A polyolefin-based reverse osmosis membrane and a method for preparing the same

Abstract

The application discloses a polyolefin-based reverse osmosis membrane and a preparation method thereof. The reverse osmosis membrane comprises a polyolefin substrate, a main separation layer and an ultrathin functional dense layer formed on the polyolefin substrate in sequence. The main separation layer is formed by reverse interfacial polymerization, and the ultrathin functional dense layer is formed in situ on the main separation layer by secondary interfacial polymerization. The preparation comprises the following steps: S1, reverse interfacial polymerization: the polyolefin substrate is immersed in a first organic phase solution, and then drained after being taken out, and then contacted with a first water phase solution to perform an interfacial polymerization reaction, and then solidified through first heat treatment to obtain a nascent composite membrane with the main separation layer; S2, secondary interfacial polymerization: the nascent composite membrane is contacted with a second water phase solution, and then contacted with a second organic phase solution, and then solidified through second heat treatment to form the ultrathin functional dense layer on the main separation layer; and S3, post-treatment: the membrane after the secondary interfacial polymerization is washed with water and dried to obtain the polyolefin-based reverse osmosis membrane.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to a polyolefin-based reverse osmosis membrane and its preparation method. Background Technology

[0002] Reverse osmosis (RO) membranes have been widely used in seawater desalination, brackish water desalination, and industrial wastewater reuse. The current mainstream reverse osmosis membrane product is the polyamide thin-layer composite membrane (PA-TFC), which consists of a non-woven fabric reinforcing layer, a polysulfone (PSF) porous support layer, and a polyamide separation layer formed by interfacial polymerization (IP) of m-phenylenediamine (MPD) and trimesoyl chloride (TMC).

[0003] In recent years, research on replacing traditional polysulfone-based membranes with polyolefin microporous membranes (PE, PP, especially commercial lithium-ion battery separators) has received widespread attention. PE / PP separators have advantages such as low cost, good chemical stability, and high mechanical strength. Patent CN118356814A (Beijing University of Technology) further proposes a support structure that combines a nonwoven fabric bottom layer with a polyolefin microporous membrane through hot pressing, balancing mechanical strength and the uniform porous surface required for interfacial polymerization. Regarding the interfacial polymerization process, a reverse interfacial polymerization method that prioritizes organic phase contact instead of the traditional aqueous phase can form a polyamide layer with a special gradient structure on the polyolefin-based membrane. However, due to the inherent hydrophobicity of polyolefin materials, reverse interfacial polymerization faces the following prominent problems in practical operation: ① Uneven spreading of the organic phase on the hydrophobic polyolefin surface leads to local monomer concentration differences; ② Rapid evaporation of organic solvents from the porous polyolefin-based membrane causes instability at the reaction interface; ③ The above factors result in nanoscale pinhole defects in the formed polyamide layer, which become channels for salt ion leakage, severely restricting the desalination rate.

[0004] To repair defects in polyamide layers, various strategies have been proposed in the prior art. For example, Chinese patent CN119857374A uses the method of grafting functional monomers after oxidation activation to repair deteriorated films; patent CN111359455A uses cyclodextrin and residual acyl chloride to undergo secondary interfacial polymerization for surface modification; patent CN119015909A modifies the film surface through secondary interfacial polymerization of fluorine-containing molecules; and patent CN114682102A repairs film surface defects by grafting amine compounds after carboxyl activation. However, these methods are all aimed at polyamide films formed on polysulfone substrates by traditional forward interfacial polymerization and are mainly for surface functionalization (anti-fouling, chlorine resistance), rather than targeting the nanoscale pinhole defects formed on hydrophobic polyolefin substrates by reverse interfacial polymerization.

[0005] In addition, patent CN110124517A discloses a method for preparing nanofiber-based composite nanofiltration membranes by low-temperature reverse interfacial polymerization, but this method only involves a single reverse interfacial polymerization and does not involve a secondary repair step. Patent CN121669013A discloses a system for preparing polyolefin nanofiltration membranes by reverse interfacial polymerization, which also does not involve secondary interfacial polymerization. Patent CN112473391A discloses a repair method that uses dye molecules to physically block defects, but this method is a physical filling rather than a chemical bonding repair, and its long-term operational stability is questionable.

[0006] In summary, the existing technologies have the following shortcomings: ① The polyamide layer formed by reverse interfacial polymerization on polyolefin-based membranes has nanoscale pinhole defects, which seriously affect the desalination rate, and there is currently no effective targeted repair solution; ② Existing secondary interfacial polymerization technologies are all used for the surface functionalization modification of conventional interfacial polymerization (IP) membranes on traditional polysulfone-based membranes, and have not yet been applied to repair the defects formed by reverse IP on polyolefin-based membranes; ③ There are no reports on synergistic combination schemes of reverse interfacial polymerization and secondary interfacial polymerization on the same polyolefin substrate, and the synergistic effect of the two in terms of chemical mechanism and structural complementarity has not been recognized. Summary of the Invention

[0007] The purpose of this invention is to provide a polyolefin-based reverse osmosis membrane and its preparation method, aiming to overcome the shortcomings of the existing technology.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution: The present invention provides a polyolefin-based reverse osmosis membrane, which includes a polyolefin substrate and a main separation layer and an ultrathin functional dense layer sequentially formed on the polyolefin substrate; wherein the main separation layer is formed by reverse interfacial polymerization, and the ultrathin functional dense layer is formed in situ on the main separation layer by secondary interfacial polymerization.

[0009] Specifically, the present invention provides a polyolefin-based reverse osmosis membrane that repairs defects based on the synergistic effect of reverse interfacial polymerization and secondary interfacial polymerization.

[0010] Furthermore, a polyolefin-based reverse osmosis membrane: the polyolefin substrate is a hydrophobic polyolefin microporous membrane or a polyolefin composite membrane; wherein, the hydrophobic polyolefin microporous membrane is selected from any one of polyethylene (PE) microporous membrane, polypropylene (PP) microporous membrane, poly(4-methyl-1-pentene) (PMP) microporous membrane, and cyclic olefin copolymer microporous membrane, and the polyolefin composite membrane is formed by hot-pressing a non-woven fabric backing with any hydrophobic polyolefin microporous membrane.

[0011] Specifically, the polyolefin-based reverse osmosis membrane provided by this invention is a polyolefin-based reverse osmosis composite membrane that uses a polyolefin microporous membrane or a polyolefin composite membrane as a substrate and employs reverse interfacial polymerization and secondary interfacial polymerization to synergistically repair nanoscale pinhole defects.

[0012] Furthermore, a polyolefin-based reverse osmosis membrane: the thickness of the ultrathin functional dense layer is less than 30.0 nm. Preferably, the thickness of the ultrathin functional dense layer is less than 15.0 nm.

[0013] This invention also provides a method for preparing a polyolefin-based reverse osmosis membrane, the method comprising the following steps: S1. Reverse interfacial polymerization—construction of the main separation layer: The polyolefin substrate is immersed in the first organic phase solution, taken out and drained, and then contacted with the first aqueous phase solution to carry out interfacial polymerization reaction. After the first heat treatment, it is cured to obtain a nascent composite film with a main separation layer. S2, Secondary interface polymerization—targeted repair of defects and construction of functional dense layer: The nascent composite membrane is contacted with the second aqueous phase solution, and then with the second organic phase solution. After a second heat treatment and curing, an ultra-thin functional dense layer is formed on the main separation layer. S3. Post-treatment: The membrane after secondary interfacial polymerization is washed with water and dried to obtain a polyolefin-based reverse osmosis membrane. The first organic phase solution and the second organic phase solution each contain polycyclic aromatic acyl chlorides, and the concentration of the first organic phase solution is higher than that of the second organic phase solution. The first aqueous phase solution and the second aqueous phase solution each contain polyamine monomers, and the concentration of the first aqueous phase solution is higher than that of the second aqueous phase solution.

[0014] Specifically, the contact methods between the second aqueous solution and the nascent composite membrane are single-sided coating, membrane-side-down floating contact, spraying, dip coating, or slot coating, so that only the main separation layer (polyamide layer) is in contact with the second aqueous solution.

[0015] Furthermore, a method for preparing a polyolefin-based reverse osmosis membrane: Step S1, reverse interfacial polymerization: Immerse the polyolefin substrate in the first organic phase solution for 20-180 seconds, remove and drain for 10-120 seconds, then contact it with the first aqueous phase solution at 15-40°C for interfacial polymerization reaction for 15-180 seconds, and then cure it under a first heat treatment at 50-100°C for 1-15 minutes to obtain a nascent composite membrane with a main separation layer.

[0016] Furthermore, a method for preparing a polyolefin-based reverse osmosis membrane includes: Step S2, secondary interfacial polymerization: the nascent composite membrane is contacted with a second aqueous solution at 10–40°C for 10–120 seconds, and then contacted with a second organic solution at 10–40°C for 10–120 seconds, followed by a second heat treatment at 40–90°C for 1–8 minutes to solidify, thereby forming an ultrathin functional dense layer on the main separation layer.

[0017] Furthermore, a method for preparing a polyolefin-based reverse osmosis membrane includes the following post-treatment process in step S3: acid washing; the membrane after secondary interfacial polymerization is washed with water, acid washed, and dried to obtain a polyolefin-based reverse osmosis membrane; wherein, the acid washing is performed by immersing the membrane in an aqueous solution containing polyhydroxycarboxylic acid at a pH of 2-6 at a temperature of 15-50°C for 1-60 minutes, and the drying temperature is 40-80°C for 1-20 minutes.

[0018] Furthermore, a method for preparing a polyolefin-based reverse osmosis membrane: the polyhydroxycarboxylic acid is selected from one or more of citric acid, tartaric acid, malic acid, gluconic acid, succinic acid, malonic acid, maleic acid, and phytic acid, and its concentration in aqueous solution is 0.2 to 5.0 wt%.

[0019] Furthermore, a method for preparing a polyolefin-based reverse osmosis membrane: the first organic phase solution and the second organic phase solution are respectively obtained by dissolving polypropylene aromatic acyl chlorides in an organic solvent, wherein the polypropylene aromatic acyl chlorides are selected from one or more of trimesoyl chloride (TMC), isophthaloyl chloride (IPC), and terephthaloyl chloride (TPC), and the organic solvents are selected from one or more of n-hexane, n-heptane, Isopar G, Isopar L, and cyclohexane, wherein the concentration of the polypropylene aromatic acyl chlorides in the first organic phase solution is 0.05–1.0 wt%, and the concentration of the polypropylene aromatic acyl chlorides in the second organic phase solution is 0.01–0 wt%. 0.3 wt%; the first aqueous phase solution includes 1.0 to 5.0 wt% of polyamine monomers, and the second aqueous phase solution includes 0.05 to 2.0 wt% of polyamine monomers, wherein the polyamine monomers are selected from one or more of m-phenylenediamine (MPD), piperazine (PIP), m-aminobenzenesulfonic acid, 3,5-diaminobenzoic acid, polyethyleneimine (molecular weight 600 to 10000 Da), bis(3-aminopropyl)polydimethylsiloxane, ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), p-phenylenediamine (PPD), chitosan oligomers, and dopamine.

[0020] Furthermore, a method for preparing a polyolefin-based reverse osmosis membrane includes: the first aqueous phase solution further comprising an acid absorbent at a concentration of 0.05–1.0 wt% and / or a surfactant at a concentration of 0.005–0.2 wt%; the first aqueous phase solution may or may not include an alcohol-based auxiliary agent, if included, at a concentration of 0.1–10 wt%. The acid absorbent is selected from at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, trisodium phosphate, disodium hydrogen phosphate, triethylamine, N,N-dimethylethanolamine, and triethanolamine; the surfactant is selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, lauryl polyoxyethylene ether, Tween-20, Tween-80, Triton X-100, and dodecyl dimethyl betaine; the alcohol auxiliaries are selected from at least one of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, ethylene glycol, propylene glycol, glycerol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, diethylene glycol, and triethylene glycol.

[0021] Preferably, the acid absorbent is sodium carbonate or trisodium phosphate, and its concentration is preferably 0.1–0.5 wt%. Preferably, the surfactant is sodium dodecyl sulfate or lauryl alcohol polyoxyethylene ether, and its concentration is preferably 0.01–0.1 wt%. Preferably, the alcohol auxiliary is ethanol, isopropanol, ethylene glycol, or polyethylene glycol 400; when ethanol or isopropanol is used as the alcohol auxiliary, its concentration is 0.5–5.0 wt%; when ethylene glycol or polyethylene glycol 400 is used as the alcohol auxiliary, its concentration is 1.0–10.0 wt%.

[0022] Specifically, when the first aqueous solution simultaneously contains an acid absorbent, a surfactant, and an alcohol auxiliary agent, the concentrations are 0.1–0.5 wt% for the acid absorbent, 0.01–0.05 wt% for the surfactant, and 1.0–5.0 wt% for the alcohol auxiliary agent.

[0023] The beneficial effects of this invention are: This invention relates to a polyolefin-based reverse osmosis membrane, which has the following technical advantages: ① Significantly improved desalination rate: By targeting and repairing the nanoscale pinhole defects formed by the reverse interface polymer membrane, the NaCl desalination rate is increased from 96.5-98.5% before repair to ≥99.0%; ② High flux retention: The secondary interface polymer repair layer (ultra-thin functional dense layer) is thin, with extremely low mass transfer resistance, and the water flux decreases only slightly compared to before repair, resulting in a high water flux retention rate; ③ Breakthrough of traditional trade-offs: While basically maintaining the high flux advantage of the reverse interface polymer membrane, it achieves an ultra-high desalination rate; ④ Good process compatibility: Secondary interfacial polymerization can be continuously completed on the same production line as reverse interfacial polymerization, ensuring process compatibility; ⑤ Wide substrate adaptability: Applicable to various polyolefin substrates such as PE microporous membranes, PP microporous membranes, and nonwoven / polyolefin composite membranes; ⑥ Customizable functionality: By selecting different polyamine monomers, additional functions (such as chlorine resistance, antifouling, and hydrophilicity) can be imparted to the membrane surface while repairing defects; Good long-term storage stability: After stabilization treatment with polyhydroxycarboxylic acids, the desalination rate of the membrane decreases significantly less during long-term storage than that of the untreated control membrane, thus extending the membrane's shelf life. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Example 1

[0026] This embodiment 1 provides a polyolefin-based reverse osmosis membrane, which includes a polyethylene (PE) microporous membrane (polyolefin substrate) with a thickness of 9.0 μm, a porosity of 45%, and an average pore size of 0.06 μm, and a main separation layer and an ultrathin functional dense layer sequentially formed on the polyethylene microporous membrane; the main separation layer is formed by reverse interfacial polymerization, and the ultrathin functional dense layer is formed in situ on the main separation layer by secondary interfacial polymerization; The preparation method of this polyolefin-based reverse osmosis membrane includes the following specific steps: S1. Reverse interfacial polymerization—construction of the main separation layer (polyamide layer): Trimethylbenzene chloride (TMC) is dissolved in n-hexane to prepare a first organic phase solution with a concentration of 0.15 wt%. m-phenylenediamine (MPD) is dissolved in water to prepare a first aqueous phase solution with a concentration of 2.0 wt%. A polyethylene microporous membrane is immersed in the first organic phase solution for 60 seconds, drained for 30 seconds, and then contacted in the first aqueous phase solution at 25°C for interfacial polymerization reaction for 60 seconds. After that, it is cured by a first heat treatment at 80°C for 5 minutes to obtain a nascent composite membrane with a main separation layer. S2, Secondary Interface Polymerization—Targeted Repair of Defects and Construction of Functional Dense Layer: Trimethylbenzene chloride (TMC) is dissolved in n-hexane to prepare a second organic phase solution with a concentration of 0.05 wt%, and piperazine (PIP) is dissolved in water to prepare a second aqueous phase solution with a concentration of 0.5 wt%. The main separation layer of the nascent composite membrane is contacted with the second aqueous phase solution at a temperature of 20°C for 30 seconds. After removal, excess aqueous phase liquid is removed by gently pressing with a rubber roller. Then, the main separation layer is contacted with the second organic phase solution at a temperature of 20°C for another 30 seconds. Then, it is cured by a second heat treatment at 60°C for 3 minutes to form an ultrathin functional dense layer on the main separation layer. S3. Post-treatment: The membrane after secondary interfacial polymerization is first rinsed with deionized water at 25°C for 10 minutes, then soaked in citric acid solution (pH 4.0) at 30°C with a concentration of 2.0wt% for 15 minutes, and finally dried at 60°C for 10 minutes to obtain a polyolefin-based reverse osmosis membrane based on the synergistic repair of defects by reverse interfacial polymerization and secondary interfacial polymerization.

[0027] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the reverse osmosis membrane in Example 1 was 58.2L / (m²·h), and the NaCl desalination rate was 99.38%.

[0028] Example 2

[0029] In Example 2, the polyolefin substrate was a polypropylene microporous membrane with a thickness of 16.0 μm, a porosity of 42%, and an average pore size of 0.08 μm. In Example 2, the second aqueous phase solution was a 0.8 wt% aqueous solution of m-aminobenzenesulfonic acid (pH adjusted to 7.0 with NaOH). The contact time between the main separation layer of the nascent composite membrane and the second aqueous phase solution was 40 seconds and the temperature was 25°C. The second organic phase solution in Example 2 was obtained by dissolving TMC in IsoparG (concentration of 0.08 wt%), with a contact time of 30 seconds and a temperature of 25°C. In Example 2, the second heat treatment curing temperature was 70°C and the time was 5 minutes; the remaining conditions in Example 2 were the same as in Example 1.

[0030] Performance testing (under test conditions of 2000ppm NaCl, 1.55MPa, and 25℃): The water flux of the reverse osmosis membrane in Example 2 was 52.7L / (m²·h), and the NaCl desalination rate was 99.42%. In Example 2, the intermediate aminobenzenesulfonic acid introduced sulfonic acid groups while repairing defects, which enhanced the hydrophilicity and negative charge density of the membrane surface.

[0031] Example 3

[0032] In Example 3, the second aqueous solution was a 0.3 wt% aqueous solution of polyethyleneimine (PEI, molecular weight 1800 Da). The contact time between the main separation layer of the nascent composite membrane and the second aqueous solution was 25 seconds and the temperature was 20°C. The second organic phase solution in Example 3 was obtained by dissolving TMC in n-hexane (concentration of 0.03 wt%), with a contact time of 25 seconds and a temperature of 20°C. In Example 3, the second heat treatment curing temperature was 65°C and the time was 2 minutes; the remaining conditions in Example 3 were the same as in Example 1.

[0033] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the reverse osmosis membrane in Example 3 was 55.1L / (m²·h), and the NaCl desalination rate was 99.35%.

[0034] Example 4

[0035] In Example 4, the polyolefin substrate was a nonwoven fabric / PE microporous membrane composite base membrane (PE microporous membrane thickness 20.0 μm, pore size 0.06 μm, porosity 40%, nonwoven fabric basis weight 40 g / m², thickness 60.0 μm); the remaining conditions in Example 4 were the same as in Example 1.

[0036] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the reverse osmosis membrane in Example 4 was 53.8L / (m²·h), and the NaCl desalination rate was 99.31%.

[0037] Example 5

[0038] In Example 5, the polyolefin substrate used was a nonwoven fabric / PP microporous membrane composite membrane (PP microporous membrane thickness 16.0 μm, pore size 0.04 μm, porosity 44%, nonwoven fabric basis weight 50 g / m², thickness 70.0 μm); the second aqueous phase solution in Example 5 was a 0.6 wt% aqueous solution of 3,5-diaminobenzoic acid (pH adjusted to 8.0 with NaOH), and the contact time between the main separation layer of the nascent composite membrane and the second aqueous phase solution was 30 seconds at a temperature of 25°C. The second organic phase solution in Example 5 was obtained by dissolving TMC in n-hexane (concentration of 0.06 wt%), with a contact time of 30 seconds and a temperature of 25°C. In Example 5, the second heat treatment curing temperature was 65°C and the time was 3 minutes; the remaining conditions in Example 5 were the same as in Example 1.

[0039] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the reverse osmosis membrane in Example 5 was 49.2L / (m²·h), and the NaCl desalination rate was 99.45%.

[0040] Example 6

[0041] In Example 6, the second aqueous solution was 0.5 wt% bis(3-aminopropyl)polydimethylsiloxane dissolved in a 50 vol% ethanol / water mixed solvent. The contact time between the main separation layer of the nascent composite membrane and the second aqueous solution was 20 seconds and the temperature was 25°C. The second organic phase solution in Example 6 was obtained by dissolving TMC in n-hexane (concentration of 0.04 wt%), with a contact time of 20 seconds and a temperature of 25°C. In Example 6, the second heat treatment curing temperature was 60°C and the time was 2 minutes; the remaining conditions in Example 3 were the same as in Example 1.

[0042] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the reverse osmosis membrane in Example 6 was 56.3L / (m²·h), and the NaCl desalination rate was 99.28%.

[0043] Comparative Example 1

[0044] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not perform the secondary interface aggregation in step S2, while the other conditions are the same as in Example 1.

[0045] Performance testing (under test conditions of 2000ppm NaCl, 1.55MPa, and 25℃): The water flux of the membrane in Comparative Example 1 was 67.8 L / (m²·h), and the NaCl desalination rate was 97.5%. The comparison between Comparative Example 1 and Example 1 shows that the desalination rate of Example 1 improved by 1.88% after secondary interfacial polymerization repair, while the flux decreased by only 14.2%.

[0046] Comparative Example 2

[0047] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 uses a PE microporous membrane as a substrate and adopts the traditional forward interfacial polymerization method of first impregnating the aqueous phase and then contacting the organic phase (oil phase). Then, it undergoes the same secondary interfacial polymerization repair as Example 1. The only difference between Comparative Example 2 and Example 1 is that Example 1 uses the reverse interfacial polymerization method of first impregnating the substrate with the organic phase and then contacting the aqueous phase, while Comparative Example 2 uses the traditional forward interfacial polymerization method of first impregnating the aqueous phase and then contacting the organic phase. All other conditions are the same as in Example 1.

[0048] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the membrane in Comparative Example 2 was 41.5L / (m²·h), and the NaCl desalination rate was 98.8%.

[0049] Comparative Example 3

[0050] The difference between Comparative Example 3 and Example 1 is that in step S2 of Comparative Example 3, the main separation layer is contacted with a 0.5wt% PIP aqueous solution (second aqueous phase solution) for 30 seconds and then directly rinsed and dried with deionized water without contacting the second organic phase solution. The other conditions are the same as in Example 1.

[0051] Performance testing (under test conditions of 2000ppm NaCl, 1.55MPa, and 25℃): The water flux of the membrane in Comparative Example 3 was 66.2L / (m²·h), and the NaCl desalination rate was 97.8%. The results indicate that immersion in a simple amine solution cannot form a repair layer.

[0052] Comparative Example 4

[0053] The difference between Comparative Example 4 and Example 1 is that the concentration of the second organic phase solution in Comparative Example 4 was increased to 0.5 wt%, while the other conditions were the same.

[0054] Performance test (under test conditions of 2000ppm NaCl, 1.55MPa, 25℃): The water flux of the membrane in Comparative Example 4 was 35.8L / (m²·h), and the NaCl desalination rate was 99.48%. It can be seen that the desalination rate of Comparative Example 4 is relatively high, but the flux decrease is much larger (45%). This indicates that if the organic phase concentration is too high during secondary interfacial polymerization, an excessively thick functional dense layer will be formed, which will lose the high flux advantage of reverse interfacial polymerization.

[0055] Comparative Example 5

[0056] The difference between Comparative Example 5 and Example 4 is that Comparative Example 5 does not perform the secondary interface aggregation in step S2, while the other conditions are the same as in Example 4.

[0057] Performance testing (under test conditions of 2000ppm NaCl, 1.55MPa, and 25℃): The water flux of the membrane in Comparative Example 5 was 63.5 L / (m²·h), and the NaCl desalination rate was 97.8%.

[0058] The performance test results of the membranes in Examples 1-6 and Comparative Examples 1-5 are shown in the table below:

[0059] Specifically, the technical principle of this invention is based on the following synergistic mechanisms: ① Structural complementarity: The main separation layer (polyamide layer) formed by reverse interfacial polymerization provides a high-flux water transport channel, but has nanoscale pinhole defects; the ultrathin dense layer constructed by secondary interfacial polymerization precisely covers and fills these defects, achieving a high desalination rate without significantly increasing the total mass transfer resistance; ② Defect coverage and sealing: The low-concentration polyamine and low-concentration polyacrylamide chloride of secondary interfacial polymerization undergo interfacial polymerization on the entire surface of the nascent composite membrane, forming an ultrathin dense layer that fully covers the membrane surface. At the same time, the polyamine monomer can penetrate into the pinhole defect area of ​​the main separation layer, and the polymerized polyamide fills the defect pores, achieving defect sealing and improved desalination rate; ③ Interfacial reinforcement: The polyamide network formed by secondary interfacial polymerization is tightly bonded to the main separation layer through hydrogen bonds, ionic crosslinking (between amine groups and carboxyl groups on the surface of the main separation layer), and physical interpenetration; the interpenetrating network structure of the secondary interfacial polymerization layer in the pores on the surface of the main separation layer enhances the interlayer bonding force, making the repair layer less prone to peeling off during use.

[0060] The above-described preferred embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of the invention. Any obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A polyolefin-based reverse osmosis membrane, characterized by, The reverse osmosis membrane includes a polyolefin substrate and a main separation layer and an ultrathin functional dense layer sequentially formed on the polyolefin substrate; wherein the main separation layer is formed by reverse interfacial polymerization, and the ultrathin functional dense layer is formed in situ on the main separation layer by secondary interfacial polymerization.

2. The polyolefin-based reverse osmosis membrane according to claim 1, characterized in that, The polyolefin substrate is a hydrophobic polyolefin microporous membrane or a polyolefin composite membrane. The hydrophobic polyolefin microporous membrane is selected from any one of polyethylene microporous membrane, polypropylene microporous membrane, poly4-methyl-1-pentene microporous membrane, and cyclic olefin copolymer microporous membrane. The polyolefin composite base membrane is formed by hot pressing a non-woven fabric bottom layer with any hydrophobic polyolefin microporous membrane.

3. The polyolefin-based reverse osmosis membrane according to claim 1, characterized in that, The thickness of the ultrathin functional dense layer is less than 30.0 nm.

4. A method for preparing a polyolefin-based reverse osmosis membrane according to any one of claims 1 to 3, characterized in that, The method includes the following steps: S1. Reverse interfacial polymerization: The polyolefin substrate is immersed in the first organic phase solution, taken out and drained, and then contacted with the first aqueous phase solution to carry out interfacial polymerization reaction. After the first heat treatment, it is cured to obtain a nascent composite membrane with a main separation layer. S2, Secondary interfacial polymerization: The nascent composite membrane is brought into contact with a second aqueous solution, and then into a second organic solution. After a second heat treatment, it is cured to form an ultrathin functional dense layer on the main separation layer. S3. Post-treatment: The membrane after secondary interfacial polymerization is washed with water and dried to obtain a polyolefin-based reverse osmosis membrane. The first organic phase solution and the second organic phase solution each contain polycyclic aromatic acyl chlorides, and the concentration of the first organic phase solution is higher than that of the second organic phase solution. The first aqueous phase solution and the second aqueous phase solution each contain polyamine monomers, and the concentration of the first aqueous phase solution is higher than that of the second aqueous phase solution.

5. The method for preparing a polyolefin-based reverse osmosis membrane according to claim 4, characterized in that, Step S1, Reverse interfacial polymerization: Immerse the polyolefin substrate in the first organic phase solution for 20-180 seconds, remove and drain for 10-120 seconds, then contact it with the first aqueous phase solution at 15-40℃ for interfacial polymerization reaction for 15-180 seconds, and then cure it under a first heat treatment at 50-100℃ for 1-15 minutes to obtain a nascent composite membrane with a main separation layer.

6. The method for preparing a polyolefin-based reverse osmosis membrane according to claim 4, characterized in that, Step S2, Secondary Interface Polymerization: The nascent composite membrane is contacted with the second aqueous solution at 10-40°C for 10-120 seconds, and then contacted with the second organic solution at 10-40°C for 10-120 seconds. Then, it is cured by a second heat treatment at 40-90°C for 1-8 minutes to form an ultrathin functional dense layer on the main separation layer.

7. The method for preparing a polyolefin-based reverse osmosis membrane according to claim 4, characterized in that, The post-processing in step S3 also includes pickling; The membrane after secondary interfacial polymerization is washed with water, acid-washed, and dried to obtain a polyolefin-based reverse osmosis membrane. The acid washing is performed by immersing the membrane in an aqueous solution containing polyhydroxycarboxylic acid at a pH of 2-6 for 1-60 minutes at a temperature of 15-50°C. The drying temperature is 40-80°C and the drying time is 1-20 minutes.

8. The method for preparing a polyolefin-based reverse osmosis membrane according to claim 7, characterized in that, The polyhydroxycarboxylic acid is selected from one or more of citric acid, tartaric acid, malic acid, gluconic acid, succinic acid, malonic acid, maleic acid, and phytic acid, and its concentration in aqueous solution is 0.2 to 5.0 wt%.

9. The method for preparing a polyolefin-based reverse osmosis membrane according to claim 4, characterized in that, The first organic phase solution and the second organic phase solution are obtained by dissolving polycyclic aromatic acyl chlorides in organic solvents. The polycyclic aromatic acyl chlorides are selected from one or more of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride. The organic solvents are selected from one or more of n-hexane, n-heptane, Isopar G, Isopar L, and cyclohexane. The concentration of the polycyclic aromatic acyl chloride in the first organic phase solution is 0.05–1.0 wt%, and the concentration of the polycyclic aromatic acyl chloride in the second organic phase solution is 0.01–0.3 wt%. The first aqueous solution contains 1.0 to 5.0 wt% of a polyamine monomer, and the second aqueous solution contains 0.05 to 2.0 wt% of a polyamine monomer. The polyamine monomer is selected from one or more of m-phenylenediamine, piperazine, m-aminobenzenesulfonic acid, 3,5-diaminobenzoic acid, polyethyleneimine, bis(3-aminopropyl)polydimethylsiloxane, ethylenediamine, diethylenetriamine, triethylenetetramine, p-phenylenediamine, chitosan oligomers, and dopamine.

10. The method for preparing a polyolefin-based reverse osmosis membrane according to claim 9, characterized in that, The first aqueous solution further includes an acid absorbent at a concentration of 0.05–1.0 wt% and / or a surfactant at a concentration of 0.005–0.2 wt%. The first aqueous solution may or may not include alcohol auxiliaries; if included, the concentration is 0.1–10 wt%. The acid absorbent is selected from at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, trisodium phosphate, disodium hydrogen phosphate, triethylamine, N,N-dimethylethanolamine, and triethanolamine. The surfactant is selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, lauryl alcohol polyoxyethylene ether, Tween-20, Tween-80, Triton X-100, and dodecyl dimethyl betaine; The alcohol additive is selected from at least one of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, ethylene glycol, propylene glycol, glycerol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, diethylene glycol, and triethylene glycol.

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