CN109621742B - A kind of method for preparing porous membrane from vinylidene fluoride copolymer resin - Google Patents
A kind of method for preparing porous membrane from vinylidene fluoride copolymer resin Download PDFInfo
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- CN109621742B CN109621742B CN201910035260.5A CN201910035260A CN109621742B CN 109621742 B CN109621742 B CN 109621742B CN 201910035260 A CN201910035260 A CN 201910035260A CN 109621742 B CN109621742 B CN 109621742B
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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- B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
- B01D71/06—Organic material
- B01D71/30—Polyalkenyl halides
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- B01D71/34—Polyvinylidene fluoride
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Abstract
本发明公开了一种偏氟乙烯共聚物树脂制备多孔膜的方法，通过配制具有低临界溶解温度的高分子铸膜液体系，采用相分离法制膜，获得结构易于调控的偏氟乙烯共聚物多孔膜。本发明的高分子铸膜液在低温下是均相稳定的，但在高温下发生相分离，相应的制膜工艺不同于传统制膜工艺，经过两道不同凝胶浴工序及适当的后处理工艺制备的偏氟乙烯共聚物树脂多孔膜没有表皮致密层，主体是高孔隙率、近似对称的海绵状孔结构，适用于膜蒸馏的应用要求，具有通量大、阻力低、疏水性强的特性。
The invention discloses a method for preparing a porous film from a vinylidene fluoride copolymer resin. By preparing a polymer film casting liquid system with a low critical solution temperature, and using a phase separation method to make a film, a porous vinylidene fluoride copolymer with an easily regulated structure is obtained. membrane. The polymer film casting liquid of the present invention is homogeneously stable at low temperature, but phase separation occurs at high temperature, and the corresponding film-making process is different from the traditional film-making process. It undergoes two different gel bath processes and appropriate post-treatment The vinylidene fluoride copolymer resin porous membrane prepared by the process has no skin dense layer, and the main body is a high porosity and approximately symmetrical sponge-like pore structure, which is suitable for the application requirements of membrane distillation. It has large flux, low resistance and strong hydrophobicity. characteristic.
Description
Technical Field
The invention belongs to the technical field of polymer membrane separation, and particularly relates to a method for preparing a porous membrane from vinylidene fluoride copolymer resin.
Background
Membrane Distillation (MD) is a membrane separation process using a hydrophobic microporous membrane and taking the steam pressure difference on two sides of the membrane as a mass transfer driving force, wherein two sides of the membrane directly contacting with membrane distillation (DCMD) are respectively in direct contact with high-temperature feed liquid and low-temperature penetrating fluid, and steam permeates through membrane pores and is directly condensed on the penetrating fluid side. The membrane distillation almost completely intercepts non-volatile components such as inorganic salt, macromolecules and the like, can utilize cheap energy sources such as solar energy, industrial waste heat and the like, and has wide application prospect in the aspects of high-salt wastewater, high-value component recovery and the like. The vinylidene fluoride copolymer is a copolymer formed by vinylidene fluoride and at least one of vinyl fluoride, tetrafluoroethylene, hexafluoropropylene and chlorotrifluoroethylene. The porous membrane prepared from the vinylidene fluoride copolymer resin has the advantages of strong hydrophobicity, high temperature resistance, solvent resistance, acid and alkali resistance, outstanding advantages, suitability for the membrane distillation process and wide attention. The method for preparing the porous membrane by the vinylidene fluoride copolymer resin is mainly a phase separation method. The phase separation method can be generally classified into a non-solvent induced phase separation (NIPS) and a Thermal Induced Phase Separation (TIPS). The NIPS method takes the phase separation and film formation of a high molecular solution caused by the exchange of a solvent and a non-solvent formed by concentration difference as a mechanism, and the TIPS method takes the phase separation and film formation of the high molecular solution caused by the thermal induction of temperature difference as a mechanism. In the TIPS method currently used for preparing the vinylidene fluoride copolymer porous membrane, the polymer solution is usually an Upper Critical Solution Temperature (UCST) system, i.e. when the temperature of the polymer solution is reduced from high temperature to its critical solution temperature, phase separation occurs to further form the porous membrane; on the other hand, a Low Critical Solution Temperature (LCST) system, i.e., a system in which a polymer solution undergoes phase separation when the temperature thereof is increased from a low temperature to its critical solution temperature to form a porous vinylidene fluoride copolymer film, has not been reported. In view of this, the present invention provides a method for preparing a porous membrane based on a vinylidene fluoride copolymer resin casting solution system having a Low Critical Solution Temperature (LCST).
Disclosure of Invention
The invention aims to overcome the defects of the existing preparation technology of the vinylidene fluoride copolymer resin porous membrane, and provides a method for preparing the porous membrane by using the vinylidene fluoride copolymer resin.
The technical scheme adopted by the invention for solving the technical problems is as follows: a method for preparing a porous membrane by using vinylidene fluoride copolymer resin is characterized by comprising the following steps:
1) preparing a vinylidene fluoride copolymer resin casting solution with a low critical solution temperature: preparing a vinylidene fluoride copolymer resin casting solution with proper concentration according to the mass percentage of 5-40% of vinylidene fluoride copolymer resin, 1-20% of second macromolecule, 50-85% of solvent and 1-20% of micromolecule pore-forming agent, firstly completely dissolving the micromolecule pore-forming agent in the solvent, then adding the vinylidene fluoride copolymer resin and the second macromolecule, stirring, heating and dissolving to form a completely dissolved vinylidene fluoride copolymer resin casting solution system with Low Critical Solution Temperature (LCST), and standing and defoaming for later use;
2) scraping the prepared vinylidene fluoride copolymer resin casting solution at a certain temperature, pouring the scraped film on a glass plate, scraping the film into a nascent state film with a certain thickness by using a film scraping machine, quickly placing the nascent state film plate into a first gel bath at a certain temperature for 5-20 seconds, then placing the nascent state film plate into a second gel bath at normal temperature, allowing the nascent state film to undergo phase separation, and curing the film from a liquid phase to form a film;
3) and (3) placing the membrane in water to further completely dissolve out a solvent, a small molecular pore-forming agent and the like in the membrane, then carrying out heat treatment on the membrane by using an ammonium persulfate aqueous solution at a certain temperature, soaking the membrane by using distilled water, and finally freeze-drying the membrane to obtain the vinylidene fluoride copolymer resin porous membrane.
The vinylidene fluoride copolymer resin casting solution in the step 1) has a Lower Critical Solution Temperature (LCST), the casting solution is a uniform high molecular solution at a temperature lower than the LCST, and the original uniform vinylidene fluoride copolymer resin solution is subjected to phase separation at a temperature higher than the LCST, and the Lower Critical Solution Temperature (LCST) is between 1 and 125 ℃, preferably between 35 and 80 ℃.
The vinylidene fluoride copolymer resin casting solution in the step 1) is a system composed of four or more components, and comprises vinylidene fluoride copolymer resin, a second polymer, a solvent and a small molecular pore-forming agent, wherein the small molecular pore-forming agent must contain magnesium chloride, but other small molecular compounds can be added at the same time, and the small molecular pore-forming agent magnesium chloride can be anhydrous magnesium chloride or magnesium chloride containing crystal water.
The second polymer in step 1) is a polymer material having good compatibility with the vinylidene fluoride copolymer resin, and may be a non-water-soluble polymer or a water-soluble polymer, and is preferably a polymer having a carbonyl group in a structural unit, such as polyvinylpyrrolidone (PVP), but is not limited thereto.
The solvent in the step 1) is a solvent capable of dissolving vinylidene fluoride copolymer resin, such as Dimethylformamide (DMF), dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP), and can also be a mixed solvent containing the solvent, but not limited to the solvent, and the solvent preferably has a carbonyl group in a structural unit, and a solvated magnesium complex ion structure capable of performing electron donor-donor interaction with a small molecular pore-forming agent magnesium chloride to form a multi-solvent molecule, such as [ Mg (DMAc) ]6]2+And a difference in solubility parameter Delta between the vinylidene fluoride copolymer resin and the vinylidene fluoride copolymer resins-pIs less than3MPa0.5Such as dimethylacetamide (DMAc).
The micromolecular pore-foaming agent magnesium chloride in the step 1) can generate an electron donor-donor interaction effect with the vinylidene fluoride copolymer resin and the second macromolecule in the step 1).
The heating dissolution temperature in the step 1) is lower than 130 ℃, and simultaneously can be higher than the LCST temperature, but preferably is lower than the LCST temperature for dissolution, and the temperature of the polymer casting solution after complete dissolution is kept to be lower than the LCST temperature for standby.
And (3) scraping the membrane at a temperature lower than the LCST temperature in the step 2) to keep the membrane casting solution in a thermodynamic stable state, preferably 5-10 ℃ lower than the LCST temperature.
The first gel bath in step 2) is polyethylene glycol 200 at a temperature of 90-140 ℃ and higher than the LCST temperature, and the second gel bath is a non-solvent for vinylidene fluoride copolymer resin, such as water, ethanol, and aqueous solution containing a solvent, but not limited thereto.
The phase separation mechanism of the nascent membrane in the step 2) in the first gel bath at the temperature higher than the LCST is mainly a TIPS mechanism, namely the nascent membrane is subjected to thermally induced phase separation under the action of the temperature difference between the temperature of the first gel bath and the temperature of the cast membrane liquid, and then is cured in the second gel bath to form the membrane.
The mass percentage of the ammonium persulfate in the ammonium persulfate aqueous solution in the step 3) is 8-10%, the heat treatment temperature is 75-85 ℃, the heat treatment time is 3-4 hours, and freeze drying and forming are carried out.
The vinylidene fluoride copolymer resin film obtained in the step 3) is a porous film, the thickness of the film is 20-200 mu m, the average pore diameter is 0.1-1.0 mu m, the vinylidene fluoride copolymer resin porous film prepared by sequentially carrying out two different gel bath procedures and ammonium persulfate post-treatment at the temperature higher than LCST has no compact skin layer, the main body is a sponge-shaped pore structure with high porosity and approximate symmetry, and meanwhile, the hydrophobicity is strong, so that the polyvinylidene fluoride copolymer resin porous film is suitable for membrane distillation application.
Compared with the prior art, the invention has the characteristics and advantages that:
1. the invention adopts a vinylidene fluoride copolymer resin casting solution system based on a Low Critical Solution Temperature (LCST), and no report with the Low Critical Solution Temperature (LCST) is available at present. The invention successfully forms a casting solution system with LCST through a quaternary system which simultaneously comprises vinylidene fluoride copolymer resin, a second macromolecule, a specific micromolecule pore-forming agent magnesium chloride and a solvent matched with the magnesium chloride, and opens up a new way for preparing the vinylidene fluoride copolymer resin porous membrane based on the quaternary system.
2. The mechanism of forming the low critical solution temperature of the vinylidene fluoride copolymer resin casting solution system based on the Low Critical Solution Temperature (LCST) is not phase separation caused by the fact that the hydrogen bond action of the system is damaged in the temperature rise process, nor is the mechanism of forming the low critical solution temperature due to the existence of temperature sensitive polymers, but because magnesium chloride can generate electron donor-donor interaction with a specific solvent and a second polymer of the system, solvated magnesium complex ions of formed specific multi-solvent molecules can generate structural change when the temperature rises and cause the interaction change of different components in a quaternary system, so that the dissolving capacity of the polymers in the system is reduced, and the phase separation is promoted. If lithium chloride or zinc chloride is used to replace magnesium chloride and other components are not changed in the casting solution system with Low Critical Solution Temperature (LCST), the new system has no Low Critical Solution Temperature (LCST) in the temperature range; further, when the second polymer or magnesium chloride is removed from the system and the system is changed from the quaternary system to the ternary system, the ternary system does not have a Lower Critical Solution Temperature (LCST) in the above temperature range.
3. According to the method for preparing the porous membrane from the vinylidene fluoride copolymer resin, TIPS phase separation involved in the membrane preparation process is different from the traditional TIPS phase separation process. In the traditional TIPS phase separation process, a vinylidene fluoride copolymer solution is stable in a high-temperature state, and phase separation occurs at a low temperature, and corresponds to a UCST system; in the TIPS phase separation process, the vinylidene fluoride copolymer solution is stable in a low-temperature state and is subjected to phase separation at a high temperature, and the process is different from the conventional method for preparing the vinylidene fluoride copolymer film corresponding to an LCST system.
4. Compared with the single NIPS method for preparing the membrane, the method for preparing the porous membrane by the vinylidene fluoride copolymer resin does not need to regulate and control a plurality of process parameters, but the aperture is easier to regulate and control; compared with the single traditional TIPS method for preparing the membrane, the membrane casting solution with the melting point temperature (more than 180 ℃) higher than that of the vinylidene fluoride copolymer resin does not need to be operated, the energy consumption is low, and the process is simple. The polyvinylidene fluoride copolymer resin porous membrane prepared by sequentially carrying out two different gel bath procedures at the temperature higher than LCST (lower temperature than lower temperature.
Drawings
FIG. 1 is an SEM image showing the cross-sectional structure of a porous membrane of vinylidene fluoride copolymer resin prepared in example 1 of the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
Example 1
Preparing a PVDF-HFP casting solution according to the mass percentage of vinylidene fluoride copolymer resin- - -polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) of 12 percent, the mass percentage of second polymer polyvinylpyrrolidone (PVP) of 12 percent, the mass percentage of solvent dimethylacetamide (DMAc) of 68 percent and the mass percentage of small molecular pore-forming agent magnesium chloride of 8 percent, firstly completely dissolving magnesium chloride in DMAc, then adding PVDF-HFP and PVP, stirring and heating at a constant temperature of 45 ℃ to dissolve the PVDF-HFP casting solution to form a completely uniform PVDF-HFP casting solution, and standing and defoaming. Then, scraping the membrane casting solution at 45 ℃, pouring the membrane casting solution on a glass plate, scraping the membrane casting solution into a nascent-state membrane by using a membrane scraping machine, quickly putting the nascent-state membrane into polyethylene glycol 200 at 97 ℃ for 15 seconds, transferring the nascent-state membrane into normal-temperature water, allowing the nascent-state membrane to undergo phase separation, and solidifying the nascent-state membrane from a liquid phase to form a membrane; placing the membrane in water to further completely dissolve out the solvent and the micromolecular pore-forming agent in the membrane, then carrying out heat treatment on the membrane at 80 ℃ for 4 hours by using an ammonium persulfate aqueous solution with the mass percentage of 9 percent, soaking the membrane in distilled water for one day, and finally carrying out freeze drying on the membrane at-50 ℃ to obtain the meta-fluorine of the inventionAn ethylene copolymer resin porous film. The PVDF-HFP casting solution system has a Lower Critical Solution Temperature (LCST) of 52 ℃. The first gel bath temperature is 45 ℃ higher than the LCST temperature. The membrane structure prepared was a sponge-like pore structure without skin layer, as shown in fig. 1. The thickness of the membrane was 80 μm, the contact angle was 87 °, and the average pore diameter was 0.2 μm. Placing the porous membrane in a direct contact type membrane distillation device, taking 3.5% sodium chloride aqueous solution as feed liquid by mass percent, introducing 20 ℃ pure water at the permeation side of the feed liquid at the temperature of 60 ℃, and measuring the pure water permeation flux of 38L/m in the membrane distillation process2Hr, salt rejection rate is greater than 99.9%.
Example 2
Preparing a PVDF-HFP membrane casting solution according to the mass percent of vinylidene fluoride copolymer resin- -PVDF-HFP is 16 percent, PVP is 4 percent, a solvent DMAc is 72 percent, and a small molecular pore-forming agent magnesium chloride is 8 percent, firstly completely dissolving magnesium chloride in DMAc, then adding PVDF-HFP and PVP, stirring and heating to 47 ℃ for constant temperature dissolution to form a completely uniform PVDF-HFP membrane casting solution, and standing and defoaming. Then, scraping the film casting solution at 47 ℃, pouring the film casting solution on a glass plate, scraping the film casting solution into a nascent-state film by using a film scraping machine, quickly putting the nascent-state film into polyethylene glycol 200 at 110 ℃ for 8 seconds, transferring the nascent-state film into normal-temperature water, allowing the nascent-state film to undergo phase separation, and solidifying the nascent-state film from a liquid phase to form a film; and (2) placing the membrane in water to further completely dissolve out a solvent, a small molecular pore-forming agent and the like in the membrane, then carrying out heat treatment on the membrane at 85 ℃ for 3 hours by using an ammonium persulfate aqueous solution with the mass percentage of 10%, soaking the membrane in distilled water for one day, and finally carrying out freeze drying on the membrane at-50 ℃ to obtain the vinylidene fluoride copolymer resin porous membrane. The PVDF-HFP casting solution system has a Lower Critical Solution Temperature (LCST) of 52 ℃. The temperature of the first gel bath is 58 ℃ higher than the LCST temperature, and the prepared film structure is a sponge-shaped pore structure without a skin layer. The membrane had a thickness of 90 μm and an average pore diameter of 0.12. mu.m. Placing the porous membrane in a direct contact type membrane distillation device, taking 3.5% sodium chloride aqueous solution as feed liquid by mass percent, introducing 20 ℃ pure water into the permeation side at the feed liquid temperature of 70 ℃, and measuring the pure water permeation flux in the membrane distillation process to be 67L/m2Hr, salt rejection rate is greater than 99.9%.
Example 3
Preparing a P (VDF-CTFE) casting solution according to the mass percentage of vinylidene fluoride copolymer resin- - -polychlorotrifluoroethylene-vinylidene fluoride copolymer P (VDF-CTFE) of 13 percent, PVP of 3 percent, solvent N-methylpyrrolidone (NMP) of 74 percent and small molecular pore-forming agent magnesium chloride of 10 percent, firstly completely dissolving magnesium chloride in NMP, then adding P (VDF-CTFE) and PVP, stirring and heating to 70 ℃ for dissolving to form a completely uniform P (VDF-CTFE) casting solution, and standing and defoaming. Then, scraping the membrane casting solution at 70 ℃, pouring the membrane casting solution on a glass plate, scraping the membrane casting solution into a nascent-state membrane by using a membrane scraping machine, quickly putting the nascent-state membrane into 130 ℃ polyethylene glycol 200 for 10 seconds, transferring the nascent-state membrane into normal-temperature water, allowing the nascent-state membrane to be separated, and solidifying the nascent-state membrane from a liquid phase to form a membrane; and (3) placing the membrane in water to further completely dissolve out a solvent, a small molecular pore-forming agent and the like in the membrane, then carrying out heat treatment on the membrane at 75 ℃ for 3.5 hours by using an ammonium persulfate aqueous solution with the mass percentage of 8%, soaking the membrane in distilled water for one day, and finally carrying out freeze drying on the membrane at-50 ℃ to obtain the vinylidene fluoride copolymer resin porous membrane. The Lower Critical Solution Temperature (LCST) of the P (VDF-CTFE) casting solution system is 75 ℃. The temperature of the first gel bath is 55 ℃ higher than the LCST temperature, and the prepared film structure is a sponge-shaped pore structure without a skin layer. The membrane had a thickness of 100 μm and an average pore diameter of 0.3. mu.m. Placing the porous membrane in a direct contact type membrane distillation device, taking 3.5% sodium chloride aqueous solution as feed liquid by mass percent, introducing pure water at 20 ℃ at the permeation side of the feed liquid at the temperature of 80 ℃, and measuring the pure water permeation flux in the membrane distillation process to be 87L/m2Hr, salt rejection rate is greater than 99.9%.
Comparative example 1
The nascent membrane was directly placed in a unique gelling bath, i.e., 20 ℃ water bath, and the polyvinylidene fluoride membrane was prepared following the formulation and procedure of example 1. The Lower Critical Solution Temperature (LCST) of the vinylidene fluoride copolymer casting solution system is 52 ℃. The gel bath temperature was 32 ℃ lower than the LCST temperature. Because the gel is formed at the temperature lower than the LCST and no TIPS phase separation exists, the prepared vinylidene fluoride copolymer membrane only has a NIPS phase separation process, and the prepared membrane structure is a typical finger-shaped pore structure. The membrane had a thickness of 90 μm and an average pore diameter of 0.03 μm, and the pure water permeation flux during membrane distillation was only 23% of that of the membrane obtained in example 1.
Comparative example 2
The nascent state membrane was directly placed in a unique gelling bath, i.e., 30 ℃ water bath, without ammonium persulfate treatment, and the polyvinylidene fluoride membrane was prepared following the same formulation and procedure of example 1. The Lower Critical Solution Temperature (LCST) of the vinylidene fluoride copolymer casting solution system is 52 ℃. The gel bath temperature was 22 ℃ lower than the LCST temperature. Because the gel is formed at the temperature lower than the LCST and no TIPS phase separation exists, the prepared vinylidene fluoride copolymer membrane only has a NIPS phase separation process, and the prepared membrane structure is a typical finger-shaped pore structure. The thickness of the membrane was 90 μm and sodium chloride leakage occurred during membrane distillation.
Comparative example 3
Preparing a vinylidene fluoride copolymer casting solution by using 12 mass percent of PVDF-HFP, 0 mass percent of second high-molecular polyvinylpyrrolidone (PVP), 80 mass percent of solvent DMAc and 8 mass percent of small-molecular pore-forming agent magnesium chloride, and dissolving the vinylidene fluoride copolymer casting solution to form a completely uniform vinylidene fluoride copolymer casting solution in the same way as the step of the example 1. The vinylidene fluoride copolymer casting film liquid system does not have Low Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the ternary system cannot form the LCST system.
Comparative example 4
Preparing a vinylidene fluoride copolymer casting solution by using 12 mass percent of PVDF-HFP, 12 mass percent of second high-molecular polyvinylpyrrolidone (PVP), 76 mass percent of solvent DMAc and 0 mass percent of small-molecular pore-forming agent magnesium chloride, and dissolving the vinylidene fluoride copolymer casting solution to form a completely uniform vinylidene fluoride copolymer casting solution in the same way as the step of the example 1. The vinylidene fluoride copolymer casting film liquid system does not have Low Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the ternary system cannot form the LCST system.
Comparative example 5
Preparing a vinylidene fluoride copolymer casting solution by using 12% of P (VDF-CTFE) by mass, 11.5% of second high-molecular polyvinylpyrrolidone (PVP) by mass, 68% of solvent DMAc by mass and 8.5% of lithium chloride as a small-molecular pore-forming agent by mass, and dissolving the vinylidene fluoride copolymer casting solution to form a completely uniform vinylidene fluoride copolymer casting solution by the same steps as the step 1. The vinylidene fluoride copolymer casting film liquid system does not have Low Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the quaternary system cannot form the LCST system.
Comparative example 6
Preparing a vinylidene fluoride copolymer membrane casting solution by using 12% of P (VDF-CTFE) by mass, 12% of second high-molecular polyvinylpyrrolidone (PVP) by mass, 71% of solvent DMAc by mass and 5% of zinc chloride by mass as a small-molecular pore-forming agent, and dissolving the vinylidene fluoride copolymer membrane casting solution to form the completely uniform vinylidene fluoride copolymer membrane casting solution by the same steps as the step of the example 1. The vinylidene fluoride copolymer casting film liquid system does not have Low Critical Solution Temperature (LCST) between 1 ℃ and 125 ℃, and the quaternary system cannot form the LCST system.
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