Composite ultrafiltration membrane and method for producing the same

Abstract

The application discloses a composite ultrafiltration membrane and a preparation method thereof. The composite ultrafiltration membrane comprises a polymer base film, a pore-forming agent and a first cross-linking agent in the polymer base film, a first cross-linking layer arranged on one surface of the polymer base film, a polyamide layer arranged on a surface of the first cross-linking layer away from the polymer base film, the first cross-linking layer being obtained by cross-linking reaction of the first cross-linking agent and the polyamide layer, and a second cross-linking layer arranged on a surface of the polyamide layer away from the polymer base film, cross-linking being present between the second cross-linking layer and the polyamide layer. Thus, cross-linking of the surfaces on both sides of the polyamide layer effectively improves the bonding force between the polyamide layer and the polymer base film and the bonding force of the second cross-linking layer, that is, the overall structural stability of the composite ultrafiltration membrane is improved, and adverse effects such as peeling and scratching of the functional layer of the polyamide layer after cross-linking are prevented.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, specifically to a composite ultrafiltration membrane and its preparation method. Background Technology

[0002] Membrane science is a novel and highly efficient separation technology applied across multiple disciplines. Membrane separation technology boasts excellent advantages such as high separation efficiency, low energy consumption, good economic benefits, simple processes, no phase change, operation at room temperature, and environmental friendliness. With the continuous development of the pharmaceutical, food, and environmental protection industries, the market demand for low molecular weight cutoff ultrafiltration membranes is increasing. Currently, there are generally three methods for manufacturing low molecular weight ultrafiltration membranes on the market. One method involves the traditional immersion precipitation phase inversion method. This type of ultrafiltration membrane requires a special coagulation bath and additives, increasing the production steps and being environmentally unfriendly. Moreover, it has a thick skin layer, low flux, a hydrophobic membrane surface, and is prone to compaction. Another method is the chemical crosslinking method, which uses two or more organic substances to generate a dense polymer film with a network or three-dimensional structure with specific properties under the action of light, heat, radiation, and crosslinking agents. This film is then adhered to a large-pore ultrafiltration base membrane. The membrane produced by this method has poor adhesion of the functional layer, is prone to peeling and scratching, and has a short service life. A third method is the interfacial polymerization method, which involves coating a polyamide layer onto a large-pore ultrafiltration base membrane. The manufacturing process and materials used are the same as those used for nanofiltration or reverse osmosis membranes, only the formulation is different. This method produces membranes with smaller pore sizes and a molecular weight cutoff generally below 500 Da.

[0003] Therefore, it is particularly urgent to study an ultrafiltration membrane with high flux, low damage resistance, and low molecular weight cutoff. Summary of the Invention

[0004] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, one object of this invention is to provide a composite ultrafiltration membrane. This composite ultrafiltration membrane exhibits good stability and strong connections between its layers.

[0005] In one aspect of the present invention, a composite ultrafiltration membrane is provided. According to an embodiment of the present invention, the composite ultrafiltration membrane comprises: a polymer base membrane having a pore-forming agent and a first crosslinking agent; a first crosslinking layer disposed on one surface of the polymer base membrane; a polyamide layer disposed on the surface of the first crosslinking layer away from the polymer base membrane, the first crosslinking layer being obtained by a crosslinking reaction between the first crosslinking agent and the polyamide layer; and a second crosslinking layer disposed on the surface of the polyamide layer away from the polymer base membrane, the second crosslinking layer being crosslinked with the polyamide layer. Thus, cross-linking reactions occur on both sides of the polyamide layer. On one hand, the polyamide layer reacts with the first cross-linking agent in the polymer base membrane to form the first cross-linked layer, thereby strengthening the bond between the polyamide layer and the polymer base membrane. On the other hand, the polyamide layer also reacts with the cross-linking agent in the second cross-linked layer, thereby strengthening the bond between the polyamide layer and the second cross-linked layer. In this way, the cross-linking of the surfaces on both sides of the polyamide layer effectively improves the bonding force between the polyamide layer and the polymer base membrane, as well as the bonding force of the second cross-linked layer. This improves the overall structural stability of the composite ultrafiltration membrane and prevents adverse effects such as the polyamide functional layer falling off or being scratched after cross-linking.

[0006] According to an embodiment of the present invention, the first crosslinking agent is a sulfonic acid crosslinking agent, and the second crosslinking agent in the second crosslinking layer is a sulfonic acid crosslinking agent.

[0007] According to embodiments of the present invention, the first crosslinking agent and the second crosslinking agent respectively include at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid. According to embodiments of the present invention, at least one of the following conditions is satisfied: the thickness of the polymer base film is 30–60 μm; the thickness of the first crosslinking layer is 40–80 nm; the thickness of the polyamide layer is 50–100 nm; and the thickness of the second crosslinking layer is 60–100 nm.

[0008] In another aspect, the present invention provides a method for preparing the aforementioned composite ultrafiltration membrane. According to an embodiment of the present invention, a casting solution is prepared by dissolving a polymer, a pore-forming agent, and a first crosslinking agent in an organic solvent; a polyamide solution is prepared for later use; an aqueous crosslinking agent solution is prepared for later use; the casting solution is coated onto a carrier, and the casting solution is allowed to solidify into a membrane to obtain a polymeric base membrane; one side of the polymeric base membrane is immersed in the polyamide solution for a period of time, then removed and dried to obtain a semi-finished product; the semi-finished product is immersed in the aqueous crosslinking agent solution for a period of time, then removed and subjected to a crosslinking reaction at a predetermined temperature to obtain the composite ultrafiltration membrane. Therefore, in the above preparation method, cross-linking reactions occur on both sides of the polyamide layer. On the one hand, the polyamide layer reacts with the first cross-linking agent in the polymer base membrane to form the first cross-linked layer, thereby strengthening the bond between the polyamide layer and the polymer base membrane. On the other hand, the polyamide layer also reacts with the cross-linking agent in the cross-linking agent aqueous solution, thereby strengthening the bond between the polyamide layer and the second cross-linked layer. Thus, the cross-linking on both sides of the polyamide layer effectively improves the bonding force between the polyamide layer and the polymer base membrane, as well as the bonding force of the second cross-linked layer, thereby improving the overall structural stability of the composite ultrafiltration membrane and preventing adverse effects such as peeling and scratching of the functional polyamide layer after cross-linking. Moreover, the above preparation method is relatively simple and easy to implement and mass-produce industrially.

[0009] According to an embodiment of the present invention, based on the total mass of the casting solution, the casting solution comprises, by mass percentage: 10wt% to 28wt% of the polymer; 0.5wt% to 8wt% of the pore-forming agent; 1wt% to 5wt% of the first crosslinking agent; and the balance being an organic solvent.

[0010] According to an embodiment of the present invention, the polymer is selected from at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, and sulfonated polysulfone.

[0011] According to an embodiment of the present invention, the pore-forming agent is selected from at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, ethanol, propanol, acetic acid, ethylene glycol methyl ether, and diethylene glycol dimethyl ether.

[0012] According to an embodiment of the present invention, the first crosslinking agent is selected from at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid.

[0013] According to embodiments of the present invention, the organic solvent is selected from at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0014] According to an embodiment of the present invention, the polyamide solution is prepared by the following method: dissolving polyamine and acyl chloride separately and mixing them to allow them to undergo a polymerization reaction to generate polyamide, then filtering and drying, and dissolving the polyamide in a multi-component solvent to obtain the polyamide solution.

[0015] According to an embodiment of the present invention, the concentration of the polyamide solution is 2% to 8%.

[0016] According to embodiments of the present invention, the polyamine is selected from at least one of triethylamine, triethylamine hydrochloride, piperazine, m-phenylenediamine, polyethyleneimine, triethanolamine, and methyldiethanolamine; the acyl chloride is selected from at least one of trimesoyl chloride, terephthaloyl chloride, isophthaloyl chloride, biphenyl dicarboxylate chloride, naphthalene dicarboxylate chloride, naphthalene sulfonyl chloride, benzene disulfonyl chloride, and benzene trisulfonyl chloride; and the multi-element solvent is selected from at least two mixtures selected from N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, ethanol, propanol, butanol, ethylene glycol, chloroethanol, propylene glycol, formic acid, and sulfuric acid.

[0017] According to an embodiment of the present invention, the polymer base film is immersed in the polyamide solution for 10s to 120s.

[0018] According to an embodiment of the present invention, the concentration of the crosslinking agent aqueous solution is 5% to 15%.

[0019] According to an embodiment of the present invention, the semi-finished product is soaked in the crosslinking agent aqueous solution for 5s to 90s.

[0020] According to an embodiment of the present invention, the predetermined temperature is 30°C to 90°C.

[0021] According to an embodiment of the present invention, the crosslinking reaction takes 0.5 min to 10 min. Attached Figure Description

[0022] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0023] Figure 1 This is a schematic diagram of a composite ultrafiltration membrane structure according to an embodiment of the present invention;

[0024] Figure 2 This is a schematic flowchart of a method for preparing a composite ultrafiltration membrane according to an embodiment of the present invention.

[0025] Explanation of reference numerals in the attached figures:

[0026] 1000: Composite ultrafiltration membrane; 120: Polymer base membrane; 130: Polyamide layer; 141: First crosslinking layer; 142: Second crosslinking layer. Detailed Implementation

[0027] The embodiments of the present invention are described in detail below. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0028] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0029] In one aspect, the present invention provides a composite ultrafiltration membrane. (See reference...) Figure 1 According to an embodiment of the present invention, the composite ultrafiltration membrane 1000 includes: a polymer base membrane 120, wherein the polymer base membrane 120 has a pore-forming agent and a first crosslinking agent; a first crosslinking layer 141, wherein the first crosslinking layer 141 is disposed on one surface of the polymer base membrane 120; a polyamide layer 130, wherein the polyamide layer 130 is disposed on the surface of the first crosslinking layer 141 away from the polymer base membrane 120, wherein the first crosslinking layer 141 is obtained by a crosslinking reaction between the first crosslinking agent and the polyamide layer 130; and a second crosslinking layer 142, wherein the second crosslinking layer 142 is disposed on the surface of the polyamide layer 130 away from the polymer base membrane 120, wherein the second crosslinking layer 142 is crosslinked with the polyamide layer 130. Thus, cross-linking reactions occur on both sides of the polyamide layer. On one hand, the polyamide layer reacts with the first cross-linking agent in the polymer base membrane to form the first cross-linked layer, thereby strengthening the bond between the polyamide layer and the polymer base membrane. On the other hand, the polyamide layer also reacts with the cross-linking agent in the second cross-linked layer, thereby strengthening the bond between the polyamide layer and the second cross-linked layer. In this way, the cross-linking of the surfaces on both sides of the polyamide layer effectively improves the bonding force between the polyamide layer and the polymer base membrane, as well as the bonding force of the second cross-linked layer. This improves the overall structural stability of the composite ultrafiltration membrane and prevents adverse effects such as the polyamide functional layer falling off or being scratched after cross-linking.

[0030] Furthermore, the thickness of the polymer base membrane is 30–60 μm; the thickness of the first cross-linked layer is 40–80 nm; the thickness of the polyamide layer is 50–100 nm; and the thickness of the second cross-linked layer is 60–100 nm. These layer thicknesses effectively ensure the superior filtration performance of the ultrafiltration composite membrane.

[0031] According to embodiments of the present invention, the polymer used to form the polymeric base membrane can be selected from at least one of polysulfone (PSF), polyethersulfone (PES), polyvinylidene fluoride (PVDF), and sulfonated polysulfone (SPSF); the pore-forming agent can be selected from at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, ethanol, propanol, acetic acid, ethylene glycol methyl ether, and diethylene glycol dimethyl ether; and the first crosslinking agent can be selected from at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid. Therefore, the selection of the above materials allows the pore-forming agent to form relatively uniform pores in the polymeric base membrane, ensuring good performance of the composite ultrafiltration membrane. The first crosslinking agent has good compatibility with the polymeric base membrane, thereby allowing the first crosslinking agent to be uniformly dispersed in the polymeric base membrane, avoiding uneven crosslinking between the first crosslinking agent and the polyamide layer.

[0032] According to embodiments of the present invention, the first crosslinking agent is a sulfonic acid crosslinking agent, and the second crosslinking agent in the second crosslinking layer is also a sulfonic acid crosslinking agent. The inventors have discovered that crosslinking amides and sulfonic acids can form a dense separation layer with very small pore sizes, exhibiting strong acid resistance, corrosion resistance, and solvent resistance, while also possessing strong hydrophilicity due to the sulfonic acid groups. Therefore, a composite ultrafiltration membrane with strong hydrophilicity, acid resistance, solvent resistance, and corrosion resistance can be obtained through crosslinking, making the composite ultrafiltration membrane of the present invention suitable as a separation membrane with a molecular weight cutoff of 1000–4000 Da.

[0033] Furthermore, the first crosslinking agent and the second crosslinking agent each include at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid. Thus, the sulfonic acid crosslinking agent and the polyamide can be crosslinked smoothly, effectively preventing the functional layer from detaching or scratching after crosslinking, resulting in a stable composite ultrafiltration membrane. In addition, the first and second crosslinking agents can be the same or different. For example, in some embodiments, both the first and second crosslinking agents are aminosulfonic acid; in other embodiments, the first crosslinking agent is aminosulfonic acid and the second crosslinking agent is sodium aminosulfonate; in still other embodiments, the first crosslinking agent is aminosulfonic acid and potassium aminosulfonate, and the second crosslinking agent is sodium aminosulfonate and p-aminobenzenesulfonic acid.

[0034] In another aspect, the present invention provides a method for preparing the aforementioned composite ultrafiltration membrane. According to an embodiment of the present invention, the method for preparing the composite ultrafiltration membrane includes:

[0035] S100: A casting solution is prepared by dissolving a polymer, a pore-forming agent, and a first crosslinking agent in an organic solvent.

[0036] According to embodiments of the present invention, a casting solution is prepared by dissolving a polymer, a pore-forming agent, and a first crosslinking agent in an organic solvent. In this step, the polymer can be selected from at least one of polysulfone (PSF), polyethersulfone (PES), polyvinylidene fluoride (PVDF), and sulfonated polysulfone (SPSF); the pore-forming agent can be selected from at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, ethanol, propanol, acetic acid, ethylene glycol methyl ether, and diethylene glycol dimethyl ether; the first crosslinking agent can be selected from at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid. Therefore, the selection of the above materials allows the pore-forming agent to form relatively uniform pores in the polymer, ensuring good performance of the composite ultrafiltration membrane. The first crosslinking agent has good compatibility with the polymer, allowing it to be uniformly dispersed in the polymer, thus avoiding uneven crosslinking between the first crosslinking agent and the polyamide layer.

[0037] According to embodiments of the present invention, the organic solvent may be selected from at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide. Thus, such solvents can uniformly disperse the aforementioned polymer, pore-forming agent, and first crosslinking agent, thereby further improving the performance of the prepared ultrafiltration membrane.

[0038] According to embodiments of the present invention, in the casting solution, based on the total mass of the casting solution, the content of the polymer can be 10wt% to 28wt%, for example, 10wt%, 12wt%, 15wt%, 18wt%, 20wt%, 25wt%, 28wt%, etc., by mass percentage; the content of the pore-forming agent can be 0.5wt% to 8wt%, for example, 0.5wt%, 1wt%, 3wt%, 5wt%, 8wt%, etc.; the content of the first crosslinking agent can be 1wt% to 5wt%, for example, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, etc.; and the balance is organic solvent. The inventors have found that by controlling the contents of the polymer, pore-forming agent, and crosslinking agent within the above ranges, the viscosity of the casting solution is more suitable, and the polymer, pore-forming agent, and crosslinking agent are more uniformly dispersed, which is beneficial for the subsequent preparation of the composite ultrafiltration membrane. Furthermore, the formed composite ultrafiltration membrane possesses a large number of pores with suitable pore sizes, which is beneficial for improving the water flux and retention rate of the composite ultrafiltration membrane. If the content of the polymer is too high, the porosity of the ultrafiltration membrane will be relatively low, resulting in a relatively low water flux of the composite ultrafiltration membrane, failing to meet the requirements for practical use. It will also cause the viscosity of the casting solution to be relatively high, potentially generating microbubbles during coating and causing surface defects in the composite ultrafiltration membrane. Conversely, if the polymer content is too low, the viscosity of the casting solution will be too low, potentially causing the casting solution to flow onto the substrate. This also results in an excessively large pore size in the composite ultrafiltration membrane, reducing the rejection rate. If the pore-forming agent content is too high, the ultrafiltration membrane will have more pores and a higher porosity, leading to a relatively low rejection rate. Conversely, if the pore-forming agent content is too low, the composite ultrafiltration membrane will have fewer pores and a lower porosity, resulting in a relatively low water flux. The content of the first crosslinking agent has the same effect as the polymer content. Therefore, within the above parameter ranges, a composite ultrafiltration membrane with moderate viscosity, high rejection rate, and high flux can ultimately be prepared.

[0039] S200: Prepare a polyamide solution for later use.

[0040] According to an embodiment of the present invention, a polyamine and an acyl chloride are dissolved separately and then mixed to polymerize and generate a polyamide. The mixture is then filtered, dried, and dissolved in a multi-component solvent to obtain a polyamide solution. In this step, the polyamine can be selected from at least one of triethylamine, triethylamine hydrochloride, piperazine, m-phenylenediamine, polyethyleneimine, triethanolamine, and methyldiethanolamine; the acyl chloride can be selected from at least one of trimesoyl chloride, terephthaloyl chloride, isophthaloyl chloride, biphenyl dicarboxylate chloride, naphthalene dicarboxylate chloride, naphthalene sulfonyl chloride, benzene disulfonyl chloride, and benzene trisulfonyl chloride. Therefore, the selection of the above-mentioned raw materials and the specific preparation method of the polyamide solution can improve the yield of polyamide preparation.

[0041] According to embodiments of the present invention, the aforementioned multi-element solvent can be selected from at least three mixtures of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, ethanol, propanol, butanol, ethylene glycol, chloroethanol, propylene glycol, formic acid, and sulfuric acid. Thus, such solvents can uniformly disperse the polyamide to form a polyamide solution, thereby further improving the performance of the prepared ultrafiltration membrane. Furthermore, the aforementioned solvents are relatively volatile, facilitating curing into a film during the preparation of the polyamide layer.

[0042] The concentration of the polyamide solution is 2% to 8%. Therefore, polyamide solutions of the above concentrations can effectively prepare polyamide layers, which are relatively easy to cure into films during the curing process, and the film quality is good.

[0043] S300: Prepare an aqueous solution of the crosslinking agent for later use.

[0044] According to some embodiments of the present invention, the second crosslinking agent is dissolved in water to prepare an aqueous crosslinking agent solution for later use. The concentration of the aqueous crosslinking agent solution is 5% to 15%. Furthermore, water can be used as a solvent to uniformly disperse the second crosslinking agent, thus preparing a crosslinking agent solution, thereby further improving the performance of the prepared ultrafiltration membrane.

[0045] The first crosslinking agent and the second crosslinking agent are consistent with the requirements for the specific types of the first crosslinking agent and the second crosslinking agent mentioned above, that is, the first crosslinking agent is a sulfonic acid crosslinking agent, and the second crosslinking agent in the second crosslinking layer is a sulfonic acid crosslinking agent.

[0046] Furthermore, in actual operation, there is no strict order of operation for the above steps S100, S200 and S300. Those skilled in the art can flexibly choose the order of implementation of the above three steps according to the actual situation.

[0047] S400: The casting solution is coated onto the carrier and then cured to form a film, thus obtaining a polymer base film.

[0048] According to an embodiment of the present invention, the casting solution obtained in step S100 is coated onto a carrier, immersed in a coagulation bath to form a phase-conversion membrane, and excess solvent is washed away to obtain a polymer-based membrane for later use. Thus, by using both the coating method and the method of immersion in a coagulation bath for curing, a polymer-based membrane with relatively uniform thickness can be obtained, which can further improve the performance of the prepared ultrafiltration membrane.

[0049] In the above steps, there are no special requirements for the specific method of coating and film formation; those skilled in the art can choose flexibly according to the actual situation. In some embodiments, the specific coating method includes, but is not limited to, dip coating, comma blade coating, smooth roller coating, anilox roller coating, wire rod coating, slot extrusion coating, slit coating, and spray coating. The above coating methods are mature processes, easy to industrialize, and the resulting composite ultrafiltration membrane has uniform thickness and stable performance.

[0050] There are no special requirements regarding the specific type and thickness of the carrier; those skilled in the art can choose flexibly according to actual needs. No restrictions are imposed here; for example, it can be a non-woven fabric, PET (polyethylene terephthalate) carrier, OPP (oriented polypropylene film) carrier, BOPP (biaxially oriented polypropylene film) carrier, etc.

[0051] S500: Immerse one side of the polymer base film in a polyamide solution for a period of time, then remove and air dry to obtain a semi-finished product, that is, prepare a polyamide layer on the surface of the polymer base film.

[0052] According to an embodiment of the present invention, the polymer base membrane obtained in step S400 is immersed on one side in the polyamide solution obtained in step S200 for a period of time, and then removed and dried to obtain a semi-finished product. Thus, by using the dip-coating method to prepare the polyamide layer, a polyamide layer with relatively uniform thickness can be formed, and the polyamide solution can fully react with the first crosslinking agent on the surface of the polymer base membrane to obtain a uniformly distributed first crosslinking layer. This further improves the bonding strength between the polymer base membrane and the polyamide layer, thereby enhancing the performance of the prepared ultrafiltration membrane.

[0053] In this step, the immersion time of one side of the polymer base film in the polyamide solution can be from 10s to 120s, for example, 10s, 30s, 50s, 80s, 100s, 120s, etc. After being removed and dried, a semi-finished product is obtained. Within the above time, the polyamide solution can fully react with the first crosslinking agent on the surface of the polymer base film to obtain a uniformly distributed first crosslinked layer.

[0054] S600: The semi-finished product is immersed in a crosslinking agent aqueous solution for a period of time, then removed and subjected to a crosslinking reaction at a predetermined temperature to obtain a composite ultrafiltration membrane.

[0055] According to an embodiment of the present invention, the semi-finished product obtained in step S500 is immersed in the crosslinking agent aqueous solution prepared in step S300 for a period of time, then removed and placed in an oven for crosslinking reaction at a predetermined temperature to obtain a second crosslinked layer, thus obtaining a composite ultrafiltration membrane. The dip-coating method for preparing the second crosslinked layer can form a second crosslinked layer with a relatively uniform thickness, and allows the polyamide layer to fully react with the second crosslinking agent in the crosslinking agent aqueous solution, resulting in a uniformly distributed second crosslinked layer. This further improves the bonding strength between the second crosslinked layer and the polyamide layer, thereby enhancing the performance of the prepared ultrafiltration membrane.

[0056] In this step, the semi-finished product is immersed in the crosslinking agent aqueous solution for a time ranging from 5s to 90s, such as 5s, 10s, 20s, 50s, 70s, 90s, etc. After being removed, it is placed in an oven for crosslinking. The predetermined oven temperature can be from 30℃ to 90℃, such as 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, etc. The crosslinking reaction time (or oven drying time) can be from 0.5min to 10min, such as 0.5min, 2min, 4min, 6min, 8min, 10min, etc. The inventors have discovered that by controlling the immersion time in the polyamide solution and the crosslinking aqueous solution, the predetermined oven temperature, and the crosslinking reaction time within the above ranges, a second crosslinked layer of uniform thickness can be obtained, ensuring sufficient crosslinking reaction between the second crosslinking agent and the polyamide layer, guaranteeing the strong bond between the polyamide layer and the second crosslinked layer, and further improving the performance of the prepared composite ultrafiltration membrane.

[0057] According to embodiments of the present invention, using the above-described preparation method, cross-linking reactions occur on both sides of the polyamide layer. On one hand, the polyamide layer undergoes a cross-linking reaction with the first cross-linking agent in the polymer base membrane to form a first cross-linked layer, thereby strengthening the bond between the polyamide layer and the polymer base membrane. On the other hand, the polyamide layer also undergoes a cross-linking reaction with the cross-linking agent in the cross-linking agent aqueous solution, thereby strengthening the bond between the polyamide layer and the second cross-linked layer. Thus, the cross-linking of the surfaces on both sides of the polyamide layer effectively improves the bonding force between the polyamide layer and the polymer base membrane, as well as the bonding force of the second cross-linked layer, thereby improving the overall structural stability of the composite ultrafiltration membrane and preventing adverse effects such as detachment and scratches of the functional polyamide layer after cross-linking. Moreover, the above preparation method is relatively simple, easy to implement, and suitable for industrial mass production. Furthermore, both the first and second cross-linking agents are sulfonic acid cross-linking agents. The cross-linking of amides and sulfonic acids can form a dense separation layer with very small pores, which has strong acid resistance, corrosion resistance, and solvent resistance, and also has strong hydrophilicity due to the sulfonic acid groups. Thus, a composite ultrafiltration membrane with strong hydrophilicity, acid resistance, solvent resistance and corrosion resistance can be obtained by crosslinking, and the composite ultrafiltration membrane of the present invention can be used as a separation membrane with a molecular weight cutoff of 1000 to 4000 Da.

[0058] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0059] Example

[0060] Example 1

[0061] Ultrafiltration membranes were prepared according to the following method:

[0062] (1) Polysulfone (polymer), polyvinylpyrrolidone (pore-forming agent) and aminosulfonic acid (first crosslinking agent) are dissolved in N,N-dimethylacetamide (DMAC) to prepare a casting solution, wherein the polysulfone content is 20%wt, the polyvinylpyrrolidone content is 10%wt, the aminosulfonic acid content is 1%wt, and the balance is the solvent N,N-dimethylacetamide;

[0063] (2) Piperazine and pyromellitic chloride were dissolved in water and n-hexane respectively and then mixed to polymerize them to produce polyamide. After filtration and drying, the polyamide was dissolved in a mixed solvent of DMAC, ethanol and 10% sulfuric acid to obtain a polyamide solution for later use. The ratio of DMAC, ethanol and sulfuric acid was 2:2:1.

[0064] (3) Dissolve aminobenzenesulfonic acid in water to prepare a crosslinking agent solution for later use, wherein the aminobenzenesulfonic acid content is 1%wt;

[0065] (4) The casting solution is coated on the nonwoven fabric, immersed in the coagulation bath to transform into a film, and the excess solvent is washed away to make a polymer base film for later use.

[0066] (5) Immerse one side of the polymer base film in the polyamide solution for 60 seconds, then remove and air dry to obtain a semi-finished product;

[0067] (6) The semi-finished product is then immersed in the crosslinking agent aqueous solution for 60 seconds and then taken out and placed in an oven at 50°C for 2 minutes to crosslink and form a composite ultrafiltration membrane.

[0068] Comparative Example 1

[0069] The difference from Example 1 is that after the polyamide solution is coated onto the polymer base film by dip coating, a polyamide layer is obtained. The crosslinking agent aqueous solution is not dip coated on the upper layer. That is, the polyamide solution directly crosslinks with the crosslinking agent in the polymer base film to form a functional layer (that is, step (6) of Example 1 is not performed in Comparative Example 1).

[0070] Example 2

[0071] (1) Polysulfone, polyvinylpyrrolidone and aminosulfonic acid are dissolved in DMAC to prepare a casting solution, wherein the polysulfone content is 20%wt, the polyvinylpyrrolidone content is 10%wt, the aminosulfonic acid content is 5%wt, and the balance is the solvent N,N-dimethylacetamide.

[0072] (2) Piperazine and pyromellitic chloride were dissolved in water and n-hexane respectively and then mixed to polymerize them to produce polyamide. After filtration and drying, the polyamide was dissolved in a mixed solvent of DMAC, ethanol and 10% sulfuric acid to obtain a polyamide solution for later use. The ratio of DMAC, ethanol and sulfuric acid was 2:2:1.

[0073] (3) Dissolve aminobenzenesulfonic acid in water to prepare a solution for later use, wherein the aminobenzenesulfonic acid content is 5%wt;

[0074] (4) The casting solution is coated on the nonwoven fabric, immersed in the coagulation bath to transform into a film, and the excess solvent is washed away to make a polymer base film for later use.

[0075] (5) Immerse one side of the polymer base film in the polyamide solution for 60 seconds, then remove and air dry to obtain a semi-finished product;

[0076] (6) The semi-finished product is then immersed in the cross-linking agent solution for 60 seconds and then taken out and placed in an oven at 50°C for 2 minutes to cross-link and form a composite ultrafiltration membrane.

[0077] Comparative Example 2

[0078] A polyamide layer is prepared directly on a polymer base film without a crosslinking agent using an existing interfacial polymerization method. The polyamide layer is then crosslinked with a crosslinking agent to form a functional layer.

[0079] Membrane durability test

[0080] Performance testing

[0081] The pure water flux of the composite ultrafiltration membranes in Examples 1 and 2 and Comparative Examples 1 and 2 were tested, as well as their retention performance for polyethylene glycol (PEG) and magnesium sulfate (MgSO4) at 1000 Da and 2000 Da, respectively.

[0082] Specifically, the test pressure was 0.5 MPa, the mass fraction of polyethylene glycol for 1000 Da and 2000 Da was 1 g / L, and the concentration of MgSO4 was 2000 PPM.

[0083] Table 1

[0084] Water flux (LMH) Retention rate 1000 Da Retention rate 2000Da <![CDATA[Retention rate of MgSO4]]> Example 1 62 23％ 94％ 2％ Example 2 33 89％ 98％ 7％ Comparative Example 1 110 4％ 8％ 0 Comparative Example 2 7 99％ 99％ 91％

[0085] As can be seen from Table 1, by adjusting the type and amount of crosslinking agent in Examples 1-2, composite ultrafiltration membranes with different retention properties can be obtained (e.g., Examples 1 and 2 can be used as separation membranes of 2000 Da and 1000 Da, respectively). However, if a second crosslinking is not performed (Comparative Example 1) or polyamide is generated directly on a polymer base membrane without crosslinking agent through polymerization (Comparative Example 2), the former will have extremely poor retention performance due to the large membrane pores, while the latter becomes a denser nanofiltration membrane.

[0086] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0087] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A composite ultrafiltration membrane, characterized in that, include: A polymer-based membrane, wherein the polymer-based membrane contains a pore-forming agent and a first crosslinking agent; A first crosslinking layer is disposed on one surface of the polymer base film; A polyamide layer is disposed on the surface of the first crosslinking layer away from the polymer base film, and the first crosslinking layer is obtained by a crosslinking reaction between the first crosslinking agent and the polyamide layer; A second crosslinking layer is disposed on the surface of the polyamide layer away from the polymer base film, and there is crosslinking between the second crosslinking layer and the polyamide layer; The first crosslinking agent is a sulfonic acid crosslinking agent, and the second crosslinking agent in the second crosslinking layer is a sulfonic acid crosslinking agent.

2. The composite ultrafiltration membrane according to claim 1, characterized in that, The first crosslinking agent and the second crosslinking agent each comprise at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid.

3. The composite ultrafiltration membrane according to claim 1, characterized in that, At least one of the following conditions must be met: The thickness of the polymer base film is 30~60μm; The thickness of the first cross-linking layer is 40~80nm; The thickness of the polyamide layer is 50~100nm; The thickness of the second cross-linking layer is 60~100nm.

4. A method for preparing the composite ultrafiltration membrane according to any one of claims 1 to 3, characterized in that, include: A casting solution is prepared by dissolving a polymer, a pore-forming agent, and a first crosslinking agent in an organic solvent. Prepare a polyamide solution for later use; Prepare an aqueous solution of the crosslinking agent for later use; The casting solution is coated onto a carrier and then cured to form a film, thereby obtaining a polymer-based film. The polymer base film is immersed in the polyamide solution for a period of time, then removed and dried to obtain a semi-finished product; The semi-finished product is immersed in the crosslinking agent aqueous solution for a period of time and then taken out, and a crosslinking reaction is carried out at a predetermined temperature to obtain the composite ultrafiltration membrane.

5. The method according to claim 4, characterized in that, Based on the total mass of the casting solution, the casting solution comprises, by mass percentage: The polymer is 10wt% to 28wt%; The pore-forming agent is 0.5 wt% to 8 wt%; The first crosslinking agent is 1 wt% to 5 wt%; and The remaining organic solvent.

6. The method according to claim 4 or 5, characterized in that, The polymer is selected from at least one of polysulfone, polyethersulfone, polyvinylidene fluoride, and sulfonated polysulfone; The pore-forming agent is selected from at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, ethanol, propanol, acetic acid, ethylene glycol methyl ether, and diethylene glycol dimethyl ether; The first crosslinking agent is selected from at least one of aminosulfonic acid, sodium aminosulfonate, potassium aminosulfonate, aminobenzenesulfonic acid, and p-aminobenzenesulfonic acid; The organic solvent is selected from at least one of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.

7. The method according to claim 4, characterized in that, The polyamide solution is prepared by the following method: dissolving a polyamine and an acyl chloride separately, mixing them to allow them to polymerize and form a polyamide, then filtering and drying the mixture, and finally dissolving the polyamide in a multi-component solvent to obtain the polyamide solution. Optionally, the concentration of the polyamide solution is 2% to 8%. Optionally, the polyamine is selected from at least one of triethylamine, triethylamine hydrochloride, piperazine, m-phenylenediamine, polyethyleneimine, triethanolamine, and methyldiethanolamine; the acyl chloride is selected from at least one of trimesoyl chloride, terephthaloyl chloride, isophthaloyl chloride, biphenyl dicarboxylate chloride, naphthalene dicarboxylate chloride, naphthalene sulfonyl chloride, benzene disulfonyl chloride, and benzene trisulfonyl chloride; and the multi-element solvent is selected from at least two mixtures of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, ethanol, propanol, butanol, ethylene glycol, chloroethanol, propylene glycol, formic acid, and sulfuric acid.

8. The method according to claim 4, characterized in that, The polymer base film is immersed in the polyamide solution for 10s to 120s.

9. The method according to claim 4, characterized in that, It also meets at least one of the following conditions: The concentration of the crosslinking agent aqueous solution is 5%~15%; The semi-finished product is soaked in the crosslinking agent aqueous solution for 5s to 90s; The preset temperature is 30℃~90℃; The cross-linking reaction takes 0.5 min to 10 min.

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