An unsupported flat ultrafiltration membrane and its preparation method

Abstract

This application relates to the technical field of membrane materials, and specifically provides a supported flat ultrafiltration membrane and a preparation method thereof. The steps include: preparing a uniform and transparent polymer solution; uniformly coating the polymer solution on the surface of polypropylene non-woven fabric, installing a gas-induced phase inversion device on the back of the polypropylene non-woven fabric coated with the polymer solution, and performing gas-induced phase inversion; immersing the polypropylene non-woven fabric that has completed gas-induced phase inversion into a coagulation bath containing a non-solvent to perform immersion precipitation phase inversion; after waiting for the immersion precipitation phase inversion to be completed, peeling the membrane formed on the polypropylene non-woven fabric from the polypropylene non-woven fabric to obtain a supported flat ultrafiltration membrane. This method uses the supported polypropylene non-woven fabric as a peelable layer, adjusts the phase separation degree on the side of the casting solution through temperature and humidity regulation in the non-woven fabric layer, and manufactures a supported flat ultrafiltration membrane with high mechanical strength, high flux, controllable cross-sectional pore size gradient, strong retention effect and anti-fouling performance.

Description

Technical Field

[0001] The present application relates to the technical field of membrane materials, and specifically to a supported flat ultrafiltration membrane and a preparation method thereof. Background Art

[0002] With the rapid development of domestic biopharmaceuticals and life science research, ultrafiltration membranes, as the core components in the purification and concentration processes, have been widely used in products such as membrane cartridges and single-use filters. The pore size range of ultrafiltration membranes is between 0.01 µm and 0.1 µm, and the concentration filtration of soluble macromolecules is achieved by pressure driving. Although products of foreign companies such as Merck and PALL dominate the market, domestic research and development of ultrafiltration membranes are also actively promoted. However, there is still a certain gap between the performance of domestic products and that of excellent foreign products in actual applications, and the research and development of high-performance ultrafiltration membranes remains an important task at present.

[0003] The membranes used in current market ultrafiltration membrane cartridges are mainly divided into supported membranes and unsupported membranes. Supported membranes are formed by directly coating a casting solution on a supporting non-woven fabric through immersion precipitation phase inversion. However, this method is prone to form a finger-like pore structure, resulting in a decrease in the mechanical strength of the membrane, and it is necessary to reinforce with a supporting non-woven fabric. Unsupported membranes are divided into single-layer and multi-layer coating schemes. By controlling the temperature and solid content of the casting solution, the formation of finger-like pores is avoided while ensuring the mechanical strength of the membrane.

[0004] However, the existing supported ultrafiltration membrane forming technology has the problem of low flux. For the single-layer coating method of unsupported ultrafiltration membranes, in order to avoid the formation of finger-like pores, the single-layer thickness is relatively thin, and it is generally used after two layers are laminated, which may reduce the flux due to the existence of a double retention layer. The synchronous multi-layer scraping method has extremely high requirements for processes and equipment. Therefore, how to construct an unsupported ultrafiltration membrane with high flux and high mechanical strength while maintaining the rejection rate of the ultrafiltration membrane is an urgent problem to be solved. Summary of the Invention

[0005] To solve the problem that the existing supported ultrafiltration membrane forming technology has low flux, while the single-layer coating method of unsupported ultrafiltration membranes may reduce the flux due to the existence of a double retention layer, and the synchronous multi-layer scraping method has extremely high requirements for processes and equipment.

[0006] On the one hand, the present application provides a preparation method of a supported flat ultrafiltration membrane, including: preparing a homogeneous and transparent polymer solution;

[0007] uniformly coating the polymer solution on the surface of a polypropylene non-woven fabric;

[0008] installing a gas-phase induced phase inversion device on the back of the polypropylene non-woven fabric coated with the polymer solution, and performing gas-phase induced phase inversion;

[0009] The polypropylene nonwoven fabric that has completed the gas phase induced phase transformation process is immersed in a coagulation bath containing a non-solvent to perform immersion precipitation phase transformation;

[0010] After the immersion precipitation phase conversion process is completed, the membrane formed on the polypropylene non-woven fabric is peeled off from the polypropylene non-woven fabric to obtain an unsupported flat ultrafiltration membrane.

[0011] In a feasible implementation, the gas phase induced phase transformation comprises the steps of:

[0012] The back side of the polypropylene nonwoven fabric having the polymer solution is placed under pure water circulating steam conditions for a preset time.

[0013] In a feasible implementation, the preparation of a uniform and transparent polymer solution comprises the steps of:

[0014] Selecting a polymer raw material, a solvent and a hydrophilic additive, wherein the polymer raw material includes one or more of polyethersulfone, polysulfone and polyvinylidene fluoride;

[0015] Adding the polymer raw material, solvent and hydrophilic additive into a mixing container in a predetermined ratio, heating the mixture to a temperature in the range of 60-80° C., and stirring the mixture sufficiently until all the ingredients are evenly mixed to form an initial polymer solution;

[0016] Cooling the initial polymer solution to 40° C., adding a non-solvent to the initial polymer solution;

[0017] The initial polymer solution is continuously stirred until the non-solvent is completely dissolved and becomes clear and transparent, thereby obtaining the polymer solution.

[0018] In a feasible implementation, the hydrophilic additive includes one or more of polyvinyl pyrrolidone, polyethylene glycol, block polyether, sulfonated polyether sulfone, sulfonated polysulfone and hydroxylated polyether sulfone.

[0019] In a feasible implementation, the solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

[0020] In a feasible implementation, the non-solvent includes one or more of 1,2-propylene glycol, 1,3-propylene glycol, glycerol, ethylene glycol and pure water.

[0021] In a feasible implementation, in the polymer solution: the mass percentage of the polymer raw material is 15-25%, the mass percentage of the solvent is 49%-81%, and the mass percentage of the non-solvent is 0-10%.

[0022] In a feasible implementation manner, the temperature range of the gas-phase induced phase inversion is: 20 - 60 °C, and the humidity range is: 40 - 90%RH.

[0023] In a feasible implementation manner, the temperature range of the immersion precipitation phase inversion is: 20 - 60 °C, and the solution in the coagulation bath is pure water.

[0024] On the other hand, the present application provides a supported flat ultrafiltration membrane, which is prepared by the preparation method described in any one of the above, and includes: an immersion precipitation phase inversion surface layer and a gas-phase induced phase inversion sublayer;

[0025] The mechanical strength of the supported flat ultrafiltration membrane is not less than 900 cN / 15 mm, the BSA rejection rate of the supported flat ultrafiltration membrane is not less than 99.5%, and the pure water flux is not less than 400 L / m 2 ·h@0.1 Mpa.

[0026] The present application provides a supported flat ultrafiltration membrane and a preparation method thereof. The steps include: preparing a uniform and transparent polymer solution; uniformly coating the polymer solution on the surface of a polypropylene non-woven fabric, installing a gas-phase induced phase inversion device on the back of the polypropylene non-woven fabric coated with the polymer solution, and performing gas-phase induced phase inversion; immersing the polypropylene non-woven fabric that has completed the gas-phase induced phase inversion process in a coagulation bath containing a non-solvent to perform immersion precipitation phase inversion; after waiting for the immersion precipitation phase inversion process to be completed, peeling the membrane formed on the polypropylene non-woven fabric from the polypropylene non-woven fabric to obtain a supported flat ultrafiltration membrane. This method uses a supported polypropylene non-woven fabric as a peelable layer, adjusts the phase separation degree on one side of the casting solution through temperature and humidity in the non-woven fabric layer, and manufactures a supported flat ultrafiltration membrane with high mechanical strength, high flux, controllable cross-sectional pore size gradient, strong rejection effect and anti-fouling performance. The process is simple, the range of condition parameters is wide, the control conditions are not harsh, it is easy to operate, has low requirements for equipment, and is easy to implement industrially. The supported flat ultrafiltration membrane obtained by the above method has a densified upper surface structure, a controllable cross-sectional porosity gradient, a highly porous sublayer structure, a mechanical strength not less than 900 cN / 15 mm, a BSA rejection rate not less than 99.5%, and a pure water flux not less than 400 L / m 2 ·h@0.1 Mpa. Description of the Drawings

[0027] The accompanying drawings herein are incorporated into and constitute a part of this specification, showing embodiments consistent with the implementation of the present invention, and are used together with the specification to explain the principles of the embodiments of the present invention. Obviously, the accompanying drawings in the following description are only some embodiments of the implementation of the present invention, and those of ordinary skill in the art can obtain other drawings based on these drawings without creative efforts.

[0028] Figure 1 is a schematic flow chart of a method for preparing a supported flat ultrafiltration membrane shown in an embodiment of the present application;

[0029] Figure 2 is a scanning electron microscope (SEM) image of the cross-section of Embodiment 1 of the present application, with a magnification of 500, a magnification of 10.00k for the upper part of the cross-section, and a magnification of 10.00k for the lower part of the cross-section;

[0030] Figure 3 is a scanning electron microscope (SEM) image of the cross-section of Embodiment 2 of the present application, with a magnification of 500, a magnification of 10.00k for the upper part of the cross-section, and a magnification of 10.00k for the lower part of the cross-section;

[0031] Figure 4 is a scanning electron microscope (SEM) image of the cross-section of Comparative Example 1 of the present application, with a magnification of 500;

[0032] Figure 5 is a scanning electron microscope (SEM) image of the cross-section of Comparative Example 2 of the present application, with a magnification of 500. Detailed Embodiments

[0033] Example embodiments will now be described more fully with reference to the accompanying drawings. However, the example embodiments can be implemented in various forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this disclosure will be more complete and comprehensive, and will fully convey the concept of the example embodiments to those skilled in the art. The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of the embodiments of the present invention.

[0034] With the continuous development of the current domestic biopharmaceutical and life science research fields, products such as membrane cartridges and single-use filters with ultrafiltration membranes as the core components, which are essential units in the purification and concentration processes, are being applied more and more. Ultrafiltration membranes are usually defined as membrane materials with a pore size range of 0.01 µm - 0.1 µm, and mainly concentrate and filter soluble macromolecules such as proteins, DNA, and starch through pressure driving. Among the several recognized membrane cartridges in the market, in addition to products of foreign companies such as Merck and Pall, domestic ones basically use foreign ultrafiltration membranes as components and are sold after encapsulation. Currently, many domestic companies have also carried out research and development on ultrafiltration membranes, but there are few that can be actually applied. Some can be applied in certain fields, but there is still a gap between the actual performance and excellent foreign products. The research and development of high-performance ultrafiltration membranes is still an urgent problem to be solved.

[0035] In several commonly used ultrafiltration membrane cartridges in the current market, the ultrafiltration membranes are basically of a support system, that is, when coating, the casting solution is coated on the supporting non-woven fabric, and after immersion precipitation phase inversion molding, the non-woven fabric and the casting solution become one. The reason for choosing this method is that the ultrafiltration membrane has a small pore size and requires a casting solution system with a high solid content to form a membrane. In immersion precipitation phase inversion, as the solid content of the casting solution increases, the double diffusion speed slows down, and the relaxation time of the polymer is less than the time required for the system to phase separate, resulting in a finger-like pore structure. The position where the finger-like pores form will change with the solid content and the degree of phase separation of the system. The formation of finger-like pores will cause a significant decrease in the mechanical strength of the membrane. Therefore, a 100KD ultrafiltration membrane of PALL company has a support structure containing finger-like pores. Although a 50KD ultrafiltration membrane of Merck is a support structure without finger-like pores, it can be clearly seen that it is a two-layer cast film, and the film thickness of each layer does not exceed 70 µm. Because once the cast film thickness is too thick, a finger-like pore structure will form at the bottom.

[0036] Another part of the commercial ultrafiltration membranes is of a non-support system. As mentioned above, for non-support membrane materials to ensure sufficient mechanical strength, it is necessary to avoid the formation of finger-like pore structures inside the membrane. A ultrafiltration membrane of PALL company is used by stacking two layers together, and the thickness of each layer does not exceed 77 µm. In addition, such as the patent US2014 / 0221629A1 of Merck company, a two-layer or multi-layer coating scheme is adopted. The lower layer uses a casting solution with a low solid content, and through reverse thermally induced phase separation, the temperature of the casting solution is controlled to prepare a porous sublayer to play a supporting role. The upper layer uses a casting solution with a high solid content, and through direct immersion precipitation phase inversion, a dense rejection layer is directly formed.

[0037] The existing supported ultrafiltration membrane forming technology has a lower overall membrane flux than the unsupported ultrafiltration membrane because the upper and lower surfaces of the casting liquid are directly in contact with the coagulation bath when the immersion precipitation phase is transformed. The unsupported ultrafiltration membrane formed by scraping a thinner casting liquid and stacking two layers increases the process time. The two layers are superimposed, resulting in the existence of a double retention layer, which reduces the flux while maintaining the retention rate and increasing the mechanical properties. When using the synchronous multi-layer scraping method, the upper and lower layers of the liquid will affect the pore size and pore size uniformity due to double diffusion and scraping film thickness, which has extremely high requirements and extremely low tolerances for the operating process and equipment.

[0038] In order to solve the above problems, the present invention provides a method for preparing an unsupported flat ultrafiltration membrane. Figure 1 As shown, the embodiment of the present application includes the following steps:

[0039] S100: Prepare a uniform, transparent polymer solution.

[0040] In some embodiments of the present application, specifically, the following steps are included:

[0041] S110: selecting a polymer raw material, a solvent and a hydrophilic additive, wherein the polymer raw material includes one or more of polyethersulfone, polysulfone and polyvinylidene fluoride.

[0042] In some embodiments of the present application, the solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone; the non-solvent includes one or more of 1,2-propylene glycol, 1,3-propylene glycol, glycerol, ethylene glycol and pure water; the hydrophilic additive includes one or more of polyvinylpyrrolidone, polyethylene glycol, block polyether, sulfonated polyethersulfone, sulfonated polysulfone and hydroxylated polyethersulfone.

[0043] S120: Add the polymer raw material, solvent and hydrophilic additive into a mixing container according to a predetermined ratio, heat to a temperature range of 60-80° C., and fully stir the mixture until all the ingredients are evenly mixed to form an initial polymer solution.

[0044] S130: Cool the initial polymer solution to 40° C. and add a non-solvent to the initial polymer solution.

[0045] S140: Continue stirring the initial polymer solution until the non-solvent is completely dissolved and becomes clear and transparent, thereby obtaining a polymer solution.

[0046] In some embodiments of the present application, in the polymer solution: the mass percentage of the polymer raw material is 15-25%, the mass percentage of the solvent is 49%-81%, and the mass percentage of the non-solvent is 0-10%.

[0047] S200: Uniformly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0048] The pore uniformity and fiber compactness of the non-woven fabric play a crucial role in the gas-phase induced phase inversion process. Specifically, the uniform pore distribution can ensure the uniform diffusion of water vapor through the non-woven fabric at the casting solution interface, avoiding local over-phase separation or insufficient phase separation, thus forming a stable transition structure between the dense surface layer and the porous sub-layer. Secondly, the dense arrangement of fibers provides a flat support interface for the polymer solution, reducing coating defects; at the same time, during the peeling process, through the dispersion of interfacial stress, it prevents the film layer from breaking. Through the above synergistic effects, the non-woven fabric not only serves as a temporary carrier but also becomes a dynamic regulation medium for the formation of the phase separation gradient, ultimately achieving the technical effects of controllable pore size gradient across the film layer cross-section and significantly improved mechanical strength.

[0049] S300: Install a gas-phase induced phase inversion device on the back of the polypropylene non-woven fabric coated with the polymer solution and perform gas-phase induced phase inversion.

[0050] In some embodiments of the present application, the specific operation of gas-phase induced phase inversion is: expose the back of the polypropylene non-woven fabric with the polymer solution to pure water circulating steam for a preset time.

[0051] In some embodiments of the present application, the temperature range of gas-phase induced phase inversion is: 20 - 60 °C, and the humidity range is: 40 - 90%RH.

[0052] S400: Immerse the polypropylene non-woven fabric that has completed the gas-phase induced phase inversion process in a coagulation bath containing a non-solvent to perform immersion precipitation phase inversion.

[0053] In some embodiments of the present application, the temperature range of immersion precipitation phase inversion is: 20 - 60 °C, and the solution in the coagulation bath is pure water.

[0054] S500: After waiting for the immersion precipitation phase inversion process to complete, peel the film formed on the polypropylene non-woven fabric from the polypropylene non-woven fabric to obtain a self-supporting flat ultrafiltration membrane.

[0055] Example 1

[0056] This example provides a method for preparing a self-supporting flat ultrafiltration membrane, and its preparation method includes the following steps:

[0057] Step 1: Prepare a homogeneous polymer solution: Disperse 20% polyethersulfone powder and 1% polyvinylpyrrolidone in N,N-dimethylacetamide, blend and stir evenly at 60 - 80 °C, cool down to below 40 °C, add 3% pure water, and stir until clear and transparent to obtain the polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0058] Step 2: Uniformly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0059] Step 3: After scraping the polymer solution of 320 µm on the polypropylene non-woven fabric, place the back side in a water vapor atmosphere at a temperature of 30 °C and a humidity of 80% RH for a preset time.

[0060] Step 4: Immerse the polypropylene non-woven fabric that has completed the gas-phase induced phase inversion process in a coagulation bath at 40 °C until the phase inversion is complete to obtain a flat membrane. The coagulation bath is a pure aqueous solution.

[0061] Step 5: After washing the polypropylene non-woven fabric with pure water at 80 °C, wind and peel it respectively to obtain a self-supporting flat ultrafiltration membrane.

[0062] Figure 2 The cross-sectional microstructure of the self-supporting flat ultrafiltration membrane of this example is shown. Two sub-layers with different microtopographies are shown, namely the gas-phase induced phase inversion sub-layer (near the lower surface at the bottom) and the immersion precipitation phase inversion sub-layer (other parts). Figure 2 On the right side, a further comparison of the microtopographies of the two sub-layers is provided. It can be seen from the figure that its microstructure rapidly densifies near the upper surface to form a rejection layer capable of retaining the BSA solution. This structure is caused by the rapid diffusion of the non-solvent during the immersion precipitation phase inversion process; the lower sub-layer structure is loose, which is due to the improvement of the phase separation degree through the gas-phase induced phase inversion process. The polypropylene non-woven fabric needs to have a uniform pore distribution and a dense fiber structure, so that water vapor uniformly penetrates through the pores to the casting solution interface, promoting the formation of a dense surface layer (rejection layer) and a highly porous sub-layer (support layer) during the gas-phase induced phase inversion process.

[0063] Through Figure 2 display, the pore size of the surface layer rapidly shrinks to the nanometer level, while the sub-layer presents an open pore structure, verifying the gradient phase separation effect. The obtained self-supporting flat ultrafiltration membrane product is tested for performance and obtained: the mechanical strength is 950 cN / 15 mm, the BSA rejection rate of the self-supporting flat ultrafiltration membrane is 99.6%, and the pure water flux is 450 L / m 2 ·h@0.1 Mpa.

[0064] Example 2

[0065] Step 1: Prepare a homogeneous polymer solution: Disperse 20% polyethersulfone powder and 1% polyvinylpyrrolidone in N,N-dimethylacetamide, blend and stir evenly at 60 - 80 °C, cool down to below 40 °C, add 3% pure water, and stir until clear and transparent to obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0066] Step 2: Uniformly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0067] Step 3: After scraping a 320-µm polymer solution on the polypropylene nonwoven fabric, place the back side in a water vapor atmosphere at a temperature of 30 °C and a humidity of 90% RH for a preset time.

[0068] Step 4: Immerse the polypropylene nonwoven fabric that has completed the gas-phase induced phase inversion process in a coagulation bath at 40 °C until the phase inversion is complete to obtain a flat membrane. The coagulation bath is an aqueous solution.

[0069] Step 5: Wash the polypropylene nonwoven fabric with pure water at 80 °C and then wind and peel it separately to obtain a self-supporting flat ultrafiltration membrane.

[0070] Figure 3 The cross-sectional microstructure of the self-supporting flat ultrafiltration membrane of this example is shown. And Figure 2 In comparison, the sublayer is looser because the increase in humidity during the gas-phase induction process causes an increase in the phase separation degree of the system.

[0071] The finished product of the self-supporting flat ultrafiltration membrane prepared in this example was tested for performance, and the results were as follows: the mechanical strength was 1000 cN / 15 mm, the BSA rejection rate of the self-supporting flat ultrafiltration membrane was 99.6%, and the pure water flux was 500 L / m 2 ·h@0.1 Mpa.

[0072] Example 3

[0073] The differences between the preparation methods of this example and Example 1 are as follows:

[0074] Step 1: Disperse 22% polyethersulfone powder and 2% polyvinylpyrrolidone in N,N-dimethylacetamide, mix and stir evenly at 60 - 80 °C, cool to below 40 °C, add 1% pure water, and stir until clear and transparent to obtain a polymer solution; the above percentages are the mass percentages of each component in the polymer solution.

[0075] Step 2: Coat the polymer solution evenly on the surface of the polypropylene nonwoven fabric.

[0076] Step 3: After scraping a 330-µm polymer solution on the polypropylene nonwoven fabric, place the back side in a water vapor atmosphere at a temperature of 25 °C and a humidity of 85% RH for a preset time.

[0077] Step 4: Immerse the polypropylene nonwoven fabric that has completed the gas-phase induced phase inversion process in a coagulation bath at 40 °C until the phase inversion is complete to obtain a flat membrane. The coagulation bath is an aqueous solution.

[0078] Step 5: Wash the polypropylene nonwoven fabric with pure water at 80 °C and then wind and peel it separately to obtain a self-supporting flat ultrafiltration membrane.

[0079] The finished product of the unsupported flat ultrafiltration membrane prepared in this example was tested for performance, and the following results were obtained: the mechanical strength was 1050 cN / 15 mm, the BSA rejection rate of the unsupported flat ultrafiltration membrane was 99.8%, and the pure water flux was 420 L / m 2 ·h@0.1 Mpa.

[0080] Example 4

[0081] The differences between the preparation methods of this example and Example 1 are as follows:

[0082] Step 1: Disperse 25% polyethersulfone powder and 1% polyvinylpyrrolidone in N,N-dimethylacetamide, mix and stir evenly at 60-80 °C, cool down to below 40 °C, add 0.5% pure water, and stir until clear and transparent to obtain a polymer solution; the above percentages are the mass percentages of each component in the polymer solution.

[0083] Step 2: Uniformly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0084] Step 3: After scraping the polymer solution at 280 µm on the polypropylene non-woven fabric, place the back side in a water vapor atmosphere at a temperature of 40 °C and a humidity of 90% RH for a preset time.

[0085] Step 4: Immerse the polypropylene non-woven fabric that has completed the gas-phase induced phase inversion process in a coagulation bath at 40 °C until the phase inversion is complete to obtain a flat membrane. The coagulation bath is an aqueous solution of pure water.

[0086] Step 5: Wash the polypropylene non-woven fabric with pure water at 80 °C, then wind and peel it off respectively to obtain an unsupported flat ultrafiltration membrane.

[0087] The finished product of the unsupported flat ultrafiltration membrane prepared in this example was tested for performance, and the following results were obtained: the mechanical strength was 1100 cN / 15 mm, the BSA rejection rate of the unsupported flat ultrafiltration membrane was 99.8%, and the pure water flux was 410 L / m 2 ·h@0.1 Mpa.

[0088] Comparative Example 1

[0089] This comparative example is used to compare and verify the function of the gas-phase induced phase inversion device on the non-woven fabric side. This comparative example is compared with Example 1, and the steps of this Comparative Example 1 are as follows:

[0090] Step 1: Prepare a uniform polymer solution: Disperse 20% polyethersulfone powder and 1% polyvinylpyrrolidone in N,N-dimethylacetamide, mix and stir evenly at 60-80 °C, cool down to below 40 °C, add 3% pure water, and stir until clear and transparent to obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0091] Step 2: Uniformly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0092] Step 3: After scraping a 320-µm polymer solution on the polypropylene non-woven fabric, immerse it in a coagulation bath at 40 °C until phase inversion is complete to obtain a flat membrane. The coagulation bath is an aqueous solution.

[0093] Step 4: Wash the polypropylene non-woven fabric with pure water at 80 °C and then wind and peel it respectively to obtain a supported flat ultrafiltration membrane.

[0094] Figure 4 The microscopic structure of the cross-section of the polyethersulfone flat membrane of this comparative example is shown. Without using a gas-induced phase inversion device, the degree of phase separation on the non-woven fabric side of the casting solution is low, tending to form a finger-like pore structure. The mechanical strength after peeling in this comparative example is very low, so it cannot play the role of intercepting BSA.

[0095] The polyethersulfone flat membrane prepared in this comparative example was tested for performance and obtained: the mechanical strength was 450 cN / 15 mm, and the strength was insufficient.

[0096] Comparative Example 2

[0097] This comparative example is used to compare and verify the role of the gas-induced phase inversion device on the non-woven fabric side. This comparative example is compared with Example 2. The steps of Comparative Example 2 are as follows:

[0098] Step 1: Prepare a uniform polymer solution: Disperse 20% polyethersulfone powder and 1% polyvinylpyrrolidone in N,N-dimethylacetamide, blend and stir evenly at 60 - 80 °C, cool to below 40 °C, add 3% pure water, and stir until clear and transparent to obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0099] Step 2: Uniformly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0100] Step 3: After scraping a 320-µm polymer solution on the polypropylene non-woven fabric, place the back side in a water vapor atmosphere at 20 °C and 60% RH for a preset time.

[0101] Step 4: Immerse the polypropylene non-woven fabric that has completed the gas-induced phase inversion process in a coagulation bath at 40 °C until phase inversion is complete to obtain a flat membrane. The coagulation bath is an aqueous solution.

[0102] Step 5: Wash the polypropylene non-woven fabric with pure water at 80 °C and then wind and peel it respectively to obtain a supported flat ultrafiltration membrane.

[0103] Figure 5The cross-sectional microstructure of the polyethersulfone flat membrane of this comparative example is shown. The gas-phase induced phase inversion device was added, but after passing through at lower temperature and humidity, the finger-like pores in the lower part of the cross-section were significantly shorter, but still existed. The cross-section of this comparative example failed to form an asymmetric structure without finger-like pores as in Example 1, and the mechanical strength was still insufficient, so the retention filtration experiment could not be carried out.

[0104] The polyethersulfone flat membrane prepared in this comparative example was tested for performance and obtained: the mechanical strength was 600 cN / 15 mm, the strength was insufficient, and there was a risk of leakage of the retained substances.

[0105] On the other hand, this application provides a self-supporting flat ultrafiltration membrane prepared by the preparation method of any one of the above embodiments, including: an immersion precipitation phase inversion surface layer and a gas-phase induced phase inversion sublayer; according to the content of the above embodiments, it can be known that the mechanical strength of the self-supporting flat ultrafiltration membrane is not less than 900 cN / 15 mm, the BSA retention rate of the self-supporting flat ultrafiltration membrane is not less than 99.5%, and the pure water flux is not less than 400 L / m 2 ·h@0.1 Mpa.

[0106] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. For example, other hydrophilic additives can be added to the polymer, including polyethylene glycol, block polyether, sulfonated polyethersulfone, sulfonated polysulfone, hydroxylated polyethersulfone, etc.; other high-boiling organic solvents can be selected / added to the polymer solution and the coagulation bath, such as N,N-dimethylformamide and N-methylpyrrolidone, etc.; the nonsolvent can also adopt components such as 1,3-propanediol, glycerol, ethylene glycol, and water.

[0107] Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claimed claims.

[0108] After considering the disclosure of the specification and the embodiments, those skilled in the art will easily think of other embodiments of the present disclosure. This application aims to cover any variations, uses, or adaptive changes of the present disclosure, and these variations, uses, or adaptive changes follow the general principles of the present disclosure and include the common general knowledge or conventional technical means in the technical field not disclosed in the present disclosure.

Claims

1. A method for preparing an unsupported flat ultrafiltration membrane, characterized in that: The steps include: Prepare uniform, transparent polymer solutions; The polymer solution is uniformly coated on the surface of a polypropylene non-woven fabric, wherein the pore structure of the polypropylene non-woven fabric guides the penetration distribution of the polymer solution; A gas-phase induced phase conversion device is installed on the back of the polypropylene non-woven fabric coated with the polymer solution to transfer water vapor through the pores of the polypropylene non-woven fabric to regulate the phase separation behavior at the interface of the casting liquid to form a gradient structure transitioning from a dense surface layer to a loose sublayer; The polypropylene nonwoven fabric that has completed the gas phase induced phase transformation process is immersed in a coagulation bath containing a non-solvent to perform immersion precipitation phase transformation to solidify the membrane structure; After the immersion precipitation phase conversion process is completed, the membrane formed on the polypropylene non-woven fabric is peeled off from the polypropylene non-woven fabric to obtain an unsupported flat ultrafiltration membrane.

2. The method for preparing an unsupported flat ultrafiltration membrane according to claim 1, characterized in that: The gas phase induced phase conversion comprises the steps of: The back side of the polypropylene nonwoven fabric having the polymer solution is placed under pure water circulating steam conditions for a preset time.

3. The method for preparing an unsupported flat ultrafiltration membrane according to claim 1, characterized in that: The method for preparing a uniform and transparent polymer solution comprises the following steps: Selecting a polymer raw material, a solvent and a hydrophilic additive, wherein the polymer raw material includes one or more of polyethersulfone, polysulfone and polyvinylidene fluoride; Adding the polymer raw material, solvent and hydrophilic additive into a mixing container in a predetermined ratio, heating the mixture to a temperature in the range of 60-80° C., and stirring the mixture sufficiently until all the ingredients are evenly mixed to form an initial polymer solution; Cooling the initial polymer solution to 40° C., adding a non-solvent to the initial polymer solution; The initial polymer solution is continuously stirred until the non-solvent is completely dissolved and becomes clear and transparent, thereby obtaining the polymer solution.

4. The method for preparing an unsupported flat ultrafiltration membrane according to claim 3, characterized in that: The hydrophilic additive includes one or more of polyvinyl pyrrolidone, polyethylene glycol, block polyether, sulfonated polyether sulfone, sulfonated polysulfone and hydroxylated polyether sulfone.

5. The method for preparing an unsupported flat ultrafiltration membrane according to claim 3, characterized in that: The solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

6. The method for preparing an unsupported flat ultrafiltration membrane according to claim 3, characterized in that: The non-solvent includes one or more of 1,2-propylene glycol, 1,3-propylene glycol, glycerol, ethylene glycol and pure water.

7. The method for preparing an unsupported flat ultrafiltration membrane according to claim 3, characterized in that: In the polymer solution: the mass percentage of the polymer raw material is 15-25%, the mass percentage of the solvent is 49%-81%, and the mass percentage of the non-solvent is 0-10%.

8. The method for preparing an unsupported flat ultrafiltration membrane according to claim 3, characterized in that: The temperature range of the gas phase induced phase conversion is: 20-60°C, and the humidity range is: 40-90%RH.

9. The method for preparing an unsupported flat ultrafiltration membrane according to claim 3, characterized in that: The temperature range of the immersion precipitation phase transformation is: 20-60°C, and the solution in the coagulation bath is pure water.

10. An unsupported flat ultrafiltration membrane, characterized in that: Prepared by the preparation method according to any one of claims 1 to 9, comprising: immersing a precipitation phase transformation surface layer and a gas phase induced phase transformation sublayer; The mechanical strength of the unsupported flat ultrafiltration membrane is not less than 900 cN / 15 mm, the BSA retention rate of the unsupported flat ultrafiltration membrane is not less than 99.5%, and the pure water flux is not less than 400 L / m 2 ·h@0.1Mpa.

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