A hydrophilic tubular membrane, a preparation method and application thereof

Abstract

The application provides a kind of hydrophilic tubular membrane, including support layer and separation layer successively from inside to outside;The pore size of the support layer is 0.5-1.5 μm, the pore size of the separation layer is 30-80 μm, and the support layer and the separation layer are diffused and intertwined at the interface, forming a continuous gradient structure without obvious interface.The separation layer contains modified nanosilica by silane coupling agent.The application effectively reduces the mass transfer resistance by gradient structure, significantly improves the hydrophilicity and anti-pollution performance of the membrane surface by using modified nanosilica, and has strong interlayer bonding strength, which is suitable for high-pollution industrial wastewater treatment and other fields.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and more specifically, to a hydrophilic tubular membrane, its preparation method, and its application. Background Technology

[0002] Tubular membranes have been widely used in industrial wastewater treatment, landfill leachate treatment, special separation, and food fermentation due to their significant advantages such as wide flow channels, resistance to high suspended solids, ease of cleaning, and high mechanical strength.

[0003] Currently, most commercially available tubular membranes are prepared using a submersion precipitation method from polymeric materials such as polyvinylidene fluoride, polyethersulfone, or polysulfone. However, in practical applications, existing tubular membrane technologies still face the following core challenges: Membrane fouling is a serious problem: commonly used membrane materials are highly hydrophobic. When treating feed solutions containing proteins, oils, or organic matter, contaminants easily deposit on the membrane surface or within the pores through hydrophobic interactions, leading to a rapid decrease in membrane flux, shortening membrane lifespan, and increasing operating energy consumption.

[0004] The trade-off between separation accuracy and flux: In order to ensure separation accuracy, traditional single-layer tubular membranes often result in increased overall membrane resistance and lower permeate flux. Although increasing porosity can improve flux, this often sacrifices the mechanical support strength of the membrane.

[0005] Poor interlayer bonding stability: Although existing multilayer composite membranes attempt to construct gradient structures through step-by-step coating, due to interfacial compatibility issues between different materials, coating peeling and detachment are prone to occur during long-term high-pressure rinsing or frequent backwashing, which seriously affects the stable operation of the system.

[0006] Therefore, developing a tubular membrane that possesses excellent hydrophilic and antifouling properties, a scientifically designed gradient structure to reduce mass transfer resistance, and tight interlayer bonding has become a key technical problem that urgently needs to be solved in the field of ultrafiltration membranes.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The purpose of this invention is to provide a hydrophilic tubular membrane and its preparation method. Addressing the problems of insufficient hydrophilicity, susceptibility to deep fouling, and poor bonding between the support layer and the separation layer in existing tubular membranes, this invention introduces a hydrophilic nano-modifier into the separation layer and combines it with a simultaneous phase inversion process to prepare a gradient structure hydrophilic tubular membrane that combines high flux, high retention, and long lifetime.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A hydrophilic tubular membrane, characterized in that it comprises a support layer and a separation layer from the inside out; The support layer has a thickness of 200-500 μm and a pore size of 0.5-1.5 μm, and is formed by non-solvent-induced phase separation of a support layer casting solution containing polyvinylidene fluoride resin and a pore-forming agent. The separation layer has a thickness of 50-150 μm and a pore size of 30-80 μm. It is formed by non-solvent-induced phase separation of a separation layer casting solution containing polyvinylidene fluoride resin, nano-silica modified with silane coupling agent and pore-forming agent. The support layer and the separation layer permeate each other at the interface, forming a continuous gradient structure without a clear interface.

[0010] By adopting a double-layer structure consisting of a support layer and a separation layer from the inside out, and limiting the pore size of the support layer to 0.5-1.5 μm and the pore size of the separation layer to 30-80 μm, a gradient pore size structure is formed, transitioning from a large-pore support layer to a smaller-pore separation layer, which effectively reduces the overall mass transfer resistance of the membrane. At the same time, the support layer and the separation layer interpenetrate at the interface to form a continuous gradient structure without obvious interface, eliminating stress concentration points caused by the physical interface in traditional composite membranes, significantly improving the interlayer bonding strength, and avoiding delamination during high-pressure operation or frequent backwashing.

[0011] Preferably, as a further specific embodiment, the casting solution of the support layer comprises, by mass percentage: 12-15% polyvinylidene fluoride resin, 10-15% pore-forming agent A, and the balance being solvent; The separation layer casting solution comprises: 18-22% polyvinylidene fluoride resin, 2-5% nano-silica modified with silane coupling agent, 5-8% pore-forming agent B, and the remainder is solvent. The solvent is N-methylpyrrolidone.

[0012] This invention limits the mass percentage of polyvinylidene fluoride resin in the casting solution of the support layer to 12-15% and the mass percentage of polyvinylidene fluoride resin in the casting solution of the separation layer to 18-22%. This results in a lower polymer concentration in the support layer to facilitate the formation of a macroporous structure and a higher polymer concentration in the separation layer to facilitate the formation of a dense separation layer. The two work synergistically to achieve a balance between high throughput and high rejection rate. The amount of nano-silica modified with silane coupling agent added to the casting solution of the separation layer is 2-5%, which ensures the effective performance of the modification while avoiding a sudden increase in the viscosity of the casting solution and agglomeration of nanoparticles due to excessive addition. Using N-methylpyrrolidone as a solvent ensures the stability and compatibility of the two casting solutions during the coating process.

[0013] Preferably, as a further specific embodiment, the pore-forming agent A in the casting solution of the support layer is polyethylene glycol 4000; The pore-forming agent B in the casting solution of the separation layer is polyvinylpyrrolidone K30.

[0014] This invention selects polyethylene glycol 4000 as the pore-forming agent A in the casting solution for the support layer, utilizing its high molecular weight to form a large pore structure during phase transformation, thus ensuring the large pore size characteristics of the support layer; and selects polyvinylpyrrolidone K30 as the pore-forming agent B in the casting solution for the separation layer, utilizing its suitable molecular weight and pore-forming ability to form a relatively dense skin structure. The selection and combination of these two pore-forming agents provide a material basis for constructing a gradient pore size structure from the support layer to the separation layer.

[0015] Preferably, as a further specific embodiment, the nano-silica modified with silane coupling agent has a particle size of 30-50 nm, and its surface is grafted with silane coupling agent KH-550.

[0016] This invention modifies the surface of nano-silica by grafting the silane coupling agent KH-550 onto the surface of the nano-silica, transforming it from a hydrophilic to an amphiphilic material with organic functional groups. This improves the compatibility and dispersion stability of nano-silica and polyvinylidene fluoride resin in the casting solution, preventing the agglomeration and sedimentation of nanoparticles. By limiting the particle size of the nano-silica to 30-50 nm, it can be uniformly embedded in the membrane pore walls and surface of the separation layer, imparting durable hydrophilic properties to the separation layer without significantly increasing mass transfer resistance.

[0017] The present invention also provides a method for preparing the aforementioned hydrophilic tubular membrane, comprising the following steps: Nano-silica is surface-modified with a silane coupling agent to obtain modified nano-silica. The modified nano-silica is dispersed in a first solvent and ultrasonically treated to obtain a modified nano liquid. Polyvinylidene fluoride resin and porogen B are added to the modified nano liquid, and stirred at a constant temperature of 60-80℃ for 12-24h. After standing to remove bubbles, the separation layer casting liquid is obtained. Polyvinylidene fluoride resin and porogen A are added to the second part of the solvent, and stirred at a constant temperature of 60-80℃ for 12-24h. After standing to remove bubbles, the support layer casting liquid is obtained. A double-layer annular extruder head is used to simultaneously coat the support layer casting liquid and the separation layer casting liquid onto the surface of the braided tube support, and the extrusion speed ratio of the two layers of casting liquid is controlled to be 1:(1.2-1.5). The coated membrane tube is placed in an aqueous coagulation bath at 25-45℃, and a membrane is formed by non-solvent-induced phase separation. After film formation, the membrane tubes are immersed in pure water at 40-60℃ to remove residual solvent, and then immersed in a 20-30% glycerol aqueous solution for moisturizing treatment.

[0018] This invention employs a double-layer annular extruder head to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the braided tube support, achieving sequential molding of the two casting solutions and simplifying the preparation process. By controlling the extrusion speed ratio of the two casting solutions to 1:(1.2-1.5), uniform coating of the support layer by the separation layer and an appropriate thickness ratio are ensured. After coating, the two layers are simultaneously introduced into a coagulation bath for non-solvent-induced phase separation, allowing sufficient molecular diffusion and physical entanglement to occur at the interface before phase transformation. After phase transformation, a continuous gradient structure is formed, fundamentally solving the problem of insufficient interlayer bonding force in the distributed coating method. Subsequently, the membrane product undergoes immersion in pure water to remove residual solvent and glycerol aqueous solution for wetting, ensuring the performance and storage stability of the membrane product.

[0019] Preferably, as a further specific embodiment, the surface modification treatment specifically involves: placing nano-silica in an ethanol solution, adding 1-2% by mass of silane coupling agent KH-550, refluxing at 60°C for 4 hours, and then centrifuging, washing, and drying to obtain modified nano-silica.

[0020] This invention employs a silane coupling agent, KH-550, to chemically modify the surface of nano-silica through grafting. The ethoxy group at one end of the coupling agent undergoes a hydrolytic condensation reaction with the silanol groups on the nano-silica surface to form covalent bonds, while the amino group at the other end imparts good affinity between the nano-silica and polyvinylidene fluoride resin and organic solvents. This allows the modified nano-silica to achieve monodispersity in the separation layer casting solution at its native particle size. This modification process provides a prerequisite for the uniform distribution of nano-silica in the separation layer and the stable performance of the hydrophilic modification effect, avoiding membrane structure defects and performance degradation caused by nanoparticle aggregation.

[0021] Preferably, as a further specific implementation, after the synchronous coating is completed, the membrane tube is left in the air for 3-5 seconds before entering the coagulation bath.

[0022] By allowing the membrane tube to remain in the air for 3-5 seconds after synchronous coating and before entering the coagulation bath, the solvent on the surface of the coated membrane tube will evaporate moderately, and the surface concentration will increase locally. This is beneficial for forming a denser ultrathin skin on the separation layer surface, further improving the retention accuracy. At the same time, this short air exposure time provides conditions for molecular interdiffusion at the interface of the two casting solutions.

[0023] Preferably, as a further specific embodiment, the coagulation bath is a pure water coagulation bath, and the soaking time is 12-36 hours.

[0024] This invention uses pure water as the coagulation bath and controls the soaking time to 12-36 hours, which allows the double diffusion process between the non-solvent and the solvent to proceed fully, ensuring the completeness of the phase transformation reaction and facilitating the formation of a stable membrane pore structure. The pure water coagulation bath avoids interference from other components on the phase transformation process, ensuring the uniformity of the membrane structure and batch stability.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention forms a gradient pore size structure that transitions from a large-pore support layer to a smaller-pore separation layer by sequentially setting a support layer and a separation layer from the inside out, and limiting the pore size of the support layer to 0.5-1.5μm and the pore size of the separation layer to 30-80μm respectively. The large-pore support layer provides low hydraulic mass transfer resistance and the small-pore separation layer ensures retention accuracy, effectively solving the contradiction between flux and retention in traditional single-layer membranes. At the same time, the support layer and the separation layer interpenetrate at the interface to form a continuous gradient structure without obvious interface, eliminating the risk of stress concentration and delamination caused by the physical interface of traditional composite membranes, and significantly improving the interlayer bonding stability. In addition, the separation layer contains nano-silica modified with silane coupling agent, which is uniformly distributed on the surface and pore walls of the separation layer, effectively reducing the water contact angle of the membrane surface, weakening the interaction between hydrophobic pollutants and the membrane surface, and giving the membrane a long-lasting hydrophilic antifouling performance.

[0026] (2) The preparation method of this invention achieves one-time molding of the bilayer film liquid, allowing the two layers of casting liquid to fully diffuse and entangle at the interface before phase transformation, forming a continuous gradient structure after solidification. This fundamentally avoids the defect of insufficient interlayer bonding in the step-by-step coating method, ensuring the uniformity of the film structure and its stability in use. Simultaneously, by pre-treating the nano-silica with silane coupling agent surface grafting modification and ultrasonic dispersion treatment, the problem of nanoparticle aggregation in organic solvents is effectively solved, ensuring the uniform distribution and long-term stable performance of the hydrophilic modified components in the separation layer. The above preparation method has simplified procedures, a reasonable process window, and good feasibility for industrial production. Detailed Implementation The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] To more clearly illustrate the technical solutions in this invention, specific embodiments are described below.

[0028] Example 1 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.5g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0029] (2) Preparation of the casting solution for separating the film 3g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 70℃ for 18h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0030] (3) Preparation of casting solution for the support layer 14g of polyvinylidene fluoride resin and 12g of polyethylene glycol 4000 were added to 74g of N-methylpyrrolidone and stirred at 70℃ for 18h. After standing to remove bubbles, the support layer casting solution was obtained.

[0031] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.3, and the coating thickness of the separation layer was controlled at 100 μm and the coating thickness of the support layer at 300 μm. After coating, the membrane tube was left in air for 3 seconds and then immersed in a 35°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 24 hours.

[0032] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0033] Example 2 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.5g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0034] (2) Preparation of the casting solution for separating the film 5g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 18g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 60℃ for 24h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0035] (3) Preparation of casting solution for the support layer 12g of polyvinylidene fluoride resin and 15g of polyethylene glycol 4000 were added to 73g of N-methylpyrrolidone and stirred at 60℃ for 18h. After standing to remove bubbles, the support layer casting solution was obtained.

[0036] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.3, and the coating thickness of the separation layer was controlled at 100 μm and the coating thickness of the support layer at 300 μm. After coating, the membrane tube was left in air for 3 seconds and then immersed in a 35°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 24 hours.

[0037] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0038] Example 3 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.5g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0039] (2) Preparation of the casting solution for separating the film 2g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 22g of polyvinylidene fluoride resin and 6g of polyvinylpyrrolidone K30 were added and stirred at 80℃ for 18h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0040] (3) Preparation of casting solution for the support layer Add 15g of polyvinylidene fluoride resin and 10g of polyethylene glycol 4000 to 75g of N-methylpyrrolidone, stir at 80℃ for 24h, let stand to remove bubbles, and obtain the support layer casting solution.

[0041] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.3, and the coating thickness of the separation layer was controlled at 100 μm and the coating thickness of the support layer at 300 μm. After coating, the membrane tube was left in air for 3 seconds and then immersed in a 35°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 24 hours.

[0042] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0043] Example 4 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.5g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0044] (2) Preparation of the casting solution for separating the film 3g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 70℃ for 12h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0045] (3) Preparation of casting solution for the support layer 14g of polyvinylidene fluoride resin and 12g of polyethylene glycol 4000 were added to 74g of N-methylpyrrolidone and stirred at 70℃ for 12h. After standing to remove bubbles, the support layer casting solution was obtained.

[0046] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of a polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.2, and the coating thickness of the separation layer was controlled at 50 μm and the coating thickness of the support layer at 200 μm. After coating, the tube was left in air for 4 seconds and then immersed in a 25°C pure water coagulation bath for non-solvent-induced phase separation and film formation, followed by immersion for 24 hours.

[0047] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 40°C for 24 hours to remove residual solvent, and then immersed in a 20% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0048] Example 5 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.5g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0049] (2) Preparation of the casting solution for separating the film 3g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 70℃ for 18h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0050] (3) Preparation of casting solution for the support layer 14g of polyvinylidene fluoride resin and 12g of polyethylene glycol 4000 were added to 74g of N-methylpyrrolidone and stirred at 70℃ for 18h. After standing to remove bubbles, the support layer casting solution was obtained.

[0051] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.5, and the coating thickness of the separation layer was controlled at 150 μm and the coating thickness of the support layer at 500 μm. After coating, the tube was left in air for 5 seconds and then immersed in a 45°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 24 hours.

[0052] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 60°C for 24 hours to remove residual solvent, and then immersed in a 30% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0053] Example 6 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.0g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0054] (2) Preparation of the casting solution for separating the film 3g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 60℃ for 24h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0055] (3) Preparation of casting solution for the support layer 14g of polyvinylidene fluoride resin and 12g of polyethylene glycol 4000 were added to 74g of N-methylpyrrolidone and stirred at 60℃ for 24h. After standing to remove bubbles, the support layer casting solution was obtained.

[0056] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.3, and the coating thickness of the separation layer was controlled at 100 μm and the coating thickness of the support layer at 300 μm. After coating, the membrane tube was left in air for 3 seconds and then immersed in a 35°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 12 hours.

[0057] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0058] Example 7 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 2.0g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0059] (2) Preparation of the casting solution for separating the film 3g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 80℃ for 12h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0060] (3) Preparation of casting solution for the support layer 14g of polyvinylidene fluoride resin and 12g of polyethylene glycol 4000 were added to 74g of N-methylpyrrolidone and stirred at 80℃ for 12h. After standing to remove bubbles, the support layer casting solution was obtained.

[0061] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.3, and the coating thickness of the separation layer was controlled at 100 μm and the coating thickness of the support layer at 300 μm. After coating, the tube was left in air for 3 seconds and then immersed in a 35°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 36 hours.

[0062] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0063] Example 8 (1) Preparation of modified nano-silica Take 100g of nano-silica, place it in an ethanol solution, add 1.5g of silane coupling agent KH-550, reflux at 60℃ for 4h, and after centrifugation, washing and drying, obtain modified nano-silica.

[0064] (2) Preparation of the casting solution for separating the film 3g of modified nano-silica was dispersed in 70g of N-methylpyrrolidone and ultrasonically treated for 1h. Then, 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 were added and stirred at 70℃ for 18h. After standing to remove bubbles, the separation layer casting solution was obtained.

[0065] (3) Preparation of casting solution for the support layer Add 15g of polyvinylidene fluoride resin and 15g of polyethylene glycol 4000 to 70g of N-methylpyrrolidone, stir at 70℃ for 18h, let stand to remove bubbles, and obtain the support layer casting solution.

[0066] (4) Simultaneous coating on phase transformation molding A double-layer annular extruder head was used to simultaneously coat the support layer casting solution and the separation layer casting solution onto the surface of the polyester braided tube. The extrusion speed ratio of the two casting solutions was controlled at 1:1.4, and the coating thickness of the separation layer was controlled at 100 μm and the coating thickness of the support layer at 300 μm. After coating, the membrane tube was left in air for 4 seconds and then immersed in a 40°C pure water coagulation bath for non-solvent-induced phase separation and film formation for 24 hours.

[0067] (5) Post-processing The membrane tubes after film formation were soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a hydrophilic tubular membrane.

[0068] Comparative Example 1 It adopts a single-layer structure, using only one layer of casting solution to coat the film, without setting a double-layer gradient structure with a support layer and a separation layer.

[0069] (1) Preparation of casting solution Add 20g of polyvinylidene fluoride resin and 7g of polyvinylpyrrolidone K30 to 73g of N-methylpyrrolidone, stir at 70℃ for 18h, let stand to remove bubbles, and obtain a single-layer casting solution.

[0070] (2) Coating and phase transformation molding A single-layer annular extruder head was used to coat the surface of a polyester braided tube with the aforementioned single-layer casting solution, controlling the coating thickness to be 400 μm. After coating, the tube was left in air for 3 seconds and then immersed in a 35°C pure water coagulation bath for non-solvent-induced phase separation to form a film, followed by soaking for 24 hours.

[0071] (3) Post-processing The membrane tube after film formation was soaked in pure water at 50°C for 24 hours to remove residual solvent, and then immersed in a 25% glycerol aqueous solution for wetting treatment to obtain a single-layer tubular membrane.

[0072] Comparative Example 2 The specific preparation steps in this embodiment are the same as in Example 1, except that no modified nano-silica is added.

[0073] Experiment Example 1 Performance Testing The hydrophilic tubular membranes prepared in Examples 1-8 and Comparative Examples 1-2 were subjected to performance testing. Pure water flux test: Under an operating pressure of 0.1 MPa, using deionized water as the feed solution, the permeable water volume per unit time and unit area is tested. The calculation formula is J = V / (A·t), where J is the pure water flux (L / m²). 2 ·h), V is the permeate water volume (L), and A is the effective membrane area (m²). 2 ), where t is the test time (h).

[0074] BAS Retention Rate Test: A bovine serum albumin aqueous solution with a concentration of 1 g / L was used as the feed solution and filtered under an operating pressure of 0.1 MPa. The absorbance values ​​of the stock solution and the permeate were measured at 280 nm using a UV spectrophotometer. The retention rate R = (1 - Cp / Cf) × 100%, where Cp is the concentration of the permeate and Cf is the concentration of the stock solution.

[0075] Water contact angle test: The static water contact angle of the membrane surface was measured at room temperature using a contact angle meter and deionized water as the test solution. Five different locations were tested for each sample and the average value was taken.

[0076] Flux recovery rate test: The initial pure water flux J0 of the membrane was measured at 0.1 MPa. Then, after filtering with 1 g / L bovine serum albumin aqueous solution for 1 h, the flux J1 was measured. The fouled membrane was backwashed with deionized water at 0.2 MPa for 10 min, and the pure water flux J2 was measured again. Flux recovery rate FRR = (J2 / J0) × 100%.

[0077] The final results are shown in Table 1 below.

[0078] Table 1

[0079] As can be seen from Table 1: Examples 1-8 show that the pure water flux is 520-750 L / (m³). 2The invention achieved good performance in four indicators: BSA rejection rate (95.5%-99.1%), water contact angle (65.6°-72.3°), and flux recovery rate (85.6%-90.8%), verifying that the technical solution of the present invention has stable implementation effect in a wide range of formulations and process parameters.

[0080] Examples 1-8 show a significant advantage in flux compared to Comparative Example 1 with a single-layer structure, indicating that the introduction of the large-pore support layer effectively reduces the mass transfer resistance of the membrane. At the same time, the rejection rate of each example did not decrease due to the increase in flux, verifying the rationality of the functional division design of "the support layer undertakes mass transfer and the separation layer undertakes rejection".

[0081] Compared with the unmodified Comparative Example 2, the hydrophilic modifications in Examples 1-8 showed significant advantages in contact angle and flux recovery rate, indicating that the nano-silica modified with silane coupling agent can effectively improve the hydrophilicity of the membrane surface and enhance antifouling performance. The flux recovery rate of all examples reached more than 85%, verifying the effectiveness of hydrophilic modification in extending membrane life and reducing cleaning frequency.

[0082] Example 1 achieves the best balance among various performance indicators: its pure water flux (680 L / (m²·h)) and BSA rejection rate (98.2%) are both at excellent levels, and its water contact angle (69.4°) and flux recovery rate (88.5%) are good. It has the best overall performance and is the optimal implementation of the present invention.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A hydrophilic tubular membrane, characterized in that, From the inside out, it includes a support layer and a separation layer; The support layer has a thickness of 200-500 μm and a pore size of 0.5-1.5 μm, and is formed by non-solvent-induced phase separation of a support layer casting solution containing polyvinylidene fluoride resin and a pore-forming agent. The separation layer has a thickness of 50-150 μm and a pore size of 30-80 μm. It is formed by non-solvent-induced phase separation of a separation layer casting solution containing polyvinylidene fluoride resin, nano-silica modified with silane coupling agent and pore-forming agent. The support layer and the separation layer permeate each other at the interface, forming a continuous gradient structure without a clear interface.

2. The hydrophilic tubular membrane according to claim 1, characterized in that, The support layer casting solution comprises, by weight percentage: 12-15% polyvinylidene fluoride resin, 10-15% pore-forming agent A, and the balance being solvent; The separation layer casting solution comprises: 18-22% polyvinylidene fluoride resin, 2-5% nano-silica modified with silane coupling agent, 5-8% pore-forming agent B, and the remainder is solvent. The solvent is N-methylpyrrolidone.

3. The hydrophilic tubular membrane according to claim 2, characterized in that, The pore-forming agent A in the casting solution of the support layer is polyethylene glycol 4000; The pore-forming agent B in the casting solution of the separation layer is polyvinylpyrrolidone K30.

4. The hydrophilic tubular membrane according to claim 2, characterized in that, The nano-silica modified with silane coupling agent has a particle size of 30-50 nm, and its surface is grafted with silane coupling agent KH-550.

5. A method for preparing a hydrophilic tubular membrane as described in any one of claims 1-4, characterized in that, Includes the following steps: Nano-silica is surface-modified with a silane coupling agent to obtain modified nano-silica. The modified nano-silica is dispersed in a first solvent and ultrasonically treated to obtain a modified nano liquid. Polyvinylidene fluoride resin and porogen B are added to the modified nano liquid, and stirred at a constant temperature of 60-80℃ for 12-24h. After standing to remove bubbles, the separation layer casting liquid is obtained. Polyvinylidene fluoride resin and porogen A are added to the second part of the solvent, and stirred at a constant temperature of 60-80℃ for 12-24h. After standing to remove bubbles, the support layer casting liquid is obtained. A double-layer annular extruder head is used to simultaneously coat the support layer casting liquid and the separation layer casting liquid onto the surface of the braided tube support, and the extrusion speed ratio of the two layers of casting liquid is controlled to be 1:(1.2-1.5). The coated membrane tube is placed in an aqueous coagulation bath at 25-45℃, and a membrane is formed by non-solvent-induced phase separation. After film formation, the membrane tubes are immersed in pure water at 40-60℃ to remove residual solvent, and then immersed in a 20-30% glycerol aqueous solution for moisturizing treatment.

6. The preparation method according to claim 5, characterized in that, The surface modification treatment specifically involves: placing nano-silica in an ethanol solution, adding 1-2% by mass of silane coupling agent KH-550, refluxing at 60°C for 4 hours, and then centrifuging, washing, and drying to obtain modified nano-silica.

7. The preparation method according to claim 5, characterized in that, After synchronous coating is completed, the membrane tube is left to stand in the air for 3-5 seconds before entering the coagulation bath.

8. The preparation method according to claim 5, characterized in that, The coagulation bath is a pure water coagulation bath, and the soaking time is 12-36 hours.

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