CN117181007A - Multilayer composite filtering membrane and preparation method thereof - Google Patents
Multilayer composite filtering membrane and preparation method thereof Download PDFInfo
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- Separation Using Semi-Permeable Membranes (AREA)
Abstract
The application relates to the field of multi-layer filtering films, in particular to a multi-layer composite filtering film, which comprises a porous main body, wherein the porous main body comprises a multi-layer single film; the porous main body has an orientation direction, and the porous main body meets the condition that the fiber ratio of which the included angle is within 30 degrees with the orientation direction is not less than 60 percent in any line segment which is perpendicular to the orientation direction and at least cuts more than 40 fibers on an SEM (scanning electron microscope) image of the liquid inlet surface or the liquid outlet surface of the porous main body; the average diameter of the fibers in the porous body is along the porous bodyThe thickness direction of the body gradually decreases from the liquid inlet level to the liquid outlet level; the porous body has a flow rate of 1in per pass as measured by a gurley flow rate tester 2 The gas amount of 100ml of the porous main body with the filtering area is 3-40 s; the filtering precision of the porous main body is 0.1-10 mu m. The multi-layer composite filter membrane provided by the application has the effect of still having better filtering flow rate and filtering capacity under higher filtering precision.
Description
Technical Field
The application relates to the field of filtration, in particular to a multi-layer composite filtration membrane and a preparation method thereof.
Background
Melt blown nonwoven technology is a process for producing ultra-fine fibers by blowing a polymer melt by means of a high-velocity high-temperature air stream to rapidly stretch the polymer melt. The melt-blown filter material is a filter material prepared by a melt-blown nonwoven technology, and is mainly applied to the fields of air filtration and liquid filtration, such as a composite filter material in an air purifier; filtration of pharmaceutical preparations, biological preparations and synthetic plasma; filtering beverage, beer and syrup in food industry; filtering industrial wastewater, and the like.
Currently, melt blown filter materials are made substantially from polypropylene, and the specific process includes: the polymer master batch is extruded after being heated and melted in a screw extruder, is filtered by a filter screen, and is conveyed to a melt distribution cavity in a die head by the accurate measurement of a metering pump, and the flow rate and the flow velocity of the melt in the melt cavity are consistent and the pressure distribution is uniform, so that the melt is ensured to be uniformly distributed along the width direction; then the melt is sent to a spinning melt pool and sprayed out through a spinneret orifice on a spinneret plate, and meanwhile, a blower supplies high-speed hot air on two sides of the spinneret orifice, and the sprayed melt is stretched and thinned under the drafting action of the hot air; and rapidly fly to and adhere to the net curtain under the guidance of the suction air, intertwine with each other through the thermal bonding effect, and cool and solidify under the action of outside cold air to form the melt-blown non-woven fabric.
Compared with other non-woven materials, the melt-blown filter material prepared by the melt-blowing method has larger specific surface area, smaller pore diameter and higher porosity. However, in the fields of beer filtration and the like in foods, clarification filtration of vaccines in biological medicine and the like, clarification filtration of other chemical products and the like, higher accuracy, higher filtration efficiency and higher filtration capacity are desired.
In a Chinese patent of invention with publication number CN114307393A, published by Jiangsu Bochi New Material science and technology Co., ltd, a blood filter material is provided, which comprises a first filter material layer, a second filter material layer, a third filter material layer and a fourth filter material layer which are sequentially overlapped; the first filter material layer to the fourth filter material layer are porous non-woven fabrics prepared by melt blending polycarbonate and silk fibroin and then spinning and pressing; and from the first filter material layer to the fourth filter material layer, the pore diameter is sequentially reduced, and the thickness is sequentially reduced. The preparation method disclosed in the patent mainly comprises the following steps: according to the respective component proportion of the first filter material layer, the second filter material layer, the third filter material layer and the fourth filter material layer, polycarbonate and silk fibroin are put into a screw extruder to be mixed and melted, extruded from a spinneret and stretched to form fiber bundles; then spreading the fiber bundles in a mould, and hot-pressing to form a first filter material layer, a second filter material layer, a third filter material layer and a fourth filter material layer; preparing fibers with different diameters by adjusting the aperture and the stretching ratio of a spinneret, and using filter material layers with different apertures formed by hot pressing the fibers with different diameters, wherein the finer the fibers, the smaller the aperture of the filter material layers formed by hot pressing; and respectively preparing a first filter material layer, a second filter material layer, a third filter material layer and a fourth filter material layer, sequentially overlapping in a mould, and performing secondary hot-pressing bonding to obtain the blood filter material.
The above-disclosed filter material is obtained by hot-pressing the individual filter material layers, respectively, and then laminating the filter material layers, and then hot-pressing the filter material layers, wherein the porosity in the filter material may have more loss, and the fiber mesh structure in the filter material may be damaged too much, so that the accuracy of the filter material is greatly improved, but the loading capacity and flux of the filter material are greatly reduced compared with those of the filter material with single layers with different accuracies.
Disclosure of Invention
Aiming at the problems existing in the prior art, the invention aims to provide a multi-layer composite filtering membrane and a preparation method thereof.
In order to achieve the above purpose, the present invention provides the following technical solutions:
a multi-layer composite filtering membrane comprises a porous main body, wherein one side of the porous main body is a liquid inlet surface, the other side of the porous main body is a liquid outlet surface,
the porous body comprises a multilayer single membrane;
the porous main body has an orientation direction, and the porous main body meets the condition that the fiber ratio of which the included angle is within 30 degrees with the orientation direction is not less than 60 percent in any line segment which is perpendicular to the orientation direction and at least cuts more than 40 fibers on an SEM (scanning electron microscope) image of the liquid inlet surface or the liquid outlet surface of the porous main body;
the average diameter of the fibers in the porous main body gradually decreases from the liquid inlet level to the liquid outlet level along the thickness direction of the porous main body;
The porous body has a flow rate of 1in per pass as measured by a gurley flow rate tester 2 The gas amount of 100ml of the porous main body with the filtering area is 3-40 s;
the filtering precision of the porous main body is 0.1-10 mu m.
The invention adopts a plurality of single-layer film stacks formed by melt blowing to be compounded into a multi-layer composite filter film with certain orientation, so as to obtain higher filter precision and simultaneously give consideration to higher filter flow rate and loading capacity.
The "filtration accuracy" in the present invention is the accuracy of the filtration membrane by supplying a polystyrene microsphere suspension having a uniform particle diameter (the polystyrene microsphere suspension includes 5wt% of polystyrene microspheres, 0.5 to 1mol/L of sodium chloride and pure water) to the filtration membrane, and the minimum polystyrene microsphere particle diameter when the filtration membrane can retain 95% or more of the particles in the polystyrene microsphere suspension is the filtration membrane.
First, for conventional melt blown filtration membranes, the higher the accuracy of the melt blown filtration membrane, the greater the attenuation in flow rate and load can occur, because conventional ways to increase the accuracy of the melt blown filtration membrane are to use finer melt blown fibers to form smaller pores or to hot press at higher temperatures to form smaller pores, both of which lose the loft structure of the melt blown membrane itself, thereby greatly decreasing the flow rate and load of the melt blown membrane.
In the invention, the multilayer single-layer films are arranged for compounding, and the orientation degree of the compounded filtering film is controlled, so that higher precision is expected to be obtained on the premise of not reducing the flow and the loading of the filtering film. Specifically, when the multilayer film is laminated and then melt-blown into a film, the fibers on the heated side of the filtration membrane are more tightly formed to form smaller pores when the multilayer film is simultaneously hot pressed and stretched, but undesirable melt-bonding or yarn-bonding between the fibers in the entire film does not occur, as compared with the case where the multilayer film is melt-blown into a film as a whole. Moreover, the high precision of the composite filtering membrane is obtained by more tightly arranging the fibers close to one side of the heating source and controlling the orientation degree of the filtering membrane, so that the overall fluffiness degree, the porosity or the hole area ratio of the filtering membrane can still be maintained at a good level, and the filtering membrane is maintained to have high flow rate and loading capacity.
The term "degree of orientation" in the present invention means that the porous body satisfies that the ratio of fibers having an angle of 30 ° or less to the orientation direction is not less than 60% in any line segment perpendicular to the orientation direction and cutting at least 40 fibers in the SEM electron microscope image of the liquid inlet surface or the liquid outlet surface. The multilayer composite filtration membrane disclosed in the present invention has a significant degree of orientation such that the direction of extension of some of the fibers in the multilayer composite filtration membrane of the present invention is substantially uniform, thereby providing the benefit that the pore structure formed by the fibers also has an elongated orientation, the orientation direction being substantially consistent with the fibers. When the pore structure of the multi-layer composite filtering membrane has certain orientation, the particle size of impurity particles which can be intercepted by the pore structure with orientation is smaller than that of the pore structure which does not have orientation, because the long and narrow pore structure enables only particles with the particle size smaller than the minimum width of the pore to easily penetrate the pore, and other particles with the particle size slightly larger than the minimum width of the pore are easily intercepted. Secondly, because the multi-layer composite filtering membrane has a certain thickness, the pore structures with orientation in the thickness direction cannot be in one-to-one correspondence, even if part of impurities with the particle size slightly larger than the minimum width of the pores pass through the pores on the surface of the filtering membrane, the impurities are easily intercepted by the following pore structures with orientation, so that the filtering precision of the filtering membrane is greatly improved. In addition, the tensile deformation of the pore structure of the membrane does not greatly reduce the loss of the porosity of the membrane and the reduction of the pore area, so that the attenuation amplitude of the flow rate and the loading of the filtering membrane is reduced.
The invention also controls the thickness of the fiber in the filtering membrane to be in gradient change in the thickness direction of the filtering membrane, and specifically, the thickness of the fiber is in gradient decrease from the liquid inlet surface to the liquid outlet surface in the thickness direction of the filtering membrane. The reason for this is that: for melt blown films, the thickness of the fibers will determine to some extent the filtration accuracy of the film; therefore, the gradient distribution of the fiber thickness is used for controlling the accuracy of the filtering membrane to be increased from the liquid inlet level to the liquid outlet level in a gradient manner in the thickness direction, the filtering effect of the filtering membrane is improved, and a prefilter area with a certain thickness is formed on one side of the filtering membrane close to the liquid inlet, so that a good depth filtering effect is formed. Secondly, controlling the direction of the gradient attenuation of the thickness of the fiber and the opposite direction of heat transfer during compounding can help to form a stable fluffy area inside the filter membrane, because the thicker the fiber, the less likely it is to form a bonding area with other fibers in the hot pressing process that loses void volume. In addition, the coarse fibers can also play a role in providing strength in the multi-layer composite filter membrane, so that the mechanical strength of the filter membrane is improved.
Further, the SEM average diameter of the fibers of the porous body near the liquid inlet surface side is 0.4-8 μm; the SEM average diameter of the fiber of the porous main body near the liquid outlet surface is 0.1-4 mu m; the ratio of the SEM average diameter of the fiber on the side of the porous body close to the liquid inlet surface to the SEM average diameter of the fiber on the side of the porous body close to the liquid outlet surface is 1.3-20:1.
In the invention, the measuring mode of the average diameter of the fiber in the porous main body can be used for carrying out morphology characterization on the structure of the filtering membrane by using a scanning electron microscope, then computer software (such as Matlab, NIS-Elements and the like) or manual measurement is used for carrying out corresponding calculation; in practice, the surface (or cross section) of the filter membrane can be characterized by electron microscopy to obtain corresponding SEM image, and selecting a certain area such as 10 4 μm 2 (100 μm by 100 μm) or 100 μm 2 (10 μm by 10 μm), the specific area size is determined according to the actual situation, the diameters of all coarse fibers on the area are measured by corresponding computer software or manually, and then calculation is carried out to obtain the average diameter of the area; of course, the person skilled in the art can also obtain the above parameters by other measuring means, which are only used as reference.
According to the invention, the SEM average diameter of the fiber at the side of the porous main body close to the liquid inlet surface is controlled within the range, so that the problem that the pore of the filtering membrane at the side of the porous main body close to the liquid inlet surface is too large due to too thick fiber at the side of the porous main body close to the liquid inlet surface is avoided, and impurities in the feed liquid can not be intercepted almost by means of a physical interception principle at the side of the porous main body close to the liquid inlet surface; meanwhile, the phenomenon that small particle impurities in the feed liquid are easily adsorbed and accumulated on one side of the porous main body close to the liquid inlet surface due to too fine fibers on one side of the porous main body close to the liquid inlet surface is avoided, the porous main body is blocked, the pre-filtering area of the filtering membrane is undesirably lost, and the loading capacity of the filtering membrane is undesirably lost is avoided.
The present invention also controls the SEM average diameter of the fibers on the side of the porous body close to the liquid surface within the above range because if the diameter of the fibers on the side close to the liquid surface of the porous body is too small, various adverse effects are caused, such as that fine fibers on the side of the porous body close to the liquid surface are liable to fall off at a large flow rate or when filtering a feed liquid of a high viscosity to contaminate the filtered feed liquid. If the fiber diameter near the liquid surface side of the filtering membrane is too large, the filtering membrane is difficult to achieve high precision.
The invention also defines that a certain proportion relation exists between the SEM average diameter of the fiber on the side close to the liquid inlet surface of the porous main body and the SEM average diameter of the fiber on the side close to the liquid outlet surface, so that the diameter gradient change amplitude of the fiber in the porous main body in the thickness direction of the porous main body is expected to be moderate. Because if the gradient change amplitude of the fiber diameter in the thickness direction of the porous main body is too large, the pore size of the porous main body in the thickness direction is easy to be suddenly changed, the filtering of the feed liquid can quickly enter smaller pores from larger pores, and impurity particles with the middle particle size can only be intercepted by small pores, so that the area with the suddenly changed pore size is easy to reach the loading capacity or be blocked, and the two sides of the area with the suddenly changed pore size are likely to not reach the loading capacity, thus the pores of each area of the porous main body can not be effectively utilized during the filtering, and a larger gap is generated between the actual loading capacity and the theoretical loading capacity of the whole porous main body.
If the magnitude of the gradient change of the fiber diameter in the thickness direction of the porous body is too small, particles which should be intercepted on the side of the porous body close to the liquid inlet surface easily enter the side of the porous body close to the liquid outlet surface, especially for particles with certain flexibility, so that more impurity particles with slightly larger particle sizes are easily accepted on the side of the porous body close to the liquid outlet surface, and thus the side of the porous body close to the liquid outlet surface is easily blocked faster, so that the porous body can reach the loading capacity faster.
Further, the porous body satisfies that the ratio of fibers having an included angle of 30 ° or less to the orientation direction in any line segment perpendicular to the orientation direction and cutting at least 40 fibers in the SEM electron microscope image of the liquid inlet surface or the liquid outlet surface thereof is 60 to 92%.
Through the limitation of the upper limit of the fiber proportion of which the included angle is within 30 degrees with the orientation direction, the problem that the degree of order of the fibers in the porous main body is too high to reduce the comprehensive complexity of the flow channel, so that the filtration under larger pressure is easy to cause series flow or penetration of impurities, and the filtration effect is poor is avoided.
Further, the porous body satisfies that the ratio of the fibers with an included angle of 20 degrees or less to the orientation direction in any line segment of at least 40 cut-off fibers perpendicular to the orientation direction on the SEM electron microscope image of the liquid inlet surface or the liquid outlet surface is not less than 30%.
By further defining the degree of orientation of the fibers in the porous body as described above, the orientation of the fibers in the porous body is made more pronounced and the degree of orientation is higher, thereby allowing the pores in the porous body to also have a higher degree of orientation. A higher degree of orientation of the filter apertures may result in smaller minimum widths of the filter apertures, thereby resulting in further improved filtration accuracy. In addition, the composite filter membrane is obtained by laminating and compounding single-layer membranes, and the fiber diameters of the filter membranes in the thickness direction are different, so that the control of the whole filter membrane to have higher orientation degree is beneficial to forming the filter membrane with more obvious orientation degree in the thickness direction. Because the closer to the heating source and the finer the fibers are, the more easily it is to achieve better orientation during hot-press stretching, and conversely, the farther from the heating source and the thicker the fibers are, the lower the degree to which the fibers are oriented during hot-press stretching, and the less pronounced this difference is when the film as a whole is at a lower degree of orientation, and the more pronounced this difference is when the degree of orientation of the whole film is controlled to be higher. The greater the difference of the orientation degree of the fibers in the thickness direction of the filtering membrane, the better the precision improvement effect of the filtering membrane caused by the fiber orientation, and the higher the filtering precision and the better the filtering flow rate and the filtering capacity of the filtering membrane can be considered.
Further, in at least any line segment of the SEM image of the porous main body liquid inlet surface, which is perpendicular to the orientation direction and cuts more than 40 fibers, the fiber ratio within 20 degrees of the orientation direction is A in ；
In at least 40 or more arbitrary line segments of the SEM electron microscope image of the liquid surface of the porous main body, which are vertical to the orientation direction, the fiber ratio A with an included angle of 20 degrees or less with the orientation direction is out ；
And A is in /A out ＝0.6～0.9。
Through A above in /A out The ratio of the degree of orientation on the inlet level and the outlet level of the filtration membrane can be indirectly controlled by limiting the value of (a). In general, when the fiber diameters are the same, the degree of orientation is higher, and the precision of forming voids between fibers is higher. Therefore, the invention controls the proportion of the orientation degree of the liquid inlet surface and the liquid outlet surface of the composite filtering membrane so that the fiber orientation degree of the liquid outlet surface is higher than that of the liquid inlet surface, and the difference is controlled within a certain range, thereby ensuring that the composite filtering membrane has a better gradient distribution precision structure and improving the filtering effect of the filtering membrane.
Further, the porous body has a total thickness of 50 μm to 400 μm.
When the overall thickness of the porous body is controlled within the above range, the filtration membrane can have a sufficient space to accommodate impurity particles, provide an effective fouling receiving space, and have a high space utilization rate in the filtration membrane. If the overall thickness of the porous body is too large, a large filtration resistance is easily formed, and a large pressure drop is required for stable filtration; and when the thickness of the porous main body is too large and the partial area of the porous main body is easy to reach the loading capacity, other areas also have larger sewage containing space, so that the sewage containing space in the filtering membrane is wasted. If the overall thickness of the porous body is too small, penetration of impurities in the filtration membrane is likely to occur, and the overall loading of the filtration membrane is too low.
Further, the thickness of the single-layer film peeled off from the porous body is 10 to 70 μm.
The thickness of the single-layer film peeled off from the porous body means the thickness of the single-layer film after hot pressing and stretching, and also means the thickness of the single-layer film in the porous body after molding. Therefore, when the thickness of the single-layer film is controlled within the above range, it is explained that the ratio of the thickness of the single-layer film to the thickness of the whole filtering film is controlled within a certain range, so that the damage to a large extent of pore structures on the single-layer film caused by the overlarge degree of compression and stretching of the single-layer film is avoided.
Further, at least one side surface of the porous body is provided with a doubling area.
Further, the surface of one side of the porous main body comprises a doubling area accounting for 10-50%.
The invention controls the liquid outlet surface of the filtering membrane to be provided with a doubling area, wherein the doubling area is at least two areas where fibers are combined and arranged or interweaved with each other, and the areas are formed by hot melt bonding among the fibers, the at least two areas where fibers are combined and arranged refer to the fibers which are mutually bonded after being melted side by side to form a whole, and the interweaving among the fibers refers to the part where the interweaving part among the fibers forms a whole due to the hot melt bonding.
On one hand, the existence of the doubling area can enable the porosity on the liquid outlet surface of the filtering membrane to be smaller and the precision to be higher; on the other hand, the formed doubling area can enable the liquid outlet surface of the filtering membrane to be smoother, so that fine fibers on the liquid surface are combined in the filtering membrane more stably, and the liquid is not easy to drop to pollute the liquid. The invention further limits the duty ratio of the doubling area, avoids the excessive reduction of the filtering efficiency caused by the overlarge area of the doubling area, and simultaneously avoids the incapability of achieving the effect caused by the overlarge area of the doubling area.
Further, the accuracy of the porous body is increased in a gradient from the liquid inlet level to the liquid outlet level in the thickness direction.
Further, the porous main body is divided into a first area, a second area and a third area from the liquid inlet surface to the liquid outlet surface in sequence; the porous bodies are respectively supplied with styrene microsphere suspension liquid with particle size near the filtration precision of the porous bodies at normal pressure;
the number of the trapped styrene microspheres in the first area, the second area and the third area is A respectively 1 、A 2 、A 3 ；
Wherein A is 3 ＞A 2 ＞A 1 。
The invention further limits the trend that the precision of the filtering membrane is gradually increased in the thickness direction of the filtering membrane, so that a prefilter area is formed on one side of the composite filtering membrane, which is close to the liquid inlet surface, and a high-precision area is formed on one side of the composite filtering membrane, which is close to the liquid outlet surface, thereby improving the filtering precision, the filtering effect and the filtering capacity of the filtering membrane.
Secondly, the filtering membrane in the invention can also control the interception capability of the filtering membrane to particles with different particle diameters in different areas in the thickness direction. Specifically, in the direction from the liquid inlet surface to the liquid outlet surface of the filtering membrane, the impurities of the large particles are mainly intercepted on one side of the filtering membrane close to the liquid inlet surface, and the impurities of the small particles are mainly intercepted on one side of the filtering membrane close to the liquid outlet surface, so that the phenomenon that the side of the composite filtering membrane close to the liquid inlet surface is prematurely blocked due to the fact that more adsorption interception occurs on one side of the small particles close to the liquid inlet surface of the composite filtering membrane is avoided, and the service life of the filtering membrane is shortened.
Further, the porosity P of the porous main body is 40-75%; wherein,v is the volume of the porous body, M is the mass of the porous body, ρ is the fiber material of the porous bodyAnd (5) material density.
The specific calculation method of the porosity is described above, wherein the porosity of the whole filtering membrane is limited. The size of the porosity means that the void fraction in the composite filtration membrane is macroscopically larger, and the higher the void fraction inside the filtration membrane is, the larger the filtration capacity and filtration flow rate of the filtration membrane should be in an ideal case. However, the present invention still needs to limit the porosity within a certain range, because when the porosity is beyond the range described in the present invention, the filtration membrane is difficult to regulate to an optimal state, that is, the highest accuracy, the optimal filtration flow rate and the optimal filtration capacity are combined. When the porosity is too large, the mechanical strength of the filtering membrane is obviously reduced, and the filtering membrane is easily damaged when filtering feed liquid with high viscosity or high pressure drop. When the porosity is too small, the upper limit of the filtration capacity of the filtration membrane is limited to be improved, and the filtration membrane is difficult to be suitable for the filtration of various feed liquid with high impurity content.
Further, the porous body has a BET specific surface area of 0.1 to 0.2cm 3 /g。
Further, the transverse tensile strength of the porous main body is 2-6 MPa, and the longitudinal tensile strength of the porous main body is 2.5-8 MPa.
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, selecting at least 2 layers of single-layer films for stacking;
s2, carrying out hot pressing and traction on the stacked single-layer films simultaneously, so that the single-layer films are combined to form a composite film; wherein the hot pressing temperature is 30-120 ℃, the hot pressing pressure is 0.2-1 MPa, and the stretching ratio is 1.05-1.2;
s3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 20-25 ℃, and the multi-layer composite filtering membrane is obtained.
The multi-layer filter composite film with the required orientation degree is prepared by controlling the technological parameters of hot pressing and stretching after the lamination of the single-layer film and synchronously controlling the hot pressing and the stretching. In the hot pressing process, the fibers in the single-layer film can be softened under the condition of heating, and the bonding effect of the fibers between the single-layer film and the single-layer film can be achieved under the action of pressure, so that the single-layer film and the single-layer film are mutually combined to form the composite filtering film. And the fiber is stretched to a certain extent in the hot pressing process, so that the heated and softened fiber is oriented under the action of the tensile force, and the prepared multi-layer composite filtering membrane has a good orientation degree.
Further, controlling the compression amount of the hot pressing in the step S2 to be 100-500 mu m;
the compression amount refers to the difference between the sum of the thicknesses of the single-layer films and the thickness of the composite film.
In the preparation process of the composite filtering membrane, the compression amount of hot pressing is a critical process parameter, because the degree of hot pressing can influence the orientation degree of single-layer membrane fibers, the size of bonding areas among the fibers and the like, the single-layer membranes are expected to be effectively bonded to form the composite membrane with certain structural strength, but too many agglomeration bonding areas are not expected to exist in the composite filtering membrane, so that the flow rate and the loading capacity of the filtering membrane are lost.
Further, the thickness of the single-layer film is 80-500 μm.
The thickness selection of the single-layer film is also an important parameter for preparing the multi-layer composite filtering film, and the thickness of the single-layer film can influence the fusion degree between fibers in the film and the composite strength between the single-layer films in the hot pressing process. The single-layer film is selected to be too thick, and a stable bonding effect can be achieved between the single-layer film and the single-layer film only by using larger pressure and higher temperature during hot pressing, but more parallel silk bonding phenomena can easily occur between fibers in the single-layer film under the larger hot pressing pressure and higher hot pressing temperature, so that the reduction of porosity is likely to be caused, and the flow rate and the loading capacity of the combined filtering film are reduced.
Further, preheating and pre-stretching are performed before hot pressing in the step S2, wherein the preheating temperature is 50-120 ℃, and the stretching ratio is 1.1-1.3.
The stacked single-layer films are also preheated and prestretched before step S2, which facilitates more pronounced orientation of the fibers in the single-layer films and a certain degree of variability in orientation between different single-layer films. Because the hot pressing is not carried out between the single-layer films at this time, the resistance between the single-layer films and between the fibers in the single-layer films is smaller, so that the pre-stretching of the films can have a better orientation effect under the heating condition, so that the single-layer films are all subjected to certain pre-orientation, and the orientation degree of the obtained multi-layer composite filtering film is better after the hot pressing and stretching process is carried out conveniently.
In summary, the invention has the following beneficial effects:
the first, the invention adopts the single-layer membrane stack formed by a plurality of melt-blown to compound into the compound filter membrane with certain orientation, so as to obtain higher filter precision and simultaneously give consideration to higher filter flow rate and loading capacity.
Secondly, the invention also reduces the bonding and doubling area in the multi-layer composite filter membrane by controlling the orientation degree of the multi-layer composite filter membrane and the distribution condition of the thickness of the fibers in the multi-layer composite filter membrane, and maintains the stability of the three-dimensional structure of the multi-layer composite filter membrane while improving the precision, thereby ensuring that the multi-layer composite filter membrane has higher filtration precision and better filtration flow rate and loading capacity.
Drawings
FIG. 1 is an SEM image of the inlet level of a multilayer composite filtration membrane of example 1 of the present application, at 500X magnification;
FIG. 2 is an SEM image of the liquid surface of the multilayer composite filtration membrane of example 1 of the present application, at 500X magnification.
Detailed Description
In order that the above-recited objects, features and advantages of the present application will be more clearly understood, a more particular description of the application will be rendered by reference to the appended drawings and appended detailed description. It should be noted that, without conflict, the embodiments of the present application and features in the embodiments may be combined with each other.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application, however, the present application may be practiced in other ways than those described herein, and therefore the scope of the present application is not limited to the specific embodiments disclosed below.
Examples
Example 1
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, stacking 5 single-layer films;
wherein, the monolayer film is polypropylene melt-blown film, and the thickness of the monolayer film is 100+/-5 mu m; the single-layer films with fiber diameters of 0.5 μm, 0.6 μm, 0.8 μm, 1.1 μm and 1.3 μm are respectively selected, and stacked in the thickness direction from the liquid inlet to the liquid outlet. The size of the filtering precision between the single-layer membranes corresponds to the fiber diameter of the single-layer membranes, the filtering precision of the single-layer membranes with large fiber diameters is lower, and the filtering precision of the single-layer membranes with small fiber diameters is higher. In this example, the filtration accuracy of the single-layer film was 0.6 μm, 0.7 μm, 1 μm, 1.5 μm, and 1.8 μm in this order from the smaller fiber diameter to the larger fiber diameter.
S2, preheating and prestretching the stacked single-layer films, wherein the preheating temperature is 110 ℃, the stretching ratio is 1.2, and then simultaneously hot-pressing and traction are carried out, so that the single-layer films are compounded to form a composite film, and the composite film is oriented;
wherein the hot pressing temperature is controlled to 120 ℃, the hot pressing pressure is 0.7MPa, the compression amount is 250+/-2 mu m, and the traction and stretching ratio is 1.1. The traction and stretching ratio is obtained by adjusting the tension of the hot press roller and the wind-up roller on the composite film.
And S3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the multi-layer composite filtering membrane can be obtained after shaping.
In this example, the thickness of the whole membrane of the molded multilayer composite filtration membrane was 358. Mu.m, and the thickness of the peeled single membrane was 71.+ -. 1. Mu.m.
Comparative example 1
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, stacking 5 single-layer films;
wherein, the monolayer film is polypropylene melt-blown film, and the thickness of the monolayer film is 100+/-5 mu m; the single-layer films with fiber diameters of 0.5 μm, 0.6 μm, 0.8 μm, 1.1 μm and 1.3 μm are respectively selected, and stacked in the thickness direction from the liquid inlet to the liquid outlet. The size of the filtering precision between the single-layer membranes corresponds to the fiber diameter of the single-layer membranes, the filtering precision of the single-layer membranes with large fiber diameters is lower, and the filtering precision of the single-layer membranes with small fiber diameters is higher. The filtration accuracy of the single-layer film was 0.6 μm, 0.7 μm, 1 μm, 1.5 μm, and 1.8 μm in this order from the smaller fiber diameter to the larger fiber diameter.
S2, hot-pressing the stacked single-layer films to enable the single-layer films to be combined to form a composite film;
wherein the hot pressing temperature is controlled to 120 ℃, the hot pressing pressure is 0.7MPa, and the compression amount is 250+/-2 mu m.
And S3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the multi-layer composite filtering membrane can be obtained after shaping.
In this example, the thickness of the whole membrane of the molded multilayer composite filtration membrane was 382. Mu.m, and the thickness of the peeled single membrane was 76.+ -.1. Mu.m.
Comparative example 2
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, stacking 5 single-layer films;
wherein, the monolayer film is polypropylene melt-blown film, and the thickness of the monolayer film is 100+/-5 mu m; the fiber diameter of the single-layer film was 0.5. Mu.m, and the filtration accuracy was 0.6. Mu.m.
S2, preheating and prestretching the stacked single-layer films, wherein the preheating temperature is 110 ℃, the stretching ratio is 1.2, and then simultaneously hot-pressing and traction are carried out, so that the single-layer films are compounded to form a composite film, and the composite film is oriented;
wherein the hot pressing temperature is controlled to 120 ℃, the hot pressing pressure is 0.7MPa, the compression amount is 250+/-2 mu m, and the traction and stretching ratio is 1.1.
And S3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the multi-layer composite filtering membrane can be obtained after shaping.
In this example, the whole membrane thickness of the molded multilayer composite filtration membrane was 321. Mu.m, and the thickness of the peeled single-layer membrane was 64.+ -. 1. Mu.m.
Comparative example 3
A method for preparing a single-layer filtration membrane, comprising the steps of:
s1, selecting a melt-blown film with the thickness of 500 mu m +/-2 mu m and the fiber thickness in gradient distribution, wherein the gradient distribution of the fiber thickness is similar to that of the single-layer film in the embodiment 1 after stacking, and the filtering precision is 0.6 mu m;
s2, preheating and prestretching the melt-blown film, wherein the preheating temperature is 110 ℃, the stretching ratio is 1.2, and then simultaneously hot-pressing and traction are carried out to enable the filter film to have orientation;
wherein the hot pressing temperature is controlled to 120 ℃, the hot pressing pressure is 0.7MPa, the compression amount is 250+/-2 mu m, and the traction and stretching ratio is 1.1.
S3, shaping the filter membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the filter membrane can be obtained after shaping.
In this example, the whole membrane thickness of the filtration membrane after molding was 366. Mu.m, and a single layer membrane could not be peeled from the whole membrane.
The following test was conducted on the composite films prepared in example 1 and comparative examples 1 to 3, and the test results are shown in table 1.
Detecting parameters:
(1) Fiber ratio test in the range of the required included angle with the orientation direction: the fiber orientation degree measurement mode can be that after the morphology of the structure of the filtering membrane is characterized by using a scanning electron microscope, computer software (such as Matlab, NIS-Elements and the like) or manual measurement is utilized, and corresponding calculation is carried out; when the measurement is actually carried out, the surface (or the section) of the filtering membrane can be characterized by an electron microscope to obtain a corresponding SEM image; then drawing a straight line segment which is vertical to the orientation direction and cuts at least 40 fibers in an SEM image (for example, 50 fibers are cut, the specific number of the cut fibers is determined according to the actual situation), and then recording the included angle between the extending direction and the orientation direction of the cut fibers;
selecting the ratio of the fibers with the included angles in the required range to the number of all the fibers, namely the ratio of the fibers with the included angles in the required range to the orientation direction; the fiber ratio in the range of the required included angle with the orientation direction can be obtained by respectively testing the liquid inlet surface and the liquid outlet surface of the filtering membrane and then taking an average value. Of course, the person skilled in the art can also obtain the above parameters by other measuring means, which are only used as reference.
In the present invention, the fiber ratio in the range of 30 ° with respect to the orientation direction and the fiber ratio in the range of 20 ° with respect to the orientation direction were tested.
In the SEM image of the liquid inlet surface, the fiber ratio within 20 degrees with the orientation direction is A in The method comprises the steps of carrying out a first treatment on the surface of the In the SEM image of the liquid level, the fiber proportion within 20 degrees of the orientation direction is A out ；
(2) Measurement of average fiber diameter: in the invention, the measuring mode of the average diameter of the fiber in the porous main body can be used for carrying out morphology characterization on the structure of the filtering membrane by using a scanning electron microscope, then computer software (such as Matlab, NIS-Elements and the like) or manual measurement is used for carrying out corresponding calculation; in practice, the surface (or cross section) of the filter membrane can be characterized by electron microscopy to obtain corresponding SEM image, and selecting a certain area such as 100 μm 2 (10 μm by 10 μm) or 625 μm 2 (25 μm by 25 μm), the specific area size is determined according to the actual situation, the diameters of all coarse fibers on the area are measured by corresponding computer software or manually, and then calculation is performed to obtain the average diameter of the area; of course, the person skilled in the art can also obtain the above parameters by other measuring means, which are only used as reference.
The ratio 1 is the ratio of the average diameter of the inlet face fibers SEM to the average diameter of the outlet face fibers SEM.
(3) Film thickness and monolayer film thickness after stripping: the whole film thickness can be obtained through testing by a thickness tester; the single-layer film is peeled from the whole film, and the thickness of the single-layer film is measured by a thickness tester with the single-layer film peeled completely as a standard.
(4) BET specific surface area test: the specific surface area of the filtration membrane was measured using a BET specific surface area tester.
(5) Porosity test: porosity of the porous materialV is the volume of the porous body, M is the mass of the porous body, ρ is the density of the fibrous material of the porous body.
(6) Area test of the doubling area: after the structure of the filtering membrane is subjected to morphological characterization by using a scanning electron microscope, the doubling area in the SEM image is marked with white with higher brightness, and then the image J test is utilized to calculate the duty ratio of the doubling area.
(7) gurley flow rate test: the flow rate measured using a gurley flow rate tester was an amount of 100ml of gas per porous body passing through a 1in2 filtration area.
(8) Load testing: filtering tap water by using a filter membrane disc with the diameter of 47mm, filtering a filter membrane with the filtering precision higher than 1 mu m by using a flow rate with the initial flow rate of 250ml/min, filtering a filter membrane with the filtering precision not higher than 1 mu m by using a flow rate of 1100ml/min, stopping filtering after the flow rate is attenuated to be 1/10 of the initial flow rate, and recording the total water quantity of the filtering to obtain the filtering capacity.
TABLE 1
The filtration accuracy of the filtration membranes prepared in example 1 and comparative examples 1 to 3 was within the range of 0.5.+ -. 0.05. Mu.m, and they were all multi-layer composite filtration membranes of higher accuracy.
Conclusion: referring to fig. 1 and 2, it is apparent from comparison of the detection results of the above-mentioned example 1 and comparative examples 1 to 3 that the present invention can control the degree of orientation of the fibers and the distribution of the fiber thickness in the thickness direction of the filtration membrane in the multi-layer composite filtration membrane, and can produce a filtration membrane having a high accuracy, a high flow rate and a high load more easily than a single-layer melt-blown membrane. Specifically, the higher the orientation degree of the fibers in the multi-layer composite membrane is controlled, and when the thickness change of the fibers in the thickness direction of the filtering membrane is in gradient taper, the multi-layer composite filtering membrane prepared by the invention has high precision, and meanwhile, the porosity in the filtering membrane is higher, the fluffiness condition of the filtering membrane is better, so that the loss of the flow and the loading of the filtering membrane can be reduced to a greater extent.
Example 2
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, stacking 4 layers of single-layer films;
wherein, the monolayer film is polypropylene melt-blown film, and the thickness of the monolayer film is 100+/-5 mu m; the single-layer films with fiber diameters of 0.5 μm, 0.6 μm, 0.8 μm and 1.1 μm are respectively selected, and stacked from thick to thin in the thickness direction of the fiber from the liquid inlet surface to the liquid outlet surface. The filtration accuracy of the single-layer membrane was 0.6 μm, 0.7 μm, 1 μm, and 1.5 μm in this order from the smaller fiber diameter to the larger fiber diameter.
S2, hot-pressing and simultaneously drawing the stacked single-layer films to enable the single-layer films to be combined to form a composite film, and enabling the composite film to be oriented;
wherein the hot pressing temperature is controlled to be 110 ℃, the hot pressing pressure is 0.8MPa, the compression amount is 140+/-2 mu m, and the traction and stretching ratio is 1.05.
And S3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the multi-layer composite filtering membrane can be obtained after shaping.
In this example, the thickness of the whole membrane of the molded multilayer composite filtration membrane was 260. Mu.m, and the thickness of the peeled single membrane was 65.+ -. 1. Mu.m.
Example 3
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, selecting 3 layers of single-layer films for stacking;
wherein, the monolayer film is polypropylene melt-blown film, and the thickness of the monolayer film is 100+/-5 mu m; the single-layer films with fiber diameters of 2 μm, 4 μm and 8 μm are respectively selected, and stacked in the thickness direction from the liquid inlet surface to the liquid outlet surface. The filtration accuracy of the single-layer membrane was 11 μm, 13 μm, and 15 μm in this order from the smaller fiber diameter to the larger fiber diameter.
S2, hot-pressing and simultaneously drawing the stacked single-layer films to enable the single-layer films to be combined to form a composite film, and enabling the composite film to be oriented;
Wherein the hot pressing temperature is controlled to be 100 ℃, the hot pressing pressure is 0.6MPa, the compression amount is 160+/-2 mu m, and the traction and stretching ratio is 1.1.
And S3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the multi-layer composite filtering membrane can be obtained after shaping.
In this example, the thickness of the whole membrane of the molded multilayer composite filtration membrane was 140. Mu.m, and the thickness of the peeled single membrane was 47.+ -. 1. Mu.m.
Filtration membranes were prepared in examples 2 to 3 and the same detection as in comparative example in example 1 was performed, and the detection results are shown in Table 2 below.
TABLE 2
| Examples | Example 2 | Example 3 |
| Oriented fiber ratio-30 ° (%) | 63 | 89 |
| Oriented fiber ratio-20 ° (%) | 38 | 61 |
| A in /A out | 0.79 | 0.65 |
| Liquid inlet level fiber SEM average diameter (mum) | 0.5 | 2.4 |
| Liquid surface fiber SEM average diameter (mum) | 0.1 | 0.8 |
| Ratio 1 | 5.0 | 3.0 |
| Thickness of porous body (μm) | 260 | 140 |
| Stripping film thickness (mu m) | 65 | 47 |
| BET specific surface area (cm) 3 /g) | 0.15 | 0.19 |
| Porosity (%) | 68 | 54 |
| Area ratio of doubling area (%) | 16 | 23 |
| Gurley flow Rate(s) | 19 | 8 |
| Filtration capacity (kg) | 25 | 67 |
Conclusion: the multi-layer composite filter membrane is obtained by compounding the single-layer membranes with different layers in the embodiment 2 and the embodiment 3, the filtering precision can still be kept in a higher precision range, the filtering precision in the embodiment 2 is 0.45 μm, the filtering precision in the embodiment 3 is 10 μm, and the filtering flow rate and the filtering capacity can still be maintained at a better level, so the number of layers of the multi-layer composite filter membrane can be selected according to actual needs.
The process parameters of examples 4 to 16 are different from those of example 1 as shown in Table 3 below.
TABLE 3 Table 3
Filtration membranes were prepared in examples 4 to 16, and the same detection as in comparative example in example 1 was performed, and the detection results are shown in Table 4 below.
TABLE 4-1
TABLE 4-2
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TABLE 4-3
The thickness of each of the single-layer films prepared in examples 4 to 16 was 100.+ -. 1. Mu.m, and 5-layer single-layer film stacks were used. The thickness of the finally prepared composite filtering membrane is 360+/-2 mu m. The filtration accuracy of the multilayer composite filtration membranes prepared in examples 4 to 16 was within the range of 0.5.+ -. 0.1. Mu.m, except that the filtration accuracy of the multilayer composite filtration membrane prepared in example 5 was higher than 0.22. Mu.m.
Conclusion: as is clear from the test results of examples 4 to 16, the specific distribution of the fiber thickness in the thickness direction of the filtration membrane is specifically controlled, and when the thickness distribution meets the preferred conditions of the invention, the permeability and the fluffiness in the composite filtration membrane are better, and the loss of the composite filtration membrane on the load and the flow rate is less. Secondly, the more the fiber ratio in the direction of orientation is, the better the bulk and permeability of the composite filtration membrane will be. Under the condition of the determination, the fluffiness and the permeability of the composite filter membrane can be further adjusted by adjusting the overall specific surface area, the porosity, the duty ratio of the doubling area and the like of the composite filter membrane, so that the filter membrane has higher flow velocity and higher loading capacity under higher precision.
Example 17
The preparation method of the multi-layer composite filtering membrane comprises the following steps:
s1, stacking 5 single-layer films;
wherein, the monolayer film is polypropylene melt-blown film, and the thickness of the monolayer film is 100+/-5 mu m; the single-layer films with fiber diameters of 0.5 μm, 0.6 μm, 0.5 μm, 0.8 μm and 1.3 μm are respectively selected, and stacked in the thickness direction from the liquid inlet to the liquid outlet.
S2, carrying out hot pressing and traction on the stacked single-layer films simultaneously, so that the single-layer films are compounded to form a composite film, and the composite film is oriented;
wherein the hot pressing temperature is controlled to 120 ℃, the hot pressing pressure is 0.8MPa, the compression amount is 250+/-2 mu m, and the traction and stretching ratio is 1.2. The traction and stretching ratio is obtained by adjusting the tension of the hot press roller and the wind-up roller on the composite film.
And S3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 25 ℃, and the multi-layer composite filtering membrane can be obtained after shaping.
In this example, the thickness of the whole membrane of the molded multilayer composite filtration membrane was 361. Mu.m, and the thickness of the peeled single membrane was 72.+ -. 1. Mu.m.
The multilayer composite films prepared in example 1 and example 17 were supplied with a styrene microsphere suspension (mainly comprising 5wt% of polystyrene microspheres, 0.5 to 1mol/L of sodium chloride and pure water) having a particle size of 0.5 μm at normal pressure, respectively, and filtration was terminated when the filtration flow rate was attenuated to 1/2 of the initial flow rate. And carrying out morphology characterization on the cross-sectional structure of the filtering membrane by using a scanning electron microscope, and equally dividing the filtering membrane into a first area, a second area and a third area from the liquid inlet level to the liquid outlet level along the thickness direction. The number of the trapped styrene microspheres in the first area, the second area and the third area is A respectively 1 、A 2 、A 3 ；
Wherein, example 1: a is that 3 ＞A 2 ＞A 1 The method comprises the steps of carrying out a first treatment on the surface of the Example 2: a is that 2 ＞A 1 ＞A 3 ；
And the actual loading test was performed on the multilayer composite films prepared in example 1 and example 17, the loading of example 17 being 14kg, which is much smaller than that of example 1.
The present embodiment is only for explanation of the present invention and is not to be construed as limiting the present invention, and modifications to the present embodiment, which may not creatively contribute to the present invention as required by those skilled in the art after reading the present specification, are all protected by patent laws within the scope of claims of the present invention.
Claims (18)
1. The multi-layer composite filtering membrane comprises a porous main body, wherein one side of the porous main body is a liquid inlet surface, the other side of the porous main body is a liquid outlet surface, and is characterized in that,
the porous body comprises a multilayer single membrane;
the porous main body has an orientation direction, and the porous main body meets the condition that the fiber ratio of which the included angle is within 30 degrees with the orientation direction is not less than 60 percent in any line segment which is perpendicular to the orientation direction and at least cuts more than 40 fibers on an SEM (scanning electron microscope) image of the liquid inlet surface or the liquid outlet surface of the porous main body;
the average diameter of the fibers in the porous main body gradually decreases from the liquid inlet level to the liquid outlet level along the thickness direction of the porous main body;
The porous body has a flow rate of 1in per pass as measured by a gurley flow rate tester 2 The gas amount of 100ml of the porous main body with the filtering area is 3-40 s;
the filtering precision of the porous main body is 0.1-10 mu m.
2. The multilayer composite filtration membrane according to claim 1, wherein the SEM average diameter of the porous body on the side near the liquid inlet surface is 0.4 to 8 μm; the SEM average diameter of the fiber of the porous main body near the liquid outlet surface is 0.1-4 mu m; the ratio of the SEM average diameter of the fiber on the side of the porous body close to the liquid inlet surface to the SEM average diameter of the fiber on the side of the porous body close to the liquid outlet surface is 1.3-20:1.
3. The multilayer composite filtration membrane according to claim 1, wherein the porous body satisfies a fiber ratio of 60 to 92% in an arbitrary line segment perpendicular to the orientation direction and cutting at least 40 fibers in the SEM electron microscope of the liquid inlet surface or the liquid outlet surface, the line segment having an angle of 30 ° or less with respect to the orientation direction.
4. The multilayer composite filtration membrane according to claim 1, wherein the porous body satisfies that the proportion of fibers having an included angle of 20 ° or less with respect to the orientation direction is not less than 30% in any line segment of at least 40 fibers cut off perpendicularly to the orientation direction on the SEM electron microscope image of the liquid inlet surface or the liquid outlet surface thereof.
5. A multi-layer composite filtration membrane according to claim 4,
in at least 40 or more arbitrary line segments of the SEM electron microscope image of the liquid inlet surface of the porous main body, which are perpendicular to the orientation direction, the fiber ratio of which the included angle with the orientation direction is within 20 DEG is A in ；
In at least 40 or more arbitrary line segments of the SEM electron microscope image of the liquid surface of the porous main body, which are vertical to the orientation direction, the fiber ratio A with an included angle of 20 degrees or less with the orientation direction is out ；
And A is in /A out ＝0.6～0.8。
6. A multilayer composite filtration membrane according to claim 1, wherein the porous body has a total thickness of 50 μm to 400 μm.
7. The multi-layer composite filtration membrane according to claim 6, wherein the thickness of the single-layer membrane peeled off from the porous body is 10 to 70. Mu.m.
8. A multi-layer composite filtration membrane according to claim 1, wherein the porous body has a doubling zone on the outlet surface.
9. A multi-layered composite filtration membrane according to claim 8, wherein said porous body has a side surface comprising a parallel wire region in an amount of 10 to 50%.
10. The multilayer composite filtration membrane of claim 1, wherein the porous body has a gradient of increasing accuracy from the inlet level to the outlet level in the thickness direction.
11. The multilayer composite filtration membrane of claim 10, wherein the porous body is divided into a first region, a second region and a third region in sequence from the liquid inlet surface to the liquid outlet surface; the porous bodies are respectively supplied with styrene microsphere suspension liquid with particle size near the filtration precision of the porous bodies at normal pressure;
the number of the trapped styrene microspheres in the first area, the second area and the third area is A respectively 1 、A 2 、A 3 ；
Wherein A is 3 ＞A 2 ＞A 1 。
12. The multilayer composite filtration membrane of claim 1, wherein the porous body has a porosity P of 40-75%; wherein,v is the volume of the porous body, M is the mass of the porous body, ρ is the density of the fibrous material of the porous body.
13. The multilayer composite filtration membrane according to claim 1, wherein the porous body has a BET specific surface area of 0.1 to 0.2cm 3 /g。
14. The multilayer composite filtration membrane of claim 1, wherein the porous body has a transverse tensile strength of 2 to 6MPa and a longitudinal tensile strength of 2.5 to 8MPa.
15. A method for producing a multi-layer composite filtration membrane according to any one of claims 1 to 14, comprising the steps of:
S1, selecting at least 2 layers of single-layer films for stacking;
s2, carrying out hot pressing and traction on the stacked single-layer films simultaneously, so that the single-layer films are combined to form a composite film; wherein the hot pressing temperature is 30-120 ℃, the hot pressing pressure is 0.2-1 MPa, and the stretching ratio is 1.05-1.2;
s3, shaping the composite membrane after hot pressing, wherein the shaping temperature is 20-25 ℃, and the multi-layer composite filtering membrane is obtained.
16. The method for producing a multi-layered composite filtration membrane according to claim 14, wherein the compression amount of the hot press in the step S2 is controlled to be 100 to 500 μm;
the compression amount refers to the difference between the sum of the thicknesses of the single-layer films and the thickness of the composite film.
17. A method for producing a multi-layered composite filtration membrane according to claim 15, wherein the thickness of said single-layered membrane is 80 μm to 500 μm.
18. The method for preparing a multi-layered composite filtering membrane according to claim 15, wherein the preheating and pre-stretching are further performed before the hot pressing in the step S2, the preheating temperature is 50-120 ℃, and the stretching ratio is 1.1-1.3.
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