A method for preparing a flexible ion exchange membrane for water purification desalination, and products and applications thereof

Abstract

The application belongs to the technical field of functional polymer membrane materials, and particularly relates to a preparation method of a flexible ion exchange membrane for water purification and desalination, a product thereof and application, comprising melt blending, online compounding and calendering of cation exchange resin powder, a flexible binder containing maleic anhydride grafted polypropylene and thermoplastic polyurethane, and a reinforcing mesh cloth to form a heterogeneous support base film with a mesh reinforcing structure; preparing an anion exchange layer coating liquid containing perfluorosulfonic acid resin and a cation exchange layer coating liquid; respectively casting and coating the anion exchange layer coating liquid and the cation exchange layer coating liquid on two surfaces of the heterogeneous support base film, and after drying and heat treatment, forming a homogeneous functional surface layer which is tightly combined with the base film. The application introduces continuous and dense homogeneous conductive layers on the two sides of the anion and the cation respectively while maintaining the mechanical support and dimensional stability of the heterogeneous membrane, shortens the ion migration path, reduces the membrane surface resistance and improves the interface bonding strength.

Description

Technical Field

[0001] This invention belongs to the field of functional polymer membrane materials technology, specifically relating to a method for preparing a flexible ion exchange membrane for water purification and desalination, as well as its products and applications. Background Technology

[0002] Heterogeneous ion exchange membranes refer to multiphase composite membranes made by using ion exchange resin particles as the ion-conducting phase, a polymer binder as the continuous or quasi-continuous matrix phase, and, when necessary, a porous reinforcing framework (such as polyolefin mesh or nonwoven fabric), through hot pressing, cold pressing lamination, melt mixing, calendering, extrusion, or molding. Unlike homogeneous membranes where the polymer itself is the continuous ion exchange phase, heterogeneous membranes include at least two phases: a resin particle phase and a binder phase. The resin particles are distributed as discrete phases around the binder and framework, achieving mechanical bonding and overall membrane formation through the binder phase. In existing heterogeneous ion exchange membranes, the binder is typically selected from polyolefins such as polyethylene, polyisobutylene, and polyvinyl chloride, or chlorinated polymers. These binders have high crystallinity or restricted chain segment movement, resulting in high elastic modulus and limited ductility, forming a relatively rigid continuous matrix phase after membrane formation. On the one hand, the aforementioned polyethylene, polyisobutylene, and polyvinyl chloride bonding system forms a rigid coating layer around the reinforcing skeleton, which is prone to stress concentration under winding, bending, and thermal cycling conditions, causing microcracks to form on the membrane along the skeleton interface or inside the bonding layer, or even brittle failure. On the other hand, the rigid binder has limited wetting and penetration capabilities relative to the resin particles. The resin particles are mainly connected by point-like "rigid binder bridges," making it difficult to form a continuous and flexible three-dimensional interlocking structure. This results in a high overall flexural modulus and insufficient flexibility of the heterogeneous membrane, making it difficult to meet the requirements of long-term stable operation of spiral wound components, small-space winding, and pressure fluctuations in household water purification and desalination equipment.

[0003] Ion exchange membranes and bipolar membranes are widely used in electrodialysis for salt production, advanced wastewater treatment, and desalination. Ion exchange membranes are functional membrane materials with selective ion conductivity, formed by polymer resins with fixed ion groups in a solvent or binder system. Ion conductivity, mechanical flexibility, and interfacial stability are the main indicators for evaluating membrane material performance. Existing homogeneous resin membranes, such as the homogeneous cation exchange membrane developed by Zhao et al. (X. Zhao, L. Liu, X. Zhang, X. Cheng, J. Sun, J. Pan, Ind. Eng. Chem. Res. 2023, 62, 5945-5953), still require multiple steps of 60 °C swelling-5 h polymerization-sulfonation. This process relies on specific functional monomers and lengthy chemical modifications, resulting in a long process chain, high acid resistance requirements for equipment, and high costs, making it difficult to simultaneously meet the needs of large-scale production and low-cost household applications. Heterogeneous resin membranes are mostly commercially available thick-film structures with high hardness and rigidity. For example, as reported in the literature W. Wang, T. Liang, H. Bai, W. Dong, X. Liu, Carbohydr. Polym. 2018, 179, 297-304, Wang et al. demonstrated with a 180 µm cellulose diacetate / nanofibrillar cellulose (CDA / NFC) heterogeneous membrane that stress concentration occurs during bending due to the interlocking of the rigid skeleton and the reinforcing mesh, and microcracks appear in 15 wt% NFC. Therefore, it is difficult to flexibly assemble and operate stably for a long time in household water purification and desalination equipment with limited space and significant pressure fluctuations. Moreover, during the winding or bending assembly process, stress concentration is easily generated near the reinforcing skeleton, inducing microcracks or even brittle failure. Meanwhile, according to the report in M. Hasan, B. Shrimant, CB Waters, CA Gorski, CG Arges, Small Structures 2024, 5, 2400090, the resin particles inside the heterogeneous membrane are connected by a binder phase, and the ion migration path is a tortuous channel of "particle-binder-particle". The channel is discontinuous and the area resistivity is relatively high (Hasan et al. found at 2 mA / cm). 2 The measured current density is approximately 2.2 Ω·cm. 2 This results in limited overall conduction efficiency; although micropatterning increases the interface area by 2.3 times and reduces the voltage by 1000 mV, it still confirms that the heterogeneous membrane's overall conduction efficiency is limited due to path interruption.

[0004] Therefore, how to construct ion exchange membranes or bipolar membranes with high flexibility, low resistance and high interfacial strength through reasonable formulation design and preparation process has become a key issue in the technological development of this field. Summary of the Invention

[0005] To address the above technical problems, the present invention provides the following technical solution:

[0006] As one aspect of the present invention, the present invention provides a method for preparing a flexible ion exchange membrane for water purification and desalination, which includes the following steps:

[0007] (1) Preparation of heterogeneous support membrane: Ion exchange resin powder is melt-blended, online composited and calendered with a flexible binder containing maleic anhydride-grafted polypropylene and thermoplastic polyurethane, and reinforcing mesh to form a heterogeneous support base membrane with a mesh-reinforced structure; wherein, the ion exchange resin powder is a hydrogen-form strong acid cation exchange resin; the content of the ion exchange resin powder is 40-50 wt% based on the total mass of the raw materials of the heterogeneous support membrane; the mass ratio of maleic anhydride-grafted polypropylene to thermoplastic polyurethane is (0.5-2):1;

[0008] (2) Preparation of coating solution: Prepare anion exchange layer coating solution and cation exchange layer coating solution containing perfluorosulfonic acid resin respectively; wherein, the anion exchange layer coating solution also contains quaternizing agent and crosslinking agent, and the cation exchange layer coating solution also contains chelating sulfonic acid resin.

[0009] (3) Forming a composite membrane: The anion exchange layer coating liquid and the cation exchange layer coating liquid are respectively cast and coated on the two surfaces of the heterogeneous support base membrane. After drying and heat treatment, a homogeneous functional surface layer that is tightly bonded to the base membrane is formed, and the flexible ion exchange membrane for water purification and desalination is obtained.

[0010] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: in step (1), the particle size distribution of the ion exchange resin powder is 10-100 µm; the ion exchange resin includes a gel-type styrene-based hydrogen-type strong acid cation exchange resin with a crosslinking degree of 7%.

[0011] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: in step (1), the melt flow rate of the maleic anhydride-grafted polypropylene in the flexible binder is 25-30 g / 10 min; the melt flow rate of the thermoplastic polyurethane is 18-20 g / 10 min.

[0012] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: in step (1), the melt blending is carried out by a twin-screw extruder, and the online composite is to combine the preheated reinforcing mesh with the melt extruded from the extruder die on a roller pressing device; the reinforcing mesh is made of polyolefin material with an areal density of 50-120 g / m³. 2 .

[0013] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: in step (2), the quaternizing agent is quaternized benzyltriphenylphosphine chloride, the crosslinking agent is N,N,N′,N′-tetramethyl-1,6-hexanediamine, and the chelating sulfonic acid resin is chelating sulfonic acid resin D403.

[0014] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: In step (2), based on the total mass of the anion exchange layer coating liquid, the solid content of the perfluorosulfonic acid resin is 3-8 wt%, the content of the quaternizing agent is 5-15 wt%, and the content of the crosslinking agent is 0.5-2 wt%; based on the total mass of the cation exchange layer coating liquid, the solid content of the perfluorosulfonic acid resin is 3-8 wt%, and the content of the chelating sulfonic acid resin is 3.5-7 wt%.

[0015] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: in step (3), the casting coating adopts slit coating or blade coating; the drying temperature is 50-80 ℃ and the time is 5-20 min; the heat treatment temperature is 100-140 ℃ and the time is 10-30 min; the dry thickness of each homogeneous functional surface layer is 10-20 µm, and the thickness of the heterogeneous support base membrane is 180-220 µm.

[0016] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination according to the present invention: in step (3), before coating the coating liquid, the method further includes a step of solvent wiping and / or corona treatment of the surface of the heterogeneous support base membrane.

[0017] As a preferred embodiment of the preparation method of the flexible ion exchange membrane for water purification and desalination described in this invention: In step (1), the processing temperature of the twin-screw extruder is set sequentially from the feeding section to the die head as 160 ℃, 180 ℃, 190 ℃, 200 ℃, 200 ℃, 195 ℃, the die head temperature is 200 ℃, the screw speed is 120 rpm, and the melt pressure is 8-10 MPa.

[0018] The present invention also provides a flexible ion exchange membrane prepared by the method described above for preparing a water purification and desalination membrane.

[0019] The present invention also provides the application of the flexible ion exchange membrane prepared by the method described above for water purification and desalination in water treatment.

[0020] The beneficial effects of this invention are as follows: The flexible ion exchange membrane of this invention significantly improves flexibility and winding adaptability, significantly improves the bending durability of the membrane, and makes the membrane suitable for spiral wound modules and pressure fluctuation conditions, solving the problems of easy cracking and difficult winding of traditional heterogeneous membranes; The ion conduction resistance of this invention is reduced: The thin homogeneous functional surface layer provides a continuous high ion exchange capacity (IEC) and high water content ion conduction channel, especially the conductivity on the anion exchange side is significantly improved, which improves the defects of discontinuous channels and high resistance of traditional heterogeneous membranes.

[0021] The present invention improves the interface strength and service life: a flexible transition interface is formed between the flexible heterogeneous support layer and the homogeneous functional surface layer, which alleviates the stress concentration caused by expansion mismatch and improves the interface stability and service life of the ion exchange membrane under long-term water purification and desalination operation conditions.

[0022] Therefore, through the above structural design and material selection, the present invention achieves a comprehensive technical effect of flexibility, low resistance and high interface strength while maintaining ion selectivity and mechanical strength. It is significantly different from the existing technology and has significant engineering applicability and promotion value. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below, wherein:

[0024] Figure 1 A simplified schematic diagram of various ion exchange membranes; Figure 1 (a) in the figure represents a homogeneous membrane; Figure 1 (b) in the diagram represents a heterogeneous membrane; Figure 1 (c) in the figure represents a semi-homogeneous ion exchange membrane; Figure 1 (d) in the figure represents a single-sided semi-homogeneous bipolar film; Figure 1 (e) in the figure represents the bipolar membrane prepared in Example 1 of this invention.

[0025] Figure 2 Scanning electron microscope images of cross-sections of resin films with different structures; Figure 2 (a) in the figure represents the cross-section of the heterogeneous resin film; Figure 2 (b) in the figure represents the cross-section of a homogeneous resin film; Figure 2 (c) in the figure represents the cross-section of the resin film with an internal heterogeneous phase and an external homogeneous phase prepared in Example 1 of the present invention.

[0026] Figure 3 The Fourier transform infrared (FTIR) spectrum of the flexible heterogeneous bipolar film in Example 1 is shown.

[0027] Figure 4 The electrochemical performance of the flexible heterogeneous bipolar membrane in Example 1 and the commercial bipolar membranes from Lanran and Tianwei under forward bias is shown. Figure 4(a) in the figure is the current density-voltage curve; Figure 4 (b) in the figure represents the electrochemical impedance spectroscopy (EIS).

[0028] Figure 5 The graph shows the change in desalination rate of the flexible heterogeneous bipolar membrane in Example 1. Detailed Implementation

[0029] To make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples.

[0030] Example 1:

[0031] This invention provides a flexible bipolar ion exchange membrane for water purification and desalination. The resin content in both the cation exchange layer and anion exchange layer is controlled at 45 wt%. The resin powder used for the ion exchange membrane (H-type 001×7 resin, Shanghai Resin Factory Co., Ltd.) has a particle size range of 10-100 µm. The binder phase is a blend of modified polypropylene (PP-g-MAH, maleic anhydride-grafted polypropylene, melt flow rate (MFR) of 25 g / 10 min, 230 ℃ / 2.16 kg) and thermoplastic polyurethane elastomer (TPU, Shore A hardness measured at 85, MFR of 18 g / 10 min), with an areal density of 80 g / m³. 2 Polypropylene (PP) polyolefin mesh (plain weave, pore size 0.25 mm, filament diameter 0.12 mm) was used as a reinforcing skeleton and extruded into a heterogeneous support membrane with a skeleton by melt mixing. Then, a homogeneous resin solution prepared with perfluorosulfonic acid resin (PFSA, equivalent weight EW of 980 g / mol) was cast and coated on both sides and heat-treated to form a thin homogeneous functional surface layer with a thickness of 10 µm. Among them, quaternized benzyltriphenylphosphine chloride (BPP, CAS No.: 1100-88-5, manufacturer: McLean) and a small amount of crosslinking agent N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA) were introduced into the homogeneous layer on the anion exchange layer side, and chelated sulfonic acid resin D403 was partially introduced into the homogeneous layer on the cation exchange layer side.

[0032] The specific preparation steps, methods, and parameters are as follows:

[0033] 1. Extrusion of heterogeneous supported membranes:

[0034] (1) 45 wt% H-type 001×7 cation exchange resin powder (particle size range 10-100 µm, vacuum dried at 120 ℃ for 4 h), 30 wt% modified polypropylene (PP-g-MAH), 24.5 wt% TPU, 0.3 wt% antioxidant 1010 / 168 (antioxidant 1010 and antioxidant 168 are mixed at a mass ratio of 1:1) and 0.2 wt% calcium stearate are stirred and mixed at 800 rpm for 5 min.

[0035] (2) Step (1) The product is extruded by a twin-screw extruder: length-to-diameter ratio L / D=48, screw diameter Ø=35 mm, the processing temperature of the twin-screw extruder is set sequentially from the feeding section to the die head as 160 ℃, 180 ℃, 190 ℃, 200 ℃, 200 ℃, 195 ℃, the die head temperature is 200 ℃, the screw speed is 120 rpm, the melt pressure is 8-10 MPa, and the melt is obtained.

[0036] (3) Online unwinding: After the PP polyolefin mesh is preheated to 180 °C, it is combined with the melt obtained in step (2). Specifically, the melt sheet extruded by the twin screw die is immediately combined with the mesh at the clamping roller.

[0037] (4) Three-roll calendering: The temperature settings of the three-roll calender are: upper roll 85 ℃, middle roll 25 ℃, lower roll 45 ℃, linear speed 2 m / min, and final product thickness 180 µm.

[0038] (5) Trim and roll up to obtain a pure heterogeneous support film with a width of 190 mm.

[0039] 2. Anion exchange homogeneous laminar flow casting solution on the outer surface:

[0040] 5 g of PFSA (5 wt% by volume, N,N-dimethylacetamide and ethanol mixed in a volume ratio of 4:1), 0.5 g of BPP (quaternization degree 98%) and 0.05 g of TMHDA were mixed and stirred at room temperature for 2 h, and then degassed under vacuum for 30 min to obtain anion exchange homogeneous laminar flow casting solution.

[0041] 3. Homogeneous laminar flow solution with cation exchange on the outer surface:

[0042] 5 g of 5 wt% PFSA (solvent is a mixture of N,N-dimethylacetamide and ethanol at a volume ratio of 4:1) and 0.3 g of chelating sulfonic acid resin D403 (particle size ≤5 µm, ion exchange capacity of resin is 1.8 mmol / g) were ultrasonicated for 30 min and degassed under vacuum to obtain a cation exchange homogeneous laminar flow casting solution.

[0043] 4. Outer surface casting and heat treatment:

[0044] (1) The base film was wiped with ethanol at 60 °C and then subjected to corona treatment. The power of the corona treatment was set to 50 W / m.

[0045] (2) Coating of side A (anion exchange side): The heterogeneous support membrane is fed into the slot coater with one side facing upward. The coating gap is controlled so that the wet film thickness is 20 µm. It is dried in a 60 °C oven for 10 min and then heat-treated at 120 °C for 15 min to obtain a dense anion exchange homogeneous layer with a thickness of 10 µm.

[0046] (3) Coating of side B (cation exchange side): Turn the membrane over so that the other side is facing up, and coat the cation exchange layer with the same slit coating parameters as side A (20 µm wet thickness); blow air at 60 °C for 10 min and heat treat at 120 °C for 15 min to form a dense homogeneous cation exchange layer with a thickness of 10 µm on the other side.

[0047] (4) Cooling and winding: After being cooled to room temperature by a 25 ℃ cooling roller, the edges are cut and the film is wound up to obtain an anion exchange homogeneous layer with a thickness of 10 µm, a heterogeneous support layer with a thickness of 180 µm, and a cation exchange homogeneous layer with a thickness of 10 µm. The three layers are stacked in sequence to form a bipolar film with a total thickness of 200 µm.

[0048] The obtained bipolar membrane, measured at 25 °C in 0.5 M sodium chloride solution, exhibited a cation exchange capacity of 1.25 mmol / g, an anion exchange capacity of 1.18 mmol / g, and a surface resistivity of 0.18 Ω·cm. 2 After being repeatedly bent from 0 to 180° 1000 times, there is no delamination.

[0049] Example 2:

[0050] 1. Extrusion of heterogeneous supported membranes:

[0051] (1) 40 wt% H-type 001×7 cation exchange resin powder (particle size range 10-100 µm, vacuum dried at 120 ℃ for 4 h), 25 wt% modified polypropylene (PP-g-MAH, MFR of 30 g / 10 min), 34.5 wt% TPU (MFR of 18 g / 10 min), 0.3 wt% antioxidant 1010 / 168 (antioxidant 1010 and antioxidant 168 are mixed at a mass ratio of 1:1) and 0.2 wt% calcium stearate are stirred and mixed at 800 rpm for 5 min.

[0052] (2) Step (1) The product is extruded by a twin-screw extruder: length-to-diameter ratio L / D=48, screw diameter Ø=35 mm, the processing temperature of the twin-screw extruder is set sequentially from the feeding section to the die head as 160 ℃, 180 ℃, 190 ℃, 200 ℃, 200 ℃, 195 ℃, the die head temperature is 200 ℃, the screw speed is 120 rpm, the melt pressure is 8-10 MPa, and the melt is obtained.

[0053] (3) Online unwinding: After the PP polyolefin mesh is preheated to 180 °C, it is combined with the melt obtained in step (2). Specifically, the melt sheet extruded by the twin screw die is immediately combined with the mesh at the clamping roller.

[0054] (4) Three-roll calendering: The temperature settings of the three-roll calender are: upper roll 85 ℃, middle roll 25 ℃, lower roll 45 ℃, linear speed 2 m / min, and final product thickness 160 µm.

[0055] (5) Trim and roll up to obtain a pure heterogeneous support film with a width of 190 mm.

[0056] 2. Anion-exchange homogeneous laminar flow casting solution on the outer surface:

[0057] 5 g of PFSA (3.5 wt% by volume, N,N-dimethylacetamide and ethanol mixed in a volume ratio of 4:1), 0.3 g of BPP (quaternization degree 98%) and 0.05 g of TMHDA were mixed and stirred at room temperature for 2 h, and then degassed under vacuum for 30 min to obtain anion exchange homogeneous laminar flow casting solution.

[0058] 3. Homogeneous laminar flow casting solution with cation exchange on the outer surface:

[0059] 5 g of 5 wt% PFSA (solvent is a mixture of N,N-dimethylacetamide and ethanol at a volume ratio of 4:1) and 0.2 g of chelating sulfonic acid resin D403 (particle size ≤5 µm, ion exchange capacity of resin is 1.8 mmol / g) were ultrasonicated for 30 min and degassed under vacuum to obtain a cation exchange homogeneous laminar flow casting solution.

[0060] 4. Outer surface casting and heat treatment:

[0061] (1) The base film was wiped with ethanol at 60 °C and then subjected to corona treatment. The power of the corona treatment was set to 50 W / m.

[0062] (2) Coating of side A (anion exchange side): The heterogeneous support membrane is fed into the slot coater with one side facing upward, dried in a 50 ℃ drying tunnel for 20 min, and then heat-treated at 100 ℃ for 30 min to obtain a dense anion exchange homogeneous layer with a thickness of about 10 µm.

[0063] (3) Coating of side B (cation exchange side): Turn the membrane over so that the other side is facing up, and coat the cation exchange layer using the same slit coating parameters as side A; blow air at 50 ℃ for 20 min and heat treat at 100 ℃ for 30 min to form a dense homogeneous cation exchange layer with a thickness of about 10 µm on the other side.

[0064] (4) Cooling and winding: After being cooled to room temperature by a 25 ℃ cooling roller, the edges are cut and the film is wound up to obtain an anion exchange homogeneous layer with a thickness of 10 µm, a heterogeneous support layer with a thickness of 160 µm, and a cation exchange homogeneous layer with a thickness of 10 µm. The three layers are stacked in sequence to form a bipolar film with a total thickness of 180 µm.

[0065] Example 3:

[0066] 1. Extrusion of heterogeneous supported membranes:

[0067] (1) Mix 50 wt% H-type 001×7 cation exchange resin powder (particle size range 10-100 µm, vacuum dried at 120 ℃ for 4 h), 32 wt% modified polypropylene (PP-g-MAH, MFR of 25 g / 10 min), 17.5 wt% TPU (MFR of 20 g / 10 min), 0.3 wt% antioxidant 1010 / 168 (antioxidant 1010 and antioxidant 168 are mixed at a mass ratio of 1:1) and 0.2 wt% calcium stearate at 800 rpm for 5 min.

[0068] (2) Step (1) The product is extruded by a twin-screw extruder: length-to-diameter ratio L / D=48, screw diameter Ø=35 mm, the processing temperature of the twin-screw extruder is set sequentially from the feeding section to the die head as 160 ℃, 180 ℃, 190 ℃, 200 ℃, 200 ℃, 195 ℃, the die head temperature is 200 ℃, the screw speed is 120 rpm, the melt pressure is 8-10 MPa, and the melt is obtained.

[0069] (3) Online unwinding: After the PP polyolefin mesh is preheated to 180 °C, it is combined with the melt obtained in step (2). Specifically, the melt sheet extruded by the twin screw die is immediately combined with the mesh at the clamping roller.

[0070] (4) Three-roll calendering: The temperature settings of the three-roll calender are: upper roll 85 ℃, middle roll 25 ℃, lower roll 45 ℃, linear speed 2 m / min, and final product thickness 180 µm.

[0071] (5) Trim and roll up to obtain a pure heterogeneous support film with a width of 190 mm.

[0072] 2. Anion-exchange homogeneous laminar flow casting solution on the outer surface:

[0073] 5 g of PFSA (8 wt% by volume, N,N-dimethylacetamide and ethanol mixed in a volume ratio of 4:1), 0.8 g of BPP (quaternization degree 98%) and 0.1 g of TMHDA were mixed and stirred at room temperature for 2 h, and then degassed under vacuum for 30 min to obtain anion exchange homogeneous laminar flow casting solution.

[0074] 3. Homogeneous laminar flow casting solution with cation exchange on the outer surface:

[0075] 5 g of PFSA (8 wt% concentration, with N,N-dimethylacetamide and ethanol mixed in a volume ratio of 4:1) and 0.35 g of chelating sulfonic acid resin D403 (particle size ≤ 5 µm, ion exchange capacity of resin 1.8 mmol / g) were ultrasonicated for 30 min and degassed under vacuum to obtain a cation exchange homogeneous laminar flow casting solution.

[0076] 4. Outer surface casting and heat treatment:

[0077] (1) The base film was wiped with ethanol at 60 °C and then subjected to corona treatment. The power of the corona treatment was set to 50 W / m. -1 .

[0078] (2) Coating of side A (anion exchange side): The heterogeneous support membrane is fed into the slot coater with one side facing upward. The coating gap is controlled and the membrane is dried in an 80 ℃ drying tunnel for 5 min. Then it is heat-treated at 140 ℃ for 10 min to obtain a dense anion exchange homogeneous layer with a thickness of about 20 µm.

[0079] (3) Coating of side B (cation exchange side): Turn the membrane over so that the other side is facing up, and coat the cation exchange layer using the same slit coating parameters as side A; blow air at 80 ℃ for 5 min and heat treat at 140 ℃ for 10 min to form a dense homogeneous cation exchange layer with a thickness of about 20 µm on the other side.

[0080] (4) Cooling and winding: After cooling to room temperature by a 25 ℃ cooling roller, the edges are cut and wound up to obtain an anion exchange homogeneous layer with a thickness of 20 µm, a heterogeneous support layer with a thickness of 180 µm, and a cation exchange homogeneous layer with a thickness of 20 µm. The three layers are stacked in sequence to form a bipolar film with a total thickness of 220 µm.

[0081] Comparative Example 1:

[0082] In Example 1, the H-type 001×7 resin was replaced with Dow. TM Marathon TM C sulfonic acid gel resin (IEC≈2.4mmol / g, equivalent weight EW=800 g / mol), particle size range 10-100 µm; other preparation methods are the same as in Example 1.

[0083] Comparative Example 2:

[0084] The H-type 001×7 resin in Example 1 was replaced with Purolite® NRW160 macroporous sulfonic acid resin (IEC≈2.2mmol / g), with a particle size range of 10-100 µm; all other preparation methods were the same as in Example 1.

[0085] Comparative Example 3:

[0086] In Example 1, PP-g-MAH was replaced with an equal mass of PP-g-GMA (glycidyl methacrylate grafted polypropylene, epoxy functional group content 0.8 wt%), and all other preparation methods were the same as in Example 1.

[0087] Comparative Example 4:

[0088] In Example 1, PP-g-MAH was replaced with PP-g-IA (polypropylene grafted with itaconic acid, carboxylic acid functional group content 1.0 wt%), and all other preparation methods were the same as in Example 1.

[0089] Comparative Example 5:

[0090] Preparation of homogeneous bipolar films:

[0091] Ingredients: 5 g of 5 wt% PFSA (solvent is N,N-dimethylacetamide and ethanol mixed at a volume ratio of 4:1), 0.5 g of BPP and 0.05 g of TMHDA are blended to form a homogeneous anion exchange membrane solution; 5 g of 5 wt% PFSA (solvent is N,N-dimethylacetamide and ethanol mixed at a volume ratio of 4:1) is blended with 0.3 g of D403 to form a homogeneous cation exchange membrane solution; no resin powder, PP-g-MAH / TPU, or PP mesh are added.

[0092] The preparation method is as follows: a 20 µm homogeneous anion-exchange membrane solution is first slit-coated onto a polyethylene terephthalate (PET) release film, followed by forced air blowing at 60 ℃ for 10 min and heat treatment at 120 ℃ for 15 min to obtain a 10 µm homogeneous anion membrane layer; then, a cation-exchange homogeneous membrane solution is coated on the same parameters to obtain a 10 µm homogeneous cation membrane layer; the total thickness is 20 µm. Test results: the membrane broke after 200 bends.

[0093] Comparative Example 6:

[0094] Preparation of heterogeneous bipolar films:

[0095] Ingredients: 45 wt% H-type 001×7 cation exchange resin powder (particle size range 10-100µm), 30 wt% PP-g-MAH, 25 wt% TPU and antioxidant are blended and extruded to form a 180 µm reinforced base film; PFSA, BPP and D403 are not added, and surface casting is not performed.

[0096] Preparation: Following the same steps as in Example 1, extrusion-composite-calendering was performed to obtain a 180 µm pure heterogeneous film; the sheet resistivity was measured to be 0.45 Ω·cm. 2 It has a cation exchange capacity of 1.25 mmol / g and anion exchange capacity of 1.18 mmol / g. It does not separate into layers after 1000 bends, but its water dissociation voltage is relatively high at 0.8 V.

[0097] Materials characterization and testing:

[0098] Table 1 compares the mechanical strength of flexible bipolar membranes with different resin-binder systems.

[0099] Table 1

[0100]

[0101] The samples obtained in each embodiment or comparative example were tested according to GB / T 1040.3-2006, under conditions of 23℃ and 50%RH, stretched at 5 mm / min until fracture, and their tensile strength and elongation at break were tested. Then, according to GB / T 232-2010, they were subjected to 1000 bends from 0-180° and their appearance was observed. The mechanical strength of each sample is shown in Table 1. As can be seen from Table 1, Example 1 maintains a balanced high level in tensile strength (28.5 MPa), elongation at break (210%), and "no delamination, no cracks" after 1000 bends, balancing stiffness and toughness, meeting the dual requirements of high-tension winding and long-term fatigue operation of the coiled assembly. Examples 1 and 3 both exhibit superior performance. In contrast, Comparative Example 1 (high IEC gel resin Marathon C) and Comparative Example 2 (high IEC macroporous resin NRW160) showed decreased tensile strengths of 23.1 MPa and 19.4 MPa, respectively, due to reduced resin crosslinking density and weakened particle-binder interface. Microcracks and even obvious cracks appeared after bending, indicating that while simply increasing IEC can improve electrochemical capacity, it comes at the cost of sacrificing mechanical reliability. This conversely proves the balanced advantage of the 001×7 resin selected in Example 1. Comparative Example 3 (using epoxy functional group PP-g-GMA) and Comparative Example 4 (using carboxylic acid functional group PP-g-IA) increased tensile strength to 32.0 MPa and 36.8 MPa through secondary crosslinking, but the elongation at break decreased to 180% and 165%, respectively. The membrane rigidity increased while toughness decreased, making it prone to brittle fracture under repeated winding conditions. Therefore, Examples 1, 2, and 3 achieved optimal performance in the three-dimensional "strength-toughness-electrochemical" index, while simultaneously meeting the requirements of high elongation and no fatigue damage.

[0102] Table 2 compares the stability, ion exchange capacity, and surface roughness of bipolar membranes with different membrane configurations.

[0103] Table 2

[0104]

[0105] According to GB / T 5760-2025, the exchange capacity (mmol / g) of each sample was tested in Table 2. Wet bending tests were conducted according to GB / T 1040.3-2006. Accelerated aging weight loss (%) was obtained under aging conditions of 168 h at 120℃ according to GB / T 2951.12-2008. Surface resistivity (Ω·cm) was measured using the bridge method according to GB / T 3930-2008. 2The results are shown in Table 2. Table 2 shows that the "internal heterogeneous phase - external dual homogeneous phase" configuration used in Example 1 achieved the best balance between wet bending lifetime, chemical stability, total exchange capacity, and sheet resistance: after 1000 0-180° bends, there was no delamination or cracking, which is superior to the 200 fractures of Comparative Example 5; the accelerated aging weight loss was only 1.8%, far lower than the 4.2% of the pure homogeneous membrane, indicating that the dense homogeneous functional outer layer effectively blocked the erosion of the internal heterogeneous region by water and oxidants; the total exchange capacity reached 3.32 mmol / g, significantly higher than the two comparative examples, while the sheet resistance remained at 0.18 Ω·cm. 2 The low resistance of the sample balances high throughput with low energy consumption. In contrast, Comparative Example 5, while having the lowest resistance, suffers from high mechanical brittleness and weight loss, while Comparative Example 6, despite its good bending life and chemical stability, exhibits a high resistance of 0.45 Ω·cm. 2 None of them can meet the four requirements of "flexibility, high capacity, low resistance, and long life" for spiral electric water purification components.

[0106] In this invention, Figure 1 The typical structural forms of various ion exchange membranes and bipolar membranes are illustrated. Figure 1 (a) is a traditional homogeneous membrane, the entire membrane is composed of a single ion-exchange polymer, and the interior is a continuous and homogeneous ion-conducting phase; Figure 1 (b) is a heterogeneous membrane, in which ion exchange resin is dispersed in the form of particles around the polymer binder and reinforcing skeleton, forming a multiphase conduction channel connected in series of "resin particles - binder - resin particles". Figure 1 (c) is a semi-homogeneous ion exchange membrane. A dense homogeneous ion exchange resin layer is coated on one side of the heterogeneous support membrane to form a composite structure of "heterogeneous support layer + thin homogeneous functional layer". Figure 1 (d) is a single-sided semi-homogeneous bipolar membrane, which consists of a bipolar interface formed by mutually bonded cation and anion exchange heterogeneous layers, and a thin homogeneous functional layer is further introduced on the outer surface of one side. Figure 1 (e) in the figure represents the bipolar membrane of this invention, in which a thin homogeneous functional layer is formed on the outer surface of both the cation and anion exchange layers, so that both sides of the bipolar membrane have a continuous and dense homogeneous functional surface layer. This invention retains the advantages of the mechanical support and flexible framework of heterogeneous membranes while introducing a homogeneous functional surface layer to shorten the ion migration path and reduce the membrane surface resistance, providing a structural basis for constructing a highly stable flexible ion exchange membrane.

[0107] Figure 2 (a) is a cross-section of a traditional heterogeneous resin membrane. It can be seen that the interior is composed of irregular resin clumps and binder phase, with a large number of interconnected macropores and gaps. The interface is rough and the channels are tortuous, which reflects the typical multiphase structure characteristics of "particle-binder-particle". Figure 2(b) in the figure is a cross-section of a homogeneous resin film. It has a dense and uniform continuous phase morphology and basically no obvious pores and particle boundaries. It is a homogeneous structure with a single ion exchange polymer as the main component. Figure 2 (c) shows the cross-section of the bipolar film of this invention, which has an internal heterogeneous phase and an external homogeneous phase. The lower part is clearly distinguishable as a porous heterogeneous support layer, and the upper part as a continuous and dense homogeneous functional surface layer. The interface between the two is tightly bonded and the transition is smooth. Compared with a single heterogeneous film, the bipolar film of this invention retains the flexible support and rollable characteristics of the porous heterogeneous layer while constructing a continuous, high-water-content ion conduction channel through the outer homogeneous functional surface layer, significantly shortening the ion migration path and reducing the sheet resistivity. Compared with a single homogeneous thick film, it avoids the problem of brittleness that easily occurs during winding assembly and long-term operation.

[0108] Figure 3 The FTIR spectrum of the self-made flexible heterogeneous bipolar film of this invention is shown. Example 1 of this invention shows the flexible heterogeneous bipolar film at 1439 cm⁻¹. -1 1080 cm -1 and 1026 cm -1 Three distinct absorption peaks appeared at 1439 cm⁻¹, all characteristic vibrations of the O=S=O group in the sulfonic acid group (-SO₃H). -1 This is an asymmetric stretching vibration with O=S=O, 1080 cm. -1 and 1026 cm -1 These correspond to the symmetric stretching and SO stretching vibrations, respectively. This indicates that sulfonic acid functional groups were successfully introduced into the cation exchange membrane framework of the self-made flexible heterogeneous bipolar membrane, thereby forming fixed cation exchange sites within the membrane. Furthermore, apart from the characteristic sulfonic acid peaks, the remaining absorption peaks mainly originate from the skeletal vibrations of the matrix polymer, with no obvious impurity peaks observed, indicating that the sulfonation modification did not destroy the original polymer backbone structure.

[0109] Figure 4 The electrochemical performance of the bipolar membrane prepared in Example 1 under forward bias, compared with commercial bipolar membranes from Lanran (model: AIO-BL2T-50-180, Hangzhou Lanran Technology Co., Ltd.) and Tianwei (model: TWEDBM, Shandong Tianwei Membrane Technology Co., Ltd.), was measured. The bipolar membrane prepared in Example 1, along with the commercial bipolar membranes from Lanran and Tianwei, were tested in an H-type electrolytic cell using 1 M sodium chloride solution as the electrolyte. Figure 4 As shown in (a), the bipolar film prepared in Example 1 achieved a maximum current density of 16 mA·cm at a voltage of 1.0 V (the negative sign in the figure represents the forward bias). -2 Commercial bipolar films, however, exhibit a certain decrease in polarization performance. According to... Figure 4 In the EIS test results of (b) in the example, the bipolar film prepared in Example 1 has the lowest resistance value, indicating that its conductivity is better than that of commercial bipolar films, which corresponds to its polarization performance.

[0110] Figure 5 The demonstration showed that tap water was used as the raw water and the membrane area was 1 m². 2 The desalination rate of the bipolar membrane prepared in Example 1 of this invention was observed under a flow rate of 2 L / min. As the volume of water treated increased, the desalination rate gradually decreased, indicating an increasing burden on the membrane. This could lead to surface fouling or a decrease in ion conductivity, thus affecting desalination performance. The fact that the desalination rate remained above 50% after purifying 20 L of water demonstrates the excellent antifouling properties of the bipolar membrane prepared in this invention.

[0111] In summary, this invention involves coating a layer of homogeneous ion exchange resin onto the surface of a flexible heterogeneous support membrane, preferably forming a continuous and dense homogeneous anion conduction layer on the surface of anion exchange heterogeneous membrane. Anion exchange layer coating solution and cation exchange layer coating solution are respectively cast and coated onto the two surfaces of the heterogeneous support base membrane. After drying and heat treatment, a homogeneous functional surface layer tightly bonded to the base membrane is formed, thereby constructing a composite membrane structure of "flexible heterogeneous support layer + thin homogeneous functional surface layer". While maintaining the mechanical support and dimensional stability of the heterogeneous membrane, continuous and dense homogeneous functional surface layers are introduced to both the anion and cation sides, shortening the ion migration path, reducing membrane surface resistance, and improving interfacial bonding strength. This is expected to realize a flexible, rollable, low-resistance, and highly stable ion exchange membrane or bipolar membrane for use in household water purification and desalination equipment.

[0112] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a flexible ion exchange membrane for water purification and desalination, characterized in that: Includes the following steps: (1) Preparation of heterogeneous support membrane: Ion exchange resin powder is melt-blended, online composited and calendered with a flexible binder containing maleic anhydride-grafted polypropylene and thermoplastic polyurethane, and reinforcing mesh to form a heterogeneous support base membrane with a mesh-reinforced structure; wherein, the ion exchange resin powder is a hydrogen-form strong acid cation exchange resin; the content of the ion exchange resin powder is 40-50 wt% based on the total mass of the raw materials of the heterogeneous support membrane; the mass ratio of maleic anhydride-grafted polypropylene to thermoplastic polyurethane is (0.5-2):1; (2) Preparation of coating solution: Prepare anion exchange layer coating solution and cation exchange layer coating solution containing perfluorosulfonic acid resin respectively; wherein, the anion exchange layer coating solution also contains quaternizing agent and crosslinking agent, and the cation exchange layer coating solution also contains chelating sulfonic acid resin. (3) Forming a composite membrane: The anion exchange layer coating liquid and the cation exchange layer coating liquid are respectively cast and coated on the two surfaces of the heterogeneous support base membrane. After drying and heat treatment, a homogeneous functional surface layer that is tightly bonded to the base membrane is formed, and the flexible ion exchange membrane for water purification and desalination is obtained.

2. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1, characterized in that: In step (1), the particle size distribution of the ion exchange resin powder is 10-100 µm; the hydrogen-form strong acid cation exchange resin includes a gel-type styrene-based hydrogen-form strong acid cation exchange resin with a crosslinking degree of 7%.

3. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1 or 2, characterized in that: In step (1), the melt flow rate of the maleic anhydride-grafted polypropylene in the flexible adhesive is 25-30 g / 10 min; the melt flow rate of the thermoplastic polyurethane is 18-20 g / 10 min.

4. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1 or 2, characterized in that: In step (1), the melt blending is carried out using a twin-screw extruder, and the online compounding is performed by compounding the preheated reinforcing mesh with the melt extruded from the extruder die on a roller pressing device; the reinforcing mesh is made of polyolefin material with an areal density of 50-120 g / m². 2 .

5. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1 or 2, characterized in that: In step (2), the quaternizing agent is quaternized benzyltriphenylphosphine chloride, the crosslinking agent is N,N,N′,N′-tetramethyl-1,6-hexanediamine, and the chelating sulfonic acid resin is chelating sulfonic acid resin D403.

6. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1 or 2, characterized in that: In step (2), based on the total mass of the anion exchange layer coating solution, the solid content of the perfluorosulfonic acid resin is 3-8 wt%, the content of the quaternizing agent is 5-15 wt%, and the content of the crosslinking agent is 0.5-2 wt%; based on the total mass of the cation exchange layer coating solution, the solid content of the perfluorosulfonic acid resin is 3-8 wt%, and the content of the chelating sulfonic acid resin is 3.5-7 wt%.

7. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1 or 2, characterized in that: In step (3), the casting coating is carried out by slit coating or blade coating; the drying temperature is 50-80 ℃ and the time is 5-20 min; the heat treatment temperature is 100-140 ℃ and the time is 10-30 min; the dry thickness of each homogeneous functional surface layer is 10-20 µm, and the thickness of the heterogeneous support base film is 180-220 µm. Before applying the coating liquid, the process includes a step of solvent wiping and / or corona treatment of the surface of the heterogeneous support substrate film.

8. The method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 4, characterized in that: In step (1), the processing temperature of the twin-screw extruder is set sequentially from the feeding section to the die head as 160 ℃, 180 ℃, 190 ℃, 200 ℃, 200 ℃, 195 ℃, the die head temperature is 200 ℃, the screw speed is 120 rpm, and the melt pressure is 8-10 MPa.

9. The flexible ion exchange membrane prepared by the method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1.

10. The application of the flexible ion exchange membrane prepared by the method for preparing a flexible ion exchange membrane for water purification and desalination according to claim 1 in water treatment.

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