Structure-controllable asymmetric composite ultrafiltration membrane, preparation method and application thereof

Abstract

The application provides a structure-controllable asymmetric composite ultrafiltration membrane, a preparation method and application thereof, and comprises an ultrafiltration separation layer and a microporous support layer, wherein the average pore diameter of the microporous support layer is greater than that of the ultrafiltration separation layer, the ultrafiltration separation layer comprises at least two interpenetrating pore diameter zones with increasing pore diameters from top to bottom, the microporous support layer comprises at least two interpenetrating pore diameter zones with increasing pore diameters from top to bottom, and the pores of the transition from the ultrafiltration separation layer to the microporous support layer are through structures. The structure-controllable asymmetric composite ultrafiltration membrane is prepared by a double-layer coating method, has high filtration precision and high flow rate, and has wide application in the field of ultrafiltration.

Description

Technical Field

[0001] This invention relates to the field of ultrafiltration membrane technology, and in particular to a structurally controllable asymmetric composite ultrafiltration membrane, its preparation method, and its application. Background Technology

[0002] Ultrafiltration is a pressure-driven membrane separation technology widely used in food, pharmaceuticals, chemicals, and water treatment. To meet the needs of different industries, various types of ultrafiltration membranes have been commercialized. Ultrafiltration membranes primarily rely on a dense surface separation layer to achieve separation. While this dense layer results in a high rejection rate, it also increases the membrane's mass transfer resistance, leading to a decrease in permeate flux. In other words, for current ultrafiltration membranes, high filtration accuracy and high flow rate are often mutually exclusive.

[0003] To obtain ultrafiltration membranes that balance high filtration accuracy and high flow rate, ultraasymmetric membranes with a dense surface and a porous bottom are a major research focus in the field of membrane separation. Traditional finger-pore ultrafiltration membranes have a large cavity structure at the bottom, which disadvantages them in terms of biosafety and mechanical strength. Furthermore, due to the limitations imposed by the dense skin layer of the ultrafiltration membrane, the fabricated sponge-pore ultrafiltration membrane exhibits relatively low structural asymmetry. Therefore, the development of a high-performance ultraasymmetric ultrafiltration membrane with a pore size gradient distribution is of great research significance.

[0004] Currently, the most common method for preparing ultrafiltration membranes on the market is single-layer coating and integral membrane formation. Ultrafiltration membranes prepared using this process are highly dependent on polymer properties and processing techniques, and have limited tunable asymmetric structures, making them less advantageous for preparing ultra-asymmetric ultrafiltration membranes. Over the past decade, research interest in co-coating or bi-layer coating for preparing multilayer filtration membranes has increased significantly. Multilayer filtration membranes refer to membranes formed by coating multiple layers of membrane solutions, thus retaining all the advantages of each coating layer and maximizing membrane performance. Currently, research on co-coating or bi-layer coating is still in its early stages. The main technical challenges in preparing multilayer filtration membranes using this technology are: layer delamination; the easy formation of skin layers between layers with clear boundaries; and the increased complexity and difficulty in controlling the phase separation process when managing the multilayer structure. Nevertheless, if a suitable scheme can be found to prepare composite membranes by coating casting solutions with different solid contents using multilayer coating technology, achieving the effect of using the top ultrafiltration layer to control filtration precision and the bottom microporous layer to control filtration speed, the performance of the resulting composite ultrafiltration membrane will be significantly improved.

[0005] Of course, existing technologies also include research on the preparation of multilayer filtration membranes through co-coating or double-layer coating. For example, CN1759924B provides a composite ultrafiltration membrane prepared by co-coating. The co-coating technology utilizes a co-casting coating head, and the coating process has no time interval. Both solutions cast into layers on the carrier surface have lower critical dissolution temperatures (LCSTs), with the microporous layer having a lower LCST. First, the cast multilayer liquid sheet is heated to a temperature higher than the LCST of the microporous layer but lower than the LCST of the ultrafiltration layer. This causes the materials in the microporous layer to become immiscible, forming a phase-separated solidification. The ultrafiltration layer then undergoes phase-separated solidification in a subsequent coagulation bath, ultimately forming a wet multilayer ultrafiltration membrane. The phase separation mechanisms of the two liquid membranes are completely different. However, this preparation process has many limitations. The microporous layer must be solidified first during preparation, and the ultrafiltration membrane layer remains in a liquid state for a long time, which may lead to a decrease in the uniformity of the ultrafiltration membrane layer. This results in poor pore structure stability after phase-separated solidification, which is not conducive to industrial scale-up.

[0006] CN115770490B discloses an asymmetric cellulose virus-removing filter membrane and its preparation process. The filter membrane uses hydrophilic cellulose as the film-forming material and employs a double-layer coating process, using casting solutions with different solid contents to form pore structures with small and large pore sizes respectively. Since the two casting solutions are largely identical, a pretreatment bath composed of a solvent-water solution is used after casting. The dilution effect of the solvent in the pretreatment bath significantly reduces the possibility of the second casting solution forming a skin layer. However, the solvent in the pretreatment bath also affects the first casting solution, making scale-up of the process difficult. Furthermore, this patent uses cellulose to prepare the membrane, but cellulose materials have poor mechanical strength, which limits its practical application.

[0007] In summary, for ultrafiltration membranes, it is often difficult to achieve both high filtration accuracy and high flow rate simultaneously. Composite ultrafiltration membranes that balance filtration accuracy and flow rate can be prepared using co-coating or sequential coating techniques. However, the technology for preparing multilayer filtration membranes using co-coating or sequential coating techniques is still in its early stages and cannot meet the needs of practical applications. Summary of the Invention

[0008] The purpose of this invention is to provide a structure-controllable asymmetric composite ultrafiltration membrane, its preparation method, and its application. The structure-controllable asymmetric composite ultrafiltration membrane, comprising a structure-controllable ultrafiltration layer and a microporous layer, is prepared using polyethersulfone as the film-forming polymer and a multilayer coating process. Both the ultrafiltration layer and the microporous layer include multiple sub-regions with different pore sizes to achieve both high filtration accuracy and high flow rate.

[0009] Firstly, this solution provides a structurally controllable asymmetric composite ultrafiltration membrane, comprising:

[0010] The ultrafiltration separation layer and the microporous support layer are provided, wherein the average pore size of the microporous support layer is larger than the average pore size of the ultrafiltration separation layer, and the ultrafiltration separation layer includes at least two interconnected pore size regions with pore size increasing sequentially from top to bottom, and the microporous support layer includes at least two interconnected pore size regions with pore size increasing sequentially from top to bottom, and the pores in the transition from the ultrafiltration separation layer to the microporous support layer are through-hole structures.

[0011] In some embodiments, the ultrafiltration separation layer includes interconnected first pore size region A1, second pore size region A2, and third pore size region A3 with pore sizes increasing sequentially from top to bottom, and the microporous support layer includes interconnected fourth pore size region A4 and fifth pore size region A5 with pore sizes increasing sequentially from top to bottom. The pore size of the third pore size region A3 is larger than the pore size of the fourth pore size region A4, and the pores of the fourth pore size region A4 penetrate through the pores of the third pore size region A3 to the pores of the fifth pore size region A5.

[0012] In some preferred embodiments, the aperture of the first aperture region A1 is 1-20 nm, the aperture of the second aperture region A2 is 10-70 nm, the aperture of the third aperture region A3 is 60-300 nm, the aperture of the fourth aperture region A4 is 200-1000 nm, and the aperture of the fifth aperture region A5 is 500-2000 nm.

[0013] In this scheme, the fourth pore area A4, which is placed on the upper layer of the microporous support layer, serves as a transition layer. This filter layer has an interconnected pore structure, which transitions the pore size of the third pore area A3 to the pore size of the fifth pore area A4, thus avoiding the formation of a boundary line between the microporous support layer and the ultrafiltration separation layer.

[0014] In this design, the pore size of the ultrafiltration separation layer is smaller than that of the microporous support layer. This ensures the membrane's filtration accuracy through the small pore size of the ultrafiltration separation layer, while the porous, interconnected macropores of the microporous support layer increase the membrane's flow rate. Consequently, the structurally controllable asymmetric composite ultrafiltration membrane provided by this design possesses both high filtration accuracy and high flow rate. Furthermore, both the ultrafiltration separation layer and the microporous support layer comprise multiple pore size regions, resulting in a gradient separation of pore sizes in the overall structurally controllable asymmetric composite ultrafiltration membrane, with the pore size increasing from the ultrafiltration separation layer side towards the microporous support layer side.

[0015] In some embodiments, the thickness of the ultrafiltration separation layer is 30%-60% of the total thickness of the structurally controllable asymmetric composite ultrafiltration membrane.

[0016] In some embodiments, the thickness of the ultrafiltration separation layer is 50-300 micrometers, the first pore region A1 is a dense separation layer with a thickness of 5-30 micrometers, the second pore region A2 has a thickness of 10-50 micrometers, and the third pore region A3 has a thickness of 10-100 micrometers.

[0017] In some embodiments, the thickness of the microporous support layer is 50-300 micrometers, the fourth pore area A4 is a transition layer with a thickness of 10-50 micrometers, and the fifth pore area A5 is a bottom support layer with a thickness of 100-200 micrometers.

[0018] like Figure 2 The illustration shows a specific embodiment. The porous matrix of the controllable asymmetric composite ultrafiltration membrane provided in this solution is interconnected. The pores of the ultrafiltration separation layer are interconnected with each other and with the pores of the transition layer. The microporous support layer regions are interconnected with each other and with the pores of the transition layer. The pores of the ultrafiltration separation layer have an average pore size that increases from top to bottom. The pores of the transition layer connect the downstream of the ultrafiltration separation layer and the upstream of the microporous support layer. The average pore size of the microporous support layer is larger than that of the ultrafiltration separation layer. The pore size of the porous matrix of the controllable asymmetric composite ultrafiltration membrane exhibits a gradient distribution, and the overall pores of the membrane are interconnected, resulting in a high porosity. The dense skin of the ultrafiltration separation layer can retain large molecules in the feed solution. Water molecules passing through the dense skin layer are rapidly filtered out through the interconnected pores within the porous matrix. The controllable asymmetric composite ultrafiltration membrane matrix with different interconnected pore structures can further intercept substances in the feed solution, reducing the possibility of local membrane clogging. The membrane's fouling resistance is superior to that of symmetric membranes.

[0019] The structure-controllable asymmetric composite ultrafiltration membrane provided in this solution is prepared using a membrane-forming polymer, represented by polyethersulfone, to obtain the membrane solution. The membrane solution is prepared using a multi-layer coating process. The polyethersulfone-based membrane-forming polymer exhibits excellent comprehensive properties, including both mechanical and chemical properties. Furthermore, the pore structure of the ultrafiltration separation layer and the microporous support layer of the structure-controllable asymmetric composite membrane is controlled by adjusting the ratio of the two-layer casting solution. The thickness of the ultrafiltration separation layer and the microporous support layer is controlled by the distance between the membrane tank containing the casting solution and the carrier, the coating amount, and the scraping process.

[0020] Specifically, the pore structure of the structure-controllable asymmetric composite ultrafiltration membrane is comprehensively controlled by the solid content of the casting liquid, the ratio of solvent to non-solvent, and pre-phase separation. The thickness of each pore region of the structure-controllable asymmetric composite ultrafiltration membrane is controlled by the distance from the membrane groove kerf to the surface of the receiving coating, the coating amount, and the coating speed.

[0021] Unlike traditional double-layer coated composite membrane processes, which use a higher solids content liquid as the first casting liquid and a lower solids content liquid as the second casting liquid, this method involves coating the first casting liquid with the higher solids content first, followed by the second casting liquid with the lower solids content. In this approach, the smaller pore size of the higher solids content liquid film makes it easy for a skin layer to form between the first and second casting liquid layers. The boundary between the layers is very obvious, leading to a sharp increase in mass transfer resistance at the boundary and consequently a decrease in the overall performance of the composite membrane. Furthermore, the second casting liquid, with its larger pore structure, is applied to the first casting liquid, which has a smaller pore structure, easily clogging the boundary structure and further degrading performance.

[0022] This scheme employs different technical methods: the ultrafiltration separation layer is obtained by coating with a second casting solution, and the microporous support layer is obtained by coating with a first casting solution, wherein the solid content of the first casting solution is lower than that of the second casting solution. Because this scheme coats the microporous support layer first and then the ultrafiltration separation layer last, the dense skin of the ultrafiltration membrane separation layer can easily affect the phase separation and solidification of the underlying microporous support layer. This can easily lead to the formation of finger-like pore structures after the microporous support layer is immersed in water, or even the lower layer failing to solidify, which is detrimental to biosafety and results in uncontrollable pore size. Therefore, this scheme appropriately increases the viscosity of the first casting solution by adding additives, slowing down the molecular chain movement during phase separation. A slower phase separation rate is beneficial for generating a sponge-like pore structure.

[0023] Common microporous support layer systems with lower solids content have lower viscosity. When immersed in a coagulation bath, the rapid infiltration of non-solvents into the casting solution causes instantaneous phase separation, resulting in large cavity structures. Therefore, this approach increases the viscosity of the microporous support layer with lower solids content, making it more difficult for non-solvents to diffuse into the casting solution, increasing the phase separation delay time, curing time, and gelation speed, which helps to generate a loose, porous, sponge-like structure. When preparing a single-layer membrane, the viscosity of the membrane solution system can often be effectively improved by simply adding various additives such as alcohols and salts, effectively generating a complete and undamaged porous structure. However, because a layer of membrane solution is coated on top of the microporous support layer, the liquid film replacement at the interface between the non-solvent and the membrane is slow, easily forming a dense skin layer. Direct immersion of the bottom layer in the coagulation bath will quickly form a cavity structure. To address the problem of the lower layer of the microporous support layer not being able to cure, this application uses a two-layer casting solution system with similar composition and the same phase separation mechanism.

[0024] Furthermore, the second casting solution designed in this scheme has a high viscosity to ensure that it does not rapidly penetrate to the bottom microporous support layer and damage the underlying structure during application. Simultaneously, this scheme uses casting solutions with similar systems as the first and second casting solutions to ensure that the microporous support layer and the ultrafiltration separation layer have similar or identical systems and thus the same separation mechanism. It should be noted that because the second casting solution has a higher solids content, its viscosity is also greater than that of the first casting solution.

[0025] In addition, in some embodiments, the film-forming polymers of the first casting solution and the second casting solution are selected from one of polyvinylidene fluoride, polyethersulfone, polysulfone, polyarylsulfone, polyphenylsulfone, polyvinyl chloride, polycarbonate, cellulose, and regenerated cellulose, preferably polyethersulfone.

[0026] In existing technologies, the prepared composite ultrafiltration membrane forms a clear boundary line between the two layers. The abrupt drop in pore size from macropores to micropores leads to higher interfacial resistance, causing a rapid decrease in membrane flux. This solution employs a double-layer coating technique combined with the membrane formulation and process to eliminate the formation of a skin layer or abrupt pore size changes at the interface. This solution generates a fourth pore size region A4 with interconnected pores as a transition layer. The upper and lower transition regions of this transition layer have gradient pore sizes, avoiding the probability of a sharp increase in mass transfer resistance in the transition layer. Simultaneously, the thickness of the structurally controllable asymmetric composite ultrafiltration membrane can be adjusted through process control. The double-layer coating process in this solution includes co-coating and sequential coating; this solution preferably uses a sequential multilayer coating process. At this point, if... Figure 1 As shown, the coating equipment consists of two roller-lined doctor blade coating devices or other pressure coating devices, and the interval between the doctor blades is adjustable.

[0027] Secondly, this solution provides a method for preparing a structure-controllable asymmetric composite ultrafiltration membrane, including:

[0028] A first casting solution and a second casting solution are prepared using polymers, solvents and additives as components, wherein the solid content of the first casting solution is lower than the solid content of the second casting solution.

[0029] The first casting solution and the second casting solution are sequentially cast onto the carrier. The first casting solution is first quantitatively coated to form a first liquid film, and then the second casting solution is quantitatively coated on the first liquid film to form a second liquid film. The coated first liquid film and the second liquid film are pre-separated for 0-300s at 35-55℃ and 30-90% humidity.

[0030] The first liquid film and the second liquid film are immersed in a coagulation bath to solidify and obtain a solidified film.

[0031] Washing and curing the membrane yields a structurally controllable asymmetric composite ultrafiltration membrane.

[0032] The casting systems of the first and second casting solutions in this scheme are similar or identical, and the polymer is selected from one of polyvinylidene fluoride, polyethersulfone, polysulfone, polyarylsulfone, polyphenylsulfone, polyvinyl chloride, polycarbonate, cellulose, and regenerated cellulose. In some embodiments, the polymer is preferably polyethersulfone.

[0033] The solvent is selected from one or more of N-dimethylformamide, N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, and butyl lactate.

[0034] The additives are used to modify and control the final pore size and porosity of the membrane, including the percentage of porosity and pore size distribution. The additives include one or more of polyvinylpyrrolidone, polyethylene glycol, water, methanol, ethanol, pentanol, hexanol, heptanol, acetone, butyl ether, ethyl acetate, calcium chloride, magnesium chloride, and lithium chloride. It is particularly important that the thermal stability of the two liquid membranes be similar. Under high temperature and humidity conditions, simply adding common pore-forming agents such as alcohols and salts can lead to excessively slow phase separation during the pre-phase separation process of the microporous support layer. Direct immersion in the coagulation bath can prevent the bottom of the microporous support layer from solidifying. To improve this problem, the preferred additive in this application is a mixture of alcohol, water, and polyvinylpyrrolidone. The addition of alcohol can effectively control the viscosity of the system, restrict the movement of polymer molecular chains, and effectively inhibit the formation of macropores during the phase separation process. The addition of water can regulate the stability of the casting solution system, causing the cloud point of the casting solution to move closer to the binode line, thereby regulating the phase formation rate in the pre-evaporation process section, and improving the compatibility between hydrophilic additives.

[0035] It should be noted that this scheme employs a double-layer coating casting solution preparation process, with the second casting solution serving as the top layer. In a high-temperature, high-humidity air medium, the skin layer of the second casting solution is formed at the air interface during the pre-evaporation stage. At this interface, the solvent exchanges with water vapor in the air medium. At this point, the replacement of non-solvent and solvent is relatively slow, and the phase separation rate is slower than that of the formation of a small and dense skin layer. At this stage, the liquid film has a slightly whitish surface, and the dense skin layer formed is region A1. The pre-phase separated film is then immersed in a coagulation bath. The solvent in the bottom liquid film is rapidly replaced by water in the coagulation bath, resulting in a faster phase separation rate and the formation of region A3 corresponding to large pores. However, due to the already formed skin layer, the transition layer from the skin layer to the bottom is affected by the skin layer resistance, causing a decrease in the mass transfer rate of the solvent and coagulation bath medium in the support layer. The phase separation rate is slower than that of the bottom layer, forming pores even smaller than those of the bottom layer, which is region A2.

[0036] In some embodiments, the distance between the second film tank containing the second casting liquid and the carrier in the coating apparatus is greater than the distance between the first film tank containing the first casting liquid and the carrier, to prevent the second liquid film from damaging the first liquid film. For example... Figure 1As shown, the coating equipment consists of two pressurized coating devices. The coating amount of the first and second casting solutions can be adjusted according to the pressure for quantitative coating. The coating thickness is controlled by the distance between the outlet of the film tank and the carrier, as well as the coating amount. The coating interval is controlled by the coating speed and the distance between the doctor blades. The preferred coating speed is 0.1-5 m / min, a further preferred coating speed is 0.1-1 m / min, and the coating thickness is 100-800 micrometers, with a more preferred coating thickness of 100-400 micrometers. The coating thickness of the second casting solution is greater than that of the first casting solution, so that the side and back of the second doctor blade will not drag over the first cast coating layer and damage the surface of the first cast coating. There is a delay of approximately 1-50 seconds between the casting of the first and second concentrated liquid layers.

[0037] like Figure 1 As shown, the coating equipment includes a first coating device for coating a first casting liquid, a second coating device for coating a second casting liquid, and a carrier. The first coating device contains a first film tank 3 and a first scraper, and the second coating device contains a second film tank 4 and a second scraper. A scraper gap 5 is provided between the first scraper and the second scraper. The first casting liquid enters the first film tank 3 at a first casting liquid feed rate 1 and forms a first liquid film 6 on the carrier 8 at a coating rate 9. The second casting liquid enters the second film tank 4 at a second casting liquid feed rate 2 and forms a second liquid film 7 on the carrier 8 at a coating rate 9.

[0038] In some embodiments, the first and second liquid films after casting are preferably pre-separated for 30-90 seconds under high temperature and high humidity conditions of 35-55°C and 60-80%. This is because volatile solvents are more likely to escape under high temperature conditions, while water molecules in the environment under high humidity conditions first replace the top surface of the ultrafiltration separation layer to form a dense skin with high porosity. Under high temperature conditions, the amount of solvent evaporating from the casting solution decreases sequentially from the ultrafiltration separation layer, the transition layer, and the microporous support layer. After a period of pre-separation, an ultrathin skin first forms on the surface of the ultrafiltration separation layer. After the high-temperature solvent evaporates, the solid content of the transition layer decreases, reducing the possibility of a sharp drop in interfacial pore size.

[0039] In some embodiments, the first and second liquid films after pre-phase separation enter a coagulation bath for phase separation and solidification, with a solidification time of approximately 10 minutes. The initial mass transfer rate between the solvent and the coagulation bath medium in the liquid film corresponding to the bottom microporous support layer is very fast, resulting in a rapid phase separation rate and the formation of large pores. However, due to the resistance affecting the transition support layer from the already formed dense skin layer to the bottom layer, the mass transfer rate between the solvent and the coagulation bath medium in the support layer decreases, and the phase separation rate is slower than that of the bottom layer. The resulting overall structure is a sponge-like structure, and the formed pores are interconnected.

[0040] In some embodiments, the cured membrane is immersed in pure water for 30 minutes to rinse off the residual solvent, and then the washed composite is stored in 20-30% glycerol to complete the preparation of the finished product of the structure-controllable asymmetric composite ultrafiltration membrane.

[0041] Thirdly, this solution provides a structure-controlled asymmetric composite ultrafiltration membrane with the above-described structure, exhibiting high filtration accuracy and high flow rate, and is applicable in the field of ultrafiltration. In some embodiments, the flow rate of the structure-controlled asymmetric composite ultrafiltration membrane of this solution is 500 LMH / bar, and the BSA rejection rate reaches over 97%.

[0042] This application addresses the problem that existing single-layer coated membranes cannot simultaneously achieve high filtration accuracy and high filtration flow rate. It employs a multi-layer coating technique to prepare a composite ultrafiltration membrane. The prepared composite ultrafiltration membrane contains at least one ultrafiltration separation layer and at least one microporous support layer. This composite ultrafiltration membrane possesses an ultra-asymmetric structure and exhibits excellent overall performance. Traditional two-layer coating techniques have many drawbacks. In particular, the most common coating sequence currently involves first coating a high-solids-content ultrafiltration layer followed by a low-solids-content microfiltration layer. This results in rapid formation of small, dense pores on the ultrafiltration layer surface. Applying the microfiltration layer under this sequence easily leads to the formation of a skin layer or a sharp drop in pore size, causing a rapid increase in mass transfer resistance at the interface and a rapid decrease in membrane permeation rate. In contrast, the composite ultrafiltration membrane structure of this application exhibits a gradient pore size distribution at the interface, and the porous core of the composite ultrafiltration membrane is interconnected, effectively solving the aforementioned problems. Furthermore, this application uses a first casting liquid with low solid content to coat the microporous support layer and a second casting liquid with high solid content to coat the ultrafiltration separation layer. In order to prevent the first liquid membrane from being immersed in the coagulation bath and rapidly displacing with the coagulation bath to form a large finger-like pore structure, this application makes corresponding improvements to the formulation of the first and second casting liquids and the process for forming the bilayer structure.

[0043] Common casting systems with lower solids content microporous support layers have lower viscosity. When immersed in a coagulation bath, the non-solvent rapidly enters the casting solution, resulting in instantaneous phase separation and the formation of large cavity structures. Therefore, this application increases the viscosity of the casting system with lower solids content microporous support layers, making it more difficult for the non-solvent to diffuse into the casting solution, increasing the phase separation delay time, curing time, and gelation speed, which helps to generate a loose, porous, sponge-like structure. When preparing single-layer membranes, the viscosity of the membrane solution system can often be effectively improved by simply adding various pore-forming agents such as alcohols and salts, effectively generating a complete and undamaged porous structure. However, because a liquid film is coated on the microporous support layer, the replacement of the non-solvent with the liquid film at the interface is slow, easily forming a dense skin layer, leading to performance degradation. To address this problem, this application uses a two-layer casting solution system with similar composition and the same phase separation mechanism.

[0044] Compared with the prior art, this application has the following beneficial effects:

[0045] 1. The composite ultrafiltration membrane prepared by double-coating retains both the filtration precision of the top ultrafiltration separation layer and the loose, porous structure of the microporous support layer. The pore structure of the transition layer connecting the ultrafiltration separation layer and the microporous support layer is interconnected. By controlling the ratio of the casting solution and the membrane preparation process, the membrane performance degradation caused by the steep pore size drop at the interface in traditional double-coating methods is overcome. The composite ultrafiltration membrane prepared in this application exhibits excellent performance in terms of filtration precision and filtration speed.

[0046] 2. In the controllable asymmetric composite ultrafiltration membrane, the pore sizes of multiple pore regions within the ultrafiltration separation layer increase sequentially, and the pores are interconnected. The transition layer connecting the bottom of the ultrafiltration separation layer and the top of the microporous support layer is also interconnected. Near the transition layer, the pore size gradually increases and connects with the pores of the microporous support layer. The pore size of the microporous support layer of the composite ultrafiltration membrane increases sequentially, and the pores are interconnected. The average pore size of the ultrafiltration separation layer is much smaller than the average pore size of the microporous support layer. The porous structure of the composite ultrafiltration membrane is ultra-asymmetric, and the overall channels of the porous structure are interconnected. The pore size of the porous structure of the composite ultrafiltration membrane exhibits a gradient distribution, resulting in excellent membrane fouling resistance.

[0047] 3. The pore structure of the ultrafiltration separation layer and microporous support layer of the composite ultrafiltration membrane can be controlled through coating and membrane fabrication processes. The coating amount can be controlled according to the membrane thickness, and the membrane solution can be quantitatively applied to make the structure of the composite ultrafiltration membrane controllable. This application not only has applications in the field of ultrafiltration but also provides inspiration for the preparation of ultraasymmetric microfiltration membranes. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of a coating equipment for double-layer coating.

[0049] Figure 2 The image shown is a scanning electron microscope image of the porous bulk of the structure-controllable asymmetric composite ultrafiltration membrane of Example 1.

[0050] Figure 3 The image shown is a scanning electron microscope image of the ultrafiltration layer of the structure-controllable asymmetric composite ultrafiltration membrane in Implementation Case 1.

[0051] Figure 4 The image shown is a scanning electron microscope image of the transition layer of the structure-controllable asymmetric composite ultrafiltration membrane in Implementation Case 1.

[0052] Figure 5 The image shown is a scanning electron microscope (SEM) image of the structure-controllable asymmetric composite ultrafiltration membrane of Comparative Example 1.

[0053] Figure 6 The image shown is a scanning electron microscope image of the structure-controllable asymmetric composite ultrafiltration membrane in Implementation Case 2;

[0054] Figure 7This is a schematic diagram showing five pore size regions marked with different pore size distributions;

[0055] In the figure: 1-First casting solution feed rate, 2-Second casting solution feed rate, 3-First film tank, 4-Second film tank, 5-Scraper gap, 6-First liquid film, 7-Second liquid film, 8-Carrier, 9-Coating rate. Detailed Implementation

[0056] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0057] Those skilled in the art should understand that, in the disclosure of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limiting this invention.

[0058] Implementation Case 1

[0059] S1. Prepare the first casting solution and the second casting solution according to Table 1. The polymer of both the first and second casting solutions is polyethersulfone, the solvent of both the first and second casting solutions is N-methylpyrrolidone, and the additives of both the first and second casting solutions are a mixture of polyethylene glycol 400, water, and polyvinylpyrrolidone.

[0060] S2. Casting. The first and second casting solutions are sequentially cast onto the carrier to form a bilayer liquid film. The first casting solution is applied first, followed by the second casting solution. The coating is done quantitatively, with the first doctor blade having a thickness of 200 micrometers and the second doctor blade having a thickness of 300 micrometers. The ratio of the ultrafiltration layer thickness to the microporous layer thickness is 1:2. The coated bilayer nascent liquid film is pre-separated for 90 seconds at 35°C and 80% humidity.

[0061] S3. Immerse the membrane liquid in a coagulation bath, which is water, so that the curing liquid penetrates into the membrane liquid and gradually diffuses inward to solidify into a film. The coagulation time is 10 minutes.

[0062] S4. Wash the solidified membrane with pure water and store it in glycerol with a glycerol content of 30% to complete the preparation of the structure-controllable composite ultrafiltration membrane.

[0063] Casting solutions with higher solid content form a dense skin with smaller pores more quickly, while casting solutions with lower solid content form a microporous support layer more slowly in a stable phase separation state. This easily leads to the problem that the ultrafiltration separation layer is completely solidified, but the microporous support layer has not yet solidified. To solve this problem, this application adds non-solvent water as one of the additives to the first casting solution. The addition of water makes the membrane solution state closer to the binode line, and the system is in a thermally unstable state, which shortens the phase separation time and effectively improves the phenomenon that the microporous layer as the bottom layer has too slow phase separation, resulting in uncontrollable pore size. Figures 2 to 4 To implement the composite ultrafiltration membrane prepared in Case 1, the thickness ratio of the ultrafiltration separation layer and the microporous support layer is close to 1:2, and the pore size of the transition layer between the layers is interconnected, which greatly improves the performance of the prepared membrane. Figure 2-4 These are the overall structure diagram of the composite membrane, the ultrafiltration layer of the composite membrane, and the transition layer between the ultrafiltration layer and the microporous support layer. It can be seen that the composite ultrafiltration membrane prepared by this method has a pore size gradient distribution, and the pores in the transition layer are interconnected. From... Figure 2 As can be seen, the composite ultrafiltration membrane is defect-free, highly asymmetric, and has an overall sponge-like pore structure. Figure 3 As can be seen, the ultrafiltration separation layer of the composite ultrafiltration membrane has a gradient pore size distribution. Figure 4 As can be seen, the pores at the bottom of the transition layer and ultrafiltration separation layer and the top of the microporous support layer are interconnected, which ensures the high flow rate of the composite membrane.

[0064] Comparison Case 1:

[0065] S1. Prepare the first casting solution and the second casting solution according to Table 1. The polymer of both the first casting solution and the second casting solution is polyethersulfone, the solvent of both the first casting solution and the second casting solution is N-methylpyrrolidone, and the additives of both the first casting solution and the second casting solution are a mixture of polyethylene glycol 400 and polyvinylpyrrolidone.

[0066] S2. Casting: The first and second casting solutions are sequentially cast onto the carrier to form a double-layer liquid film. Specifically, the second casting solution is applied after the first, and the application is quantitative. The thickness of the first doctor blade is 200 micrometers, and the thickness of the second doctor blade is 300 micrometers. The coated double-layer nascent liquid film is pre-separated for 90 seconds at 35°C and 80% humidity.

[0067] S3. Immerse the membrane liquid in a coagulation bath, which is water, so that the curing liquid penetrates into the membrane liquid and gradually diffuses inward to solidify into a film. The coagulation time is 10 minutes.

[0068] S4. Wash the solidified membrane with pure water and store it in glycerol with a glycerol content of 30% to complete the preparation of the structure-controllable composite ultrafiltration membrane.

[0069] Compared to the first casting solution in Implementation Case 1, the casting solution system is more stable due to the reduced addition of non-solvents. After a brief pre-phase separation process, a dense skin has formed on the surface of the ultrafiltration separation layer. After immersion in the coagulation bath for displacement, the already formed skin slows down the exchange of solvents and non-solvents in the system, resulting in progressively increasing pore sizes. The microfiltration support layer remains stable after pre-phase separation. After immersion in the coagulation bath, the bottom liquid film rapidly displaces the non-solvents, forming a large cavity structure. This structure does not offer any practical advantages. Figure 5 To compare the electron microscope images of the composite ultrafiltration membrane generated in Case 1, it is clearly observed from the electron microscope images that the ultrafiltration separation layer of the generated composite ultrafiltration membrane is relatively complete, but the bottom of the microporous support layer has a very large cavity structure, which further illustrates the effectiveness of this application.

[0070] Implementation Case 2

[0071] S1. Prepare the first casting solution and the second casting solution according to Table 1. The polymer of both the first and second casting solutions is polyethersulfone, the solvent of both the first and second casting solutions is N-methylpyrrolidone, and the additives of both the first and second casting solutions are a mixture of propylene glycol, polyvinylpyrrolidone, and water.

[0072] S2. Casting: The first and second casting solutions are sequentially cast onto the carrier to form a double-layer liquid film. Specifically, the second casting solution is applied after the first, using a quantitative method. The thickness of the first doctor blade is 200 micrometers, and the thickness of the second doctor blade is 400 micrometers. The coated double-layer nascent liquid film is pre-separated for 120 seconds at 35°C and 80% humidity.

[0073] S3. Immerse the membrane liquid in a coagulation bath, which is water, so that the curing liquid penetrates into the membrane liquid and gradually diffuses inward to solidify into a film. The coagulation time is 10 minutes.

[0074] S4. Wash the cured membrane with pure water and store it in glycerol with a glycerol content of 30%, thus completing the preparation of the structure-controllable composite ultrafiltration membrane.

[0075] Figure 6 The electron microscope (EM) image of the composite membrane prepared according to Example 2 shows that the composite ultrafiltration membrane prepared according to Example 2 has a significant bilayer structure, with the ratio of the ultrafiltration separation layer and the microporous support layer being 1:1. The EEM image further illustrates the controllability of the structure and the repeatability of the process of this invention.

[0076] Implementation Case 3

[0077] S1. Prepare the first casting solution and the second casting solution according to Table 1. The polymer of both the first and second casting solutions is polyethersulfone. The solvent of both the first and second casting solutions is N,N-dimethylformamide. The additive of the first casting solution is a mixture of polyethylene glycol and polyvinylpyrrolidone. The additive of the second casting solution is a mixture of methanol, polyvinylpyrrolidone, and water.

[0078] S2. Casting: The first and second casting solutions are sequentially cast onto the carrier to form a double-layer liquid film. Specifically, the second casting solution is applied after the first, using a quantitative method. The thickness of the first doctor blade is 200 micrometers, and the thickness of the second doctor blade is 300 micrometers. The coated double-layer nascent liquid film is pre-separated for 100 seconds at 45°C and 60% humidity.

[0079] S3. Immerse the membrane liquid in a coagulation bath, which is water, so that the curing liquid penetrates into the membrane liquid and gradually diffuses inward to solidify into a film. The coagulation time is 10 minutes.

[0080] S4. Wash the solidified membrane with pure water and store it in glycerol with a glycerol content of 30% to complete the preparation of the structure-controllable composite ultrafiltration membrane.

[0081] Implementation Case 4

[0082] S1. Prepare the first casting solution and the second casting solution according to Table 1. The polymer of both the first and second casting solutions is polyethersulfone, the solvent of both the first and second casting solutions is N,N dimethylacetamide, and the additives of both the first and second casting solutions are a mixture of triethylene glycol, polyvinylpyrrolidone, and water.

[0083] S2. Casting: The first and second casting solutions are sequentially cast onto the carrier to form a double-layer liquid film. Specifically, the second casting solution is applied after the first, using a quantitative method. The thickness of the first doctor blade is 100 micrometers, and the thickness of the second doctor blade is 300 micrometers. The coated nascent double-layer liquid film is pre-separated for 90 seconds at 45°C and 70% humidity.

[0084] S3. Immerse the membrane liquid in a coagulation bath, which is water, so that the curing liquid penetrates into the membrane liquid and gradually diffuses inward to solidify into a film. The coagulation time is 10 minutes.

[0085] S4. Wash the solidified membrane with pure water and store it in glycerol with a glycerol content of 30% to complete the preparation of the structure-controllable composite ultrafiltration membrane.

[0086] Implementation Case 5

[0087] S1. Prepare the first casting solution and the second casting solution according to Table 1. The polymer of both the first casting solution and the second casting solution is polyethersulfone, the solvent of both the first casting solution and the second casting solution is dimethyl sulfoxide, and the additives of both the first casting solution and the second casting solution are a mixture of polyethylene glycol 400, polyvinylpyrrolidone, and water.

[0088] S2. Casting: The first and second casting solutions are sequentially cast onto the carrier to form a double-layer liquid film. Specifically, the second casting solution is applied after the first, using a quantitative method. The thickness of the first doctor blade is 150 micrometers, and the thickness of the second doctor blade is 300 micrometers. The coated double-layer nascent liquid film is pre-separated for 70 seconds at 55°C and 60% humidity.

[0089] S3. Immerse the membrane liquid in a coagulation bath, which is water, so that the curing liquid penetrates into the membrane liquid and gradually diffuses inward to solidify into a film. The coagulation time is 10 minutes.

[0090] S4. Wash the solidified membrane with pure water and store it in glycerol with a glycerol content of 30% to complete the preparation of the structure-controllable composite ultrafiltration membrane.

[0091] Table 1. Formulation parameters of casting solutions in Examples 1-5

[0092]

[0093] To further illustrate the separation performance of the obtained composite ultrafiltration membrane, tangential flow filtration was used to characterize its separation performance, and the results are shown in Table 2 below:

[0094] Table 2. Separation performance characterization results

[0095]

[0096]

[0097] It is evident that the flux of the asymmetric ultrafiltration membrane prepared in this application is significantly higher than that of existing ultrafiltration membranes for pure water, while the retention rates are all above 97%. Furthermore, the prepared asymmetric ultrafiltration membrane also exhibits excellent strength. The prepared structure-controllable asymmetric ultrafiltration membrane effectively balances separation performance and mechanical strength.

[0098] In addition, such as Figure 7As shown, this scheme uses the controllable asymmetric composite ultrafiltration membrane from Implementation Case 1 for scanning electron microscopy (SEM) examination and SEM testing to obtain the pore sizes of different pore size regions of the controllable asymmetric composite ultrafiltration membrane. The thickness of the ultrafiltration separation layer and the microporous support layer of the composite ultrafiltration membrane is 50-300 micrometers. For further analysis, the ultrafiltration separation layer is divided into three regions with different pore sizes: region A1, which has a dense separation layer and a thickness of 5-30 micrometers; region A2, which has a larger pore size and a thickness of approximately 10-50 micrometers, behind the dense skin layer; and region A3, which has a thickness of approximately 10-100 micrometers at the bottom of the ultrafiltration separation layer. The pore size of the transition region below region A3 is interconnected with the pores near the transition region on the microporous support layer, forming the transition region A4 of the composite porous layer, with a thickness of approximately 10-50 micrometers. The bottom microporous support layer constitutes region A5, which is the porous body of the composite membrane, with a thickness of approximately 100-200 micrometers. The composite membrane porous matrix exhibits a gradient pore size distribution, which is divided into five regions based on this gradient pore structure. These five regions have different pore size distributions, with the pore size increasing sequentially from region A1 to A5. Specifically, the pore size in the first pore size region A1 is 1-20 nm, in the second pore size region A2 it is 10-70 nm, in the third pore size region A3 it is 60-300 nm, in the fourth pore size region A4 it is 200-1000 nm, and in the fifth pore size region A5 it is 500-2000 nm.

[0099] This invention is not limited to the preferred embodiments described above. Anyone can derive other products in various forms under the guidance of this invention. However, regardless of any changes in shape or structure, any technical solution that is the same as or similar to this application falls within the protection scope of this invention.

Claims

1. A method for preparing a structure-controllable asymmetric composite ultrafiltration membrane, characterized in that, include: A first casting solution and a second casting solution are prepared using polymers, solvents, and additives as components. The solid content of the first casting solution is lower than that of the second casting solution. The components of the first casting solution and the second casting solution are the same. The viscosity of the second casting solution is greater than that of the first casting solution. The additives include one or more of polyvinylpyrrolidone, polyethylene glycol, water, methanol, ethanol, pentanol, hexanol, heptanol, acetone, butyl ether, ethyl acetate, calcium chloride, magnesium chloride, and lithium chloride. The first casting solution and the second casting solution are sequentially cast onto the carrier. The first casting solution is first quantitatively coated to form a first liquid film, and then the second casting solution is quantitatively coated on the first liquid film to form a second liquid film. The coated first liquid film and the second liquid film are pre-separated for 0-300s at 35-55℃ and 30-90% humidity. The first liquid film and the second liquid film are immersed in a coagulation bath to solidify and obtain a solidified film. Washing and curing the membrane yields a structurally controllable asymmetric composite ultrafiltration membrane; The pore structure of the ultrafiltration separation layer and the microporous support layer is controlled by adjusting the ratio of the double-layer casting solution and the liquid membrane pre-phase separation conditions. The thickness of the ultrafiltration separation layer and the microporous support layer is controlled by the distance between the casting solution tank and the carrier, the coating amount, and the scraping process.

2. The method for preparing a structure-controllable asymmetric composite ultrafiltration membrane according to claim 1, characterized in that, include: An ultrafiltration separation layer and a microporous support layer are provided, wherein the average pore size of the microporous support layer is larger than the average pore size of the ultrafiltration separation layer, and the ultrafiltration separation layer includes at least two interconnected pore size regions with progressively increasing pore size from top to bottom. The microporous support layer also includes at least two interconnected pore size regions with progressively increasing pore size from top to bottom. The pores in the transition from the ultrafiltration separation layer to the microporous support layer are interconnected. A first casting solution and a second casting solution are prepared using polymers, solvents, and additives as components. The ultrafiltration separation layer is obtained by coating the second casting solution, and the microporous support layer is obtained by coating the first casting solution.

3. The method for preparing a structure-controllable asymmetric composite ultrafiltration membrane according to claim 2, characterized in that, The ultrafiltration separation layer includes interconnected first pore size region A1, second pore size region A2, and third pore size region A3 with pore sizes increasing sequentially from top to bottom. The microporous support layer includes interconnected fourth pore size region A4 and fifth pore size region A5 with pore sizes increasing sequentially from top to bottom. The pore size of the third pore size region A3 is larger than the pore size of the fourth pore size region A4, and the pores of the fourth pore size region A4 penetrate through the pores of the third pore size region A3 to the pores of the fifth pore size region A5.

4. The method for preparing a structure-controllable asymmetric composite ultrafiltration membrane according to claim 3, characterized in that, The aperture of the first aperture region A1 is 1-20nm, the aperture of the second aperture region A2 is 10-70nm, the aperture of the third aperture region A3 is 60-300nm, the aperture of the fourth aperture region A4 is 200-1000nm, and the aperture of the fifth aperture region A5 is 500-2000nm.

5. The method for preparing a structure-controllable asymmetric composite ultrafiltration membrane according to claim 3, characterized in that, The thickness of the ultrafiltration separation layer is 50-300 micrometers, the thickness of the first pore region A1 is 5-30 micrometers, the thickness of the second pore region A2 is 10-50 micrometers, and the thickness of the third pore region A3 is 10-100 micrometers; the thickness of the microporous support layer is 50-300 micrometers, the thickness of the fourth pore region A4 is 10-50 micrometers, and the thickness of the fifth pore region A5 is 100-200 micrometers.

6. The method for preparing a structure-controllable asymmetric composite ultrafiltration membrane according to claim 1, characterized in that, The thickness of the ultrafiltration separation layer is 30%-60% of the total thickness of the structurally controllable asymmetric composite ultrafiltration membrane.

7. The method for preparing a structure-controllable asymmetric composite ultrafiltration membrane according to claim 1, characterized in that, The polymer is selected from one of polyvinylidene fluoride, polyether sulfate, polysulfone, polyaryl sulfate, polyphenyl sulfate, polyvinyl chloride, polycarbonate, cellulose, and regenerated cellulose. The additives include one or more of polyvinylpyrrolidone, polyethylene glycol, water, methanol, ethanol, pentanol, hexanol, heptanol, acetone, butyl ether, ethyl acetate, calcium chloride, magnesium chloride, and lithium chloride.

8. A structure-controllable asymmetric composite ultrafiltration membrane, characterized in that, The structure-controllable asymmetric composite ultrafiltration membrane is prepared according to any one of claims 1 to 7.

9. An application of the structure-controllable asymmetric composite ultrafiltration membrane according to claim 8, characterized in that, It is used in the field of ultrafiltration.

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