Method for preparing a pvdf ultrafiltration membrane using a fumed silica as a porogen

Abstract

The application belongs to the technical field of PVDF ultrafiltration membrane, and relates to a method for preparing a PVDF ultrafiltration membrane by using fumed silica as a pore-forming agent. First, PVDF powder and fumed silica are respectively dried and then added to N,N-dimethylformamide to prepare a casting solution. Then, the casting solution is degassed and scraped to be immersed in a coagulation bath to prepare a PVDF flat membrane. Finally, the fumed silica in the PVDF flat membrane is washed away by using a tetrabutylammonium fluoride aqueous solution to realize pore-forming, so that the PVDF ultrafiltration membrane is prepared. In the application, the fumed silica in the membrane is washed away by using the tetrabutylammonium fluoride aqueous solution, so that the pore size and porosity of the PVDF ultrafiltration membrane are improved, and the pore size is uniform.

Description

Technical Field

[0001] This invention belongs to the field of PVDF ultrafiltration membrane technology, and relates to a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent. Background Technology

[0002] Ultrafiltration membranes are molecular sieve membranes with pore sizes ranging from 1 to 100 nm. They rely on the pressure difference across the membrane as the mass transfer driving force for sieving and retention, primarily separating large organic molecules such as proteins. They have wide applications in medicine, food, and biotechnology. Different membrane materials are selected based on the properties of the separated substances and the operating conditions. Materials used to prepare membranes are generally classified into two main categories based on their properties: inorganic membranes and organic membranes. Compared to inorganic membranes, organic membranes offer advantages such as better control over pore formation mechanisms, greater flexibility, smaller installation space, and lower cost. Common organic membrane materials include polysulfone (PSF) and polyvinylidene fluoride (PVDF).

[0003] The main method for preparing organic membranes is the phase inversion method. Based on the initiation mechanism of phase separation, the phase inversion method can be divided into solvent-inducible phase separation (NIPS), thermally induced phase separation (TIPS), and vapor-induced phase separation (VIPS). Among these, the NIPS method is the most commonly used. Specifically, the polymer and other additives are dissolved in a solvent to form a homogeneous casting solution. This solution is then cast onto a suitable substrate or glass plate and immersed in a solvent-free coagulation bath. Through the mutual diffusion and exchange between the solvent and the non-solvent, an asymmetric membrane structure is formed.

[0004] PVDF porous membranes possess excellent chemical resistance, good thermal stability, and good processability and mechanical properties, leading to their widespread application in filtration processes. Currently, the main method for preparing PVDF porous membranes is the NIPS method. The film-forming mechanism and process of PVDF porous membranes prepared by the NIPS method are influenced by the thermodynamic equilibrium of the casting solution system and the dynamic process of solvent / non-solvent diffusion. During film formation, the lower the thermodynamic stability of the system, the faster the mass transfer rate, and the more likely the film-forming system will experience instantaneous liquid-liquid separation. This easily leads to the development of a dense layer and an asymmetric membrane structure characterized by finger-like pores. Conversely, the better the thermodynamic stability, the slower the mass transfer rate, and the film-forming system tends to delay liquid-liquid phase separation and easily form an asymmetric membrane with a sponge-like pore bottom layer. Changes in the concentration and chemical composition of the casting solution, as well as variations in additives, solvent composition, casting solution temperature, coagulation bath temperature, and evaporation time, all affect thermodynamic stability and mass transfer kinetics, resulting in changes in pore structure, crystal structure, permeability, hydrophilicity, mechanical properties, and other related characteristics. The study of various additives to enhance the structural integrity and functionality of membranes has always been an important research area, especially the exploration of porogens. Commonly used porogens include polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), non-solvents, and surfactants.

[0005] Patent CN106560231B discloses a method for preparing PVC microfiltration membranes using fumed silica as a pore-forming agent. It discloses a method for preparing microfiltration membranes via a phase inversion process using hydrophobically modified polydimethylsiloxane fumed silica as a pore-forming agent, PVC as the membrane material, and DMAc as a solvent. This method improves the pore size of the prepared PVC microfiltration membrane, resulting in uniform pore size that meets the requirements for the wide application of microfiltration membranes. The principle behind using fumed silica as a pore-forming agent in this patent is that fumed silica is insoluble in water, while N,N-dimethylacetamide in the casting solution is hydrophilic. This causes the fumed silica to form a discontinuous phase in the membrane material within the coagulation bath. When there are many discontinuous phases that can communicate with each other, filter pores are formed that penetrate the cross-section of the membrane. Simultaneously, the large molecular particles are immiscible with the high molecular polymer PVC, breaking the polymer linkages and causing the homogeneous liquid membrane of the casting solution to change from a continuous phase to a discontinuous phase, thereby increasing the pore size of the microfiltration membrane. Fumed silica can effectively regulate the changes in the pore structure of PVC microfiltration membranes, making its application in the regulation of pore structure in PVDF porous membranes highly valuable for research.

[0006] Pore-forming agents are substances or methods introduced into polymer casting solutions or precursors during membrane preparation and subsequently removed in some way, thereby leaving pores (or increasing porosity) inside and on the surface of the membrane. Using fumed silica as a pore-forming agent in PVDF ultrafiltration membranes first requires a suitable method to remove the fumed silica from the membrane without affecting the PVDF membrane matrix. Current research has reported methods such as using hydrofluoric acid (HF) and sodium hydroxide (NaOH) solutions to wash away fumed silica from PVDF membranes, but these methods involve hazardous chemicals or may affect the PVDF substrate.

[0007] Therefore, it is of great significance to study a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent in order to solve the problems existing in the prior art. Summary of the Invention

[0008] The purpose of this invention is to solve the problems existing in the prior art and to provide a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent involves first drying PVDF powder and fumed silica separately, then adding them to N,N-dimethylformamide (DMF) to prepare a casting solution; then, after degassing and scraping, the casting solution is immersed in a coagulation bath to form a PVDF flat sheet membrane; finally, tetrabutylammonium fluoride aqueous solution is used to wash away the fumed silica in the PVDF flat sheet membrane to achieve pore formation, thus obtaining the PVDF ultrafiltration membrane.

[0011] As a preferred technical solution:

[0012] The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, as described above, comprises the following specific steps:

[0013] (1) Heat and dry PVDF powder and fumed silica at 60~70 °C for more than 24 h;

[0014] (2) Add dried PVDF powder and fumed silica to N,N-dimethylformamide to prepare casting solution;

[0015] (3) Stir the casting solution until the PVDF powder and fumed silica are completely dissolved to obtain a homogenized casting solution;

[0016] (4) Degas the homogenized casting liquid in step (3), scrape the degassed casting liquid into a uniform liquid film, and then immerse it in a coagulation bath to obtain a PVDF flat sheet membrane.

[0017] (5) Using tetrabutylammonium fluoride aqueous solution to clean and remove fumed silica in PVDF flat sheet membrane to achieve pore formation. Cleaning does not affect PVDF. Sodium hydroxide aqueous solution can also wash away fumed silica, but the washing conditions are harsh and will cause PVDF to undergo defluorination and hydrogen degradation reaction, which macroscopically manifests as membrane color change and unstable performance.

[0018] (6) The PVDF sheet membrane after pore formation in step (5) is shaken and cleaned in a water bath at 30~50 ℃ for 1~2 h to remove the tetrabutylammonium fluoride adhering to the surface and obtain a PVDF ultrafiltration membrane.

[0019] The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent as described above, wherein the fumed silica in step (1) is unmodified hydrophilic fumed silica or hydrophobic fumed silica modified with dimethyl dichlorosilane.

[0020] As described above, in a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, in step (2), dried fumed silica is first added to N,N-dimethylformamide, and then PVDF powder is added after ultrasonic dispersion treatment. The mixture is then dissolved in an oil bath at 70 °C. Ultrasonic dispersion treatment is performed using an ultrasonic cell disruptor, model JY92-WⅡ, with a power of 390W and an amplitude transformer diameter of 6 mm.

[0021] In the method for preparing PVDF ultrafiltration membrane using fumed silica as a pore-forming agent as described above, the mass ratio of PVDF powder, N,N-dimethylformamide and fumed silica in step (2) is 0.15~17:0.79~0.82:0.02~0.05.

[0022] In the method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent as described above, the degassing treatment in step (4) is carried out in a vacuum oven at 40 °C for 1~1.5 h, the liquid film thickness is 150~250 µm, and the membrane scraping speed is 20~30 mm / s.

[0023] In the above-mentioned method for preparing PVDF ultrafiltration membrane using fumed silica as a pore-forming agent, the coagulation bath in step (4) is deionized water, the coagulation bath temperature is room temperature (25 ℃), and the coagulation time is more than 3 min.

[0024] The method for preparing PVDF ultrafiltration membrane using fumed silica as a pore-forming agent as described above, in step (5), the concentration of tetrabutylammonium fluoride aqueous solution is 5~10 wt%, the cleaning temperature is 25~40 ℃ (cleaning can be done at room temperature, but the temperature can be appropriately increased, 25~40℃ is more suitable), and the cleaning time is 45~90 min.

[0025] The principle of this invention is:

[0026] In the preparation of PVDF ultrafiltration membranes, adding fumed silica to the casting solution and uniformly dispersing it serves as an inducing nucleus for microphase separation. During non-solvent-induced phase inversion (NIPS), the well-dispersed fumed silica interferes with the ordered arrangement of PVDF molecular chains, thereby inhibiting polymer crystal growth. Due to the limited crystal growth, the polymer network inside the membrane becomes more open, ultimately forming an ultrafiltration membrane with an interconnected pore structure. After membrane formation, silica particles are removed from the membrane using a tetrabutylammonium fluoride aqueous solution to create a more porous membrane structure and improve water permeability.

[0027] In aqueous solution, tetrabutylammonium fluoride (TBAF) completely dissociates into tetrabutylammonium cations (TBA). + ) and fluoride ions (F - Subsequently, protons in the water are transferred to fluoride ions, which are rapidly converted into hydrogen difluoride (HF2). - In acidic media (tetrabutylammonium fluoride aqueous solution is acidic), HF2 - It undergoes a nucleophilic substitution reaction with silicon dioxide (SiO2), F - The OH group in the substituted silanol (Si-OH) - The process generates Si-F. Once the first Si-OH group becomes Si-F, the remaining three nucleophilic substitution reactions rapidly form Si-F bonds because the barrier energies of the other three substitution reactions are consistently lower than those of the first reaction, with the last reaction having the smallest barrier energy. (The lower barrier energy of the remaining three substitution reactions is due to the occurrence of the first substitution reaction. This is because after the first substitution reaction, due to the strong electron affinity of the fluoride ion, compared to the silicon-oxygen (Si-O) bond, the reaction results in more electrons transferring to the fluoride ion side. Therefore, the remaining bonds become weaker and more susceptible to attack by the fluoride ion, naturally reducing the barrier energy.) After the first substitution, due to the F... - The strong electron affinity of F leads to the reaction - The electron transfer rate is higher in the Si-O bond than in the Si-O bond. Therefore, the remaining bonds become weaker and more susceptible to Fo. - As the silicon dioxide permeates, the potential barrier naturally decreases. Finally, SiF4 is formed, thereby removing the fumed silica from the PVDF film and achieving a porous structure.

[0028] In tetrahydrofuran (THF) solution, THF, as an aprotic solvent, cannot provide protons (H+). + ) F - Convert to HF2 - TBAF in THF as ion pairs (TBA) + F - ) or "naked F" - It exists in the form of "F". -It is a strong base and a strong nucleophile. It reacts with HF2. - In comparison, F - Its nucleophilicity and basicity are relatively strong, resulting in poor reaction selectivity. Meanwhile, in THF, the strongly basic F... - It attacks the β-hydrogen atom on the methylene (-CH2-CF2-) group adjacent to the fluorine atom in the PVDF polymer chain, initiating an elimination reaction, removing the HF molecule, and forming a carbon-carbon double bond (C=C) on the polymer chain, affecting the structure and properties of PVDF. Therefore, tetrabutylammonium fluoride tetrahydrofuran solution not only fails to remove fumed silica, but also adversely affects the PVDF membrane substrate.

[0029] Beneficial effects:

[0030] (1) The present invention provides a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent. The fumed silica in the membrane can be effectively washed away by tetrabutylammonium fluoride aqueous solution without affecting the membrane material PVDF, thereby forming a pore-forming effect. The mechanical properties of the PVDF membrane remain almost unchanged after being washed by tetrabutylammonium fluoride aqueous solution.

[0031] (2) A method for preparing PVDF ultrafiltration membrane using fumed silica as a pore-forming agent according to the present invention controls the changes in the pore structure of PVDF porous membrane by controlling the hydrophilicity and hydrophobicity of fumed silica and its content in the casting solution system, thereby improving the pore size and porosity of PVDF ultrafiltration membrane to varying degrees.

[0032] (3) The present invention provides a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, which constructs PVDF ultrafiltration membranes with stable performance and controllable structure in a mild and simple manner, effectively removing fumed silica from the membrane without affecting the PVDF substrate, thereby achieving the regulation of the structure and performance of PVDF ultrafiltration membranes.

[0033] (4) The present invention provides a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, which improves the pore size and porosity of PVDF membranes and produces uniform pore size, meeting the requirements for the wide application of ultrafiltration membranes; at the same time, fumed silica and casting liquid have good compatibility and do not produce precipitation or other phase separation, and the preparation process is simple.

[0034] (5) The present invention provides a method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent. The membrane has excellent membrane-forming performance. The prepared PVDF ultrafiltration membrane has obvious finger-like pores and sponge-like structure. By changing the amount and type of fumed silica, the ratio of the two structures and the number of pores can be controlled, so that the membrane can significantly improve the pure water flux and porosity of the membrane while maintaining high BSA rejection. In addition, the pore size distribution is uniform and the mechanical properties are good. Attached Figure Description

[0035] Figure 1 The diagram shows the surface (left), cross-section (middle), and magnified pore structure (right) of the pure PVDF ultrafiltration membrane without fumed silica pores in Example 3.

[0036] Figure 2 The diagram shows the surface (left), cross-section (middle), and magnified pore structure (right) of the fumed silica porous PVDF ultrafiltration membrane without washing with tetrabutylammonium fluoride aqueous solution in Example 3.

[0037] Figure 3 The diagram shows the surface (left), cross-section (middle), and magnified pore structure (right) of the fumed silica porous PVDF ultrafiltration membrane in Example 3. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0039] The test methods involved in the performance indicators in the embodiments and comparative examples of this invention are as follows:

[0040] Pure water flux: Pure water flux was tested using a cross-flow filtration device. First, the membrane sample was pre-pressurized at 0.2 MPa for 1 h, then the volume of pure water passing through the membrane was measured at 0.1 MPa for 1 h. The pure water flux (J) can be calculated using the following formula:

[0041] ;

[0042] J is the pure water flux (L / m³) -2 h -1 V is The volume of pure water passing through the membrane in time t (L), where A is the effective area of ​​the membrane (m²). 2 ), t represents the test time (h).

[0043] Retention rate: The retention rate of the PVDF ultrafiltration membrane was determined using bovine serum albumin as a contaminant. First, the membrane sample was pre-pressurized in deionized water at 0.2 MPa for 1 h. Then, the permeate of BSA solution (concentration 1 g / L) was collected at 0.2 MPa. The absorbance of the original solution and the permeate was measured at 280 nm using a UV-Vis spectrophotometer. The concentration was calculated according to the BSA concentration-absorbance standard curve. The retention rate R was calculated using the following formula:

[0044] ;

[0045] Where C f and C p These represent the stock solution concentration and the permeate concentration of the BSA solution, respectively.

[0046] Example 1

[0047] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, comprising the following specific steps:

[0048] (1) PVDF powder (Zhejiang Juhua Co., Ltd., JHS-7010) and unmodified hydrophilic fumed silica (CAB-O-SIL® LM-150, particle size 0.015~0.030 μm; BET specific surface area: 130.0~170.0 m²) were mixed. 2 (g) was heated and dried at 60 °C for 48 h;

[0049] (2) Add the dried fumed silica from step (1) to N,N-dimethylformamide, disperse it by ultrasonication, then add PVDF powder, and then dissolve it in an oil bath at 70 °C to prepare a casting solution;

[0050] The mass ratio of PVDF powder, N,N-dimethylformamide, and fumed silica is 0.16:0.82:0.02.

[0051] (3) Stir the casting solution until the PVDF powder and silica are completely dissolved to obtain a homogenized casting solution;

[0052] (4) The homogenized casting solution in step (3) is degassed in a vacuum oven at 40 °C for 1 h. The degassed casting solution is scraped into a uniform liquid film and then immersed in a coagulation bath to obtain a PVDF flat sheet film.

[0053] The liquid film thickness was 150 µm, the film scraping speed was 20 mm / s, the coagulation bath was deionized water, the coagulation bath temperature was room temperature, the coagulation time was 3 min, and the film was stored in deionized water after coagulation.

[0054] (5) At 25 °C, the PVDF flat sheet membrane was cleaned with a 5 wt% tetrabutylammonium fluoride aqueous solution for 45 min to remove silica and achieve pore formation.

[0055] (6) The PVDF flat sheet membrane after pore formation in step (5) is shaken and cleaned in a water bath at 25°C for 2 hours to remove the tetrabutylammonium fluoride adhering to the surface, thus obtaining a PVDF ultrafiltration membrane.

[0056] The final PVDF ultrafiltration membrane had an average pore size of 42.9 nm, a pore size distribution of 10–100 nm, a porosity of 77.80%, and a pure water flux of 167.2 L / (m²). 2 (·h), the retention rate of BSA protein was 88.50%.

[0057] Comparative Example 1

[0058] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent is basically the same as in Example 1, except that the tetrabutylammonium fluoride aqueous solution is replaced with a tetrabutylammonium fluoride THF solution.

[0059] In Comparative Example 1, the THF solution of tetrabutylammonium fluoride failed to remove the fumed silica, did not react with the fumed silica, and caused the PVDF to turn black and undergo a degradation reaction.

[0060] Comparing Comparative Example 1 and Example 1, it can be seen that the color of the PVDF membrane in Comparative Example 1 gradually changed from white to black, and the membrane performance deteriorated. The fumed silica distributed on the membrane surface and cross-section was not washed away. This is because the THF solution of tetrabutylammonium fluoride used in the solution cannot wash away the fumed silica and will quickly corrode the PVDF.

[0061] Comparative Example 2

[0062] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent is basically the same as in Example 1, except that tetrabutylammonium fluoride aqueous solution is replaced with HF.

[0063] The final PVDF ultrafiltration membrane had a pure water flux of 108.7 L / (m²). 2 (·h), the retention rate of BSA protein was 78.6%.

[0064] Comparing Comparative Example 2 and Example 1, it can be seen that under the same washing process, the performance of the PVDF ultrafiltration membrane obtained by washing with HF in Comparative Example 2 is lower than that of the PVDF ultrafiltration membrane washed with tetrabutylammonium fluoride aqueous solution. This is because the concentration required for washing fumed silica with HF is higher than that of tetrabutylammonium fluoride aqueous solution. 5 wt% HF cannot completely wash away fumed silica, so some fumed silica will remain in the membrane, affecting the performance of the PVDF ultrafiltration membrane.

[0065] Comparative Example 3

[0066] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent is basically the same as in Example 1, except that the tetrabutylammonium fluoride aqueous solution is replaced with NaOH solution.

[0067] The final PVDF ultrafiltration membrane had a pure water flux of 85.8 L / (m³). 2 (·h), the retention rate of BSA protein was 76.4%.

[0068] Comparing Comparative Example 3 and Example 1, it can be seen that under the same washing process, the performance of the PVDF ultrafiltration membrane washed with HF is lower than that of the PVDF ultrafiltration membrane washed with tetrabutylammonium fluoride aqueous solution. Furthermore, the color of the PVDF membrane gradually changes from white to brown. This is because the washing conditions with NaOH aqueous solution are demanding, requiring a temperature above 60°C to remove the fumed silica and a longer washing time. Under the conditions of Example 1, the fumed silica in the membrane was not removed. In addition, NaOH causes the PVDF to undergo a defluorination hydrogen degradation reaction, which leads to the membrane color change and performance degradation.

[0069] Example 2

[0070] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, comprising the following specific steps:

[0071] (1) PVDF powder (Zhejiang Juhua Co., Ltd., JHS-7010) and unmodified hydrophilic fumed silica (CAB-O-SIL® LM-150, particle size 0.015~0.030 μm; BET specific surface area: 130.0~170.0 m²) were mixed. 2 (g) was heated and dried at 62 °C for 42 h;

[0072] (2) Add the dried fumed silica from step (1) to N,N-dimethylformamide, disperse it by ultrasonication, then add PVDF powder, and then dissolve it in an oil bath at 70 °C to prepare a casting solution;

[0073] The mass ratio of PVDF powder, N,N-dimethylformamide, and fumed silica is 0.16:0.80:0.04.

[0074] (3) Stir the casting solution until the PVDF powder and fumed silica are completely dissolved to obtain a homogenized casting solution;

[0075] (4) The homogenized casting solution in step (3) was degassed in a vacuum oven at 40 °C for 1.1 h. The degassed casting solution was scraped into a uniform liquid film and then immersed in a coagulation bath to obtain a PVDF flat sheet film.

[0076] The liquid film thickness was 175 µm, the film scraping speed was 23 mm / s, the coagulation bath was deionized water, the coagulation bath temperature was room temperature, the coagulation time was 5 min, and after coagulation treatment, it was stored in deionized water.

[0077] (5) At 25 °C, the PVDF flat sheet membrane was cleaned with a 6 wt% tetrabutylammonium fluoride aqueous solution for 60 min to remove silica and achieve pore formation.

[0078] (6) The PVDF sheet membrane after pore formation in step (5) is shaken and cleaned in a 30 °C water bath for 1.75 h to remove the tetrabutylammonium fluoride adhering to the surface, and a PVDF ultrafiltration membrane is obtained.

[0079] The final PVDF ultrafiltration membrane had an average pore size of 40.2 nm, a pore size distribution of 10–90 nm, a porosity of 79.90%, and a pure water flux of 132.6 L / (m²). 2 (·h), the retention rate of BSA protein was 91.30%.

[0080] Example 3

[0081] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, comprising the following specific steps:

[0082] (1) PVDF powder (Zhejiang Juhua Co., Ltd., JHS-7010) and dimethyldichlorosilane-modified hydrophobic fumed silica (CAB-O-SIL® TS-610, supplier: CABOT, particle size 0.020~0.030 μm; BET specific surface area: 105.0~145.0 m²) were mixed. 2 (g) was heated and dried at 65 °C for 36 h;

[0083] (2) Add dried fumed silica to N,N-dimethylformamide, disperse it by ultrasonication, then add PVDF powder, and then dissolve it in an oil bath at 70 °C to prepare casting solution;

[0084] The mass ratio of PVDF powder, N,N-dimethylformamide, and silica is 0.16:0.79:0.05.

[0085] (3) Stir the casting solution until the PVDF powder and fumed silica are completely dissolved to obtain a homogenized casting solution;

[0086] (4) The homogenized casting solution in step (3) was degassed in a vacuum oven at 40 °C for 1.25 h. The degassed casting solution was scraped into a uniform liquid film and then immersed in a coagulation bath to obtain a PVDF flat sheet film.

[0087] The liquid film thickness was 200 µm, the film scraping speed was 25 mm / s, the coagulation bath was deionized water, the coagulation bath temperature was room temperature, the coagulation time was 10 min, and after coagulation treatment, it was placed in deionized water for storage.

[0088] (5) At 25 °C, the PVDF flat sheet membrane was cleaned with a 7 wt% tetrabutylammonium fluoride aqueous solution for 70 min to remove the fumed silica and achieve pore formation.

[0089] (6) The PVDF sheet membrane after pore formation in step (5) is shaken and cleaned in a 40 °C water bath for 1.5 h to remove the tetrabutylammonium fluoride adhering to the surface, and a PVDF ultrafiltration membrane is obtained.

[0090] The final PVDF ultrafiltration membrane had an average pore size of 25.7 nm, a pore size distribution of 5–80 nm, a porosity of 69.90%, and a pure water flux of 78.8 L / (m²). 2 (·h), the retention rate of BSA protein was 94.70%.

[0091] like Figure 1 As shown, pure PVDF ultrafiltration membranes without fumed silica pores have mostly closed pores, poor connectivity, few pores, and poor performance.

[0092] like Figure 2 As shown, the surface and cross-section of the PVDF ultrafiltration membrane with fumed silica that has not been cleaned with tetrabutylammonium fluoride aqueous solution have a large amount of silica, which blocks the pores on the surface and the pores in the cross-section of the PVDF ultrafiltration membrane, thus limiting the membrane's performance.

[0093] like Figure 3 As shown, after washing with tetrabutylammonium fluoride aqueous solution, the fumed silica on the surface and cross-section of the unmodified hydrophilic amorphous silica-porous PVDF ultrafiltration membrane is effectively removed, resulting in a large number of pores and voids in the membrane, which improves the connectivity of the membrane pores. It also has a large finger-like pore and sponge-like structure, which enables the membrane to effectively increase the pure water flux while maintaining a high BSA protein rejection rate.

[0094] Example 4

[0095] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, comprising the following specific steps:

[0096] (1) PVDF powder (Zhejiang Juhua Co., Ltd., JHS-7010) and dimethyldichlorosilane-modified hydrophobic fumed silica (CAB-O-SIL® TS-610, particle size 0.020~0.030 μm; BET specific surface area: 105.0~145.0 m²) were mixed. 2 (g) was heated and dried at 68 °C for 30 h;

[0097] (2) Add dried fumed silica to N,N-dimethylformamide, disperse it by ultrasonication, then add PVDF powder, and then dissolve it in an oil bath at 70 °C to prepare casting solution;

[0098] The mass ratio of PVDF powder, N,N-dimethylformamide, and fumed silica is 0.15:0.80:0.05.

[0099] (3) Stir the casting solution until the PVDF powder and fumed silica are completely dissolved to obtain a homogenized casting solution;

[0100] (4) The homogenized casting solution in step (3) is degassed in a vacuum oven at 40 °C for 1.4 h. The degassed casting solution is scraped into a uniform liquid film and then immersed in a coagulation bath to obtain a PVDF flat sheet film.

[0101] The liquid film thickness was 225 µm, the film scraping speed was 27 mm / s, the coagulation bath was deionized water, the coagulation bath temperature was room temperature, the coagulation time was 20 min, and after coagulation treatment, it was stored in deionized water.

[0102] (5) At 25 °C, the PVDF flat sheet membrane was cleaned with an 8 wt% tetrabutylammonium fluoride aqueous solution for 80 min to remove the fumed silica and achieve pore formation.

[0103] (6) The PVDF sheet membrane after pore formation in step (5) is shaken and cleaned in a 50 °C water bath for 1.25 h to remove the tetrabutylammonium fluoride adhering to the surface, thus obtaining a PVDF ultrafiltration membrane.

[0104] The final PVDF ultrafiltration membrane had an average pore size of 28.6 nm, a pore size distribution of 3–90 nm, a porosity of 72.90%, and a pure water flux of 88.3 L / (m²). 2 (·h), the retention rate of BSA protein was 92.80%.

[0105] Example 5

[0106] A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, comprising the following specific steps:

[0107] (1) PVDF powder (Zhejiang Juhua Co., Ltd., JHS-7010) and dimethyldichlorosilane-modified hydrophobic fumed silica (CAB-O-SIL® TS-610, particle size 0.020~0.030 μm; BET specific surface area: 105.0~145.0 m²) were mixed. 2 (g) was heated and dried at 70 °C for 24 h;

[0108] (2) Add dried fumed silica to N,N-dimethylformamide, disperse it by ultrasonication, then add PVDF powder, and then dissolve it in an oil bath at 70 °C to prepare casting solution;

[0109] The mass ratio of PVDF powder, N,N-dimethylformamide, and fumed silica is 0.17:0.81:0.02.

[0110] (3) Stir the casting solution until the PVDF powder and fumed silica are completely dissolved to obtain a homogenized casting solution;

[0111] (4) The homogenized casting solution in step (3) is degassed in a vacuum oven at 40 °C for 1.5 h. The degassed casting solution is scraped into a uniform liquid film and then immersed in a coagulation bath to obtain a PVDF flat sheet film.

[0112] The liquid film thickness was 250 µm, the film scraping speed was 30 mm / s, the coagulation bath was deionized water, the coagulation bath temperature was room temperature, the coagulation time was 30 min, and after coagulation treatment, it was stored in deionized water.

[0113] (5) At 25 °C, the PVDF flat sheet membrane was cleaned for 90 min with a 10 wt% tetrabutylammonium fluoride aqueous solution to remove fumed silica and achieve pore formation.

[0114] (6) The PVDF sheet membrane after pore formation in step (5) is shaken and cleaned in a 60 °C water bath for 1 h to remove the tetrabutylammonium fluoride adhering to the surface, thus obtaining a PVDF ultrafiltration membrane.

[0115] The final PVDF ultrafiltration membrane had an average pore size of 22.5 nm, a pore size distribution of 3–70 nm, a porosity of 52.90%, and a pure water flux of 50.9 L / (m²). 2 (·h), the retention rate of BSA protein was 96.20%.

Claims

1. A method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent, characterized in that: First, PVDF powder and fumed silica are dried separately and then added to N,N-dimethylformamide to prepare a casting solution. Then, after degassing and scraping, the casting solution is immersed in a coagulation bath to form a PVDF flat sheet membrane. Finally, tetrabutylammonium fluoride aqueous solution is used to wash away the fumed silica in the PVDF flat sheet membrane to achieve pore formation, thus obtaining a PVDF ultrafiltration membrane.

2. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 1, characterized in that, The specific steps are as follows: (1) Heat and dry PVDF powder and fumed silica at 60~70 °C for more than 24 h; (2) Add dried PVDF powder and fumed silica to N,N-dimethylformamide to prepare casting solution; (3) Stir the casting solution until the PVDF powder and fumed silica are completely dissolved to obtain a homogenized casting solution; (4) Degas the homogenized casting liquid in step (3), scrape the degassed casting liquid into a uniform liquid film, and then immerse it in a coagulation bath to obtain a PVDF flat sheet membrane. (5) Pore formation is achieved by cleaning the PVDF flat sheet membrane with a tetrabutylammonium fluoride aqueous solution to remove fumed silica; (6) The PVDF flat sheet membrane after pore formation in step (5) is shaken and cleaned in a water bath at 30~50 ℃ for 1~2 h to remove the tetrabutylammonium fluoride adhering to the surface and obtain a PVDF ultrafiltration membrane.

3. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 2, characterized in that, In step (1), the fumed silica is unmodified hydrophilic fumed silica or hydrophobic fumed silica modified with dimethyldichlorosilane.

4. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 2, characterized in that, In step (2), dried fumed silica is first added to N,N-dimethylformamide, and then PVDF powder is added after ultrasonic dispersion treatment. Finally, it is dissolved in an oil bath at 70 °C.

5. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 2, characterized in that, In step (2), the mass ratio of PVDF powder, N,N-dimethylformamide and fumed silica is 0.15~0.17:0.79~0.82:0.02~0.

05.

6. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 2, characterized in that, In step (4), the degassing treatment is carried out in a vacuum oven at 40 ℃ for 1~2 h, the liquid film thickness is 150~250 µm, and the film scraping speed is 20~30 mm / s.

7. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 2, characterized in that, In step (4), the coagulation bath is deionized water, the coagulation bath temperature is room temperature, and the coagulation time is more than 3 minutes.

8. The method for preparing PVDF ultrafiltration membranes using fumed silica as a pore-forming agent according to claim 2, characterized in that, In step (5), the concentration of the tetrabutylammonium fluoride aqueous solution is 5~10 wt%, the cleaning temperature is 25~40 ℃, and the cleaning time is 45~90 min.

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