A polytetrafluoroethylene hollow fiber membrane having an asymmetric structure, a method for preparing the same, and a membrane contactor using the same

Abstract

The application discloses an asymmetric structure polytetrafluoroethylene hollow fiber membrane and a preparation method and application thereof, relates to the field of membrane separation technology, and aims to solve the problem that the existing polytetrafluoroethylene hollow fiber membrane is difficult to consider high porosity and small pore diameter. The membrane is asymmetric in the cross-section direction, the average pore diameter of the outer surface is smaller than that of the main body and the inner surface, and the preparation method comprises the steps of mixing, extruding, degreasing, stretching, sintering and asymmetric heat treatment. The core innovation lies in that a dense layer is formed on the outer surface of the membrane through the asymmetric heat treatment process, compared with the fluorine coating method and the wrapping method, no additional materials and complex processes are needed, and the method is easy to scale up. The polytetrafluoroethylene hollow fiber membrane prepared by the application has the characteristics of high porosity and high liquid penetration pressure, the smaller pore structure of the outer surface endows the membrane with better wet resistance, and the high porosity of the main body makes the membrane have lower mass transfer resistance, and the membrane is suitable for a membrane contactor process with liquid in the shell.

Description

Technical Field

[0001] This application relates to a polytetrafluoroethylene hollow fiber membrane with an asymmetric structure, its preparation method, and its application in membrane contactors, belonging to the field of membrane separation technology. Background Technology

[0002] Membrane technology boasts numerous advantages, including high efficiency and environmental friendliness, and has experienced rapid development in recent years. Currently, membrane separation processes are widely used in water resource development and utilization, environmental pollution control, energy conservation and emission reduction, and have become a key technology supporting the upgrading of traditional industries and the transformation of industrial processes, playing a vital role in promoting the sustainable development of the national economy.

[0003] Membrane contactors are a coupling technology between membranes and traditional chemical unit operations, enabling interphase mass transfer in the absence of direct contact between gas-liquid or liquid-liquid phases. Common membrane contactor processes include membrane absorption, membrane distillation, membrane ammonia removal, and membrane extraction. Due to its advantages such as large specific surface area, high efficiency, compact equipment structure, and modular design, membrane contactor technology has promising application prospects in fields such as acid gas removal, pollutant treatment, and process intensification.

[0004] Unlike traditional membrane separation processes, the membrane in a membrane contactor process is typically a microporous membrane. It lacks separation selectivity and merely serves as an interface between two phases, with mass transfer occurring through the numerous micropores within the membrane. An ideal microporous membrane for a membrane contactor should possess the following characteristics: excellent hydrophobicity to prevent liquid phase from entering the pores and wetting the membrane; excellent physicochemical stability to avoid corrosion and degradation by the contacting liquid medium during long-term operation; high porosity, resulting in low mass transfer resistance; and small average pore size, leading to high liquid phase penetration pressure and ensuring long-term stable operation of the membrane contactor.

[0005] Polytetrafluoroethylene (PTFE) possesses excellent hydrophobicity, temperature resistance, and physicochemical stability, making it an ideal material for membrane contactor processes. However, its "insoluble and infusible" nature prevents the use of thermally induced phase inversion methods for membrane fabrication. Currently, membranes prepared using mainstream paste extrusion processes typically exhibit high porosity but also large pore sizes, making it difficult to balance high porosity and small pore size requirements. To address these issues, some researchers have employed post-processing methods. The main idea is to add a dense skin layer to the surface of the PTFE hollow fiber membrane to reduce the pore size while maintaining the overall high porosity of the membrane to the maximum extent possible. Examples include the fluorine coating method (Zhang Huapeng, Guo Yuhai, Chen Jianyong, A method for controlling the pore size of PTFE hollow fiber membranes, 201010504784.3) and the wrapping method (Wu Yier, Qian Jianbin, Chen Xiaomei, A PTFE hollow fiber membrane and its preparation method, 201110153322.6), etc.

[0006] However, both the fluorine coating and wrapping methods mentioned above require additional materials and processing steps, making them complex and difficult to scale up for continuous production. Therefore, there is a need to develop a simpler, more easily scalable method for preparing polytetrafluoroethylene hollow fiber membranes to meet the technical requirements of high porosity and small pore size. Summary of the Invention

[0007] The purpose of this invention is to provide an asymmetric polytetrafluoroethylene hollow fiber membrane, its preparation method, and its application, in order to solve the problems in the prior art where polytetrafluoroethylene hollow fiber membranes are difficult to balance high porosity and small pore size, and where existing pore size control methods are complex and difficult to scale up.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: According to one aspect of this application, a polytetrafluoroethylene hollow fiber membrane with an asymmetric structure is provided, characterized in that the polytetrafluoroethylene hollow fiber membrane has an asymmetric structure in the cross-sectional direction, and the average pore size of its outer surface is smaller than that of the body and the inner surface.

[0009] Optionally, the hollow fiber membrane has an inner diameter of 0.4~0.8 mm, an outer diameter of 0.8~1.6 mm, and a porosity of not less than 45%. Preferably, the porosity of the hollow fiber membrane is 45% to 70%.

[0010] Optionally, the outer surface of the hollow fiber membrane has a dense layer with smaller pore size.

[0011] Optionally, the polytetrafluoroethylene hollow fiber membrane is a hydrophobic membrane with a water contact angle greater than 100°; Preferably, the water contact angle of the polytetrafluoroethylene hollow fiber membrane is 110°~120°.

[0012] According to another aspect of this application, a method for preparing the polytetrafluoroethylene hollow fiber membrane with the asymmetric structure is provided, comprising the following steps: S1. Mixing: Mix polytetrafluoroethylene dispersion resin with extrusion aid to obtain polytetrafluoroethylene paste; S2. Extrusion: The polytetrafluoroethylene paste obtained in step S1 is cured, pressed, and then extruded to obtain a natural hollow fiber membrane. S3. Degreasing: The original hollow fiber membrane obtained in step S2 is degreased to remove extrusion aids; S4. Stretching: Stretching the degreased hollow fiber membrane obtained in step S3; S5. Sintering: The stretched hollow fiber membrane obtained in step S4 is sintered and shaped. S6. Asymmetric heat treatment: The hollow fiber membrane obtained in step S5 is subjected to asymmetric heat treatment.

[0013] Preferably, in S1, the molecular weight of the polytetrafluoroethylene dispersion resin is 2 million to 10 million, the extrusion aid is aviation kerosene or isoparaffin, and the mass fraction of the extrusion aid is 17% to 25% wt%. Preferably, in step S2, the extrusion temperature is 40~80℃; Preferably, in S3, the degreasing temperature is 20-50°C higher than the boiling point of the extrusion aid; Preferably, in step S4, the stretching temperature is 150~250℃, and the stretching ratio is 2.0~5.0; Preferably, in step S5, the sintering temperature is 350~380℃ and the sintering time is 0.5~3.0min; Preferably, in S6, the asymmetric heat treatment temperature is 450~600℃, and the heat treatment time is 2~20s; Preferably, in S6, the asymmetric heat treatment temperature is 500~580℃, and the heat treatment time is 2~10s.

[0014] Optionally, in S6, the asymmetric heat treatment is carried out using a trumpet-shaped copper heater. The effective heating straight tube section of the trumpet-shaped copper heater has a length of 4~10cm and an inner diameter that is equivalent to the outer diameter of the polytetrafluoroethylene hollow fiber membrane.

[0015] Optionally, in S5, after sintering and shaping, the polytetrafluoroethylene hollow fiber membrane is rapidly cooled to room temperature using an air knife.

[0016] The asymmetric heat treatment selectively melts and rearranges the outer surface of the polytetrafluoroethylene hollow fiber membrane through localized high temperatures (450-600℃), forming a dense skin layer in a very short time (2-20 seconds), while the interior of the membrane retains its original porous network structure due to insufficient heat conduction time. This rapid heat treatment avoids the collapse of the overall structure and achieves an organic combination of small pores on the outer surface and high porosity in the main body. According to another aspect of this application, the application of the polytetrafluoroethylene hollow fiber membrane in a membrane contactor is provided. The polytetrafluoroethylene hollow fiber membrane is woven and then sealed in a membrane element with epoxy resin for use in a membrane contactor process where the shell side is in the liquid phase.

[0017] By precisely controlling the above process parameters, this invention can prepare polytetrafluoroethylene hollow fiber membranes that simultaneously meet the requirements of high porosity and high critical water penetration pressure, successfully solving the technical contradiction in the prior art of achieving both high porosity and small pore size.

[0018] In this application, "porosity" refers to the percentage of pore volume in the membrane to the total membrane volume; "Critical water penetration pressure" refers to the lowest pressure at which water begins to penetrate the membrane pores; The "water contact angle" refers to the contact angle between a water droplet and the membrane surface, and is used to characterize the hydrophobic properties of the membrane.

[0019] The beneficial effects that this application can produce include: 1) Successfully solved the technical contradiction of balancing high porosity and small pore size. The prepared membrane porosity reached over 45%, while the critical water penetration pressure was significantly improved. The dense outer surface layer provided high liquid penetration pressure and excellent wettability, while the high porosity of the main body maintained low mass transfer resistance. 2) Compared to existing fluorine coating and wrapping methods, this invention requires no additional materials or complex process steps, and the preparation process is simple, achieving the construction of asymmetric structures through a single asymmetric heat treatment (2-20 seconds). Compared to the fluorine coating method, it avoids the use of expensive reagents and the risk of coating peeling; compared to the wrapping method, it eliminates the complex wrapping process and specialized equipment, facilitating continuous production and scale-up. 3) The prepared polytetrafluoroethylene hollow fiber membrane maintains excellent hydrophobicity and dimensional stability, and is particularly suitable for membrane contactor processes in which the shell side is liquid. It has significant advantages in industrial applications such as membrane absorption, membrane distillation, membrane deammoniation, and membrane extraction. It can effectively prevent liquid phase from entering the membrane pores and causing membrane wetting problems, and ensure the long-term stable operation of the membrane contactor. Attached Figure Description

[0020] Figure 1 A schematic diagram of the preparation steps for asymmetric polytetrafluoroethylene hollow fiber membranes; Figure 2 Microstructure diagrams of the outer and inner surfaces of an asymmetric polytetrafluoroethylene hollow fiber membrane; Figure 3 A schematic diagram of a trumpet-shaped copper heater used for asymmetric heat treatment. Detailed Implementation

[0021] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0022] Unless otherwise specified, the raw materials used in the embodiments of this application are all purchased commercially and used directly without any special treatment.

[0023] Example 1 Polytetrafluoroethylene dispersion resin (number average molecular weight 6 million) and isoparaffin Isopar G were mixed evenly at a mass fraction of 80% and 20% respectively to prepare a polytetrafluoroethylene paste, which was then heated at 40°C. After maturing at 60°C for 24 hours, the material is pressed into a paste-like preform in a pre-compression device. The extrusion die is a concentric annular die with an inner diameter of 0.6 mm and an outer diameter of 1.2 mm. The preform is extruded at 60°C, degreased at 200°C, and then... C stretches 2.5 times, 360 degrees After sintering and shaping for 2 minutes, asymmetric heat treatment was performed. The asymmetric heat treatment used a 10cm long, 1.2mm inner diameter straight tube section of a horn-shaped copper heater. The heat treatment temperature was 550℃, and the treatment time was 3 seconds. The resulting asymmetric polytetrafluoroethylene hollow fiber membrane had an inner diameter of 0.6mm, an outer diameter of 1.2mm, and a static water contact angle of 112°. It has a porosity of 52% and a critical water penetration pressure of 0.40 MPa.

[0024] The prepared asymmetric polytetrafluoroethylene hollow fiber membrane can be used in membrane contactor processes where the shell side is liquid. In use, multiple hollow fiber membranes are braided and sealed within a membrane element with epoxy resin. Gas or liquid phase A is introduced into the tube side, and liquid phase B is introduced into the shell side. Mass transfer between the two phases occurs through the micropores in the membrane, but they do not mix directly. The dense outer surface layer of the membrane contacts the liquid phase in the shell side, and the small-pore structure provides high liquid penetration pressure, preventing liquid phase from entering the membrane pores and causing membrane wetting. The high porosity of the membrane ensures low mass transfer resistance and improves mass transfer efficiency. This membrane is particularly suitable for membrane contactor processes such as membrane absorption (e.g., CO2 absorption), membrane distillation (e.g., seawater desalination), membrane ammonia removal (e.g., wastewater treatment), and membrane extraction (e.g., metal ion extraction).

[0025] Example 2 Polytetrafluoroethylene (PTFE) dispersion resin (number average molecular weight 8 million) and aviation kerosene were mixed uniformly at a mass fraction of 78% and 22% respectively to form a PTFE paste. After maturing at 40℃ for 24 hours, the paste was pressed into a pre-pressing device to form a preform. A concentric annular die with an inner diameter of 0.5 mm and an outer diameter of 1.0 mm was used for extrusion. The preform was extruded at 80℃, degreased at 220℃, stretched 4.0 times at 200℃, and sintered at 370℃ for 1 minute before undergoing asymmetric heat treatment. The asymmetric heat treatment used a horn-shaped copper heater with a straight tube section length of 5 cm and an inner diameter of 1.0 mm. The asymmetric heat treatment temperature was 600℃, and the heat treatment time was 2 seconds. The resulting asymmetric PTFE hollow fiber membrane had an inner diameter of 0.5 mm, an outer diameter of 1.0 mm, and a static water contact angle of 116°. It has a porosity of 68% and a critical water penetration pressure of 0.12 MPa.

[0026] Although the prepared asymmetric polytetrafluoroethylene hollow fiber membrane has a relatively low critical water penetration pressure, its porosity reaches 68%. This high porosity characteristic makes it particularly suitable for applications sensitive to mass transfer resistance, such as membrane deammoniation processes where the tube side is liquid and the shell side is gas.

[0027] Comparative Example 1 Polytetrafluoroethylene (PTFE) dispersion resin (number average molecular weight 6 million) and isoparaffin Isopar G were mixed uniformly at a mass fraction of 80% and 20% respectively to form a PTFE paste. After curing at 40℃ for 24 hours, the paste was pressed into a pre-pressing device to form a paste-like preform. The preform was extruded using a concentric annular die with an inner diameter of 0.6 mm and an outer diameter of 1.2 mm. The preform was extruded at 60℃, degreased at 200℃, stretched 2.5 times at 200℃, and sintered at 360℃ for 2 minutes to obtain a homogeneous PTFE hollow fiber membrane with an inner diameter of 0.6 mm, an outer diameter of 1.2 mm, and a static water contact angle of 112°. It has a porosity of 53% and a critical water penetration pressure of 0.25 MPa.

[0028] Comparative Example 2 Polytetrafluoroethylene (PTFE) dispersion resin (number average molecular weight 6 million) and isoparaffin Isopar G were mixed uniformly at a mass fraction of 80% and 20% respectively to form a PTFE paste. After curing at 40°C for 24 hours, the paste was pressed into a pre-pressing device to form a paste-like preform. The preform was extruded using a concentric annular die with an inner diameter of 0.6 mm and an outer diameter of 1.2 mm. The preform was extruded at 60°C, degreased at 200°C, stretched 2.5 times at 200°C, and sintered at 360°C for 2 minutes to obtain a homogeneous PTFE hollow fiber membrane. Subsequently, a fluorine coating method was used for surface treatment: the PTFE hollow fiber membrane obtained above was immersed in a fluorinated silane ethanol solution (concentration 5 wt%) for 30 minutes, removed, dried at 80°C for 2 hours, and then cured at 150°C for 1 hour to finally obtain a PTFE hollow fiber membrane with a dense surface coating. The resulting membrane has an inner diameter of 0.6 mm, an outer diameter of 1.2 mm, a static water contact angle of 118°, a porosity of 46%, and a critical water penetration pressure of 0.38 MPa. This method has a preparation cycle of over 8 hours and requires expensive fluorinated silane reagents, resulting in high costs. Furthermore, the coating layer has limited adhesion to the PTFE matrix, posing a risk of detachment during long-term operation.

[0029] Comparative Example 3 Polytetrafluoroethylene dispersion resin (number average molecular weight 6 million) and isoparaffin Isopar G were mixed evenly at a mass fraction of 80% and 20% respectively to form a polytetrafluoroethylene paste. After maturing at 40°C for 24 hours, the paste was pressed into a pre-pressing device. The extrusion die was a concentric annular die with an inner diameter of 0.6 mm and an outer diameter of 1.2 mm. The preform was extruded at 60°C, degreased at 200°C, stretched 2.5 times at 200°C, and sintered at 360°C for 2 minutes to obtain a homogeneous polytetrafluoroethylene hollow fiber membrane. Subsequently, a surface densification treatment was performed using a wrapping method: the PTFE hollow fiber membrane prepared above was used as the core fiber, and ultrafine PTFE fibers (approximately 1-2 μm in diameter) were tightly wrapped around the outer surface of the membrane in a spiral manner, with the wrapping density controlled at 15-20 turns per centimeter. Then, a secondary sintering process was carried out at 380℃ for 10 minutes to fuse the wrapped fibers with the base membrane to form a dense outer layer. The resulting composite membrane has an inner diameter of 0.6 mm, an outer diameter of approximately 1.4 mm (due to the increased thickness from the wrapping layer), a static water contact angle of 115°, a porosity of 42% (including the overall porosity after the wrapping layer), and a critical water penetration pressure of 0.42 MPa. This method is complex, requires specialized wrapping equipment, is difficult to scale up for continuous production, and the wrapping process can easily cause unevenness on the membrane surface, affecting mass transfer performance.

[0030] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A polytetrafluoroethylene hollow fiber membrane with an asymmetric structure, characterized in that, The polytetrafluoroethylene hollow fiber membrane has an asymmetrical structure in the cross-sectional direction. Its average pore size on the outer surface is smaller than that on the main body and the inner surface, and the outer surface has a dense layer.

2. The polytetrafluoroethylene hollow fiber membrane with an asymmetric structure according to claim 1, characterized in that, Porosity not less than 45%.

3. The polytetrafluoroethylene hollow fiber membrane with an asymmetric structure according to claim 1, characterized in that, The hollow fiber membrane has an inner diameter of 0.4~0.8 mm and an outer diameter of 0.8~1.6 mm.

4. The polytetrafluoroethylene hollow fiber membrane with an asymmetric structure according to claim 1, characterized in that, The polytetrafluoroethylene hollow fiber membrane is a hydrophobic membrane with a water contact angle greater than 100°.

5. A method for preparing a polytetrafluoroethylene hollow fiber membrane with an asymmetric structure, characterized in that, Includes the following steps: S1. Mixing: Mix polytetrafluoroethylene dispersion resin with extrusion aid to obtain polytetrafluoroethylene paste; S2. Extrusion: The polytetrafluoroethylene paste obtained in step S1 is cured, pressed, and then extruded to obtain a natural hollow fiber membrane. S3. Degreasing: The original hollow fiber membrane obtained in step S2 is degreased to remove extrusion aids; S4. Stretching: Stretching the degreased hollow fiber membrane obtained in step S3; S5. Sintering: The stretched hollow fiber membrane obtained in step S4 is sintered and shaped. S6. Asymmetric heat treatment: The hollow fiber membrane obtained in step S5 is subjected to asymmetric heat treatment.

6. The preparation method according to claim 5, characterized in that, In S1, the molecular weight of the polytetrafluoroethylene dispersion resin is 2 million to 10 million, and the extrusion aid is aviation kerosene or isoparaffin, with a mass fraction of 17% to 25%; Preferably, in step S2, the extrusion temperature is 40~80℃; Preferably, in S3, the degreasing temperature is 20-50°C higher than the boiling point of the extrusion aid; Preferably, in step S4, the stretching temperature is 150~250℃, and the stretching ratio is 2.0~5.0; Preferably, in step S5, the sintering temperature is 350~380℃ and the sintering time is 0.5~3.0min; Preferably, in S6, the asymmetric heat treatment temperature is 450~600℃, and the heat treatment time is 2~20s.

7. The preparation method according to claim 5, characterized in that, In S6, the asymmetric heat treatment temperature is 500~580℃, and the heat treatment time is 2~10s.

8. The preparation method according to claim 5, characterized in that, In S6, asymmetric heat treatment is carried out using a trumpet-shaped copper heater. The effective heating straight tube section of the trumpet-shaped copper heater is 4~10cm long, and its inner diameter is equivalent to the outer diameter of the polytetrafluoroethylene hollow fiber membrane.

9. The preparation method according to claim 5, characterized in that, In S5, after sintering and shaping, the polytetrafluoroethylene hollow fiber membrane is rapidly cooled to room temperature using an air knife.

10. The application of the polytetrafluoroethylene hollow fiber membrane according to claim 1 in a membrane contactor, characterized in that, Polytetrafluoroethylene hollow fiber membranes are woven and then sealed inside membrane elements with epoxy resin for use in membrane contactor processes where the shell side is liquid.

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