Support-free flat ultrafiltration membrane and preparation method thereof

Abstract

The invention relates to the technical field of membrane materials, and particularly provides an unsupported flat ultrafiltration membrane and a preparation method thereof.The preparation method comprises the steps that a uniform and transparent polymer solution is prepared; uniformly coating the polymer solution on the surface of the polypropylene non-woven fabric, and mounting a gas-phase induced phase inversion device on the back surface of the polypropylene non-woven fabric coated with the polymer solution to carry out gas-phase induced phase inversion; immersing the polypropylene non-woven fabric subjected to gas-phase induced phase inversion into a coagulating bath containing a non-solvent, and carrying out immersion precipitation phase inversion; and after the immersion precipitation phase conversion is completed, stripping the membrane formed on the polypropylene non-woven fabric from the polypropylene non-woven fabric to obtain the support-free flat ultrafiltration membrane. According to the method, the non-support polypropylene non-woven fabric is used as a peelable layer, and the phase splitting degree of one side of the membrane casting solution is adjusted on the non-woven fabric layer through temperature and humidity, so that the non-support flat ultrafiltration membrane which is high in mechanical strength, high in flux, controllable in section aperture gradient and relatively strong in interception effect and anti-pollution performance is manufactured.

Description

Technical Field

[0001] The present application relates to the technical field of membrane materials, and in particular to an unsupported 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 core components in the purification and concentration process, have been widely used in products such as membrane packages and disposable filters. The pore size of ultrafiltration membranes ranges from 0.01µm to 0.1µm, and the concentration and filtration of soluble macromolecules are achieved through pressure-driven. Although the products of foreign companies such as Merck and PALL have dominated the market, China is also actively promoting the research and development of ultrafiltration membranes. However, the performance of domestic products in actual applications is still somewhat behind that of excellent foreign products, and the research and development of high-performance ultrafiltration membranes is still an important task at present.

[0003] The membranes used in ultrafiltration membrane packages on the current market are mainly divided into supported membranes and unsupported membranes. Supported membranes are formed by coating the casting liquid on a supporting non-woven fabric and directly forming it through immersion precipitation phase transformation. However, this method is prone to forming a finger-like pore structure, which leads to a decrease in the mechanical strength of the membrane and must be reinforced 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 liquid, the formation of finger-like pores can be avoided while ensuring the mechanical strength of the membrane.

[0004] However, the existing supported ultrafiltration membrane forming technology has the problem of low flux, and the single-layer coating method of unsupported ultrafiltration membrane has a relatively thin single layer thickness to avoid the formation of finger-like holes. Generally, two layers are stacked and used, which may reduce the flux due to the presence of double retention layers. The synchronous multi-layer scraping method has extremely high requirements for process and equipment. Therefore, how to construct an unsupported ultrafiltration membrane with high flux and high mechanical strength while maintaining the ultrafiltration membrane retention rate is an urgent problem to be solved. Summary of the invention

[0005] To solve the problem of low flux in the existing supported ultrafiltration membrane forming technology, the single-layer coating method of the unsupported ultrafiltration membrane may reduce the flux due to the presence of the double retention layer, and the synchronous multi-layer scraping method has extremely high requirements on the process and equipment.

[0006] In one aspect, the present application provides a method for preparing an unsupported flat ultrafiltration membrane, comprising: preparing a uniform and transparent polymer solution; The polymer solution is evenly coated on the surface of the polypropylene nonwoven fabric; Installing a gas-phase induced phase inversion device on the back side of the polypropylene non-woven fabric coated with the polymer solution to perform gas-phase induced phase inversion; 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; 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.

[0007] In a feasible implementation, 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.

[0008] In a feasible implementation, the preparation of a uniform and transparent polymer solution comprises the steps of: 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.

[0009] 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.

[0010] In a feasible implementation, the solvent includes one or more of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

[0011] 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.

[0012] 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%.

[0013] In a feasible implementation, the temperature range of the gas phase induced phase transformation is: 20-60°C, and the humidity range is: 40-90%RH.

[0014] In a feasible implementation, the temperature range of the immersion precipitation phase transformation is: 20-60°C, and the solution in the coagulation bath is pure water.

[0015] Another aspect of the present application provides an unsupported flat ultrafiltration membrane, which is prepared by any of the preparation methods described above, comprising: an immersed precipitation phase conversion surface layer and a gas phase induced phase conversion 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.

[0016] The present application provides an unsupported flat ultrafiltration membrane and a preparation method thereof, the steps comprising: 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 transformation device on the back of the polypropylene non-woven fabric coated with the polymer solution, and performing a gas-phase induced phase transformation; immersing the polypropylene non-woven fabric that has completed the gas-phase induced phase transformation process in a coagulation bath containing a non-solvent to perform an immersion precipitation phase transformation; after waiting for the immersion precipitation phase transformation process to be 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. The method uses a supported polypropylene non-woven fabric as a peelable layer, and adjusts the phase separation degree of the casting liquid on one side of the non-woven fabric layer by temperature and humidity to manufacture an unsupported flat ultrafiltration membrane with high mechanical strength, high flux, controllable cross-sectional pore size gradient, strong interception effect and anti-pollution performance. The process is simple, the range of condition parameters is wide, the control conditions are not harsh, it is easy to operate, the equipment requirements are low, and it is easy to implement industrially. The unsupported flat ultrafiltration membrane obtained by the above method has a densified upper surface structure, a controllable cross-sectional porosity gradient, a highly loose sublayer structure, a mechanical strength of not less than 900cN / 15mm, a BSA retention rate of not less than 99.5%, and a pure water flux of not less than 400L / m 2 ·h@0.1Mpa. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings herein are incorporated into and constitute a part of the specification, illustrate embodiments consistent with the implementation of the present invention, and together with the specification are used to explain the principles of the embodiments of the present invention. Obviously, the accompanying drawings described below are only some embodiments of the implementation of the present invention, and for ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without creative work.

[0018] Figure 1 It is a schematic diagram of a process for preparing an unsupported flat ultrafiltration membrane according to an embodiment of the present application; Figure 2 is a scanning electron microscope (SEM) image of the cross section of Example 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; Figure 3 is a scanning electron microscope (SEM) image of the cross section of Example 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; 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; Figure 5 This 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 DESCRIPTION

[0019] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in a variety of forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that the embodiments of the present invention will be more comprehensive and complete and fully convey the concepts of the example embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, many specific details are provided to provide a full understanding of the implementation of the embodiments of the present invention.

[0020] With the continuous development of the current domestic biopharmaceutical field and life science research field, products such as membrane packages and disposable filters with ultrafiltration membrane as the core component are used more and more as essential units in the purification and concentration process. Ultrafiltration membranes are usually defined as membrane materials with a pore size range of 0.01µm-0.1µm, which are mainly driven by pressure to concentrate and filter soluble macromolecules such as proteins, DNA, starch and other substances. In addition to the products of foreign companies such as Merck and Pall, the domestic market also basically uses foreign ultrafiltration membranes as components, which are packaged and sold. At present, many domestic companies have also conducted research and development on ultrafiltration membranes, but there are still few practical applications. Some can be applied in some fields, but the actual performance is still far behind that of excellent foreign products. The research and development of high-performance ultrafiltration membranes is still an urgent problem to be solved.

[0021] The ultrafiltration membranes in several commonly used ultrafiltration membrane packages on the market are basically support systems, that is, the casting liquid is coated on the supporting non-woven fabric during coating, and the non-woven fabric and the casting liquid become one after the immersion precipitation phase is transformed and formed. The reason for choosing this method is that the pore size of the ultrafiltration membrane is small, and a casting liquid system with a high solid content is required to form the membrane. In the immersion precipitation phase transformation, as the solid content of the casting liquid increases, the double diffusion rate slows down, and the relaxation time of the polymer is less than the time required for the system phase separation, a finger-like pore structure will be formed. The position of the finger-like pores will change with the solid content and the degree of phase separation of the system. The formation of finger-like pores will cause the mechanical strength of the membrane to decrease significantly, so a 100KD ultrafiltration membrane of PALL company is a support structure containing finger-like pores. Although a 50KD ultrafiltration membrane of Merck does not contain a support structure with a finger-like pore structure, it can be clearly seen that it is a two-layer scraped membrane, and the thickness of each layer does not exceed 70µm. Because once the scraped membrane thickness is too thick, a finger-like pore structure will be formed at the bottom.

[0022] Another part of the commercialized ultrafiltration membrane is an unsupported system. As mentioned above, unsupported membrane materials must ensure sufficient mechanical strength and avoid the appearance of finger-like pore structures inside the membrane. An ultrafiltration membrane of PALL is used in two layers stacked together, and the thickness of each layer does not exceed 77µm. In addition, as in Merck's patent US2014 / 0221629A1, a two-layer or multi-layer coating scheme is adopted. The lower layer uses a low-solid content casting liquid, and through reverse thermal phase separation, the casting liquid temperature is controlled to prepare a loose sublayer to play a supporting role. The upper layer uses a high-solid content casting liquid, and through direct immersion precipitation phase transformation, a dense retention layer is directly formed.

[0023] 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.

[0024] 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: S100: Prepare a uniform, transparent polymer solution.

[0025] In some embodiments of the present application, specifically, the following steps are included: 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.

[0026] 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.

[0027] 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.

[0028] S130: Cool the initial polymer solution to 40° C. and add a non-solvent to the initial polymer solution.

[0029] S140: Continue stirring the initial polymer solution until the non-solvent is completely dissolved and becomes clear and transparent, thereby obtaining a polymer solution.

[0030] 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%.

[0031] S200: evenly coating the polymer solution on the surface of the polypropylene non-woven fabric.

[0032] The pore uniformity and fiber density of non-woven fabrics play a key role in the gas-induced phase transformation process. Specifically, the uniform pore distribution ensures that water vapor diffuses evenly at the casting liquid interface when passing through the non-woven fabric, avoiding local excessive or insufficient phase separation, thereby forming a stable transition structure of dense surface layer and loose sublayer. Secondly, the dense arrangement of fibers provides a flat support interface for the polymer solution, reducing coating defects; at the same time, during the stripping process, the interfacial stress is dispersed to prevent the film from breaking. Through the above-mentioned synergistic effect, the non-woven fabric not only serves as a temporary carrier, but also becomes a dynamic regulating medium for the formation of phase gradient, ultimately achieving the technical effect of controllable pore gradient in the cross-section of the membrane layer and significantly improved mechanical strength.

[0033] S300: Installing a gas-phase induced phase conversion device on the back of the polypropylene non-woven fabric coated with the polymer solution to perform gas-phase induced phase conversion.

[0034] In some embodiments of the present application, the specific operation of gas-phase induced phase transformation is: subjecting the back side of the polypropylene non-woven fabric having the polymer solution to pure water circulating steam conditions for a preset time.

[0035] In some embodiments of the present application, the temperature range of the gas-phase induced phase transformation is: 20-60°C, and the humidity range is: 40-90%RH.

[0036] S400: immersing the polypropylene nonwoven fabric that has completed the gas phase induced phase transformation process in a coagulation bath containing a non-solvent to perform immersion precipitation phase transformation.

[0037] In some embodiments of the present application, the temperature range of the immersion precipitation phase transformation is: 20-60°C, and the solution in the coagulation bath is pure water.

[0038] S500: After waiting for the immersion precipitation phase conversion process to be 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.

[0039] Embodiment 1 This example provides a method for preparing an unsupported flat ultrafiltration membrane, and the preparation method comprises the following steps: 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 to below 40°C, add 3% pure water, stir until clear and transparent, and obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0040] Step 2: Evenly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0041] Step 3: After scraping a 320µm film of polymer solution onto a 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.

[0042] 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, and the coagulation bath is a pure aqueous solution.

[0043] Step 5: The polypropylene non-woven fabric is washed with 80° C. pure water, and then rolled up and peeled off to obtain an unsupported flat ultrafiltration membrane.

[0044] Figure 2 The cross-sectional microstructure of the unsupported flat ultrafiltration membrane of this embodiment is shown. Two sublayers with different microscopic morphologies are shown, namely the gas-phase induced phase transformation sublayer (the lower part near the lower surface) and the immersed precipitation phase transformation sublayer (the other parts). Figure 2The right side of the figure provides a further comparison of the microscopic morphology of the two sublayers. It can be seen from the figure that the microstructure quickly densifies near the upper surface to form a retention layer that can retain the BSA solution. This structure is caused by the rapid diffusion of the non-solvent during the immersion precipitation phase transformation process; the lower sublayer has a loose structure, which is caused by the increase in the degree of phase separation during the gas-induced phase transformation process. Polypropylene non-woven fabrics must have a uniform pore distribution and a dense fiber structure, so that water vapor can evenly penetrate through the pores to the casting liquid interface, prompting the formation of a dense surface layer (retention layer) and a highly loose sublayer (support layer) during the gas-induced phase transformation process.

[0045] pass Figure 2 The surface pore size rapidly shrank to the nanometer level, while the sublayer showed an open pore structure, verifying the gradient phase separation effect. The obtained unsupported flat ultrafiltration membrane was tested for performance, and the results showed that the mechanical strength was 950cN / 15mm, the BSA retention rate of the unsupported flat ultrafiltration membrane was 99.6%, and the pure water flux was 450L / m 2 ·h@0.1Mpa.

[0046] Embodiment 2 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 to below 40°C, add 3% pure water, stir until clear and transparent, and obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0047] Step 2: Evenly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0048] Step 3: After scraping a 320µm film of polymer solution onto a polypropylene non-woven 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.

[0049] 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, and the coagulation bath is a pure aqueous solution.

[0050] Step 5: The polypropylene non-woven fabric is washed with 80° C. pure water, and then rolled up and peeled off to obtain an unsupported flat ultrafiltration membrane.

[0051] Figure 3 The cross-sectional microstructure of the unsupported flat ultrafiltration membrane of this embodiment is shown. Figure 2 In contrast, the sublayer is more loose, which is due to the increase in humidity during the gas phase induction process, which causes the degree of phase separation in the system to increase.

[0052] The unsupported flat ultrafiltration membrane prepared in this example was tested for performance, and the results showed that the mechanical strength was 1000 cN / 15 mm, the BSA retention rate of the unsupported flat ultrafiltration membrane was 99.6%, and the pure water flux was 500 L / m 2 ·h@0.1Mpa.

[0053] Embodiment 3 The differences between the preparation method of this embodiment and that of embodiment 1 are as follows: 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, stir until clear and transparent to obtain a polymer solution; the above percentages are the mass percentages of each component in the polymer solution.

[0054] Step 2: Evenly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0055] Step 3: After scraping a 330µm film of polymer solution onto a polypropylene non-woven 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.

[0056] 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, and the coagulation bath is a pure aqueous solution.

[0057] Step 5: The polypropylene non-woven fabric is washed with 80° C. pure water, and then rolled up and peeled off to obtain an unsupported flat ultrafiltration membrane.

[0058] The unsupported flat ultrafiltration membrane product prepared in this example was tested for performance and found to have a mechanical strength of 1050 cN / 15 mm, a BSA retention rate of 99.8%, and a pure water flux of 420 L / m 2 ·h@0.1Mpa.

[0059] Embodiment 4 The differences between the preparation method of this embodiment and that of embodiment 1 are as follows: Step 1: Disperse 25% polyethersulfone powder and 1% polyvinylpyrrolidone in N,N-dimethylacetamide, mix and stir evenly at 60-80°C, cool to below 40°C, add 0.5% pure water, stir until clear and transparent to obtain a polymer solution; the above percentages are the mass percentages of each component in the polymer solution.

[0060] Step 2: Evenly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0061] Step 3: After scraping a 280µm film of polymer solution onto a polypropylene non-woven fabric, place the back side in a water vapor atmosphere at 40°C and 90%RH for a preset time.

[0062] 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, and the coagulation bath is a pure aqueous solution.

[0063] Step 5: The polypropylene non-woven fabric is washed with 80° C. pure water, and then rolled up and peeled off to obtain an unsupported flat ultrafiltration membrane.

[0064] The unsupported flat ultrafiltration membrane product prepared in this embodiment was tested for performance and found to have a mechanical strength of 1100 cN / 15 mm, a BSA retention rate of 99.8%, and a pure water flux of 410 L / m 2 ·h@0.1Mpa.

[0065] Comparative Example 1 This comparative example is used to compare and verify the effect of the gas phase induced phase conversion device on the non-woven fabric side. This comparative example is compared with Example 1. The steps of this comparative example 1 are as follows: 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 to below 40°C, add 3% pure water, stir until clear and transparent, and obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

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

[0067] Step 3: After scraping a 320µm film of the polymer solution on a polypropylene non-woven fabric, immerse it 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.

[0068] Step 4: The polypropylene non-woven fabric is washed with 80° C. pure water, and then rolled up and peeled off to obtain an unsupported flat ultrafiltration membrane.

[0069] Figure 4 The microstructure of the cross section of the polyethersulfone flat membrane of this comparative example is shown. Without the use of a gas-phase induced phase conversion device, the non-woven fabric side of the casting liquid has a low degree of phase separation and tends to form a finger-like pore structure. The mechanical strength of this comparative example after peeling is very low, so it cannot play the role of retaining BSA.

[0070] The performance test of the polyethersulfone flat membrane prepared in this comparative example showed that the mechanical strength was 450 cN / 15 mm, which was insufficient.

[0071] Comparative Example 2 This comparative example is used to compare and verify the effect of the gas phase induced phase conversion device on the non-woven fabric side. This comparative example is compared with Example 2. The steps of this comparative example 2 are as follows: 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 to below 40°C, add 3% pure water, stir until clear and transparent, and obtain a polymer solution. The above percentages are the mass percentages of each component in the polymer solution.

[0072] Step 2: Evenly coat the polymer solution on the surface of the polypropylene non-woven fabric.

[0073] Step 3: After scraping a 320µm film of polymer solution onto a polypropylene non-woven fabric, place the back side in a water vapor atmosphere at a temperature of 20°C and a humidity of 60%RH for a preset time.

[0074] 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, and the coagulation bath is a pure aqueous solution.

[0075] Step 5: The polypropylene non-woven fabric is washed with 80° C. pure water, and then rolled up and peeled off to obtain an unsupported flat ultrafiltration membrane.

[0076] Figure 5 The cross section of the polyethersulfone flat membrane of this comparative example is shown. The gas phase induced phase conversion device is added, but after the lower temperature and humidity, the finger-like holes in the lower part of the cross section are obviously shortened, but there are still. The cross section of this comparative example failed to form an asymmetric structure without finger-like holes as in Example 1, and the mechanical strength is still insufficient, so the interception and filtration experiment cannot be carried out.

[0077] The performance test of the polyethersulfone flat membrane prepared in this comparative example showed that the mechanical strength was 600 cN / 15 mm, which was insufficient and there was a risk of leakage of the retained substances.

[0078] On the other hand, the present application provides an unsupported flat ultrafiltration membrane, which is prepared by any one of the preparation methods in the above embodiments, including: an immersed precipitation phase conversion surface layer and a gas phase induced phase conversion sublayer; according to the above embodiments, it can be seen that the mechanical strength of the unsupported flat ultrafiltration membrane is not less than 900cN / 15mm, 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 400L / m 2 ·h@0.1Mpa.

[0079] It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention. For example, other hydrophilic additives can be added to the polymer, including polyethylene glycol, block polyether, sulfonated polyether sulfone, sulfonated polysulfone, hydroxylated polyether sulfone, etc.; other high-boiling point organic solvents, such as N,N-dimethylformamide and N-methylpyrrolidone, etc., can be selected / added to the polymer solution and the coagulation bath; and non-solvents can also be 1,3-propylene glycol, glycerol, ethylene glycol, water, etc.

[0080] Therefore, no matter from which point of view, the embodiments should be regarded as illustrative and non-restrictive, and the scope of the present invention is limited by the appended claims rather than the above description, so it is intended that all changes falling within the meaning and range of equivalent elements of the claims are included in the present invention. Any reference sign in the claims should not be regarded as limiting the claims involved.

[0081] Those skilled in the art will readily appreciate other embodiments of the present disclosure after considering the disclosure of the specification and the examples. This application is intended to cover any modifications, uses or adaptations of the present disclosure, which follow the general principles of the present disclosure and include common knowledge or customary technical means in the art that are 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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