Preparation method of polyvinyl chloride bipolar membrane

Abstract

The invention relates to a method for preparing a bipolar membrane through physical and chemical modification on the basis of polyvinyl chloride (PVC), and aims to provide a process method for preparing the bipolar membrane by taking an important product polyvinyl chloride in the chlor-alkali industry as a main raw material. According to the method, the accumulated polyvinyl chloride is effectively utilized, and meanwhile, the bipolar membrane which is widely used in the industry is prepared. The polyvinyl chloride-based bipolar membrane has relatively small membrane surface resistance and relatively large conductivity.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane synthesis and preparation, and particularly relates to a bipolar membrane and a preparation method thereof. Background Art

[0002] As a new type of ion exchange composite membrane, the bipolar membrane is mainly applied to the electrodialysis process, and can efficiently dissociate water into hydrogen ions and hydroxide ions, and then convert the salt in the aqueous solution into the corresponding acid and base. This process is not only energy-saving, efficient and environmentally friendly, but also has broad application prospects in the fields of pollution control, resource recovery and chemical production. The preparation method of the bipolar membrane has gone through multiple development stages, from the early simple pressing to the modern integrated membrane research, the technology is becoming increasingly mature and the performance is continuously optimized. In the process of exploring the preparation and application of the bipolar membrane (BPM), an important innovation is to use polyvinyl chloride (PVC) as the raw material. PVC, as an important product of the chlor-alkali industry, has accumulated an excessive stock due to its production process and application, posing challenges to the environment and resource utilization. By converting PVC into the substrate for preparing the bipolar membrane, not only a new and environmentally friendly use is provided for this ubiquitous synthetic material, but also the development of the circular economy is promoted, effectively alleviating the problem of PVC surplus in the chlor-alkali industry.

[0003] In this context, it is particularly important to use PVC as the raw material of the bipolar membrane. PVC not only has a relatively low cost, but also has good chemical stability and processing performance, making it an ideal choice for preparing the bipolar membrane. Using PVC as the substrate of the bipolar membrane can effectively reduce the preparation cost, and at the same time utilize its excellent mechanical properties and chemical resistance to improve the stability and service life of the bipolar membrane. In terms of environmental protection, the process of converting PVC into the raw material of the bipolar membrane not only helps to reduce the problem of PVC surplus in the chlor-alkali industry, but also reduces the demand for new raw materials, thereby reducing the environmental pressure. This way of recycling reflects the concept of sustainable development, converting industrial by-products into valuable resources and achieving the environmental protection goal of "waste utilization". In addition, using PVC as the raw material of the bipolar membrane also has potential technical advantages. The plasticity and adjustability of PVC make it a flexible material, which can be modified by chemical or physical methods to optimize the performance of the bipolar membrane. For example, by adding specific additives or performing surface modification, the hydrophilicity, conductivity and ion selectivity of the PVC-based bipolar membrane can be improved, thereby improving the efficiency of the electrodialysis process. Summary of the Invention

[0004] The object of the present invention is to provide a process for preparing bipolar membranes using polyvinyl chloride, an important product of the chlor-alkali industry, as the main raw material. This method realizes the effective utilization of accumulated polyvinyl chloride and simultaneously prepares bipolar membranes widely used in industry. Such polyvinyl chloride-based bipolar membranes have a small membrane surface resistance and a large conductivity.

[0005] The present invention adopts the following scheme: the preparation of anion exchange membranes, the preparation of cation exchange membranes, and the preparation of bipolar membranes, including the following steps:

[0006] (1) Preparation of anion exchange membranes

[0007] Take a certain amount of polyvinyl chloride (PVC), polyethyleneimine (PEI), and N,N-dimethylacetamide (DMAc), stir until clear and transparent, then add a certain amount of nano-TiO2 powder and ultrasonicate for 30 min. Heat under reflux at 80 °C for 3 h, cast in a glass petri dish, dry in an oven at 80 °C for 3 h, and take out the membrane.

[0008] Where mPVC∶mPEI = 1∶(1 - 4), mDMAc∶m(PVC + PEI) = 21∶4, mTiO2∶m(PVC + PEI) = (0 - 1)∶100

[0009] (2) Preparation of cation exchange membranes

[0010] Take a certain amount of polyvinyl chloride (PVC), N,N-dimethylacetamide (DMAc), and polyethylene glycol (PEG), heat under reflux at 70 °C for 2 h, then add a certain amount of cation exchange resin powder and stir for 3 h. Scrape the membrane solution with a 200-μm steel knife. Immerse the glass plate in deionized water and take out the membrane and store it in deionized water.

[0011] Where mPVC∶m resin powder = (3∶7 - 7∶3), mDMAc∶m(PVC + resin powder)∶mPEG = 4∶1∶(0 - 0.2)

[0012] (3) Preparation of bipolar membranes

[0013] Take the prepared anion exchange membrane and cation exchange membrane, trim them to similar sizes, and place them between polytetrafluoroethylene plates for hot pressing. The temperature of the hot press is 60 °C - 100 °C, the pressure is 8 MPa, and the duration is 20 min. Take out the membrane after the device cools and store it in deionized water.

[0014] Preferably, in the preparation process of the anion exchange membrane, mPVC∶mPEI = 1∶2, and mTiO2∶m(PVC + PEI) = 0.5∶100.

[0015] Preferably, during the preparation of the cation exchange membrane, mPVC∶m resin powder = 4∶6, and mDMAc∶m(PVC + resin powder)∶mPEG = 4∶1∶0.1.

[0016] Preferably, the cation exchange resin powder used in the preparation of the cation exchange membrane needs to be prepared by ball milling the cation exchange resin after cleaning and activation, and then screening through a 200-mesh sieve.

[0017] Preferably, the temperature during the hot pressing of the bipolar membrane should be 80 °C. Description of the Drawings Figure 1 Cation exchange membrane surface of the bipolar membrane in Example 3 Figure 2 Anion exchange membrane surface of the bipolar membrane in Example 3 Figure 3 Effect of the bipolar membrane in Example 3 for electrocatalytic reduction of CO2 for 10 h in an H-type electrolytic cell at a potential of -0.9 V (vs. RHE) Detailed Description of the Invention

[0018] In order to better illustrate the existing technical solutions of the present invention, the preparation process of the present invention will be described in detail through examples below.

[0019] Example 1

[0020] Prepare a polyvinyl chloride-based bipolar membrane by the hot pressing method. The specific steps are as follows:

[0021] 1. Preparation of the anion exchange membrane

[0022] Put a fixed ratio of polyvinyl chloride (PVC), polyethyleneimine (PEI), and N,N-dimethylacetamide (DMAc) into a flask, and stir the mixed solution until it is clear and transparent. Then add a certain amount of nano-TiO2 powder, and ultrasonicate in an ultrasonic cleaner for 30 min until the nano-particles are evenly distributed in the mixed solution. Preheat the water bath to 80 °C, put the flask in and keep stirring, and reflux and heat at 80 °C for 3 h for sufficient crosslinking. After the membrane solution cools, cast it on a clean glass petri dish and dry it in an 80 °C oven for 3 h. After the membrane cools, take it out and soak it in deionized water to remove uncrosslinked substances.

[0023] Among them, mPVC∶mPEI = 1∶1, mDMAc∶m(PVC + PEI) = 21∶4, and mTiO2∶m(PVC + PEI) = 1∶100.

[0024] 2. Preparation of the cation exchange membrane

[0025] Put a fixed ratio of polyvinyl chloride (PVC), N,N-dimethylacetamide (DMAc), and polyethylene glycol (PEG) into a flask. Preheat a water bath to 70 °C, put the flask in it and keep stirring, and reflux and heat at 70 °C for 2 h for sufficient dissolution. After the solution cools, add a certain amount of cation exchange resin powder and keep stirring for 3 h. Drop the membrane solution onto a clean glass plate and use a 200-μm doctor blade to scrape the membrane. Immerse the glass plate with the scraped membrane in deionized water, take out the membrane after the solvent exchange is completed, and store it in deionized water.

[0026] mPVC∶m resin powder = 5∶5, mDMAc∶m(PVC + resin powder)∶mPEG = 4∶1∶0.05.

[0027] 3. Preparation of bipolar membrane

[0028] Take the anion exchange membrane and cation exchange membrane prepared above. To prevent the separation and displacement of the anion and cation exchange membranes during the hot pressing process, a scalpel is needed to trim the two membranes into similar sizes. Align the anion exchange membrane with the cation exchange membrane, place it between polytetrafluoroethylene plates and clamp it, and place it in a hot pressing plate for fixation. Set the temperature of the hot press to 100 °C, the pressure to 8 MPa, and the hot pressing duration to 20 min. After hot pressing is completed, take out the membrane after the device cools and store it in deionized water.

[0029] After hot pressing treatment, the bipolar membrane is tightly combined and will not crack. The average membrane thickness is 0.2723 mm, as shown in Table 1. After testing, the membrane surface resistance of this membrane is 2.69 Ω·cm 2 , and the conductivity is 7.28 mS cm -1 . The conductivity of this membrane is relatively low, probably because there is less polyethyleneimine in the anion exchange membrane and at the same time the amount of nano-TiO2 powder is too much. Too high a hot pressing temperature may also damage the cross-linked structure in the membrane.

[0030] Example 2

[0031] Prepare a polyvinyl chloride-based bipolar membrane by the hot pressing method. The specific steps are as follows:

[0032] 1. Preparation of anion exchange membrane

[0033] Put a fixed ratio of polyvinyl chloride (PVC), polyethyleneimine (PEI), and N,N-dimethylacetamide (DMAc) into a flask, and stir the mixed solution until it is clear and transparent. Then add a certain amount of nano-TiO₂ powder, and ultrasonicate in an ultrasonic cleaner for 30 min until the nanoparticles are evenly distributed in the mixed solution. Preheat a water bath to 80 °C, put the flask in and keep stirring, and reflux and heat at 80 °C for 3 h for sufficient cross-linking. After the membrane solution cools down, cast it on a clean glass petri dish and dry it in an oven at 80 °C for 3 h. After the membrane cools down, take it out and soak it in deionized water to remove uncrosslinked substances.

[0034] Where mPVC∶mPEI = 1∶3, mDMAc∶m(PVC + PEI) = 21∶4, mTiO₂∶m(PVC + PEI) = 0∶100.

[0035] 2. Preparation of cation exchange membrane

[0036] Put a fixed ratio of polyvinyl chloride (PVC), N,N-dimethylacetamide (DMAc), and polyethylene glycol (PEG) into a flask, preheat a water bath to 70 °C, put the flask in and keep stirring, and reflux and heat at 70 °C for 2 h for sufficient dissolution. After the solution cools down, add a certain amount of cation exchange resin powder and stir continuously for 3 h. Drop the membrane solution on a clean glass plate and use a 200-μm doctor blade to doctor the film. Immerse the glass plate with the doctored film in deionized water, take out the membrane after the solvent exchange is completed, and store it in deionized water.

[0037] mPVC∶m resin powder = 6∶4, mDMAc∶m(PVC + resin powder)∶mPEG = 4∶1∶0.2.

[0038] 3. Preparation of bipolar membrane

[0039] Take the anion exchange membrane and cation exchange membrane prepared above. To prevent the separation and displacement of the anion and cation exchange membranes during the hot pressing process, a scalpel is needed to trim the two membranes into similar sizes. Align the anion exchange membrane and the cation exchange membrane, place them between polytetrafluoroethylene plates and clamp them, and place them in a hot pressing plate for fixation. Set the temperature of the hot press to 60 °C, the pressure to 8 MPa, and the hot pressing duration to 20 min. After hot pressing is completed, take out the membrane after the device cools down and store it in deionized water.

[0040] After the hot pressing treatment, the bipolar membrane occasionally cracks, which may be due to insufficient hot pressing temperature. The average thickness of the membrane is 0.2335 mm, as shown in Table 1. After testing, the membrane surface resistance of this membrane is 6.90 Ω·cm 2 , and the conductivity is 8.34 mS cm -1。The membrane surface resistance is relatively large, which may be due to the low proportion of resin powder in the cation exchange membrane. At the same time, excessive polyethyleneimine in the anion exchange membrane and excessive polyethylene glycol in the cation exchange membrane may be the reasons for the decrease in conductivity, because the water absorption rates of the anion and cation exchange membranes increase with the increase of polyethyleneimine and polyethylene glycol.

[0041] Example 3

[0042] Prepare a polyvinyl chloride-based bipolar membrane by the hot pressing method. The specific steps are as follows:

[0043] 1. Preparation of anion exchange membrane

[0044] Put a fixed proportion of polyvinyl chloride (PVC), polyethyleneimine (PEI), and N,N-dimethylacetamide (DMAc) into a flask, stir the mixed solution until it is clear and transparent. Then add a certain amount of nano-TiO2 powder and ultrasonicate it in an ultrasonic cleaner for 30 min until the nanoparticles are evenly distributed in the mixed solution. Preheat the water bath to 80 °C, put the flask in and keep stirring, and reflux and heat at 80 °C for 3 h for sufficient crosslinking. After the membrane solution cools down, cast it on a clean glass petri dish and dry it in an oven at 80 °C for 3 h. Take out the membrane after it cools down and soak it in deionized water to remove uncrosslinked substances.

[0045] Where mPVC∶mPEI = 1∶2, mDMAc∶m(PVC + PEI) = 21∶4, mTiO2∶m(PVC + PEI) = 0.5∶100.

[0046] 2. Preparation of cation exchange membrane

[0047] Put a fixed proportion of polyvinyl chloride (PVC), N,N-dimethylacetamide (DMAc), and polyethylene glycol (PEG) into a flask, preheat the water bath to 70 °C, put the flask in and keep stirring, and reflux and heat at 70 °C for 2 h for sufficient dissolution. After the solution cools down, add a certain amount of cation exchange resin powder and continue stirring for 3 h. Drop the membrane solution on a clean glass plate and use a 200-μm doctor blade for scraping. Immerse the glass plate with the scraped membrane in deionized water, take out the membrane after the solvent exchange is completed, and store it in deionized water.

[0048] Where mPVC∶m resin powder = 4∶6, mDMAc∶m(PVC + resin powder)∶mPEG = 4∶1∶0.1.

[0049] 3. Preparation of bipolar membrane

[0050] Take the anion exchange membrane and cation exchange membrane prepared above. To prevent the separation and displacement of the anion and cation exchange membranes during the hot pressing process, a scalpel is needed to trim the two membranes into similar sizes. Align the anion exchange membrane with the cation exchange membrane, place it between polytetrafluoroethylene plates and clamp it, and then place it in a hot pressing plate for fixation. The temperature of the hot press is set at 80 °C, the pressure is 8 MPa, and the hot pressing duration is 20 min. After hot pressing, take out the membrane after the device cools down and store it in deionized water.

[0051] After the hot pressing treatment, the bipolar membrane is tightly combined and will not crack. The average thickness of the membrane is 0.2480 mm, as shown in Table 1. The cation exchange membrane surface of the bipolar membrane has more pores, and at the same time, it can be seen that more cation exchange resin powders are evenly distributed on the membrane surface, as Figure 1 ; the anion exchange membrane surface has fewer pores and is smoother overall, as Figure 2 . After testing, the membrane surface resistance of this membrane is 1.65 Ω·cm 2 , and the conductivity is 13.88 mS cm -1 . The test data show that the polyvinyl chloride-based bipolar membrane prepared by this scheme has the lowest membrane surface resistance and conductivity. In the life test, a self-made porous silver catalyst was used at a potential of -0.9 V (vs. RHE), and a H-type electrolytic cell with 0.5 M potassium bicarbonate as the electrolyte was used as the electrolytic cell for continuous 10 h of electrocatalytic reduction of carbon dioxide. As Figure 3 shown, the results show that this bipolar membrane has good stability. Excluding the deactivation factor of the catalyst, the whole device can maintain a stable current density, and the Faraday efficiency of carbon monoxide can be maintained at about 50%.

[0052]

[0053] Table 1 Thickness of the bipolar membrane in the examples

[0054] The above is only the preferred embodiment of the present invention, which is illustrative rather than restrictive to the present invention; those of ordinary skill in the art understand that many changes, modifications, and even equivalent changes can be made within the spirit and scope defined by the claims of the present invention, but all will fall within the protection scope of the present invention.

Claims

1. A method for preparing a bipolar membrane based on polyvinyl chloride (PVC), characterized in that: Through physical and chemical modification, PVC is converted into the main raw material of bipolar membranes, realizing the efficient recycling of by-products in the chlor-alkali industry.

2. The preparation method of the bipolar membrane according to claim 1, characterized in that: This method includes the preparation of anion exchange membranes, the preparation of cation exchange membranes, and the final assembly of bipolar membranes.

3. The preparation method of the bipolar membrane according to claim 2, characterized in that: During the preparation of anion exchange membranes, polyvinyl chloride with a mass ratio of 1:1 to 1:4, polyethyleneimine, and 84% N,N-dimethylacetamide are mixed, and 0-1% of nano-TiO2 powder based on m(PVC+PEI) is added to improve the hydrophilicity and conductivity of the membrane.

4. The preparation method of the bipolar membrane according to claim 2, characterized in that: During the preparation of cation exchange membranes, polyvinyl chloride with a mass ratio of 7:3 to 3:7, cation exchange resin powder, and 80% N,N-dimethylacetamide are mixed, and further 0-20% of polyethylene glycol based on m(PVC+resin powder) is added to optimize the ion selectivity of the membrane.

5. The preparation method of the bipolar membrane according to claim 2, wherein: In a hot press, at a temperature range of 60-100°C, a pressure of 8 MPa, and a hot pressing time of 20 min, under specific temperature and pressure conditions, the anion and cation exchange membranes are tightly combined to form a stable and efficient bipolar membrane.

6. The method for preparing a bipolar membrane according to claims 3 and 4, characterized in that: Specific polymer, additive, and solvent ratios can be adjusted according to the performance requirements of the desired membrane. During the preparation of anion exchange membranes, the mass ratio of polyvinyl chloride to polyethyleneimine is 1:2, and the nano-TiO2 powder accounts for 0.5% of m(PVC+PEI); during the preparation of cation exchange membranes, the mass ratio of polyvinyl chloride to cation exchange resin powder is 4:6, and polyethylene glycol accounts for 10% of m(PVC+resin powder); the hot pressing temperature is 80°C to achieve the best electrochemical performance and mechanical stability.

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