A method for preparing a PVDF hollow fiber ultrafiltration membrane based on an amphiphilic fluorine-containing organosilane modification

Abstract

The application relates to a preparation method of an amphiphilic fluorine-containing organosilane modified PVDF hollow fiber ultrafiltration membrane, and fluorine-containing silane coupling agents are introduced in a phase transition process, so that the PVDF ultrafiltration membrane surface has strong hydrophilicity, is easy to combine with water molecules to form a hydration layer, the hydrophilicity of the PVDF ultrafiltration membrane is improved, and the possibility of adsorption of pollutants to the PVDF ultrafiltration membrane surface is reduced. After physical blending of silane and PVDF is completed, gradient ethanol replacement, freezing and vacuum drying are combined, the silane alkoxy is fixed in the PVDF network skeleton, the problem of gradual loss of silane with water flow caused by residual solvents and water in the membrane hole is avoided, and the long-acting property of hydrophilic modification is ensured.

Description

Technical Field

[0001] This invention relates to a method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified with PVDF, and belongs to the field of ultrafiltration membrane modification technology. Background Technology

[0002] Ultrafiltration (UF) is a membrane separation technology that falls between microfiltration and nanofiltration. It utilizes ultrafiltration membranes with pore sizes of approximately 0.01-0.1 micrometers, driven by pressure difference, to separate and retain large molecules such as particles and colloids in liquids, allowing only small molecules such as ions and water to pass through. This achieves the purpose of separation, purification, and concentration. UF membranes can separate substances of different sizes by controlling different operating pressures and are widely used in water treatment, pharmaceuticals, food processing, and chemical industries. Currently, the main UF materials are polyethersulfone (PES), polyvinyl chloride (PVC), polycarbonate (PCC), polyacrylonitrile (PAC), and polyvinylidene fluoride (PVDF). Among these, PVDF has many advantages, including excellent chemical resistance, high-temperature resistance, and oxidation resistance. Furthermore, its good mechanical properties allow it to withstand higher transmembrane pressures. PVDF ultrafiltration membranes can generate higher fluxes at lower transmembrane pressures, making it a widely used UF membrane material.

[0003] Although polyvinylidene fluoride (PVDF) is widely used in the preparation of ultrafiltration membranes due to its excellent properties, its poor hydrophilicity makes it prone to adsorbing organic macromolecules, such as proteins or organic pollutants, during the filtration process. This can easily lead to membrane clogging and fouling, affecting the permeate flux of the PVDF ultrafiltration membrane, and consequently its filtration performance and operating costs.

[0004] Coal chemical wastewater contains a large amount of organic pollutants, high concentrations of salts, heavy metals, and other pollutants, which can easily cause clogging and fouling of polyvinylidene fluoride (PVDF) ultrafiltration membranes, leading to a decline in their performance. Therefore, hydrophilic modification of PVDF ultrafiltration membranes is of great significance for their application in coal chemical wastewater treatment. Currently, hydrophilic modification of PVDF ultrafiltration membranes mainly employs two methods: blending modification and surface modification. Blending modification typically improves the hydrophilicity of the PVDF membrane by mixing silica particles into the casting solution. However, introducing silica particles through blending can easily reduce the porosity of the PVDF ultrafiltration membrane. Surface modification includes surface coating and surface grafting. Surface coating refers to applying or depositing a very thin functional hydrophilic layer on the surface of the PVDF membrane. For example, application CN109692576A discloses a method for preparing a hollow fiber ultrafiltration membrane through interfacial crosslinking modification. The method includes the following steps: ① Preparation of casting solution: The raw materials are mixed and heated to 30-70°C in a reactor to dissolve, and a vacuum is drawn to remove air bubbles from the solution, thus preparing a casting solution; ② Preparation of hollow fiber base membrane: The casting solution and core solution are extruded through a spinneret into a coagulation bath and subjected to phase inversion to obtain a hollow fiber base membrane; ③ Interfacial crosslinking modification: The hollow fiber base membrane is immersed in an adsorbent monomer solution, then removed and placed in a crosslinking agent monomer solution. Through interfacial crosslinking modification, a crosslinking reaction occurs on the surface of the base membrane, resulting in a layer membrane with a certain thickness and gel adsorption function. The layer membrane is then washed with pure water, immersed in glycerol, and dried to obtain a modified hollow fiber ultrafiltration membrane. However, this method is essentially a surface coating and suffers from the defect of an unstable surface hydrophilic layer that is easily washed away during membrane application.

[0005] Therefore, there is a need to provide a new method for preparing PVDF hollow fiber ultrafiltration membranes to improve the overall hydrophilicity of PVDF hollow fiber ultrafiltration membranes and further enhance their performance in coal chemical wastewater treatment. Summary of the Invention

[0006] This invention provides a method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified with a fluorinated silane. The method uses a fluorinated silane coupling agent to modify the PVDF ultrafiltration membrane to improve its hydrophilicity and mechanical strength, thus solving the problem that PVDF ultrafiltration membranes have poor hydrophilicity and are easily affected by pollutants.

[0007] The technical solution adopted by this invention to solve its technical problem is: A method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilanes includes the following steps: Step S1: Polyvinylidene fluoride, additives, and organic solvents are mixed, heated, and stirred evenly in a reaction vessel to prepare a PVDF hollow fiber casting solution; the PVDF hollow fiber casting solution comprises, by mass percentage: 10wt%~30wt% polyvinylidene fluoride, 20wt%~40wt% additives, and 30wt%~60wt% organic solvents; wherein, the additives contain a fluorinated organosilane coupling agent; Step S2: The cast film solution is poured into a degassing tank for vacuum degassing to form a homogeneous cast film solution. Step S3: The homogeneous casting solution formed in step S2 is squeezed from the degassing tank into the metering pump, and then pumped from the metering pump into the spinning nozzle; the casting solution flows out from the hole in the middle of the spinning nozzle at a distance of 10~20cm from the coagulation bath, and the core liquid flows out from the inner hole of the spinning nozzle; wherein, the spinning temperature of the spinning nozzle is ≤ the temperature of the coagulation bath. In step S4, the casting solution and the core solution flow out of the spinning nozzle at the same time and polymerize to form a mixed solution. The formed film fibers are then placed in the air section and enter the coagulation bath for phase separation and transformation to form PVDF film fibers. Step S5: The PVDF membrane fibers formed in step S4 are placed in a rinsing tank for multiple washes, and the blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. Step S6: The blended modified PVDF hollow fiber membrane fibers are sequentially immersed in pure water and ethanol; wherein, the ethanol includes ethanol of different concentration gradients. Step S7: After processing the PVDF hollow fiber membrane fibers in a freeze dryer, they are further dried in a vacuum drying oven to form a fluorinated organosilicon modified PVDF hollow fiber ultrafiltration membrane. Furthermore, in step S1, the additive is one or more of polypropylene glycol, polyvinyl alcohol, and urea. The fluorinated organosilane coupling agent is one or more of the following: trifluoropropyltriethoxysilane, trimethyl-tritrifluoropropyl-cyclotrisiloxane, perfluorooctyltriethoxysilane, and perfluorodecyltriethoxysilane. The organic solvent is one or more of methylpyrrolidone and dimethylamide; Furthermore, the additive contains a fluorinated organosilane coupling agent at a mass percentage of 0.5 wt% to 5 wt%. Furthermore, in step S2, the casting solution is allowed to stand in a degassing tank for 6 to 24 hours to degas, and the temperature inside the degassing tank is set to 30°C to 60°C. Furthermore, in step S3, when the homogenized casting solution is extruded from the degassing tank, the pressure is set to 0.5 bar to 3 bar; The spinning nozzle has a spinning temperature of 50℃~80℃, with a temperature variation range of ±0.5℃; the coagulation bath has a temperature of 60℃~80℃, with a temperature variation range of ±0.5℃. Furthermore, in step S4, the flow rate of the casting solution and the core solution flowing out of the spinning nozzle is 1~5 mL / min; the time the formed film filaments remain in the air is 5~8 s; The coagulation bath is filled with pure water at a temperature of 60℃~80℃, and the immersion time is 5~10 minutes. Furthermore, in step S5, the rinsing tank is filled with pure water at a temperature of 50℃~70℃, and the immersion time is 5~10 minutes. Furthermore, in step S6, the PVDF hollow fiber membrane fibers are soaked in ethanol solutions of different concentrations, with the ethanol solution concentration ranging from 5 wt% to 30 wt%. Before immersing in one gradient concentration ethanol solution to the next gradient concentration ethanol solution, drain the previous gradient concentration ethanol solution, and immerse in each gradient concentration ethanol solution for 5~10 hours; Furthermore, three different concentration gradient ethanol solutions were prepared, and the samples were soaked sequentially from high to low concentration, namely 30wt%, 15wt%, and 5wt%. Furthermore, in step S7, the PVDF hollow fiber membrane fibers that have been washed multiple times are placed in a freeze dryer for 4 to 12 hours. Set the temperature of the vacuum drying oven to 80℃~120℃ and dry for 1h~4h.

[0008] By employing the above technical solutions, the present invention has the following beneficial effects compared to the prior art: 1. The preparation method of PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilane modification provided by the present invention introduces fluorinated silane coupling agent during the phase transition process, so that the surface of PVDF ultrafiltration membrane has strong hydrophilicity, which makes it easy to combine with water molecules to form a hydration layer, thereby improving the hydrophilicity of PVDF ultrafiltration membrane, reducing the possibility of pollutants adsorbing onto the surface of PVDF ultrafiltration membrane, and thus improving its anti-pollution performance in coal chemical wastewater. 2. The preparation method of PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified by the present invention controls the phase separation thermodynamics by adjusting the temperature, air section and coagulation bath, so that the modifier migrates to the membrane surface, which makes it easy to construct highly permeable pores and ensures the consistency of the degree of modification. 3. The preparation method of PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified by the present invention involves physically blending silane and PVDF, followed by gradient ethanol replacement and freeze-drying to fix silane alkoxy groups in the PVDF network framework. This avoids the problem of residual solvent in the membrane pores and the loss of silane with water flow, thus ensuring the long-term effectiveness of hydrophilic modification. Attached Figure Description

[0009] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0010] Figure 1 This is a surface hydrophilicity diagram of the modified PVDF hollow fiber ultrafiltration membrane in a preferred embodiment of the present invention; Figure 2 In the image, 2a is a surface electron microscope image of the unmodified PVDF hollow fiber ultrafiltration membrane of Comparative Example 1, and 2b is a surface electron microscope image of the modified PVDF hollow fiber ultrafiltration membrane of Example 1. Figure 3 In the diagram, 3a is a cross-sectional electron microscope image of the unmodified PVDF hollow fiber ultrafiltration membrane of Comparative Example 1, and 3b is a cross-sectional electron microscope image of the modified PVDF hollow fiber ultrafiltration membrane of Example 1. Detailed Implementation

[0011] As described in the background section, the current hydrophilic modification of PVDF ultrafiltration membranes mainly adopts two methods: blending modification and surface modification. Blending modification introduces silica particles, which can easily reduce the porosity of polyvinylidene fluoride ultrafiltration membranes. Surface modification has the drawback of unstable surface hydrophilic layer, which is easily washed away during membrane application.

[0012] To address the aforementioned issues, this application provides a method for preparing a PVDF hollow fiber ultrafiltration membrane modified with an amphiphilic fluorinated organosilane. The method involves blending a fluorinated silane coupling agent into the casting solution, causing it to segregate onto the surface of the PVDF ultrafiltration membrane, thereby introducing hydroxyl groups to enhance the membrane's hydrophilicity. The amphiphilic fluorinated silane coupling agent exhibits good compatibility, effectively addressing the defects mentioned in the background art, improving the hydrophilicity of the PVDF ultrafiltration membrane, and simultaneously enhancing its bonding with PVDF, thus improving the overall hydrophilicity of the PVDF hollow fiber membrane and enhancing its overall performance in coal chemical wastewater treatment.

[0013] The preparation method provided in this application includes the following steps: Step S1: Polyvinylidene fluoride, additives, and organic solvents are mixed, heated, and stirred evenly in a reaction vessel to prepare a PVDF hollow fiber casting solution. The PVDF hollow fiber casting solution comprises, by mass percentage: 10wt%~30wt% polyvinylidene fluoride, 20wt%~40wt% additives, and 30wt%~60wt% organic solvents. The additives include a fluorinated organosilane coupling agent.

[0014] The polyvinylidene fluoride (PVDF) content range is set to balance the tensile strength of the membrane fibers and the ultrafiltration permeate flux, making it particularly suitable for subsequent dual-pore hollow spinneret molding. The most innovative aspect of this step is the in-situ blending of 20wt%~40wt% additives with 0.5wt%~5wt% fluorinated organosilane coupling agents. The fluorinated silane coupling agents possess a dual-functional structure with both hydrolyzable and fluorinated groups. During hydrophilic modification, inorganic-organic interfacial bonding and surface energy regulation are achieved. Its core advantage lies in the condensation reaction of hydroxyl groups generated by the hydrolysis of hydrolyzable groups during hydrophilic modification, which covalently anchors the hydrophilic components. Simultaneously, the compatibility of the fluorinated groups with the polymer matrix optimizes dispersibility, ultimately balancing hydrophobic and hydrophilic properties through structural design. Figure 1 As shown in the figure, it is suitable for surface functionalization modification of hydrophobic materials such as PVDF. The advantage of using its blending modification is to improve compatibility and enhance the hydrophilicity of PVDF membrane fibers.

[0015] The additive's inclusion of a fluorinated organosilane coupling agent ensures rapid dissolution during wet phase separation, increasing pure water flux. Simultaneously, the low dosage of the fluorinated silane coupling agent prevents silane agglomeration and pore blockage, imparting amphiphilic properties to the membrane while maintaining excellent durability and long-term antifouling effects. Preferably, the additive is one or more of polypropylene glycol, polyvinyl alcohol, and urea; the fluorinated organosilane coupling agent is one or more of trifluoropropyltriethoxysilane, trimethyl-tritrifluoropropyl-cyclotrisiloxane, perfluorooctyltriethoxysilane, and perfluorodecyltriethoxysilane. Finally, the organic solvent ensures the casting solution is uniform and transparent; preferably, the organic solvent is one or more of methylpyrrolidone and dimethylamide.

[0016] The implementation of fluorinated organosilane coupling agents simplifies subsequent treatment methods and reduces equipment requirements, thereby reducing treatment costs and facilitating industrial production.

[0017] High temperature accelerates PVDF dissolution. After the additives are uniformly dispersed in the casting solution, in step S2, the prepared casting solution is placed in a degassing tank for vacuum degassing to form a homogeneous casting solution. The casting solution is allowed to stand in the degassing tank for 6 to 24 hours for degassing, and the temperature inside the degassing tank is set at 30°C to 60°C.

[0018] In step S3, the homogeneous casting solution formed in step S2 is extruded from the deaerator into the metering pump. The pressure is set to 0.5 bar to 3 bar when the homogeneous casting solution is extruded from the deaerator. Then, it is pumped into the spinning nozzle from the metering pump. The casting solution flows out from the central hole of the spinning nozzle at a position 10 cm to 20 cm away from the coagulation bath, and the core liquid flows out from the inner hole of the spinning nozzle. The spinning temperature of the spinning nozzle is ≤ the temperature of the coagulation bath, forming a positive temperature gradient to delay the instantaneous rapid cooling and phase separation of the casting solution, thus providing time for the additives to migrate to the membrane surface and improving the hydrophilic modification effect of the membrane surface. Preferably, the spinning temperature of the spinning nozzle is 50℃ to 80℃, with a temperature variation range of ±0.5℃; the temperature of the coagulation bath is 60℃ to 80℃, with a temperature variation range of ±0.5℃.

[0019] In step S4, the casting solution and core solution simultaneously flow out of the spinning nozzle and polymerize to form a mixed solution, forming a hollow tubular structure in one step. The outflow rate is set to 1~5 mL / min. The formed membrane fibers are then placed in an air section for 5~8 seconds before entering the coagulation bath for phase separation and transformation to form PVDF membrane fibers. There are two key points in the air section: first, the spinning nozzle is positioned 10cm~20cm away from the coagulation bath; second, the air section is held for 5s~8s. The solvent in the air evaporates slightly, pre-forming a thin and dense separation skin. The air section also promotes the migration of fluorinated silanes to the membrane surface, enhancing the hydrophilic and antifouling capabilities of the membrane surface.

[0020] The coagulation bath is filled with pure water at a temperature of 60℃~80℃, and the immersion time is 5~10 minutes. The high temperature setting in the coagulation bath slows down the exchange rate between the solvent and water, and forms through-holes inside to significantly increase the water flow rate.

[0021] Step S5 involves placing the PVDF membrane fibers formed in step S4 into a rinsing tank for multiple washes, collecting the blended modified PVDF hollow fiber membrane fibers using a winding wheel. The rinsing tank is filled with pure water at a temperature of 50℃~70℃, and the immersion time is 5min~10min. This high-temperature rinsing setting rapidly dissolves any remaining free solvent within the membrane pores, preventing flux reduction during later use. Multiple washes further prevent damage to the formed pore structure caused by rapid cooling.

[0022] Step S6: The blended modified PVDF hollow fiber membrane fibers are sequentially immersed in pure water and ethanol; wherein the ethanol includes ethanol of different gradient concentrations, all ranging from 5wt% to 30wt%; immersing the PVDF hollow fiber membrane fibers in ethanol solutions of different gradient concentrations gradually replaces the water in the membrane pores, avoiding membrane pore collapse and shrinkage caused by rapid water evaporation due to direct immersion in anhydrous ethanol. Preferably, three different gradient concentration ethanol solutions are used, from high to low concentration, namely 30wt%, 15wt%, and 5wt%.

[0023] It is important to note that before immersing in the next gradient concentration ethanol solution, the previous gradient concentration ethanol solution should be drained to prevent the washing liquid from carrying in ethanol and diluting the ethanol concentration. In order to ensure the displacement effect, the immersion time in each gradient concentration ethanol solution should be 5~10 hours.

[0024] Step S7: After multiple cleanings, the PVDF hollow fiber membrane fibers are placed in a freeze dryer for 4 to 12 hours. Freeze drying freezes the water in the pores into ice crystals, preserving the interconnected porous structure. Then, the membrane is placed in a vacuum drying oven for further drying. The temperature of the vacuum drying oven is set to 80°C to 120°C, and the drying time is 1 to 4 hours to form a fluorinated organosilicon modified PVDF hollow fiber ultrafiltration membrane.

[0025] The entire process described above begins with the design of the casting solution formulation, which is then matched and adjusted with a high-temperature coagulation bath to control the rate of phase separation. Next, the membrane filaments are cleaned and dried. The casting solution formulation avoids filament breakage during molding, and the molding process is controlled to prevent incomplete replacement due to dense pores during performance curing. Ultimately, this avoids problems such as low throughput and easy contamination in the finished product.

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the design scheme and corresponding preparation process of this invention will be described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Example

[0027] The first step is to prepare the PVDF hollow fiber casting solution: The PVDF hollow fiber casting solution consists of 15 wt% polyvinylidene fluoride, 30 wt% additives (including 2 wt% trifluoropropyltrimethoxysilane coupling agent), and 55 wt% organic solvent. The various reagents are mixed and injected into the reaction vessel and stirred and heated at a certain temperature to form the casting solution. After the casting solution is dissolved, it is transferred to a degassing tank and allowed to stand at 40°C for 6 hours to degas, thus obtaining a homogeneous casting solution.

[0028] The second step involves squeezing the deaerated casting solution from the deaerator in the first step into a metering pump at 2 bar, and then pumping it into the spinning nozzle. The spinning nozzle temperature is controlled at 60°C, and the coagulation bath temperature is 70°C. The PVDF casting solution flows out from the middle hole of the double-hole spinneret, while the core solution flows out from the inner hole of the spinneret.

[0029] The third step involves maintaining the PVDF casting solution and core solution at a flow rate of 2L / min. The two solutions polymerize into a single mixed solution, which flows out of the spinning nozzle. The resulting membrane fibers pass through an air section at a certain speed for 8 seconds, and then enter a 70℃ coagulation bath at a certain speed for phase separation and molding, allowing the fluorinated silane coupling agent to segregate onto the surface of the PVDF hollow fiber membrane fibers.

[0030] The fourth step involves washing the PVDF membrane fibers formed by phase separation and conversion in the third step multiple times in a 60°C rinsing tank. The resulting blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. The collected membrane fibers are then soaked in pure water and ethanol of different concentrations for 7 hours in sequence to remove various reagents remaining on the surface of the PVDF hollow fiber membrane fibers.

[0031] The fifth step involves placing the PVDF hollow fiber membrane fibers, which have undergone multiple cleaning steps in the fourth step, into a freeze dryer for 7 hours, and then into a vacuum drying oven at 100°C for 1.5 hours to form an amphiphilic silane coupling agent modified PVDF hollow fiber ultrafiltration membrane. Example

[0032] The first step is to prepare the PVDF hollow fiber casting solution: The PVDF hollow fiber casting solution consists of 17 wt% polyvinylidene fluoride, 35 wt% additives (including 3 wt% trifluoropropyltriethoxysilane coupling agent) and 48 wt% organic solvent. The various reagents are mixed and injected into the reaction vessel and stirred and heated at a certain temperature to form the casting solution. After the casting solution is dissolved, it is transferred to a degassing tank and allowed to stand at 45°C for 10 h to degas, thus obtaining a homogeneous casting solution.

[0033] The second step involves squeezing the deaerated casting solution from the deaerator in the first step into a metering pump at 1 bar, and then pumping it into the spinning nozzle. The spinning nozzle temperature is controlled at 55°C, and the coagulation bath temperature is 75°C. The PVDF casting solution flows out from the middle hole of the double-hole spinneret, while the core solution flows out from the inner hole of the spinneret.

[0034] The third step involves maintaining the PVDF casting solution and core solution at a flow rate of 4 mL / min. The two solutions polymerize into a single mixed solution, which flows out of the spinning nozzle. The resulting membrane fibers pass through an air section at a certain speed for 6 seconds, and then enter a 60°C coagulation bath at a certain speed for phase separation and molding, allowing the fluorinated silane coupling agent to segregate onto the surface of the PVDF hollow fiber membrane fibers.

[0035] The fourth step involves washing the PVDF membrane fibers formed by phase separation and conversion in the third step multiple times in a 55°C rinsing tank. The resulting blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. The collected membrane fibers are then soaked in pure water and ethanol of different concentrations for 9 hours in sequence to remove various reagents remaining on the surface of the PVDF hollow fiber membrane fibers.

[0036] The fifth step involves placing the PVDF hollow fiber membrane fibers, which have undergone multiple cleaning steps in the fourth step, into a freeze dryer for 7 hours, and then into a vacuum drying oven at 100°C for 2 hours to form an amphiphilic silane coupling agent modified PVDF hollow fiber ultrafiltration membrane. Example

[0037] The first step is to prepare the PVDF hollow fiber casting solution: The PVDF hollow fiber casting solution consists of 19 wt% polyvinylidene fluoride, 40 wt% additives (including 3.5 wt% trimethyl-trisfluoropropyl-cyclotrisiloxane coupling agent) and 41 wt% organic solvent. The various reagents are mixed and injected into the reaction vessel and stirred and heated at a certain temperature to form the casting solution. After the casting solution is dissolved, it is transferred to a degassing tank and allowed to stand at 55°C for 20 h to degas, thus obtaining a homogeneous casting solution.

[0038] The second step involves squeezing the deaerated casting solution from the first step into a metering pump at 2.5 bar, and then pumping it into the spinning nozzle. The spinning nozzle temperature is controlled at 80°C, and the coagulation bath temperature is 60°C. The PVDF casting solution flows out from the middle hole of the double-hole spinneret, while the core solution flows out from the inner hole of the spinneret.

[0039] The third step involves maintaining the PVDF casting solution and core solution at a flow rate of 2 mL / min. The two solutions polymerize into a single mixed solution, which flows out of the spinning nozzle. The resulting membrane fibers pass through an air section at a certain speed for 10 seconds, and then enter an 80°C coagulation bath at a certain speed for phase separation and molding, allowing the fluorinated silane coupling agent to segregate onto the surface of the PVDF hollow fiber membrane fibers.

[0040] The fourth step involves washing the PVDF membrane fibers formed by phase separation and conversion in the third step multiple times in a 70°C rinsing tank. The resulting blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. The collected membrane fibers are then soaked in pure water and ethanol of different concentrations for 6.5 hours in sequence to remove various reagents remaining on the surface of the PVDF hollow fiber membrane fibers.

[0041] The fifth step involves placing the PVDF hollow fiber membrane fibers, which have undergone multiple cleaning steps in the fourth step, into a freeze dryer for 8.5 hours, and then into a vacuum drying oven at 110°C for 2 hours to form an amphiphilic silane coupling agent modified PVDF hollow fiber ultrafiltration membrane. Example

[0042] The first step is to prepare the PVDF hollow fiber casting solution: The PVDF hollow fiber casting solution consists of 20 wt% polyvinylidene fluoride, 38 wt% additives (including 5 wt% perfluorooctyltriethoxysilane coupling agent) and 42 wt% organic solvent. The various reagents are mixed and injected into the reaction vessel and stirred and heated at a certain temperature to form the casting solution. After the casting solution is dissolved, it is transferred to a degassing tank and allowed to stand at 60°C for 24 hours to degas, thus obtaining a homogeneous casting solution.

[0043] The second step involves squeezing the deaerated casting solution from the first step into a metering pump at 3 bar, and then pumping it into the spinning nozzle. The spinning nozzle temperature is controlled at 50°C, and the coagulation bath temperature is 60°C. The PVDF casting solution flows out from the middle hole of the double-hole spinneret, while the core solution flows out from the inner hole of the spinneret.

[0044] The third step involves maintaining the PVDF casting solution and core solution at a flow rate of 4 L / min. The two solutions polymerize into a single mixed solution, which flows out of the spinning nozzle. The resulting membrane fibers pass through an air section at a certain speed for 9 seconds, and then enter an 80°C coagulation bath at a certain speed for phase separation and molding, allowing the fluorinated silane coupling agent to segregate onto the surface of the PVDF hollow fiber membrane fibers.

[0045] The fourth step involves washing the PVDF membrane fibers formed by phase separation and conversion in the third step multiple times in a 75°C rinsing tank. The resulting blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. The collected membrane fibers are then soaked in pure water and ethanol of different concentrations for 5-10 hours to remove various reagents remaining on the surface of the PVDF hollow fiber membrane fibers.

[0046] The fifth step involves placing the PVDF hollow fiber membrane fibers, which have undergone multiple cleaning steps in the fourth step, into a freeze dryer for 10 hours, and then into a vacuum drying oven at 120°C for 2 hours to form an amphiphilic silane coupling agent modified PVDF hollow fiber ultrafiltration membrane.

[0047] This application further provides comparative example 1 to verify the feasibility and superiority of this application.

[0048] Comparative Example 1: The first step is to prepare the PVDF hollow fiber casting solution: The PVDF hollow fiber casting solution consists of 18 wt% polyvinylidene fluoride, 30 wt% additives, and 52 wt% organic solvent. The various reagents are mixed and injected into the reaction vessel and stirred and heated at a certain temperature to form the casting solution. After the casting solution is dissolved, it is transferred to a degassing tank and allowed to stand at 40°C for 6 hours to degas, thus obtaining a homogeneous casting solution.

[0049] The second step involves squeezing the deaerated casting solution from the deaerator in the first step into a metering pump at 2 bar, and then pumping it into the spinning nozzle. The spinning nozzle temperature is controlled at 60°C, and the coagulation bath temperature is 70°C. The PVDF casting solution flows out from the middle hole of the double-hole spinneret, while the core solution flows out from the inner hole of the spinneret.

[0050] The third step involves maintaining the PVDF casting solution and core solution at a flow rate of 2L / min. The two solutions polymerize into a single mixed solution, which flows out of the spinning nozzle. The resulting membrane fibers pass through an air section at a certain speed for 8 seconds, and then enter a 70℃ coagulation bath at a certain speed for phase separation and molding, allowing the fluorinated silane coupling agent to segregate onto the surface of the PVDF hollow fiber membrane fibers.

[0051] The fourth step involves washing the PVDF membrane fibers formed by phase separation and conversion in the third step multiple times in a 60°C rinsing tank. The resulting blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. The collected membrane fibers are then soaked in pure water and ethanol of different concentrations for 7 hours in sequence to remove various reagents remaining on the surface of the PVDF hollow fiber membrane fibers.

[0052] In the fifth step, the PVDF hollow fiber membrane fibers, after being washed multiple times in the fourth step, were placed in a freeze dryer for 7 hours, and then dried in a vacuum drying oven at 100°C for 1.5 hours to form an amphiphilic silane coupling agent modified PVDF hollow fiber ultrafiltration membrane. The performance of the amphiphilic silane coupling agent modified PVDF hollow fiber ultrafiltration membranes formed in Examples 1-4 and Comparative Example 1 was tested and analyzed. The testing methods included scanning electron microscopy, mechanical strength testing, pure water flux testing, retention performance testing, and antifouling testing.

[0053] Regarding scanning electron microscopy (SEM) testing, a comparison chart of the results from Example 1 and Comparative Example 1 is provided here. Figure 2 In the image, 2a is a surface electron microscope image of the unmodified PVDF hollow fiber ultrafiltration membrane of Comparative Example 1, and 2b is a surface electron microscope image of the modified PVDF hollow fiber ultrafiltration membrane of Example 1. Figure 3 In the figures, 3a is a cross-sectional electron microscope image of the unmodified PVDF hollow fiber ultrafiltration membrane of Comparative Example 1, and 3b is a cross-sectional electron microscope image of the modified PVDF hollow fiber ultrafiltration membrane of Example 1. As can be seen from the surface structure of the PVDF ultrafiltration membrane, the outer surface of the hydrophilically modified PVDF hollow fiber ultrafiltration membrane is relatively dense, the pore size distribution is relatively uniform, and there are no obvious pore size defects. At the same time, the outer surface has an obvious layered structure. After the surface segregation of the fluorosilane coupling agent, a hydrophilic structure is formed on the surface, which improves the hydrophilicity of the PVDF ultrafiltration membrane. Pollutants on the outer surface of the membrane fibers are not easily adsorbed, thus improving the antifouling performance of the membrane fibers. As can be seen from the cross-sectional structure of the PVDF ultrafiltration membrane, the cross-sectional structure of the hydrophilically enhanced PVDF hollow fiber ultrafiltration membrane is a sponge or honeycomb porous layered structure, and the membrane fibers have good mechanical strength and permeate flux.

[0054] The mechanical strength test is shown in Table 1.

[0055] Table 1 Mechanical Strength of Membrane Filaments

[0056] The pure water flux test results are shown in Table 2.

[0057] Table 2. Membrane fiber pure water flux

[0058] The retention performance test is shown in Table 3.

[0059] Table 3. Membrane fiber retention performance test

[0060] The anti-pollution test results are shown in Table 4.

[0061] Table 4. Membrane filament antifouling test

[0062] As shown in Examples 1 and 2, the difference between using fluorinated methoxysilane coupling agents and fluorinated ethoxysilane coupling agents to prepare blended modified PVDF ultrafiltration membranes is that the former has less steric hindrance and a faster hydrolysis reaction rate. During hydrophilic modification, it can rapidly hydrolyze, accelerating the condensation reaction of surface hydroxyl groups to form Si-O-inorganic bonds. The latter has greater steric hindrance and a relatively slower hydrolysis rate. The reaction process is more controllable and can reduce self-polymerization side reactions caused by excessively rapid hydrolysis, thereby improving coupling efficiency. Both achieve ultrafiltration membrane modification through silanol bonding and fluorinated chain combination, but the ethoxy type has better stability. Its slow hydrolysis characteristics can ensure sufficient reaction of the substrate, reduce interface defects, and make it suitable for systems containing trace amounts of water.

[0063] Examples 1 and 3 show that, compared with cyclic fluorinated silanes, chain-like fluorinated silanes form a more continuous and dense modified layer structure, which can more efficiently reduce surface energy, has better chemical stability, and can improve hydrophilicity without reducing the regularity of PVDF molecular chains. It has little impact on the mechanical properties of PVDF hollow fiber membrane filaments, and can improve interfacial bonding force, with a more stable hydrophilic modification effect.

[0064] Examples 1 and 4 show that, compared to chain-like fluorinated silanes, perfluorinated silanes, due to their extremely high degree of molecular fluorine substitution, can reduce the surface energy of PVDF hollow fiber membranes to an extremely low level, thus significantly improving their antifouling performance. The compatibility between perfluorinated silanes and PVDF is better, improving the uniformity of material dispersion and enhancing the modification effect of PVDF membranes. The modified PVDF membranes exhibit long-term stability, enhancing the overall antifouling performance of PVDF hollow fiber ultrafiltration membranes.

[0065] Examples 1-4 and Comparative Example 1 show that the PVDF ultrafiltration membrane modified by blending with amphiphilic fluorinated organosilicon can form silanol groups after hydrolysis, thereby improving the hydrophilicity of the PVDF ultrafiltration membrane. Moreover, the fluorinated groups have a synergistic effect, which can regulate the hydrophilic-hydrophobic balance on the surface of the PVDF membrane fibers and improve the overall hydrophilicity and antifouling properties of the material.

[0066] In summary, the silicon-modified PVDF ultrafiltration membrane prepared by the method of amphiphilic fluorinated organosilicon-modified PVDF hollow fiber ultrafiltration membrane provided in this application forms a membrane fiber surface structure with a relatively uniform pore size distribution. The outer surface of the membrane is relatively dense and free of obvious cracks and defects. The surface segregation of the fluorinated silane coupling agent in the PVDF ultrafiltration membrane produces a hydrophilic layered structure, which enhances the hydrophilicity of the membrane fiber. While improving the pure water flux of the membrane fiber, the antifouling performance can also be improved. Moreover, by utilizing the compatibility between the fluorinated groups and the polymer matrix to optimize the dispersibility, it is possible to design a balance between hydrophobic and hydrophilic properties, thereby improving the overall hydrophilicity and antifouling performance of the material, and thus making it better suited for the treatment of coal chemical wastewater.

[0067] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0068] The meaning of "and / or" as used in this application includes situations where each exists alone or both exist simultaneously.

[0069] The term "connection" as used in this application can mean a direct connection between components or an indirect connection between components through other components.

[0070] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified with PVDF, characterized in that: Includes the following steps: Step S1: Polyvinylidene fluoride, additives and organic solvents are mixed, heated and stirred evenly in a reaction vessel to prepare PVDF hollow fiber casting solution. The PVDF hollow fiber casting solution comprises, by mass percentage: 10wt%~30wt% polyvinylidene fluoride, 20wt%~40wt% additives, and 30wt%~60wt% organic solvent; wherein the additives include a fluorinated organosilane coupling agent. Step S2: The cast film solution is poured into a degassing tank for vacuum degassing to form a homogeneous cast film solution. Step S3: The homogeneous casting solution formed in step S2 is squeezed from the degassing tank into the metering pump, and then pumped from the metering pump into the spinning nozzle; the casting solution flows out from the hole in the middle of the spinning nozzle at a distance of 10~20cm from the coagulation bath, and the core liquid flows out from the inner hole of the spinning nozzle; wherein, the spinning temperature of the spinning nozzle is ≤ the temperature of the coagulation bath. In step S4, the casting solution and the core solution flow out of the spinning nozzle at the same time and polymerize to form a mixed solution. The formed film fibers are then placed in the air section and enter the coagulation bath for phase separation and transformation to form PVDF film fibers. Step S5: The PVDF membrane fibers formed in step S4 are placed in a rinsing tank for multiple washes, and the blended modified PVDF hollow fiber membrane fibers are collected by a winding wheel. Step S6: The blended modified PVDF hollow fiber membrane fibers are sequentially immersed in pure water and ethanol; wherein, the ethanol includes ethanol of different concentration gradients. Step S7: After processing the PVDF hollow fiber membrane fibers in a freeze dryer, they are further dried in a vacuum drying oven to form a fluorinated organosilicon modified PVDF hollow fiber ultrafiltration membrane.

2. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S1, the additive is one or more of polypropylene glycol, polyvinyl alcohol, and urea. The fluorinated organosilane coupling agent is one or more of the following: trifluoropropyltriethoxysilane, trimethyl-tritrifluoropropyl-cyclotrisiloxane, perfluorooctyltriethoxysilane, and perfluorodecyltriethoxysilane. The organic solvent is one or more of methylpyrrolidone and dimethylamide.

3. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 2, characterized in that: The additive contains fluorinated organosilane coupling agents at a mass percentage of 0.5wt% to 5wt%.

4. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S2, the casting solution is allowed to stand in a degassing tank for 6 to 24 hours to degas, and the temperature inside the degassing tank is set to 30°C to 60°C.

5. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S3, when the homogenized casting solution is extruded from the degassing tank, the pressure is set to 0.5 bar to 3 bar; The spinning nozzle has a spinning temperature of 50℃~80℃, with a temperature variation range of ±0.5℃; the coagulation bath has a temperature of 60℃~80℃, with a temperature variation range of ±0.5℃.

6. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S4, the flow rate of the casting solution and the core solution from the spinning nozzle is 1~5 mL / min; the time the formed film fibers remain in the air is 5~8 s; The coagulation bath is filled with pure water at a temperature of 60℃~80℃, and the immersion time is 5~10 minutes.

7. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S5, the rinsing tank is filled with pure water at a temperature of 50℃~70℃, and the immersion time is 5~10 minutes.

8. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S6, PVDF hollow fiber membrane fibers are soaked in ethanol solutions of different concentrations, with the ethanol solution concentration ranging from 5 wt% to 30 wt%. Before immersing in the next gradient concentration ethanol solution, drain the previous gradient concentration ethanol solution. The immersion time in each gradient concentration ethanol solution is 5~10 hours.

9. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 8, characterized in that: Three different gradient concentrations of ethanol solution were set up, and the soaking was carried out sequentially from high concentration to low concentration, namely 30wt%, 15wt% and 5wt%.

10. The method for preparing a PVDF hollow fiber ultrafiltration membrane based on amphiphilic fluorinated organosilicon modified according to claim 1, characterized in that: In step S7, the PVDF hollow fiber membrane fibers that have been washed multiple times are placed in a freeze dryer for 4 to 12 hours. Set the temperature of the vacuum drying oven to 80℃~120℃ and the drying time to 1h~4h.

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