiber formation difficult, resulting in a smaller number of transverse fibers. Consequently, the porous membrane exhibits a high degree of pore orientation, still producing a structure similar to long nodes and fibrils. While such a membrane structure, with a similar thickness (as in Example 1), has only slightly lower air permeability, its waterproof performance is clearly inferior.

[0155] Although the porous membrane prepared in Comparative Example 3 used high- and low-crystallinity PTFE resins as film-forming resins, the low-crystallinity PTFE resin accounted for a higher proportion of the film-forming resins. Furthermore, it employed a lower ratio of transverse stretching and a higher ratio of transverse stretching, making fiber formation difficult during transverse stretching and resulting in fewer transverse fibers. Consequently, the porous membrane exhibited a high degree of pore orientation, still producing a structure similar to long nodes and fibrils. Such a membrane structure, with a similar thickness (as in Example 6), exhibits significantly lower air permeability and waterproof performance.

[0156] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A PTFE porous membrane having a low degree of orientation pore structure, characterized by: The film body comprises a first outer surface and a second outer surface, both of which comprise surface fibers and blocky surface nodes, the surface fibers being connected to the surface nodes or to adjacent surface fibers, the gaps between adjacent surface fibers forming long strip-shaped holes; The holes are distributed radially around the surface nodes, and the fiber structures are cross-laminated in the thickness direction of the film body to form connected flow paths; The thickness of the film body is 3-40 μm; The average diameter of the surface fibers is Z, the surface fibers include reinforcing fibers with a diameter not less than 1.2 times Z and distribution fibers with a diameter not greater than 0.6 times Z, and the reinforcing fibers account for 10-40% of all surface fibers; The air permeation rate of the porous membrane is not less than 2 x 10 4 ml / min / cm 2 @7 kPa.

2. The PTFE porous membrane having a low degree of orientation pore structure according to claim 1, characterized by: The relative deviation of the width tensile strength of the film body and the film tensile strength is not greater than 60%, and the soap water penetration time of the PTFE porous film is not less than 10 min.

3. The PTFE porous membrane having a low degree of orientation pore structure according to claim 1, characterized by: The distribution fibers are dispersed between the reinforcing fibers, and the distribution fibers account for 10-60% of all surface fibers.

4. The PTFE porous membrane having a low degree of orientation pore structure according to claim 1, wherein: The aspect ratio of the reinforcing fibers is 60-150, and the aspect ratio of the distribution fibers is 450-650.

5. The PTFE porous membrane having a low degree of orientation pore structure according to claim 1, wherein: The distribution density of the surface nodes is 10-30 / 10000μm 2 The surface nodes have node aggregates with an area not less than 150μm 2 The surface nodes have node aggregates with an area not less than 150μm 2 The number of the node aggregates is less than the number of the dispersed nodes.

6. The PTFE porous membrane having a low degree of orientation pore structure according to claim 5, characterized by: The distribution density of the node aggregate is 0.2-2 / 10000 μm 2 The distribution density of the dispersed node is 5-25 / 10000 μm 2 .

7. The PTFE porous membrane having a low degree of orientation pore structure according to claim 1, wherein: The surface fibers include longitudinal fibers having an angle with the longitudinal stretch direction not greater than 55° and transverse fibers having an angle with the transverse stretch direction less than 35°, the longitudinal fibers having an SEM-measured average length greater than an SEM-measured average length of the transverse fibers; the porous membrane having an air permeation rate not greater than 25 x 10 4 ml / min / cm 2 @ 7 kPa.

8. The PTFE porous membrane having a low degree of orientation pore structure according to claim 7, wherein: The average distance between adjacent longitudinal fibers is 2.5-7.5 μm, and the average distance between adjacent transverse fibers is 0.5-3 μm.

9. The PTFE porous membrane having a low degree of orientation pore structure according to claim 7, wherein: The ratio of the number of longitudinal fibers to the number of transverse fibers is 1.5-6, and the ratio of the SEM-measured average length of the longitudinal fibers to the SEM-measured average length of the transverse fibers is 1.5-4.

5.

10. The PTFE porous membrane having a low degree of orientation pore structure according to claim 7, wherein: The gap between adjacent longitudinal fibers is a longitudinal hole, and the gap between adjacent transverse fibers is a transverse hole, the SEM-measured length-diameter of the longitudinal hole is greater than the SEM-measured length-diameter of the transverse hole, the SEM-measured length-diameter of the longitudinal hole is 60-130 μm, and the average length-diameter of the transverse hole is 25-65 μm.

11. The PTFE porous membrane having a low degree of orientation pore structure according to claim 1, wherein: The water contact angle of the first outer surface and the second outer surface is 110 to 150°, and the areal density of the film body is 1.2 to 7.5 g / m 2 .

12. The process for producing a PTFE porous membrane having a low degree of oriented pore structure according to any one of claims 1 to 11, characterized by: The process comprises the following steps: S1, mixing and swelling, uniformly mixing a film-forming resin and an auxiliary oil and swelling at a temperature of 30-60 °C for 8-24 h to obtain a paste-like mixture, the film-forming resin being obtained by mixing a low-crystallinity PTFE resin with a crystallinity of 90-93% and a high-crystallinity PTFE resin with a crystallinity not less than 94% at a mass ratio of 1:1-1:4; S2, pre-film forming, extruding the paste-like mixture to form an oil-containing base tape with a thickness of 0.15-0.45 mm; S3, oil removal, heating the oil-containing base tape to remove the auxiliary oil and obtain an oil-free base tape; S4, longitudinal stretching, placing the oil-free base tape in an environment at a temperature of 200-300 °C to perform longitudinal stretching at a stretching ratio of 8-20 times to obtain a unidirectional stretched film; S5, heat treatment, placing the unidirectional stretched film in a temperature of 300-360 °C for heat treatment for 0.5-2 min to obtain a disentangled film; S6, transverse stretching, transversely stretching the disentangled film to obtain a bidirectional stretched film, wherein the transverse stretching ratio is 1.2-2 times the longitudinal stretching ratio, and the stretching temperature is 20-60 °C lower than the longitudinal stretching temperature. S7, heat setting, heat setting the biaxially stretched film at a temperature of 330-390℃ for 1-10 minutes.

13. The process for producing a PTFE porous membrane having a low degree of oriented pore structure according to claim 12, characterized by: In the step S1, the solid content of the paste mixture is 70%-85%; the auxiliary oil is at least one of petroleum ether, solvent oil and aviation kerosene.

14. The process for producing a PTFE porous membrane having a low degree of oriented pore structure according to claim 12, characterized by: In the step S4, the longitudinal stretching is divided into 4-20 steps; In the step S7, the biaxially stretched film is stretched by a stretching ratio of 1.1-1.5 during heat setting, and an intermittent stretching process is adopted, specifically, the stretching force is removed and kept for 10 seconds every 20 seconds of stretching, and the finished PTFE film is obtained after heat setting.

15. The process for producing a PTFE porous membrane having a low degree of oriented pore structure according to claim 12, characterized by: The step S2 specifically comprises the following process steps: S21, preforming, preforming the paste mixture at a temperature of 30-45℃ to obtain a cylindrical preform; S22, primary calendering, extruding and calendering the preform to form a sheet with a thickness of 2-5mm, the temperature of primary calendering being 35-55℃; S23, secondary calendering, further calendering the sheet to form a base tape with a thickness of 0.15-0.45mm, the temperature of secondary calendering being 50-80℃ and the temperature of secondary calendering being not lower than that of primary calendering.

16. The process for producing a PTFE porous membrane having a low degree of oriented pore structure according to claim 12, characterized by: In the step S6, when the heat treatment temperature is lowered to the transverse stretching temperature, the lowering speed is 3-15℃ / 10min, and the temperature is kept for 10 minutes every 50℃ of lowering.

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