Acid and alkali resistant high-strength nanofiltration membrane and preparation method thereof

Abstract

The invention relates to the technical field of polyamide nanofiltration membranes, in particular to an acid and alkali resistant high-strength nanofiltration membrane and a preparation method thereof. The problem that the strength and the acid and alkali resistance of a polyamide nanofiltration membrane in the prior art cannot be considered at the same time is solved. The base membrane material of the polyamide nanofiltration membrane is modified polyethersulfone and is obtained by chemically bonding a carbon nanotube subjected to double surface modification by titanium dioxide sol-gel and a KH550 coupling agent in a polyester non-woven fabric, and the modulus of the material is improved by utilizing an inorganic-organic hybrid network; the functional layer of the polyamide nanofiltration membrane is made of modified polyamide, and the modified polyamide functional layer is formed by forming a polyamide matrix through polymerization reaction of 2, 2 '-disulfonic acid-4, 4'-diaminobiphenyl, piperazine and trimesoyl chloride and introducing 4-amino-2, 2, 6, 7-tetramethyl-1, 3-pentanediol at the same time. And the chemical resistance is improved by a free radical capturing group generated by 2, 2, 6, 6-tetramethylpiperidine and octa-isobutyl cage type silsesquioxane.

Description

Technical Field

[0001] This invention relates to the field of polyamide nanofiltration membrane technology, specifically to an acid and alkali resistant high-strength nanofiltration membrane and its preparation method. Background Technology

[0002] Polyamide nanofiltration membrane technology is widely used in water treatment, and its core lies in the performance of the membrane material. Currently, commercial nanofiltration membranes mainly use polyethersulfone as the support material and polyamide as the separation layer material. Among them, the modified polyamide functional layer is mostly formed by the chemical reaction of m-phenylenediamine and trimesoyl chloride, and it exhibits excellent desalination performance due to the dense network structure in its molecular chain. However, existing polymer membrane materials have significant chemical and physical stability defects under special operating conditions. Traditional polyamide / polyethersulfone materials are prone to conformational rearrangement and physical creep under prolonged high pressure, leading to overall material densification, decreased membrane porosity, and irreversible flux degradation. Furthermore, purely physically blended inorganic fillers lack chemical bonding, making it difficult to restrict the microscopic movement of polymer chains, resulting in poor overall strength of polyamide nanofiltration membranes. Because the polyamide molecular backbone contains numerous amide bonds, these bonds are highly susceptible to hydrolysis under strong acid or alkali conditions, leading to polymer degradation. In addition, the nitrogen atoms on the amide bonds possess lone pairs of electrons, making them highly vulnerable to electrophilic attacks from oxidants, resulting in N-chlorination reactions, which in turn trigger benzene ring rearrangement and backbone breakage, causing the membrane material to lose its separation function.

[0003] In summary, existing technologies alleviate the above problems by optimizing the physical structure, but fail to fundamentally solve the inherent shortcomings of polyamide materials in terms of weather resistance, making it difficult to balance high mechanical strength with excellent acid and alkali resistance.

[0004] To address this, an acid and alkali resistant high-strength nanofiltration membrane and its preparation method are proposed. Summary of the Invention

[0005] The purpose of this invention is to provide an acid and alkali resistant, high-strength nanofiltration membrane and its preparation method. The nanofiltration membrane of this invention comprises a modified polyethersulfone base membrane and a modified polyamide functional layer; the base membrane contains titanium dioxide / KH550-coated carbon nanotubes, constructing a rigid framework and eliminating interfacial defects; the functional layer is formed through interfacial polymerization, containing a crosslinked network of 2,2'-disulfonic-4,4'-diaminobiphenyl, piperazine, and trimesoyl chloride, and introducing 4-amino-2,2,6,6-tetramethylpiperidine and octaisobutyl cage-like silsesquioxane; this invention significantly improves the pressure resistance and separation stability of the nanofiltration membrane under extreme conditions through inorganic framework reinforcement, intermediate layer chemical anchoring, and synergistic regulation of functional monomer charge steric hindrance, making it suitable for treating highly challenging industrial wastewater.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing an acid and alkali resistant, high-strength nanofiltration membrane, comprising the following steps: The casting solution was coated onto the surface of a polyester nonwoven fabric with a surface density of 80-100 g / m², an ambient temperature of 25°C, and a humidity of 45%. After coating, the liquid film was left to stand in the air for 15 seconds before entering the coagulation bath. The film was then smoothly and obliquely inserted into the coagulation bath and immersed in pure water at 25°C for coagulation. It was then transferred to a rinsing tank and soaked in warm water at 40-50°C to remove residual solvent and PVP, thus obtaining the modified base film. Rinse the modified base film with deionized water for 1-2 minutes, fix it on a glass plate, pour the coating solution onto the film surface, keep the liquid surface covered, and let it stand at room temperature for 30-60 minutes. After the reaction is complete, drain the coating solution and rinse the film surface with deionized water for 3-5 minutes. After rinsing, the coated base film is obtained. The aqueous phase solution is dip-coated onto the surface of the base membrane to obtain the treated base membrane. The coating contact time is controlled at 1.5-2.5 min. Excess aqueous phase solution on the membrane surface is removed using a precision air knife, maintaining an air knife pressure of 0.2-0.4 MPa and a distance of 1-2 cm, so that the membrane surface is free of visible droplets and presents a matte, moist state. The oil phase solution is uniformly coated onto the surface of the treated base membrane using a flexible spray coating method, controlling the reaction time at 40-60 s to obtain the membrane sheet. The membrane sheet is placed vertically and allowed to drain off excess oil phase solution naturally. The membrane sheet undergoes a two-stage heat treatment: first, it is pre-cured at a low temperature, then heated to 60℃ and pre-cured for 3 min; then, it is cross-linked and cured at 90-105℃ for 5-8 min to obtain the cured composite membrane. The cured composite membrane is immersed in a 0.2% sodium carbonate aqueous solution at 40-50℃ for 10 min; then immersed in a 5% ethanol aqueous solution at 30℃ for 10 min; finally, it is immersed in a 1% sodium bisulfite solution and sealed in packaging to obtain a high-strength nanofiltration membrane.

[0007] Preferably, the preparation of the casting solution includes the following steps: 0.8-1.5% of modified filler is added to a reaction vessel, NMP is added at room temperature, and ultrasonic dispersion is carried out for 60-90 minutes at a power of 400-600W. After dispersion, the mixture is switched to paddle mechanical stirring at a speed of 300-500 rpm to obtain a suspension. 4-6% of PVP K30 is added to the suspension in three equal batches, the temperature is raised to 45-50℃, and the mixture is stirred at 200 rpm for 1-2 hours to obtain a transparent solution. The temperature is raised to 60-65℃, 20-25% of PES is slowly added, the stirring speed is reduced to 60-100 rpm, and the mixture is stirred at a constant temperature for 10-12 hours. The mixture is then transferred to a sealed container, kept at 40-45℃ for 24 hours, and then vacuumed for 2-4 hours at 50℃ under a vacuum of -0.08 to -0.1 MPa to obtain the casting solution. The percentage dosage is calculated relative to the total mass of the casting solution, with the remainder being NMP.

[0008] Preferably, the preparation of the modified filler includes the following steps: 1g of multi-walled carbon nanotubes are added to 100ml of a mixed acid solution, wherein the volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed acid is 3:1, and the mixture is ultrasonically treated at 60℃ for 2-4h; deionized water is added for dilution, centrifugation is performed, and the mixture is washed with deionized water until neutral, and then vacuum dried at 80℃ for 2h to obtain acidified carbon nanotubes; the acidified carbon nanotubes are dispersed in anhydrous ethanol and ultrasonically dispersed for 30min to obtain a dispersion system; wherein the ultrasonic power is 400-600W and the ultrasonic frequency is 40KHz; tetrabutyl titanate and glacial acetic acid are added to the dispersion system, wherein the mass ratio of tetrabutyl titanate to carbon nanotubes is 3-5:1. A 75% ethanol aqueous solution was slowly added dropwise at 500 rpm to control the hydrolysis rate and promote the heterogeneous nucleation hydrolysis reaction for 2 hours. After the reaction was completed, the mixture was centrifuged and calcined at 400-500℃ under air protection to obtain the reactant. The reactant was dispersed in a 95% ethanol aqueous solution, and KH550 was added, with the mass of KH550 being 10-15% of the carbon nanotubes. The pH was adjusted to 4-5, the temperature was raised to 70-80℃, and the mixture was refluxed for 2 hours. After separation and centrifugation, the mixture was vacuum dried at 80℃ for 2 hours to obtain the modified filler, in which a titanium dioxide coating layer with a thickness of 5-10 nm was formed to coat the acidified carbon nanotubes.

[0009] Preferably, the preparation of the coating solution includes the following steps: adding deionized water and tris(hydroxymethyl)aminomethane to a mixing tank, stirring to dissolve and controlling the volume concentration to 3.5 g / L; then slowly adding 10% dilute hydrochloric acid to adjust the pH to 8.3-8.8 to obtain a buffer solution; adding polyethyleneimine with a molecular weight of 1000-2000 Da to the buffer solution, controlling the volume concentration to 0.3-0.8 g / L, stirring at 100 rpm for 5-10 min to obtain a mixed solution, with the volume concentration calculated relative to the volume of the buffer solution; before coating, adding dopamine hydrochloride powder to the mixed solution, maintaining its volume concentration at 1.8-2.5 g / L, stirring at 200 rpm for 3-5 min to obtain the coating solution, with this volume concentration calculated based on the total volume of the coating solution.

[0010] Preferably, the preparation of the aqueous solution includes the following steps: adding deionized water to a mixing tank, starting the stirrer, adding triethylamine, stirring at 100 rpm until homogeneous, adjusting the pH to 10-11, adding 0.4%-0.8% of 2,2'-disulfonic-4,4'-diaminobiphenyl (a soluble salt will be formed during the addition process), stirring at 200 rpm for 10-20 min until the solution changes from turbid to clear and bright yellow; then adding 1.2%-1.8% of piperazine and 0.1%-0.3% of 4-amino-2,2,6,6-tetramethylpiperidine sequentially, stirring for 10 min to dissolve; adding 0.05-0.1% of sodium dodecyl sulfate, stirring at 50 rpm for 5 min, filtering with a 0.45 μm filter cartridge to remove undissolved particles, and obtaining the aqueous solution; the percentage amounts are calculated relative to the total mass of the aqueous solution.

[0011] Preferably, the preparation of the oil phase solution includes the following steps: adding isoparaffin solvent Isopar G to a dry container, slowly adding 0.12-0.18% trimesoyl chloride; then adding 0.02% octaisobutyl cage-type silsesquioxane, controlling the relative humidity to be less than 40%, purging with nitrogen for protection, and stirring at 200 rpm for 30 min to obtain the oil phase solution; the percentage amount is calculated relative to the total mass of the oil phase solution; humidity has a significant impact on trimesoyl chloride, and it is necessary to strictly control the humidity to be less than 40% during the preparation process, and add it to a dry container.

[0012] The present invention also provides an acid and alkali resistant high-strength nanofiltration membrane, comprising a modified polyethersulfone base membrane forming a support layer, a modified polyamide functional network forming a functional layer, and an intermediate crosslinking layer composed of polyethyleneimine and polydopamine; the support layer comprises polyethersulfone, multi-walled carbon nanotubes, titanium dioxide, and 3-aminopropyltriethoxysilane; the intermediate crosslinking layer comprises polyethyleneimine, dopamine hydrochloride, and tris(hydroxymethyl)aminomethane; the modified polyamide functional network comprises piperazine, triethylamine, sodium dodecyl sulfate, 2,2'-disulfonic-4,4'-diaminobiphenyl, 4-amino-2,2,6,6-tetramethylpiperidine, isoparaffin solvent, trimesoyl chloride, and octaisobutylsilsesquioxane; the functional layer is anchored to the surface of the modified polyethersulfone base membrane through the intermediate crosslinking layer.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention introduces titanium dioxide and KH550-modified CNTs into the matrix. The abundant hydroxyl groups on the surface of titanium dioxide not only improve the hydrophilicity of the membrane, but also act as an intermediate medium to eliminate non-selective voids at the organic-inorganic interface. In addition, hydrophobic POSS opens up nanoscale voids in the dense polyamide network, constructing rapid water channels and offsetting the resistance caused by dense crosslinking. At the same time, the sulfonic acid groups introduced by DSDA endow the membrane surface with a high density of negative charges. Through the triple mechanism of hydrophilic substrate, nanopores and charge repulsion, the balance between permeability and selectivity is effectively solved.

[0014] 2. This invention addresses the stress concentration defects caused by the agglomeration of fillers in traditional doped membranes. It utilizes a sol-gel method to grow a titanium dioxide layer in situ on the surface of CNTs, achieving stable chemical bonding through a KH550 coupling agent. The modified CNTs are uniformly dispersed in the PES matrix. When the membrane is subjected to tensile force, the stress is efficiently transferred to the high-modulus carbon nanotubes through chemical bonds, preventing interfacial slippage and the propagation of microcracks. This solves the brittleness problem caused by poor compatibility of inorganic fillers, improving the processing toughness and service life of nanofiltration membranes.

[0015] 3. This invention regulates the phase transformation rate by adding PVP in stages, forming a uniform sponge-like pore structure. Combined with the rigid microstructure constructed by modified CNTs, it directly supports the membrane pore walls and restricts the displacement and rearrangement of polymer chain segments under pressure. The resulting nanofiltration membrane has extremely excellent dimensional stability and pressure resistance, and is suitable for high-pressure reverse osmosis pretreatment and deep concentration processes.

[0016] 4. On the one hand, this invention utilizes the large steric hindrance framework and strongly negatively charged sulfonic acid groups of DSDA to construct an electrostatic shielding layer, effectively blocking the nucleophilic attack of acid and base ions on the amide bonds; on the other hand, it utilizes the strong adhesion and abundant amine groups of the PDA / PEI interlayer to chemically bond the ultrathin functional layer to the surface of the base membrane, preventing the skin layer from peeling off under the shear force of cross-flow cleaning; it exhibits excellent chemical cleaning resistance and significantly reduces the replacement frequency of nanofiltration membranes in industrial applications.

[0017] 5. This invention grafts 4-amino-2,2,6,6-tetramethylpiperidine into the interfacial polymerization network. The oxidative environment is converted into nitryl radicals, which act as highly efficient hindered amine radical scavengers. These radicals preferentially react with the active radicals generated by hypochlorous acid, sacrificing themselves to protect the polyamide backbone from breakage and rearrangement. After immersion in 1000ppm sodium hypochlorite solution, the retention performance of this invention is almost undamaged, solving the industry pain point that existing nanofiltration membranes cannot tolerate chlorinated wastewater or require frequent dechlorination, and expanding its application potential in the treatment of harsh wastewater from printing and dyeing, papermaking and other industries. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the high-pressure pure water flux change rate of the nanofiltration membranes obtained in Embodiment 1 and Comparative Examples 7-11 of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The hydrophobic POSS is octaisobutyl cage-type silsesquioxane; the multi-walled carbon nanotubes have an outer diameter of 10-20 nm, a length of 10-30 μm, and a purity >95%; PES is polyethersulfone with a weight-average molecular weight of 55,000-65,000 Da; PVP K30 has a K value of 27-32 and a weight-average molecular weight of 45,000-55,000 Da; PEI is polyethyleneimine, exhibiting a branched structure, with a weight-average molecular weight of 1800 Da; the isoparaffin solvent of this invention is Isopar G with a flash point greater than 60 °C; DSDA is 2,2'-disulfonic acid-4,4'-diaminobiphenyl; KH550 is 3-aminopropyltriethoxysilane; 2,2'-disulfonic acid-4,4'-diaminobiphenyl is 4,4'-diaminobiphenyl-2,2'-disulfonic acid, CAS number 117-61-3.

[0021] Please see Figure 1 This invention provides an acid and alkali resistant high-strength nanofiltration membrane and its preparation method, the technical solution of which is as follows: Example

[0022] 1 g of multi-walled carbon nanotubes (MWCNTs) were added to 100 ml of a mixed acid solution (the volume ratio of concentrated sulfuric acid to concentrated nitric acid was 3:1). The mixture was sonicated at 60 °C for 3 h. After dilution with deionized water, the mixture was centrifuged, washed with deionized water until neutral, and vacuum dried at 80 °C for 2 h to obtain acidified carbon nanotubes. The acidified carbon nanotubes were dispersed in anhydrous ethanol and sonicated for 30 min to obtain a dispersion system (ultrasonic power 500 W, ultrasonic frequency 40 kHz). Tetrabutyl titanate was added to the dispersion system (mass ratio of tetrabutyl titanate to carbon nanotubes 4:1), and a 75% ethanol aqueous solution was slowly added dropwise. The mixture was hydrolyzed for 2 h. After the reaction was complete, the mixture was centrifuged and calcined at 400 °C under air protection to obtain the reactant. The reactant was dispersed in a 95% ethanol aqueous solution, and KH550 (12% of the carbon nanotubes by mass) was added. The pH was adjusted to 4, the temperature was raised to 80 °C, and the mixture was refluxed for 2 h. After separation and centrifugation, the mixture was vacuum dried at 80 °C for 2 h to obtain the modified filler. 1% of the modified filler was added to the reactor, and NMP was added at room temperature. The mixture was ultrasonically dispersed for 90 minutes at a power of 400W. After dispersion, the mixture was switched to a paddle mixer at 300 rpm to obtain a suspension. 6% of PVP K30 was added to the suspension in three equal batches. The mixture was heated to 50℃ and stirred at 200 rpm for 2 hours to obtain a transparent solution. The mixture was then heated to 65℃, and 22% of PES was slowly added. The stirring speed was reduced to 80 rpm, and the mixture was stirred at a constant temperature for 10 hours. The mixture was then transferred to a sealed container and kept at 40℃ for 24 hours. Under a vacuum of -0.1 MPa, the mixture was kept at 50℃ and vacuumed for 3 hours to remove bubbles and obtain the casting solution. Using a continuous coating machine, the casting solution was coated onto the surface of a polyester nonwoven fabric at an ambient temperature of 25°C and a humidity of 45%. After coating, the liquid film was left to stand in the air for 15 seconds before entering the coagulation bath. The film was then smoothly and obliquely inserted into the coagulation bath and immersed in 25°C pure water for coagulation. It was then transferred to a rinsing tank and rinsed with 45°C warm water to remove residual solvent and PVP, thus obtaining the modified base film. Add deionized water and tris(hydroxymethyl)aminomethane to a mixing tank, stir to dissolve, and control the volume concentration to 3.5 g / L. Then slowly add 10% dilute hydrochloric acid to adjust the pH to 8.5 to obtain a buffer solution. Add polyethyleneimine (molecular weight 1800 Da) to the buffer solution and control the mass concentration to 0.5 g / L. Stir at 100 rpm for 8 min to obtain a mixture. Before coating, add dopamine hydrochloride powder to the mixture to maintain its concentration at 2.0 g / L. Stir at 200 rpm for 5 min to obtain a coating solution. Rinse the modified base film with deionized water for 2 min, fix it on a glass plate, pour the coating solution onto the film surface, keep the liquid surface covered, and let it stand at room temperature for 40 min. After the reaction is complete, drain the coating solution, rinse the film surface with deionized water for 5 min, and obtain the coated base film. Add deionized water to the mixing tank, start stirring, add triethylamine, stir at 100 rpm until homogeneous, adjust the pH to 10, add 0.6% 2,2'-disulfonic-4,4'-diaminobiphenyl, stir at 200 rpm for 10 min until the solution changes from turbid to clear bright yellow; then add 1.5% piperazine and 0.2% 4-amino-2,2,6,6-tetramethylpiperidine sequentially, stir for 10 min to dissolve; add 0.08% sodium dodecyl sulfate, stir at 50 rpm for 5 min, filter using a 0.45 μm filter cartridge to remove undissolved particles, and obtain a clear aqueous solution; Add Isopar G solvent to a dry container, slowly add 0.15% trimesoyl chloride; then add 0.02% octaisobutyl cage silsesquioxane, and stir at 200 rpm for 30 min under sealed conditions to obtain an oil phase solution. The sealed conditions require humidity control to be less than 40% and nitrogen protection. Aqueous solution was dip-coated onto the surface of a substrate membrane to obtain a treated substrate membrane. The coating contact time was controlled at 2 minutes. Excess aqueous solution on the membrane surface was removed using a precision air knife, maintaining an air knife pressure of 0.3 MPa and a distance of 1.5 cm, resulting in a matte, wet surface free of visible droplets. Oil solution was then uniformly coated onto the treated substrate membrane surface using a flexible spray coating method, with a reaction time controlled at 60 seconds to obtain a membrane sheet. The membrane sheet was placed vertically and allowed to drain excess oil solution naturally. The membrane sheet underwent a two-stage heat treatment: first, low-temperature pre-curing was performed by raising the temperature to 60°C and pre-curing for 3 minutes; then, cross-linking curing was performed at 100°C for 6 minutes to obtain a cured composite membrane. The cured composite membrane was then immersed in a 0.2% sodium carbonate aqueous solution at 50°C for 10 minutes; then immersed in a 5% ethanol aqueous solution at 30°C for 10 minutes; finally, it was immersed in a 1% sodium bisulfite solution and sealed in packaging to obtain a high-strength nanofiltration membrane.

[0023] Examples 2-4 follow the same preparation method and parameters as Example 1, with differences shown in Table 1.

[0024] Table 1. Parameter variations in Examples 1-4 Example Mass ratio of tetrabutyl titanate to carbon nanotubes KH550 as a percentage of carbon nanotube usage Modified filler content in casting solution (%) Polyethersulfone as a percentage of casting solution (by mass) Polyethyleneimine mass concentration / g / L Dopamine hydrochloride concentration (g / L) Example 1 4:1 12 1 22 0.5 2.0 Example 2 3:1 10 0.8 20 0.3 1.8 Example 3 5:1 15 1.5 25 0.8 2.5 Example 4 4:1 14 1.2 24 0.6 2.2 Comparative Example 1: Refer to Example 1, except that tetrabutyl titanate is not introduced into the modified filler, while the other amounts remain unchanged.

[0025] Comparative Example 2 is the same as Example 1, except that the casting solution is not prepared by stepwise mixing and addition, but by direct mixing and stirring, followed by vacuum degassing.

[0026] Comparative Example 3 is the same as Example 1, except that only polyethyleneimine is used in the preparation of the coating liquid, and dopamine hydrochloride is not added.

[0027] Comparative Example 4 is the same as Example 1, except that only dopamine hydrochloride is used in the preparation of the coating liquid, and polyethyleneimine is not used.

[0028] Comparative Example 5 is the same as Example 1, except that 4,4'-diaminobiphenyl is used instead of 2,2'-disulfonic-4,4'-diaminobiphenyl in the aqueous solution, with the dosage adjusted accordingly.

[0029] Comparative Example 6 is the same as Example 1, except that octaisobutylcage-type silsesquioxane is not added to the oil phase solution, while the amounts of the other components remain unchanged.

[0030] Experiment Example 1: Water Flux and Retention Rate Test The high-strength nanofiltration membranes obtained in Examples 1-4 and Comparative Examples 1-6 were tested for water flux and rejection rate according to GB / T GB / T 36137-2018. A sodium sulfate aqueous solution of 2000 mg / L was used, the test temperature was 25℃, the operating pressure was 0.6 MPa, and the membrane was pre-pressed at 1.0 MPa for 30 min before being adjusted to the test pressure. The test results are shown in Table 2.

[0031] Table 2 Test Results of Examples and Comparative Examples Example Water flux / L / m²·h Retention rate / % Example 1 58.5 98.8 Example 2 52.1 97.5 Example 3 51.8 99.2 Example 4 55.2 98.4 Comparative Example 1 42.3 94.2 Comparative Example 2 35.6 88.5 Comparative Example 3 50.1 91.3 Comparative Example 4 45.5 93.6 Comparative Example 5 28.4 85.7 Comparative Example 6 32.8 98.1 As shown in Table 2, the performance of the high-strength nanofiltration membrane obtained in the comparative example, obtained by adjusting the components and process, was significantly lower than that in the example. In Comparative Example 1, without titanium dioxide as an intermediate medium, the efficiency of KH550 directly grafted onto CNTs was much lower than that grafted onto the hydroxyl-rich titanium dioxide surface, resulting in poor interfacial compatibility between CNTs and the PES matrix, and the formation of non-selective micropores at the interface, leading to a significant decrease in the retention rate. Tetrabutyl titanate was used to coat the surface of carbon nanotubes with titanium dioxide, forming a hydrophilic nanocoating. Combined with the grafting effect of the silane coupling agent, this improved the filler's performance. Dispersibility and membrane hydrophilicity; In Comparative Example 2, without a stepwise mixing process, due to the extremely high specific surface area and van der Waals forces of the nanomaterials, they are prone to agglomeration. The filler forms micron-sized agglomerates in the casting solution, which not only fail to enhance membrane strength but also become large defect points, resulting in incomplete skin coverage, a coarse microstructure of the modified base membrane, and loss of the high-precision separation function of the nanofiltration membrane; The results of Comparative Example 3 show that dopamine hydrochloride self-polymerizes to form a polydopamine layer, providing anchoring points to enhance the adhesion of polyethyleneimine. Without the addition of dopamine hydrochloride, PEI is difficult to firmly adsorb onto the relatively inert PES base membrane surface, resulting in interfacial... During polymerization, the bonding force between the PA layer and the base film is weak. Under high pressure testing, the skin layer is prone to local peeling or slippage, resulting in unstable and low retention rates. Combined with the results of Comparative Example 4, it can be seen that PEI is rich in amine groups, acting as both a crosslinking agent for the intermediate layer and participating in interfacial polymerization. Lacking polyethyleneimine and relying solely on dopamine, the intermediate layer lacks density and sufficient amine monomer reserves, leading to an excessively thin polyamide layer with insufficient crosslinking, decreased retention rate, and poorer compressive strength. In Comparative Example 5, DSDA contains two strongly hydrophilic sulfonic acid groups. Replacing them with hydrophobic common benzidine significantly reduces the hydrophilicity of the membrane surface, allowing water molecules to permeate. The resistance increases dramatically; at the same time, the nanofiltration membrane mainly relies on the repulsion effect to retain divalent salts. DSDA imparts a high density of negative charges to the membrane surface, strongly repelling negatively charged sulfate ions. The loss of sulfonic acid groups weakens the membrane's electrical properties and significantly degrades its retention capacity. The results of Comparative Example 6 show that during the interfacial polymerization process, octaisobutyl cage-like silsesquioxane, due to its incompatibility with hydrophilic monomers, opens up nanoscale pores in the dense polyamide layer, forming water channels. Without the addition of octaisobutyl cage-like silsesquioxane, the polyamide layer is too dense. Although the retention rate is maintained slightly, the higher water flux is sacrificed, and the nanofiltration membrane loses its application value.

[0032] In summary, this invention significantly improves the compressibility of the PES-based membrane by using modified CNTs as a rigid framework, ensuring that the flow channel does not collapse under high pressure. The stepwise chemical bonding of titanium dioxide / KH550 / intermediate layer solves the compatibility problem of the inorganic / organic interface, eliminates interface defects, and ensures a high retention base. In addition, the sulfonic acid groups introduced by DSDA provide strong negative charges, which efficiently retain divalent anions by utilizing electrostatic repulsion. The dense network structure formed by PIP / TMC, combined with the steric hindrance filling of 4-amino-2,2,6,6-tetramethylpiperidine, provides precise pore size sieving. Through the hydrophilic bridging of the modified filler, the anchoring enhancement of the PDA / PEI double coating, the charge network regulation of the water / oil phase, and the orderly integration of the stepwise process, a synergistic effect of hydrophilic flux and dense retention is achieved. It is suitable for the treatment of high-load dye wastewater and has broad application potential.

[0033] Comparative Example 7 is the same as Example 1, except that KH550 modification treatment is not introduced during the preparation of the modified filler.

[0034] Comparative Example 8 is the same as Example 1, except that PVP K30 is not added during the preparation of the casting solution, while the other components remain unchanged.

[0035] Comparative Example 9 is the same as Example 1, except that the amount of PES added during the preparation of the casting solution is 10%.

[0036] Comparative Example 10 is the same as Example 1, except that the amount of PES added during the preparation of the casting solution is 30%.

[0037] Comparative Example 11 is the same as Example 1, except that only acidified carbon nanotubes are used in the modified filler, and no tetrabutyl titanate and KH550 modification treatment are introduced.

[0038] Experiment Example 2 Strength Test The high-strength nanofiltration membranes prepared in Examples 1-4, Comparative Examples 1-2, and Comparative Examples 7-11 were subjected to strength tests. The membranes were cut into 150mm × 15mm strips according to GB / T 1040.4-2006. A universal testing machine was used, with a tensile rate of 50mm / min, and the maximum stress value at the moment of membrane strip breakage was recorded to test the fracture strength. A hydraulic burst strength testing machine was used, with the nanofiltration membrane fixed in a circular fixture with an effective diameter of 30mm. Hydraulic pressure was applied at a constant rate until the membrane ruptured, and the maximum pressure value at the moment of rupture was recorded to test the burst strength. Under a high pressure of 1.5MPa, the membranes were continuously operated for 24 hours, and the rate of change in pure water flux before and after the rupture was calculated to test the high-pressure compaction flux attenuation rate. The test results are shown in Table 3, where the rate of change in high-pressure pure water flux in Examples 1 and Comparative Examples 7-11 is as follows: Figure 1 As shown.

[0039] Table 3 Test Results of Examples and Comparative Examples Example Fracture strength / MPa Explosive strength / MPa High pressure compaction flux attenuation rate / % Example 1 8.45 1.85 4.2 Example 2 7.92 1.68 5.8 Example 3 8.61 1.84 3.9 Example 4 8.23 1.76 4.8 Comparative Example 1 6.85 1.42 12.5 Comparative Example 2 5.24 1.15 18.6 Comparative Example 7 6.55 1.38 10.8 Comparative Example 8 5.84 1.25 15.4 Comparative Example 9 4.25 0.85 32.5 Comparative Example 10 8.86 1.92 3.1 Comparative Example 11 5.45 1.18 16.2 As shown in Table 2, the nanofiltration membranes obtained in the comparative examples, through adjustments to the components and processes, exhibited significantly lower strength compared to the examples. In Comparative Example 1, the absence of a titanium dioxide coating layer, coupled with the smooth surface and high chemical inertness of the carbon nanotubes, made efficient grafting with KH550 difficult despite acidification. Without titanium dioxide as an intermediate medium, the bonding force at the organic-inorganic interface decreased significantly, leading to reduced fracture strength. Combined with the results of Comparative Example 7, it is evident that one end of KH550 can react with the hydroxyl groups on the surface of titanium dioxide, while the organic chains at the other end can physically entangle or react with the PES molecular chains. Without KH550, the inorganic filler and organic matrix are only connected by weak van der Waals forces, lacking a molecular bridge. Under high pressure, polymer segments easily bypass the filler, resulting in interfacial slippage and increased compaction attenuation rate. Conversely, in Comparative Example 11, the absence of titanium dioxide coating significantly reduced the strength of the nanofiltration membranes compared to the examples. The provided roughness and hydroxyl sites, without the coupling effect of KH550, make carbon nanotubes prone to migration in the matrix, unable to effectively bear the load; in Comparative Example 2, carbon nanotubes have extremely high specific surface area and are prone to agglomeration. Direct addition of the filler results in micron-sized agglomerates in the casting solution, which not only fail to provide reinforcement but also become stress concentration points. Under stress, cracks preferentially originate from the edges of the agglomerates, leading to a significant reduction in burst strength; in Comparative Example 8, PVP is not only a pore-forming agent but also a thickener and rheology modifier of the casting solution. The absence of PVP leads to an excessively fast phase transformation rate, forming a porous structure instead of a uniform sponge-like structure, which makes it prone to collapse under high pressure, resulting in severe compaction attenuation; in Comparative Examples 9-10, when the polyethersulfone concentration is too low, the formed film wall is too thin, the physical support skeleton is insufficient, and the strength difference cannot meet the requirements; conversely, combined with Figure 1 The results show that higher concentrations will increase the intensity, but this will lead to excessively high viscosity of the casting solution, making it difficult to degas and coat. Furthermore, the resulting membrane will be too dense, resulting in a decrease in water flux and negating the application significance of nanofiltration membranes, thus failing to provide comprehensive advantages.

[0040] In summary, this invention utilizes modified CNTs as high-modulus reinforcing steel dispersed within a PES matrix. The carbon nanotubes directly bear the primary mechanical stress, limiting the creep behavior of polymer segments under pressure. Furthermore, an in-situ titanium dioxide layer is grown on the surface of the carbon nanotubes, preventing agglomeration and providing abundant hydroxyl reaction sites. A silane coupling agent locks the titanium dioxide on one side and wraps around the PES on the other, achieving chemical bonding between the inorganic and organic phases and eliminating microscopic interface defects. Finally, stepwise dispersion and mixing ensure the monodisperse state of the nanofiller before film formation, avoiding agglomeration defects and guaranteeing the uniformity and stability of mechanical properties. Through the stepwise bridging of the filler, the stepwise toughening of PVP / PES, the interfacial network regulation of PDA / PEI-POSS, and the synergistic effect of ordered processes, a synergistic effect of nano-reinforcement, flexibility balance, and compressive stability is achieved, resulting in improved overall strength and suitability for treating high-load dye wastewater.

[0041] Examples 5-7 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 4.

[0042] Table 4 Parameter changes in Examples 1 and 5-7 Example 2,2'-Disulfonic-4,4'-Diaminobiphenyl (mass dosage) / % Piperazine dosage (%) 4-Amino-2,2,6,6-Tetramethylpiperidine (mass dosage) / % Mass dosage of pyromellitic acid chloride / % Crosslinking curing temperature / ℃ Crosslinking curing time / min Example 1 0.6 1.5 0.2 0.15 100 6 Example 5 0.4 1.2 0.1 0.12 90 5 Example 6 0.8 1.8 0.3 0.18 105 8 Example 7 0.5 1.4 0.15 0.14 95 7 Comparative Example 12 is the same as Example 1, except that the intermediate layer is not coated with a coating solution during the nanofiltration membrane preparation process, while the other components remain unchanged.

[0043] Comparative Example 13 is the same as Example 1, except that ordinary polyamide material without DSDA / POSS / TEMPO is used as the functional layer in the nanofiltration membrane preparation process, while the other components remain unchanged.

[0044] Comparative Example 14 is the same as Example 1, except that 4-amino-2,2,6,6-tetramethylpiperidine is not used in the aqueous solution, while the amounts of the other components remain the same.

[0045] Comparative Example 15 is the same as Example 1, except that it does not undergo two-stage heat treatment and is directly pre-cured at 60°C for 9 minutes.

[0046] Comparative Example 16 is the same as Example 1, except that it does not undergo two-stage heat treatment, but is directly cross-linked and cured at 100°C for 9 minutes.

[0047] Experiment Example 3: Acid and Alkali Resistance and Chlorine Resistance Test HCl and NaOH solutions with pH values ​​of 1 and 13, respectively, were prepared and placed in HCl and NaOH tanks, respectively, as acid and alkali cleaning solutions. Acid cleaning performance test: The nanofiltration membranes prepared in Examples 1, 5-7, Comparative Examples 5, and Comparative Examples 12-16 were cross-flow rinsed with HCl solution at pH 1 at 45℃ for 24 hours, and the difference in flux before and after rinsing was measured and calculated. Alkali cleaning performance test: The nanofiltration membranes were cross-flow rinsed with NaOH solution at pH 13 at 35℃ for 24 hours, and the difference in flux before and after rinsing was measured and calculated, and the water flux attenuation rate was calculated. Additionally, a 1000 ppm sodium hypochlorite solution with pH 8.0 was prepared to simulate disinfection conditions, and the nanofiltration membranes were cross-flow rinsed at 25℃ for 24 hours, and the difference in flux before and after rinsing was measured and calculated, and the water flux attenuation rate was calculated. The test results are shown in Table 5.

[0048] Table 5 Test Results of Examples and Comparative Examples Example Water flux / L / m²·h Acid resistance degradation / % Alkali resistance degradation / % Chlorine resistance degradation / % Example 1 58.5 3.2 4.1 2.8 Example 5 54.2 3.8 4.7 3.2 Example 6 52.8 2.6 3.5 2.3 Example 7 56.7 3.1 3.9 2.6 Comparative Example 5 28.4 8.5 9.2 7.1 Comparative Example 12 42.1 7.3 8.0 6.4 Comparative Example 13 35.7 12.6 11.8 10.5 Comparative Example 14 40.9 6.8 7.5 5.9 Comparative Example 15 45.2 5.4 6.1 4.7 Comparative Example 16 43.8 6.2 6.9 5.3 As shown in Table 5, the acid and alkali resistance and chlorine resistance of the high-strength nanofiltration membrane obtained in the comparative examples, obtained by adjusting the components and processes, were significantly reduced compared to the examples. In Comparative Example 5, the introduction of sulfonic acid groups into DSDA provided negative charge shielding against acid / alkali / chlorine ion erosion and enhanced hydrophilic stability of the hydration layer. After replacement, the membrane surface became hydrophobic, and the amide bonds broke rapidly under strong acid and alkali conditions, making nucleophilic attacks more likely, leading to membrane pore enlargement, abnormally increased flux, and a decrease in overall acid and alkali resistance. In Comparative Example 12, the lack of chemical anchoring points provided by the PDA / PEI intermediate layer resulted in insufficient bonding between the PA skin and the PES base membrane under the shear force of cross-flow rinsing, leading to local detachment or peeling, causing the resistance layer to fail, membrane damage, and a surge in flux. In Comparative Example 13, although the ordinary polyamide material had acceptable chemical stability, it was prone to degradation under strong oxidizing environments, leading to polymer aging and brittleness, and a decrease in overall performance. The results of Example 14 show that active chlorine readily attacks the nitrogen atoms on the amide bond, causing an N-chlorination reaction, which in turn leads to rearrangement of the benzene ring and breakage of the polymer chain. 4-Amino-2,2,6,6-Tetramethylpiperidine, as a hindered amine free radical scavenger, can preferentially capture the free radicals generated by active chlorine, sacrificing itself to protect the polyamide backbone. Without its addition, the membrane material is directly exposed to oxidative attack, resulting in rapid structural degradation. The results of Comparative Examples 15-16 show that pre-curing at 60℃ alone results in incomplete interfacial polymerization and low crosslinking degree, causing the polymer chain segments to easily swell in acid and alkali solutions, loosening the polymer chain, and significantly reducing the resistance to chemical cleaning. Conversely, crosslinking curing promotes the complete reaction of residual amines and acyl chlorides, increasing the crosslinking density. The dense network structure itself has better chemical penetration resistance than the loose structure. However, high temperature in a single segment causes excessive crosslinking, resulting in stress concentration, forming a brittle network, and affecting the overall acid and alkali resistance.

[0049] In summary, the DSDA introduced in this invention utilizes the strong negative charge and large steric hindrance of its sulfonic acid group to construct the first line of defense, effectively resisting the hydrolytic attack of acid and base ions on the amide bond. 4-Amino-2,2,6,6-Tetramethylpiperidine, as a free radical scavenging structure grafted onto the membrane surface, constructs the second line of defense, specifically neutralizing the oxidative damage of active chlorine and significantly extending the lifespan of the nanofiltration membrane in chlorine-containing environments. Simultaneously, the strong adhesion of dopamine and the cross-linking network of PEI firmly hold the functional skin layer onto the base membrane, preventing delamination under chemical cleaning. Pre-curing removes solvents to prevent defects, and cross-linking curing promotes the complete reaction of residual amines and acyl chlorides, increasing cross-linking density. Through the charge-steric hindrance protection of DSDA and 4-amino-2,2,6,6-tetramethylpiperidine, the anchoring buffer of PDA / PEI, the network optimization controlled by POSS, and the stepwise stabilization integration of two-stage heat treatment, a synergistic effect of chemical shielding, interface protection, and structural anti-degradation is achieved, significantly improving acid and alkali resistance and making it suitable for treating high-load dye wastewater.

[0050] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An acid and alkali resistant, high-strength nanofiltration membrane, characterized in that, The invention comprises a modified polyethersulfone matrix component, a modified polyamide functional network, and a crosslinking system. The modified polyethersulfone matrix component includes polyethersulfone and modified fillers. The modified fillers include multi-walled carbon nanotubes, titanium dioxide, and 3-aminopropyltriethoxysilane. The crosslinking system consists of polyethyleneimine, polydopamine, and tris(hydroxymethyl)aminomethane. The modified polyamide functional network is formed by the polymerization reaction of amine monomers in an aqueous solution and acyl chloride monomers in an oil phase solution. The aqueous solution includes piperazine, triethylamine, sodium dodecyl sulfate, 2,2'-disulfonic-4,4'-diaminobiphenyl, and 4-amino-2,2,6,6-tetramethylpiperidine. The oil phase solution includes isoparaffin solvents, pyromellitic tricarboxylic acid chloride, and octaisobutylsilsesquioxane. The polydopamine is formed by in-situ polymerization of dopamine hydrochloride.

2. A method for preparing an acid and alkali resistant, high-strength nanofiltration membrane, characterized in that, The preparation of the nanofiltration membrane as described in claim 1 includes the following steps: coating a casting solution onto the surface of a polyester nonwoven fabric; obtaining a modified base membrane through a coagulation bath; pouring a coating solution onto the surface of the modified base membrane and allowing it to stand to obtain a coated base membrane; applying an aqueous solution to the surface of the coated base membrane through immersion coating to obtain a treated base membrane; applying an oil phase solution to the surface of the treated base membrane through flexible spray coating, and reacting to obtain a membrane sheet; draining the membrane sheet and subjecting it to a two-stage heat treatment to obtain a cured composite membrane; and sequentially passing the cured composite membrane through an aqueous sodium carbonate solution, an aqueous ethanol solution, and a sodium bisulfite solution, and then sealing and packaging it to obtain the high-strength nanofiltration membrane.

3. The method for preparing an acid and alkali resistant high-strength nanofiltration membrane according to claim 2, characterized in that, The preparation of the casting solution includes the following steps: adding NMP to the modified filler and ultrasonically dispersing it to obtain a suspension; adding PVP K30 to the suspension and stirring to obtain a transparent solution; then adding PES and stirring at a constant temperature, followed by vacuum degassing to obtain the casting solution.

4. The method for preparing an acid and alkali resistant high-strength nanofiltration membrane according to claim 3, characterized in that, The preparation of the modified filler includes the following steps: Multi-walled carbon nanotubes were added to a mixed acid solution to obtain acidified carbon nanotubes; the acidified carbon nanotubes were dispersed in anhydrous ethanol and ultrasonically dispersed. Then add tetrabutyl titanate, add dropwise ethanol aqueous solution, perform hydrolysis reaction, and calcine under air protection to obtain the reactant; then add KH550 and reflux reaction to obtain the modified filler.

5. The method for preparing an acid and alkali resistant high-strength nanofiltration membrane according to claim 2, characterized in that, The preparation of the coating liquid includes the following steps: Add deionized water and tris(hydroxymethyl)aminomethane to the mixing tank; then add dilute hydrochloric acid to adjust the pH value; then add polyethyleneimine and stir to obtain a mixture; before coating, add dopamine hydrochloride powder to the mixture and stir to obtain the coating solution.

6. The method for preparing an acid and alkali resistant high-strength nanofiltration membrane according to claim 2, characterized in that, The preparation of the aqueous solution includes the following steps: adding deionized water and triethylamine to a mixing tank, stirring until homogeneous, adjusting the pH value, and adding 2,2'-disulfonic-4,4'-diaminobiphenyl; then adding piperazine and 4-amino-2,2,6,6-tetramethylpiperidine in sequence; continuing to add sodium dodecyl sulfate, and filtering to obtain the aqueous solution.

7. The method for preparing an acid and alkali resistant high-strength nanofiltration membrane according to claim 2, characterized in that, The preparation of the oil phase solution includes the following steps: adding trimesoyl chloride to an isoparaffin solvent; then adding octaisobutylsilsesquioxane, and sealing and stirring to obtain the oil phase solution.

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