A braided tube reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, and a preparation method and application thereof

Abstract

This invention belongs to the field of hollow fiber membrane preparation technology, and discloses a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, its preparation method, and its application. The polyvinylidene fluoride hollow fiber hydrophobic membrane includes a braided tube and a hydrophobic membrane layer formed on the surface of the braided tube. The outer diameter of the braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane is 0.3-3 mm, the tensile breaking strength is greater than 100 N, and the pure water permeability is 200-7500 L / (m²). 2 •h)@0.1MPa 20℃. The preparation method includes: preparing a casting solution by combining polyvinylidene fluoride resin, organic solvent, composite pore-forming agent, and optionally polytetrafluoroethylene microparticles; extruding the casting solution onto the outer surface of a braided tube using a hollow fiber membrane spinning machine, and forming a membrane through phase separation in a coagulation bath. This invention significantly improves the mechanical strength of the membrane through braided tube reinforcement and achieves high flux and high hydrophobicity using a water-inorganic-organic composite pore-forming agent, making it suitable for high-performance separation processes such as membrane distillation, membrane contactors, and gas separation.

Description

Technical Field

[0001] This invention belongs to the field of hollow fiber membrane preparation technology, and relates to a braided tube reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, its preparation method and application. Background Technology

[0002] Hydrophobic hollow fiber porous membranes (hereinafter referred to as hydrophobic membranes) are commonly used in membrane distillation, membrane absorption, membrane aeration, gas separation, air purification and other applications.

[0003] Early commercially available hydrophobic membranes were made of polypropylene plastic, prepared using melt stretching or thermally induced phase separation methods. However, due to the poor stain and oxidation resistance of polypropylene and the relatively large pore size of the resulting hydrophobic membranes, the application range of polypropylene hydrophobic membranes has been significantly reduced. Polytetrafluoroethylene (PTFE) possesses excellent acid and alkali resistance, hydrophobicity, stain and oxidation resistance; however, as PTFE is a non-melting and insoluble plastic, it is difficult to prepare membranes using conventional methods. Currently, the commonly used method for preparing PTFE separation membranes involves high-pressure extrusion, stretching, and then high-temperature sintering. For example, fine PTFE powder paste can be extruded and stretched under high pressure, followed by high-temperature sintering at temperatures above 320°C to produce PTFE hollow fiber membranes. Alternatively, a two-step membrane preparation method can be used: first, PTFE resin is mixed with other materials, utilizing the extensibility and processability of the matrix material to pre-form a hollow fiber membrane shape; then, the matrix polymer material is ablated to prepare the PTFE hydrophobic membrane. Due to limitations in the membrane fabrication method, the resulting PTFE hydrophobic membrane has high manufacturing costs and a large separation pore size, limiting its application range. Furthermore, due to the difficult-to-bond nature of PTFE material, there are significant production difficulties when encapsulating PTFE hydrophobic membranes into membrane modules using epoxy resin.

[0004] Polyvinylidene fluoride (PVDF) is an excellent hydrophobic membrane material. Hydrophobic membranes can be prepared using conventional solution-phase separation or thermally induced phase separation methods, covering a pore size range comparable to that of ultrafiltration membranes, and the pore size is easily controllable. PVDF hydrophobic membranes exhibit excellent resistance to fouling and oxidation, thus showing great promise for future applications.

[0005] However, current polyvinylidene fluoride hollow fiber hydrophobic membrane products struggle to balance mechanical strength, separation flux, and hydrophobicity. Hollow fiber membranes formed from pure PVDF resin through phase separation have limited bulk strength, making them prone to fiber breakage and rupture during use, and unable to withstand high operating pressures and mechanical loads. To achieve ideal separation flux and hydrophobicity, organic small-molecule pore-forming agents (such as alcohols, ethers, and ketones) are added to the casting solution. While this can form porous structures through solvent exchange, the pore-forming efficiency is often limited, resulting in only a limited increase in water permeability.

[0006] Therefore, there is a need to prepare a hollow fiber hydrophobic membrane that combines high strength and high throughput while meeting the requirements of high-intensity operating conditions and high-efficiency separation processes, which greatly limits its promotion and application in many harsh industrial scenarios. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention proposes a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, its preparation method, and its application. First, a rigid skeleton formed by using braided tubes is used to ensure the mechanical strength of the membrane. At the same time, a composite pore-forming agent without hydrophilic polymer materials is added to the casting solution to improve the pore size of the separation membrane while maintaining high hydrophobicity.

[0008] The formation mechanism of the polyvinylidene fluoride hollow fiber hydrophobic membrane of the present invention is as follows: Polyvinylidene fluoride resin, organic solvent, composite pore-forming agent, and polytetrafluoroethylene microparticles are mixed in a certain proportion to obtain a casting solution. The organic solvent and low-molecular-weight pore-forming agent in the casting solution are extracted by water, and solution phase separation occurs in the casting solution. The solvent and organic low-molecular-weight pore-forming agent in the casting solution enter the coagulant aqueous phase, and the polyvinylidene fluoride resin precipitates and solidifies, thereby preparing the polyvinylidene fluoride hollow fiber hydrophobic membrane.

[0009] The technical solution of this invention is: The first aspect of this invention provides a method for preparing a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, comprising the following steps: (1) Mix polyvinylidene fluoride resin with organic solvent, composite pore-forming agent and polytetrafluoroethylene microparticles and stir evenly, then let stand to remove bubbles to obtain casting solution; the mass fraction of polyvinylidene fluoride resin in the casting solution is 15-35wt%, the mass fraction of organic solvent is 50-80wt%, the mass fraction of composite pore-forming agent is 2.6-30wt%, the mass fraction of polytetrafluoroethylene microparticles is 0-10wt%, the composite pore-forming agent is composed of inorganic particulate pore-forming agent, organic low molecular weight pore-forming agent and water, the mass fraction of water in the casting solution is 0.1-3wt%, the mass fraction of organic low molecular weight pore-forming agent in the casting solution is 2-27wt%, and the mass fraction of inorganic particulate pore-forming agent in the casting solution is 0.5-25wt%.

[0010] (2) Using a hollow fiber membrane spinning machine, the casting liquid is extruded from the spinning nozzle in a ring shape and evenly coated on the outer surface of the braided tube drawn from the central tube of the spinning nozzle. Then it enters the coagulation bath, where the casting liquid undergoes phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane.

[0011] Preferably, the temperature for static degassing is 35-85℃, and the time is 8-15 hours. The coagulation bath temperature is 30-50℃.

[0012] Preferably, the polyvinylidene fluoride resin includes one or more of polyvinylidene fluoride homopolymer, polyvinylidene fluoride-trifluorochloroethylene copolymer, and polyvinylidene fluoride copolymer. The polyvinylidene fluoride copolymer is mainly composed of vinylidene fluoride repeating units and is selected from one of vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, or vinylidene fluoride-trifluorochloroethylene copolymer. In this invention, the content of vinylidene fluoride repeating units in the polyvinylidene fluoride copolymer is above 70%. If the content of vinylidene fluoride repeating units in the copolymer is too low, the copolymer has low solubility in solvents, making solution phase transfer spinning impossible. In this invention, the hydrophobicity and alkali resistance of the polyvinylidene fluoride copolymer are superior to those of the polyvinylidene fluoride homopolymer resin, making it more advantageous for preparing hydrophobic films. The disadvantage of polyvinylidene fluoride copolymers is that their mechanical properties are inferior to those of polyvinylidene fluoride homopolymers, resulting in poor membrane strength when used alone, which is detrimental to their application. However, through the mechanical property enhancement method of this invention, the mechanical strength of the hollow fiber membrane is provided by the braided tube, resulting in a hollow fiber hydrophobic membrane with better mechanical properties, separation performance, alkali resistance, and hydrophobicity. The polyvinylidene fluoride resin content in the casting solution is 20-30 wt%.

[0013] Preferably, the organic solvent is one of dimethylformamide (DMF), dimethylacetamide (DMAc), N-methylpyrrolidone, triethyl phosphate, sulfolane, and dimethyl sulfoxide. The mass fraction of the organic solvent is 60-75 wt%.

[0014] Preferably, the inorganic particulate pore-forming agent is selected from one or more of lithium nitrate, sodium chloride, calcium chloride, calcium carbonate, calcium nitrate, silicon dioxide, aluminum oxide, and kaolin. The particle size of the inorganic particulate pore-forming agent is less than 10 micrometers, preferably nano-sized particles. After spinning, the inorganic particulate pore-forming agent is dissolved from the membrane using alkali, acid, water, etc. The preferred mass fraction of the inorganic particulate pore-forming agent in the casting solution is 1-10 wt%.

[0015] Preferably, the water content in the casting solution is 0.2-1.5 wt%. Adding water can induce differential phase reaction in the casting solution, which can synergistically interact with organic low-molecular-weight pore-forming agents and / or inorganic particulate pore-forming agents to achieve excellent pore-forming effect, thus facilitating the preparation of large-pore hollow fiber hydrophobic membranes.

[0016] Preferably, the organic low-molecular-weight pore-forming agent is one or more of the following organic low-molecular-weight substances with pore-forming properties: polyethylene glycol-400, dioxane, methyl ethyl ketone, acetone, ethylene glycol monomethyl ether, diethylene glycol methyl ether, glycerol, etc. The preferred mass fraction of the organic low-molecular-weight pore-forming agent in the casting solution is 5-20 wt%, and its molecular weight is below 600 Daltons.

[0017] Preferably, the coagulation bath is composed of water.

[0018] Polyvinylidene fluoride (PVDF) resin and organic low-molecular-weight pore-forming agent are uniformly dissolved in an organic solvent, while inorganic particulate pore-forming agent and polytetrafluoroethylene (PTFE) microparticles are uniformly dispersed in a suspension in the casting solution. When the inorganic particulate pore-forming agent and the organic low-molecular-weight pore-forming agent are used simultaneously, the inorganic particulate pore-forming agent acts as a nucleating agent during phase transfer and film formation when the casting solution enters the coagulation bath. After film formation, the organic low-molecular-weight pore-forming agent dissolves out of the membrane. In this way, the two pore-forming agents work together to promote the formation of more interconnected membrane pores, which is beneficial for obtaining high-flux hollow fiber hydrophobic membranes.

[0019] Preferably, the polytetrafluoroethylene (PTFE) microparticles have a particle size of 1-5 μm. The PTFE microparticles are insoluble in the casting solution and exist in a suspended, dispersed state, which can increase the water contact angle of the membrane surface after film formation, thereby improving the hydrophobicity and fouling resistance of the membrane.

[0020] Preferably, the braided tube is one of polyester fiber braided tube, nylon braided tube, or aromatic polyamide braided tube, and the outer diameter of the braided tube is 0.3-3 mm. Polyester, nylon, aromatic polyamide, and other materials can be produced by blending long and short fibers, which facilitates the appropriate penetration of the casting solution into the cross-sectional structure of the braided tube during spinning, improving the mechanical bonding strength between the polyvinylidene fluoride (PVDF) sheath material and the braided tube substrate, and enhancing the stability of the membrane performance. The PVDF material is anchored within the braided tube substrate. The surface of the fiber material of the braided tube can also be treated with coupling agents, surfactants, etc., to further improve the mechanical bonding strength between the PVDF sheath material and the braided tube substrate.

[0021] A second aspect of this invention provides a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane obtained by the above-described preparation method, comprising a braided tube and a hydrophobic membrane layer formed on the surface of the braided tube, the thickness of the hydrophobic membrane layer being 0.05-1 mm and the membrane separation pore size ranging from 0.01-5 μm; the outer diameter of the braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane is 0.3-3 mm, the tensile breaking strength is greater than 100 N, and the pure water permeability is 200-7500 L / (m²). 2 ·h)@0.1MPa 20℃.

[0022] The casting solution of the reinforced hydrophobic polyvinylidene fluoride hollow fiber membrane of the present invention does not contain hydrophilic polymers. The organic low-molecular-weight pore-forming agent can be easily dissolved in water during the solution phase separation method of membrane preparation, so as to ensure the hydrophobicity of the hollow fiber membrane. In addition to conventional polyester materials, the fibers of the braided tube can also be made of materials such as nylon and aromatic polyamide with better chemical resistance, temperature resistance and mechanical strength, so as to expand the application range of the hydrophobic membrane.

[0023] A third aspect of the present invention provides the application of the braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane in membrane distillation, membrane aeration, gas separation or air purification.

[0024] The advantages and beneficial effects of this invention are: (1) This invention uses braided tubing as a rigid reinforcing skeleton and employs a composite pore-forming agent composed of water, inorganic particulate pore-forming agent and organic pore-forming agent to achieve a significant increase in membrane pore size while ensuring high strength. Furthermore, polytetrafluoroethylene (PTFE) microparticles are added to impart stable high hydrophobicity to the hydrophobic membrane, with a pure water contact angle of up to 105°.

[0025] (2) This invention is based on a mature solution phase separation spinning process and does not require complex special equipment. The process parameters (such as temperature, concentration, and coagulation bath conditions) are all within the conventional control range in the field, which makes it easy to achieve stable and mass production, controllable production costs, and conducive to the commercialization of high-performance products.

[0026] In summary, this invention successfully prepared a polyvinylidene fluoride hollow fiber membrane that combines high strength, high flux, and excellent hydrophobicity, providing a reliable foundation for its large-scale preparation and widespread application. Attached Figure Description

[0027] Figure 1 The image shows an electron microscope image of the braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane prepared in Example 1.

[0028] Figure 2 A magnified electron microscope image of a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane prepared in Example 1.

[0029] Figure 3 Electron micrograph of a braided tube-reinforced hydrophobic polyvinylidene fluoride hollow fiber membrane prepared for Comparative Example 1.

[0030] Figure 4 Electron micrograph of a braided tube-reinforced hydrophobic polyvinylidene fluoride hollow fiber membrane prepared for Comparative Example 3.

[0031] Figure 5 A magnified electron microscope image of a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane prepared for Comparative Example 3. Detailed Implementation

[0032] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0033] Example 1 A method for preparing a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane includes the following steps: (1) Mix 2 kg of vinylidene fluoride-hexafluoropropylene copolymer (vinylidene fluoride repeating units account for 80% of the total number of repeating units of copolymer), 1 kg of polyethylene glycol-400 and 800 g of 2-micron calcium carbonate particles, add them to 5 kg of dimethylacetamide solvent and stir evenly, then add 2 kg of dimethylacetamide and 30 g of water, continue to mix and dissolve, and then let stand at 70°C for 15 hours to remove bubbles to obtain casting solution; (2) A hollow fiber membrane spinning machine is used to extrude the casting solution in a ring from the spinning nozzle and evenly coat it on the outer surface of a polyester fiber braided tube with an outer diameter of 1.8 mm, which is drawn from the central tube of the spinning nozzle. Then, the tube enters a coagulation bath at 40°C, where the casting solution undergoes solution-phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0034] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.9 mm, a tensile breaking strength of 150 N, an internal pressure bursting pressure of 2.5 MPa, and a pure water permeability of 7500 L / (m²). 2 The membrane separation pore size is 1.5 μm, the pure water contact angle of the hollow fiber membrane is 98°, and the membrane fibers are hydrophobic.

[0035] Figure 1 Electron micrograph of the braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane prepared in Example 1: Figure 2 The magnified view shows that, under the combined action of water molecules and inorganic particulate pore-forming agents, the expected macropores are stably generated on the outer surface of the membrane fibers, achieving the goal of increasing air permeability and reducing air permeability pressure.

[0036] Comparative Example 1 The only difference from Example 1 is that water and inorganic particulate pore-forming agent are not added. The specific steps are as follows: (1) Mix 2 kg of vinylidene fluoride-hexafluoropropylene copolymer (vinylidene fluoride repeating units account for 80% of the total number of repeating units of copolymer) and 1 kg of polyethylene glycol-400, then add 5 kg of dimethylacetamide solvent and stir evenly. Then add 2 kg of dimethylacetamide and continue to mix and dissolve. Then let stand at 70°C for 15 hours to remove bubbles and obtain casting solution.

[0037] (2) A hollow fiber membrane spinning machine is used to extrude the casting solution from the spinning nozzle in a ring shape and evenly coat it on the outer surface of a polyester fiber braided tube with an outer diameter of 1.8 mm that is drawn from the central tube of the spinning nozzle. Then, it enters a coagulation bath at 40°C. The casting solution undergoes solution phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0038] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.9 mm, a tensile breaking strength of 150 N, an internal pressure bursting pressure of 2.5 MPa, and a pure water permeability of 150 L / (m²). 2 The membrane separation pore size is 0.16 μm, the hollow fiber membrane has a pure water contact angle of 75°, and the membrane fibers are hydrophilic.

[0039] Figure 3 To prepare a braided tube-reinforced hydrophobic polyvinylidene fluoride hollow fiber membrane for Comparative Example 1, it can be seen that without the addition of inorganic particulate pore-forming agent and water, only the organic low molecular weight pore-forming agent is used, and the membrane fiber surface has only dense micropores.

[0040] Comparative Example 2 The only difference from Example 1 is that inorganic particles are not added. The specific steps are as follows: (1) Mix 2 kg of vinylidene fluoride-hexafluoropropylene copolymer (vinylidene fluoride repeating units account for 80% of the total number of repeating units in the copolymer) and 1 kg of polyethylene glycol-400, then add 5 kg of dimethylacetamide solvent and stir evenly. Then add 2 kg of dimethylacetamide and 30 g of water and continue to mix and dissolve. Then let it stand at 70°C for 15 hours to remove bubbles and obtain the casting solution; (2) A hollow fiber membrane spinning machine is used to extrude the casting solution from the spinning nozzle in a ring shape and evenly coat it on the outer surface of a polyester fiber braided tube with an outer diameter of 1.8 mm that is drawn from the central tube of the spinning nozzle. Then, it enters a coagulation bath at 40°C. The casting solution undergoes solution phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0041] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.9 mm, a tensile breaking strength of 150 N, an internal pressure bursting pressure of 2.5 MPa, and a pure water permeability of 310 L / (m²). 2 •h)@0.1MPa 20 ℃, membrane separation pore size 0.2μm, hollow fiber membrane pure water contact angle is 90°.

[0042] Comparative Example 3 The only difference from Example 1 is that no water is added. The specific steps are as follows: (1) Mix 2 kg of vinylidene fluoride-hexafluoropropylene copolymer (vinylidene fluoride repeating units account for 80% of the total number of repeating units of copolymer), 1 kg of polyethylene glycol-400 and 800 g of 2-micron calcium carbonate particles, add them to 5 kg of dimethylacetamide solvent and stir evenly, then add 2 kg of dimethylacetamide and continue to mix and dissolve, then let stand for 15 hours to remove bubbles to obtain casting solution; (2) A hollow fiber membrane spinning machine is used to extrude the casting solution from the spinning nozzle in a ring shape and evenly coat it on the outer surface of a polyester fiber braided tube with an outer diameter of 1.8 mm that is drawn from the central tube of the spinning nozzle. Then it enters the coagulation bath water tank, where the casting solution undergoes solution phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0043] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.9 mm, a tensile breaking strength of 150 N, an internal pressure bursting pressure of 2.5 MPa, and a pure water permeability of 5500 L / (m²). 2 The membrane separation pore size is 0.8 μm, and the hollow fiber membrane pure water contact angle is 93°.

[0044] Figure 4 Electron micrograph of a braided tube-reinforced hydrophobic polyvinylidene fluoride hollow fiber membrane prepared for Comparative Example 3. Figure 5 For local magnification electron microscopy, when the additive contains no water, irregular macropores are formed on the surface of the membrane fibers under the action of inorganic particles. Figure 4 As can be seen, the direction of some macropores is almost parallel to the direction of the membrane tube, making it impossible to stably form normal macropores.

[0045] Example 2 A method for preparing a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane includes the following steps: (1) Mix 2 kg of vinylidene fluoride-hexafluoropropylene copolymer (vinylidene fluoride repeating units account for 80% of the total number of repeating units of copolymer), 1 kg of polyethylene glycol-400, and 800 g of 2-micron calcium carbonate particles, and then add them to 5 kg of dimethylacetamide solvent and stir evenly. Then add 2 kg of dimethylacetamide and 30 g of water, and continue to mix and dissolve. After the mixture is completed, 300 g of 2-micron polytetrafluoroethylene microparticles are evenly dispersed in the mixture under high-speed stirring. After standing at 70 °C for 15 hours to remove bubbles, the casting solution is obtained.

[0046] (2) A hollow fiber membrane spinning machine is used to extrude the casting solution from the spinning nozzle in a ring shape and evenly coat it on the outer surface of a polyester fiber braided tube with an outer diameter of 1.8 mm that is drawn from the central tube of the spinning nozzle. Then, it enters a 40°C coagulation bath water tank, where the casting solution undergoes solution phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0047] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.9 mm, a tensile breaking strength of 145 N, an internal pressure bursting pressure of 2.5 MPa, and a pure water permeability of 7200 L / (m²). 2 ·h)@0.1MPa 20℃, membrane separation pore size is 2μm, hollow fiber membrane pure water contact angle is 105°, membrane fibers are hydrophobic.

[0048] The pure water contact angle in Example 2 is greater than that in Example 1, which shows that polytetrafluoroethylene microparticles can increase the water contact angle on the membrane surface after film formation.

[0049] Example 3 A method for preparing a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane includes the following steps: (1) In 7kg N-methylpyrrolidone solvent, add 2kg vinylidene fluoride-tetrafluoroethylene copolymer resin (vinylidene fluoride repeating 80%), 1kg polyethylene glycol-400, 150g of 2-micron calcium carbonate particles and 50g of water, stir to dissolve evenly, and then let stand at 70℃ for 15 hours to remove bubbles to obtain casting solution. (2) A hollow fiber membrane spinning machine is used to extrude the casting solution from the spinning nozzle in a ring shape and evenly coat it on the outer surface of a nylon-6 braided tube with an outer diameter of 1.0 mm that is drawn out from the central tube of the spinning nozzle. Then, it enters a coagulation bath at 40°C. The casting solution undergoes solution phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0050] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.2 mm, a tensile breaking strength of 220 N, an internal pressure bursting pressure of 1.7 MPa, and a pure water permeability of 750 L / (m²). 2 ·h)@0.1MPa 20℃, membrane separation pore size is 0.7μm, hollow fiber membrane pure water contact angle is 90°.

[0051] Comparative Example 4 The only difference from Example 3 is that nylon braided tubing is not used. The specific steps are as follows: (1) In 7kg N-methylpyrrolidone solvent, add 2kg vinylidene fluoride-tetrafluoroethylene copolymer resin (vinylidene fluoride repeating 80%), 1kg polyethylene glycol-400, 150g of 2-micron calcium carbonate particles and 50g of water, stir to dissolve evenly, and let stand at 70℃ for 15 hours to remove bubbles to obtain casting solution. (2) A hollow fiber membrane spinning machine is used to extrude the casting liquid from the spinning nozzle in a ring shape. The core liquid is water. The casting liquid enters the coagulation bath water tank and phase separation occurs to obtain a polyvinylidene fluoride hollow fiber hydrophobic membrane. The coagulant in the coagulation bath is water.

[0052] The aforementioned hollow fiber hydrophobic membrane has an outer diameter of 1.2 mm, a tensile breaking strength of 1.5 N, an internal pressure bursting pressure of 0.2 MPa, and a pure water permeability of 720 L / (m²). 2 •h)@0.1MPa 20 ℃, membrane separation pore size is 0.7μm, hollow fiber membrane pure water contact angle is 90°.

[0053] The examples and comparative data are shown in Table 1. Table 1

[0054] As can be seen from the table, the separation flux of the hollow fiber hydrophobic membrane prepared in Example 1 was significantly improved. In Example 2, polytetrafluoroethylene (PTFE) microparticles were added to impart stable and tunable hydrophobicity to the hydrophobic membrane, with a pure water contact angle reaching up to 105°.

[0055] Comparative Example 1 used only organic low-molecular-weight pore-forming agents, without adding water or calcium carbonate, and had a low membrane flux of 150 L / (m²). 2 The membrane exhibits hydrophilicity with a contact angle of only 75° at 0.1 MPa and 20°C. This indicates that only organic low-molecular-weight pore-forming agents (polyethylene glycol-400) can form small, interconnected membrane pores.

[0056] Comparative Example 2, with only water and an organic low-molecular-weight pore-forming agent added, but without calcium carbonate, achieved a flux of 310 L / (m²). 2 At 0.1 MPa and 20 °C, with a contact angle of 90°, compared to the results in Comparative Example 1, the hydrophobicity of the membrane fibers and the separation pore size in this example are improved, but they are still not as good as the film-forming hydrophobicity and membrane fiber pore size in Example 1.

[0057] Comparative Example 3 only added inorganic particles and organic low-molecular-weight pore-forming agents, without water, and the flux was 5500 L / (m²). 2 The result of adding water at 0.1MPa and 20℃ indicates that the addition of water can cause the casting solution to produce a differential phase effect, which can play a synergistic role with organic low molecular weight pore-forming agents or (and) inorganic particulate pore-forming agents to achieve a good pore-forming effect, which is beneficial to obtaining hollow fiber hydrophobic membranes with large pore size.

[0058] Example 3 illustrates that the film-forming method provided by the present invention is also effective for systems using N-methylpyrrolidone solvent and vinylidene fluoride-tetrafluoroethylene copolymer resin.

[0059] Comparing the results of Example 3 and Comparative Example 4, it can be seen that using a woven tubular membrane can significantly increase the tensile breaking strength and internal pressure rupture pressure of the formed film.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the inventive concept, and these all fall within the protection scope of the present invention.

Claims

1. A method for preparing a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, characterized in that, Includes the following steps: (1) Polyvinylidene fluoride resin is mixed with organic solvent, composite pore-forming agent and polytetrafluoroethylene microparticles and stirred evenly, and then allowed to stand to remove bubbles to obtain casting solution; the mass fraction of polyvinylidene fluoride resin in the casting solution is 15-35 wt%, the mass fraction of organic solvent is 50-80 wt%, the mass fraction of composite pore-forming agent is 2.6-30 wt%, the mass fraction of polytetrafluoroethylene microparticles is 0-10 wt%, and the composite pore-forming agent is composed of inorganic particulate pore-forming agent, organic low molecular weight pore-forming agent and water; the mass fraction of water in the casting solution is 0.1-3 wt%, the mass fraction of organic low molecular weight pore-forming agent in the casting solution is 2-27 wt%, and the mass fraction of inorganic particulate pore-forming agent in the casting solution is 0.5-25 wt%; (2) Using a hollow fiber membrane spinning machine, the casting solution is squeezed out from the spinning nozzle and evenly coated on the outer surface of the braided tube drawn from the central tube of the spinning nozzle. Then it enters the coagulation bath, where the casting solution undergoes phase separation to obtain a braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane.

2. The preparation method according to claim 1, characterized in that, The polyvinylidene fluoride resin is one or more of polyvinylidene fluoride homopolymer, polyvinylidene fluoride-chlorotrifluoroethylene copolymer, and polyvinylidene fluoride copolymer, wherein the polyvinylidene fluoride copolymer is mainly composed of vinylidene fluoride repeating units, and the content of vinylidene fluoride repeating units is more than 70%.

3. The preparation method according to claim 1, characterized in that, The organic solvent is one of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, triethyl phosphate, sulfolane, and dimethyl sulfoxide.

4. The preparation method according to claim 1, characterized in that, The inorganic particulate pore-forming agent is selected from one or more of lithium nitrate, sodium chloride, calcium chloride, calcium carbonate, calcium nitrate, silicon dioxide, aluminum oxide, and kaolin.

5. The preparation method according to claim 1, characterized in that, The organic low-molecular-weight pore-forming agent is selected from one or more of polyethylene glycol-400, dioxane, methyl ethyl ketone, acetone, ethylene glycol monomethyl ether, diethylene glycol methyl ether, and glycerin.

6. The preparation method according to claim 1, characterized in that, The coagulant in the coagulation bath is water.

7. The preparation method according to claim 1, characterized in that, The braided tube is one of polyester fiber braided tube, nylon braided tube, or aromatic polyamide braided tube.

8. A braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane, characterized in that, The preparation method according to any one of claims 1-7 yields a product comprising a braided tube and a hydrophobic membrane layer formed on the surface of the braided tube, wherein the thickness of the hydrophobic membrane layer is 0.05-1 mm and the membrane separation pore size ranges from 0.01-5 μm; the outer diameter of the braided tube-reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane is 0.3-3 mm, the tensile breaking strength is greater than 100 N, and the pure water permeability is 200-7500 L / (m²). 2 ·h)@0.1MPa20℃.

9. The application of a braided tube reinforced polyvinylidene fluoride hollow fiber hydrophobic membrane as described in claim 8 in membrane distillation, membrane aeration, gas separation or air purification.

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