Polyvinylidene fluoride filter membrane and method for producing the same

Abstract

This invention relates to the field of membrane materials and discloses a polyvinylidene fluoride (PVDF) filter membrane and its preparation method. The method for preparing the PVDF filter membrane includes coating a casting solution onto a substrate surface, followed by pre-evaporation, film formation, and curing. The casting solution contains PVDF, polyamide, and a pore-forming agent in a mass ratio of 100:(5-15):(10-25). This invention also discloses the PVDF filter membrane obtained by the above method. Through the above technical solution, this invention obtains a PVDF filter membrane with superior hydrophilicity, retention rate, thermal stability, and chemical stability.

Description

Technical Field

[0001] This invention relates to the field of membrane materials, and more specifically, to polyvinylidene fluoride filter membranes and their preparation methods. Background Technology

[0002] Membrane separation technology is green and efficient, and is widely used in water purification, chemical separation, and biopharmaceutical fields. Ultrafiltration membranes, in particular, can effectively remove biomolecules, polymers, and colloidal substances, and offer advantages such as high packing density, low energy consumption, and ease of operation. Therefore, ultrafiltration membranes are widely used in wastewater treatment, chemical, biological, and pharmaceutical industries.

[0003] Polyvinylidene fluoride (PVDF) ultrafiltration membranes have attracted much attention due to their excellent thermal stability, chemical stability, thermal radiation, and superior mechanical properties. However, the abundant CF and CH bonds on the surface of PVDF ultrafiltration membranes result in poor hydrophilicity. Water molecules have difficulty adhering to the PVDF membrane surface, leading to low water flux. On the other hand, organic matter readily adsorbs onto the PVDF membrane surface, reducing membrane lifespan. Generally, the hydrophilicity of the membrane can be improved through physical and chemical methods. Modification methods are mainly divided into membrane surface modification and membrane material modification.

[0004] Membrane surface modification involves introducing polar groups and hydrophilic molecules into the surface and pores of the prepared PVDF membrane. Modification methods include surface coating, chemical modification, and plasma modification. Membrane material modification includes chemical modification and blending modification. Among these, blending modification is simple to operate, and the hydrophilic groups are not easily detached, making it easy to promote on a large scale. CN101703897A describes blending polyethersulfone with PVDF to improve the membrane's anti-shrinkage properties and hydrophilicity. CN102716677A describes blending cellulose acetate and polymethyl methacrylate to improve the membrane's hydrophilicity and toughness. CN101905123A describes blending hydrophilic nanoparticles with PVDF to improve the membrane's hydrophilicity. However, ultrafiltration membranes obtained by the above methods are difficult to simultaneously meet the requirements of hydrophilicity, retention rate, thermal stability, and chemical stability, thus limiting their application. Therefore, it is of great significance to improve the retention rate, thermal stability, and chemical stability of the membrane while improving its hydrophilicity. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem that existing technologies cannot simultaneously ensure hydrophilicity, retention rate, thermal stability and chemical stability, and to provide a PVDF filter membrane and its preparation method.

[0006] To achieve the above objectives, the present invention provides a method for preparing a PVDF filter membrane, the method comprising coating a casting solution onto the surface of a substrate, followed by pre-evaporation, film formation, and curing, wherein the casting solution contains polyvinylidene fluoride, polyamide, and a pore-forming agent in a mass ratio of 100:(5-15):(10-25).

[0007] The present invention also provides a polyvinylidene fluoride filter membrane prepared by the above method.

[0008] Through the above technical solution, the present invention obtains a polyvinylidene fluoride filter membrane with superior hydrophilicity, retention rate, thermal stability and chemical stability, even when the casting solution does not contain any surfactants. Detailed Implementation

[0009] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0010] The method for preparing polyvinylidene fluoride filter membrane provided by the present invention includes coating a casting solution onto the surface of a substrate, followed by pre-evaporation, film formation, and curing. The casting solution contains polyvinylidene fluoride, polyamide, and a pore-forming agent in a mass ratio of 100:(5-15):(10-25).

[0011] In this invention, the preferred mass ratio between polyvinylidene fluoride and polyamide is 100:(8-12).

[0012] In this invention, the preferred mass ratio of polyvinylidene fluoride to porogen is 100:(22-25).

[0013] In this invention, the polyvinylidene fluoride can be the polyvinylidene fluoride commonly used in the art for preparing filter membranes. Preferably, the number average molecular weight of the polyvinylidene fluoride is 400,000 to 800,000.

[0014] In this invention, the polyamide can be a common polymer in the art, containing amide groups in the repeating units of the macromolecular backbone. Preferably, the polyamide is at least one of aliphatic polyamide and aromatic polyamide, and more preferably, poly(m-phenylene isophthalamide). The weight-average molecular weight of poly(m-phenylene isophthalamide) can be 1.2-1.8 million.

[0015] In this invention, the porogen can be a substance commonly used in the art that helps the membrane form a uniform pore structure during phase separation. Preferably, the porogen is polyvinylpyrrolidone (PVP) and / or polyethylene glycol. The polyethylene glycol is preferably polyethylene glycol 200 (PEG-200) and / or polyethylene glycol 400 (PEG-400).

[0016] In this invention, there are no particular requirements for the concentration of polyvinylidene fluoride in the casting solution. Preferably, the mass ratio of solvent to polyvinylidene fluoride in the casting solution is (250-350):100, more preferably (280-320):100. Even more preferably, the solvent in the casting solution is at least one of dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP).

[0017] This invention enables the production of high-performance filter membranes without the presence of any surfactants in the casting solution. Therefore, the casting solution is free of surfactants (including various common carboxylates, sulfates, quaternary ammonium salts, and polyethylene oxide derivatives).

[0018] In this invention, the casting solution can be obtained through various conventional methods. However, the inventors have discovered that dissolving polyvinylidene fluoride in a solvent first, followed by the addition of polyamide and a pore-forming agent, can result in a filter membrane with better performance. Therefore, preferably, the casting solution is prepared by the following method:

[0019] At 50-80°C, polyvinylidene fluoride (PVDF) is dissolved in a solvent, and the resulting solution is then mixed with polyamide and a porogen. There is no particular requirement for the mixing order of the polyamide and porogen; the solution can be mixed with the polyamide first, or with the porogen first, or the solution can be mixed with both the polyamide and the porogen simultaneously. According to a specific embodiment, the method for preparing the casting solution includes: dissolving PVDF in a solvent at 50-80°C and stirring for 1-5 hours until completely dissolved to obtain blend A; adding polyamide to blend A and stirring for 1-5 hours to obtain blend B; adding a porogen to blend B at 50-80°C and stirring for 1-3 hours to obtain the casting solution.

[0020] In this invention, the casting solution needs to be degassed before coating to remove air bubbles, thereby facilitating uniform coating. Degasting can be performed using conventional methods, preferably including: allowing the casting solution to stand at 20-40°C for 12-24 hours and / or evacuating it under vacuum for 3-5 hours.

[0021] In this invention, coating can be performed using conventional methods, such as by using a doctor blade or a coating rod. The thickness of the casting solution coating is preferably 100-500 μm, more preferably 150-300 μm.

[0022] In this invention, a skin structure is formed during pre-evaporation due to solvent evaporation. Preferred pre-evaporation conditions include a temperature of 20-35°C, more preferably 25-30°C. Further preferred pre-evaporation conditions include a relative humidity of 50-80%, more preferably 60-70%. Even more preferably, pre-evaporation conditions include a time of 20 seconds to 5 minutes, more preferably 1-2 minutes.

[0023] In this invention, the film formation can be carried out under conventional conditions, such as by water bath.

[0024] Preferably, the conditions for the water bath (film formation) include a temperature of 20-40°C. More preferably, the conditions for the water bath (film formation) also include a time of 3-10 minutes.

[0025] In this invention, the curing can be carried out under conventional conditions, such as by using a glycerol bath.

[0026] Preferably, the conditions for the glycerol bath (curing) include a temperature of 20-40°C. More preferably, the conditions for the glycerol bath (curing) also include a time of 5-20 hours. The glycerol bath (curing) can be performed using an aqueous glycerol solution with a glycerol mass content of 20-40%.

[0027] The present invention also provides a polyvinylidene fluoride filter membrane prepared by the method described above.

[0028] According to the present invention, the polyvinylidene fluoride filter membrane has an average pore size of 0.01-0.04 μm and a pure water flux of 510 L·m³. -2 ·h -1 (e.g., 440-510 L·m) -2 ·h -1 Therefore, it can be used as an ultrafiltration membrane.

[0029] The present invention will be described in detail below through examples. In the following examples, PVDF is a commercially available product of Solvay AG, Belgium, with brand name 6020 and a number average molecular weight of 800,000; PA is an industrial product of poly(m-phenylene isophthalamide) provided by Saint-Ou Aramid Co., Ltd., with a weight average molecular weight of 1.4 million; PVP (K30), PEG-200 and PEG-400 are commercially available products of Sinopharm Chemical Reagent Co., Ltd.

[0030] Example 1

[0031] (1) Control the mass ratio of PVDF, PA, DMAc and PVP to 20:2:60:5. Stir at 70℃ for 2 hours to completely dissolve PVDF in DMAc to obtain a homogeneous solution A. Add PA to solution A and continue stirring at 70℃ for 2 hours to obtain a homogeneous solution B.

[0032] (2) PVP was added to solution B and stirred at 70°C for 2 hours to obtain the casting solution. The casting solution was then allowed to stand in a 30°C constant temperature oven for 20 hours to remove bubbles.

[0033] (3) Use a scraper to evenly coat the casting solution onto a clean glass plate. Place the glass plate coated with the casting solution in a constant temperature and humidity chamber at 30℃ and 60% relative humidity for 1 min for pre-evaporation. Then quickly immerse it in a constant temperature water bath at 30℃ for 5 min; then soak it in a 20wt% glycerol aqueous solution (30℃) for 10 h, and then take it out and let it air dry to obtain the PVDF ultrafiltration membrane.

[0034] Example 2

[0035] (1) The mass ratio of PVDF, PA, DMAc and PEG-200 was controlled to be 20:1:70:4. The PVDF was completely dissolved in DMAc and a homogeneous solution A was obtained by stirring at 50°C for 5 hours. PA was added to solution A and the mixture was stirred at 50°C for another 5 hours to obtain a homogeneous solution B.

[0036] (2) Add PEG-200 to solution B and stir at 50°C for 3 hours to obtain casting solution. Let the casting solution stand in a 40°C constant temperature oven for 12 hours to remove bubbles.

[0037] (3) Use a scraper to evenly coat the casting solution onto a clean glass plate. Place the glass plate coated with the casting solution in a constant temperature and humidity chamber at 35℃ and 50% relative humidity for 20s for pre-evaporation. Then quickly immerse it in a constant temperature water bath at 20℃ for 10min; then soak it in a 40wt% glycerol aqueous solution (20℃) for 20h. After that, take it out and let it air dry to obtain the PVDF ultrafiltration membrane.

[0038] Example 3

[0039] (1) The mass ratio of PVDF, PA, DMAc and PEG-400 was controlled to be 20:3:50:2. The PVDF was completely dissolved in DMF and a homogeneous solution A was obtained by stirring at 80°C for 1 hour. PA was added to solution A and the mixture was stirred at 80°C for another hour to obtain a homogeneous solution B.

[0040] (2) Add PEG-400 to solution B and stir at 80°C for 2 hours to obtain casting solution. Let the casting solution stand in a constant temperature oven at 20°C for 24 hours to remove bubbles.

[0041] (3) Use a scraper to evenly coat the casting solution onto a clean glass plate. Place the glass plate coated with the casting solution in a constant temperature and humidity chamber at 20℃ and 80% relative humidity for 5 minutes for pre-evaporation. Then quickly immerse it in a constant temperature water bath at 40℃ for 3 minutes; then soak it in a 30wt% glycerol aqueous solution (40℃) for 5 hours. After that, take it out and let it air dry to obtain the PVDF ultrafiltration membrane.

[0042] Example 4

[0043] The filter membrane was prepared according to the method of Example 2, except that in step (1), PVDF and PA were simultaneously added to the solvent DMAc for dissolution. Specifically:

[0044] At 50°C, PVDF and PA were simultaneously added to the solvent DMAc for dissolution, and the mixture was stirred for 13 hours to obtain a homogeneous solution.

[0045] Example 5

[0046] The filter membrane was prepared according to the method of Example 2, except that the pre-evaporation temperature was controlled at 15°C and the time was 15 seconds.

[0047] Example 6

[0048] The filter membrane was prepared according to the method in Example 2, except that the water bath time was 30 min and the glycerol soaking time was 4 h.

[0049] Comparative Example 1

[0050] (1) Control the mass ratio of PVDF, DMAc and PVP to 4:12:1. Stir at 70°C for 4 hours to completely dissolve PVDF and PVP in DMAc to obtain the film-forming solution.

[0051] (2) Let the casting solution stand in a 30℃ constant temperature box for 20 hours to remove bubbles.

[0052] (3) Use a scraper to evenly coat the casting solution onto a clean glass plate. Place the glass plate coated with the casting solution in a constant temperature and humidity chamber at 30℃ and 60% relative humidity for 1 min for pre-evaporation. Then quickly immerse it in a constant temperature water bath at 30℃ for 5 min; then soak it in a 20wt% glycerol aqueous solution (30℃) for 10 h, and then take it out and let it air dry to obtain the PVDF ultrafiltration membrane.

[0053] Comparative Example 2

[0054] The filter membrane was prepared according to the method of Example 2, except that the mass ratio of PVDF to PA was 7:5.

[0055] Comparative Example 3

[0056] The filter membrane was prepared according to the method of Example 2, except that PA was replaced with polyimide (synthesized from pyromellitic tetracarboxylate chloride and m-phenylenediamine, with a weight-average molecular weight of 1.4 million).

[0057] Comparative Example 4

[0058] The filter membrane was prepared according to the method of Example 2, except that PA was replaced with polyethersulfone (a commercially available product from Solvay, brand name A-100).

[0059] Test Example 1

[0060] The performance of the filter membranes prepared in the above embodiments and comparative examples was tested according to the following methods:

[0061] The average pore size of the membrane was measured using a 3H-2000PBL multifunctional filter membrane pore size analyzer.

[0062] The pure water flux and rejection rate of the membrane were measured according to the method described in HY / T050-1999. The prepared filter membrane was soaked in pure water at 80℃ for 3 months and then dried. The pure water flux and rejection rate were tested again to examine the thermal stability of the filter membrane.

[0063] The test results are shown in Table 1.

[0064] Table 1 Membrane performance test results

[0065]

[0066] As can be seen from the results in Table 1, the filter membrane prepared by the method of the present invention exhibits excellent hydrophilicity, retention rate, and thermal stability. While the filter membrane prepared in Comparative Example 3 also possesses relatively excellent hydrophilicity, retention rate, and thermal stability, the polyimide is easily hydrolyzed under alkaline conditions, limiting its application range and the availability of backwashing reagents during use. In particular, a comparison of Example 2 with Examples 4-6 shows that preparing the casting solution and performing pre-evaporation or curing using the preferred embodiments can further improve the performance of the resulting filter membrane.

[0067] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a polyvinylidene fluoride (PVDF) filter membrane, the method comprising coating a degassed casting solution onto a substrate surface, followed by pre-evaporation, film formation, and curing, characterized in that, The casting solution contains polyvinylidene fluoride, polyamide, and a pore-forming agent in a mass ratio of 100:(5-15):(10-25). The casting solution is prepared by the following method: At 50-80℃, polyvinylidene fluoride is dissolved in a solvent, and then the resulting solution is mixed with polyamide and a porogen. The pre-evaporation conditions include: a temperature of 20-35℃, a relative humidity of 50-80%, and a time of 20s-5min; The film formation is carried out in a water bath, and the film formation conditions include: temperature of 20-40℃ and time of 3-10min.

2. The method according to claim 1, wherein, The mass ratio of polyvinylidene fluoride, polyamide and porogen is 100:(8-12):(22-25).

3. The method according to claim 1 or 2, wherein, The number-average molecular weight of the polyvinylidene fluoride is 400,000 to 800,000. And / or, the polyamide is an aliphatic polyamide and / or an aromatic polyamide; And / or, the pore-forming agent is polyvinylpyrrolidone and / or polyethylene glycol.

4. The method according to claim 3, wherein, The polyamide is poly(m-phenylene isophthalamide); And / or, the polyethylene glycol is polyethylene glycol 200 and / or polyethylene glycol 400.

5. The method according to claim 1, wherein, The mass ratio of polyvinylidene fluoride to solvent in the casting solution is 100:(250-350); And / or, the solvent in the casting solution is at least one of dimethylformamide, dimethylacetamide, and N-methylpyrrolidone.

6. The method according to claim 1, wherein, The casting solution is defoamed using the following method: Let the casting solution stand at 20-40℃ for 12-24 hours and / or evacuate under vacuum for 3-5 hours.

7. The method according to claim 1, wherein, The curing is carried out in a glycerol bath with a glycerol content of 20-40 wt%, and the curing conditions include a temperature of 20-40℃ and a time of 5-20h.

8. A polyvinylidene fluoride filter membrane prepared by the method according to any one of claims 1-7.

9. The polyvinylidene fluoride filter membrane according to claim 8, wherein, The average pore size of the polyvinylidene fluoride filter membrane is 0.01-0.04 μm.

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