A composite cellulose virus removal filter membrane and its preparation process, and virus removal membrane assembly

Abstract

The present application relates to a composite cellulose virus removal filter membrane and its preparation process, and a virus removal membrane assembly. The filter membrane includes a nylon microporous membrane and a cellulose layer. One side of the nylon microporous membrane is a liquid inlet surface, and the area in the nylon microporous membrane where the cellulose layer is permeated is a permeation zone. The thickness of the permeation zone is 10 to 50 μm. The liquid inlet surface includes liquid inlet fibers and liquid inlet holes. The hole area ratio of the liquid inlet holes is 15 to 55%, the surface tension is 42 to 58 dyne / cm, and the thickness of the nylon microporous membrane is 40 to 120 μm. The flux of the filter membrane is not less than 40 L / h / m 2 This application further discloses a process for preparing the aforementioned filter membrane and a virus removal membrane assembly comprising the aforementioned filter membrane. The filter membrane is supported by a nylon microporous membrane with an appropriate pore area ratio and surface tension. The thickness of the permeable zone is controlled to improve the integrity of the composite membrane and reduce the possibility of peeling between the two. This allows the composite membrane to possess both high peel strength and high flux, which are difficult to achieve simultaneously.

Description

Technical Field

[0001] The present application relates to the field of membrane separation technology, and in particular to a composite cellulose virus removal filter membrane and its preparation process, and a virus removal membrane assembly. Background Art

[0002] Ensuring viral safety is paramount in the production of various biologics. Both the new edition of the Chinese Pharmacopoeia and the document "ICH Q5A: Biotechnology Products - Viral Safety Evaluation" clearly stipulate high standards for the viral safety of biologics. Consequently, viral clearance and / or inactivation steps have become essential during the preparation of biologics.

[0003] Membrane separation technology is widely used in the virus removal process of various biological agents due to its advantages, such as high separation efficiency, easy process scale-up, and low risk of denaturation of active substances during the separation process. When membrane separation technology is applied to virus filtration processes, dead-end filtration is often used. Since the primary filtration driving force of dead-end filtration is the pressure difference between the two ends of the filter membrane after the pressurized liquid to be filtered, further increasing the pressure of the liquid to be filtered is expected to improve filtration efficiency. However, higher pressure liquid to be filtered places higher pressure resistance requirements on the filter membrane to avoid degradation of virus retention performance or even damage to the filter membrane under high pressure.

[0004] For example, the Chinese invention patent application with application publication number CN105980038A (filed by Asahi Kasei Corporation) discloses a virus removal membrane comprising cellulose, having a first surface for supplying a protein-containing solution and a second surface for discharging the permeate passing through the virus removal membrane. The membrane achieves a logarithmic removal rate of 4 or greater (LRV>4) for porcine parvovirus (approximately 18-26 nm). The membrane is a cellulose filter membrane, and its excellent hydrophilicity imparts low protein adsorption. However, the inherently soft texture of cellulose materials dictates that the three-dimensional network structure formed by the cellulose fibers within the membrane has poor mechanical strength, resulting in insufficient self-supporting properties. The cellulose membrane described in this application can only withstand a pressure of 15 psi and is difficult to withstand higher pressures. During use, the filter membrane is likely to be impacted by the feed liquid. Whether it is the pressure of the feed liquid itself, which already has a high pressure, on the filter membrane during the virus filtration process, or the impact caused by equipment vibration and other reasons, it puts higher demands on the pressure resistance of the filter membrane. Filter membranes that are difficult to withstand high pressure may not only have a shorter service life, but in some special application scenarios, they may also lead to the risk of virus leakage due to insufficient stability.

[0005] To improve the pressure resistance of cellulose-based filter membranes, a support membrane layer with higher pressure resistance can be introduced on top of the cellulose layer to form a composite membrane structure. This support membrane layer is often made of a microporous membrane or nonwoven substrate made of a polymer material. Nonwoven substrates have high pore openness, making it easier to form a high-integrity composite membrane structure with the casting solution. However, the surface flatness of nonwoven substrates is relatively low, and nonwoven fibers can easily penetrate and damage the casting solution, increasing the risk of virus leakage. Therefore, microporous membrane substrates with high intrinsic integrity are widely used.

[0006] For example, Chinese invention patent application publication number CN114173911A (applied by Sartorius) discloses a mechanically stable ultrafiltration membrane and its preparation method. The method involves coating a first polymer solution and a second polymer solution on a support layer. The first polymer solution is briefly treated with a non-solvent-containing gas to form a damping region after the first polymer solution solidifies. This damping region imparts strong impact resistance (i.e., pressure resistance) to the composite membrane. The support layer can be a nonwoven mesh, a manufactured fabric, or a polyester nonwoven, or a polypropylene, polyethylene, polyfiber, polyethersulfone, or cellulose microfiltration membrane. This ultrafiltration membrane achieves enhanced impact resistance by forming a dense damping structure with loose pores on both sides. However, this membrane pore structure is not suitable for virus removal filtration membranes. This is because, unlike ultrafiltration membranes, which often use tangential flow filtration (the feed flows parallel to the membrane surface), virus removal membranes often use dead-end filtration (the feed flows perpendicular to the membrane surface, also known as transmembrane filtration). The dense damping structure within the membrane is likely to result in concentrated retention of particulate impurities in the feed, leading to a rapid decrease in membrane flux and a shortened membrane lifespan. Furthermore, the patent application does not explain the effects of different support layers on the membrane, although support layers do have a significant impact on the membrane.

[0007] For example, the U.S. patent application document (MILLIPORE CORP application) with application publication number US4824568A discloses a high-bubble-point filter membrane. This membrane is formed by coating PVDF or PES onto a 0.22μm PVDF microporous substrate to form a composite membrane. During coating, the solvent in the casting solution can soften a portion of the PVDF substrate, thereby significantly improving the peel strength of the composite membrane. However, organic solvents erode PVDF rapidly. For example, a PVDF microporous substrate membrane completely loses its mechanical and permeability properties after soaking in NMP for 10 seconds and completely dissolves within 2 minutes. This means that the high peel strength of the composite membrane achieved by softening the substrate often comes at the expense of reduced permeability (i.e., flux), making it difficult to achieve both.

[0008] PES materials also have the problem of not being resistant to highly polar organic solvents such as ketones, esters, halogenated hydrocarbons, dimethyl sulfoxide, etc. Therefore, the composite membrane structure with a PES microporous base membrane also has the above problems. Cellulose materials are inherently soft in texture. If cellulose materials are used as base membranes, the mechanical properties of the composite membrane may not meet the requirements, and the solvents in the cellulose casting solution may also soften the cellulose base membrane, resulting in the same problem of decreased permeability. Compared with the aforementioned materials, nylon materials have a wider range of organic solvent resistance. Therefore, using a nylon microporous membrane as a base can effectively solve the problem of reduced flux caused by the softening of the microporous base membrane by the solvent. However, the inability to soften the nylon base means a decrease in the peel strength of the composite membrane and a decrease in the overall integrity. Therefore, it is difficult to achieve both flux and peel strength of the composite membrane.

[0009] How to obtain a composite membrane with both high flux and high peel strength is a problem that needs to be solved urgently but is difficult to solve. Summary of the Invention

[0010] The present application provides a composite cellulose virus-removal filter membrane, a preparation process thereof, and a virus-removal membrane assembly. The virus-removal filter membrane of the present application uses a nylon microporous membrane as a support for the cellulose layer to improve the pressure resistance of the composite membrane. Since the nylon microporous membrane has a wider resistance to organic solvents, it is not easily softened by the organic solvents in the cellulose casting liquid, and thus is not easy to form a local dense structure. Furthermore, by controlling the nylon microporous membrane to have a suitable pore area ratio and surface tension, it is ensured that the cellulose casting liquid can properly penetrate into the nylon microporous membrane, forming a penetration zone with a thickness of 10 to 50 μm. The penetration zone can greatly improve the integrity of the cellulose layer and the nylon microporous membrane, and reduce the possibility of peeling between the two, so that the composite membrane has both high peel strength and high flux, which are difficult to obtain at the same time.

[0011] In a first aspect, the present application provides a supported composite cellulose virus removal filter membrane, which adopts the following technical solution:

[0012] A supported composite cellulose virus removal filter membrane comprises a nylon microporous membrane and a cellulose layer at least partially permeated into the nylon microporous membrane, wherein the side of the nylon microporous membrane away from the cellulose layer is a liquid inlet surface, the side of the cellulose layer away from the nylon microporous membrane is a liquid outlet surface, and the area of ​​the nylon microporous membrane where the cellulose layer permeates is a permeation zone;

[0013] The thickness of the permeable zone is 10 to 50 μm;

[0014] The liquid inlet surface includes liquid inlet fibers and liquid inlet holes, the liquid inlet fibers are mutually surrounded to form the liquid inlet holes, the hole area ratio of the liquid inlet holes is 15-55%, the surface tension of the liquid inlet surface is 42-58 dyne / cm, and the thickness of the nylon microporous membrane is 40-120 μm;

[0015] The flux of the filter membrane is not less than 40L / h / m 2 @30psi.

[0016] Optionally, the hole area ratio of the liquid inlet hole is 20%~50%; further optionally, the hole area ratio of the liquid inlet hole is 25%~45%.

[0017] Optionally, the surface tension of the liquid inlet surface is 45~55dyne / cm; further optionally, the surface tension of the liquid inlet surface is 50~55dyne / cm.

[0018] Optionally, the flux of the filter membrane is not less than 50 L / h / m 2 @30psi; further optionally, the flux of the filter membrane is not less than 60L / h / m 2 @30psi; further optionally, the flux of the filter membrane is not less than 70L / h / m 2 @30psi; further optionally, the flux of the filter membrane is not less than 80L / h / m 2 @30psi; further optionally, the flux of the filter membrane is not less than 90L / h / m 2 @30psi; further optionally, the flux of the filter membrane is not less than 100L / h / m 2 @30psi.

[0019] By adopting the above technical solution, the present application obtains a composite membrane by combining nylon microporous membranes with a cellulose layer, and the cellulose layer in the composite membrane, which plays the main role in virus interception, adopts a cellulose-based membrane-forming material with good hydrophilicity, so that the composite membrane has a high protein yield and ensures the concentration and purity of active proteins in the filtrate. The introduction of the nylon microporous membrane greatly improves the inherent defect of the soft texture of the cellulose-based membrane-forming material. For the virus removal filtration process that uses pressure as the filtration driving force, this means that the filtration efficiency can be improved by further increasing the filtration pressure, thereby reducing production costs, thus having a very positive effect. However, as mentioned above, the introduction of nylon microporous membranes often brings about the problem that it is difficult to achieve both flux and anti-delamination performance. At present, general composite membranes often need to make trade-offs; if the nylon microporous membrane is softened by solvent, although the anti-delamination performance and pressure resistance of the composite membrane can be improved, the local softening of the nylon microporous membrane will lead to a significant increase in the local solid content of the casting liquid, and a denser membrane structure will be obtained after re-phase solidification, resulting in a simultaneous decrease in flux and load; if the solvent in the casting liquid cannot soften the nylon microporous membrane, which already has a high integrity (compared to the more open non-woven substrate), it is often difficult to obtain a composite membrane with a high integrity. The nylon microporous membrane and the cellulose layer are prone to peeling under higher operating pressure, which brings unnecessary and unexpected risks to the virus filtration process.

[0020] The inventors of this application have found that by specifically selecting a nylon microporous membrane with good organic solvent resistance and thus not easily softened by the solvent as the substrate, and by controlling the pore area ratio of the nylon microporous membrane at the liquid inlet surface to 15-55% and the surface tension to 42-58 dyne / cm, a permeation zone with a thickness of 10-50 μm can be obtained, thereby making the composite membrane have a flow rate of not less than 40 L / h / m 2 @30psi flux, also has high anti-delamination performance.

[0021] This may be because, for a roughly symmetrical nylon microporous membrane (roughly symmetrical means that the pore size maintains a small change gradient along the membrane thickness direction, such as less than 5nm / μm), the morphology of the surfaces on both sides of the membrane has a small difference. Therefore, it is only necessary to observe the morphology of the liquid inlet surface to know the approximate morphology of the surfaces on both sides. The characterization of the morphological parameters of the liquid inlet surface also characterizes the morphological parameters of the side of the nylon microporous membrane in contact with the cellulose casting liquid to a certain extent.

[0022] By controlling the pore area ratio of the liquid inlet surface of the nylon microporous membrane to be no less than 15% and the surface tension to be no less than 42 dyne / cm, it is possible to ensure that the cellulose casting liquid and the nylon microporous membrane are easily wetted, and the casting liquid does not have difficulty in penetrating into the nylon microporous membrane due to excessive wetting resistance. On the basis of ensuring the wettability of the casting liquid and the nylon microporous membrane, the surface of the nylon microporous membrane has sufficient microporous structure to produce a "capillary phenomenon" on the cellulose casting liquid, thereby promoting the cellulose casting liquid to penetrate into the nylon microporous membrane, obtaining a penetration area with a thickness of no less than 10 μm, and ensuring that the prepared composite membrane has good anti-delamination performance.

[0023] However, the higher the pore area ratio and surface tension of the liquid inlet surface of the nylon microporous membrane, the better. The inventors of this application unexpectedly discovered that as the pore area ratio and surface tension of the liquid inlet surface of the nylon microporous membrane increase, the thickness of the permeable zone in the composite membrane continues to increase. As the thickness of the permeable zone continues to increase, the anti-delamination performance of the virus removal membrane does not increase linearly, but rather generally shows a trend of first rapidly increasing and then slowing down the rate of increase. That is, the thickness of the permeable zone has a marginal diminishing effect on the anti-delamination performance of the composite membrane. This may be because when peeling the composite layer structure, the area most prone to peeling is often located near the composite interface of the layer structure, that is, the anti-delamination performance of the composite membrane mainly depends on the structure near the composite interface of the layer structure. Therefore, the farther the distance from the composite interface, the smaller the impact on the anti-delamination performance between the layer structures. For this reason, when the thickness of the permeable zone is too large (such as greater than 50 μm), it is too far away from the composite interface, and the impact on the anti-delamination performance of the composite membrane is already small. Further increase in the thickness of the permeable zone will inevitably lead to an increase in the resistance of the liquid in the permeable zone.

[0024] Correspondingly, the flux and load of the composite membrane first decreased slightly, then decreased at a faster rate. When the pore area ratio of the nylon microporous membrane's liquid inlet surface was controlled to no less than 55% and the surface tension was no less than 58 dyne / cm, the composite membrane's flux and load decreased significantly while maintaining essentially unchanged anti-stratification performance. This may be because the excessively high pore area ratio and surface tension at the liquid inlet surface can further promote the penetration of a large amount of casting liquid into the nylon microporous membrane, resulting in a permeation zone with excessive thickness (e.g., greater than 50μm). This excessively thick permeation zone causes excessive blockage of the pore structure within the nylon microporous membrane, resulting in a greater resistance and lower contaminant holding space in the permeation zone, leading to a decrease in flux and load.

[0025] Further controlling the thickness of the nylon microporous membrane to 40~120μm can not only ensure that the composite membrane has the required high pressure resistance, but also reduce the possibility of increased liquid resistance due to excessive thickness of the nylon microporous membrane, ensuring that the composite membrane has a higher flux on the basis of high pressure resistance.

[0026] It is understandable that the measurement method of various surface morphological parameters of composite membranes (such as thickness, fiber diameter, pore size, pore area ratio, etc.) 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; in the preparation process 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 measured to reflect the overall average pore size on the plane. When actually measuring, the membrane surface (or cross-section) can be characterized by an electron microscope first, the corresponding SEM image can be obtained, and a certain area can be selected, such as 1μm 2 (1μm multiplied by 1μm) or 25μm 2 (5μm multiplied by 5μm), the specific area size depends on the actual situation, and then the pore size, fiber diameter and other morphological parameters of all holes in the area are measured using corresponding computer software or manually, and then calculated to obtain the average pore size, average fiber diameter and other morphological parameters of the area; 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.

[0027] Optionally, the average diameter of the liquid-inlet fibers is 150-450 nm, and the average pore size of the liquid-inlet holes measured by SEM is 700-2000 nm.

[0028] By adopting the above technical solution, on the basis of a pore area ratio of 15-55% and a surface tension of 42-58 dyne / cm on the liquid inlet surface, liquid inlet fibers with an average diameter of 150-450 nm and liquid inlet holes with an average pore size of 700-2000 nm measured by SEM are preferably used, which not only enables the nylon microporous membrane to have a further preferred liquid resistance and the composite membrane to have a further preferred flux; it also enables the nylon microporous membrane to have a further preferred casting liquid resistance and a further preferred internal pore wall structure (the pore wall of the pore structure is actually the surface of the fiber structure, so the thicker the fiber, the lower the specific surface area generally), to ensure that the casting liquid is easy to penetrate and interacts with the nylon microporous membrane with more pore wall structures and the casting liquid that penetrates into the nylon microporous membrane, so as to improve the anti-delamination performance of the composite membrane; but the casting liquid is not difficult to penetrate due to the small and dense pore structure, resulting in a decrease in anti-delamination performance.

[0029] For example, for a nylon microporous membrane with a specific pore area ratio and surface tension, if the average diameter of the liquid-influent fibers is too large (e.g., greater than 450 nm) and the average pore size of the liquid-influent pores is too large (e.g., greater than 2000 nm), it indicates that the liquid-influent surface has a macroporous, coarse fiber structure and a small number of liquid-influent pores (based on a certain pore area ratio, the larger the area of ​​a single pore, the fewer the pores). Although the macroporous structure of the liquid-influent surface can promote the penetration of the casting solution into the interior of the nylon microporous membrane, thereby improving the integrity of the composite membrane, the coarse fibers and small amount of macroporous structure also mean that the nylon microporous membrane has a low specific surface area. Even if the casting solution penetrates the nylon microporous membrane, there is less pore wall structure for the casting solution to adhere to and interact with the casting solution. As a result, even if there is a permeable area, it does not have further preferred anti-delamination performance. If the average size of the liquid-feeding fibers is small (e.g., less than 150 nm) and the average pore size of the liquid-feeding pores is too small (e.g., less than 700 nm), this indicates that the liquid-feeding surface has a small-pore, fine-fiber structure. This small-pore, fine-fiber structure implies a large number of liquid-feeding pores and a relatively large specific surface area within the nylon microporous membrane. While a larger specific surface area often means more interaction forces with the casting solution penetrating the nylon microporous membrane, an excessively small pore size often increases the difficulty of the casting solution penetrating, resulting in suboptimal anti-delamination performance. Furthermore, a small-pore, fine-fiber structure often implies a denser pore structure within the nylon microporous membrane. These pore structures form a more tortuous feed liquid flow path. A more tortuous feed liquid path often means higher feed liquid resistance, resulting in a composite membrane with suboptimal flux.

[0030] Therefore, for a nylon microporous membrane with a liquid inlet surface having a pore area ratio of 15-55% and a surface tension of 42-58 dyne / cm, the average diameter of the liquid inlet fiber can be preferably controlled to be 150-450 nm, and the average pore size of the liquid inlet hole measured by SEM can be controlled to be 700-2000 nm to obtain further preferred comprehensive performance.

[0031] Optionally, in the liquid inlet surface, the liquid inlet fibers having a diameter not less than 1.3 times the average diameter are coarse fibers, and the proportion of coarse fibers in the liquid inlet fibers does not exceed 25%.

[0032] By adopting the above technical solution, the coarse fibers can, on the one hand, play a role in local structural reinforcement and improve the pressure resistance of the composite membrane. On the other hand, the presence of coarse fibers often means a reduction in local specific surface area, providing pore walls for the casting liquid to adhere to and generate interaction forces with the casting liquid.

[0033] Optionally, the connection points between adjacent liquid-influent fibers are connection points, the average diameter of the connection points is larger than the average diameter of the liquid-influent fibers, and on average, 2.5 to 4.5 liquid-influent fibers are connected to each connection point.

[0034] By adopting the above technical solution, the fibers do not exist independently of each other, but form a three-dimensional network structure, wherein the fiber structure constitutes the skeleton part of the three-dimensional network structure, the pores of the three-dimensional network structure are the membrane pore structure, and the connection between the fibers is the connection point; and the morphology structure of the liquid inlet surface can show the entire three-dimensional network structure to a certain extent.

[0035] The connection point, as the connecting structure between fibers, has a larger diameter than the fibers. The larger diameter of the connection point means that it has a good connection and support effect on the local fiber structure, thereby improving the local stability of the three-dimensional network structure after pressure, and improving the pressure resistance of the nylon microporous membrane. Correspondingly, the connection point has a larger diameter than the fiber and thus has a greater resistance to the feed liquid. The presence of the connection point will increase the irregularity of the membrane pore structure, thereby increasing the resistance of the membrane pore structure to the feed liquid. Therefore, the number of liquid-feeding fibers connected to each connection point should not be too many, so as to avoid insufficient reinforcement effect of the connection point; the number of liquid-feeding fibers connected to each connection point should not be too few, so as to avoid large resistance to the feed liquid and reduce the flux of the composite membrane.

[0036] Optionally, the zeta potential of the liquid inlet surface at a pH of 7 is -2mV to -40mV.

[0037] By adopting the above technical solution, and based on the roughly identical surface morphology of nylon microporous membranes, the inventors of this application unexpectedly discovered that the zeta potential of the nylon microporous membrane also affects the anti-delamination performance and flux of the resulting composite membrane. Using a nylon microporous membrane with a zeta potential of -2mV to -40mV can further optimize the composite membrane to achieve excellent anti-delamination performance. It should be noted that while nylon is known to have a certain inherent negative charge, the ability to achieve even better anti-delamination performance in the composite membrane by optimally controlling the zeta potential of the nylon material is a highly unexpected result.

[0038] This may be because cellulose materials have a large number of hydroxyl groups. Therefore, when cellulose materials phase separate in a non-solvent coagulation bath (such as water), the hydroxyl groups on the cellulose material may ionize and become slightly negatively charged. If the nylon microporous membrane has an excessive negative charge (such as a zeta potential less than -40mV), excessive electrostatic forces will be generated between the negatively charged nylon microporous membrane and the casting solution, reducing the bonding strength between the casting solution and the nylon microporous membrane pore wall. Even if the permeation zone has the same thickness, the cellulose layer and the nylon microporous membrane are more likely to peel off, thereby reducing the anti-delamination performance of the composite membrane.

[0039] However, if the zeta potential of a nylon microporous membrane needs to be controlled to be greater than -2mV, or even positively charged, it is often necessary to modify the nylon microporous membrane to be positively charged. In virus removal applications, the feed solution often contains negatively charged active protein substances. Nylon microporous membranes with less negative charge or even positive charge are likely to adsorb protein active substances, resulting in a decrease in protein yield, which is an undesirable result for expensive protein active substances. Therefore, in this application, a nylon microporous membrane with a zeta potential of -2mV to -40mV is preferably used to ensure low adsorption of protein in the feed solution while ensuring the anti-delamination performance of the composite membrane.

[0040] It is understandable that the aforementioned cellulose material can be a regenerated cellulose material obtained by hydrolyzing a cellulose derivative, or a cellulose derivative that already has hydroxyl groups.

[0041] Optionally, in the liquid inlet surface, the liquid inlet holes with an SEM-measured pore size not less than 1.3 times the SEM-measured average pore size are large pores, and the liquid inlet holes with an SEM-measured pore size not greater than 0.5 times the SEM-measured average pore size are small pores. Among all the liquid inlet holes, the number of large pores accounts for 10~30%, and the proportion of small pores does not exceed 25%.

[0042] By adopting the above technical solution, the large pores in the liquid inlet surface can promote the penetration of the casting liquid into the nylon microporous membrane. Therefore, the proportion of large pores should not be too small (such as less than 10%). However, excessive large pores (such as greater than 30%) will lead to too low specific surface area of ​​the nylon microporous membrane, thereby reducing the interaction area between the nylon microporous membrane and the casting liquid, and reducing the anti-delamination performance of the composite membrane. Similarly, a certain amount of small pores can appropriately increase the specific surface area of ​​the nylon microporous membrane, thereby improving the anti-delamination performance of the composite membrane. However, the proportion of small pores should not be too high (such as greater than 25%), so as to avoid excessive small pores not only generating excessive resistance to the feed liquid, affecting the flux of the composite membrane, but also affecting the penetration of the casting liquid.

[0043] Optionally, the liquid inlet surface is taken as the position where the thickness of the nylon microporous membrane is 0, and the surface away from the liquid inlet surface is taken as the position where the thickness of the nylon microporous membrane is 1, and the pore size variation coefficient X of the nylon microporous membrane is ≤ 0.4, where X is calculated by the following formula:

[0044] ;

[0045] In the above formula, D 0.1 D1 is the average pore size measured by SEM within the 0~0.1mm thickness range of the nylon microporous membrane, D2 is the average pore size measured by SEM within the 0.9~1mm thickness range of the nylon microporous membrane, and D 均 D 0.1 and the average value of D1.

[0046] By adopting the above technical solution, the pore size variation coefficient of the nylon microporous membrane, to a certain extent, characterizes the degree of pore size variation of the nylon microporous membrane in the thickness direction. In this application, a nylon microporous membrane with a pore size variation coefficient of no more than 0.4 is selected, indicating that the nylon microporous membrane has a small pore size variation in the thickness direction and has a roughly symmetrical or slightly asymmetrical structure. A roughly symmetrical nylon microporous membrane can reduce the variation in permeation resistance caused by membrane pore size variation, thereby achieving more uniform anti-delamination performance across the composite membrane.

[0047] If the pore size variation coefficient of the nylon microporous membrane is too large, it means that the pore structure of the nylon microporous membrane has a large variation gradient along the membrane thickness direction. Starting from the position of membrane thickness 1 (that is, the side of the nylon microporous membrane in contact with the casting liquid), if the membrane pores of the nylon microporous membrane rapidly increase in size, it means that the area near the membrane thickness 1 of the nylon microporous membrane has a high-density pore structure, and the casting liquid is difficult to penetrate. Moreover, the casting liquid that penetrates into the nylon microporous membrane is difficult to fill the membrane pore structure of the nylon microporous membrane with an enlarged pore size. Therefore, it is difficult to ensure a good interaction between the casting liquid and the pore wall structure of the nylon microporous membrane, resulting in a decrease in the anti-delamination performance of the composite membrane. If the membrane pores of the nylon microporous membrane become smaller rapidly, it means that the area near the thickness of the nylon microporous membrane 1 has a pore structure with a higher degree of porosity, and naturally has a lower specific surface area. The interaction area between the casting liquid and the pore wall structure of the nylon microporous membrane near this area is smaller, which will also lead to a decrease in the anti-delamination performance of the composite membrane; in addition, the rapid decrease in the membrane pores of the nylon microporous membrane also means a rapid increase in the permeation resistance of the casting liquid, which can easily lead to too low a thickness of the permeation zone, which will also affect the anti-delamination performance of the composite membrane.

[0048] Optionally, the ratio of the average fiber diameter in the nylon microporous membrane thickness range of 0~0.1 to the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 is 0.7~1.3; the ratio of the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 to the average fiber diameter in the nylon microporous membrane thickness range of 0.9~1 is 0.7~1.3.

[0049] Optionally, the difference between the average fiber diameter in the nylon microporous membrane thickness range of 0~0.1 and the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 is not greater than 50nm; the difference between the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 and the average fiber diameter in the nylon microporous membrane thickness range of 0.9~1 is not greater than 50nm.

[0050] By using the above technical solution, the pore size variation coefficient of the nylon microporous membrane can characterize the overall pore structure variation trend of the nylon microporous membrane, but the variation trend of the internal morphology structure of the nylon microporous membrane is difficult to clearly characterize. However, if the nylon microporous membrane is divided into two parts according to the membrane thickness and the morphology structure of these two parts is characterized separately, the variation trend of the internal morphology structure of the nylon microporous membrane can be further characterized.

[0051] There is no significant mutation in the average fiber diameters of the upper and lower membrane structures of the nylon microporous membrane, indicating that the upper and lower membrane structures of the nylon microporous membrane have a roughly symmetrical structure, thereby further ensuring the composite uniformity of the composite membrane throughout and ensuring that the composite membrane has relatively uniform anti-stratification performance.

[0052] It can be understood that whether it is the difference between the average fiber diameter in the nylon microporous membrane thickness area of ​​0~0.1 and the average fiber diameter in the nylon microporous membrane thickness area of ​​0.4~0.5, or the difference between the average fiber diameter in the nylon microporous membrane thickness area of ​​0.4~0.5 and the average fiber diameter in the nylon microporous membrane thickness area of ​​0.9~1, both refer to the difference in the average fiber diameters in the two areas, and both values ​​are positive numbers.

[0053] Optionally, the permeation zone includes nylon fibers and cellulose fibers, the nylon fibers surround each other to form primary pores, the cellulose fibers are located in the primary pores, the cellulose fibers surround each other and separate the primary pores into several secondary pores, and the average pore size of the secondary pores measured by SEM is smaller than the average pore size of the primary pores measured by SEM.

[0054] By adopting the above technical solution, based on a permeation zone thickness of 10-50μm, the casting solution's penetration into the pore structure of the nylon microporous membrane will inevitably affect the membrane's porosity, thereby increasing the membrane's resistance. Of course, given the same coating thickness, as the casting solution's penetration into the nylon microporous membrane increases (i.e., the thickness of the permeation zone increases), the total thickness of the composite membrane will decrease. This is because the thickness of the pure cellulose layer that has not penetrated the nylon microporous membrane will decrease, and a thinner pure cellulose layer tends to have lower resistance to the liquid. Therefore, the casting solution's penetration process is accompanied by an increase in the nylon microporous membrane's resistance to the liquid and a decrease in the pure cellulose layer's resistance to the liquid.

[0055] The present application further preferably introduces a secondary pore structure into the permeation zone. These secondary pore structures are formed by the cellulose fibers within the permeation zone, which intertwine and entangle with each other, separating the pore structure (i.e., primary pores) of the nylon microporous membrane, thereby forming secondary pores. These secondary pores create a large number of pathways for the feed liquid to flow within the primary pores within the permeation zone. Compared to primary pores without secondary pores, this unique large-pore-within-small-pore structure significantly reduces the feed liquid resistance in the permeation zone. As previously mentioned, as the thickness of the permeation zone increases, the thickness of the pure cellulose layer decreases, and the feed liquid resistance of the pure cellulose layer also decreases. In short, the introduction of secondary pores in the permeation zone reduces the resistance of the pure cellulose layer while only slightly increasing the resistance of the permeation zone. This allows the composite membrane to achieve superior flux while maintaining good anti-delamination performance (as guaranteed by the thickness of the permeation zone).

[0056] Optionally, the area of ​​the permeation zone with a thickness of 10 μm close to the cellulose layer is the junction area, the area of ​​the junction area with a thickness of 5 μm close to the liquid inlet surface is the upper junction area, and the area of ​​the junction area with a thickness of 5 μm close to the liquid outlet surface is the lower junction area, the SEM-measured average pore size of the secondary pores in the permeation zone is larger than the SEM-measured average pore size of the secondary pores in the junction area; the average diameter of the cellulose fibers in the junction area is not less than 50 nm.

[0057] By adopting the above technical solution, as previously mentioned, different locations in the permeation zone have different effects on the anti-delamination performance of the composite membrane. The region with the greatest impact on the anti-delamination performance of the composite membrane is located near the composite interface of each membrane structure. The inventors of this application have discovered that for the composite membrane of the specific structure of this application, the region of the permeation zone approximately 10 μm thick near the composite interface (i.e., the interface region) has a greater impact on the anti-delamination performance of the virus removal membrane.

[0058] The present application further prefers that the average pore size of the secondary pores measured by SEM in the permeation zone is larger than the average pore size of the secondary pores measured by SEM in the junction zone. This means that the secondary pores in the junction zone have a smaller pore size and a higher density. Although the cellulose fibers in the junction zone have a smaller diameter, the denser and more entangled cellulose fibers in the junction zone can still ensure that the composite membrane has sufficiently high anti-stratification performance. Of course, although the cellulose fibers with smaller diameters in the junction zone have less flow resistance to the slurry, cellulose fibers with too small diameters (such as less than 50 nm) may still cause the cellulose fibers at the junction, which has the greatest impact on the anti-stratification performance of the composite membrane, to break easily, thereby reducing the anti-stratification performance of the composite membrane. Therefore, the diameter of the cellulose fibers in the junction zone should not be less than 50 nm.

[0059] Therefore, the present application controls the boundary area to have a denser three-dimensional cellulose fiber skeleton structure than the entire permeation area, and coordinates the diameter of the cellulose fibers in the boundary area to be no less than 50 nm, so that the virus removal membrane not only has higher anti-stratification performance, but also has higher flux.

[0060] Optionally, the average pore size of the secondary pores in the lower boundary region measured by SEM is 100-200 nm; the average pore size of the secondary pores in the upper boundary region measured by SEM is 120-220 nm.

[0061] By employing this technical solution, the average pore size of the nylon microporous membrane at the liquid inlet surface, measured by SEM, is 700-2000 nm, and the membrane is generally symmetrical. Furthermore, the pore size of the secondary pores in the lower and upper interface regions is controlled. In the interface region of a unique large-pore-within-small-pore structure, while the primary pore size is roughly defined, the size of the secondary pores significantly influences the composite membrane's anti-delamination performance and the feed-liquid resistance in the permeation zone.

[0062] As mentioned above, the area with the greatest impact on the anti-stratification performance of the composite membrane is located near the composite interface. That is, the interface area in the permeation zone has the greatest impact on the anti-stratification performance of the virus removal membrane, and the lower interface area in the interface area has the greatest impact on the anti-stratification performance of the virus removal membrane. And because the secondary pores in the permeation zone of the composite membrane in this application have a roughly asymmetric structure, the secondary pores with the smallest pore size in the entire permeation zone are located in the interface area and near the lower interface area. Therefore, the lower interface area and the upper interface area have the greatest impact on the flux of the composite membrane in the permeation zone.

[0063] Because the lower interface region has a relatively small secondary pore structure, to ensure that the entire permeation zone has a minimal impact on the composite membrane flux, it is necessary to ensure that the lower interface region has a low feed-liquid resistance pore structure of "large pores within small pores." However, to ensure that the composite membrane still has high anti-demixing performance, the secondary pore structure in this "large pores within small pores" structure should not have an excessively large pore diameter, as this will prevent the cellulose fiber skeleton from becoming insufficiently dense and prone to demixing. Therefore, the secondary pore diameter in the lower interface region is preferably controlled to 100-200 nm to ensure that the composite membrane has both good anti-demixing performance and high flux.

[0064] Similarly, the morphology of the upper interface region also has a significant impact on the resistance to the liquid in the permeation zone and the anti-delamination performance of the composite membrane. If the pore size of the secondary pores in the upper interface region is too large (e.g., greater than 220nm), it indicates that the density of the cellulose fiber skeleton in the upper interface region is low. Although the resistance to the liquid in the upper interface region is reduced, the mutual entanglement of the cellulose fibers between the upper and lower interface regions is reduced. The cellulose fibers in the upper interface region are unable to inhibit the breakage of the cellulose fibers in the lower interface region, resulting in insufficient anti-delamination performance of the composite membrane. In addition, based on the secondary pore structure with a smaller pore size in the lower interface region, if the pore size of the secondary pore structure in the upper interface region is too large, it means that the secondary pores in the interface region have a large pore size gradient along the thickness direction, which easily leads to the concentrated retention of impurity particles (concentrated retention refers to the retention of large and small particles in a small area). The concentrated retention of impurity particles often means that the local pore structure of the composite membrane is rapidly clogged, resulting in a too short service life of the composite membrane. If the pore size of the secondary pores in the upper boundary region is too small (e.g., less than 120 nm), although the denser cellulose fiber skeleton in the upper boundary region can further improve the anti-stratification performance of the composite membrane by forming entanglements with the cellulose fiber skeleton in the lower boundary region; however, on the basis of the lower boundary region having a denser secondary pore structure, further introducing an upper boundary region having a denser secondary pore structure will make the entire boundary region have a denser secondary pore structure, forming a larger material-liquid resistance, resulting in a lower flux of the composite membrane. Therefore, on the basis of the lower boundary region having a denser secondary pore structure, by further optimizing and controlling the pore size of the secondary pore structure of the upper boundary region, the composite membrane can have further optimized flux and anti-stratification performance.

[0065] Optionally, the ratio of the average pore size of the primary holes measured by SEM to the average pore size of the secondary holes in the lower boundary area is 4-10; the ratio of the average pore size of the primary holes measured by SEM to the average pore size of the secondary holes in the upper boundary area is 3-9.

[0066] By adopting the above technical solution, both the upper and lower boundary regions have a special structure of large holes within small holes. Among them, the larger primary holes determine the looseness of the nylon microporous membrane. In addition to the aforementioned impact on the permeation resistance of the casting solution, it also largely determines the flux and support capacity of the nylon microporous membrane. If the aperture of the primary hole is too large, although the flux of the nylon microporous membrane is greater, it often has a lower support capacity. If the aperture of the primary hole is too small, although the nylon microporous membrane has a better support effect, it cannot guide the slurry to pass through with a larger flux and enter the cellulose layer.

[0067] In addition, although the size of the primary pores largely determines the flux of the microporous membrane, for the entire virus removal membrane, the size of the flux is still determined by the superposition of the resistance of the feed liquid at various locations, and in the interface area, in addition to the nylon fibers with larger fiber sizes, there are also secondary pores with relatively small pore sizes that have greater resistance to the feed liquid. By limiting the pore size ratio of the primary pores and the secondary pores, it can largely reflect the density of the nylon fibers with greater resistance to the feed liquid in the primary pores, and it can also reflect the density of the cellulose fibers with greater resistance to the feed liquid in the secondary pores. Among them, whether it is the upper interface area or the lower interface area, the pore size ratio of the primary pores and the secondary pores is too large, which often means that the primary pores are too large to obtain sufficient support effect and the primary pores have a secondary pore structure with too small pore size. The secondary pores with too small pore size will produce greater resistance to the feed liquid. The ratio of the pore size of the primary pore to the secondary pore is too small, which often means that the primary pore is too small and has a denser nylon fiber, while the pore size of the secondary pore within the primary pore is larger. Although the secondary pore with a larger pore size has less resistance to the feed liquid, the overly loose cellulose fiber structure will lead to insufficient anti-stratification performance of the virus removal membrane, and the denser nylon fiber structure will also form a greater resistance to the feed liquid, resulting in a decrease in the flux of the virus removal membrane.

[0068] Optionally, the average diameter of the cellulose fibers in the lower junction area is not less than 40 nm, and the ratio of the average pore size of the primary pores measured by SEM in the lower junction area to the average diameter of the cellulose fibers is 10~25; the average diameter of the cellulose fibers in the upper junction area is not less than 60 nm, and the ratio of the average pore size of the primary pores measured by SEM in the upper junction area to the average diameter of the cellulose fibers is 8~20.

[0069] By adopting this technical solution, the morphology of small pores and fine fibers in the lower interface region, through a dense three-dimensional network skeleton structure, can significantly improve the composite membrane's anti-delamination performance. However, if the cellulose fiber diameter in the lower interface region is too small, since the cross-sectional area will be reduced to one-quarter as the cellulose fiber diameter is halved, the fiber's mechanical properties will not decrease linearly but exponentially. Therefore, even if the skeleton structure formed by cellulose fibers with too small a diameter (e.g., less than 40nm) is relatively dense, it will not achieve the required mechanical properties and will still be prone to fiber breakage under external forces, leading to delamination.

[0070] Furthermore, the specific secondary pore structure within the primary pores in this application creates a unique large-pore-within-small-pore structure within the lower interface region, along with a fine fiber structure within the large pores. For the lower interface region, which already has larger primary pores, the presence of the secondary pores combined with the fine fibers reduces the resistance the feed experiences within the secondary pores. Therefore, this unique large-pore-within-small-pore structure within the lower interface region not only imparts excellent anti-stratification properties to the virus removal membrane, but also offers minimal resistance to the feed, resulting in a higher flux.

[0071] Since the upper junction area has a secondary pore structure with a larger pore size than that of the lower junction area, even if there are cellulose fibers with relatively larger diameters (such as not less than 60nm), the larger pore size secondary pore structure and the larger pore size primary pore structure (the ratio of 8 to 20 to the diameter of the cellulose fiber) can ensure that there are sufficient channels for the feed liquid to flow in the upper junction area. The upper junction area has a special structure of large pores within small pores (the small pores are larger than the small pores in the lower junction area) and fine fibers within large pores (the fiber diameter is larger than the fiber diameter in the lower junction area), which can ensure that the upper junction area has less resistance to the feed liquid.

[0072] In addition, the larger secondary pore structure in the upper junction area relative to the lower junction area means that the three-dimensional network skeleton structure formed by the cellulose fibers in the upper junction area is looser, and the looser three-dimensional network skeleton combined with the cellulose fibers with relatively larger diameters can ensure that the cellulose fibers in the upper junction area still have good breaking strength even if the looseness is higher; thereby, the upper junction area closer to the composite interface not only has low material-liquid resistance, but also can further improve the anti-stratification performance of the composite membrane.

[0073] Optionally, the filter membrane has a capacity of not less than 200 L / m 2 The peel strength retention rate of the filter membrane is not less than 0.8@(30psi, 2h).

[0074] By adopting the above technical solution, the load capacity of the filter membrane largely reflects the service life of the filter membrane. Although the filter membrane in this application is a composite membrane, it has a higher load capacity while ensuring high anti-stratification performance.

[0075] Currently, the common method for evaluating the delamination resistance of composite membranes is to test their 180° peel strength. However, the 180° peel strength only reflects the delamination resistance of the composite membrane in its initial state. Virus removal membranes are subjected to continuous pressure, a process that is likely accompanied by changes in the morphology and structure of the composite membrane's interface (e.g., cellulose fibers with excessively fine diameters in the interface region may break after prolonged pressure). These changes in the morphology and structure of the composite membrane's interface are likely to lead to changes in the composite membrane's flux and virus retention capacity. Therefore, limiting the peel strength retention rate of the composite membrane after prolonged exposure to liquid pressure is a better indicator of its performance in actual applications.

[0076] In this application, by limiting the peel strength retention rate of the filter membrane after being treated with a liquid at a pressure of 30 psi for 2 hours to no less than 0.8, it can be ensured that the filter membrane has stable and good anti-delamination performance and flux in actual application.

[0077] In a second aspect, the present application provides a virus removal membrane assembly, which adopts the following technical solution:

[0078] A virus removal membrane assembly comprises any one of the aforementioned filter membranes, wherein the number of filter membranes is 1 to 3 layers, and the retention capacity of the membrane assembly for PP7 bacteriophage reaches LRV>4.

[0079] Optionally, the membrane assembly includes one layer of the aforementioned filter membrane, and the retention capacity for PP7 phage reaches LRV>5; preferably, the retention capacity for PP7 phage of the membrane assembly reaches LRV>6.

[0080] Optionally, the membrane assembly comprises two layers of the aforementioned filter membrane, and the retention capacity for PP7 bacteriophage reaches LRV>6; preferably, the retention capacity for PP7 bacteriophage of the membrane assembly reaches LRV>7.

[0081] In a third aspect, the present application provides a preparation process for a supported composite cellulose virus removal filter membrane, which adopts the following technical solution:

[0082] A preparation process of a supported composite cellulose virus removal filter membrane comprises the following steps:

[0083] S1. Preparing a casting solution, wherein the casting solution comprises at least a cellulose polymer, a first good solvent, and a first non-solvent, wherein the mass ratio of the first non-solvent is 2-4%, the solid content of the casting solution is 10-25%, the viscosity is 5000-20000 cps, and the first non-solvent is a small molecule alcohol;

[0084] S2. Casting: Casting the casting liquid onto the carrier to form a liquid membrane, and compounding the nylon microporous membrane on the liquid membrane at a compounding speed of 0.5-2.5 m / s. Then, the carrier is placed in a negative pressure environment with an air pressure of 0.7-0.85 bar for 15-30 seconds to obtain a composite semi-finished membrane;

[0085] S3, curing to form a film, immersing the carrier in a curing bath, curing for at least 5 minutes to obtain a green film, wherein the curing bath comprises a second good solvent and a second non-solvent, and the second good solvent accounts for 5-15 wt% of the curing bath;

[0086] S4, cellulose regeneration, placing the raw membrane in a regeneration bath to hydrolyze and regenerate the raw membrane to obtain a cellulose virus-removing membrane.

[0087] By adopting the above technical solution, the current membrane preparation process of the composite membrane with a support layer generally involves directly coating the casting liquid on the microporous membrane support layer, and then obtaining the desired composite membrane through a series of post-processing. This film-making process mainly relies on the gravity of the casting liquid itself to allow the casting liquid to partially penetrate into the substrate, and there is no other driving force to drive the casting liquid to penetrate into the substrate. In fact, since the substrate is located on the carrier side and the liquid casting liquid is located on the upper surface of the substrate, even if an external force is applied, it will first act on the casting liquid. Under the action of external forces, the liquid casting liquid is likely to produce unexpected defects, etc., so this film-making process has high difficulty in applying external driving force; in addition, when the casting liquid penetrates into the substrate, it will inevitably squeeze the air in the pore structure of the substrate and force the air to be discharged, which obviously increases the difficulty of penetration of the casting liquid. Moreover, since the lower surface of the substrate is in direct contact with the carrier, the pore structure of the substrate is largely shielded, and the air in the substrate is difficult to be discharged from the lower surface of the substrate. If the air is discharged upward, it needs to penetrate the casting liquid with higher viscosity. If the air in the substrate does not completely penetrate the casting liquid and remains in the liquid film, it will likely cause the cellulose layer after phase separation and solidification to have large pore defects caused by bubbles.

[0088] In this application, a film-making process of single-layer casting + microporous membrane back cover is specifically adopted, that is, the casting liquid is first cast on the carrier, and then the nylon microporous membrane is covered on the liquid film that has been cast. Compared with the process of coating the casting liquid on the microporous membrane substrate, the process of the microporous membrane back cover allows the casting liquid to penetrate into the microporous membrane, and the air in the microporous membrane can be simply discharged upward. On the one hand, it is not easy to form resistance to the penetration of the casting liquid, and on the other hand, it is not easy to leave bubbles in the casting liquid, thereby causing defects. In addition, the specific microporous membrane back cover process of this application makes it possible to further improve the permeability of the casting liquid in the nylon microporous membrane. The microporous membrane back cover process is combined with the negative pressure treatment after the nylon microporous membrane is compounded, which can promote the casting liquid to penetrate into the nylon microporous membrane and is not easy to introduce defects in the cellulose layer. This is because when a liquid membrane composited with a nylon microporous membrane is placed in a negative pressure environment (negative pressure refers to an ambient pressure less than atmospheric pressure), the negative pressure first acts on the nylon microporous membrane, extracting air from within. The resulting negative pressure then draws the casting liquid back into the membrane, creating a permeable zone of the desired thickness. Furthermore, the liquid membrane is located on the carrier side, making it less susceptible to defects caused by overall stress, which could lead to instability in the liquid membrane.

[0089] To ensure that the cellulose layer formed after phase separation and solidification of the casting solution has good virus retention, the solid content of the casting solution must be controlled. If the solid content of the casting solution is less than 10%, the cellulose layer obtained by phase separation and solidification is likely to be too loose. Although the composite membrane has a higher flux, the risk of virus leakage is too high. If the solid content of the casting solution is higher than 25%, the higher solid content casting solution often produces a denser cellulose layer after phase separation and solidification. Although the risk of virus leakage is lower, the flux of the composite membrane will decrease, resulting in reduced filtration efficiency. By controlling the solid content of the casting solution to 10-25% and using a mixed solvent containing a good solvent and a non-solvent as the curing bath during the curing and membrane formation stage to regulate the phase separation rate of the casting solution (the addition of a good solvent can slightly delay the gel phase separation of the casting solution), the casting solution in the permeation zone can phase separate at a slower rate, reducing the possibility of forming a dense pore structure in the permeation zone, thereby reducing the feed liquid resistance in the permeation zone and achieving composite membranes with both high flux and high anti-delamination properties.

[0090] In addition, in order to promote the penetration of the casting liquid into the nylon microporous membrane and obtain the required penetration zone that can improve the anti-delamination performance of the composite membrane, the casting liquid of the present application is added with a mass ratio of 2~4% of small molecule alcohol. The small molecule alcohol has a low surface tension, and thus can reduce the surface tension of the casting liquid to a certain extent, and improve the wettability of the casting liquid and the nylon microporous membrane. Therefore, the amount of small molecule alcohol added to the casting liquid should not be less than 2%; however, small molecule alcohol, as a non-solvent for cellulose polymer, will increase the viscosity of the casting liquid after being added to the casting liquid, resulting in excessive penetration resistance of the casting liquid, making it difficult to form a penetration zone of sufficient thickness. Therefore, the amount of small molecule alcohol added to the casting liquid should not be higher than 4%. By controlling the addition amount of small molecule alcohol to 2-4% and controlling the solid content of the casting liquid to 10-25%, the viscosity of the casting liquid can be controlled to 5000-20000 cps (at 25°C) so that the permeation resistance of the casting liquid is not too large, the permeation area of ​​the required thickness is obtained, and it can also ensure that the prepared composite membrane has good virus retention performance.

[0091] In summary, the present application uses a specific microporous membrane back cover process combined with a specific casting liquid added with small molecule alcohols, a negative pressure treatment process, and the addition of a good solvent to the curing bath to control the phase separation rate. This not only ensures that the casting liquid with higher viscosity can penetrate well into the nylon microporous membrane to form a permeation zone of the required thickness, but also that the pore structure in the permeation zone is not too dense, thereby affecting the flux.

[0092] Optionally, before immersing the composite semi-finished film in the curing bath, the composite semi-finished film is pre-gelated. Specifically, the composite semi-finished film is allowed to stand for 20 to 30 seconds and then immersed in a pre-gel bath. The pre-gel time is 20 to 30 seconds to obtain a pre-gel film. The pre-gel bath includes at least a third good solvent and a third non-solvent, and the proportion of the third non-solvent is 5 to 20 wt%.

[0093] By adopting the above technical solution, the present application further introduces a pre-gel bath to pre-treat the composite semi-finished membrane, and controls the proportion of non-solvent in the pre-gel bath to 5-20% (i.e., the proportion of good solvent is high, and the effect of promoting gel phase separation is poor). The pre-gel bath can dilute the casting liquid in the permeation zone, and the casting liquid in the permeation zone can form a solid content gradient change in the thickness direction (because the closer the mass transfer process in the pre-gel bath is to the composite interface, the greater the resistance, and therefore, the closer the permeation zone is to the composite interface, the higher the solid content). Combined with the specific use of a curing bath with a good solvent proportion of 5-15%, a clear large-pore-in-small-pore structure can be formed in the permeation zone. The presence of secondary pores greatly reduces the resistance of the permeation zone to the feed liquid, so that the permeation zone can not only improve the anti-stratification performance of the composite membrane, but also has little impact on the flux.

[0094] It should be noted that it is more difficult to open pores on the surface of cellulose raw materials (this is related to its own characteristics. The phase separation speed in the solidification liquid is faster, and the rapid phase separation easily leads to a small pore structure with a higher density). The preparation process of coating the casting liquid on the surface means that even if a certain pretreatment method is used to pretreat the casting liquid, since the microporous membrane support layer is located on the carrier side, it is difficult for the pretreatment system to penetrate the microporous membrane support layer whose pores are blocked. The pretreatment system needs to penetrate the entire layer of casting liquid to pretreat the junction of the casting liquid and the microporous membrane support layer, which is not realistic. Therefore, even if pretreatment is performed, a larger pore structure can only be obtained on the side of the casting liquid away from the microporous membrane support layer, which makes it easier to form a permeation zone with a higher density, resulting in a decrease in flux and load. Therefore, the pregel treatment in this application needs to be combined with the microporous membrane back cover process to obtain a further preferred secondary pore structure. It can be understood that even without pre-gel treatment, the present application regulates the phase separation rate of the permeation zone by controlling the ratio of the solidification bath. Although the permeation zone does not have a preferred secondary pore structure, it still has a higher flux than the general composite membrane because the pore structure is not dense.

[0095] Optionally, the temperature of the pregel bath is 50-70° C., a penetrant having a surface tension not higher than 20 dyne / cm is further added to the pregel bath, and the concentration of the penetrant in the pregel bath is 1-3 wt %.

[0096] Optionally, the penetrant is at least one of hexafluoroisopropanol or trifluoroethanol.

[0097] By adopting the above technical solution, by adding a small amount of low surface tension penetrant to the pre-gel bath and controlling the temperature of the pre-gel bath to a relatively high temperature of 50-70°C, the penetration capacity of the pre-gel bath can be improved, the pre-gel bath has a further preferred pretreatment effect, and the secondary pore structure in the permeation zone is more stable.

[0098] Optionally, in step S2, the nylon microporous membrane is firstly temperature-controlled in an environment of 50-70° C. for 1-3 minutes before being compounded with the liquid membrane.

[0099] By adopting the above technical solution, the higher the temperature of the casting liquid, the lower the viscosity. On the basis of the solid content being constant and the addition of small molecule alcohol to the casting liquid to improve the wetting properties of the casting liquid, appropriately reducing the viscosity of the casting liquid can further promote the casting liquid to penetrate into the nylon membrane. However, if the viscosity of the casting liquid as a whole is too low, the dimensional stability of the liquid film cast on the carrier is poor, and therefore, it is not appropriate to heat the entire liquid film to a higher temperature. Based on the aforementioned problems, in the present application, a specific method is used in which the nylon microporous membrane is subjected to a temperature adjustment treatment, and a higher temperature is provided before the nylon microporous membrane is composited. Therefore, after the nylon microporous membrane is composited to the liquid membrane, the temperature at the composite interface of the liquid membrane and the nylon microporous membrane is appropriately increased, and the casting liquid viscosity at the composite interface is reduced. The casting liquid having better wetting properties is thus added with small molecule alcohol, and the penetration resistance of the casting liquid can be further reduced, thereby obtaining the required permeation zone.

[0100] Optionally, the cellulose polymer is at least one of cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose acetate phthalate, cellulose acetate butyrate, and cellulose acetate propionate;

[0101] The first good solvent and the second good solvent are at least one of acetone, dioxane, dimethylacetamide, N-methylpyrrolidone, acetic acid, propionic acid, butyric acid and valeric acid;

[0102] The small molecule alcohol is at least one of ethanol, 1-propanol, isopropanol, n-butanol, and isobutanol.

[0103] In summary, this application includes at least one of the following beneficial technical effects:

[0104] The present application specifically uses a nylon microporous membrane as a support for the cellulose layer, which can significantly improve the pressure resistance of the composite membrane. Since the nylon microporous membrane has good solvent resistance, it is not easily softened by the organic solvent in the cellulose casting solution, and thus it is not easy to form a local dense structure. In addition, by controlling the nylon microporous membrane to have an appropriate pore area ratio and surface tension, it is ensured that the cellulose casting solution can properly penetrate into the nylon microporous membrane, forming a permeation zone with a thickness of 10 to 50 μm. This permeation zone can greatly improve the integrity of the cellulose layer and the nylon microporous membrane, reducing the possibility of peeling between the two, thereby making the composite membrane have both high peel strength and high flux, which are difficult to obtain at the same time.

[0105] The present application unexpectedly discovered that by using a nylon microporous membrane with a zeta potential of -2mV~-40mV, the bonding strength between the nylon microporous membrane and the cellulose material in the casting solution can be improved, thereby improving the anti-delamination performance of the composite membrane;

[0106] The present application further introduces a secondary pore structure in the permeation zone to form a special large-pore-in-small-pore structure, forming a cellulose skeleton structure of the secondary pores and an internal entanglement of the primary pores, so that the composite membrane has a higher anti-stratification performance. Due to the existence of the secondary pores, the permeation zone has a large number of passages for the flow of feed liquid, so that the feed liquid resistance in the permeation zone is relatively low. Therefore, the composite membrane not only has a higher anti-stratification performance, but also has a higher flux. BRIEF DESCRIPTION OF THE DRAWINGS

[0107] Figure 1 This is a scanning electron microscope image of the entire cross-section of the composite membrane of Example 1 of the present application. The upper layer in the image is a nylon microporous membrane, the lower layer is a cellulose layer, and the magnification in the image is 500×.

[0108] Figure 2 This is a scanning electron microscope image of the liquid inlet surface of the composite membrane of Example 1 of the present application, and the magnification in the image is 5000×.

[0109] Figure 3 This is a further magnified scanning electron microscope image of the liquid inlet surface of the composite membrane of Example 1 of the present application, and the magnification in the image is 20000×.

[0110] Figure 4 A scanning electron microscope image of a cross section of the nylon microporous membrane near the liquid inlet surface in the composite membrane of Example 1 of the present application, with a magnification of 10000×.

[0111] Figure 5 A scanning electron microscope image of a cross section of the nylon microporous membrane in the composite membrane of Example 1 of the present application along the middle of the thickness direction, with a magnification of 10000×.

[0112] Figure 6 A scanning electron microscope image of a cross section of the nylon microporous membrane in the composite membrane of Example 1 of the present application, close to the permeation zone, with a magnification of 10000×.

[0113] Figure 7 A cross-sectional scanning electron microscope image of the permeation region of the composite membrane of Example 1 of the present application, with a magnification of 5000×.

[0114] Figure 8 The cross-section of the permeation zone in the composite membrane of Example 1 of the present application is further enlarged by a scanning electron microscope image, and the magnification in the image is 10000×.

[0115] Figure 9The cross section of the permeation zone in the composite membrane of Example 1 of the present application is further enlarged in a scanning electron microscope image, and the magnification in the image is 20,000×. DETAILED DESCRIPTION

[0116] The following is combined with Figures 1 to 9 This application is described in further detail.

[0117] The embodiments of the present application provide a composite cellulose virus removal filter membrane, a preparation process thereof, and a virus removal membrane assembly. Example 1

[0118] This embodiment discloses a preparation process of a composite cellulose virus removal filter membrane, comprising the following process steps:

[0119] S1. Preparation of a casting solution. The casting solution is prepared from a cellulose polymer, a first good solvent, and a first non-solvent. After preparation, the solution is degassed to obtain the casting solution. The casting solution comprises 3% by weight of the first non-solvent, 18% by weight of the cellulose polymer (i.e., a solids content of 18%), and a viscosity of 13,000 cps. The first non-solvent is a low-molecule alcohol, which is ethanol. In this embodiment, the cellulose polymer is cellulose diacetate, and the first good solvent is dimethylacetamide.

[0120] S2. Casting: The casting solution is cast onto a support (a steel strip in this example) to form a liquid membrane. A nylon microporous membrane is then laminated onto the liquid membrane. Prior to lamination, the membrane is conditioned at 60°C for 2 minutes at a lamination speed of 1.5 m / s. The support is then placed in a negative pressure environment at 0.78 bar for 20 seconds to produce a composite semi-finished membrane. The composite semi-finished membrane is then allowed to rest for 25 seconds before being immersed in a pregel bath composed of 15 wt% of a third non-solvent, 2 wt% of a penetrant, and 83 wt% of a third good solvent. The pregel bath temperature is 60°C, and the pregel treatment lasts for 25 seconds. After pretreatment, a pregel membrane is obtained. In this example, the third non-solvent is water, the third good solvent is dimethylacetamide, and the penetrant is trifluoroethanol. The morphological parameters and other parameters of the nylon microporous membrane used in this example are detailed in Table 1.

[0121] S3, curing to form a film, immersing the carrier in a curing bath for 7 minutes to obtain a green film, wherein the curing bath includes a second good solvent and a second non-solvent, and the second good solvent accounts for 10 wt % of the curing bath. In this embodiment, the second good solvent is dimethylacetamide, and the second non-solvent is water;

[0122] S4, cellulose regeneration, placing the raw membrane in a regeneration bath to hydrolyze and regenerate the raw membrane to obtain a cellulose virus-removing membrane; in this embodiment, the regeneration bath uses a sodium hydroxide aqueous solution with a concentration of 0.1 mol / L and a temperature of 40°C, and the raw membrane is hydrolyzed and regenerated in the regeneration bath for 80 minutes. Example 2

[0123] The main difference between Example 2 and Example 1 is that the same nylon microporous membrane is used in this example, but it is reversed based on Example 1 and the casting solution formula and various process parameters are adjusted. The various morphological parameters and other options of the nylon microporous membrane used in this example are detailed in Table 1. The differences in the casting solution formula and the adjustments to the various process parameters are detailed in Table 2. Example 3

[0124] Example 3 differs primarily from Example 1 in that a higher-density nylon microporous membrane (i.e., smaller pore size) is used in this example, and the casting solution formulation and various process parameters are adjusted. The various morphological parameters and other options for the nylon microporous membrane used in this example are detailed in Table 1. The differences in the casting solution formulation and the adjustments to the various process parameters are detailed in Table 2. Example 4

[0125] Example 4 differs primarily from Example 1 in that a lower-density nylon microporous membrane (i.e., larger pore size) is used in this example, and the casting solution formulation and various process parameters are adjusted. The various morphological parameters and other options for the nylon microporous membrane used in this example are detailed in Table 1. The differences in the casting solution formulation and the adjustments to the various process parameters are detailed in Table 2. Example 5

[0126] The main difference between Example 5 and Example 1 is that this example uses a denser nylon microporous membrane than Example 3, and the liquid inlet surface of the nylon microporous membrane has liquid inlet fibers with a lower average diameter and liquid inlet holes with a smaller average pore size as measured by SEM. In addition, this example does not perform temperature adjustment on the nylon microporous membrane before lamination, that is, the nylon microporous membrane is at room temperature during lamination, and the formula of the casting solution and various process parameters are adjusted. The various morphological parameters and other options of the nylon microporous membrane used in this example are detailed in Table 1, and the differences in the casting solution formula and the adjustments to the various process parameters are detailed in Table 2. Example 6

[0127] The main difference between Example 6 and Example 1 is that this example uses a nylon microporous membrane with a lower density than that of Example 4. The liquid inlet surface of the nylon microporous membrane has liquid inlet fibers with a higher average diameter and liquid inlet holes with a larger average pore size as measured by SEM. The casting solution formula and various process parameters are adjusted. The various morphological parameters and other options of the nylon microporous membrane used in this example are detailed in Table 1. The differences in the casting solution formula and the adjustments to the various process parameters are detailed in Table 2. Example 7

[0128] Example 7 differs primarily from Example 1 in that a nylon microporous membrane with a lower zeta potential is selected (the larger the absolute value of the negative number, the lower the zeta potential), and adjustments are made to the casting solution formulation and various process parameters. The various morphological parameters and other options for the nylon microporous membrane used in this example are detailed in Table 1. The differences in the casting solution formulation and adjustments to the various process parameters are detailed in Table 2. Example 8

[0129] Example 8 differs from Example 1 primarily in that no pre-gel treatment was performed, and the casting solution formulation and various process parameters were adjusted. The various morphological parameters and other options for the nylon microporous membrane used in this example are detailed in Table 1, and the differences in the casting solution formulation and adjustments to the various process parameters are detailed in Table 2.

[0130] Comparative Example

[0131] Comparative Example 1

[0132] The main difference between Comparative Example 1 and Example 1 is that the microporous support layer in this comparative example uses a PVDF microporous membrane instead of a nylon microporous membrane. In addition, this comparative example adopts a preparation process of casting a membrane casting liquid on a PVDF microporous membrane, and does not adopt a preparation process of a microporous membrane back cover. The specific process steps are as follows:

[0133] S1. Preparation of a casting solution. The casting solution is prepared from a cellulose polymer, a first good solvent, and a first non-solvent. After preparation, the solution is degassed to obtain the casting solution. The casting solution comprises 3% by weight of the first non-solvent, 18% by weight of the cellulose polymer (i.e., a solids content of 18%), and a viscosity of 13,000 cps. The first non-solvent is a low-molecule alcohol, which is ethanol. In this embodiment, the cellulose polymer is cellulose diacetate, and the first good solvent is dimethylacetamide.

[0134] S2, casting, first place the PVDF microporous membrane on the carrier and place the carrier loaded with the PVDF microporous membrane in an environment of 70°C for 3 minutes, then cast the casting liquid onto the PVDF microporous membrane at a casting speed of 1.5m / s to obtain a PVDF microporous membrane loaded with a liquid membrane, that is, a composite semi-finished membrane. The composite semi-finished membrane is then placed in a pregel bath, which is prepared by 5wt% water, 2wt% trifluoroethanol and 93wt% dimethylacetamide. The temperature of the pregel bath is 65°C, and the pregel treatment time is 25s. After the pretreatment is completed, a pregel membrane is obtained.

[0135] S3, solidifying and forming a film, immersing the carrier into a curing bath, the curing treatment time is 7 minutes, and a green film is obtained. The curing bath is prepared by water and dimethylacetamide, and the proportion of dimethylacetamide in the curing bath is 10wt%.

[0136] S4, cellulose regeneration, placing the raw membrane in a regeneration bath to hydrolyze and regenerate the raw membrane to obtain a cellulose virus-removing membrane; in this embodiment, the regeneration bath uses a sodium hydroxide aqueous solution with a concentration of 0.1 mol / L and a temperature of 40°C, and the raw membrane is hydrolyzed and regenerated in the regeneration bath for 80 minutes.

[0137] Comparative Example 2

[0138] Comparative Example 2 differs primarily from Example 1 in that this comparative example uses a nylon microporous membrane with excessive density and does not undergo negative pressure treatment after laminating the nylon microporous membrane to the liquid membrane. Furthermore, the pre-gel bath in this comparative example uses a mixture of pure benign solvent and penetrant, without the addition of a non-solvent. The casting solution formulation and various process parameters were adjusted. The various morphological parameters and other options for the nylon microporous membrane used in this comparative example are detailed in Table 1. The differences in the casting solution formulation and the adjustments to the various process parameters are detailed in Table 2.

[0139] Comparative Example 3

[0140] Comparative Example 3 differs from Example 1 primarily in that this comparative example uses a nylon microporous membrane with an excessively low density and undergoes excessive negative pressure treatment after laminating the nylon microporous membrane to the liquid membrane. Furthermore, this comparative example does not perform a pre-gel treatment, and the casting solution formulation and various process parameters are adjusted. The various morphological parameters and other options for the nylon microporous membrane used in this comparative example are detailed in Table 1. The differences in the casting solution formulation and adjustments to the various process parameters are detailed in Table 2.

[0141]

[0142]

[0143] Table 1 Morphological parameters of the microporous membranes used in various examples and comparative examples

[0144] Table 2 Casting solution ratios and process parameters for each embodiment and comparative example

[0145]

[0146]

[0147] Performance testing and performance data

[0148] Viral challenge test

[0149] The virus challenge test method is carried out in accordance with the relevant provisions of PDA TR41, with the model virus being PP7 phage or hepatitis B virus, the model protein being IVIG, and the buffer being PBS. During the test, the changes in flux and load over time are recorded to obtain the LRV, flux, and load of the filter membrane.

[0150] Anti-delamination performance

[0151] The unused composite films obtained in each embodiment and comparative example were tested for 180° peel strength using a universal tensile tester, and the test result was recorded as B0. Subsequently, the composite films treated with a slurry pressure of 30 psi for 2 h were tested for 180° peel strength, and the test result was recorded as B1. The peel strength retention rate was B1 / B0.

[0152] The morphological parameters of the composite membranes in the various embodiments and comparative examples are detailed in Table 3; the virus retention performance and anti-delamination performance of the composite membranes in the various embodiments and comparative examples are detailed in Table 4.

[0153] Table 3 Morphological parameters of the composite films in various embodiments and comparative examples

[0154]

[0155] Table 4 Virus retention performance and anti-delamination performance of the composite membranes in the examples and comparative examples

[0156]

[0157] in conclusion

[0158] By comparing the technical solutions of Examples 1 to 4 and the various performance parameters in the above table, it is not difficult to find that on the basis of selecting a nylon microporous membrane as the substrate, by controlling the morphological parameters of the nylon microporous membrane (such as the liquid inlet pore area ratio, surface tension, etc.), the final composite membrane can not only have high anti-delamination performance, but also have good flux and loading capacity.

[0159] By comparing the technical solutions of Example 1 and Examples 5-6 and the various performance parameters in the above table, it is not difficult to find that even if a non-preferred nylon microporous membrane is selected (such as the nylon microporous membrane with higher density in Example 5 and the nylon microporous membrane with lower density in Example 6), it still has higher anti-delamination performance, flux and loading capacity.

[0160] By comparing the technical solutions of Example 1 and Example 7 and the various performance parameters in the above table, it is not difficult to find that although Example 7 has a significantly larger permeation zone thickness than Example 1, the anti-delamination performance of the two is similar. This may be because the nylon microporous membrane used in Example 7 has a smaller zeta potential, which generates a larger electrostatic force between it and the casting liquid, resulting in a decrease in anti-delamination performance, thereby having anti-delamination performance similar to that of Example 1 with a significantly lower permeation zone thickness.

[0161] Comparing the technical solutions of Example 1 and Example 8 and the various performance parameters in the table above, it is readily apparent that, even without the pre-gel treatment, which reduces the secondary pore diameter and reduces the flux and loading capacity of the composite membrane, it still exhibits relatively high flux, loading capacity, and anti-delamination performance. In other words, while the solution in Example 8 is not a preferred solution, it still exhibits relatively good overall performance.

[0162] By comparing the technical solutions of Example 1 and Comparative Example 1 and the various performance parameters in the above table, it is not difficult to find that, compared with Example 1 using a nylon microporous membrane, Comparative Example 1 using a PVDF microporous membrane and not using a microporous membrane back cover process has better anti-stratification performance, but its flux and load performance are poor, and it has basically no practical value. This may be because the good solvent in the casting solution is not only a good solvent for diacetate fiber, but also a good solvent for PVDF, which can soften and even partially dissolve the PVDF microporous membrane, resulting in a cellulose-PVDF mixed casting solution with a higher solid content at the composite interface. After phase separation and solidification, a pore structure with a higher density will be produced. Although this can improve the bonding strength between the cellulose layer and the PVDF microporous membrane layer, it has a greater impact on the flux and load of the composite membrane. In addition, a certain concentration of alkaline solution is required for hydrolysis and regeneration, and PVDF is not alkali-resistant, which may also destroy the mechanical properties and permeability of the PVDF micropores.

[0163] By comparing the technical solutions of Example 1 and Comparative Examples 2-3 and the various performance parameters in the above table, it is not difficult to find that even if a nylon microporous membrane is used as the substrate for the composite membrane, if a nylon microporous membrane with specific morphology and physicochemical parameters is not selected, a composite membrane with both high anti-delamination performance and high flux and loading capacity cannot be obtained. For example, in Comparative Example 2, the nylon microporous membrane has an excessively low liquid inlet pore area ratio and surface tension, resulting in a composite membrane with an excessively thin permeation zone (less than 10 μm, only 5 μm). As a result, the anti-delamination performance is unsatisfactory (its strength retention rate is 0, meaning that it exhibits significant delamination at 30 psi), making it unsuitable for practical application. For example, in Comparative Example 3, the nylon microporous membrane has an excessively high liquid inlet pore area ratio and surface tension, allowing the casting solution to easily wet and penetrate the nylon microporous membrane, forming an excessively thick permeation zone (greater than 50 μm, reaching 68 μm). Although the resulting composite membrane has good anti-delamination performance, its flux and loading capacity are both low, making it unsuitable for practical application.

[0164] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A composite cellulose virus removal filter membrane, characterized in that: The invention comprises a nylon microporous membrane and a cellulose layer at least partially permeated into the nylon microporous membrane, wherein the side of the nylon microporous membrane away from the cellulose layer is a liquid inlet surface, the side of the cellulose layer away from the nylon microporous membrane is a liquid outlet surface, and the area of ​​the nylon microporous membrane where the cellulose layer permeates is a permeation zone; The thickness of the permeable zone is 10 to 50 μm; The liquid inlet surface includes liquid inlet fibers and liquid inlet holes, the liquid inlet fibers are mutually surrounded to form the liquid inlet holes, the hole area ratio of the liquid inlet holes is 15-55%, the surface tension of the liquid inlet surface is 42-58 dyne / cm, and the thickness of the nylon microporous membrane is 40-120 μm; The flux of the filter membrane is not less than 40L / h / m 2 @30psi.

2. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: The average diameter of the liquid-inlet fiber is 150-450 nm, and the average pore size of the liquid-inlet hole measured by SEM is 700-2000 nm.

3. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: In the liquid inlet surface, the liquid inlet fibers having a diameter not less than 1.3 times the average diameter are coarse fibers, and the proportion of coarse fibers in the liquid inlet fibers does not exceed 25%.

4. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: The connection points of adjacent liquid-influent fibers are connection points. The average diameter of the connection points is greater than the average diameter of the liquid-influent fibers. On average, 2.5 to 4.5 liquid-influent fibers are connected to each connection point.

5. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: The zeta potential of the liquid inlet surface at a pH of 7 is -2mV to -40mV.

6. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: In the liquid inlet surface, the liquid inlet holes with an SEM-measured pore size not less than 1.3 times the SEM-measured average pore size are large pores, and the liquid inlet holes with an SEM-measured pore size not greater than 0.5 times the SEM-measured average pore size are small pores. Among all the liquid inlet holes, the number of large pores accounts for 10-30%, and the proportion of small pores does not exceed 25%.

7. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: The liquid inlet surface is taken as the position where the thickness of the nylon microporous membrane is 0, and the surface away from the liquid inlet surface is taken as the position where the thickness of the nylon microporous membrane is 1. The pore size variation coefficient X of the nylon microporous membrane is ≤ 0.4, and X is calculated by the following formula: ； In the above formula, D 0.1 D1 is the average pore size measured by SEM within the 0~0.1mm thickness range of the nylon microporous membrane, D2 is the average pore size measured by SEM within the 0.9~1mm thickness range of the nylon microporous membrane, and D 均 D 0.1 and the average value of D1.

8. The composite cellulose virus removal filter membrane according to claim 7, characterized in that: The ratio of the average fiber diameter in the nylon microporous membrane thickness range of 0~0.1 to the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 is 0.7~1.3; the ratio of the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 to the average fiber diameter in the nylon microporous membrane thickness range of 0.9~1 is 0.7~1.

3.

9. The composite cellulose virus removal filter membrane according to claim 7, characterized in that: The difference between the average fiber diameter in the nylon microporous membrane thickness range of 0~0.1 and the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 is no more than 50nm; the difference between the average fiber diameter in the nylon microporous membrane thickness range of 0.4~0.5 and the average fiber diameter in the nylon microporous membrane thickness range of 0.9~1 is no more than 50nm.

10. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: The permeation zone includes nylon fibers and cellulose fibers, the nylon fibers surround each other to form primary pores, the cellulose fibers are located in the primary pores, the cellulose fibers surround each other and separate the primary pores into several secondary pores, and the average pore size of the secondary pores measured by SEM is smaller than the average pore size of the primary pores measured by SEM.

11. The composite cellulose virus removal filter membrane according to claim 10, characterized in that: The area of ​​the permeation zone with a thickness of 10 μm close to the cellulose layer is the junction area, the area of ​​the junction area with a thickness of 5 μm close to the liquid inlet surface is the upper junction area, and the area of ​​the junction area with a thickness of 5 μm close to the liquid outlet surface is the lower junction area. The average pore size of the secondary pores in the permeation zone measured by SEM is larger than the average pore size of the secondary pores in the junction area measured by SEM; the average diameter of the cellulose fibers in the junction area is not less than 50 nm.

12. The composite cellulose virus removal filter membrane according to claim 11, characterized in that: The average pore size of the secondary pores in the lower boundary region measured by SEM is 100~200nm; the average pore size of the secondary pores in the upper boundary region measured by SEM is 120~220nm.

13. The composite cellulose virus removal filter membrane according to claim 11, characterized in that: The ratio of the average pore size of the primary pores measured by SEM to that of the secondary pores in the lower boundary area is 4-10; the ratio of the average pore size of the primary pores measured by SEM to that of the secondary pores in the upper boundary area is 3-9.

14. The composite cellulose virus removal filter membrane according to claim 11, characterized in that: The average diameter of the cellulose fibers in the lower junction area is not less than 40 nm, and the ratio of the average pore size of the primary pores measured by SEM in the lower junction area to the average diameter of the cellulose fibers is 10-25; the average diameter of the cellulose fibers in the upper junction area is not less than 60 nm, and the ratio of the average pore size of the primary pores measured by SEM in the upper junction area to the average diameter of the cellulose fibers is 8-20.

15. The composite cellulose virus removal filter membrane according to claim 1, characterized in that: The filter membrane has a capacity of not less than 200 L / m 2 The peel strength retention rate of the filter membrane is not less than 0.8@(30psi, 2h).

16. A virus removal membrane assembly, characterized in that: The filter membrane comprises the filter membrane according to any one of claims 1 to 15, wherein the number of the filter membranes is 1 to 3 layers, and the retention capacity of the membrane assembly for PP7 bacteriophage reaches LRV>4.

17. The process for preparing the composite cellulose virus removal filter membrane according to any one of claims 1 to 15, characterized in that: The following steps are involved: S1. Preparing a casting solution, wherein the casting solution comprises at least a cellulose polymer, a first good solvent, and a first non-solvent, wherein the mass ratio of the first non-solvent is 2-4%, the solid content of the casting solution is 10-25%, the viscosity is 5000-20000 cps, and the first non-solvent is a small molecule alcohol; S2. Casting: Casting the casting liquid onto the carrier to form a liquid membrane, and compounding the nylon microporous membrane on the liquid membrane at a compounding speed of 0.5-2.5 m / s. Then, the carrier is placed in a negative pressure environment with an air pressure of 0.7-0.85 bar for 15-30 seconds to obtain a composite semi-finished membrane; S3, curing to form a film, immersing the carrier in a curing bath, curing for at least 5 minutes to obtain a green film, wherein the curing bath comprises a second good solvent and a second non-solvent, and the second good solvent accounts for 5-15 wt% of the curing bath; S4, cellulose regeneration, placing the raw membrane in a regeneration bath to hydrolyze and regenerate the raw membrane to obtain a cellulose virus-removing membrane.

18. The process for preparing the composite cellulose virus removal filter membrane according to claim 17, characterized in that: Before immersing the composite semi-finished film in a curing bath, the composite semi-finished film is pre-gelated. Specifically, the composite semi-finished film is allowed to stand for 20 to 30 seconds and then immersed in a pre-gel bath for 20 to 30 seconds to obtain a pre-gel film. The pre-gel bath includes at least a third good solvent and a third non-solvent, and the proportion of the third non-solvent is 5 to 20 wt%.

19. The process for preparing the composite cellulose virus removal filter membrane according to claim 18, characterized in that: The temperature of the pregel bath is 50-70° C., and a penetrant with a surface tension not higher than 20 dyne / cm is added to the pregel bath. The concentration of the penetrant in the pregel bath is 1-3 wt %.

20. The process for preparing the composite cellulose virus removal filter membrane according to claim 17, wherein: In the step S2, the nylon microporous membrane is firstly temperature-controlled in an environment of 50-70° C. for 1-3 minutes before being compounded with the liquid membrane.

21. The process for preparing the composite cellulose virus removal filter membrane according to claim 17, wherein: The cellulose polymer is at least one of cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose acetate phthalate, cellulose acetate butyrate, and cellulose acetate propionate; The first good solvent and the second good solvent are at least one of acetone, dioxane, dimethylacetamide, N-methylpyrrolidone, acetic acid, propionic acid, butyric acid and valeric acid; The small molecule alcohol is at least one of ethanol, 1-propanol, isopropanol, n-butanol, and isobutanol.

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