CN115569528A

CN115569528A - Asymmetric hydrophilic PVDF (polyvinylidene fluoride) filter membrane for virus removal, preparation process thereof and membrane filter

Info

Publication number: CN115569528A
Application number: CN202211214499.7A
Authority: CN
Inventors: 贾建东; 卢红星
Original assignee: Hangzhou Cobetter Filtration Equipment Co Ltd
Current assignee: Hangzhou Cobetter Filtration Equipment Co Ltd
Priority date: 2022-09-30
Filing date: 2022-09-30
Publication date: 2023-01-06

Abstract

The application relates to an asymmetric hydrophilic PVDF filter membrane for virus removal, a preparation process thereof and a membrane filter. The filter membrane comprises a porous main body, wherein a non-directional tortuous passage is formed in the porous main body, the porous main body comprises a liquid inlet surface and a liquid outlet surface, the porous main body is in transition by continuous fibers, the SEM measurement average pore size of the porous main body is gradually reduced from the liquid inlet surface to the liquid outlet surface, the porous main body comprises a pre-filtering layer and a separating layer used for intercepting colloidal gold with the diameter of 20nm, the separating layer comprises the liquid outlet surface, the SEM measurement average pore size of the liquid outlet surface is 60-160 nm, and the thickness of the separating layer is 15-45 mu m. The application also discloses a preparation process of the filter membrane, and further discloses a membrane filter comprising the filter membrane. The separation layer of the filter membrane has larger pore diameter and thickness, the high interception effect on small-size viruses is ensured through a longer tortuous path, and the protein adsorption rate of the separation layer with larger average pore diameter is greatly reduced, so that higher virus interception effect and protein yield are obtained.

Description

Asymmetric hydrophilic PVDF (polyvinylidene fluoride) filter membrane for virus removal, preparation process thereof and membrane filter

Technical Field

The application relates to the field of membrane separation technology, in particular to an asymmetric hydrophilic PVDF filter membrane for virus removal, a preparation process thereof and a membrane filter.

Background

The membrane separation technology is a separation technology having a separation membrane as a core and using external pressure, concentration difference, or the like as a driving force, and is capable of separating, concentrating, purifying, or the like components in a feed liquid. Compared with the conventional separation technology, the membrane separation technology has high separation efficiency, does not need additional reagents in the separation process, and can separate systems which cannot be separated by the conventional separation technology. More importantly, the membrane separation technology is a pure physical separation technology with mild separation conditions, and in the field of biomedicine, the characteristic makes the separation process not easy to cause the denaturation of active substances in the feed liquid, and the production cost of the active substances is high, so the membrane separation technology is particularly suitable for the separation process in the field of biomedicine.

The production process of various biological agents is very complex, and the procedures of culture, purification, cleaning, passivation, extraction, freezing, freeze-drying and the like are usually required, but the introduction of various viruses is difficult to avoid in the procedures, and once the viruses are injected into a patient along with the biological agents, the consequences are unimaginable. Therefore, the virus safety of biological agents is definitely required in the relevant documents such as the Chinese pharmacopoeia of the new edition, the Q5A virus safety evaluation of biological products issued by the International harmonization Institute (ICH) for the registration of drugs for human use, and the like. In the case of drug application, a virus safety evaluation test result report of the biological agent must be attached to prove the virus safety of the biological agent. The virus removal necessity of biological agents is combined with the characteristic that the membrane separation technology is not easy to cause the denaturation of active substances when viruses are filtered, so that the membrane separation technology is widely applied to biological medicine enterprises.

At present, common membrane making materials of the virus-removing filter membrane comprise cellulose, PES, PVDF and the like, and the PVDF (polyvinylidene fluoride) is a semi-crystalline polymer and has good mechanical property, weather resistance and chemical stability, so that the PVDF is one of the membrane making materials of the mainstream ultrafiltration membrane and the microfiltration membrane at present. Compared with cellulose filter membranes, PVDF filter membranes tend to have better mechanical properties, and the easy shrinkage of cellulose filter membranes after drying makes them necessary to be stored and transported in a wet state, which is costly and cumbersome. Compared with PES (polyether sulfone) filter membranes, PVDF (polyvinylidene fluoride) filter membranes have higher protein yield, because PVDF materials subjected to hydrophilic modification have better hydrophilicity and lower protein adsorption rate compared with PES filter membranes subjected to hydrophilic modification, which is an important advantage for biopharmaceutical enterprises.

For example, japanese patent application laid-open No. JP1984204911A discloses a regenerated cellulose film (RC film) which has a good ability to remove AIDS virus (about 100 nm), but has a poor ability to remove viruses of smaller sizes such as hepatitis B virus (about 42 nm), nAnB type hepatitis virus (30 to 60 nm) and murine parvovirus (about 20 nm) having a size of 20 to 100nm, and has failed to satisfy the current stringent virus removal requirements. In addition, the complicated storage and transportation conditions of cellulose filter membranes limit the development and application of the cellulose filter membranes.

For example, chinese patent application publication No. CN113842792A discloses an asymmetric PES filter membrane for virus removal, which comprises a main body including a pre-filter layer and a separation layer for trapping viruses, wherein the other side of the pre-filter layer and the other side of the separation layer are transited by continuous fibers. The PES membrane filters larger particles of substances through a large-aperture pre-filtering layer, and further intercepts viruses through a small-aperture separating layer, so that the PES membrane has good virus intercepting capability, and the log removal rate of the viruses can reach more than 4. However, the poor hydrophilicity of PES material determines that the protein yield is often low, and the production cost of protein is very high, so that the low protein yield is an unacceptable defect for biological medicine enterprises.

For example, chinese patent No. CN105980037B discloses a virus-removing membrane comprising a hydrophilized synthetic polymer for removing viruses from a protein-containing solution, the virus-removing membrane having: and a second side surface from which a permeated liquid that has permeated through the virus-removed membrane is discharged, wherein in a cross section of the virus-removed membrane in a wet state, a portion that captures colloidal gold having a diameter of 20nm is a region that is 25% or more and 85% or less of the thickness of the membrane from the first side, and a portion that captures colloidal gold having a diameter of 15nm is a region that is 60% or more and 100% or less of the thickness of the membrane from the first side, and does not capture colloidal gold having a diameter of 10nm. The filter membrane has a constant reduced pore size and a most dense layer near the second side to produce good rejection of colloidal gold of 20nm or even 15nm diameter. However, although the filter does not capture 10nm gold colloid, the logarithmic removal rate of the filter with respect to 10nm gold colloid still reaches an LRV of 0.03 to 0.09 (i.e., the yield of 10nm gold colloid is about 81 to 93%, as described in the table of fig. 5), and since 10nm gold colloid is mainly used for the purpose of characterizing the retention effect of the filter with respect to proteins (igG proteins, about 10 nm), the yield of 10nm gold colloid of only 81 to 93% means that the yield of igG proteins is also low. It is believed that the yield of igG protein is lower than that of 10nm colloidal gold, because PVDF has a certain hydrophobicity even after hydrophilic modification, and binds to the region with smaller pore size of the filter membrane, so that it can strongly adsorb protein, resulting in a decrease in protein yield. In addition, the filter membrane is prepared by adopting a pure TIPS method, the intrinsic characteristic of the pure TIPS method determines that the filter membrane is always low in porosity, and for feed liquid with high protein concentration, the filter membrane is easily blocked due to the high protein retention rate and the high protein concentration, so that the flux and the service life are reduced.

Based on the foregoing problems, it is an urgent problem to obtain a filter membrane having a high virus retention effect for small-sized viruses and a high protein yield.

Disclosure of Invention

The asymmetric hydrophilic PVDF filter membrane for removing the viruses, the preparation process thereof and the membrane filter are provided, a separation layer of the filter membrane has larger average pore size and thickness, a high interception effect on the small-size viruses is ensured through a longer tortuous path, and the adsorption rate of the separation layer with the larger average pore size on the proteins is greatly reduced, so that a higher virus interception effect and a higher protein yield are obtained.

The application provides an asymmetric hydrophilic PVDF filter membrane for virus removal and a preparation process thereof, and a membrane filter adopts the following technical scheme:

in a first aspect, the present application provides an asymmetric hydrophilic PVDF filter membrane for virus removal, which adopts the following technical scheme:

the utility model provides a remove virus with asymmetric hydrophilic PVDF filter membrane, contains porous main part, the unoriented tortuous passageway has in the porous main part, one side that porous main part is close to the supply feed liquid is the feed liquor face, one side that the supply feed liquid was kept away from to porous main part is for going out the liquid face, porous main part uses continuous fibers transition in the membrane thickness direction, the SEM measurement average pore size of porous main part by the feed liquor is towards going out the liquid face and reducing gradually, porous main part includes prefilter layer and is used for intercepting the separating layer of diameter 20nm colloidal gold, the separating layer includes it is 60 ~ 160nm to go out the liquid face, the thickness of separating layer is 15 ~ 45 mu m.

Preferably, the protein yield of the filter membrane is not less than 95%. More preferably, the protein yield of the filter is not less than 98%.

By adopting the technical scheme, although the hydrophilicity of the hydrophilic modified PVDF filter membrane is superior to that of the commercialized hydrophilic modified PES filter membrane, the hydrophilicity of the hydrophilic modified PVDF filter membrane is still not the same as that of a cellulose filter membrane, so that the hydrophilic PVDF filter membrane still can adsorb proteins, and the protein yield is reduced. In addition, after the filter membrane adsorbs protein, the pore structure of the filter membrane is easily blocked, and thus the flux of the filter membrane is rapidly reduced. For the biomedical enterprise, the high production cost of protein actives makes low protein yields unacceptable, and the impact of rapid flux loss on production efficiency is also to be avoided.

On the basis of the above, for virus removal membranes, in order to reduce the risk of virus leakage, the filtration capability of the filter membrane for viruses needs to be improved, and the filtration capability of the filter membrane for small-sized viruses becomes more important as the virus risk control requirement is improved. However, the size of small size viruses and the size of proteins have been relatively close, e.g., the diameter of a typical parvoviral murine parvovirus is about 20nm, and the diameter of a typical small size viral PP7 bacteriophage is about 27nm; whereas typical proteins such as igG (immunoglobulin) have a diameter of about 10nm. Therefore, if the filtering capability of the filter membrane for small-size viruses is higher, the filtering capability for proteins is also higher, and the yield of the proteins is reduced; the PVDF combined with the hydrophilic modification still has certain hydrophobicity and has certain adsorption effect on proteins, so that the improvement of the virus filtering capacity of the PVDF filter membrane is usually at the cost of the reduction of the protein yield.

In view of the foregoing problems, the inventors of the present application have unexpectedly found that, for a hydrophilic PVDF filter membrane, when the filter membrane has an asymmetric structure (the average pore size measured by SEM at the inlet side is the largest, and the average pore size measured by SEM at the outlet side is the smallest), the pore size of the outlet side with the smallest average pore size measured by SEM is larger, and is about 60 to 160nm; the thickness of the filter membrane separation layer is larger and is about 15-45 mu m; the filter membrane not only has higher interception effect on small-size viruses, but also can obtain higher protein yield. This is in contrast to general knowledge that for PVDF filters, in order to efficiently retain small size viruses around 20nm, the pore size of the pore structure of the filter should be small, and at least the pore size at the exit level with the smallest pores should not be larger than 40nm. If the average pore diameter of the SEM of the liquid outlet surface of the filter membrane is larger than 40nm and even reaches 60-160 nm (the pore diameter of the separation layer should be larger due to the gradient change of the pore diameter of the filter membrane), the interception efficiency of the filter membrane to small-size viruses is often poor, and the actual use requirements cannot be met. That is, the filters in this application have high viral retention and high protein yield, which are generally considered difficult to achieve simultaneously with PVDF filters, are quite unexpected results.

This is probably because, after hydrophilic treatment, PVDF material has better hydrophilic effect (better than hydrophilic modified PES material), but still has certain hydrophobicity, so the pore walls of the tortuous path inside the filter membrane still have certain adsorption capacity for protein in the feed liquid (this is still only about 81-93% of the protein yield after hydrophilic modification of PVDF filter membrane in CN 105980037B), thereby making it very difficult to improve the protein yield.

Generally, the actual pore size of the virus-removing filter is larger than the size of the protein, and therefore, the filter does not have a significant pore size sieving effect on the smaller protein, and even if the filter has a tortuous path inside the filter, the sieving retention of the protein is still small. However, in order to obtain efficient retention of small size viruses (colloidal gold), the filter must have a region with a smaller average pore size as measured by SEM, as in CN105980037B, which is capable of retaining colloidal gold even at 15nm, which is already very similar to that of proteins. Although these SEM measurements show that the smaller average pore size regions are still larger than the protein size, these smaller pore size regions are more likely to produce significant adsorption of the protein, resulting in a decrease in protein yield. That is, for the hydrophilic PVDF filter, the reduction of the filter pore size can improve the virus retention effect, but as the pore size is further reduced, the adsorption of proteins by the pore structure with smaller filter pore size may be rapidly increased, resulting in a rapid decrease in protein yield.

When the effluent surface with the smallest average pore diameter measured by the filter membrane SEM also has a larger SEM measured average pore diameter (60-160 nm), the pore diameter of the pore structure in the filter membrane is larger, so that the protein screening effect is not easy to generate, and the adsorption capacity for the protein is also obviously reduced, and the protein can be adsorbed on the pore wall only if the adsorption capacity of the filter membrane for the protein is more than a certain threshold value, therefore, the adsorption capacity is reduced due to the increase of the average pore diameter measured by the effluent surface SEM, the protein yield is obviously improved to reach 95% or even more than 98%, which is a great improvement for the current common PVDF filter membrane.

It should be noted that, for the hydrophilic PVDF filter having a certain protein adsorption rate, the protein yield is increased closer to 100% and the difficulty is increased, which is not a linear increase, because the adsorption of the protein by the filter is inevitably present. For example, the difficulty of increasing the protein yield from 80% to 90% is relatively low, the difficulty of increasing from 90% to 95% is sharply increased, the difficulty of increasing from 95% to 98% is further increased, and when the protein yield reaches more than 98%, the difficulty of increasing is doubled.

On the basis of high protein yield, the thickness of a separation layer with virus interception capacity is increased, the path of a tortuous passage in the separation layer is prolonged, a plurality of layers of pore structures with larger sizes are continuously stacked on the thickness of the membrane, and the size of the misaligned part of the stacked pore structures is far smaller than the average pore size measured by SEM (the solid part of the lower layer can form a barrier to the pore structure of the upper layer so as to prevent feed liquid and various particulate matters in the feed liquid from passing through, so that the size of an effective passage for viruses to pass through is often smaller than the actual size of the pores of the filter membrane). The effective path of the feed liquid formed by the laminated pore structure is far smaller than the average aperture measured by SEM, so that the thickness of the separation layer is increased, the aperture of the separation layer is properly increased, the path of the laminated pore structure is further increased, the low retention rate of the laminated pore structure on protein is ensured, the high retention rate on viruses with slightly larger sizes is ensured, and the effect is that the aperture of the effective path after lamination and the length of the path after lamination are simultaneously increased, so that the high protein yield and the high virus retention rate are simultaneously obtained. This is quite unexpected, unlike the generally accepted inability of large pore hydrophilic PVDF filters to reject small size viruses.

Certainly, the larger the SEM measurement average pore diameter of the liquid outlet surface is, the better the SEM measurement average pore diameter of the liquid outlet surface is, and along with the improvement of the SEM measurement average pore diameter of the liquid outlet surface, on one hand, the whole SEM measurement average pore diameter of the filter membrane is also improved, and even if the path of the tortuous path is prolonged and the multi-layer pore structures are continuously superposed on the membrane thickness, the overlarge SEM measurement average pore diameter still cannot ensure that the size of the laminated effective path is small enough to intercept small-size viruses, and the filtering effect on the small-size viruses cannot be ensured; the further improvement of the average pore diameter measured by the SEM has a marginal decreasing effect on the influence of the adsorption force of the protein, so the further improvement of the average pore diameter measured by the SEM has no obvious effect on the improvement of the yield of the protein; on the other hand, as the average pore diameter of the filter membrane measured by SEM is increased, the mechanical strength of the filter membrane is likely to be reduced, and the filter membrane is subjected to a large pressure of a filtered liquid in use, so that the internal pore structure of the filter membrane is likely to be deformed due to compression deformation, and even the filter membrane is likely to be blocked or cracked. Therefore, for a specific hydrophilic PVDF filter membrane, on the basis of comprehensively considering the protein yield and the retention effect for small-size viruses, the average pore diameter measured by SEM of the effluent surface of the filter membrane is preferably 60-160 nm, which is quite different from the common technical route that the average pore diameter measured by SEM of the effluent surface of the filter membrane is controlled to be less than 40nm in order to obtain high virus retention rate, and is also quite unexpected.

In the present application, the definition of the separation layer is a region capable of retaining 20nm colloidal gold, and the ability to retain 20nm colloidal gold means that the prepared filter membrane retains 20nm colloidal gold, and the distribution of 20nm colloidal gold in the membrane thickness direction is measured, and the measurement of the distribution result can be performed according to the test method in the membrane for removing viruses of chinese patent CN 105980038B-: slicing the filter membrane after trapping the colloidal gold, and determining the brightness distribution of a plurality of sites of a part stained by the colloidal gold in an interface of the slice; the colloidal gold is opaque, so the displacement value of the brightness can be used for representing the capture amount of the colloidal gold. The background noise can be removed from the luminance distribution as necessary. Then, a graph in which the horizontal axis shows the film thickness and the vertical axis shows the luminance shift is prepared; this resulted in a region where 20nm colloidal gold was cut off in the film thickness direction, and the schematic view is shown in FIG. 6. Furthermore, by observing the SEM image of the thick cross section of the filter membrane after the 20nm colloidal gold rejection experiment, it can also be seen that the separation layer area of the filter membrane is significantly blocked by 20nm colloidal gold, resulting in the disappearance of the pore structure, and the schematic diagram is detailed in FIG. 7.

For various reasons (such as adsorption and blind hole trapping), even the region such as the prefilter layer is likely to have a very small amount of gold remaining, but the region cannot be considered to have an effective trapping for 20nm gold. Therefore, in the present application, when the shift value of the luminance in the film thickness direction is measured, the luminance shift spectrum in which the ratio of the maximum shift peak, which is the region where the cutoff amount of the 20nm colloidal gold is the maximum, to the maximum shift peak of the spectrum is less than 10%, is considered to be only a small amount of residue or error of the colloidal gold, and is not considered to be the region where the 20nm colloidal gold is actually cut off, is obtained by the difference between the luminance constant 255 and the measured luminance distribution.

That is, in the direction along the film thickness, although there are some regions where a small amount of 20nm colloidal gold is retained, the amount of 20nm colloidal gold retained by these regions is extremely small, and it is not considered as a region where 20nm colloidal gold is truly retained, and it is merely an error or a small amount of residue; therefore, the separation layer in the filter should be a large number of regions that trap 20nm colloidal gold that are continuous in the film thickness direction.

It should be understood that the term "non-directional tortuous path" as used herein refers to a structure having randomly oriented grooves and/or discretely distributed holes in the porous body, and these non-directional tortuous paths are interconnected, so that the feed liquid can penetrate the filter membrane through the interconnected paths, and viruses, large particulate matters, etc. in the feed liquid are trapped in the liquid inlet surface of the filter membrane or the non-directional tortuous path inside the porous body, thereby achieving the effect of filtering viruses.

The term "continuous fiber transition" as used herein means that all the fibers of the porous body in the thickness direction of the membrane are integrally formed, all the fibers are integrally connected to each other, and the fibers are not bonded to each other by using an additional adhesive or the like, and the fibers in the form of a three-dimensional network are not separated from each other unless they are torn off by an external force. At the same time, the continuous transition three-dimensional network-like fibers are interconnected with the first outer surface and the second outer surface.

The parameters of the pore diameter, the thickness of the layer structure, the fiber diameter and the like in the application refer to the average value calculated after the film structure is subjected to morphology characterization by using a scanning electron microscope and then is measured by using computer software (such as Matlab, NIS-Elements and the like) or manually, and the average value is not considered for the part with obviously smaller or larger size in the measurement process. It should be added that the porosity measurement method can also be calculated by computer software (such as Matlab, NIS-Elements, etc.), or the porosity of the membrane can be measured by gravimetric method. In the aspect of the test for measuring the average pore diameter by SEM, the average pore diameter can be measured by directly analyzing each layer SEM by SEM measurement average pore diameter distributor, measuring the average pore diameter by bubble pressure test SEM, and the like, in addition to the measurement analysis by SEM image. The above methods for measuring the parameters are only examples, and it is understood that the parameters can be obtained by other measuring means by those skilled in the art.

Optionally, the separation layer comprises separation fibers, the separation fibers are connected with each other to form a three-dimensional network structure of the separation layer, and the average diameter of the separation fibers measured by SEM is 30-50 nm; the pre-filter layer comprises supporting fibers, the supporting fibers are connected with one another to form a three-dimensional network structure of the pre-filter layer, and the average diameter of the supporting fibers measured by SEM is 30-55 nm.

By adopting the technical scheme, the average pore diameter of the SEM measurement of the liquid outlet surface is larger, and correspondingly, the average pore diameter of the SEM measurement of the separation layer and the pre-filtering layer is also larger. For the separation layer, if the average diameter of the separation fiber measured by SEM is too small, it is likely to result in insufficient self-supporting property of the separation layer, collapse of pore structure upon external pressure, and loss of virus-retaining ability of the separation layer (either of crushing and clogging or breaking of the separation layer) upon collapse of the pore structure of the separation layer.

Furthermore, the primary entrapment of proteins and viruses in the filter is the separation layer, while the primary adsorption of proteins in the separation layer is their solid portion. The separation fibers of the solid part in the separation layer are mutually crossed and connected, so that a three-dimensional network structure with a pore structure is formed, and the separation fibers are the pore walls of the pore structure in the separation layer. When the pore structure of the separation layer is not deformed, the distance between the separation fibers is longer, the pore diameter of the formed pore structure is larger, and the threshold value of protein adsorption is not easily reached, so that the adsorption of the separation layer on the protein is reduced, and the protein yield is improved. Once the pore structure of the separation layer collapses, the distance between the separation fibers is greatly shortened to form a pore structure with a smaller pore diameter, and even if the separation layer still has a good virus interception effect, the protein yield is also greatly reduced.

If the average diameter of the separation fibers measured by SEM is too large, although the pore structure of the separation layer can be better supported, when the feed liquid flows through the filter membrane, the feed liquid is mainly subjected to resistance of the solid part, and the average diameter of the separation fibers measured by SEM is too large, which may cause the feed liquid to be subjected to too large resistance, so that the flux of the filter membrane is reduced. Therefore, the SEM measurement of the mean diameter of the separated fibers should not be too large or too small.

The pre-filter layer has a lower rejection rate for both viruses and proteins than the separation layer due to the larger SEM-measured average pore size; however, the larger size of the pore structure of the pre-filter layer also means that the mechanical properties of the pre-filter layer are poorer, and the pre-filter layer is easily deformed under pressure as a region of the filter membrane directly contacting with the feed liquid. Once the pre-filtering layer deforms due to compression, the pore structure of the pre-filtering layer is likely to collapse, so that the flux and the loading capacity of the filter membrane are reduced; because the distance between the supporting fibers is reduced after the pore structure is collapsed, more remarkable interception and adsorption to protein are formed, and the adsorption force reaches the threshold value of protein adsorption, the protein yield of the filter membrane is reduced, therefore, the average diameter of SEM measurement of the supporting fibers is not too small, and the pore structure of the pre-filter layer is supported better. However, the average diameter of the support fiber measured by SEM is not too large, because too large average diameter of the support fiber measured by SEM usually means that the resistance of the pre-filter layer to feed liquid is increased, resulting in the decrease of flux; in addition, the average diameter measured by SEM of the supporting fiber means that the proportion of the solid part of the pre-filtering layer is higher, the proportion of the pore structure for intercepting and containing large-particle substances is naturally reduced, and as the filter membrane in the application has the separating layer with larger thickness, once the large-particle substances leak from the pre-filtering layer, the large-particle substances are likely to be screened and intercepted by the separating layer or intercepted by the tortuous path with longer path, so that the blockage of the separating layer is caused, and the rapid reduction of the flux and the reduction of the loading capacity of the filter membrane are caused. Thus, the SEM measured average diameter of the support fibers should also not be too large or too small.

Alternatively, the ratio of the SEM measured average diameter of the supporting fibers to the SEM measured average diameter of the separating fibers is 0.8 to 1.5.

By adopting the above technical scheme, it is generally considered that there is a positive correlation between the size of the pore structure of the filter membrane and the size of the fiber structure constituting the filter membrane, and therefore, the pre-filter layer with larger average pore size measured by SEM should have a fiber structure with larger size, and the separation layer with smaller average pore size measured by SEM should have a fiber structure with smaller size, that is, the pore size of the filter membrane is asymmetrically similar to that of the filter membrane, and the fiber diameter of the filter membrane should also be asymmetrically distributed. However, the inventors of the present application have surprisingly found that the filter membranes of the present application have substantially the same fiber size in the direction of the membrane thickness, and the difference in fiber size is small, i.e. the SEM-measured average diameter of the pre-filter layer supporting fibers is substantially the same as the SEM-measured average diameter of the separation layer separating fibers, and do not exhibit a significant asymmetric structure. This may be one of the reasons for the high protein yield of the filter membrane, and the fiber size is gradually reduced from the pre-filtering layer to the separation layer compared with the common filter membrane, so as to form a more obvious asymmetric structure; the fiber size of the filter membrane in this application does not vary significantly from the pre-filter layer to the separation layer, and therefore the separation layer in this application has a larger-sized fiber structure.

Since the finer the fibers, the larger the specific surface area, the higher the adsorption rate for proteins, the separation layer composed of the separation fibers having a larger average diameter as measured by SEM of the present application had a lower protein adsorption rate, resulting in a further increase in the protein yield of the filtration membrane. In addition, the specific separation layer structure with large thickness and large pore diameter in the application provides higher requirements for the self-supporting performance of the separation layer, and the separation fibers with larger average diameter measured by SEM can form a three-dimensional network structure with a thicker framework and stronger self-supporting capability, so that the mechanical strength of the separation layer and even the filter membrane is improved, and the pressure resistance of the filter membrane is further improved.

Optionally, the separation fiber has an average length of 80 to 150nm as measured by SEM, and the support fiber has an average length of 120 to 180nm as measured by SEM.

By adopting the technical scheme, the pore structure of the separation layer is mainly formed by the separation fibers, and the SEM measurement average length of the separation fibers can represent the pore structure size of the separation layer to a certain extent; likewise, the pore structure of the pre-filter layer is primarily formed by the support fibers, and SEM measurements of the average length of the support fibers can also characterize the pore structure size of the pre-filter layer to some extent. Thus, the SEM measured average lengths of the separation fibers and the SEM measured average lengths of the support fibers are both capable of characterizing the pore structure of the filter membrane to some extent.

In addition, it should be noted that, because the average diameters of the separation fibers and the support fibers measured by SEM are slightly different, and the average lengths of the separation fibers and the support fibers measured by SEM are also slightly different, the aspect ratio of the separation fibers and the aspect ratio of the support fibers are not greatly different, which means that the self-supporting performance of the filter membrane in the membrane thickness direction is slightly changed, there are no obvious weak points in the mechanical properties at various positions in the membrane thickness direction, the filter membrane is not easily deformed by the weak points when being subjected to external pressure, so as to obtain higher pressure resistance, and the separation fibers and the support fibers are not easily compressed to be close by pressure, so that the pore structure is collapsed, and the possibility of the increase of the protein adsorption force due to the collapse of the pore structure is reduced, so as to obtain higher protein yield.

Optionally, the ratio of the SEM-measured average pore diameter of the separation layer to the SEM-measured average diameter of the separation fibers is 2 to 7; the ratio of the SEM-measured average pore size of the pre-filter layer to the SEM-measured average diameter of the support fibers is 3.5 to 8.

Preferably, the ratio of the SEM-measured average pore diameter of the separation layer to the SEM-measured average diameter of the separation fiber is 3 to 5; the ratio of the SEM-measured average pore size of the pre-filter layer to the SEM-measured average diameter of the support fibers is 4.5 to 6.5.

By adopting the technical scheme, the average pore diameter measured by SEM (scanning electron microscope) is larger, and the virus removal filter membrane is subjected to larger pressure (30 psi or even 50 psi) of feed liquid when in use, so that the separation layer and the pre-filter layer both need stronger supporting force to ensure the dimensional stability of the pore structure when being subjected to larger pressure. As the fiber structure plays a main supporting role for the pore structure, the ratio of the size of the pore structure to the size of the fibers forming the pore structure represents the pressure resistance of the pore structure to a great extent. This is because larger sized void structures tend to require greater support forces, while greater support forces tend to require thicker fiber structures, and thus larger sized void structures tend to require thicker fiber structures.

In the case of the separation layer, if the ratio of the SEM-measured average pore diameter of the separation layer to the SEM-measured average diameter of the separation fiber is too high, the pore structure of the separation layer may not be supported well and may be easily deformed by pressure, and since the separation layer mainly affects the virus-rejection effect and the protein yield, the pore structure of the separation layer may not have sufficient pressure resistance, the virus-rejection effect of the filter membrane and the protein yield may be easily decreased. If the ratio of the average pore diameter measured by SEM of the separation layer to the average diameter measured by SEM of the separation fibers is too low, the solid part of the separation layer accounts for too high, and when the feed liquid flows in the separation layer, the feed liquid is subjected to larger resistance of the solid part, so that the flux is easily reduced greatly, and the filtration efficiency is influenced.

For the pre-filtering layer, if the ratio of the average pore diameter measured by SEM of the pre-filtering layer to the average diameter measured by SEM of the supporting fibers is too high, the pore structure of the pre-filtering layer cannot be supported well, and once the pore structure of the pre-filtering layer collapses, the interception capability of large particulate matters in the feed liquid is lost, and the filter membrane is easily blocked. If the ratio of the SEM-measured mean pore size of the pre-filter layer to the SEM-measured mean diameter of the support fibers is too low, it is also indicated that the solid portion of the pre-filter layer has too high a ratio, resulting in insufficient loading of the pre-filter layer, failure to trap and hold large particulate matter in the feed solution, and a decrease in flux.

Optionally, the ratio of the SEM-measured average pore diameter of the separation layer to the SEM-measured average length of the separation fiber is 1.1 to 1.4; the ratio of the SEM measured average pore size of the pre-filter layer to the SEM measured average length of the support fibers is 1.2 to 1.7.

By adopting the technical scheme, the pore structure is mainly formed by the fiber structure in a surrounding way, and for the pore structure, the more the number of fibers forming the pores is, the shorter the length of the fibers is (the outer edge of the pores can be considered as a polygonal structure, and on the premise that the size of the pores is not changed, the more the polygonal edges forming the pores are, the shorter the length of the edges is), the larger the ratio of the size of the pore structure to the length of the fibers is, correspondingly, the more the pore structure is close to a circle, and the larger the flux of the filter membrane is due to the smaller resistance of the fluid in the circular channel; in addition, for the pores having the same area, the circumference of the circle is smaller than that of the polygon, and therefore, the closer the pore structure is to the circle, the less the feed liquid contacts the pore wall, the lower the protein adsorption rate, and the higher the protein yield of the filter membrane. Therefore, the ratio of pore size to fiber length of the pore structure, whether it be a separation layer or a prefilter layer, should not be too low to achieve higher flux and protein yield.

However, the ratio of the pore diameter of the pore structure to the fiber length is also not suitable to be too high, whether the separation layer or the pre-filtration layer is the separation layer or the pre-filtration layer, which may be because if the ratio of the pore diameter of the pore structure to the fiber length is too high, it indicates that the number of fibers constituting the pores is large, although the pore structure is closer to a circle, the fiber proportion of the solid portion is often increased, and when the feed liquid flows through the filtration membrane, although the resistance of the pores closer to a circle to the feed liquid is reduced, the feed liquid is subjected to the larger resistance of the fiber structure of the solid portion, and the flux is reduced instead. Thus, the ratio of pore size to fiber length of the pore structure, whether it be a separation layer or a pre-filter layer, needs to be controlled within a reasonable range, which is about 1.1 to 1.4 for the separation layer and about 1.2 to 1.7 for the pre-filter layer.

Optionally, the average pore diameter of the liquid inlet surface measured by SEM is 200 to 500nm, the ratio of the average pore diameter of the liquid inlet surface measured by SEM to the average pore diameter of the liquid outlet surface measured by SEM is 1.2 to 6, and the specific surface area of the porous body is 6 to 12m ² /g。

By adopting the technical scheme, the asymmetry degree of the filter membrane in the membrane thickness direction is represented to a great extent by the ratio of the SEM measured average pore diameter of the liquid inlet surface of the filter membrane to the SEM measured average pore diameter of the liquid outlet surface of the filter membrane, and for the filter membrane with a large-thickness and large-pore-diameter separation layer, the influence of the asymmetry degree of the filter membrane in the membrane thickness direction on the performance of the filter membrane is great. If the asymmetry degree of the filter membrane is too high (if the ratio of the SEM measured average pore diameter of the liquid inlet surface to the SEM measured average pore diameter of the liquid outlet surface is more than 5), the size of the pore structure on the side, close to the liquid outlet surface, of the filter membrane is too large; on one hand, on the basis of larger average pore diameter of SEM measurement of the liquid outlet surface of the filter membrane, the interception effect of the filter membrane on small-size viruses is probably not ensured; on the other hand, the whole pore structure of the filter membrane has larger size, and the structure is likely to collapse when a larger external force is applied, so that the virus interception effect of the filter membrane and the protein yield are reduced. If the degree of asymmetry of the filter membrane is too low (e.g., the ratio of the SEM-measured mean pore size at the inlet to the SEM-measured mean pore size at the outlet is less than 1.5), it is likely that the pore structure of the pre-filter layer of the filter membrane will be of a smaller size, which will not ensure the retention and containment of large particulate matter, resulting in clogging of the filter membrane and reduced flux. Therefore, the ratio of the average pore diameter measured by SEM of the liquid inlet surface and the liquid outlet surface of the filter membrane must be strictly controlled between 1.2 and 6, effective interception of virus is formed through a separation layer with small gradient, large pore diameter and large thickness, the adsorption of protein is reduced, and the filter membrane with high virus interception efficiency and high protein yield is obtained.

In addition, because the pore walls of the pore structure mainly adsorb proteins, the specific surface area of the filter membrane can greatly represent the size of the pore walls of the filter membrane. If the specific surface area of the filter membrane is too large, the adhesion of the filter membrane to the protein is improved, and the yield of the protein is reduced; the control of the specific surface area of the filter membrane in a relatively small range can ensure that the filter membrane has high protein yield, and the filter membrane with the low specific surface area has low protein adsorption and is not easy to block, so the flux attenuation is slow, and high flux can be maintained for a long time.

It is to be noted that the specific surface area in the present application is measured by a BET specific surface area test method.

Optionally, the SEM-measured average pore size variation gradient of the separation layer is not greater than 8nm/μm, and the SEM-measured average pore size of the separation layer is 100 to 200nm.

By adopting the technical scheme, the average pore diameter of the liquid level SEM measurement of the filter membrane is larger, and the average pore diameter of the liquid level SEM measurement of the filter membrane is smaller, so that the integral symmetry of the filter membrane is reflected to a certain extent.

On the basis that the average pore diameter measured by the SEM of the separation layer is too large, small-sized viruses still easily penetrate through the laminated pore structure, and the virus interception effect of the filter membrane cannot meet the use requirement; if the average pore diameter of the separation layer is too small as measured by SEM, although good entrapment of small-sized viruses can be achieved, the difference in size between the protein and the small-sized viruses is not large, and the porous structure with a long path and a small pore diameter in which the separation layer is laminated is liable to form excessive entrapment of the protein, resulting in a decrease in the yield of the protein. Therefore, when the average pore diameter measured by SEM is 100-200 nm in combination with the thickness of the separation layer of 15-45 μm, the filter membrane can be ensured to obtain high virus retention rate and protein yield at the same time.

In addition, the SEM-measured average pore diameter of the separation layer is large, and on this basis, the gradient of variation in the average pore diameter of the separation layer as a whole is small and is not more than 8nm/μm, which means that there is no mutation region in the pore diameter in the membrane thickness direction of the separation layer, and the pore diameter mutation region generates extensive entrapment of particles having different particle diameters such as viruses and proteins due to having a dense, rapidly varying laminated pore structure, resulting in a decrease in flux and a decrease in protein yield, and therefore, the separation layer having a small gradient of variation in the average pore diameter as a SEM-measured average pore diameter is almost free of a high adsorption region for proteins in the membrane thickness direction, to ensure a high protein yield of the filter membrane.

It is understood that the SEM-measured gradient of the change in the average pore diameter = (first porous surface SEM-measured average pore diameter-second porous surface SEM-measured average pore diameter)/thickness, the larger the value, the faster the change in the pore diameter, and the smaller the value, the smaller the change in the pore diameter.

Optionally, one side of the separation layer close to the liquid inlet surface is regarded as 0%, the liquid outlet surface is regarded as 100%, the separation layer is divided into ten equal parts along the thickness direction of the separation layer, the ratio of the SEM-measured average pore size of the region where the thickness position of the separation layer is 0-10% to the SEM-measured average pore size of the region where the thickness position of the separation layer is 40-50% is K1, and K1 is not more than 2.2.

By adopting the technical scheme, the pore structure of the separation layer has a large influence on the overall performance of the filter membrane, and if the pore structure of the separation layer has a rapid pore size change region, the rapid pore size change region is positioned above or below the separation layer, which may cause great reduction in the flux of the filter membrane and the protein yield. Therefore, although the gradient of the variation in the mean pore diameter of the separation layer as a whole is small in SEM measurement, if there are a region of rapid variation in the pore diameter and a region of slow variation in the separation layer in the thickness direction of the membrane, the flux of the filter membrane and the protein yield may be greatly reduced although both have a small gradient of the mean pore diameter of SEM measurement in combination.

The region from 0 to 10% of the thickness of the separation layer is regarded as the 10% of the thickness of the separation layer above the separation layer, and the region from 40 to 50% of the thickness of the separation layer is regarded as the 50% of the thickness of the separation layer in the middle, and the ratio K1 of the average pore diameters in SEM measurement of the two can characterize the variation tendency of the average pore diameter in SEM above and in the middle of the separation layer. Since K1 is not more than 2.2, it means that the pore diameter change speed of the upper half of the separation layer is slow, and there is no region where the pore diameter changes rapidly, so as to ensure that there is no high-retardation region for the feed liquid and high-adsorption region for the protein in the upper half of the separation layer. The combination of a small SEM mean pore size variation gradient over the separation layer can greatly reduce the potential for filter flux and protein yield reduction in the separation layer due to the rapidly varying region of pore size.

It will be understood that the side of the membrane adjacent to the inlet side is considered "above" and the side adjacent to the outlet side is considered "below", i.e.upstream in the direction of flow of the feed liquid is "above" and downstream in the direction of flow of the feed liquid is "below".

Optionally, the ratio of the SEM-measured average pore diameter of the region where the thickness position of the separation layer is 40 to 50% to the SEM-measured average pore diameter of the liquid outlet surface is K2, and K2 is not more than 1.5.

By adopting the technical scheme, similarly, the area of 40-50% of the thickness position of the separation layer is regarded as the 50% of the thickness area above the separation layer, the liquid outlet surface is regarded as the 100% of the thickness area in the middle of the separation layer, and the ratio K2 of the average pore diameters measured by SEM of the two can represent the variation trend of the average pore diameters of SEM in the middle and at the bottom of the separation layer. Since K2 is not more than 1.5, it means that the pore diameter change speed of the lower half of the separation layer is slow, and there is no rapid pore diameter change region, and it is reported that the lower half of the separation layer does not have a high-retardation region for the feed liquid and a high-adsorption region for the protein. By combining the small SEM average pore size change gradient of the whole separation layer and the slow pore size change speed of the upper half part of the separation layer, the possibility that the filter membrane flux and the protein yield are reduced due to the existence of the rapid pore size change region in each region of the separation layer can be greatly reduced.

Optionally, K1 is greater than K2, and the ratio of K1 to K2 is 1.05 to 1.6.

By adopting the technical scheme, the K1 is larger than the K2, the ratio of the K1 to the K2 is 1.05-1.6, the change speed of the average pore diameter measured by the SEM of the separation layer is high before low, the integral phase difference is not large, the change gradient of the average pore diameter measured by combining the integral SEM of the separation layer is small, the integral symmetry of the separation layer is high, and the asymmetry of the small gradient is only exists.

In addition, the SEM measurement of the separation layer reduces the average pore diameter at a relatively fast speed, the size of an effective passage formed by the laminated pore structure reduces at a fast speed, so that a good interception effect on small-size viruses can be formed, the filter membrane has a high virus interception rate, and the size of the effective passage formed by the laminated pore structure is still large, so that smaller-size proteins can still pass through smoothly; as the average pore diameter measured by the SEM of the separation layer gradually decreases, the size of the effective passage formed by the stacked pore structure also gradually decreases, and once the size of the effective passage formed by the stacked pore structure is too small, not only the entrapment of the virus but also the entrapment of the protein is likely to occur, resulting in a decrease in the yield of the protein. The slower pore diameter change speed of the lower half of the separation layer means that the symmetry of the lower half of the separation layer is further improved, and the effective path formed by the laminated pore structure has smaller size change, so that the interception of protein is reduced on the basis of ensuring that the virus can be well intercepted. So that the filter membrane has good protein yield and higher virus retention rate.

Optionally, the pre-filter layer comprises the liquid inlet surface, the thickness of the pre-filter layer is 1-15 μm, the SEM-measured average pore size of the pre-filter layer is 150-300 nm, and the SEM-measured average pore size variation gradient of the pre-filter layer is 5-20 nm/μm.

By adopting the technical scheme, the thickness of the pre-filtering layer of the filter membrane is relatively small (compared with a separating layer with larger thickness), and the difference between the SEM measured average pore diameter of the pre-filtering layer and the SEM measured average pore diameter of the separating layer is not large. The reason is that the separation layer in the present application has a large thickness, and when the feed liquid flows through the pore structure of the separation layer with a long path, the feed liquid is subjected to a large resistance by the solid part in the separation layer, which results in a decrease in flux; if the thickness of the pre-filter layer is too large, the flow path of the feed liquid is further extended, the resistance is larger, and the flux may be greatly reduced, so that the thickness of the pre-filter layer should be reduced on the basis of the larger thickness of the separation layer in order to make the filter membrane have higher flux.

On this basis, it is necessary to ensure effective retention of large particulate matter by the pre-filter layer to reduce the potential for clogging of the filter membrane. And the separation layer is great near one side pore structure size of prefiltering layer, equally can be better hold the impurity of bigger granule, that is to say, specific large aperture separation layer has certain prefiltering effect concurrently in this application, can form supplementary and additional action to the prefiltering layer, consequently, even the prefiltering layer of filter membrane is relatively thin in this application, under the cooperative fit of the separation layer of specific big thickness, large aperture, still can form better holding back and do not cause the quick jam of filter membrane to the large granule material.

In addition, the average pore size (150-300 nm) measured by SEM of the pre-filtering layer is not large, and the change gradient (5-20 nm/mum) of the average pore size measured by SEM is relatively large, which shows that the pore size of the pre-filtering layer is relatively fast reduced at the side close to the liquid outlet surface, and the area with the fast reduced pore size is matched with the pore structure (compared with large-particle substances) with the small pore size, so that the large-particle substances can be effectively intercepted; since the pore size of this region is still large (compared to viruses and proteins), there is substantially no entrapment of smaller sized viruses and proteins. And the large particle substances which are not partially trapped by the upper half part of the pre-filtering layer are trapped by the lower half part of the pre-filtering layer with smaller pore size with high probability so as to reduce the influence on the separation layer.

It should be noted that, since the thickness of the pre-filter layer is relatively small, it is inevitably difficult to avoid leakage of a small amount of large particulate matter from the pre-filter layer, the first half of the separation layer having a large pore size (smaller than that of large particulate matter compared to viruses and proteins) and a small variation gradient can trap and hold the large particulate matter leaked from the pre-filter layer, and since the pore size of the side of the separation layer close to the pre-filter layer is large, the loading is large, and the small amount of large particulate matter does not block the separation layer.

Optionally, the flux of the filter membrane is not less than 50 L.h ^-1 ·m ^-2 @30psi, log removal rate of the filter for PP7 phage greater than 2; the tensile strength of the filter membrane is 5-20 MPa.

Optionally, the elongation at break of the filter membrane is 50-200%.

By adopting the technical scheme, the currently internationally accepted standard is that the log removal rate of the membrane filter for small-size viruses (such as typical PP7 bacteriophage, and indicator viruses of small-size viruses in PDA instruction file TR 41) needs to be more than 4, and the number of filter membranes in the current membrane filter is not limited to only 1. Aiming at the membrane filter with only 1 filter membrane, the filter membrane in the application can obtain higher logarithmic removal rate by adjusting parameters such as thickness, pore size and the like of a separation layer, for example, the logarithmic removal rate of PP7 is not less than 4; for a membrane filter with a plurality of filter membranes, the filter membranes in the application can obtain relatively low logarithmic removal rate (the logarithmic removal rate can still be ensured to reach more than 4 by using a plurality of filter membranes in series) and relatively high flux by adjusting parameters such as thickness, pore diameter and the like of a separation layer.

In addition, it is generally considered that the separation layer occupies a larger membrane, and the flux tends to be smaller, and the filtration efficiency is lower, because the resistance of the pore structure with smaller pore diameter to the feed liquid is larger, which results in the flux of the membrane being reduced. However, the specific large pore size, large thickness separation layer of the present application incorporates a pre-filter layer structure of relatively small thickness, such that the filter membrane has a relatively high flux even with a relatively large thickness separation layer. This is probably because, although the separation layer has a large thickness, the average pore diameter variation gradient (not more than 8 nm/. Mu.m) and the relatively stable pore diameter variation gradient (the ratio of K1 to K2 is 1.05 to 1.6) were measured in combination with the SEM having a small thickness of the whole separation layer, indicating that there is no pore diameter abrupt region having a large influence on the flux in the separation layer, and therefore, the flow rate of the feed liquid in each region of the separation layer is relatively stable, and there is no "bottleneck region" having a large influence on the flux. In combination with the porous structure of the separation layer, which has a large pore size, although the stacked porous structure can form a good retention of viruses, the effective path formed by the stacked porous structure is relatively small for the feed liquid itself. Since the whole large-pore separation layer has a small retardation with respect to the feed liquid and a region having a rapidly changing pore diameter, which significantly affects the flux, does not exist in the separation layer, the high flux is obtained even if the separation layer has a large thickness.

In addition, the thickness of the separation layer of the filter membrane is high, and the fiber structure is hardly reduced along with the reduction of the pore diameter of the pore structure of the separation layer, so that the separation layer has a three-dimensional network structure with a thick framework and good self-supporting performance, and the pressure resistance of the separation layer and the filter membrane is good. The PVDF filter membrane has high tensile strength (5-20 MPa, the tensile strength of the common PVDF filter membrane is about 3.5-8 MPa), good elongation at break and good mechanical properties, and is not easy to generate mechanical damage in the membrane preparation process or the subsequent storage, transportation and use processes, so that the virus leakage risk is reduced.

It should be noted that, when the membrane filter is assembled to the membrane filter, membrane damage is likely to occur due to mechanical collision, and since the thickness of the separation layer of the membrane filter is large in the present application, even if a small amount of damage is generated on the surface of the separation layer, the separation layer area with large thickness can retain viruses, and therefore, the risk of virus leakage caused by mechanical damage can be significantly reduced by the membrane filter with a large-thickness separation layer in the present application.

Optionally, the average pore diameter of the effluent surface measured by SEM is 60-120 nm, the thickness of the separation layer is 25-45 μm, and the logarithmic removal rate of the filter membrane on the PP7 bacteriophage is not less than 4.

By adopting the technical scheme, when the thickness of the separation layer takes a larger value and is matched with the SEM measurement average pore diameter of the liquid outlet surface to take a smaller value, the three-dimensional network structure of the separation layer forms an effective passage with a longer path and a smaller pore diameter after stacking, and can form long-acting interception of viruses in the feed liquid so as to obtain a higher virus interception effect and further reduce virus leakage risk.

In addition, although the separation layer forms an effective path having a long path and a small pore diameter, since the pore structure of the separation layer has a large pore diameter, even if an effective path having a relatively small pore diameter is formed by the laminated structure of the three-dimensional network structure, the pore diameter of the three-dimensional network structure itself is still large, and it is difficult to form high adsorption of protein. In addition, the hydrophilic PVDF has better hydrophilicity, and the adsorption force of the pore walls to the protein is smaller, so that the adsorption force of the pore walls of the pore structure with large pore diameter to the protein is probably unable to reach the threshold value of protein adsorption, and thus, the protein which is smaller in size and is not easy to be adsorbed can still better pass through. Therefore, the filter membrane of the application has a larger value by the thickness of the separation layer and a smaller value by the SEM measurement of the average pore diameter of the liquid outlet surface, and can still keep higher protein yield on the basis of obtaining high virus retention rate.

Optionally, the average pore diameter of the effluent surface is 90-160 nm through SEM measurement, the thickness of the separation layer is 15-30 μm, and the logarithmic removal rate of the filter membrane on PP7 bacteriophage is greater than 2 and less than 4.

By adopting the technical scheme, when the thickness of the separation layer takes a small value and the average pore diameter of the SEM measurement of the liquid level takes a large value, the three-dimensional network structure of the separation layer forms an effective passage with a slightly short path and a large pore diameter after lamination, so that the resistance borne by the feed liquid is greatly reduced, and a larger flux is obtained. Although the effect of the filter membrane with the structure on the interception of the viruses is reduced, the lower virus leakage risk can be ensured by using the filter membrane in multiple layers, and the filter membrane has higher applicability to certain specific filtering working conditions.

In a second aspect, the present application provides a process for preparing a filter membrane, which adopts the following technical scheme:

a preparation process of a filter membrane comprises the following process steps:

s1, preparing a casting solution, and casting the casting solution on a carrier to form a liquid film; the casting solution comprises the following substances in parts by weight: 18-30 parts of PVDF resin and 5-25 parts of hydrophilic additive; 20-50 parts of small molecular additive and 20-40 parts of good solvent; the mass percent of the PVDF resin in the casting solution is more than 20 percent;

s2, blowing air flow with the relative humidity of 60-90% onto the surface of the liquid film for treatment to obtain a raw film; wherein the relative speed between the air flow and the liquid film is 0.1-6 m/s, and the duration is not less than 5s; and the temperature of the airflow is 5-20 ℃ lower than that of the casting solution;

s3, immersing the raw membrane into an extraction bath for further extraction and solidification to obtain a solid membrane;

and S4, carrying out hydrophilic post-treatment on the solid membrane to obtain a finished membrane.

Optionally, the number average molecular weight of the PVDF resin is 30 to 120 ten thousand; the hydrophilic additive is at least one of polyethylene glycol, polyvinylpyrrolidone and polyvinyl alcohol; the small molecular additive is LiCl or NH ₄ At least one of Cl, nano-silica, acetone, butanone and tetrahydrofuran.

Optionally, the polyethylene glycol is one of PEG-2000, PEG-4000 and PEG-6000; the polyvinylpyrrolidone is one of PVP (K30), PVP (K60) and PVP (K90); the polyvinyl alcohol is one of PVA-117, PVA-205 and PVA-103 of Coli, japan.

Optionally, the mass percent of the PVDF resin in the membrane casting solution is 20-35%

Optionally, in step S3, the treatment time is not less than 3min, and the extraction bath includes at least one of water and small molecule alcohol.

Optionally, the good solvent is at least one of N-methyl pyrrolidone, dimethylformamide, dimethylacetamide, trimethyl phosphate, triethyl phosphate and r-butyrolactone; the small molecular alcohol is at least one of ethanol and isopropanol.

By adopting the technical scheme, when the PVDF virus-removing membrane is prepared, a membrane casting solution is prepared firstly, wherein the membrane casting solution comprises PVDF resin, a hydrophilic additive, a small-molecule additive and a good solvent; the PVDF resin is a film-forming polymer which has good film-forming processability, and the finally prepared film has good mechanical property and pollution resistance, and is suitable for being applied to the field of virus removal. In addition, the solid content of the PVDF resin in the membrane casting solution of the present application needs to be strictly controlled, and the solid content needs to be more than 20%, because the inventors of the present application found that in the membrane making system of the present application, if the solid content of the PVDF resin in the membrane casting solution is too low, it is difficult to obtain a virus removing membrane with an ideal membrane structure. The good solvent is at least one of N-methyl pyrrolidone, dimethylformamide, dimethylacetamide, trimethyl phosphate, triethyl phosphate and r-butyrolactone (one of the solvents can be used, and a mixed solvent obtained by mixing a plurality of solvents can be used); the good solvent is used for fully dissolving the PVDF resin, so that a uniform, stable and clear casting solution is formed, and a filter membrane with an ideal aperture is formed through subsequent processes such as phase-splitting solidification and the like.

Preferably, the PVDF resin has a number average molecular weight of 30 to 120 ten thousand, which is advantageous for forming a uniform and stable casting solution with high solid content and for obtaining a film with high mechanical properties. Meanwhile, a small molecular additive is added into the casting solution, and the small molecular additive is LiCl or NH ₄ At least one of Cl, nano-silica, acetone, butanone and tetrahydrofuran; the small molecular additive is favorable for leading the finally prepared filter membrane to have ideal pore diameter, obtaining larger flux and keeping high-efficiency interception of virus, and simultaneously can improve the uniformity of the pore structure of the filter membrane so as to improve the filter membraneTensile strength and other mechanical properties. In addition, a hydrophilic additive is added into the membrane casting solution, the hydrophilic additive is at least one of polyethylene glycol, polyvinylpyrrolidone and polyvinyl alcohol, on one hand, the hydrophilic additive can enable the filter membrane to have better hydrophilicity, can promote phase splitting of the membrane casting solution and improve porosity of membrane pores, and the hydrophilic additive in the membrane casting solution and the extraction liquid are cooperatively matched to promote more reasonable phase splitting, extraction and solidification of the membrane casting solution, so that an ideal membrane pore structure can be obtained by membrane formation, defects and macropores are not easy to occur, and the pore diameters of the pores are relatively uniform. It should be noted that, the membrane-forming processes of different filter membranes are greatly different, the membrane-casting solution system in the present application is only suitable for PVDF membrane formation, and is not suitable for polyethersulfone and cellulose filter membranes, and the inventors guess that there is a certain relationship between this and the properties of the membrane-forming material itself.

The reasonable formula of the casting solution can greatly influence the structure and performance of the finally formed filter membrane, such as the pore size distribution, the thickness, the flow rate (flux) and the like of the filter membrane; the reasonable formula of the casting solution ensures that the finally prepared filter membrane has proper thickness and obtains ideal aperture. In addition, the higher solids casting solutions herein are reasonably viscous and easy to handle, can be cast manually (e.g., by hand pouring, casting, or spreading onto a casting surface) or automatically (e.g., poured or otherwise cast onto a moving bed); various apparatus known in the art may be used for casting. Casting equipment includes, for example, mechanical coaters, including doctor blades, or spray/pressurized systems. As is known in the art, a variety of casting speeds are suitable, such as casting speeds of about 2 to 6 feet per minute (fpm), and the like, as the case may be.

After the casting solution is cast into a liquid film, the liquid film is subjected to phase splitting treatment, and air flow with high humidity and relatively low temperature is blown to the surface of the liquid film to promote phase splitting of the liquid film so as to form a raw film; as is well known, the faster the phase separation speed, the smaller the pores formed; in the invention, the liquid film is relatively and quickly split in the air side under the synergistic cooperation of the airflow humidity (60-90%), the airflow temperature (5-20 ℃ lower than the temperature of the casting film liquid) and a proper formula of the casting film liquid, so that holes with small apertures are formed in the air side (however, because the content of water vapor in the airflow is limited, the film holes cannot be too small, and holes with the aperture of about 20nm cannot be formed in the formed film), and a relatively thick separation layer is formed; in addition, the relative speed between the air flow and the liquid film is 0.1-6 m/s, and the duration is not less than 5s; after the phase separation treatment, on one hand, the ideal pore size and thickness of the filter membrane are obtained; at the same time, the ideal membrane structure, namely the ideal fiber, is obtained.

Then immersing the raw membrane into an extraction bath for further extraction and solidification, wherein the extraction bath is at least one of water and small molecular alcohol (C1-C4 small molecular alcohol), and the small molecular alcohol is preferably at least one of ethanol and isopropanol; under the action of the extraction bath, PVDF can be more completely separated out, and the treatment time is not less than 3min, so that a solid film with an ideal film structure is ensured.

Then, in order to increase the hydrophilicity of the solid membrane, the solid membrane is subjected to hydrophilic treatment by a grafting method. The hydrophilic treatment liquid comprises 8% of hydroxypropyl acrylate, 23% of 3-butanol and the balance of water according to volume percentage. The hydrophilic treatment liquid is stirred by bubbling nitrogen to remove oxygen, the treatment time is 20min, and the temperature of the hydrophilic treatment liquid is kept at 45 ℃ during treatment. Subsequently, the solid film was placed in a nitrogen atmosphere, the temperature of the solid film was lowered to-60 ℃ or lower, and the solid film was irradiated with gamma rays of at least 25kGy using cobalt-60 as a radiation source. And after irradiation, standing the solid membrane for 15min at a low pressure of below 13.4Pa, carrying out contact reaction on the hydrophilic treatment solution after stirring and deoxygenation and the solid membrane, standing for 1h, washing with 2-propanol after reaction is finished, and carrying out vacuum drying at the temperature of 60 ℃ to obtain the finished membrane.

The finally prepared PVDF filter membrane has an ideal membrane structure, is large in flux and high in tensile strength, can efficiently intercept viruses, and has high protein yield.

In a third aspect, the present application provides a membrane filter, which adopts the following technical solutions:

the membrane filter comprises the filter membrane, and meets the following conditions:

A. the log removal rate of PP7 bacteriophage of the filter membrane is more than or equal to 4, and 1-2 filter membranes are included in the membrane filter;

B. the log removal rate of PP7 bacteriophage of the filter membrane is less than 4, and 2-3 filter membranes are arranged in the membrane filter.

Preferably, the membrane filter may be a capsule filter, a needle filter.

By adopting the technical scheme, for the filter membrane with the logarithmic removal rate of more than or equal to 4, 1 filter membrane can be directly made into the membrane filter or 2 filter membranes can be laminated to be used for making the membrane filter, so that the virus leakage risk is further reduced.

For the filter membrane with the logarithmic removal rate less than 4, 2 to 3 filter membranes can be used in a stacked manner to reduce the virus leakage risk in order to achieve the logarithmic removal rate more than 4.

In summary, the present application includes at least one of the following beneficial technical effects:

1. the filter membrane in the application forms a laminated pore structure with a longer path in the membrane thickness direction by controlling the separation layer with a larger pore diameter and a larger thickness, the size of an effective channel formed by a plurality of layers of pore structures for viruses to pass through is far smaller than the actual size of the pore structure, so that the high-efficiency interception of small-size viruses is realized, and even if the pore structure with a smaller size is laminated, the interception and adsorption of a smaller-size protein layer are less, so that the high protein yield and the high virus interception rate are simultaneously obtained; this is in contrast to the general belief that large pore size hydrophilic PVDF filters are difficult to reject small size viruses, and that a very unexpected result is obtained, as well, as the separation layer needs to be controlled to have a smaller pore size to achieve high viral retention.

2. The fiber structure of the filter membrane in the application has no obvious size change from the pre-filtering layer to the separating layer, which is different from the cognition that the pore diameter of the filter membrane is larger and the fiber is thicker, and the specific fiber structure not only ensures the low adsorption of the fiber structure to protein, but also has better mechanical property because the pressure resistance of the filter membrane is relatively uniform and no weak point exists.

3. The filter membrane has low overall symmetry and low symmetry of the separation layer, not only can ensure the uniformity of the overall mechanical property of the filter membrane, but also shows that the filter membrane has no mutation area of the pore diameter in the membrane thickness direction, and the mutation area of the pore diameter has a dense and rapidly-changed laminated pore structure, so that high interception and high adsorption of protein are likely to be generated, and the protein yield is reduced.

4. According to the preparation process of the filter membrane, the mass percent of the PVDF resin is controlled to be more than 20% by controlling the formula of the membrane casting solution, the hydrophilic additive and the small-molecule additive are added into the membrane casting solution, the phase splitting process is promoted, the hydrophilicity of the PVDF resin is improved, and the subsequent wet air sweeping, extraction and solidification and hydrophilic post-treatment are combined, so that the filter membrane with an ideal pore diameter structure can be obtained, and high virus retention rate, high protein yield and high mechanical performance can be obtained at the same time.

Drawings

FIG. 1 is an SEM photograph showing a cross-section of a filter membrane of example 1 at a magnification of 10K x.

FIG. 2 is a SEM image of the cross-section of the filter membrane of example 1 at 10K ×.

FIG. 3 is an SEM photograph of the liquid inlet side of the filter membrane obtained in example 3 of the present application, at a magnification of 10K ×.

FIG. 4 is an SEM photograph of the effluent surface side of the filter membrane obtained in example 3 of the present application, at a magnification of 10K x.

FIG. 5 is a SEM image of a cross-section of a filter membrane prepared in example 3 of the present application, at a magnification of 2K.

FIG. 6 is a graph showing the distribution of 20nm colloidal gold in the thickness direction of a filter membrane according to the present application, wherein the lower side of the filter membrane is a liquid inlet surface and the upper side is a liquid outlet surface; and the figure is only schematic and not a filter membrane of one embodiment.

FIG. 7 is an SEM image of the cross section of the filter membrane after a 20nm colloidal gold interception experiment is performed on the filter membrane of the present application, wherein the magnification is 5 Kx, the upper side of the filter membrane is a liquid inlet surface, and the lower side of the filter membrane is a liquid outlet surface; and the figure is only schematic and not a filter membrane of one embodiment.

Detailed Description

The present application is described in further detail below with reference to fig. 1-7.

In order to more clearly explain the overall concept of the present application, the following detailed description is given by way of example. In the following examples and comparative examples, the raw materials and equipment used were all commercially available in a usual manner, unless otherwise specified. Wherein the structural morphology of the filter membrane is characterized by adopting a scanning electron microscope with the model number of S-5500 provided by Hitachi company.

Example 1

A preparation process of an asymmetric hydrophilic PVDF filter membrane for virus removal comprises the following process steps:

s1, preparing a casting solution, and casting the prepared casting solution on a carrier to form a liquid film; the casting solution consists of the following substances in parts by weight: 23 parts of PVDF resin and 15 parts of hydrophilic additive; 35 parts of small molecular additive and 30 parts of good solvent; in the casting solution, the mass percentage of the PVDF resin is 22.3%.

In the casting solution, the number average molecular weight of PVDF resin is 80 ten thousand, the hydrophilic additive is a mixture of PEG-4000 and PVP (K30) in equal mass ratio, and the small molecular additive is a mixture of LiCl and acetone in equal mass ratio; the good solvent is a mixture of N-methyl pyrrolidone and dimethylformamide with equal volume.

S2, blowing airflow with the relative humidity of 68% onto the surface of the liquid film for treatment to obtain a raw film; the relative speed between the air flow and the liquid film is 1.5m/s, the duration is 24s, and the temperature of the air flow is 10 ℃ lower than that of the casting film liquid (namely the liquid film).

And S3, immersing the prepared green film into an extraction bath for further extraction and solidification to obtain a solid film. Wherein the extraction bath is a mixture of deionized water and small molecular alcohol at equal volume ratio, the small molecular alcohol is ethanol, and the treatment time is 6min.

And S4, performing hydrophilic post-treatment on the obtained solid membrane, specifically performing hydrophilic treatment on the solid membrane by using a grafting method. The hydrophilic treatment liquid comprises 8% of hydroxypropyl acrylate, 23% of 3-butanol and the balance of water according to volume percentage. The hydrophilic treatment liquid is stirred by bubbling nitrogen gas for deoxygenation, the treatment time is 20min, and the temperature of the hydrophilic treatment liquid is kept at 45 ℃ during treatment. Subsequently, the solid film was placed in a nitrogen atmosphere, and the temperature of the solid film was lowered to about-65 ℃ and the solid film was irradiated with about 27kGy of gamma rays using cobalt-60 as a radiation source. And after irradiation, standing the solid membrane for 15min at a low pressure of below 13.4Pa, carrying out contact reaction on the hydrophilic treatment solution after stirring and deoxygenation and the solid membrane, standing for 1h, washing with 2-propanol after reaction is finished, and carrying out vacuum drying at the temperature of 60 ℃ to obtain the finished membrane.

Examples 2 to 7

The main difference between examples 2 to 7 and example 1 is that the formulation of the casting solution and the process parameters of each step are different, and the details are shown in the following table:

in the casting solution, the number average molecular weight of PVDF resin is 80W, the hydrophilic additive is a mixture of PEG-4000 and PVP (K30) in equal mass ratio, and the small molecular additive is a mixture of LiCl and acetone in equal mass ratio; the good solvent is a mixture of N-methyl pyrrolidone and dimethylformamide with equal volume.

Wherein the content of the first and second substances,

in the casting solution of example 2, the hydrophilic additive is a mixture of PEG-2000 and PVP (K60) in a mass ratio, and the small molecule additive is a mixture of nano-silica and acetone in a mass ratio; the good solvent is a mixture of N-methyl pyrrolidone and dimethylacetamide in equal volume.

In the casting solution of example 3, the hydrophilic additive was PEG-6000 and the small molecule additive was NH ₄ Cl; the good solvent is N-methyl pyrrolidone, and the small molecular alcohol is ethanol.

In the casting solution of example 4, the hydrophilic additive is a mixture of PEG-2000 and PVP (K30) in a mass ratio, and the small-molecule additive is a mixture of LiCl and butanone in a mass ratio; the good solvent is a mixture of N-methyl pyrrolidone and trimethyl phosphate with equal volume, and the micromolecular alcohol is ethanol.

In the casting solution of example 5, the hydrophilic additive was a mixture of PEG-2000 and PVA-117 in equal mass ratio, and the small molecule additive was a mixture of LiCl and acetone in equal mass ratio; the good solvent is a mixture of N-methyl pyrrolidone and dimethylformamide with equal volume, and the small molecular alcohol is isopropanol.

In the casting solution of example 6, the hydrophilic additive was a mixture of PEG-6000 and PVA-103 in equal mass ratio, and the small molecule additive was a mixture of LiCl and acetone in equal mass ratio; the good solvent is a mixture of N-methylpyrrolidone and dimethylformamide with equal volume, and the small molecular alcohol is ethanol.

In the casting solution of example 7, the hydrophilic additive was a mixture of PVP (K30) and PVA-103 in equal mass ratio, and the small molecule additive was a mixture of LiCl and acetone in equal mass ratio; the good solvent is a mixture of N-methyl pyrrolidone and dimethylformamide with equal volume, and the small molecular alcohol is isopropanol.

Comparative example

Comparative example 1

s1, preparing a casting solution, and casting the prepared casting solution on a carrier to form a liquid film; the casting solution consists of the following substances in parts by weight: 40 parts of PVDF resin and 10 parts of hydrophilic additive; 10 parts of small molecular additive and 30 parts of good solvent; in the casting solution, the mass percentage of the PVDF resin is 44.4 percent.

S2, immersing the liquid membrane into a pretreatment solution for pretreatment, wherein the pretreatment solution is prepared from deionized water and N-methylpyrrolidone according to a volume ratio of 3:2, the pretreatment time is 10s, the temperature of the pretreatment liquid is the same as that of the liquid film, and the raw film is obtained after pretreatment.

And S3, immersing the prepared raw film into an extraction bath for further extraction and solidification to obtain a solid film. Wherein the extraction bath is a mixture of deionized water and small molecular alcohol in equal volume ratio, and the small molecular alcohol is ethanol.

And S4, performing hydrophilic post-treatment on the obtained solid membrane, specifically performing hydrophilic treatment on the solid membrane by using a grafting method. The hydrophilic treatment liquid comprises 8% of hydroxypropyl acrylate, 23% of 3-butanol and the balance of water according to volume percentage. The hydrophilic treatment liquid is stirred by bubbling nitrogen gas for deoxygenation, the treatment time is 20min, and the temperature of the hydrophilic treatment liquid is kept at 45 ℃ during treatment. Subsequently, the solid film was placed in a nitrogen atmosphere, and the solid film was cooled to about-65 ℃ and irradiated with about 27kGy of gamma rays using cobalt-60 as a radiation source. And after irradiation, standing the solid membrane for 15min at a low pressure of below 13.4Pa, carrying out contact reaction on the hydrophilic treatment solution after stirring and deoxygenation and the solid membrane, standing for 1h, washing with 2-propanol after reaction is finished, and carrying out vacuum drying at the temperature of 60 ℃ to obtain the finished membrane.

Comparative example 2

s1, preparing a casting solution, and casting the prepared casting solution on a carrier to form a liquid film; the casting solution consists of the following substances in parts by weight: 15 parts of PVDF resin and 15 parts of hydrophilic additive; 35 parts of small molecular additive and 30 parts of good solvent; in the casting solution, the mass percentage of the PVDF resin is 15.8%.

S2, blowing air flow with the relative humidity of 40% onto the surface of the liquid film for treatment to obtain a raw film; the relative speed between the air flow and the liquid film is 7m/s, the duration is 60s, and the air flow temperature is 25 ℃ lower than the temperature of the casting film liquid (namely the liquid film).

And S3, immersing the prepared green film into an extraction bath for further extraction and solidification to obtain a solid film. Wherein the extraction bath is a mixture of deionized water and small molecular alcohol at equal volume ratio, the small molecular alcohol is ethanol, and the treatment time is 20min.

And S4, carrying out hydrophilic post-treatment on the obtained solid membrane, specifically carrying out hydrophilic treatment on the solid membrane by using a grafting method. The hydrophilic treatment liquid comprises 8% of hydroxypropyl acrylate, 23% of 3-butanol and the balance of water according to volume percentage. The hydrophilic treatment liquid is stirred by bubbling nitrogen to remove oxygen, the treatment time is 20min, and the temperature of the hydrophilic treatment liquid is kept at 45 ℃ during treatment. Subsequently, the solid film was placed in a nitrogen atmosphere, and the solid film was cooled to about-65 ℃ and irradiated with about 27kGy of gamma rays using cobalt-60 as a radiation source. And after irradiation, standing the solid membrane for 15min at a low pressure of below 13.4Pa, carrying out contact reaction on the hydrophilic treatment solution after stirring and deoxygenation and the solid membrane, standing for 1h, washing with 2-propanol after reaction is finished, and carrying out vacuum drying at the temperature of 60 ℃ to obtain the finished membrane.

Performance detection and performance parameters

Firstly, the following steps: structural characterization

Performing morphology characterization on the membrane structures of the filter membranes obtained in the embodiments 1-7 and the comparative examples 1-2 by using a scanning electron microscope, and thus obtaining required data; wherein, the shape parameters of the liquid inlet surface, the liquid outlet surface and the pre-filtering layer structure of the examples 1-7 and the comparative examples 1-2 are represented as the following table:

the morphological parameters of the separation layer structures of examples 1 to 7 and comparative examples 1 to 2 are given in the following table:

it should be noted that, in the above table, since the whole pore size of the filter membrane prepared in comparative example 2 is large and cannot trap colloidal gold of 20nm well, there is no definite separation layer and pre-filter layer structure, and therefore there is no relevant morphological parameter, and only the surface morphologies of the liquid inlet surface and the liquid outlet surface are characterized.

2. Viral retention capacity

The filters prepared in the examples or comparative examples were used as samples for virus challenge tests, in which the LRV detection method of the filter was performed with reference to the instruction document TR41 issued by PDA, in which the test was performed with PP7 phage as the virus-trapping material, material flow as immunoglobulin IVIG, buffer system as PBS, and the pressure of the feed liquid was 30psi in the virus challenge test. Calculating LRV by measuring the titer of PP7 phage in the challenge and filtrate; calculating the protein yield by detecting the concentration of the protein in the challenge solution and the filtrate; the flux was calculated by recording the flow and time.

It should be noted that in performing the virus challenge assay, the assay is first run with a single filter sample; if the LRV measured by the single-layer filter membrane is less than 4, a double-layer filter membrane sample is used for carrying out the test, and the LRV and the protein yield are further measured when the double-layer filter membrane is used.

3. Mechanical Properties

The tensile properties of the filter membranes prepared in the examples and comparative examples were measured by a universal tensile testing machine to determine the tensile strength of the filter membranes.

The mechanical properties and the virus retention capacity of the filters of examples 1 to 7 and comparative examples 1 to 2 are given in the following table:

it should be noted that the examples 5-7 and comparative example 1 did not have the double-layer filter LRV data and the double-layer filter protein yield data, because the LRV > 4 (virus retention rate > 99.99%) of the PP7 bacteriophage in examples 5-7 and comparative example 1 achieved the virus retention rate required for international approval and no virus challenge test was performed on the double-layer filter.

The LRV of the filter membrane prepared in comparative example 2 for the PP7 phage was < 1, and although the flux and the protein yield were high, the risk of virus leakage was great and it was not of practical value. No further double-filter virus challenge experiments were performed as well, and therefore, comparative example 2 also had no double-filter LRV data and no double-filter protein yield data.

Conclusion

By comparing the data of examples 1-7 and comparative example 1, it can be seen that the specific filter membrane with large-thickness and large-aperture separation layer in the present application can not only obtain high virus retention rate (for filter membranes with LRV of 2-4, high LRV can be obtained by connecting 2-3 filter membranes in series), but also obtain high protein yield (for examples 1-4, even if connecting 2 filter membranes in series, the protein yield still reaches 95% or even more than 98%). Despite the filters of comparative example 1 and CN105980037B, the protein yield was low, only about 80-90%, although higher virus retention rates (LRV > 6 and even LRV > 7) could be achieved with a small pore size separation layer structure.

Furthermore, by further separating the properties of the filter membrane obtained in comparative example 1, it can be seen that the filter membrane has significantly poor mechanical properties, probably because the ratio of the diameters of the fibrous structures of the pre-filtration layer and the separation layer reaches about 3.9, i.e. the fibrous structures of the filter membrane have a significantly asymmetric distribution in the direction of the membrane thickness, which means that the mechanical properties of the filter membrane are not uniform in the thickness direction and are prone to be weakened, leading to a decrease in mechanical properties. This is a close approximation to the filter membrane of CN105980037B which can only be used at a relatively low pressure of 15 psi.

Furthermore, the membrane flux of comparative example 1 is also clearly lower, while the region that mainly affects the membrane flux is the separation layer, the ratio of the SEM-measured mean pore size of the separation layer to the SEM-measured mean diameter of the separation fibers is smaller, only about 1.8, which makes the separation layer more resistant to the feed solution, plus the smaller pore size of the membrane separation layer of comparative example 1, resulting in a significant decrease in flux and a significant decrease in protein yield. This suggests that high viral retention is often accompanied by a decrease in protein yield for the usual hydrophilic PVDF filters with higher asymmetry.

The present embodiment is only for explaining the present application, and it is not limited to the present application, and those skilled in the art can make modifications of the present embodiment without inventive contribution as needed after reading the present specification, but all of them are protected by patent law within the scope of the claims of the present application.

Claims

1. An asymmetric hydrophilic PVDF filter membrane for virus removal, comprising a porous body having a non-directional tortuous path therein, the porous body having an inlet side on a side thereof close to a feed solution and an outlet side on a side thereof remote from the feed solution, the porous body transitioning in the direction of membrane thickness as continuous fibers, characterized in that: the average pore size of SEM measurement of porous main part by the inlet surface is towards going out the liquid level and is reduced gradually, porous main part includes prefilter layer and is used for intercepting 20nm diameter colloidal gold's separating layer, the separating layer includes the liquid level, the average pore size of SEM measurement of liquid level is 60 ~ 160nm, the thickness of separating layer is 15 ~ 45 mu m.

2. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 1, wherein: the separation layer comprises separation fibers, the separation fibers are connected with each other to form a three-dimensional network structure of the separation layer, and the average diameter of the separation fibers measured by SEM is 30-50 nm; the pre-filter layer comprises support fibers, the support fibers are connected with each other to form a three-dimensional network structure of the pre-filter layer, and the average diameter of the support fibers measured by SEM is 30-55 nm.

3. The asymmetric hydrophilic PVDF filter membrane for virus removal as set forth in claim 2, wherein: the ratio of the SEM measured average diameter of the supporting fibers to the SEM measured average diameter of the separating fibers is 0.8 to 1.5.

4. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 2, wherein: the separation fiber has an average length of 80-150 nm measured by SEM, and the support fiber has an average length of 120-180 nm measured by SEM.

5. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 2, wherein: the ratio of the SEM measured average pore diameter of the separation layer to the SEM measured average diameter of the separation fibers is 2-7; the ratio of the SEM-measured average pore size of the pre-filter layer to the SEM-measured average diameter of the support fibers is 3.5 to 8.

6. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 2, wherein: the ratio of the SEM measured average pore diameter of the separation layer to the SEM measured average length of the separation fiber is 1.1-1.4; the ratio of the SEM measured average pore size of the pre-filter layer to the SEM measured average length of the support fibers is 1.2 to 1.7.

7. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 1, wherein: the average pore diameter measured by SEM of the liquid inlet surface is 200-500 nm, the ratio of the average pore diameter measured by SEM of the liquid inlet surface to the average pore diameter measured by SEM of the liquid outlet surface is 1.2-6, and the specific surface area of the porous main body is 6-12 m ² /g。

8. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 1, wherein: the SEM measurement average pore diameter variation gradient of the separation layer is not more than 8 nm/mum, and the SEM measurement average pore diameter of the separation layer is 100-200 nm.

9. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 1, wherein: the side, close to the liquid inlet face, of the separation layer is regarded as 0%, the liquid outlet face is regarded as 100%, the separation layer is divided into ten equal parts along the thickness direction of the separation layer, the ratio of the SEM measured average pore diameter of a region with the thickness position of the separation layer being 0-10% to the SEM measured average pore diameter of a region with the thickness position of the separation layer being 40-50% is K1, and K1 is not more than 2.2.

10. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 9, wherein: the ratio of the SEM measured average pore diameter of the area with the film thickness position of the separation layer of 40-50% to the SEM measured average pore diameter of the liquid outlet surface is K2, and K2 is not more than 1.5.

11. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 10, wherein: the K1 is larger than the K2, and the ratio of the K1 to the K2 is 1.05-1.6.

12. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 1, wherein: the pre-filtering layer comprises the liquid inlet surface, the thickness of the pre-filtering layer is 1-15 mu m, the SEM measurement average pore diameter of the pre-filtering layer is 150-300 nm, and the SEM measurement average pore diameter change gradient of the pre-filtering layer is 5-20 nm/mu m.

13. The asymmetric hydrophilic PVDF filter membrane for virus removal according to claim 1, wherein: the flux of the filter membrane is not less than 50 L.h ^-1 ·m ^-2 @30psi, log removal rate of the filter for PP7 phage is greater than 2; the tensile strength of the filter membrane is 5-20 MPa.

14. The asymmetric hydrophilic PVDF filter membrane for virus removal according to any one of claims 1 to 13, wherein: the average pore diameter of the effluent surface measured by SEM is 60-120 nm, the thickness of the separation layer is 25-45 mu m, and the logarithmic removal rate of the filter membrane to PP7 bacteriophage is not less than 4.

15. The asymmetric hydrophilic PVDF filter membrane for virus removal according to any one of claims 1 to 13, wherein: the average pore diameter of the effluent surface is 90-160 nm through SEM measurement, the thickness of the separation layer is 15-30 mu m, and the logarithmic removal rate of the filter membrane to PP7 bacteriophage is more than 2 and less than 4.

16. A process for the preparation of a filter membrane according to any one of claims 1 to 15, wherein: the method comprises the following process steps:

s2, blowing air flow with relative humidity of 60-90% onto the surface of the liquid film for treatment to obtain a raw film; wherein the relative speed between the air flow and the liquid film is 0.1-6 m/s, and the duration is not less than 5s; and the temperature of the airflow is 5-20 ℃ lower than that of the casting solution;

17. The process for the preparation of a filtration membrane according to claim 16, wherein: the number average molecular weight of the PVDF resin is 30-120 ten thousand;

the hydrophilic additive is at least one of polyethylene glycol, polyvinylpyrrolidone and polyvinyl alcohol;

the small molecular additive is LiCl or NH ₄ At least one of Cl, nano-silica, acetone, butanone and tetrahydrofuran.

18. The process for the preparation of a filtration membrane according to claim 16, characterized in that: in the step S3, the treatment time is not less than 3min, and the extraction bath comprises at least one of water and small molecular alcohol.

19. The process for the preparation of a filtration membrane according to claim 16, characterized in that:

the good solvent is at least one of N-methyl pyrrolidone, dimethylformamide, dimethylacetamide, trimethyl phosphate, triethyl phosphate and r-butyrolactone;

the small molecular alcohol is at least one of ethanol and isopropanol.

20. A membrane filter, characterized by: comprising a filter membrane according to any one of claims 1 to 15, and which membrane filter satisfies the following condition:

A. the log removal rate of the PP7 bacteriophage of the filter membrane is more than or equal to 4, and 1-2 filter membranes are included in the membrane filter;

B. the log removal rate of the PP7 bacteriophage of the filter membrane is less than 4, and 2-3 filter membranes are arranged in the membrane filter.