Tubular composite ultrafiltration membrane, and preparation method and application thereof

Abstract

The application relates to the field of membrane separation technology, in particular to a tubular composite ultrafiltration membrane and a preparation method and application thereof. The tubular composite ultrafiltration membrane comprises a support base material layer, a support transition layer and a polyester separation layer. The application further provides a preparation method of the tubular composite ultrafiltration membrane, and the steps are as follows: the support base material is made into a tubular support base material layer; a casting solution is coated on the inner surface of the tubular support base material layer, pre-evaporated, solidified, and an organic tubular membrane is obtained; the organic tubular membrane is filled with a dendritic polyester polyol solution, then the two ends of the organic tubular membrane are sealed, the organic tubular membrane is immersed into a crosslinking agent solution, taken out and drained, and the tubular composite ultrafiltration membrane is obtained. The application further provides application of the tubular composite ultrafiltration membrane in landfill leachate treatment and biological fermentation liquid treatment. The application solves the problems of easy pollution and low separation precision of the existing composite membrane filter, and solves the problems of large manufacturing difficulty, high cost and difficulty in industrialization of the preparation method of the existing composite membrane.

Description

Technical Field

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

[0002] Organic tubular membranes, as a novel type of separation membrane, feature wide flow channels that allow for turbulent flow of the filtered material within the membrane. This reduces contaminant deposition on the membrane surface, making it relatively less susceptible to fouling. Furthermore, their high mechanical strength facilitates backwashing and allows them to withstand higher filtration pressures and various filtration methods, such as cross-flow filtration and full-flow filtration. Consequently, they are widely used in landfill leachate treatment, specialty chemical wastewater treatment, oil-water separation, food processing, and biopharmaceuticals.

[0003] Organic tubular membranes typically consist of a porous tubular support substrate and an organic separation layer. The preparation process mainly includes the fabrication of the support tubes, the coating of the organic polymer layer, and the curing of the polymer solution into a membrane through a phase inversion process. The porous substrate primarily serves a supporting function, while the membrane's separation efficiency is mainly determined by the organic separation layer on its inner surface.

[0004] Composite membrane structures refer to bilayer or multilayer membrane structures made of two or more materials, and are commonly found in flat sheet membranes and hollow fiber membranes. Organic tubular membranes themselves are composite structures composed of a supporting substrate and an organic separation layer. However, currently, their separation layer is usually a single-layer structure made of a single polymer material through a phase inversion method. Among commercially available organic tubular membrane products, the widely used polymer materials are polyvinylidene fluoride (PVDF) and polyethersulfone (PES).

[0005] However, most organic separation membranes prepared by phase inversion methods exhibit asymmetric structures. Under an electron microscope, they typically consist of an extremely thin (approximately 0.1–1.0 µm) dense layer and a porous layer (approximately 30–150 µm) with a sponge-like or finger-like pore structure. This structure results in uneven pore size distribution on the membrane surface, a tendency to generate defective pores, and the porous layer structure collapsing and deforming with increasing operating pressure and time. Consequently, it leads to decreased membrane retention accuracy, accumulation of contaminants on the membrane surface, and a tendency for water flux to decrease, severely impacting the performance and lifespan of organic tubular membranes.

[0006] Existing technologies include methods for preparing support layer casting solutions and functional layer casting solutions separately using polymers, solvents, and additives. After degassing, these solutions are simultaneously coated onto a nonwoven fabric substrate and then solidified in a coagulation bath to form an organic tubular membrane. While this method achieves simultaneous coating of the two casting solutions, it requires a specialized scraper head to achieve the composite of the support layer and functional layer casting solutions. Manufacturing this scraper head is challenging, and its long-term performance is uncertain. Another existing technology uses a coating scraper to perform primary and secondary coatings on the inner side of the support substrate to ensure uniformity and integrity. However, the drying time after the casting solution is uniformly cast and solidified into a film is as long as 22–26 hours, resulting in low preparation efficiency. Existing technologies also improve the membrane's microstructure by adding a small amount of modified halloysite to the casting solution, thereby enhancing separation efficiency and antifouling properties. However, the preparation process of modified halloysite in this method is very time-consuming and cumbersome, making industrial-scale implementation difficult. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a tubular composite ultrafiltration membrane, its preparation method and application, so as to solve the problems of easy fouling and low separation accuracy of existing composite membrane filters, as well as the problems of low mechanical properties of existing composite filter membranes, and the problems of high manufacturing difficulty, high cost and difficulty in industrialization of existing composite membrane preparation methods.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A tubular composite ultrafiltration membrane includes a supporting substrate layer, a supporting transition layer, and a polyester separation layer;

[0010] The support transition layer is formed by phase transformation of the casting liquid coated on the inner surface of the support substrate layer;

[0011] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution on the surface of the supporting transition layer.

[0012] By setting a supporting transition layer formed by phase transformation of the casting solution on the inner surface of the supporting substrate layer, not only is good porosity and permeability ensured, enabling preliminary filtration of large particulate impurities, but a uniform adhesion base is also provided for the polyester separation layer. The polyester separation layer is formed by a thermal reaction of a dendritic polyester polyol solution on the surface of the supporting transition layer. The dendritic structure of the polyester polyol has high branching and abundant active sites, which gives the polyester separation layer higher selectivity and separation precision, effectively retaining small molecule impurities and thus improving the separation effect. Furthermore, the dendritic structure enhances the membrane surface's antifouling ability, reduces the adsorption and accumulation of pollutants, extends the membrane's lifespan, and improves filtration efficiency. This tubular composite ultrafiltration membrane, through its unique three-layer structure design, effectively solves the problems of easy fouling and low separation precision found in existing composite membrane filters.

[0013] The supporting transition layer is formed by a non-solvent-induced phase transformation of a casting solution coated on the inner surface of the supporting substrate layer.

[0014] The polyester separation layer is formed by a chemical cross-linking reaction of a dendritic polyester polyol solution on the surface of the supporting transition layer.

[0015] Preferably, the casting solution comprises polyvinylidene fluoride, additives, and a polar solvent.

[0016] Preferably, the additive is selected from at least one of polyvinylpyrrolidone, Tween, polyethylene glycol, chitosan, chitin, polyvinyl alcohol, and lithium chloride.

[0017] Preferably, the polar solvent is selected from N,N-dimethylacetamide or N,N-dimethylformamide.

[0018] Preferably, the casting solution comprises, by mass percentage: 10-16% polyvinylidene fluoride, 1-10% additives, and the remainder being a polar solvent.

[0019] Preferably, the dendritic polyester polyol solution comprises dendritic polyester polyol, graphene oxide, alcohol solvent, and water.

[0020] Preferably, the dendritic polyester polyol is selected from hyperbranched polyesters with terminal hydroxyl groups.

[0021] Preferably, the theoretical molecular weight of the dendritic polyester polyol is between 3,000 and 8,000 Daltons, and the hydroxyl value is between 460 and 540.

[0022] Preferably, the alcohol solvent is selected from at least one of ethanol, isopropanol, and n-butanol.

[0023] Preferably, the dendritic polyester polyol solution comprises, by mass percentage: 1-5% dendritic polyester polyol, 0.1-0.5% graphene oxide, 5-25% alcohol solvent, and the remainder being water.

[0024] By adding two-dimensional sheet-like graphene oxide to a dendritic polyester polyol solution, the overall mechanical properties of the tubular composite ultrafiltration membrane are effectively enhanced.

[0025] Preferably, the graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0026] Preferably, the pore size of the polyester separation layer is 2~10nm.

[0027] Measurements show that the pore size of the polyester separation layer on the inner surface of the tubular composite ultrafiltration membrane is 2~10nm, which has the advantages of high separation accuracy and good resistance to fouling.

[0028] Preferably, the supporting substrate layer is composed of nonwoven fabric.

[0029] Preferably, the supporting substrate layer is composed of a double-layer polypropylene nonwoven fabric.

[0030] The present invention also provides a method for preparing a tubular composite ultrafiltration membrane as described herein, comprising the following steps:

[0031] S1. The supporting substrate is made into a tubular supporting substrate layer;

[0032] S2. The casting solution is coated on the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane.

[0033] S3. Fill the organic tubular membrane with dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in the crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain the tubular composite ultrafiltration membrane.

[0034] Based on the aforementioned technical means, firstly, a tubular support substrate layer is fabricated. This step is simple and easy to perform, laying the foundation for subsequent processes. Next, a casting solution is coated onto the inner surface of the tubular support substrate layer. Through pre-evaporation, curing, and cleaning, a stable organic tubular membrane is formed. This process is easy to control and ensures membrane quality. Finally, the organic tubular membrane is filled with a dendritic polyester polyol solution, its ends are sealed, and it is immersed in a crosslinking agent solution for reaction. After cleaning and drying, a polyester separation layer with excellent performance is formed. The entire preparation process is simple to operate, operates under mild conditions, is easy to mass-produce, reduces production costs, improves production efficiency, and has good prospects for industrial application. The tubular composite ultrafiltration membrane preparation method of this invention solves the problems of high manufacturing difficulty, high cost, and difficulty in industrialization of existing composite membrane preparation methods through a step-by-step process design.

[0035] Preferably, step S1 includes: welding a double-layer polypropylene nonwoven fabric into a tubular support substrate layer using an ultrasonic welding machine.

[0036] Preferably, step S2 includes: coating the inner surface of the tubular support substrate layer with a casting solution, then pre-evaporating it at a temperature of 20-25°C for 20-60 seconds, followed by immersing it in water at a temperature of 20-40°C for phase transformation for 20-40 seconds to solidify and form a support transition layer, and then cleaning it to obtain an organic tubular membrane.

[0037] Preferably, step S3 includes: filling an organic tubular membrane with a dendritic polyester polyol solution at a temperature of 40-60°C, then sealing both ends of the organic tubular membrane with pistons, immersing it in a crosslinking agent solution for 10-120 minutes, removing it, draining the dendritic polyester polyol solution, washing it, and air-drying it at room temperature to obtain a tubular composite ultrafiltration membrane.

[0038] Preferably, the casting solution is prepared by adding polyvinylidene fluoride and additives into a polar solvent, mechanically stirring at a temperature of 60-80°C for 4-6 hours, then letting it stand for 8-12 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0039] Preferably, the dendritic polyester polyol solution is prepared by sequentially adding dendritic polyester polyol, alcohol solvent and graphene oxide to water, mechanically stirring for 1 to 3 hours at a temperature of 60 to 80°C, and then keeping warm at 60°C for later use.

[0040] Preferably, the crosslinking agent is selected from glutaraldehyde.

[0041] The present invention also provides an application of the tubular composite ultrafiltration membrane as described in the present invention, wherein the tubular composite ultrafiltration membrane is used in the treatment of high COD and high turbidity water, such as landfill leachate and bio-fermentation broth.

[0042] Preferably, the tubular composite ultrafiltration membrane, when used as a filter membrane in the treatment of landfill leachate and bio-fermentation liquid, can efficiently filter out small molecule organic matter, improve the retention effect, and reduce the frequency of cleaning.

[0043] The beneficial effects of this invention are:

[0044] 1) The tubular composite ultrafiltration membrane of the present invention is composed of a polyvinylidene fluoride support transition layer formed by phase inversion and a polyester separation layer formed by thermal reaction polymerization. The surface of the polyester separation layer is dense and smooth under electron microscopy, with no defects or pores. It has a high rejection rate for small molecule organic matter and can effectively resist the deposition of pollutants on the membrane surface, thus ensuring the long-term performance of the organic composite tubular membrane in high COD and high turbidity treatment scenarios.

[0045] 2) In the tubular composite ultrafiltration membrane of the present invention, during the formation of the polyester separation layer, the dendritic polyester polyol, due to its large molecular weight and unique three-dimensional network structure, adsorbs and aggregates together with sheet-like graphene oxide on one side of the polyvinylidene fluoride (PVDF) support transition layer. Meanwhile, the crosslinking agent glutaraldehyde, with its smaller molecular weight, diffuses through the PVDF support transition layer and reacts thermally with the spherical dendritic polyester polyol on the surface of the PVDF support transition layer to form the polyester separation layer. This unique thermal reaction method allows the formed polyester separation layer to be firmly embedded within the PVDF support transition layer. The gradient composite structure effectively improves the pressure resistance of the support transition layer, ensuring that the tubular membrane does not experience pore collapse or corresponding compaction effects during use, thus avoiding a sudden drop in membrane performance.

[0046] 3) The preparation method of the tubular composite ultrafiltration membrane of the present invention involves firstly fabricating a tubular support substrate layer, a simple and easy step that lays the foundation for subsequent processes. Next, a casting solution is coated onto the inner surface of the tubular support substrate layer. Through pre-evaporation, curing, and cleaning, a stable organic tubular membrane is formed. This process is easy to control and ensures membrane quality. Finally, the organic tubular membrane is filled with a dendritic polyester polyol solution, its ends are sealed, and it is immersed in a crosslinking agent solution for reaction. After cleaning and drying, a polyester separation layer with excellent performance is formed. The entire preparation process is simple to operate, operates under mild conditions, is easy to mass-produce, reduces production costs, improves production efficiency, and has good prospects for industrial application. It has significant application value in the field of membrane separation technology. Attached Figure Description

[0047] Figure 1 This is a scanning electron microscope image of the separation layer on the inner surface of the tubular composite ultrafiltration membrane prepared in Example 1;

[0048] Figure 2 This is a scanning electron microscope image of the polyvinylidene fluoride support layer on the inner surface of the ordinary tubular filter membrane prepared in Comparative Example 1. Detailed Implementation

[0049] The following description, with reference to preferred embodiments, illustrates the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are merely illustrative of the present invention and not intended to limit the scope of protection of the present invention.

[0050] Where specific techniques or conditions are not specified in the detailed embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0051] Example 1

[0052] A tubular composite ultrafiltration membrane includes a nonwoven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0053] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0054] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0055] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 3% terminal hydroxyl hyperbranched polyester, 0.2% graphene oxide, 6% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0056] The method for preparing the tubular composite ultrafiltration membrane in this embodiment includes the following steps:

[0057] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0058] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0059] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0060] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support transition layer. After washing with pure water, an organic tubular membrane is obtained.

[0061] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0062] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, mechanically stir at 60°C for 2 hours, and then keep warm at 60°C for later use.

[0063] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 1.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0064] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31. Then, seal both ends of the organic tubular membrane with a piston and immerse it in the glutaraldehyde solution at 60°C in S32 for 30 minutes. After that, remove the organic tubular membrane and drain the dendritic polyester polyol solution. Wash with pure water and air dry at room temperature to obtain the tubular composite ultrafiltration membrane.

[0065] Example 2

[0066] A tubular composite ultrafiltration membrane includes a nonwoven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0067] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0068] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 16% polyvinylidene fluoride, 7% additive polyvinylpyrrolidone (K60), 3% additive Tween 80 and 74% polar solvent N,N-dimethylacetamide.

[0069] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 5% terminal hydroxyl hyperbranched polyester, 0.1% graphene oxide, 10% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0070] The method for preparing the tubular composite ultrafiltration membrane in this embodiment includes the following steps:

[0071] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0072] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0073] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K60) and Tween 80 are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 6 hours, then allowed to stand for 12 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0074] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 40 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 40 seconds to solidify and form a polyvinylidene fluoride support transition layer. After washing with pure water, an organic tubular membrane is obtained.

[0075] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0076] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, stir mechanically for 3 hours at 60°C, and then keep warm at 60°C for later use.

[0077] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 2.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0078] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31. Then, seal both ends of the organic tubular membrane with a piston and immerse it in the glutaraldehyde solution at 60°C in S32 for 30 minutes. After that, remove the organic tubular membrane and drain the dendritic polyester polyol solution. Wash with pure water and air dry at room temperature to obtain the tubular composite ultrafiltration membrane.

[0079] Example 3

[0080] A tubular composite ultrafiltration membrane includes a nonwoven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0081] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0082] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 12% polyvinylidene fluoride, 5% additive polyvinylpyrrolidone (K30), 1% additive polyethylene glycol 400 and 82% polar solvent N,N-dimethylacetamide.

[0083] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 1% terminal hydroxyl hyperbranched polyester, 0.5% graphene oxide, 2% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0084] The method for preparing the tubular composite ultrafiltration membrane in this embodiment includes the following steps:

[0085] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0086] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0087] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and polyethylene glycol 400 are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 8 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0088] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 25 seconds to solidify and form a polyvinylidene fluoride support transition layer. After washing with pure water, an organic tubular membrane is obtained.

[0089] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0090] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, mechanically stir at 60°C for 2 hours, and then keep warm at 60°C for later use.

[0091] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 0.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0092] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31. Then, seal both ends of the organic tubular membrane with a piston and immerse it in the glutaraldehyde solution at 60°C in S32 for 30 minutes. After that, remove the organic tubular membrane and drain the dendritic polyester polyol solution. Wash with pure water and air dry at room temperature to obtain the tubular composite ultrafiltration membrane.

[0093] Example 4

[0094] A tubular composite ultrafiltration membrane includes a nonwoven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0095] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0096] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0097] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 3% terminal hydroxyl hyperbranched polyester, 0.5% graphene oxide, 6% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0098] The method for preparing the tubular composite ultrafiltration membrane in this embodiment includes the following steps:

[0099] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0100] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0101] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0102] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support transition layer. After washing with pure water, an organic tubular membrane is obtained.

[0103] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0104] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, stir mechanically for 2 hours at 60°C, and then keep warm at 60°C for later use.

[0105] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 1.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0106] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31, then seal both ends of the organic tubular membrane, and then immerse it in the glutaraldehyde solution at 60°C in S32 for 30 minutes. After that, remove the organic tubular membrane, drain the dendritic polyester polyol solution, wash with pure water, and air dry at room temperature to obtain the tubular composite ultrafiltration membrane.

[0107] Example 5

[0108] A tubular composite ultrafiltration membrane includes a nonwoven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0109] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0110] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0111] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 3% terminal hydroxyl hyperbranched polyester, 0.2% graphene oxide, 6% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0112] The method for preparing the tubular composite ultrafiltration membrane in this embodiment includes the following steps:

[0113] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0114] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0115] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0116] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support transition layer. After washing with pure water, an organic tubular membrane is obtained.

[0117] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0118] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, stir mechanically for 2 hours at 60°C, and then keep warm at 60°C for later use.

[0119] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 1.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0120] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31, then seal both ends of the organic tubular membrane, and then immerse it in the glutaraldehyde solution at 60°C in S32 for 10 minutes. After that, remove the organic tubular membrane, drain the dendritic polyester polyol solution, wash with pure water, and air dry at room temperature to obtain the tubular composite ultrafiltration membrane.

[0121] Example 6

[0122] A tubular composite ultrafiltration membrane includes a nonwoven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0123] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0124] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0125] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 3% terminal hydroxyl hyperbranched polyester, 0.2% graphene oxide, 6% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0126] The method for preparing the tubular composite ultrafiltration membrane in this embodiment includes the following steps:

[0127] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0128] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0129] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0130] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support transition layer. After washing with pure water, an organic tubular membrane is obtained.

[0131] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0132] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, stir mechanically for 2 hours at 60°C, and then keep warm at 60°C for later use.

[0133] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 1.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0134] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31, then seal both ends of the organic tubular membrane, and then immerse it in the glutaraldehyde solution at 60°C in S32 for 120 minutes. After that, remove the organic tubular membrane, drain the dendritic polyester polyol solution, wash with pure water, and air dry at room temperature to obtain the tubular composite ultrafiltration membrane.

[0135] Comparative Example 1

[0136] A common tubular filter membrane includes a nonwoven fabric support substrate layer and a polyvinylidene fluoride support layer;

[0137] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0138] The polyvinylidene fluoride support layer is formed by phase transformation of a casting solution coated on the inner surface of a nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0139] The preparation method of the ordinary tubular filter membrane in this embodiment includes the following steps:

[0140] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0141] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0142] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0143] S22. The casting solution obtained in S21 is coated on the inner surface of the tubular support substrate layer obtained in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support layer. After washing with pure water, a common tubular filter membrane is obtained.

[0144] Comparative Example 2

[0145] A common tubular composite filter membrane includes a non-woven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0146] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0147] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0148] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer; the dendritic polyester polyol solution comprises the following components by mass percentage: 3% terminal hydroxyl hyperbranched polyester, 6% isopropanol, and the remainder is water.

[0149] The preparation method of the ordinary tubular composite filter membrane in this embodiment includes the following steps:

[0150] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0151] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0152] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0153] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support transition layer. After cleaning, an organic tubular membrane is obtained.

[0154] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash with pure water, and air dry to obtain a tubular composite ultrafiltration membrane, specifically including:

[0155] S31. Add dendritic polyester polyol and isopropanol to water in the above proportions, stir mechanically for 2 hours at 60°C, and then keep warm at 60°C for later use.

[0156] S32. Add glutaraldehyde to room temperature water and stir to obtain a glutaraldehyde solution with a mass fraction of 1.5%. Then heat the glutaraldehyde solution to 60°C and keep it warm for later use.

[0157] S33. Fill the organic tubular membrane prepared in S2 with the dendritic polyester polyol solution at 60°C in S31. Then, seal both ends of the organic tubular membrane with a piston and immerse it in the glutaraldehyde solution at 60°C in S32 for 30 minutes. After that, remove the organic tubular membrane and drain the dendritic polyester polyol solution. Wash with pure water and air dry at room temperature to obtain a common tubular composite filter membrane.

[0158] Comparative Example 3

[0159] A common tubular composite filter membrane includes a non-woven fabric support substrate layer, a polyvinylidene fluoride support transition layer, and a polyester separation layer.

[0160] The nonwoven fabric support substrate layer is composed of polypropylene nonwoven fabric;

[0161] The polyvinylidene fluoride support transition layer is formed by phase transformation of the casting solution coated on the inner surface of the nonwoven support substrate layer; the casting solution includes the following components by mass percentage: 14% polyvinylidene fluoride, 6% additive polyvinylpyrrolidone (K30), 2% additive lithium chloride and 78% polar solvent N,N-dimethylacetamide.

[0162] The polyester separation layer is formed by thermal reaction of a dendritic polyester polyol solution in a polyvinylidene fluoride support transition layer. The dendritic polyester polyol solution includes the following components by mass percentage: 3% terminal hydroxyl hyperbranched polyester, 0.2% graphene oxide, 6% isopropanol, and the remainder is water. Graphene oxide is a two-dimensional sheet-like inorganic material with a certain amount of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl groups on its surface.

[0163] The preparation method of the ordinary tubular composite filter membrane in this embodiment includes the following steps:

[0164] S1. The supporting substrate is made into a tubular supporting substrate layer, specifically including: welding double-layer polypropylene nonwoven fabric into a tubular supporting substrate layer using an ultrasonic welding machine.

[0165] S2. The casting solution is coated onto the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane, specifically including:

[0166] S21. Polyvinylidene fluoride, polyvinylpyrrolidone (K30) and lithium chloride are added to N,N-dimethylacetamide in the above proportions. The mixture is mechanically stirred at 70°C for 4 hours, then allowed to stand for 10 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

[0167] S22. The casting solution prepared in S21 is coated on the inner surface of the tubular support substrate layer prepared in S1, and then pre-evaporated in air for 30 seconds. After that, it is immersed in a water bath at a temperature of 25°C for phase transformation for 30 seconds to solidify and form a polyvinylidene fluoride support transition layer. After cleaning, an organic tubular membrane is obtained.

[0168] S3. Fill the organic tubular membrane with a dendritic polyester polyol solution, then seal both ends of the organic tubular membrane, immerse it in a crosslinking agent solution for reaction, remove it, drain the dendritic polyester polyol solution, wash it, and dry it to obtain a tubular composite ultrafiltration membrane, specifically including:

[0169] S31. Add dendritic polyester polyol, isopropanol and graphene oxide to water in the above proportions, stir mechanically for 2 hours at 60°C, and then keep warm at 60°C for later use.

[0170] S33. Fill the organic tubular membrane obtained in S2 with the dendritic polyester polyol solution at 60°C in S31, then seal both ends of the organic tubular membrane with a piston, place it directly for 30 minutes, drain the dendritic polyester polyol solution, wash with pure water, and air dry at room temperature to obtain a common tubular composite filter membrane.

[0171] Detection and Analysis

[0172] 1) Scanning electron microscopy analysis

[0173] The inner surface separation layer of the tubular composite ultrafiltration membrane prepared in Example 1 was observed by scanning electron microscopy, as was the inner surface polyvinylidene fluoride support layer of the ordinary tubular filter membrane prepared in Comparative Example 1. The results are as follows: Figure 1 and Figure 2 As shown.

[0174] from Figure 1 As can be observed, the inner surface of the tubular composite ultrafiltration membrane prepared in Example 1 is uniformly covered with a polyester separation layer that plays a key role in improving the membrane separation performance. The membrane surface skin is dense and flat, without defects or pores, and the introduced graphene oxide can be uniformly and completely embedded into the polyester separation layer.

[0175] And from Figure 2 As can be observed, the surface of the ordinary tubular membrane prepared in Comparative Example 1 has obvious open-pore structure and uneven pore size distribution.

[0176] 2) Performance Testing

[0177] (1) Membrane pore size: The pore size was tested using a bubble point method pore size analyzer. The specific operating steps are as follows:

[0178] Instrument preparation: Check that all components of the bubble point pore size analyzer are working properly, including the pressure control system, gas flow meter, and detection sensors, to ensure that the instrument is in good working condition.

[0179] Sample preparation: The tubular composite ultrafiltration membranes prepared in Examples 1 to 6, the ordinary tubular filter membranes prepared in Comparative Example 1, and the ordinary tubular composite filter membranes prepared in Comparative Examples 2 to 3 are used as membrane samples, and they are cut into appropriate sizes. Generally, the sample surface should be flat, undamaged, and wrinkle-free.

[0180] Preparation of wetting solution: Isopropanol is selected as the wetting solution to ensure that the wetting solution can completely wet the membrane sample.

[0181] Test sample

[0182] Sample wetting: Immerse the membrane sample completely in the wetting solution to ensure that the membrane pores are fully filled with the wetting solution.

[0183] Sample Installation: Install the impregnated membrane sample into the testing apparatus and seal it securely. Ensure a good seal between the sample and the apparatus to prevent gas leakage.

[0184] Test operation

[0185] Gas introduction: Slowly introduce nitrogen gas that does not react with the wetting solution or the sample, gradually increasing the gas pressure. The pressure increase should be uniform and slow to ensure accurate detection.

[0186] Record the bubble point pressure: Observe the membrane surface closely as the gas pressure gradually increases. When the first continuous bubble appears, record the pressure value at this point; this is the bubble point pressure.

[0187] Record the pressure-flow curve: Continue to increase the gas pressure and record the gas flow rate at different pressures in real time to obtain the pressure-flow curve.

[0188] Calculate the aperture: Calculate the maximum aperture based on the bubble point pressure and the Yang-Laplace equation.

[0189] The formula for calculating the maximum aperture is: Where D represents the maximum pore size in meters (m), γ represents the surface tension of the liquid in N / m, θ represents the contact angle in degrees (°), and P represents the bubble point pressure in Pa.

[0190] Finally, the unit of the maximum aperture was converted to nm, and the results are shown in Table 1.

[0191] (2) Tensile strength at break: The tubular composite ultrafiltration membranes prepared in Examples 1 to 6, the ordinary tubular filter membranes prepared in Comparative Example 1, and the ordinary tubular composite filter membranes prepared in Comparative Examples 2 to 3 were prepared into samples with a length of 240 mm. The tensile strength at break of each sample was tested using a tensile testing machine. Five samples were prepared for each example and comparative example for testing, and the average value of the tensile strength at break was taken. The test results are shown in Table 1.

[0192] (3) Pure water flux: The tubular composite ultrafiltration membranes prepared in Examples 1 to 6, the ordinary tubular filter membrane prepared in Comparative Example 1, and the ordinary tubular composite filter membranes prepared in Comparative Examples 2 to 3 were sealed at one end, and pure water entered from the other side of the membrane tube. At an inlet water pressure of 0.2 MPa and a water temperature of 25°C, the volume of water that permeates through the effective membrane area in 1 minute was measured, and the pure water flux of the tubular membrane was calculated. The test results are shown in Table 1.

[0193] Table 1 shows the performance test results of the filter membrane.

[0194]

[0195] As can be seen from the comparative analysis in Table 1, the polyester separation layer prepared by chemical cross-linking reaction can significantly reduce the pore size of the polyvinylidene fluoride support transition layer. The graphite oxide introduced into the polyester separation layer can greatly improve the tensile strength of the tubular membrane at break, and at the same time has a certain swelling effect on the pore structure of the polyester cross-linking, further improving the pure water flux of the membrane. In the process of polysulfone separation layer formation, glutaraldehyde, as a cross-linking agent, greatly promotes the formation of the polyester cross-linking structure and the densification of the surface skin.

[0196] In summary, compared with traditional organic tubular membranes (traditional tubular membranes refer to organic tubular filter membranes composed of a non-woven fabric support substrate layer and a polyvinylidene fluoride support layer as separation layers), the tubular composite ultrafiltration membrane prepared by the present invention is composed of a polyvinylidene fluoride support transition layer formed by phase inversion and a polyester separation layer formed by thermal reaction polymerization. The surface of the polyester separation layer is dense and smooth under electron microscopy, with no defects or pores. It has a high rejection rate for small molecule organic matter and can effectively resist the deposition of pollutants on the membrane surface, ensuring the long-term performance of the organic composite tubular membrane in high COD and high turbidity treatment scenarios.

[0197] In the tubular composite ultrafiltration membrane of this invention, during the formation of the polyester separation layer, the dendritic polyester polyol, due to its large molecular weight and unique three-dimensional network structure, adsorbs and aggregates together with sheet-like graphene oxide on one side of the polyvinylidene fluoride (PVDF) support transition layer. Meanwhile, the crosslinking agent glutaraldehyde, with its smaller molecular weight, diffuses through the PVDF support transition layer and reacts thermally with the spherical dendritic polyester polyol on the surface of the PVDF support transition layer to form the polyester separation layer. This unique thermal reaction method allows the formed polyester separation layer to be firmly embedded within the PVDF support transition layer. The gradient composite structure effectively improves the pressure resistance of the support transition layer, ensuring that the tubular membrane does not experience pore collapse or corresponding compaction effects during use, thus preventing a sudden drop in membrane performance.

[0198] The tubular composite ultrafiltration membrane of this invention differs from the traditional method of directly adding inorganic materials such as graphene oxide to the casting solution. In this invention, graphene oxide with a two-dimensional sheet structure is added to a dendritic polyester polyol solution. On the one hand, graphene oxide has good dispersibility in the polyester polyol solution, avoiding the problem of easy agglomeration and delamination of inorganic materials in the casting solution of the supporting transition layer. On the other hand, the graphene oxide introduced into the polyester separation layer has a certain swelling effect on the cross-linked pore structure of the polyester, which can further improve the water permeation efficiency while ensuring the rigidity and pressure resistance of the separation layer. The prepared tubular composite ultrafiltration membrane has a pure water flux of more than 400 LMH (liters / square meter / hour) under extremely low pressure (0.2 MPa).

[0199] The method for preparing the tubular composite ultrafiltration membrane of this invention involves firstly fabricating a tubular support substrate layer. This simple and easy step lays the foundation for subsequent processes. Next, a casting solution is coated onto the inner surface of the tubular support substrate layer. Through pre-evaporation, curing, and cleaning, a stable organic tubular membrane is formed. This process is easy to control and ensures membrane quality. Finally, the organic tubular membrane is filled with a dendritic polyester polyol solution, its ends are sealed, and it is immersed in a crosslinking agent solution for reaction. After cleaning and drying, a polyester separation layer with excellent performance is formed. The entire preparation process is simple, low-cost, and easy to industrialize. The prepared tubular composite membrane has a dense surface skin, good antifouling performance, good pressure resistance, and separation accuracy, showing promising prospects for industrial application and possessing significant application value in the field of membrane separation technology.

[0200] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

Claims

1. A method for preparing a tubular composite ultrafiltration membrane, characterized in that, Includes the following steps: S1. The supporting substrate is made into a tubular supporting substrate layer; S2. The casting solution is coated on the inner surface of the tubular support substrate layer, pre-evaporated, cured, and cleaned to obtain an organic tubular membrane. S3. Fill the organic tubular membrane with a dendritic polyester polyol solution at a temperature of 40~60℃, then seal both ends of the organic tubular membrane with a piston, immerse it in the crosslinking agent solution and react for 10~120 minutes, take it out, drain the dendritic polyester polyol solution, wash it, and air dry it at room temperature to obtain a tubular composite ultrafiltration membrane. The preparation method of the dendritic polyester polyol solution is as follows: Hydroxyl-terminated hyperbranched polyester, alcohol solvent, and graphene oxide are sequentially added to water, stirred at 60–80°C for 1–3 hours, and then kept at 60°C for later use. The graphene oxide introduced into the polyester separation layer can significantly improve the tensile strength at break of the tubular membrane, and at the same time has a certain swelling effect on the cross-linked pore structure of the polyester, further improving the pure water flux of the membrane; the graphene oxide is a two-dimensional sheet-like inorganic material with oxygen-containing functional groups on its surface. The tubular composite ultrafiltration membrane includes a supporting substrate layer, a supporting transition layer, and a polyester separation layer; The support transition layer is formed by phase transformation of the casting liquid coated on the inner surface of the support substrate layer; The polyester separation layer is formed by a dendritic polyester polyol solution through a thermal reaction on the surface of the supporting transition layer. During the formation of the polyester separation layer, the dendritic polyester polyol and sheet graphene oxide are adsorbed and aggregated on one side of the polyvinylidene fluoride supporting transition layer. Meanwhile, the crosslinking agent glutaraldehyde diffuses through the polyvinylidene fluoride supporting transition layer and reacts thermally with the spherical dendritic polyester polyol on the surface of the polyvinylidene fluoride supporting transition layer to form the polyester separation layer. This thermal reaction method allows the formed polyester separation layer to be firmly embedded in the polyvinylidene fluoride supporting transition layer. The gradient composite structure effectively improves the pressure resistance of the supporting transition layer, ensuring that the tubular membrane will not experience pore collapse or corresponding compaction effect during use, thus avoiding a sudden drop in membrane performance. The pore size of the polyester separation layer is 2~10nm.

2. The method for preparing the tubular composite ultrafiltration membrane according to claim 1, characterized in that, The casting solution comprises polyvinylidene fluoride, additives, and polar solvents.

3. The method for preparing the tubular composite ultrafiltration membrane according to claim 2, characterized in that, The additive is selected from at least one of polyvinylpyrrolidone, Tween, polyethylene glycol, chitosan, chitin, polyvinyl alcohol, and lithium chloride; And / or, the polar solvent is selected from N,N-dimethylacetamide or N,N-dimethylformamide; And / or, the casting solution comprises, by mass percentage: 10-16% polyvinylidene fluoride, 1-10% additives, and the remainder being a polar solvent.

4. The method for preparing the tubular composite ultrafiltration membrane according to claim 1, characterized in that, The alcohol solvent is selected from at least one of ethanol, isopropanol, and n-butanol; And / or, the dendritic polyester polyol solution comprises, by mass percentage: 1-5% dendritic polyester polyol, 0.1-0.5% graphene oxide, 5-25% alcohol solvent, and the remainder being water.

5. The method for preparing the tubular composite ultrafiltration membrane according to claim 1, characterized in that, The supporting substrate layer is composed of non-woven fabric.

6. The method for preparing the tubular composite ultrafiltration membrane according to claim 1, characterized in that, S1 includes: welding double-layer polypropylene nonwoven fabric into a tubular support substrate layer using an ultrasonic welding machine. And / or, S2 includes: coating the inner surface of the tubular support substrate layer with a casting solution, then pre-evaporating it at a temperature of 20~25°C for 20~60 seconds, then immersing it in water at a temperature of 20~40°C for phase transformation for 20~40 seconds to solidify and form a support transition layer, and then cleaning it to obtain an organic tubular membrane.

7. The method for preparing the tubular composite ultrafiltration membrane according to claim 1, characterized in that, The casting solution is prepared by adding polyvinylidene fluoride and additives into a polar solvent, stirring at a temperature of 60-80°C for 4-6 hours, then letting it stand for 8-12 hours, followed by vacuum degassing for 2 hours to obtain the casting solution.

8. An application of a tubular composite ultrafiltration membrane prepared by the preparation method according to any one of claims 1 to 7, characterized in that, Application of the tubular composite ultrafiltration membrane in landfill leachate treatment and bio-fermentation broth treatment.

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