A high-strength self-supporting polyvinylidene fluoride hollow fiber membrane and a method for preparing the same

Abstract

This invention discloses a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, comprising an outer porous layer, a middle porous wall layer, and an inner porous layer. The thickness of the outer porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and its porosity is greater than 75%. The thickness of the inner porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and its porosity is greater than 75%. The thickness of the middle porous wall layer accounts for more than 90% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and its porosity is greater than 75%. This invention also discloses a method for preparing the above-mentioned hollow fiber membrane. Compared with the prior art, this invention has the following advantages: the tensile strength at break of the hollow fiber membrane is greater than 10 MPa, and the elongation can reach more than 100%, making it particularly suitable for applications in water treatment, pharmaceuticals, food, and other fields where high membrane strength, membrane separation accuracy, and membrane separation scale are required.

Description

Technical Field

[0001] This invention belongs to the field of polymer hollow fiber membrane preparation technology, and particularly relates to a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane and its preparation method. Background Technology

[0002] Phase separation is currently the most commonly used method for preparing polymer microporous membranes. Based on the difference in the driving force of phase separation, it can be mainly divided into thermally induced phase separation (TIPS) and non-solvent-induced phase separation (NIPS). NIPS typically involves a homogeneous casting solution formed between the polymer and solvent near a relatively low temperature. When immersed in a non-solvent, mass transfer at the solvent-non-solvent interface leads to phase separation and polymer solidification, resulting in a porous membrane. It was the earliest proposed and applied phase inversion process. Its advantages include operation at room temperature and pressure and a wide range of solvents. However, it suffers from complex preparation systems, poor process stability, and low membrane strength, often requiring composite reinforcement using braided or knitted tubing. Compared to the NIPS method, the TIPS method is based on the formation of a homogeneous casting solution by polymer and high-boiling-point diluent at high temperature. Then, during the cooling process, phase separation and polymer solidification occur due to the decrease in thermodynamic compatibility of the system. Finally, the diluent is extracted to obtain a membrane material with a porous structure. Its process is highly controllable, and the prepared membrane usually has a more regular microporous structure, better separation and permeation performance, and higher mechanical strength.

[0003] Polyvinylidene fluoride (PVDF) possesses strong resistance to biodegradation, good thermal stability, high mechanical strength, resistance to chemical oxidation, and resistance to UV aging. It meets the requirements of membranes for advanced processing in separation and purification engineering, making it one of the most favored membrane materials in the current membrane manufacturing field. Early PVDF microporous membranes were typically prepared using the NIPS method, which suffered from problems such as wide pore size distribution, low separation accuracy, numerous structural defects, poor mechanical properties, low product quality, and homogeneous competition, negatively impacting the expansion of the membrane industry and its applications. In contrast, PVDF microporous membranes prepared by the TIPS method typically exhibit higher mechanical properties, narrower pore size distribution, more regular pore structure, and more precise separation performance. Besides its widespread application in traditional water treatment, PVDF hollow fiber membranes prepared using TIPS membrane separation technology are increasingly attracting attention for their application in material separation processes in the pharmaceutical and food industries.

[0004] However, most publicly available TIPS membrane fabrication methods are based on organic ester diluent systems, which are prone to polymer spheroidization during the cooling and phase separation process, leading to a decrease in the mechanical properties of the prepared membrane material. The tensile strength of most publicly reported thermo-induced PVDF hollow fiber membranes is below 8 MPa, with only a very few reports achieving tensile strengths of 10 MPa through high-concentration PVDF, but at the cost of sacrificing membrane separation scale and permeation performance. For example, patent 109621745B discloses a method for preparing microfiltration membranes through dry-wet phase conversion. Example 2 shows that the prepared thermo-induced phase PVDF membrane has an average pore size of 0.45 micrometers, but a tensile strength of only 3.7 MPa. Patent CN101342468B discloses a method for preparing β-crystalline phase polyvinylidene fluoride hollow fiber porous membranes. Example 3 shows that the prepared thermo-induced phase PVDF hollow fiber membrane has a tensile strength of 3.9 MPa and an average pore size of 0.3 micrometers. Patent CN112191108B discloses a polyvinylidene fluoride hollow fiber ultrafiltration membrane capable of retaining heavy metal ions. The preparation method, as shown in Example 1, yielded a thermo-induced PVDF hollow fiber membrane with a strength of 5.12 MPa and a heavy metal rejection rate of only about 50%. Patent CN101362057B discloses a method for preparing polyvinylidene fluoride porous membranes; Example 3 shows that the prepared PVDF membrane has a sponge-like pore structure and a tensile strength of 10.1 MPa, but the PVDF mass percentage in its formulation is as high as 60 wt%. The tensile strengths of the membranes obtained in the other examples are all less than 7 MPa, especially those with a PVDF mass percentage below 35%, and the tensile strengths of the membranes obtained in the aforementioned examples are generally below 6 MPa. The preparation technology of thermo-induced PVDF hollow fiber membranes with high strength, high separation accuracy, ultrafiltration separation scale, and high permeability is urgently needed. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane and its preparation method, which solves the problems of low tensile strength and large separation scale that are common in polyvinylidene fluoride hollow fiber membranes prepared by the existing thermally induced phase method.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, characterized in that it comprises an outer porous layer, a middle porous wall layer, and an inner porous layer, wherein the thickness of the outer porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the outer porous layer is greater than 75%; the thickness of the inner porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the inner porous layer is greater than 75%; and the thickness of the middle porous wall layer accounts for more than 90% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the middle porous wall layer is greater than 75%.

[0007] Preferably, the pore skeleton of the outer porous layer is <100nm, the pore skeleton of the inner porous layer is <100nm, and the pore skeleton of the middle porous wall layer is 200-350nm.

[0008] Preferably, the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane has a thickness of 100-300 μm, a tensile strength > 10 MPa, and an elongation at break > 100%. The polyvinylidene fluoride hollow fiber membrane prepared by this invention has excellent mechanical strength, with a tensile strength greater than 10 MPa and an elongation at break greater than 100%.

[0009] Another objective of this invention is to provide a method for preparing a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, the method specifically comprising the following steps:

[0010] S1. Mix polyvinylidene fluoride powder, caprolactam, small molecule ethylene glycol condensate, high molecular weight polyethylene glycol and processing aids evenly to obtain a mixture.

[0011] S2. The mixture obtained in step S1 is extruded and internally supported by a twin-screw extruder to form hollow fibers;

[0012] S3. Place the hollow fibers obtained in step S2 in a gel bath to solidify them, and obtain the polyvinylidene fluoride hollow fiber membrane precursor.

[0013] S4. The polyvinylidene fluoride hollow fiber membrane precursor obtained in step S3 is subjected to restretching, hot annealing and heat setting processes to finally obtain a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane.

[0014] This invention innovatively designs the film-forming formulation and polymer thermally induced phase separation extrusion spinning process, and innovates the formulation of components to induce phase separation growth in the membrane structure and the process of inducing phase separation by partitioning the membrane cross-section. At high temperature, high molecular weight polyethylene glycol (PEG) and polyvinylidene fluoride (PVDF) form macromolecular chain entanglements. Simultaneously, through the repulsive interaction between the strong hydrophilicity of PEG and the strong hydrophobicity of PVDF molecules, the solid phase pore walls of PVDF are induced to grow to a larger size during the cooling phase separation process, resulting in a high-strength, large-sized membrane pore framework. Simultaneously, through the condensation of small-molecule ethylene glycol, the hydrophilic interface barrier formed by PEG during the cooling phase separation process is broken down, achieving double continuity of the large-sized PVDF solid phase pore walls. Furthermore, by combining the mass exchange of the interfacial micro-regions between the water-soluble diluent and the aqueous gel bath, a membrane separation layer micro-region with high porosity and high separation precision is constructed, enabling the preparation of PVDF hollow fiber membranes that possess high strength, high separation precision, ultrafiltration separation scale, and high permeability.

[0015] Preferably, in step S1, the mass percentage of the raw materials used to prepare the mixture is as follows: polyvinylidene fluoride powder: 25-35%, caprolactam: 30-40%, small molecule ethylene glycol condensate: 15-25%, high molecular weight polyethylene glycol: 10-20%, and processing aids: 1-3%.

[0016] Preferably, in step S1, the small molecule ethylene glycol condensate is selected from one or more of diethylene glycol, triethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400, and polyethylene glycol 600.

[0017] Preferably, the high molecular weight polyethylene glycol is selected from one or more polyethylene glycols with a molecular weight of 20,000 to 200,000.

[0018] Preferably, the processing aid is a mixture of an antioxidant and a lubricant, and the mass ratio of the antioxidant to the lubricant is (5-10):1. The processing aid of this invention can be used to improve the properties of twin-screw extrusion and the apparent smoothness and macroscopic dimensional uniformity of polyvinylidene fluoride hollow fiber membranes.

[0019] Preferably, in step S2, the inner cavity support is provided by inert gas support or core fluid support.

[0020] Preferably, the inert gas is nitrogen or argon.

[0021] Preferably, the solution supporting the core fluid is polyethylene glycol or a mixture of polyethylene glycol and a polar solvent.

[0022] Preferably, the oligomeric polyethylene glycol is selected from one or more of polyethylene glycol, ethylene glycol, diethylene glycol, and triethylene glycol.

[0023] Preferably, the polar solvent is selected from one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, trimethyl phosphate, methylpyrrolidone, and dimethyl sulfoxide.

[0024] Preferably, the volume percentage of oligoethylene glycol in the mixture is 20-99%.

[0025] Preferably, in step S3, the solution in the gel bath is selected from water or an aqueous solution.

[0026] This invention, through gel bath compounding and regulation, can not only prepare the previously reported macroporous microfiltration thermotropic polyvinylidene fluoride hollow fiber membrane, but also prepare ultrafiltration polyvinylidene fluoride hollow fiber membranes with a pore size of less than 50 nanometers and high permeation flux, which have narrow pore size distribution and high separation accuracy.

[0027] Preferably, the solvent in the aqueous solution is selected from one or more of autolactam, oligoethylene glycol, and polar solvents.

[0028] Preferably, in step S4, the parameters for re-stretching are as follows: ambient temperature of 50-90℃ and stretching ratio of ≤2.5 times.

[0029] Preferably, the heat annealing environment is a hot air environment with a temperature of 100-150℃ and a time of ≤45s.

[0030] Preferably, the heat setting stabilization temperature is 50-90℃, and the time is ≤20s. This invention, through post-processing steps such as re-stretching, heat annealing, and heat setting, can further adjust the layer ratio, pore size, porosity, and hollow skeleton size of the spun polyvinylidene fluoride hollow fiber membrane. Simultaneously, it can adjust the polymer crystallization morphology of the spun polyvinylidene fluoride hollow fiber membrane, thereby achieving special functions such as piezoelectric and magnetoelectric properties of the membrane material.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. This invention starts with the design of membrane-forming formulation components. Through innovative membrane structure phase separation growth induction component compounding design, a large skeleton structure of membrane main pore wall is constructed. This avoids the accumulation of polyvinylidene fluoride spherulites that are easily formed in the traditional conventional thermally induced phase process, as well as the low-strength fine pore skeleton formed by some disclosed improvement technologies. This can significantly improve the tensile strength of hollow fiber membrane materials at break, and avoid the problem of low mechanical strength of polyvinylidene fluoride hollow fiber membranes prepared by traditional thermally induced phase processes. The prepared polyvinylidene fluoride hollow fiber membrane can achieve high strength, high separation accuracy, ultrafiltration separation scale and high permeability performance.

[0033] 2. Unlike the traditional single-membrane pore structure control process in conventional thermally induced phase separation, this invention utilizes high-molecular-weight, strongly hydrophilic polyethylene glycol to form a phase separation interface barrier during the cooling phase separation process. This induces the enrichment and growth of polyvinylidene fluoride (PVDF) molecular chains, leading to the formation of a large-size pore framework. Simultaneously, hydrophilic small-molecule ethylene glycol condensates penetrate the hydrophilic interface barrier, achieving bicontinuous connectivity of the large-framework PVDF solid phase pore walls. This simultaneously enables the construction of a high-strength membrane pore framework and a highly permeable bicontinuous membrane pore structure.

[0034] 3. Unlike the thermally induced phase separation process of traditional organic ester diluent systems, the present invention uses a water-soluble diluent system combined with an aqueous gel bath to simultaneously achieve the thermally induced phase separation process of cooling and the mass exchange of interfacial microphases in the micro-regions on the outer surface of the membrane, thereby realizing the construction of a high-precision micro-region separation pore layer structure on the outer surface of the membrane with typical high porosity and fine pore size distribution.

[0035] 4. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane prepared by this invention has a tensile strength at break greater than 10 MPa and an elongation of over 100%, making it particularly suitable for applications in water treatment, pharmaceuticals, food, and other fields where high membrane strength, membrane separation accuracy, and membrane separation scale are required. Attached Figure Description

[0036] Figure 1 A panoramic electron micrograph of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Example 1 of the present invention.

[0037] Figure 2 This is an electron micrograph of the outer porous layer of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Example 1 of the present invention.

[0038] Figure 3 This is an electron micrograph of the inner porous layer of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Example 1 of the present invention.

[0039] Figure 4 This is an electron micrograph of the porous intermediate wall of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Example 1 of the present invention.

[0040] Figure 5 This is an electron micrograph of the porous intermediate wall of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Example 5 of the present invention.

[0041] Figure 6 This is an electron micrograph of the porous intermediate wall of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Example 6 of the present invention.

[0042] Figure 7 This is a magnified electron microscope image of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Comparative Example 1 of the present invention.

[0043] Figure 8 This is a magnified electron microscope image of the cross-section of the polyvinylidene fluoride hollow fiber membrane prepared in Comparative Example 2 of this invention. Detailed Implementation

[0044] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0045] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0046] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This application specification and embodiments are merely exemplary.

[0047] This invention provides a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, comprising an outer porous layer, a middle porous wall layer, and an inner porous layer. The thickness of the outer porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the outer porous layer is greater than 75%. The thickness of the inner porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the inner porous layer is greater than 75%. The thickness of the middle porous wall layer accounts for more than 90% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the middle porous wall layer is greater than 75%. The pore skeleton of the outer porous layer is less than 100 nm, the pore skeleton of the inner porous layer is less than 100 nm, and the pore skeleton of the middle porous wall layer is 200-350 nm.

[0048] Another objective of this invention is to provide a method for preparing a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, which specifically includes the following steps:

[0049] S1. Polyvinylidene fluoride powder, caprolactam, small molecule ethylene glycol condensate, high molecular weight polyethylene glycol, and processing aids are mixed evenly to obtain a mixture. The mass percentage of the raw materials used to prepare the mixture is as follows: polyvinylidene fluoride powder: 25-35%, caprolactam: 30-40%, small molecule ethylene glycol condensate: 15-25%, high molecular weight polyethylene glycol: 10-20%, processing aids: 1-3%. The small molecule ethylene glycol condensate is selected from one or more of diethylene glycol, triethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400, and polyethylene glycol 600. The high molecular weight polyethylene glycol is selected from one or more of polyethylene glycols with a molecular weight of 20,000 to 200,000. The processing aids are a mixture of antioxidants and lubricants, and the mass ratio of antioxidants to lubricants is (5-10):1.

[0050] S2. The mixture obtained in step S1 is extruded and internally supported by a twin-screw extruder to form hollow fibers. The internal support adopts inert gas support or core liquid support. The inert gas is nitrogen or argon. The solution of the core liquid support is polyethylene glycol or a mixture of polyethylene glycol and a polar solvent. The polyethylene glycol is selected from one or more of polyethylene glycol, ethylene glycol, diethylene glycol, and triethylene glycol. The polar solvent is selected from one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, trimethyl phosphate, methylpyrrolidone, and dimethyl sulfoxide. The volume percentage of polyethylene glycol in the mixture is 20-99%.

[0051] S3. Place the hollow fibers obtained in step S2 in a gel bath for curing to obtain a polyvinylidene fluoride hollow fiber membrane precursor. The solution in the gel bath is selected from water or aqueous solution. The solvent in the aqueous solution is selected from one or more of the following: lactam, oligomeric polyethylene glycol, and polar solvent.

[0052] S4. The polyvinylidene fluoride hollow fiber membrane precursor obtained in step S3 is subjected to re-stretching, hot annealing, and heat setting processes to finally obtain a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane. The re-stretching parameters are as follows: ambient temperature is 50-90℃, stretching ratio is ≤2.5 times, hot annealing environment is hot air environment, temperature is 100-150℃, time is ≤45s, and heat setting stabilization temperature is 50-90℃, time is ≤20s.

[0053] This invention innovatively designs the film-forming formulation and polymer thermally induced phase separation extrusion spinning process, and innovates the formulation of components to induce phase separation growth in the membrane structure and the process of inducing phase separation by partitioning the membrane cross-section. At high temperature, high molecular weight polyethylene glycol (PEG) and polyvinylidene fluoride (PVDF) form macromolecular chain entanglements. Simultaneously, through the repulsive interaction between the strong hydrophilicity of PEG and the strong hydrophobicity of PVDF molecules, the solid phase pore walls of PVDF are induced to grow to a larger size during the cooling phase separation process, resulting in a high-strength, large-sized membrane pore framework. Simultaneously, through the condensation of small-molecule ethylene glycol, the hydrophilic interface barrier formed by PEG during the cooling phase separation process is broken down, achieving double continuity of the large-sized PVDF solid phase pore walls. Furthermore, by combining the mass exchange of the interfacial micro-regions between the water-soluble diluent and the aqueous gel bath, a membrane separation layer micro-region with high porosity and high separation precision is constructed, enabling the preparation of PVDF hollow fiber membranes that possess high strength, high separation precision, ultrafiltration separation scale, and high permeability.

[0054] The technical effects of the present invention will be described below with reference to specific embodiments.

[0055] Example 1

[0056] S1. Add 30% polyvinylidene fluoride, 35.8% caprolactam, 15% triethylene glycol, 18% polyethylene glycol 100000, 1% antioxidant 1010 and 0.2% stearamide by mass to a high-speed mixer and mix evenly.

[0057] S2. Set the temperature of each section of the twin-screw extruder to 150℃-160℃ and the twin-screw speed to 230 rpm. Pass the uniformly mixed mixture from step S1 through the twin-screw extruder and extrude it into hollow fibers through the hollow fiber die. The core liquid is a blend solution of triethylene glycol and polyethylene glycol 200 in a volume ratio of 5:5.

[0058] S3. Introduce the hollow fiber obtained in step S2 into a gel bath for curing. The air gap is 15 cm. The gel bath is a mixed solution of caprolactam and water with a mass fraction of 50% and the temperature is 10°C.

[0059] S4. Transfer the solidified hollow fiber from step S3 to water at 25°C, wash and dry to obtain a polyvinylidene fluoride hollow fiber membrane.

[0060] The prepared polyvinylidene fluoride hollow fiber membrane was used for pure water flux testing, and the results showed that the pure water flux of the hollow fiber membrane was approximately 610 L / m³. 2 ·h·bar.

[0061] like Figure 1 As shown, this is a cross-sectional panoramic electron microscope image of the prepared polyvinylidene fluoride hollow fiber membrane, revealing a differentiated three-layer porous structure. The porous layer on the outer surface is approximately 11 micrometers thick, the porous layer on the inner surface is approximately 8 micrometers thick, and the intermediate porous wall is approximately 231 micrometers thick. Figure 2 As shown, this is a magnified photograph of the porous layer on the outer surface. Through image integration, its porosity is approximately 78%, and its pore structure size is approximately 89 nanometers. Figure 3 As shown, this is a magnified photograph of the porous layer on the inner surface. Through image integration, its porosity is calculated to be approximately 83%, and its pore structure size is approximately 73 nanometers. Figure 4 As shown in the magnified photograph of its porous intermediate wall, by integrating the image, its porosity is approximately 70%, and the size of its main pore skeleton is approximately 330 nanometers.

[0062] Tensile testing revealed that the average tensile breaking force of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 10.6 N, and the average elongation at break was approximately 230%.

[0063] The initial water droplet contact angle of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was measured to be approximately 69 degrees through contact angle testing.

[0064] Analysis using a pore size analyzer revealed that the average pore size of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 66 nm.

[0065] The prepared polyvinylidene fluoride hollow fiber membrane was used for bovine serum albumin solution retention test, and the results showed that the hollow fiber membrane had a retention rate of more than 95% for bovine serum albumin.

[0066] Example 2

[0067] The polyvinylidene fluoride hollow fiber membrane prepared in Example 1 was stretched by 1.0 times in a hot water bath at 70°C, and then set in a hot oven at 70°C for 8 seconds to obtain a re-stretched polyvinylidene fluoride hollow fiber membrane.

[0068] The polyvinylidene fluoride hollow fiber membrane obtained in this embodiment was used for pure water flux testing, and the result was: the pure water flux of the hollow fiber membrane was approximately 1580 L / m³. 2 ·h·bar.

[0069] Tensile testing revealed that the average tensile breaking force of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 12.1 N, and the average elongation at break was approximately 100%.

[0070] The initial water droplet contact angle of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was measured to be approximately 65 degrees through contact angle testing.

[0071] Analysis using a pore size analyzer revealed that the average pore size of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 100 nm.

[0072] The polyvinylidene fluoride hollow fiber membrane obtained in this embodiment was used for bovine serum albumin solution retention test, and the result was that the hollow fiber membrane achieved a retention rate of more than 92% for bovine serum albumin.

[0073] Example 3

[0074] The polyvinylidene fluoride hollow fiber membrane prepared in Example 1 was stretched by 1.0 times in a hot water bath at 70°C, then annealed at 150°C for 30 seconds, and then shaped in a hot oven at 70°C for 8 seconds to obtain a re-stretched polyvinylidene fluoride hollow fiber membrane.

[0075] Infrared spectroscopy was performed on the prepared polyvinylidene fluoride hollow fiber membrane, and the PVDF crystallization peaks in the spectrum were analyzed and calculated. The content of β crystal form 6 was approximately 83%.

[0076] Tensile testing revealed that the average tensile breaking force of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 12.7 N, and the average elongation at break was approximately 100%.

[0077] The initial water droplet contact angle of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was measured to be approximately 61 degrees through contact angle testing.

[0078] Example 4

[0079] S1: Add 25% polyvinylidene fluoride, 36% caprolactam, 18% triethylene glycol, 20% polyethylene glycol 100000, 0.9% antioxidant 1010 and 0.1% stearamide by mass to a high-speed mixer and mix evenly.

[0080] S2: Set the temperature of each section of the twin-screw extruder to 145℃-155℃ and the twin-screw speed to 200 rpm. Pass the uniformly mixed mixture through the twin-screw extruder and extrude it into hollow fibers through the hollow fiber die. The supporting core liquid is a blend solution of polyethylene glycol 400 and dimethylacetamide with a volume ratio of 9:1.

[0081] S3: Hollow fibers are introduced into a gel bath for curing, with an air gap of 5 cm. The gel bath is a mixed solution of 30% polyethylene glycol 400, 50% dimethylacetamide, and water, at a temperature of 5°C.

[0082] S4: The cured hollow fiber is transferred to water at 25°C for cleaning, then stretched 0.5 times in a hot water bath at 50°C, then annealed at 100°C for 45 seconds, and then shaped in a hot oven at 50°C for 20 seconds to obtain a restretched polyvinylidene fluoride hollow fiber membrane.

[0083] The prepared polyvinylidene fluoride hollow fiber membrane was used for pure water flux testing, and the results showed that the pure water flux of the hollow fiber membrane was approximately 1070 L / m³. 2 ·h·bar.

[0084] Electron microscopy characterization revealed that the porous layer on the outer surface was approximately 7 micrometers thick, the porous layer on the inner surface was approximately 6 micrometers thick, and the intermediate porous wall was approximately 235 micrometers thick. Magnified electron microscopy analysis of the outer surface porous layer, combined with image integration, showed a porosity of approximately 81% and a pore framework size of approximately 77 nanometers. Magnified electron microscopy analysis of the inner surface porous layer, combined with image integration, showed a porosity of approximately 80% and a pore framework size of approximately 77 nanometers. Magnified electron microscopy analysis of the intermediate porous wall, combined with image integration, showed a porosity of approximately 75% and a main pore framework size of approximately 230 nanometers.

[0085] Tensile testing revealed that the average tensile breaking force of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 10.1 N, and the average elongation at break was approximately 130%.

[0086] The initial water droplet contact angle of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was measured to be approximately 66 degrees through contact angle testing.

[0087] Analysis using a pore size analyzer revealed that the average pore size of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 53 nm.

[0088] The prepared polyvinylidene fluoride hollow fiber membrane was used for bovine serum albumin solution retention test, and the results showed that the hollow fiber membrane had a retention rate of more than 96% for bovine serum albumin.

[0089] Example 5

[0090] S1: Add 35% polyvinylidene fluoride, 30.6% caprolactam, 16% triethylene glycol, 17% polyethylene glycol 100000, 1.2% antioxidant 1010 and 0.2% stearamide by mass to a high-speed mixer and mix evenly.

[0091] S2: Set the temperature of each section of the twin-screw extruder to 155℃-170℃ and the twin-screw speed to 250 rpm. Pass the uniformly mixed mixture through the twin-screw extruder and extrude it into hollow fibers through the hollow fiber die. The core liquid is nitrogen.

[0092] S3: Hollow fibers are introduced into a gel bath for curing, with an air gap of 1 cm. The gel bath water temperature is 0℃.

[0093] S4: Transfer the cured hollow fiber to water at 25°C, wash and dry to obtain polyvinylidene fluoride hollow fiber membrane.

[0094] The prepared polyvinylidene fluoride hollow fiber membrane was used for pure water flux testing, and the result was that the pure water flux of the hollow fiber membrane was approximately 390 L / m2·h·bar.

[0095] like Figure 5 As shown in the magnified photograph of its porous intermediate wall, by integrating the image, its porosity is approximately 65%, and the average size of its main pore skeleton is approximately 480 nanometers.

[0096] Tensile testing revealed that the average tensile breaking force of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 11.9 N, and the average elongation at break was approximately 280%.

[0097] The initial water droplet contact angle of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was measured to be approximately 69 degrees through contact angle testing.

[0098] Analysis using a pore size analyzer revealed that the average pore size of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 35 nm.

[0099] The prepared polyvinylidene fluoride hollow fiber membrane was used for bovine serum albumin solution retention test, and the results showed that the hollow fiber membrane had a retention rate of more than 99% for bovine serum albumin.

[0100] Example 6

[0101] The polyvinylidene fluoride hollow fiber membrane prepared in Example 5 was stretched 2.5 times in a 90°C hot water bath, then annealed at 150°C for 25 seconds, and then set in a 90°C hot oven for 10 seconds to obtain a re-stretched polyvinylidene fluoride hollow fiber membrane.

[0102] The prepared polyvinylidene fluoride hollow fiber membrane was used for pure water flux testing, and the result was that the pure water flux of the hollow fiber membrane was approximately 1640 L / m2·h·bar.

[0103] Electron microscopy characterization revealed that the porous layer on the outer surface was approximately 5 micrometers thick, the porous layer on the inner surface was approximately 2 micrometers thick, and the intermediate porous wall thickness was approximately 200 micrometers. Magnified electron microscopy analysis of the outer surface porous layer, combined with image integration, showed a porosity of approximately 83% and a pore structure size of approximately 61 nanometers. Magnified electron microscopy analysis of the inner surface porous layer, combined with image integration, showed a porosity of approximately 79% and a pore structure size of approximately 71 nanometers. Figure 6 As shown, its porous intermediate wall was characterized by electron microscopy and the porosity was calculated by integrating the images. The average size of its main pore skeleton was about 210 nanometers.

[0104] Tensile testing revealed that the average tensile breaking force of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 14.3 N, and the average elongation at break was approximately 100%.

[0105] The initial water droplet contact angle of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was measured to be approximately 71 degrees through contact angle testing.

[0106] Analysis using a pore size analyzer revealed that the average pore size of the polyvinylidene fluoride hollow fiber membrane prepared in this embodiment was approximately 113 nm.

[0107] The prepared polyvinylidene fluoride hollow fiber membrane was used for bovine serum albumin solution retention test, and the results showed that the hollow fiber membrane had a retention rate of more than 86% for bovine serum albumin.

[0108] Comparative Example 1

[0109] S1: Add 30% polyvinylidene fluoride, 68.8% caprolactam, 1% antioxidant 1010 and 0.2% stearamide by mass to a high-speed mixer and mix evenly.

[0110] S2: Set the temperature of each section of the twin-screw extruder to 150℃-160℃ and the twin-screw speed to 230 rpm. Pass the uniformly mixed mixture through the twin-screw extruder and extrude it into hollow fibers through the hollow fiber die. The supporting core liquid is a blend solution of triethylene glycol and polyethylene glycol 200 in a volume ratio of 5:5.

[0111] S3: Hollow fibers are introduced into a gel bath for curing, with an air gap of 15 cm. The gel bath is a mixed solution of caprolactam and water with a mass fraction of 50%, and the temperature is 10℃.

[0112] S4: Transfer the cured hollow fiber to water at 25°C and soak and clean it for 72 hours, changing the water 5 times during the process, to obtain polyvinylidene fluoride hollow fiber membrane.

[0113] The prepared polyvinylidene fluoride hollow fiber membrane was used for pure water flux testing, and the results showed that the pure water flux of the hollow fiber membrane was approximately 33 L / m³. 2 ·h·bar.

[0114] The mechanical properties of the polyvinylidene fluoride hollow fiber membrane were tested by tensile testing. The average tensile breaking force was approximately 3.1 N, and the average elongation at break was approximately 60%.

[0115] The hydrophilicity of the polyvinylidene fluoride hollow fiber membrane was tested by contact angle, with an initial water droplet contact angle of 98 degrees.

[0116] Analysis using a pore size analyzer showed that the average pore size of the polyvinylidene fluoride hollow fiber membrane was approximately 63 nanometers, with a porosity of 71%.

[0117] like Figure 5 As shown, this is a magnified electron microscope image of the cross section of the prepared polyvinylidene fluoride hollow fiber membrane, which shows that there is a typical polyvinylidene fluoride spherulite cluster structure in the inner region of the membrane.

[0118] In summary, the thermo-induced phase membrane fabrication technology in Comparative Example 1, which uses caprolactam as a diluent, produces polyvinylidene fluoride hollow fiber membranes that are mainly composed of polyvinylidene fluoride spherulites, resulting in problems such as low flux and poor mechanical properties.

[0119] Comparative Example 2

[0120] S1: Add 30% polyvinylidene fluoride and 70% diphenyl carbonate by mass to a high-speed mixer and mix evenly.

[0121] S2: Set the temperature of each section of the twin-screw extruder to 200℃-220℃ and the twin-screw speed to 250 rpm. Pass the uniformly mixed mixture through the twin-screw extruder and extrude it into hollow fibers through the hollow fiber die. The core liquid is glycerol.

[0122] S3: Hollow fibers are introduced into a gel bath for curing, with an air gap of 15 cm. The gel bath is water, and the temperature is 30℃.

[0123] S4: The cured hollow fiber is transferred to room temperature ethanol, and after multiple extractions, it is dried to obtain a polyvinylidene fluoride hollow fiber membrane.

[0124] The prepared polyvinylidene fluoride hollow fiber membrane was used for pure water flux testing, and the results showed that the pure water flux of the hollow fiber membrane was approximately 1730 L / m³. 2 ·h·bar.

[0125] The mechanical properties of the polyvinylidene fluoride hollow fiber membrane were tested by tensile testing. The average tensile breaking force was approximately 5.2 N, and the average elongation at break was approximately 35%.

[0126] The hydrophilicity of the polyvinylidene fluoride hollow fiber membrane was tested by contact angle, with an initial water droplet contact angle of 97 degrees.

[0127] Analysis using a pore size analyzer showed that the average pore size of the polyvinylidene fluoride hollow fiber membrane was approximately 0.5 micrometers, with a porosity of 76%.

[0128] like Figure 6 As shown, it is a magnified electron microscope image of the cross section of the prepared polyvinylidene fluoride hollow fiber membrane. It can be seen that the internal region of the membrane has a honeycomb sponge membrane pore structure, and the size of its membrane pore channel skeleton is [value missing].

[0129] Comparative Example 2 uses a membrane preparation method that integrates the technologies described in the published patents CN101362057B and CN101342468B. The prepared polyvinylidene fluoride hollow fiber membrane exhibits a double continuous membrane pore structure, but the membrane pore channel skeleton is relatively thin, and the tensile strength of the hollow fiber membrane is relatively low.

[0130] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, characterized in that, The membrane comprises an outer porous layer, a middle porous wall layer, and an inner porous layer. The thickness of the outer porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the outer porous layer is greater than 75%. The thickness of the inner porous layer accounts for less than 5% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the inner porous layer is greater than 75%. The thickness of the middle porous wall layer accounts for more than 90% of the total thickness of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane, and the porosity of the middle porous wall layer is greater than 75%. The preparation method of the high-strength self-supporting polyvinylidene fluoride hollow fiber membrane specifically includes the following steps: S1. Mix polyvinylidene fluoride powder, caprolactam, small molecule ethylene glycol condensate, high molecular weight polyethylene glycol and processing aids evenly to obtain a mixture. S2. The mixture obtained in step S1 is extruded and internally supported by a twin-screw extruder to form hollow fibers; S3. Place the hollow fibers obtained in step S2 in a gel bath to solidify them, and obtain the polyvinylidene fluoride hollow fiber membrane precursor. S4. The polyvinylidene fluoride hollow fiber membrane precursor obtained in step S3 is subjected to restretching, hot annealing and heat setting processes to finally obtain a high-strength self-supporting polyvinylidene fluoride hollow fiber membrane. In step S1, the small molecule ethylene glycol condensate is selected from one or more of diethylene glycol, triethylene glycol, polyethylene glycol 200, polyethylene glycol 300, polyethylene glycol 400, and polyethylene glycol 600, and the high molecular weight polyethylene glycol is selected from one or more of polyethylene glycols with a molecular weight of 20,000 to 200,000.

2. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 1, characterized in that, The pore skeleton of the outer porous layer is <100nm, the pore skeleton of the inner porous layer is <100nm, and the pore skeleton of the middle porous wall layer is 200-350nm.

3. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 1, characterized in that, The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane has a thickness of 100-300 μm, a tensile strength >10 MPa, and an elongation at break >100%.

4. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 1, characterized in that, In step S1, the mass percentage of the raw materials used to prepare the mixture is as follows: polyvinylidene fluoride powder: 25-35%, caprolactam: 30-40%, small molecule ethylene glycol condensate: 15-25%, high molecular weight polyethylene glycol: 10-20%, processing aids: 1-3%.

5. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 4, characterized in that, The processing aid is a mixture of antioxidant and lubricant, and the mass ratio of the antioxidant to the lubricant is (5-10):

1.

6. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 1, characterized in that, In step S2, the inner cavity support is provided by either inert gas support or core fluid support.

7. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 6, characterized in that, The inert gas is nitrogen or argon.

8. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 6, characterized in that, In step S3, the solution in the gel bath is selected from water or an aqueous solution, and the solvent in the aqueous solution is selected from one or more of autolactam, oligoethylene glycol, and polar solvent.

9. The high-strength self-supporting polyvinylidene fluoride hollow fiber membrane as described in claim 6, characterized in that, In step S4, the parameters for re-stretching are as follows: ambient temperature is 50-90℃, and the stretching ratio is ≤2.5 times; The heat annealing environment is a hot air environment with a temperature of 100-150℃ and a time of ≤45s; The heat setting stabilization temperature is 50-90℃, and the time is ≤20s.

Images