Method for preparing PVDF (Polyvinylidene Fluoride) microporous membrane by multistage low-temperature coagulating bath

Abstract

The invention relates to a preparation method of a PVDF (Polyvinylidene Fluoride) microporous membrane, in particular to a preparation method of a PVDF microporous membrane by a multi-stage low-temperature coagulating bath, which comprises the following steps: heating and dissolving a PVDF raw material, a solvent and a non-solvent additive to prepare a membrane liquid, scraping the membrane liquid on a support plate by using a scraper to prepare a membrane, and entering the multi-stage low-temperature coagulating bath to achieve the effect of gradually delaying phase separation. Compared with a conventional method, a membrane with a smaller microstructure and higher filtering precision can be obtained, the porous PVDF microporous membrane is obtained after drying, and the PVDF microporous membrane prepared through the method is of a spongy structure under the microscale and is suitable for being applied to the field of filtering separation in the biological medicine industry.

Description

Technical Field

[0001] The present invention relates to the field of membrane science and technology, in particular to a method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath. Background Art

[0002] PVDF, the raw material, is a polymer material with excellent comprehensive properties, including good chemical and thermal stability, physical and mechanical properties, acid and alkali resistance, and blood compatibility. It has been applied in areas such as gas separation, seawater desalination, ultrapure water production, waste treatment, artificial organ manufacturing, medicine, food, agriculture, and chemical industry, and has broad prospects for application in biomedical filtration.

[0003] Polymer membranes achieve filtration and screening by adjusting their surface and internal pore structures. Therefore, controlling membrane pore structure is crucial for obtaining high-quality membranes. During the exchange of the nonsolvent and solvent in the coagulation bath, different conditions lead to different pore structures. Sponge pores are interconnected pore structures that allow fluids to penetrate with high permeability, resulting in microporous membranes with sponge-like structures exhibiting faster flow rates. Therefore, process conditions must eliminate or minimize the presence of occluded pores such as cellular pores. By modifying the coagulation bath conditions, microporous membranes with higher flow rates can be produced. Summary of the Invention

[0004] The present invention adopts the NIPS method to prepare PVDF microporous filter membranes. The casting solution in the present invention mostly adopts a mixed solvent, and a single solvent is rarely used. The liquid film scraped from the membrane liquid is immersed in a non-solvent, and the uniform liquid layer is separated into a polymer-rich phase and a polymer-poor phase. As the non-solvent in the coagulation bath enters the polymer membrane, the solvent in the membrane also diffuses outward. When the polymer chain gradually solidifies from the solvent to the non-solvent-rich phase, the membrane pore structure is formed. There are many factors that affect the membrane pore structure. The coagulation bath, as the most important link in the process conditions, has a greater impact on it. In order to obtain a more uniform membrane pore structure with higher filtration accuracy, it is necessary to delay the exchange process between the solvent and the non-solvent in the coagulation bath. We achieve this process by using a multi-stage soft coagulation bath and a low-temperature coagulation bath, which can effectively eliminate the closed pore structure and dense cortex. The mixing of different solvents can shorten the time that the membrane liquid is in a metastable state after nucleation, thereby reducing the formation of closed pore structure. The desired microporous filter membrane morphology can be obtained by combining different solvents.

[0005] The above-mentioned closed pore structure and dense cortex limit the performance of the membrane and do not have an application environment. The present invention solves the above-mentioned technical problems and prepares a sponge-like PVDF microporous membrane with a suitable membrane pore structure, a large water flux, an initial bubble point range of 0.1Mpa-0.25Mpa, and a uniform bubble point range of 0.15Mpa-0.3Mpa.

[0006] As a feature of the present invention, the ratio of the initial bubble point to the uniform bubble point is above 0.85.

[0007] As a feature of the present invention, the average pore diameter is above 0.1 μm and the porosity is 60-70%. Preferably, the average pore diameter is around 0.2 μm and the porosity is above 65%.

[0008] As a feature of the present invention, the filter membrane has a thickness of 120-150 μm, and preferably, the filter membrane has a thickness of 125-140 μm.

[0009] The sponge-like PVDF microporous membrane as described above is characterized in that the tensile strength is 3.5-4.5 MPa.

[0010] The preparation process of the present invention adopts the following technical scheme, and the preparation process is as follows:

[0011] 1. Prepare a casting solution. The solvent used may include one or more mixed solvents. A non-solvent is added to the casting solution as a pore-forming agent.

[0012] 2. Use a scraper to scrape the film on the support plate.

[0013] 3. Immerse in the coagulation bath, and then enter the first and second low-temperature soft coagulation baths in turn. The temperature of the second coagulation bath should be lower than that of the first coagulation bath. Finally, use the deionized water coagulation bath to solidify the membrane structure, and then use deionized water to wash away the solvent in the membrane pores.

[0014] 4. Prepare a sponge-like PVDF microporous membrane. The membrane can be used for practical use after drying.

[0015] As a further improvement to the preparation process of the present invention, the solvents used to prepare the solution in step 1 are N-methylpyrrolidone (NMP), N-dimethylformamide (DMF), N-dimethylacetamide (DMAc), and triethyl phosphate (TEP). The pore-forming agent used to prepare the solution is typically PEG200, PEG400, PEG600, PEG800, etc.

[0016] As a further improvement on the preparation process of the present invention, the mass fraction of polyethersulfone in step 1 is 20-25 wt %.

[0017] As a further improvement on the preparation process of the present invention, the thickness of the liquid film scraped in step 2 is between 400-600 μm.

[0018] As a further improvement on the preparation process of the present invention, in step 3, the content of the first-level coagulation bath water is less than 20%, the content of the second-level coagulation bath water is less than 50%, the coagulation bath is a low-temperature coagulation bath, the temperature of the coagulation bath is 0-20 ° C, the temperature of the second-level coagulation bath is lower than the temperature of the first-level coagulation bath, the soft coagulation bath is usually ethanol and deionized water, and the final deionized water coagulation bath solidifies the membrane structure, and then the solvent in the membrane pores is washed away with deionized water.

[0019] The present invention can prepare a sponge-like PVDF microporous membrane with a distinct sponge-like morphology. The PVDF microporous membrane prepared by the present invention has the properties of high specific surface area and high porosity and is suitable for high-flow rate liquid filtration occasions.

[0020] Preferred drawings:

[0021] Figure 1 This is a scanning electron microscope cross-sectional view of the PVDF microporous membrane prepared in Example 2 of the present invention;

[0022] Figure 2 This is a scanning electron microscope air surface image of the PVDF microporous membrane prepared in Example 2 of the present invention;

[0023] Figure 3 This is a scanning electron microscope image of the PVDF microporous membrane carrier prepared in Example 2 of the present invention; DETAILED DESCRIPTION

[0024] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0025] Example 1

[0026] A method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath, the method comprising preparing a membrane solution, scraping it on a support plate, and immersing the membrane in a multi-stage low-temperature coagulation bath to prepare a sponge-like PVDF microporous membrane, wherein:

[0027] Step 1: Weigh 38 parts NMP, 37 parts DMF, and 5 parts PEG400 and stir to form a solution. Then add 20 parts PVDF and stir to obtain a homogeneous casting solution.

[0028] Step 2: The obtained casting solution is allowed to stand and degas for 24 hours;

[0029] Step 3: The casting liquid is scraped onto the support plate with a scraper to a thickness of 450 μm. The air temperature in the scraping chamber is 20°C and the relative humidity is 80%.

[0030] Step 4: Immerse the membrane in a primary coagulation bath containing 80% ethanol at 10°C for 2 minutes for preliminary curing. Remove the membrane and place it in a secondary coagulation bath containing 50% ethanol at 0°C for 5 minutes. Finally, place it in a deionized water coagulation bath for complete curing. Rinse the membrane in 20°C deionized water to remove any remaining solvent and precipitates. After drying, collect the membrane.

[0031] Example 2

[0032] A method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath, the method comprising preparing a membrane solution, scraping it on a support plate, and immersing the membrane in a multi-stage low-temperature coagulation bath to prepare a sponge-like PVDF microporous membrane, wherein:

[0033] Step 1: Weigh 48 parts NMP, 22 parts DMF, and 5 parts PEG800 and stir to form a solution. Then add 25 parts PVDF and stir to obtain a homogeneous casting solution.

[0034] Step 2: The obtained casting solution is allowed to stand and degas for 24 hours;

[0035] Step 3: The casting liquid is scraped onto the support plate with a scraper to a thickness of 500 μm. The air temperature in the climate chamber is 20°C and the relative humidity is 70%.

[0036] Step 4: Immerse the membrane in a primary coagulation bath containing 80% ethanol at 15°C for 5 minutes for preliminary curing. Remove the membrane and place it in a secondary coagulation bath containing 60% ethanol at 10°C for 3 minutes. Finally, place it in a deionized water coagulation bath for complete curing. Rinse the membrane in 20°C deionized water to remove any remaining solvent and precipitates. After drying, collect the membrane.

[0037] Example 3

[0038] A method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath, the method comprising preparing a membrane solution, scraping it on a support plate, and immersing the membrane in a multi-stage low-temperature coagulation bath to prepare a sponge-like PVDF microporous membrane, wherein:

[0039] Step 1: Weigh 55 parts DMAc, 22 parts DMF, and 3 parts PEG200 and stir to form a solution. Then add 20 parts PVDF and stir to obtain a homogeneous casting solution.

[0040] Step 2: The obtained casting solution is allowed to stand and degas for 24 hours;

[0041] Step 3: The casting liquid is scraped onto the support plate with a scraper. The thickness of the liquid film is 400 μm. The air temperature in the climate chamber is 20°C and the relative humidity is 80%.

[0042] Step 4: Immerse the membrane in a primary coagulation bath containing 80% ethanol at 20°C for 2 minutes for preliminary curing. Remove the membrane and place it in a secondary coagulation bath containing 50% ethanol at 10°C for 5 minutes. Finally, place it in a deionized water coagulation bath for complete curing. Rinse the membrane in 20°C deionized water to remove any remaining solvent and precipitates. After drying, collect the membrane.

[0043] Example 4

[0044] A method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath, the method comprising preparing a membrane solution, scraping it on a support plate, and immersing the membrane in a multi-stage low-temperature coagulation bath to prepare a sponge-like PVDF microporous membrane, wherein:

[0045] Step 1: Weigh 75 parts of TEP and 5 parts of PEG600 and stir to form a solution. Then add 20 parts of PVDF and stir to obtain a homogeneous casting solution.

[0046] Step 2: The obtained casting solution is allowed to stand and degas for 24 hours;

[0047] Step 3: The casting liquid is scraped onto the support plate with a scraper to a thickness of 500 μm. The air temperature in the scraping chamber is 20°C and the relative humidity is 80%.

[0048] Step 4: Immerse the membrane in a primary coagulation bath containing 80% ethanol at 10°C for 10 minutes for preliminary curing. Remove the membrane and place it in a secondary coagulation bath containing 50% ethanol at 0°C for 10 minutes. Finally, place it in a deionized water coagulation bath for complete curing. Rinse with 20°C deionized water to remove any remaining solvent and precipitates. After drying, collect the membrane.

[0049] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements to the present invention based on the present invention are intended to fall within the scope of protection of the present invention.

[0050] Sample Thickness / μm Pore ​​diameter / μm Porosity / % Example 1 125(±3) 0.2 68 Example 2 120(±2) 0.22 72 Example 3 140(±3) 0.13 62 Example 4 125(±3) 0.2 69

[0051] As can be seen from the table, the PVDF microporous membrane has a thickness, pore size, and porosity suitable for high flow rate scenarios. This manufacturing process achieves the sponge-like pore structure of the PVDF microporous membrane, ensuring sufficient filtration and sufficient impurity retention, while also greatly improving water flux and high filtration efficiency.

[0052] 1. Bubble point test

[0053] Testing the alcohol initial bubble point and alcohol uniform bubble point of microporous filter membrane

[0054] The bubble point is related to the pore size. By testing the bubble point, we can calculate the pore size. The industry usually uses the bubble point to represent the pore size. The sample results are as follows:

[0055] Sample Initial bubble point / MPa Uniform bubble point / MPa Ratio of initial bubble point to average bubble point Example 1 0.1658 0.1825 0.908 Example 2 0.1357 0.1534 0.884 Example 3 0.2415 0.2811 0.859 Example 4 0.1574 0.1764 0.892

[0056] As can be seen from the above table, the filter membrane prepared by the present invention has a suitable bubble point, thereby ensuring that the filter membrane is suitable for use in scenarios with relatively high flow rates in the biopharmaceutical filtration industry.

[0057] 2. Water flux test

[0058] The water flux test simulates the flux of a real fluid environment and can provide more direct feedback on the performance of the membrane flux. The water flux results of the sample are as follows:

[0059]

[0060]

[0061] It can be seen more intuitively from the above table that the PVDF microporous filter membrane prepared in the present invention has a large flow rate and is suitable for use in high flow rate scenarios.

[0062] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements to the present invention based on the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath, characterized in that: The following steps are involved: (1) Add the raw material PVDF, solvent, and non-solvent additive into a container in sequence, heat and stir to dissolve, and form a membrane solution; (2) scraping the membrane liquid onto the support plate with a scraper to form a membrane; (3) Place the support plate in a multi-stage low-temperature coagulation bath until it is fully formed, rinse, and dry.

2. The method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath according to claim 1, characterized in that: The solvent used in preparing the solution in step (1) is one or more of TEG, NMP, DMAc, and DMF, with a mass fraction of 70 wt% to 80 wt%.

3. The method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath according to claim 1, wherein: The mass fraction of PVDF in step (1) is 18wt% to 25wt%.

4. The method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath according to claim 1, wherein: The mass fraction of the additive in step (1) is 2wt% to 5wt%, and the types of additives include PEG200, PEG400, PEG800, triethylene glycol, etc.

5. The method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath according to claim 1, wherein: The size of the scraper in step (2) is 400-600 μm.

6. The method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath according to claim 1, wherein: In step (2), the air humidity is 70-80%.

7. The method for preparing a PVDF microporous membrane using a multi-stage low-temperature coagulation bath according to claim 1, wherein: The coagulation bath in step (3) is a multi-stage coagulation bath, the water content of the first-stage coagulation bath is below 20%, and the water content of the second-stage coagulation bath is below 50%.

8. The method for preparing a network polyvinylidene fluoride film according to claim 1, wherein: The coagulation bath in step (3) is a low-temperature coagulation bath with a temperature of 0-20° C. The temperature of the secondary coagulation bath is lower than that of the primary coagulation bath.

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