A novel double-layer composite base film, its preparation method and application

Abstract

The present invention provides a double-layer composite base film, a preparation method thereof and an application. The double-layer composite base film is composed of a support body, a first modified layer located on the surface of the support body, and a second modified layer located on the surface of the first modified layer. The material of the first modified layer is a first polymer selected from one or more of polysulfone, polyethersulfone, polyacrylonitrile, polyimide and polyvinylidene fluoride. The material of the second modified layer is a second polymer selected from one or more of PSf-b-PEO block copolymer and PSf-b-PEG block copolymer. In the present invention, two casting solutions are simultaneously coated on the surface of the support layer, and phase inversion is carried out on the two solutions simultaneously. The present invention can regulate the pore size and pore size distribution on the surface of the base film, improve the surface porosity, form relatively regular pores on the surface, and there is no obvious dense separation skin layer, and the cost is low.

Description

Technical Field

[0001] The present invention belongs to the field of polymer separation and relates to a novel double-layer composite substrate membrane, a preparation method thereof, and an application thereof. Background Art

[0002] In order to prepare highly efficient reverse osmosis (RO) / nanofiltration (NF) membranes, it is necessary to use polymer ultrafiltration membranes as support substrates. The pore structure of the ultrafiltration membrane has a great influence on the subsequent interfacial polymerization composite membrane. If the pore size of the ultrafiltration membrane is too large, it is difficult to form a defect-free selective layer through interfacial polymerization on this basis, thus affecting the selectivity. The L-S phase inversion method is a relatively simple membrane preparation method, with simple process, convenient operation and wide application.

[0003] Existing polysulfone substrate membranes are directly prepared by phase inversion. However, the polysulfone substrate membranes prepared by this method have defects such as wide pore size distribution and low surface porosity, which will affect the stability and separation performance of the composite membrane formed by subsequent interfacial polymerization.

[0004] There are also literatures that use PSf-b-PEG block polymers to directly prepare membranes by phase inversion. For example, Liu Yaping of Tianjin Polytechnic University (Research on the Preparation and Pore Structure Regulation Mechanism of PSf-b-PEG Separation Membranes [D]. Tianjin Polytechnic University, 2021) prepared separation membranes using PSf-b-PEG materials. Compared with PSf / PEG blend membranes, the surface porosity and overall porosity of PSf-b-PEG membranes were significantly improved, and the thickness of the dense skin layer was significantly thinned. However, this method is mainly used to directly prepare nanofiltration membranes for separating dyes and salts. The disadvantage is that PSf-b-PEG will form its own skin layer, increasing the water permeation resistance and consuming more materials, resulting in high costs. The literature "Preparation and Characterization of Pressure-Resistant Polysulfone Amphiphilic Block Copolymer Ultrafiltration Membranes" (Chen Wei et al. Preparation and Characterization of Pressure-Resistant Polysulfone Amphiphilic Block Copolymer Ultrafiltration Membranes [J]. Journal of Membrane Science and Technology, 2018, 38, 27) used 25% solid content of PSf-b-PEG as raw material to prepare ultrafiltration membranes, forming a dense layer. Summary of the Invention

[0005] To address the drawbacks of existing polysulfone-based RO / NF membranes, such as wide pore size distribution, difficulty in precisely adjusting the surface pore size, and low surface porosity, the present invention provides a novel double-layer composite substrate membrane. In the present invention, a solution of a first polymer (such as polysulfone) and a solution of a block polymer are simultaneously coated on the surface of the support layer, and the two layers of solutions undergo phase inversion simultaneously to prepare the double-layer composite substrate membrane. The present invention can regulate the pore size and pore size distribution on the surface of the substrate membrane, increase the surface porosity, form relatively regular pores on the surface, have no obvious dense separation skin layer, and have low cost. The preparation process of the present invention is realized by a mature multi-solution coating slot-die coater in the market, with simple operation and precise quantitative coating to prepare the required substrate membrane. Preferably, both solutions have a polysulfone (PSf) chain segment, with good bonding force and no problem of peeling off.

[0006] Specifically, the present invention provides a double-layer composite substrate membrane, which is composed of a support, a first modified layer on the surface of the support, and a second modified layer on the surface of the first modified layer. The material of the first modified layer is a first polymer selected from one or more of polysulfone, polyethersulfone, polyacrylonitrile, polyimide, and polyvinylidene fluoride, and the material of the second modified layer is a second polymer selected from one or more of PSf-b-PEO block copolymer and PSf-b-PEG block copolymer.

[0007] In one or more embodiments, the material of the first modified layer is polysulfone.

[0008] In one or more embodiments, the material of the second modified layer is PSf-b-PEO block copolymer.

[0009] In one or more embodiments, the pore size of the double-layer composite substrate membrane is 5-25 nm.

[0010] In one or more embodiments, the support is non-woven fabric.

[0011] The present invention also provides a method for preparing the double-layer composite substrate membrane according to any one of the embodiments herein, and the method includes the following steps:

[0012] (1) Prepare a first casting solution containing the first polymer and a second casting solution containing the second polymer;

[0013] (2) Simultaneously coat the first casting solution and the second casting solution onto the support by a simultaneous slot coating process, and undergo phase inversion to form a membrane after passing through a coagulation bath.

[0014] In one or more embodiments, the content of the first polymer in the first casting solution is 15 wt% to 25 wt%, such as 16±1 wt%.

[0015] In one or more embodiments, the solvent of the first casting solution is N,N-dimethylformamide.

[0016] In one or more embodiments, the content of the second polymer in the second casting solution is 1 wt% to 5 wt%.

[0017] In one or more embodiments, the solvent of the second casting solution is N,N-dimethylformamide.

[0018] In one or more embodiments, the second casting solution further contains additives, and the additives are ethylene glycol dimethyl ether and tetrahydrofuran.

[0019] In one or more embodiments, the content of the additives in the second casting solution is 5 wt% to 15 wt%, such as 10±1 wt%.

[0020] In one or more embodiments, in the additives, the mass ratio of ethylene glycol dimethyl ether to tetrahydrofuran is 1:2 to 2:1.

[0021] In one or more embodiments, in step (2), a slot die coater capable of coating two or more liquids simultaneously is used for coating.

[0022] In one or more embodiments, the coagulation bath is water.

[0023] The present invention also provides a double-layer composite base film prepared by the method described in any one of the embodiments herein.

[0024] The present invention also provides a reverse osmosis composite membrane or a nanofiltration composite membrane. The reverse osmosis composite membrane or the nanofiltration composite membrane includes a base film and a separation layer located on the surface of the base film. The base film is the double-layer composite base film described in any one of the embodiments herein; preferably, the separation layer is a polyamide separation layer. Description of the Drawings

[0025] Figure 1 It is a schematic structural diagram of the double-layer composite base film in some embodiments of the present invention.

[0026] Figure 2 It is a schematic process diagram of coating a polysulfone solution and a block polymer solution on a support layer in some embodiments of the present invention. Detailed Embodiments

[0027] To enable those skilled in the art to understand the features and effects of the present invention, the following provides a general description and definition of the terms and phrases mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meaning understood by those skilled in the art for the present invention. In case of conflict, the definition in this specification shall prevail.

[0028] The theories or mechanisms described and disclosed herein, whether right or wrong, shall not limit the scope of the present invention in any way, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0029] In this text, terms such as "comprising", "including", "containing" and similar terms cover the meanings of "consisting essentially of" and "consisting of". For example, when it is disclosed herein that "A comprises B and C", "A consists essentially of B and C" and "A consists of B and C" should be considered to have been disclosed herein.

[0030] In this text, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents and concentrations, are only for the sake of brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the ranges (including integers and fractions).

[0031] In this text, unless otherwise specified, percentages refer to mass percentages and ratios refer to mass ratios.

[0032] In this text, when describing embodiments or examples, it should be understood that they are not used to limit the present invention to these embodiments or examples. On the contrary, all alternatives, modifications and equivalents of the methods and materials described in the present invention can be covered within the scope defined by the claims.

[0033] In this text, for the sake of concise description, all possible combinations of all technical features in each embodiment or example are not described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered to be within the scope described in this specification.

[0034] The double-layer composite base film of the present invention comprises a support, a first modified layer on the surface of the support and a second modified layer on the surface of the first modified layer, that is, the second modified layer is the surface layer of the double-layer composite base film, and the first modified layer is located between the support and the second modified layer.

[0035] The support applicable to the present invention can be non-woven fabric.

[0036] The material of the first modified layer can be polysulfone, polyethersulfone, polyacrylonitrile, polyimide, polyvinylidene fluoride, etc. The polymer in the first modified layer is referred to as the first polymer in the present invention. In some preferred embodiments, the material of the first modified layer is polysulfone.

[0037] The material of the second modified layer can be a polysulfone-block-polyethylene oxide (PSf-b-PEO) block copolymer and / or a polysulfone-block-polyethylene glycol (PSf-b-PEG) block copolymer. In the present invention, the polymer in the second modified layer is referred to as the second polymer. In some preferred embodiments, the material of the second modified layer is a PSf-b-PEO block copolymer.

[0038] In some embodiments, as Figure 1 shown, the support of the double-layer composite base film of the present invention is a non-woven fabric, the first modified layer is a polysulfone layer, and the second modified layer is a block polymer layer.

[0039] The pore size of the double-layer composite base film of the present invention can be controlled to be 5-25 nm, such as 8 nm, 10 nm, 15 nm, 20 nm.

[0040] The double-layer composite base film of the present invention is prepared by a method comprising the following steps:

[0041] (1) Prepare a first casting solution containing a first polymer and a second casting solution containing a second polymer;

[0042] (2) Simultaneously coat the first casting solution and the second casting solution onto the support by a simultaneous slot coating process, and phase-invert them into a film after passing through a coagulation bath.

[0043] In the present invention, the first casting solution is used to form the first modified layer. The first casting solution contains a first polymer and a solvent, or consists of a first polymer and a solvent. The content of the first polymer in the first casting solution is preferably 15 wt% - 25 wt%, such as 16 wt%, 17 wt%, 18 wt%, 20 wt%, 22 wt%. The solvent of the first casting solution can be N,N-dimethylformamide (DMF).

[0044] In the present invention, the second casting solution is used to form the second modified layer. The second casting solution contains a second polymer and a solvent. The content of the second polymer in the second casting solution is preferably 1 wt% - 5 wt%, such as 2 wt%, 3 wt%, 4 wt%. The solvent of the second casting solution can be N,N-dimethylformamide. Preferably, the second casting solution further contains an additive. The additive is preferably ethylene glycol dimethyl ether and tetrahydrofuran (THF). Preferably, the content of the additive in the second casting solution is 5 wt% - 15 wt%, such as 7 wt%, 9 wt%, 10 wt%, 11 wt%, 13 wt%. Preferably, in the additive, the mass ratio of ethylene glycol dimethyl ether to tetrahydrofuran is 2:1 - 1:2, such as 1:1.

[0045] In the present invention, the two casting solutions are simultaneously coated onto the support by a simultaneous slot coating process, for example, using a slot die head that can simultaneously coat two or more liquids for coating. In some embodiments, asFigure 2 As shown, a slit coater is used to simultaneously coat a first casting solution (such as a polysulfone solution) and a second casting solution (such as a block copolymer solution) onto a support. Preferably, a closed-loop system is used for coating in the present invention. Preferably, a manifold dedicated to uniformly coating the liquid is used in the present invention.

[0046] In the present invention, the coating amount of the first casting solution on the support can be 60 - 150 g / m 2 , for example, 80 g / m 2 , 100 g / m 2 , 120 g / m 2 . The coating amount of the second casting solution on the support can be 5 - 50 g / m 2 , for example, 10 g / m 2 , 20 g / m 2 , 30 g / m 2 , 40 g / m 2 .

[0047] After coating, a coagulation bath is used to simultaneously cause phase inversion of the first casting solution and the second casting solution, thereby forming a first modified layer and a second modified layer of the double-layer composite substrate membrane. The coagulation bath applicable to the present invention can be pure water (RO water).

[0048] The double-layer composite substrate membrane of the present invention is suitable as the substrate membrane of a reverse osmosis membrane and a nanofiltration composite membrane. After contacting the double-layer composite substrate membrane of the present invention with an aqueous solution containing an amine monomer, removing the excess aqueous solution on the surface, and then contacting it with an organic solution containing an acyl chloride monomer for an interfacial polymerization reaction, followed by washing and drying, a reverse osmosis membrane and a nanofiltration composite membrane containing a polyamide separation layer of the double-layer composite substrate membrane of the present invention can be obtained.

[0049] When preparing a reverse osmosis membrane, the amine monomer in the aqueous solution can be metaphenylenediamine (MPD). The content of the amine monomer in the aqueous solution can be 1 wt% - 5 wt%, for example, 2.5 wt%. The aqueous solution can also contain 1 wt% - 5 wt%, for example, 3 wt% of camphorsulfonic acid (CSA) and 1 wt% - 2 wt%, for example, 1.5 wt% of triethylamine (TEA). The acyl chloride monomer in the organic solution can be trimesoyl chloride (TMC). The content of the acyl chloride monomer in the organic solution can be 0.1 wt% - 0.2 wt%, for example, 0.13 wt%. The organic solution can also contain 0.01 wt% - 0.1 wt%, for example, 0.05 wt% of isopropanol (IPA). The solvent of the organic solution can be an alkane solvent, such as IsoparE. The drying temperature can be 40 - 100 °C, for example, 50 °C.

[0050] In some embodiments, the present invention prepares a reverse osmosis membrane by the following method:

[0051] (a) Mix the first polymer with a solvent, and after complete dissolution, let it stand for degassing at room temperature to obtain the first casting solution; mix the second polymer, the additive with the solvent, and after complete dissolution, let it stand for degassing at room temperature to obtain the second casting solution;

[0052] (b) Use the simultaneous slot coating technique to coat the first casting solution and the second casting solution onto a support, and after passing through a coagulation bath, transform them into a film to obtain a base film;

[0053] (c) Immerse the base film in an aqueous solution of polyamine. After the immersion ends and the film surface is dried, immerse it in an organic phase solution containing trimesoyl chloride. After the reaction ends, take it out, perform pure water rinsing, and dry it at 40 - 100 °C to obtain the reverse osmosis membrane.

[0054] The present invention has the following beneficial technical effects:

[0055] (1) The present invention prepares an excellent RO / NF base film, which has the advantages of narrow pore size distribution, adjustable surface pore size, and regular pore morphology on the surface, solving the disadvantages of the polysulfone-based membrane currently used in RO / NF, such as wide pore size distribution, difficulty in precisely adjusting the surface pore size, and low surface porosity.

[0056] (2) The preparation process of the present invention is simple and is achieved by using a slot-die coating head that can coat multiple solutions simultaneously; the preparation process is optimized, and the relatively mature multi-layer liquid simultaneous slot coating technique is adopted, which can prepare a block polymer / polysulfone composite membrane base film. This method is simple to prepare and can be industrialized;

[0057] (3) By using block polymers (such as PSf-b-PEO, PSf-b-PEG, etc.) to composite with the polysulfone membrane to prepare the base film, various properties of the base film can be conveniently adjusted;

[0058] (4) The RO / NF base film prepared by the present invention has the advantages of adjustable and narrow surface pore size distribution, many surface pores, and uniform pore size. It has good compressive performance, can better adjust the properties of the base film, can well match the preparation of RO / NF membranes, and can solve the deficiencies of the base film prepared by the traditional phase inversion method (wide membrane pore size distribution and the need to change many process parameters to adjust the pore size);

[0059] (5) The present invention uses a coating die head that can coat multiple solutions simultaneously, which can coat one or more layers of fluid at high speed with only one operation, reducing production costs, and can also achieve excellent uniformity and quality, improving the quality of the final product; the present invention uses a closed-loop system to reduce pollution and emissions of volatile substances, and according to the rheology of the fluid, a manifold dedicated to uniformly coating the liquid is adopted.

[0060] The present invention will be described below by way of specific embodiments. It should be understood that these embodiments are merely illustrative and are not intended to limit the scope of the present invention. The methods, reagents, and materials used in the embodiments are conventional methods, reagents, and materials in the art unless otherwise specified. The starting compounds in the embodiments can all be obtained through commercial channels.

[0061] Example 1

[0062] Prepare Solution A with a polysulfone content of 16 wt% and the rest being DMF, and prepare Solution B with a PSf-b-PEO block polymer content of 1 wt%, ethylene glycol dimethyl ether content of 5 wt%, THF content of 5 wt%, and the rest being DMF. Use a multi-layer liquid simultaneous slot coater to coat Solution A and Solution B on the non-woven fabric. The coating amount of Solution A is 100 g / m 2 , and the coating amount of Solution B is 20 g / m 2 . Solution A is in contact with the non-woven fabric, and Solution B is above Solution A. Immerse the coated non-woven fabric in the coagulation bath RO water for phase inversion to form a film, and then rinse it to prepare the base film.

[0063] Example 2

[0064] Prepare Solution A with a polysulfone content of 16 wt% and the rest being DMF, and prepare Solution B with a PSf-b-PEO block polymer content of 5 wt%, ethylene glycol dimethyl ether content of 5 wt%, THF content of 5 wt%, and the rest being DMF. Use a multi-layer liquid simultaneous slot coater to coat Solution A and Solution B on the non-woven fabric. The coating amount of Solution A is 100 g / m 2 , and the coating amount of Solution B is 20 g / m 2 . Solution A is in contact with the non-woven fabric, and Solution B is above Solution A. Immerse the coated non-woven fabric in the coagulation bath RO water for phase inversion to form a film, and then rinse it to prepare the base film.

[0065] Example 3

[0066] Prepare Solution A with a polysulfone content of 16 wt% and the rest being DMF, and prepare Solution B with a PSf-b-PEO block polymer content of 1 wt%, ethylene glycol dimethyl ether content of 5 wt%, THF content of 5 wt%, and the rest being DMF. Use a multi-layer liquid simultaneous slot coater to coat Solution A and Solution B on the non-woven fabric. The coating amount of Solution A is 100 g / m 2 , and the coating amount of Solution B is 20 g / m 2, Solution A is in contact with the non-woven fabric, and Solution B is on top of Solution A. The coated non-woven fabric is immersed in the coagulation bath RO water for phase inversion to form a membrane, and then it is rinsed to obtain the base membrane. The base membrane is immersed in an aqueous solution containing 2.5 wt% m-phenylenediamine (MPD), 3 wt% camphorsulfonic acid (CSA), and 1.5 wt% triethylamine (TEA). After taking it out and draining the residual solution on the surface, it is then immersed in an Isopar E organic phase solution containing 0.13 wt% trimesoyl chloride (TMC) and 0.05 wt% isopropyl alcohol (IPA). After the reaction, it is taken out, the residual solution on the surface is drained, and it is placed in an oven at 50 °C for 4 min and then taken out to obtain the reverse osmosis membrane.

[0067] Example 4

[0068] Prepare Solution A with a polysulfone content of 16 wt% and the rest being DMF, and prepare Solution B with a PSf-b-PEO block polymer content of 5 wt%, ethylene glycol dimethyl ether content of 5 wt%, THF content of 5 wt%, and the rest being DMF. Use a multi-layer liquid simultaneous slot coater to coat Solution A and Solution B on the non-woven fabric. The coating amount of Solution A is 100 g / m 2 , and the coating amount of Solution B is 20 g / m 2 , Solution A is in contact with the non-woven fabric, and Solution B is on top of Solution A. The coated non-woven fabric is immersed in the coagulation bath RO water for phase inversion to form a membrane, and then it is rinsed to obtain the base membrane. The base membrane is immersed in an aqueous solution containing 2.5 wt% MPD, 3 wt% CSA, and 1.5 wt% TEA. After taking it out and draining the residual solution on the surface, it is then immersed in an Isopar E organic phase solution containing 0.13 wt% TMC and 0.05 wt% IPA. After the reaction, it is taken out, the residual solution on the surface is drained, and it is placed in an oven at 50 °C for 4 min and then taken out to obtain the reverse osmosis membrane.

[0069] Comparative Example 1

[0070] Prepare Solution A with a polysulfone content of 16 wt% and the rest being DMF. Use a single-layer liquid slot coater to coat Solution A on the non-woven fabric. The coated non-woven fabric is immersed in the coagulation bath RO water for phase inversion to form a membrane, and then it is rinsed to obtain the base membrane.

[0071] Comparative Example 2

[0072] Prepare solution A with 16 wt% polysulfone and the rest DMF. Coat solution A on the non-woven fabric using a single-layer liquid slit coater. Immerse the coated non-woven fabric in the coagulation bath RO water for phase inversion to form a membrane, and then prepare the base membrane after rinsing. Immerse the base membrane in an aqueous solution containing 2.5 wt% MPD, 3 wt% CSA, and 1.5 wt% TEA, take it out and drain the residual solution on the surface, then immerse it in an organic phase solution of IsoparE containing 0.13 wt% TMC and 0.05 wt% IPA, take it out after reaction, drain the residual solution on the surface, and place it in an oven at 50 °C for 4 min and then take it out to obtain a reverse osmosis membrane.

[0073] Test example

[0074] Test the performance of the base membrane and the reverse osmosis membrane according to the following method, and the results are shown in Table 1:

[0075] Test method for the performance of the base membrane: First, pre-press the membrane sample at a pressure of 0.15 MPa for 30 min, then adjust the test pressure to 0.15 MPa, and record the mass change of pure water passing through the ultrafiltration membrane at this pressure. The test process lasts for 30 min.

[0076] The calculation method of the pure water flux (Jw) is as follows:

[0077] Jw = V / St

[0078] Where, Jw is the pure water flux of the membrane sample (unit: LMH), V is the volume of the permeated water (unit: L), S is the area of the membrane sample (unit: m 2 ), and t is the test time (unit: h).

[0079] Test method for the performance of the reverse osmosis membrane: Place the reverse osmosis membrane on the cross-flow membrane sheet detection platform for membrane sheet performance testing. Under the test conditions of an operating pressure of 150 psi, a sodium chloride raw aqueous solution concentration of 500 ppm, a solution temperature of 25 °C, and a pH value of 6.5 - 7.5, measure the water flux and rejection rate after the membrane sheet has been operating for 30 min.

[0080] Pore size: Measured by the liquid-liquid method using a PMI pore size analyzer. The specific operation is to cut a membrane sheet of a certain size, first soak it in n-butanol for a certain period of time for pretreatment, and then install it in the pore size analyzer for analysis and testing.

[0081] Table 1: Performance of the base membrane and the reverse osmosis membrane

[0082]

[0083] As can be seen from Table 1, the fluxes of the base membranes in Example 1 and Example 2 are significantly improved compared to the base membrane flux in Comparative Example 1, and the fluxes of the reverse osmosis membranes in Example 3 and Example 4 are significantly improved compared to the reverse osmosis membrane flux in Comparative Example 2.

Claims

1. A double-layer composite base film, characterized in that, The double-layer composite base membrane is composed of a support body, a first modified layer located on the surface of the support body, and a second modified layer located on the surface of the first modified layer. The material of the first modified layer is polysulfone, and the material of the second modified layer is a PSf-b-PEO block copolymer.

2. The double-layer composite base film according to claim 1, wherein, The pore size of the double-layer composite base membrane is 5-25 nm; and / or the support body is a non-woven fabric.

3. A method for preparing the double-layer composite base film according to claim 1 or 2, characterized in that, The method includes the following steps: (1) Prepare a first casting solution containing polysulfone and a second casting solution containing a PSf-b-PEO block copolymer; (2) Use a simultaneous slot coating process to coat the first casting solution and the second casting solution onto the support body at the same time, and phase-invert to form a membrane through a coagulation bath.

4. The method according to claim 3, wherein The content of polysulfone in the first casting solution is 15 wt% to 25 wt%; and / or the solvent of the first casting solution is N,N-dimethylformamide.

5. The method according to claim 3, characterized in that The content of polysulfone in the first casting solution is 16 ± 1 wt%.

6. The method according to claim 3, characterized in that, The content of the PSf-b-PEO block copolymer in the second casting solution is 1 wt% to 5 wt%; and / or the solvent of the second casting solution is N,N-dimethylformamide.

7. The method according to claim 3, characterized in that, The second casting solution further contains additives, and the additives are ethylene glycol dimethyl ether and tetrahydrofuran.

8. The method according to claim 7, wherein The content of the additives in the second casting solution is 5 wt% to 15 wt%.

9. The method according to claim 7, wherein The content of the additives in the second casting solution is 10 ± 1 wt%.

10. The method according to claim 7, wherein In the additives, the mass ratio of ethylene glycol dimethyl ether to tetrahydrofuran is 1:2 to 2:

1.

11. The method according to claim 3, characterized in that, In step (2), a slot die head capable of coating two or more liquids simultaneously is used for coating; and / or the coagulation bath is water.

12. A double-layer composite base membrane prepared by the method according to any one of claims 3-11.

13. A reverse osmosis composite membrane or a nanofiltration composite membrane, characterized in that, The reverse osmosis composite membrane or nanofiltration composite membrane includes a base membrane and a separation layer located on the surface of the base membrane, and the base membrane is the double-layer composite base membrane according to claim 1, 2 or 12.

14. The reverse osmosis composite membrane or nanofiltration composite membrane according to claim 13, wherein The separation layer is a polyamide separation layer.

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