A polytetrafluoroethylene porous membrane and preparation and use thereof

Abstract

The application discloses a kind of polytetrafluoroethylene porous and preparation method thereof.The method comprises the following steps: water-soluble polymer spinning carrier solution, dispersion emulsion, regulator or reinforcing body solution is mixed in certain proportion respectively to obtain porous support layer, middle layer and surface layer spinning solution, then sequentially spin to obtain nascent film, and then solidify in coagulation bath, sintering heat treatment in muffle furnace, cleaning and drying, to obtain multistage porous structure PTFE porous membrane.The application does not use fluorosilane, hydrophobic nanomaterial and other reagents, reduces reagent pollution and recovery, avoids nanoparticle shedding, is integrally formed by sintering process PTFE in-situ mutual adhesion, without layering, high strength, with mutually interpenetrating open structure, good permeability, resistant to high temperature and chemical reagent, with self-supporting, structure controllable, thickness adjustable.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, specifically to a polytetrafluoroethylene porous membrane and its preparation method. Background Technology

[0002] Water scarcity and water pollution have become bottlenecks restricting social progress and economic development. Developing efficient water treatment technologies is a realistic option for solving the global water crisis. Membrane distillation is a novel separation technology that combines membrane technology with traditional distillation techniques. It has low energy consumption, can be carried out at normal pressure and low temperature, and theoretically retains 100% of non-volatile components. It can fully utilize low-quality heat sources such as industrial waste heat and has broad application prospects in seawater desalination, wastewater treatment, and other fields.

[0003] Membrane distillation is a membrane process that uses a hydrophobic microporous membrane as the separation interface and, under thermally driven conditions, utilizes the vapor pressure difference of volatile components across the membrane as the mass transfer driving force to achieve selective separation of the feed liquid. The porosity and hydrophobicity of the membrane used in membrane distillation are crucial parameters. Improving the membrane's hydrophobicity can effectively solve the membrane wetting phenomenon, enhancing its antifouling ability and the efficiency of the membrane distillation process. The key to preparing superhydrophobic membranes lies in increasing the membrane surface roughness while reducing its surface energy. A common method is to obtain a superhydrophobic membrane by coating the surface with inorganic nanoparticles and then modifying the surface. However, surface-coated inorganic nanoparticles suffer from insufficient stability and are prone to clogging the membrane pores, and a single rough structure is insufficient to achieve a sustained and stable superhydrophobic effect. Therefore, increasing the membrane porosity while ensuring the membrane remains unwetted can effectively increase the membrane distillation flux.

[0004] Traditional phase inversion membrane fabrication suffers from thick skin layers and poor permeability, while stretching methods result in large pore sizes and insufficient separation precision. Compared to traditional non-solvent-induced phase separation, thermally induced phase separation, and melt-stretching membrane preparation processes, electrospinning produces nanofiber membranes with high porosity (>80%), excellent through-pore structure, and no blind pores, making it easier to achieve higher fluxes in membrane distillation processes.

[0005] In existing technologies, patent 202011239575.0 uses hydrophobic polymers and inorganic nanoparticles as spinning solutions, and volatile and non-volatile solvents as mixed solvents, to prepare superhydrophobic membrane materials with various surface morphologies such as smooth fibers, porous fibers, and porous microspheres through electrospinning technology. However, using a blending method, inorganic particles are easily embedded by the polymer, and the porous structure can affect the strength of the fiber membrane to some extent. Patent 201410757998.X uses atomic layer deposition to coat PTFE mesh fibers with micro-nano particles, then treats the PTFE membrane surface with plasma, and finally places it in a perfluorinated monomer for plasma grafting treatment to obtain a superhydrophobic polytetrafluoroethylene membrane. However, this method has a relatively complex process and is not easily scaled up. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a polytetrafluoroethylene (PTFE) porous membrane and its preparation method. The prepared PTFE porous membrane has a multi-layered, multi-level pore structure. The bottom porous support layer provides high strength and large mass transfer flux, the middle layer imparts high separation accuracy to the membrane, and the surface rough microsphere structure imparts strong and stable hydrophobicity to the membrane. The three layers are bonded together by PTFE through a sintering process, without delamination, exhibiting high strength and an interconnected open-pore structure, significantly reducing steam mass transfer resistance and increasing water flux. The three layers work synergistically to provide excellent durability, and have broad application prospects in waterproofing, moisture permeability, sound permeability, membrane distillation, and other fields.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:

[0008] Step 1: Mix the water-soluble polymer spinning carrier solution, polytetrafluoroethylene (PTFE) dispersion emulsion, and regulator in a certain proportion to obtain spinning solution 1;

[0009] The water-soluble polymer spinning carrier is one of polyvinyl alcohol (PVA), pullulan, sodium alginate, gelatin, polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP), preferably PVA. Further, the PVA solution concentration is 5-20 wt%; the PTFE dispersion emulsion has a solid content of 60 wt%.

[0010] The spinning solution 1 is composed of a spinning carrier polymer and polytetrafluoroethylene dry weight ratio of 1:1 to 14, and a regulator of 0.005 to 0.3 wt% of the total mass; further, the regulator is one of boric acid, calcium chloride, and glutaraldehyde, which can crosslink with PVA and adjust the viscosity of the spinning solution.

[0011] Step 2: Mix the water-soluble polymer spinning carrier solution, PTFE dispersion emulsion, and tetrafluoroethylene-perfluoropropyl vinyl ether copolymer (PFA) dispersion emulsion in a certain proportion to obtain spinning solution 2;

[0012] The spinning solution 2 is composed of: spinning carrier polymer: PTFE: PFA dry weight ratio of 1:1 to 14:1 to 8; the PFA dispersion emulsion has a solid content of 60 wt%, and PFA is a meltable perfluoropolymer that melts and flows during the sintering process, and they bond together, which plays a role in reducing and optimizing the pore structure.

[0013] Step 3: Mix the water-soluble polymer spinning carrier solution, PTFE dispersion emulsion, and PFA dispersion emulsion in a certain proportion to obtain spinning solution 3;

[0014] The spinning solution 3 is composed of: spinning carrier polymer: PTFE: PFA dry weight ratio of 1:15~30:1~5; PFA plays a role in mutual bonding and improving the stability of PTFE microspheres.

[0015] Step 4: Spinning solution 1 is spun into a porous support layer by gel electrospinning. Then, spinning solution 2 is spun into an intermediate layer by electrospinning. Then, spinning solution 3 is electrostatically sprayed to obtain a surface layer. The resulting nascent membrane is then placed in a coagulation bath to solidify and form a solid. After sintering and heat treatment in a muffle furnace, it is cleaned and dried to obtain the PTFE porous membrane.

[0016] The porous support layer is spun using a spinning solution of another polymer, which is spun in parallel with spinning solution 1 using a dual-needle spinning process. Further, the polymer is polyacrylonitrile (PAN), which is used as a reinforcement. During the spinning process, the fibers formed by the PAN and spinning solution 1 interweave to improve the strength of the nascent film. During the sintering process, PAN undergoes pre-oxidation cyclization, which plays a supporting role and prevents severe shrinkage during the sintering process. The molecular weight is 150,000 to 300,000, and the concentration of the spinning solution is 8 to 20 wt%.

[0017] The three layers of the porous support layer, intermediate layer and surface layer have the same spinning parameters: positive voltage 18-30kV, negative voltage -3 to -8kV, spinning distance 10-20cm, transverse oscillation speed 40-200mm / s, receiver is a roller or mandrel, respectively to prepare flat sheet membranes or hollow membranes, rotation speed 300-1000r / min, extrusion rate 0.6-1.5ml / h, needle-to-receiver angle 30-60°, temperature 20-30℃, humidity 10-40%;

[0018] The spinning / spraying parameters for the three layers of the nascent membrane—the porous support layer, the intermediate layer, and the surface layer—are consistent: the porous support layer is spun for 0.1–6 hours with a membrane thickness of 10–120 μm; the intermediate layer is spun for 0.1–6 hours with a membrane thickness of 10–120 μm; and the surface layer is sprayed for 0.1–2 hours with a membrane thickness of 0.5–30 μm.

[0019] The coagulation bath is a sodium sulfate solution, preferably with a concentration of 25-50 wt%, and the solidification time is 0.1-60 min;

[0020] The sintering heat treatment temperature is 350–410℃, and the time is 10–300 min;

[0021] The thickness of the substrate support layer is 10–80 μm, with an average pore size of 0.80–2.00 μm; the thickness of the intermediate layer is 10–80 μm, with an average pore size of 0.45–0.75 μm; the thickness of the surface layer is 0.1–20 μm; and the overall average pore size of the membrane is 0.20–0.65 μm.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] (1) The PTFE porous membrane prepared by the present invention has a multi-level and multi-grade pore structure. The rough structure of PTFE micro-nano microspheres / microfibers on the surface gives the membrane excellent superhydrophobicity, and the regular circular pore structure in the middle layer gives the membrane excellent separation performance. The high-strength substrate and the fiber-interwoven macroporous support layer give the membrane good permeability and strength.

[0024] (2) This invention does not use reagents such as fluorosilanes and hydrophobic nanomaterials, reducing reagent pollution and recycling, and avoiding nanoparticle shedding. By utilizing the low surface energy of PTFE itself, and combining it with the multi-level rough structure of PTFE nanofiber membranes, a stable and durable hydrophobic membrane is constructed, solving the problem of difficult modification of PTFE. No secondary modification is required, the process is simple, and it is suitable for large-scale preparation.

[0025] (3) The three-layer membrane of the present invention is formed by in-situ bonding of PTFE in the sintering process, without delamination, with high strength, with an open structure that is interconnected, good permeability, resistance to high temperature and chemical reagents, self-supporting, controllable structure, and adjustable thickness. Attached Figure Description

[0026] Figure 1 These are digital photographs of the nascent PTFE porous membrane prepared in Example 1 of this invention.

[0027] Figure 2 This is a digital photograph of the sintered PTFE porous membrane obtained in Example 1 of this invention.

[0028] Figure 3 This is a scanning electron microscope image of the cross-section of the PTFE porous membrane obtained in Example 1 of the present invention;

[0029] Figure 4 This is a scanning electron microscope image of the PTFE porous membrane surface obtained in Example 1 of the present invention;

[0030] Figure 5 This is a digital photograph of the water droplet state on the surface of the PTFE porous membrane obtained in Embodiment 1 of the present invention;

[0031] Figure 6 This is a static water contact angle image of the PTFE porous membrane surface obtained in Embodiment 1 of the present invention. Detailed Implementation

[0032] The following detailed description of a polytetrafluoroethylene porous membrane and its preparation method according to the present invention, with reference to the accompanying drawings and specific embodiments, is provided. The specific embodiments are only used to further illustrate the present invention and do not limit the scope of protection of the claims of this application.

[0033] Unless otherwise specified, the test reagents used in the following examples are all commercially available conventional chemical reagents, and the test methods used are all conventional methods.

[0034] Example 1

[0035] 10wt% PVA (model 1788, the same below) and PTFE dispersion emulsion (solid content 60wt%, the same below) were mixed at a PVA to PTFE dry weight ratio of 1:8. Boric acid with a final concentration of 0.1wt% was added and stirred for 6 hours to obtain spinning solution 1. 12wt% PAN (molecular weight 250,000) was prepared as a reinforcing solution using N,N-dimethylformamide (DMF) as solvent.

[0036] 10wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion (solid content 60wt%, the same below) were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:6:2 and stirred for 6h to obtain spinning solution 2;

[0037] 10wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:15:1 and stirred for 6 hours to obtain spinning solution 3.

[0038] The Tongli Micro-Nano TL-Pro electrospinning equipment was used with the following parameters set: positive voltage 23kV, negative voltage -3kV, spinning distance 20cm, lateral oscillation speed 50mm / s, receiver roller with a rotation speed of 500r / min, extrusion rate 1ml / h, needle-receiver angle of 45°, temperature 30℃, and humidity 40%. Spinning solution 1 and reinforcing solution were spun through parallel double needles for 2 hours to obtain a porous support layer. Then, spinning solution 2 was electrospun through parallel double needles for 2 hours to obtain an intermediate layer. Finally, spinning solution 3 was electrostatically sprayed for 0.5 hours to obtain a surface layer.

[0039] The mass ratio of PAN to (the sum of water-soluble polymer and PTFE) in the spun porous support layer is 1:1; the thickness of the porous support layer is 75 μm, the thickness of the intermediate layer is 70 μm, and the thickness of the surface layer is 8 μm.

[0040] The resulting nascent membrane was then placed in a 30℃ 42wt% sodium sulfate solution coagulation bath to solidify for 30 seconds, and then fired in a muffle furnace at 395℃ for 100 minutes. After sintering, it was removed, soaked in deionized water for 3 hours, rinsed, and dried to obtain a PTFE flat porous membrane.

[0041] The porous support layer was tested to have a thickness of 58 μm and an average pore size of 1.22 μm (pore size distribution 1.10–1.35 μm). The intermediate layer was also tested to have a thickness of 58 μm and an average pore size of 0.67 μm (pore size distribution 0.55–0.75 μm). The surface layer was 5 μm thick, resulting in an overall average pore size of 0.54 μm (pore size distribution 0.4–0.65 μm). The static water contact angle was 153°, and the porosity was 81%. After being immersed in 3.5 wt% sodium chloride solution, 10 wt% HCl, and 10 wt% NaOH solution for one week, the membrane performance remained essentially unchanged, maintaining stability under harsh conditions. Using a 3.5 wt% sodium chloride aqueous solution to simulate seawater, the membrane was tested for desalination under reduced pressure. Under conditions of 80°C on the feed side (3.5 wt% NaCl aqueous solution) and a vacuum of -0.08 MPa, the membrane distillation permeate flux reached 31 L / m³. 2 With a sodium chloride salt rejection rate of 99.96%, no infiltration or penetration will occur after 12 hours of operation.

[0042] Example 2

[0043] 15wt% PVA and PTFE dispersion emulsions were mixed at a PVA to PTFE dry weight ratio of 1:12, and boric acid with a final concentration of 0.02wt% was added. The mixture was stirred for 6 hours to obtain spinning solution 1. 14wt% PAN (molecular weight 150,000) was prepared as a reinforcing solution using DMF as a solvent.

[0044] 15wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:10:1 and stirred for 6 hours to obtain spinning solution 2.

[0045] 15wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:20:1 and stirred for 6 hours to obtain spinning solution 3.

[0046] The Tongli Micro-Nano TL-Pro electrospinning equipment was used with the following parameters set: positive voltage 28kV, negative voltage -2kV, spinning distance 10cm, lateral oscillation speed 40mm / s, receiver roller at 800r / min, extrusion rate 1.2ml / h, needle-receiver angle 45°, temperature 30℃, and humidity 40%. Spinning solution 1 and reinforcing solution were spun using parallel double needles for 1 hour to obtain a porous support layer. Then, spinning solution 2 was electrospun using parallel double needles for 1 hour to obtain an intermediate layer. Finally, spinning solution 3 was electrostatically sprayed for 0.1 hours to obtain a surface layer.

[0047] The mass ratio of PAN to (the sum of water-soluble polymer and PTFE) in the spun porous support layer is 1:1; the thickness of the porous support layer is 45 μm, the thickness of the intermediate layer is 40 μm, and the thickness of the surface layer is 3 μm.

[0048] The resulting nascent membrane was then placed in a 30℃ 35wt% sodium sulfate solution coagulation bath to solidify for 2 minutes, and then fired in a muffle furnace at 375℃ for 2 hours. After sintering, it was taken out, soaked in deionized water for 3 hours, rinsed, and dried to obtain a PTFE flat porous membrane.

[0049] The porous support layer was tested to have a thickness of 30 μm and an average pore size of 1.51 μm, the intermediate layer to have a thickness of 30 μm and an average pore size of 0.70 μm, and the surface layer to have a thickness of 1 μm. The overall average pore size of the membrane was 0.52 μm, the static water contact angle was 152°, and the porosity was 79%. After being immersed in 3.5 wt% sodium chloride solution, 10 wt% HCl, and 10 wt% NaOH solution for one week, the membrane performance remained essentially unchanged, maintaining stability under harsh conditions. Using a 3.5 wt% sodium chloride aqueous solution to simulate seawater, the membrane was tested for desalination under reduced pressure. Under conditions of 80°C on the feed side (3.5 wt% NaCl aqueous solution) and a vacuum of -0.08 MPa, the membrane distillation permeate flux reached 34 L / m³. 2 With a sodium chloride salt rejection rate of 99.96%, no infiltration or penetration will occur after 12 hours of operation.

[0050] Example 3

[0051] 10wt% PVA and PTFE dispersion emulsions were mixed at a PVA to PTFE dry weight ratio of 1:8, and boric acid with a final concentration of 0.01wt% was added. The mixture was stirred for 6 hours to obtain spinning solution 1. 14wt% PAN (molecular weight 150,000) was prepared as a reinforcing solution using N,N-dimethylformamide (DMF) as solvent.

[0052] 10wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:6:3 and stirred for 6 hours to obtain spinning solution 2;

[0053] 10wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:18:1 and stirred for 6 hours to obtain spinning solution 3;

[0054] The Tongli Micro-Nano TL-Pro electrospinning equipment was used with the following parameters set: positive voltage 23kV, negative voltage -3kV, spinning distance 20cm, lateral oscillation speed 50mm / s, receiver as mandrel (outer diameter 2mm), rotation speed 100r / min, extrusion rate 0.6ml / h, needle-receiver angle 60°, temperature 30℃, and humidity 40%. Spinning solution 1 and reinforcing solution were spun side-by-side at both ends for 0.1h to obtain a porous support layer. Then, spinning solution 2 was spun side-by-side at two needles for 0.1h to obtain an intermediate layer. Finally, spinning solution 3 was electrostatically sprayed for 0.1h to obtain a surface layer.

[0055] The mass ratio of PAN to (the sum of water-soluble polymer and PTFE) in the spun porous support layer is 1:1; the thickness of the porous support layer is 35 μm, the thickness of the intermediate layer is 30 μm, and the thickness of the surface layer is 7 μm.

[0056] The resulting primary membrane was then placed in a 35℃ 40wt% sodium sulfate solution coagulation bath to solidify for 5 minutes. After molding, it was fired in a muffle furnace at 375℃ for 1 hour. After being removed, it was soaked in deionized water for 3 hours, rinsed, and dried to obtain a PTFE flat porous membrane.

[0057] The porous support layer was tested to have a thickness of 20 μm and a pore size of 0.95 μm, the intermediate layer to have a thickness of 20 μm and an average pore size of 0.59 μm, and the surface layer to have a thickness of 4 μm. The overall average pore size of the membrane was 0.45 μm, the static water contact angle was 153°, and the porosity was 75%. After being immersed in 3.5 wt% sodium chloride solution, 10 wt% HCl, and 10 wt% NaOH solution for one week, the membrane performance remained essentially unchanged, maintaining stability under harsh conditions. Using a 3.5 wt% sodium chloride aqueous solution to simulate seawater, the membrane was tested for depressurization distillation in seawater desalination. At a feed side temperature (3.5 wt% NaCl aqueous solution) of 80℃ and a pressure of -0.08 MPa, the membrane distillation permeate flux reached 22 L / m³. 2 With a sodium chloride salt rejection rate of 99.95%, no infiltration or penetration will occur after 12 hours of operation.

[0058] Example 4

[0059] 12wt% PVA and PTFE dispersion emulsions were mixed at a PVA to PTFE dry weight ratio of 1:9, and 0.01wt% glutaraldehyde was added. The mixture was stirred for 6 hours to obtain spinning solution 1. 8wt% PAN (molecular weight 250,000) was prepared as a reinforcing solution using DMF as solvent.

[0060] 12wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:9:5 and stirred for 6 hours to obtain spinning solution 2;

[0061] 12wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:20:1 and stirred for 6 hours to obtain spinning solution 3;

[0062] The parameters were set as follows: positive voltage 23kV, negative voltage -3kV, spinning distance 20cm, lateral oscillation speed 50mm / s, receiver is a roller with a rotation speed of 300r / min, extrusion rate 1.5ml / h, needle-to-receiver angle 45°, temperature 30℃, humidity 40%. Spinning solution 1 and reinforcing solution were spun through parallel double needles for 1.5h to obtain a porous support layer. Then, spinning solution 2 was electrospun through parallel double needles for 1h to obtain an intermediate layer. Finally, spinning solution 3 was electrostatically sprayed for 1h to obtain a surface layer.

[0063] The mass ratio of PAN to (the sum of water-soluble polymer and PTFE) in the spun porous support layer is 1:1; the thickness of the porous support layer is 85 μm, the thickness of the intermediate layer is 40 μm, and the thickness of the surface layer is 20 μm.

[0064] The resulting nascent membrane was then placed in a coagulation bath of 25°C and 35wt% sodium sulfate solution for 20 minutes to solidify and form a membrane. After sintering at 380°C for 2.5 hours in a muffle furnace, it was removed, soaked in deionized water for 3 hours, rinsed, and dried to obtain a PTFE flat porous membrane.

[0065] The porous support layer was tested to have a thickness of 70 μm and an average pore size of 1.52 μm, the intermediate layer to have a thickness of 31 μm and an average pore size of 0.55 μm, and the surface layer to have a thickness of 15 μm. The overall average pore size of the membrane was 0.38 μm, the static water contact angle was 152°, and the porosity was 75%. After being immersed in 3.5 wt% sodium chloride solution, 10 wt% HCl, and 10 wt% NaOH solution for one week, the membrane performance remained essentially unchanged, maintaining stability under harsh conditions. Using a 3.5 wt% sodium chloride aqueous solution to simulate seawater, the membrane was tested for desalination under reduced pressure. At a feed side temperature (3.5 wt% NaCl aqueous solution) of 80℃ and a pressure of -0.08 MPa, the membrane distillation permeate flux reached 26 L / m³. 2 With a sodium chloride salt rejection rate of 99.92%, no infiltration or penetration will occur after 12 hours of operation.

[0066] Example 5

[0067] 11 wt% PVA and PTFE dispersion emulsions were mixed at a PVA to PTFE dry weight ratio of 1:9, and 0.01 wt% glutaraldehyde was added. The mixture was stirred for 6 hours to obtain spinning solution 1. 8 wt% PAN (molecular weight 250,000) was prepared as a reinforcing solution using DMF as solvent.

[0068] 11wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:9:5 and stirred for 6 hours to obtain spinning solution 2.

[0069] 11wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:20:1 and stirred for 6 hours to obtain spinning solution 3;

[0070] The parameters were set as follows: positive voltage 20kV, negative voltage -3kV, spinning distance 15cm, lateral oscillation speed 50mm / s, receiver is a mandrel with an outer diameter of 2mm, rotation speed 100r / min, extrusion rate 1.0ml / h, needle-to-receiver angle 45°, temperature 30℃, humidity 40%. The three sets of spinning solutions 1 and reinforcing solutions were spun through a four-needle spinneret for 0.1h to obtain a porous support layer. Then, spinning solution 2 was spun through a two-needle electrospinning process for 0.2h to obtain an intermediate layer. Finally, spinning solution 3 was electrostatically sprayed for 0.2h to obtain a surface layer.

[0071] The mass ratio of PAN to (the sum of water-soluble polymer and PTFE) in the spun porous support layer is 1:3; the thickness of the porous support layer is 82 μm, the thickness of the intermediate layer is 85 μm, and the thickness of the surface layer is 20 μm.

[0072] The resulting nascent membrane was then placed in a coagulation bath of 25°C and 35wt% sodium sulfate solution for 60 minutes to solidify and form a membrane. After sintering at 390°C for 2.5 hours in a muffle furnace, it was removed, soaked in deionized water for 3 hours, rinsed, and dried to obtain a PTFE flat porous membrane.

[0073] The porous support layer was tested to have a thickness of 45 μm and an average pore size of 0.70 μm, the intermediate layer to have a thickness of 46 μm and an average pore size of 0.45 μm, and the surface layer to have a thickness of 15 μm. The overall average pore size of the membrane was 0.39 μm, the static water contact angle was 150°, and the porosity was 71%. After being immersed in 3.5 wt% sodium chloride solution, 10 wt% HCl, and 10 wt% NaOH solution for one week, the membrane performance remained essentially unchanged, maintaining stability under harsh conditions. Using a 3.5 wt% sodium chloride aqueous solution to simulate seawater, the membrane was tested for depressurization distillation in seawater desalination. At a feed side temperature (3.5 wt% NaCl aqueous solution) of 80℃ and a pressure of -0.08 MPa, the membrane distillation permeate flux reached 18 L / m³. 2 With a sodium chloride salt rejection rate of 99.92%, no infiltration or penetration will occur after 12 hours of operation.

[0074] Example 6

[0075] 10wt% PVA and PTFE dispersion emulsions were mixed at a PVA to PTFE dry weight ratio of 1:9, and 0.01wt% glutaraldehyde was added. The mixture was stirred for 6 hours to obtain spinning solution 1. 8wt% PAN (molecular weight 250,000) was prepared as a reinforcing solution using DMF as solvent.

[0076] 10wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:9:5 and stirred for 6 hours to obtain spinning solution 2;

[0077] 10wt% PVA, PTFE dispersion emulsion and PFA dispersion emulsion were mixed at a dry weight ratio of PVA:PTFE:PFA of 1:20:1 and stirred for 6 hours to obtain spinning solution 3;

[0078] The parameters were set as follows: positive voltage 20kV, negative voltage -3kV, spinning distance 15cm, lateral oscillation speed 50mm / s, receiver is a roller with a rotation speed of 200r / min, extrusion rate 1.2ml / h, needle-to-receiver angle 45°, temperature 30℃, humidity 40%. Spinning solution 1 and reinforcing solution were spun through parallel double needles for 0.5h to obtain a porous support layer. Then, spinning solution 2 was electrospun through parallel double needles for 0.5h to obtain an intermediate layer. Finally, spinning solution 3 was electrostatically sprayed for 1h to obtain a surface layer.

[0079] The mass ratio of PAN to (the sum of water-soluble polymer and PTFE) in the spun porous support layer is 1:1; the thickness of the porous support layer is 31 μm, the thickness of the intermediate layer is 102 μm, and the thickness of the surface layer is 20 μm.

[0080] The resulting nascent membrane was then placed in a coagulation bath of 25°C and 35wt% sodium sulfate solution for 60 minutes to solidify and form a membrane. After sintering at 380°C for 2.5 hours in a muffle furnace, it was removed, soaked in deionized water for 3 hours, rinsed, and dried to obtain a PTFE flat porous membrane.

[0081] The porous support layer was tested to have a thickness of 15 μm and an average pore size of 1.8 μm; the intermediate layer was 80 μm thick and had an average pore size of 0.72 μm; the surface layer was 15 μm thick; the overall average pore size of the membrane was 0.65 μm; the static water contact angle was 152°; and the porosity was 75%. After being immersed in 3.5 wt% sodium chloride solution, 10 wt% HCl, and 10 wt% NaOH solution for one week, the membrane performance remained essentially unchanged, maintaining stability under harsh conditions. Using a 3.5 wt% sodium chloride aqueous solution to simulate seawater, the membrane was tested for depressurization distillation in seawater desalination. At a feed side temperature (3.5 wt% NaCl aqueous solution) of 80℃ and a pressure of -0.08 MPa, the membrane distillation permeate flux reached 35 L / m³. 2 With a sodium chloride salt rejection rate of 99.85%, no infiltration or penetration will occur after 12 hours of operation.

[0082] Comparative Example 1

[0083] PTFE porous membranes were prepared using the method described in Example 1, except that the spinning solution 1 and the reinforcing solution were spun using a pair of needles in parallel for 4.1 hours.

[0084] The obtained PTFE porous membrane was tested and found to be 125 μm thick with an average pore size of 0.92 μm (pore size distribution 0.8–1.35 μm). Due to the lack of an intermediate layer to optimize the pore size, the overall pore size of the membrane was too large, and wetting and permeation occurred during the membrane distillation test.

[0085] Comparative Example 2

[0086] PTFE porous membranes were prepared using the method described in Example 1, except that the spinning solution 3 was spun through a pair of parallel needles for 4.1 hours.

[0087] The sintered film has low strength, is easily broken, and has poor integrity, making it unsuitable for testing and application.

[0088] Comparative Example 3

[0089] PTFE porous membranes were prepared using the method described in Example 1, except that spinning solution 2 was spun by electrospinning with parallel double needles for 4.1 hours.

[0090] The obtained PTFE porous membrane was tested and found to have a thickness of 128 μm, an average pore size of 0.68 μm (pore size distribution 0.58–0.72 μm), a porosity of 70%, and a membrane distillation permeate flux of 18 L / m³. 2 The salt rejection rate exceeds 99.85%. However, due to the overall homogeneity of the membrane, the bottom layer has high permeation resistance, resulting in a lower permeation flux than in Example 1. Furthermore, the static water contact angle is 143°, lower than in Example 1, due to the absence of a rough surface layer.

[0091] Comparative Example 4

[0092] PTFE porous membranes were prepared using the method described in Example 1, except that no reinforcing solution was used in the porous support layer.

[0093] After sintering, the film shrinks significantly, resulting in numerous cracks. Since PAN acts as a reinforcing agent, it strengthens the nascent film and acts as a framework during sintering, slowing down the shrinkage process and preventing crack formation.

[0094] Comparative Example 5

[0095] PTFE porous membranes were prepared using the method described in Example 1, except that the nascent membrane was not placed in a coagulation bath for curing.

[0096] After sintering, many cracks are generated. This is because, on the one hand, the nascent film contains a large amount of moisture, and in the initial stage of the sintering process, PVA swells and dissolves, resulting in poor fiber uniformity. On the other hand, due to incomplete drying, the shrinkage space during the sintering process is small, leading to breakage.

[0097] Comparative analysis revealed that the performance of Examples 1-6 was superior to that of the PTFE porous membranes obtained in Comparative Examples 1-5. It can be seen that the PTFE porous membranes prepared by the present invention have a multi-level and multi-grade pore structure. The rough structure of PTFE micro / nano microspheres / microfibers on the surface endows the membrane with excellent superhydrophobicity, while the regular circular pore structure in the middle layer endows the membrane with excellent separation performance. The high-strength substrate and the fiber-interwoven macroporous support layer endow the membrane with good permeability and strength.

[0098] The present invention has been described in detail above with reference to the embodiments. However, those skilled in the art will understand that, without departing from the spirit of the present invention, various specific parameters in the above embodiments can be changed to form multiple specific embodiments, all of which are common variations of the present invention, and will not be described in detail here.

[0099] This invention does not use reagents such as fluorosilanes and hydrophobic nanomaterials, reducing reagent pollution and recycling, and avoiding nanoparticle detachment. Through the sintering process, PTFE is bonded in situ to form an integral shape without delamination, with high strength, an interconnected open-pore structure, good permeability, resistance to high temperature and chemical reagents, self-supporting properties, controllable structure, and adjustable thickness.

Claims

1. A method for preparing a polytetrafluoroethylene porous membrane, characterized in that... The method includes the following steps: Step 1: Mix the water-soluble polymer spinning carrier solution, polytetrafluoroethylene (PTFE) dispersion emulsion, and regulator to obtain spinning solution 1. Step 2: Mix the water-soluble polymer spinning carrier solution, PTFE dispersion emulsion, and tetrafluoroethylene-perfluoropropyl vinyl ether copolymer (PFA) dispersion emulsion to obtain spinning solution 2. Step 3: Mix the water-soluble polymer spinning carrier solution, PTFE dispersion emulsion, and PFA dispersion emulsion to obtain spinning solution 3, and obtain the surface layer by electrostatic spraying; Step 4: Spinning solution 1 is spun into a porous support layer by gel electrospinning. Then, spinning solution 2 is spun into an intermediate layer by electrospinning. Next, spinning solution 3 is electrostatically sprayed to obtain a surface layer. The resulting nascent membrane is then placed in a coagulation bath for curing and shaping. After sintering and heat treatment in a muffle furnace, it is cleaned and dried to obtain the polytetrafluoroethylene porous membrane. In step 4, the spinning of the porous support layer also includes spinning a spinning solution of another polymer with spinning solution 1 in parallel with the spinning solution 1 using a dual-needle spinning method. The other polymer is polyacrylonitrile (PAN), used as a reinforcing agent, with a molecular weight of 150,000 to 300,000 and a spinning solution concentration of 8 to 20 wt%. The coagulation bath in step 4 is a sodium sulfate solution with a concentration of 25 to 50 wt%. The curing time is 0.1 to 60 minutes, and the curing temperature is 20 to 60°C.

2. The preparation method according to claim 1, characterized in that: The water-soluble polymer spinning carrier is one or more of polyvinyl alcohol (PVA), pullulan, sodium alginate, gelatin, polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP), and the concentration of the water-soluble polymer spinning carrier solution is 5-20 wt%. The solid content of the PTFE dispersion emulsion is 40~70wt%.

3. The preparation method according to claim 1 or 2, characterized in that, In the spinning solution 1 described in step 1, the dry weight ratio of the water-soluble polymer to polytetrafluoroethylene is 1:1~14, and the amount of regulator is 0.005~0.3 wt% of the total mass of the spinning solution 1; the regulator is one or more of boric acid, calcium chloride, and glutaraldehyde.

4. The preparation method according to claim 1 or 2, characterized in that, In the spinning solution 2 described in step 2, the dry weight ratio of the water-soluble polymers PTFE and PFA (tetrafluoroethylene-perfluoropropyl vinyl ether copolymer) is 1:1~14:1~8. The solid content of the tetrafluoroethylene-perfluoropropyl vinyl ether copolymer (PFA) dispersion emulsion is 40~70wt%.

5. The preparation method according to claim 1 or 2, characterized in that, In the spinning solution 3 described in step 3, the dry weight ratio of the water-soluble polymers PTFE and PFA (tetrafluoroethylene-perfluoropropyl vinyl ether copolymer) is 1:15~30:1~5. The solid content of the tetrafluoroethylene-perfluoropropyl vinyl ether copolymer (PFA) dispersion emulsion is 50-60 wt%.

6. The preparation method according to claim 1, characterized in that: In step 4, the concentration of the spinning solution is 8-15 wt%; The mass ratio of PAN to the water-soluble polymer and PTFE in the spun porous support layer is 1:1~10; The coagulation bath in step 4 is a sodium sulfate solution with a concentration of 30-45 wt%; the curing time is 0.5-30 min; and the curing temperature is 30-50℃. The cleaning process involves soaking in water for 1-10 hours followed by rinsing.

7. The preparation method according to claim 1, characterized in that, The electrospinning / spraying parameters in step 4 are as follows: positive voltage 18~30kV, negative voltage -2~-8kV, spinning distance 10~20cm, lateral oscillation speed 40~200mm / s, receiver is a roller or mandrel rotating around its own axis, mandrel diameter is 2~6mm, flat sheet membrane or hollow membrane is prepared respectively, rotation speed is 100~1000r / min, extrusion rate is 0.6~1.5ml / h, the angle between the needle and the receiver is 30~60°, temperature is 20~30℃, humidity is 10~40%; The spinning / spraying parameters for the three layers of the nascent membrane—the porous support layer, the intermediate layer, and the surface layer—are consistent: the porous support layer is spun for 0.1–6 h with a membrane thickness of 10–120 μm; the intermediate layer is spun for 0.1–6 h with a membrane thickness of 10–120 μm; and the surface layer is sprayed for 0.1–2 h with a membrane thickness of 0.5–30 μm. The sintering heat treatment temperature is 350~410℃, and the time is 10~300min.

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

9. The polytetrafluoroethylene porous membrane according to claim 8, characterized in that: The PTFE porous membrane has a porous support layer with a thickness of 10~80μm and an average pore size of 0.80~2.00μm; an intermediate layer with a thickness of 10~80μm and an average pore size of 0.45~0.75μm; a surface layer with a thickness of 0.1~20μm; and an overall average pore size of 0.20~0.65μm.

10. The application of the polytetrafluoroethylene porous membrane according to claim 8 or 9 in membrane distillation seawater desalination, waterproof and breathable or waterproof and sound-permeable electronic equipment.

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