Anisotropic microfiltration membrane, preparation method therefor and application thereof

Abstract

The present application relates to the technical field of membrane separation, and particularly to an anisotropic microfiltration membrane, a preparation method therefor and application thereof. A separation layer is arranged inside a microfiltration membrane, and a large-aperture supporting layer and a macroporous layer are provided on two sides of the separation layer, such that the flux is increased. By means of limiting the thickness of the separation layer, a high flux is achieved, the production efficiency is improved, a high interception effect can still be maintained even if the outer skin is damaged due to an external force, and the risk of low interception caused by scratches in a membrane surface caused by the external force is reduced; and by means of limiting asymmetry coefficients of the macroporous layer and the supporting layer, pore structures of three layers of microfiltration membranes are distributed in a large-small-large manner, such that the high flux is ensured, and an excellent screening performance is also achieved. The present application can obtain the anisotropic microfiltration membrane by means of an integrally-forming method by using a single membrane casting solution without requiring compounding, the preparation method is simple, there are only structural differences between the layers, the transition is smooth, the phenomenon of tearing of different filtration membrane layers does not occur, and the mechanical strength is higher.

Description

Anisotropic microfiltration membrane and its preparation method and application

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on January 2, 2024, with application number 202410002270.X and invention name “Anisotropic microfiltration membrane, preparation method and application thereof”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the field of membrane separation technology, and specifically relates to an anisotropic microfiltration membrane and a preparation method and application thereof. Background Art

[0004] Microfiltration membranes have an effective pore size range of 0.1 to 10 microns and can be used to remove particles, such as solid particles, bacteria, and microorganisms, from liquid streams. It is known in the art that microporous membranes have isotropic (symmetrical) and anisotropic (asymmetric) structures. Symmetric membranes are membranes in which the measured micropore values ​​do not vary substantially in the thickness direction, and the micropore diameters on both sides are essentially the same. Asymmetric membranes, on the other hand, are membranes in which the micropore diameter varies continuously or discontinuously in the thickness direction. However, traditional symmetric membranes are no longer suitable for certain applications because they offer greater resistance to fluid flow and have slower flow rates than asymmetric membranes with the same retention properties.

[0005] Flux performance is becoming increasingly important for filtration applications. One way to increase the flux of filtration membranes without compromising their retention properties is to optimize the membrane structure; another is to increase the porosity. Membranes with multi-zone structures are scientifically more attractive because each zone can be fine-tuned to achieve overall improved performance. The prior art discloses a gradient structure having the most common asymmetric membrane, in which the pore size gradually and continuously increases from one surface to the other; the prior art also discloses an asymmetric membrane comprising a skin and a highly porous asymmetric carrier, the skin containing pores with an average pore size of about 0.005 to about 3.0 microns, the asymmetric carrier comprising a reticular structure containing pores with an average pore size of about 10 to about 20,000 times the average pore size of the surface, and this structure is often accompanied by the risk of reduced retention effect due to surface scratches; the prior art also discloses an "hourglass" type asymmetric microporous membrane, the pore size is distributed in the thickness direction, and the minimum pore layer is arranged in the membrane to increase the filtration flow and extend the life of the filter, but its preparation process requires controlling the amount of non-solvent vapor in contact with the surface of the developing solution and the blowing rate, the depth of the minimum pore layer and the pore diameter therein are difficult to adjust, and the minimum pore layer is relatively thin.

[0006] In the construction of anisotropic microfiltration structures, the stacked membranes obtained by stacking multiple conventional single-layer membranes, along with subsequent modification or additional erosion and solvation, can increase membrane costs. Co-casting anisotropic membranes using two casting solutions requires more complex processes and procedures than preparing anisotropic membranes using a single casting solution. Furthermore, existing microfiltration membranes have fixed inlet and outlet surfaces, resulting in poor adaptability.

[0007] Summary of the Invention

[0008] Therefore, the technical problem to be solved by the present application is to overcome the above-mentioned defects of the asymmetric microfiltration membrane in the prior art, thereby providing an anisotropic microfiltration membrane and a preparation method and application thereof.

[0009] To this end, this application provides the following technical solutions:

[0010] The present application provides an anisotropic microfiltration membrane comprising a porous body having a first surface on one side and a second surface on the other side. The porous body comprises a support layer having a pore size that gradually decreases from the first surface to the second surface, a separation layer having a pore size that tends to be uniform, and a macroporous layer having a pore size that gradually increases. The separation layer occupies 14-30% of the thickness of the porous body (the entire microfiltration membrane). The thickness ratio of the support layer to the macroporous layer is (1.0-11):1, the asymmetry coefficient of the support layer is 1-16, and the asymmetry coefficient of the macroporous layer is 5-38.

[0011] The separation layer is disposed between the support layer and the macroporous layer, which can protect the separation layer and prevent damage to the pore structure of the separation layer. By controlling the thickness ratio of the separation layer to 14-30%, the microfiltration membrane can be guaranteed to have an excellent retention effect. Moreover, both the support layer and the macroporous layer have an asymmetric structure, which can increase the thickness of the separation layer to improve the separation effect while also achieving excellent flux. The asymmetry coefficient of the support layer has a small tendency to change within the membrane, which can provide better support inside the membrane and ensure better compressive performance of the membrane. The macroporous layer, as the outer side of the separation layer, has a higher asymmetry coefficient, which can not only protect the separation layer but also provide a pre-filtration effect. By limiting the thickness ratio and asymmetry coefficient of the support layer and the macroporous layer, both the first surface and the second surface can be used as the liquid inlet surface for filtration operations, without having to limit a specific surface as the liquid inlet surface. This solves the problem in the prior art of requiring a specific liquid inlet surface and the difficulty in distinguishing between the first and second surfaces in actual use.

[0012] The average pore size of the first surface of the anisotropic microfiltration membrane is 0.2-1.1 μm, and the pore ratio is 12%-35%; the average pore size of the second surface of the anisotropic microfiltration membrane is 0.35-1.8 μm, and the pore ratio is 10%-30%.

[0013] In this application, the pore ratio refers to the ratio of the pore area on the first surface to the surface area on the second surface. A higher pore ratio indicates that more liquid enters the membrane at the same time point, ensuring better flux.

[0014] The average pore diameter of the separation layer is 170-500 nm.

[0015] The separation layer plays the primary role in separation, and its pore size tends to be consistent and symmetrical. Maintaining a pore size of 170nm to 500nm in the separation layer can remove most bacteria and microorganisms, meeting chromatographic requirements for filtering routine samples and mobile phases. To address different feeds, separation layers with different average pore sizes can be selected, including 200nm, 300nm, 400nm, or 500nm.

[0016] In the present application, the asymmetry coefficient of the small pore layer is 2 to 53; the asymmetry coefficient represents the pore size variation trend within each layer. The higher the asymmetry coefficient of the small pore layer, the smaller the pore size close to the first surface, which is conducive to better retention.

[0017] As an optional embodiment, the anisotropic microfiltration membrane described in the present application not only includes a three-layer structure, but also has a small pore layer designed on the first surface side, so that the microfiltration membrane body has a small pore layer with a pore size gradually increasing from the first surface to the second surface, a support layer with a pore size gradually decreasing, a separation layer with a consistent pore size, and a large pore layer with a pore size gradually increasing.

[0018] The separation layer with a consistent pore size is a symmetrical layer with a small pore size, which plays a role in filtering and intercepting, and the separation layer is arranged inside the microfiltration membrane to prevent the separation layer from being damaged by external forces, which affects the interception efficiency. The small pore layer, the support layer and the large pore layer are all arranged in an asymmetric structure to increase the flux size of the microfiltration membrane. The microfiltration membrane of the present application is widely used and is sufficient to meet the high requirements for flux and interception in practical applications. It can not only be used for conventional dead-end filtration, but also can achieve better results when applied to tangential flow filtration. The microfiltration membrane of the present application has strong adaptability. The first surface and the second surface can both be used as the liquid inlet surface. When the second surface is used as the liquid inlet surface, the separation layer serves as the main interception area. Due to the presence of the small pore layer and by limiting the thickness of the small pore layer and the asymmetry coefficient, it can play a role of secondary interception. When the first surface is used as the liquid inlet surface, the pore size on the first surface is smaller than that on the second surface, but the number of holes and the hole ratio are significantly increased, which can pre-filter large particles of impurities in the liquid and prevent them from entering the microfiltration membrane, significantly prolong the clogging time of the small pore layer and the support layer, and improve the service life of the filter membrane. Especially when used for tangential flow filtration, the large particles of impurities on the first surface will be washed away by the flowing liquid, and the effect is more prominent.

[0019] In this application, the asymmetry coefficient is expressed as the gradient of the average pore size in different layers. The asymmetry coefficient A is calculated by the following formula: A=|D 大孔 -D 小孔 | / L 厚度 ;

[0020] In the above formula, D 大孔 is the average pore size of the largest pore area in the selected layer, D 小孔 is the average pore size of the minimum pore size area in the selected layer, L 厚度 A is the distance from the maximum pore size region to the minimum pore size region. The main unit of A is nm / (μm). For example, the asymmetry coefficient of the pinhole layer is calculated as the difference in average pore size on both sides of the pinhole layer / the thickness of the pinhole layer.

[0021] The average pore size of the macroporous layer is greater than the average pore size of the microporous layer and the support layer;

[0022] The average pore size of the separation layer is smaller than the average pore size of the small pore layer and the support layer;

[0023] When the microporous layer is provided, the average pore size of the first surface ranges from 0.25 to 0.65 μm.

[0024] Wherein, the porous membrane includes a small pore layer, and the ratio of the thickness of the small pore layer to the thickness of the separation layer is (0.05-0.8):1.

[0025] Optionally, the anisotropic microfiltration membrane comprises a small pore layer, wherein the average pore size of the small pore layer is 250 to 600 nm, and the thickness range of the small pore layer accounts for 2 to 18% of the thickness of the porous body; the average pore size of the support layer is 500 to 1000 nm, and the thickness range of the support layer accounts for 20 to 60% of the thickness of the porous body; the thickness range of the separation layer accounts for 14 to 30% of the overall thickness of the membrane; the average pore size of the large pore layer is 650 to 1200 nm, and the thickness range of the large pore layer accounts for 5 to 35% of the thickness of the porous body.

[0026] The parameters in this application, such as the average pore size, thickness of different layer structures, pore ratio, fiber diameter, etc., can be calculated by using a scanning electron microscope to characterize the membrane structure morphology, and then using computer software (such as Matlab, NIS-Elements, etc.) or manual measurement to calculate the average value. When measuring, the parts that are obviously smaller or larger are not taken into account. When actually measuring, the membrane surface (or cross section) can be characterized by an electron microscope first, and the corresponding SEM image can be obtained, and a certain area, such as 1μm, can be selected. 2 (1 μm times 1 μm) or 25 μm 2(5μm multiplied by 5μm), the specific area size depends on the actual situation, and then the pore size of all holes in the area is measured by corresponding computer software or manually, and then calculated to obtain the average pore size of the area (that is, the average pore size measured by SEM). In terms of the average pore size test, in addition to being able to measure and analyze the SEM image, the average pore size of each layer can also be directly analyzed by an average pore size distribution instrument. For example, a membrane of a certain size is first cut, and the water in the original wet membrane is replaced with ethanol of different concentrations. Then, it is wetted with a low surface tension solvent, and then placed in a test tank. Finally, the PMI average pore size is obtained through the dry-wet line. The above methods for measuring each parameter are only examples. It can be understood that those skilled in the art can also obtain the above parameters by other measurement means.

[0027] Optionally, the total thickness of the anisotropic microfiltration membrane is in the range of (100-180) μm, the porosity of the anisotropic microfiltration membrane is 60-80%; the bubble point of the anisotropic microfiltration membrane is (18-60) psi; the water flux of the anisotropic microfiltration membrane at 14.5 psi is (5-60) mL / cm 2 ·min.

[0028] Optionally, the average diameter of the fibers in the small pore layer of the anisotropic microfiltration membrane is in the range of (100-500) nm, and the ratio of the average pore diameter (i.e., average pore size) in the small pore layer to its average fiber diameter is maintained at (0.5-6.0):1; the average diameter of the fibers in the support layer of the microfiltration membrane is in the range of (120-450) nm, and the ratio of the average pore diameter in the support layer to its average fiber diameter is maintained at (1.1-8.5):1; the average diameter of the fibers in the separation layer of the microfiltration membrane is in the range of (80-350) nm, and the ratio of the average pore diameter in the separation layer to its average fiber diameter is maintained at (0.5-6.3):1; the average diameter of the fibers in the large pore layer of the microfiltration membrane is in the range of (200-550) nm, and the ratio of the average pore diameter in the large pore layer to its average fiber diameter is maintained at (1.1-6.0):1.

[0029] The beneficial effect of the above scheme is that when the ratio of the average pore size of the macroporous layer to the average fiber diameter is maintained at (1.1-6.0):1, the coarser fibers can protect the separation layer and have better pressure resistance, ensuring the smooth entry or permeation of the feed liquid; and the ratio of the average pore size to the average fiber diameter in the separation layer is higher, which contributes to the permeability of the internal structure of the membrane, ensuring retention while ensuring good permeability of the area; the main function of the support layer is to provide support and permeation, and it can achieve better results when the ratio of the average pore size to the average fiber diameter is maintained at (1.1-8.5):1; the small pore layer, as one of the important links in the multiple retention process, also has good permeability.

[0030] The ratio of the average pore size of the above-mentioned different regions to the average fiber diameter thereof refers to the ratio of the diameter of the fibers surrounding the pores to the average pore size of the pores formed.

[0031] Optionally, the microfiltration membrane is made of at least one of polyethersulfone, polysulfone, cellulose acetate, regenerated cellulose, polytetrafluoroethylene and polyvinylidene fluoride.

[0032] In this application, the nitrogen content of the first surface of the anisotropic microfiltration membrane is 2-6%, and the water contact angle of the first surface is 25-55°; the nitrogen content of the second surface is 3.5-9%, and the water contact angle of the second surface is 18-50°. The nitrogen content in the microfiltration membrane primarily accounts for the proportion of the total carbon, nitrogen, oxygen, and sulfur elements.

[0033] In this application, the membrane structure is primarily asymmetric. Due to surface segregation, the enrichment of the hydrophilic polymer within the membrane varies. The additive contains a large number of -NH2 groups, resulting in a difference in nitrogen content between the first and second surfaces. Furthermore, the -NH2 groups can form hydrogen bonds with water molecules, exhibiting enhanced hydrophilicity, thereby improving the membrane's flux, anti-fouling properties, and anti-attenuation performance.

[0034] In this application, the wet phase transformation process to form an asymmetric membrane structure is primarily driven by the water concentration gradient near the interface between the casting solution and the coagulation bath. Due to surface segregation, the hydrophilic additive migrates toward the water / membrane interface, resulting in its enrichment near the membrane / water interface. The primary hydrophilic additives used in this application are acrylamide monomers and their derivatives, which can increase the nitrogen content on the membrane surface.

[0035] The anisotropic microfiltration membrane described in this application is an integrated membrane prepared from a single casting liquid, which is different from the microfiltration membrane prepared from multiple casting liquids. The advantage of this is that the preparation method is simple, and there are only structural differences between the layers, the transition is smooth, there will be no tearing of different layers of the filter membrane, and the mechanical strength is higher.

[0036] The present application provides a method for preparing an anisotropic microfiltration membrane, comprising the following steps:

[0037] 1) preparing a casting solution; dissolving a polymer in a solvent, and adding an additive after dissolution to obtain a casting solution, wherein the casting solution has a polymer solid content of 14 to 20%, a solvent content of 40 to 70%, and an additive content of 20 to 35% by weight;

[0038] 2) Controlling the carrier to a predetermined temperature, applying the casting solution to the carrier surface to form a liquid film, and placing it in a specific environment for a period of time to obtain a primary film;

[0039] 3) Immersing the native membrane in a first coagulation bath for phase separation to obtain an anisotropic microfiltration membrane.

[0040] Optionally, the polymer in step 1) is selected from one or more of polyethersulfone, polysulfone, cellulose acetate, regenerated cellulose, polytetrafluoroethylene and polyvinylidene fluoride.

[0041] Optionally, in step 1), the solvent is selected from one or more of dimethylformamide, dimethylacetamide, tetrahydrofuran, N-methylpyrrolidone, triethyl phosphate, chloroform, dimethyl sulfoxide and γ-butyrolactone.

[0042] Optionally, the additives in step 1) include a porogen and a hydrophilic additive, wherein the porogen is selected from alkanes, such as nonane, heptane, and octane; various polyethylene glycols, such as PEG-200 and PEG-800; various polyvinyl pyrrolidones, such as PVP-K30 and PVP-K60; various alcohols, such as tripropylene glycol, triethylene glycol, diethylene glycol, n-butanol, and tert-amyl alcohol; and various salts, such as lithium chloride, potassium dihydrogen phosphate, potassium bicarbonate, sodium bicarbonate; and mixtures thereof; and the hydrophilic additive is selected from one or more of acrylamide monomers and their derivatives.

[0043] Optionally, the viscosity of the casting solution in step 1) at 25° C. is 3000 to 13000 mPa.s.

[0044] Optionally, in step 2), the surface temperature of the carrier is controlled to be 25-80°C.

[0045] Optionally, the specific environment in step 2) can be a high humidity environment with the humidity controlled at 60-90% RH; or at room temperature with the air flow rate set at 0.3-1 m / s to form an air flow on the membrane.

[0046] Optionally, the placement time in step 2) is the time the native film is exposed to a high humidity environment or an air flow environment, and the exposure time is controlled to be 10 to 120 seconds.

[0047] In the present application, when a microporous layer is included, the step of immersing the substrate in a second coagulation bath is further included, wherein the immersion time in the first coagulation bath is controlled to be within 5 seconds, and the surface tension of the second coagulation bath is less than the surface tension of the first coagulation bath;

[0048] Alternatively, when the pinhole layer is not included, the time of immersion in the first coagulation bath is controlled to be greater than 5 seconds. Optionally, the time of immersion in the first coagulation bath is controlled to be greater than 5 seconds and less than 120 seconds.

[0049] Optionally, the temperature of the first coagulation bath is 25-35°C, and the temperature of the second coagulation bath is 20-25°C.

[0050] Optionally, in step 3), the first coagulation bath is water or a mixed solution of an organic solvent and water, the organic solvent is mainly at least one of alcohols, ethers or ketone compounds, and the mass ratio of the organic solvent to water is (0-0.15):1; the second coagulation bath is a mixed solution of an organic solvent and water, the organic solvent also includes silicon- and fluorine-containing solvents, such as organosiloxane, hexafluoroisopropanol, and trifluoroethanol, and the mass ratio of the organic solvent to water is (0.005-0.1):1.

[0051] Alternatively, in step 3), the method for controlling the appearance of a pinhole layer in the final membrane structure is to control the immersion time in the first coagulation bath to less than 5 seconds, then remove the membrane and immerse it in the second coagulation bath for 5-120 seconds. Alternatively, in step 3), controlling the immersion time in the first coagulation bath to greater than 5 seconds and 120 seconds can obtain a membrane without a pinhole layer.

[0052] Through the above-mentioned technical solution, the traditional wet phase transformation method is combined with the thermally induced phase separation method or the steam-induced phase separation method to prepare a multilayer microfiltration membrane. In a high humidity environment, the continuous diffusion of water vapor on the surface of the native membrane promotes the phase separation at the membrane-air interface to form a large pore area. After immersion in a coagulation bath, bidirectional diffusion between the solvent and the non-solvent occurs, promoting the formation of a separation layer and a support layer. Before the phase transformation is completed, the membrane is transferred to a second coagulation bath (with lower surface tension). Rapid bidirectional diffusion occurs on one side of the membrane-support interface, causing the membrane structure in this area to tend towards a dense structure, forming a small pore layer.

[0053] Using the same raw material ratio, we reduced the step of immersing in the second coagulation bath, allowing the native membrane to undergo phase transformation directly in the first coagulation bath. By controlling the immersion time to greater than 5 seconds, a three-layer anisotropic microfiltration membrane can be obtained.

[0054] The anisotropic microfiltration membrane of the present application can be used for dead-end filtration and tangential flow filtration, and is not limited to the liquid inlet surface. Both the second surface and the first surface can be used as the liquid inlet surface. When the second surface is used as the liquid inlet surface, the separation layer serves as the main retention area, but due to the presence of the first surface, it can play a role of secondary retention. When the first surface is used as the liquid inlet surface, the pore size on the first surface is smaller than the pore size on the second surface, but the number of holes and the hole ratio are significantly increased, which can play a role of pre-filtration for large particle impurities in the feed liquid, prolong the clogging time of the small pore area and the middle area, and increase the service life of the filter membrane. Especially when used for tangential flow filtration, the large particle impurities on the first surface will be washed away by the flowing feed liquid, and the effect is more prominent. However, conventional microfiltration membranes in the prior art are prone to low flux and rapid attenuation during dead-end filtration; due to the large pore size, impurities remain in the membrane pores and are not suitable for tangential flow filtration.

[0055] In this application, dead-end filtration and tangential flow filtration are two operating modes used during the operation of microfiltration membrane filtration. Dead-end filtration is to place the fluid to be filtered upstream of the membrane. Under the influence of the pressure difference, the fluid and particles smaller than the membrane pores pass through the membrane, and particles larger than the membrane pores are retained by the membrane. The pressure difference can be formed by pressurizing the fluid to be filtered or by vacuuming the filtrate side. With the extension of the filtration time, the retained particles will form a contamination layer on the membrane surface, which increases the filtration resistance. When the operating pressure remains unchanged, the filtration permeability of the membrane will decrease. Therefore, dead-end filtration can only be carried out intermittently, and the contamination layer on the membrane surface must be periodically removed or the membrane must be replaced. When tangential flow filtration is running, the fluid to be filtered generates two components of force on the membrane surface. One is the normal force perpendicular to the membrane surface, which causes the fluid to pass through the membrane surface, and the other is the tangential force parallel to the membrane surface, which washes away the retained matter on the membrane surface. When the permeability of tangential flow filtration decreases, the membrane can be effectively cleaned and restored to its original performance by reducing the normal force on the membrane surface and increasing the tangential force on the membrane surface. Therefore, the membrane surface of tangential flow filtration is less prone to concentration polarization and scaling, and the permeability decays more slowly.

[0056] The present application provides an application of an anisotropic microfiltration membrane in removing insoluble particles or microbial impurities from a liquid flow.

[0057] The technical solution of this application has the following advantages:

[0058] The anisotropic microfiltration membrane provided by the present application includes a porous body, one side of the porous body is a first surface, and the other side is a second surface; the porous body includes a support layer with a pore size gradually decreasing from the first surface to the second surface, a separation layer with a pore size tending to be uniform, and a macroporous layer with a pore size gradually increasing; the asymmetry coefficient of the support layer is 1-16, the asymmetry coefficient of the macroporous layer is 5-38, the thickness ratio of the separation layer to the porous body is 14-30%, and the thickness ratio of the support layer to the macroporous layer is 1.0-11:1. Among them, the separation layer is arranged inside the microfiltration membrane, and the large-pore support layer and macroporous layer are arranged on both sides of the separation layer, which increases the flux. By setting the thickness of the separation layer to 14-30% of the total thickness of the membrane, high flux is achieved and production efficiency is improved. Even if the outer skin is damaged due to external force, the high retention effect can still be maintained, and this structure reduces the risk of low retention caused by scratches on the membrane surface caused by external force. By limiting the asymmetric coefficients of the macroporous layer and the support layer, the three-layer microfiltration membrane's pore structure exhibits a uniform distribution of large and small pores, ensuring both high flux and excellent screening performance. Conventional microfiltration membranes, however, have uniform pore sizes, which can cause foreign matter to be captured prematurely on the inlet side, leading to blockage and impacting performance.

[0059] The anisotropic microfiltration membrane provided in the present application can further improve the flux of the membrane while ensuring the filtration efficiency by optimizing the average pore size of the first surface and the average pore size of the second surface.

[0060] The anisotropic microfiltration membrane provided by the present application further includes a small pore layer on one side of the support layer. The small pore layer can play a certain selective separation role. By introducing a double separation layer structure and a porous support layer located between them, the separation capacity can be further improved and a higher flux can be achieved. In addition, the double separation layer structure improves the retention rate of bacteria and microorganisms in intermittent filtration, thereby improving production efficiency. In addition, the four-layer structure of the small pore layer, support layer, separation layer and macroporous layer and the limitation of the asymmetric coefficient make the overall pore size of the microfiltration membrane present a "small, large, large" distribution, which expands its application. It can be used for both dead-end filtration and tangential flow filtration. The small pore layer can block large particles on the membrane surface and capture small particles in the separation layer, greatly playing a role in the thickness direction of the membrane, and further improving the efficiency of the microfiltration membrane. That is, the small pore layer acts as a pre-filtration layer, and its structure is an "inverted" asymmetric structure. This design can intercept the feed liquid in advance, isolate larger impurities outside the membrane, reduce the risk of clogging of the pores in the membrane, and increase the filtration volume in practical applications.

[0061] The preparation method of the anisotropic microfiltration membrane provided by the present application is different from the prior art method of using two casting liquids to cast together to obtain a multilayer membrane. Compared with the preparation of a single casting liquid in the present application, the process is more complicated and the process is more complex. In addition, the microfiltration membrane in the prior art has fixed liquid inlet and outlet surfaces, and the adaptability of the filter membrane is poor. The present application is a method for preparing a multilayer microfiltration membrane by a single casting liquid. The membrane structure consists of at least three regions, and its separation layer is mainly inside the membrane, and the separation layer occupies 14 to 30% of the overall thickness of the membrane. The areas bordering the separation layer are all macroporous structures, which ensure the flux performance of the membrane and reduce the risk of scratches on the membrane surface caused by external forces that reduce the interception effect. The anisotropic microfiltration membrane can be obtained by a single casting liquid integral molding method without the need for compounding. Unlike the prior art method of preparing a multilayer membrane with multiple casting liquids, the advantage is that the preparation method is simple, and there is only a structural difference between the layers, the transition is smooth, there is no tearing phenomenon between different layers of the filter membrane, and the mechanical strength is higher.

[0062] The anisotropic microfiltration membrane provided by this application can be used for dead-end filtration and tangential flow filtration, and is not limited to the liquid inlet surface. Both the second surface and the first surface can serve as the liquid inlet surface. When the second surface serves as the liquid inlet surface, the separation layer serves as the primary retention area, but due to the presence of the first surface, it can play a secondary retention role. When the first surface is used as the liquid inlet surface, the pore size on the first surface is smaller than the pore size on the second surface, but the number of holes and the hole ratio are significantly increased, which can play a pre-filtration role for large particle impurities in the feed liquid, prolong the clogging time of the small pore area and the intermediate area, and increase the service life of the filter membrane. Especially when used for tangential flow filtration, the large particle impurities on the first surface will be washed away by the flowing feed liquid, and the effect is more prominent. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0064] FIG1 is a schematic structural diagram of the anisotropic microfiltration membrane provided in Example 1 of the present application;

[0065] FIG2 is a SEM image of a cross section of the microfiltration membrane in Example 1 of the present application magnified 800 times;

[0066] FIG3 is a SEM image of a cross section of the microfiltration membrane in Example 2 of the present application magnified 800 times;

[0067] FIG4 is a SEM image of a cross section of the microfiltration membrane in Example 7 of the present application magnified 800 times;

[0068] FIG5 is a SEM image of a cross section of the microfiltration membrane in Example 8 of the present application magnified 800 times;

[0069] Reference numerals: 1. small pore layer; 2. support layer; 3. separation layer; 4. large pore layer. DETAILED DESCRIPTION

[0070] The following examples are provided to further better understand the present application, but are not limited to the best implementation mode described herein, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the scope of protection of the present application.

[0071] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0072] Example 1

[0073] This embodiment provides an anisotropic microfiltration membrane, the structure of which is shown in FIG1 . From bottom to top, it includes a small pore layer 1, a support layer 2, a separation layer 3, and a macroporous layer 4. The specific preparation method is as follows:

[0074] First, PES (E6020P) was dissolved in DMF, and then additives N-hydroxyethyl acrylamide, lithium chloride and PVP-K60 were added. Finally, the porogen diethylene glycol was added and mixed. The mass ratio of PES, DMF, N-hydroxyethyl acrylamide, lithium chloride, PVP-K60 and diethylene glycol was 15:59.5:3:0.1:2:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0075] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. The membrane was kept in the air section for 30 seconds under a relative humidity of 75% RH, immersed in a first coagulation bath at 25°C (10 wt% ethanol aqueous solution), and immediately transferred to a second coagulation bath at 25°C (5 wt% hexafluoroisopropanol aqueous solution) for 10 seconds after 3 seconds to perform complete phase separation to obtain an anisotropic microfiltration membrane (a).

[0076] Figure 2 is an SEM image of the cross-section of the microfiltration membrane obtained in this embodiment magnified 800 times. It can be seen from the figure that the microfiltration membrane clearly has a four-layer structure, the lower side is the first surface, the upper side is the second surface, and the separation layer is located inside the entire microfiltration membrane. The thickness of the macroporous layer is about 18um, the thickness of the separation layer is about 20um, the thickness of the support layer is about 63um, and the thickness of the small pore layer is about 9um.

[0077] Example 2

[0078] This embodiment provides an anisotropic microfiltration membrane, the structure of which is shown in FIG1 . From bottom to top, it includes a small pore layer 1, a support layer 2, a separation layer 3, and a macroporous layer 4. The specific preparation method is as follows:

[0079] First, PES (5900P) was dissolved in NMP, and then additives acrylamide, potassium dihydrogen phosphate and PVP-K30 were added. Finally, the porogen polyethylene glycol (PEG 200) was added and mixed. The mass ratio of PES, NMP, acrylamide, potassium dihydrogen phosphate, PVP-K30 and polyethylene glycol was 19:51:4:0.2:5:25. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0080] The scraper thickness was adjusted to 250 μm, and the casting solution was cast on a mobile carrier that was rapidly heated to 50° C. The solution was allowed to remain in the air section for 40 seconds under a relative humidity of 80% RH, and then immersed in a first coagulation bath at 35° C. (3 wt % isopropyl alcohol aqueous solution). After 3 seconds, the solution was immediately transferred to a second coagulation bath at 25° C. (containing an aqueous solution of 20 wt % ethanol and 0.5 wt % hexamethylcyclotrisiloxane) for 5 seconds to undergo complete phase separation to obtain an anisotropic microfiltration membrane (b).

[0081] Figure 3 is an SEM image of the cross-section of the microfiltration membrane obtained in this embodiment magnified 800 times. It can be seen from the figure that the microfiltration membrane clearly has a four-layer structure, the lower side is the first surface, the upper side is the second surface, and the separation layer is located inside the entire microfiltration membrane. The thickness of the macroporous layer is about 8um, the thickness of the separation layer is about 26um, the thickness of the support layer is about 75um, and the thickness of the small pore layer is about 11um.

[0082] Example 3

[0083] This embodiment provides an anisotropic microfiltration membrane, the structure of which is shown in FIG1 . From bottom to top, it includes a small pore layer 1, a support layer 2, a separation layer 3, and a macroporous layer 4. The specific preparation method is as follows:

[0084] First, PES (E6020P) was dissolved in a mixed solvent of DMF and triethyl phosphate, then additives acrylamide and PVP-K30 were added, and finally the porogen triethylene glycol was added and mixed. The mass ratio of PES, DMF, triethyl phosphate, acrylamide, PVP-K30 and triethylene glycol was 16:45:9:4:4:22. The mixture was stirred at 45°C until clear to obtain a casting solution.

[0085] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. The membrane was allowed to remain in the air section for 20 seconds under a relative humidity of 70% RH, and then immersed in a first coagulation bath at 30°C (5 wt% isopropanol aqueous solution). After 2 seconds, the membrane was immediately transferred to a second coagulation bath at 20°C (3 wt% trifluoroethanol aqueous solution) for 60 seconds to undergo complete phase separation to obtain an anisotropic microfiltration membrane (c).

[0086] Example 4

[0087] This embodiment provides an anisotropic microfiltration membrane, the structure of which is shown in FIG1 . From bottom to top, it includes a small pore layer 1, a support layer 2, a separation layer 3, and a macroporous layer 4. The specific preparation method is as follows:

[0088] First, PES (5900P) was dissolved in γ-butyrolactone, and then additives caprolactam, acrylamide and PVP-K30 were added. Finally, the porogen polyethylene glycol (polyethylene glycol 400) was added and mixed. The mass ratio of PES, γ-butyrolactone, caprolactam, acrylamide, PVP-K30 and polyethylene glycol was 19:51:4:2:2:22. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0089] The scraper thickness was adjusted to 200 μm, and the casting solution was cast on a mobile carrier that was rapidly heated to 60°C. In an environment with a relative humidity of 60% RH, the air flow rate was set at 0.5 m / s, and the native membrane was allowed to stay in the air section for 60 seconds. It was then immersed in a first coagulation bath at 30°C (2 wt% acetone aqueous solution) and immediately transferred to a second coagulation bath at 25°C (5 wt% trifluoroethanol aqueous solution) after 3 seconds for 120 seconds to perform complete phase separation, thereby obtaining an anisotropic microfiltration membrane (d).

[0090] Example 5

[0091] This embodiment provides an anisotropic microfiltration membrane, the structure of which is shown in FIG1 . From bottom to top, it includes a small pore layer 1, a support layer 2, a separation layer 3, and a macroporous layer 4. The specific preparation method is as follows:

[0092] First, PES (5200P) was dissolved in DMF, and then additives N-hydroxyethyl acrylamide, acrylamide and PVP-K60 were added. Finally, the porogen diethylene glycol was added and mixed. The mass ratio of PES, DMF, N-hydroxyethyl acrylamide, acrylamide, PVP-K60 and diethylene glycol was 17:56:4:1:2:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0093] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 70°C. In an environment with a relative humidity of 65% RH, the air flow rate was set at 1 m / s, and the native membrane was allowed to stay in the air section for 10 seconds. It was then immersed in a first coagulation bath at 35°C (5 wt% ethanol aqueous solution), and immediately transferred to a second coagulation bath at 20°C (3 wt% hexafluoroisopropanol aqueous solution) after 3 seconds for 80 seconds to perform complete phase separation to obtain an anisotropic microfiltration membrane (e).

[0094] Example 6

[0095] This embodiment provides an anisotropic microfiltration membrane, the structure of which is shown in FIG1 . From bottom to top, it includes a small pore layer 1, a support layer 2, a separation layer 3, and a macroporous layer 4. The specific preparation method is as follows:

[0096] First, PES (E6020P) was dissolved in a mixed solvent of NMP and tetrahydrofuran, and then additives N-hydroxymethyl acrylamide and lithium chloride were added. Finally, a porogen polyethylene glycol (PEG 400) was added and mixed. The mass ratio of PES, NMP, tetrahydrofuran, N-hydroxymethyl acrylamide, lithium chloride and polyethylene glycol was 18:47.8:4:5:0.2:25. The mixture was stirred at 50°C until clear to obtain a casting solution.

[0097] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 80°C. The membrane was kept in the air section for 25 seconds under a relative humidity of 80% RH, immersed in a first coagulation bath at 30°C (3 wt% isopropanol aqueous solution), and immediately transferred to a second coagulation bath at 20°C (5 wt% trifluoroethanol aqueous solution) for 20 seconds after 2 seconds to perform complete phase separation to obtain an anisotropic microfiltration membrane (f).

[0098] Example 7

[0099] This embodiment provides an anisotropic microfiltration membrane, which includes a support layer 2, a separation layer 3, and a macroporous layer 4 arranged in sequence from bottom to top. The specific preparation method is as follows:

[0100] First, PES (E6020P) was dissolved in DMF, and then additives N-hydroxyethyl acrylamide, lithium chloride and PVP-K60 were added. Finally, the porogen diethylene glycol was added and mixed. The mass ratio of PES, DMF, N-hydroxyethyl acrylamide, lithium chloride, PVP-K60 and diethylene glycol was 15:59.5:3:0.1:2:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0101] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. The membrane stayed in the air section for 30 seconds under a relative humidity of 75% RH, and then immersed in a first coagulation bath at 25°C (10 wt% ethanol aqueous solution). After 10 seconds, the membrane was transferred to a second coagulation bath at 25°C (5 wt% hexafluoroisopropanol aqueous solution) for 10 seconds to perform complete phase separation to obtain an anisotropic microfiltration membrane (g).

[0102] Figure 4 is an SEM image of the cross-section of the microfiltration membrane obtained in this embodiment magnified 800 times. It can be seen from the figure that the microfiltration membrane clearly has a three-layer structure, the lower side is the first surface, and the upper side is the second surface. There are only macroporous layers and support layers on both sides of the separation layer of the prepared microfiltration membrane, and there is no microporous layer. The thickness of the macroporous layer is about 24um, the thickness of the separation layer is about 24um, and the thickness of the support layer is about 70um.

[0103] Example 8

[0104] This embodiment provides an anisotropic microfiltration membrane, which includes a support layer 2, a separation layer 3, and a macroporous layer 4 arranged in sequence from bottom to top. The specific preparation method is as follows:

[0105] First, PES (E6020P) was dissolved in DMF, and then additives N-hydroxyethyl acrylamide, lithium chloride, and PVP-K60 were added. Finally, the porogen diethylene glycol was added and mixed. The mass ratio of PES, DMF, N-hydroxyethyl acrylamide, lithium chloride, PVP-K60, and diethylene glycol was 15:59.5:3:0.1:2:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0106] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. The membrane stayed in the air section for 30 seconds under a relative humidity of 75% RH and was immersed in a first coagulation bath (10 wt% ethanol aqueous solution) at 25°C for 7 seconds to perform complete phase separation to obtain an anisotropic microfiltration membrane (h).

[0107] Figure 5 is an SEM image of the cross-section of the microfiltration membrane obtained in this embodiment magnified 800 times. It can be seen from the figure that the microfiltration membrane clearly has a three-layer structure, the lower side is the first surface, and the upper side is the second surface. There are only macroporous layers and support layers on both sides of the separation layer of the prepared microfiltration membrane, and there is no microporous layer. The thickness of the macroporous layer is about 24um, the thickness of the separation layer is about 35um, and the thickness of the support layer is about 81um.

[0108] Example 9

[0109] This embodiment provides an anisotropic microfiltration membrane, and the specific preparation method thereof is as follows:

[0110] First, cellulose acetate was dissolved in DMF, and then additives N-hydroxyethyl acrylamide, lithium chloride and PVP-K60 were added. Finally, the porogen diethylene glycol was added and mixed. The mass ratio of cellulose acetate, DMF, N-hydroxyethyl acrylamide, lithium chloride, PVP-K60 and diethylene glycol was 15:59.5:3:0.1:2:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0111] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. The membrane stayed in the air section for 30 seconds under a relative humidity of 75% RH, and then immersed in a first coagulation bath at 25°C (10 wt% ethanol aqueous solution). After 3 seconds, the membrane was immediately transferred to a second coagulation bath at 25°C (5 wt% hexafluoroisopropanol aqueous solution) for 50 seconds to perform complete phase separation to obtain an anisotropic microfiltration membrane (k).

[0112] Example 10

[0113] This embodiment provides an anisotropic microfiltration membrane. Compared with Example 1, the only difference is that an air flow environment is used instead of an environment with a relative humidity of 75%. That is, under an air flow rate of 0.8 m / s, the membrane is kept in the air section for 30 seconds and then immersed in a first coagulation bath (10 wt % ethanol aqueous solution) at 25°C. After 3 seconds, the membrane is immediately transferred to a second coagulation bath (5 wt % hexafluoroisopropanol aqueous solution) at 25°C for 20 seconds to achieve complete phase separation, thereby obtaining an anisotropic microfiltration membrane (1).

[0114] Example 11

[0115] This embodiment provides an anisotropic microfiltration membrane, which is formed by stacking two double-layer membranes. The specific preparation method is as follows:

[0116] First, PES (E6020P) was dissolved in DMF, and then additives N-hydroxyethyl acrylamide, lithium chloride and PVP-K60 were added. Finally, the porogen diethylene glycol was added and mixed. The mass ratio of PES, DMF, N-hydroxyethyl acrylamide, lithium chloride, PVP-K60 and diethylene glycol was 15:59.5:3:0.1:2:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0117] The scraper thickness was adjusted to 150 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. In an environment with a relative humidity of 65% RH, it stayed in the air section for 10 seconds and was immersed in a coagulation bath at 45°C (3 wt% ethanol aqueous solution) for 20 seconds to obtain an asymmetric microfiltration membrane. Then, by stacking and compounding two asymmetric microfiltration membranes, a four-layer anisotropic microfiltration membrane (m) was obtained.

[0118] Example 12

[0119] This embodiment provides an anisotropic microfiltration membrane. Unlike the previous single casting solution casting method, two casting solutions are used for casting. The specific preparation method is as follows:

[0120] First, the casting solution in Example 8 was used as the first casting solution, and the casting solution in Example 6 was used as the second casting solution for standby use. The first scraper thickness was adjusted to 180 μm, and the first casting solution was cast on a mobile carrier that was rapidly heated to 60°C. The solution was allowed to remain in the air zone for 30 seconds under a relative humidity of 75%. Simultaneously, the scraper thickness was adjusted to 120 μm, and the second casting solution was evenly scraped onto the primary membrane formed by the first casting solution. The solution remained in the air zone for 10 seconds, and the membrane was immersed in a coagulation bath (10 wt% aqueous ethanol) at 25°C for 10 seconds to undergo complete phase separation, thereby obtaining an anisotropic microfiltration membrane (n).

[0121] Comparative Example 1

[0122] This comparative example provides an anisotropic microfiltration membrane, and its specific preparation method is as follows:

[0123] First, PES (E6020P) was dissolved in NMP, and then additives acrylamide, lithium chloride and PVP-K30 were added. Finally, the porogen triethylene glycol was added and mixed. The mass ratio of PES, NMP, acrylamide, lithium chloride, PVP-K30 and triethylene glycol was 13:54.9:8:0.1:4:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0124] The scraper thickness was adjusted to 300 μm, and the casting solution was cast on a mobile carrier that was rapidly heated to 60°C. In an environment with a relative humidity of 75% RH, the air flow rate was set at 1 m / s, and the native membrane was allowed to stay in the air section for 30 seconds. The membrane was then immersed in a coagulation bath (5 wt% isopropanol aqueous solution) for complete phase separation to obtain an anisotropic microfiltration membrane (i).

[0125] Comparative Example 2

[0126] This comparative example provides an anisotropic microfiltration membrane, and its specific preparation method is as follows:

[0127] First, PES (E6020P) was dissolved in NMP, and then additives acrylamide, lithium chloride and PVP-K30 were added. Finally, the porogen triethylene glycol was added and mixed. The mass ratio of PES, NMP, acrylamide, lithium chloride, PVP-K30 and triethylene glycol was 22:45.9:4:0.1:8:20. The mixture was stirred at 60°C until clear to obtain a casting solution.

[0128] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 60°C. In an environment with a relative humidity of 75% RH, the air flow rate was set at 1 m / s, and the native membrane was allowed to stay in the air section for 30 seconds. It was then immersed in a coagulation bath (5 wt% isopropanol) for complete phase separation to obtain an anisotropic microfiltration membrane (j).

[0129] Comparative Example 3

[0130] This comparative example provides an anisotropic microfiltration membrane, and its specific preparation method is as follows:

[0131] First, PES (E6020P) was dissolved in a mixed solvent of NMP and tetrahydrofuran, and then additives N-hydroxymethyl acrylamide and lithium chloride were added. Finally, a porogen polyethylene glycol (PEG 400) was added and mixed. The mass ratio of PES, NMP, tetrahydrofuran, N-hydroxymethyl acrylamide, lithium chloride and polyethylene glycol was 21:45.8:4:4:0.2:25. The mixture was stirred at 50°C until clear to obtain a casting solution.

[0132] The scraper thickness was adjusted to 250 μm, and the casting liquid was cast on a mobile carrier that was rapidly heated to 80°C. The membrane was kept in the air section for 5 seconds under a relative humidity of 45% RH, immersed in a first coagulation bath at 25°C (3 wt% isopropanol aqueous solution), and immediately transferred to a second coagulation bath at 25°C (5 wt% trifluoroethanol aqueous solution) after 2 seconds for complete phase separation to obtain an anisotropic microfiltration membrane (o).

[0133] Test Method

[0134] The following performance tests were performed on the microfiltration membranes provided in the examples and comparative examples of the present application:

[0135] 1. Bubble point test

[0136] Bubble Point Pore Size is the bubble point value representing the maximum effective pore size in a sample, measured in microns, as measured in accordance with ASTM F-316-03 using IPA:H2O (60:40) with a surface energy of 23 dynes / cm as the wetting fluid and a 47 mm disc as the sample size.

[0137] 2. Flux testing

[0138] Membrane water flux is determined by measuring the amount of water that passes through a membrane sample in a given time. Membrane samples are cut into 47 mm diameter discs and pre-wetted with Milli-Q water or IPA. The sample is placed in a negative pressure device at 14.5 psi. The time required for the membrane to pass 50 mL of water at this pressure is recorded, and the flow rate per minute at this pressure is calculated.

[0139] 3. Water contact angle test

[0140] Water contact angles were measured using a DropMeter A-100P contact angle / surface tension tester. A 2 cm x 2 cm square membrane was secured to a glass slide with tape and placed on a sample stage for testing. The initial contact angle of the water droplet upon impact was recorded. The experiment was repeated five times and the average value was calculated.

[0141] 4. Retention test

[0142] The prepared microfiltration membrane was tested for bacterial retention using Brevundimonas diminuta (ATCC19146) and Serratia marcescens (ATCC8100). 0.9% normal saline was prepared and the corresponding bacteria were dissolved in it so that the number of bacteria per milliliter of solution was not less than 10 7 cfu, select effective area 13.8cm 2 Finally, the number of microorganisms remaining per square centimeter of effective filtration area was calculated.

[0143] 5. Average pore size, pore ratio and asymmetry coefficient test

[0144] In this application, the average pore size, pore ratio and asymmetry coefficient can be obtained by characterizing the membrane structure using a scanning electron microscope, and then using computer software (such as Matlab, NIS-Elements, etc.) or manual mapping and calculation analysis. When measuring, the parts that are obviously smaller or larger are not taken into account. When calculating the average pore size, the membrane surface can be characterized by an electron microscope first, and the corresponding SEM image can be obtained. A certain area, such as 100 μm, can be selected. 2 (10 μm times 10 μm) or 25 μm 2 (5 μm multiplied by 5 μm), the specific area size depends on the actual situation, and then the corresponding computer software (Image J) is used for calculation to obtain the average pore size of the area (that is, the average pore size measured by SEM).

[0145] 6. Viscosity test

[0146] The casting solution was stored in a constant temperature water bath at 25°C for 24 hours, and the viscosity of the casting solution was measured using a rotational viscometer.

[0147] 7. Nitrogen content test

[0148] The nitrogen element on the membrane surface was analyzed by XPS (VGESCALAB MARKⅡ) with MgKα as the X-ray source (1253.6eV) and a power of 300W; the full spectrum scanning range was 0-1060ev, and the contents of C (1s), N (1s), O (1s) and S (2p) on the membrane surface were tested. In this application, the content of N element mainly accounts for the proportion of C, N, O and S elements.

[0149] 8. Mechanical strength test

[0150] The tensile strength of the membranes was measured at room temperature using a universal testing machine (Instron 5943). All samples were dried at 60°C for 2 hours to eliminate the effects of residual moisture on the membrane's mechanical properties. The membranes were cut to 10 mm × 0.1 mm × 70 mm. The universal testing machine was used at a tensile rate of 20 mm / min. Ten replicates were performed, and the maximum and minimum values ​​were removed to calculate the average.

[0151] 9. Flux decay test

[0152] Prepare a 0.3 g / L whey (Sigma) solution by adding 50 ml of 20× PBS buffer to 950 ml of pure water, ultrasonically vibrate for 10 minutes, and add 0.3 g of whey (Sigma-W1500) to the mixed buffer to a concentration of 0.3 g / L. Then, use an effective area of ​​13.4 cm 2The flux attenuation test was carried out using an ultrafiltration cup (Merck), and the whey solution was filtered at 14.5 psi. The flux and filtration volume of the anisotropic microfiltration membranes with different liquid inlet surfaces were compared when the flux attenuation was 80%.

[0153] The specific test results are shown in the table below:

[0154] Table 1

[0155] Table 2

[0156] As can be seen from the above data, the present application limits the asymmetric coefficient of the microfiltration membrane, the proportion of the separation layer, and the ratio of the thickness of the support layer and the macroporous layer, so that the membrane has a higher flux and better filtration performance. By selecting the specific preparation method of the present application, the mechanical properties of the membrane can be significantly improved. The membrane in Table 2 is filtered through a dead end and measured with the second surface (macroporous end) as the liquid inlet surface. Combined with the above data, it can be seen that by rationally controlling the surface pore size, the proportion of surface holes, the proportion of the separation layer, and the ratio of the thickness of the support layer to the thickness of the macroporous layer, a microfiltration membrane with high flux and high retention effect can be prepared. Among them, the difference in N elements between the first surface and the second surface represents the surface segregation phenomenon of the hydrophilic additive toward the coagulation bath side during the phase inversion process, so that the N element content of the second surface close to the air end is higher, and the hydrophilicity of the second surface is better than that of the first surface. This phenomenon is reflected in the water contact angle. The comparative example illustrates the importance of the thickness of the separation layer for membrane flux and retention performance. When the thickness of the separation layer is lower than 14% of the overall membrane thickness, a better flux can be obtained, but a part of the separation performance is sacrificed, and the cost of subsequent processing is increased; and when the thickness of the separation layer is higher than 30% of the overall membrane thickness, the flux of the membrane will drop sharply, reducing production efficiency. Comparative Example 3 shows that when the asymmetry coefficient exceeds the specified range of this application, it is easy to cause a decline in performance, especially the membrane flux will be lower than the microfiltration membrane of other embodiments, and it is not competitive compared with other embodiments. By comparing Example 2, Example 4 and Example 6 with other embodiments, it can be seen that the average pore size of the first and second surfaces is within the preferred range, which can further improve the flux. Compared with other embodiments, Examples 11 and 12 show that this application adopts a method of integrally molding a casting liquid, which can significantly improve the mechanical strength of the membrane.

[0157] Table 3

[0158] To adapt to a wider range of applications and broaden the scope of application of anisotropic microfiltration membranes, further investigations were conducted on the microfiltration membranes provided in some examples. Experiments were conducted using tangential flow filtration and flux attenuation using either the first or second surface of the anisotropic microfiltration membrane as the liquid inlet surface. The attenuation observed for the four-layer structures in Examples 1 and 2 showed significant differences. The total filtration yield at 80% attenuation using the membrane with the first surface's pores as the liquid inlet was significantly higher than that achieved using the second surface as the liquid inlet surface. In contrast, the differences in flux and filtration yield observed in Examples 7 and 8, which have three layers, were not significant, demonstrating that the four-layer anisotropic microfiltration membranes described herein are applicable to a wider range of applications.

[0159] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of this application.

Claims

1. An anisotropic microfiltration membrane, characterized in that, It includes a porous body, with one side of the porous body being the first surface and the other side being the second surface; The porous body includes a support layer with gradually decreasing pore diameters from the first surface to the second surface, a separation layer with consistent pore diameters, and a macroporous layer with gradually increasing pore diameters; The asymmetry coefficient of the support layer is 1 - 16, the asymmetry coefficient of the macroporous layer is 5 - 38, the thickness ratio of the separation layer to the porous body is 14 - 30%, and the thickness ratio of the support layer to the macroporous layer is (1.0 - 11):

1.

2. The anisotropic microfiltration membrane according to claim 1, wherein The average pore diameter of the first surface of the anisotropic microfiltration membrane is 0.2 - 1.1 μm, and the average pore diameter of the second surface is 0.35 - 1.8 μm.

3. The anisotropic microfiltration membrane according to claim 1 or 2, characterized in that, The hole occupancy ratio of the first surface of the anisotropic microfiltration membrane is 12% - 35%; the hole occupancy ratio of the second surface is 10% - 30%; And / or, the average pore diameter of the separation layer is 170 - 500 nm.

4. The anisotropic microfiltration membrane according to claim 1 or 2, characterized in that, A small - hole layer is further included between the support layer and the first surface, and the average pore diameter of the small - hole layer gradually increases from the first surface to the second surface direction; And / or, the asymmetry coefficient of the small - hole layer is 2 - 53.

5. The anisotropic microfiltration membrane according to claim 4, wherein The average pore diameter of the macroporous layer is larger than the average pore diameters of the small - hole layer and the support layer; And / or, the average pore diameter of the separation layer is smaller than the average pore diameters of the small - hole layer and the support layer; And / or, when there is a small - hole layer, the average pore diameter range of the first surface is 0.25 - 0.65 μm; And / or, the thickness ratio of the small - hole layer to the separation layer is (0.05 - 0.8):

1.

6. The anisotropic microfiltration membrane according to claim 4, wherein The average pore diameter of the small - hole layer is 250 - 600 nm, and the thickness range of the small - hole layer accounts for 2 - 18% of the thickness of the porous body; And / or, the average pore diameter of the support layer is 500 - 1000 nm, and the thickness range of the support layer accounts for 20 - 60% of the thickness of the porous body; And / or, the average pore diameter of the macroporous layer is 650 - 1200 nm, and the thickness range of the macroporous layer accounts for 5 - 35% of the thickness of each porous body.

7. The anisotropic microfiltration membrane according to claim 4, characterized in that, The average fiber diameter of the small - hole layer is 100 - 500 nm, and the ratio of the average pore diameter of the small - hole layer to its average fiber diameter is (0.5 - 6.0):1; And / or, the average fiber diameter of the support layer is 120 - 450 nm, and the ratio of the average pore diameter of the support layer to its average fiber diameter is (1.1 - 8.5):1; And / or, the average fiber diameter of the separation layer is 80 - 350 nm, and the ratio of the average pore diameter of the separation layer to its average fiber diameter is (0.5 - 6.3):1; And / or, the average fiber diameter of the macroporous layer is 200 - 550 nm, and the ratio of the average pore diameter of the macroporous layer to its average fiber diameter is (1.1 - 6.0):

1.

8. The anisotropic microfiltration membrane according to claim 4, wherein The total thickness range of the anisotropic microfiltration membrane is 100 - 180 μm, and the porosity of the anisotropic microfiltration membrane is 60 - 80%; and / or, the bubble point of the anisotropic microfiltration membrane is 18 to 60 psi, and the water flux of the anisotropic microfiltration membrane at 14.5 psi is 5 to 60 mL / cm 2 ·min; And / or, the material of the anisotropic microfiltration membrane is at least one of polyethersulfone, polysulfone, cellulose acetate, regenerated cellulose, polytetrafluoroethylene, and polyvinylidene fluoride; And / or, the nitrogen element content on the first surface of the anisotropic microfiltration membrane is 2-6%, and the water contact angle on the first surface is 25-55°; the nitrogen element content on the second surface is 3.5-9%, and the water contact angle on the second surface is 18-50°.

9. A method for preparing an anisotropic microfiltration membrane according to any one of claims 1-3, characterized in that, Comprising the following steps: S1, preparing a casting solution; S2, scraping the casting solution on the surface of the carrier, controlling the temperature of the carrier to be 25-80°C, and placing it for 10-20 s in an environment with a humidity of 60-90% RH or an air flow rate of 0.3-1 m / s to obtain a primary membrane; S3, immersing the primary membrane in a first coagulation bath to obtain an anisotropic microfiltration membrane.

10. The preparation method of the anisotropic microfiltration membrane according to claim 9, wherein It further includes the step of immersing in a second coagulation bath, controlling the immersion time in the first coagulation bath to be within 5 s, and the surface tension of the second coagulation bath is less than that of the first coagulation bath to obtain a microfiltration membrane including a small pore layer; Or, controlling the immersion time in the first coagulation bath to be greater than 5 s to obtain a microfiltration membrane without a small pore layer.

11. The method for preparing an anisotropic microfiltration membrane according to claim 9 or 10, characterized in that, The viscosity of the casting solution at 25°C is 3000-13000 mPa·s; And / or, the immersion time in the second coagulation bath is 5-120 s.

12. Use of the anisotropic microfiltration membrane according to any one of claims 1-8 or the anisotropic microfiltration membrane prepared by the preparation method according to any one of claims 9-11 for removing insoluble particles or microbial impurities in a liquid stream.

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