Preparation method and application of polyvinyl alcohol composite membrane for enhancing solvent resistance

Abstract

The invention discloses a preparation method and application of a polyvinyl alcohol composite membrane for enhancing solvent resistance, and the preparation method comprises the following steps: carrying out inert gas replacement and preheating on hexafluoropropylene, a perfluoro solvent and perfluoro-2-methylene-4-methyl-1, 3-dioxolane; adding an initiator into the monomer mixed solution, and carrying out thermal insulation polymerization; carrying out reduced pressure distillation treatment on the reaction mixed solution; adding a cosolvent into the first perfluoropolymer solution, and stirring; curing and purifying the second perfluoropolymer solution; crosslinking the polyvinyl alcohol solution, tetraethyl orthosilicate and hydrogen chloride; the preparation method comprises the following steps: dissolving perfluorodecyl triethoxy silane into absolute ethyl alcohol; and coating and drying the polyvinylidene fluoride base film or the polytetrafluoroethylene base film to obtain the polyvinyl alcohol composite film. According to the embodiment, the structural stability of the polyvinyl alcohol composite film in a strong polar solvent such as tetrahydrofuran can be improved, the interface bonding force between the perfluorinated protective layer and the polyvinyl alcohol active layer is improved, and the preparation process is simplified.

Description

Technical Field

[0001] The embodiments disclosed herein relate to the field of membrane separation technology, specifically to a method for preparing and applying polyvinyl alcohol composite membranes for enhancing solvent resistance. Background Technology

[0002] Separating tetrahydrofuran aqueous solutions using polyvinyl alcohol (PVA) dehydration membranes is a method for azeotropic dehydration with low energy consumption, leveraging the preferential adsorption and permeation properties of PVA for water molecules. Currently, the common approach involves coating a microfiltration membrane with a cross-linked PVA active layer to create a composite membrane for pervaporation dehydration. When handling highly polar solvents such as tetrahydrofuran, inorganic fillers are added to the PVA cross-linking system or a protective layer is coated onto the surface of the PVA active layer to improve its solvent resistance.

[0003] However, in practice, the following technical problems are frequently encountered when preparing solvent-resistant polyvinyl alcohol membranes for tetrahydrofuran dehydration: The polyvinyl alcohol (PVA) in the cross-linked active layer coated on the microfiltration base membrane is highly hydrophilic and easily swells in strong polar solvents such as tetrahydrofuran, causing the cross-linked network to be destroyed. This can lead to the PVA active layer peeling off from the base membrane or structural collapse, resulting in a decrease in the dehydration selectivity of the composite membrane or even failure. This results in frequent replacement of the membrane module and increased application costs. The extremely low surface energy and chemical inertness of the protective layer make its interfacial compatibility with the hydrophilic PVA active layer poor and the bonding force weak. Simple physical coating can cause the protective layer to peel off easily during operation, making it unable to provide long-term protection for the PVA active layer. To achieve a firm bond between the protective layer and the PVA active layer, complex surface pretreatment or multi-layer transition coating processes are required, resulting in cumbersome processes and high difficulty in process control.

[0004] The information disclosed in this background section is only intended to enhance the understanding of the background of the present disclosure concept, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] The summary portion of this disclosure is intended to provide a brief overview of the concepts, which will be described in detail in the detailed description portion. This summary portion is not intended to identify key or essential features of the claimed technical solutions, nor is it intended to limit the scope of the claimed technical solutions.

[0006] Some embodiments of this disclosure provide a method for preparing and applying polyvinyl alcohol composite films with enhanced solvent resistance, in order to solve the technical problems mentioned in the background section above.

[0007] In a first aspect, some embodiments of this disclosure provide a method for preparing a polyvinyl alcohol composite film with enhanced solvent resistance, comprising: subjecting hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane to inert gas purging and preheating to obtain a monomer mixed solution; adding an initiator to the monomer mixed solution and performing a heat-preserving polymerization reaction to obtain a reaction mixture; subjecting the reaction mixture to vacuum distillation to obtain a first perfluorinated polymer; and dissolving the first perfluorinated polymer in a perfluorinated solvent to obtain a first perfluorinated polymer solution. A co-solvent is added to the first perfluoropolymer solution and stirred to obtain a second perfluoropolymer solution; the second perfluoropolymer solution is then aged and purified to obtain a first coating solution; a polyvinyl alcohol solution, tetraethyl orthosilicate, and hydrogen chloride are crosslinked to obtain a second coating solution; perfluorodecyltriethoxysilane is dissolved in anhydrous ethanol to obtain a third coating solution; based on the first, second, and third coating solutions, a polyvinylidene fluoride film or a polytetrafluoroethylene film is coated and dried to obtain a polyvinyl alcohol composite film.

[0008] Secondly, some embodiments of this disclosure provide a polyvinyl alcohol composite film for enhancing solvent resistance, wherein the polyvinyl alcohol composite film for enhancing solvent resistance is as described in the first aspect above.

[0009] Thirdly, some embodiments of this disclosure provide the application of polyvinyl alcohol composite membranes for enhancing solvent resistance in the field of membrane separation technology, wherein the polyvinyl alcohol composite membranes for enhancing solvent resistance are as described in the first aspect above.

[0010] The above-described embodiments of this disclosure have the following beneficial effects: the preparation method of polyvinyl alcohol composite film for enhancing solvent resistance according to some embodiments of this disclosure can improve the structural stability of polyvinyl alcohol composite film in strong polar solvents such as tetrahydrofuran, improve the interfacial bonding force between the perfluorinated protective layer and the polyvinyl alcohol active layer, and simplify the preparation process. The reasons for the poor solvent resistance, easy peeling of the protective layer, and complex process of existing polyvinyl alcohol composite membranes are as follows: The polyvinyl alcohol in the cross-linked polyvinyl alcohol active layer coated on the microfiltration base membrane is highly hydrophilic and easily swells in strong polar solvents such as tetrahydrofuran, causing the cross-linked network to be destroyed. This leads to the polyvinyl alcohol active layer peeling off from the base membrane or structural collapse, resulting in decreased dehydration selectivity or even failure of the composite membrane. This results in frequent replacement of the membrane module and increased application costs. The extremely low surface energy and chemical inertness of the protective layer make its interfacial compatibility with the hydrophilic polyvinyl alcohol active layer poor, and the bonding force is weak. Simple physical coating makes the protective layer easy to peel off during operation, failing to provide durable protection for the polyvinyl alcohol active layer. To achieve a strong bond between the protective layer and the polyvinyl alcohol active layer, complex surface pretreatment or multi-layer transition coating processes are required, resulting in cumbersome processes and high difficulty in process control. Based on this, the method for preparing a polyvinyl alcohol composite film for enhancing solvent resistance disclosed herein includes: subjecting hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane to inert gas replacement and preheating treatment to obtain a monomer mixed solution; adding an initiator to the above monomer mixed solution and carrying out a heat-preserving polymerization reaction to obtain a reaction mixture; subjecting the above reaction mixture to vacuum distillation to obtain a first perfluorinated polymer; dissolving the above first perfluorinated polymer in a perfluorinated solvent to obtain a first perfluorinated polymer solution; and adding a co-solvent... The first perfluoropolymer solution is added to the above-mentioned perfluoropolymer solution, and the mixture is stirred to obtain a second perfluoropolymer solution. The second perfluoropolymer solution is then aged and purified to obtain a first coating solution. A crosslinking treatment is performed on a polyvinylidene fluoride solution, tetraethyl orthosilicate, and hydrogen chloride to obtain a second coating solution. Perfluorodecyltriethoxysilane is dissolved in anhydrous ethanol to obtain a third coating solution. Based on the first, second, and third coating solutions, a polyvinylidene fluoride film or a polytetrafluoroethylene film is coated and dried to obtain a polyvinyl alcohol composite film. Because the perfluorodecyltriethoxysilane used has a siloxane group at one end that can hydrolyze and condense with the hydroxyl groups on the surface of the polyvinyl alcohol layer to form Si-O-Si covalent bonds, and its perfluorocarbon chain at the other end can be highly compatible with the outermost perfluoropolymer coating through hydrophobic interactions and van der Waals forces, it can effectively enhance the interfacial bonding between the perfluoroprotective layer and the polyvinyl alcohol active layer, reducing the peeling of the protective layer.Furthermore, after coating with a perfluorinated layer, irradiation with ultraviolet light of a specific wavelength allows the ozone generated in the air to synergistically oxidize the polyvinyl alcohol (PVA) layer surface, generating more hydroxyl functional groups and enhancing interfacial bonding. This improves the structural stability and dehydration performance retention of the composite film in harsh solvent environments. Also, because this invention introduces a silane coupling agent with an amphiphilic structure to achieve strong bonding between the PVA and perfluorinated layers, it reduces the need for complex surface activation processes or multi-layer gradient coating processes typically required in existing technologies to improve interfacial bonding. Therefore, it simplifies the operation and reduces the difficulty of process control. Attached Figure Description

[0011] The above and other features, advantages, and aspects of the embodiments of this disclosure will become more apparent from the accompanying drawings and the following detailed description. Throughout the drawings, the same or similar reference numerals denote the same or similar elements. It should be understood that the drawings are schematic, and elements are not necessarily drawn to scale.

[0012] Figure 1 This is a flowchart of some embodiments of the method for preparing a polyvinyl alcohol composite film with enhanced solvent resistance according to the present disclosure; Figure 2 The figure shows the experimental results of testing the separation performance of the polyvinyl alcohol composite membrane on a self-designed pervaporation experimental platform according to the preparation method of the polyvinyl alcohol composite membrane for enhancing solvent resistance disclosed herein. Figure 3 These are comparative test images of the surface state of the polyvinyl alcohol composite film prepared according to the method for preparing a polyvinyl alcohol composite film for enhancing solvent resistance according to this disclosure after being immersed in solvent. Figure 4 This is a graph showing the separation performance of the polyvinyl alcohol composite membrane prepared according to the method for preparing a polyvinyl alcohol composite membrane with enhanced solvent resistance according to this disclosure, tested on a self-designed pervaporation experimental platform. Figure 5 This is a photograph of a membrane separation apparatus used in an actual industrial separation internal test of the polyvinyl alcohol composite membrane prepared according to the method for preparing a polyvinyl alcohol composite membrane for enhancing solvent resistance according to this disclosure. Detailed Implementation

[0013] Embodiments of this disclosure will now be described in more detail with reference to the accompanying drawings. While some embodiments of this disclosure are shown in the drawings, it should be understood that this disclosure can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this disclosure. It should be understood that the accompanying drawings and embodiments of this disclosure are for illustrative purposes only and are not intended to limit the scope of protection of this disclosure.

[0014] It should also be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings. Unless otherwise specified, the embodiments and features described in this disclosure can be combined with each other.

[0015] It should be noted that the concepts of "first" and "second" mentioned in this disclosure are used only to distinguish different devices, modules or units, and are not used to limit the order of functions performed by these devices, modules or units or their interdependencies.

[0016] It should be noted that the terms "a" and "a plurality of" used in this disclosure are illustrative rather than restrictive, and those skilled in the art should understand that, unless otherwise expressly indicated in the context, they should be understood as "one or more".

[0017] The names of messages or information exchanged between multiple devices in the embodiments of this disclosure are for illustrative purposes only and are not intended to limit the scope of such messages or information.

[0018] This disclosure will now be described in detail with reference to the accompanying drawings and embodiments.

[0019] Figure 1 A process 100 is shown illustrating some embodiments of a method for preparing a polyvinyl alcohol composite film for enhancing solvent resistance according to the present disclosure. The method for preparing the polyvinyl alcohol composite film includes the following steps: Step 101: Hexafluoropropylene, perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane are subjected to inert gas replacement and preheating treatment to obtain a monomer mixed solution.

[0020] In some embodiments, hexafluoropropylene, perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane can be subjected to inert gas replacement and preheating treatment to obtain a monomer mixed solution.

[0021] In some optional implementations of certain embodiments, a monomer mixed solution can be obtained by subjecting hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane to inert gas purging and preheating treatment via the following steps: The first step involves mixing hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane in a pressure vessel to obtain an initial mixture. This initial mixture characterizes a solution containing the aforementioned hexafluoropropylene, perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane. The mass ratio of perfluoro-2-methylene-4-methyl-1,3-dioxolane, hexafluoropropylene, and perfluorinated solvent is (3~5):1:(3~5). The pressure vessel can be a mechanically stirred high-pressure reactor. The perfluorinated solvent can be perfluoro-2-butyltetrahydrofuran. In practice, firstly, hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane can be added to the aforementioned pressure vessel. Then, the stirring device of the pressure vessel is turned on, and the mixture is stirred at 100 r / min at 20°C until no visible suspended matter or stratification occurs, obtaining an initial mixture. The stirring device can be a turbine stirrer. It should be noted that the mass ratio of perfluoro-2-methylene-4-methyl-1,3-dioxolane, hexafluoropropylene, and the perfluorinated solvent in Example 1 can be 3:1:3. The mass ratio of perfluoro-2-methylene-4-methyl-1,3-dioxolane, hexafluoropropylene, and the perfluorinated solvent in Example 2 can be 2:1:4. The mass ratio of perfluoro-2-methylene-4-methyl-1,3-dioxolane, hexafluoropropylene, and the perfluorinated solvent in Example 3 can be 4:1:5. In Example 4, the mass ratio of the above-mentioned perfluoro-2-methylene-4-methyl-1,3-dioxolane, the above-mentioned hexafluoropropylene, and the above-mentioned perfluoro solvent can be 5:1:5.

[0022] The second step involves replacing the initial mixture using a nitrogen purging system to obtain a monomeric mixed solution. This nitrogen purging system can be a nitrogen purging system. The monomeric mixed solution can characterize a uniformly mixed solution containing hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane. In practice, firstly, nitrogen gas with a purity of ≥99.999% can be connected to the inlet of the pressure vessel. By repeatedly filling the pressure vessel with nitrogen and then venting the gas 3-5 times, the monomeric mixed solution is obtained.

[0023] Step 102: Add the initiator to the monomer mixture solution and carry out a heat-preserving polymerization reaction to obtain a reaction mixture.

[0024] In some embodiments, an initiator may be added to the monomer mixture solution and subjected to a heat-insulated polymerization reaction to obtain a reaction mixture.

[0025] In some optional implementations of certain embodiments, the initiator can be added to the monomer mixture solution and subjected to a heat-maintained polymerization reaction to obtain the reaction mixture: The first step involves heating the monomer mixture to a first preset temperature (30°C) to obtain a polymerization precursor solution. In practice, the monomer mixture can be uniformly heated and stabilized at 30±1°C using a pressure vessel at a heating rate of 1~2°C / min to obtain the polymerization precursor solution.

[0026] The second step involves treating the aforementioned polymerization precursor solution with a polymerization reaction according to a second preset temperature and an initiator to obtain a reaction mixture. The second preset temperature can be 40-70°C. The initiator can be diisopropyl peroxide dicarbonate. The amount of initiator added can be 1-2% of the mass of the perfluoro-2-methylene-4-methyl-1,3-dioxolane and hexafluoropropylene. In practice, firstly, the initiator can be added dropwise to the pressure vessel through a syringe interface, and then stirred at 200 r / min. Subsequently, the material in the pressure vessel can be heated to the second preset temperature at a rate of 0.5°C / min, and the pressure in the reaction vessel can be maintained at 0.3-0.8 MPa for a preset reaction time to obtain the reaction mixture. The preset reaction time can be 4-8 hours. It should be noted that the amount of initiator added in Example 1 can be 1%. The amount of initiator added in Example 2 can be 1.5%. The amount of initiator added in Example 3 can be 2%. The amount of initiator added in Example 4 can be 2%. The second preset temperature in Example 1 can be 40°C. The second preset temperature in Example 2 can be 50°C. The second preset temperature in Example 3 can be 60°C. The second preset temperature in Example 4 can be 70°C. The pressure inside the reactor in Example 1 can be 0.3 MPa. The pressure inside the reactor in Example 2 can be 0.4 MPa. The pressure inside the reactor in Example 3 can be 0.6 MPa. The pressure inside the reactor in Example 4 can be 0.8 MPa. The second preset reaction time in Example 1 can be 4 hours. The second preset reaction time in Example 2 can be 5 hours. The second preset reaction time in Example 3 can be 6 hours. The second preset reaction time in Example 4 can be 8 hours.

[0027] Step 103: The reaction mixture is subjected to vacuum distillation to obtain the first perfluorinated polymer.

[0028] In some embodiments, the reaction mixture can be subjected to vacuum distillation to obtain a first perfluoropolymer.

[0029] In some optional implementations of certain embodiments, the reaction mixture can be subjected to vacuum distillation to obtain the first perfluoropolymer by the following steps: The above-mentioned reaction mixture can be subjected to vacuum distillation using a distillation apparatus to obtain a first perfluoropolymer. The temperature for vacuum distillation of the reaction mixture can be 80-90°C, and the vacuum degree can be -0.09 to -0.099 MPa. The distillation apparatus can be a rotary evaporator. In practice, the reaction mixture can be transferred from the pressure vessel to the distillation flask of the distillation apparatus. The pressure of the distillation apparatus is reduced to the aforementioned vacuum degree using a rotary vane vacuum pump and a cold trap. Subsequently, the water bath of the distillation apparatus is turned on, and the temperature of the material inside the distillation apparatus is controlled at 80-90°C for distillation until no liquid drips from the distillation apparatus, thus obtaining the first perfluoropolymer. It should be noted that the vacuum distillation temperature in Example 1 is 80°C, and the vacuum degree is -0.09 MPa. The vacuum distillation temperature in Example 2 is 85°C, and the vacuum degree is -0.099 MPa. The vacuum distillation temperature in Example 3 is 90°C, and the vacuum degree is -0.095 MPa. In Example 4, the temperature of the vacuum distillation process was 90°C and the vacuum degree was -0.099 MPa.

[0030] Step 104: Dissolve the first perfluorinated polymer in a perfluorinated solvent to obtain a first perfluorinated polymer solution.

[0031] In some embodiments, the first perfluoropolymer can be dissolved in a perfluoro solvent to obtain a first perfluoropolymer solution. The mass percentage concentration of the first perfluoropolymer solution ranges from 1.0% to 5.0%. In practice, firstly, the perfluoro solvent can be added to a clean container equipped with a stirrer. Then, under stirring at a target dissolution temperature and a target dissolution speed, the first perfluoropolymer is added in batches to the perfluoro solvent for dissolution to obtain a first perfluoropolymer solution. The concentration of the first perfluoropolymer in the solvent can be controlled to reach 1.0% to 5.0%. The clean container can be a three-necked flask equipped with a mechanical stirrer. The target dissolution temperature can be 40-50°C. The target dissolution speed can be 200-300 r / min. It should be noted that the concentration of the first perfluoropolymer solution in Example 1 can be 1.0%. The concentration of the first perfluoropolymer solution in Example 2 can be 2.0%. The concentration of the first perfluoropolymer solution in Example 3 can be 4.0%. The concentration of the first perfluoropolymer solution in Example 4 can be 5.0%. In Example 1, the target dissolution temperature and target dissolution speed can be 40℃ and 200 r / min, respectively. In Example 2, the target dissolution temperature and target dissolution speed can be 45℃ and 250 r / min, respectively. In Example 3, the target dissolution temperature and target dissolution speed can be 48℃ and 300 r / min, respectively. In Example 4, the target dissolution temperature and target dissolution speed can be 50℃ and 300 r / min, respectively.

[0032] Step 105: Add a co-solvent to the first perfluoropolymer solution and stir to obtain a second perfluoropolymer solution.

[0033] In some embodiments, a co-solvent can be added to the first perfluoropolymer solution, followed by stirring to obtain a second perfluoropolymer solution. The co-solvent can be perfluorotoluene. The amount of co-solvent added can be 3% to 5% of the mass of the first perfluoropolymer solution. In practice, firstly, the second perfluoropolymer solution can be added to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Then, while stirring, the co-solvent is slowly added to the three-necked flask. Subsequently, the three-necked flask is placed in an oil bath or heating mantle, heated to a target aging temperature, and stirred continuously within this temperature range for a target preset time to obtain a homogeneous solution. The target aging temperature can be 80 to 90°C. Finally, the homogeneous solution is cooled to room temperature (20°C) to obtain the second perfluoropolymer solution. The target preset time is in the range of 1 to 3 hours. Adding the co-solvent to the first perfluoropolymer solution can change the solubility parameters and solvent and solvation capabilities of the first perfluoropolymer solution, thereby directly affecting the leveling and wetting properties of the first coating liquid in step 108. It should be noted that the target curing temperature in Example 1 can be 80°C. The target curing temperature in Example 2 can be 85°C. The target curing temperature in Example 3 can be 88°C. The target curing temperature in Example 4 can be 90°C. The target preset time in Example 1 can be 1 hour. The target preset time in Example 2 can be 2 hours. The target preset time in Example 3 can be 2 hours. The target preset time in Example 4 can be 3 hours.

[0034] Step 106: The second perfluoropolymer solution is aged and purified to obtain the first coating solution.

[0035] In some embodiments, the second perfluoropolymer solution can be aged and purified to obtain the first coating solution.

[0036] In some optional implementations of certain embodiments, the second perfluoropolymer solution can be aged and purified in the following manner to obtain the first coating liquid: The first step involves heating the second perfluoropolymer solution to a preset temperature and time to obtain a coating solution for heat treatment. The preset temperature is 80-90°C, and the preset time is 1-3 hours. In practice, the second perfluoropolymer solution is first added to a heatable reaction vessel (such as a three-necked flask) equipped with a stirrer and a reflux condenser. Stirring is then started, and the reaction vessel is heated using an oil bath or heating mantle, maintaining a stable temperature within the range of 80-90°C. Finally, the second perfluoropolymer solution is continuously stirred for the preset time to obtain the coating solution for heat treatment. The preset time can be 1-3 hours. It should be noted that in Example 1, the preset temperature can be 80°C and the preset time can be 1 hour. In Example 2, the preset temperature can be 85°C and the preset time can be 2 hours. In Example 3, the preset temperature can be 88°C and the preset time can be 2 hours. In Example 4, the third preset temperature can be 90°C and the preset duration can be 3 hours.

[0037] The second step is to cool and filter the coating solution from the heat treatment process to obtain the first coating solution.

[0038] In addressing the aforementioned technical problems in the application scenario—specifically, the purification of ultra-high purity solvents such as electronic-grade tetrafluorohydroran and acetonitrile—often presents a second technical problem: during the coating process to prepare the perfluorinated protective layer, dust particles from the environment or gel particles not completely removed from the coating solution may be introduced and adhere to the membrane surface or interior, forming physical defects in the perfluorinated protective layer. These defects, under long-term penetration by high-purity solvents, become the starting point for structural damage, leading to localized failure of the perfluorinated protective layer and a decrease in the separation performance of the polyvinyl alcohol composite membrane. Considering the following requirements for this application scenario—adaptability to operation in an ultra-clean environment and deep removal of particulate matter and gel from the coating solution—we have decided to adopt the following solution: In some optional implementations of certain embodiments, the coating liquid during heat treatment can be cooled and filtered to obtain the first coating liquid: The first step involves a controlled slow cooling process using a circulating water cooling system and a programmable temperature-controlled circulating water bath to obtain a primary coolant. In practice, the heat-treated coating solution in a three-necked flask containing the solution can be cooled to 60°C at a rate of 0.2°C / min or less using a circulating water cooling system. Subsequently, the solution is cooled to 24-26°C at a rate of 0.2-0.5°C / min using a programmable temperature-controlled circulating water bath to obtain the primary coolant. This controlled slow cooling process allows for the orderly rearrangement of the perfluoropolymer's molecular chains, releasing any residual internal stress that may have accumulated during the temperature drop from high temperature (80-90°C) to room temperature (20°C) due to thermal expansion or contraction. This reduces defects such as cracking, micropores, or localized structural loosening induced by uneven thermal stress during subsequent film formation, thus minimizing these defects at the source of the manufacturing process.

[0039] The second step involves pre-filtration and degassing of the primary coolant in a cleanroom environment to obtain a coarsely filtered clarified liquid. The cleanroom environment can be characterized by a laminar flow hood. In practice, a three-necked flask containing the primary coolant can be transferred to the laminar flow hood and allowed to stand for 30-60 minutes to remove large air bubbles. Subsequently, a 5.0 μm PTFE membrane can be used for filtration under a nitrogen atmosphere of 0.05-0.1 MPa to remove larger gel particles and dust from the environment, yielding the coarsely filtered clarified liquid. The large air bubbles mentioned above can be characterized by bubbles larger than 0.5 mm in diameter.

[0040] The third step is to subject the coarsely filtered clarified liquid to ultrasonic-assisted homogenization to obtain a homogeneous dispersion. In practice, the coarsely filtered clarified liquid can be placed in an ultrasonic cell disruptor and treated for 15-25 minutes at a constant temperature of 25°C and an output power of 300-400W using a pulsed operating mode (e.g., ultrasonic emission for 2-4 seconds followed by an interval of 1-2 seconds) to break up the submicron-scale (100 nanometers to 1 micrometer) aggregates or associations remaining in the coarsely filtered clarified liquid. This improves the uniformity and stability of the homogeneous dispersion, resulting in a homogeneous dispersion.

[0041] The fourth step involves fine passivation of the homogeneous dispersion using a series gradient precision filtration device to obtain a refined filtrate. In practice, the homogeneous dispersion can be passed sequentially through the series gradient precision filtration device to obtain the refined filtrate. The series gradient precision filtration device can be a three-stage filter support equipped with three polytetrafluoroethylene (PTFE) membranes of different pore sizes. The pore sizes of the PTFE membranes can be 1.0 μm, 0.45 μm, and 0.22 μm, respectively, and the operating pressures can be 0.08–0.12 MPa, 0.15–0.20 MPa, and 0.20–0.25 MPa, respectively. This series gradient precision filtration device can trap and remove solid particulate impurities ranging from micrometers to submicrometers, reducing the introduction of defects into the perfluoropolymer protective layer due to impurities in the coating solution, thus ensuring the long-term stability of the prepared polyvinyl alcohol composite membrane in electronic-grade solvent separation.

[0042] The fifth step involves terminal ultrafiltration and sterilization of the purified filtrate using the ultrafiltration membrane module and sterile filter membrane to obtain an ultra-clean coating solution. In practice, the purified filtrate can be passed through an ultrafiltration membrane module in a clean bench for 20-40 minutes in a cross-flow filtration manner to deeply remove any possible nanoscale agglomerates or metastable crystal nuclei. Subsequently, a sterile filter membrane made of polyethersulfone with a pore size of 0.22 μm can be used for filtration to obtain the ultra-clean coating solution, ensuring its sterility and cleanliness. The filter medium of the ultrafiltration membrane module can be a polytetrafluoroethylene hollow fiber membrane. The molecular weight cutoff of the ultrafiltration membrane module can be 100,000 to 300,000 Daltons.

[0043] Step 6: Using an inert atmosphere, the above-mentioned ultra-clean coating solution is dispensed and sealed to obtain the dispensed finished solution. In practice, under an ultra-clean environment, the ultra-clean coating solution can be dispensed into pre-cleaned brown glass reagent bottles that have been dried at 180°C for 4 hours and purged three times with high-purity nitrogen (purity greater than 99.999%). Then, the space above the liquid surface in the brown glass reagent bottle can be filled with the aforementioned inert atmosphere, and the bottle cap lined with a polytetrafluoroethylene gasket can be used to isolate oxygen and atmospheric pollution, maintaining the purity of the ultra-clean coating solution inside the brown glass reagent bottle, thus obtaining the dispensed finished solution. The aforementioned inert atmosphere can be high-purity nitrogen.

[0044] Step 7: Under a low-temperature, light-protected environment, the above-packaged finished liquid undergoes a stabilization aging treatment to obtain the first coating solution. In practice, the sealed brown glass reagent bottle containing the above-packaged finished liquid can be tightly wrapped with aluminum foil and then immediately transferred to a refrigerator set at 4±1℃. It should then be stored in the refrigerator without any light source for 48-72 hours to obtain the first coating solution. This allows the first coating solution to reach physicochemical equilibrium in a clean and isolated state, thus maintaining optimal stability before coating.

[0045] The above-described technical solution, as an inventive point of this disclosure, solves technical problem two: the decline in the separation performance of polyvinyl alcohol composite membranes. The reasons for the decline in the separation performance of polyvinyl alcohol composite membranes in the prior art are as follows: During the coating process of the perfluorinated protective layer, dust particles from the environment or gel particles not completely removed from the coating solution may be introduced and adhere to the membrane surface or interior, forming physical defects in the perfluorinated protective layer. These defects, under long-term penetration by high-purity solvents, become the starting point for structural damage, leading to localized failure of the perfluorinated protective layer and a decline in the separation performance of the polyvinyl alcohol composite membrane. Solving these factors can reduce the decline in the separation performance of polyvinyl alcohol composite membranes. To achieve this effect, some embodiments of this disclosure eliminate thermal stress in the coating solution through programmed slow cooling, reducing cracking and micropore defects during film formation; large particles and bubbles are removed in an ultra-clean environment through pre-filtration and degassing treatment, and submicron-scale aggregates or associative compounds are broken up through ultrasonic-assisted homogenization, thus improving the uniformity and stability of the homogeneous dispersion; micron to submicron particle impurities are removed through the aforementioned series gradient precision filtration device; at the same time, oxygen and atmospheric pollution are isolated due to inert atmosphere dispensing and sealing; finally, the physicochemical equilibrium of the coating solution is achieved through low-temperature light-proof aging, thereby achieving high purity, homogeneity and stability of the coating solution. Therefore, a dense and defect-free perfluorinated protective layer can be formed after coating, thereby reducing the decline in the separation performance of polyvinyl alcohol composite membranes.

[0046] Step 107: Crosslink the polyvinyl alcohol solution, tetraethyl orthosilicate, and hydrogen chloride to obtain the second coating solution.

[0047] In some embodiments, the polyvinyl alcohol solution, tetraethyl orthosilicate, and hydrogen chloride can be crosslinked to obtain a second coating solution.

[0048] In some alternative implementations of certain embodiments, the second coating liquid can be obtained by crosslinking the polyvinyl alcohol solution, tetraethyl orthosilicate, and hydrogen chloride in the following manner: The first step is to add a measured amount of deionized water to the reaction vessel, turn on the stirrer, and heat the deionized water using an oil bath or water bath until the temperature rises and stabilizes at 85-95°C. The reaction vessel can be a three-necked flask equipped with a mechanical stirrer, thermometer, and reflux condenser.

[0049] The second step involves slowly and gradually adding polyvinyl alcohol powder to deionized water at 85-95°C, continuously stirring until the polyvinyl alcohol powder is completely dissolved, yielding a polyvinyl alcohol solution with a mass percentage of 4%-7%. It should be noted that in Example 1, the temperature of the deionized water can be 85°C, and the resulting polyvinyl alcohol solution has a mass percentage of 4%. In Example 2, the temperature of the deionized water can be 90°C, and the resulting polyvinyl alcohol solution has a mass percentage of 5%. In Example 3, the temperature of the deionized water can be 90°C, and the resulting polyvinyl alcohol solution has a mass percentage of 6%. In Example 4, the temperature of the deionized water can be 95°C, and the resulting polyvinyl alcohol solution has a mass percentage of 7%.

[0050] The third step involves maintaining the polyvinyl alcohol solution at 85-95°C with continuous stirring. Tetraethyl orthosilicate is slowly added and dispersed evenly. Then, concentrated hydrochloric acid aqueous solution is added as a catalyst. The amount of pure hydrogen chloride added to the concentrated hydrochloric acid aqueous solution is 0.3%-0.8% of the total mass of the polyvinyl alcohol solution. The amount of tetraethyl orthosilicate added can be 4%-8% of the total mass of the polyvinyl alcohol solution. It should be noted that in Example 1, the amount of tetraethyl orthosilicate added is 4% of the total mass of the polyvinyl alcohol solution. In Example 2, the amount of tetraethyl orthosilicate added is 4.5% of the total mass of the polyvinyl alcohol solution. In Example 3, the amount of tetraethyl orthosilicate added is 6% of the total mass of the polyvinyl alcohol solution. In Example 4, the amount of tetraethyl orthosilicate added is 8% of the total mass of the polyvinyl alcohol solution. The mass fraction of pure hydrogen chloride added to the concentrated hydrochloric acid aqueous solution in Example 1 is 0.3%. The mass fraction of pure hydrogen chloride added to the concentrated hydrochloric acid aqueous solution in Example 2 is 0.5%. In Example 3, the mass fraction of pure hydrogen chloride added to the concentrated hydrochloric acid aqueous solution can be 0.6%. In Example 4, the mass fraction of pure hydrogen chloride added to the concentrated hydrochloric acid aqueous solution can be 0.8%.

[0051] The fourth step involves maintaining the mixture at 85-95°C with continuous stirring for 2-4 hours to obtain the second coating solution. In practice, the mixture of the above-mentioned polyvinyl alcohol solution, tetraethyl orthosilicate, and concentrated hydrochloric acid aqueous solution can be continuously stirred at 85-95°C for 2-4 hours to obtain the second coating solution.

[0052] Step 108: Dissolve perfluorodecyltriethoxysilane in anhydrous ethanol to obtain the third coating solution.

[0053] In some embodiments, perfluorodecyltriethoxysilane can be dissolved in anhydrous ethanol to obtain a third coating solution.

[0054] In the process of adopting technical solutions to solve the technical problems mentioned above, for the application scenarios: in large-scale continuous production in the fields of chemical, pharmaceutical and new energy materials (such as the production and refining of methyl tert-butyl ether, which requires efficient separation and recovery of methanol from the azeotrope formed with methanol), the following technical problem often arises: Existing technologies mainly rely on extractive distillation or pressure swing distillation, which requires operation under high pressure and high temperature, resulting in a continuous high energy consumption load, complex process flow and huge equipment investment; and when conventional polyvinyl alcohol pervaporation membranes used for solvent dehydration are applied to high-concentration, highly polar organic solvents, the polyvinyl alcohol active layer will swell excessively due to strong solvent permeation, resulting in a decrease in the separation selectivity of the polyvinyl alcohol active layer. Furthermore, under continuous temperature fluctuations and material scouring, the protective layer formed by simple physical coating is prone to peeling off from the active layer due to weak bonding, causing the overall structure of the membrane to fail, which cannot meet the long-term, high-stability operation requirements of industrial equipment for membrane elements. To meet the following requirements for this application scenario: adaptability to efficient separation to significantly save energy, adaptability to resist long-term corrosion by highly polar solvents, and adaptability to maintain the interlayer structure of the membrane to ensure long-term stable operation, we have decided to adopt the following solution: In some alternative implementations of certain embodiments, the third coating solution can be obtained by dissolving perfluorodecyltriethoxysilane in anhydrous ethanol in the following manner: The first step involves mixing and dispersing perfluorodecyltriethoxysilane with anhydrous ethanol according to a first preset mass ratio to obtain a silane-ethanol dispersion. The first preset mass ratio is in the range of 1:(5~15), and the anhydrous ethanol has a water content of less than 50 ppm. This ensures a pure and uniform starting point for the reaction between the perfluorodecyltriethoxysilane and anhydrous ethanol, reducing the risk of localized and premature hydrolysis due to residual moisture. In practice, under the protection of dry nitrogen, the perfluorodecyltriethoxysilane and anhydrous ethanol can be added to a three-necked flask according to the first preset mass ratio and mechanically stirred until a clear, uniform silane-ethanol dispersion without visible layering is formed. Mechanical stirring to form a uniform silane-ethanol dispersion reduces microscopic defects at the interface between the polyvinyl alcohol active layer and the perfluorinated protective layer caused by heterogeneity, thus preventing weak interfacial bonding.

[0055] The second step involves adding an aqueous glacial acetic acid solution in batches to the aforementioned silane-ethanol dispersion and stirring the mixture to obtain a gradient acid-catalyzed hydrolysis solution. In practice, a 0.5-2% (w / w) aqueous glacial acetic acid solution can be prepared first. Then, this solution is slowly added dropwise to the silane-ethanol dispersion in two batches. The first batch can contain 30-50% of the aqueous glacial acetic acid solution, reacting at 25-30°C for 10-30 minutes. The second batch can contain the remaining aqueous glacial acetic acid solution, with the temperature raised to 30-40°C and the reaction continued at this temperature for 20-40 minutes to obtain the gradient acid-catalyzed hydrolysis solution.

[0056] The third step involves detecting the degree of hydrolysis and adjusting the pH of the gradient acid-catalyzed hydrolysate to obtain a pH-optimized hydrolysate. In practice, a precision pH meter can be used to monitor the pH of the gradient acid-catalyzed hydrolysate in real time. Then, based on the monitored pH results, the pH of the gradient acid-catalyzed hydrolysate is controlled within the range of 3.5 to 4.5 to obtain a pH-optimized hydrolysate.

[0057] Fourth, deionized water is added to the pH-optimized hydrolysate, and the mixture is stirred at a constant temperature in the dark to obtain an oligomeric silanol solution. Steps three, four, and five above can generate a structurally controllable oligomeric silanol. Therefore, this oligomeric silanol can form a cross-linked and dense Si-O-Si network on the surface of polyvinyl alcohol in subsequent steps, reducing swelling by strongly polar solvents. This significantly slows down the penetration and attack of strongly polar solvents on the internal polyvinyl alcohol active layer, thereby inhibiting swelling and protecting the separation selectivity of the polyvinyl alcohol active layer.

[0058] The fifth step involves cooling and allowing the oligomeric silanol solution to stand for aging to obtain an aged silanol solution. In practice, the oligomeric silanol solution can first be placed in a programmable temperature-controlled bath and cooled to 10-20°C at a rate of 0.5-1°C / min. Then, it can be allowed to stand at 10-20°C for 2-6 hours to obtain the aged silanol solution.

[0059] Step 6: According to the preset mass, add a chelating agent to the above-mentioned hydrated silanol solution and stir to dissolve it, thereby obtaining a stable silanol solution. The preset mass can be 0.01~0.1% of the total mass of the hydrated silanol solution. In practice, the preset mass of chelating agent can be added to the above-mentioned hydrated silanol solution, and stirring can be continued at room temperature (20°C) until the chelating agent is completely dissolved and uniformly dispersed in the hydrated silanol solution, thus obtaining a stable silanol solution. The specific type of chelating agent is not limited here and can be selected according to the actual situation. For example, the chelating agent can be ethylenediaminetetraacetic acid (EDTA).

[0060] Step 7: Using a polytetrafluoroethylene (PTFE) filter membrane, the stabilized silanol solution is pressure filtered and then refrigerated to obtain the third coating solution. In practice, firstly, the stabilized silanol solution can be pressure filtered through a filtration device equipped with a PTFE aluminum membrane with a pore size of 0.1 μm under a positive pressure of 0.5~0.3 MPa. Then, the filtrate obtained after pressure filtration can be immediately dispensed into brown glass reagent bottles, and nitrogen gas is introduced to replace the air in the brown glass reagent bottles and the bottles are sealed. Subsequently, the bottles are transferred to a refrigerator set at 5°C for storage to obtain the third coating solution. Steps 6, 7, and 8 above ensure that the performance of the prepared third coating solution does not degrade before coating and can be uniformly and purely coated to form a defect-free interface layer.

[0061] The above-described technical solution, as an inventive point of this disclosure, solves technical problem three: the existing technology suffers from high energy consumption, excessive swelling of the polyethylene active layer, and easy failure of the overall membrane structure. The reasons for these problems are as follows: the existing technology mainly relies on extractive distillation or pressure swing distillation, requiring operation under high pressure and high temperature, resulting in a continuous high energy consumption, complex process flow, and huge equipment investment; furthermore, when conventional polyvinyl alcohol pervaporation membranes used for solvent dehydration are applied to high-concentration, highly polar organic solvents, the polyvinyl alcohol active layer undergoes excessive swelling due to strong solvent permeation, reducing the separation selectivity of the polyvinyl alcohol active layer. Moreover, under continuous temperature fluctuations and material scouring, the protective layer formed by simple physical coating is prone to peeling from the active layer due to weak adhesion, causing overall membrane structure failure and failing to meet the long-term, high-stability operation requirements of industrial equipment. Solving these factors can reduce the problems of high energy consumption, excessive swelling of the polyethylene active layer, and easy failure of the overall membrane structure in the existing technology. To achieve this effect, some embodiments of this disclosure employ an energy-efficient pervaporation membrane separation process as a solution. This solution requires the membrane material to withstand long-term erosion by highly polar organic solvents. Furthermore, by performing pH adjustment on the hydrolysate, adding deionized water for constant-temperature, light-protected stirring, cooling and settling, adding a chelating agent, and finally filtering and refrigerating, a highly active and stable third coating solution is prepared. This third coating solution is used to construct a robust interface layer between the polyvinyl alcohol active layer and the perfluoropolymer protective layer. This interface layer, through strong chemical bonds and a dense network structure, can inhibit the permeation and swelling of the polyvinyl alcohol active layer by highly polar solvents and reduce the problem of easy peeling between physical coating layers. This allows the prepared composite membrane to achieve the structural stability and separation performance retention required for long-term operation in harsh solvent environments. Moreover, this composite membrane enables the low-power pervaporation process to be reliably applied to the industrial separation of azeotropic systems such as methyl tert-butyl ether / methanol, thereby reducing the energy consumption of the separation process.

[0062] In the process of adopting technical solutions to solve the technical problems mentioned above, for the application scenario: in the large-scale, long-cycle, continuous industrial production of key raw materials such as dimethyl carbonate and lithium battery electrolytes, the following technical problem often arises: When polyvinyl alcohol composite membranes are used to replace traditional high-energy-consuming distillation towers, and the polyvinyl alcohol composite membranes are integrated into industrial plants with an annual processing capacity of up to 100,000 tons for continuous pervaporation separation, the industrial-grade azeotropic mixture to be processed contains water and aldehyde impurities that cannot be completely removed. These impurities will continuously and irreversibly adsorb and accumulate on the surface of the polyvinyl alcohol composite membrane, causing chronic swelling of the polyvinyl alcohol active layer and resulting in an irreversible decrease in the separation flux of the polyvinyl alcohol composite membrane. The inherent pressure and temperature fluctuations of the industrial plant will amplify the micro-stress fatigue effect caused by the difference in the thermal expansion coefficient of the materials between the layers of the polyvinyl alcohol composite membrane, resulting in fluctuations and substandard product purity. To address the following requirements for this application scenario: adaptability to inherent pressure and temperature fluctuations in industrial equipment, a separation flux attenuation rate of less than 15% for the polyvinyl alcohol composite membrane, and tolerance of trace impurities in the raw materials to the polyvinyl alcohol composite membrane, we have decided to adopt the following solution: In some optional implementations of certain embodiments, deionized water can be added to the pH-optimized hydrolysate, followed by constant-temperature, light-protected stirring treatment to obtain an oligomeric silanol solution: The first step involves surface hydroxylation pretreatment of nano-silica powder to obtain activated nano-silica. This activated nano-silica characterizes activated nano-silica with a surface rich in active silanol groups. In practice, the nano-silica powder is first placed in a muffle furnace and calcined at 450-500°C for 2-3 hours, then cooled to room temperature to obtain calcined nano-silica. The calcined nano-silica is then dispersed in a 10-15% hydrochloric acid solution and refluxed at 80-85°C for 3-4 hours to obtain an acid-treated nano-silica slurry. This slurry is then centrifuged and washed with deionized water until the washing solution is neutral. Finally, it is dried at 100-120°C for 2 hours to obtain activated silica. The room temperature can be 20°C.

[0063] The second step involves ultrasonically treating the aforementioned active nano-silica and perfluorodecyltriethoxysilane according to a second preset mass ratio and ammonium polyacrylate to obtain a nano-silica aqueous dispersion slurry. The second preset mass ratio can be 100:(1~5). In practice, the aforementioned active nano-silica and perfluorodecyltriethoxysilane can be weighed according to the second preset mass ratio and dispersed in a measured amount of plasma water to obtain a preliminary nano-silica / silane dispersion. Subsequently, ammonium polyacrylate, accounting for 0.05~0.2% of the total mass of the preliminary nano-silica / silane dispersion, can be used as a dispersant. The preliminary nano-silica / silane dispersion and the ammonium polyacrylate are ultrasonically treated in an ultrasonic cell disruptor at 300~400W power for 20~40 minutes to obtain the nano-silica aqueous dispersion slurry.

[0064] The third step involves premixing the pH-optimized hydrolysate with the nano-silica aqueous dispersion slurry to obtain the reaction precursor solution. In practice, the pH-optimized hydrolysate and the nano-silica aqueous dispersion slurry can be mixed in a mechanical stirrer at 200-300 rpm for 15-30 minutes at 25-30°C under an inert atmosphere to obtain the reaction precursor solution. The inert atmosphere can be nitrogen.

[0065] Fourth, according to the preset molar ratio, add deionized water to the above reaction precursor solution to obtain the reaction system. The preset molar ratio can be (2.0~4.0):1. In practice, deionized water can be added to the above reaction precursor solution to achieve the preset molar ratio of deionized water to perfluorodecyltriethoxysilane, resulting in a mixture. Then, the mixture can be transferred to a three-necked flask, placed in a 40°C constant temperature oil bath and protected from light with aluminum foil, and stirred continuously for 1.5 hours to obtain the reaction system.

[0066] The fifth step involves subjecting the above reaction system to uniform heating and stirring in the dark, along with sampling and monitoring, to obtain the target viscosity reaction solution. In practice, the temperature of the above reaction system can be uniformly raised to 55-60℃ and maintained at 55-60℃ while stirring in the dark for 4-8 hours to obtain the target viscosity reaction solution. During this process, the viscosity of the reaction solution can be monitored by sampling. When the viscosity of the above reaction solution increases to 2-3 times the initial value, the product obtained from the above reaction can be identified as the target viscosity reaction solution. For example, the initial value can be 15 mPa·s.

[0067] Step 6: Cool and adjust the pH of the target viscosity reaction solution to obtain a hybrid sol. In practice, the target viscosity reaction solution can be cooled to below 30°C. Subsequently, use a dilute ammonia solution with a concentration of 0.1~0.5 mol / L to adjust and stabilize the pH of the target viscosity reaction solution within the range of 4.0~4.3 while stirring. Continue stirring and stabilizing for 20~30 minutes to obtain the hybrid sol.

[0068] Step 7: Using a programmed temperature-controlled constant temperature bath, the hybrid sol is subjected to programmed cooling to obtain an aged hybrid sol. In practice, the hybrid sol can be transferred to a programmed temperature-controlled constant temperature bath and cooled from 30°C to 5°C at a rate of 0.3~0.5°C / min, and maintained at 5°C for 4~8 hours. Then, the temperature can be slowly increased to 20°C at a rate of 0.1~0.2°C / min, and maintained at 20°C for 2~4 hours to obtain an aged hybrid sol.

[0069] Step 8: The aged hybrid sol is purified and homogenized to obtain an oligomeric silanol solution. In practice, the aged hybrid sol can be restored to atmospheric pressure at 20-25°C. First, it can be treated with a high-shear disperser at 2000-3000 rpm for 5-10 minutes to obtain a homogeneous hybrid sol. Then, the homogeneous hybrid sol is allowed to stand for degassing for 15-30 minutes to obtain a degassed hybrid sol. Finally, the degassed hybrid sol can be filtered through a 0.5 μm PTFE membrane under an inert gas positive pressure of 0.05-0.1 MPa to obtain the oligomeric silanol solution.

[0070] The above-described technical solution, as an inventive point of this disclosure, solves technical problem four: irreversible decline in the separation flux of polyvinyl alcohol composite membranes and fluctuations and substandard product purity. The reasons for the irreversible decline in the separation flux of polyvinyl alcohol composite membranes and fluctuations and substandard product purity in the prior art are as follows: When polyvinyl alcohol composite membranes are used to replace traditional high-energy-consuming distillation columns and integrated into industrial plants with an annual processing capacity of 100,000 tons for continuous pervaporation separation, the industrial-grade azeotropic mixture to be processed contains water and aldehyde impurities that cannot be completely removed. These impurities continuously and irreversibly adsorb and accumulate on the surface of the polyvinyl alcohol composite membrane, causing chronic swelling of the polyvinyl alcohol active layer and resulting in irreversible decline in the separation flux of the polyvinyl alcohol composite membrane. Furthermore, the inherent pressure and temperature fluctuations in industrial plants amplify the micro-stress fatigue effect caused by the difference in the thermal expansion coefficients of the materials between the layers of the polyvinyl alcohol composite membrane, leading to fluctuations and substandard product purity. If the above factors are addressed, it is possible to reduce the irreversible decline in separation flux of polyvinyl alcohol composite membranes and the fluctuations and substandardities in product purity. To achieve this effect, some embodiments of this disclosure introduce a Si-O-Si-nano silica network structure formed by nano-silica and silane. This network structure can more effectively block the penetration and adsorption of impurities such as moisture and aldehydes from industrial raw materials into the polyvinyl alcohol active layer, alleviating chronic swelling caused by impurity accumulation and thus maintaining long-term stability of separation flux. Furthermore, the introduction of nano-silica particles significantly enhances the mechanical strength and modulus of the interface layer, and the programmed cooling treatment gives the impurity layer good elasticity and toughness. This effectively absorbs and disperses the thermal and mechanical stresses generated between different material layers due to temperature and pressure fluctuations in industrial equipment, thereby greatly reducing micro-stress fatigue effects and minimizing the decrease in separation selectivity caused by interlayer micro-peeling or deformation. Consequently, the composite membrane can maintain stable separation performance under fluctuating operating conditions, ensuring the compliance and stability of product purity.

[0071] Step 109: Apply the polyvinylidene fluoride film or polytetrafluoroethylene film to the first coating liquid, the second coating liquid and the third coating liquid, and perform a coating and drying process to obtain a polyvinyl alcohol composite film.

[0072] In some embodiments, a polyvinylidene fluoride film or a polytetrafluoroethylene film can be coated and dried according to the first coating liquid, the second coating liquid, and the third coating liquid to obtain a polyvinyl alcohol composite film.

[0073] In some optional implementations of certain embodiments, a polyvinylidene fluoride (PVDF) film or a polytetrafluoroethylene (PTFE) film can be coated and dried using the first coating liquid, the second coating liquid, and the third coating liquid described above to obtain a polyvinyl alcohol (PVA) composite film. The first step involves applying the second coating solution onto the polyvinylidene fluoride (PVDF) membrane or the polytetrafluoroethylene (PTFE) membrane, followed by a first drying process to obtain a first coated membrane. The coating thickness is in the range of 70-80 μm, the drying temperature is in the range of 80-100°C, and the drying time is in the range of 0.5-1.5 hours. A coating thickness less than 70 μm will result in an excessively thin coating on the first membrane, leading to a decreased selectivity of the formed polyvinyl alcohol (PVA) composite membrane for separating organic matter. A coating thickness greater than 80 μm will result in an excessively low permeation flux of the final PVA composite membrane. In practice, a clean PVDF or PTFE membrane can be laid flat on a smooth glass plate. The coating solution can then be evenly applied to the surface of the PVDF or PTFE membrane using a doctor blade to achieve a coating thickness in the range of 70-80 μm. Subsequently, the coated polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE) film can be immediately transferred to a preheated drying oven at 80-100°C for a first drying treatment of 0.5-1.5 hours. After drying, the PVDF or PTFE film after the first drying treatment is removed and cooled to room temperature in air to obtain the first coated film. It should be noted that in Example 1, the coating thickness can be 70 μm, the first drying temperature can be 80°C, and the first drying time can be 0.5 hours. In Example 2, the coating thickness can be 75 μm, the first drying temperature can be 90°C, and the first drying time can be 1 hour. In Example 3, the coating thickness can be 75 μm, the first drying temperature can be 100°C, and the first drying time can be 1.2 hours. In Example 4, the coating thickness can be 80 μm, the first drying temperature can be 100°C, and the first drying time can be 1.5 hours.

[0074] The second step involves subjecting the first coated film to ultraviolet (UV) light irradiation to obtain a primary composite film. This UV irradiation uses a 150W dual-band UV lamp, with the lamp positioned 30-50mm from the film surface and the irradiation time ranging from 1 to 3 minutes. In practice, the first coated film can be immediately placed under the 150W dual-band UV lamp, and the vertical distance between the lamp and the surface of the coated second film can be adjusted to ensure the lamp is 30-50mm away. The lamp is then turned on, and the surface of the second coated film is irradiated for 1-3 minutes. Finally, the lamp is turned off to obtain the primary composite film. It should be noted that in Example 1, the distance between the dual-band UV lamp and the film surface can be 30mm, and the irradiation time can be 1 minute. In Example 2, the distance between the dual-band UV lamp and the film surface can be 40 mm, and the illumination time can be 2 minutes. In Example 3, the distance between the dual-band UV lamp and the film surface can be 50 mm, and the illumination time can be 2 minutes. In Example 4, the distance between the dual-band UV lamp and the film surface can be 50 mm, and the illumination time can be 3 minutes.

[0075] The third step involves coating and drying the primary composite film using the third coating solution to obtain a second coated film. The coating thickness ranges from 10 to 30 μm. This coating thickness allows for the formation of a suitable and uniformly continuous second coated film on the surface of the first coated film. In practice, the primary composite film can first be cooled to room temperature in air. Then, the third coating solution can be uniformly coated onto the surface of the primary composite film using a doctor blade coating method to achieve a coating thickness of 10 to 30 μm, resulting in a coated film. The coated film can then be naturally dried in a clean, room-temperature environment until no liquid residue remains on its surface, thus obtaining the second coated film. It should be noted that the coating thickness in Example 1 can be 10 μm. The coating thickness in Example 2 can be 20 μm. The coating thickness in Example 3 can be 25 μm. The coating thickness in Example 4 can be 30 μm.

[0076] The fourth step involves applying the first coating solution onto the second coating film to obtain a composite film. In practice, the second coating film can be flattened and fixed onto a clean glass plate. An automatic coating machine is used to apply the first coating solution to the entire surface of the second coating film. By controlling the blade gap of the automatic coating machine or the coating machine itself, the thickness of the resulting composite film can be maintained between 50 and 60 μm. It should be noted that the thickness of the composite film obtained in Example 1 can be 50 μm. The thickness of the composite film obtained in Example 2 can be 55 μm. The thickness of the composite film obtained in Example 3 can be 60 μm. The thickness of the composite film obtained in Example 4 can be 60 μm.

[0077] The fifth step involves subjecting the composite film to a second drying treatment to obtain a polyvinyl alcohol composite film. The temperature range for this second drying treatment is 110℃~130℃, and the drying time is 4~6 hours. In practice, the composite film treated with ultraviolet light can first be transferred to an oven and subjected to a second drying treatment at 110℃~130℃ for 4~6 hours. Then, the film after the second drying treatment can be allowed to cool naturally to room temperature to obtain the polyvinyl alcohol composite film. It should be noted that in Example 1, the temperature for the second drying treatment can be 110℃, and the drying time can be 4 hours. In Example 2, the temperature for the second drying treatment can be 120℃, and the drying time can be 5 hours. In Example 3, the temperature for the second drying treatment can be 130℃, and the drying time can be 6 hours. In Example 4, the temperature for the second drying treatment can be 130℃, and the drying time can be 6 hours.

[0078] from Figure 2 The data shows that conventional polyvinyl alcohol (PVA) membranes exhibit decreased dehydration performance for ethanol-water after immersion in tetrahydrofuran, and the ethanol concentration in the permeate significantly increased. However, the PVA composite membrane used to enhance solvent resistance, while showing a slight increase in permeate flux after immersion in tetrahydrofuran, did not show a significant increase in the ethanol concentration in the permeate, thus maintaining its dehydration performance for ethanol-water. This experiment demonstrates that the PVA composite membrane used to enhance solvent resistance, after modification with a perfluorinated coating, has a longer service life in a tetrahydrofuran system compared to conventional PVA membranes.

[0079] from Figure 3 As can be seen above, the conventional polyvinyl alcohol composite film exhibited significant peeling of the polyvinyl alcohol layer after being soaked in tetrahydrofuran for 90 days. The fully modified PVA film in Example 3, which is the polyvinyl alcohol composite film proposed in this invention for enhancing solvent resistance, showed no abnormalities on its surface after being soaked in tetrahydrofuran. This demonstrates that the polyvinyl alcohol composite film for enhancing solvent resistance has extremely strong solvent resistance to tetrahydrofuran.

[0080] from Figure 4 As can be seen above, conventional polyvinyl alcohol (PVA) membranes significantly lose their separation performance in the methyl acetate / methanol azeotropic system after immersion in methyl acetate. The methanol content in the permeate is essentially the same as that in the feed solution, indicating that the conventional PVA membrane no longer possesses separation selectivity. In contrast, the PVA composite membrane used to enhance solvent resistance, after immersion under the same conditions, although the permeate flux slightly increases, the methanol content in the permeate does not show a significant decrease, indicating that this membrane still maintains stable separation efficiency in the methyl acetate / methanol azeotropic system. These experimental results demonstrate that perfluorinated coating modification can significantly improve the tolerance and service life of PVA composite membranes in the methyl acetate / methanol mixed solvent azeotropic system, showing a clear advantage over conventional PVA membranes.

[0081] from Figure 5 The above demonstrates the feasibility of using polyvinyl alcohol composite membranes, which enhance solvent resistance, for separation and purification in organic solvent systems.

[0082] The above-described embodiments of this disclosure have the following beneficial effects: the preparation method of polyvinyl alcohol composite film for enhancing solvent resistance according to some embodiments of this disclosure can improve the structural stability of polyvinyl alcohol composite film in strong polar solvents such as tetrahydrofuran, improve the interfacial bonding force between the perfluorinated protective layer and the polyvinyl alcohol active layer, and simplify the preparation process. The reasons for the poor solvent resistance, easy peeling of the protective layer, and complex process of existing polyvinyl alcohol composite membranes are as follows: The polyvinyl alcohol in the cross-linked polyvinyl alcohol active layer coated on the microfiltration base membrane is highly hydrophilic and easily swells in strong polar solvents such as tetrahydrofuran, causing the cross-linked network to be destroyed. This leads to the polyvinyl alcohol active layer peeling off from the base membrane or structural collapse, resulting in decreased dehydration selectivity or even failure of the composite membrane. This results in frequent replacement of the membrane module and increased application costs. The extremely low surface energy and chemical inertness of the protective layer make its interfacial compatibility with the hydrophilic polyvinyl alcohol active layer poor, and the bonding force is weak. Simple physical coating makes the protective layer easy to peel off during operation, failing to provide durable protection for the polyvinyl alcohol active layer. To achieve a strong bond between the protective layer and the polyvinyl alcohol active layer, complex surface pretreatment or multi-layer transition coating processes are required, resulting in cumbersome processes and high difficulty in process control. Based on this, the method for preparing a polyvinyl alcohol composite film for enhancing solvent resistance disclosed herein includes: subjecting hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane to inert gas replacement and preheating treatment to obtain a monomer mixed solution; adding an initiator to the above monomer mixed solution and carrying out a heat-preserving polymerization reaction to obtain a reaction mixture; subjecting the above reaction mixture to vacuum distillation to obtain a first perfluorinated polymer; dissolving the above first perfluorinated polymer in a perfluorinated solvent to obtain a first perfluorinated polymer solution; and adding a co-solvent... The first perfluoropolymer solution is added to the above-mentioned perfluoropolymer solution, and the mixture is stirred to obtain a second perfluoropolymer solution. The second perfluoropolymer solution is then aged and purified to obtain a first coating solution. A crosslinking treatment is performed on a polyvinylidene fluoride solution, tetraethyl orthosilicate, and hydrogen chloride to obtain a second coating solution. Perfluorodecyltriethoxysilane is dissolved in anhydrous ethanol to obtain a third coating solution. Based on the first, second, and third coating solutions, a polyvinylidene fluoride film or a polytetrafluoroethylene film is coated and dried to obtain a polyvinyl alcohol composite film. Because the perfluorodecyltriethoxysilane used has a siloxane group at one end that can hydrolyze and condense with the hydroxyl groups on the surface of the polyvinyl alcohol layer to form Si-O-Si covalent bonds, and its perfluorocarbon chain at the other end can be highly compatible with the outermost perfluoropolymer coating through hydrophobic interactions and van der Waals forces, it can effectively enhance the interfacial bonding between the perfluoroprotective layer and the polyvinyl alcohol active layer, reducing the peeling of the protective layer.Furthermore, after coating with a perfluorinated layer, irradiation with ultraviolet light of a specific wavelength allows the ozone generated in the air to synergistically oxidize the polyvinyl alcohol (PVA) layer surface, generating more hydroxyl functional groups and enhancing interfacial bonding. This improves the structural stability and dehydration performance retention of the composite film in harsh solvent environments. Also, because this invention introduces a silane coupling agent with an amphiphilic structure to achieve strong bonding between the PVA and perfluorinated layers, it reduces the need for complex surface activation processes or multi-layer gradient coating processes typically required in existing technologies to improve interfacial bonding. Therefore, it simplifies the operation and reduces the difficulty of process control.

[0083] The above description is merely a selection of preferred embodiments of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in the embodiments of this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in the embodiments of this disclosure.

Claims

1. A method for preparing a polyvinyl alcohol composite film for enhancing solvent resistance, comprising: Hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane were subjected to inert gas purging and preheating to obtain a monomer mixed solution. An initiator is added to the monomer mixture solution, and a heat-insulated polymerization reaction is carried out to obtain a reaction mixture. The reaction mixture was subjected to vacuum distillation to obtain a first perfluoropolymer; The first perfluoropolymer is dissolved in a perfluoro solvent to obtain a first perfluoropolymer solution; A co-solvent is added to the first perfluoropolymer solution, and the solution is stirred to obtain a second perfluoropolymer solution. The second perfluoropolymer solution is subjected to aging and purification treatment to obtain the first coating solution; A second coating solution is obtained by crosslinking a polyvinyl alcohol solution, tetraethyl orthosilicate, and hydrogen chloride. Dissolve perfluorodecyltriethoxysilane in anhydrous ethanol to obtain the third coating solution; Polyvinylidene fluoride (PVDF) film or polytetrafluoroethylene (PTFE) film is coated and dried using the first coating liquid, the second coating liquid, and the third coating liquid to obtain a polyvinyl alcohol (PVA) composite film.

2. The method according to claim 1, wherein, The process involves inert gas purging and preheating of hexafluoropropylene, a perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane to obtain a monomeric mixed solution, comprising: According to the pressure vessel, hexafluoropropylene, perfluorinated solvent, and perfluoro-2-methylene-4-methyl-1,3-dioxolane are stirred and mixed to obtain an initial mixture, wherein the mass ratio of perfluoro-2-methylene-4-methyl-1,3-dioxolane, hexafluoropropylene, and perfluorinated solvent is (3~5):1:(3~5). The initial mixture was subjected to a nitrogen displacement system to obtain a monomeric mixed solution.

3. The method according to claim 1, wherein, The step of adding the initiator to the monomer mixture solution and carrying out a heat-maintained polymerization reaction to obtain a reaction mixture includes: The monomer mixture is heated to a first preset temperature to obtain a polymerization precursor liquid, wherein the first preset temperature is 30°C. According to the second preset temperature and the initiator, the polymerization precursor liquid is subjected to polymerization reaction treatment to obtain a reaction mixture, wherein the second preset temperature is 40~70℃, the initiator is diisopropyl peroxide dicarbonate, and the amount of the initiator added is 1~2% of the mass of perfluoro-2-methylene-4-methyl-1,3-dioxolane and hexafluoropropylene.

4. The method according to claim 1, wherein, The reaction mixture is subjected to vacuum distillation to obtain a first perfluoropolymer, comprising: The reaction mixture is subjected to vacuum distillation using a distillation apparatus to obtain a first perfluoropolymer, wherein the vacuum distillation temperature of the reaction mixture is 80~90℃ and the vacuum degree is -0.09~-0.099MPa.

5. The method according to claim 1, wherein, The first coating solution is obtained by aging and purifying the second perfluoropolymer solution, comprising: The second perfluoropolymer solution is heated according to a third preset temperature and a preset duration to obtain a heat treatment coating liquid, wherein the third preset temperature is 80~90℃ and the preset duration is 1~3 hours; The coating solution in the heat treatment is cooled and filtered to obtain the first coating solution.

6. The method according to claim 1, wherein, The process of coating and drying a polyvinylidene fluoride (PVDF) film or a polytetrafluoroethylene (PTFE) film using the first coating liquid, the second coating liquid, and the third coating liquid to obtain a polyvinyl alcohol (PVA) composite film includes: The second coating liquid is applied to a polyvinylidene fluoride membrane or a polytetrafluoroethylene membrane, and a first drying process is performed to obtain a first coated film. The first coated film is subjected to ultraviolet light irradiation to obtain a primary composite film; According to the third coating liquid, the primary composite film is coated and dried to obtain a second coated film, wherein the coating thickness after coating and drying is in the range of 10~30μm; The first coating liquid is applied onto the second coating film to obtain a composite film; The composite film is subjected to a second drying process to obtain a polyvinyl alcohol composite film.

7. The method according to claim 1, wherein, The mass percentage concentration of the first perfluoropolymer solution is in the range of 1.0% to 5.0%, and the amount of the co-solvent added is 3% to 5% of the mass of the second perfluoropolymer solution.

8. A polyvinyl alcohol composite film for enhancing solvent resistance, wherein, The polyvinyl alcohol composite film for enhancing solvent resistance is the polyvinyl alcohol composite film for enhancing solvent resistance as described in any one of claims 1-7.

9. An application of a polyvinyl alcohol composite membrane for enhancing solvent resistance in the field of membrane separation technology, wherein, The solvent-resistant polyvinyl alcohol composite film is the polyvinyl alcohol composite film for enhancing solvent resistance as described in any one of claims 1-7.

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