Hollow fiber microfiltration membrane and method for producing the same

Abstract

The application discloses a kind of hollow fiber microfiltration membrane and its preparation method, including the following steps: spinning solution and core liquid are co-extruded by hollow fiber spinning spinneret, preliminary phase conversion is formed, and primary hollow fiber membrane filament is obtained;The primary hollow fiber membrane filament is washed by multiple stages, and the hollow fiber microfiltration membrane is obtained after drying.The gradient temperature difference of washing and the residence time of primary hollow fiber membrane filament in each stage of washing are controlled, so that the full elution of pore-forming agent and the directional migration and enrichment of hydrophilic additive to the membrane surface can be realized simultaneously, the membrane filament after washing can be directly dried, no additional elution post-processing step is needed in the whole process, the loss of hydrophilic additive is not caused, and the obtained hollow fiber microfiltration membrane can maintain good hydrophilicity and reduce membrane pollution phenomenon.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, and in particular to a hollow fiber microfiltration membrane and its preparation method. Background Technology

[0002] Hollow fiber microfiltration technology is one of the earliest separation technologies applied in membrane process industry. Its operating pressure range is approximately 0.01 MPa to 0.02 MPa, and the pore size of the separation membrane used is 0.1 μm to 10 μm. It can retain solid particles, microorganisms, colloids, etc., in suspensions while allowing macromolecules and dissolved substances to pass through, ultimately achieving the purpose of separation and purification. Hollow fiber microfiltration technology has wide applications in the separation of high-value materials in food, biopharmaceuticals, medical engineering, and other fields.

[0003] Hollow fiber microfiltration membranes are the core component of hollow fiber microfiltration technology. They can be prepared using the non-solvent phase-induced separation (NIPS) method. This method controls the membrane pore size, structure, and performance by manipulating the thermodynamic properties of the polymer and solvent, as well as the dual diffusion kinetics of solvent and non-solvent during phase separation. Because hollow fiber microfiltration membranes require large pore sizes and hydrophilicity, a large amount of porogen and hydrophilic additives are typically added during their preparation to ensure micron-sized pores and excellent hydrophilicity. However, excessive porogen can lead to incomplete elution, necessitating post-treatment to remove the porogen.

[0004] In existing technologies, patent CN105771702A requires the spun microfiltration membrane fibers to be soaked sequentially in sodium hypochlorite aqueous solution and pure water for 24-72 hours to elute free pore-forming agents; patent CN115475536B requires the microfiltration membrane fibers to be soaked in isopropanol for 2 hours and rinsed in pure water for 24 hours before drying; patent CN118059690A requires the spun hollow fibers to be soaked in water at 30℃-42℃ for 6-24 hours for post-treatment, during which the water needs to be changed continuously. After centrifugation and dehydration, the fibers also need to undergo pore-preserving treatment to finally obtain hollow fiber microfiltration membranes with pore sizes of 0.01μm-1μm. Although the above post-treatment processes can ensure the pure water flux of the membrane fibers, the harsh elution conditions may cause the hydrophilic additives to be lost rapidly at once, reducing the hydrophilicity of the membrane fibers. This will lead to increased membrane fouling in certain applications. In addition, cumbersome post-processing steps may affect the stability of the membrane fiber structure, reduce production efficiency, and increase production costs. Summary of the Invention

[0005] In view of this, this application provides a hollow fiber microfiltration membrane and its preparation method. The preparation method provided by this application does not require complicated post-processing steps, and while ensuring sufficient elution of the pore-forming agent, it does not cause excessive loss of hydrophilic additives, thus ensuring the hydrophilicity of the hollow fiber microfiltration membrane and improving production efficiency.

[0006] This application provides a method for preparing a hollow fiber microfiltration membrane, comprising the following steps:

[0007] The spinning solution and core solution are co-extruded through a hollow fiber spinning spinneret and formed into nascent hollow fiber membrane filaments through preliminary phase inversion. The spinning solution includes a polymer, a pore-forming agent, a hydrophilic additive, a good solvent, and a poor solvent. The core solution includes a good solvent and a poor solvent.

[0008] The nascent hollow fiber membrane fibers are subjected to multiple stages of washing and drying to obtain a hollow fiber microfiltration membrane. The washing temperature at each stage is increased sequentially along the fiber feeding direction to synergistically control the residence time of the nascent hollow fiber membrane fibers in each stage of washing, thereby regulating the directional migration and enrichment of hydrophilic additives to the membrane surface.

[0009] In some specific implementations, the temperature of each stage of water washing is 50℃~100℃;

[0010] The residence time for each stage of the water washing is independently 20s~70s;

[0011] The temperature difference between two adjacent washing stages in the multi-stage washing process is 10℃~30℃.

[0012] In some specific implementations, the multi-stage washing is a three-stage washing.

[0013] In some specific implementations, the temperature of the first stage water wash is 50℃~75℃, the temperature of the second stage water wash is 60℃~85℃, and the temperature of the third stage water wash is 70℃~100℃.

[0014] In some specific implementations, the spinning solution comprises, by weight, 12 to 18 parts of polymer, 3 to 12 parts of pore-forming agent, 1.5 to 5 parts of hydrophilic additive, 60 to 80 parts of good solvent, and 1 to 7 parts of poor solvent.

[0015] In some specific implementations, the polymer includes one or more of polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polyamide, or polyimide;

[0016] The pore-forming agent includes one or more of polyvinylpyrrolidone, copolyvinyl ketone, zinc chloride, or lithium chloride.

[0017] The hydrophilic additive includes one or more of polyethylene glycol, polyoxyethylene-b-polyoxypropylene-b-polyoxyethylene, polyethylene oxide-b-polyoxypropylene, poly(4-vinylpyridine-co-methacryloyloxyethylphosphocholine), poly(2-methacryloyloxyethylphosphocholine), poly(sulfobetaine methacrylate), or poly(carboxybetaine methacrylate).

[0018] The good solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, or dioxane;

[0019] The unsuitable solvents include one or more small molecule alcohols such as deionized water, methanol, ethanol, or isopropanol.

[0020] In some specific implementations, the weight percentage of good solvent in the core fluid is 50 to 80 parts, and the weight percentage of bad solvent is 20 to 50 parts.

[0021] In some specific implementations, the preliminary phase transformation specifically includes:

[0022] After filtering the spinning solution and core solution, the fibers are co-extruded through a hollow fiber spinning spinneret and then enter a coagulation bath for preliminary phase transformation after passing through an air gap.

[0023] In some specific implementations, the spinneret temperature is 40℃~50℃;

[0024] The air gap height is 8cm~30cm, the air gap temperature is 50℃~65℃, and the air humidity is 80~90%RH%.

[0025] The coagulation bath is water or a mixture of water and a good solvent, wherein the water is 80 to 100 parts by weight.

[0026] The temperature of the coagulation bath is 50℃~95℃.

[0027] Furthermore, this application provides a hollow fiber microfiltration membrane, which is prepared by the above-described method;

[0028] The water contact angle of the hollow fiber microfiltration membrane is 32°~53°.

[0029] The method for preparing hollow fiber microfiltration membrane disclosed in this application includes the following steps: co-extruding spinning solution and core solution through a hollow fiber spinning spinneret, and forming them through preliminary phase inversion to obtain nascent hollow fiber membrane fibers; the spinning solution includes a polymer, a pore-forming agent, a hydrophilic additive, a good solvent, and a poor solvent; the core solution includes a good solvent and a poor solvent; the nascent hollow fiber membrane fibers are washed in a multi-stage washing tank and dried to obtain a hollow fiber microfiltration membrane; the temperature of each stage of washing is increased sequentially along the fiber feeding direction to synergistically control the residence time of the nascent hollow fiber membrane fibers in each stage of washing and regulate the directional migration and enrichment of the hydrophilic additive to the membrane surface. This application involves co-extruding the spinning solution and core solution through a hollow fiber spinneret, followed by preliminary phase inversion to obtain nascent hollow fiber membrane filaments. These nascent hollow fiber membrane filaments undergo multi-stage washing, with the temperature of each wash progressively increasing along the fiber feeding direction. Simultaneously, the residence time of the nascent hollow fiber membrane filaments in each wash stage is controlled. By controlling the gradient temperature difference and the residence time of the nascent hollow fiber membrane filaments in each wash stage, the complete elution of the pore-forming agent and the directional migration and enrichment of hydrophilic additives to the membrane surface can be achieved simultaneously. The washed membrane filaments can be directly dried and collected without any additional post-washing processing steps, preventing excessive loss of hydrophilic additives. The resulting hollow fiber microfiltration membrane maintains good hydrophilicity and reduces membrane fouling. Furthermore, multi-stage washing does not affect the stability of the hollow fiber microfiltration membrane structure, improving production efficiency and reducing production costs.

[0030] Experimental results show that the hollow fiber microfiltration membrane prepared by this invention has high pure water permeability, excellent hydrophilicity and controllable pore size distribution. Under the premise of completely eliminating the traditional post-elution treatment process, the water contact angle can be as low as 32° and the pure water permeability can be as high as 23497 L·m⁻²h⁻¹bar⁻¹. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the preparation process of the hollow fiber microfiltration membrane described in this application. Detailed Implementation

[0032] It should be understood that the expression “one or more of…” individually includes each of the objects described after the expression, as well as various different combinations of two or more of the described objects, unless otherwise understood from the context and usage. The expression “and / or” combined with three or more described objects should be understood to have the same meaning, unless otherwise understood from the context.

[0033] The terms “including,” “having,” or “containing,” including the use of their grammatical synonyms, should generally be understood as open-ended and non-restrictive, for example, not excluding other unstated elements or steps, unless otherwise specifically stated or understood from the context.

[0034] It should be understood that the order of steps or the sequence of actions is not important as long as this application remains operational. Furthermore, two or more steps or actions can be performed simultaneously.

[0035] This application provides a method for preparing a hollow fiber microfiltration membrane, comprising the following steps:

[0036] The spinning solution and core solution are co-extruded through a hollow fiber spinning spinneret and formed by preliminary phase inversion to obtain nascent hollow fiber membrane filaments; the spinning solution includes a polymer, a pore-forming agent, a hydrophilic additive, a good solvent, and a poor solvent; the core solution includes a good solvent and a poor solvent.

[0037] The nascent hollow fiber membrane fibers are washed in multiple stages of water washing tanks and dried to obtain a hollow fiber microfiltration membrane. The temperature of each stage of water washing is increased sequentially along the fiber feeding direction, which synergistically controls the residence time of the nascent hollow fiber membrane fibers in each stage of water washing and regulates the directional migration and enrichment of hydrophilic additives to the membrane surface.

[0038] This application first co-extrudes the spinning solution and core solution through a hollow fiber spinning spinneret, and then performs preliminary phase inversion to obtain nascent hollow fiber membrane filaments. The spinning solution comprises: a polymer, a pore-forming agent, a hydrophilic additive, a good solvent, and a poor solvent. In some specific implementations, the spinning solution, by weight, comprises: 12-18 parts polymer, preferably 13-18 parts; 3-12 parts pore-forming agent, preferably 5-11 parts; 1.5-5 parts hydrophilic additive, preferably 2-5 parts; 60-80 parts good solvent, preferably 65-75 parts; and 1-7 parts poor solvent, preferably 1-6 parts.

[0039] In some specific implementations, the polymer includes one or more of polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polyamide, or polyimide, preferably polysulfone, polyethersulfone, or polyacrylonitrile; the pore-forming agent includes one or more of polyvinylpyrrolidone, copovidone, zinc chloride, or lithium chloride, preferably polyvinylpyrrolidone K30, polyvinylpyrrolidone K90, copovidone, or lithium chloride; the hydrophilic additive includes polyethylene glycol, polyoxyethylene-b-polyoxypropylene-b-polyoxyethylene, polyethylene oxide-b-polyoxypropylene, poly(4-vinylpyridine-co-methacryloyloxyethylphosphocholine), and poly(2-methacryloyloxy... The solvent is selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, or dioxane, preferably N,N-dimethylacetamide, N-methylpyrrolidone, or dimethyl sulfoxide; the solvent is selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, or dioxane, preferably N,N-dimethylacetamide, N-methylpyrrolidone, or dimethyl sulfoxide; the solvent is selected from deionized water, methanol, ethanol, or isopropanol, preferably deionized water or ethanol.

[0040] In this application, the porogen has good water compatibility. During the washing process, the porogen in the hollow fiber membrane gradually migrates into the water. The longer the washing time, the more porogen migrates, resulting in larger pore sizes in the hollow fiber membrane and higher pure water permeability. The hydrophilic additive has highly flexible hydrophilic segments, which can induce surface segregation through both temperature and solvent pathways. Under elevated temperatures, the mobility of the hydrophilic segments increases (entropy increase effect), driving them to migrate from the interior of the hydrophobic matrix to the surface to reduce the system's free energy. Simultaneously, its inherent thermal responsiveness allows for a reversible change in the hydrophilicity and hydrophobicity of the segments, thereby reconstructing the surface structure. The solvent induces surface segregation by regulating the dissolution state of the segments and phase separation kinetics. In solvents with hydrophilic groups, such as water and ethanol, the hydrophilic segments easily extend and segregate to the surface.

[0041] Surface segregation refers to the spontaneous migration of a component in a composite material to the surface or interface of the material under the induction of external conditions. Its essence is the process by which molecular chains migrate to the surface or interface and accumulate under the combined action of thermodynamic driving and kinetic processes.

[0042] This application does not specifically limit the preparation method of the spinning solution. Specifically, the preparation steps involve thoroughly stirring the polymer, pore-forming agent, hydrophilic additive, good solvent, and poor solvent, followed by standing and degassing to obtain a bubble-free, clear, and transparent spinning solution. In some specific implementations, the stirring temperature is 60℃~100℃, preferably 60℃, 75℃, or 95℃; the stirring time is 12h~72h, preferably 12h, 24h, or 72h; and the standing and degassing conditions are standing and degassing for 8-12h under a pressure of -0.04 to 1 MPa.

[0043] The core fluid described in this application includes a good solvent and a bad solvent. In some specific implementations, the good solvent is 50 to 80 parts by weight, preferably 55 to 80 parts, more preferably 55, 70 or 77 parts; the bad solvent is 20 to 50 parts by weight, preferably 20 to 45 parts, more preferably 23, 30 or 45 parts. This application does not impose any special restrictions on the good and bad solvents used in the core fluid, as long as they are the same types as the good and bad solvents used in the spinning solution. The types of good and bad solvents are as described above, and will not be repeated here.

[0044] The preparation steps of the core liquid in this application specifically include mixing a good solvent and a poor solvent, stirring evenly, and then fully degassing under vacuum to obtain the core liquid. This application does not impose specific restrictions on the parameters of the stirring and vacuum degassing processes, as long as the core liquid can be obtained.

[0045] This application, after obtaining the spinning solution and core solution, filters them and then co-extrudes them through an air gap into a coagulation bath for preliminary phase inversion. In some specific implementations, the specific steps of the preliminary phase inversion include: filtering the spinning solution and core solution separately; co-extruding the filtered spinning solution and core solution through the outer annular slit and the central inner hole of a hollow fiber spinning spinneret composed of two concentric circular channels; and then, after passing through an air gap, vertically falling into a coagulation bath located directly below the spinneret, where preliminary phase inversion occurs to form nascent hollow fiber membrane filaments. This application does not specifically limit the filtration method, but preferably uses a stainless steel filter element with a pore size of 20 μm.

[0046] In some specific implementations, the spinneret temperature is 40℃~50℃, the spinning solution tank temperature is 50℃~80℃, preferably 60℃~80℃, and the tank pressure is 0.25MPa~0.35MPa.

[0047] In some specific implementations, the air gap is the distance between the spinneret outlet and the coagulation bath liquid surface, the air gap height is 8cm~30cm, the air gap temperature is 50℃~65℃, and the air humidity is 80~90RH.

[0048] In some specific implementations, the coagulation bath is water or a mixture of water and a good solvent. This application does not have specific restrictions on the type of good solvent, as long as it is the same as the type of good solvent mentioned above. This application will not elaborate further here. The water is preferably purified water, and the weight parts of the water are 80 to 100 parts, preferably 90 to 100 parts. The temperature of the coagulation bath is 50℃ to 95℃, preferably 65℃ to 90℃.

[0049] This application involves preparing nascent hollow fiber membrane fibers, followed by multi-stage washing and drying to obtain a hollow fiber microfiltration membrane. The temperature of each washing stage increases sequentially along the fiber feeding direction. By controlling the residence time of each washing stage, the pore-forming agent is fully eluted, and the hydrophilic additives are fully migrated and enriched onto the membrane fiber surface. This multi-stage washing process effectively controls the elution of the pore-forming agent and the surface segregation behavior of the hydrophilic additives, avoiding excessive loss of hydrophilic additives due to prolonged post-treatment soaking.

[0050] In some specific implementations, the temperature of each stage of water washing is 50℃~100℃, preferably 50℃~95℃; the residence time of each stage of water washing is independently 20s~70s, preferably 36s~66s; the temperature difference between two adjacent stages of water washing in the multi-stage water washing is 10℃~30℃, preferably 10℃~20℃, more preferably 10℃, 12℃, 16℃, 20℃.

[0051] In some specific implementations, the multi-stage washing is a three-stage washing process; the temperature of the first stage washing is 50℃~75℃, preferably 50℃~55℃; the temperature of the second stage washing is 60℃~85℃, preferably 70℃~75℃; and the temperature of the third stage washing is 70℃~100℃, preferably 90℃~95℃. It is understood that although there is some overlap in the selection of washing temperatures at each stage in this application, it does not mean that the temperatures of each stage of washing can be the same. The temperature of the third stage washing must be greater than the temperature of the second stage washing, which in turn must be greater than the temperature of the first stage washing to ensure a strict temperature increase and create a temperature difference that effectively drives the migration of the hydrophilic additives.

[0052] Specifically, this application performs multi-stage water washing according to the following method. In some specific implementations, this application adopts gradient temperature difference online water washing, in which the obtained nascent hollow fiber membrane filaments are sequentially pulled by upper and lower rollers and continuously passed through a water washing tank with the temperature increasing sequentially along the wire feeding direction for gradient temperature difference online water washing, so that the nascent hollow fiber membrane filaments complete phase transformation during the water washing process to obtain hollow fiber microfiltration membrane.

[0053] In this application, the temperature difference between each washing tank can control the temperature-induced migration effect of hydrophilic additives. The greater the temperature difference, the faster the migration speed of the hydrophilic additives, resulting in the enrichment of hydrophilic additives on the membrane fiber surface, a decrease in the water contact angle of the membrane fiber surface, and enhanced hydrophilicity. This application controls the residence time of nascent hollow fiber membrane fibers in each washing tank by setting the roller speed. The residence time of nascent hollow fiber membrane fibers in the washing tank can control the solvent-induced migration effect of hydrophilic additives and the elution of pore-forming agents. The longer the residence time, the more pore-forming agents are eluted, the larger the membrane fiber pore size, and the higher the pure water permeability. On the other hand, the longer the residence time, the more hydrophilic additives migrate to the membrane fiber surface, the lower the water contact angle of the membrane fiber surface, and the enhanced hydrophilicity.

[0054] See Figure 1 , Figure 1 As shown in the typical preparation process flow diagram, the spinning solution and core solution are co-extruded from the spinneret, pass through the air gap, and fall into the coagulation bath. After preliminary phase transformation, nascent hollow fiber membrane filaments are obtained. The nascent hollow fiber membrane filaments are then subjected to three stages of washing: a first-stage washing tank, a second-stage washing tank, and a third-stage washing tank. The washing temperature in the first-stage washing tank is lower than that in the second-stage washing tank, and the washing temperature in the second-stage washing tank is lower than that in the third-stage washing tank. The residence time in the three washing tanks is controlled. After the membrane filaments are washed, they are dried and wound up to obtain the hollow fiber microfiltration membrane.

[0055] Furthermore, this application provides a hollow fiber microfiltration membrane, which is prepared by the above-described method;

[0056] The water contact angle of the hollow fiber microfiltration membrane is 32°~53°.

[0057] The method for preparing hollow fiber microfiltration membrane disclosed in this application includes the following steps: co-extruding spinning solution and core solution through a hollow fiber spinning spinneret, and forming them through preliminary phase inversion to obtain nascent hollow fiber membrane fibers; the spinning solution includes a polymer, a pore-forming agent, a hydrophilic additive, a good solvent, and a poor solvent; the core solution includes a good solvent and a poor solvent; subjecting the nascent hollow fiber membrane fibers to multiple stages of water washing and drying to obtain a hollow fiber microfiltration membrane; the temperature of each stage of water washing is increased sequentially along the fiber feeding direction to synergistically control the residence time of the nascent hollow fiber membrane fibers in each stage of water washing, thereby regulating the directional migration and enrichment of the hydrophilic additive to the membrane surface. This application involves co-extruding the spinning solution and core solution through a hollow fiber spinneret, followed by preliminary phase inversion to obtain nascent hollow fiber membrane filaments. These nascent hollow fiber membrane filaments undergo multi-stage washing, with the temperature of each wash progressively increasing along the fiber feeding direction. Simultaneously, the residence time of the nascent hollow fiber membrane filaments in each wash stage is controlled. By controlling the gradient temperature difference during washing and the residence time of the nascent hollow fiber membrane filaments in each wash stage, the complete elution of the pore-forming agent and the directional migration and enrichment of hydrophilic additives to the membrane surface can be achieved simultaneously. The washed membrane filaments can be directly dried and collected without any additional post-washing processing steps, preventing excessive loss of hydrophilic additives. The resulting hollow fiber microfiltration membrane maintains good hydrophilicity and reduces membrane fouling. Furthermore, multi-stage washing does not affect the stability of the hollow fiber microfiltration membrane structure, improving production efficiency and reducing production costs.

[0058] Experimental results show that the hollow fiber microfiltration membrane prepared by this invention has high pure water permeability, excellent hydrophilicity and controllable pore size distribution. Under the premise of completely eliminating the traditional post-elution treatment process, the water contact angle can be as low as 32° and the pure water permeability can be as high as 23497 L·m⁻²h⁻¹bar⁻¹.

[0059] The present application is further illustrated below with reference to embodiments. The scope of protection of the present application is not limited to the following embodiments.

[0060] Example 1

[0061] 1) Add 15.3 parts of polyethersulfone, 8.6 parts of polyvinylpyrrolidone K30, 2.1 parts of polyvinylpyrrolidone K90, 1.8 parts of polyethylene glycol, and 5.7 parts of deionized water to 66.5 parts of N,N-dimethylacetamide, stir thoroughly at 75°C for 24 h, and then let stand under -0.04 MPa pressure for 10 h to degas, to obtain a bubble-free, clear and transparent spinning solution;

[0062] 2) Mix 70 parts of N,N-dimethylacetamide and 30 parts of deionized water, stir evenly and fully degas under vacuum to prepare the core solution;

[0063] 3) The spinning solution and core solution are introduced into a stainless steel filter element with a pore size of 20μm through a delivery pump. After filtration, the spinning solution and core solution are extruded from the outer ring seam and the central inner hole of a hollow fiber spinning spinneret composed of two concentric circular channels, respectively. The temperature inside the spinning solution tank is maintained at 60℃~70℃, the pressure inside the tank is 0.28 MPa~0.32 MPa, and the core solution temperature is 25℃~30℃. Circulating water is used to control the temperature of the spinneret, and the spinneret temperature is set at 45℃~50℃. After passing through the air gap, the extruded spinning solution and core solution fall vertically into a coagulation bath containing 100wt% purified water at a temperature of 65℃~75℃ located directly below the spinneret for preliminary phase inversion to obtain nascent hollow fiber membrane filaments. The air gap is the distance from the spinneret to the surface of the coagulation bath water, the air gap height is maintained at 13cm, the air humidity is maintained at 80~90 RH%, and the air gap temperature is maintained at 50℃~65℃.

[0064] 4) The nascent hollow fiber membrane fibers are sequentially drawn through upper and lower rollers into three washing tanks containing 100 wt% purified water, with the temperature increasing sequentially and maintaining a fixed temperature difference of 20℃. Specifically, the washing temperature of the first washing tank is 50℃~55℃, the washing temperature of the second washing tank is 70℃~75℃, and the washing temperature of the third washing tank is 90℃~95℃. The drawing speed is adjusted so that the residence time of the membrane fibers in each washing tank is maintained at 46s~48s, so that the hydrophilic additives with surface segregation properties gradually migrate to the surface of the membrane fibers under the solvent-temperature induction.

[0065] 5) The membrane fibers that have completed the gradient temperature difference water washing are not dried. The membrane fibers are directly wound up while still moist. Then, the membrane fibers are placed in a fiber storage box with a temperature of 80℃ and a composition of 100wt% purified water for elution and post-treatment. The soaking time is 24h.

[0066] 6) After post-processing, the membrane fibers are naturally dried and packaged into bundles as required to obtain hollow fiber microfiltration membranes.

[0067] Example 2

[0068] 1) Add 15.3 parts of polyethersulfone, 8.6 parts of polyvinylpyrrolidone K30, 2.1 parts of polyvinylpyrrolidone K90, 1.8 parts of polyethylene glycol, and 5.7 parts of deionized water to 66.5 parts of N,N-dimethylacetamide, stir thoroughly at 75°C for 24 h, and then let stand under -0.04 MPa pressure for 10 h to degas, to obtain a bubble-free, clear and transparent spinning solution;

[0069] 2) Mix 70 parts of N,N-dimethylacetamide and 30 parts of deionized water, stir evenly and fully degas under vacuum to prepare the core solution;

[0070] 3) The spinning solution and core solution are introduced into a stainless steel filter with a pore size of 20μm by a delivery pump. After filtration, the spinning solution and core solution are extruded from the outer ring seam and the central inner hole of a hollow fiber spinning spinneret composed of two concentric circular channels, respectively. The temperature inside the spinning solution tank is maintained at 60℃~70℃, the pressure inside the tank is 0.28 MPa~0.32 MPa, and the core solution temperature is 25℃~30℃. The temperature of the spinneret is controlled by circulating water and set at 45℃~50℃. After passing through the air gap, the extruded spinning solution and core solution fall vertically into a coagulation bath containing 100wt% purified water at a temperature of 65℃~75℃ located directly below the spinneret for preliminary phase transformation to form nascent hollow fiber membrane filaments. The air gap is the distance from the spinneret to the surface of the coagulation bath water. The air gap height is maintained at 13cm, the air humidity is maintained at 80~90 RH%, and the air gap temperature is maintained at 50℃~65℃.

[0071] 4) The nascent hollow fiber membrane fibers are sequentially drawn through upper and lower rollers into three washing tanks containing 100 wt% purified water, with the temperature increasing sequentially and maintaining a fixed temperature difference of 20℃. Specifically, the washing temperature of the first washing tank is 50℃~55℃, the washing temperature of the second washing tank is 70℃~75℃, and the washing temperature of the third washing tank is 90℃~95℃. The drawing speed is adjusted so that the residence time of the membrane fibers in each washing tank is maintained at 46s~48s, so that the hydrophilic additives with surface segregation properties gradually migrate to the surface of the membrane fibers under the solvent-temperature induction.

[0072] 5) The washed membrane fibers are dried in a circulating hot air environment. After the moisture is removed, the membrane fibers are wound up by a winding wheel and packaged into bundles as required to obtain hollow fiber microfiltration membranes.

[0073] Example 3

[0074] The difference between this embodiment and Embodiment 2 is the stretching speed, which keeps the residence time of the membrane fibers in each washing tank at 36s~38s. The other raw materials and steps are exactly the same.

[0075] Example 4

[0076] The difference between this embodiment and Embodiment 2 is the stretching speed, which keeps the residence time of the membrane fibers in each washing tank at 26s~28s. The other raw materials and steps are exactly the same.

[0077] Example 5

[0078] 1) Add 15.3 parts of polyethersulfone, 8.6 parts of polyvinylpyrrolidone K30, 2.1 parts of polyvinylpyrrolidone K90, 1.8 parts of polyethylene glycol, and 5.7 parts of deionized water to 66.5 parts of N,N-dimethylacetamide, stir thoroughly at 75°C for 24 h, and then let stand under -0.04 MPa pressure for 10 h to degas, to obtain a bubble-free, clear and transparent spinning solution;

[0079] 2) Mix 70 parts of N,N-dimethylacetamide and 30 parts of deionized water, stir evenly and fully degas under vacuum to prepare the core solution;

[0080] 3) The spinning solution and core solution are introduced into a stainless steel filter with a pore size of 20μm by a delivery pump. After filtration, the spinning solution and core solution are extruded from the outer ring seam and the central inner hole of a hollow fiber spinning spinneret composed of two concentric circular channels, respectively. The temperature inside the spinning solution tank is maintained at 60℃~70℃, the pressure inside the tank is 0.28 MPa~0.32 MPa, and the core solution temperature is 25℃~30℃. The temperature of the spinneret is controlled by circulating water and set at 45℃~50℃. After passing through the air gap, the extruded spinning solution and core solution fall vertically into a coagulation bath containing 100wt% purified water at a temperature of 65℃~75℃ located directly below the spinneret for preliminary phase transformation to form nascent hollow fiber membrane filaments. The air gap is the distance from the spinneret to the surface of the coagulation bath water. The air gap height is maintained at 13cm, the air humidity is maintained at 80~90 RH%, and the air gap temperature is maintained at 50℃~65℃.

[0081] 4) The nascent hollow fiber membrane fibers are sequentially drawn through upper and lower rollers into three washing tanks containing 100 wt% purified water, with the temperature increasing sequentially and maintaining a fixed temperature difference of 10℃. Specifically, the washing temperature of the first washing tank is 50℃~55℃, the washing temperature of the second washing tank is 60℃~65℃, and the washing temperature of the third washing tank is 70℃~75℃. The drawing speed is adjusted so that the residence time of the membrane fibers in each washing tank is maintained at 26s~28s, so that the hydrophilic additives with surface segregation properties gradually migrate to the surface of the membrane fibers under the solvent-temperature induction.

[0082] 5) The washed membrane fibers are dried in a circulating hot air environment. After the moisture is removed, the membrane fibers are wound up by a winding wheel and packaged into bundles as required to obtain hollow fiber microfiltration membranes.

[0083] Example 6

[0084] The difference between this embodiment and Embodiment 5 is the stretching speed, which keeps the residence time of the membrane fibers in each washing tank at 46s~48s. The other raw materials and steps are exactly the same.

[0085] Example 7

[0086] 1) 17.6 parts polyacrylonitrile, 0.9 parts lithium chloride, 3.9 parts polyvinylpyrrolidone K90, 2.4 parts polyoxyethylene-b-polyoxypropylene-b-polyoxyethylene, and 1.2 parts ethanol were added to 74 parts N-methylpyrrolidone and stirred thoroughly at 95°C for 12 h. Then, the mixture was allowed to stand under a pressure of -0.06 MPa for 12 h to remove bubbles, resulting in a clear and transparent spinning solution without bubbles.

[0087] 2) Mix 55 parts of N-methylpyrrolidone and 45 parts of deionized water, stir evenly and fully degas under vacuum to prepare the core solution;

[0088] 3) The spinning solution and core solution are introduced into a stainless steel filter element with a pore size of 20μm via a delivery pump. After filtration, the spinning solution and core solution are extruded from the outer ring slit and the central inner hole of a hollow fiber spinning spinneret composed of two concentric flow channels, respectively. The temperature inside the spinning solution tank is maintained at 70℃~80℃, the pressure inside the tank is 0.32MPa~0.35MPa, and the core solution temperature is 25℃~30℃. Circulating water is used to control the temperature of the spinneret, which is set at 40℃~45℃. After passing through an air gap, the extruded spinning solution and core solution fall vertically into a coagulation bath located directly below the spinneret. The temperature is 85℃~90℃ and it contains 95wt% purified water and 5wt% dioxane for preliminary phase transformation, forming nascent hollow fiber membrane filaments. The air gap is the distance from the spinneret to the surface of the coagulation bath, and the air gap height is maintained at 30cm. The air humidity is maintained at 80~90%. RH%, air gap temperature maintained at 50℃~65℃;

[0089] 4) The nascent hollow fiber membrane fibers are sequentially drawn through upper and lower rollers into three washing tanks containing 100 wt% purified water, with the temperature increasing sequentially and maintaining a fixed temperature difference of 16℃. Specifically, the washing temperature of the first washing tank is 56℃~61℃, the washing temperature of the second washing tank is 72℃~77℃, and the washing temperature of the third washing tank is 88℃~93℃. The drawing speed is adjusted so that the residence time of the membrane fibers in each washing tank is maintained at 63s~65s, so that the hydrophilic additives with surface segregation properties gradually migrate to the surface of the membrane fibers under the solvent-temperature induction.

[0090] 5) The washed membrane fibers are dried in a circulating hot air environment. After the moisture is removed, the membrane fibers are wound up by a winding wheel and packaged into bundles as required to obtain hollow fiber microfiltration membranes.

[0091] Example 8

[0092] 1) Add 13.3 parts of polysulfone, 1.2 parts of sulfonated polyethersulfone, 5.2 parts of copolyvinylpyrrolidone, 4.8 parts of poly(4-vinylpyridine-co-methacryloyloxyethylphosphocholine), and 4.6 parts of deionized water to 70.9 parts of dimethyl sulfoxide. Stir thoroughly at 60°C for 72 hours, and then let stand under -1 MPa pressure for 8 hours to remove bubbles, to obtain a bubble-free, clear, and transparent spinning solution.

[0093] 2) Mix 77 parts of dimethyl sulfoxide and 23 parts of deionized water, stir evenly and fully degas under vacuum to prepare the core solution;

[0094] 3) The spinning solution and core solution are introduced into a stainless steel filter element with a pore size of 20μm through a delivery pump. After filtration, the spinning solution and core solution are extruded from the outer ring seam and the central inner hole of a hollow fiber spinning spinneret composed of two concentric circular channels, respectively. The temperature inside the spinning solution tank is maintained at 60-70℃, the pressure inside the tank is 0.25Mpa-0.28MPa, and the core solution temperature is 25℃-30℃. The temperature of the spinneret is controlled by circulating water and set at 40℃-45℃. After passing through the air gap, the extruded spinning solution and core solution fall vertically into a coagulation bath containing 100wt% purified water at a temperature of 65℃-68℃ located directly below the spinneret for preliminary phase transformation to form nascent hollow fiber membrane filaments. The air gap is the distance from the spinneret to the surface of the coagulation bath water. The air gap height is maintained at 8cm, the air humidity is maintained at 80-90% RH%, and the air gap temperature is maintained at 50℃-65℃.

[0095] 4) The nascent hollow fiber membrane fibers are sequentially drawn through upper and lower rollers into three washing tanks containing 100 wt% purified water, with the temperature increasing sequentially and maintaining a fixed temperature difference of 12℃. Specifically, the washing temperature of the first washing tank is 66℃~71℃, the washing temperature of the second washing tank is 78℃~83℃, and the washing temperature of the third washing tank is 90℃~95℃. The drawing speed is adjusted so that the residence time of the membrane fibers in each washing tank is maintained at 56s~58s, so that the hydrophilic additives with surface segregation properties gradually migrate to the surface of the membrane fibers under the solvent-temperature induction.

[0096] 5) The washed membrane fibers are dried in a circulating hot air environment. After the moisture is removed, the membrane fibers are wound up by a winding wheel and packaged into bundles as required to obtain hollow fiber microfiltration membranes.

[0097] Comparative Example 1

[0098] The difference between this comparative example and Example 1 is that the first to third stage washing tanks are at a constant temperature with no temperature difference setting, all at 50℃~55℃. The other raw materials and steps are exactly the same.

[0099] Comparative Example 2

[0100] The difference between this comparative example and Example 1 is that the first to third stage washing tanks are at a constant temperature with no temperature difference setting, all at 90℃~95℃. The other raw materials and steps are exactly the same.

[0101] Comparative Example 3

[0102] The difference between this comparative example and Example 2 is that the residence time of the membrane fibers in each washing tank is maintained at 13s~15s, while the other raw materials and steps are exactly the same.

[0103] Comparative Example 4

[0104] The difference between this comparative example and Example 6 is that the residence time of the membrane fibers in each washing tank is maintained at 76s~78s, while the other raw materials and steps are exactly the same.

[0105] Performance testing:

[0106] 1) Pure water permeability test. Pure water permeability is one of the key indicators for measuring the separation efficiency of microfiltration membranes. It is defined as the volume or mass of water passing through per unit time, unit area, and unit pressure. The higher the value, the higher the membrane separation efficiency. The test method is carried out in accordance with the standard HY / T 051-1999 "Test Method for Hollow Fiber Microporous Filter Membranes".

[0107] 2) Water Contact Angle Test. The water contact angle is a direct description of the hydrophilicity of the membrane fibers; the smaller the water contact angle, the stronger the hydrophilicity of the membrane fibers. The water contact angle of the membrane fibers was measured using a German Dataphysics OCA25 video optical contact angle meter. The droplet volume was set to 1 μL, and recording was started to record the changes of the water droplet on the membrane fiber surface. The contact angle value when the water droplet completely landed on the membrane fiber surface was taken as the water contact angle.

[0108] 3) Most probable pore size. The most probable pore size is the pore size corresponding to the peak value in the membrane pore size distribution curve, representing the size range where the pores are most concentrated in the separation membrane. The most probable pore size of the hollow fiber membrane was tested using a Bestech BSD-PB gas-liquid displacement bubble pressure filter membrane pore size analyzer. The wetting liquid was BSD16, and the gas-liquid interfacial tension was 16.00 dyn / cm. The test results are shown in Table 1.

[0109] Table 1 shows the experimental data for the proportions and examples.

[0110]

[0111] Under the premise of retaining 24h post-elution treatment, Example 1, which uses a fixed gradient temperature difference water washing process at 20℃, showed no significant difference in pure water permeability, most probable pore size, and water contact angle compared to Comparative Examples 1 and 2, which use constant temperature water washing without temperature difference. This proves that the presence of post-elution treatment masks the performance gains brought by the gradient temperature difference water washing process. However, Example 2, which uses the same gradient temperature difference water washing process as Example 1 but completely eliminates post-elution treatment, achieved a pure water permeability of 21774 L·m⁻²h⁻¹bar⁻¹ and a most probable pore size of 0.520 μm, which is basically the same as Comparative Examples 1, 2, and Example 1. This proves that the gradient temperature difference water washing process of the present invention can complete the sufficient elution of pore-forming agents during the membrane fiber formation process without the need for additional post-elution treatment. At the same time, the water contact angle of Example 2 was only 32°, compared to 58° of Example 1. This significantly reduces the amount of hydrophilicity of the membrane fibers, thereby fundamentally avoiding the problem of excessive loss of hydrophilic additives caused by traditional high-temperature elution and post-treatment.

[0112] This invention, through comparative experiments, clarified the control mechanism of two core process parameters—gradient temperature difference in the washing tank and single-tank washing residence time of nascent hollow fiber membrane fibers—on the performance of hollow fiber microfiltration membranes, and also verified the multi-system adaptability of the process. Experimental results show that the washing residence time and gradient temperature difference in the washing tank synergistically affect the elution of pore-forming agents and the surface migration of hydrophilic additives in nascent hollow fiber membrane fibers. Too short a washing residence time, even under high gradient temperature conditions, will result in incomplete elution of pore-forming agents and insufficient migration of hydrophilic additives to the membrane surface, leading to low pure water permeability and poor hydrophilicity of the hollow fiber microfiltration membrane. For example, in Comparative Example 3, compared to Example 2, the washing residence time was reduced to 13-15 seconds, and the pure water permeability of the hollow fiber microfiltration membrane increased from 21774 L·m⁻¹. -2 h -1 bar -1 Reduced to 7349 L·m -2 h -1 bar -1The water contact angle increased from 32° to 60°, and the hydrophilicity decreased significantly. Excessive water washing time, even under low gradient temperature difference conditions, can cause excessive migration and loss of hydrophilic additives, resulting in an increase in the water contact angle, i.e., a decrease in the hydrophilicity of the hollow fiber microfiltration membrane. For example, compared with Example 6, the water washing time in Comparative Example 4 was extended to 76~78s, and the water contact angle increased from 44° to 59°, resulting in a significant decrease in the hydrophilicity of the hollow fiber microfiltration membrane. With a fixed gradient temperature difference, appropriately extending the residence time of the membrane fibers in the washing tank helps to more fully elute the pore-forming agent and promotes the migration of hydrophilic additives to the membrane surface. This results in the hollow fiber microfiltration membrane exhibiting higher pure water permeability, lower water contact angle, and larger most probable pore size. For example, in Examples 2-4, when the residence time was increased from 26-28s to 46-48s, the pure water permeability of the hollow fiber microfiltration membrane increased from 11723 L·m⁻²h⁻¹bar⁻¹ to 21774 L·m⁻²h⁻¹bar⁻¹, and the water contact angle decreased from 48° to 32°. On the other hand, under the same residence time, increasing the gradient temperature difference of the washing tank can enhance the temperature-induced migration effect of hydrophilic additives, thereby improving the hydrophilicity and pure water permeability of the hollow fiber microfiltration membrane. At a residence time of 46s~48s, Example 2 with a temperature difference of 20℃ exhibited better hydrophilicity and pure water permeability than Example 6 with a temperature difference of 10℃. In addition, the gradient temperature difference water washing process of the present invention can be adapted to a variety of mainstream film-forming polymer spinning systems. Example 7, which uses a polyacrylonitrile spinning system, and Example 8, which uses a polysulfone / sulfonated polyethersulfone composite spinning system, both adopted this process and completely eliminated post-washing treatment. The hollow fiber microfiltration membranes prepared in the end all showed excellent comprehensive performance.

[0113] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in this application, based on the technical solution and application concept of this application, should be included within the scope of protection of this application.

Claims

1. A method for preparing a hollow fiber microfiltration membrane, characterized in that, Includes the following steps: The spinning solution and core solution are co-extruded through a hollow fiber spinning spinneret and formed by preliminary phase inversion to obtain nascent hollow fiber membrane filaments; the spinning solution includes a polymer, a pore-forming agent, a hydrophilic additive, a good solvent, and a poor solvent; the core solution includes a good solvent and a poor solvent. The nascent hollow fiber membrane fibers are subjected to multiple stages of washing and drying to obtain a hollow fiber microfiltration membrane. The temperature of each stage of washing is increased sequentially along the fiber feeding direction to synergistically control the residence time of the nascent hollow fiber membrane fibers in each stage of washing, thereby regulating the directional migration and enrichment of hydrophilic additives to the membrane surface.

2. The preparation method according to claim 1, characterized in that, The temperature of each stage of water washing is 50℃~100℃; The residence time for each stage of the water washing is independently 20s~70s; The temperature difference between two adjacent water washing stages in the multi-stage water washing is 10℃~30℃.

3. The preparation method according to any one of claims 1 or 2, characterized in that, The multi-stage washing is a three-stage washing process.

4. The preparation method according to claim 3, characterized in that, The temperature for the first stage of washing is 50℃~75℃, the temperature for the second stage of washing is 60℃~85℃, and the temperature for the third stage of washing is 70℃~100℃.

5. The preparation method according to claim 1, characterized in that, The spinning solution comprises, by weight, 12 to 18 parts of polymer, 3 to 12 parts of pore-forming agent, 1.5 to 5 parts of hydrophilic additive, 60 to 80 parts of good solvent, and 1 to 7 parts of poor solvent.

6. The preparation method according to claim 5, characterized in that, The polymer includes one or more of polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polyamide, or polyimide; The pore-forming agent includes one or more of polyvinylpyrrolidone, copolyvinyl ketone, zinc chloride, or lithium chloride. The hydrophilic additive includes one or more of polyethylene glycol, polyoxyethylene-b-polyoxypropylene-b-polyoxyethylene, polyethylene oxide-b-polyoxypropylene, poly(4-vinylpyridine-co-methacryloyloxyethylphosphocholine), poly(2-methacryloyloxyethylphosphocholine), poly(sulfobetaine methacrylate), or poly(carboxybetaine methacrylate). The good solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, or dioxane; The unsuitable solvents include one or more small molecule alcohols such as deionized water, methanol, ethanol, or isopropanol.

7. The preparation method according to claim 1, characterized in that, The core fluid contains 50 to 80 parts by weight of good solvent and 20 to 50 parts by weight of bad solvent.

8. The preparation method according to claim 1, characterized in that, The preliminary phase transformation specifically includes: After filtering the spinning solution and core solution, the fibers are co-extruded through a hollow fiber spinning spinneret and then enter a coagulation bath for preliminary phase transformation after passing through an air gap.

9. The preparation method according to claim 8, characterized in that, The spinneret temperature is 40℃~50℃; The air gap height is 8cm~30cm, the air gap temperature is 50℃~65℃, and the air humidity is 80~90%RH%. The coagulation bath is water or a mixture of water and a good solvent, wherein the water is 80 to 100 parts by weight. The temperature of the coagulation bath is 50℃~95℃.

10. A hollow fiber microfiltration membrane, characterized in that, Prepared by the preparation method according to any one of claims 1 to 9; The water contact angle of the hollow fiber microfiltration membrane is 32°~53°.

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