Preparation method of high-flux ultrafiltration membrane

Abstract

The invention relates to a preparation method of a high-flux ultrafiltration membrane, which is characterized by comprising the following steps of: scraping or spinning a polymer solution, standing for a certain time at low temperature in a solvent atmosphere, and immersing into a non-solvent of a polymer to obtain the ultrafiltration membrane, or, the polymer solution is subjected to membrane scraping or spinning and then placed in high-concentration non-solvent steam to stand for a certain time to obtain the ultrafiltration membrane, and the ultrafiltration membrane can also be obtained after the polymer solution is immersed in the non-solvent of the polymer, taken out and dried. The obtained membrane is very high in flux, belongs to an ultrafiltration membrane, and can meet the technical index requirements of the ultrafiltration membrane.

Description

Technical Field

[0001] The invention relates to the technical field of membrane materials, and in particular to a method for preparing a high-flux ultrafiltration membrane. Background Art

[0002] Polymer ultrafiltration membranes are mainly prepared by phase inversion method, which changes the thermodynamic state of polymer solution to make the solution phase separate and transform from homogeneous phase to gel structure. Non-solvent induced phase separation is also called immersion precipitation method, which is to immerse the wet membrane into a coagulation bath to solidify the polymer from the homogeneous solution into a membrane. High-flux membranes can improve water production efficiency and are of great significance in the fields of microfiltration and ultrafiltration. It has been reported in the literature that the flux of anti-pollution GO-TiO2 / PES ultrafiltration membrane is 194.5L m -2 h -1 (Journal of Applied Polymer Science, 2021, 138, 39). The flux of the high-flux and high-rejection TiO2 nanofiber ultrafiltration membrane with porous titanium as the carrier is 940Lm -2 h -1 (Separation and Purification Technology, 2020, 248). However, the flux of these ultrafiltration membranes is still not very high. Summary of the invention

[0003] In order to further improve the ultrafiltration membrane flux, the present invention provides a method for preparing a high-flux ultrafiltration membrane.

[0004] The technical solution of the present invention is: a method for preparing a high-flux ultrafiltration membrane, characterized in that:

[0005] (1) After the polymer solution is scraped or spun, it is allowed to stand at low temperature for a certain period of time in a solvent atmosphere, and then immersed in a non-solvent for the polymer to obtain an ultrafiltration membrane;

[0006] (2) Alternatively, the polymer solution may be scraped or spun and then placed in a high concentration of non-solvent vapor for a certain period of time to directly obtain an ultrafiltration membrane. Alternatively, the polymer solution may be immersed in a non-solvent for the polymer and then taken out and dried to obtain an ultrafiltration membrane.

[0007] In the above characteristics, the polymer is a crystalline or semi-crystalline polymer, preferably polyvinylidene fluoride, polyether sulfone, polyacrylonitrile, cellulose acetate. The low temperature refers to a temperature lower than 10°C, but equal to or higher than the freezing point of the solvent. The solvent atmosphere refers to a temperature at which the solvent in the gas phase approaches or reaches the saturated vapor pressure of the solvent at that temperature, preferably 80%-100% of the saturated vapor pressure of the solvent. The low temperature is left to stand for a certain period of time, preferably 0.25 hours to 2.5 hours.

[0008] The non-solvent for the polymer cannot dissolve the polymer, but is miscible with the solvent in the polymer solution.

[0009] The non-solvent is one or a mixture of several solvents.

[0010] The high concentration of non-solvent vapor has a concentration of 30%-100% of the saturated vapor pressure of the non-solvent, preferably 50%-100%. The non-solvent vapor is allowed to stand for a certain period of time.

[0011] Preferably, it is 0.25 hours to 3 hours.

[0012] The invention has the following advantages: the obtained membrane has high flux, belongs to an ultrafiltration membrane, and can meet the technical index requirements of the ultrafiltration membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a surface SEM photo of the PVDF ultrafiltration membrane obtained under -15°C static conditions in Example 1 of the present invention.

[0014] Figure 2 This is a surface SEM photo of the PVDF ultrafiltration membrane obtained under 20°C static conditions in Example 1 of the present invention.

[0015] Figure 3 The porosity of the PVDF ultrafiltration membrane obtained under different standing temperature conditions in Example 1 of the present invention.

[0016] Figure 4 The water flux of the PVDF ultrafiltration membrane obtained under different standing temperature conditions in Example 1 of the present invention.

[0017] Figure 5 It is the retention rate of the PVDF ultrafiltration membrane obtained under different standing temperature conditions in Example 1 of the present invention.

[0018] Figure 6 This is a surface SEM photo of the PVDF ultrafiltration membrane obtained under a relative humidity of 60% in Example 2 of the present invention.

[0019] Figure 7 This is a surface SEM photo of the PVDF ultrafiltration membrane obtained under a relative humidity of 80% in Example 2 of the present invention.

[0020] Figure 8 The porosity of the PVDF ultrafiltration membrane obtained under different relative humidity conditions in Example 2 of the present invention.

[0021] Fig. 9 This is the water flux of the PVDF ultrafiltration membrane obtained under different relative humidity conditions in Example 2 of the present invention.

[0022] Fig.10It is the retention rate of the PVDF ultrafiltration membrane obtained under different relative humidity conditions in Example 2 of the present invention.

[0023] Fig.11 The water flux of the PVDF ultrafiltration membrane obtained under different concentration conditions in Example 3 of the present invention.

[0024] Fig.12 The retention rate of the PVDF ultrafiltration membrane obtained under different concentration conditions in Example 3 of the present invention. DETAILED DESCRIPTION

[0025] Several specific embodiments of the present invention are described in detail below. The described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Example 1

[0027] Weigh a certain amount of N,N-dimethylformamide (DMF) in a beaker, add a certain amount of dried polyvinylidene fluoride (PVDF) powder, stir at 80℃ for 1h to completely dissolve PVDF, and obtain a 20% PVDF solution, and let it stand for about 4h for degassing. Pour the casting solution on a glass plate with non-woven fabric, scrape the surface of the membrane with a 500μm scraper, put it in a closed DMF atmosphere at -15℃, 2℃, and 20℃, let it stand for 2h, and then immerse it in deionized water for 6h to perform phase inversion to obtain a membrane.

[0028] The SEM images show that the PVDF membrane obtained by immersion precipitation after standing at -15℃ for 2h has a loose surface and spherulite stacking pores ( Figure 1 ). Since the solvent volatilization can be ignored when it is placed in DMF atmosphere, the solubility of PVDF decreases at low temperature, and the polymer nucleates and grows to form a spherulite stacking structure. As the standing temperature increases, the solubility of PVDF in DMF increases, the nucleation rate slows down, and the phase transformation after immersion precipitation occurs, the surface tends to be flat, and the pore size becomes smaller ( Figure 2 , 20℃ for 2h to obtain the membrane surface). In addition, with the increase of the static temperature, the porosity decreased from 63% to 47% ( Figure 3 ), the flux also decreases ( Figure 4 ), the water flux of the membrane obtained at -15℃ was 4765L h -1 m -2 bar -1 , which is 5 times the membrane flux obtained by standing at 20℃.

[0029] like Figure 5As shown in the figure, the retention rate of the membrane obtained at -15℃ for bovine serum albumin is 41.56%, and the retention rate of the membrane obtained at 20℃ for bovine serum albumin is 66.95%. With the increase of the standing temperature, the retention rate of the membrane for 60,000 molecular weight dextran increases from 45% to 65%, the retention rate for 100,000 molecular weight dextran increases from 91.44% to 95.38%, and the retention rate for 200,000 molecular weight dextran increases from 96.62% to 96.37%. According to the standard of 90% retention rate, the molecular weight cutoffs of the membranes obtained at -15℃, 2℃ and 20℃ are 99,500, 95,700 and 90,000 respectively, all of which belong to high-flux ultrafiltration membranes.

[0030] Example 2

[0031] A 20% PVDF DMF solution was prepared and coated into a wet film. The film was placed in a constant temperature and humidity chamber at 25°C and relative humidity of 60%, 70%, 80%, and 90% for phase transfer for 2 hours to obtain a PVDF membrane. When the ambient humidity is low, the phase conversion rate of the PVDF casting solution is slow, and PVDF forms a spherical stacking structure ( Figure 6 , the membrane surface obtained when the relative humidity is 60%); as the humidity increases, the film formation speed is accelerated, the structure coarsening time in the polymer solution is reduced, the spherical particles on the membrane surface are reduced, and the surface density is increased ( Figure 7 , the membrane surface obtained when the relative humidity is 80%).

[0032] As the relative humidity (60%-90%) increases, the porosity of the resulting membrane gradually decreases (76%-65%, Figure 8 ), the flux also decreases ( Fig. 9 ), when the relative humidity was 60%, the membrane water flux reached 39112 L h -1 m -2 bar -1 , the water flux is 23596 L h at relative humidity of 90% -1 m -2 bar -1 In addition, the retention rate of the membrane for standard substances is improved ( Fig.10 ). Based on a 90% retention rate, the molecular weight cutoffs of the membranes obtained at relative humidity of 60%, 70%, 80%, and 90% were 195,000, 192,600, 170,400, and 157,800, respectively, all of which belong to ultrafiltration membranes, but the flux is significantly higher than that of ultrafiltration membranes with the same molecular weight cutoff.

[0033] Example 3

[0034] PVDF casting solutions with concentrations of 10%, 15%, 20%, and 25% were prepared by scraping and coating into wet films, and placed in a constant temperature and humidity chamber at 25°C and 60% relative humidity for phase inversion for 2 hours to prepare PVDF membranes. As the concentration of the casting solution increased, the pure water flux of the PVDF membrane showed a decreasing trend ( Fig.11 ), when the concentration of the casting solution was 10%, the water flux of the membrane reached 50065 L h -1 m -2 bar -1 When the concentration of the casting solution is 25%, the water flux of the membrane reaches 26570 L h -1 m -2 bar -1 With the increase of casting solution concentration, the retention rate of PVDF membrane for BSA showed an increasing trend ( Fig.12 ), when the concentration of the casting solution is 10%, the retention rate of the membrane for bovine serum albumin is 17.46%, and when the concentration is 15%, the retention rate is 28.09%. According to the standard of 90% retention rate, when the PVDF casting solution is 10%, 15%, 20%, and 25%, the retention molecular weight of the membrane is 209,200, 195,000, 183,100, and 180,200 respectively, which is an ultrafiltration membrane.

[0035] The above description of the disclosed embodiments enables professionals and technicians in the field to implement or use the present invention. Various modifications to these embodiments will be obvious to professionals and technicians in the field, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. The above description is only an individual embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a high-flux ultrafiltration membrane, characterized in that: (1) After the polymer solution is scraped or spun, it is allowed to stand at low temperature for a certain period of time in a solvent atmosphere, and then immersed in a non-solvent for the polymer to obtain an ultrafiltration membrane; (2) Alternatively, the polymer solution may be scraped or spun, and then placed in a high concentration of non-solvent vapor for a certain period of time to obtain an ultrafiltration membrane. Alternatively, the polymer solution may be immersed in a non-solvent for the polymer and then taken out and dried to obtain an ultrafiltration membrane.

2. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The polymer is a crystalline or semi-crystalline macromolecule, preferably polyvinylidene fluoride, polyether sulfone, polyacrylonitrile, or cellulose acetate.

3. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The low temperature is lower than 10°C but equal to or higher than the freezing point of the solvent.

4. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The low temperature standing time is preferably 0.25 hours to 2.5 hours.

5. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The solvent atmosphere refers to a situation where the partial pressure of the solvent in the gas phase is close to or reaches the saturated vapor pressure of the solvent at that temperature, preferably reaching 80%-100% of the saturated vapor pressure of the solvent.

6. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The non-solvent for the polymer cannot dissolve the polymer, but is miscible with the solvent in the polymer solution, and the non-solvent is a mixture of one or more solvents.

7. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The high-concentration non-solvent vapor has a gas phase concentration of 30%-100% of the saturated vapor pressure of the non-solvent, preferably 50%-100%.

8. The method for preparing a high-flux ultrafiltration membrane according to claim 1, characterized in that: The non-solvent vapor is allowed to stand for a certain period of time, preferably 0.25 hours to 3 hours.

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