Gradient aperture PTFE hollow fiber membrane and preparation method thereof

Abstract

The invention belongs to the technical field of polymer separation membranes, and particularly relates to a gradient aperture PTFE hollow fiber membrane and a preparation method thereof. The gradient aperture PTFE hollow fiber membrane is composed of an inner layer membrane and an outer layer membrane which are tightly combined, and the average aperture of the inner layer membrane is larger than that of the outer layer membrane; the porosity of the inner-layer film is greater than or equal to that of the outer-layer film. The inner-layer film is prepared from a first PTFE (Polytetrafluoroethylene) paste, and the outer-layer film is prepared from a second PTFE paste. The weight-average molecular weight or / and compression ratio of first PTFE powder adopted by the first PTFE paste is higher than that of second PTFE powder adopted by the second PTFE paste. The gradient aperture PTFE hollow fiber membrane has a unique asymmetric structure of which the aperture gradient is reduced from inside to outside, so that the interception precision and the anti-pollution capacity are remarkably improved while high flux is kept. According to the invention, the technical contradiction that the traditional single PTFE hollow fiber membrane is difficult to consider both high flux and high precision at the same time is solved.

Description

Technical Field

[0001] This invention belongs to the field of polymer separation membrane technology, specifically relating to a gradient pore size PTFE hollow fiber membrane and its preparation method. Background Technology

[0002] Polytetrafluoroethylene (PTFE) hollow fiber membranes are widely used in membrane distillation, membrane contactors, gas filtration, and liquid separation due to their excellent chemical stability, hydrophobicity, and high-temperature resistance. The current mainstream preparation method is the paste extrusion-stretching method, which involves mixing fine PTFE powder with a lubricant to form a paste, extruding it through a plunger to form a preform tube, and then degreasing, longitudinally stretching, and laterally expanding it to form a porous structure.

[0003] However, the membranes produced by this method are typically symmetrical with a relatively uniform pore size distribution. In practical applications, membrane materials often need to possess an asymmetric gradient pore size structure: for example, a larger pore size on the feed side for pre-filtration and to reduce membrane fouling, while a smaller pore size on the permeate side to ensure high separation accuracy. Existing single-material, one-step stretching processes struggle to achieve such a controllable gradient structure. Although multilayer membranes can be prepared through subsequent lamination and coating, these methods suffer from weak interlayer bonding, complex processes, and high costs.

[0004] Therefore, developing a PTFE hollow fiber membrane preparation technology that can be extruded in one step, has a robust gradient structure, and is simple to process is of great practical significance. Summary of the Invention

[0005] In view of the problems and shortcomings of the existing technology, the purpose of this invention is to provide a gradient pore size PTFE hollow fiber membrane and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of the present invention provides a gradient pore size PTFE hollow fiber membrane, wherein the membrane wall of the gradient pore size PTFE hollow fiber membrane is composed of a tightly bonded inner membrane and an outer membrane, wherein the average pore size of the inner membrane is larger than the average pore size of the outer membrane; the inner membrane is prepared from a first PTFE paste, and the outer membrane is prepared from a second PTFE paste.

[0007] Preferably, the porosity of the inner membrane is greater than or equal to the porosity of the outer membrane.

[0008] Preferably, the inner membrane has an average pore size of 0.5–2.0 μm and a porosity of 70%–85%; the outer membrane has an average pore size of 0.1–0.5 μm and a porosity of 50%–70%.

[0009] Preferably, the first PTFE paste is formed by mixing a first PTFE powder and an additive, and the second PTFE paste is formed by mixing a second PTFE powder and an additive; the weight-average molecular weight and / or compression ratio of the first PTFE powder is higher than that of the second PTFE powder.

[0010] Preferably, the first PTFE resin has a weight-average molecular weight of 5-10 million and a compression ratio of 1500-2500:1. The first PTFE resin is a high molecular weight and high compression ratio PTFE resin. High molecular weight resins form a more robust crystal network during stretching, tending to form longer, stronger fibrils and larger nodes, thus tending to form larger pores. High compression ratio pastes have higher orientation during extrusion, which also facilitates the subsequent formation of larger pores.

[0011] Preferably, the second PTFE resin has a weight-average molecular weight of 2-5 million and a compression ratio of 800-1500:1. The second PTFE resin is a low molecular weight and low compression ratio PTFE resin. Low molecular weight resins have relatively small crystalline regions, resulting in finer fibrils and nodes formed during stretching, thus tending to form smaller pore sizes. Low compression ratio pastes have a relatively loose structure.

[0012] Preferably, the additive is petroleum ether.

[0013] Preferably, the thickness ratio of the inner layer to the outer layer is 1:10 to 10:1.

[0014] The second aspect of this invention provides a method for preparing the gradient pore size PTFE hollow fiber membrane described in the first aspect above, comprising the following steps: (1) The first PTFE paste is added into the inner ring cavity of the concentric double-channel extrusion die, and the second PTFE paste is added into the outer ring cavity of the concentric double-channel extrusion die. At an extrusion temperature of 30 to 80°C, the first PTFE paste and the second PTFE paste are co-extruded using the concentric double-channel extrusion die to obtain a hollow composite preform with an inner and outer double-layer film structure. (2) Dry the hollow composite preform at 50-80°C, and then stretch the dried hollow composite preform in one or two directions at 200-280°C to obtain a double-layer hollow composite membrane. (3) Remove the additives from the double-layer hollow composite membrane material, and then heat-set the double-layer hollow composite membrane at 330-380℃ to obtain the gradient pore size PTFE hollow fiber membrane.

[0015] Preferably, in step (1), the concentric dual-channel extrusion die includes an outer die body, one end of which is provided with a detachable connecting plate, and the other end is provided with a detachable die; the outer die body is provided with a concentric dual-channel extrusion structure that cooperates with the die, the concentric dual-channel extrusion structure includes an outer cavity fitted inside the outer die body, an intermediate core mold is provided inside the outer cavity, and an outer annular cavity is formed between the intermediate core mold and the outer die body; an inner cavity is fitted inside the intermediate core mold, and a hollow central core rod is provided inside the inner cavity, and an inner annular cavity is formed between the central core rod and the intermediate core mold; a preparation outlet is provided at the center of the side of the die away from the outer die body; an extruder that cooperates with the outer annular cavity and the inner annular cavity is provided at the end of the outer die body away from the die; an air pipe that is clearance-fitted with the preparation outlet is provided at the end of the central core rod near the die; compressed air is continuously introduced into the hollow cavity inside the central core rod during co-extrusion to maintain the hollow shape of the membrane tube.

[0016] Preferably, the extruder includes an outer ring-shaped push block slidably connected to an outer ring-shaped cavity, with external pressure rods evenly distributed on the side of the outer ring-shaped push block away from the die, and a pressure plate provided at the end of the external pressure rod; an inner ring-shaped push block is sleeved inside the outer ring-shaped push block and slidably connected to an inner ring-shaped cavity, with internal pressure rods evenly distributed on the side of the inner ring-shaped push block away from the die and connected to the pressure plate, and the inner circle of the inner ring-shaped push block is slidably connected to the central mandrel.

[0017] Preferably, the connecting plate has an outer ring groove that is sealed to the outer mold body, an inner ring groove that is sealed to the intermediate core mold, and a core rod groove that is located at the center of the outer ring groove and the inner ring groove and connected to the central core rod; the connecting plate has an outer pressure hole that is slidably connected to the outer pressure rod and an inner pressure hole that is slidably connected to the inner pressure rod.

[0018] Preferably, the die has a conical extrusion cavity with its tip connected to the preparation outlet; the end of the intermediate core mold has a conical guide surface, and the end of the conical guide surface is clearance-fitted with the end of the conical extrusion cavity near the preparation outlet.

[0019] Preferably, in step (3), the specific operation for removing the additives is to completely remove the residual additives at 250-320°C.

[0020] Preferably, in step (2), the specific stretching conditions are: stretching with a longitudinal stretching ratio of 50% to 300%.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The gradient pore size PTFE hollow fiber membrane of the present invention is composed of two coaxially nested inner and outer membranes. The inner membrane is made of high molecular weight, high compression ratio PTFE raw material, and the outer membrane is made of low molecular weight, low compression ratio PTFE raw material. The preform is formed by extrusion in one step through a specially designed concentric dual-channel paste extrusion die, and then degreased, stretched and heat-set. The membrane has a unique asymmetric structure with a pore size gradient decreasing from the inside to the outside. The pore size of the inner membrane is approximately 0.5-2.0 μm, and the pore size of the outer membrane is approximately 0.1-0.5 μm. Thus, while maintaining high throughput, it significantly improves retention accuracy and antifouling ability. The corresponding preparation device and process are simple and efficient, suitable for continuous production, and solve the technical contradiction that traditional single PTFE hollow fiber membranes cannot achieve both high throughput and high precision.

[0022] (2) In this invention, the first PTFE paste and the second PTFE paste are formed into a double-layer preform in one step by concentric co-extrusion technology. Under the subsequent basically the same stretching process, due to the difference in the internal microstructure of the two materials, they will respond differently to external forces, thus "spontaneously" forming differentiated pore structures under the same macroscopic process conditions. Finally, the inner membrane forms a support layer with large pore size and high porosity, and the outer membrane forms a separation layer with small pore size and relatively dense. The two are firmly bonded at the interface through the mutual diffusion of molecular chains and sintering during the heat setting process to form an integral gradient membrane.

[0023] (3) The average pore size of the inner layer of the gradient pore size PTFE hollow fiber membrane of the present invention is 0.5-2μm, and the average pore size of the outer layer is 0.1-0.5μm. This structure is particularly suitable for processes such as membrane distillation: the macroporous layer on the feed liquid side (assuming it is the inner cavity) can effectively reduce the permeation resistance and pre-intercept large particulate pollutants; the vapor needs to pass through the dense microporous layer on the outside, which provides the main hydrophobic barrier and separation selectivity.

[0024] (4) The present invention ensures that the two pastes remain independent during the extrusion process through the precise mechanical design of the concentric dual-channel extrusion die, and only come into contact and adhere momentarily before they merge at the die outlet. This minimizes the premixing of the two layers of materials under high temperature and high shear, ensures the clarity of the gradient structure, and enables stable and continuous co-extrusion production.

[0025] (5) The present invention provides a process for preparing gradient pore size PTFE hollow fiber membrane. The process route is clear. Based on the traditional paste extrusion process, the main addition is the two-component paste preparation and co-extrusion steps. The subsequent degreasing, stretching and heat setting processes are compatible with the traditional process, and it is easy to carry out technical transformation and industrialization on existing production lines.

[0026] (6) This invention combines the design of the material’s own properties with the co-extrusion process to construct a robust asymmetric structure with a gradient pore size inside the PTFE hollow fiber membrane. This gradient structure can effectively coordinate the permeate flux and retention accuracy (or separation selectivity) in the filtration process, reduce the membrane fouling rate, and improve process efficiency. The device and process provided are improvements on the mature paste extrusion method, with good technology inheritance, relatively low modification difficulty and cost, and easy to achieve large-scale production. Such gradient membranes have broad application prospects in membrane water treatment, chemical separation, biomedicine and other fields. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the cross-sectional structure of the gradient pore size PTFE hollow fiber membrane in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the structure of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the internal structure of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the internal structure of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of the internal structure of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 6 This is a side view of the outer ring pusher and inner ring pusher of the concentric dual-channel extrusion die in Embodiment 1 of the present invention. Figure 7 This is a schematic diagram of the outer ring pusher and inner ring pusher of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 8 This is a schematic diagram of the structure of the concentric dual-channel extrusion die orifice in Embodiment 1 of the present invention; Figure 9 This is a schematic diagram of the top surface structure of the concentric dual-channel extrusion die connecting plate in Embodiment 1 of the present invention; Figure 10 This is a schematic diagram of the bottom structure of the concentric dual-channel extrusion die connecting plate of Embodiment 1 of the present invention; Figure 11 This is a schematic diagram of the structure of the connecting plate of the concentric dual-channel extrusion die of Embodiment 1 of the present invention; Figure 12 This is a schematic diagram of the structure of the intermediate core mold of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 13 This is a schematic diagram of the structure of the outer mold body of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 14 This is a schematic diagram of the structure of the central core rod of the concentric dual-channel extrusion die in Embodiment 1 of the present invention; Figure 15 This is a schematic diagram of the cross-sectional structure of the concentric dual-channel extrusion die in Embodiment 1 of the present invention.

[0028] In the diagram: 1. Outer mold; 2. Die; 3. Preparation outlet; 4. Connecting plate; 5. Pressure plate; 6. External pressure rod; 7. Internal pressure rod; 8. Outer ring push block; 9. Intermediate core mold; 10. Central core rod; 11. Inner ring push block; 12. Conical extrusion cavity; 13. Outer ring groove; 14. External pressure hole; 15. Inner ring groove; 16. Core rod groove; 17. Inner pressure hole; 18. Conical guide surface; 19. Inner cavity; 20. Outer cavity; 21. Central air pipe; 22. Confluence area; 23. Outer ring cavity; 24. Inner ring cavity; 25. Small diameter outer layer; 26. Large diameter inner layer. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] (I) Discussion on the weight-average molecular weight of PTFE resin To verify the decisive role of the gradient design of "high molecular weight inner layer and low molecular weight outer layer" in forming the target gradient pore size structure, this invention conducted Example 1, Comparative Example 1, and Comparative Example 2. The specific details of Example 1, Comparative Example 1, and Comparative Example 2 are as follows: Example 1: A gradient pore size PTFE hollow fiber membrane, such as Figure 1 As shown, its membrane wall consists of a tightly bonded inner membrane and an outer membrane, with the average pore size of the inner membrane being larger than that of the outer membrane. The inner membrane is prepared from a first PTFE paste, and the outer membrane is prepared from a second PTFE paste. The compositions of the first and second PTFE pastes are shown below: First PTFE paste: PTFE fine powder (DuPont 7AJ model) with a weight-average molecular weight of approximately 8 million and a compression ratio of approximately 2000:1 was selected. 100 parts by weight of this fine powder were mixed with 22 parts by weight of petroleum ether in a sealed container at 25°C for 30 minutes to obtain the first PTFE paste. Second PTFE paste: PTFE fine powder (Asahi Glass L-169 model) with a weight-average molecular weight of approximately 3.5 million and a compression ratio of approximately 1200:1 was selected. 100 parts by weight of this fine powder were mixed with 20 parts by weight of petroleum ether in a sealed container at 25°C for 30 minutes to obtain the second PTFE paste.

[0031] The specific steps for preparing the gradient pore size PTFE hollow fiber membrane are as follows: (1) Open the connecting plate 4 of the concentric dual-channel extrusion die, add the first PTFE paste into the inner ring cavity 24 of the concentric dual-channel extrusion die, add the second PTFE paste into the outer ring cavity 23 of the concentric dual-channel extrusion die, and co-extrude the first PTFE paste and the second PTFE paste using the concentric dual-channel extrusion die at an extrusion temperature of 60°C. At the same time, compressed air is introduced into the hollow cavity inside the central core rod 10 to maintain the hollow shape of the membrane tube, and a hollow composite preform with an inner diameter of about 4 mm and a wall thickness of about 1 mm (of which the inner layer 26 with a large pore diameter is about 0.9 mm thick and the outer layer 25 with a small pore diameter is about 0.1 mm thick) with an inner and outer double-layer membrane structure is obtained. (2) Dry the hollow composite preform at 70°C for 5 minutes to remove about 60% of the lubricant; then put the dried hollow composite preform into a stretching oven at 260°C and stretch it longitudinally at a stretching ratio of 100%. At the same time, complete the removal of the remaining lubricant and the initial shaping of the film are completed in a sintering furnace at 300°C to obtain a double-layer hollow composite film. (3) The double-layer hollow composite membrane is quickly heat-set in a high-temperature oven at 360°C for about 30 seconds, so that the PTFE resin is fully sintered, the large-pore inner membrane 26 and the small-pore outer membrane 25 are completely fused at the interface, and the pore structure is fixed; then it is wound up to obtain a gradient pore size PTFE hollow fiber membrane.

[0032] The specific results of the concentric dual-channel extrusion die are as follows: Figure 2-15 As shown, the device includes an outer mold body 1, with a detachable connecting plate 4 at one end and a detachable die 2 at the other end. The outer mold body 1 contains a concentric dual-channel extrusion structure that mates with the die 2. The concentric dual-channel extrusion structure includes inner and outer cavities 20 fitted within the outer mold body 1. An intermediate core mold 9 is located within the outer cavity 20, forming an outer annular channel 23 between the intermediate core mold 9 and the outer mold body 1. An inner cavity 19 is fitted within the intermediate core mold 9, containing a hollow central core rod 10, forming an inner annular channel 24 between the central core rod 10 and the intermediate core mold 9. A preparation outlet 3 is located at the center of the die 2 on the side furthest from the outer mold body 1. An extruder mates with the outer annular channel 23 and the inner annular channel 24 at the end of the outer mold body 1 furthest from the die 2.

[0033] The extruded part includes an outer ring-shaped push block 8 that is slidably connected to an outer ring-shaped cavity 23. External pressure rods 6 are evenly distributed on the side of the outer ring-shaped push block 8 away from the die 2, and a pressure plate 5 is provided at the end of each external pressure rod 6. An inner ring-shaped push block 11, which is slidably connected to an inner ring-shaped cavity 24, is fitted inside the outer ring-shaped push block 8. Internal pressure rods 7, which are connected to the pressure plate 5, are evenly distributed on the side of the inner ring-shaped push block 11 away from the die 2. The inner circle of the inner ring-shaped push block 11 is slidably connected to a central mandrel 10.

[0034] The connecting plate 4 has an outer annular groove 13 that is sealed to the outer mold body 1, an inner annular groove 15 that is sealed to the intermediate core mold 9, and a core rod groove 16 that is located at the center of the outer annular groove 13 and the inner annular groove 15 and connected to the central core rod 10. The connecting plate 4 has an outer pressure hole 14 that is slidably connected to the outer pressure rod 6 and an inner pressure hole 17 that is slidably connected to the inner pressure rod 7.

[0035] The die 2 has a conical extrusion cavity 12 with its tip connected to the preparation outlet 3; the intermediate core die 9 has a conical guide surface 18 at its end, and the end of the conical guide surface 18 is in clearance fit with the end of the conical extrusion cavity 12 near the preparation outlet 3. The central mandrel 10 has an air tube 21 at its end near the die 2, which is in clearance fit with the preparation outlet 3.

[0036] The specific steps for co-extruding the first PTFE paste and the second PTFE paste using the aforementioned concentric dual-channel extrusion die to obtain a hollow composite preform with an inner and outer double-layer film structure are as follows: The pressure plate 5 pushes the outer pressure rod 6 and the inner pressure rod 7 to move towards the connecting plate 4. The outer pressure rod 6 moves towards the outer mold body 1 along the outer pressure hole 14, and the inner pressure rod 7 moves towards the outer mold body 1 along the inner pressure hole 17. The outer pressure rod 6 pushes the outer ring-shaped push block 8 to move along the outer ring-shaped cavity 23. The outer ring-shaped push block 8 squeezes the second PTFE paste in the outer ring-shaped cavity 23 and squeezes the second PTFE paste towards the conical guide surface 18. At the same time, the inner pressure rod 7 pushes the inner ring-shaped push block 11 along the inner ring-shaped cavity. 24 moves, the inner ring pusher 11 squeezes the first PTFE paste in the inner ring cavity 24, and squeezes the first PTFE paste toward the air tube 21. The first PTFE paste and the second PTFE paste come into contact and bond instantly in the confluence area 22. While co-extruding, compressed air is continuously introduced into the hollow cavity inside the central core rod 10 to maintain the hollow shape of the membrane tube. After compression and shearing, they are co-extruded into a preform and flow out through the preparation outlet 3 of the die 2 to form a composite preform tube.

[0037] The outer ring groove 13 on the connecting plate 4 is used to seal and fix the outer mold body 1, the inner ring groove 15 is used to seal and fix the intermediate core mold 9, and the core rod groove 16 is used to fix the central core rod 10; the outer ring push block 8 is sleeved on the intermediate core mold 9, and the inner ring push block 11 is sleeved on the central core rod 10; the conical extrusion cavity 12 and the conical guide surface 18 are fitted with a clearance to facilitate the bonding of the second PTFE paste and the first PTFE paste after they come together; the outer cavity 20 is used to place the intermediate core mold 9 and forms an outer ring cavity 23 on the outer periphery of the intermediate core mold 9; the inner cavity 19 is used to place the central core rod 10 and forms an inner ring cavity 24 on the outer periphery of the central core rod 10.

[0038] The morphology of the gradient pore size PTFE hollow fiber membrane prepared in the embodiments of the present invention was characterized as follows: The gradient pore size PTFE hollow fiber membrane prepared in this embodiment was observed using a scanning electron microscope, as shown below. Figure 1 As shown, the inner membrane 26 and the outer membrane 25 are clearly visible on the tube wall.

[0039] The pore size and porosity of the gradient pore size PTFE hollow fiber membrane prepared in this embodiment were measured using the bubble point method and mercury porosimetry. The inner layer membrane 26 had an average pore size of approximately 0.8 μm and a porosity of approximately 78%; the outer layer membrane 25 had an average pore size of approximately 0.1 μm and a porosity of approximately 52%. No obvious interface defects were found at the junction of the two layers.

[0040] Comparative Example 1: The content of Comparative Example 1 is basically the same as that of Example 1, except that in step S1, the resin in the first PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 3.5 million and a compression ratio of about 1200:1, and the resin in the second PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 8 million and a compression ratio of about 2000:1.

[0041] Comparative Example 2: The content of Comparative Example 2 is basically the same as that of Example 1, except that in step S1, the resin in the first PTFE paste and the second PTFE paste is selected as PTFE resin fine powder with a weight average molecular weight of about 5 million and a compression ratio of about 2000:1.

[0042] The average pore size, pure water flux, and rejection rate of the gradient pore size PTFE hollow fiber membranes prepared in Example 1, Comparative Example 1, and Comparative Example 2 were tested. The test methods are as follows: 1. Inner / outer layer average pore size test: The test was conducted using a scanning electron microscope (SEM) and image analysis software (such as ImageJ). The specific test steps were as follows: (1) The hollow fiber membrane sample was quenched in liquid nitrogen to obtain a clean cross-section; (2) The cross-section was sputtered with gold and observed under an SEM. Three high-resolution images were taken in the inner and outer regions respectively; (3) The images were imported into the image analysis software, and the pores were identified by grayscale threshold segmentation. The equivalent circle diameter of 100 pores was counted; (4) The arithmetic mean of the diameters of all statistical pores in the inner and outer layers were calculated respectively, which are the average pore diameters of the inner and outer layers.

[0043] 2. Pure water flux test: A cross-flow filtration device, including a feed tank, a horizontal flow pump, a pressure gauge, a membrane module (with known effective membrane area), an electronic balance, and a timer testing device, was used for testing. The test conditions were: temperature 25±1℃, operating pressure 0.10 MPa (1.0 bar). The specific test steps were: (1) Encapsulate the membrane fibers into a module and pre-pressurize it with pure water at 0.10 MPa for 30 minutes until the flux stabilized; (2) Adjust the system pressure to 0.10 MPa, collect the permeate for a certain period of time (e.g., 2 minutes), and weigh it accurately with an electronic balance; (3) Calculate the flux based on the permeate mass (converted to volume), collection time, and effective membrane area. Repeat 3 times and take the average value. The formula for calculating the pure water flux (J, LMH / bar) is as follows: Pure water flux = V / (A*t*ΔP), In the formula, V is the permeate volume (L), A is the effective membrane area (m²), t is the collection time (h), and ΔP is the transmembrane pressure (bar).

[0044] 3. Retention Rate Test: The testing apparatus used is the same as that used for pure water flux testing, and is also equipped with an ultraviolet spectrophotometer or a particle counter. The specific testing steps are as follows: (1) Prepare a 0.1% suspension of 200nm polystyrene (PS) particles and disperse it evenly by ultrasonication; (2) Perform cross-flow filtration at 0.10 MPa, and after the operation is stable, collect the permeate and feed liquid samples at the same time; (3) Use an ultraviolet spectrophotometer to measure the absorbance of the feed liquid and permeate at a specific wavelength (such as the characteristic absorption wavelength of PS particles), or use a particle counter to measure the particle concentration. The formula for calculating the retention rate (R,%) is as follows: Retention rate = (1 - Cp / Cf) * 100%, In the formula, Cp is the permeate concentration and Cf is the feed solution concentration.

[0045] The performance results of the gradient pore size PTFE hollow fiber membranes prepared in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention are shown in Table 1.

[0046] Table 1 Performance indicators of Example 1, Comparative Example 1 and Comparative Example 2 As shown in Table 1, the gradient direction of the resin molecular weight is the decisive intrinsic factor in the formation of the gradient pore structure. Only when a positive gradient combination of "higher inside and lower outside" is adopted can the intrinsic properties of the material be utilized to "spontaneously generate" an ideal gradient pore structure with a larger inside and a smaller outside under the same stretching conditions. The reverse gradient (Comparative Example 1) cannot achieve the goal and may even lead to performance degradation.

[0047] (II) Discussion on the compression ratio of PTFE resin To investigate the regulatory effect of compression ratio differences on the degree of gradient structure and overall performance under a fixed positive molecular weight gradient, this invention conducted Examples 2, 3, and Comparative Example 3. The specific details of Examples 2, 3, and Comparative Example 3 are as follows: Example 2: Example 2 is basically the same as Example 1, except that in step S1, the resin in the first PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 8 million and a compression ratio of about 1800:1, and the resin in the second PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 3.5 million and a compression ratio of about 1000:1.

[0048] Example 3: The content of Example 3 is basically the same as that of Example 1, except that in step S1, the resin in the first PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 8 million and a compression ratio of about 2200:1, and the resin in the second PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 3.5 million and a compression ratio of about 1400:1.

[0049] Comparative Example 3: The content of Comparative Example 3 is basically the same as that of Example 1, except that in step S1, the resin in the first PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 8 million and a compression ratio of about 1500:1, and the resin in the second PTFE paste is selected from PTFE resin fine powder with a weight average molecular weight of about 3.5 million and a compression ratio of about 1500:1.

[0050] The performance of the gradient pore size PTFE hollow fiber membranes prepared in Example 2, Example 3 and Comparative Example 3 was tested using the test method described above in this invention. The test results are shown in Table 2.

[0051] Table 2 Performance indicators of Example 2, Example 3 and Comparative Example 3 Table 2 shows that the compression ratio gradient difference positively affects the magnitude of the pore size gradient, but there is an optimal range. The combination in Example 1 (2000:1 / 1200:1) achieves the best balance between ensuring a significant pore size gradient, excellent permeability flux, and good mechanical strength. When there is no compression ratio gradient (Comparative Example 3), a pore size gradient cannot be formed.

[0052] (III) Performance Testing 1. Gas-liquid mass transfer coefficient (Kov) test: The gas-liquid mass transfer coefficients of the gradient pore size PTFE hollow fiber membranes prepared in Examples 1 and 2 of this invention were compared with those of the homogeneous membrane prepared in Comparative Example 2. A 2% NaOH aqueous solution was used as the absorbent, and pure CO2 was used as the gas source. The mass transfer coefficients were tested under the same membrane module and operating conditions. The test results are shown in Table 3.

[0053] Table 3 Performance test results of Example 1, Comparative Example 1 and Comparative Example 2 As shown in Table 3, the overall volumetric mass transfer coefficient (Kov) of the gradient pore size PTFE hollow fiber membrane in Example 1 of the present invention is about 28% higher than that of the homogeneous membrane. This is because the macroporous structure of the large-pore inner membrane 26 in the gradient pore size PTFE hollow fiber membrane of Example 1 of the present invention can effectively reduce gas diffusion resistance, while the dense structure of the small-pore outer membrane 25 can ensure good liquid barrier properties and a stable interface. The synergistic effect of the two effectively improves the performance of the PTFE hollow fiber membrane.

[0054] 2. Stain resistance test: Yeast suspension was filtered using gradient pore size PTFE hollow fiber membranes prepared in Example 1 and Comparative Example 1 of this invention, and the flux recovery rate after filtering the yeast suspension was measured. The test results are shown in Table 4.

[0055] Table 4 Performance test results of Example 1 and Comparative Example 1 As shown in Table 4, the flux recovery rate of Example 1 of the present invention is 90%. This is because the dense pores of the surface membrane of the gradient pore size PTFE hollow fiber membrane in Example 1 of the present invention prevent pollutants from entering the deep layer, while the large pores of the inner membrane facilitate backwashing. Therefore, it can still maintain a high flux recovery rate after contamination, demonstrating good contamination resistance and easy cleaning performance.

[0056] 3. Bubble Point Pressure Test: The maximum pore size of the outer layer of the gradient pore size PTFE hollow fiber membrane prepared in Example 1 of this invention was determined using the isopropanol bubble point method. The maximum pore size of the outer layer membrane was found to be ~0.25 μm. This result confirms that the outer layer membrane structure of the gradient pore size PTFE hollow fiber membrane can provide precise separation accuracy.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Those skilled in the art can modify or make equivalent substitutions to the technical solutions of the present invention based on the concept of the present invention, without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A gradient pore size PTFE hollow fiber membrane, characterized in that, The membrane wall of the gradient pore size PTFE hollow fiber membrane is composed of a tightly bonded inner membrane and an outer membrane, with the average pore size of the inner membrane being larger than that of the outer membrane; the inner membrane is prepared from a first PTFE paste, and the outer membrane is prepared from a second PTFE paste.

2. The gradient pore size PTFE hollow fiber membrane according to claim 1, characterized in that, The porosity of the inner membrane is greater than or equal to that of the outer membrane.

3. The gradient pore size PTFE hollow fiber membrane according to claim 1 or 2, characterized in that, The inner membrane has an average pore size of 0.5–2.0 μm and a porosity of 70%–85%; the outer membrane has an average pore size of 0.1–0.5 μm and a porosity of 50%–70%.

4. The gradient pore size PTFE hollow fiber membrane according to claim 1, characterized in that, The first PTFE paste is made by mixing a first PTFE powder and additives, and the second PTFE paste is made by mixing a second PTFE powder and additives; the weight-average molecular weight and / or compression ratio of the first PTFE powder is higher than that of the second PTFE powder.

5. The gradient pore size PTFE hollow fiber membrane according to claim 1, characterized in that, The first PTFE powder has a weight-average molecular weight of 5 million to 10 million and a compression ratio of 1,500 to 2,500:1; the second PTFE powder has a weight-average molecular weight of 2 million to 5 million and a compression ratio of 800 to 1,500:

1.

6. The gradient pore size PTFE hollow fiber membrane according to claim 1, characterized in that, The thickness ratio of the inner layer to the outer layer is 1:10 to 10:

1.

7. The method for preparing the gradient pore size PTFE hollow fiber membrane according to any one of claims 1-4, characterized in that, Includes the following steps: (1) The first PTFE paste is added into the inner ring cavity of the concentric double-channel extrusion die, and the second PTFE paste is added into the outer ring cavity of the concentric double-channel extrusion die. At an extrusion temperature of 30 to 80°C, the first PTFE paste and the second PTFE paste are co-extruded using the concentric double-channel extrusion die to obtain a hollow composite preform with an inner and outer double-layer film structure. (2) Dry the hollow composite preform at 50-80°C, and then stretch the dried hollow composite preform in one or two directions at 200-280°C to obtain a double-layer hollow composite membrane. (3) Remove the additives from the double-layer hollow composite membrane material, and then heat-set the double-layer hollow composite membrane at 330-380℃ to obtain the gradient pore size PTFE hollow fiber membrane.

8. The method for preparing a gradient pore size PTFE hollow fiber membrane according to claim 7, characterized in that, In step (1), the concentric dual-channel extrusion die includes an outer die body, one end of which is provided with a detachable connecting plate, and the other end is provided with a detachable die. The outer die body is provided with a concentric dual-channel extrusion structure that cooperates with the die. The concentric dual-channel extrusion structure includes an outer cavity fitted inside the outer die body, an intermediate core mold inside the outer cavity, and an outer annular cavity formed between the intermediate core mold and the outer die body. An inner cavity is fitted inside the intermediate core mold, and a hollow central core rod is provided inside the inner cavity. An inner annular cavity is formed between the central core rod and the intermediate core mold. A preparation outlet is provided at the center of the die on the side away from the outer die body. An extruder that cooperates with the outer annular cavity and the inner annular cavity is provided at the end of the outer die body away from the die. An air pipe that is clearance-fitted with the preparation outlet is provided at the end of the central core rod near the die. Compressed air is continuously introduced into the hollow cavity inside the central core rod during co-extrusion.

9. The method for preparing a gradient pore size PTFE hollow fiber membrane according to claim 8, characterized in that, The extruder includes an outer ring-shaped push block that is slidably connected to an outer ring-shaped cavity. External pressure rods are evenly distributed on the side of the outer ring-shaped push block away from the die, and a pressure plate is provided at the end of the external pressure rod. An inner ring-shaped push block that is slidably connected to an inner ring-shaped cavity is sleeved inside the outer ring-shaped push block. Internal pressure rods that are connected to the pressure plate are evenly distributed on the side of the inner ring-shaped push block away from the die. The inner circle of the inner ring-shaped push block is slidably connected to the central mandrel.

10. The method for preparing a gradient pore size PTFE hollow fiber membrane according to claim 8, characterized in that, The connecting plate has an outer annular groove that is sealed to the outer mold body, an inner annular groove that is sealed to the intermediate core mold, and a core rod groove that is located at the center of the outer and inner annular grooves and connected to the central core rod. The connecting plate has an outer pressure hole that is slidably connected to the outer pressure rod and an inner pressure hole that is slidably connected to the inner pressure rod. The die has a conical extrusion cavity with its tip connected to the preparation outlet. The end of the intermediate core mold has a conical guide surface, and the end of the conical guide surface is clearance-fitted with the end of the conical extrusion cavity near the preparation outlet.

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