Ultrafiltration supporting layer, composite nanofiltration membrane, preparation method of composite nanofiltration membrane and application of composite nanofiltration membrane

Abstract

The invention relates to the technical field of membranes, in particular to an ultrafiltration supporting layer, a composite nanofiltration membrane, a preparation method of the composite nanofiltration membrane and application of the composite nanofiltration membrane, and the preparation method of the ultrafiltration supporting layer comprises the following steps: (1) preparing nanocrystalline cellulose and copolyimide composite slurry; and (2) coating the composite slurry in the step (1) on a non-woven fabric base material, and carrying out phase inversion, washing and drying to prepare the ultrafiltration support layer. According to the composite nanofiltration membrane prepared by using the ultrafiltration support layer, the coordination of a high-strength support layer and a high-performance desalination layer is realized, and the long-term operation stability and the separation efficiency of the composite nanofiltration membrane are improved.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, specifically to an ultrafiltration support layer, a composite nanofiltration membrane, and a method for preparing the same. Background Technology

[0002] Composite nanofiltration membranes, with their approximately 1 nm pore size, achieve a synergistic balance between selectivity and permeability, making them a core functional material in the field of high-efficiency separation technology. Their typical structure employs a three-layer composite design: non-woven fabric, ultrafiltration support layer, and polyamide desalination layer. Each layer works synergistically to support the overall separation performance of the membrane. The ultrafiltration support layer must simultaneously meet three core technical requirements: first, it must possess excellent mechanical strength to resist compaction under high pressure; second, it must form a uniform pore structure to accommodate the preparation and performance of the upper polyamide desalination layer; and third, it must have good antifouling properties to reduce pollutant adsorption. The separation performance of the polyamide desalination layer, on the other hand, is highly dependent on the precise control of interfacial polymerization process parameters, including key conditions such as monomer concentration and reaction time. Even slight fluctuations in these process parameters can significantly affect the performance of the desalination layer.

[0003] The mainstream polysulfone or polyethersulfone ultrafiltration support layers used in current commercial composite nanofiltration membranes have significant performance shortcomings, making it difficult to meet the requirements for long-term stable operation. Regarding mechanical strength and high-pressure resistance, the tensile strength of the skin layer of this type of support layer is generally ≤2.5MPa, and the porosity is ≤70%. Under normal high-pressure operating conditions of 0.5-1.5MPa, irreversible compaction easily occurs, leading to a membrane thickness reduction exceeding 0.3mil, which in turn causes a decrease in pure water flux exceeding 20%, severely affecting separation efficiency. In terms of antifouling performance, its poor hydrophilicity and water contact angle >80° make it prone to adsorbing pollutants such as proteins and colloids in wastewater systems. Frequent chemical or physical cleaning is required to maintain operation, directly shortening the service life of the membrane module and increasing maintenance costs and operational complexity.

[0004] In the field of polyimide-based ultrafiltration support layers, although pure polyimide material possesses excellent acid and alkali chemical stability, its hydrophilicity and antifouling properties still do not meet the ideal application requirements of commercial composite nanofiltration membranes. Existing modification schemes for polyimide support layers mostly focus on improving a single performance without introducing nano-reinforcing phases for multi-performance synergistic optimization. This makes it difficult to simultaneously improve the mechanical strength and antifouling properties of the support layer, limiting its large-scale application. At the same time, the m-phenylenediamine (MPD) / trimethylammonium chloride (TMC) monomer system widely used in commercial composite nanofiltration membranes is prone to problems such as excessively high monomer concentration or excessively long reaction time due to the lack of precise process control methods. This leads to over- or under-crosslinking of the polyamide desalination layer, ultimately resulting in a membrane molecular weight cutoff (MWCO) fluctuation range of 500-1000 Da. The retention rate of divalent salts is unstable and generally <90%, which cannot meet the stringent requirements of high-precision separation scenarios, thus restricting the application expansion of composite nanofiltration membranes in high-end separation fields.

[0005] In summary, how to improve the performance shortcomings of the support layer and the defects of the interfacial polymerization process of composite nanofiltration membranes, thereby enhancing the overall strength of the membrane and stabilizing its high desalination performance, is a key issue that urgently needs to be addressed in this field. Summary of the Invention

[0006] To address the problems of poor pressure resistance and weak antifouling properties of the ultrafiltration support layer in existing composite nanofiltration membranes, as well as the performance fluctuations of the desalination layer caused by suboptimal interfacial polymerization conditions, this invention provides a composite nanofiltration membrane with polyimide / nanocellulose as the ultrafiltration support layer and its preparation method, achieving synergy between a "high-strength support layer + high-performance desalination layer" and improving the long-term operational stability and separation efficiency of the composite nanofiltration membrane.

[0007] The first aspect of this invention provides a method for preparing an ultrafiltration support layer, comprising the following steps:

[0008] (1) Prepare a composite slurry of nanocrystalline cellulose and copolyimide;

[0009] (2) The composite slurry from step (1) is applied to a nonwoven fabric substrate, phase-inverted, washed, and dried to obtain an ultrafiltration support layer.

[0010] This invention prepares an ultrafiltration support layer using a composite slurry formulated with nanocrystalline cellulose and copolyimide. This not only improves the mechanical strength of the ultrafiltration support layer but also simultaneously enhances its antifouling ability. Nanocrystalline cellulose, with its high crystallinity and high specific surface area, can be uniformly dispersed in the composite system and exert a nano-reinforcing effect, effectively bearing stress and hindering crack propagation. Simultaneously, its abundant hydroxyl groups can form hydrogen bonds with the copolyimide, strengthening the synergistic load-bearing capacity of both nanocrystalline cellulose and copolyimide, thus jointly improving the overall strength of the support layer. This composite slurry can also modify the surface properties of the support layer, enhancing surface hydrophilicity to reduce the adsorption of hydrophobic pollutants, reducing surface roughness to weaken the possibility of pollutant adhesion, and leveraging the chemical stability of the copolyimide to improve the ultrafiltration support layer's resistance to chemical erosion and reduce membrane fouling. This achieves a simultaneous and efficient improvement in the mechanical properties and antifouling performance of the ultrafiltration support layer.

[0011] Furthermore, the dry film thickness of the nonwoven fabric substrate is 60 μm, and the wet film thickness is 80~120 μm.

[0012] Furthermore, the nanocrystalline cellulose is prepared by acid hydrolysis of microcrystalline cellulose at a temperature of 40-50°C for 2-3 hours.

[0013] In some embodiments of the present invention, the preparation process of nanocrystalline cellulose by acid hydrolysis of microcrystalline cellulose is as follows: microcrystalline cellulose is added to a sulfuric acid solution with a mass concentration of 60-65%, wherein the solid-liquid ratio of microcrystalline cellulose to sulfuric acid solution is 1g:(10-15)mL, and the mixture is heated to 40-50℃ and stirred to hydrolyze the microcrystalline cellulose for 2-3 hours; after hydrolysis, deionized water is added to the solution after reaction to dilute it, centrifuge it, and collect the precipitate; the precipitate is repeatedly washed with deionized water until the pH value of the washing solution is 5-6, and after washing, it is freeze-dried and pulverized to obtain NCC powder.

[0014] Preferably, the stirring speed is 300-400 r / min;

[0015] Preferably, the volume of the deionized water added is 5 to 8 times that of the sulfuric acid solution;

[0016] Preferably, the centrifugation speed is 8000-10000 r / min;

[0017] Preferably, the freeze-drying temperature is -50 to -40°C, the drying vacuum degree is <10Pa, and the drying time is 24 to 36 hours.

[0018] Furthermore, the particle size of the nanocrystalline cellulose is 50~100nm.

[0019] Nanocrystalline cellulose prepared by microcrystalline cellulose hydrolysis possesses a high specific surface area (greater than 200 m² / g) and uniform particle size, preferably within the range of 50–100 nm. This allows for more uniform dispersion in copolyimide slurries, avoiding performance limitations caused by agglomeration. The high specific surface area significantly increases the exposure of hydroxyl groups on the surface of the nanocrystalline cellulose, substantially enhancing the probability and number of hydrogen bonds formed with the copolyimide molecular chains. This stronger interfacial interaction not only strengthens the internal bonding force of the composite system through hydrogen bonding, thereby improving the tensile strength of the ultrafiltration support layer, but also improves the surface properties and optimizes the pore structure of the support layer through the hydrophilicity of hydroxyl groups, achieving a simultaneous improvement in the hydrophilicity, tensile strength, and porosity of the support layer.

[0020] Furthermore, the composite slurry is prepared by adding nanocrystalline cellulose to a copolyimide slurry, wherein the nanocrystalline cellulose is 0.3~0.7 wt% of the composite slurry, stirring, ultrasonically dispersing, and filtering impurities through a filter membrane to obtain the composite slurry.

[0021] Preferably, the stirring is performed at 25-30°C at a speed of 500-600 r / min for 1-2 hours.

[0022] Preferably, the ultrasonic power of the ultrasonic dispersion is 300~500W and the ultrasonic time is 40~60min.

[0023] Preferably, the filter membrane is a 1.0~2.0μm polytetrafluoroethylene filter membrane.

[0024] In some embodiments of the present invention, the copolyimide slurry is prepared in the following manner:

[0025] 1) Preparation of polyamic acid (PAA) solution: Under inert gas protection, bisphenol A diamine (BAPP, CAS: 1547-73-1) is added to a polar aprotic solvent. The temperature is maintained at 20~25℃, and the mixture is stirred until the bisphenol A diamine is completely dissolved. The temperature is then lowered to 0~5℃, and decahydronaphthalenetetracarboxylic dianhydride (CAS: 106-90-7) is slowly added dropwise. The molar ratio of the amount of decahydronaphthalenetetracarboxylic dianhydride added to the bisphenol A diamine is 1:(0.98~1.02), and the addition time is 30~45 min. After the addition is completed, the mixture is kept at 0~5℃ for 20~24 h to obtain a transparent polyamic acid solution.

[0026] 2) Preparation of copolyimide (CPI) slurry: Add dehydrating agent and catalyst to the polyamic acid solution obtained in step 1), and stir and react at 23~27℃ for 18~24h to obtain a light yellow copolyimide slurry.

[0027] Preferably, in step 1), the inert gas is at least one of nitrogen or argon with a purity ≥ 99.999%.

[0028] Preferably, in step 1), the polar aprotic solvent is N,N-dimethylacetamide (DMAC) or N-methylpyrrolidone (NMP).

[0029] Preferably, in step 1), the stirring speed is 300~400 r / min;

[0030] In step 1), the concentration of the polyamic acid solution obtained is 12-15 wt%.

[0031] Preferably, the dehydrating agent in step 2) is acetic anhydride.

[0032] Preferably, the catalyst in step 2) is pyridine or triethylamine.

[0033] Preferably, in step 2), the molar ratio of the dehydrating agent to polyamic acid is 0.1:1.

[0034] Preferably, in step 2), the molar ratio of the catalyst to the dehydrating agent is (0.5~1):1.

[0035] Preferably, in step 2), the stirring speed is 300~400 r / min.

[0036] In some specific embodiments of the present invention, step (2) specifically involves: uniformly coating the nanocrystalline cellulose and copolyimide composite slurry onto a nonwoven fabric substrate, letting it stand for 5-10 seconds, immersing it in deionized water at 30-80°C for phase inversion for 15-60 minutes, removing the coated nonwoven fabric substrate, washing it with deionized water 2-3 times to remove residual solvent, and then drying it at 60-80°C and a vacuum degree <0.1 kPa for 4-6 hours to obtain an ultrafiltration support layer.

[0037] A second aspect of the present invention provides an ultrafiltration support layer, which is prepared by the above-described method for preparing an ultrafiltration support layer.

[0038] Furthermore, the water contact angle of the ultrafiltration support layer is ≤57.1°, the skin tensile strength is ≥4.01MPa, the porosity is ≥85.7%, and the molecular weight cutoff is 1000~10000Da; the ultrafiltration support layer has mechanical strength, uniform pore structure and antifouling properties.

[0039] A third aspect of this invention provides a method for preparing a composite nanofiltration membrane, comprising the following steps:

[0040] (1) Immerse the ultrafiltration support layer prepared above in an aqueous solution containing aqueous monomers for 1-2 minutes, then remove and drain excess aqueous solution from the surface.

[0041] (2) Immerse it in an oil phase solution containing oil phase monomers, and perform interfacial polymerization reaction at a temperature of 23~27℃ for 15~30s. Dry it to obtain a composite nanofiltration membrane.

[0042] Furthermore, the mass fraction of the aqueous monomer in the aqueous solution is 2-4 wt%.

[0043] Furthermore, the aqueous monomer is at least one selected from piperazine, o-phenylenediamine, diethylenetriamine, meta-phenylenediamine, triethylenetetramine, triethanolamine, and methyldiethanolamine.

[0044] Furthermore, the mass fraction of the oil phase monomer in the oil phase solution is 0.1~0.2wt%;

[0045] Furthermore, the oil phase monomer is at least one of pyromellitic chloroformyl chloride, phthaloyl chloride, isophthaloyl chloride, and terephthaloyl chloride;

[0046] Furthermore, the solvent of the oil phase solution is at least one selected from hexane, trifluorotrichloroethane, cyclohexane, and thiocarbamate.

[0047] Furthermore, the drying temperature is 60~80℃, and the drying time is 10~15min.

[0048] A fourth aspect of the present invention provides a composite nanofiltration membrane, which is prepared by the above-described method for preparing composite nanofiltration membranes.

[0049] The composite nanofiltration membrane prepared by the above method according to the present invention has the following performance indicators:

[0050] Molecular weight cutoff (MWCO): 600 Da (≥92% for PEG600, ≥78% for PEG400);

[0051] Salt resistance: ≥92% rejection rate for 2000ppm Na2SO4, ≥88% rejection rate for 2000ppm MgCl2 (test conditions: 0.6MPa, 25℃);

[0052] Pressure resistance: After operating at 0.6 MPa for 24 hours, the membrane thickness change is ≤0.2 mil;

[0053] Antifouling resistance: After filtration of bovine serum albumin (BSA) solution (1 g / L) for 8 hours, the flux attenuation rate was ≤5%;

[0054] Hydrophilicity: water contact angle ≤37°, surface potential ≥-8mV.

[0055] The fifth aspect of the present invention provides the application of the composite nanofiltration membrane prepared above in wastewater treatment, brine denitrification or pharmaceutical purification.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] 1) Significantly improved performance of the support layer: This invention prepares an ultrafiltration support layer using a composite slurry formulated with nanocrystalline cellulose and copolyimide. Through the nano-reinforcing effect of nanocrystalline cellulose and the strong interfacial bond formed by the copolyimide, the tensile strength of the support layer is ≥4.01 MPa. Compared with the polyethersulfone membrane support layer commonly used in the prior art, the tensile strength is increased by 78.22%. At the same time, the porous structure of nanocrystalline cellulose optimizes the pore distribution of the support layer, making the porosity of the support layer reach 85.70% or more. This invention solves the problem of poor pressure resistance of traditional support layers from the aspects of structural strengthening and pore optimization.

[0058] 2) Excellent pressure resistance of nanofiltration membranes: The composite nanofiltration membrane prepared using the support layer of this invention achieves synergy between a "high-strength support layer + high-performance desalination layer," improving the long-term operational stability and separation efficiency of the composite nanofiltration membrane. For example, the composite nanofiltration membrane (NF2) prepared with a support layer containing 0.5% NCC has a thickness change of only 0.2 mil after being compressed at 0.6 MPa, which is superior to the commercial membrane VNF-K (0.3 mil), and exhibits higher long-term flux stability.

[0059] 3) Outstanding antifouling properties: The composite nanofiltration membrane surface exhibits enhanced hydrophilicity due to the introduction of hydroxyl groups from nanocrystalline cellulose, reducing adsorption sites for hydrophobic pollutants. Simultaneously, the more uniform pore structure lowers the probability of pollutant clogging. For example, the composite nanofiltration membrane NF2 exhibits a significantly lower flux decline rate for bovine serum albumin (BSA) filtration (4.31%) compared to the commonly used commercial membrane, the Walton VF2 (9.17%), reducing cleaning frequency and extending membrane lifespan.

[0060] 4) Stable retention performance: The composite nanofiltration membrane uses nanocrystalline cellulose to regulate the size and distribution of the pore size in the skin layer, precisely controlling the molecular weight cutoff (MWCO) of the composite nanofiltration membrane within the range of 500~800 Da. At the same time, the chemical stability of the copolyimide greatly reduces the probability of membrane swelling or deformation during filtration. The retention mechanism for divalent salts is stable, with a retention rate of 92% or higher, which can reliably meet the requirements for retention accuracy in the desalination process of high-salt wastewater.

[0061] 5) High process repeatability: The entire preparation process can be standardized and is suitable for mass production.

[0062] 6) Wide applicability: The composite nanofiltration membrane prepared by this invention can be applied to various wastewater treatment, brine denitrification and pharmaceutical purification processes. Attached Figure Description

[0063] Figure 1 The curves show the effect of piperazine (PIP) concentration on the water flux and 2000 ppm Na2SO4 rejection rate of the composite nanofiltration membrane.

[0064] Figure 2 The curves show the effect of piperazine (PIP) concentration on the water flux and 2000 ppm MgCl2 rejection rate of the composite nanofiltration membrane.

[0065] Figure 3 The curves show the effect of trimethylbenzene chloride (TMC) concentration on the water flux and 2000 ppm Na2SO4 rejection rate of the composite nanofiltration membrane.

[0066] Figure 4 The curves show the effect of pyromellitic acid chloride concentration on the water flux and 2000 ppm MgCl2 rejection rate of the composite nanofiltration membrane.

[0067] Figure 5 The curves show the effect of interfacial polymerization time on the water flux and 2000ppm Na2SO4 rejection rate of the composite nanofiltration membrane.

[0068] Figure 6 The curves show the effect of interfacial polymerization time on the water flux and 2000 ppm MgCl2 rejection rate of the composite nanofiltration membrane.

[0069] Figure 7 Comparison of FTIR spectra of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 (showing the double peaks of NH 1577, CN 1237, and COC 1147 / 1103).

[0070] Figure 8 Comparison of surface SEM images of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 (magnified 100,000 times, a is Comparative Example 1, b is Example 1, both showing dense and rough surfaces).

[0071] Figure 9 Comparison of cross-sectional SEM images of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 (magnified 10,000 times, a is Comparative Example 1, b is Example 1, showing the thin desalination layer on the surface of the support layer).

[0072] Figure 10 Comparison of AFM patterns of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 (a is Comparative Example 1, roughness 11.9; b is Example 1, roughness 10.1).

[0073] Figure 11 Comparative atomic force scanning electron microscope (AFM) images of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 (a is Comparative Example 1; b is Example 1).

[0074] Figure 12 The graph shows a comparison of the rejection rate of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 for 2000 ppm MgCl2 and the water flux.

[0075] Figure 13 The flux change curves (8h) of the composite nanofiltration membranes prepared in Example 1 and Comparative Example 1 and the commercial membrane VNF-K in the BSA antifouling test are shown. Detailed Implementation

[0076] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments thereof. 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 protection scope of the present invention.

[0077] In the embodiments and comparative examples of this invention, the methods for detecting performance parameters are as follows:

[0078] Water contact angle: HY / T 266—2018 "Contact angle method for testing the hydrophilicity of the surface of external pressure hollow fiber ultrafiltration membrane";

[0079] Tensile strength: Tested using a standard tensile testing machine, in accordance with ISO 527 standard;

[0080] Porosity: GB / T 20103-2006 "Terminology for Membrane Separation Technology" and GB / T 1966-1996 "Test Methods for Apparent Porosity and Bulk Density of Porous Ceramics" and other relevant standards;

[0081] Water flux: Tested according to GB / T 32360-2015 "Test Methods for Ultrafiltration Membranes";

[0082] Salt rejection rate: Salt ion concentration after filtration / Salt ion concentration before filtration.

[0083] Example 1

[0084] The method for preparing a composite nanofiltration membrane according to this embodiment includes the following steps:

[0085] (1) Preparation of nanocrystalline cellulose powder: 10g of microcrystalline cellulose was added to 120mL of 64% sulfuric acid solution (solid-liquid ratio 1g:12mL), and stirred at 45℃ and 350r / min for 2.5 hours for hydrolysis; 600mL of deionized water was added to terminate the reaction, and the mixture was centrifuged at 9000r / min for 12 minutes to collect the precipitate; the precipitate was washed to pH 5.5, freeze-dried at -45℃ and 5Pa for 30 hours, and pulverized to obtain NCC powder with a particle size of 70-80nm;

[0086] (2) Preparation of copolyimide slurry:

[0087] Nitrogen gas was introduced into a 500 mL three-necked flask, and 200 mL of N,N-dimethylacetamide (DMAC) solvent was added. 17.72 g (0.05 mol) of bisphenol A diamine (BAPP) was added to the solvent. The mixture was heated to 25 °C and stirred at 350 r / min until BAPP was completely dissolved to form a composite solution. The temperature was lowered to 3 °C, and 14.21 g (0.05 mol) of decahydronaphthalenetetracarboxylic acid dianhydride was slowly added dropwise to the composite solution over a period of 40 min. The mixture was kept at 3 °C for 22 hours to obtain a 13 wt% polyamic acid (PAA) solution.

[0088] 12.25 g (0.12 mol) of acetic anhydride and 9.49 g (0.12 mol) of pyridine were added to a polyamic acid (PAA) solution and stirred at 25 °C for 20 hours to obtain a copolyimide (CPI) slurry.

[0089] (3) Preparation of nanocrystalline cellulose / copolyimide composite slurry: 0.6g of nanocrystalline cellulose powder (accounting for 0.5% of the mass fraction of nanocrystalline cellulose / copolyimide composite slurry) was added to the copolyimide (CPI) slurry prepared in step (2), stirred at 550r / min for 1.5 hours at 28℃, ultrasonically dispersed at 400W for 50 minutes, and impurities were filtered through a 1.5μm filter membrane to obtain nanocrystalline cellulose / copolyimide composite slurry;

[0090] (4) Preparation of ultrafiltration support layer: The composite slurry was uniformly coated on a nonwoven fabric with a wet membrane thickness of 100 μm. After standing for 8 seconds, it was immersed in a 60℃ water bath for 30 min. After washing, it was dried at 70℃ and 0.05 kPa for 5 hours to obtain the support layer. The performance of the ultrafiltration support layer was tested, and its water contact angle was measured to be 57.1°, tensile strength was 4.01 MPa, porosity was 85.70%, and pure water flux was 714.33 GFD.

[0091] (5) Preparation of composite nanofiltration membrane: Prepare an aqueous solution containing 3 wt% piperazine and an oil solution containing 0.15 wt% trimesoyl chloride; immerse the ultrafiltration support layer prepared in step (4) in the aqueous solution of piperazine for 1.5 min, remove it, and drain the surface moisture; then immerse it in the oil solution of trimesoyl chloride at 25°C for 20 s to undergo interfacial polymerization reaction, remove it, and dry it at 70°C for 12 min to obtain the composite nanofiltration membrane (labeled as NF2). The performance of the NF2 composite nanofiltration membrane was tested.

[0092] Example 2

[0093] The preparation method of the composite nanofiltration membrane in this embodiment is basically the same as that in Example 1, except that the nanocellulose accounts for 0.3 wt% of the composite slurry, and the other steps are the same.

[0094] Example 3

[0095] The preparation method of the composite nanofiltration membrane in this embodiment is basically the same as that in Example 1, except that the nanocellulose accounts for 0.7wt% of the composite slurry, and the other steps are the same.

[0096] Example 4

[0097] The preparation method of the composite nanofiltration membrane in this embodiment is basically the same as that in Example 1, except that the nanofiltration cellulose accounts for 0.2 wt% of the composite slurry, and the other steps are the same.

[0098] Example 5

[0099] The preparation method of the composite nanofiltration membrane in this embodiment is basically the same as that in Example 1, except that the nanofiltration cellulose accounts for 0.8 wt% of the composite slurry, and the other steps are the same.

[0100] Comparative Example 1

[0101] The difference between the preparation method of the composite nanofiltration membrane in Comparative Example 1 and Example 1 is that no nanocellulose powder is added to the copolyimide composite slurry, and it is directly coated on the nonwoven fabric. The other steps are the same.

[0102] Comparative Example 2

[0103] The difference between the preparation method of the composite nanofiltration membrane in Comparative Example 1 and Example 1 is that the nanocellulose suspension is poured onto the polysulfone ultrafiltration membrane and filtered to obtain the ultrafiltration support layer; the remaining steps are the same.

[0104] Comparative Example 3

[0105] Commercially available composite nanofiltration membranes (Times Wharton VF-2) are purchased from the market.

[0106] The performance of the ultrafiltration support layers prepared in Examples 1-13 and Comparative Examples 1-2 was tested, with the following parameters: water contact angle, tensile strength, porosity, and pure water flux. The performance of the composite nanofiltration membranes prepared in Examples 1-13 and Comparative Examples 1-3 was also tested, with the following parameters: water contact angle, surface potential, molecular weight cutoff, Na₂SO₄ rejection rate (2000 ppm), MgCl₂ rejection rate (2000 ppm), thickness change after 0.6 MPa compaction, and bovine serum albumin (BSA) flux decay rate (8 h). The test results are shown in Table 1.

[0107] Table 1

[0108]

[0109] Comparative analysis of the data from Examples 1-5 and Comparative Examples 1-3 in Table 1 Figure 1-13 Analysis shows that the present invention constructs a novel ultrafiltration support layer by uniformly compositing nanocrystalline cellulose onto a copolyimide matrix, and then fabricates a polyamide desalination layer on this support layer by precisely controlling the interfacial polymerization parameters. The resulting high-performance composite nanofiltration membrane based on the nanocomposite-reinforced ultrafiltration support layer has significantly improved and optimized mechanical properties, structural stability, separation accuracy and selectivity, and its surface properties and antifouling ability have also been improved.

[0110] The core NCC / CPI composite support layer of this invention (Example 1) exhibits a skin tensile strength of 4.01 MPa and a porosity of 85.7%, representing a strength increase of over 78% compared to the unmodified CPI support layer (Comparative Example 1, 2.25 MPa, 72.3%) and commercial membranes (Comparative Example 3, ~2.1 MPa, ~70%), with significantly optimized pore structure. This directly translates into excellent pressure resistance of the membrane module. The nanofiltration membrane prepared in Example 1 showed a thickness change of only 0.2 mil after operation at 0.6 MPa, far lower than the 0.9 mil of Comparative Example 1 and the 1.3 mil of the commercial membrane. Under optimized interfacial polymerization, the composite nanofiltration membrane (Example 1) prepared based on the support layer of this invention achieved a PEG600 rejection rate of 93.29%, corresponding to a molecular weight cutoff of approximately 600 Da, and its rejection performance for divalent salts remained consistently high (Na2SO4: 94.5%, MgCl2: 96.7%). This performance is superior to all comparative examples (comparative example 1 corresponding values ​​are 82%, 87.4%, and 90%), demonstrating that the "high-strength support layer" provides an excellent substrate for forming a more uniform, dense, and stable desalination layer. The introduction of NCC greatly enhances the hydrophilicity of the membrane surface. The water contact angle of the support layer and nanofiltration membrane in Example 1 is reduced to 57.1° and 36.9°, respectively, and the surface potential is more negative (-7.68 mV). This directly leads to a qualitative change in antifouling performance: in the BSA fouling test, the flux decay rate of Example 1 is only 4.31%, which is more than half lower than that of comparative example 1 (9.2%) and commercial membrane VF2 (9.17%), greatly extending the operating cycle and membrane life.

[0111] Depend on Figure 8-9 As can be seen from the SEM images, comparative example 1 ( Figure 8 The interface between the desalination layer and the support layer in Example 1 (a, 9a) is relatively blurred, while that in Example 1 ( Figure 8 (b,9b) Thanks to the more uniform and hydrophilic support surface after NCC modification, the polyamide desalination layer formed on it is more continuous and dense and tightly bonded to the support layer. This optimization of the microstructure directly translates into an improvement in the performance of the composite nanofilm.

[0112] Depend on Figure 12 As shown, Example 1 exhibited a significantly higher MgCl2 rejection rate (96.7%) and water flux than Comparative Example 1 (90%). Figure 5-6 This indicates that its laboratory-optimized process window (reaction time 20 s) brings the performance to its peak, ensuring high throughput with high rejection rate. At the same time, its pressure resistance (thickness variation of 0.2 mil) is far superior to that of the commercial membrane VNF-K (1.3 mil, Table 1).

[0113] Depend on Figure 10 AFM analysis showed that the surface roughness of the membrane in Example 1 (10.1 nm) was lower than that in Comparative Example 1 (11.9 nm). Combined with its superior hydrophilicity (water contact angle 36.9° vs. 57.1°), this effectively reduced the number of pollutant adsorption sites. This is directly reflected in... Figure 13 In the anti-fouling test: Example 1 had the flattest flux decay curve, with a flux decay rate of only 4.31% after 8 hours, which was much lower than Comparative Example 1 (9.2%) and commercial membrane VF2 (9.17%).

[0114] Comparative analysis of data from Examples 1-3 and Examples 4-5 in Table 1 and appendices Figure 1-4 Analysis revealed that Example 1 (0.5 wt%) exhibited peak performance, with the highest tensile strength (4.01 MPa) and porosity (85.7%) in its support layer, and the best hydrophilicity (water contact angle 57.1°). The corresponding nanofiltration membrane was optimal in all key indicators. When the NCC content deviated from this preferred range, the performance showed a regular decline: the data for Examples 4 (0.2 wt%) and 5 (0.8 wt%) were significantly worse than those for Examples 1-3. For example, the tensile strength of the support layer decreased to approximately 3.1 MPa, and the water contact angle increased to over 67°; the corresponding nanofiltration membrane's Na2SO4 rejection rate decreased to 91.2%-90.2%, and the BSA flux attenuation rate increased to over 7%. This clear "peak-shaped" performance trend indicates that an NCC addition of 0.3-0.7 wt% is the optimal balance point for achieving the nano-reinforcement effect (improving strength and regulating pore structure) while maintaining good dispersibility and avoiding the negative effects of agglomeration. Below this amount, the reinforcing effect is insufficient; above this amount, it may disrupt the continuity of the matrix or clog pores, thus impairing performance. This provides a precise and reliable parameter window for the implementation of the process of this invention.

[0115] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the nature and scope of the present invention. Various modifications made to the above embodiments by those skilled in the art after reading this specification are all within the scope of protection of the present invention.

Claims

1. A method for preparing an ultrafiltration support layer, characterized by, It comprises the following steps: (1) preparing nanocrystalline cellulose and copolymer polyimide composite slurry; (2) applying the composite slurry in step (1) to a non-woven fabric substrate, phase inversion, washing, drying, and obtaining an ultrafiltration support layer.

2. The method of claim 1, wherein the ultrafiltration support layer is prepared by the steps of: The nanocrystalline cellulose is prepared by acid hydrolysis of microcrystalline cellulose, the hydrolysis temperature is 40-50℃, the hydrolysis time is 2-3h, centrifugation, washing, and freeze-drying to obtain nanocrystalline cellulose powder; and / or, the particle size of the nanocrystalline cellulose powder is 50-100nm.

3. The method for preparing an ultrafiltration support layer according to claim 1, characterized in that, The preparation method of the composite slurry is to add nanocrystalline cellulose to the copolymer polyimide slurry, the nanocrystalline cellulose accounts for 0.3-0.7wt% of the composite slurry, stirring, ultrasonic dispersion, and filtering out impurities on the filter membrane to obtain the composite slurry.

4. An ultrafiltration support layer, characterized in that, The ultrafiltration support layer is prepared by the preparation method of any one of claims 1-3.

5. An ultrafiltration support layer according to claim 4, characterized in that: The water contact angle of the ultrafiltration support layer is ≤57.1°, the skin layer tensile strength is ≥4.01MPa, the porosity is ≥85.7%, and the molecular weight cut-off is 1000-10000Da.

6. A method for preparing a composite nanofiltration membrane, characterized by, It comprises the following steps: (1) soaking the ultrafiltration support layer prepared in claim 4 in an aqueous phase solution containing an aqueous phase monomer for 1-2min, and draining the excess aqueous phase solution on the surface; (2) immersing it in an oil phase solution containing an oil phase monomer, and carrying out interfacial polymerization reaction at a temperature of 23-27℃ for 15-30s, and drying to obtain a composite nanofiltration membrane.

7. The method of claim 6, wherein the composite nanofiltration membrane is prepared by the steps of: The mass fraction of the aqueous phase monomer in the aqueous phase solution is 2-4wt%; and / or, The aqueous phase monomer is at least one of piperazine, o-phenylenediamine, diethylenetriamine, m-phenylenediamine, triethylenetetramine, triethanolammonium, and methyldiethanolamine.

8. The method of claim 6, wherein the composite nanofiltration membrane is prepared by the steps of: The mass fraction of the oil phase monomer in the oil phase solution is 0.1-0.2wt%; and / or, The oil phase monomer is at least one of trimesoyl chloride, phthaloyl chloride, isophthaloyl chloride, and terephthaloyl chloride; and the solvent of the oil phase solution is at least one of n-hexane, trifluorotrichloroethane, cyclohexane, and heptane.

9. A composite nanofiltration membrane, characterized by, The composite nanofiltration membrane is prepared by the preparation method of any one of claims 6-8.

10. The application of the composite nanofiltration membrane of claim 9 in sewage treatment, brine desalination, or pharmaceutical purification.

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