Microwave sintering-based polytetrafluoroethylene / ceramic composite membrane as well as preparation method and application thereof

Abstract

The invention discloses a polytetrafluoroethylene ceramic membrane and a preparation method and application thereof, and the preparation method comprises the following steps: (1) adding polytetrafluoroethylene and a binder into water, and carrying out first mixing to obtain polytetrafluoroethylene slurry; adding a carbon nanotube and a dispersing agent into water, and carrying out secondary mixing, so as to obtain a carbon nanotube dispersion liquid; (2) carrying out third mixing on the carbon nanotube dispersion liquid and the polytetrafluoroethylene slurry to obtain a membrane preparation liquid; and (3) coating the surface of a ceramic support with the membrane preparation liquid, drying, and carrying out microwave sintering. The PTFE / ceramic composite membrane is prepared by microwave sintering, so that the sintering time and the production period are obviously shortened, and the production efficiency and the energy utilization rate of the product are obviously improved; the prepared PTFE / ceramic composite membrane is narrow in pore diameter and uniform in pore diameter distribution, has good hydrophobicity and oil-water separation performance, can effectively intercept 30-200nm nanoparticles, and has excellent anti-pollution capacity and stable separation performance for water-in-oil emulsions with different concentrations.

Description

Technical Field

[0001] The present invention relates to the field of ceramic membrane preparation, and particularly to a polytetrafluoroethylene / ceramic composite membrane based on microwave sintering, a preparation method thereof, and an application thereof. Background Art

[0002] As a new generation of common separation technology, membrane separation technology generally has the advantages of simple process, compact equipment, environmental friendliness, etc. In recent years, great progress has been made in the development of hydrophobic membranes, which have been widely used in the fields of wastewater treatment, carbon capture, gas separation, etc. and shown broad development prospects. Among them, membrane contactors, as a mature and environmentally friendly technology, have become the main application points of hydrophobic membranes. Membrane wetting is an inherent problem of membrane contactors. The penetration and wetting of membrane pores by liquid absorbents will lead to an increase in mass transfer resistance, deterioration of membrane flux and overall long-term stability performance. Most of the liquid absorbents used in membrane contactors are aqueous solutions. Therefore, the use of hydrophobic membranes can minimize membrane wetting and effectively prevent the destruction of mass transfer channels caused by the wetting or corrosion of membrane pores by absorbents, thereby improving the separation performance of membrane contactors and extending their service life. Low surface energy organic materials represented by polytetrafluoroethylene (PTFE) have good chemical stability and thermal stability, and are excellent choices for preparing hydrophobic membranes.

[0003] However, PTFE itself has limited mechanical strength and is easy to wear, and usually requires a carrier for support during preparation. And PTFE porous membranes usually require a sintering process at about 300 - 400 °C to bond particles and decompose the remaining organic substances during preparation, which makes common organic carriers unable to meet the requirements. In recent years, using ceramic materials as carriers for organic membranes has become a trend. Due to the hydrophilicity of ceramic membranes themselves, it limits their application in the fields of crude oil dehydration, waste oil purification, removal of organic pollutants, etc. Using PTFE as a hydrophobic modification material is a feasible means. Therefore, combining PTFE and ceramics to prepare a hydrophobic PTFE-ceramic composite membrane is a two-way choice.

[0004] Due to the high solubility resistance and melt creep viscosity of PTFE, PTFE particles cannot flow uniformly at temperatures above its melting point. Instead, they sinter like powder metals and ceramics, adhering to each other at the contact points to form a porous membrane. This makes it impossible to process PTFE using phase inversion or melt stretching techniques. The PTFE membranes prepared by the mechanical stretching method have the characteristics of a node-fiber pore structure and a wide pore size distribution, which is not conducive to precise separation. However, under traditional sintering methods, the long-term heating and heat preservation of PTFE membranes in the temperature field make the membrane layer prone to densification and difficult to maintain a good pore structure. And traditional muffle furnace sintering heats through heat transfer, from the surface to the inside, which is easy to generate a temperature gradient. It is possible that the PTFE particles on the surface have already undergone molten bonding, while the bonding between the bottom and the support has not started yet.

[0005] Microwaves can almost instantaneously heat the object to be heated, and can significantly reduce the sintering temperature, shorten the sintering time and production cycle, and remarkably improve the production efficiency of products. Microwave energy can be directly absorbed by materials. If the insulation system is well designed, there is almost no heat loss, and the energy utilization rate is very high, saving 50% - 80% energy compared with conventional sintering. However, different materials have quite different microwave absorption capabilities. For example, some ceramic materials (aluminum oxide) hardly absorb microwaves under normal circumstances, and PTFE, as a hydrophobic modification material, also does not have microwave absorption ability.

[0006] In the case where a PTEF / ceramic composite membrane with a well-maintained pore structure cannot be obtained by traditional muffle furnace sintering, applying microwave sintering to the preparation of PTEF / ceramic composite membranes is a viable option. However, currently, microwave sintering technology is mainly applied in the field of material preparation such as inorganic materials and composite materials to improve the strength and toughness of materials. When using microwave sintering to prepare materials, there is no need to consider its impact on the pore structure of the materials. However, composite ceramic membranes are usually used in fields such as filtration and separation, requiring them to have a uniformly distributed pore structure. Their functions and microstructural characteristics are different from those of conventional coatings in the material field, and the impact of the sintering process on their membrane pore structure must be considered during their preparation process. Therefore, how to apply microwave sintering to the preparation of PTEF / ceramic composite membranes and ensure that the microporous structure of the prepared composite membrane meets the product requirements is a technical problem that is difficult to solve.

[0007] Therefore, there is an urgent need to provide an effective preparation method to combine PTFE and a ceramic support to prepare a PTFE / ceramic composite membrane with good hydrophobicity and separation performance. Summary of the Invention

[0008] The object of the present invention is to address the problem that the prior art cannot well combine PTFE and a ceramic support to prepare a PTFE / ceramic composite membrane with good hydrophobicity and separation performance, and to provide a preparation method of a PTFE / ceramic composite membrane based on microwave sintering, a PTFE / ceramic composite membrane prepared by using this preparation method, and its application in oil purification. The PTFE / ceramic composite membrane prepared by the present invention using the microwave sintering method has good liquid penetration performance, and a very good surface hydrophobic effect is achieved by using PTFE. The entire ceramic membrane has a good oil-water separation effect due to the PTFE layer added on the surface of the ceramic support.

[0009] To achieve the above object, the present invention discloses a preparation method of a polytetrafluoroethylene / ceramic composite membrane based on microwave sintering, which includes the following steps:

[0010] (1) Add polytetrafluoroethylene and a binder to water for the first mixing, and mix evenly to obtain a polytetrafluoroethylene slurry; add carbon nanotubes and a dispersant to water for the second mixing, and mix evenly to obtain a carbon nanotube dispersion.

[0011] (2) Conduct the third mixing of the carbon nanotube dispersion and the polytetrafluoroethylene slurry, and mix evenly to obtain a film-forming solution.

[0012] (3) Coat the film-forming solution on the surface of a ceramic support, dry it, and sinter it by microwave to obtain the polytetrafluoroethylene / ceramic composite membrane.

[0013] In step (1), the polytetrafluoroethylene is a polytetrafluoroethylene concentrated dispersion or polytetrafluoroethylene powder, preferably a polytetrafluoroethylene concentrated dispersion, and the polytetrafluoroethylene concentrated dispersion can be obtained by purchasing from the market; the solid content of polytetrafluoroethylene in the polytetrafluoroethylene concentrated dispersion is about 60 wt%; the particle size of the polytetrafluoroethylene is 0.15 - 1.5 μm.

[0014] In step (1), the binder is any one or a combination of polyvinyl alcohol (PVA) aqueous solution, polyvinyl butyral, polyvinylpyrrolidone, sodium polyacrylate, sodium carboxymethylcellulose, sodium dodecylbenzenesulfonate, polysiloxane, and fatty alcohol polyoxyethylene ether (AEO), preferably a combination of polyvinyl alcohol aqueous solution or polyvinyl alcohol aqueous solution and fatty alcohol polyoxyethylene ether; the concentration of the polyvinyl alcohol aqueous solution is 1 - 10 wt%; the polyvinyl alcohol is any one of polyvinyl alcohol 1788 type, polyvinyl alcohol 2088 type, and polyvinyl alcohol 1799 type, preferably polyvinyl alcohol 1788 type; preferably, the preparation method of the polyvinyl alcohol aqueous solution is: heat water to 70 - 90 °C, and under stirring conditions, add polyvinyl alcohol in 2 - 3 batches to water (add the next batch of polyvinyl alcohol after the previous batch has dissolved), after adding all the polyvinyl alcohol, continue to keep it warm at 70 - 90 °C for 10 - 14 h under stirring, cool it to room temperature and then store it in a refrigerator at 1 - 10 °C.

[0015] In step (1), the carbon nanotubes are any one or a combination of single-walled carbon nanotubes, multi-walled carbon nanotubes, double-walled carbon nanotubes, amino-functionalized carbon nanotubes, and carboxyl-functionalized carbon nanotubes, preferably multi-walled carbon nanotubes; the outer diameter of the carbon nanotubes is 8 - 80 nm, and the length is 0.5 - 50 μm.

[0016] In step (1), the dispersant is any one or a combination of polyoxyethylene polyoxypropylene ether triblock copolymer (F127), poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (P123), sodium dodecylbenzenesulfonate, polyacrylic acid, and polyvinyl alcohol, preferably F127.

[0017] In step (1), the mass percentage of the polytetrafluoroethylene in the polytetrafluoroethylene slurry is 0.01% to 2%; the concentration of the carbon nanotubes in the carbon nanotube dispersion is 0.02 to 0.2 mg / mL.

[0018] In step (2), the mass ratio of the dispersant, binder, carbon nanotubes and polytetrafluoroethylene in the film-forming solution is (0.2 to 3):(1 to 25):(1 to 3):100, preferably (0.2 to 3):1:(1 to 3):100 or (0.2 to 3):25:(1 to 3):100.

[0019] Among them, for the first mixing, second mixing and third mixing, the mixing method is stirring mixing or ultrasonic mixing; preferably, the method of the first mixing is stirring for 2 to 6 h; the method of the second mixing is ultrasonic wave at 300 to 600 W for 10 to 20 minutes to uniformly disperse the carbon nanotubes in water; the method of the third mixing is ultrasonic wave at 300 to 600 W for 10 to 20 minutes and then stirring for 1 to 2 h.

[0020] In step (3), the material of the ceramic support is alumina, zirconia, silica or silicon nitride, preferably alumina; the average pore diameter of the ceramic support is 100 nm to 3 μm; the ceramic support can be obtained by purchasing from the market; the coating method is vacuum filtration, dip coating, spraying or spin coating, preferably vacuum filtration.

[0021] In step (3), the film-forming solution is coated on the surface of the ceramic support so that the amount of polytetrafluoroethylene loaded on the surface of the ceramic support is 0.2 to 2.0 mg / cm 2 .

[0022] In step (3), the drying is carried out at 70 to 110 °C for 4 to 8 h; preferably, the drying is first carried out at room temperature for 1 to 2 h, and then successively dried at 70 °C and 110 °C for 2 to 3 h each.

[0023] In step (3), the power of the microwave sintering is 80 to 1000 W, and the time is 5 to 40 minutes.

[0024] The present invention also claims to protect the polytetrafluoroethylene / ceramic composite membrane prepared by the above preparation method; the thickness of the polytetrafluoroethylene membrane layer in the polytetrafluoroethylene / ceramic composite membrane is 0.2 to 5 μm, and the average membrane pore diameter is 75 to 85 nm.

[0025] The present invention also provides the application of the above polytetrafluoroethylene / ceramic composite membrane in oil purification.

[0026] Among them, when using the polytetrafluoroethylene / ceramic composite membrane for oil purification, the membrane surface flow rate is 1 to 3 m·s -1, the transmembrane pressure difference is 2 - 4 bar, and the temperature is 20 - 60 °C.

[0027] Microwave sintering can directly act on the interior of the material to achieve bulk heating, resulting in a very small temperature gradient within the material, enabling the particles in the film layer to be uniformly heated during sintering, thereby making the bonding between particles more orderly and the pore distribution more uniform; the selective heating of microwave sintering on residual organic matter accelerates its decomposition, which can avoid the pore collapse caused by the slow decomposition of traditional sintering for a long time; the microwave sintering time is short, which can inhibit the excessive growth of grains, retain more pore structures formed by the initial particle packing, and at the same time, the microwave electromagnetic field enhances the ion mobility, preferentially promoting volume diffusion to enable rapid bonding between particles through point contact, which is conducive to the formation of an open pore network. The present invention creatively applies the microwave sintering technology to the preparation of PTFE / ceramic composite membranes by utilizing the above characteristics of microwave sintering.

[0028] Different ceramic materials have significant differences in their microwave absorption capabilities. Some ceramics, such as alumina, hardly absorb microwaves under normal circumstances, and at the same time, PTFE, which is the main material of the hydrophobic ceramic membrane of the present invention, also does not have the ability to absorb microwaves. Therefore, in the present invention, carbon nanotubes are uniformly mixed with polytetrafluoroethylene and then uniformly formed into a film on the ceramic membrane carrier in the form of particle packing. The uniformly distributed carbon nanotubes absorb microwaves to achieve uniform bulk heating, effectively increasing the ability of the PTFE film layer to absorb microwaves, enabling the coated PTFE film layer to absorb microwaves under microwave irradiation, generate heat inside the film layer, and complete sintering, thereby realizing the preparation of PTFE / ceramic composite membranes with good performance.

[0029] Although microwave heating can rapidly increase the temperature, it is difficult to control the temperature of the material during the heating process, which may cause local overheating or incomplete sintering of the material, thereby affecting the performance of the membrane; moreover, the selection and addition amount of carbon nanotube (CNT) materials will also directly affect the microstructure and hydrophobicity of the separation membrane. Therefore, the control of the pore structure is also the key to successfully preparing PTFE / ceramic composite membranes with good hydrophobic performance. The present invention has obtained a method for successfully preparing PTFE / ceramic composite membranes with good hydrophobic and separation performance by regulating a large number of experimental parameters of the microwave sintering process, the chemical composition and dosage parameters in the membrane-forming solution.

[0030] Beneficial effects:

[0031] 1. The PTFE / ceramic composite membrane of the present invention enhances the dielectric loss constant of the material by adding carbon nanotubes to the PTFE layer, solves the problem that the PTFE layer cannot absorb microwaves, realizes the preparation of PTFE / ceramic composite membranes by microwave sintering. Compared with traditional muffle sintering, it can effectively reduce the temperature gradient inside the material, significantly shorten the sintering time and production cycle, and significantly improve the production efficiency and energy utilization rate of the product.

[0032] 2. The PTFE / ceramic composite membrane prepared by the present invention has a narrow pore size and a uniform distribution, with good hydrophobicity, oil-water separation performance and oil purification ability. The rejection rate for nanoparticles with a size of 30 - 200 nm is as high as 99.9%; it has stable oil-water separation performance and excellent anti-pollution ability for oil-in-water emulsions with different concentrations, and the water phase rejection rate is above 90%. After the PTFE / ceramic composite membrane is subjected to oil-water separation and then backwashed with petroleum ether, its oil-water separation performance remains stable. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The following further specific description of the present invention will be made in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0034] Figure 1 SEM image of the surface of the ceramic support used in Example 1.

[0035] Figure 2 SEM image of the surface of the PTFE / ceramic composite membrane prepared in Example 1.

[0036] Figure 3 SEM cross-sectional image of the PTFE / ceramic composite membrane prepared in Example 1.

[0037] Figure 4 SEM image of the surface of the PTFE / ceramic composite membrane prepared in Comparative Example 1.

[0038] Figure 5 SEM image of the surface of the PTFE / ceramic composite membrane prepared in Comparative Example 2.

[0039] Figure 6 Wet-dry line graph of the PTFE / ceramic composite membrane measured by the bubble pressure method in Example 5.

[0040] Figure 7 Pore size distribution graph of the PTFE / ceramic composite membrane measured by the bubble pressure method in Example 5.

[0041] Figure 8 Normalized flux statistical graph of the PTFE / ceramic composite membrane when filtering PS nanoparticle dispersions with different particle sizes in Example 6.

[0042] Figure 9 Separation efficiency statistical graph of the PTFE / ceramic composite membrane for PS nanoparticle dispersions with different particle sizes in Example 6.

[0043] Figure 10 Separation effect statistical graph of the PTFE / ceramic composite membrane for soybean oil-in-water emulsion in Example 7.

[0044] Figure 11Statistical chart of the separation effect of the PTFE / ceramic composite membrane on removing particles in the white oil / water system in Example 8

[0045] Figure 12 Statistical chart of the separation effect of the PTFE / ceramic composite membrane on water-in-oil emulsions with different ratios in Example 9. Detailed implementation mode

[0046] The present invention will be further described according to the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention, rather than limiting the present invention.

[0047] For specific technologies or conditions not specified in the embodiments, they shall be carried out according to the technologies or conditions described in the literature in this field or according to the product instructions. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be purchased through regular channels.

[0048] The PTFE concentrated dispersion used in the following examples and comparative examples was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., product number: P-111312-500g, in which the PTFE solid content was 60 wt% and the average particle size was 0.15 - 0.17 μm.

[0049] Example 1

[0050] Prepare an aqueous PVA solution: Turn on the water bath and set the temperature to 80°C. Place a glass bottle containing 180 g of pure water in the water bath and heat it to 80°C. Weigh 20 g of polyvinyl alcohol type 1788 and divide it into 3 batches and add it to the water under stirring conditions (add the next batch of polyvinyl alcohol after the previous batch has dissolved). After adding all the polyvinyl alcohol, continue to keep it warm at 80°C with stirring for 12 h, cool it to room temperature and then store it in a 4°C refrigerator to obtain an aqueous PVA solution with a mass fraction of 10%.

[0051] At room temperature, mix a PTFE concentrated dispersion with a PTFE solid content of 60 wt% and an average particle size of 0.15 - 0.17 μm and an aqueous PVA solution with a mass fraction of 10% into pure water according to the mass ratio of PTFE to PVA of 100:1, and stir for 3 h to obtain a PTFE slurry with a PTFE solid content of 0.1 wt%.

[0052] Add multi-walled carbon nanotubes with an outer diameter of 30 - 80 nm and a length less than 10 μm and F127 to pure water according to a mass ratio of 1:0.8. The content of carbon nanotubes in pure water is 0.2 mg / mL. Use an ultrasonic cell disruptor to ultrasonicate for 15 minutes at a power of 500 W to make the carbon nanotubes disperse evenly in pure water to obtain a carbon nanotube dispersion.

[0053] Mix the carbon nanotube dispersion with the PTFE slurry. The mass ratio of carbon nanotubes to PTFE in the mixed solution is 2:100. After ultrasonic treatment at a power of 500 W for 10 min, stir for 2 h to obtain a film-forming solution;

[0054] Coat the film-forming solution evenly on an alumina ceramic support with an average pore size of 100 nm by vacuum filtration. Dry it at room temperature, 70 °C, and 110 °C for 2 h each in turn, and then put it into a microwave oven and sinter it at a power of 400 W for 10 minutes to obtain the PTFE / ceramic composite membrane of the present invention.

[0055] Example 2

[0056] Prepare an aqueous PVA solution: Turn on the water bath and set the temperature to 80 °C. Place a glass bottle containing 20 g of pure water in the water bath and heat it to 80 °C. Weigh 180 g of polyvinyl alcohol type 1788, and under stirring conditions, divide it into 3 batches and add it to the water (add the next batch of polyvinyl alcohol after the previous batch has dissolved). After adding all the polyvinyl alcohol, continue to keep it warm at 80 °C with stirring for 12 h. Cool it to room temperature and store it in a 4 °C refrigerator to obtain an aqueous PVA solution with a mass fraction of 10%.

[0057] At room temperature, mix PTFE powder with an average particle size of 1.5 μm and a PTFE concentrated dispersion with a PTFE solid content of 60 wt% and an average particle size of 0.15 - 0.17 μm in a ratio of 9:1 by mass of PTFE powder to PTFE in the concentrated dispersion. Then add an aqueous PVA solution with a mass fraction of 10% (control the mass ratio of total PTFE to PVA to be 100:5) and AEO (control the mass ratio of total PTFE to AEO to be 100:20), and finally add pure water and stir for 3 h to prepare a PTFE slurry with a PTFE solid content of 0.1 wt%;

[0058] Add oriented multi-walled carbon nanotubes with an outer diameter of 8 - 15 nm and a length of 30 - 50 μm and F127 to pure water in a mass ratio of 1:1. The content of carbon nanotubes in pure water is 0.15 mg / mL. Use an ultrasonic cell disruptor to ultrasonically treat at a power of 600 W for 10 minutes to uniformly disperse the carbon nanotubes in pure water and obtain a carbon nanotube dispersion;

[0059] Mix the carbon nanotube dispersion with the PTFE slurry. The mass ratio of carbon nanotubes to PTFE in the mixed solution is 1:100. After ultrasonic treatment at a power of 600 W for 15 min, stir for 2 h to obtain a film-forming solution;

[0060] The film-forming solution was uniformly coated on an alumina ceramic support with an average pore diameter of 1 μm by vacuum filtration. It was first dried at room temperature for 2 h, then successively dried at 70 °C and 110 °C for 3 h each, and then placed in a microwave oven and sintered at a power of 800 W for 5 minutes to obtain the PTFE / ceramic composite film of the present invention.

[0061] Example 3

[0062] Prepare an aqueous PVA solution: Turn on the water bath and set the temperature to 80 °C. Place a glass bottle containing 20 g of pure water in the water bath and heat it to 80 °C. Weigh 180 g of polyvinyl alcohol type 1788, and under stirring conditions, divide it into 3 batches and add it to the water (add the next batch of polyvinyl alcohol after the previous batch has dissolved). After all the polyvinyl alcohol has been added, continue to keep it warm at 80 °C with stirring for 12 h, cool it to room temperature, and then store it in a 4 °C refrigerator to obtain an aqueous PVA solution with a mass fraction of 10%.

[0063] At room temperature, a PTFE concentrated dispersion with a PTFE solid content of 60 wt% and an average particle size of 0.15 - 0.17 μm and an aqueous PVA solution with a concentration of 10 wt% were added to pure water in a ratio of 100:1 by mass of PTFE to PVA, and stirred for 3 h to obtain a PTFE slurry with a PTFE solid content of 0.01 wt%.

[0064] Double-walled carbon nanotubes with an outer diameter of 10 - 20 nm and a length of 0.5 - 2 μm and F127 were added to pure water in a mass ratio of 1:0.5. The content of carbon nanotubes in pure water was 0.1 mg / mL, and an ultrasonic cell disruptor was used to ultrasonicate at a power of 400 W for 20 minutes to uniformly disperse the carbon nanotubes in pure water to obtain a carbon nanotube dispersion.

[0065] The carbon nanotube dispersion was mixed with the PTFE slurry. The mass ratio of carbon nanotubes to PTFE in the mixed solution was 3:100. After ultrasonication at a power of 500 W for 15 min, it was stirred for 2 h to obtain a film-forming solution.

[0066] The film-forming solution was uniformly coated on an alumina ceramic support with an average pore diameter of 100 nm by vacuum filtration. It was first dried at room temperature for 1 h, then successively dried at 70 °C and 110 °C for 2 h each, and then placed in a microwave oven and sintered at a power of 80 W for 25 minutes to obtain the PTFE / ceramic composite film of the present invention.

[0067] Comparative Example 1

[0068] At room temperature, a PTFE concentrated dispersion with a PTFE solid content of 60 wt% and an average particle size of 0.15 - 0.17 μm was mixed with a 10% by mass PVA aqueous solution (prepared in the same way as in Example 1) in pure water at a ratio of the mass ratio of PTFE to PVA of 100:1, and stirred for 3 h to obtain a PTFE slurry with a PTFE solid content of 0.1 wt%.

[0069] The above PTFE slurry was coated on an alumina ceramic support with an average pore size of 100 nm by vacuum filtration, dried at room temperature, 70 °C, and 110 °C for 2 h each, and then sintered in a microwave oven at a power of 400 W for 10 minutes to obtain the product.

[0070] Comparative Example 2

[0071] At room temperature, a PTFE concentrated dispersion with a PTFE solid content of 60% (w / w) and an average particle size of 0.15 - 0.17 μm was mixed with a 10% by mass PVA aqueous solution (prepared in the same way as in Example 1) in pure water at a ratio of the mass ratio of PTFE to PVA of 100:1, and stirred for 3 h to obtain a PTFE slurry with a PTFE solid content of 0.1 wt%.

[0072] The above PTFE slurry was uniformly coated on an alumina ceramic support with an average pore size of 100 nm by vacuum filtration, dried at room temperature, 70 °C, and 110 °C for 2 h each, and then calcined in a muffle furnace at 330 °C for 30 min with a heating rate of 5 °C / min. After cooling to room temperature, the product was obtained.

[0073] Example 4

[0074] The surface of the ceramic support used in Example 1, the surface and cross-section of the PTFE / ceramic composite membrane prepared in Example 1, and the surfaces of the PTFE / ceramic composite membranes prepared in Comparative Example 1 and Comparative Example 2 were observed using a scanning electron microscope, and the results are as follows Figures 1 to 5 shown.

[0075] Figure 1 It is a scanning electron microscope (SEM) image of the surface of an alumina ceramic support with an average pore size of 100 nm. It can be seen that the surface of the ceramic support is not coated with PTFE particles at this time; Figure 2 It is an SEM image of the surface of the PTFE / ceramic composite membrane prepared in Example 1. It can be seen that on the surface of the ceramic support, carbon nanotubes are evenly dispersed in the PTFE film layer, and there are obvious sintering marks on the PTFE particles; Figure 3 It is a cross-section SEM image of the PTFE / ceramic composite membrane prepared in Example 1. It can be seen that the PTFE film layer is tightly bonded to the underlying ceramic film layer; Figure 4SEM image of the surface of the PTFE / ceramic composite membrane prepared in Comparative Example 1. When preparing the PTFE / ceramic composite membrane in Comparative Example 1, carbon nanotubes were not added. No sintering traces of PTFE particles could be observed in the prepared PTFE / ceramic composite membrane under the electron microscope, indicating that the presence of carbon nanotubes in the PTFE membrane layer is the key for the membrane layer to absorb microwaves for sintering; Figure 5 SEM image of the surface of the PTFE / ceramic composite membrane prepared in Comparative Example 2. The PTFE / ceramic composite membrane was prepared by sintering in a muffle furnace. It can be seen that the PTFE particles are melted into a dense state, and there are only slit-shaped pore structures on the membrane surface, indicating that the traditional sintering method has uneven heating, long heating and heat preservation times, and is prone to densification of the membrane surface.

[0076] Example 5

[0077] The pore size of the PTFE membrane layer of the PTFE / ceramic composite membrane prepared in Example 1 was measured using the bubble pressure method.

[0078] Add isobutanol to a suction flask, immerse the PTFE / ceramic composite membrane in it, turn on the vacuum filter and filter until there are no bubbles in the suction flask. At this time, the membrane pores are filled with isobutanol. Use a porous material pore size analyzer (American PMI multi-functional pore size analyzer ipore 1500) to measure the gas flux of the PTFE / ceramic composite membrane under different transmembrane pressure differences, summarize the data, and calculate the pore size and distribution of the membrane.

[0079] The experimental results are as Figure 6 and Figure 7 shown. The pore size of the PTFE / ceramic composite membrane is concentrated in the range of 60 - 85 nm, and the average pore size is 78 nm, which is very close to the average pore size of 79 nm calculated from the intersection of the semi-trunk line and the wet line. This indicates that the pore size distribution of the PTFE / ceramic composite membrane prepared in Example 1 is very uniform.

[0080] Example 6

[0081] The pore size screening performance of the PTFE / ceramic composite membrane prepared in Example 1 was tested. The specific method is as follows: Monodisperse polystyrene (PS) nanoparticles with four particle sizes (30 nm, 50 nm, 100 nm, and 200 nm) were respectively dispersed in anhydrous ethanol to form a dispersion. The concentration of the PS nanoparticles with the four particle sizes in the dispersion was 50 ppm. The above PS nanoparticle dispersion was subjected to cross-flow filtration for 1 h using the PTFE / ceramic composite membrane prepared in Example 1. The filtration process was carried out at a membrane surface flow rate of 25 °C and 2 m·s -1 and a transmembrane pressure difference of 1.5 bar. The membrane flux was measured during the filtration process, and the permeate was collected. The absorbance and turbidity of the permeate were respectively detected using an ultraviolet spectrophotometer and a turbidimeter to test the retention performance of the PTFE / ceramic composite membrane for the PS nanoparticle dispersions with different particle sizes.

[0082] Membrane permeation flux where J is the membrane permeation flux (L·m -2 ·h -1 ·bar -1 ), V is the volume of the permeate (L), A is the membrane area (m 2 ), t is the filtration time (h), and ΔP l is the transmembrane pressure difference (bar).

[0083] The rejection performance of the PTFE / ceramic composite membrane was analyzed by measuring the concentrations of the target substance (PS nanoparticles) in the feed (initial PS nanoparticle dispersion) and the permeate. The concentration of PS nanoparticles in the permeate was detected using a UV-visible spectrophotometer (absorption wavelength: 260 nm), and the turbidity of the permeate was measured using a turbidimeter. The calculation methods for the corresponding rejection rates of PS nanoparticles were as follows: Rejection rate (UV) = (1 - N p / N f ) × 100%; Rejection rate (turbidity) = (1 - T p / T f ) × 100%; where N f and T f are the concentration and turbidity of PS nanoparticles in the feed, respectively; N p and T p are the concentration and turbidity of PS nanoparticles in the permeate, respectively.

[0084] Normalized flux calculation formula: J norm = J t / J0, where J norm is the normalized flux; J t is the membrane permeation flux at filtration time t; J0 is the membrane permeation flux at the beginning of filtration.

[0085] The experimental results are shown in Figure 8 and Figure 9 . Figure 8 is the statistical chart of the normalized flux of the PTFE / ceramic composite membrane when filtering PS nanoparticle dispersions with different particle sizes. By normalizing the membrane flux, the influence of operating conditions on the evaluation of membrane performance was eliminated. It can be seen from the figure that during the 60-minute membrane separation process of PS nanoparticle dispersions with different particle sizes, the flux of the PTFE / ceramic composite membrane only decreased by 10% - 20%. The decrease in membrane flux was mainly due to the formation of agglomerates of PS nanoparticles on the membrane surface.

[0086] Figure 9It is a statistical chart of the separation efficiency of the PTFE / ceramic composite membrane for PS nanoparticle dispersions with different particle sizes. As can be seen from the figure, after 60 minutes of membrane separation of PS nanoparticle dispersions with different particle sizes using the PTFE / ceramic composite membrane, the permeation flux of the PTFE / ceramic composite membrane still remains at 110 L·m -2 ·h -1 ·bar -1 or above. The permeate was detected using an ultraviolet spectrophotometer and a turbidimeter respectively. The results showed that there was almost no light-absorbing substance in the permeate of the PS nanoparticle dispersions of each particle size, and its turbidity was also close to that of pure ethanol solvent. The PS nanoparticle rejection rate was as high as 99.9%. This indicates that almost all of the PS nanoparticles with diameters of 30 - 200 nm were retained by the PTFE / ceramic composite membrane.

[0087] Example 7

[0088] The PTFE / ceramic composite membrane prepared in Example 1 was tested for its performance in dehydrating soybean oil. The specific method was as follows: A mixed liquid with a mass ratio of soybean oil to water of 4:1 was prepared, and the PTFE / ceramic composite membrane prepared in Example 1 was used to perform cross-flow filtration on it for 1 h continuously. The filtration process was carried out at 30°C, a membrane surface velocity of 3 m·s -1 and a transmembrane pressure difference of 2.0 bar. The rejection rate of water and the permeation flux of soybean oil during this process were recorded, and the calculation method was referred to Example 6.

[0089] The results are as Figure 10 shown. During the continuous 1-hour filtration process, the permeation flux of the PTFE / ceramic composite membrane decreased slowly, and only decreased by 23%. The rejection rate was stable above 95%, indicating that the PTFE / ceramic composite membrane of the present invention has good performance in dehydrating oil products.

[0090] Example 8

[0091] The PTFE / ceramic composite membrane prepared in Example 1 was tested for its performance in separating powders in a white oil / water system. The specific method was as follows: Alumina powders with particle sizes of 1 μm and 0.5 μm were respectively added to a white oil / water system (mass ratio of white oil to water of 5:1). The white oil / water system containing alumina powders was subjected to continuous 1-hour cross-flow filtration at 30°C, a membrane surface velocity of 3 m·s -1 and a transmembrane pressure difference of 2.0 bar. The rejection rate of the powder was calculated by measuring the turbidity of the permeate, and the permeation flux during the filtration process was calculated. The calculation method was referred to Example 6. The experimental results are as Figure 11 shown. During the separation process of the white oil / water system by the PTFE / ceramic composite membrane, the permeation performance of white oil was good, and 100% rejection of the powder could be achieved.

[0092] Example 9

[0093] Separation performance test of the water-in-oil emulsion in the extraction process using the PTFE / ceramic composite membrane prepared in Example 1.

[0094] Prepare a water-in-oil emulsion by simulating the extraction process: Prepare feed solutions with volume ratios of soybean oil to water of 6:1, 4:1, and 2:1 respectively. Select ethyl acetate as the extractant and extract the feed solutions respectively. The volume ratio of ethyl acetate to the feed solution is 1:1 to obtain a water-in-oil emulsion.

[0095] Use the PTFE / ceramic composite membrane prepared in Example 1 to conduct cross-flow filtration on the water-in-oil emulsion for 2 consecutive hours. The cross-flow filtration is carried out at 30 °C, a membrane surface velocity of 3 m·s -1 and a transmembrane pressure difference of 2.0 bar, and measure the rejection rate of water and the permeation flux of soybean oil by the PTFE / ceramic composite membrane during the filtration process.

[0096] The experimental results are as Figure 12 shown. During the 2-hour continuous cross-flow filtration process, the flux of the PTFE / ceramic composite membrane decreases slowly, and the rejection rate is stable at about 90%.

[0097] After the filtration is completed, rinse the PTFE / ceramic composite membrane with petroleum ether and dry it, and then use it again to conduct the separation performance test on the above water-in-oil emulsion. The results show that its separation performance is still stable, indicating that the PTFE / ceramic composite membrane of the present invention has good anti-fouling performance. This shows that the PTFE membrane layer of the heat-treated PTFE / ceramic composite membrane is stably combined with the ceramic support and can be recycled without affecting the separation effect.

[0098] The present invention provides an idea and method for a polytetrafluoroethylene / ceramic composite membrane based on microwave sintering, its preparation method and application. There are many ways to specifically implement this technical solution. The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be implemented by existing technologies.

Claims

1. A preparation method of a polytetrafluoroethylene / ceramic composite membrane based on microwave sintering, characterized in that, It includes the following steps: (1) Add polytetrafluoroethylene and a binder to water for the first mixing, and mix evenly to obtain a polytetrafluoroethylene slurry; add carbon nanotubes and a dispersant to water for the second mixing, and mix evenly to obtain a carbon nanotube dispersion; (2) Conduct the third mixing of the carbon nanotube dispersion and the polytetrafluoroethylene slurry, and mix evenly to obtain a film-forming solution; (3) Coat the film-forming solution on the surface of a ceramic support, dry it, and sinter it by microwave to obtain the polytetrafluoroethylene / ceramic composite membrane.

2. The preparation method according to claim 1, characterized in that, In step (1), the polytetrafluoroethylene is a polytetrafluoroethylene concentrated dispersion or polytetrafluoroethylene powder; the particle size of the polytetrafluoroethylene is 0.15 - 1.5 μm; The binder is any one or a combination of polyvinyl alcohol aqueous solution, polyvinyl butyral, polyvinylpyrrolidone, sodium polyacrylate, sodium carboxymethyl cellulose, sodium dodecylbenzenesulfonate, polysiloxane, and fatty alcohol polyoxyethylene ether; the concentration of the polyvinyl alcohol aqueous solution is 1% - 10%; the polyvinyl alcohol is any one of polyvinyl alcohol 1788 type, polyvinyl alcohol 2088 type, and polyvinyl alcohol 1799 type; The carbon nanotubes are any one or a combination of single-walled carbon nanotubes, multi-walled carbon nanotubes, double-walled carbon nanotubes, amino-functionalized carbon nanotubes, and carboxyl-functionalized carbon nanotubes; the outer diameter of the carbon nanotubes is 8 - 80 nm, and the length is 0.5 - 50 μm; The dispersant is any one or a combination of polyoxyethylene polyoxypropylene ether triblock copolymer, polyethylene oxide - polypropylene oxide - polyethylene oxide triblock copolymer, sodium dodecylbenzenesulfonate, polyacrylic acid, and polyvinyl alcohol.

3. The preparation method according to claim 1, characterized in that, In step (1), the mass percentage of the polytetrafluoroethylene in the polytetrafluoroethylene slurry is 0.01% - 2%, and the concentration of the carbon nanotubes in the carbon nanotube dispersion is 0.02 - 0.2 mg / mL; in step (2), the mass ratio of the dispersant, binder, carbon nanotubes, and polytetrafluoroethylene in the film-forming solution is (0.2 - 3):(1 - 25):(1 - 3):

100.

4. The preparation method according to claim 1, characterized in that, For the first mixing, second mixing, and third mixing, the mixing method is stirring mixing or ultrasonic mixing.

5. The preparation method according to claim 1, characterized in that, In step (3), the material of the ceramic support is alumina, zirconia, silica, or silicon nitride; the coating method is vacuum filtration, dip coating, spraying, or spin coating.

6. The preparation method according to claim 1, characterized in that, In step (3), the film-forming solution is coated on the surface of the ceramic support so that the amount of polytetrafluoroethylene loaded on the surface of the ceramic support is 0.2 to 2.0 mg / cm 2 .

7. The preparation method according to claim 1, characterized in that, In step (3), the drying temperature is 70 - 110 °C, and the time is 4 - 8 h; the power of the microwave sintering is 80 - 1000 W, and the time is 5 - 40 minutes.

8. The polytetrafluoroethylene / ceramic composite membrane prepared by the preparation method according to any one of claims 1 - 7.

9. The application of the polytetrafluoroethylene / ceramic composite membrane according to claim 8 in oil purification.

10. The application according to claim 9, characterized in that, When using the polytetrafluoroethylene / ceramic composite membrane for oil purification, the flow velocity on the membrane surface is 1 to 3 m·s -1 , the transmembrane pressure difference is 2 to 4 bar, and the temperature is 20 to 60 °C.

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