A method for preparing long-lasting hydrophilic pvdf hollow fiber membranes

Abstract

This invention relates to the field of polymer membrane separation technology and discloses a method for preparing a long-lasting hydrophilic PVDF hollow fiber membrane, comprising the following steps: fully dissolving membrane raw materials, including polyvinylidene fluoride, polyvinylpyrrolidone, and ammonium persulfate microcapsules, in a solvent to prepare a casting solution, wherein the core material of the ammonium persulfate microcapsules is an ammonium persulfate solution, and the wall material is a polyacrylate cross-linked with DA bonds; after preparing the hollow fiber membrane using the casting solution, washing with water to form pores to obtain a porous hollow fiber membrane; during the above process, the temperature is controlled not to exceed 80°C; the porous hollow fiber membrane is treated at 120~125°C and 0.20~0.25MPa for 1~2 hours, and then treated at 70~90°C for 1~2 hours. This invention can improve the hydrophilic durability of the PVDF hollow fiber membrane while ensuring its water permeability, enabling it to maintain high water flux and antifouling properties even after long-term use.

Description

Technical Field

[0001] This invention relates to the field of polymer membrane separation technology, and in particular to a method for preparing a long-lasting hydrophilic PVDF hollow fiber membrane. Background Technology

[0002] Hydrophilic modification of PVDF membranes can enhance their antifouling properties, helping to prevent hydrophobic pollutants from clogging the membrane pores and causing a decline in membrane permeability during wastewater treatment. Furthermore, hydrophilic modification of PVDF membranes can also increase their water flux. While existing technologies offer numerous methods to effectively improve the hydrophilicity of PVDF membranes, few studies focus on whether this hydrophilicity can be stably maintained under long-term chemical cleaning.

[0003] Introducing cross-linked polyvinylpyrrolidone (PVPP) into PVDF membranes can improve their hydrophilicity. While PVP is a common pore-forming agent in membrane fabrication and possesses excellent hydrophilic properties, it is highly soluble in water and is rapidly eluted during filtration, causing the membrane to lose its hydrophilicity. However, PVPP, the cross-linked product of PVP, is insoluble in water and exhibits strong chemical resistance. Therefore, compared to PVP alone, using PVPP results in more durable hydrophilicity in PVDF membranes.

[0004] Regarding the methods of introducing PVPP into PVDF membranes, PVP crosslinking can be performed before PVDF membrane fabrication (such as during the preparation of casting solution), or post-crosslinking can be performed after PVDF membrane fabrication to form PVPP. For example, patent CN113368704A uses a pre-fabrication crosslinking method, where the vinylpyrrolidone monomer is first synthesized into PVP during the batching process and then crosslinked, so that the crosslinked product PVPP is directly present in the casting solution. This method requires high control of the batching process, the degree of crosslinking is difficult to control, and the resulting crosslinked product is insoluble in the system, which will affect the phase separation process and the film structure, resulting in a low water flux of the PVDF membrane. Patent CN106132521A uses a post-crosslinking method. The specific process is to first prepare a PVDF membrane containing PVP through a thermal method, and then place the membrane in a 1% hydrogen peroxide solution to react and generate crosslinked insoluble PVPP. This method can overcome the shortcomings of pre-crosslinking, but due to the limited permeability of hydrogen peroxide solution inside the membrane, it is difficult to guarantee the PVPP content inside the membrane. Therefore, the effect of improving the hydrophilic durability of the PVDF membrane is limited, resulting in the difficulty of maintaining the water flux and antifouling properties of the PVDF membrane at a high level after long-term use. Summary of the Invention

[0005] To address the aforementioned technical problem—that existing hydrophilic modified PVDF membranes lack sustained hydrophilicity and struggle to maintain good hydrophilicity under prolonged chemical cleaning—this invention provides a method for preparing a long-lasting hydrophilic PVDF hollow fiber membrane. This invention employs special ammonium persulfate microcapsules in the PVDF hollow fiber membrane preparation process. This avoids premature cross-linking of PVP in the casting solution, which would affect the water permeability of the PVDF hollow fiber membrane, while simultaneously improving the durability of the PVDF hollow fiber membrane's hydrophilicity. This allows it to maintain good hydrophilicity even after long-term use, thereby maintaining high water flux and antifouling properties.

[0006] The specific technical solution of this invention is as follows:

[0007] A method for preparing a long-lasting hydrophilic PVDF hollow fiber membrane includes the following steps:

[0008] (1) The membrane raw materials, including polyvinylidene fluoride, polyvinylpyrrolidone and ammonium persulfate microcapsules, are added to a solvent and fully dissolved to prepare a casting solution; the core material of the ammonium persulfate microcapsules is ammonium persulfate solution, and the wall material is polyacrylate crosslinked with DA bonds.

[0009] (2) Hollow fiber membranes were prepared using a casting solution;

[0010] (3) The hollow fiber membrane is washed with water to form pores, thereby obtaining a porous hollow fiber membrane;

[0011] In steps (1) to (3), the temperature is controlled to be no higher than 80℃;

[0012] (4) The porous hollow fiber membrane is treated at 120-125℃ and 0.20-0.25MPa for 1-2 hours, and then treated at 70-90℃ for 1-2 hours to obtain a long-lasting hydrophilic PVDF hollow fiber membrane.

[0013] In the preparation process of PVDF hollow fiber membrane, the present invention uses ammonium persulfate microcapsules with specific core and wall materials (i.e., the core material is ammonium persulfate solution and the wall material is polyacrylate crosslinked with DA bonds), which can play the following roles: In the process of preparing porous hollow fiber membrane (i.e., steps (1) to (3)), the temperature is lower than the bond breaking temperature of DA bonds, the polyacrylate molecular chains in the shell of the microcapsule are crosslinked, the shell is relatively dense, which can slow down the swelling of the shell and thus prevent the release of ammonium persulfate solution inside the microcapsule; In the process of post-crosslinking (i.e. step (4)), on the one hand, the increase in temperature can accelerate the swelling of the shell, and on the other hand, the temperature reaches the bond breaking temperature of DA bonds, the crosslinking between polyacrylate molecular chains breaks, which can further accelerate the swelling of the shell and the dispersion of the molecular chains therein, thereby causing the shell to crack faster and quickly release the ammonium persulfate solution inside the microcapsule, triggering the crosslinking reaction of PVP.

[0014] Therefore, ammonium persulfate microcapsules can be added to the casting solution and distributed relatively evenly within the porous hollow fiber membrane. This results in a higher concentration of PVPP within the final PVDF hollow fiber membrane, enabling it to maintain good hydrophilicity after long-term use, thereby maintaining high water flux and antifouling properties. Furthermore, since very little ammonium persulfate solution is released from the microcapsules in the casting solution, premature cross-linking of PVP can be avoided, which would affect the phase separation process and membrane structure. This results in the PVDF hollow fiber membrane exhibiting good water permeability.

[0015] Preferably, in step (1), the preparation process of the ammonium persulfate microcapsules includes the following steps:

[0016] (1.1) Furfuryl methacrylate, methyl methacrylate, polymerization initiator, emulsifier and dichloromethane are thoroughly mixed to obtain an oil phase; ammonium persulfate and acrylic acid are dissolved in water to obtain an aqueous phase; the aqueous phase is added to the oil phase and emulsified to prepare a W / O emulsion;

[0017] (1.2) The W / O emulsion was stirred and heated at 70-80°C for 1.5-2 hours, and a dichloromethane solution of bismaleimide was added. The mixture was stirred and heated at 60-70°C until the dichloromethane was completely evaporated, thus obtaining ammonium persulfate microcapsules.

[0018] This invention employs a single-emulsion solvent evaporation method to prepare ammonium persulfate microcapsules, and utilizes DA bond crosslinking in the wall material. The specific preparation mechanism is as follows: In the W / O emulsion formed by emulsification, the aqueous phase is dispersed in the oil phase in the form of droplets; during the stirring and heating of the W / O emulsion at 70-80°C, the oil phase solvent (dichloromethane) evaporates, and the furfuryl methacrylate and methyl methacrylate in it gradually transfer to the surface of the aqueous phase droplets. Simultaneously, the acrylic acid in the aqueous phase gradually transfers to the interface between the two phases. Under the action of the polymerization initiator, the furfuryl methacrylate and methyl methacrylate... Methyl ester and acrylic acid undergo a polymerization reaction to form polyacrylate with furan groups on the side chains. After the addition of bismaleimide, during the process of stirring and heating at 60-70°C, the furan groups in the polyacrylate undergo a Diels-Alder reaction with the bismaleimide to form DA bonds, thereby covalently crosslinking the polyacrylate molecular chains through DA bonds. At the same time, during this process, the polyacrylate molecular chains are further elongated, and dichloromethane continues to volatilize. Finally, the polyacrylate precipitates out and encapsulates on the surface of the aqueous droplets to form microcapsules.

[0019] Further, in step (1.1), the mass ratio of furfuryl methacrylate, methyl methacrylate and acrylic acid is 1:1 to 1.5:0.3 to 0.5; the mass-volume ratio of ammonium persulfate and water is 5 to 10 g:100 mL.

[0020] When the concentration of ammonium persulfate in the aqueous phase is too low, it is difficult for PVP in the porous hollow fiber membrane to be fully cross-linked, which is not conducive to the long-term hydrophilicity of the PVDF hollow fiber membrane. When the concentration of ammonium persulfate in the aqueous phase is too high, it will cause excessive cross-linking of PVP in the porous hollow fiber membrane, resulting in severe membrane pore blockage and thus low water flux.

[0021] Further, in step (1.1), the mass-to-volume ratio of the emulsifier, furfuryl methacrylate, polymerization initiator, and dichloromethane is 4-6 g: 3-5 g: 0.05-0.08 g: 100 mL.

[0022] Further, in step (1.1), the volume ratio of the aqueous phase to the oil phase is 1:7 to 15.

[0023] Further, the amount of bismaleimide used in step (1.2) is 25-50% of the mass of furfuryl methacrylate used in step (1.1).

[0024] Further, in step (1.2), the concentration of the dichloromethane solution of the bismaleimide is 10-20 wt%.

[0025] Preferably, in step (1), the casting solution is composed of the following components in mass percentage: 10-30 wt% polyvinylidene fluoride, 1-10 wt% polyvinylpyrrolidone, 0.5-2.5 wt% ammonium persulfate microcapsules, 0-20 wt% additives, and the balance being solvent.

[0026] Further, in step (1), the casting solution is composed of the following components in mass percentage: 15-25 wt% polyvinylidene fluoride, 1-10 wt% polyvinylpyrrolidone, 0.5-2.5 wt% ammonium persulfate microcapsules, 0.5-20 wt% additives, and the balance being solvent.

[0027] Preferably, in step (1), the viscosity characteristic value (i.e., K value) of the polyvinylpyrrolidone is 12-100, more preferably 17-90, and even more preferably 30-90; the weight-average molecular weight of the polyvinylidene fluoride is 4×10⁻⁶. 5 ~1×10 6 Da, further preferably 5×10 5 ~8×10 5 Da.

[0028] Preferably, the additive is one or more of polyoxyethylene, polyethylene glycol, water, ethanol, ethylene glycol, diethylene glycol, and ethylene glycol monomethyl ether.

[0029] Furthermore, the weight-average molecular weight of the polyoxyethylene is 1×10⁻⁶. 4 ~1×10 6Da, the weight-average molecular weight of the polyethylene glycol is 2 × 10⁻⁶. 2 ~2×10 4 Da.

[0030] Preferably, in step (1), the solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone.

[0031] Preferably, in step (3), the residual amount of polyvinylpyrrolidone in the membrane is 0.1-5% after washing (i.e., the content of polyvinylpyrrolidone in the membrane is 0.1-5% after washing).

[0032] Furthermore, the residual amount of polyvinylpyrrolidone was determined by infrared spectroscopy.

[0033] Preferably, in step (1), the method for achieving complete dissolution is to stir at 60-80°C for 4-5 hours.

[0034] Preferably, in step (2), the casting solution is degassed before preparing the hollow fiber membrane.

[0035] As a preferred embodiment, the specific process of step (2) includes the following steps: controlling the temperature of the casting solution at 20-80°C, coating it onto the hollow braided rope, and then gelling and solidifying it in a coagulation bath at 20-80°C to form a hollow fiber membrane.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] (1) In the preparation process of PVDF hollow fiber membrane, the present invention adds special ammonium persulfate microcapsules to the casting solution, which can prevent PVP from cross-linking in the casting solution in advance and affecting the water permeability of PVDF hollow fiber membrane, while improving the hydrophilic durability of PVDF hollow fiber membrane, so that it can maintain a high water flux and antifouling properties after long-term use.

[0038] (2) In the preparation process of ammonium persulfate microcapsules, the present invention can make the PVDF hollow fiber membrane have a high water flux and good hydrophilic durability by controlling the concentration of ammonium persulfate in the aqueous phase. Detailed Implementation

[0039] The present invention will be further described below with reference to embodiments.

[0040] General Implementation Examples

[0041] A method for preparing a long-lasting hydrophilic PVDF hollow fiber membrane includes the following steps:

[0042] (1) The membrane raw materials, including polyvinylidene fluoride, polyvinylpyrrolidone and ammonium persulfate microcapsules, are added to a solvent and fully dissolved to prepare a casting solution; the core material of the ammonium persulfate microcapsules is ammonium persulfate solution, and the wall material is polyacrylate crosslinked with DA bonds.

[0043] (2) Hollow fiber membranes were prepared using a casting solution;

[0044] (3) The hollow fiber membrane is washed with water to form pores, thereby obtaining a porous hollow fiber membrane;

[0045] In steps (1) to (3), the temperature is controlled to be no higher than 80℃;

[0046] (4) The porous hollow fiber membrane is treated at 120-125℃ and 0.20-0.25MPa for 1-2 hours, and then treated at 70-90℃ for 1-2 hours to obtain a long-lasting hydrophilic PVDF hollow fiber membrane.

[0047] In one specific implementation, step (1) involves the following steps in the preparation of the ammonium persulfate microcapsules:

[0048] (1.1) Furfuryl methacrylate, methyl methacrylate, polymerization initiator, emulsifier and dichloromethane are thoroughly mixed to obtain an oil phase; ammonium persulfate and acrylic acid are dissolved in water to obtain an aqueous phase; the aqueous phase is added to the oil phase and emulsified to prepare a W / O emulsion; the mass ratio of furfuryl methacrylate, methyl methacrylate and acrylic acid is 1:1~1.5:0.3~0.5, the mass-volume ratio of ammonium persulfate and water is 5~10g:100mL, the mass-volume ratio of emulsifier, furfuryl methacrylate, polymerization initiator and dichloromethane is 4~6g:3~5g:0.05~0.08g:100mL, and the volume ratio of aqueous phase to oil phase is 1:7~15;

[0049] (1.2) The W / O emulsion is stirred and heated at 70-80°C for 1.5-2 hours, and a dichloromethane solution of 10-20 wt% bismaleimide is added. The mixture is stirred and heated at 60-70°C until the dichloromethane is completely evaporated to obtain ammonium persulfate microcapsules. The amount of bismaleimide used in step (1.2) is 25-50% of the mass of furfuryl methacrylate used in step (1.1).

[0050] In one specific embodiment, in step (1), the casting solution is composed of the following components by mass percentage: 10-30 wt% polyvinylidene fluoride, 1-10 wt% polyvinylpyrrolidone, 0.5-2.5 wt% ammonium persulfate microcapsules, 0-20 wt% additives, and the balance being solvent.

[0051] In one specific embodiment, in step (1), the viscosity characteristic value (i.e., K value) of the polyvinylpyrrolidone is 12 to 100; the weight-average molecular weight of the polyvinylidene fluoride is 4 × 10⁻⁶. 5 ~1×10 6 Da.

[0052] In one specific embodiment, the additive is one or more selected from polyoxyethylene, polyethylene glycol, water, ethanol, ethylene glycol, diethylene glycol, and ethylene glycol monomethyl ether; the weight-average molecular weight of the polyoxyethylene is 1×10⁻⁶. 4 ~1×10 6 Da, the weight-average molecular weight of the polyethylene glycol is 2 × 10⁻⁶. 2 ~2×10 4 Da.

[0053] In one specific embodiment, in step (1), the solvent is one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), and N-methylpyrrolidone (NMP).

[0054] In one specific implementation, in step (3), the water is washed until the residual amount of polyvinylpyrrolidone in the membrane is 0.1-5%.

[0055] As one specific implementation method, in step (1), the method for fully dissolving is to stir at 60-80°C for 4-5 hours.

[0056] As a specific implementation method, the specific process of step (2) includes the following steps: controlling the temperature of the casting liquid at 20-80°C, degassing it, coating it onto the hollow braided rope, and then gelling and solidifying it in a coagulation bath at 20-80°C to form a hollow fiber membrane.

[0057] Example 1

[0058] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0059] (1) Preparation of ammonium persulfate microcapsules:

[0060] 5 g of N,N'-(4,4'-methylenediphenyl)bismaleimide (BMI) was added to 30 mL of dichloromethane and stirred to dissolve, yielding a BMI solution. 20 g of furfuryl methacrylate, 20 g of methyl methacrylate, 0.2 g of azobisisobutyronitrile (AIBN), and 16 g of Span-60 were added to 400 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain the oil phase. 2 g of ammonium persulfate and 10 g of acrylic acid were added to 40 mL of water and stirred to dissolve, yielding the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was heated and stirred at 70 °C for 2 h, then the BMI solution was added, and the mixture was heated and stirred at 60 °C until the dichloromethane was completely evaporated. The mixture was washed three times with water to obtain ammonium persulfate microcapsules.

[0061] The ammonium persulfate microcapsules prepared in this example were placed in deionized water, and the release of ammonium persulfate was monitored by conductivity. The results are as follows: the initial release time of the microcapsules in deionized water at 80℃ was 32h, and the 80% release time was 35h; the initial release time in deionized water at 122℃ (0.25MPa pressure) was 1h, and the 80% release time was 110min.

[0062] (2) Preparation of porous hollow fiber membranes:

[0063] 320g of weight-average molecular weight 6×10 5 Da's PVDF resin, 100g PVP K60 (i.e., polyvinylpyrrolidone with a K value of 60), 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether and 1150g DMF were uniformly mixed at 80℃, stirred to dissolve, degassed, and coated on polyester hollow braided rope with an outer diameter of 1.85~1.90mm at 80℃. After gel curing in a 45wt% DMF coagulation bath (a mixed solution of DMF and water) at 65℃ for 40s, a lining-reinforced hollow fiber membrane with an outer diameter of 2.1mm was obtained. The lining-reinforced hollow fiber membrane was sent to an online cleaning equipment and washed with water through a continuous cleaning tube by a stainless steel active roller until the residual amount of PVP in the membrane was 1.52%. Then it was wound and collected by a winding wheel and cut to obtain bundled membrane fibers, i.e., porous hollow fiber membrane. (3) Post-crosslinking:

[0064] The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 2 hours, and then the pressure was reduced to normal and treated at 80℃ for another hour. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0065] Example 2

[0066] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0067] (1) Preparation of ammonium persulfate microcapsules:

[0068] 5 g of N,N'-(4,4'-methylenediphenyl)bismaleimide (BMI) was added to 30 mL of dichloromethane and stirred to dissolve, yielding a BMI solution. 20 g of furfuryl methacrylate, 20 g of methyl methacrylate, 0.2 g of azobisisobutyronitrile (AIBN), and 16 g of Span-60 were added to 400 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain the oil phase. 2 g of ammonium persulfate and 10 g of acrylic acid were added to 40 mL of water and stirred to dissolve, yielding the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was heated and stirred at 70 °C for 2 h, then the BMI solution was added, and the mixture was heated and stirred at 60 °C until the dichloromethane was completely evaporated. The mixture was washed three times with water to obtain ammonium persulfate microcapsules.

[0069] (2) Preparation of porous hollow fiber membranes:

[0070] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed with water through a cleaning tube until the residual PVP content inside the membrane was 1.56%. The membrane was then collected by a winding wheel and cut into bundles of membrane fibers, forming the porous hollow fiber membrane.

[0071] (3) Post-crosslinking:

[0072] The porous hollow fiber membrane was treated at 130℃ and 0.25MPa for 2 hours, and then the pressure was reduced to normal and treated at 90℃ for another hour. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0073] Example 3

[0074] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0075] (1) Preparation of ammonium persulfate microcapsules:

[0076] 5 g of N,N'-(4,4'-methylenediphenyl)bismaleimide (BMI) was added to 30 mL of dichloromethane and stirred to dissolve, yielding a BMI solution. 20 g of furfuryl methacrylate, 20 g of methyl methacrylate, 0.2 g of azobisisobutyronitrile (AIBN), and 16 g of Span-60 were added to 400 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain the oil phase. 2 g of ammonium persulfate and 10 g of acrylic acid were added to 40 mL of water and stirred to dissolve, yielding the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was heated and stirred at 70 °C for 2 h, then the BMI solution was added, and the mixture was heated and stirred at 60 °C until the dichloromethane was completely evaporated. The mixture was washed three times with water to obtain ammonium persulfate microcapsules.

[0077] (2) Preparation of porous hollow fiber membranes:

[0078] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. The reinforced hollow fiber membrane was fed into an online cleaning device and washed with water through a continuous cleaning tube driven by a stainless steel active roller until the residual PVP content inside the membrane was 1.63%. The membrane was then wound and collected by a winding wheel and cut into bundles of membrane fibers, i.e., porous hollow fiber membranes.

[0079] (3) Post-crosslinking:

[0080] The porous hollow fiber membrane was treated at 120℃ and 0.25MPa for 2 hours, and then the pressure was reduced to normal and treated at 70℃ for another hour. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0081] Example 4

[0082] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0083] (1) Preparation of ammonium persulfate microcapsules:

[0084] 9.5 g of BMI was added to 50 mL of dichloromethane and stirred to dissolve, yielding a BMI solution. 19.2 g of furfuryl methacrylate, 23.1 g of methyl methacrylate, 0.38 g of AIBN, and 24 g of Span-60 were added to 480 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain the oil phase. 5 g of ammonium persulfate and 7.7 g of acrylic acid were added to 60 mL of water and stirred to dissolve, yielding the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was stirred and heated at 80 °C for 1.5 h, and then the BMI solution was added. The mixture was then stirred and heated at 70 °C until the dichloromethane was completely evaporated. After washing three times with water, ammonium persulfate microcapsules were obtained.

[0085] The ammonium persulfate microcapsules prepared in this example were placed in deionized water, and the release of ammonium persulfate was monitored by conductivity. The results are as follows: the initial release time of the microcapsules in deionized water at 80℃ was 29h, and the 80% release time was 31h; the initial release time in deionized water at 122℃ (0.25MPa pressure) was 40min, and the 80% release time was 90min.

[0086] (2) Preparation of porous hollow fiber membranes:

[0087] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed through a cleaning tube until the residual PVP content inside the membrane was 1.45%. The membrane was then wound and collected by a winding wheel, and cut into bundles of membrane fibers, thus forming the porous hollow fiber membrane.

[0088] (3) Post-crosslinking:

[0089] The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 1.5h, and then the pressure was reduced to normal and treated at 80℃ for another 1.5h. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0090] Example 5

[0091] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0092] (1) Preparation of ammonium persulfate microcapsules:

[0093] 6 g of BMI was added to 40 mL of dichloromethane and stirred to dissolve, thus preparing a BMI solution. 17.8 g of furfuryl methacrylate, 26.8 g of methyl methacrylate, 0.46 g of AIBN, and 23.2 g of Span-60 were added to 580 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain an oil phase. 4 g of ammonium persulfate and 5.4 g of acrylic acid were added to 40 mL of water and stirred to dissolve, thus preparing an aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was stirred and heated at 70 °C for 2 h, and then the BMI solution was added. The mixture was then stirred and heated at 60 °C until the dichloromethane was completely evaporated. After washing three times with water, ammonium persulfate microcapsules were obtained.

[0094] The ammonium persulfate microcapsules prepared in this example were placed in deionized water, and the release of ammonium persulfate was monitored by conductivity. The results are as follows: the initial release time of the microcapsules in deionized water at 80℃ was 28h, and the 80% release time was 30h; the initial release time in deionized water at 122℃ (0.25MPa pressure) was 40min, and the 80% release time was 80min.

[0095] (2) Preparation of porous hollow fiber membranes:

[0096] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed through a cleaning tube until the residual PVP content inside the membrane was 1.59%. The membrane was then wound and collected by a winding wheel, and cut into bundles of membrane fibers, thus forming the porous hollow fiber membrane.

[0097] (3) Post-crosslinking:

[0098] The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 1.5h, and then the pressure was reduced to normal and treated at 80℃ for another 1.5h. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0099] Example 6

[0100] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0101] (1) Preparation of ammonium persulfate microcapsules:

[0102] 5 g of BMI was added to 30 mL of dichloromethane and stirred to dissolve, yielding a BMI solution. 20 g of furfuryl methacrylate, 20 g of methyl methacrylate, 0.2 g of AIBN, and 16 g of Span-60 were added to 400 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain the oil phase. 1 g of ammonium persulfate and 10 g of acrylic acid were added to 40 mL of water and stirred to dissolve, yielding the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was stirred and heated at 70 °C for 2 h, then the BMI solution was added, and the mixture was stirred and heated at 60 °C until the dichloromethane was completely evaporated. The mixture was washed three times with water to obtain ammonium persulfate microcapsules.

[0103] (2) Preparation of porous hollow fiber membranes:

[0104] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed through a cleaning tube until the residual PVP content inside the membrane was 1.44%. The membrane was then wound and collected by a winding wheel, and cut into bundles of membrane fibers, thus forming the porous hollow fiber membrane.

[0105] (3) Post-crosslinking:

[0106] The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 2 hours, and then the pressure was reduced to normal and treated at 80℃ for another hour. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0107] Example 7

[0108] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0109] (1) Preparation of ammonium persulfate microcapsules:

[0110] 6 g of BMI was added to 40 mL of dichloromethane and stirred to dissolve, yielding a BMI solution. 17.8 g of furfuryl methacrylate, 26.8 g of methyl methacrylate, 0.46 g of AIBN, and 23.2 g of Span-60 were added to 580 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain the oil phase. 8 g of ammonium persulfate and 5.4 g of acrylic acid were added to 40 mL of water and stirred to dissolve, yielding the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was stirred and heated at 70 °C for 2 h, then the BMI solution was added, and the mixture was stirred and heated at 60 °C until the dichloromethane was completely evaporated. The mixture was washed three times with water to obtain ammonium persulfate microcapsules.

[0111] (2) Preparation of porous hollow fiber membranes:

[0112] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed with water through a cleaning tube until the residual PVP content inside the membrane was 1.52%. The membrane was then wound and collected by a winding wheel, and cut into bundles of membrane fibers, thus forming the porous hollow fiber membrane.

[0113] (3) Post-crosslinking:

[0114] The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 1.5h, and then the pressure was reduced to normal and treated at 80℃ for another 1.5h. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0115] Comparative Example 1

[0116] Hydrophilic PVDF hollow fiber membranes were prepared using the following steps:

[0117] 320g of weight-average molecular weight 6×10 5A mixture of PVDF resin, 100g PVP K60, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled in a 45 wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. The reinforced hollow fiber membrane was fed into an online cleaning device and washed with water through a continuous cleaning tube driven by a stainless steel active roller until the residual PVP content inside the membrane was 1.51%. The membrane was then wound and collected by a winding wheel, cut into bundles, and dried to obtain a hydrophilic PVDF hollow fiber membrane.

[0118] Comparative Example 2

[0119] Hydrophilic PVDF hollow fiber membranes were prepared using the following steps:

[0120] (1) Preparation of porous hollow fiber membranes:

[0121] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60 (polyvinylpyrrolidone with a K value of 60), 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45 wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed through a cleaning tube until the residual PVP content inside the membrane was 1.60%. The membrane was then wound and collected by a winding wheel, and cut into bundles of membrane fibers, forming the porous hollow fiber membrane.

[0122] (2) Post-crosslinking:

[0123] The porous hollow fiber membrane was immersed in a 5 wt% ammonium persulfate aqueous solution and treated at 122℃ and 0.25 MPa for 2 hours. Then the pressure was reduced to atmospheric pressure and the membrane was treated at 80℃ for another 1 hour. After drying, a hydrophilic PVDF hollow fiber membrane was obtained.

[0124] Comparative Example 3

[0125] (1) Preparation of ammonium persulfate microcapsules:

[0126] 5 g of N,N'-(4,4'-methylenediphenyl)bismaleimide (BMI) was added to 30 mL of dichloromethane and stirred to dissolve, thus preparing a BMI solution. 20 g of furfuryl methacrylate, 20 g of methyl methacrylate, 0.2 g of AIBN, and 16 g of Span-60 were added to 400 mL of dichloromethane and stirred at 1500 rpm for 30 min to obtain an oil phase. 2 g of ammonium persulfate and 10 g of acrylic acid were added to 40 mL of water and stirred to dissolve, thus preparing an aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500 rpm for 30 min to obtain a W / O emulsion. The W / O emulsion was stirred and heated at 70 °C for 2 h, and then stirred and heated at 60 °C until the dichloromethane was completely evaporated. The emulsion was washed three times with water to obtain ammonium persulfate microcapsules.

[0127] The ammonium persulfate microcapsules prepared in this example were placed in deionized water, and the release of ammonium persulfate was monitored by conductivity. The results are as follows: the initial release time of the microcapsules in deionized water at 80℃ was 2.5h, and the 80% release time was 3.5h; the initial release time in deionized water at 122℃ (0.25MPa pressure) was 40min, and the 80% release time was 80min.

[0128] (2) Preparation of porous hollow fiber membranes:

[0129] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60 (polyvinylpyrrolidone with a K value of 60), 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured for 40 seconds in a 45 wt% DMF coagulation bath (a mixed solution of DMF and water) at 65°C to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning device, driven by a stainless steel active roller, and continuously washed with water through a cleaning tube until the residual PVP content inside the membrane was 1.55%. The membrane was then collected by a winding wheel and cut into bundles of membrane fibers, forming the porous hollow fiber membrane.

[0130] (3) Post-crosslinking:

[0131] The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 2 hours, and then the pressure was reduced to normal and treated at 80℃ for another hour. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0132] Comparative Example 4

[0133] The following steps are used to prepare a long-lasting hydrophilic PVDF hollow fiber membrane:

[0134] (1) Preparation of ammonium persulfate microcapsules:

[0135] 20g of furfuryl methacrylate, 20g of methyl methacrylate, 3.4g of N,N'-methylenebisacrylamide, 0.2g of AIBN, and 16g of Span-60 were added to 400mL of dichloromethane and stirred at 1500rpm for 30min to obtain the oil phase. 2g of ammonium persulfate and 10g of acrylic acid were added to 40mL of water and stirred to dissolve, obtaining the aqueous phase. The aqueous phase was added to the oil phase and stirred at 4500rpm for 30min to obtain the W / O emulsion. The W / O emulsion was stirred and heated at 70℃ for 2h, and then stirred and heated at 60℃ until the dichloromethane was completely evaporated. After washing three times with water, ammonium persulfate microcapsules were obtained.

[0136] The ammonium persulfate microcapsules prepared in this example were placed in deionized water, and the release of ammonium persulfate was monitored by conductivity. The results are as follows: the initial release time of the microcapsules in deionized water at 80℃ was 32h, and the 80% release time was 35h; the initial release time in deionized water at 122℃ (0.25MPa pressure) was 20h, and the 80% release time was 22h.

[0137] (2) Preparation of porous hollow fiber membranes:

[0138] 320g of weight-average molecular weight 6×10 5 A mixture of PVDF resin, 100g PVP K60, 30g ammonium persulfate microcapsules, 100g polyethylene glycol, 300g ethylene glycol monomethyl ether, and 1150g DMF was uniformly mixed at 80°C, stirred to dissolve, and degassed. The mixture was then coated onto polyester hollow braided ropes with an outer diameter of 1.85–1.90 mm at 80°C. The mixture was then gelled and cured in a 45wt% DMF coagulation bath at 65°C for 40 seconds to obtain a 2.1 mm outer diameter reinforced hollow fiber membrane. This membrane was then fed into an online cleaning system, driven by a stainless steel active roller, and continuously washed through a cleaning tube until the residual PVP content inside the membrane was 1.59%. The membrane was then wound and collected by a winding wheel, and cut into bundles of membrane fibers, thus forming the porous hollow fiber membrane. (3) Post-crosslinking: The porous hollow fiber membrane was treated at 122℃ and 0.25MPa for 2 hours, and then the pressure was reduced to normal pressure and treated at 80℃ for another 1 hour. After drying, a long-lasting hydrophilic PVDF hollow fiber membrane was obtained.

[0139] Test Example 1: Repeated Immersion and Drying Test of PVDF Hollow Fiber Membrane

[0140] The PVDF hollow fiber membranes prepared in each example and comparative example were used to test their water flux (referred to as "initial water flux"). After repeated soaking and drying 20 times, the water flux was tested again, and the water flux decay rate was calculated. The results are shown in Table 1.

[0141] Table 1 Results of repeated immersion and drying tests on PVDF hollow fiber membranes

[0142]

[0143] Test Example 2: Hydrophilicity and Chemical Resistance Test of PVDF Hollow Fiber Membranes

[0144] The PVDF hollow fiber membranes prepared in each example and comparative example were continuously immersed in HCl solution (pH=1), NaOH solution (pH=12), and 2000ppm NaClO solution for 45 days, respectively. Samples were taken every 3 days to measure the water contact angle and wetting time. The changes in hydrophilicity at different times were compared, with the acceptable standards being ≤90° for contact angle and ≤120s for wetting time. The results are shown in Table 2, where "○" indicates that both contact angle and wetting time meet the standards in all three solutions; "×" indicates that one or more solutions have either a substandard contact angle or a substandard wetting time.

[0145] Table 2. Results of hydrophilicity and chemical resistance tests on PVDF hollow fiber membranes.

[0146]

[0147] Based on the test results in Tables 1 and 2, we can conclude that:

[0148] (1) By comparing Example 1 and Comparative Example 1, it can be seen that after the porous hollow fiber membrane is made, the post-crosslinking treatment is carried out to crosslink the PVP in the membrane to form PVPP, which can improve the hydrophilic durability of the PVDF hollow fiber membrane, so that it can still maintain good hydrophilicity and high water flux after long-term use.

[0149] (2) By comparing Example 1 and Comparative Example 2, it can be seen that: compared with the method of soaking in ammonium persulfate aqueous solution for post-crosslinking, the method of using ammonium persulfate microcapsules of the present invention, adding them to the casting solution, and releasing the ammonium persulfate solution after forming the hollow fiber membrane can improve the hydrophilic durability of PVDF hollow fiber membrane to a greater extent.

[0150] (3) A comparison between Example 1 and Comparative Example 3 shows that, in ammonium persulfate microcapsules, compared to the non-crosslinked polyacrylate molecular chains in the shell, the use of a shell with molecular chains crosslinked by DA bonds can effectively improve the water flux of the PVDF hollow fiber membrane. This is because, if the polyacrylate molecular chains in the shell are not crosslinked, excessive ammonium persulfate will be released during the preparation of the casting solution and its coating onto the polyester hollow braided rope, causing premature crosslinking of PVP, which in turn affects the phase separation process and the film formation structure, resulting in poor water permeability of the PVDF hollow fiber membrane.

[0151] (4) A comparison between Example 1 and Comparative Example 4 shows that, in ammonium persulfate microcapsules, compared to the crosslinking of polyacrylate molecular chains in the shell by the conventional crosslinking agent N,N'-methylenebisacrylamide, the use of DA bonds for crosslinking can effectively improve the hydrophilic durability of the PVDF hollow fiber membrane. This is because: if the polyacrylate molecular chains in the shell are crosslinked by N,N'-methylenebisacrylamide, the crosslinking between the polyacrylate molecular chains cannot be broken during post-crosslinking, resulting in slow release of the ammonium persulfate solution inside the microcapsule and difficulty in achieving effective crosslinking of PVP; while if the polyacrylate molecular chains in the shell are crosslinked by DA bonds, the DA bonds can be broken by appropriate heating treatment during post-crosslinking, accelerating the swelling and rupture of the shell, thereby allowing the ammonium persulfate solution to be released quickly and achieving effective crosslinking of PVP inside the membrane.

[0152] (5) Through the comparison of Examples 1, 5 to 7, it can be seen that in the process of preparing ammonium persulfate microcapsules, when the mass-volume ratio of ammonium persulfate to water in the core material reaches 5g:100mL or more, the PVDF hollow fiber membrane can be endowed with good hydrophilic durability; when the mass-volume ratio exceeds 10g:100mL, it will cause excessive cross-linking of PVP, serious membrane pore blockage, and a decrease in the good water flux of the PVDF hollow fiber membrane.

[0153] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0154] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing long-lasting hydrophilic PVDF hollow fiber membranes, characterized by, Includes the following steps: (1) The membrane raw materials, including polyvinylidene fluoride, polyvinylpyrrolidone and ammonium persulfate microcapsules, are added to a solvent and fully dissolved to prepare a casting solution; the core material of the ammonium persulfate microcapsules is ammonium persulfate solution, and the wall material is polyacrylate crosslinked with DA bonds. (2) Hollow fiber membranes were prepared using a casting solution; (3) The hollow fiber membrane is washed with water to form pores, thereby obtaining a porous hollow fiber membrane; In steps (1) to (3), the temperature is controlled to be no higher than 80℃; (4) The porous hollow fiber membrane is treated at 120~125℃ and 0.20~0.25MPa for 1~2h, and then treated at 70~90℃ for 1~2h to obtain a long-lasting hydrophilic PVDF hollow fiber membrane.

2. The production method according to claim 1, characterized by, In step (1), the preparation process of the ammonium persulfate microcapsules includes the following steps: (1.1) Furfuryl methacrylate, methyl methacrylate, polymerization initiator, emulsifier and dichloromethane are thoroughly mixed to obtain an oil phase; ammonium persulfate and acrylic acid are dissolved in water to obtain an aqueous phase; the aqueous phase is added to the oil phase and emulsified to prepare a W / O emulsion; (1.2) The W / O emulsion was stirred and heated at 70~80℃ for 1.5~2h, and a dichloromethane solution of bismaleimide was added. The mixture was stirred and heated at 60~70℃ until the dichloromethane was completely evaporated to obtain ammonium persulfate microcapsules.

3. The method of claim 2, wherein, In step (1.1), the mass ratio of furfuryl methacrylate, methyl methacrylate and acrylic acid is 1:1~1.5:0.3~0.5; the mass-volume ratio of ammonium persulfate and water is 5~10g:100mL.

4. The production method according to claim 2 or 3, characterized by, The amount of bismaleimide used in step (1.2) is 25-50% of the mass of furfuryl methacrylate used in step (1.1).

5. The preparation method according to claim 1, characterized in that, In step (1), the casting solution is composed of the following components by mass percentage: 10-30 wt% polyvinylidene fluoride, 1-10 wt% polyvinylpyrrolidone, 0.5-2.5 wt% ammonium persulfate microcapsules, 0-20 wt% additives, and the balance being solvent.

6. The production method according to claim 1 or 5, characterized by, In step (1), the polyvinylpyrrolidone has a viscosity property value of 12 to 100; the polyvinylidene fluoride has a weight average molecular weight of 4 x 10 5 ~1 x 10 6 Da.

7. The preparation method according to claim 5, characterized in that, The additive is one or more of the following: polyoxyethylene, polyethylene glycol, water, ethanol, ethylene glycol, diethylene glycol, and ethylene glycol monomethyl ether.

8. The method of claim 1, wherein, In step (3), the water is washed until the residual amount of polyvinylpyrrolidone in the membrane is 0.1~5%.

9. The method of claim 1, wherein, In step (1), the method for achieving complete dissolution is to stir at 60~80℃ for 4~5 h.

10. The method of claim 1, wherein, The specific process of step (2) includes the following steps: the temperature of the casting solution is controlled at 20~80℃, coated onto the hollow braided rope, and then gelled and solidified in a coagulation bath at 20~80℃ to form a hollow fiber membrane.

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