Post-treatment modification method of polyester film, modified composite film and application

Abstract

The application discloses a post-treatment modification method of a polyester membrane, a modified composite membrane and application, and relates to the technical field of membrane separation. The post-treatment modification method comprises the following steps: providing a polyester membrane to be modified containing residual phenolic hydroxyl groups; immersing the polyester membrane to be modified in an alkaline aqueous solution, so that at least part of the residual phenolic hydroxyl groups are sequentially converted into phenolic oxygen negative ions and quinone active sites; removing the alkaline aqueous solution attached to the surface of the polyester membrane to be modified, and directly transferring the polyester membrane to be modified to an amino polymer solution for immersion treatment for a second preset time, so as to graft the amino polymer to the polyester membrane to be modified; and washing the polyester membrane to be modified treated through the amino grafting step, to obtain a modified composite membrane. Through alkaline activation and oxidation of the residual phenolic hydroxyl groups and further grafting of polyethylene imine, the stability of active sites is stably closed while excessive hydrolysis of a polyester skeleton is inhibited, so that the structural stability of the polyester membrane in a tap water environment and the salt retention performance maintenance rate are improved.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a post-treatment modification method for polyester membranes, modified composite membranes, and their applications. Background Technology

[0002] Polyester separation membranes can be formed by interfacial polymerization of polyphenolic compounds and polyacrylamide chlorides on the surface of a porous supported membrane. In this process, the phenolic hydroxyl groups in the polyphenolic compounds react with the acrylamide chloride groups to form a polyester network, thereby endowing the membrane with selective separation properties. Due to factors such as reactant diffusion, reaction rate, and steric hindrance at the interface, some unreacted residual phenolic hydroxyl groups usually remain in the polyester membrane formed by interfacial polymerization. Although these residual phenolic hydroxyl groups can improve the hydrophilicity of the membrane surface, their high chemical reactivity makes them prone to oxidation, complexation, or other reactions during water treatment, especially in tap water environments containing active chlorine, metal ions, and other dissolved components. This can cause local structural changes in the polyester network, thereby affecting the membrane's compactness and salt retention performance.

[0003] In the prior art, when post-treating polyester membranes, if only alkaline media are used, although the reactivity of residual phenolic hydroxyl groups can be improved, the alkaline media may also hydrolyze the polyester bonds, resulting in a loose membrane structure or decreased separation performance. If only amino-containing substances are used to treat the polyester membrane, the covalent reaction ability between the unactivated residual phenolic hydroxyl groups and amino groups is limited. The amino-containing substances mainly adhere to the membrane layer through physical adsorption, hydrogen bonding or electrostatic interaction, making it difficult to form a stable binding structure. Therefore, the degree of improvement on the water environment stability of the polyester membrane is limited.

[0004] Therefore, existing polyester membrane post-treatment technologies are insufficient to effectively block residual phenolic hydroxyl-related active sites while maintaining the integrity of the polyester backbone structure and initial separation performance, and to improve the long-term structural stability and salt retention rate of polyester membranes in complex water environments such as tap water. Summary of the Invention

[0005] One objective of the first aspect of this invention is to provide a post-treatment modification method for polyester membranes, which solves the technical problems in the prior art where residual phenolic hydroxyl groups in polyester membranes are difficult to effectively and stably seal, and alkaline treatment easily causes hydrolysis of the polyester skeleton, resulting in insufficient structural stability and salt retention performance of polyester membranes in tap water environments.

[0006] Another objective of the first aspect of this invention is to balance the formation efficiency of quinone active sites with the structural integrity of the polyester film to be modified.

[0007] The second aspect of this invention aims to provide a modified composite film prepared by the post-processing modification method of the above-mentioned polyester film.

[0008] The third aspect of this invention aims to provide the application of the modified composite membrane described above in drinking water treatment.

[0009] According to a first aspect of the present invention, the present invention provides a post-treatment modification method for polyester film, comprising the following steps: A polyester film to be modified is provided, wherein the polyester film to be modified is prepared by interfacial polymerization of polyphenolic compounds and polyacryl chlorides, and the polyester film to be modified contains residual phenolic hydroxyl groups; The polyester film to be modified is immersed in an alkaline aqueous solution containing dissolved oxygen at a pH of 8-9 for a first preset time, so that at least a portion of the residual phenolic hydroxyl groups are converted into phenolic anions under alkaline conditions, and at least a portion of the phenolic anions are oxidized under the action of dissolved oxygen to form quinone active sites. The polyester film to be modified, which has been treated with the alkaline aqueous solution, is taken out of the alkaline aqueous solution, the alkaline aqueous solution adhering to the surface of the polyester film to be modified is removed, and the polyester film to be modified is directly transferred to an amino polymer solution for immersion treatment for a second preset time, so that the amino groups in the amino polymer react with the quinone active sites to graft the amino polymer onto the polyester film to be modified. The modified polyester film treated by the amino grafting step is washed with water to obtain a modified composite film. Wherein, the amino polymer is polyethyleneimine, the amino polymer solution is an aqueous solution of polyethyleneimine, the mass fraction of polyethyleneimine in the aqueous solution of polyethyleneimine is 0.1%-2%, and the pH of the aqueous solution of polyethyleneimine is any value between 8 and 9.

[0010] Optionally, the solute in the alkaline aqueous solution is one or more of sodium hydroxide, potassium hydroxide, and ammonia, and the first preset time is any value between 0.5 min and 2 min.

[0011] Optionally, the second preset time is any value between 1 min and 5 min.

[0012] Optionally, the reaction temperature between the amino polymer solution and the polyester film to be modified is any value between 20°C and 40°C.

[0013] Optionally, the weight-average molecular weight of the polyethyleneimine is 600-70000.

[0014] Optionally, the method for preparing the polyester film includes the following steps: The polysulfone ultrafiltration membrane is reacted with an aqueous solution containing the aforementioned polyphenolic compound; After removing the aqueous solution from the surface of the polysulfone ultrafiltration membrane, the polysulfone ultrafiltration membrane is reacted with an organic phase solution containing the polyacryl chloride, so that the polyphenolic compound and the polyacryl chloride undergo an interfacial polymerization reaction on the polysulfone ultrafiltration membrane. The polysulfone ultrafiltration membrane that has undergone the interfacial polymerization reaction is subjected to heat treatment to obtain the polyester membrane.

[0015] Optionally, the mass fraction of the polyphenolic compound in the aqueous solution is any value between 0.5 wt% and 2.5 wt%.

[0016] Optionally, the mass fraction of the polyacrylamide chloride in the organic phase solution is any value between 0.05wt% and 0.5wt%.

[0017] According to a second aspect of the present invention, the present invention also provides a modified composite film, said modified composite film being prepared by a post-treatment modification method of the polyester film described in any one of the preceding claims, said modified composite film containing polyethyleneimine segments grafted onto the polyester film.

[0018] According to a third aspect of the present invention, the present invention also provides the application of the modified composite membrane described above in drinking water treatment.

[0019] This invention converts residual phenolic hydroxyl groups into highly reactive phenolic anions through alkaline activation treatment, and further oxidizes these anions into quinone active sites using dissolved oxygen in an alkaline aqueous solution, which is beneficial for improving the subsequent grafting efficiency of polyethyleneimine. The amino groups in polyethyleneimine can undergo addition and / or condensation reactions with quinone active sites to form stable CN bonds, which can further stabilize the polyester membrane structure through hydrogen bonding or electrostatic interactions. At the same time, by promptly removing the adhering liquid on the membrane surface after alkaline treatment and transferring it to a polyethyleneimine aqueous solution for grafting treatment, the continuous hydrolysis of the polyester skeleton by the alkaline medium can be reduced. Thus, while sealing the residual phenolic hydroxyl groups, the structural integrity of the polyester membrane is maintained, improving the structural stability and salt rejection performance retention rate of the obtained modified composite membrane in tap water environments.

[0020] Furthermore, the solute in the alkaline aqueous solution of the present invention is one or more of sodium hydroxide, potassium hydroxide, and ammonia, and the first preset time is any value between 0.5 min and 2 min. By selecting the above-mentioned alkaline substance and controlling the first preset time, at least some of the residual phenolic hydroxyl groups in the polyester film to be modified can be fully deprotonated to form phenolic oxygen anions, and further form quinone active sites under the action of dissolved oxygen. At the same time, the continuous hydrolysis of polyester bonds by the alkaline medium is limited. The alkaline substance and the first preset time work together to balance the formation efficiency of quinone active sites and the structural integrity of the polyester film to be modified.

[0021] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0022] The following sections will describe some specific embodiments of the invention in detail by way of example and not limitation, with reference to the accompanying drawings. The same reference numerals in the drawings denote the same or similar parts or portions. Those skilled in the art should understand that these drawings are not necessarily drawn to scale. In the drawings: Figure 1 This is a schematic flowchart of a post-treatment modification method for a polyester film according to an embodiment of the present invention; Figure 2 These are scanning electron microscope (SEM) images of the surface and cross-section of the membrane products prepared according to Example 1 and Comparative Example 1 of the present invention; Figure 3 The X-ray photoelectron spectroscopy (XPS) spectra of the membrane products prepared according to Example 1 and Comparative Example 1 of the present invention are shown. Detailed Implementation

[0023] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0024] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, it should be noted that, for ease of description, only the parts relevant to this application are shown in the accompanying drawings, not the entire structure. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0025] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0026] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0027] Figure 1 This is a schematic flowchart of a post-treatment modification method for polyester film according to an embodiment of the present invention.

[0028] like Figure 1 As shown, the present invention provides a post-treatment modification method for polyester film, comprising the following steps: Step S100: Provide a polyester film to be modified. The polyester film to be modified is prepared by interfacial polymerization of polyphenolic compounds and polyacryl chlorides. The polyester film to be modified contains residual phenolic hydroxyl groups. Step S200: Immerse the polyester film to be modified in an alkaline aqueous solution with a pH of 8-9 and containing dissolved oxygen for a first preset time, so that at least some of the residual phenolic hydroxyl groups are converted into phenolic anions under alkaline conditions, and at least some of the phenolic anions are oxidized under the action of dissolved oxygen to form quinone active sites. Step S300: The polyester film to be modified, which has been treated with the alkaline aqueous solution, is taken out from the alkaline aqueous solution, the alkaline aqueous solution adhering to the surface of the polyester film to be modified is removed, and the polyester film to be modified is directly transferred to the amino polymer solution for immersion treatment for a second preset time, so that the amino groups in the amino polymer react with the quinone active sites to graft the amino polymer onto the polyester film to be modified. Step S400: The modified polyester membrane treated in the amino grafting step is washed with water to obtain a modified composite membrane; wherein, the amino polymer is polyethyleneimine, the amino polymer solution is an aqueous solution of polyethyleneimine, the mass fraction of polyethyleneimine in the aqueous solution of polyethyleneimine is 0.1%-2%, and the pH of the aqueous solution of polyethyleneimine is any value between 8 and 9. Here, the mass fraction of polyethyleneimine in the aqueous solution of polyethyleneimine can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, or 2.0%, or any other value within the range of 0.1%-2%, and the pH of the aqueous solution of polyethyleneimine can be 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, or 11.0, or any other value within the range of 8-9.

[0029] This embodiment provides a post-treatment modification method for a polyester film, comprising the following steps: providing a polyester film to be modified prepared by interfacial polymerization of a polyphenolic compound and a polyacrylamide chloride, wherein the polyester film contains residual phenolic hydroxyl groups; immersing the polyester film to be modified in an alkaline aqueous solution containing dissolved oxygen at a pH of 8-9 for alkaline activation and oxidation treatment, thereby converting at least some of the residual phenolic hydroxyl groups into phenolic anions and quinone active sites sequentially; removing the treated polyester film from the alkaline aqueous solution, removing the alkaline aqueous solution adhering to its surface, and directly transferring it to a polyethyleneimine aqueous solution for immersion treatment, thereby reacting the amino groups in the polyethyleneimine with the quinone active sites to graft polyethyleneimine onto the polyester film to be modified; subsequently washing the polyester film treated with the polyethyleneimine aqueous solution to obtain a modified composite film, wherein the mass fraction of polyethyleneimine in the polyethyleneimine aqueous solution is 0.1%-2%, and the pH of the polyethyleneimine aqueous solution is any value between 8 and 9.

[0030] In this embodiment, residual phenolic hydroxyl groups are converted into highly reactive phenolic anions through alkaline activation treatment, and dissolved oxygen in the alkaline aqueous solution is used to further oxidize the phenolic anions into quinone active sites, which is beneficial to improving the subsequent grafting efficiency of polyethyleneimine. The amino groups in polyethyleneimine can undergo addition and / or condensation reactions with quinone active sites to form stable CN bonds, and can further stabilize the polyester membrane structure through hydrogen bonding or electrostatic interactions. At the same time, by promptly removing the adhering liquid on the membrane surface after alkaline treatment and transferring it to a polyethyleneimine aqueous solution for grafting treatment, the continuous hydrolysis of the polyester skeleton by the alkaline medium can be reduced. Thus, while sealing the residual phenolic hydroxyl groups, the structural integrity of the polyester membrane is maintained, improving the structural stability and salt rejection performance retention rate of the obtained modified composite membrane in the tap water environment.

[0031] In a preferred embodiment, the polyphenolic compound is tannic acid, and the polyacrylamide chloride is trimesoyl chloride. Tannic acid and trimesoyl chloride form a polyester film to be modified through interfacial polymerization. The polyester film to be modified is immersed in an aqueous sodium hydroxide solution at pH 8.5. At least some of the residual phenolic hydroxyl groups in the polyester film are deprotonated under alkaline conditions to form phenolic anions, which are then oxidized by dissolved oxygen in the sodium hydroxide solution to form quinone active sites. Subsequently, the polyester film to be modified is transferred to an aqueous polyethyleneimine solution at pH 8.5. The primary and / or secondary amino groups in polyethyleneimine can act as nucleophiles, undergoing a Michael addition reaction on the conjugated unsaturated structure of the quinone active sites to form CN bonds, thereby grafting polyethyleneimine segments onto the polyester film to be modified.

[0032] In another preferred reaction pathway, the polyphenolic compound is tannic acid, and the polyacrylamide chloride is trimesoyl chloride. After the polyester membrane to be modified is treated with an aqueous sodium hydroxide solution at pH 8.5, at least some of the residual phenolic hydroxyl groups are converted to form quinone active sites. When the polyester membrane to be modified is contacted with an aqueous polyethyleneimine solution at pH 8.5, the primary amino groups in the polyethyleneimine can undergo nucleophilic addition with the carbonyl groups in the quinone active sites, and then undergo dehydration condensation to form an imine linkage structure containing C=N bonds, thereby grafting the polyethyleneimine segments onto the polyester membrane to be modified.

[0033] In the reaction system of this embodiment, the Michael addition reaction and the Schiff base condensation reaction can occur separately, in parallel, or competitively; this application does not limit this. Through the above reactions, the amino groups in polyethyleneimine can form nitrogen-containing linkage structures with the quinone active sites in the polyester membrane to be modified, thereby grafting polyethyleneimine segments onto the polyester membrane. Furthermore, the amino groups that do not participate in the covalent reaction can also form hydrogen bonds and / or electrostatic interactions with residual phenolic hydroxyl groups or phenoxy anions. Covalent grafting and non-covalent interactions together reduce the exposure of residual phenolic hydroxyl groups and limit structural changes of the polyester network in the aquatic environment, thereby improving the structural stability and salt rejection performance retention rate of the resulting modified composite membrane in tap water environments.

[0034] In this embodiment, by setting the mass fraction of polyethyleneimine in the polyethyleneimine aqueous solution to 0.1%-2% and the pH of the polyethyleneimine aqueous solution to any value between 8 and 9, the synergistic effect of the mass fraction range and pH range allows polyethyleneimine to possess suitable amino supply, reactivity, and chain segment migration ability. This is beneficial for improving the grafting and blocking degree of quinone active sites by polyethyleneimine, while also taking into account the structural integrity and separation performance of the polyester membrane. As a result, the structural stability and salt retention performance retention rate of the obtained modified composite membrane in the tap water environment are improved. Specifically, controlling the mass fraction of polyethyleneimine to 0.1%-2% provides a sufficient number of amino reaction sites, allowing the amino groups in polyethyleneimine to undergo addition and / or condensation reactions with the quinone active sites in the polyester film. This avoids excessively high polyethyleneimine concentrations that could lead to increased solution viscosity, restricted chain diffusion, or the formation of an excessively thick polymer layer on the film surface. Controlling the pH of the polyethyleneimine aqueous solution to 8-9 ensures that at least some of the amino groups in polyethyleneimine remain in a suitable unprotonated state, enhancing their nucleophilic reactivity and reducing the continuous hydrolysis of polyester bonds caused by excessively alkaline conditions.

[0035] In a further embodiment, the solute in the alkaline aqueous solution is one or more of sodium hydroxide, potassium hydroxide, and ammonia. The first preset time is any value between 0.5 min and 2 min, for example, 0.5 min, 0.6 min, 0.8 min, 1.0 min, 1.2 min, 1.4 min, 1.5 min, 1.6 min, 1.8 min, or 2.0 min, or any other value within the range of 0.5 min to 2 min. By selecting the above-mentioned alkaline substance and controlling the first preset time, at least some of the residual phenolic hydroxyl groups in the polyester film to be modified can be fully deprotonated to form phenolic anions, and further form quinone active sites under the action of dissolved oxygen. At the same time, the continuous hydrolysis of polyester bonds by the alkaline medium is limited. The alkaline substance and the first preset time work together to balance the formation efficiency of quinone active sites and the structural integrity of the polyester film to be modified.

[0036] In other embodiments, if the first preset time is too short, the activation and oxidation of the residual phenolic hydroxyl groups may be insufficient; if the first preset time is too long, it may increase the risk of polyester skeleton hydrolysis and membrane structure loosening.

[0037] In a further embodiment, the second preset time is any value within the range of 1 min to 5 min, for example, it can be 1.0 min, 1.2 min, 1.5 min, 1.8 min, 2.0 min, 2.5 min, 3.0 min, 3.5 min, 4.0 min, 4.5 min, or 5.0 min, or any other value within the range of 1 min to 5 min. Controlling the second preset time within the above range provides suitable reaction time for the migration of polyethyleneimine segments to the surface and interior of the polyester membrane to be modified, allowing the amino groups in polyethyleneimine to undergo addition and / or condensation reactions with the quinone active sites, and further blocking residual phenolic hydroxyl groups through hydrogen bonding or electrostatic interactions. This helps to improve the degree of blocking of active sites while maintaining the separation performance of the modified composite membrane.

[0038] In other embodiments, if the second preset time is too short, the grafting degree of polyethyleneimine may be insufficient; if the second preset time is too long, it may increase the possibility of non-reactive adsorption of polyethyleneimine on the membrane surface or the formation of an excessively thick polymer layer, thereby affecting the mass transfer performance of the membrane.

[0039] In a further embodiment, the reaction temperature between the amino polymer solution and the polyester film to be modified is any value within the range of 20°C to 40°C, for example, 20°C, 22°C, 24°C, 25°C, 26°C, 28°C, 30°C, 32°C, 34°C, 35°C, 36°C, 38°C, or 40°C, or any other value within the range of 20°C to 40°C. The above reaction temperature can regulate the diffusion rate of the polyethyleneimine segments and the reaction rate between the amino groups and the quinone active sites. This allows the polyethyleneimine to maintain suitable segment mobility and nucleophilic reactivity while avoiding excessively high temperatures that could lead to uncontrollable reaction rates, excessive polymer layer thickening, or adverse changes in the polyester film structure. This is beneficial for forming a more uniformly distributed polyethyleneimine graft structure, while balancing modification efficiency and film structure stability.

[0040] In a further embodiment, the weight-average molecular weight of polyethyleneimine is 600-70000, for example, it can be 600, 800, 1000, 1200, 1800, 5000, 10000, 25000, 40000, 50000, 60000, or 70000, or any other value within the range of 600-70000. The weight-average molecular weight of polyethyleneimine can adjust its segment size, number of amino groups, diffusion ability, grafting density, and steric hindrance. Polyethyleneimine with a relatively low weight-average molecular weight has better diffusion ability, making it easier to enter the surface and interior of the polyester film to be modified and react with quinone active sites; polyethyleneimine with a relatively high weight-average molecular weight has more amino reaction sites and stronger segment entanglement and steric hindrance, which is beneficial for forming multi-point grafting structures and restricting structural changes in the polyester network. By controlling the weight-average molecular weight within the above range, a balance can be achieved between the mass transfer capacity of polyethyleneimine and the grafting stabilization effect, thereby improving the blocking degree of residual phenolic hydroxyl-related active sites and the structural stability and salt rejection performance retention rate of the modified composite membrane in the aqueous environment.

[0041] In a further embodiment, the method for preparing the polyester film includes the following steps: The polysulfone ultrafiltration membrane was reacted with an aqueous solution containing polyphenolic compounds; After removing the aqueous solution from the surface of the polysulfone ultrafiltration membrane, the polysulfone ultrafiltration membrane is reacted with an organic phase solution containing polyacryl chlorides, so that the polyphenolic compounds and polyacryl chlorides undergo interfacial polymerization on the polysulfone ultrafiltration membrane. Polysulfone ultrafiltration membranes that have undergone interfacial polymerization are heat-treated to obtain polyester membranes.

[0042] In this embodiment, by first contacting the polysulfone ultrafiltration membrane with an aqueous solution containing polyphenolic compounds, the polyphenolic compounds can be distributed on the surface of the polysulfone ultrafiltration membrane and near its pore openings. After removing excess aqueous solution from the membrane surface, it is then contacted with an organic solution containing polyacrylamide chlorides. This confines the interfacial polymerization reaction to the interface between the aqueous and organic phases, reducing the impact of excess aqueous solution on the thickness and uniformity of the reaction layer. This allows the polyphenolic compounds and polyacrylamide chlorides to form a polyester separation layer on the polysulfone ultrafiltration membrane. Subsequent heat treatment promotes the continued reaction of unreacted functional groups and solidifies the formed polyester separation layer, improving the bonding stability between the polyester separation layer and the polysulfone ultrafiltration membrane. The polysulfone ultrafiltration membrane provides mechanical support and moisture transport channels for the polyester separation layer, while the polyester separation layer formed by interfacial polymerization is used to achieve selective separation. Simultaneously, it retains residual phenolic hydroxyl groups that can be used for subsequent alkali activation, oxidation, and amino grafting treatments, thus balancing the structural integrity, separation performance, and post-processing modification capabilities of the polyester membrane.

[0043] In a further embodiment, the mass fraction of the polyphenolic compound in the aqueous solution is any value within the range of 0.5 wt% to 2.5 wt%, for example, it can be 0.5 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.4 wt%, or 2.5 wt%, or any other value within the range of 0.5 wt% to 2.5 wt%. By controlling the concentration of the polyphenolic compound within the above range, an appropriate amount of polyphenolic compound can be adsorbed and distributed on the surface of the polysulfone ultrafiltration membrane, providing sufficient phenolic hydroxyl reaction sites for subsequent interfacial polymerization. This is beneficial for controlling the continuity, thickness, and crosslinking structure of the polyester separation layer, and allows the resulting polyester membrane to balance separation performance and water transport performance, while retaining an appropriate amount of residual phenolic hydroxyl groups for subsequent alkali activation, oxidation, and amino grafting treatments.

[0044] In other embodiments, if the concentration of polyphenolic compounds is too low, the amount of reactants participating in interfacial polymerization may be insufficient, making it difficult to form a continuous and complete polyester separation layer. If the concentration is too high, it may lead to the enrichment of aqueous monomers on the membrane surface, resulting in an increased thickness or an overly dense structure of the formed polyester separation layer.

[0045] In a further embodiment, the mass fraction of the polyacrylamide chloride in the organic phase solution is any value within the range of 0.05 wt% to 0.5 wt%, for example, it can be 0.05 wt%, 0.06 wt%, 0.08 wt%, 0.10 wt%, 0.12 wt%, 0.15 wt%, 0.18 wt%, 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt%, or 0.50 wt%, or any other value within the range of 0.05 wt% to 0.5 wt%. By controlling the concentration of the polyacrylamide chloride within the above range, a suitable number of acrylamide groups can be provided for the interfacial polymerization between the polyacrylamide chloride and the polyphenolic compound, and the polymerization rate, degree of crosslinking, and density of the polyester release layer can be adjusted.

[0046] The present invention also provides a modified composite membrane, which is prepared by the post-processing modification method of the above-mentioned polyester membrane, and the modified composite membrane contains polyethyleneimine segments grafted onto the polyester membrane.

[0047] The present invention also provides the application of the above-mentioned modified composite membrane in drinking water treatment.

[0048] The technical solution of this application will be further described below with reference to specific embodiments.

[0049] Example 1 This embodiment provides a post-treatment modification method for polyester film, including the following steps: Step S100: Tannic acid is dissolved in deionized water to prepare an aqueous solution with a tannic acid content of 1.2 wt%, and hexadecyltrimethylammonium chloride is added to make the concentration of hexadecyltrimethylammonium chloride 1200 μmol / L. Tristylacetyl chloride is dissolved in an isoalkane solvent to prepare an organic solution with a tristylacetyl chloride content of 0.2 wt%. A polysulfone ultrafiltration membrane is selected as the supporting membrane. The polysulfone ultrafiltration membrane is immersed in the aqueous solution for 0.5 min, and after removal, the aqueous droplets on the membrane surface are removed using an air knife. Then, the polysulfone ultrafiltration membrane is immediately immersed in the organic solution for 1 min to allow the tannic acid and tristylacetyl chloride to undergo an interfacial polymerization reaction on the polysulfone ultrafiltration membrane. After the reaction is completed, the polysulfone ultrafiltration membrane is removed and heat-treated in an oven at 60°C for 2 min to obtain the polyester membrane to be modified. The polyester membrane to be modified contains residual phenolic hydroxyl groups. Step S200: Immerse the polyester film to be modified in an aqueous sodium hydroxide solution with a pH of 8.5, and perform alkali activation treatment for 1 min and oxidation treatment in sequence, so that at least some of the residual phenolic hydroxyl groups are successively converted into phenolic anions and quinone active sites. Step S300: The polyester film to be modified after alkali activation and oxidation treatment is taken out from the sodium hydroxide aqueous solution, the sodium hydroxide aqueous solution adhering to the surface of the polyester film to be modified is removed, and the polyester film to be modified is directly transferred to a polyethyleneimine (PEI) aqueous solution with pH 8.5 and a mass fraction of 1 wt% for 2 min and treated at a temperature of 25°C. The weight average molecular weight of polyethyleneimine is 70,000, so that the amino groups in the amino polymer solution react with the quinone active sites to graft the amino polymer onto the polyester film to be modified. Step S400: The modified polyester film treated by the amino grafting step is washed with water to obtain the modified composite film.

[0050] Example 2 The only difference between Example 2 and Example 1 is that the pH of the sodium hydroxide aqueous solution is 10.5.

[0051] Example 3 The only difference between Example 3 and Example 1 is that the treatment time of immersing the polyester film to be modified in an aqueous sodium hydroxide solution with a pH of 8.5 is adjusted to 0.5 min.

[0052] Example 4 The only difference between Example 4 and Example 1 is that the treatment time for immersing the polyester film to be modified in a sodium hydroxide aqueous solution with a pH of 8.5 is adjusted to 2 minutes.

[0053] Example 5 The only difference between Example 5 and Example 1 is that the treatment time of the polyethyleneimine aqueous solution is 1 min.

[0054] Example 6 The only difference between Example 6 and Example 1 is that the treatment time of the polyethyleneimine aqueous solution is 5 minutes.

[0055] Example 7 The only difference between Example 7 and Example 1 is that the weight-average molecular weight of polyethyleneimine is 600.

[0056] Example 8 The only difference between Example 8 and Example 1 is that the weight-average molecular weight of polyethyleneimine is 1800.

[0057] Example 9 The only difference between Example 9 and Example 1 is that the weight-average molecular weight of polyethyleneimine is 25,000.

[0058] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that the polyester film to be modified was not post-treated.

[0059] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that Comparative Example 2 was subjected to alkaline activation and oxidation treatment in sodium hydroxide aqueous solution for 1 min, without PEI grafting treatment.

[0060] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that Comparative Example 3 was not subjected to alkali activation and oxidation treatment, but was directly immersed in a 1.0 wt% PEI solution for 2 min.

[0061] Comparative Example 4 The only difference between Comparative Example 4 and Example 1 is that the polyester film to be modified was placed in a 0.1 wt% sodium hydroxide aqueous solution for 10 min.

[0062] Figure 2 These are scanning electron microscope (SEM) images of the surface and cross-section of the membrane products prepared according to Example 1 and Comparative Example 1 of the present invention. Figure 3 The X-ray photoelectron spectroscopy (XPS) spectra of the membrane products prepared according to Example 1 and Comparative Example 1 of the present invention are shown.

[0063] To evaluate the impact of post-treatment modification on the microstructure and aqueous environment stability of the polyester membrane, the membrane samples obtained in Example 1 and Comparative Example 1 were characterized by scanning electron microscopy, and the NaCl retention performance of each example and comparative example was tested. The test results are as follows: Figure 2 And as shown in Table 2-4. Among them, Figure 2 a and Figure 2 b in the figure shows the surface morphology and cross-sectional morphology of the polyester film to be modified obtained in Comparative Example 1, respectively. Figure 2 c and Figure 2 The 'd' values ​​in Example 1 show the surface morphology and cross-sectional morphology of the modified composite film obtained in Example 1.

[0064] like Figure 2 As shown, the membrane surfaces of Comparative Example 1 and Example 1 maintained a continuous morphology, with no obvious through cracks or large-area peeling observed. This indicates that the short-time alkaline activation oxidation treatment and polyethyleneimine grafting treatment used in this application did not significantly damage the overall continuity of the membrane layer. The thickness of the polyester separation layer in Comparative Example 1 was approximately 24 nm, while the thickness of the membrane layer in Example 1 after post-treatment modification was approximately 35 nm, an increase of approximately 11 nm. This thickness change is consistent with the result of the grafting and deposition of polyethyleneimine segments on the surface and near-surface of the polyester membrane, indicating that the post-treatment can form modified regions containing polyethyleneimine segments while maintaining the continuity of the original polyester separation layer.

[0065] To characterize the effect of PEI post-treatment on the chemical composition of the membrane surface, XPS tests were performed on the membrane samples prepared in Example 1 and Comparative Example 1, and the results were as follows: Figure 3 The X-ray photoelectron spectrum is shown in the figure, and the surface element content is shown in Table 1.

[0066] Table 1. Atomic percentage of surface elements in the membrane samples prepared in Example 1 and Comparative Example 1

[0067] like Figure 3 As shown in Table 1, the N 1s signal intensity in the XPS total spectrum of the polyester film after PEI post-treatment was significantly enhanced. The untreated polyester film had a low N content, with C and O as the main elements, consistent with the polyester structure. After PEI post-treatment, the N content on the film surface increased significantly to 15.96 at%, and the O / N ratio changed significantly. This is attributed to the introduction of a large number of nitrogen-containing groups by PEI grafting, indicating that PEI was successfully grafted onto the film surface according to XPS results.

[0068] Table 2. Salt rejection rates of the membrane products prepared in Example 1 and Comparative Example 1 in different media.

[0069] Table 2 shows that the modified composite membrane obtained in Example 1, after being stored in pure water for 48 hours, had a NaCl retention rate of 96.7%, which did not decrease compared to the initial value; after being stored in tap water for 48 hours, the NaCl retention rate remained at 96.2%, a decrease of only 0.5%. The untreated polyester membrane obtained in Comparative Example 1, after being stored in pure water for 48 hours, had a NaCl retention rate of 94.1%, a decrease of 2.3% compared to the initial value, indicating that the unmodified polyester membrane could still basically maintain its separation structure in a pure water environment; however, after being stored in tap water for 48 hours, the NaCl retention rate of Comparative Example 1 decreased to 31%, a decrease of 68%. These results indicate that the performance degradation of the unmodified polyester membrane was not mainly caused by simple water immersion, but was related to the influence of active chlorine, metal ions, and other dissolved components in tap water on the residual phenolic hydroxyl groups and the polyester network structure. Example 1 demonstrates that by alkali-activated oxidation of residual phenolic hydroxyl groups and grafting blockage with polyethyleneimine, the membrane can maintain NaCl retention performance close to its initial state in tap water, indicating that this post-treatment method can improve the tolerance of polyester membranes to complex aquatic environments.

[0070] Table 3. NaCl rejection rate of membrane products from Examples 1-9 and Comparative Example 1 after soaking in tap water for 48 hours.

[0071] Table 3 further reflects the effects of pH of the alkali treatment solution, alkali treatment time, and polyethyleneimine treatment time on the modification effect. Under otherwise unchanged conditions, Example 2 only changed the pH of the alkali treatment solution to 10.5. The results show that the NaCl rejection rate of Example 2 after soaking in tap water for 48 h was 92%, which, although lower than Example 1, is far higher than Comparative Example 1, indicating that any pH value within the range of 8-9 can achieve post-treatment modification of the polyester film. With a polyethyleneimine treatment time of 2 min, the alkali treatment times for Examples 3, 1, and 4 were 0.5 min, 1 min, and 2 min, respectively, and their NaCl rejection rates after soaking in tap water for 48 h were 88%, 96%, and 94%, respectively. When the alkali treatment time was 0.5 min, although the NaCl retention rate of the resulting membrane was improved compared to 31% in Comparative Example 1, it was still lower than that of the samples treated with alkali for 1 min and 2 min. This indicates that when the alkali treatment time is short, the deprotonation and subsequent oxidation of residual phenolic hydroxyl groups may be insufficient, resulting in a limited number of quinone active sites and thus limiting the grafting and blocking of polyethyleneimine. When the alkali treatment time was extended to 1 min, the NaCl retention rate increased to 96%, indicating that this treatment time can provide suitable conditions for the activation and oxidation of residual phenolic hydroxyl groups. After further extending to 2 min, the NaCl retention rate was 94%, which was still at a high level, but slightly lower than that of 1 min. This indicates that when the active sites have been sufficiently formed, further extending the alkali treatment time has limited effect on promoting the grafting effect and may increase the degree of polyester bond hydrolysis. Therefore, controlling the alkali treatment time within the range of 0.5 min to 2 min can achieve a balance between the formation of quinone active sites and maintaining the structural integrity of the polyester separation layer, with 1 min showing the best overall effect.

[0072] Under the condition that the alkali treatment time was 1 min, the polyethyleneimine treatment times for Examples 5, 1, and 6 were 1 min, 2 min, and 5 min, respectively. Their NaCl retention rates after soaking in tap water for 48 h were 89%, 96%, and 96%, respectively. When the polyethyleneimine treatment time was 1 min, the NaCl retention rate was higher than that of the untreated Comparative Example 1, but still lower than that of the samples treated for 2 min and 5 min. This indicates that when the treatment time was short, the polyethyleneimine segments had not fully diffused to the surface and near-surface of the polyester film, and some quinone active sites were not sufficiently grafted or blocked. When the treatment time was increased to 2 min, the NaCl retention rate increased to 96%; after further extending to 5 min, the NaCl retention rate remained at 96%, without further improvement. This shows that under the polyethyleneimine concentration, pH, and reaction temperature conditions used in this example, a treatment time of 2 min was sufficient for the main quinone active sites to react adequately, and the effect of further extending the treatment time on improving salt retention performance tended to stabilize. Therefore, the second preset time of 1 min to 5 min can improve the water stability of the polyester film, and 2 min can take into account both the grafting effect and the processing efficiency.

[0073] Furthermore, Examples 7-9 and Example 1 used polyethyleneimine with weight-average molecular weights of 600, 1800, 25000 and 70000, respectively. The NaCl rejection rates of the modified composite membranes after soaking in tap water for 48 hours were 92.4%, 93.7%, 93.2% and 96%, respectively, all higher than those of Comparative Example 1 without post-treatment. This indicates that the polyethyleneimine with different weight-average molecular weights can improve the salt rejection performance retention rate of the polyester membrane in tap water environment.

[0074] Table 4. NaCl rejection rates of membrane products prepared in Example 1 and Comparative Examples 1-4

[0075] Table 4 shows that the initial NaCl rejection rate of Example 1 and the NaCl rejection rate after soaking in tap water for 48 hours were both 96.2%, indicating that the initial separation performance of the membrane was not adversely affected after short-term alkaline activation oxidation and polyethyleneimine grafting treatment, and it could remain stable in the tap water environment. Comparative Example 1, without any post-treatment, saw its NaCl rejection rate decrease from the initial 96.7% to 31%, indicating that although the polyester membrane formed by tannic acid and trimesoyl chloride has high initial salt rejection performance, the residual phenolic hydroxyl groups and related polyester network in the membrane are not stable enough in tap water.

[0076] Comparative Example 2, which underwent only alkali activation oxidation treatment without polyethyleneimine grafting, had an initial NaCl rejection rate of 93%, which decreased to 2% after soaking in tap water for 48 hours, lower than that of the untreated Comparative Example 1. This result indicates that alkali activation oxidation can convert residual phenolic hydroxyl groups into highly reactive phenoxy anions and quinone active sites. However, if the formed active sites are not promptly blocked by subsequent amino reactions, alkali treatment may simultaneously cause partial polyester bond hydrolysis, and the formed quinone active sites may continue to react in the aqueous environment. Therefore, alkali treatment alone cannot improve the long-term stability of the membrane.

[0077] Comparative Example 3 was not subjected to alkali activation oxidation treatment but was directly treated with polyethyleneimine. Its initial NaCl rejection rate was 96%, which decreased to 74% after soaking in tap water for 48 hours. Compared to Comparative Example 1, the NaCl rejection rate of Comparative Example 3 increased by 43%, indicating that polyethyleneimine can exert a certain stabilizing effect on the polyester film through physical adsorption, hydrogen bonding, and electrostatic interactions. However, due to the low reactivity of the residual phenolic hydroxyl groups that were not subjected to alkali activation oxidation, the number of quinone active sites capable of covalently reacting with the amino groups of polyethyleneimine is limited, resulting in insufficient binding stability between polyethyleneimine and the polyester film. Therefore, its water stability is still lower than that of Example 1.

[0078] Comparative Example 4 was treated with a 0.1 wt% sodium hydroxide aqueous solution for 10 min before being treated with polyethyleneimine. Its initial NaCl retention rate was 87%, lower than that of Example 1 and other comparative examples. After soaking in tap water for 48 h, the NaCl retention rate was 55%. These results indicate that excessive alkali treatment may lead to significant hydrolysis of ester bonds in the polyester separation layer, affecting the film's density and structural integrity before polyethyleneimine grafting. While subsequent polyethyleneimine treatment can block some active sites, it cannot completely compensate for the aforementioned structural damage. Therefore, this application does not achieve stabilization by increasing the intensity of alkali treatment, but rather by limiting alkaline conditions and a shorter treatment time to moderately activate and oxidize residual phenolic hydroxyl groups, which are then directly transferred to a polyethyleneimine aqueous solution for grafting after removal.

[0079] In summary, the treatment method combining short-term alkaline activation oxidation and polyethyleneimine grafting in this application can maintain the continuity of the membrane and its initial salt rejection performance, while also improving the retention rate of the membrane's salt rejection performance in tap water. This demonstrates the synergistic effect of each treatment step in terms of active site formation, grafting sealing, and polyester structure protection.

[0080] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0081] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A post-treatment modification method for polyester film, characterized in that, Includes the following steps: A polyester film to be modified is provided, wherein the polyester film to be modified is prepared by interfacial polymerization of polyphenolic compounds and polyacryl chlorides, and the polyester film to be modified contains residual phenolic hydroxyl groups; The polyester film to be modified is immersed in an alkaline aqueous solution containing dissolved oxygen at a pH of 8-9 for a first preset time, so that at least a portion of the residual phenolic hydroxyl groups are converted into phenolic anions under alkaline conditions, and at least a portion of the phenolic anions are oxidized under the action of dissolved oxygen to form quinone active sites. The polyester film to be modified, which has been treated with the alkaline aqueous solution, is taken out of the alkaline aqueous solution, the alkaline aqueous solution adhering to the surface of the polyester film to be modified is removed, and the polyester film to be modified is directly transferred to an amino polymer solution for immersion treatment for a second preset time, so that the amino groups in the amino polymer react with the quinone active sites to graft the amino polymer onto the polyester film to be modified. The modified polyester film, after undergoing the amino grafting step, is washed with water to obtain a modified composite film. Wherein, the amino polymer is polyethyleneimine, the amino polymer solution is an aqueous solution of polyethyleneimine, the mass fraction of polyethyleneimine in the aqueous solution of polyethyleneimine is 0.1%-2%, the pH of the aqueous solution of polyethyleneimine is any value between 8 and 9, the solute of the alkaline aqueous solution is one or more of sodium hydroxide, potassium hydroxide and ammonia, and the first preset time is any value between 0.5 min and 2 min.

2. The post-treatment modification method for polyester film according to claim 1, characterized in that, The second preset time is any value between 1 min and 5 min.

3. The post-treatment modification method for polyester film according to claim 2, characterized in that, The reaction temperature between the amino polymer solution and the polyester film to be modified is any value between 20℃ and 40℃.

4. The post-treatment modification method for polyester film according to claim 1, characterized in that, The weight-average molecular weight of the polyethyleneimine is 600-70000.

5. The post-treatment modification method for polyester film according to claim 1, characterized in that, The method for preparing the polyester film includes the following steps: The polysulfone ultrafiltration membrane is reacted with an aqueous solution containing the aforementioned polyphenolic compound; After removing the aqueous solution from the surface of the polysulfone ultrafiltration membrane, the polysulfone ultrafiltration membrane is reacted with an organic phase solution containing the polyacryl chloride, so that the polyphenolic compound and the polyacryl chloride undergo an interfacial polymerization reaction on the polysulfone ultrafiltration membrane. The polysulfone ultrafiltration membrane that has undergone the interfacial polymerization reaction is subjected to heat treatment to obtain the polyester membrane.

6. The post-treatment modification method for polyester film according to claim 5, characterized in that, The mass fraction of the polyphenolic compound in the aqueous solution is any value between 0.5wt% and 2.5wt%.

7. The post-treatment modification method for polyester film according to claim 5, characterized in that, The mass fraction of the polyacrylamide chloride in the organic phase solution is any value between 0.05wt% and 0.5wt%.

8. A modified composite membrane, characterized in that, The modified composite film is prepared by the post-processing modification method of the polyester film according to any one of claims 1-7, and the modified composite film contains polyethyleneimine segments grafted onto the polyester film.

9. The application of the modified composite membrane according to claim 8 in drinking water treatment.

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