A cellulose ultrafiltration membrane and its preparation method

Abstract

The invention discloses a cellulose ultrafiltration membrane and a preparation method thereof, relating to the technical field of membrane materials. The membrane comprises a main body, wherein an ultrafiltration layer, a support layer and a base layer are sequentially arranged along the direction of fluid flow; the ultrafiltration layer and the support layer comprise a cellulose polymer layer, the base layer comprises a polytetrafluoroethylene layer, and the PMI average pore size of the base layer is greater than 1 μm; the polytetrafluoroethylene layer is a hydrophilic polytetrafluoroethylene layer; the cellulose polymer layer and the polytetrafluoroethylene layer are osmotically bonded to form a bonding layer; the SEM average pore size of the first side surface is 1-90 nm; in the present invention, the surface of the polytetrafluoroethylene layer is relatively flat and has strong solvent resistance, and the defects of the cellulose polymer layer in the prepared ultrafiltration membrane are relatively small, so that the integrity of the ultrafiltration membrane is relatively good, and the cellulose polymer can penetrate into the polytetrafluoroethylene layer to form a bonding layer, thereby eliminating the solute accumulation phenomenon, so that the prepared ultrafiltration membrane has good flux and composite performance.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane materials, and more particularly to a cellulose ultrafiltration membrane and a preparation method thereof. Background Art

[0002] Membrane technology is a new, highly efficient separation technology. Compared to traditional distillation and rectification techniques, it offers advantages such as high separation efficiency, low energy consumption, and a small footprint. The core of membrane separation technology is the separation membrane. Polymer membranes are a type of separation membrane made from organic polymers using a specific process. Depending on the type of polymer, polymer membranes can be further categorized into cellulose, polyamide, sulfone, and polytetrafluoroethylene polymers. Furthermore, based on pore size, membranes can be further divided into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes.

[0003] Ultrafiltration membranes are semipermeable polymer membranes used in ultrafiltration processes to separate large-sized polymer colloids or suspended particles from solutions. They are widely used in the advanced treatment of industrial wastewater and process water, such as the concentration, purification, and separation of macromolecular substances in the chemical, food, and pharmaceutical industries; sterilization of biological solutions; separation of dyes from printing and dyeing wastewater; glycerin recovery from petrochemical wastewater; silver recovery from photographic chemical wastewater; and the preparation of ultrapure water. They can also be used for sludge concentration and dehydration.

[0004] Among polymer ultrafiltration membranes made of different materials, cellulose polymer ultrafiltration membranes have high hydrophilicity and therefore have low nonspecific adsorption during protein concentration, purification and separation, thereby preventing the adsorption of protein molecules. On the one hand, this prevents the reduction of protein yield, and on the other hand, it prevents the ultrafiltration membrane from clogging too quickly.

[0005] However, due to the low strength of cellulose polymer ultrafiltration membranes, composite cellulose ultrafiltration membranes with a substrate layer have appeared on the market, which give the membrane a higher mechanical strength as a whole and increase its practical practicality. Generally, the composite ultrafiltration membranes on the market use non-woven fabrics as the substrate layer, that is, cellulose polymers are coated on the surface of non-woven fabrics to form a composite ultrafiltration membrane. The preparation process is relatively simple, but the surface of non-woven fabrics is relatively rough. When cellulose is coated on the non-woven fabric substrate layer, the fiber protrusions and the raised ends of the fibers on the surface can easily puncture the cellulose ultrafiltration layer in the ultrafiltration membrane, causing damage to the ultrafiltration membrane surface and seriously affecting the integrity of the membrane.

[0006] The microporous membrane has a relatively flat surface and has good integrity as a base layer for preparing a composite ultrafiltration membrane. For example, Xiamen University's patent CN103877867B discloses a cellulose ultrafiltration membrane composed of a microfiltration membrane and a cellulose cortex layer using a 0.2 μm polytetrafluoroethylene microfiltration membrane as a support layer. However, the preparation process involves filtering a cellulose membrane-making liquid onto the microfiltration membrane and then freely stacking to form a nanoporous cellulose cortex layer, which does not penetrate into the polytetrafluoroethylene microfiltration membrane. Furthermore, the pore size of the polytetrafluoroethylene microfiltration membrane used is too small (0.2 μm), making it easy for solutes to rapidly aggregate and accumulate at the microfiltration membrane interface. Although the membrane has a relatively good retention rate, it results in a low overall membrane flux, resulting in a long filtration time in actual use, reducing efficiency. Summary of the Invention

[0007] The purpose of the present invention is to provide a cellulose ultrafiltration membrane and a preparation method thereof. The ultrafiltration membrane uses a microporous membrane containing a polytetrafluoroethylene layer as a base layer and a cellulose polymer as an ultrafiltration layer. It has a retention molecular weight of 1-750K, and at the same time has fewer defects, high integrity, uniform pores on the surface of the ultrafiltration layer, fast flow rate, high flux, and short filtration time.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A cellulose ultrafiltration membrane comprises a main body, wherein the main body has:

[0010] a first side surface for supplying liquid to be filtered, and

[0011] a second side surface for discharging permeate through the body;

[0012] The main body comprises an ultrafiltration layer, a support layer and a base layer in sequence along the direction of fluid flow;

[0013] The ultrafiltration layer and the support layer include a cellulose polymer layer, and the base layer includes a polytetrafluoroethylene layer.

[0014] The average pore size of the PMI of the base layer is greater than 0.8 μm;

[0015] The polytetrafluoroethylene layer is a hydrophilic polytetrafluoroethylene layer;

[0016] The cellulose polymer layer and the polytetrafluoroethylene layer are infiltrated and bonded to form a bonding layer;

[0017] The SEM average pore size of the first side surface is 1-90 nm.

[0018] In the present invention, the cellulose ultrafiltration membrane is a composite membrane, which is composed of a cellulose polymer layer and a polytetrafluoroethylene layer. Compared with the ultrafiltration membrane with an integrally formed structure, the composite ultrafiltration membrane can optimize the functions of each layer, thereby making the performance of the entire membrane more ideal.

[0019] The main body of the cellulose ultrafiltration membrane of the present invention is composed of an ultrafiltration layer, a support layer and a base layer in the direction of fluid flow, wherein the ultrafiltration layer mainly plays a role in retaining substances. One side of the ultrafiltration layer is a first side surface for supplying the liquid to be filtered. There are holes on this surface with relatively small pore size, which play a good retaining role. It is well known that the size of the pore size is a key factor in the ability to retain substances. Different pore sizes can retain substances of different particle sizes. After research, it was found that when the SEM average pore size of the first side surface is 1-90nm, such a pore size is suitable for intercepting various biomolecules (such as antibodies) with a molecular weight of 1K-750K, which is conducive to obtaining a higher retention efficiency. At the same time, various biological protein products are concentrated and purified in the form of tangential flow, and the retained particles on the membrane surface can be flushed and removed to prevent clogging of the membrane surface. The presence of the support layer plays a protective role for the ultrafiltration layer, preventing the fiber structure such as burrs on the binding layer from affecting or even destroying the membrane pore structure of the ultrafiltration layer, thereby affecting the overall retention efficiency of the membrane, further ensuring that the ultrafiltration layer can efficiently retain the corresponding substances.

[0020] In the present invention, both the ultrafiltration layer and the transition layer are prepared by phase inversion of cellulose polymers, and are cellulose polymer layers. Therefore, the ultrafiltration layer has strong hydrophilicity and is not easy to adsorb biological molecules such as antibodies, thereby ensuring a high yield of the product; and there is only one film-forming polymer (i.e., cellulose polymer) in the ultrafiltration layer and the support layer, and no other film-forming polymers exist. Of course, the presence of very small amounts of solvents, pore-forming agents, and other substances is not excluded.

[0021] In the present invention, a microporous membrane including a polytetrafluoroethylene layer is used as the base layer. First, polytetrafluoroethylene has good anti-pollution and chemical resistance. During preparation, since the present invention adopts a preparation method of casting liquid after dissolution and phase separation, some organic reagents (such as acetone, dioxane, etc.) that are also highly soluble in the substrate layer are used in the preparation of the casting liquid. Therefore, when the base layer is not resistant to chemical reagents, it is easy to cause the solvent in the casting liquid to have a certain dissolving effect on the base layer, resulting in the originally relatively flat surface of the base layer having more potholes and the surface becoming uneven, resulting in the final prepared cellulose ultrafiltration membrane having more surface defects and relatively poor integrity. The base layer includes a polytetrafluoroethylene layer, and when the surface of the polytetrafluoroethylene layer is in contact with the cellulose polymer layer, this is because the surface of the polytetrafluoroethylene layer is relatively flat, and the good solvent resistance can prevent the surface from being partially dissolved to form potholes. Therefore, the defects of the cellulose polymer layer in the prepared ultrafiltration membrane are relatively small, and the integrity of the ultrafiltration membrane is relatively good; moreover, the base layer can play a supporting role for the cellulose polymer layer, ensuring that the membrane as a whole has good mechanical strength and high compressive strength, and is suitable for long-term stable filtration under high pressure; at the same time, the base layer uses a microporous membrane to ensure that the membrane as a whole has a high flux and a fast flow rate, and the filtration speed is fast.

[0022] However, it was also found that when the cellulose polymer casting liquid is applied to the surface of the polytetrafluoroethylene layer for phase transformation, solutes will rapidly accumulate and accumulate at the interface of the polytetrafluoroethylene layer, which can easily cause a significant decrease in flow rate. In the present invention, the average pore size of the base layer PMI is limited to greater than 0.8 μm, and the polytetrafluoroethylene layer is a hydrophilic polytetrafluoroethylene layer. On this basis, during the preparation process, the cellulose polymer can penetrate into the polytetrafluoroethylene layer to form a bonding layer, thereby eliminating the solute accumulation phenomenon and making the prepared ultrafiltration membrane have good flux. At the same time, the bonding layer can also impart better composite properties between the cellulose polymer layer and the polytetrafluoroethylene layer, that is, improve the peel strength between the two, and prevent peeling during use or between the cellulose polymer layer and the polytetrafluoroethylene layer.

[0023] If the average pore size of the base layer PMI in the present invention is too small, or the polytetrafluoroethylene layer is too hydrophobic, it is easy for the cellulose polymer to be unable to penetrate into the polytetrafluoroethylene layer to form a binding layer, thereby being unable to eliminate the solute accumulation phenomenon, resulting in a decrease in flux; at the same time, the inventors surprisingly found that when a large-pore and hydrophilic polytetrafluoroethylene layer is used as the base layer, the pores on the surface of the ultrafiltration layer can be made more uniform, making the filtration performance of the ultrafiltration membrane more stable and uniform; this may be because in the actual production process, due to large-scale production, the solid content or viscosity of the cellulose polymer casting solution cannot be kept uniform and stable everywhere, resulting in a relatively non-uniform pore size on the surface of the ultrafiltration layer finally prepared; when a small-pore hydrophobic polytetrafluoroethylene layer is used as the base, during the phase separation process, the small-pore hydrophobic polytetrafluoroethylene layer coagulation bath penetrates slowly, and during phase separation, the phase separation first starts from the cellulose polymer. The phase separation starts from the upper surface of the composite casting liquid (when immersed in the coagulation bath, this is the first contact place) and gradually moves inward. After the ultrafiltration layer is formed, due to the small pore size of the ultrafiltration layer surface, the coagulation bath penetrates slowly. When it contacts the cellulose polymer casting liquid that has penetrated into the polytetrafluoroethylene layer, the cellulose polymer casting liquid in the polytetrafluoroethylene layer is basically phase-separated. The macroporous hydrophilic polytetrafluoroethylene layer used in the present invention can make it easier for the coagulation bath to enter from the polytetrafluoroethylene layer side, thereby contacting and separating the cellulose polymer casting liquid that has penetrated into the polytetrafluoroethylene layer faster. Therefore, at this time, the cellulose polymer in the casting liquid outside the polytetrafluoroethylene layer interface will be snatched away. Moreover, due to the instability of the solid content or viscosity of the cellulose polymer casting liquid, the snatching makes the solid content and viscosity of the casting liquid outside the polytetrafluoroethylene layer interface more uniform and stable, thereby making the surface pores more uniform. At the same time, robbing the solute in the casting solution outside the interface of the polytetrafluoroethylene layer can also alleviate the solute accumulation phenomenon. Moreover, due to the reduction of the solute outside the interface of the polytetrafluoroethylene layer, the support layer becomes relatively thinner, thereby improving the flux.

[0024] The SEM average pore size measurement method of the first side surface can be achieved by using a scanning electron microscope to characterize the morphology of the membrane structure, and then using computer software (such as Matlab, NIS-Elements, etc.) or manually to measure and perform corresponding calculations; during the preparation of the membrane, in the direction perpendicular to the thickness of the membrane (if the membrane is in the form of a flat membrane, this direction is the planar direction; if the membrane is in the form of a hollow fiber membrane, this direction is perpendicular to the radial direction), its various characteristics such as pore size distribution are roughly uniform and basically consistent; therefore, the average pore size of a part of the area on the corresponding plane can be used to reflect the overall average pore size on the plane. When actually measuring, the membrane surface can be characterized by an electron microscope to obtain the corresponding SEM image. Since the holes on the membrane surface are roughly uniform, a certain area can be selected, such as 1μm 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 using corresponding computer software or manually, and then calculated to obtain the average pore size of the surface; the pore area ratio of the inner surface is the ratio of the sum of the areas of all holes on the surface to the area of ​​the surface; of course, those skilled in the art can also obtain the above parameters by other measurement methods, and the above measurement methods are for reference only.

[0025] In the present invention, the PMI pore size is obtained by testing with a PMI pore size tester. The PMI pore size of the base layer can be obtained by directly measuring the base layer, or by dissolving the cellulose ultrafiltration membrane with a solvent (for example, using NMMO, ionic liquid, alkali / urea system) or enzymatically hydrolyzing cellulose to obtain the base layer, and then testing it with a PMI pore size tester.

[0026] Furthermore, the coefficient of variation of the average pore size of the first side surface measured by SEM is less than 0.5.

[0027] In the present invention, the average pore size of the first side surface measured by SEM is relatively uniform, and the coefficient of variation is less than 0.5. This allows the ultrafiltration layer in different areas to have a relatively uniform molecular weight cutoff during use, thereby preventing deviations between the same membrane or batches of membranes, which would result in uneven filtration performance.

[0028] Furthermore, the base layer includes a substrate layer arranged on the polytetrafluoroethylene layer away from the cellulose polymer layer, the surface of the substrate layer away from the polytetrafluoroethylene layer forms a second side surface, the substrate layer includes a non-woven fabric, the thickness of the non-woven fabric accounts for 30-85% of the thickness of the entire film, and the thickness of the non-woven fabric is 60-300 μm.

[0029] In the present invention, although the mutual penetration between the base layer and the cellulose polymer layer can increase the overall flux and peel strength and increase the service life, due to the relatively low surface strength of the cellulose ultrafiltration membrane, during the subsequent membrane hydrolysis and cleaning during preparation, or during the preparation of the filtration product (such as an ultrafiltration membrane package or filter element), the surface of the cellulose ultrafiltration membrane will inevitably come into contact with some grids with a certain strength. During preparation and filtration, when conditions permit, high pressure is often used for filtration (the greater the pressure during membrane filtration, the faster the filtration speed and the higher the economic benefit per unit time), which can easily cause the surface of the ultrafiltration membrane to be squeezed by the grid, resulting in rupture of the ultrafiltration membrane surface and affecting its integrity. The present invention uses a non-woven fabric as the base layer, which can make the ultrafiltration membrane have higher strength. In addition, the non-woven fabric base layer has a certain compressibility. During high-pressure filtration, the non-woven fabric base layer is compressed, thereby playing a good buffering role, preventing the ultrafiltration membrane surface from being subjected to greater force, causing its integrity to be damaged, thereby resulting in poor filtration effect. Among them, the thickness of the non-woven fabric accounts for 30-85% of the thickness of the entire membrane, and the thickness of the non-woven fabric is 60-300μm. This is because the thickness of the non-woven fabric should not be too small as a percentage of the thickness of the entire membrane. If it is too small, a good buffering effect cannot be achieved. At the same time, the thickness of the non-woven fabric should not be too large as a percentage of the thickness of the entire membrane. If it is too large, it will easily cause the ultrafiltration membrane to be too easily compressed and deformed, and it will be difficult to recover to its original thickness after deformation, resulting in changes in the overall working conditions of the membrane during subsequent use, resulting in different filtration efficiencies between different batches.

[0030] Furthermore, the air permeability of the nonwoven fabric is greater than 50cc / cm 2 / sec, fiber thickness is 5-30μm, and weight is 15-40g / m 2 .

[0031] In the present invention, the non-woven fabric as the substrate layer will also affect the flux of the membrane to a certain extent. Therefore, the air permeability of the non-woven fabric is greater than 50cc / cm 2 / sec, the substrate layer can have a faster flow rate to prevent it from affecting the flux of the entire membrane. At the same time, the fiber thickness of the non-woven fabric is 5-30μm and the gram weight is 15-40g / m 2 , which makes the nonwoven fabric moderately compressible and ensures the integrity of the cellulose ultrafiltration membrane.

[0032] The thickness of the non-woven fabric, the fiber thickness and the thickness of the entire membrane can be calculated by characterizing the morphology of the membrane structure using a scanning electron microscope, and then using computer software (such as Matlab, NIS-Elements, etc.) or manual measurement. Of course, those skilled in the art can also obtain the above parameters by other measurement methods (such as the overall thickness of the membrane can be obtained by freeze-drying the filter membrane and then measuring it with a measuring tool). The above measurement methods are for reference only.

[0033] Furthermore, the thickness ratio of the cellulose polymer layer to the polytetrafluoroethylene layer is 0.1-3, the thickness of the cellulose polymer layer is 1.5-60 μm, and the thickness of the polytetrafluoroethylene layer is 15-90 μm.

[0034] In the present invention, due to the presence of the non-woven fabric substrate layer, the cellulose ultrafiltration membrane can be given a certain mechanical strength. At the same time, the cellulose polymer layer and the polytetrafluoroethylene layer have relatively thin thicknesses, wherein the thickness ratio of the cellulose polymer layer to the polytetrafluoroethylene layer is 0.1-3, the thickness of the cellulose polymer layer is 1-55 μm, and the thickness of the polytetrafluoroethylene layer is 15-90 μm. The thinner thickness of the cellulose polymer layer and the polytetrafluoroethylene layer can enable the cellulose ultrafiltration membrane to have a higher flux.

[0035] Furthermore, the thickness of the bonding layer accounts for 10-100% of the thickness of the polytetrafluoroethylene layer, and the thickness of the bonding layer is 10-100 μm.

[0036] In the present invention, the thickness of the binding layer is 10-100 μm, and the thickness of the binding layer accounts for more than 10% of the thickness of the polytetrafluoroethylene layer. On the one hand, it can ensure that the cellulose ultrafiltration membrane has good mechanical strength and peel strength. On the other hand, although the flux is affected by the penetration of the cellulose polymer into the polytetrafluoroethylene layer in the binding layer, due to the low thickness of the polytetrafluoroethylene layer, even if the cellulose polymer penetrates in the thickness direction of the polytetrafluoroethylene layer, the cellulose ultrafiltration membrane can still have a relatively high flux.

[0037] Furthermore, the base layer is a polytetrafluoroethylene layer, the thickness ratio of the cellulose polymer layer to the polytetrafluoroethylene layer is 0.02-1, the thickness of the cellulose polymer layer is 1.5-60 μm, and the thickness of the polytetrafluoroethylene is 100-300 μm.

[0038] Furthermore, the average pore size of the PMI of the polytetrafluoroethylene layer is 1-20 μm, and the porosity is 60-90%; the surface roughness of the polytetrafluoroethylene layer is 0.7-2 μm.

[0039] In the present invention, the average pore size of the polytetrafluoroethylene layer PMI is 1-20 μm, and the porosity is 60-90%, which can ensure that the cellulose polymer can penetrate into the polytetrafluoroethylene layer well, thereby improving the peel strength of the cellulose ultrafiltration membrane. If the pore size is too small or the porosity is too low, the cellulose polymer will penetrate too little and it will easily delaminate during use, resulting in a decrease in filtration performance. If the average pore size and porosity are too large, the casting liquid will easily penetrate completely into the polytetrafluoroethylene layer during preparation, resulting in the inability to separate the phases to form the ultrafiltration layer. At the same time, since the surface roughness of the polytetrafluoroethylene layer is 0.7-2 μm, at this roughness, the surface of the polytetrafluoroethylene layer is relatively flat, and the prepared ultrafiltration membrane layer has good integrity. At the same time, having a certain roughness can better adhere the cellulose polymer layer to the surface of the polytetrafluoroethylene layer, thereby increasing the peel strength. If the roughness is too high, the prepared ultrafiltration layer will easily have too many defects, resulting in impaired integrity.

[0040] Furthermore, the water contact angle of the surface of the polytetrafluoroethylene layer is less than 80°, and the water contact angle of the second side surface is within 50° greater than the water contact angle of the first side surface.

[0041] Furthermore, the dry film was immersed in water and wetted within 5 s.

[0042] In the present invention, in order to enable the cellulose polymer to penetrate well into the polytetrafluoroethylene layer, it is necessary to ensure that the surface of the polytetrafluoroethylene layer has good hydrophilicity. When the water contact angle of the polytetrafluoroethylene layer surface is less than 80°, or the dry membrane is immersed in water and wetted within 5 seconds, the membrane-making liquid can penetrate quickly during preparation, so that the prepared cellulose ultrafiltration membrane has good peel strength; at the same time, in the prepared cellulose ultrafiltration membrane, the water contact angle of the second side surface is greater than the water contact angle of the first side surface by less than 50°. This is because during preparation, the coagulation bath needs to penetrate from the second side surface to ensure that the cellulose ultrafiltration membrane in the binding layer can also phase separate in time. If the second side surface is too hydrophobic, it is easy to cause the phase separation in the binding layer to be too late, so that the thickness of the cellulose polymer layer finally prepared is too large, and the lack of cellulose polymer in the binding layer not only affects the flux, but also affects the composite performance of the cellulose ultrafiltration membrane.

[0043] In the present invention, water is used as the test liquid and a contact angle tester is used for testing. When 10-100 microliters of water droplets are evenly dropped onto the surface of the material, a regular contact angle is formed in an instant (within 0.4 seconds).

[0044] Furthermore, the roughness of the first side surface is 0.1-2.5 μm, the pore area ratio of the first side surface is 1-10%, and the water contact angle of the first side surface is 10-55°.

[0045] The cellulose ultrafiltration membrane of the present invention is generally used for the concentration, purification and separation of biomacromolecules, i.e., protein substances. In this process, in order to ensure the protein yield and reduce the nonspecific adsorption of the first side surface, the first side surface needs to have a certain hydrophilicity. In the present invention, the water contact angle of the first side surface of the cellulose ultrafiltration membrane is 10-55°. On this basis, the cellulose ultrafiltration membrane can ensure that it has low protein adsorption in protein product production applications and ensure a high protein yield. At the same time, the pore area ratio of the first side surface is 1-10%, which can ensure that the pores on the first side surface are relatively few, and the dense pore structure can cooperate with the smaller pore size to ensure the retention efficiency of the ultrafiltration layer of the cellulose ultrafiltration membrane; and the roughness of the first side surface measured by a roughness tester is 0.1-2.5 μm, which makes the first side surface have a certain roughness, not too smooth, preventing the occurrence of concentration polarization, avoiding clogging of the pores after a short filtration time, causing a significant decrease in flux, but the first side surface cannot be too rough. Excessive roughness will increase the shear force on the protein surface of the cellulose ultrafiltration membrane during filtration, resulting in a decrease in effective protein yield.

[0046] Furthermore, the ultrafiltration layer has ultrafiltration fibers forming a porous structure, and the SEM average diameter of the ultrafiltration fibers is 20-60 nm;

[0047] The support layer has support fibers forming a porous structure, and the SEM average diameter of the support fibers is 20-85 nm;

[0048] The ratio of the SEM average diameters of the support fibers to the ultrafiltration fibers is 1.2-2.4.

[0049] The presence of ultrafiltration fibers in the ultrafiltration layer ensures the stability of the pores inside the ultrafiltration layer and prevents the collapse or shrinkage of the pores. At the same time, the support fibers in the support layer can provide good support for the ultrafiltration layer. If the ultrafiltration fibers and support fibers are too thick, the filtration flow rate of the ultrafiltration layer will be reduced, resulting in a decrease in the overall flux. If the ultrafiltration fibers and support fibers are too thin, they will not be able to provide good support and stabilization of the pores. When the ratio of the SEM average diameters of the support fibers and the ultrafiltration fibers is within the range of 1.2-2.4, the cellulose ultrafiltration membrane has higher mechanical strength and filtration stability.

[0050] Furthermore, the thickness of the ultrafiltration layer is 0.1-5 μm, the thickness of the support layer is 0.5-50 μm, and the ratio of the thickness of the support layer to the thickness of the ultrafiltration layer is 2-13.

[0051] In order to ensure that the ultrafiltration layer plays a good retention role in filtration, the ultrafiltration layer needs to have a certain thickness. However, if the thickness of the ultrafiltration layer is too thick, not only will the retention efficiency be unable to be further improved, but the overall flux of the membrane will also decrease. The support layer plays a supporting and protective role on the ultrafiltration layer. It can not only increase the pore stability of the ultrafiltration layer, but also improve the integrity of the ultrafiltration layer. Therefore, the support layer needs to have a certain thickness. However, if the support layer is too thick, it will easily lead to a decrease in the overall flow rate of the membrane. When the ratio of the support layer thickness to the ultrafiltration layer thickness is controlled within the range of 2-13, the cellulose ultrafiltration membrane has higher mechanical strength and better integrity.

[0052] Furthermore, the SEM average pore size of the support layer gradually increases along the fluid flow direction, with a change gradient of 20-450 nm / 1 μm.

[0053] The SEM average pore size of the support layer gradually increases along the direction of fluid flow, that is, the pore size close to the ultrafiltration layer is small, which can increase the support for the ultrafiltration layer, while the pore size close to the polytetrafluoroethylene layer is larger, which can give the membrane an overall high flux. However, if the gradient of change is too large, it is easy to reduce the overall strength of the cellulose ultrafiltration membrane and shorten the filtration service life of the cellulose ultrafiltration membrane.

[0054] The thickness, pore size, and fiber diameter of each layer of the cellulose ultrafiltration membrane can be calculated by characterizing the morphology of the membrane structure using a scanning electron microscope, and then measuring it using computer software (such as Matlab, NIS-Elements, etc.) or manually. Of course, those skilled in the art can also obtain the above parameters by other measurement methods (such as the thickness of each layer can be obtained by freeze-drying the filter membrane and then measuring it with a measuring tool). The above measurement methods are for reference only.

[0055] Furthermore, the thickness of the ultrafiltration membrane is 130-420 μm;

[0056] The cut-off molecular weight of the ultrafiltration membrane is 1K-750K;

[0057] The tensile strength of the ultrafiltration membrane is not less than 10 MPa;

[0058] Under the conditions of pressure of 0.68 bar and temperature of 25℃, the water flux of 100K cellulose ultrafiltration membrane is 1-1.8mL / min / cm 2 .

[0059] When the thickness of the membrane is too small, the mechanical strength of the membrane will be low; when the thickness of the membrane is too large, the filtration time will be too long and the time cost will be too high; since the filter membrane of the present invention is a composite membrane, the pore size in most areas is relatively large. In order to ensure the mechanical properties, the overall thickness of the membrane is relatively thick. After research, its thickness is suitable for 130-420 μm, which ensures that the filter membrane not only has high mechanical strength, but also has a short filtration time and low time cost; the ultrafiltration membrane of the present invention has a retention efficiency of more than 90% for substances with a molecular weight of 1kD-750kD, and the retention efficiency is high, which shows that the membrane is particularly suitable for biological purification and meets the needs of practical applications; the protein yield of this membrane is not less than 90%, which shows that the effective substance protein in the fluid is not easily adsorbed on the membrane. On the one hand, it will not block the membrane pores, ensuring that the filter membrane still has a high service life, and on the other hand, it ensures that the content of various proteins of the effective substance in the fluid changes very little, and the protein is basically not lost, and the economic benefits are guaranteed. An important indicator for evaluating the mechanical strength of a filter membrane is the tensile strength of the filter membrane. Under certain conditions, the greater the tensile strength of the filter membrane, the better the mechanical strength of the filter membrane. The wet tensile strength of the filter membrane of the present invention is not less than 10 MPa (measured under wet membrane conditions), has a large tensile strength, good mechanical properties, and high industrial practical value, which can fully meet market demand. At the same time, by conducting a flow rate test on the filter membrane, it is shown that the flow rate of the filter membrane is large, the filtration time is short, and the time cost is low.

[0060] Furthermore, the surface of the polytetrafluoroethylene layer forming the bonding layer is a polytetrafluoroethylene layer bonding surface, and the bonding surface includes nodes and fiber filaments, and the nodes are connected to each other through fiber filaments.

[0061] The polytetrafluoroethylene layer can have different structures according to different preparation processes. However, the inventors have found that not all polytetrafluoroethylene layers have good effects as base layers, but polytetrafluoroethylene layers with nodes and interconnected by fiber filaments between the nodes have better effects as base layers. First, compared with polytetrafluoroethylene membranes with other structural forms (such as those prepared by sintering), the surface of the polytetrafluoroethylene membrane with node and fiber filament structure is smoother. This is because the fiber filaments between the nodes are obtained by stretching, so they do not have raised ends. As a base layer, it can achieve good integrity of the composite ultrafiltration membrane; secondly, the surface open porosity of the polytetrafluoroethylene membrane with node and fiber filament structure is high (the fiber filaments are thinner and directly open), which makes it easier for the cellulose polymer layer to penetrate and form a permeable layer compared to other structural forms, while increasing the composite capacity and improving the peeling strength; finally, the nodes have higher strength, while the fiber filaments are relatively easy to deform. The combination of the two can increase the strength and elasticity of the membrane as a whole.

[0062] Furthermore, the area of ​​the node occupied by the bonding surface of the polytetrafluoroethylene layer is S1;

[0063] The area of ​​the fiber filaments occupying the bonding surface of the polytetrafluoroethylene layer is S2;

[0064] The S1:S2 is 0.13-7; the S1 is 4-40%; and the S2 is 5-35%.

[0065] In the present invention, if the area S1 of the nodes in the polytetrafluoroethylene layer bonding surface accounts for too small a proportion, and the area S2 of the fiber filaments in the polytetrafluoroethylene layer bonding surface accounts for too large a proportion, the strength of the substrate layer is likely to be low. If the area S1 of the nodes in the polytetrafluoroethylene layer bonding surface accounts for too large a proportion, and the area S2 of the fiber filaments in the polytetrafluoroethylene layer bonding surface accounts for too small a proportion, the cellulose polymer layer is likely to have difficulty in penetrating, resulting in a solute accumulation effect and reduced flux. At the same time, after penetration, the peeling strength decreases due to too little bonding with the fiber filaments. In addition, a too small proportion of the fiber filaments can also lead to a decrease in the elasticity of the entire membrane, resulting in the ultrafiltration membrane being too brittle.

[0066] The node ratio, width and density, fiber ratio, width and density on the polytetrafluoroethylene bonding surface can be characterized by using a scanning electron microscope to characterize the membrane structure, and then measured using computer software (such as Matlab, NIS-Elements, etc.) or manually, and corresponding calculations can be performed; in the preparation process of the membrane, in the direction perpendicular to the membrane thickness (if the membrane is a flat membrane, the direction is the plane direction; if the membrane is a hollow fiber membrane, the direction is perpendicular to the radius direction), its various characteristics such as node width and density, fiber width and density distribution are roughly uniform and basically consistent; therefore, the node width and density, fiber width and density of a part of the corresponding plane can be used to reflect the overall node width and density on the plane. , fiber width and density; when actually measuring, the outer surface of the membrane can be characterized by an electron microscope first to obtain the corresponding SEM image. Since the node proportion, width and density, fiber width and density on the outer surface of the membrane are roughly uniform, a certain area can be selected, such as 1000μm2 (40μm multiplied by 25μm) or 10000μm2 (100μm multiplied by 100μm). The specific area size depends on the actual situation. Then use the corresponding computer software or manually measure the node width and density, fiber width and density on the area to obtain the node proportion, width and density, fiber filament proportion, width and density of the surface; of course, those skilled in the art can also obtain the above parameters by other measurement means. The above measurement means are for reference only.

[0067] Furthermore, the average width of the node is 1-6 μm, and the difference between the maximum width and the minimum width of the node is less than 7 μm; the average width of the fiber filament is 0.1-1.2 μm, and the difference between the maximum width and the minimum width of the fiber filament is less than 1.5 μm.

[0068] The average width of the node affects the mechanical strength of the polytetrafluoroethylene layer. If it is too small, the strength of the polytetrafluoroethylene as the base layer will be low, resulting in a decrease in the strength of the entire membrane. If the width of the node is too large, it will easily lead to the inability of the casting liquid to penetrate well at the node during preparation, which will not only affect the peel strength of the ultrafiltration membrane, but also lead to an increase in defects in the ultrafiltration layer and reduce its integrity.

[0069] Furthermore, the number of connected cellulose fibers within a length of 50 μm along the node direction is 15-70.

[0070] Fiber density has a great influence on membrane permeability and bonding. If the density is too low, not only will the overall strength be reduced, but there will not be enough fibers in the bonding layer to bond with the support layer, resulting in a decrease in peel strength. If the density is too high, the overall surface porosity will decrease, and the casting liquid will not be able to penetrate well during preparation, resulting in a decrease in the peel strength of the ultrafiltration membrane.

[0071] Furthermore, the material of the cellulose polymer layer includes one or more of regenerated cellulose and cellulose ester.

[0072] The present invention also provides a method for preparing a cellulose ultrafiltration membrane:

[0073] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0074] 10-30 parts of cellulose polymer; 40-60 parts of polar solvent; 20-40 parts of pore-forming agent;

[0075] S2: Casting the casting solution onto a hydrophilic substrate to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0076] The water contact angle on the surface of the polytetrafluoroethylene porous membrane is less than 80°, and the pore size of the polytetrafluoroethylene porous membrane is greater than 0.8 μm;

[0077] S3: Phase separation solidification, immersing the liquid film in a coagulation bath for phase separation solidification to obtain a film;

[0078] S4: placing the formed membrane in a sodium hydroxide aqueous solution for hydrolysis, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0079] Furthermore, the cellulose polymer is at least one of nitrocellulose, cellulose acetate and regenerated cellulose;

[0080] The cellulose acetate is selected from one or more of cellulose diacetate, cellulose triacetate, cellulose nitrate, cellulose acetate butyrate and cellulose acetate propionate.

[0081] Furthermore, the polar solvent includes at least one of acetone, dioxane, dimethylacetamide, N-methylpyrrolidone, acetic acid, propionic acid, butyric acid and valeric acid;

[0082] The pore former includes at least one of polyvinyl pyrrolidone, polyethylene glycol and polyvinyl alcohol.

[0083] Furthermore, the casting liquid has a viscosity of 6000-40000 cpa.s.

[0084] Furthermore, the phase separation coagulation duration is 5-60s, the coagulation bath is water, and the phase separation temperature is 20-40°C.

[0085] Furthermore, the concentration of the sodium hydroxide aqueous solution is between 0.01 mol / L and 1 mol / L; the hydrolysis time is between 30° C. and 80° C., and the time is 40 min to 200 min.

[0086] Furthermore, the preparation of the cellulose ultrafiltration membrane includes cross-linking;

[0087] The crosslinking is carried out by crosslinking the cellulose ultrafiltration membrane with a water-soluble crosslinking agent in an alkaline environment, the crosslinking time is 20-400 minutes, and the temperature is 30°C-60°C; the crosslinking agent is at least one of halogenated epoxides, diepoxides, dihalogenated alkanes and dihalogenated alcohols.

[0088] During the preparation process of the present invention, the casting liquid is first prepared. Among them, the cellulose polymer has relatively good hydrophilicity and low non-specific adsorption, and is therefore very suitable for the purification, concentration and separation of proteins. It can be used as the solute part of the casting liquid. The polar solvent is used to form a solvent system to dissolve the cellulose polymer and form a uniform and stable casting liquid. The pore-forming agent can not only effectively control the viscosity of the system and inhibit the formation of large pores in the membrane during the phase separation process, but also effectively improve the stability of the membrane flow rate. In addition, it can also greatly improve the hydrophilicity of the formed membrane, so that the membrane has higher hydrophilicity and reduces protein adsorption.

[0089] Subsequently, the casting liquid is cast onto the substrate to form a liquid film. In the present invention, in order to make the cellulose ultrafiltration membrane have a higher flux, the solute part in the casting liquid needs to penetrate into the microporous layer on the substrate during casting. The inventors found that the compatibility of base membranes made of different materials with cellulose polymers is quite different. When a hydrophilic polytetrafluoroethylene porous membrane is used as the substrate layer, when the water contact angle on the surface of the polytetrafluoroethylene porous membrane is less than 80° and the pore size of the polytetrafluoroethylene porous membrane is greater than 0.8 μm, the casting liquid can penetrate into the polytetrafluoroethylene porous membrane to form a binding layer, thereby eliminating the solute accumulation phenomenon, so that the prepared ultrafiltration membrane has good flux. At the same time, the binding layer can also give the cellulose polymer layer and the polytetrafluoroethylene layer in the finally prepared cellulose ultrafiltration membrane better composite performance, that is, improve the peel strength of the two, and prevent peeling during use, or between the cellulose polymer layer and the polytetrafluoroethylene layer; preferably, the polytetrafluoroethylene porous membrane has a PMI average pore size of 1-20 μm and a porosity of 60-90%; the surface roughness is 0.7-2 μm, and the surface includes nodes and fiber filaments, and the nodes are interconnected by fiber filaments. Under this condition, the casting liquid can be better penetrated, and a cellulose ultrafiltration membrane with better composite performance and filtration performance can be obtained. At the same time, in the casting step, the viscosity of the casting liquid is also one of the factors that determine whether it can penetrate well. In the present invention, the viscosity of the casting liquid is 6000-40000 cpa.s, thereby ensuring that the cellulose ultrafiltration membrane has a suitable thickness and an ideal membrane pore structure and pore size, thereby achieving good composite performance and filtration performance; this is because if the viscosity of the casting liquid is too high, the casting liquid cannot penetrate well into the substrate layer, and even the solvent may penetrate in, while the solute is not permeable, resulting in the accumulation of solutes on the surface of the substrate layer, resulting in a decrease in the final composite performance and filtration performance. If the viscosity is too low, the casting liquid will be completely penetrated, and ultimately it will be impossible to form an ultrafiltration layer that can be used for interception, and it will also be unable to meet actual needs.

[0090] The subsequent phase separation solidification is to immerse the substrate coated with the liquid film in water for phase separation solidification. The phase separation solidification lasts for 5-60s. By selecting a suitable casting liquid and a suitable phase separation time, the casting liquid is properly phase separated to ensure that the film with an ideal membrane pore size is obtained. At the same time, the present invention uses a hydrophilic and macroporous polytetrafluoroethylene porous membrane as the substrate. During phase separation, the coagulation bath is more likely to enter from the bottom surface of the substrate layer, so that the cellulose polymer that penetrates into the substrate layer can also be phase-solidified relatively earlier, thereby snatching the solute outside the upper surface of the substrate layer. On the one hand, it can alleviate the solute accumulation phenomenon. On the other hand, due to the reduction of solute outside the polytetrafluoroethylene layer, the support layer becomes relatively thinner, thereby increasing the flux.

[0091] Finally, the membrane is hydrolyzed in a sodium hydroxide aqueous solution and then washed to form a solid membrane. Depending on actual needs, cross-linking can be performed later. During the cross-linking modification process, the hydroxyl groups in the solid membrane react with functional groups such as epoxy and halogen, thereby increasing the mechanical strength of the membrane structure and making it less susceptible to swelling. This also improves the membrane's alkali resistance and extends its service life. This also facilitates the efficient retention of small molecular weight biomolecules (such as 3K and 5K biomolecules).

[0092] In the present invention, a microporous membrane comprising a polytetrafluoroethylene layer is used as the base layer. First, the surface of the polytetrafluoroethylene layer is relatively flat and has strong solvent resistance. The defects of the cellulose polymer layer in the prepared ultrafiltration membrane are relatively small, so the integrity of the ultrafiltration membrane is relatively good. The base layer can play a supporting role for the cellulose polymer layer, ensuring that the membrane as a whole has good mechanical strength and high compressive strength, and is suitable for long-term stable filtration under high pressure. At the same time, the cellulose polymer can penetrate into the polytetrafluoroethylene layer to form a binding layer, thereby eliminating the solute accumulation phenomenon, so that the prepared ultrafiltration membrane has good flux and composite performance, and prevents peeling during use. Finally, during subsequent phase separation, the coagulation bath is more likely to enter from the second side surface, so that the cellulose polymer in the binding layer can also undergo phase separation and solidification relatively earlier, thereby being able to snatch the solute outside the polytetrafluoroethylene layer, ensuring the uniformity of the SEM average pore size of the first side surface, while also alleviating the solute accumulation phenomenon and facilitating the regulation of the thickness of the support layer, making the support layer relatively thinner, thereby increasing the flux. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] The present invention will be further described below in conjunction with the accompanying drawings:

[0094] Figure 1 This is a SEM image of the first side surface of the ultrafiltration membrane prepared in Example 1 of the present invention;

[0095] Figure 2 This is a SEM image of a cross section of the ultrafiltration membrane prepared in Example 1 of the present invention;

[0096] Figure 3 This is an SEM image of the bonding surface of the polytetrafluoroethylene layer in the base layer of Example 1 of the present invention;

[0097] Figure 4 This is a SEM image of the first side surface of the ultrafiltration membrane prepared in Example 10 of the present invention;

[0098] Figure 5 This is a SEM image of a cross section of an ultrafiltration membrane prepared in Example 10 of the present invention;

[0099] Figure 6This is a SEM image of the first side surface of the ultrafiltration membrane prepared in Example 12 of the present invention;

[0100] Figure 7 This is a SEM image of a cross section of an ultrafiltration membrane prepared in Example 12 of the present invention;

[0101] Figure 8 This is the base layer used in the preparation of Example 17 of the present invention, magnification 2000×;

[0102] Figure 9 Schematic diagram of the membrane package diffusion flow testing device of the present invention. DETAILED DESCRIPTION

[0103] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that, in the absence of conflict, the embodiments of the present application and the features therein can be combined with each other.

[0104] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0105] Example 1: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0106] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0107] 10 parts of cellulose diacetate; 40 parts of polar solvent acetone; 22 parts of pore-forming agent polyvinyl alcohol; casting solution viscosity 6000cps;

[0108] S2: Casting the casting liquid onto the base layer to form a liquid film; the hydrophilic base is a polytetrafluoroethylene porous membrane; the polytetrafluoroethylene layer bonding surface in the base layer is as follows Figure 3 As shown;

[0109] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, solidifying the phases at 25°C for 10 seconds to obtain a film;

[0110] S4: placing the formed membrane in a 0.1 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 60°C for 120 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0111] S5: placing the hydrolyzed membrane in an alkaline environment of pH = 10 and cross-linking with a water-soluble cross-linking agent, and washing it after the cross-linking is completed to obtain an ultrafiltration membrane; wherein the cross-linking agent is epichlorohydrin, the concentration of the cross-linking agent in the aqueous solution is 10%, the cross-linking time is 150 minutes, and the temperature is 45°C.

[0112] The morphology of the prepared ultrafiltration membrane is as follows Figure 1-Figure 2 shown.

[0113] Example 2: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0114] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0115] 12 parts of cellulose diacetate; 43 parts of polar solvent dioxane; 23 parts of pore-forming agent polyethylene glycol; casting solution viscosity 7000 cps;

[0116] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0117] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 25°C for 15 seconds to obtain a film;

[0118] S4: placing the formed membrane in a 0.1 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 60°C for 120 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0119] S5: placing the hydrolyzed membrane in an alkaline environment of pH = 10 and cross-linking it with a water-soluble cross-linking agent, and then washing it after the cross-linking is completed to obtain an ultrafiltration membrane; wherein the cross-linking agent is epichlorohydrin, the concentration of the cross-linking agent in the aqueous solution is 10%, the cross-linking time is 100 minutes, and the temperature is 50°C.

[0120] Example 3: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0121] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0122] 10 parts of cellulose diacetate; 41 parts of dimethylacetamide, a polar solvent; 25 parts of polyvinyl pyrrolidone, a pore-forming agent; a casting solution viscosity of 6000 cps;

[0123] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0124] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, solidifying the phases at 25°C for 20 seconds to obtain a film;

[0125] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40°C for 60 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0126] S5: The hydrolyzed membrane is placed in an alkaline environment of pH = 10 and cross-linked with a water-soluble cross-linking agent, and then washed to obtain an ultrafiltration membrane; wherein the cross-linking agent is epichlorohydrin, the concentration of the cross-linking agent in the aqueous solution is 10%, the cross-linking time is 70 minutes, and the temperature is 55°C.

[0127] Example 4: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0128] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0129] 13 parts of cellulose diacetate; 45 parts of polar solvent N-methylpyrrolidone; 20 parts of pore-forming agent polyvinyl alcohol; casting solution viscosity 8000cps;

[0130] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0131] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, and solidifying the phase separation at 30°C for 10 seconds to obtain a film;

[0132] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40°C for 60 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0133] S5: placing the hydrolyzed membrane in an alkaline environment of pH = 10 and cross-linking with a water-soluble cross-linking agent, and washing it after the cross-linking is completed to obtain an ultrafiltration membrane; wherein the cross-linking agent is epichlorohydrin, the concentration of the cross-linking agent in the aqueous solution is 10%, the cross-linking time is 250 minutes, and the temperature is 35°C.

[0134] Example 5: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0135] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0136] 12 parts of cellulose diacetate; 42 parts of polar solvent acetone; 21 parts of pore-forming agent polyethylene glycol; casting solution viscosity 7000 cps;

[0137] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0138] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, and solidifying the phases at 30°C for 15 seconds to obtain a film;

[0139] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40°C for 60 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0140] S5: The hydrolyzed membrane is placed in an alkaline environment of pH = 10 and cross-linked with a water-soluble cross-linking agent, and then washed to obtain an ultrafiltration membrane; wherein the cross-linking agent is epichlorohydrin, the concentration of the cross-linking agent in the aqueous solution is 10%, the cross-linking time is 350 minutes, and the temperature is 58°C.

[0141] Example 6: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0142] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0143] 14 parts of cellulose diacetate; 44 parts of polar solvent acetone; 22 parts of pore-forming agent polyvinyl pyrrolidone; casting solution viscosity 9000cps;

[0144] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0145] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, solidifying it at 30°C for 20 seconds to obtain a film;

[0146] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40°C for 80 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0147] Example 7: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0148] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0149] 14 parts of cellulose diacetate; 43 parts of dimethylacetamide, a polar solvent; 24 parts of polyvinyl alcohol, a pore-forming agent; a casting solution viscosity of 10,000 cps;

[0150] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0151] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 30°C for 30 seconds to obtain a film;

[0152] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40°C for 90 minutes, and washing it after hydrolysis to form a cellulose ultrafiltration membrane;

[0153] Example 8: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0154] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0155] 15 parts of cellulose diacetate; 44 parts of polar solvent N-methylpyrrolidone; 27 parts of pore-forming agent polyethylene glycol; casting solution viscosity 12000 cps;

[0156] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0157] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 30°C for 40 seconds to obtain a film;

[0158] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40° C. for 100 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0159] Example 9: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0160] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0161] 17 parts of cellulose diacetate; 48 parts of polar solvent acetone; 25 parts of pore-forming agent polyvinyl pyrrolidone; casting solution viscosity 16000cps;

[0162] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0163] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, solidifying at 35 ° C for 20 seconds to obtain a film;

[0164] S4: placing the formed membrane in a 0.2 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 50°C for 120 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0165] Example 10: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0166] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0167] 19 parts of cellulose diacetate; 48 parts of dioxane, a polar solvent; 30 parts of polyvinyl alcohol, a pore-forming agent; and a casting solution viscosity of 20,000 cps.

[0168] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0169] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 35°C for 30 seconds to obtain a film;

[0170] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 50°C for 150 minutes, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0171] The morphology of the prepared ultrafiltration membrane is as follows Figure 4-Figure 5 shown.

[0172] Example 11: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0173] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0174] 19 parts of cellulose diacetate; 49 parts of dimethylacetamide, a polar solvent; 33 parts of polyethylene glycol, a pore-forming agent; a casting solution viscosity of 24,000 cps;

[0175] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0176] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 35°C for 35 seconds to obtain a film;

[0177] S4: placing the formed membrane in a 0.3 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 60°C for 170 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0178] Example 12: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0179] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0180] 21 parts of cellulose diacetate; 51 parts of polar solvent N-methylpyrrolidone; 35 parts of pore-forming agent polyvinylpyrrolidone; casting solution viscosity 27000cps;

[0181] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0182] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 35°C for 40 seconds to obtain a film;

[0183] S4: placing the formed membrane in a 0.5 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 40°C for 60 minutes, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0184] The morphology of the prepared ultrafiltration membrane is as follows Figure 6-Figure 7 shown.

[0185] Example 13: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0186] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0187] 24 parts of cellulose diacetate; 53 parts of polar solvent acetone; 35 parts of pore-forming agent polyvinyl alcohol; casting solution viscosity 29000cps;

[0188] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0189] S3: Phase separation solidification, immersing the liquid film in a coagulation bath and solidifying it at 35°C for 50 seconds to obtain a film;

[0190] S4: placing the formed membrane in a 0.5 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 70°C for 60 minutes, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0191] Example 14: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0192] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0193] 25 parts of cellulose diacetate; 52 parts of polar solvent dioxane; 36 parts of pore-forming agent polyethylene glycol; casting solution viscosity 30,000 cps;

[0194] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0195] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, and solidifying the phases at 40°C for 20 seconds to obtain a film;

[0196] S4: placing the formed membrane in a 0.5 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 50° C. for 100 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0197] Example 15: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0198] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0199] 27 parts of cellulose diacetate; 57 parts of dimethylacetamide, a polar solvent; 38 parts of polyvinyl pyrrolidone, a pore-forming agent; and a casting solution viscosity of 34,000 cps.

[0200] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0201] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, and solidifying the phase separation at 40°C for 40 seconds to obtain a film;

[0202] S4: placing the formed membrane in a 0.5 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 50°C for 120 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0203] Example 16: A method for preparing a cellulose ultrafiltration membrane, comprising the following steps:

[0204] S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight:

[0205] 29 parts of nitrocellulose; 59 parts of polar solvent N-methylpyrrolidone; 40 parts of pore-forming agent polyvinyl alcohol; casting solution viscosity 38000cps;

[0206] S2: Casting the casting liquid onto the substrate layer to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane;

[0207] S3: Phase separation solidification, immersing the liquid film in a coagulation bath, and solidifying the phases at 40 ° C for 50 seconds to obtain a film;

[0208] S4: placing the formed membrane in a 0.5 mol / L sodium hydroxide aqueous solution, hydrolyzing it at 60° C. for 100 min, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

[0209] Example 17: The difference from Example 1 is that Figure 8 The other structures shown have hydrophilic PTFE as the base layer.

[0210] Comparative Example 1: The same as Example 1, the base layer used is shown in Table 2-1 and Table 2-2 below.

[0211] Comparative Example 2: The same as Example 1, the base layer used is shown in Table 2-1 and Table 2-2 below.

[0212] 1. Structural Characterization: The surface and end face morphologies of the membrane structures of the above-mentioned embodiments and comparative examples were characterized. The specific values ​​are as follows:

[0213] Table 1: Membrane cross-sectional morphology and structure:

[0214]

[0215] Table 2-1: Base layer (PTFE layer) structure

[0216]

[0217]

[0218] Table 2-2: Base layer (non-woven fabric) structure

[0219] project Air permeability / cc / cm2 / sec Fiber thickness / μm Gram weight / g / m2 Example 1 120 14 31 Example 2 120 14 31 Example 3 120 14 31 Example 4 120 14 31 Example 5 / / / Example 6 120 14 31 Example 7 / / / Example 8 120 14 31 Example 9 120 14 31 Example 10 120 14 31 Example 11 120 14 31 Example 12 120 14 31 Example 13 160 19 27 Example 14 / / / Example 15 180 26 34 Example 16 180 26 34 Comparative Example 1 120 14 31 Comparative Example 2 120 14 31

[0220] Table 3: Membrane surface structure:

[0221]

[0222]

[0223] As can be seen from the above table, the cellulose ultrafiltration membrane prepared in the embodiment has a good membrane structure, and the defects of the cellulose polymer layer in the prepared ultrafiltration membrane are relatively small, so the integrity of the ultrafiltration membrane is relatively good; at the same time, the cellulose polymer can penetrate into the polytetrafluoroethylene layer to form a bonding layer, thereby eliminating the solute accumulation phenomenon, so that the prepared ultrafiltration membrane has good flux and composite performance, and prevents peeling during use; finally, it can also facilitate the regulation of the thickness of the support layer, making the support layer relatively thinner, thereby increasing the flux.

[0224] 2. Performance characteristics

[0225] 1. Filtration accuracy test: The specific results are shown in the following table:

[0226]

[0227]

[0228] The cellulose composite ultrafiltration membranes prepared in Examples 1-10 of the present invention purify various biomolecules by tangential flow filtration; their molecular weight cutoffs range from 3K to 750K, and their retention efficiencies are greater than 90%, ensuring efficient retention of biomolecules of various molecular weights.

[0229] 2. Flux test: At 25°C, 50 ml of deionized water was passed through a 47 mm diameter filter membrane. The time was recorded and the flux was calculated. The pressure for Examples 10 to 16 was 0.68 bar; the pressure for Examples 1 to 9 and Comparative Examples 1-2 was 3.8 bar. The specific results are shown in the following table:

[0230]

[0231]

[0232] Flow rate tests show that the membranes of the various embodiments of the present invention exhibit relatively high flux, i.e., fast flow rates, enabling rapid filtration of fluids containing biomolecules and high economic efficiency. However, Comparative Examples 1 and 2, which do not utilize the required composite substrate layer, exhibit lower flux at the same molecular weight cut-off.

[0233] 3. Mechanical strength test: The tensile strength of the wet membrane was tested. The tensile strength of the ultrafiltration membranes of Examples 1-16 was not less than 10 MPa, and the elastic modulus was greater than 200 MPa. The mechanical properties were relatively high and they had high pressure resistance during use. However, the tensile strength and elastic modulus of Example 17 were low, which did not meet the actual use requirements.

[0234] 4. Protein yield test (which can be performed according to the protein yield test method used in China CN201010154974.7 - Ultraporous membrane and its preparation method, or other methods can be used for testing). The protein yield of the ultrafiltration membrane of the embodiment is greater than 90%, which can obtain a higher protein yield and high economic benefit.

[0235] 5. Diffusion flow test: 0.11m 2 When conducting diffusion flow test on 3K membrane package, the specific method is as follows: prepare a membrane package with a filtration area of ​​0.11㎡, such as Figure 9 As shown in the figure, the test device is assembled, the feed tank 01 is connected to the liquid inlet hole on one side of the filter membrane package through the liquid inlet pipe, the liquid inlet pipe is connected to the pump 02, the drain valve 03 and the air valve 04, the waste tank 05 is connected to the liquid inlet hole on the other side of the filter membrane package through the reflux pipe, the reflux valve 06 is installed on the reflux pipe, the beaker 07 is connected to the filtrate hole of the filter membrane package through the permeation pipe, and the permeation valve 08 is installed on the permeation pipe to open and close the connection between the beaker 07 and the filtrate hole; fill the 50ml measuring cylinder 09 with water and invert it in the 500mL beaker 07 filled with water. During the test, first close the air valve 04 and set the pressure regulator to 0 bar (0 psi); then, close the feed valve and drain valve 03, open the air valve 04, reflux valve 06 and permeate valve 08, remove the water in the membrane package feed-reflux pipe, and then slowly adjust the pressure regulator to 0.35 bar (5 psi) to allow air to flow through the system until the water stops being discharged from the reflux pipe where the reflux valve 06 is located; close the reflux valve 06, allow the air pressure to remove the water in the permeate pipe from the filtrate hole, and slowly adjust the pressure regulator to 1 bar (15 psi); when the bubble rate stabilizes, record the corresponding time and the amount of air in the measuring cylinder 09; when 5-10 mL of gas is collected, record the corresponding time and air volume again; calculate the diffusion flow (mL / min / @15 psi); the results are shown in the following table.

[0236]

[0237] From the above data, it can be seen that the use of a polytetrafluoroethylene layer with a node-cellulose structure within the scope of the present invention and a non-woven fabric substrate layer as the base layer can provide the membrane with relatively better integrity during use. Although the diffusion flow of Examples 3 and 5 is larger and the integrity is relatively poor, it is also within the practical usable standard (<12 mL / min@15 psi).

[0238] While the preferred embodiments of the present invention have been described in detail above, it should be understood that, after reading the above teachings of the present invention, those skilled in the art may make various changes or modifications to the present invention. Such equivalents also fall within the scope defined by the appended claims.

Claims

1. A cellulose ultrafiltration membrane, comprising a main body, wherein the main body has: a first side surface for supplying liquid to be filtered, and a second side surface for discharging permeate through the body; The main body comprises an ultrafiltration layer, a support layer and a base layer in sequence along the direction of fluid flow; Its characteristics are: The ultrafiltration layer and the support layer include a cellulose polymer layer, which is formed by phase separation of the casting liquid on the base layer; The base layer comprises a polytetrafluoroethylene layer, The average pore size of the PMI of the base layer is greater than 0.8 μm; The polytetrafluoroethylene layer is a hydrophilic polytetrafluoroethylene layer; the water contact angle of the polytetrafluoroethylene layer surface is less than 80°, and the water contact angle of the second side surface is within 50° greater than the water contact angle of the first side surface; The cellulose polymer layer and the polytetrafluoroethylene layer are infiltrated and bonded to form a bonding layer; the surface forming the bonding layer is the bonding surface of the polytetrafluoroethylene layer, the bonding surface includes nodes and fiber filaments, and the nodes are connected to each other through the fiber filaments; The SEM average pore size of the first side surface is 1-90 nm.

2. The cellulose ultrafiltration membrane according to claim 1, characterized in that The coefficient of variation of the SEM average pore size of the first side surface is less than 0.

5.

3. The cellulose ultrafiltration membrane according to claim 1, characterized in that The base layer includes a substrate layer arranged on the polytetrafluoroethylene layer away from the cellulose polymer layer, the surface of the substrate layer away from the polytetrafluoroethylene layer forms a second side surface, the substrate layer includes a non-woven fabric, the thickness of the non-woven fabric accounts for 30-85% of the thickness of the entire film, and the thickness of the non-woven fabric is 60-300 μm.

4. The cellulose ultrafiltration membrane according to claim 3, characterized in that The air permeability of the nonwoven fabric is greater than 50cc / cm 2 / sec, fiber thickness is 5-30μm, and weight is 15-40g / m 2 .

5. The cellulose ultrafiltration membrane according to claim 3, characterized in that The thickness ratio of the cellulose polymer layer to the polytetrafluoroethylene layer is 0.1-3, the thickness of the cellulose polymer layer is 1-55 μm, and the thickness of the polytetrafluoroethylene layer is 15-90 μm.

6. The cellulose ultrafiltration membrane according to claim 3, characterized in that The thickness of the bonding layer accounts for 10-100% of the thickness of the polytetrafluoroethylene layer, and the thickness of the bonding layer is 10-100 μm.

7. The cellulose ultrafiltration membrane according to claim 1, characterized in that The base layer is a polytetrafluoroethylene layer, the thickness ratio of the cellulose polymer layer to the polytetrafluoroethylene layer is 0.02-1, the thickness of the cellulose polymer layer is 1.5-60 μm, and the thickness of the polytetrafluoroethylene is 100-300 μm.

8. The cellulose ultrafiltration membrane according to claim 1, characterized in that The average pore size of the PMI of the polytetrafluoroethylene layer is 1-20 μm, and the porosity is 60-90%; the surface roughness of the polytetrafluoroethylene layer is 0.7-2 μm.

9. The cellulose ultrafiltration membrane according to claim 1, characterized in that Immerse the dry film in water and allow it to wet within 5 seconds.

10. The cellulose ultrafiltration membrane according to claim 1, characterized in that The roughness of the first side surface is 0.1-2.5 μm, the pore area ratio of the first side surface is 1-10%, and the water contact angle of the first side surface is 10-55°.

11. The cellulose ultrafiltration membrane according to claim 1, characterized in that The ultrafiltration layer has ultrafiltration fibers forming a porous structure, and the SEM average diameter of the ultrafiltration fibers is 20-60 nm; The support layer has support fibers forming a porous structure, and the SEM average diameter of the support fibers is 20-85 nm; The ratio of the SEM average diameters of the support fibers to the ultrafiltration fibers is 1.2-2.

4.

12. The cellulose ultrafiltration membrane according to claim 1, characterized in that The thickness of the ultrafiltration layer is 0.1-5 μm, the thickness of the support layer is 0.5-50 μm, and the ratio of the thickness of the support layer to the thickness of the ultrafiltration layer is 2-13.

13. The cellulose ultrafiltration membrane according to claim 1, characterized in that The average pore size of the support layer gradually increases along the flow direction of the fluid, with a change gradient of 20-450 nm / 1 μm.

14. The cellulose ultrafiltration membrane according to claim 1, characterized in that The thickness of the ultrafiltration membrane is 130-420 μm; The cut-off molecular weight of the ultrafiltration membrane is 1K-750K; The tensile strength of the ultrafiltration membrane is not less than 10 MPa; At a pressure of 0.68 bar and a temperature of 25°C, the water flux of a 100K cellulose ultrafiltration membrane is 1-1.8 mL / min / cm 2 .

15. The cellulose ultrafiltration membrane according to claim 1, characterized in that The polytetrafluoroethylene layer is prepared by a stretching method.

16. The cellulose ultrafiltration membrane according to claim 15, characterized in that The area of ​​the node occupied by the surface of the polytetrafluoroethylene layer is S1; The area of ​​the fiber filaments on the surface of the polytetrafluoroethylene layer is S2; The S1:S2 is 0.13-7; The nodes occupy an area S1 of 4-40% of the surface of the bonding surface; the fiber filaments occupy an area S2 of 5-35% of the surface of the bonding surface.

17. The cellulose ultrafiltration membrane according to claim 15, characterized in that The average width of the node is 1-6 μm, and the difference between the maximum width and the minimum width of the node is less than 7 μm; the average width of the fiber filament is 0.1-1.2 μm, and the difference between the maximum width and the minimum width of the fiber filament is less than 1.5 μm.

18. The cellulose ultrafiltration membrane according to claim 15, characterized in that The number of connected cellulose fibers in a length of 50 μm along the node direction is 15-70.

19. The cellulose ultrafiltration membrane according to claim 1, characterized in that The material of the cellulose polymer layer includes one or more of regenerated cellulose and cellulose ester.

20. The method for preparing a cellulose ultrafiltration membrane according to claim 1, wherein: S1: preparing a casting solution, wherein the casting solution comprises the following components in parts by weight: 10-30 parts of cellulose polymer; 40-60 parts of polar solvent; 20-40 parts of pore-forming agent; S2: Casting the casting solution onto a hydrophilic substrate to form a liquid film; the hydrophilic substrate is a polytetrafluoroethylene porous membrane; The water contact angle on the surface of the polytetrafluoroethylene porous membrane is less than 80°; S3: Phase separation solidification, immersing the liquid film in a coagulation bath for phase separation solidification to obtain a film; S4: placing the formed membrane in a sodium hydroxide aqueous solution for hydrolysis, and washing it after hydrolysis to form a cellulose ultrafiltration membrane.

21. The preparation method according to claim 20, characterized in that: The cellulose polymer is at least one of nitrocellulose, cellulose acetate and regenerated cellulose; The cellulose acetate is selected from one or more of cellulose diacetate, cellulose triacetate, cellulose acetate butyrate and cellulose acetate propionate.

22. The method for preparing a cellulose ultrafiltration membrane according to claim 20, wherein: The polar solvent includes at least one of acetone, dioxane, dimethylacetamide, N-methylpyrrolidone, acetic acid, propionic acid, butyric acid and valeric acid; The pore former includes at least one of polyvinyl pyrrolidone, polyethylene glycol and polyvinyl alcohol.

23. The method for preparing a cellulose ultrafiltration membrane according to claim 20, wherein: The viscosity of the casting solution is 6000-40000 cpa.s.

24. The method for preparing a cellulose ultrafiltration membrane according to claim 20, wherein: The phase separation coagulation duration is 5-60s, the coagulation bath is water, and the phase separation temperature is 20-40°C.

25. The method for preparing a cellulose ultrafiltration membrane according to claim 20, wherein: The concentration of the sodium hydroxide aqueous solution is between 0.01 mol / L and 1 mol / L; the hydrolysis time is between 30° C. and 80° C., and the time is between 40 min and 200 min.

26. The method for preparing a cellulose ultrafiltration membrane according to claim 20, wherein: The preparation of the cellulose ultrafiltration membrane includes cross-linking; The crosslinking is carried out by crosslinking the cellulose ultrafiltration membrane with a water-soluble crosslinking agent in an alkaline environment, the crosslinking time is 20-400 minutes, and the temperature is 30°C-60°C; the crosslinking agent is at least one of halogenated epoxides, diepoxides, dihalogenated alkanes and dihalogenated alcohols.

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