Silicon dioxide reinforced high-hydrophobicity polyvinylidene fluoride membrane as well as preparation method and application thereof

Abstract

The invention discloses a silicon dioxide reinforced high-hydrophobicity polyvinylidene fluoride membrane as well as a preparation method and application thereof, SiO2 nanoparticles are taken as a mechanical reinforcing agent and a structure regulator and are directly added into a membrane casting solution system, and solid-liquid phase separation is preliminarily induced on the surface of a primary membrane in combination with non-solvent spraying ethanol, so that the high-hydrophobicity PVDF membrane is successfully prepared. A unique multi-stage micro-nano structure consisting of mutually connected spherocrystals is constructed on the surface of the membrane, the hydrophobicity, aperture, porosity and mechanical property of the membrane are effectively improved, and the membrane shows excellent and stable separation performance in performance tests of seawater desalination and erythritol concentration, and has good application prospects in seawater desalination and erythritol concentration. The method shows huge application potential in the fields of wastewater treatment, food production and the like.

Description

Technical Field

[0001] The present invention relates to a method for preparing a membrane material and its application, and in particular to a method for preparing a hydrophobic polyvinylidene fluoride membrane and its application; it belongs to the field of membrane technology. Background Art

[0002] Polyvinylidene fluoride (PVDF) is a polymer with excellent chemical resistance, thermal stability and inherent hydrophobicity, and these properties make it suitable for membrane separation processes such as wastewater treatment and product concentration. However, the hydrophobic PVDF membranes used for membrane distillation face problems such as easy wetting and low flux, which greatly limit the application of membrane distillation technology in the treatment of complex liquid mixtures. Membrane wetting caused by amphiphilic pollutants will reduce the liquid entry pressure of membrane pores, resulting in the direct passage of the liquid on the feed side through the membrane pores into the permeate side, and the rejection rate drops sharply. At the same time, during the membrane distillation process, solids such as salts will continuously accumulate in the stagnant layer on the membrane surface, further accelerating membrane fouling and wetting. Therefore, optimizing the physicochemical properties and anti-wetting performance of PVDF membrane distillation membranes has become the focus of research in recent years.

[0003] Constructing a membrane surface with superhydrophobicity and self-cleaning function is an effective way to achieve the anti-wetting performance of the membrane. Inspired by natural plants such as lotus leaves in nature, researchers have improved the hydrophobicity of the membrane surface by constructing a hierarchical micro-nano rough structure and using low-surface-energy substances. In order to prepare a highly hydrophobic membrane for membrane distillation by the traditional non-solvent induced phase separation (NIPS) method, a weak coagulation bath composed of alcohols or solvents (such as N-methyl-2-pyrrolidone (NMP)) is usually used. The main role of the coagulation bath is to reduce the exchange rate of the solvent / non-solvent, thereby delaying the phase inversion process. Pagliero et al. used a mixture of water and ethanol as the coagulation bath to prepare a porous hydrophobic membrane composed of interconnected semi-crystalline polymer spherulites and microfibers through a phase inversion process. The flux of this membrane in vacuum membrane distillation was significantly improved, reaching about 15, 50 and 100 L·m -2 ·h -1 respectively at feed temperatures of 30 °C, 50 °C and 70 °C. -2 ·h -1 Munirasu et al. used methanol as a non-solvent coagulation bath to induce phase inversion to prepare a porous, superhydrophobic flat PVDF membrane. The membrane prepared showed a flux as high as 40 L·m

[0004] In addition, membranes without a dense surface layer can also be prepared by the vapor-induced phase separation method. In the VIPS process, the newly cast membrane solution is first exposed to a gaseous nonsolvent for a certain period of time and then transferred to a coagulation bath to complete the phase inversion process. Venault et al. successfully prepared superhydrophobic membranes with a water contact angle exceeding 152° by controlling the polymer concentration, dissolution temperature, relative humidity, and vapor exposure time. Compared with commercial PTFE membranes, the prepared VIPS-PVDF membranes exhibited higher water vapor fluxes (38 LMH vs. 36 LMH), lower flux decay rates (22% vs. 39%), and higher salt rejection rates (99.99%). Ma et al. prepared porous membranes with a rough microsphere structure by the VIPS method and then combined them with a surface perfluorination modification process to construct more excellent fully hydrophobic PVDF membranes. Although the VIPS method can prepare hydrophobic membranes with micro / nano hierarchical structures and three-dimensional particle-packed interconnected pores, making them suitable for efficient and stable membrane distillation processes, and finally forming hydrophobic membranes without macropores and with multi-level rough structures to improve the anti-wetting performance, this process requires a long exposure time to form a highly hydrophobic surface, and the overall film preparation process takes a long time.

[0005] In addition, the mechanical strength of PVDF membranes with a spherulite packing structure prepared in the prior art is generally low. Therefore, the development of porous PVDF membranes with excellent mechanical properties, hydrophobic properties, and application properties remains a huge challenge. Summary of the Invention

[0006] To solve the deficiencies of the prior art, the purpose of the present invention is to provide a silica-reinforced highly hydrophobic polyvinylidene fluoride membrane, its preparation method, and application to solve the problems of easy wetting, low flux, and low mechanical properties faced by hydrophobic membranes used in membrane distillation in the prior art.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] The present invention first discloses a preparation method for a nonsolvent spray-assisted induced phase separation of a hydrophobic polyvinylidene fluoride membrane, including the following steps:

[0009] S1. Uniformly disperse SiO2 in a DMF solvent to form a homogeneous dispersion liquid, and then add PVDF powder thereto, heat and stir to obtain a clear and transparent membrane solution;

[0010] S2. Knife-coat the membrane solution on a clean glass plate with a casting gap of 250 μm;

[0011] S3. Use a spray gun to spray ethanol on the surface of the newly cast primary liquid membrane, and immerse the film sprayed several times in a water coagulation bath;

[0012] S4. Transfer the membrane to fresh deionized water and soak it for more than 12 h to ensure complete phase inversion of the membrane.

[0013] S5. First freeze the prepared membrane sample in a refrigerator, and then dry it using a freeze dryer. The dried sample is sealed and stored for subsequent use.

[0014] Preferably, in the aforementioned casting solution, the mass percentage content of SiO2 is 1%, and the mass percentage content of PVDF powder is 14% - 18%.

[0015] More preferably, the aforementioned spraying times are 1 - 5 times.

[0016] Further preferably, the aforementioned spraying times are more than 2 times to form a complete spherulite structure on the cross-section of the membrane.

[0017] Preferably, ethanol spraying is carried out using a micro air pump under the conditions of 25°C and a relative humidity of about 60%. The working pressure of the air pump is 2.0 bar, and the distance between the nozzle and the surface of the primary liquid film is 15 cm.

[0018] The present invention also discloses a hydrophobic polyvinylidene fluoride membrane prepared by the aforementioned non-solvent spray-assisted induced phase separation preparation method. The surface of the membrane is a porous structure composed of interconnected spherulites, and the cross-section of the membrane presents an interconnected spherulite structure.

[0019] Preferably, the water contact angle of the aforementioned membrane can be as high as 145°, and the membrane flux is as high as 33.4 kg·m -2 ·h -1 , and the membrane tensile stress and elongation at break are 374 kPa and 28.2% respectively.

[0020] The advantages of the present invention are as follows:

[0021] (1) In the present invention, SiO2 nanoparticles are used as mechanical reinforcing agents and structure regulators, and are directly added to the casting solution system. Combining non-solvent spray ethanol to preliminarily induce solid-liquid phase separation on the surface of the primary membrane, which affects the morphology structure of the membrane. That is, a highly hydrophobic PVDF membrane is successfully prepared by the spray-assisted non-solvent induced phase separation method (SANIPS method), and a unique multi-level micro-nano structure composed of interconnected spherulites is constructed on the membrane surface, effectively improving the hydrophobicity, pore size, porosity and mechanical properties of the membrane.

[0022] (2) The prepared hydrophobic PVDF membrane is tested for the performance of desalination of brine (such as seawater desalination, treatment of saline wastewater) and concentration of food and drug additives (such as erythritol) through the direct contact membrane distillation (DCMD) process, and all show excellent and stable separation performance, demonstrating its application potential in the fields of brine treatment and food and drug production. Description of the Drawings

[0023] Figure 1 Surface SEM image of the PVDF hydrophobic membrane prepared according to the present invention;

[0024] Figure 2 Cross-section SEM image of the PVDF hydrophobic membrane prepared according to the present invention;

[0025] Figure 3 Chemical composition analysis diagram of the PVDF hydrophobic membrane prepared according to the present invention, where: (a) XPS characterization of PVDF and PVDF-SiO2 membranes; (b) SEM image and EDS element distribution diagram of the cross-section of 18% PVDF-SiO2-E5 membrane;

[0026] Figure 4 Water contact angle of the PVDF hydrophobic membrane prepared according to the present invention in air, where: (a) 14%, (b) 16%, (c) 18%, (d) 14% - 18%;

[0027] Figure 5 Mechanical strength analysis diagram of the PVDF hydrophobic membrane prepared according to the present invention;

[0028] Figure 6 Permeation flux and rejection rate of the PVDF hydrophobic membrane prepared according to the present invention during MD operation, where: (a) Feed solution = 3.5 wt% NaCl; (b) Feed solution = 10 wt% erythritol (feed temperature = 60 °C, permeate side temperature = 20 °C);

[0029] Figure 7 Concentration performance test diagram of the PVDF hydrophobic membrane prepared according to the present invention, where: (a) Membrane distillation concentration test of erythritol with 16% PVDF-SiO2-E5 membrane and 18% PVDF-SiO2-E5 membrane (feed solution = 10 wt% erythritol; feed temperature = 60 °C, permeate side temperature = 20 °C, replenish the solution in the feed tank to 5.5 liters with deionized water every 24 hours); (b) Concentration of erythritol in the feed tank during MD process and concentration coefficient every 24 hours. Detailed implementation manners

[0030] The present invention will be specifically introduced below in conjunction with the accompanying drawings and specific embodiments.

[0031] In the present invention, unless otherwise specified, all raw materials are commercially available, and the preferred commercial sources are shown in Table 1 below.

[0032]

[0033]

[0034] Table 1 Sources of reagents and materials used in the present invention

[0035] Examples 1 to 12 and Comparative Examples 1 to 12 of the present invention all adopt an innovative process: a non-solvent spray-assisted induced phase separation method to prepare a silica-reinforced highly hydrophobic polyvinylidene fluoride membrane. The method steps are basically similar and are as follows:

[0036] S1. Prepare a casting solution:

[0037] Casting solution of the example: SiO2 was uniformly dispersed in a DMF solvent to form a homogeneous dispersion, and then PVDF powder was added thereto, and the temperature was raised and stirred to obtain a clear and transparent casting solution;

[0038] Casting solution of the comparative example: PVDF powder and DMF were mixed in different mass ratios in a reagent bottle, and then placed in an oven at 90 °C to dissolve, and the mixture was shaken regularly until a uniform and transparent casting solution was obtained.

[0039] S2. Knife-coat the casting solution on a clean glass plate, and the casting gap is 250 μm;

[0040] S3. Use a spray gun to spray ethanol on the surface of the newly knife-coated primary liquid film, and immerse the film sprayed several times (1 / 2 / 3 / 5) in a water coagulation bath for 20 to 60 min;

[0041] S4. Transfer the membrane to fresh deionized water and soak it for more than 12 h to ensure complete phase inversion of the membrane;

[0042] S5. First freeze the prepared membrane sample in a refrigerator, and then dry it using a freeze dryer. The dried sample is sealed and stored for subsequent use.

[0043] The main differences between each example and the comparative example lie in the differences in raw materials, contents, and processes. See Table 2 below for details:

[0044]

[0045]

[0046] Table 2 Raw material dosages and process parameters of Examples 1 to 12 and Comparative Examples 1 to 12

[0047] In the above table, the number of spraying times (1 / 2 / 3 / 5 times) is denoted as E1, E2, E3, and E5 respectively. The 18% PVDF-SiO2-E2 membrane refers to a membrane prepared by using PVDF / SiO2 / DMF (18 / 1 / 81 wt%) as the casting solution (casting solution of the example) and spraying ethanol twice on the surface of the primary liquid film; the 16% PVDF-E3 membrane is a membrane prepared by using PVDF / DMF = 16 / 84 wt% as the casting solution (casting solution of the comparative example) and spraying ethanol three times on the surface of the primary liquid film.

[0048] Structural characterization

[0049] (1) Membrane surface morphology characterization

[0050] The surface and cross-section morphologies of the prepared PVDF membranes were observed by field emission scanning electron microscopy (FESEM, SUPRATM55, Carl Zeiss AG, Germany). The membrane cross-section samples were prepared by liquid nitrogen quenching fracture method. Before the FESEM test, the surface of the samples was treated with a gold layer conductive treatment for 120 seconds using an ion sputtering coater (Q150R ES, USA) to obtain clear electron microscope images.

[0051] In the SANIPS method of the present invention, the non-solvent spray of ethanol induces surface phase separation of the casting solution, which greatly affects the membrane structure. As Figure 1 shown, when ethanol is sprayed once, the membrane surface is still relatively dense, but the near-surface layer is composed of finger-like pores and a spongy structure in the main body. With the increase in the number of ethanol sprays (E2 - E5), the membrane surface gradually transforms into a porous structure composed of interconnected spherulites, and the membrane cross-section also shows an interconnected spherulite structure. By comparing the morphological changes of 14%, 16% and 18% PVDF membranes, it is found that with the increase in the polymer concentration, the amount of ethanol spray required for the membrane to form a complete interconnected spherulite porous structure gradually decreases.

[0052] We also found that the addition of SiO2 also has a significant impact on the membrane structure. When ethanol is sprayed once, compared with the 14% PVDF-E1 membrane, more nucleation-like structures appear in the cross-section of the 14% PVDF-SiO2-E1 membrane, as Figure 2 shown. When ethanol is sprayed twice, the cross-section of the 14% PVDF-SiO2-E2 membrane forms a complete spherulite structure, while the 14% PVDF-E2 membrane still contains larger finger-like pores and some spongy structures. Similarly, by comparing the micro-morphologies of 16% and 18% PVDF membranes and 16% and 18% PVDF-SiO2 membranes, it is found that with the increase in the polymer concentration, their structures show similar changing rules. This indicates that the addition of SiO2 nanoparticles is beneficial to promoting the solid-liquid phase separation process of the casting solution, changing the thermodynamic and rheological properties of the polymer solution, and forming a uniform spherulite-like structure. During the phase inversion process, SiO2 nanoparticles serve as nucleation sites for the polymer, increasing the nucleation density of the polymer solution and also increasing the crystallization rate, thereby forming a porous structure with interconnected spherulites.

[0053] With the increase of PVDF concentration, the cloud point of the casting solution decreases. After adding nano-SiO2 to the casting solution system, the cloud point of the casting solution system further decreases, as shown in Table 2. This indicates that with the increase of PVDF concentration and the addition of nano-SiO2, the thermodynamic stability of the casting solution system decreases, and only a small amount of ethanol can induce phase inversion. In addition, nano-SiO2 also plays the role of pre-crystallization nuclei in the casting solution. With the addition of SiO2, the number of nucleation points in the casting solution increases, which promotes the crystallization of the polymer and helps the occurrence of solid-liquid phase separation, which is consistent with Figure 1 and Figure 2 the morphological characterization results.

[0054] Cloud point (wt%) / casting solution 14％ 16％ 18％ PVDF 3.05±0.225 1.65±0.221 0.95±0.050 <![CDATA[PVDF-SiO2]]> 1.91±0.076 1.26±0.085 0.55±0.033

[0055] Table 2 Cloud point values of different casting solution systems

[0056] The viscosity of the casting solution in Table 3 reflects the magnitude of the shear force during the mutual movement of polymer molecular segments and is one of the key factors affecting the hydrodynamics of polymer solutions. The viscosity of the casting solution affects the exchange rate between the solvent and the non-solvent, thereby affecting the rate of phase inversion. As can be seen from Table 3, with the increase of PVDF concentration, the viscosity of the casting solution also increases; after introducing nano-SiO2 into the casting solution, the viscosity further increases. Therefore, the increase in the viscosity of the casting solution system reduces the exchange rate between the solvent and the non-solvent, which further promotes the occurrence of solid-liquid phase separation.

[0057] Viscosity (Pa.s) / casting solution 14％ 16％ 18％ PVDF 6049.0±58.6 13390.3±334.5 27880.0±676.2 <![CDATA[PVDF-SiO2]]> 6906.0±38 14387.7±76.5 30259.0±310

[0058] Table 3 Viscosity values of different casting solution systems

[0059] (2) Analysis of membrane chemical composition

[0060] XPS and EDS were used to analyze the surface element composition and element distribution of the membrane. As Figure 3 shown, new peaks of O1s and Si 2p appear in the XPS spectrum of the PVDF-SiO2 membrane, indicating the successful doping of nano-SiO2. At the same time, the EDS scan of the cross-section of the 18% PVDF-SiO2-E5 membrane shows that Si and O elements are evenly distributed in the membrane without obvious agglomeration. This proves that nano-SiO2 is evenly incorporated into the membrane matrix.

[0061] (3) Membrane surface wettability

[0062] A contact angle measuring instrument (OCA-15EC, Dataphysics, Germany) was used to measure the water contact angle (WCA) of the membrane sample in air. The specific method is as follows: 3.0 μL of deionized water was dropped on the membrane surface, and the angle was measured immediately. Each sample was measured at least 5 times, and the final result was the average value.

[0063] The water contact angle (WCA) and surface roughness are important parameters determining the hydrophobicity of the membrane. Figure 4 The water contact angle of the hydrophobic membrane prepared in this invention in air. With the increase of the spraying amount of the non-solvent (i.e., ethanol), the surface hydrophobicity of both the PVDF and PVDF-SiO2 membranes is significantly improved ( Figure 4 (a)-(c)). When ethanol is sprayed 3 times, the water contact angles of both the PVDF and PVDF-SiO2 membranes exceed 140°, which may be due to the formation of the micro-nano rough structure as shown in Figure 2 the figure. This hierarchical structure generates "cavities" between water and the spherulite structure, reducing the liquid-solid contact area and increasing the contact angle. Figure 4 (d) shows that for the membranes sprayed with ethanol 5 times, the water contact angles of the PVDF membranes and PVDF-SiO2 membranes with different concentrations are about 145°, all showing characteristics close to superhydrophobicity.

[0064] (4) Characterization of membrane mechanical properties

[0065] The mechanical strength of the membrane samples was tested using a tensile strength tester (Dongguan Dongri Instrument Co., Ltd.). The membrane was cut into samples of 50 mm × 15 mm. Five positions on the sample were randomly selected to measure its thickness, and the average value was taken as the membrane thickness. The cut membrane samples were fixed on the sample clamp of the tensile strength tester. Then, parameters such as the thickness and specimen gauge length of the corresponding sample were input into the computer software, and a constant elongation rate of 20 mm / min was set for the test. The test ended until the sample broke, and the stress-strain curve data could be obtained.

[0066] Figure 5The tensile strength and elongation at break of the 16% and 18% series membranes are shown. It can be seen that both the tensile strength and elongation at break increase with the increase in PVDF concentration. After adding nano-SiO2, the mechanical properties of the membranes are significantly improved. Specifically, for the 16% PVDF-E5 membrane, the tensile stress is 42 kPa and the elongation at break is 4.3%; while for the 16% PVDF-SiO2-E5 membrane containing SiO2, the tensile stress increases to 133 kPa, which is 3.17 times higher than that of the 16% PVDF-E5 membrane without added nanoparticles. The tensile stress of the 18% PVDF-E5 membrane is 145 kPa and the elongation at break is 10.4%; the tensile stress and elongation at break of the 18% PVDF-SiO2-E5 membrane are 374 kPa and 28.2% respectively. Compared with the 18% PVDF-E5 membrane, the tensile stress and elongation at break are increased by 2.58 times and 2.71 times respectively. This indicates that adding an appropriate amount of SiO2 nanoparticles can effectively improve the mechanical strength of PVDF membranes, which may be attributed to two aspects: (1) Nano-SiO2 acts as a nucleating agent for polymer crystallization, further optimizing the crystallization morphology of PVDF membranes and forming a more perfect spherulite structure; (2) Polymer molecular chains are entangled on the surface of SiO2 nanoparticles, increasing the binding force between them. At the same time, nanoparticles have a high specific surface area and excellent mechanical properties, which can effectively disperse stress and enhance the overall strength of the membrane.

[0067] (5) Membrane pore structure characterization

[0068] Table 4 shows the pore structure parameters and membrane thickness of the NIPS membrane without spray treatment (i.e., E0 membrane) and the SANIPS membrane with spray treatment (i.e., E5 membrane). From the data, it can be seen that compared with the NIPS membrane without spray treatment (i.e., E0 membrane), the pore size and porosity of the SANIPS membranes (16% PVDF-E5 and 18% PVDF-E5) of the present invention are significantly increased. The main reason may be that the non-solvent (i.e., ethanol) spray promotes the formation of an interconnected spherulite membrane structure. By adding nano-SiO2, the pore size and porosity of the 16% PVDF-SiO2-E5 and 18% PVDF-SiO2-E5 membranes are slightly increased. It is worth noting that the thickness of the membrane also increases significantly after spray treatment, which is also attributed to the fact that the newly formed interconnected spherulite structure is looser than the initial finger-like pores and sponge-like structures, and the aforementioned SEM images also confirm this phenomenon.

[0069] Membrane number Average pore size (μm) Porosity (%) Membrane thickness (μm) 16% PVDF-E0 0.025±0.0041 61.14±3.19 77.83±4.26 16% PVDF-E5 0.687±0.0297 82.72±1.22 108.83±3.13 <![CDATA[16% PVDF - SiO2 - E5]]> 1.358±0.0839 86.40±2.84 114.84±2.12 18% PVDF-E0 0.020±0.0013 61.34±2.16 99.83±4.12 18% PVDF-E5 0.0642±0.0086 88.05±3.31 141.21±6.07 <![CDATA[18% PVDF-SiO2-E5]]> 0.766±0.0447 91.15±4.34 149.60±4.91

[0070] Table 4 Pore structure parameters and membrane thickness of the E0 membrane without spray treatment and the E5 membrane with spray treatment

[0071] (6) Membrane distillation (MD) performance test

[0072] The MD separation performance of the membrane prepared by the present invention for NaCl (brine) and erythritol solution is as follows Figure 6 shown. As can be seen from Figure 6 (a), when the same amount of non-solvent ethanol is sprayed, the flux of the 16% series of PVDF membranes is higher than that of the 18% series of membranes. After introducing nano-SiO2, the fluxes of the 16% PVDF-SiO2-E5 and 18% PVDF-SiO2-E5 membranes are increased to 39.6±2.8 kg·m -2 ·h -1 and 31.4±1.7 kg·m -2 ·h -1 respectively, and the rejection rates are 99.8% and 99.99% respectively.

[0073] Furthermore, the 16% PVDF-SiO2-E5 and 18% PVDF-SiO2-E5 membranes with relatively excellent desalination performance are selected for MD concentration of erythritol solution. Figure 6 (b) shows that the fluxes of the 16% PVDF-SiO2-E5 and 18% PVDF-SiO2-E5 membranes are 33.4±1.9 kg·m -2 ·h -1 and 28.1±0.8 kg·m -2 ·h -1 respectively, and the rejection rates both exceed 99.9%. Compared with the membrane distillation desalination test, the flux decreases slightly.

[0074] (7) Membrane concentration and long-term stability test

[0075] The polyvinylidene fluoride membrane prepared by the present invention can also be applied to the concentration of food and pharmaceutical additives. Figure 7 The separation performance and long-term operation stability of the 16% PVDF-SiO2-E5 and 18% PVDF-SiO2-E5 membranes during the erythritol concentration process are shown in the following. As can be seen from Figure 7 (a), during the 48-hour test, the flux of the 16% PVDF-SiO2-E5 membrane decreases slightly, and the rejection rate drops to 99.5%; while the flux of the 18% PVDF-SiO2-E5 membrane decreases slightly during the 48-hour test, but the rejection rate still remains above 99.9%. When steam passes through the MD membrane, the concentration of erythritol in the feed solution increases. According to Raoult's law, the vapor pressure drops, resulting in a decrease in the permeation flux. After replenishing the feed solution to the initial volume, the fluxes of both membranes increase significantly.

[0076] Figure 7(b) shows the change of erythritol concentration in the feed tank during the 48-hour test. As the MD process proceeds, the erythritol concentration in the feed liquid with an initial volume of 5.5 L gradually increases. When using the 16% PVDF-SiO2-E5 membrane, the erythritol concentration increases by 2.18 times and 1.84 times in the first 24 hours and the last 24 hours, respectively. While using the 18% PVDF-SiO2-E5 membrane, these two values are 1.88 times and 1.81 times, respectively. The higher concentration efficiency of the 16% PVDF-SiO2-E5 membrane is mainly attributed to its higher permeate flux. In addition, for the entire test process, the concentration coefficient in the first 24 hours is significantly greater than that in the last 24 hours because the average flux during the last 24-hour test process is lower, resulting in a lower concentration coefficient.

[0077] In summary, the present invention uses SiO2 nanoparticles as a mechanical strength enhancer and a structure regulator, and combines the SANIPS method to prepare a highly hydrophobic PVDF membrane, constructing a multi-level micro-nano structure composed of interconnected spherulites on the membrane surface, which effectively improves the hydrophobicity, pore size, porosity and mechanical properties of the membrane. Therefore, the membrane exhibits excellent physicochemical properties and stable separation performance during the membrane distillation process, and has potential application value in fields such as food production and drug concentration.

[0078] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form. Any technical solutions obtained by using equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for preparing a silicon dioxide reinforced highly hydrophobic polyvinylidene fluoride membrane, characterized in that: The steps include: S1, dispersing SiO2 uniformly in DMF solvent to form a homogeneous dispersion, then adding PVDF powder thereto, heating and stirring to obtain a clear and transparent casting solution; S2, apply the casting liquid on a clean glass plate with a casting gap of 250 μm; S3, using a spray gun to spray ethanol on the surface of the newly scraped primary liquid film, and immersing the film sprayed several times in a water coagulation bath; S4. Transfer the membrane to fresh deionized water and soak it for more than 12 hours to ensure complete membrane phase transformation; S5. Freeze the prepared membrane sample in a refrigerator, then dry it using a freeze dryer, and store the dried sample in a sealed container for subsequent use.

2. The method for preparing a silicon dioxide reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 1, characterized in that: In the casting solution, the mass percentage of SiO2 is 1%, and the mass percentage of PVDF powder is 14% to 18%.

3. The method for preparing a silicon dioxide reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 1, characterized in that: The number of spraying times is 1 to 5 times.

4. The method for preparing a silicon dioxide reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 1, characterized in that: The spraying frequency is 2 times or more.

5. The method for preparing a silicon dioxide reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 1, characterized in that: Ethanol spraying was performed using a micro air pump at 25° C. and a relative humidity of about 60%. The working pressure of the air pump was 2.0 bar, and the distance between the nozzle and the surface of the primary liquid film was 15 cm.

6. The silica-reinforced highly hydrophobic polyvinylidene fluoride membrane prepared by the preparation method according to claim 4, characterized in that: Its surface is a porous structure composed of interconnected spherulites, and the cross section of the membrane presents an interconnected spherulite structure.

7. The silica-reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 6, characterized in that: The water contact angle of the membrane reaches 145°.

8. The silica-reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 6, characterized in that: Membrane flux up to 33.4kg·m -2 ·h -1 .

9. The silica-reinforced highly hydrophobic polyvinylidene fluoride membrane according to claim 6, characterized in that: The tensile stress and elongation at break of the film are 374 kPa and 28.2%, respectively.

10. Use of the silica-reinforced highly hydrophobic polyvinylidene fluoride membrane as claimed in claim 6 in desalination of salt water and concentration of food and pharmaceutical additives.

Images