A three-layer structure hollow fiber ultra-microfiltration PVDF composite membrane and a preparation method thereof

Abstract

The application provides a three-layer structure hollow fiber ultra-microfiltration PVDF composite membrane and a preparation method thereof, and the preparation method comprises the following steps: S1, preparing a PVDF resin spinning solution; S2, preparing a second resin spinning solution, and the film-forming resin is at least one of polyvinyl chloride and polyvinyl butyral; S3, using a fiber braided tube as a lining, and sequentially attaching the second resin spinning solution and the PVDF resin spinning solution on the fiber braided tube through a spinning process; and S4, performing a phase inversion reaction in a coagulation bath to make the resin gel solidify into a film, so as to obtain the three-layer structure hollow fiber ultra-microfiltration PVDF composite membrane. The composite membrane uses the fiber braided tube as the lining, and the outer layer resin is the PVDF resin, so that the rejection rate is high, the toughness and oxidation resistance are good, the inner layer resin is other non-PVDF resin with low price, and the large-pore structure is beneficial to the rapid passing of small molecules and good mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of hollow fiber membrane technology, specifically relating to a three-layer hollow fiber ultrafiltration PVDF composite membrane and its preparation method. Background Technology

[0002] Ultrafiltration membranes are microporous filtration membranes with an average pore size of less than 0.1 μm, primarily used for pretreatment of polluted water at the front end of other water treatment membrane technologies. Polyvinylidene fluoride (PVDF) hollow fiber membranes are a type of ultrafiltration membrane, possessing advantages such as good oxidation resistance, chemical corrosion resistance, and high mechanical strength, making them particularly suitable for treating heavily polluted wastewater and widely used in membrane bioreactors. Currently, with the continuous rise in PVDF raw material prices and its expanding application in the lithium battery field, the application space for PVDF resin in water treatment membranes is being compressed. While other inexpensive resins possess excellent performance in certain aspects, their application in heavily polluted wastewater is limited.

[0003] There are many types of resins used to prepare hollow fiber ultrafiltration membranes, including polyvinylidene fluoride, polyethylene, polypropylene, polymethyl methacrylate, polyvinyl butyral, polyvinyl chloride, polycarbonate, and polysulfone. The selection of resin type requires consideration of both the resin's molecular structure and its pore-forming and film-forming properties. Among these materials, using a single resin to prepare hollow fiber membranes cannot fully meet the requirements of all applications. For example, resins often exhibit poor chemical stability and mechanical properties. Therefore, the performance of hollow fiber membranes is improved by mixing resins, adjusting resin content ratios, regulating the type and amount of additives, and controlling parameters such as the composition and temperature of the gel bath. In addition, the preparation of three-layer hollow fiber composite membranes, surface physicochemical modification, and the blending of inorganic materials in the spinning solution are also commonly used optimization and modification methods.

[0004] In multilayer hollow fiber membranes, self-supporting hollow fiber membranes without an inner liner are commonly used. For example, Chinese patent document CN 109621746A discloses a method for preparing a hydrophilic-phobic bilayer polyvinylidene fluoride (PVDF) membrane, in which the outer layer is a PVDF spinning solution with a low critical solution temperature (LCST) and the inner layer is a hydrophilic modified PVDF spinning solution, mainly used in the field of membrane distillation. Chinese patent document CN101642683B discloses a bilayer composite hollow fiber nanofiltration membrane, its preparation method, and special tools. It uses sulfonated polysulfone as the selective separation outer layer and polysulfone, polyacrylonitrile, and polyvinyl chloride as the mechanically supporting inner polymer layer to prepare hollow fibers with nanofiltration-level separation capabilities. However, its preparation product is a nanofiltration membrane, and its pore structure and sieving mechanism are different from those of ultrafiltration membranes.

[0005] Research on three-layer hollow fiber membranes has a long history, with most studies focusing on exploring the preparation of linerless hollow fiber membranes using different resin materials, additive types and contents, and surface modification and functionalization. However, linerless three-layer membrane materials are mostly high-cost resins, which is not conducive to reducing industrial production costs. Furthermore, the differences in the molecular structures of the two resin materials result in macroscopic differences in solubility and mechanical properties, leading to weak interlayer bonding and performance degradation of the membrane element during long-term operation. Summary of the Invention

[0006] This invention provides a three-layer hollow fiber ultrafiltration PVDF composite membrane and its preparation method. The membrane uses a fiber braided tube as the inner liner, with PVDF resin as the outer layer and other inexpensive non-PVDF resins as the inner layer. The large-pore structure of the inner layer resin facilitates the rapid passage of small molecules, provides good mechanical properties, and effectively utilizes inexpensive PVC and PVB resins. The outer PVDF membrane exhibits good toughness and oxidation resistance, significantly reducing the amount of PVDF resin used. The prepared hollow fiber membrane has high flux, and the compatibility and adhesion between the resins are excellent.

[0007] To address the aforementioned problems, a first aspect of the present invention provides a method for preparing a three-layer hollow fiber ultrafiltration PVDF composite membrane, comprising the following steps:

[0008] S1. Prepare a first resin spinning solution, wherein the first resin spinning solution is a PVDF resin spinning solution;

[0009] S2. Prepare a second resin spinning solution, wherein the film-forming resin in the second resin spinning solution is at least one of polyvinyl chloride and polyvinyl butyral;

[0010] S3. Using a fiber braided tube as an inner liner, the second resin spinning solution and the first resin spinning solution are sequentially attached to the fiber braided tube through a spinning process.

[0011] S4. The fiber braided tube with the second resin spinning solution and the first resin spinning solution attached is subjected to a phase inversion reaction in a coagulation bath to solidify the resin gel into a film, thereby obtaining the three-layer hollow fiber ultrafiltration PVDF composite membrane.

[0012] Preferably, the first resin spinning solution contains PVDF resin, a first solvent, a first pore-forming agent, and a first additive;

[0013] The second resin spinning solution contains the film-forming resin, the second solvent, the second pore-forming agent, and the second additive;

[0014] The first additive and the second additive are additives that increase the porosity of the membrane fibers.

[0015] Preferably, the first solvent and / or the second solvent is N,N-dimethylacetamide;

[0016] The first porogen and / or the second porogen are polyethylene glycol with a degree of polymerization of 400-2000;

[0017] The first additive and / or the second additive is at least one of polyvinylpyrrolidone K30 and polyvinylpyrrolidone K60;

[0018] The fiber braided tube is a PET braided tube.

[0019] Preferably, the first resin spinning solution contains 70 parts of a first solvent, 1-20 parts of a first pore-forming agent, and 8-35 parts of a first additive, and the solid content of the PVDF resin is 15%-35%, calculated by mass fraction.

[0020] The second resin spinning solution contains 70 parts by weight of a second solvent, 1-20 parts by weight of a second pore-forming agent, and 8-35 parts by weight of a second additive, and the solid content of the film-forming resin is 15%-35%.

[0021] Preferably, the viscosity of the first resin spinning solution and / or the second resin spinning solution is 30,000-80,000 mPa·s.

[0022] Preferably, step S3 specifically involves: using a three-channel spinneret, passing the fiber braided tube through the central channel of the three-channel spinneret, extruding the second resin spinning solution through the inner resin channel of the three-channel spinneret, extruding the first resin spinning solution through the outer resin channel of the three-channel spinneret, and simultaneously extruding the inner and outer resins through the spinneret and attaching them to the fiber braided tube.

[0023] Preferably, in step S3, the spinning solution temperature is 60-70℃; the extrusion speed of the fiber braided tube is 5-30m / min; the first spinning solution and / or the second spinning solution are pumped by a melt pump with a reduction ratio of 25:1 and a pump speed of 5-50Hz / min. The pump speed of the inner and outer layers varies with the running speed of the braided tube. The ratio of the pump speed of the second resin spinning solution to the pump speed of the first resin spinning solution is (1-3):1. The outer diameter of the membrane fiber is controlled at 2.0±0.2mm, and the total thickness of the membrane layer is controlled at 40-100um.

[0024] Preferably, in step S4, the gel bath contains water and N,N-dimethylacetamide, the temperature of the gel bath is 50-70°C, and the length of the air bath is 0.1-1m.

[0025] Preferably, the process further includes the following step after step S4: Step S5. Soak the obtained membrane fibers in water for 8-24 hours, then heat-treat them in water at 60-80°C for 10-60 minutes, then soak them in 10-30% glycerol for 0.2-1 hours, and finally remove and air dry them.

[0026] A second aspect of the present invention provides a three-layer hollow fiber ultrafiltration PVDF composite membrane prepared by the preparation method described above.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] The method for preparing a three-layer hollow fiber ultrafiltration PVDF composite membrane of the present invention uses a fiber braided tube as an inner liner and simultaneously coats its surface with two layers of resin material to prepare a three-layer hollow fiber ultrafiltration PVDF composite membrane. The outer thin-layer dense pore PVDF membrane has the characteristics of good toughness and good oxidation resistance, and mainly undertakes the function of sieving and filtration. The inner resin macroporous structure has the advantages of rapid water permeability and good mechanical properties. Due to the low viscosity of the inner resin, it can better penetrate into the pores of the braided tube, thereby increasing the adhesion of the resin to the braided tube. At the same time, the compatibility between the two resins can be increased by further adjusting the formulation of the two resins. In addition, unlike self-supporting hollow fibers, the composite membrane of the present invention has a braided tube supporting liner. Phase transformation begins at the spinneret. The inner resin adheres to the braided tube, which contains very little non-solvent. The phase transformation of the inner resin is very slow, reducing the phase transformation process within the inner resin. The phase transformation process mainly starts from the outer surface of the outer resin and relies on the outer layer's phase transformation to drive the inner layer's phase transformation. Therefore, the compatibility problem between the two resin layers is solved. The preparation method uses a two-resin co-coating spinning technology, which is simple. The inner layer uses a low-cost resin with good mechanical strength as raw material, effectively utilizing inexpensive PVC and PVB resins and reducing the amount of PVDF resin used. At the same time, the PVDF resin material used in the outer layer allows it to be used in heavily polluted water environments. The use of inexpensive resin in the inner layer reduces the company's production costs. Attached Figure Description

[0029] Figure 1 These are scanning electron microscope images of the surface and cross-section of the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 1 of this invention.

[0030] Figure 2 The images are scanning electron microscope (SEM) images of the surface and cross-section of the PVC single-layer hollow fiber ultrafiltration membrane prepared in Comparative Example 1.

[0031] Figure 3 This is a scanning electron microscope image of the cross-section of the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 2 of the present invention;

[0032] Figure 4 These are scanning electron microscope images of the surface and cross-section of the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 3 of this invention.

[0033] Figure 5 The images are scanning electron microscope (SEM) images of the surface and cross-section of the monolayer membrane prepared in Comparative Example 2.

[0034] Figure 6 This is a schematic longitudinal section of the three-channel spinneret used in an embodiment of the present invention;

[0035] Figure 7 This is a schematic diagram of the structure of the three-layer hollow fiber ultrafiltration PVDF composite membrane according to an embodiment of the present invention.

[0036] Wherein: 1-outer channel; 2-inner channel; 3-middle channel; 4-PET braided tube liner. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] Using a three-layer hollow fiber membrane structure, including a support tube, can improve the performance of hollow fiber membranes. However, most of the research and preparation at present are hollow fiber membranes without a liner structure. Most of the double-layer membrane materials without a liner structure are resins with high unit cost, which is not conducive to reducing the cost of industrial production. Moreover, the different molecular structures of the two resin materials make it difficult to control the phase transformation process.

[0039] Therefore, a first aspect of the present invention provides a method for preparing a three-layer hollow fiber ultrafiltration PVDF composite membrane, comprising the following steps:

[0040] S1. Prepare a first resin spinning solution, wherein the first resin spinning solution is a PVDF resin spinning solution;

[0041] S2. Prepare a second resin spinning solution, wherein the film-forming resin in the second resin spinning solution is at least one of polyvinyl chloride and polyvinyl butyral;

[0042] S3. Using a fiber braided tube as an inner liner, the second resin spinning solution and the first resin spinning solution are sequentially attached to the fiber braided tube through a spinning process.

[0043] S4. The fiber braided tube with the second resin spinning solution and the first resin spinning solution attached is subjected to a phase inversion reaction in a coagulation bath to solidify the resin gel into a film, thereby obtaining the three-layer hollow fiber ultrafiltration PVDF composite membrane.

[0044] The method for preparing a three-layer hollow fiber ultrafiltration PVDF composite membrane according to an embodiment of the present invention uses a fiber braided tube as an inner liner, and simultaneously coats its surface with two layers of resin material to prepare a three-layer hollow fiber ultrafiltration PVDF composite membrane containing a braided tube and inner and outer resin layers. The outer thin-layer dense pore PVDF membrane has the characteristics of good toughness and good oxidation resistance, and mainly undertakes the function of sieving and filtering. The inner resin has the advantages of rapid water permeability and good mechanical properties due to its large pore structure. Since the inner resin has low viscosity, it can better penetrate into the pores of the braided tube, thereby increasing the adhesion of the resin to the braided tube. At the same time, the compatibility between the two resins can be increased by further adjusting the formulation of the two resins. In addition, unlike self-supporting hollow fibers, the composite membrane of the present invention has a braided tube support. The inner liner is designed so that the resin undergoes phase transformation at the spinneret. The inner resin adheres to the braided tube, and since there are very few non-solvents inside the braided tube, the phase transformation of the inner resin is very slow, reducing the phase transformation process within the inner resin. The phase transformation process mainly starts from the outer surface of the outer resin and relies on the phase transformation of the outer layer to drive the phase transformation of the inner layer, thus solving the compatibility problem between the two resin layers. The preparation method uses a two-resin co-coating spinning technology, which is simple. The inner layer uses a low-cost resin with good mechanical strength as the raw material, effectively utilizing inexpensive PVC and PVB resins and reducing the amount of PVDF resin used. At the same time, the PVDF resin material used in the outer layer allows it to be used in heavily polluted water environments, and the use of inexpensive resin in the inner layer reduces the company's production costs.

[0045] In some embodiments, the first resin spinning solution contains PVDF resin, a first solvent, a first pore-forming agent, and a first additive; the second resin spinning solution contains the film-forming resin, a second solvent, a second pore-forming agent, and a second additive; the first additive and the second additive are additives that increase the porosity of the film fibers.

[0046] In spinning solutions, pore-forming agents can create pore structures during the phase transition of membrane fibers, which is beneficial for mass transport; additives can increase the porosity of membrane fibers, improve the connectivity between pores, and reduce the mass transfer resistance. Only by increasing porosity can the flux be effectively increased.

[0047] In some embodiments, the first solvent and the second solvent may be N,N-dimethylacetamide, N,N-dimethylformamide, methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or triethyl phosphate (TEP), respectively; preferably, the first solvent and / or the second solvent is N,N-dimethylacetamide.

[0048] In some embodiments, the first porogen and the second porogen can be polyethylene glycol, sodium chloride, phosphoric acid, glycerol, acetone, ethanol, etc., respectively; preferably, the first porogen and / or the second porogen is polyethylene glycol with a degree of polymerization of 400-2000, and the surface and cross-sectional hole structure can be adjusted by using such a porogen.

[0049] In some embodiments, the first additive and the second additive can increase the porosity of the membrane fibers. The first additive and the second additive can be at least one of polyvinylpyrrolidone K30, polyvinylpyrrolidone K60, polyvinylpyrrolidone K10, and polyvinylpyrrolidone K15, respectively. Preferably, the first additive and / or the second additive are at least one of polyvinylpyrrolidone K30 and polyvinylpyrrolidone K60. More preferably, the first additive is a mixture of polyvinylpyrrolidone K30 and polyvinylpyrrolidone K60, and the second additive is a mixture of polyvinylpyrrolidone K30 and polyvinylpyrrolidone K60.

[0050] In some embodiments, the specific material of the fiber braided tube can be PET, PAN, etc.; preferably, the fiber braided tube is a PET braided tube, which is non-toxic, acid and alkali resistant, and heat resistant, and can effectively reduce process costs.

[0051] In some embodiments, the mass fractions of each component in the first resin spinning solution are selected within a wide range and are not particularly limited. Preferably, the first resin spinning solution contains 70 parts of a first solvent, 1-20 parts of a first pore-forming agent, and 8-35 parts of a first additive, and the solid content of the PVDF resin is 15%-35%.

[0052] In some embodiments, the mass fractions of each component in the second resin spinning solution are selected within a wide range and are not particularly limited. Preferably, the second resin spinning solution contains 70 parts of a second solvent, 1-20 parts of a second pore-forming agent, and 8-35 parts of a second additive, and the solid content of the film-forming resin is 15%-35%.

[0053] In some embodiments, the viscosity of the first resin spinning solution and the second resin spinning solution is not particularly limited. Preferably, the viscosity of the first resin spinning solution and the second resin spinning solution is 30,000-80,000 mPa·s. Using spinning solutions within this viscosity range can result in less resin seepage into the inner layer and better film formation between the two resin layers.

[0054] In some embodiments, the specific steps of the spinning process are not limited, as long as the first resin spinning solution and the second resin spinning solution can be simultaneously extruded onto the liner, with the second resin spinning solution located outside the liner and the first resin spinning solution located outside the second resin spinning solution. Preferably, step S3 specifically involves: using a three-channel spinneret, passing the fiber braided tube through the central channel of the three-channel spinneret, extruding the second resin spinning solution through the inner resin channel of the three-channel spinneret, and extruding the first resin spinning solution through the outer resin channel of the three-channel spinneret, with both inner and outer resin layers being extruded from the spinneret simultaneously and adhering to the fiber braided tube.

[0055] In some embodiments, in step S1, a first resin spinning solution is prepared at 60-80°C.

[0056] In some embodiments, step S1 specifically involves mixing PVDF resin, a first solvent, a first pore-forming agent, and a first additive, then stirring for 5-10 hours, followed by degassing, degassing, and sealing for later use.

[0057] In some embodiments, in step S2, a second resin spinning solution is prepared at 60-80°C.

[0058] In some embodiments, step S2 specifically involves mixing the film-forming resin, the second solvent, the second pore-forming agent, and the second additive, then stirring for 5-10 hours, followed by degassing, degassing, and sealing for later use.

[0059] In some embodiments, in step S3, the spinning solution temperature is 60-70°C; the spinning temperature is controlled at 60-70°C; the extrusion speed of the fiber braided tube is 3-30 m / min; the first spinning solution and / or the second spinning solution are pumped using a melt pump at a speed of 5-50 Hz / min. By controlling the above process parameters, the resin can adhere well to the braided tube to form a film.

[0060] In some embodiments, in step S3, the pump speed ratio of the second resin spinning solution to the first resin spinning solution is (1-3):1, the outer diameter of the membrane filaments is controlled at 2.0±0.2 mm, and the total thickness of the membrane layer is controlled at 40-100 μm. By controlling the above parameters, the obtained composite membrane can have a higher membrane flux and a higher rejection rate.

[0061] In some embodiments, in step S4, the gel bath contains water and N,N-dimethylacetamide, and the temperature of the gel bath is 50-70°C; the length of the air bath is 0.1-1 m. After the spinning solution is extruded from the spinneret, it must first pass through a certain distance of air gap before entering the gel bath for solidification. This process is called the air bath, and the length of this air bath is also called the dry length. By controlling the above process parameters, smaller surface pores and a gradient sponge pore structure can be obtained.

[0062] In some embodiments, the process further includes the following step after step S4: Step S5. Immerse the obtained membrane fibers in water for 8-24 hours, then heat-treat them in water at 70°C for 45 minutes, then immerse them in 10-30% glycerol for 0.2-1 hours, and finally remove and air-dry them. By controlling the above process parameters, the additives inside the membrane fibers can be precipitated more quickly.

[0063] A second aspect of the present invention provides a three-layer hollow fiber ultrafiltration PVDF composite membrane prepared by the preparation method described above.

[0064] Example

[0065] The performance testing methods for the three-layer hollow fiber ultrafiltration PVDF composite membranes obtained in the following embodiments are as follows:

[0066] Membrane flux refers to the amount of water that passes through a unit membrane area per unit time under certain temperature and pressure. It is used to test the liquid permeability of membrane products and is one of the performance evaluation methods for membrane products.

[0067] The method for testing membrane fiber flux is as follows: Take 3-5 fibers of each type, each 35cm long, with an effective length of 30cm. Test them in pure water at 0.1MPa and 25℃. The calculation formula is as follows: Flux (L / m²*h) = (Flow rate L * 3600S / h) / (3.14 * Outer diameter m * Effective length m * Time s).

[0068] Membrane fiber inner and outer diameters: measured using an optical microscope to characterize the effective membrane thickness of the membrane fiber.

[0069] Membrane fiber cross-sectional structure test: Since the inner liner cannot be brittlely fractured under liquid nitrogen conditions, the membrane fibers were cut open during sample preparation. After removing the inner liner, the membrane fiber samples were prepared by liquid nitrogen brittle fracture, and the cross-section was scanned using a scanning electron microscope.

[0070] Retention rate test: Prepare a 150ppm solution of 200,000 PEO (polyethylene oxide PEO) and stir magnetically until there is no flocculent matter; take 3 membrane fibers of about 40cm for testing, aspirate under negative pressure for about 20 minutes, then collect 30-40mL of sample and mark it; after collecting the sample, immediately send it to test TOC, and calculate the retention rate by the TOC values ​​of the retained solution and the permeate.

[0071] Example 1

[0072] The preparation method of the three-layer hollow fiber ultrafiltration PVDF composite membrane described in this embodiment includes the following steps:

[0073] S1. Prepare PVDF resin spinning solution A at 60℃ by mixing 20g of PVDF resin, 70g of N,N-dimethylacetamide, 2g of polyethylene glycol with a degree of polymerization of 400, 8g of polyvinylpyrrolidone K30, and 1g of polyvinylpyrrolidone K60, and then stirring for 10h. After degassing, degassing, and sealing, the solution is ready for use.

[0074] S2. Prepare PVC resin spinning solution B at 60℃ by mixing 18g of polyvinyl chloride (PVC), 70g of N,N-dimethylacetamide, 5g of polyethylene glycol with a degree of polymerization of 400, 8g of polyvinylpyrrolidone K30, and 3g of polyvinylpyrrolidone K60, then stirring for 10h, followed by degassing, degassing, and sealing for later use.

[0075] S3. Using a PET braided tube as the inner lining, PVC resin spinning solution and PVDF resin spinning solution are sequentially attached to the fiber braided tube through a spinning process; specifically, as shown... Figure 6 This is a schematic diagram of the longitudinal section of a three-channel spinneret; a schematic diagram of the hollow fiber ultrafiltration PVDF composite membrane structure is shown below. Figure 7 A three-channel spinneret is used. The fiber braided tube passes through the central channel of the spinneret. PVC resin spinning solution B is extruded through the inner resin channel of the spinneret, while PVDF resin spinning solution A is extruded through the outer resin channel. Both inner and outer resins are extruded simultaneously and adhere to the fiber braided tube. The process parameters are as follows: spinning temperature controlled at 65℃, PET braided tube liner speed at 10m / min, inner and outer melt pumps at 1.2CC with a reduction ratio of 25:1, inner spinning solution pump speed at 10Hz / min, and outer spinning solution pump speed at 6Hz / min.

[0076] S4. The PET braided tube coated with PVC resin spinning solution and PVDF resin spinning solution is subjected to a phase inversion reaction in a coagulation bath to solidify the resin gel into a film, thereby obtaining a three-layer hollow fiber ultrafiltration PVDF composite membrane. The gel bath consists of a mixture of water and N,N-dimethylacetamide (volume ratio 3:1), the temperature is 65℃, and the length of the air bath is 0.2m.

[0077] S5. Soak the obtained membrane fibers in water for 10 hours, then heat-treat them in water at 60°C for 45 minutes, then soak them in 20% glycerol for 0.5 hours, remove them, and hang them to dry.

[0078] Comparative Example 1

[0079] This comparative example is a PVC single-layer hollow fiber ultrafiltration membrane. Compared with Example 1, the remaining preparation steps and raw material composition are the same as in Example 1. The difference is that only PVC spinning solution is attached to the outside of the PET braided tube.

[0080] Performance testing of three-layer hollow fiber PVDF ultrafiltration composite membrane:

[0081] Table 1 below shows the inner and outer diameters and pure water flux data of the hollow fiber ultrafiltration membranes prepared in Example 1 and Comparative Example 1. As shown in Table 1, the average inner and outer diameters of the PVC monolayer membrane observed under a microscope were 1.00 mm and 1.95 mm, respectively, with a pure water flux of 3354 LMH. In contrast, the flux of the three-layer hollow fiber PVDF ultrafiltration composite membrane was 3862 LMH, higher than that of the monolayer membrane, and its retention rate was higher than that of the monolayer PVC membrane.

[0082] Table 1

[0083]

[0084] Cross-sectional structure test of three-layer hollow fiber ultrafiltration PVDF composite membrane:

[0085] Figure 1 The images show scanning electron microscope (SEM) images of the surface and cross-section of the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 1. Figure 2 This is a scanning electron microscope (SEM) image of the surface and cross-section of the PVC single-layer hollow fiber ultrafiltration membrane prepared in Comparative Example 1. Figure 2 It can be seen that the surface of the PVC single-layer membrane filament has a certain number of large pores, which are unevenly distributed, resulting in poor screening effect; the cross-section shows a large cavity structure, with a large number of small pores on the inner wall of the cavity. Figure 1 It is known that the surface of the three-layer hollow fiber ultrafiltration PVDF composite membrane has a large number of micropores, which are evenly distributed. The boundary between the upper and lower layers on the cross section is not obvious, and a small number of macropores and gradient pore structure can be seen. This sponge pore structure is beneficial to mechanical properties and rapid mass transfer.

[0086] Example 2

[0087] The preparation method of the three-layer hollow fiber ultrafiltration PVDF composite membrane described in this embodiment includes the following steps:

[0088] S1. Prepare PVDF resin spinning solution A. The preparation method and formulation of PVDF resin spinning solution A are the same as in Example 1.

[0089] S2. Prepare PVC resin spinning solution B. The preparation method and formulation of PVC resin spinning solution B are the same as in Example 1.

[0090] S3. Using a PET braided tube as the inner liner, PVC resin spinning solution and PVDF resin spinning solution are sequentially attached to the PET braided tube through a spinning process. A three-channel spinneret is used, through which the fiber braided tube passes. The inner resin channel of the three-channel spinneret extrudes PVC resin spinning solution B, and the outer resin channel extrudes PVDF resin spinning solution A. Both inner and outer resins are extruded from the spinneret simultaneously and attached to the fiber braided tube. The process parameters in the spinning process are: spinning temperature controlled at 65℃, PET braided tube liner speed at 25m / min, inner and outer layer melt pumps at 1.2CC with a reduction ratio of 25:1, inner layer spinning solution pump speed at 18Hz / min, and outer layer spinning solution pump speed at 6Hz / min.

[0091] S4. The PET braided tube coated with PVC resin spinning solution and PVDF resin spinning solution is subjected to a phase inversion reaction in a coagulation bath to solidify the resin gel into a film, thereby obtaining a three-layer hollow fiber ultrafiltration PVDF composite membrane. The gel bath consists of a mixture of water and N,N-dimethylacetamide (volume ratio 3:1) at 60℃, and the length of the air bath is 0.2m.

[0092] S5. Soak the obtained membrane fibers in water for 10 hours, then heat-treat them in water at 60°C for 45 minutes, then soak them in 20% glycerol for 0.5 hours, remove them, and hang them to dry.

[0093] Performance testing of three-layer hollow fiber PVDF ultrafiltration composite membrane:

[0094] Table 2 below shows the inner and outer diameters and pure water flux data of the three-layer hollow fiber PVDF ultrafiltration composite membrane prepared in Example 2. As can be seen from Table 2, its membrane flux is 3383 LMH. Compared with the membrane fibers prepared from PVC in Comparative Example 1, the flux performance is not significantly different, and the retention performance is basically consistent with that of Example 1, but higher than that of Comparative Example 1.

[0095] Table 2

[0096]

[0097]

[0098] Cross-sectional structure test of three-layer hollow fiber ultrafiltration PVDF composite membrane:

[0099] Figure 3 This is a scanning electron microscope (SEM) image of the cross-sectional area of ​​the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 2. In Example 2, when the membrane fiber linear velocity was 25 m / min, the corresponding inner and outer layer pump speeds were also increased. Figure 3As can be seen, the cross-section clearly shows the inner and outer layer structures. The inner layer cross-section structure exhibits a loose, honeycomb-like pore structure, while the outer layer cross-section structure exhibits a relatively dense, sponge-like pore structure. Magnification at the interface reveals that the space between the two layers is not an air structure, but rather a mixture of large and small pores that permeate each other. The outer layer is relatively thin, approximately 20 μm, while the inner layer is relatively thick, approximately 30 μm, achieving the desired goal of a thinner outer and thicker inner membrane cross-section. This indicates that the thickness of the inner and outer membrane layers can be controlled by adjusting the spinning process parameters, enabling industrial-scale production.

[0100] Example 3

[0101] The preparation method of the three-layer hollow fiber ultrafiltration PVDF composite membrane described in this embodiment includes the following steps:

[0102] S1. Prepare PVDF resin spinning solution A at 70℃ by mixing 18g of PVDF resin, 70g of N,N-dimethylacetamide, 5g of polyethylene glycol with a degree of polymerization of 2000, and 20g of polyvinylpyrrolidone K30, then stirring for 10h, followed by degassing, degassing, and sealing for later use.

[0103] S2. Prepare the second resin spinning solution B at 60℃ by mixing 18g of polyvinyl butyral (PVB), 70g of N,N-dimethylacetamide, 8g of polyethylene glycol with a degree of polymerization of 2000, 15g of polyvinylpyrrolidone K30, and 5g of polyvinylpyrrolidone K60, then stirring for 10h, followed by degassing, degassing, and sealing for later use.

[0104] S3. Using a PET braided tube as the inner liner, the second resin spinning solution B and PVDF resin spinning solution are sequentially attached to the fiber braided tube through a spinning process. A three-channel spinneret is used, and the fiber braided tube is passed through the central channel of the three-channel spinneret. The second resin spinning solution B is extruded through the inner resin channel of the three-channel spinneret, and the PVDF resin spinning solution A is extruded through the outer resin channel of the three-channel spinneret. The inner and outer resins are extruded simultaneously from the spinneret and attached to the fiber braided tube. The process parameters in the spinning process are as follows: spinning temperature is controlled at 70℃, the PET braided tube liner speed is 20m / min, the inner and outer layer melt pumps are 1.2CC with a reduction ratio of 25:1, the inner layer spinning solution pump speed is 12Hz / min, and the outer layer spinning solution pump speed is 10Hz / min.

[0105] S4. The PET braided tube with the second resin spinning solution B and PVDF resin spinning solution A attached is placed in a coagulation bath for a phase inversion reaction, so that the resin gel solidifies into a film, and a three-layer hollow fiber ultrafiltration PVDF composite membrane is obtained. The gel bath consists of a mixture of water and N,N-dimethylacetamide (volume ratio 1:1), the temperature is 70℃, and the length of the air bath is 0.5m.

[0106] S5. Soak the obtained membrane fibers in water for 10 hours, then heat-treat them in water at 60°C for 45 minutes, then soak them in 20% glycerol for 0.5 hours, remove them, and hang them to dry.

[0107] Comparative Example 2

[0108] This comparative example is a single-layer hollow fiber ultrafiltration membrane. Compared with Example 3, the remaining preparation steps and raw material composition are the same as in Example 3. The difference is that only the second resin spinning solution is attached to the outside of the PET braided tube. The formula is the same as in Example 3.

[0109] Performance testing of three-layer hollow fiber PVDF ultrafiltration composite membrane:

[0110] Table 3 below shows the inner and outer diameters and pure water flux data of the hollow fiber ultrafiltration membranes prepared in Example 3 and Comparative Example 2. As shown in Table 3, the single-layer hollow fiber ultrafiltration membrane of Comparative Example 2 has a membrane flux of 1994 LMH and an overall membrane thickness of 0.925 mm; the three-layer hollow fiber PVDF ultrafiltration composite membrane of Example 3 has a flux of 3450 LMH and an overall membrane thickness of 0.94 mm. The rejection rate of the three-layer hollow fiber PVDF ultrafiltration composite membrane is slightly higher than that of the single-layer PVB membrane in Comparative Example 2, but its flux is much higher than that of the single-layer PVB membrane in Comparative Example 2.

[0111] Table 3

[0112]

[0113]

[0114] Cross-sectional structure test of three-layer hollow fiber ultrafiltration PVDF composite membrane:

[0115] Figure 4 The images show scanning electron microscope (SEM) images of the surface and cross-section of the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 3. Figure 5 The images show scanning electron microscope (SEM) images of the surface and cross-section of the monolayer membrane fabricated in Comparative Example 2. Figure 5 It can be seen that the surface porosity of the monolayer membrane fiber in Comparative Example 2 is low and the porosity is unevenly distributed. Figure 4 It can be seen that the three-layer hollow fiber ultrafiltration PVDF composite membrane prepared in Example 3 has a large number of large pores on its surface, which are evenly distributed. The cross-section shows a dense sponge pore structure, and the boundary between the inner and outer resin layers is not obvious. However, the pore size gradually increases and then decreases from top to bottom. Compared with the single-layer membrane of Comparative Example 2, it can be confirmed that the prepared membrane is a three-layer composite membrane.

[0116] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a three-layer hollow fiber ultrafiltration PVDF composite membrane, characterized in that, Includes the following steps: S1. Prepare a first resin spinning solution, wherein the first resin spinning solution is a PVDF resin spinning solution; S2. Prepare a second resin spinning solution, wherein the film-forming resin in the second resin spinning solution is at least one of polyvinyl chloride and polyvinyl butyral; S3. Using a fiber braided tube as an inner liner, the second resin spinning solution and the first resin spinning solution are sequentially attached to the fiber braided tube through a spinning process; Step S3 specifically involves: using a three-channel spinneret, the fiber braided tube is passed through the central channel of the three-channel spinneret, the second resin spinning solution is extruded from the inner resin channel of the three-channel spinneret, the first resin spinning solution is extruded from the outer resin channel of the three-channel spinneret, and the inner and outer resins are simultaneously extruded from the spinneret and attached to the fiber braided tube; S4. The fiber braided tube with the second resin spinning solution and the first resin spinning solution attached is placed in a coagulation bath to carry out a phase transformation reaction, so that the resin gel is solidified into a film to obtain the three-layer hollow fiber ultra-microfiltration PVDF composite membrane. The first resin spinning solution contains PVDF resin, a first solvent, a first pore-forming agent, and a first additive; calculated by mass parts, the first resin spinning solution contains 70 parts of the first solvent, 1-20 parts of the first pore-forming agent, and 8-35 parts of the first additive, and the solid content of the PVDF resin is 15%-35%. The second resin spinning solution contains the film-forming resin, the second solvent, the second pore-forming agent, and the second additive; calculated by mass parts, the second resin spinning solution contains 70 parts of the second solvent, 1-20 parts of the second pore-forming agent, and 8-35 parts of the second additive, and the solid content of the film-forming resin is 15%-35%. The first additive and the second additive are additives that increase the porosity of the membrane fibers; The first solvent and the second solvent are N,N-dimethylacetamide; The first porogen and the second porogen are polyethylene glycol with a degree of polymerization of 400-2000; The first additive and the second additive are at least one of polyvinylpyrrolidone K30 and polyvinylpyrrolidone K60; The fiber braided tubing is PET braided tubing; The viscosity of the first resin spinning solution and the second resin spinning solution is 30,000-80,000 mPa·s; In step S3, the spinning solution temperature is 60-70℃; the fiber braiding tube running speed is 5-30m / min; the first resin spinning solution and the second resin spinning solution are pumped by a melt pump with a reduction ratio of 25:1 and a pump speed of 5-50Hz / min; the pump speed ratio of the second resin spinning solution to the first resin spinning solution is (1-3):1; the outer diameter of the membrane fiber is controlled at 2.0±0.2mm; and the total thickness of the membrane layer is controlled at 40-100um. In step S4, the coagulation bath contains water and N,N-dimethylacetamide, and the temperature of the coagulation bath is 50-70°C; the length of the air bath is 0.1-1m. It also includes the following steps after step S4: Step S5. Soak the obtained membrane fibers in water for 8-24 hours, then heat treat them in water at 60-80°C for 10-60 minutes, then soak them in 10-30% glycerol for 0.2-1 hours, and then remove and air dry.

2. The three-layer hollow fiber ultrafiltration PVDF composite membrane prepared by the preparation method described in claim 1.

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