A method for preparing a high porosity high flux asymmetric membrane

Abstract

The application relates to a preparation method of a high-porosity high-flux asymmetric membrane, and belongs to the field of membrane separation technology. The preparation method of the high-porosity high-flux asymmetric membrane comprises the following steps: uniformly dissolving a polymer and a hydrophilic additive in an organic solvent to prepare a homogeneous casting solution; preparing a nascent membrane from the casting solution; then, performing atomization pretreatment on the nascent membrane; rapidly cooling the nascent membrane after the pretreatment; then, immersing the nascent membrane in a gel bath to form an asymmetric membrane; and performing multiple flushing and soaking on the asymmetric membrane in normal-temperature deionized water to obtain the high-porosity high-flux asymmetric membrane. The asymmetric membrane adopts a non-solvent atomization technology combined with cooling-induced crystallization, the non-solvent atomization is introduced into the surface of the nascent membrane, the solution is rapidly saturated by accurate temperature control, the crystal formation is triggered, the macroporous pores are formed, and the membrane porosity and flux are improved. The obtained asymmetric membrane has a macroporous network on the surface layer and a finger-shaped pore on the support layer, the porosity is 75% to 85%, the pure water flux is greatly improved, and the high-efficiency separation requirement is met.

Description

A method for preparing a high-porosity, high-flux asymmetric membrane Technical Field

[0001] This invention relates to a method for preparing a high-porosity, high-flux asymmetric membrane, belonging to the field of membrane separation technology. Background Technology

[0002] Membrane purification technology, as a highly efficient and environmentally friendly water treatment technology, has been widely applied in laboratory research and industrial practice. This technology, by controlling the membrane pore size, can achieve precise separation of substances of different sizes and molecular weights at room temperature. Among them, asymmetric (UF) membranes, as an important component of membrane purification technology, have shown broad application prospects in various fields such as food, pharmaceuticals, drinking water production, and industrial wastewater treatment due to their excellent separation performance, ease of operation, and ease of maintenance.

[0003] However, traditional asymmetric membranes often suffer from insufficient porosity, uneven pore size distribution, and low flux during preparation, limiting their separation efficiency and processing capacity. Asymmetric membranes have an overall asymmetric structure, with a thin outer skin and a more open internal structure. While this structure provides the necessary mechanical strength, it also affects their permeation performance to some extent. To improve the porosity and flux of asymmetric membranes, researchers have tried various methods; however, while these methods improve membrane performance, they also bring problems such as high preparation costs, complex processes, and low production efficiency. Summary of the Invention

[0004] This invention provides a method for preparing high-porosity, high-flux asymmetric membranes. This method offers advantages such as a simple preparation process, readily available raw materials, and low cost. It is easily scalable for continuous, large-scale production of separation membranes, facilitating industrial application. The method employs a preparation strategy combining solvent-free atomization with cooling-induced crystallization technology, opening a new avenue for the production of high-porosity, high-flux asymmetric membranes. This method not only significantly reduces operating costs but is also environmentally friendly. Its unique process advantages indicate that this technology has significant application potential and is expected to achieve large-scale production in the future.

[0005] A method for preparing a high-porosity, high-flux asymmetric membrane involves uniformly dissolving a polymer and a hydrophilic additive in an organic solvent to form a homogeneous casting solution; using the casting solution to form a primary membrane, followed by atomization pretreatment, rapid cooling, and then immersion in a gel bath to form an asymmetric membrane; and finally, repeatedly rinsing and soaking the asymmetric membrane in room-temperature deionized water to obtain a high-porosity, high-flux asymmetric membrane.

[0006] The atomization pretreatment is as follows: at a certain distance from the surface of the nascent membrane, an aqueous solution of inorganic salt with a mass fraction of 60% to 95% of the corresponding inorganic salt saturation concentration is uniformly sprayed onto the surface of the nascent membrane in the form of atomization for a period of time. The inorganic salt is at least one of sodium chloride, calcium chloride, magnesium sulfate, potassium chloride, ammonium sulfate, calcium nitrate, and sodium carbonate.

[0007] In the above technical solution, the concentration of the inorganic salt aqueous solution used in this invention is 60-95% of the saturation concentration of the inorganic salt aqueous solution at the corresponding temperature.

[0008] Preferably, an inorganic salt aqueous solution at a temperature of 30℃ to 70℃ is sprayed evenly onto the surface of the nascent membrane in a mist form at a distance of 3cm to 20cm from the surface of the nascent membrane for 5s to 120s, with a spray rate of 50ml / h to 350ml / h.

[0009] Preferably, the atomization method is one of pressure atomization, rotary atomization, ultrasonic atomization, and thermal atomization.

[0010] Preferably, the polymer in the casting solution has a mass fraction of 4% to 22%, the hydrophilic additive has a mass fraction of 2% to 18%, and the remainder is an organic solvent.

[0011] Preferably, the polymer is selected from at least one of polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, polyacrylic acid, polylactic acid, polyamide, chitosan, polyetherimide, polystyrene, polyolefin, polyester, polychlorotrifluoroethylene, silicone resin, and acrylonitrile-styrene copolymer.

[0012] Preferably, the hydrophilic additive is selected from at least one of polyvinylpyrrolidone, polyethylene glycol, fluoropolymers, polyoxyethylene, and polyoxypropylene ether block copolymers.

[0013] Preferably, the organic solvent is at least one selected from N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N-vinylpyrrolidone (NVP).

[0014] Preferably, the thickness of the nascent membrane is 100–500 μm.

[0015] Preferably, the nascent membrane is prepared by scraping or stretching.

[0016] Preferably, the primary membrane is a flat sheet or a composite support structure.

[0017] The preparation method is scraping or stretching; the membrane has a flat or composite support structure.

[0018] Preferably, the rapid cooling is characterized by a cooling time of 5s to 60s, cooling down to 0℃ to 20℃.

[0019] Preferably, the gel bath is one of deionized water, methanol, ethanol, glycerol, isopropanol, ethylene glycol, n-butanol, and acetone.

[0020] Furthermore, the temperature of the gel bath is 10℃~20℃.

[0021] Another object of the present invention is to provide a high porosity, high flux asymmetric membrane prepared by the above method, wherein the surface layer of the asymmetric membrane has a network-like pore structure, the support layer has finger-like pores, and the porosity is 75% to 85%.

[0022] The beneficial effects of this invention are as follows: The preparation method of the asymmetric supported membrane described in this invention employs non-solvent atomization technology combined with cooling-induced crystallization. Non-solvent atomization is introduced onto the surface of the nascent membrane, and precise temperature control rapidly saturates the solution, initiating crystal formation and creating large-pore structures, thereby increasing membrane porosity and flux. The resulting asymmetric membrane surface has a large-pore network, the support layer has finger-like pores, and the porosity is 75%–85%, significantly increasing pure water flux and meeting the requirements for high-efficiency separation. This invention innovates upon the traditional non-solvent-induced phase inversion method, requiring only the addition of atomization pretreatment and cooling-induced crystallization. This innovative method not only simplifies the preparation process but also uses readily available and cost-effective raw materials. More importantly, this method supports continuous large-scale production of separation membrane materials, making it highly suitable for industrial applications. Therefore, this invention demonstrates broad application potential and excellent prospects in various fields such as water treatment, biotechnology, pharmaceuticals, and energy. Attached Figure Description

[0023] Figure 1 is an electron microscope image of the surface of the asymmetric film prepared in Example 1;

[0024] Figure 2 is an electron microscope image of the cross-section of the asymmetric film prepared in Example 1;

[0025] Figure 3 shows an electron microscope image of the surface of the asymmetric film prepared in Comparative Example 1;

[0026] As can be seen from Figures 1 and 2, the asymmetric membrane treated by the method described in this invention has a large-pore network-like pore structure on its surface and a finger-like pore structure in its support layer. For easy comparison, Figure 3 shows that the asymmetric membrane not treated by this method has a dense sponge-like pore structure on its surface. Detailed Implementation

[0027] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0028] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0029] One specific implementation method: A method for preparing a high-porosity, high-flux asymmetric membrane, comprising dissolving a polymer and a hydrophilic additive in an organic solvent, stirring until homogeneous, and then vacuum degassing to prepare a homogeneous casting solution, which is then cast into a primary membrane; subsequently, the primary membrane is subjected to atomization pretreatment, and after a period of treatment, it is immediately placed in a cooling device for rapid cooling and induced crystallization, and then immersed in a gel bath to form an asymmetric membrane. The asymmetric membrane is then rinsed and soaked multiple times in room temperature deionized water to obtain the high-porosity, high-flux asymmetric membrane.

[0030] Example 1

[0031] 12g of polyacrylonitrile and 16g of polyethylene glycol 400 were dissolved in 72g of DMF and stirred until homogeneous. The mixture was then degassed under vacuum to prepare a homogeneous casting solution. This solution was cast and scraped to form a 200μm thick flat nascent membrane. Immediately afterward, a 40% (w / w) calcium chloride solution at 70℃ was uniformly sprayed onto the nascent membrane surface at a rated mist flow rate of 350ml / h at a distance of 5cm from the nascent membrane surface for 30s. The membrane was then immediately placed in a cooling device for 20s to rapidly cool to 5℃ for induced crystallization. Finally, it was immersed in 15℃ deionized water to form an asymmetric membrane. The asymmetric membrane was repeatedly rinsed and soaked in room temperature deionized water to obtain the high-porosity, high-flux asymmetric membrane. The surface microstructure of this membrane is shown in Figure 1, and the porosity and pure water flux are shown in Table 1.

[0032] Comparative Example 1

[0033] For ease of comparison, the experimental method of Example 1 was followed, except that the ultrafiltration membrane was prepared without the atomization pretreatment and low-temperature induced crystallization process. The surface microstructure of the membrane is shown in Figure 3, and the porosity and pure water flux are shown in Table 1.

[0034] Table 1

[0035]

[0036] Examples 2-18

[0037] The experimental method of Example 1 was followed, but the difference from Example 1 was that the types and mass fractions of polymers were changed. The polymers were polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, polyacrylic acid, polylactic acid, polyamide, chitosan, polyetherimide, polystyrene, polyolefin, polyester, polychlorotrifluoroethylene, and silicone resin. The mass fraction of the polymers ranged from 4% to 22%. The porosity and pure water flux of the asymmetric membranes obtained are shown in Table 2.

[0038] Table 2

[0039]

[0040]

[0041] Example 19

[0042] 14g of polyaryletherketone and 14g of polyethylene glycol 1000 were dissolved in 72g of DMAc and stirred until homogeneous. The mixture was then degassed under vacuum to prepare a homogeneous casting solution. This solution was cast and scraped to form a 300μm thick flat nascent membrane. Immediately afterward, a 20% sodium chloride solution at 70℃ was uniformly sprayed onto the nascent membrane surface at a rated flow rate of 300ml / h from 8cm above the surface using a rotary atomizer for 20s. The membrane was then immediately placed in a cooling device for 10s to rapidly cool to 5℃ for induced crystallization. Finally, it was immersed in 20℃ deionized water to form an asymmetric membrane. The asymmetric membrane was repeatedly rinsed and soaked in room temperature deionized water, yielding an asymmetric membrane with a porosity of 83% and a pure water flux of 4106 L / (m²). 2 ·h·bar).

[0043] Examples 20-22

[0044] The experimental method of Example 19 was followed, but the difference from Example 19 was the type and mass fraction of the hydrophilic additives. The hydrophilic additives were polyvinylpyrrolidone, polyethylene glycol, fluoropolymer, and polyoxyethylene; the mass fractions were 2% to 18%. The porosity and pure water flux of the asymmetric membranes prepared are shown in Table 3.

[0045] Table 3

[0046]

[0047] Example 23

[0048] 16g of polyimide and 8g of polyvinylpyrrolidone K30 were dissolved in 76g of DMAc and stirred until homogeneous. The mixture was then degassed under vacuum to prepare a homogeneous casting solution. This solution was cast and scraped to form a 250μm thick flat nascent membrane. Immediately afterward, a 30% magnesium sulfate solution at 70℃ was uniformly sprayed onto the nascent membrane surface at a rated flow rate of 250ml / h at a distance of 12cm using pressure atomization for 15s. The membrane was then immediately placed in a cooling device for 10s to rapidly cool to 5℃ to induce crystallization. Finally, it was immersed in 25℃ ethanol to form an asymmetric membrane. The asymmetric membrane was repeatedly rinsed and soaked in deionized water at room temperature, resulting in an asymmetric membrane with a porosity of 80% and a pure water flux of 4026L / (m²). 2 ·h·bar).

[0049] Examples 24-26

[0050] The experimental method of Example 23 was followed, but the difference from Example 23 was the type of organic solvent used: N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N-vinylpyrrolidone (NVP). The porosity and pure water flux of the resulting asymmetric membranes are shown in Table 4.

[0051] Table 4

[0052]

[0053] Example 27

[0054] 10g of polyolefin and 10g of polyoxyethylene were dissolved in 80g of DMAc and stirred until homogeneous. The mixture was then degassed under vacuum to prepare a homogeneous casting solution. This solution was cast and scraped to form a flat nascent membrane with a thickness of 180μm. Immediately afterward, a 30% magnesium sulfate solution at 70℃ was uniformly sprayed onto the nascent membrane surface at a rated flow rate of 250ml / h at a distance of 11cm from the surface using a thermal atomization method for 15s. The membrane was then immediately placed in a cooling device for 10s to rapidly cool to 5℃ to induce crystallization. Finally, it was immersed in 25℃ ethanol to form an asymmetric membrane. The asymmetric membrane was repeatedly rinsed and soaked in deionized water at room temperature, resulting in an asymmetric membrane with a porosity of 80% and a pure water flux of 4026L / (m²). 2 ·h·bar).

[0055] Examples 28-33

[0056] The experimental method of Example 27 was followed, but the difference lay in the types and mass fractions of the inorganic salt aqueous solutions. The inorganic salt aqueous solutions were sodium chloride, calcium chloride, potassium chloride, ammonium sulfate, calcium nitrate, and sodium carbonate aqueous solutions. The porosity and pure water flux of the prepared asymmetric membrane are shown in Table 5.

[0057] Table 5

[0058]

[0059] Examples 34-36

[0060] The experimental method was followed according to Example 27, but the difference from Example 27 was the atomization method, which was pressure atomization, rotary atomization, and ultrasonic atomization, respectively. The porosity and pure water flux of the obtained asymmetric membranes are shown in Table 6.

[0061] Table 6

[0062]

Claims

1. A method for preparing a high-porosity, high-flux asymmetric membrane, characterized in that, The polymer and hydrophilic additives are uniformly dissolved in an organic solvent to prepare a homogeneous casting solution. The casting solution is used to form a nascent membrane, which is then subjected to atomization pretreatment. After treatment, the membrane is rapidly cooled and then immersed in a gel bath to form an asymmetric membrane. The asymmetric membrane is repeatedly rinsed and soaked in deionized water at room temperature to obtain a high-porosity, high-flux asymmetric membrane. The atomization pretreatment involves uniformly spraying an inorganic salt aqueous solution with a mass fraction of 60% to 95% of the corresponding inorganic salt saturation concentration onto the surface of the nascent membrane at a certain distance from the surface of the nascent membrane for a period of time. The inorganic salt is at least one of sodium chloride, calcium chloride, magnesium sulfate, potassium chloride, ammonium sulfate, calcium nitrate, and sodium carbonate.

2. The method according to claim 1, characterized in that, At a distance of 3 cm to 20 cm from the surface of the nascent membrane, an inorganic salt aqueous solution at a temperature of 30℃ to 70℃ is uniformly sprayed onto the surface of the nascent membrane in the form of atomization for 5 s to 120 s, with a spray rate of 50 ml / h to 350 ml / h.

3. The method according to claim 1, characterized in that, The atomization method is one of pressure atomization, rotary atomization, ultrasonic atomization, or thermal atomization.

4. The method according to claim 1, characterized in that, The casting solution contains 4% to 22% polymer by mass, 2% to 18% hydrophilic additive by mass, and the remainder is organic solvent.

5. The method according to claim 1, characterized in that, The polymer is selected from at least one of polyvinyl chloride, polysulfone, polyethersulfone, sulfonated polyethersulfone, polyacrylonitrile, cellulose acetate, polyvinylidene fluoride, polyimide, polylactic acid, polyamide, chitosan, polyetherimide, polystyrene, polyolefin, polyester, polychlorotrifluoroethylene, silicone resin, and acrylonitrile-styrene copolymer; the hydrophilic additive is selected from at least one of polyvinylpyrrolidone, polyethylene glycol, and polyoxyethylene; the organic solvent is at least one of N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N-vinylpyrrolidone (NVP).

6. The method according to claim 1, characterized in that, The thickness of the primary membrane is 100~500 μm.

7. The method according to claim 1, characterized in that, The primary membrane is prepared by scraping or stretching; the primary membrane is a flat plate or composite support structure.

8. The method according to claim 1, characterized in that, The rapid cooling process involves a cooling time of 5 to 60 seconds, reducing the temperature to 0°C to 20°C.

9. The method according to claim 1, characterized in that, The gel bath is one of deionized water, methanol, ethanol, glycerol, isopropanol, ethylene glycol, n-butanol, and acetone, and the temperature is 10℃~20℃.

10. The high-porosity, high-flux asymmetric membrane prepared by the method according to any one of claims 1 to 9, characterized in that, The surface of the asymmetric membrane has a network-like pore structure, the support layer has finger-like pores, and the porosity is 75%~85%.

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