Polyether sulfone membrane with double-layer structure and preparation method thereof

Abstract

The invention belongs to the technical field of polyethersulfone, and particularly discloses a polyethersulfone film with a double-layer structure and a preparation method of the polyethersulfone film. The polyethersulfone membrane is prepared by adopting a single solution coating on the basis of a specific solution composition system through an upper and lower layer dual induction process, and the cross section of the membrane is in a highly asymmetric form with a double-layer structure integrally; the upper layer is of a coarse-branch continuous macroporous structure and can accommodate more intercepted particles, so that the service life of the membrane is prolonged; the lower layer is of a fine branched continuous small hole structure, so that interception of particles can be ensured, and the filtering quality is improved; the upper layer and the lower layer are communicated in aperture, the structural cohesion is good, and no fracture phenomenon occurs; the thickness ratio of the upper layer to the lower layer is (2.5-3.5): 2; the preparation technology is simple and controllable in process, easy to implement, low in equipment requirement and suitable for batch production.

Description

Technical Field

[0001] This invention relates to the field of polymer organic materials technology, particularly to the field of polyethersulfone technology, and especially to a polyethersulfone membrane with a bilayer structure and its preparation method. Background Technology

[0002] Microporous polyethersulfone (PES) membranes are polymer films with microporous structures and possess many excellent properties, such as high thermal stability, oxidation stability, and temperature stability. Therefore, they have wide applications in fields such as medicine, food, semiconductors, and water treatment.

[0003] To obtain PES microfiltration membranes that balance high filtration accuracy, high flow rate, and high dirt-holding capacity, the membrane must possess both a dense layer and a porous layer. Currently, research on highly asymmetric PES membranes with a bilayer structure is still in its early stages. Commonly used methods include co-coating or sequential coating, where two or more solutions of different properties are simultaneously or sequentially coated onto a support. Based on the phase separation of the two solution coatings and the control of the curing speed, each solution coating generates a structure with different pore sizes, which are then stacked together to form a bilayer structure.

[0004] For example, invention patents CN115038510A, CN103055709A, and CN115038510A, among others, describe the preparation of double-layer or multi-layer PES filter membranes. These membranes are created by co-coating or sequentially coating two or more solutions, and then controlling the viscosity, temperature, humidity, and other inducing conditions of the solutions to generate pore structures of different sizes for each coating solution. Ultimately, the filter membrane exhibits a double-layer or multi-layer structure. Theoretically, the preparation of asymmetric membranes is mainly based on the principle of liquid-liquid phase separation and the control of solvent-non-solvent exchange rates. The polymer is dissolved to form a homogeneous solution, and then conditions (such as temperature, humidity, and solution composition) are changed to induce liquid-liquid phase separation, forming a polymer-rich phase and a solvent-rich, solvent-poor phase. During the phase transition, the solvent-non-solvent exchange rate is controlled to generate membrane structures with different pore sizes. The preparation of bilayer or multilayer filter membranes using co-coating or sequential coating composite methods requires high precision in controlling solution composition and viscosity, precise operation of the membrane scraping process, and advanced equipment construction and accuracy. These factors are interrelated and directly affect the structure and performance of the filter membrane. Such methods are prone to problems such as layer delamination, poor pore connectivity, insufficient depth of the upper macropore layer, and limited membrane contaminant holding capacity and separation ability. Furthermore, the need to control the multilayer structure, the stringent process control, and the high precision requirements for several solution compositions and equipment make mass production difficult. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a polyethersulfone membrane with a double-layer structure and its preparation method, which solves the problems of traditional double-layer high asymmetric membrane multilayer coating preparation methods, such as easy delamination between layers, poor pore connectivity, insufficient depth of the upper macropore layer, limited membrane dirt holding capacity and separation capacity, strict process control, and difficulty in achieving mass production.

[0006] To achieve the above and other related objectives, the present invention provides a method for preparing a polyethersulfone film with a bilayer structure, comprising the following steps:

[0007] A casting solution is prepared, wherein the casting solution comprises a membrane material, a pore-forming agent, a first non-solvent, a second non-solvent, and a solvent, and the mass ratio of the first non-solvent, the second non-solvent, and the solvent in the casting solution is 2-10:38-50:25-40;

[0008] The casting solution is coated onto the support to obtain a liquid film;

[0009] The casting solution is coated onto the support to obtain a liquid film;

[0010] The lower surface of the liquid membrane is heated, while the upper surface of the liquid membrane is sequentially purged with hot air and then with moisture to obtain a nascent membrane; the hot air purging temperature is higher than the moisture purging temperature, and the hot air purging humidity is higher than the moisture purging humidity.

[0011] The nascent membrane was placed in a coagulation bath to solidify into a film, and then washed and dried to obtain the integrally formed double-layer polyethersulfone membrane.

[0012] Furthermore, the first non-solvent is selected from polyethylene glycol with a molecular weight of 400 to 2000, and the second non-solvent is selected from at least one of ethylene glycol monomethyl ether, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, or tetraethylene glycol.

[0013] Furthermore, the mass ratio of the film material, pore-forming agent, first non-solvent, second non-solvent, and solvent in the casting solution is 12-18:2-10:2-10:38-50:25-40.

[0014] Furthermore, the solid content of the casting solution is 10% to 18%. Here, solid content refers to the mass ratio of polyethersulfone resin in the casting solution.

[0015] Furthermore, the heat preservation temperature of the casting solution is 25℃~35℃.

[0016] Furthermore, the membrane material is selected from polyethersulfone.

[0017] Furthermore, the pore-forming agent is selected from hydrophilic polymers, which are selected from polyvinylpyrrolidone and / or polyethylene glycol with a molecular weight greater than 10,000.

[0018] Furthermore, the solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide, or N,N-dimethylformamide.

[0019] Furthermore, the heating temperature is 60℃~100℃.

[0020] Furthermore, the hot air purging conditions include: hot air temperature of 40℃~60℃, hot air humidity of 80%~100%, and purging time of 1s~5s.

[0021] Furthermore, the moisture purging conditions include: moisture temperature of 5℃~30℃, moisture humidity of 40%~60%, and purging duration of 10s~60s.

[0022] The present invention also provides a polyethersulfone membrane having a bilayer structure, wherein the upper layer has a coarse-branched continuous macroporous structure and the lower layer has a fine-branched continuous microporous structure, and the pore sizes of the upper and lower layers are interconnected.

[0023] The ratio of the average pore size of the upper layer to the average pore size of the lower layer is greater than or equal to 50.

[0024] The thickness ratio of the upper layer to the lower layer is 2.5 to 3.5:2;

[0025] The thickness of the highly asymmetric polyethersulfone film is 100 μm to 120 μm.

[0026] Furthermore, the average pore size of the polyethersulfone membrane is 0.01 μm to 1.0 μm.

[0027] Furthermore, the bubble point of the polyethersulfone film is greater than 0.4 MPa.

[0028] Furthermore, the flux of the polyethersulfone membrane is greater than 35 ml / min / cm. 2 .

[0029] Furthermore, the polyethersulfone membrane has a dirt-holding capacity greater than 50 ml / cm³. 2 .

[0030] Furthermore, the polyethersulfone film is prepared using the method described above. As described above, the polyethersulfone film with a bilayer structure and its preparation method of the present invention have the following beneficial effects:

[0031] This invention utilizes a dual-induction process based on a single solution coating and upper and lower layers to prepare a highly asymmetric polyethersulfone (PES) membrane with a bilayer structure, based on a specific solution composition system. During membrane preparation, the lower surface of the coating is heated under a specific solution composition system to slow down phase separation and curing. The upper surface of the coating is then purged and induced in two steps using high-temperature, high-humidity hot air followed by low-temperature, low-humidity cold air. This promotes pore opening on the upper surface and controls the pore depth, thereby controlling the solvent-non-solvent exchange rate and molding speed in different regions of the single solution cross-section. This process results in a bilayer PES membrane with high asymmetry and high porosity. The bilayer PES membrane of this invention comprises an upper layer with a coarse-branched, continuous macroporous structure, which can accommodate more trapped particles, thereby increasing the membrane's pollutant loading capacity and extending its service life. The lower layer is constructed with a fine-branched, continuous microporous structure, ensuring particle retention and enhancing the membrane's separation capacity, thus improving filtration quality. The upper and lower layers are well-connected, with interconnected pore sizes and no breakage. The depth of the upper macroporous layer is greater than that of the lower microporous layer, and this ratio is controlled within a certain range, which improves the membrane's filtration capacity while ensuring its separation capacity and service life. Furthermore, this invention only requires induced treatment of a single-layer membrane solution to achieve integrated molding of a bilayer high-asymmetric PES membrane from a single solution coating. The preparation process is simple, controllable, easy to implement, requires minimal equipment, and is suitable for mass production. Attached Figure Description

[0032] Figure 1 The image shown is a 2000x magnified cross-sectional view of the polyethersulfone film prepared in Example 1 of this invention.

[0033] Figure 2 The image shown is a 2000x magnified cross-sectional view of the polyethersulfone film prepared in Comparative Example 1 of this invention.

[0034] Figure 3 The image shown is a 2000x magnified cross-sectional view of the polyethersulfone film prepared in Comparative Example 2 of this invention.

[0035] Figure 4 The image shown is a 2000x magnified cross-sectional view of the polyethersulfone film prepared in Comparative Example 3 of this invention.

[0036] Figure 5 The image shown is a 2000x magnified cross-sectional view of the polyethersulfone film prepared in Comparative Example 4 of this invention. Detailed Implementation

[0037] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0038] In this invention, unless otherwise stated, the term "multiple" means two or more.

[0039] The character " / " indicates that the objects before and after it are in an "or" relationship. For example, A / B means: A or B.

[0040] The term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0041] One embodiment of the present invention provides a polyethersulfone membrane having a bilayer structure, wherein the upper layer has a coarse-branched continuous macroporous structure and the lower layer has a fine-branched continuous microporous structure, and the pore sizes of the upper and lower layers are interconnected.

[0042] The ratio of the average pore size of the upper layer to the average pore size of the lower layer is greater than or equal to 50.

[0043] The thickness ratio of the upper layer to the lower layer is 2.5 to 3.5:2;

[0044] The thickness of the polyethersulfone film is 100μm to 120μm.

[0045] The double-layer PES membrane of this invention has an average pore size ratio of ≥50 between the upper and lower layers, exhibiting a highly asymmetric structure; the thickness ratio of the upper and lower layers is 2.5 to 3.5:2, with the upper layer being thicker than the lower layer; the upper layer is composed of a coarse-branched continuous macroporous structure, which can accommodate more trapped particles, thus improving the membrane's pollutant loading capacity and extending its service life; the lower layer is composed of a fine-branched continuous microporous structure, which ensures particle retention, enhancing the membrane's separation capacity and thereby improving filtration quality; the upper and lower membrane structures are well-connected, with interconnected pore sizes and no breakage observed.

[0046] In some embodiments, the thickness of the upper layer is 60 μm to 72 μm, and the thickness of the lower layer is 40 μm to 48 μm.

[0047] In some embodiments, the average pore size of the polyethersulfone membrane is 0.01 μm to 1.0 μm.

[0048] In some embodiments, the bubble point of the polyethersulfone film is greater than 0.4 MPa.

[0049] In some embodiments, the flux of the polyethersulfone membrane is greater than 35 ml / min / cm. 2 .

[0050] In some embodiments, the dirt-holding capacity of the polyethersulfone membrane is greater than 50 ml / cm³. 2 .

[0051] Another embodiment of the present invention provides a method for preparing a polyethersulfone film with a bilayer structure as described above by integral molding, comprising the following steps:

[0052] A casting solution is prepared, wherein the casting solution comprises a membrane material, a pore-forming agent, a first non-solvent, a second non-solvent, and a solvent, and the mass ratio of the first non-solvent, the second non-solvent, and the solvent in the casting solution is 2-10:38-50:25-40;

[0053] The casting solution is coated onto the support to obtain a liquid film;

[0054] The lower surface of the liquid membrane is heated, while the upper surface of the liquid membrane is sequentially purged with hot air and then with moisture to obtain a nascent membrane; the hot air purging temperature is higher than the moisture purging temperature, and the hot air purging humidity is higher than the moisture purging humidity.

[0055] The nascent membrane was placed in a coagulation bath to solidify into a film, and then washed and dried to obtain the integrally formed double-layer polyethersulfone membrane.

[0056] In some embodiments, the first non-solvent is selected from polyethylene glycol with a molecular weight of 400 to 2000, wherein the polyethylene glycol with a molecular weight of 400 to 2000 is a low molecular weight polyethylene glycol, including but not limited to polyethylene glycol 200, polyethylene glycol 400, and polyethylene glycol 600; the second non-solvent is selected from at least one of ethylene glycol monomethyl ether, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, or tetraethylene glycol.

[0057] This invention utilizes a dual-induction process based on a single solution coating to prepare a highly asymmetric polyethersulfone (PES) membrane with a bilayer structure, as described above. During membrane preparation, the lower surface of the coating is heated under a specific solution composition to slow down phase separation and curing. The upper surface of the coating is then purged and induced in two steps using high-temperature, high-humidity hot air followed by low-temperature, low-humidity cold air. This promotes pore opening on the upper surface and controls the pore depth, thereby controlling the solvent-non-solvent exchange rate and molding speed in different regions of the single solution cross-section. This process results in a highly asymmetric, high-porosity bilayer PES membrane.

[0058] Unlike traditional multilayer coating methods for preparing highly asymmetric PES membranes with multilayer structures, this invention only requires induction treatment of the single-layer membrane solution to achieve integral molding of a single solution coating to prepare a highly asymmetric PES membrane with a double-layer structure. The process is simpler, more controllable, easier to implement, requires less equipment, and is suitable for mass production.

[0059] Furthermore, based on dual induction, this invention also employs a specific solution system. The specific principle is as follows: the casting solution has a higher non-solvent content than the solvent content, and contains two types of non-solvents. The first and second non-solvents are used in combination. The first non-solvent can modify the strong phase separation effect of the second non-solvent, reducing structural defects in the membrane and thus deepening the macropore depth. This makes the upper macropore layer deeper than the lower micropore layer, thereby increasing the membrane's filtration capacity while ensuring its separation capability and lifespan. Based on the solution characteristics of this system, heating the lower surface of the solution coating provides heat to suppress phase separation in the lower layer. Simultaneously, high-temperature, high-humidity hot air is used to purge the upper surface of the solution coating, causing rapid liquid-liquid phase separation and pore opening, forming a polymer-rich phase and a polymer-poor phase. Then, low-temperature, low-humidity cold air is used to purge, slowing down the solvent-non-solvent exchange rate, allowing the polymer-poor phase to grow into macropores. This avoids excessive pore opening in the lower layer and ensures a smooth transition with the lower dense micropore layer. Finally, the lower layer of the solution coating is immersed in water for phase separation and rapid solidification, forming a dense micropore layer. Through a special dual-induction treatment of a single solution, the final membrane cross-section forms a double-layer asymmetric structure with a coarse-branched continuous macroporous structure in the upper layer and a fine-branched continuous microporous structure in the lower layer, with good connection between the two layers.

[0060] In some embodiments, the mass ratio of the film material, pore-forming agent, first non-solvent, second non-solvent, and solvent in the casting solution is 12-18:2-10:2-10:38-50:25-40.

[0061] In some embodiments, the solid content of the casting solution is 12% to 18%. Here, solid content refers to the mass ratio of polyethersulfone resin in the casting solution.

[0062] In some embodiments, the holding temperature of the casting solution is 25°C to 35°C. That is, after the casting solution is prepared, it is allowed to stand and cool down to 25°C to 35°C, and then held at this temperature for later use.

[0063] In some embodiments, the membrane material is selected from polyethersulfone. The porogen is selected from a hydrophilic polymer, which is selected from polyvinylpyrrolidone and / or polyethylene glycol with a molecular weight greater than 10,000. The polyethylene glycol with a molecular weight greater than 10,000 is a high molecular weight polyethylene glycol. The solvent is selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), or N,N-dimethylformamide (DMF).

[0064] In some embodiments, the heating temperature is 60°C to 100°C, and the heating time is the sum of the hot air blowing time and the moisture blowing time.

[0065] In some embodiments, the support is, for example, a release film, a glass plate, a steel strip film, etc., wherein the release film material is, for example, polyethylene terephthalate (PET), but is not limited thereto.

[0066] In some embodiments, the hot air purging conditions include: hot air temperature of 40°C to 60°C, hot air humidity of 80% to 100%, and purging duration of 1 to 5 seconds.

[0067] In some embodiments, the moisture purging conditions include: moisture temperature of 5°C to 30°C, moisture humidity of 40% to 60%, and purging duration of 10s to 60s.

[0068] In some embodiments, the coagulation bath temperature is 20℃~80℃, and the coagulation bath liquid is water, such as deionized water, pure water, ultrapure water, etc. Specific examples are given below to illustrate the present invention in detail. It should also be understood that the following examples are only for specific illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0069] Example 1

[0070] This embodiment prepares a polyethersulfone film, and the steps are as follows:

[0071] (1) Mix 8 parts of polyvinylpyrrolidone, 5 parts of polyethylene glycol (relative molecular mass of 400), 37 parts of ethylene glycol monomethyl ether, and 36 parts of N,N-dimethylacetamide, and heat and stir at 60°C until uniform. Then add 14 parts of polyethersulfone (BASFE6020P) and continue stirring to dissolve. After 5 hours, a clear homogeneous solution is obtained. Vacuum degassing is performed and the temperature is kept constant at 25°C to obtain the final casting solution, which is ready for scraping.

[0072] (2) The casting solution is coated onto the release film using a 300μm doctor blade to obtain a liquid film;

[0073] (3) The liquid membrane is heated on a stainless steel heating plate at 60°C, while the upper surface of the liquid membrane is blown with hot air at 50°C and 85% RH for 5 seconds.

[0074] (4) After the liquid membrane is purged with hot air, it is then purged with humid air at a temperature of 20°C and a humidity of 50% RH for 20 seconds to obtain the initial ecological membrane.

[0075] (5) The nascent membrane is placed in a deionized water coagulation bath at 35°C to solidify into a membrane, and finally cleaned and dried to obtain a polyethersulfone membrane.

[0076] Example 2

[0077] The only difference between the polyethersulfone film preparation method in this embodiment and that in Example 1 is:

[0078] Step (3) Place the liquid membrane on a stainless steel heating plate at 100°C and heat it.

[0079] Example 3

[0080] The only difference between the polyethersulfone film preparation method in this embodiment and that in Example 1 is:

[0081] Step (3) involves blowing hot air at 40°C and 100% RH for 5 seconds.

[0082] Example 4

[0083] The only difference between the polyethersulfone film preparation method in this embodiment and that in Example 1 is:

[0084] Step (3) involves blowing hot air at 60°C and 80% RH for 1 second.

[0085] Example 5

[0086] The only difference between the polyethersulfone film preparation method in this embodiment and that in Example 1 is:

[0087] Step (4) involves purging with humid air at a temperature of 5°C and a humidity of 40% RH for 60 seconds.

[0088] Example 6

[0089] The only difference between the polyethersulfone film preparation method in this embodiment and that in Example 1 is:

[0090] Step (5) involves purging with humid air at a temperature of 30°C and a humidity of 40% RH for 30 seconds.

[0091] Comparative Example 1

[0092] The only difference between the polyethersulfone film preparation method in this comparative example and that in Example 1 is:

[0093] Step (1) The casting solution is prepared by 6 parts polyvinylpyrrolidone, 43 parts ethylene glycol monomethyl ether, 34 parts N-methylpyrrolidone and 17 parts polyethersulfone (BASF E6020P).

[0094] Comparative Example 2

[0095] The only difference between the polyethersulfone film preparation method in this comparative example and that in Example 1 is:

[0096] Step (1) The casting solution is prepared by 5 parts polyvinylpyrrolidone, 47 parts triethylene glycol, 33 parts N-methylpyrrolidone, and 15 parts polyethersulfone (BASF E6020P).

[0097] Comparative Example 3

[0098] The only difference between the polyethersulfone film preparation method in this comparative example and that in Example 1 is:

[0099] In step (3), the upper surface of the liquid membrane was not purged with hot air, but was directly purged with hot air at a temperature of 50°C and a humidity of 85%RH for 5 seconds.

[0100] Comparative Example 4

[0101] The only difference between the polyethersulfone film preparation method in this comparative example and that in Example 1 is:

[0102] Step (3) is: place the liquid membrane on a stainless steel heating plate at room temperature (i.e., do not heat the lower surface of the liquid membrane), and first blow the upper surface of the liquid membrane with hot air at a temperature of 45°C and a humidity of 85% RH for 3 seconds;

[0103] Step (4) is as follows: After the liquid membrane is purged with hot air, it is then purged with humid air at a temperature of 25°C and a humidity of 60% RH for 10 seconds to obtain the initial ecological membrane.

[0104] Comparative Example 5

[0105] The only difference between the polyethersulfone film preparation method in this comparative example and that in Example 1 is:

[0106] Step (4) was not performed, i.e., no moisture purging was performed. After hot air purging, a nascent membrane was obtained.

[0107] The polyethersulfone films of Examples 1-6 and Comparative Examples 1-5 were observed by scanning electron microscopy (SEM), and 2000x magnified cross-sectional images were obtained. The thickness ratio of the upper and lower layers of the polyethersulfone films of Examples 1 and Comparative Examples 1-3 was calculated based on the 2000x magnified SEM cross-sectional images, and the film thickness was measured. The results are shown in Table 1. Simultaneously, the performance of the polyethersulfone films of Examples 1-6 and Comparative Examples 1-5 was tested according to the following methods, and the test results are shown in Table 1.

[0108] 1. Average aperture test method:

[0109] Based on the electron microscope images, the pore structure of the membrane cross-section was directly observed, and the average pore size was calculated using MATLAB image analysis software.

[0110] 2. Flow rate and bubble point test methods: The flow rate (i.e., water flux) and bubble point (anhydrous ethanol / pure water) of polyethersulfone membranes were tested in accordance with GB / T 32361-2015 "Separation membrane pore size test method bubble point and average flow rate method".

[0111] 3. Dirt Holding Capacity Test: Tryptic Soy Peptone Liquid Culture Medium (TSB) was used as the filtrate. The procedure was as follows: Take 30g of TSB, add 1000ml of distilled water until completely dissolved, dispense into containers, and sterilize at 121℃ for 15 minutes. Then, cut the membrane to be tested into sheets with a diameter (d) of 47mm, moisten them, and place them in a disc filter. Filter the volume V of tryptic soy peptone liquid culture medium (TSB) at a constant pressure of 1 bar. The dirt holding capacity was calculated as follows:

[0112]

[0113] Table 1

[0114]

[0115]

[0116] from Figure 1 As shown in Table 1, the polyethersulfone membrane prepared in Example 1 has a distinct bilayer structure. The upper layer is a continuous macroporous structure formed by coarse branches, and the lower layer is a continuous dense microporous structure formed by fine branches. The thickness ratio of the upper and lower layers is 3:2. The pore size of the upper layer is significantly larger than that of the lower layer, and the pore size ratio between the upper and lower layers is greater than 50, exhibiting high asymmetry overall.

[0117] The polyethersulfone films prepared in Examples 2-6 also have a distinct bilayer structure. The upper layer is a continuous macroporous structure formed by coarse branches, and the lower layer is a continuous dense microporous structure formed by fine branches. The thickness of the upper layer is greater than that of the lower layer. The pore size of the upper layer is significantly larger than that of the lower layer, and the pore size ratio between the upper and lower layers is greater than 50, exhibiting high overall asymmetry.

[0118] from Figure 2 As shown in Table 1, the polyethersulfone membrane prepared in Comparative Example 1 has a distinct bilayer structure. The upper layer is a continuous macroporous structure formed by coarse branches, and the lower layer is a continuous dense microporous structure formed by fine branches. The thickness ratio of the upper and lower layers is 2:3. The pore size of the upper layer is significantly larger than that of the lower layer, and the pore size ratio between the upper and lower layers is greater than 50, exhibiting high asymmetry overall.

[0119] from Figure 4 As shown in Table 1, the polyethersulfone membrane prepared in Comparative Example 2 has a distinct bilayer structure. The upper layer is a continuous macroporous structure formed by coarse branches, and the lower layer is a continuous dense microporous structure formed by fine branches. The thickness ratio of the upper and lower layers is 2:3. The pore size of the upper layer is significantly larger than that of the lower layer, and the pore size ratio between the upper and lower layers is greater than 50, exhibiting high asymmetry overall.

[0120] Compared to Example 1, Comparative Examples 1 and 2 used only one non-solvent, resulting in a thinner upper layer and lower membrane dirt-holding capacity and separation ability. This indicates that the combined use of the first and second non-solvents increases the macropore depth, making the upper macropore layer deeper than the lower micropore layer. This improves the membrane's filtration capacity while maintaining its separation ability and lifespan.

[0121] Compared to Example 1, Comparative Example 3 did not undergo hot air purging. Although the prepared polyethersulfone film had a bilayer structure, the fiber branches formed in the upper layer were finer, the pore size of the macropore layer was smaller, and the overall structure exhibited low asymmetry (e.g., ...). Figure 4 As shown in the figure, the water flux is low, the membrane separation capacity is poor, and the dirt holding capacity is low.

[0122] Compared to Example 1, Comparative Example 4 did not involve under-membrane surface heating. The prepared polyethersulfone membrane had a macroporous structure on the upper layer and a finger-like pore structure on the lower layer (e.g., ...). Figure 5 As shown in the figure, it has a low bubble point and poor separation performance.

[0123] Compared to Example 1, Comparative Example 5 did not undergo moisture purging. Although the polyethersulfone membrane prepared had a bilayer structure, the difference in pore size between the upper and lower layers was very small, and no difference could be seen from the electron microscope image with the naked eye. Moreover, the bubble point and water flux were low, the separation performance was poor, and the dirt holding capacity was low.

[0124] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing a polyethersulfone film with a bilayer structure, characterized in that, Includes the following steps: A casting solution is prepared, wherein the casting solution comprises a membrane material, a pore-forming agent, a first non-solvent, a second non-solvent, and a solvent, and the mass ratio of the first non-solvent, the second non-solvent, and the solvent in the casting solution is 2-10:38-50:25-40; The casting solution is coated onto the support to obtain a liquid film; The lower surface of the liquid membrane is heated, while the upper surface of the liquid membrane is sequentially purged with hot air and then with moisture to obtain a nascent membrane; the hot air purging temperature is higher than the moisture purging temperature, and the hot air purging humidity is higher than the moisture purging humidity. The nascent membrane was placed in a coagulation bath to solidify into a film, and then washed and dried to obtain the integrally formed double-layer polyethersulfone membrane.

2. The preparation method according to claim 1, characterized in that: The first non-solvent is selected from polyethylene glycol with a molecular weight of 400 to 2000, and the second non-solvent is selected from at least one of ethylene glycol monomethyl ether, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol or tetraethylene glycol.

3. The preparation method according to claim 1, characterized in that: The mass ratio of membrane material, pore-forming agent, first non-solvent, second non-solvent and solvent in the casting solution is 12-18:2-10:2-10:38-50:25-40; And / or, the solid content of the casting solution is 12% to 18%.

4. The preparation method according to any one of claims 1 to 3, characterized in that: The membrane material is selected from polyethersulfone; Or, the pore-forming agent is selected from hydrophilic polymers; Or, the solvent is selected from at least one of N-methylpyrrolidone, N,N-dimethylacetamide, or N,N-dimethylformamide.

5. The preparation method according to any one of claims 1 to 3, characterized in that: The heating temperature is 60℃~100℃; And / or, the hot air purging conditions include: hot air temperature of 40℃~60℃, hot air humidity of 80%~100%, and purging time of 1s~5s; And / or, the moisture purging conditions include: moisture temperature of 5℃~30℃, moisture humidity of 40%~60%, and purging duration of 10s~60s.

6. A polyethersulfone film, characterized in that: It has a double-layer structure, with the upper layer being a coarse-branched continuous large-pore structure and the lower layer being a fine-branched continuous small-pore structure, and the pore diameters of the upper and lower layers are interconnected. The ratio of the average pore size of the upper layer to the average pore size of the lower layer is greater than or equal to 50. The thickness ratio of the upper layer to the lower layer is 2.5 to 3.5:2; The thickness of the polyethersulfone film is 100μm to 120μm.

7. The polyethersulfone film according to claim 6, characterized in that: The average pore size of the polyethersulfone membrane is 0.01 μm to 1.0 μm.

8. The polyethersulfone film according to claim 6, characterized in that: The bubble point of the polyethersulfone film is greater than 0.4 MPa.

9. The polyethersulfone film according to claim 6, characterized in that: The flux of the polyethersulfone membrane is greater than 35 ml / min / cm. 2 . And / or, the dirt holding capacity of the polyethersulfone membrane is greater than 50 ml / cm³. 2 .

10. The polyethersulfone film according to any one of claims 6 to 9, characterized in that: The polyethersulfone film is prepared by the method described in any one of claims 1 to 5.

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