Method for preparing polyamide film composite film by low-temperature salt crystallization method
By controlling the precipitation of inorganic salts through low-temperature salt crystallization, a polyamide composite membrane with a high nano-wrinkle structure is formed, which solves the problems of insufficient inorganic salt introduction and crystallization pressure damaging the selective layer, and achieves a balance between high water flux and high salt rejection rate.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-04
- Publication Date
- 2026-04-03
AI Technical Summary
In existing technologies, the introduction of small amounts of inorganic salts results in minimal improvement in membrane performance, while high amounts of inorganic salts can lead to crystallization pressure damage to the selective layer, affecting the performance of polyamide composite membranes.
By employing a low-temperature salt crystallization method, inorganic salts are precipitated at low temperatures through the preparation of aqueous and organic phase solutions, combined with suitable reaction conditions, to form a polyamide composite membrane with a highly nano-wrinkled structure.
A polyamide composite membrane with both high water flux and high salt rejection rate was prepared. The preparation process is stable and highly reproducible, and has industrial application value.
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Figure CN121775682A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of polyamide composite film preparation technology, and in particular to a method for preparing polyamide thin film composite films by low-temperature salt crystallization. Background Technology
[0002] Faced with the severe global drinking water shortage crisis, the development of membrane-based water purification and desalination technologies has become an inevitable trend. To alleviate this situation, membrane separation technologies such as ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) have been developed. Membrane separation technology, characterized by high energy efficiency, low carbon emissions, and high design flexibility, has become an effective and sustainable method for alleviating global water scarcity, environmental remediation, and resource recycling. Nanofiltration (NF), as a cost-effective membrane separation method, can effectively filter small molecules and multivalent ions, showing great promise in wastewater treatment, water softening, and purification processes. Polyamide membranes prepared by interfacial polymerization are the benchmark nanofiltration membranes. Interfacial polymerization is the most widely used process due to its simple operation and mature technology. Generally, interfacial polymerization involves the diffusion and rapid reaction of aqueous monomers with oil-phase monomers to deposit a polyamide active layer on a porous support layer.
[0003] However, the highly cross-linked, dense PA layer results in low water flux. Studies have shown that adding inorganic salts during interfacial polymerization introduces a small amount of inorganic salts between the polyamide layer and the support layer. Through oven heat treatment, the inorganic salts crystallize out. Washing away the inorganic salts with water can reduce the adhesion between the selective and support layers and increase the membrane specific area. However, this preparation method has significant limitations: it can only introduce a small amount of inorganic salts; excessive salt crystallization can cause crystallization pressure that damages the formed selective layer. To address this problem, this patent proposes a low-temperature salt crystallization method to prepare polyamide composite membranes, increasing the amount of inorganic salts introduced into the membrane while simultaneously producing a high-performance polyamide composite membrane.
[0004] By simultaneously carrying out inorganic salt precipitation and selective layer growth, the selective layer damage caused by crystallization pressure can be effectively avoided. Furthermore, a high content of inorganic salt can be introduced into the membrane to prepare a polyamide composite membrane with a highly nano-wrinkled structure and high nano-porosity, thereby achieving high water flux while maintaining a high salt rejection rate. Summary of the Invention
[0005] Therefore, the purpose of this invention is to provide a method for preparing polyamide thin film composite membranes by low-temperature salt crystallization, which solves the problem that the membrane performance improvement is not obvious when the amount of inorganic salt introduced is small, but the selective layer is damaged by crystallization pressure caused by high inorganic salt content.
[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:
[0007] A method for preparing polyamide thin-film composite films by low-temperature salt crystallization, the method comprising the following steps:
[0008] Preparation of aqueous solution: Dissolve piperazine and inorganic salt in hot water to obtain an aqueous solution;
[0009] Preparation of organic phase solution: An organic phase solution was prepared using n-hexane as solvent and trimesoyl chloride as solute, and the organic phase solution was cooled to below -20℃;
[0010] The polyethersulfone-based membrane is immersed in an aqueous solution, and then the excess aqueous solution on the surface of the polyethersulfone-based membrane is removed. Subsequently, an organic solution is poured onto the wetted surface of the polyethersulfone-based membrane and reacted. The excess organic solution is then poured off to obtain a polyamide composite membrane.
[0011] Furthermore, the inorganic salt is selected from one of sodium chloride, potassium chloride, lithium chloride, sodium sulfate, potassium nitrate, ammonium nitrate, sodium nitrate, sodium bicarbonate, sodium carbonate, and sodium acetate.
[0012] Preferably, the inorganic salt is selected from sodium carbonate, sodium sulfate, and potassium nitrate.
[0013] Furthermore, the concentration of the inorganic salt in the aqueous solution is 0.15–0.6 g / mL.
[0014] Furthermore, the concentration of piperazine in the aqueous solution is 2.5–8 mg / mL.
[0015] Furthermore, when preparing the aqueous solution, the hot water temperature is 50°C.
[0016] Furthermore, the mass-volume percentage of the pyromellitic trimethylol chloride in the organic phase solution is 0.1–0.15% (w / v).
[0017] Furthermore, the cooling temperature of the organic phase solution is -20℃ to -40℃.
[0018] Furthermore, when the polyethersulfone-based membrane is immersed in the aqueous solution, the immersion time is 5 minutes; the reaction time between the organic phase solution and the polyethersulfone-based membrane is 1.5 to 5 minutes.
[0019] Furthermore, the polyamide composite membrane was treated with hot water at 60°C for 30 min.
[0020] Beneficial effects:
[0021] This invention lowers the temperature of the organic phase, causing the near-saturated inorganic salts in the high-temperature aqueous phase to crystallize and precipitate during the interfacial polymerization process due to the decrease in solubility caused by the lower temperature. This low-temperature salt crystallization method creates a unique interfacial polymerization reaction system through the systematic design and synergistic control of reaction conditions and parameters. The various reaction conditions are interconnected and reinforced to work together at the reaction interface, thereby obtaining a polyamide composite membrane with both ultra-high water flux and ultra-high salt rejection rate and no defects. Moreover, the preparation process is stable, highly reproducible, and has significant industrial application value. Attached Figure Description
[0022] Figure 1 Polyamide film composite films prepared in Examples 1, 2, and 3;
[0023] Figure 2 Polyamide film composite films prepared in Examples 2, 3, 4, 5, 6, and 7. Detailed Implementation
[0024] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings:
[0025] Unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods. Unless otherwise specified, the materials and reagents used in the embodiments of this invention are commercially available, and the base film is SynderFiltration, PES LX-300.
[0026] Example 1:
[0027] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0028] Add 30g of sodium carbonate and 0.5g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0029] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0030] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0031] The polyethersulfone (PES) membrane was completely immersed in an aqueous solution for 5 minutes. Excess aqueous solution was then removed from the surface of the PES membrane. An organic solution was then poured onto the wetted PES membrane surface, and the reaction was allowed to proceed for 5 minutes. The excess organic solution was then discarded, yielding a polyamide composite membrane. This newly formed polyamide composite membrane was then heat-treated in 60°C hot water for 30 minutes, and subsequently stored in pure water for use.
[0032] Example 2:
[0033] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0034] Add 30g of sodium carbonate and 0.5g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0035] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0036] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -20 °C before use.
[0037] The reaction of the polyethersulfone-based membrane with the aqueous and organic phase solutions was the same as in Example 1.
[0038] Example 3:
[0039] Preparation of aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50℃ to obtain an aqueous solution; the specific preparation method is as follows: Add 15g sodium carbonate and 0.5g piperazine to 100mL of hot water, stir evenly to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.15g / mL;
[0040] The organic phase solution was prepared in the same way as in Example 1, but was cooled to -30°C before use.
[0041] The reaction of the polyethersulfone-based membrane with the aqueous and organic phase solutions was the same as in Example 1.
[0042] Example 4:
[0043] Preparation of the aqueous phase solution: Dissolve piperazine and sodium carbonate in hot water at 50℃ to obtain an aqueous phase solution; the specific preparation method is as follows: Add 30g of sodium carbonate and 0.8g of piperazine to 100mL of hot water, stir evenly to obtain an aqueous phase solution with a piperazine concentration of 8mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0044] The organic phase solution was prepared in the same manner as in Example 1, but after preparation, it was cooled to -30°C before use.
[0045] The reaction of the polyethersulfone-based membrane with the aqueous and organic phase solutions was the same as in Example 1.
[0046] Example 5:
[0047] Preparation of aqueous solution: Dissolve piperazine and sodium sulfate in hot water at 50℃ to obtain an aqueous solution; the specific preparation method is as follows: Add 46g of sodium sulfate and 0.25g of piperazine to 100mL of hot water, stir evenly to obtain an aqueous solution with a piperazine concentration of 2.5mg / mL and a sodium sulfate concentration of 0.46g / mL;
[0048] The organic phase solution was prepared in the same manner as in Example 1, but after preparation, it was cooled to -40°C before use.
[0049] The reaction of the polyethersulfone-based membrane with the aqueous and organic phase solutions was the same as in Example 1.
[0050] Example 6:
[0051] Preparation of the aqueous solution: Dissolve piperazine and potassium nitrate in hot water at 50℃ to obtain an aqueous solution; the specific preparation method is as follows: Add 60g of potassium nitrate and 0.25g of piperazine to 100mL of hot water, stir evenly to obtain an aqueous solution with a piperazine concentration of 2.5mg / mL and a potassium nitrate concentration of 0.6g / mL;
[0052] The organic phase solution was prepared in the same manner as in Example 1, but after preparation, it was cooled to -40°C before use.
[0053] The reaction of the polyethersulfone-based membrane with the aqueous and organic phase solutions was the same as in Example 1.
[0054] Comparative Example 1:
[0055] Preparation of aqueous solution: Add 0.5g piperazine to 100mL of water at room temperature, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL;
[0056] Preparation of organic phase solution: Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane at room temperature, stir well to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride;
[0057] The subsequent processing is the same as in Example 1.
[0058] Comparative Example 2:
[0059] Preparation of the aqueous solution: Dissolve piperazine in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0060] Add 0.5g piperazine to 100mL of hot water and stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL;
[0061] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0062] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0063] The subsequent processing is the same as in Example 1.
[0064] Comparative Example 3:
[0065] Preparation of aqueous solution: Add 30g sodium carbonate and 0.5g piperazine to 100mL of water at room temperature, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0066] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0067] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane at room temperature, and stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride.
[0068] The subsequent processing is the same as in Example 1.
[0069] Comparative Example 4:
[0070] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0071] Add 30g of sodium carbonate and 0.5g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0072] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0073] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to 0 °C before use.
[0074] The subsequent processing is the same as in Example 1.
[0075] Comparative Example 5:
[0076] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0077] Add 30g of sodium carbonate and 0.5g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0078] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0079] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -40 °C before use.
[0080] The subsequent processing is the same as in Example 1.
[0081] Comparative Example 6:
[0082] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0083] Add 5g of sodium carbonate and 0.5g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.05g / mL;
[0084] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0085] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0086] The subsequent processing is the same as in Example 1.
[0087] Comparative Example 7:
[0088] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0089] Add 45g of sodium carbonate and 0.5g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 5mg / mL and a sodium carbonate concentration of 0.45g / mL;
[0090] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0091] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0092] The subsequent processing is the same as in Example 1.
[0093] Comparative Example 8:
[0094] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 50°C to obtain an aqueous solution; the specific preparation method is as follows:
[0095] Add 30g of sodium carbonate and 0.25g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 25mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0096] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0097] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0098] The subsequent processing is the same as in Example 1.
[0099] Comparative Example 9:
[0100] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in water at 20°C to obtain an aqueous solution; the specific preparation method is as follows:
[0101] Add 30g of sodium carbonate and 0.8g of piperazine to 100mL of water at 20℃, stir well to obtain an aqueous solution with a piperazine concentration of 8mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0102] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0103] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0104] The subsequent processing is the same as in Example 1.
[0105] Comparative Example 10:
[0106] Preparation of the aqueous solution: Dissolve piperazine and sodium carbonate in hot water at 80℃ to obtain an aqueous solution; the specific preparation method is as follows:
[0107] Add 30g of sodium carbonate and 0.8g of piperazine to 100mL of hot water, stir well to obtain an aqueous solution with a piperazine concentration of 8mg / mL and a sodium carbonate concentration of 0.3g / mL;
[0108] Preparation of the organic phase solution: An organic phase solution was prepared using n-hexane as the solvent and trimesoyl chloride as the solute; the specific preparation method is as follows.
[0109] Add 0.1 g of trimesoyl chloride to 100 mL of n-hexane, stir until homogeneous to obtain an organic phase solution with a mass-volume percentage of 0.1% trimesoyl chloride, and cool the organic phase solution to -30 °C before use.
[0110] The subsequent processing is the same as in Example 1.
[0111] To make the observation more intuitive, the preparation methods of the above embodiments are represented in a table, as shown in Table 1:
[0112] Table 1
[0113]
[0114] I. Composite Membrane Performance Testing
[0115] Electron microscopy was performed on the polyamide composite films prepared in Examples 1-3 and Comparative Examples 1-7, and the results are as follows: Figure 1 , 2 As shown, analysis Figure 1 , 2 The results show that:
[0116] from Figure 1 It can be observed that although changing the two-phase temperature alone or adding inorganic salts can roughen the membrane surface, compared to Comparative Examples 1-3, Example 1, by simultaneously changing the two-phase temperature and adding inorganic salts, resulted in a rough, wrinkled structure on the membrane surface, significantly increasing the specific surface area for water mass transfer. Figure 2It can be observed that: comparing Comparative Examples 4 and 5, the lower the temperature of the organic phase, the rougher the membrane. Comparing Comparative Examples 6 and 7 with Example 1, the higher the salt content, the rougher the membrane; however, excessively high salt content leads to the formation of numerous salt crystals on the membrane surface, damaging the membrane's structural integrity. Comparing Example 1 and Example 2, the lower the temperature, the rougher the membrane surface. Comparing Example 1 and Example 3, the more salt, the larger the wrinkles on the membrane surface, and the rougher the membrane surface.
[0117] II. Testing of brine flux and sodium sulfate retention performance
[0118] To better demonstrate the performance of the polyamide composite membranes, the brine flux and sodium sulfate retention performance of the polyamide composite membranes prepared in the above examples and comparative examples were tested. The specific experimental procedures are as follows:
[0119] (1) Preparation of test solution: Prepare a sodium sulfate solution with a concentration of 1 g / L using sodium sulfate and pure water.
[0120] (2) Experimental procedure: The membrane was tested under a pressure of 6 bar and a flow rate of 40 L / h.
[0121] The formula for calculating the brine flux is: A =
[0122] Where A is the brine flux (LMH / bar), ΔV is the change in feed volume (L) during the test time Δt, and ΔP is the transmembrane pressure difference (bar). m Effective membrane area (m²) 2 ).
[0123] The formula for calculating the retention rate is:
[0124] Among them, R s C represents the salt rejection rate (%). f and C p The concentrations (mg / L) of the feed solution and the permeate are respectively.
[0125] Finally, the data for each group were tallied, and the results are shown in Table 2.
[0126] Table 2
[0127]
[0128] Data Analysis:
[0129] The comparison of the results of the example group and the comparative example group in Table 2 shows that the example group exhibits excellent and balanced performance, with water flux concentrated between 20.01-32.9 LMH / bar, and Na2SO4 rejection rate consistently maintained at a high level of 97.3-99.05%. This indicates that the polyamide composite membrane prepared in this invention achieves a good balance between high permeability and high selectivity. In contrast, the membrane performance of the comparative example group varies significantly and generally exhibits shortcomings. The water flux range is wide, between 3.14-35.32 LMH / bar, and there are instances where the rejection rate drops sharply at high fluxes, or the flux is extremely low while maintaining the rejection rate.
[0130] Analysis of Tables 1 and 2 shows that:
[0131] 1. Based on the comparative results of Examples 1 and 2, and Comparative Examples 3 and 5, it can be seen that the higher the temperature of the organic phase, the higher the reactivity and the thicker the selective layer, resulting in a less significant increase in water flux. Conversely, if the temperature of the organic phase is too low, the reactivity is poor, leading to defects in the formed selective layer and a decrease in salt rejection. This indicates that in the membrane preparation process disclosed in this invention, using a suitable temperature for the organic phase allows the aqueous and organic phases more sufficient time to react with the base membrane, forming a polyamide layer with higher crosslinking degree, greater density, and thinner thickness, ultimately yielding a high-flux, high-rejection polyamide composite membrane.
[0132] 2. Comparison of Example 1 with Comparative Examples 1 and 2 shows that the addition of inorganic salts can effectively improve the separation performance of the membrane. This also demonstrates that both inorganic salts and low temperature are indispensable. The concentration of inorganic salts also has a significant impact on the performance of the composite membrane. The performance of Comparative Example 6 (too low inorganic salt concentration of 0.05 g / mL) and Comparative Example 7 (too high inorganic salt concentration of 0.45 g / mL) were both unsatisfactory. This indicates that the low temperature environment greatly inhibits the hydrolysis rate of the organic phase monomers, providing a time window for the reaction. The addition of inorganic salts, on the one hand, enriches the aqueous phase monomers at the interface through the salting-out effect, increasing the local reaction concentration; on the other hand, it acts as an acid absorbent to neutralize the byproduct hydrochloric acid in a timely manner, maintaining an alkaline microenvironment conducive to amide bond formation. The synergistic effect of these two factors precisely controls the reaction kinetics of interfacial polymerization, promoting the formation of a highly cross-linked, extremely thin, and defect-free polyamide selective layer. Examples 5 and 6 also verify that, except for sodium carbonate, potassium nitrate and sodium sulfate can be used to prepare high-performance polyamide composite membranes under appropriate concentrations and conditions.
[0133] 3. Piperazine is a film-forming monomer, and its concentration directly affects the number of amine groups available for reaction at the interface. Both excessively high and low piperazine concentrations affect membrane performance; generally, within a certain range, increasing the concentration results in a thicker film. Comparative Example 9 demonstrates that increasing the aqueous phase temperature increases the kinetic energy and diffusion rate of piperazine molecules, allowing them to replenish the interface more quickly, which is beneficial for forming a more uniform and fully reacted film. However, excessively high temperatures, such as in Comparative Example 10, while resulting in rapid diffusion, may simultaneously exacerbate the hydrolysis and side reactions of the organic phase, disrupting the ordered interfacial polymerization process and forming a film with more defects, significantly inferior in performance to the examples under the same conditions. In the examples, the moderate aqueous phase temperature ensured that the amine monomer had sufficient diffusion kinetic energy, enabling rapid and uniform delivery to the reaction interface. Combined with the low-temperature cooling of the organic phase, this strongly suppressed side reactions at the interface, allowing the aqueous and organic phases to react fully and orderly. This ensured a perfect match between the monomer supply rate and the controllable reaction rate at the interface, achieving a globally optimal balance in the interfacial polymerization reaction kinetics, thereby guaranteeing the repeatable and stable formation of the high-performance polyamide selective layer.
[0134] In summary, this invention employs a low-temperature salt crystallization method to prepare a high-performance polyamide composite membrane. By matching the concentrations of the low-temperature organic phase solution and the monomer solution in the aqueous phase, combined with the microenvironment of inorganic salts, a perfect balance of interfacial polymerization kinetics is achieved, resulting in a high-throughput, high-retention polyamide composite membrane. Furthermore, comparisons of various embodiments demonstrate that the solution of this invention has the advantages of stable performance and high repeatability.
[0135] 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 present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention. Technical aspects, shapes, and structures not described in detail in this invention are all well-known technologies.
Claims
1. A method for preparing polyamide thin-film composite films by low-temperature salt crystallization, characterized in that, The method includes the following steps: Preparation of aqueous solution: Dissolve piperazine and inorganic salt in hot water to obtain an aqueous solution; Preparation of organic phase solution: An organic phase solution was prepared using n-hexane as solvent and trimesoyl chloride as solute, and the organic phase solution was cooled to below -20°C; The polyethersulfone-based membrane is immersed in an aqueous solution, and then the excess aqueous solution on the surface of the polyethersulfone-based membrane is removed. Subsequently, an organic solution is poured onto the wetted surface of the polyethersulfone-based membrane and reacted. The excess organic solution is then poured off to obtain a polyamide composite membrane.
2. The method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to claim 1, characterized in that, The inorganic salt is selected from one of sodium chloride, potassium chloride, lithium chloride, sodium sulfate, potassium nitrate, ammonium nitrate, sodium nitrate, sodium bicarbonate, sodium carbonate, and sodium acetate.
3. The method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to claim 2, characterized in that, The inorganic salt is selected from one of sodium carbonate, sodium sulfate, and potassium nitrate.
4. The method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to claim 3, characterized in that, The concentration of the inorganic salt in the aqueous solution is 0.15–0.6 g / mL.
5. The method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to claim 4, characterized in that, The concentration of piperazine in the aqueous solution is 2.5–8 mg / mL.
6. The method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to claim 5, characterized in that, When preparing the aqueous solution, the hot water temperature is 50°C.
7. A method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to any one of claims 1-6, characterized in that, The mass-volume percentage of the pyromellitic trimethylol chloride in the organic phase solution is 0.1–0.15% (w / v).
8. The method for preparing polyamide thin film composite membrane by low-temperature salt crystallization according to claim 7, characterized in that, The organic phase solution is cooled to a temperature of -20℃ to -40℃.
9. The method for preparing polyamide thin-film composite films by low-temperature salt crystallization according to claim 8, characterized in that, When the polyethersulfone-based membrane is immersed in the aqueous solution, the immersion time is 5 minutes; the reaction time between the organic phase solution and the polyethersulfone-based membrane is 1.5 to 5 minutes.
10. The method for preparing polyamide thin film composite membrane by low-temperature salt crystallization according to claim 9, characterized in that, The obtained polyamide composite film was treated with hot water at 60°C for 30 min.