A PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, its preparation method and application

Abstract

This invention discloses a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, its preparation method, and its applications, belonging to the field of solid material surface modification technology. By controlling the curing time of the PVDF coating, TiO2 nanoparticles are sprayed onto the coating in a semi-dry state. Utilizing the viscosity differences of PVDF at different curing degrees, the TiO2 particles are partially embedded in the PVDF matrix and partially exposed on the surface, forming a unique semi-embedded and semi-exposed structure. By precisely adjusting the curing time and optimizing the embedding depth of TiO2, a coating membrane with superhydrophilic / superoleophilic properties in air, superoleophobic properties underwater, and superhydrophobic properties underwater is obtained. This membrane, through pre-wetting to induce surface state transition, can achieve bidirectional selective separation of oil-in-water and water-in-oil emulsions, as well as the separation of immiscible oil-water mixtures, and exhibits excellent recyclability, corrosion resistance, and mechanical stability.

Description

Technical Field

[0001] This invention relates to the fields of extraction of effective components from traditional Chinese medicinal materials, preparation of cosmetics, and membrane separation technology. Specifically, it relates to a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, its preparation method, and its application. Background Technology

[0002] With the modernization of traditional Chinese medicine and the rapid development of the cosmetics industry, the demand for high-value utilization of Chinese medicinal herb extracts and precise separation of active ingredients in cosmetics is increasing. The extraction of effective components from Chinese medicinal herbs often involves various extraction methods such as water extraction, alcohol extraction, and oil extraction. The extracts often contain complex components such as oils, proteins, polysaccharides, and saponins, easily forming stable emulsion systems, posing a significant challenge to subsequent separation and purification. Simultaneously, in the preparation of cosmetics, the stability treatment and targeted separation of active ingredients in emulsion-type products such as creams, lotions, and serums place higher demands on efficient and precise separation technologies.

[0003] Traditional extraction methods, such as centrifugation, precipitation, and filtration, often suffer from drawbacks such as low separation efficiency, high energy consumption, and significant loss of active ingredients. Membrane separation technology, due to its advantages of simple operation, high separation efficiency, low energy consumption, and ability to operate at room temperature, has broad application prospects in the clarification and concentration of traditional Chinese medicine extracts and the processing of cosmetic emulsions. However, conventional polymer membrane materials, such as polyvinylidene fluoride (PVDF), have strong hydrophobicity. When processing extracts of traditional Chinese medicine containing oils or cosmetic emulsions, the membrane surface is easily contaminated by oily components, leading to a sharp drop in flux and reduced separation efficiency, thus limiting its application in the fields of traditional Chinese medicine extraction and cosmetics.

[0004] To address this deficiency, researchers have attempted to prepare organic-inorganic hybrid membranes through hydrophilic modification or the introduction of functional nanomaterials to enhance the membrane's antifouling ability and separation performance. In recent years, nano-titanium dioxide (TiO2) has been widely used in PVDF membrane modification research due to its excellent photocatalytic activity, antibacterial properties, and high hydrophilicity, showing good results in improving the membrane material's hydrophilicity and antifouling ability. However, existing modification methods mostly employ blending phase transformation or spraying techniques, making it easy for TiO2 nanoparticles to be embedded or agglomerated by polymers, hindering the formation of effective micro-nano composite structures on the membrane surface and limiting its application in complex traditional Chinese medicine extraction systems and cosmetic emulsion separation.

[0005] Furthermore, given the simultaneous presence of both oil-in-water and water-in-oil emulsions during the extraction of traditional Chinese medicinal materials, as well as the need for separation of oil-in-water and water-in-oil emulsions in cosmetic production, developing a membrane material capable of bidirectional selective separation to effectively separate oil-in-water and water-in-oil emulsion systems remains a pressing technical challenge in this field. Summary of the Invention

[0006] In view of this, the present invention provides a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, its preparation method and application. By controlling the curing time of PVDF, a structure in which titanium dioxide is partially exposed in PVDF is designed, successfully realizing the bidirectional selective separation of water-in-oil and oil-in-water emulsions as well as the separation of immiscible oil-water mixtures.

[0007] To solve the above-mentioned technical problems, this application adopts the following technical solution:

[0008] The primary objective of this application is to provide a method for preparing a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, comprising the following steps: 1) Dissolve 1,000,000 molecular weight polyvinylidene fluoride in a mixed solution of N,N-dimethylformamide and acetone, and stir magnetically until completely dissolved to obtain mixed solution A; Polyvinylidene fluoride (PVDF): mainly plays the following roles: (1) Adhesive role: After PVDF is dissolved in DMF / acetone mixed solvent, it is coated on the surface of stainless steel mesh. As the solvent evaporates, the PVDF macromolecular chains wrap around and attach to the metal wires of the stainless steel mesh, forming a uniform and firm polymer base film, which provides a basis for the subsequent loading of TiO2 nanoparticles; (2) Capture TiO2 particles role: When the TiO2 ethanol solution is sprayed in the second step, the surface of the PVDF layer in the "semi-dry" state is still sticky, which can effectively capture and fix the TiO2 nanoparticles that collide with the surface, preventing them from falling off; (3) Hydrophilic skeleton role: The strong electronegative fluorine atoms on the PVDF molecular chain give it a certain hydrophilic basis, which works with TiO2 to construct the overall hydrophilic properties of the coating.

[0009] N,N-Dimethylformamide (DMF) and acetone: As a mixed solvent for PVDF, their functions include: (1) dissolving PVDF: DMF is a good solvent for PVDF and can fully dissolve it to form a uniform polymer solution; (2) regulating the evaporation rate: acetone has a low boiling point (56℃) and evaporates quickly, while DMF has a high boiling point (153℃) and evaporates slowly. When the two are mixed in a volume ratio of 4:1, the drying process of the coating can be precisely controlled. The rapid evaporation of acetone causes the coating to quickly change from a liquid state to a gel state, preventing uneven coating due to gravity flow; the slow evaporation of DMF can keep the PVDF coating in a flowable or highly elastic state for a long time, ensuring that the PVDF layer still has enough viscosity to capture particles when spraying TiO2.

[0010] 2) Disperse TiO2 nanoparticles in an ethanol solution, sonicate, and stir until homogeneous to obtain mixed solution B; Titanium dioxide (TiO2) nanoparticles play a core role in this invention: (1) Constructing a micro-nano composite rough structure: After TiO2 is sprayed onto the PVDF surface in the form of nanoparticles, a rough layer composed of nanoscale protrusions and voids is formed, which greatly increases the actual surface area of ​​the material and is the key to amplifying the intrinsic wettability of the material; (2) Providing high surface energy: TiO2 is an inorganic material with high surface energy and excellent hydrophilicity and oleophilicity, which helps the coating to exhibit super affinity for both water and oil in the air; (3) Formation of hydration layer: The surface of TiO2 can form hydrogen bonds and Ti-O bonds with water molecules to build a stable hydration layer, which effectively blocks the adhesion of oil droplets; (4) Promoting oil droplet aggregation: TiO2 nanoparticles have the ability to promote the aggregation of small oil droplets into large oil droplets, reducing the adsorption and spreading of oil droplets on the membrane surface; (5) Antibacterial and photoactive: TiO2 has photocatalytic activity and antibacterial properties, which provide additional advantages for the membrane to resist organic pollution and maintain separation efficiency during long-term use.

[0011] Ethanol: As a dispersion medium for TiO2, its functions include: (1) Dispersing TiO2: Ethanol can effectively disperse TiO2 nanoparticles, prevent agglomeration, form a stable suspension, and ensure uniform adhesion of particles during spraying; (2) Non-solvent properties: Ethanol is a non-solvent for PVDF, and will not redissolve the pre-formed PVDF underlayer during spraying, thus protecting the integrity of the underlayer structure.

[0012] 3) The mixed solution A obtained in step 1) is uniformly coated onto the pretreated stainless steel mesh, and hot air is used to treat it to achieve a semi-dry state, so as to obtain a polyvinylidene fluoride modified stainless steel mesh. 4) Spray the polyvinylidene fluoride-modified stainless steel mesh obtained in step 3) with the mixed solution B obtained in step 2), and after drying at room temperature, the PVDF-TiO2 micro-nano bidirectional selective separation coating mesh is obtained.

[0013] The components of this invention are not simply mixed, but rather multiple synergistic effects are achieved through a specific two-step process: 1. Synergistic "anchoring-anchored" relationship between PVDF and TiO2: The semi-dry PVDF layer acts as "glue," firmly "grabbing" the sprayed TiO2 nanoparticles through physical adsorption and intermolecular forces, fixing them to the surface. This effect prevents TiO2 from falling off during use and ensures that the TiO2 nanoparticles are exposed on the outermost surface of the coating rather than being embedded inside the PVDF, which is key to achieving high roughness and high surface energy.

[0014] 2. Synergistic “stack-pore formation” among TiO2 nanoparticles: A large number of TiO2 nanoparticles are stacked on the PVDF surface, and countless nanoscale pores and protrusions are naturally formed between the particles, constructing a micro-nano composite structure that cannot be achieved by a single PVDF or TiO2 film, providing the necessary physical basis for the subsequent realization of underwater superoleophobicity and oil superhydrophobicity.

[0015] 3. Synergy between PVDF-TiO2 composite coating and wettability: (1) Super-affinity in air: Due to the high surface energy of TiO2 and the hydrophilic nature of PVDF, coupled with the rough structure brought by TiO2 nanoparticles, the coating exhibits super-affinity properties to both water and oil (contact angle ≈ 0°), and liquids can instantly spread on the surface and enter the mesh; (2) State transformation after pre-wetting: If water is used for pre-wetting first, water molecules preferentially occupy and lock in the micro-nano rough structure composed of hydrophilic PVDF and TiO2, forming a strong hydration layer, repelling low surface energy oil droplets, and achieving "underwater super-oleophobicity"; if oil is used for pre-wetting first, oil molecules quickly spread across the high surface energy rough structure to form an oil film, generating a strong repulsive force on water droplets, and achieving "oil-based superhydrophobicity".

[0016] As a preferred technical solution, in step 1), the volume ratio of N,N-dimethylformamide and acetone in the mixed solution A is 4:1; the mass percentage of polyvinylidene fluoride in the mixed solution A is 8-16%; and in step 2), the mass percentage of TiO2 in the mixed solution B is 5-12%.

[0017] As a preferred technical solution, in step 1), the mass percentage of polyvinylidene fluoride in the mixed solution A is 10-15%; in step 2), the mass percentage of TiO2 in the mixed solution B is 6-8%.

[0018] As a preferred technical solution, the wind speed of the hot air treatment in step 3) is 15-20 m / s; the hot air treatment time is 10-20 min; the hot air treatment temperature is 45-60℃; and the distance between the hot air treatment device and the stainless steel mesh surface is 1-40 cm.

[0019] As a preferred technical solution, the hot air treatment time in step 3) is 13-16 minutes, and the hot air treatment temperature is 50°C.

[0020] As a preferred technical solution, in step 4), the distance between the spray nozzle and the mesh is 30-40 cm, the spraying power is 5-15W, the number of sprays is 1-4, the time interval between each spray is 0.5-5 min, and the spraying time is 1-10 s.

[0021] As a preferred technical solution, the pretreatment in step 3) includes: soaking the stainless steel mesh in dilute nitric acid for 5-20 minutes, rinsing it with distilled water after removal, and then ultrasonically cleaning it in acetone, ethanol, and distilled water for 5-15 minutes in sequence, and then air-drying it at room temperature.

[0022] One object of this application is to provide: a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane prepared by the method.

[0023] As a preferred technical solution, the mesh has a contact angle of less than 5° with water or oil in air, exhibiting superhydrophilic and superoleophilic properties in air; a contact angle of more than 150° with oil underwater, exhibiting superoleophobic properties underwater; and a contact angle of more than 150° with water underwater, exhibiting superhydrophobic properties underwater.

[0024] The core mechanism by which the membrane of this invention achieves bidirectional selective separation can be summarized as "pre-wetting induced surface state transition": When the membrane is pre-wetted with water, water molecules rapidly penetrate and fill the micro-nano rough structure of the PVDF-TiO2 coating, forming a stable hydration layer on the membrane surface. This hydration layer has high surface energy, allowing the continuous phase water to pass through smoothly when the oil-in-water emulsion contacts the membrane surface, while the dispersed phase oil droplets are blocked from contacting the membrane surface by the hydration layer, thus achieving demulsification and separation of the oil-in-water emulsion.

[0025] When the membrane is pre-wetted with oil, oil molecules quickly spread across the entire rough structure with high surface energy, forming an oil film. This oil film exerts a strong repulsive force on water droplets. When the water-in-oil emulsion comes into contact with the membrane surface, the continuous phase oil can pass through smoothly, while the dispersed phase water droplets are blocked by the oil film and cannot pass through, thus achieving demulsification and separation of the water-in-oil emulsion.

[0026] This ability to flexibly switch between two extreme wetting states through simple pre-wetting gives the membrane the function of "bidirectional selective separation," enabling it to process both water-in-oil and oil-in-water emulsions.

[0027] One object of this application is to provide: the application of the PVDF-TiO2 micro / nano bidirectional selective separation coating membrane prepared by the method, or the PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, in the separation of water-in-oil emulsions, oil-in-water emulsions, and immiscible oil-water mixtures.

[0028] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: Bidirectional selective separation capability: The PVDF-TiO2 micro-nano coated membrane prepared in this invention has special wettability of superhydrophilic / superoleophilic in air, superoleophobic underwater and superhydrophobic in oil. It can achieve bidirectional selective separation of oil-in-water emulsion and water-in-oil emulsion by simple pre-wetting, overcoming the limitation of existing separation membranes that can only separate in one direction.

[0029] Excellent separation performance: The membrane of this invention achieves a separation efficiency of over 99% for both oil-in-water emulsions and water-in-oil emulsions, and a separation flux of up to 2.8 × 10⁻⁶ for immiscible oil-water mixtures. 5 L·m -2 ·h -1 In summary, it combines the advantages of high throughput and high efficiency.

[0030] Unique micro-nano composite structure: Through a two-step process of "coating first and then spraying", a micro-nano composite structure enriched with TiO2 nanoparticles on the surface was successfully constructed, giving full play to the multiple functions of TiO2 in building roughness, forming a hydration layer and promoting oil droplet aggregation.

[0031] Good stability: The membrane of this invention has excellent recyclability, and the separation efficiency is still above 99.3% after 10 separation cycles; it has excellent mechanical stability and retains superwetting properties after ultrasonic treatment; it has excellent resistance to acids, alkalis and salts, and its performance did not decrease significantly after soaking in a solution with pH 1-14 for 72 hours.

[0032] The preparation process is simple, low-cost, and environmentally friendly: This invention is simple and easy to operate, requiring no complex equipment; the raw materials are widely available and inexpensive; and the preparation process does not use toxic or harmful substances, meeting the requirements of green environmental protection. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0034] Figure 1 This is a scanning electron microscope image of the surface morphology of the bidirectional selective separation membrane prepared in Example 1.

[0035] Figure 2 Image showing the contact angle of the bidirectional separation membrane prepared in Example 1 with water in air.

[0036] Figure 3 The contact angle image of the bidirectional separation membrane prepared in Example 1 with n-hexane in air.

[0037] Figure 4 Image showing the water contact angle of the bidirectional separation membrane prepared in Example 1 under oil (n-hexane).

[0038] Figure 5 The image shows the contact angle of the bidirectional separation membrane prepared in Example 1 with oil (n-hexane) underwater.

[0039] Figure 6 The images show the contact angles of the bidirectional separation membrane in Example 1 under water with oil (silicone oil) and under water with oil (silicone oil).

[0040] Figure 7 The images show the contact angles of the bidirectional separation membrane in Example 1 under water with oil (xylene) and under water with oil (xylene).

[0041] Figure 8 The images show the contact angles of the bidirectional separation membrane in Example 1 under water with oil (petroleum ether) and under water with oil (petroleum ether).

[0042] Figure 9 Optical microscope images of droplets before (left) and after (right) separation of an oil-in-water emulsion using the bidirectional separation membrane prepared in Example 1.

[0043] Figure 10 Optical microscope images of droplets before (left) and after (right) separation of an oil-in-water emulsion using the bidirectional separation membrane prepared in Example 2.

[0044] Figure 11 Optical microscope images of droplets before (left) and after (right) separation of an oil-in-water emulsion using the bidirectional separation membrane prepared in Example 3.

[0045] Figure 12 Optical microscope images of droplets before (left) and after (right) separation of water-in-oil emulsion using the bidirectional separation membrane prepared in Example 1.

[0046] Figure 13 Optical microscope images of droplets before (left) and after (right) separation of water-in-oil emulsion using the bidirectional separation membrane prepared in Example 2.

[0047] Figure 14 Optical microscope images of droplets before (left) and after (right) separation of water-in-oil emulsion using the bidirectional separation membrane prepared in Example 3.

[0048] Figure 15 Images showing the contact angles (a) of the separation membrane prepared for Comparative Example 1 with water in air, (b) with oil in air, (c) with oil underwater, and (d) with water underwater.

[0049] Figure 16Images showing the contact angles (a) of the separation membrane prepared for Comparative Example 2 with water in air, (b) with oil in air, (c) with oil underwater, and (d) with water underwater. Detailed Implementation

[0050] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0051] Example 1 Preparation of PVDF-TiO2 micro / nano bidirectional selective separation coating membrane (1) Soak the stainless steel mesh in dilute nitric acid for 15 minutes, take it out and rinse it with distilled water. Then, ultrasonically clean it in acetone, ethanol and distilled water for 10 minutes in sequence to remove the oxide layer and residual grease on the surface of the stainless steel mesh. Then, let it dry at room temperature for later use.

[0052] (2) At room temperature, 16 ml of DMF and 4 ml of acetone were added to a 50 ml beaker. After stirring magnetically for 5 min, PVDF was added while stirring magnetically at 55 °C, for a total of 2.2 g. The mixture was then stirred for 12 h to ensure homogeneity. In this mixed solution, the volume ratio of DMF to acetone was 4:1, and the mass percentage of PVDF with a molecular weight of 1,000,000 was 12%.

[0053] (3) The mixed solution obtained in step (2) is evenly coated onto the stainless steel mesh that was dried in step (1). The coated stainless steel mesh is then suspended in the air and the solution is brought to a semi-dry state by using a hot air blower. The hot air blower is placed vertically above the mesh, 30 cm away from the stainless steel mesh, with the wind speed adjusted to 15 m / s, the temperature set to 50 °C, and the drying time set to 15 min.

[0054] (4) At room temperature, 100 ml of ethanol and 5.04 g of TiO2 were added to a 200 ml beaker, sonicated for 10 min and then stirred for 5 min to obtain a milky white TiO2-ethanol solution. The mass percentage of TiO2 in the solution was 6%.

[0055] (5) The solution obtained in step (4) is filled into a nano-atomizing spray gun and sprayed onto the semi-dry coating in step (3). The nozzle of the spray gun is 35 cm away from the membrane, the power of the spray gun is 10 W, and the coating is applied three times with a 1 min interval between each application. The application time for each application is set to 5 s. After the coating is completed, the membrane is dried at room temperature to successfully prepare the bidirectional selective separation membrane.

[0056] The surface morphology scanning electron microscope image of the bidirectional selective separation membrane obtained in this example is shown below. Figure 1 As shown. From Figure 1 As can be seen, TiO2 nanoparticles are uniformly covered on the coating surface, forming a continuous nanoparticle layer with abundant nanopores between the particles, thus successfully constructing a micro-nano composite rough structure.

[0057] Example 2 Preparation of PVDF-TiO2 micro / nano bidirectional selective separation coating membrane (1) Soak the stainless steel mesh in dilute nitric acid for 15 minutes, take it out and rinse it with distilled water. Then, ultrasonically clean it in acetone, ethanol and distilled water for 10 minutes in sequence to remove the oxide layer and residual grease on the surface of the stainless steel mesh. Then, let it dry at room temperature for later use.

[0058] (2) At room temperature, 16 ml of DMF and 4 ml of acetone were added to a 50 ml beaker. After stirring magnetically for 5 min, PVDF was added while stirring magnetically at 55 °C, for a total of 3.4 g. The mixture was then stirred for 12 h to ensure homogeneity. In this mixed solution, the volume ratio of DMF to acetone was 4:1, and the mass percentage of PVDF with a molecular weight of 1,000,000 was 16%.

[0059] (3) The mixed solution obtained in step (2) is evenly coated onto the stainless steel mesh that was dried in step (1). The coated stainless steel mesh is then suspended in the air and the solution is brought to a semi-dry state by using a hot air blower. The hot air blower is placed vertically above the mesh, 30 cm away from the stainless steel mesh, with the wind speed adjusted to 15 m / s, the temperature set to 50 °C, and the drying time set to 15 min.

[0060] (4) At room temperature, 100 ml of ethanol and 6.86 g of TiO2 were added to a 200 ml beaker, sonicated for 10 min and then stirred for 5 min to obtain a milky white TiO2-ethanol solution. The mass percentage of TiO2 in the solution was 8%.

[0061] (5) The solution obtained in step (4) is filled into a nano-atomizing spray gun and sprayed onto the semi-dry coating in step (3). The nozzle of the spray gun is 30 cm away from the membrane, the power of the spray gun is 10 W, and the coating is applied three times with a 1 min interval between each application. The application time for each application is set to 5 s. After the coating is completed, the membrane is dried at room temperature to successfully prepare the bidirectional selective separation membrane.

[0062] Example 3 Preparation of PVDF-TiO2 micro / nano bidirectional selective separation coating membrane (1) Soak the stainless steel mesh in dilute nitric acid for 15 minutes, take it out and rinse it with distilled water. Then, ultrasonically clean it in acetone, ethanol and distilled water for 10 minutes in sequence to remove the oxide layer and residual grease on the surface of the stainless steel mesh. Then, let it dry at room temperature for later use.

[0063] (2) At room temperature, 16 ml of DMF and 4 ml of acetone were added to a 50 ml beaker. After stirring magnetically for 5 min, PVDF was added while stirring magnetically at 55 °C, for a total of 1.6 g. The mixture was then stirred for 12 h to ensure homogeneity. In this mixed solution, the volume ratio of DMF to acetone was 4:1, and the mass percentage of PVDF with a molecular weight of 1,000,000 was 8%.

[0064] (3) The mixed solution obtained in step (2) is evenly coated onto the stainless steel mesh that was dried in step (1). The coated stainless steel mesh is then suspended in the air and the solution is dried by using a hot air blower. The hot air blower is placed vertically above the mesh, 30 cm away from the stainless steel mesh. The wind speed is adjusted to 18 m / s, the temperature is 60℃, and the drying time is 15 min.

[0065] (4) At room temperature, 100 ml of ethanol and 10.76 g of TiO2 were added to a 200 ml beaker, sonicated for 10 min and then stirred for 5 min to obtain a milky white TiO2-ethanol solution. The mass percentage of TiO2 in the solution was 12%.

[0066] (5) The solution obtained in step (4) is filled into a nano-atomizing spray gun and sprayed onto the semi-dry coating in step (3). The nozzle of the spray gun is 40 cm away from the membrane, the power of the spray gun is 10 W, and the coating is applied three times with a 1 min interval between each application. The application time for each application is set to 5 s. After the coating is completed, the membrane is dried at room temperature to successfully prepare the bidirectional selective separation membrane.

[0067] Example 4 Performance testing of PVDF-TiO2 micro / nano bidirectional selective separation coating membrane (1) Wetting performance test The wetting properties of the bidirectional selective separation membranes prepared in Examples 1, 2 and 3 were tested using a contact angle measuring instrument.

[0068] Figure 2 This image shows the contact angle between the bidirectional separation membrane of Example 1 and water in air. The results show that the contact angle between the bidirectional separation membrane and water in air is 0°, reaching a superhydrophilic state. This indicates that water molecules can rapidly spread and permeate the membrane surface.

[0069] Figure 3 This image shows the contact angle of the bidirectional separation membrane of Example 1 with n-hexane in air. The results show a contact angle of 0°, exhibiting a superoleophilic state in air. This indicates that oil molecules can also spread rapidly on the membrane surface.

[0070] Figure 4 This image shows the water contact angle of the bidirectional separation membrane in Example 1 under oil (n-hexane). The test method involved placing the membrane in an oil-phase environment and adding water droplets. The results showed that the water contact angle under oil was 152°, exhibiting superhydrophobicity in oil. This indicates that after pre-wetting with oil, the membrane surface strongly repels water.

[0071] Figure 5 This image shows the underwater oil (n-hexane) contact angle of the bidirectional separation membrane in Example 1. The test method involved placing the membrane in an aqueous environment and dropping oil droplets. The results showed an underwater oil contact angle of 151°, exhibiting underwater superoleophobicity. This indicates that after pre-wetting with water, the membrane surface strongly repels oil.

[0072] Figure 6 Images show the contact angles of the bidirectional separation membrane of Example 1 in underwater oil (silicone oil) and in oil (silicone oil) in water. The results show that the underwater oil contact angle is 155.298°, exhibiting underwater superoleophobicity in water. The oil-to-water contact angle is 154.204°, exhibiting oil-to-oil superhydrophobicity in oil.

[0073] Figure 7 Images show the contact angles of the bidirectional separation membrane of Example 1 in underwater oil (xylene) and in oil (xylene) in water. The results show that the underwater oil contact angle is 149.841°, exhibiting underwater superoleophobicity in water. The oil-to-water contact angle is 150.870°, exhibiting oil-to-oil superhydrophobicity in oil.

[0074] Figure 8 Images show the contact angles of the bidirectional separation membrane from Example 1 in underwater oil (petroleum ether) and in oil (petroleum ether) in water. The results show that the underwater oil contact angle is 145.804°, exhibiting underwater superoleophobicity in water. The oil-to-water contact angle is 151.769°, exhibiting oil-to-oil superhydrophobicity in oil.

[0075] The above results indicate that the membrane prepared by this invention has the ability to induce surface state transitions through pre-wetting, laying the foundation for achieving bidirectional selective separation.

[0076] (2) Oil-water separation performance test a) Water-in-oil emulsion separation performance test Mix 100 ml of water and 5 ml of n-hexane, add 0.2 g of Tween 80 as an emulsifier, and stir at room temperature for 10 h to obtain a stable white oil-in-water emulsion.

[0077] After fixing the membranes prepared in Examples 1, 2, and 3 onto the vacuum filtration device, the membranes were pre-wetted with a small amount of water. Then, the oil-in-water emulsions were continuously poured onto the separation membrane of the vacuum filtration device for filtration. As the oil-in-water emulsions passed through the separation membrane, they were demulsified. The aqueous phase permeated through the separation membrane and was collected in a collection bottle, while the oil phase was blocked above the separation membrane, thus achieving the separation of water and oil in the oil-in-water emulsion.

[0078] Test results show that the separation membrane in Example 1 achieved a separation efficiency of 99.7% for oil-in-water (n-hexane) emulsions, with a separation flux of 462 L·m⁻¹. -2 ·h -1 Example 2: The separation membrane achieved a separation efficiency of 99.6% for oil-in-water (n-hexane) emulsions; the separation flux was 470 L·m⁻¹. -2 ·h -1 Example 3: The separation membrane achieved a separation efficiency of 99.5% for oil-in-water (n-hexane) emulsions; the separation flux was 468 L·m⁻¹. -2 ·h -1 The filtrate after separation was collected, and the changes before and after separation were observed under an optical microscope to evaluate the separation performance of the bidirectional selective separation membrane. Figure 9 , Figure 10 and Figure 11 As shown, before separation, dense oil-in-water droplets could be clearly observed under an optical microscope in the oil-in-water emulsion, while the filtrate after separation was clear, and no oil droplets were detected under an optical microscope. This indicates that the emulsion was completely demulsified and the oil phase was effectively retained.

[0079] b) Water-in-oil emulsion separation performance test Mix 100 ml of silicone oil and 2 ml of water, add 0.2 g of Span 80 as an emulsifier, and stir at room temperature for 6 hours to obtain a stable water-in-oil emulsion.

[0080] After fixing the membranes prepared in Examples 1, 2, and 3 onto the vacuum filtration device, the membranes were pre-wetted with a small amount of silicone oil. Then, the water-in-oil emulsions were continuously poured onto the separation membrane of the vacuum filtration device for filtration. As the water-in-oil emulsions passed through the separation membrane, the emulsions were demulsified. The oil phase permeated through the separation membrane and was collected in a collection bottle, while the aqueous phase was blocked above the separation membrane, thus achieving the separation of oil and water in the water-in-oil emulsions.

[0081] Test results show that the separation membrane prepared in Example 1 achieves a separation efficiency of 99.6% for oil (silicone oil) water-in-oil emulsions, with a separation flux of 448 L·m⁻¹. -2 ·h -1 The separation membrane prepared in Example 2 achieved a separation efficiency of 99.8% for oil (silicone oil) water-in-oil emulsions, with a separation flux of 446 L·m⁻¹. -2 ·h -1 The separation membrane prepared in Example 3 achieved a separation efficiency of 99.56% for oil (silicone oil) water-in-oil emulsions, with a separation flux of 443 L·m⁻¹. -2 ·h -1 The filtrate after separation was collected, and the changes before and after separation were observed under an optical microscope to evaluate the separation performance of the bidirectional selective separation membrane. Figure 12 , Figure 13 , Figure 14 As shown, before separation, dense water-in-oil droplets could be clearly observed under an optical microscope in the water-in-oil emulsion, while the filtrate after separation was clear, and no water droplets were detected under an optical microscope. This indicates that the emulsion was completely demulsified and the aqueous phase was effectively retained.

[0082] Example 5 Performance test of separation of immiscible oil-water mixture Mix 100 ml of water and 100 ml of n-hexane in a 250 ml beaker to obtain an immiscible oil-water mixture.

[0083] The bidirectional selective separation membranes prepared in Examples 1, 2, and 3 were fixed between two glass tubes of a vacuum filtration device and clamped with fixtures, resulting in an effective separation area of ​​1.77 cm². An oil-water mixture was poured into the vacuum filtration device, and oil and water were separated by gravity. Test results showed that the separation membrane prepared in Example 1 achieved a separation efficiency of 99.6% for the hexane / water mixture, with a separation flux as high as 293,778 L·m³. -2 ·h -1 The separation membrane prepared in Example 2 achieved a separation efficiency of 99.7% for the hexane / water mixture, with a separation flux as high as 293,780 L·m⁻¹. -2 ·h -1 The separation membrane prepared in Example 3 achieved a separation efficiency of 99.9% for the hexane / water mixture, with a separation flux as high as 293,783 L·m⁻¹. -2 ·h-1 .

[0084] The above separation performance test results show that the membrane prepared by this invention can achieve bidirectional selective separation of oil-in-water emulsions and water-in-oil emulsions on the same membrane material through pre-wetting treatment, with a separation efficiency of over 99% and a separation flux of up to 2.8 × 10⁻⁶ for immiscible mixtures. 5 L·m -2 ·h -1 The above demonstrates excellent overall separation performance.

[0085] (3) Cyclic performance test After completing the emulsion separation test, the separation membranes of Examples 1, 2, and 3 were rinsed with ethanol and distilled water sequentially, and then dried with nitrogen gas for use in the next separation cycle. After 10 separation cycles, the separation efficiency of the separation membranes remained at 99.2% or higher, demonstrating good reusability. This performance is attributed to the structural design in which TiO2 nanoparticles are firmly captured by a semi-dry PVDF layer, as well as the photocatalytic self-cleaning properties of TiO2.

[0086] Table 1 Performance test results of different groups after repeated use

[0087] (4) Mechanical performance testing The separation membranes of Examples 1, 2, and 3 were ultrasonicated for 15 min, and the changes in their wettability and separation performance were tested. The results showed that after ultrasonic treatment, the contact angle of the separation membrane of Example 1 under oil was 151°, and the contact angle under water was 150°, with a separation efficiency of 99.6%. The contact angle of the separation membrane of Example 2 under oil was 150°, and the contact angle under water was 150°, with a separation efficiency of 99.3%. The contact angle of the separation membrane of Example 3 under oil was 151°, and the contact angle under water was 150°, with a separation efficiency of 99.1%. This indicates that the separation membranes can still maintain superhydrophobicity under oil and superoleophobicity under water after intense ultrasonic treatment. The separation efficiency of the ultrasonically treated separation membranes for water-in-oil and oil-in-water emulsions still reached over 99%. This indicates that the TiO2 nanoparticles are firmly anchored by the PVDF layer and are not easily detached, and the membrane structure has good mechanical stability.

[0088] (5) Corrosion resistance test Hydrochloric acid solutions with pH values ​​of 1 and 4, NaOH solutions with pH values ​​of 10 and 14, and a saturated sodium chloride solution with pH value of 7 were prepared. The bidirectional separation membrane was then immersed in these solutions for 72 hours. After immersion, the contact angles of the separation membrane in Example 1 (oil-to-water) and underwater (water-to-oil) were measured to be 152° and 151° respectively, with separation efficiencies of over 99.5% for both oil-in-water and water-in-oil emulsions. In Example 2, the contact angles of the separation membrane (oil-to-water and underwater) were both 150° and 150° respectively, with separation efficiencies of over 99.3% for both oil-in-water and water-in-oil emulsions. In Example 3, the contact angles of the separation membrane (oil-to-water and underwater) were both 151° and 150° respectively, with separation efficiencies of over 99.1% for both oil-in-water and water-in-oil emulsions. This demonstrates that the bidirectional selective separation membrane exhibits excellent resistance to acids, alkalis, and salts, and can be used long-term under harsh environments.

[0089] To verify the selection of PVDF molecular weight in this application and the effect of hot air drying time on the overall technical effect after PVDF coating on stainless steel mesh, Comparative Example 1 and Comparative Example 2 were set up to verify the technical effect of this application: Comparative Example 1 Preparation of PVDF-TiO2 micro / nano bidirectional selective separation coating membrane The remaining steps are the same as in Example 1, except that: after coating the stainless steel mesh with PVDF, it is semi-dried by hot air blowing for 5 minutes, and the PVDF solution is in a liquid state. Then, titanium dioxide solution is sprayed on and dried at room temperature.

[0090] Results analysis: The obtained coating structure shows that titanium dioxide is fully encapsulated by PVDF, and the titanium dioxide is not partially exposed on the PVDF surface, exhibiting the wettability of PVDF, namely superoleophilic and hydrophobic in air, oleophobic underwater, and superhydrophobic under oil. Figure 15 To achieve bidirectional emulsion separation, the surface contact angle must be superhydrophilic in air (contact angle is 0), superhydrophobic in oil, and superoleophobic underwater. Therefore, the coating membrane prepared in this comparative example cannot achieve bidirectional emulsion separation.

[0091] Comparative Example 2 Preparation of PVDF-TiO2 micro / nano bidirectional selective separation coating membrane The remaining steps are the same as in Example 1, except that: after coating a stainless steel mesh with PVDF of molecular weight 534,000, it is semi-dried by hot air blowing for 15 minutes, and the PVDF is in a film-forming state. Then, titanium dioxide solution is sprayed on and dried at room temperature.

[0092] Results Analysis: The obtained coating structure consists of a small amount of titanium dioxide adhering to the PVDF surface, indicating insufficient PVDF adhesion. The coating exhibits excellent wettability of the PVDF and titanium dioxide bond, demonstrating superoleophilic and hydrophilic properties in air, oleophobic underwater, and superhydrophobic underwater. Figure 16To achieve bidirectional emulsion separation, the surface contact angle must be superhydrophilic in air (contact angle is 0), superhydrophobic in oil, and superoleophobic underwater. Therefore, the coating membrane prepared in this comparative example cannot achieve bidirectional emulsion separation.

[0093] In summary, the bidirectional selective separation membrane of this invention successfully constructed a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane through a unique two-step "coating followed by spraying" process. PVDF acts as a binder, firmly adhering to the stainless steel mesh substrate and capturing TiO2 particles. The TiO2 nanoparticles form a micro / nano rough structure on the surface, and the two work synergistically to endow the membrane with special wettability: superhydrophilic / superoleophilic in air, superoleophobic underwater, and superhydrophobic under oil. Based on the pre-wetting-induced surface state transition mechanism, this membrane can achieve bidirectional selective separation of oil-in-water emulsions and water-in-oil emulsions, with separation efficiencies exceeding 99%, while also exhibiting excellent recyclability, mechanical stability, and corrosion resistance. This invention features a simple, low-cost, and environmentally friendly preparation process, and has broad application prospects in the separation of complex emulsion systems in the fields of biomedicine, cosmetics, and petrochemicals.

[0094] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. All equivalent variations and modifications made based on the content of the present invention are covered within the patent scope of the present invention.

[0095] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0096] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a PVDF-TiO2 micro / nano bidirectional selective separation coating membrane, characterized in that, Includes the following steps: 1) Dissolve 1,000,000 molecular weight polyvinylidene fluoride in a mixed solution of N,N-dimethylformamide and acetone, and stir magnetically until completely dissolved to obtain mixed solution A; 2) Disperse TiO2 nanoparticles in an ethanol solution, sonicate, and stir until homogeneous to obtain mixed solution B; 3) The mixed solution A obtained in step 1) is uniformly coated onto the pretreated stainless steel mesh, and hot air is used to treat it to achieve a semi-dry state, so as to obtain a polyvinylidene fluoride modified stainless steel mesh. 4) Spray the polyvinylidene fluoride-modified stainless steel mesh obtained in step 3) with the mixed solution B obtained in step 2), and after drying at room temperature, the PVDF-TiO2 micro-nano bidirectional selective separation coating mesh is obtained.

2. The preparation method according to claim 1, characterized in that, Step 1) The volume ratio of N,N-dimethylformamide and acetone in the mixed solution A is 4:1; the mass percentage of polyvinylidene fluoride in the mixed solution A is 8-16%; Step 2) The mass percentage of TiO2 in the mixed solution B is 5-12%.

3. The preparation method according to claim 2, characterized in that, Step 1) The mass percentage of polyvinylidene fluoride in the mixed solution A is 10-15%; Step 2) The mass percentage of TiO2 in the mixed solution B is 6-8%.

4. The preparation method according to claim 1, characterized in that, Step 3) The air velocity of the hot air treatment is 15-20 m / s; the hot air treatment time is 10-20 min; the hot air treatment temperature is 45-60℃; and the distance between the hot air treatment device and the stainless steel mesh surface is 1-40 cm.

5. The preparation method according to claim 1, characterized in that, Step 3) The hot air treatment time is 13-16 minutes, and the hot air treatment temperature is 50℃.

6. The preparation method according to claim 1, characterized in that, In step 4), the spray nozzle is 30-40 cm away from the mesh during spraying, the spraying power is 5-15 W, the number of sprays is 1-4, the time interval between each spray is 0.5-5 min, and the spraying time is 1-10 s.

7. The preparation method according to claim 1, characterized in that, Step 3) The pretreatment includes: soaking the stainless steel mesh in dilute nitric acid for 5-20 minutes, rinsing it with distilled water, and then ultrasonically cleaning it in acetone, ethanol, and distilled water for 5-15 minutes in sequence, and then air-drying it at room temperature.

8. The PVDF-TiO2 micro / nano bidirectional selective separation coating membrane prepared by the method according to any one of claims 1-7.

9. The PVDF-TiO2 micro / nano bidirectional selective separation coating membrane according to claim 8, characterized in that, The membrane exhibits superhydrophilic and superoleophilic properties in air, with contact angles with water or oil both below 5°; underwater, it exhibits superoleophobic properties, with contact angles with oil above 150°; and underwater, it exhibits superhydrophobic properties, with contact angles with water above 150°.

10. The application of the PVDF-TiO2 micro / nano bidirectional selective separation coating membrane prepared by the method of any one of claims 1-7 or the PVDF-TiO2 micro / nano bidirectional selective separation coating membrane of any one of claims 8-9 in the separation of water-in-oil emulsions, oil-in-water emulsions, and immiscible oil-water mixtures.

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