A high-precision film preparation method

Abstract

The application discloses a high-precision membrane preparation method, characterized in that a commonly used phase inversion method polyacrylonitrile ultrafiltration membrane on the market is soaked in a solution, and then wet annealing treatment is performed on the membrane, so that the shrinkage behavior and pore structure of the separation skin layer of the membrane can be regulated and controlled, the filtration precision of the membrane is significantly improved to the nanofiltration level, the pore structure of the finger-shaped pore support layer of the membrane is still maintained, and high permeation flux is ensured. The high-precision polyacrylonitrile nanofiltration membrane prepared by the method is used for treating printing and dyeing wastewater, has excellent separation performance and long-term stability, has low annealing temperature and low cost, can be prepared on the basis of a mature commercial polyacrylonitrile ultrafiltration membrane without introducing other materials for compounding, and has a large-scale popularization potential.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a high-precision polyacrylonitrile nanofiltration membrane, which is mainly used for treating printing and dyeing wastewater and belongs to the field of membrane separation technology. BACKGROUND

[0002] Printing and dyeing wastewater is an important type of wastewater in China, and has the characteristics of large treatment capacity, high pollutant content, strong adsorption and difficult separation. Membrane separation technology is an effective means for treating printing and dyeing wastewater, and has the advantages of high separation efficiency, low separation energy consumption and modular use. The nanofiltration membrane is an important component for treating printing and dyeing wastewater. Compared with the traditional reverse osmosis membrane, the nanofiltration membrane has lower operating pressure and larger permeation flux, and thus has great market prospects. The pore size of the nanofiltration membrane is generally in the nanometer range, and the nanofiltration membrane can effectively intercept small molecular dyes with a molecular weight of 400-2000 daltons. Although the nanofiltration membrane has many excellent properties, there are still problems in the preparation process, cost and stability of the nanofiltration membrane, and there is no nanofiltration membrane widely recognized by the market.

[0003] The polyacrylonitrile polymer has good solvent resistance, hydrophilicity and low cost, and is widely used for preparing separation membranes. The polyacrylonitrile is usually prepared into a separation membrane by using a phase inversion method. By changing the polymer concentration in the casting solution, the type of solvent and the conditions of the coagulation bath and other parameters, researchers can control the pore size distribution and porosity of the polyacrylonitrile filter membrane. However, due to the limited types of solvents in which polyacrylonitrile can be dissolved and the limited solubility, it is very difficult to precisely adjust the pore size of the polyacrylonitrile filter membrane to the nanofiltration level.

[0004] At present, researchers have developed some methods to reduce the pore size of the polyacrylonitrile filter membrane and improve the filtration precision. For example, Chinese patent CN111773932A discloses a preparation method of a nanofiltration membrane based on polyacrylonitrile, which uses hydrophilic particles to partially fill the pores of the polyacrylonitrile ultrafiltration membrane to reduce the pore size; Chinese patent CN111821861B discloses a preparation method of a nanofiltration membrane based on polyacrylonitrile, which uses a polyacrylonitrile ultrafiltration membrane as a substrate and uses interfacial polymerization to prepare a star-shaped compound separation layer on the substrate to achieve nanofiltration precision. It can be seen that the current methods for preparing nanofiltration membranes based on polyacrylonitrile often need to introduce composite materials to improve the separation precision, which not only increases the cost but also brings compatibility problems. If a preparation method can be developed without adding composite materials and only based on the polyacrylonitrile material itself to achieve nanofiltration separation precision, the process complexity and membrane preparation cost can be greatly reduced, which is conducive to industrialization.

[0005] Chinese patent CN112915807B discloses a modification method based on polyacrylonitrile ultrafiltration membrane, heat treatment combined with alkali treatment to improve its resistance to polar organic solvent ability. The patent uses higher temperature (> 200℃) and additional alkali treatment, the purpose is to make the membrane cyclization hydrolysis and crosslinking and other complex chemical reactions to increase the solvent resistance. And the prepared membrane still needs to form a composite separation layer by interfacial polymerization to be used for nanofiltration separation. It can be seen that how to realize the preparation of high-precision nanofiltration membrane based on polyacrylonitrile itself at lower temperature (< 120℃) is extremely challenging. SUMMARY

[0006] In order to overcome the deficiencies of the prior art, the present application provides the following technical route: the route is based on wet annealing treatment of solution infiltrated polyacrylonitrile ultrafiltration membrane, by adjusting the process parameters, the polyacrylonitrile molecular chain undergoes small molecule assisted thermoplastic behavior, and the skin layer pore structure of the original polyacrylonitrile filter membrane collapses; At this time, by further coupling the evaporation speed of the solution and the phase state of the additive, the collapse behavior of the skin layer pore structure can be controlled, and the degree of shrinkage can be precisely adjusted; then cooling down makes the polyacrylonitrile molecular chain relock, that is, the pore structure remains stable after washing with water, and finally the separation precision of the polyacrylonitrile filter membrane can be improved to the nanofiltration level.

[0007] It should be pointed out that the main research object of the present application is polyacrylonitrile ultrafiltration membrane prepared by phase inversion method. The membrane structure characteristics of polyacrylonitrile ultrafiltration membrane prepared by phase inversion method are "porous skin layer / separation layer" with pore size in the tens of nanometer level, and "finger-like pore layer / support layer" with pore size in the several micrometer level. This membrane structure feature can not only exert the high separation selectivity advantage of the porous skin layer, but also exert the high permeation flux advantage of the finger-like pore layer. Therefore, the porous polyacrylonitrile filter membrane prepared by phase inversion method is also the most widely used filter membrane type with the highest industry recognition.

[0008] The preparation method proposed by the present application mainly acts on the porous skin layer of the polyacrylonitrile ultrafiltration membrane to reduce its effective pore size and improve the filtration precision of the filter membrane. As for the finger-like pore layer, since its polyacrylonitrile internal interaction force is relatively strong, after the treatment by the method of the present application, its pore structure will not collapse obviously. Therefore, the preparation method of the present application can still maintain the asymmetric membrane structure of "small pore skin layer-large pore support layer" which is specific to the polyacrylonitrile filter membrane prepared by phase inversion method, so as to obtain its balanced permeation flux-separation selectivity advantage feature.

[0009] Further, the wet annealing process used in the preparation method proposed by the present application has a significant energy economy type, and the highest temperature does not exceed 120℃, which is much lower than the temperature used in other common heat treatment methods. The reason is that the preparation method proposed by the present application utilizes the thermoplastic process of polyacrylonitrile polymer molecular chain, and does not involve thermal cyclization or thermal crosslinking reaction, so it does not need to use higher temperature.

[0010] Further, the polyacrylonitrile nanofiltration membrane prepared by the present application is used for dye wastewater treatment application. Compared with the original polyacrylonitrile ultrafiltration membrane, the polyacrylonitrile nanofiltration membrane prepared by the present application has a nanofiltration level of filtration precision: the rejection rate of a plurality of dyes including but not limited to Congo red (CR), Coomassie brilliant blue G250 (CBB), Evans blue (EB), Alcian blue 8GX (AB-8GX) and the like is more than 97%, while maintaining a high permeation flux of 13.30-87.48 LMH bar -1 , and the performance can be continuously operated for more than 80h in cross-flow mode, which has excellent technical advancement.

[0011] The present application provides a preparation method of a high-precision polyacrylonitrile nanofiltration membrane, and the specific steps are as follows:

[0012] Step one: dissolve dry polyacrylonitrile powder in a solvent at 80-90℃, vacuum degassing for 10min, and prepare a casting solution. Preferably, the molecular weight of the polyacrylonitrile powder is 50-150kDa, and the polyacrylonitrile powder is vacuum dried; the solvent is one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methyl pyrrolidone (NMP).

[0013] Step two: cast the casting solution on a smooth glass plate with a doctor blade, the liquid layer thickness is 100-400μm, then immerse it in a water coagulation bath at room temperature, take it out after the filter membrane is formed, and wash it with water, to obtain a polyacrylonitrile ultrafiltration membrane. Preferably, the mass concentration of the casting solution is 15-21wt%.

[0014] Step three: immerse the filter membrane in an aqueous solution containing an additive, vacuum degassing for 10min, take it out and wipe off the liquid water on its surface, and put it into a glass container; put the glass container into an oven for annealing treatment, and the treatment atmosphere is one of air and nitrogen. Preferably, the additive is one of glycerol, xanthan gum, polyethylene glycol, and sodium carboxymethyl cellulose, and the additive content is 0.1-5wt%; the annealing treatment is preheating the oven to a specified temperature or heating to a specified temperature at a certain speed; the heat treatment temperature is 40-120℃; the heat treatment time is 1-24h; and then reduce to room temperature at a certain speed.

[0015] Step four: immerse the annealed filter membrane in a mixed solution of ethanol and water, the volume ratio of ethanol to water is 1:3-9, the immersion time is 12h, repeat 1 time, then wash it with deionized water for 3 times, immerse it in deionized water or a preservative solution for standby, to obtain a high-precision polyacrylonitrile nanofiltration membrane.

[0016] Further, the high precision polyacrylonitrile nanofiltration membrane is used for treating printing and dyeing wastewater. The specific test steps are as follows: the nanofiltration membrane is cleaned and cut to a suitable size, loaded into a cross-flow filtration membrane cell, the liquid circulation pipeline is connected, the flow rate is kept constant, the operating pressure is adjusted to 1.5 bar and kept for 30 min, then reduced to 1 bar, the filtrate is collected and the change of the mass of the filtrate with time and the composition of the filtrate are measured, the permeation flux of the membrane and the rejection rate of the dye are obtained. After the test is completed, the membrane is taken out and washed with deionized water, the membrane is repeatedly loaded into the membrane cell and tested to obtain the antifouling property of the membrane. Preferably, the composition of the dye wastewater is that the concentration of the dye molecules is 50-5000 ppm and the water contains one or more of the following dyes: anionic dyes: methyl orange (MO), acid orange G (OG), amino black 10B (AB-10B), Congo red (CR), coomassie brilliant blue G250 (CBB), evans blue (EB); cationic dyes: methylene blue (MB), rhodamine B (RB), tetrazolium blue (BT), alizarin blue 8GX (AB-8GX). Preferably, the method for testing the dye concentration of the dye aqueous solution is ultraviolet spectrophotometry. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 SEM surface morphology of polyacrylonitrile nanofiltration membranes under different annealing treatment temperatures. (a) is a scanning electron microscope image of the unmodified membrane in Comparative Example 1, (b) is a scanning electron microscope image of the modified membrane with an annealing treatment temperature of 40°C in Example 1, (c) is a scanning electron microscope image of the modified membrane with an annealing treatment temperature of 100°C in Example 2, and (d) is a scanning electron microscope image of the modified membrane with an annealing treatment temperature of 120°C in Example 3.

[0018] Figure 2 SEM cross-sectional morphology of polyacrylonitrile nanofiltration membranes under different annealing treatment temperatures. (a) is a scanning electron microscope image of the unmodified membrane in Comparative Example 1, (b) is a scanning electron microscope image of the modified membrane with an annealing treatment temperature of 40°C in Example 1, (c) is a scanning electron microscope image of the modified membrane with an annealing treatment temperature of 100°C in Example 2, and (d) is a scanning electron microscope image of the modified membrane with an annealing treatment temperature of 120°C in Example 3.

[0019] Figure 3 (a) N2adsorption-desorption isotherms of the annealed and modified polyacrylonitrile nanofiltration membranes in Example 1 and Example 2 at 77K, and (b) pore size distribution of the annealed and modified polyacrylonitrile nanofiltration membranes in Example 1 and Example 2.

[0020] Figure 4 Permeation flux and Congo red CR rejection rate of polyacrylonitrile nanofiltration membranes under different annealing treatment temperatures.

[0021] Figure 5Long-term performance of polyacrylonitrile nanofiltration membranes in Example 3 for separation of Congo red dye effluent.

[0022] Figure 6 Cycling performance of polyacrylonitrile nanofiltration membranes in Example 2 for separation of Congo red dye effluent.

[0023] Figure 7 Rejection performance of polyacrylonitrile nanofiltration membranes in Example 2 for different anionic and cationic dye wastewaters. Anionic dyes: methyl orange (MO), acid orange G (OG), amido black 10B (AB-10B), Congo red (CR), coomassie brilliant blue G250 (CBB), ethidium bromide (EB). Cationic dyes: methylene blue (MB), rhodamine B (RB), tetrazolium blue (BT), alizarine blue 8GX (AB-8GX). DETAILED DESCRIPTION

[0024] The application will be further described below in connection with the drawings, which are given by way of illustration. It should be noted that the examples described herein represent only some embodiments of the application and are not meant to be limiting. Other embodiments of the application, based on the examples described herein, obtained by others of ordinary skill in the art without exercising inventive faculty are protected by the application.

[0025] Example 1

[0026] A commercial polyacrylonitrile ultrafiltration membrane with a pore size of ~40 nm and a thickness of ~100 μm was selected. The pure water flux of the membrane was 200 LMH at 25 °C under 1.0 bar pressure. The membrane was cut into small pieces of 5 x 5 cm and soaked and washed with deionized water at room temperature to remove contaminants. The cleaned polyacrylonitrile ultrafiltration membrane was immersed in a 5 wt% glycerol aqueous solution, wiped to remove the surface liquid and placed in a glass container in a 40 °C air oven for 12 h. The temperature was then decreased to room temperature and the membrane was washed twice with 1:5 ethanol / water solution and once with deionized water to obtain the polyacrylonitrile modified membrane, which was designated as PAN-40.

[0027] Example 2

[0028] A polyacrylonitrile powder was dissolved in DMAc to prepare a casting solution with a mass fraction of 20 wt%. The casting solution was degassed under vacuum for 10 min and then cast on a smooth glass plate using a doctor blade. The membrane was then immersed in a deionized water coagulation bath to obtain a polyacrylonitrile ultrafiltration membrane. The membrane was removed and cut into small pieces of 5 x 5 cm and soaked and washed with deionized water. The cleaned polyacrylonitrile ultrafiltration membrane was immersed in a 3 wt% xanthan gum aqueous solution, wiped to remove the surface liquid and placed in a glass container in an air oven, which was preheated to 100 °C and maintained for 12 h. The temperature was then decreased to room temperature and the membrane was washed twice with 1:3 ethanol / water solution and once with deionized water to obtain the polyacrylonitrile modified membrane, which was designated as PAN-100.

[0029] Example 3

[0030] Polyacrylonitrile powder was dissolved in DMF to form a casting solution with a concentration of 15 wt%. After vacuum degassing for 10 min, the casting solution was cast on a smooth glass plate using a doctor blade. The cast film was then immersed in a coagulation bath of deionized water to obtain a polyacrylonitrile ultrafiltration membrane. The membrane was removed and cut into small pieces of 5 x 5 cm, which were immersed in deionized water and washed. The cleaned polyacrylonitrile ultrafiltration membrane was fully immersed in a 1 wt% polyethylene glycol aqueous solution and placed in a forced air oven. The temperature was increased to 120 °C at a rate of 2 °C / min and maintained for 12 h. The membrane was then naturally cooled to room temperature. The modified membrane was washed twice with a 1:3 ethanol / water solution and once with deionized water to obtain a polyacrylonitrile modified membrane, denoted as PAN-120.

[0031] Comparative Example 1

[0032] Polyacrylonitrile powder was dissolved in NMP to form a casting solution with a concentration of 17 wt%. After vacuum degassing for 10 min, the casting solution was cast on a smooth glass plate using a doctor blade. The cast film was then immersed in a coagulation bath of deionized water to obtain a polyacrylonitrile ultrafiltration membrane. The membrane was cut into small pieces of 5 x 5 cm, which were immersed in deionized water and washed. The cleaned polyacrylonitrile ultrafiltration membrane was fully immersed in deionized water, and the surface liquid water was wiped off before being placed in a forced air oven. The membrane was dried at room temperature for 12 h to obtain a polyacrylonitrile ultrafiltration membrane, denoted as PAN.

[0033] As shown in FIG. 1, the cross-sectional images of the scanning electron microscope show that the polyacrylonitrile modified membranes in Examples 1 to 3 all exhibit asymmetric structures. However, compared to Example 1, the pores in the separation layer of Examples 2 and 3 disappear, while the large finger-like pores are still retained. Figure 1 As shown in FIG. 1, the cross-sectional images of the scanning electron microscope show that the polyacrylonitrile modified membranes in Examples 1 to 3 all exhibit asymmetric structures. However, compared to Example 1, the pores in the separation layer of Examples 2 and 3 disappear, while the large finger-like pores are still retained.

[0034] As shown in FIG. 1, the cross-sectional images of the scanning electron microscope show that the polyacrylonitrile modified membranes in Examples 1 to 3 all exhibit asymmetric structures. However, compared to Example 1, the pores in the separation layer of Examples 2 and 3 disappear, while the large finger-like pores are still retained. Figure 2 As shown in FIG. 1, the cross-sectional images of the scanning electron microscope show that the polyacrylonitrile modified membranes in Examples 1 to 3 all exhibit asymmetric structures. However, compared to Example 1, the pores in the separation layer of Examples 2 and 3 disappear, while the large finger-like pores are still retained.

[0035] As shown in FIG. 1, the cross-sectional images of the scanning electron microscope show that the polyacrylonitrile modified membranes in Examples 1 to 3 all exhibit asymmetric structures. However, compared to Example 1, the pores in the separation layer of Examples 2 and 3 disappear, while the large finger-like pores are still retained. Figure 3As shown, the surface area and pore size distribution of the membrane were analyzed using the isothermal adsorption-desorption curve of N2 at 77K. The results show that with increasing annealing temperature, the specific surface area and pore size of the membrane decrease. Compared with Comparative Example 1, the pore size distribution of the polyacrylonitrile-modified membrane in Example 2 is significantly reduced, with the peak pore size concentrated around ~4nm, which is at the nanofiltration level.

[0036] like Figure 4 As shown, the nanofiltration performance of the membrane was evaluated using a cross-flow device. When the annealing temperature increased from 25°C to 120°C, the flux decreased from 205 to 13.3 LMH bar. -1 The rejection rate of Congo red dye increased from 65% to 98%, indicating that the polyacrylonitrile filter membrane treated by this method has good dye rejection nanofiltration separation performance.

[0037] like Figure 5 As shown, the long-term operational stability of the modified polyacrylonitrile nanofiltration membrane in Example 3 was tested. It can be seen that even after 83 hours, the modified membrane still maintained a strength of 10.76 LMH bar. -1 The permeation flux and the rejection rate are 98.90%.

[0038] like Figure 6 As shown, we tested the separation cycle performance of the modified polyacrylonitrile nanofiltration membrane in Example 2. One cycle consisted of three steps: (1) using a cross-flow device, with 50 ppm Congo red dye waste liquid as the feed liquid, the membrane was tested at 1 bar pressure for 1 h to obtain the permeate flux and dye rejection rate; (2) the membrane was removed, rinsed with deionized water for 5 min, and soaked for 1 h to remove surface contaminants; (3) the cleaned membrane was reloaded into the membrane tank, and its dye separation performance was tested according to step (1). As can be seen from the figure, the modified polyacrylonitrile nanofiltration membrane in Example 2 maintained a high rejection rate (>97%) for the Congo red dye solution in 10 cycles, indicating that it has good antifouling performance.

[0039] like Figure 7 As shown, we also tested the retention performance of the modified membrane in Example 2 for dyes of different molecular weights and charges. The retention rate of the modified membrane increased with increasing molecular weight, and for dyes with the same negative charge, it showed the best retention rate as methyl orange (MO, 327.35 Da, 78.8%).

Claims

1. A high-precision film production method, characterized by The polyacrylonitrile ultrafiltration membrane prepared based on the phase inversion method is treated by solution immersion and wet annealing process to precisely adjust the pore structure of the membrane skin layer and improve the filtration precision, and can be used in the nanofiltration membrane separation process with high flux, high retention and high stability for printing and dyeing wastewater, and specifically comprises the following steps: step one, dissolving polyacrylonitrile powder in a solvent to prepare a casting solution, coating the casting solution on a substrate by a doctor blade, and then placing the substrate in a coagulation bath, and then washing the formed membrane with water; step two, immersing the membrane in a solution containing one of xanthan gum, glycerol, polyethylene glycol and carboxymethyl cellulose sodium additives, wiping the surface liquid, and then placing the membrane in an oven for annealing treatment, the treatment temperature is 40-120℃, the heating rate is 1-5℃ / min, the annealing treatment time is 1-24h, and then washing the membrane with an ethanol / water solution to obtain a polyacrylonitrile nanofiltration membrane with high separation precision.

2. The method of claim 1, wherein: The solvent in step one is one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

3. The method of claim 1, wherein: The concentration of the casting solution in step one is 15-21wt%.

4. The method of claim 1, wherein: The polyacrylonitrile membrane in step two needs to be fully immersed in the solution, and the immersion method is a conventional method including vacuum immersion.

5. The method of claim 1, wherein: The heating mode of the annealing treatment in step two is one of preheating, stepwise heating and programmed heating.

6. The method of claim 1, wherein: The heating mode of the annealing treatment in step two is a conventional heating method including a forced air oven.

7. The method of claim 1, wherein: The cooling mode in step two includes a conventional cooling method including natural cooling.

8. The method of claim 1, wherein: The ethanol / water solution in step two includes an ethanol / water mixture with a volume ratio of 1:3-9 and pure water.

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