Preparation method of composite double-layer microporous membrane

Abstract

The invention provides a preparation method of a composite double-layer microporous membrane, and belongs to the technical field of separation membrane preparation, different lower-layer and upper-layer membrane casting solutions are prepared respectively, the separation membrane is prepared at one time by adopting a double-scraper casting and coating method, the upper layer is thermally induced phase separation, and the lower layer is thermally induced phase separation or non-solvent induced phase separation; the double-layer liquid membrane is subjected to pre-phase splitting and pore opening in a constant-temperature and constant-humidity environment, and enters a gel bath at a certain temperature after pre-phase splitting, and a microporous membrane with a certain membrane structure is formed under the action of RTIPS and NIPS. According to the invention, a co-coating method is adopted, and VIPS, RTIPS and NIPS are used for adjusting the pore structure of the membrane, so that the microporous membrane meeting different filtering requirements is prepared; the prepared membrane product is high in flux and high in yield; the preparation process is simple and conditions are mild: the microporous membrane prepared by the method is simple in process, mild in conditions and suitable for industrial production; and the membrane structures of the upper and lower membranes can be freely adjusted.

Description

Technical Field

[0001] The invention relates to the technical field of separation membrane preparation, and in particular to a method for preparing a composite double-layer microporous membrane that can meet different filtering requirements. Background Art

[0002] Membrane separation technology is an efficient, energy-saving and environmentally friendly separation process. It presents various characteristics in physical, chemical and biological properties, has many advantages, and has become one of the most important means in separation science today. The improvement of membrane flux can be achieved through methods such as adjustment of membrane structure and surface modification. Among them, the method of adjusting membrane structure has better adaptability to large-scale production. The preparation method of single-layer membrane (using a single casting liquid and a simple preparation process) determines the singleness of its structure, while the advantage of double-layer or multi-layer membranes is that each layer can not only use materials with different functions, but also fine-tune the membrane pore structure of different layers to achieve overall improved performance, thereby preparing microporous membranes with various membrane structures.

[0003] The Chinese invention patent application with publication number CN103877868A discloses a method for preparing a separation membrane and high-throughput and high-strength separation, using a co-extrusion method to extrude the membrane-making liquid of the support layer and the separation layer at the same time to form a separation membrane with a double-layer structure. Although the co-extrusion method simplifies some steps, it may require precise control of multiple parameters in actual operation, which has high requirements for equipment and increases the complexity of the process. In addition, the TIPs method requires a higher temperature to dissolve organic materials, which increases the cost of membrane production.

[0004] The Chinese invention patent application with publication number CN115038510A discloses a design and manufacturing method for a multi-zone, carrier-free, microporous, high-flux filtration membrane, which uses a casting method to cast a polymer solution as three separate zones onto a carrier to form a multi-zone liquid sheet. This method uses advanced materials and complex manufacturing processes, which may increase the production cost of the membrane, especially when using special polymers and electron beam cross-linking technology. In addition, the waste and pollutants generated during the production process still need to be properly handled.

[0005] The US invention patent with publication number US2011 / 0225940A1 discloses a method for preparing a microporous membrane, wherein multiple feed formulas are cast on a support to form a multilayer sheet having a first main surface. This method requires precise control of relative humidity, temperature, time and other conditions during the VIPS process, and there may be problems with the stability and repeatability of the production process. The phase separation time is 7.5-25 minutes, which is relatively long. In addition, step-by-step humidity control is required, which places high demands on equipment.

[0006] The existing single-layer casting microporous membranes prepared, the membrane structure depends largely on the membrane formation process, especially the air exposure process step. This makes the process less stable and may therefore produce uneven membranes. The existing multi-layer membrane preparation method is relatively complicated, with high requirements on production process parameters, and may cause environmental pollution problems and unstable microporous membrane products. Multi-layer prepared membranes may have defects and incomplete structures. Co-coating of two or more casting solutions may cause stratification due to different miscibility of the casting solutions, gelation rates and shrinkage rates of the membranes. The prepared microporous membrane has a single structure and low flux and strength. Summary of the invention

[0007] The object of the present invention is to provide a method for preparing a composite double-layer microporous membrane that can meet different filtration requirements, so as to solve at least one technical problem existing in the above-mentioned background technology.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for preparing a composite double-layer microporous membrane, comprising:

[0010] Different lower and upper layer casting solutions were prepared respectively, and the separation membrane was prepared at one time by double doctor blade casting method. The upper layer was reverse thermally induced phase separation, while the lower layer was reverse thermally induced phase separation or non-solvent induced phase separation:

[0011] Preparation of the lower layer casting solution: Add the polymer, porogen and organic solvent into the batching tank, stir to dissolve and degas;

[0012] Preparation of the upper layer casting solution: Add the polymer, porogen and organic solvent into the batching tank, stir to dissolve and degas;

[0013] The double-scraper coating method is used to prepare a double-layer separation membrane at one time: the lower and upper casting liquids are added to the double-scraper device respectively, and the upper and lower casting liquids are coated on the PET plate. The double-layer liquid membrane is pre-phase-separated and pore-formed in a constant temperature and humidity environment, and then enters a gel bath at a certain temperature after pre-phase separation. Due to the action of RTIPS and NIPS, a microporous membrane with a certain membrane structure is formed;

[0014] The prepared flat microporous membrane is immersed in deionized water to remove the porogen and solvent in the membrane.

[0015] Optionally, in the preparation of the lower layer casting solution, the solution is dissolved by stirring at 25-80°C.

[0016] Optionally, in the preparation of the upper casting solution, the solution is dissolved by stirring at 25-60°C.

[0017] Optionally, the upper layer casting liquid and the lower layer casting liquid are coated on the PET board at a pulling speed of 2-5 m / min.

[0018] Optionally, the components of the upper layer casting solution and the lower layer casting solution are 10-30 parts of polymer and 5-65 parts of porogen in parts by weight.

[0019] Optionally, the polymer is polyethersulfone.

[0020] Optionally, the porogen is one of diethylene glycol, triethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400 or polyvinyl pyrrolidone, or a combination of two thereof.

[0021] Optionally, the solvent is a combination of one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylformamide or dimethyl sulfoxide.

[0022] Terminology explanation:

[0023] Co-casting: Two or more membrane-forming solutions are cast simultaneously using a special molding mold. The membrane solution may include different polymers, solvents or additives. The resulting primary membrane undergoes phase separation through mutual exchange between solvent and non-solvent to obtain a two-layer or multi-layer membrane with an asymmetric structure.

[0024] Vapor-induced phase separation (VIPS): Exposure of a polymer solution to a vapor environment, usually humid air or other non-solvent vapor, causes the solubility of the polymer in the solvent to decrease, thereby initiating the phase separation process.

[0025] Reverse Thermally Induced Phase Separation (RTIPS): The casting solution is formed at a low temperature, and the phases are separated and formed into a membrane at a high temperature (above the cloud point). Compared with the traditional thermally induced phase separation (TIPS) method, RTIPS uses the opposite process to induce phase separation, that is, the casting solution is formed at a lower temperature, and then phase separation is carried out at a higher temperature.

[0026] Nonsolvent Induced Phase Separation (NIPS) is a commonly used method for preparing microporous membranes. Its basic principle is to dissolve the polymer in a certain solvent to form a homogeneous solution, and then slowly add a non-solvent (also called an extractant) that is more miscible with the solvent to extract the solvent, thereby forming a two-phase structure with the polymer as the continuous phase and the solvent as the dispersed phase.

[0027] Beneficial effects of the present invention: The present invention adopts a co-casting method, uses VIPS, RTIPS and NIPS to adjust the membrane pore structure, and prepares a microporous membrane that meets different filtration requirements. The present invention has a simple preparation method, a simple formula of the casting liquid, and a simple preparation process. The prepared membrane product has the advantages of high flux and high yield, and can be used in the fields of biomedicine, electronics, food and beverages, and has a wide range of applications. Simplicity of preparation process and mild conditions: The microporous membrane prepared by this method has a simple process and mild conditions, and is suitable for industrial production. The membrane structure of the upper and lower membranes can be freely adjusted.

[0028] Additional advantages of the present invention will be more clearly given in the following description or learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0030] Figure 1 This is a flow chart of the method for preparing the composite double-layer microporous membrane described in an embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of the structure of a double scraper device used for preparing a composite double-layer microporous membrane according to an embodiment of the present invention.

[0032] Figure 3 This is a cross-sectional SEM photograph of the microporous membrane prepared in Example 1 of the present invention.

[0033] Figure 4 This is a SEM photograph of the lower surface of the microporous membrane prepared in Example 1 of the present invention.

[0034] Figure 5 The upper surface SEM photograph of the microporous membrane prepared by implementing 1 of the present invention.

[0035] Figure 6 A cross-sectional SEM photograph of the microporous membrane prepared in Example 2 of the present invention.

[0036] Figure 7 This is a SEM photograph of the lower surface of the microporous membrane prepared in Example 2 of the present invention.

[0037] Figure 8 This is a SEM photograph of the upper surface of the microporous membrane prepared in Example 2 of the present invention.

[0038] Fig. 9 This is a cross-sectional SEM photograph of the microporous membrane prepared in Example 3 of the present invention.

[0039] Fig.10 This is a SEM photograph of the lower surface of the microporous membrane prepared in Example 3 of the present invention.

[0040] Fig.11 This is a SEM photograph of the upper surface of the microporous membrane prepared in Example 3 of the present invention.

[0041] Among them; 1-first scraper; 2-second scraper; 3-back scraper. DETAILED DESCRIPTION

[0042] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The embodiments described below by the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention.

[0043] It should be understood by those skilled in the art that unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this invention belongs.

[0044] It should also be understood that terms, such as those defined in commonly used dictionaries, should be understood to have a meaning consistent with that in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless as defined herein.

[0045] Those skilled in the art will appreciate that, unless otherwise stated, the singular forms "a", "an", "said" and "the" used herein may also include plural forms. It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements and / or groups thereof.

[0046] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples, unless they are contradictory.

[0047] To facilitate understanding of the present invention, the present invention is further explained below with reference to specific embodiments in conjunction with the accompanying drawings, and the specific embodiments do not constitute a limitation on the embodiments of the present invention.

[0048] Those skilled in the art should understand that the drawings are merely schematic diagrams of embodiments, and the components in the drawings are not necessarily necessary for implementing the present invention.

[0049] In view of the high preparation cost, complex preparation process, high requirements for production process parameters, possible environmental pollution and unstable preparation of microporous membrane products in the production process of microfiltration membranes. The present invention adopts a co-casting method, uses VIPS, RTIPS and NIPS to adjust the membrane pore structure, and prepares microporous membranes that meet different filtration requirements. The present invention has the advantages of simple preparation method, simple casting liquid formula, simple preparation process, and the prepared membrane products have the advantages of high flux and high yield, and can meet the filtration requirements of different fields. The preparation method provided by the present invention, two casting liquids are co-casted to prepare composite membranes: two layers of casting liquids are independently prepared, and the upper layer is reverse thermally induced phase separation (RTIPS) and the lower layer is reverse thermally induced phase separation or non-solvent induced phase separation (NIPS) are formed simultaneously, ensuring the mechanical strength and separation performance of the membrane. The membrane structure is adjusted by combining two different formulas and changing the process, so that the membrane structure is adapted to different fields and meets different filtration requirements. Simplicity of preparation process and mild conditions: The microporous membrane prepared by this method has a simple process and mild conditions, which is suitable for industrial production. The prepared separation membrane has a wide range of applications: the separation membrane can be used in biomedicine, electronics, food and beverage and other fields.

[0050] The present invention provides a casting solution formula for achieving double-layer membrane formation: the casting solution comprises 10-30 parts of polyethersulfone and 5-65 parts of porogen, the porogen can be one or two of diethylene glycol, triethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400, polyvinyl pyrrolidone (k15, k30, k60 or k90), and the solvent can be one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylformamide and dimethyl sulfoxide. The porogen can increase the hydrophilicity of the prepared membrane and improve the filtration flux.

[0051] like Figure 1 As shown, the microporous membrane preparation method of the present invention comprises preparing different lower and upper layer casting solutions respectively, and using Figure 2 The double-blade device casting method shown in the figure is used to prepare the separation membrane at one time. The upper layer of the method is reverse thermally induced phase separation (RTPIS), and the lower layer is reverse thermally induced phase separation (RTIPS) or non-solvent induced phase separation (NIPS); the specific process is as follows:

[0052] Preparation of lower layer casting solution: add lower layer polymer, porogen and organic solvent into the batching tank, stir and dissolve at 25-80℃, and degas;

[0053] Preparation of upper casting solution: add the upper polymer, porogen and organic solvent into the mixing tank, stir and dissolve at 25-60℃, and degas;

[0054] The double-scraper coating method is used to prepare a double-layer separation membrane at one time: the lower and upper casting liquids are added to the double-scraper device respectively, the casting liquid A (upper casting liquid) is between the first scraper 1 and the second scraper 2, and the casting liquid B (lower casting liquid) is between the second scraper 2 and the back scraper 3. The double-layer liquid film is coated on the PET plate at a pulling speed of 2-5m / min. The double-layer liquid film is pre-phase-separated and pore-formed in a constant temperature and humidity environment, and after pre-phase separation, it enters a gel bath (water) at a certain temperature, and forms a microporous membrane with a certain membrane structure due to the action of RTIPS and NIPS;

[0055] The prepared flat microporous membrane is immersed in deionized water to remove the porogen and solvent in the membrane.

[0056] Example 1

[0057] In this embodiment 1, a method for preparing a composite double-layer microporous membrane is provided, comprising the following steps:

[0058] Upper casting liquid: weigh 43.8 parts of N,N-dimethylacetamide, 10 parts of polyethersulfone (BASF 6020) and 20.2 parts of diethylene glycol (DEG) in a stirring kettle, heat and stir at 60°C until fully dissolved, cool to 30°C, add 26 parts of DEG, and continue stirring for 3-6 hours. The stirred casting liquid is allowed to stand for 6 hours to degas. Lower casting liquid: weigh 42 parts of N,N-dimethylacetamide, 13 parts of polyethersulfone (BASF 6020), 5 parts of polyvinyl pyrrolidone (k30) and 20 parts of diethylene glycol (DEG) in a stirring kettle, heat and stir at 60°C until fully dissolved, cool to 30°C, add 20 parts of diethylene glycol, and continue stirring for 3-6 hours. The stirred casting liquid is allowed to stand for 6 hours to degas.

[0059] Adjust the scraper that controls the thickness of the upper and lower liquid films to 270 and 130 μm respectively. Add the lower liquid into the feed tank first, and add the upper liquid into the feed tank after stabilization. The formed double-layer liquid membrane is placed in an environment with a temperature and humidity of 35°C and 10% respectively. After 15 seconds, it enters the gel bath (water). After washing at 80°C, a double-layer microporous membrane is obtained. According to "GB / T32361-2015 Separation Membrane Pore Size Test Method Bubble Point and Average Flow Method", the bubble point pressure (the test liquid is deionized water) and flux (the test liquid is deionized water) of the filter membrane prepared in Example 1 are tested. Flux bubble point test results: the pure water flux is 15000LMH@1bar, and the water bubble point is 0.36~0.44Mp. As Figure 3 The figure shows the cross-sectional SEM image of the prepared microporous membrane. Figure 4 is the SEM photo of the lower surface of the prepared microporous membrane. Figure 5 The SEM photograph of the upper surface of the prepared microporous membrane.

[0060] Example 2

[0061] In this embodiment 2, a method for preparing a composite double-layer microporous membrane is provided, comprising the following steps:

[0062] Upper casting solution: same as in Example 1

[0063] Lower layer casting solution: weigh 21.25 parts of N,N-dimethylacetamide, 15 parts of polyethersulfone (BASF 6020), and 40 parts of polyethylene glycol 200 (PEG200) in a stirring kettle, heat and stir at 60°C until fully dissolved, cool to 30°C, add 23.75 parts of PEG200, and continue stirring for 3-6 hours. Let the stirred casting solution stand for 6 hours to degas;

[0064] The scraper controlling the thickness of the upper and lower layers of liquid film was adjusted to 270 and 130 μm respectively. The lower layer of liquid was first added to the material tank, and after stabilization, the upper layer of liquid was added to the material tank. The formed double-layer liquid film was placed in an environment with a temperature and humidity of 35°C and 5%, respectively, and entered into a gel bath (water) after 15 seconds. After washing at 80°C, a double-layer microporous membrane was obtained.

[0065] Flux bubble point test results: pure water flux is 10000LMH@1bar, water bubble point is 0.40~0.50Mp. Figure 6 The cross-sectional SEM image of the prepared microporous membrane is shown in FIG. Figure 7 is the SEM photo of the lower surface of the prepared microporous membrane. Figure 8 The SEM photo of the upper surface of the prepared microporous membrane is shown below. The membrane structure has small holes on both the upper and lower layers, and double separation layers to improve the filtration accuracy.

[0066] Example 3

[0067] In this embodiment 3, a method for preparing a composite microporous membrane is provided, comprising the following steps:

[0068] Upper casting solution: Weigh 43.6 parts of N,N-dimethylacetamide, 15 parts of polyethersulfone (BASF 6020) and 26 parts of diethylene glycol (DEG) in a stirring kettle, heat and stir at 60°C until fully dissolved, cool to 30°C, add 15.4 parts of DEG, and continue stirring for 3-6 hours. Let the stirred casting solution stand for 6 hours to degas;

[0069] Lower layer casting solution: weigh 42 parts of N,N-dimethylacetamide, 13 parts of polyethersulfone (BASF 6020), 5 parts of polyvinyl pyrrolidone (k30) and 20 parts of diethylene glycol (DEG) in a stirring kettle, heat and stir at 60°C until fully dissolved, cool to 30°C, add 20 parts of DEG, and continue stirring for 3-6 hours. Let the stirred casting solution stand for 6 hours to degas;

[0070] The scraper controlling the thickness of the upper and lower layers of liquid film was adjusted to 270 and 130 μm respectively. The lower layer of liquid was first added to the material tank, and after stabilization, the upper layer of liquid was added to the material tank. The formed double-layer liquid film was placed in an environment with a temperature and humidity of 35°C and 15%, respectively, and entered into a gel bath (water) after 15 seconds. After washing at 80°C, a double-layer microporous membrane was obtained.

[0071] Flux bubble point test results: pure water flux is 8000LMH@1bar, water bubble point is 0.50-0.60Mp. Fig. 9 The cross-sectional SEM photograph of the prepared microporous membrane is shown in Figure 2. Fig.10 This is a SEM photo of the lower surface of the microporous membrane prepared in Example 3. Fig.11 This is a SEM photo of the upper surface of the microporous membrane prepared in Example 3. The lower part of the membrane structure has large pores, which mainly play a pre-filtration role, and the upper part has small pores, which mainly play a separation role.

[0072] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative work on the basis of the technical solution disclosed in the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a composite double-layer microporous membrane, characterized in that: include: Different lower and upper layer casting solutions were prepared respectively, and the separation membrane was prepared at one time by double doctor blade casting method. The upper layer was reverse thermally induced phase separation, while the lower layer was reverse thermally induced phase separation or non-solvent induced phase separation: Preparation of the lower layer casting solution: Add the polymer, porogen and organic solvent into the batching tank, stir to dissolve and degas; Preparation of the upper layer casting solution: Add the polymer, porogen and organic solvent into the batching tank, stir to dissolve and degas; The double-scraper coating method is used to prepare a double-layer separation membrane at one time: the lower and upper casting liquids are added to the double-scraper device respectively, and the upper and lower casting liquids are coated on the PET plate. The double-layer liquid membrane is pre-phase-separated and pore-formed in a constant temperature and humidity environment, and then enters a gel bath at a certain temperature after pre-phase separation. Due to the action of RTIPS and NIPS, a microporous membrane with a certain membrane structure is formed; The prepared flat microporous membrane is immersed in deionized water to remove the porogen and solvent in the membrane.

2. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: In the preparation of the lower layer casting solution, stir and dissolve at 25-80°C.

3. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: In the preparation of the upper casting solution, stir and dissolve at 25-60°C.

4. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: The double-layer film liquid is coated on the PET board at a pulling speed of 2-5m / min.

5. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: The components of the upper layer casting solution and the lower layer casting solution are calculated by weight: 10-30 parts of polymer and 5-65 parts of porogen.

6. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: The polymer is polyethersulfone.

7. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: The porogen is one of diethylene glycol, triethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400 or polyvinyl pyrrolidone, or a combination of two thereof.

8. The method for preparing a composite double-layer microporous membrane according to claim 1, characterized in that: The solvent is a combination of one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylformamide or dimethyl sulfoxide.

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