A method for preparing polyvinylidene fluoride composite hollow fiber membrane

Abstract

The present invention relates to the technical field of membrane separation and water treatment, and in particular to a method for preparing a polyvinylidene fluoride composite hollow fiber membrane, comprising: mixing polyvinylidene fluoride powder, a pore-forming agent and a first solvent according to a preset ratio to obtain a first mixed solution; stirring the first mixed solution based on a stirring parameter to obtain a spinning solution; filling a key coagulation bath and a water coagulation bath to the corresponding slots of a coagulation bath tank; vacuum degassing the spinning solution based on the degassing vacuum degree, and injecting the vacuum degassed spinning solution into a water coagulation bath through a spinneret, and then pulling it into a key coagulation bath to obtain a hollow fiber membrane; immersing the hollow fiber membrane in a key coagulation bath, and performing a performance test on the hollow fiber membrane to determine whether the immersion of the hollow fiber membrane is completed; if the immersion is completed, the hollow fiber membrane is rinsed and dried to obtain a polyvinylidene fluoride composite hollow fiber membrane. The present invention can improve the ability to resist biological contamination.

Description

Technical Field

[0001] The invention relates to the technical field of membrane separation and water treatment, and in particular to a method for preparing a polyvinylidene fluoride composite hollow fiber membrane. Background Art

[0002] Polyvinylidene fluoride (PVDF) membranes are widely used in the field of membrane separation technology due to their good mechanical properties, thermal stability and chemical stability. At present, PVDF membrane components mainly include flat plate type and hollow fiber type. Among them, hollow fiber membrane components have been widely used due to their high filling density and low price.

[0003] Membrane fouling is an unavoidable problem in the application of membrane separation technology. How to slow down membrane fouling is one of the research focuses of membrane separation technology. Among membrane fouling, membrane microbial fouling is the most complex and has the greatest impact. It is of great significance to prepare separation membranes with anti-biological fouling functions. Studies have shown that graphene has unique physical and chemical properties. It can use its sharp edges to penetrate bacteria, or use its high specific surface area and strong adsorption capacity to wrap bacteria, thereby achieving the effect of sterilization and anti-biological fouling. The physical sterilization mechanism of graphene will not allow bacteria and other microorganisms to develop drug resistance, has a long-term anti-biological fouling effect, and shows great anti-biological fouling application potential. Because the cost of graphene is high and the overall stability of large-area separation membranes is poor, it is not suitable for single membrane formation. Therefore, combining graphene materials with traditional membrane materials is expected to prepare separation membranes with good anti-biological fouling functions and practical value.

[0004] At present, there are two main ways to combine graphene materials with traditional membrane materials: one is to directly add graphene as an additive to the polymer membrane material to prepare a mixed matrix membrane; the other is to cover the membrane surface with graphene as a functional coating through filtration or functionalization to prepare a composite membrane. However, membrane microbial contamination mainly occurs on the membrane surface. In the mixed matrix membrane prepared by the first scheme, the graphene is coated in the polymer material and cannot exert its anti-biological contamination ability. In the composite membrane prepared by the second scheme, the graphene can contact well with microorganisms, thereby achieving sterilization and anti-biological contamination effects. However, graphene has good hydrophilicity, and the stability of graphene on the surface of the composite membrane has become a key factor limiting its practical application. At this stage, there is a lack of appropriate methods to prepare polyvinylidene fluoride hollow fiber membranes with stable graphene functional layers.

[0005] The discovery of the principle of non-interface adhesion of dopamine molecules has brought a turnaround to this dilemma. Dopamine can adhere to the surface of the separation membrane through complex oxidative self-polymerization to form a polydopamine nanocoating, which is stable and reliable. Then, the amino groups of dopamine react with the carboxyl groups on the surface of graphene to undergo an amidation reaction, thereby fixing the graphene on the surface of the separation membrane. Therefore, a method for preparing a separation membrane with good anti-biofouling properties using dopamine and graphene is urgently needed. Summary of the invention

[0006] To this end, the present invention provides a method for preparing a polyvinylidene fluoride composite hollow fiber membrane, so as to overcome the problem of insufficient anti-biological contamination ability of the hollow fiber membrane in the prior art.

[0007] To achieve the above object, the present invention provides a method for preparing a polyvinylidene fluoride composite hollow fiber membrane, comprising:

[0008] Step S1, mixing polyvinylidene fluoride powder, a pore-forming agent and a first solvent according to a preset ratio to obtain a first mixed solution, and obtaining a first parameter of the first mixed solution, wherein the first parameter is a mass fraction of the mass of the solute in the first mixed solution to the total mass of the first mixed solution;

[0009] Step S2, determining a stirring parameter according to the first parameter, and stirring the first mixed solution based on the stirring parameter to obtain a spinning solution, wherein the stirring parameter includes a stirring time, a stirring speed, and a stirring temperature;

[0010] Step S3, preparing a water coagulation bath and a key coagulation bath, and filling the key coagulation bath and the water coagulation bath into corresponding slots of the coagulation bath tank, wherein the key coagulation bath is prepared based on graphene oxide and dopamine hydrochloride;

[0011] Step S4, vacuum degassing the spinning solution based on the degassing vacuum degree, injecting the vacuum degassing spinning solution into the water coagulation bath through a spinneret, and then pulling it into the key coagulation bath to obtain a hollow fiber membrane;

[0012] Step S5, immersing the hollow fiber membrane in the key coagulation bath, and performing a performance test on the hollow fiber membrane, and determining whether the immersion of the hollow fiber membrane is complete based on the performance test result;

[0013] Step S6: if the soaking is completed, the hollow fiber membrane is rinsed and dried to obtain a polyvinylidene fluoride composite hollow fiber membrane.

[0014] Further, in step S3, the step of preparing the key coagulation bath includes:

[0015] Step S31, dissolving a first preset mass of graphene oxide in water, and adding a second preset mass of dopamine hydrochloride to obtain a first mixed solution;

[0016] Step S32, stirring the first mixed solution uniformly and adjusting the pH value to obtain the key coagulation bath.

[0017] Further, in step S4, the degassing vacuum degree is determined by the following steps:

[0018] Step S41, obtaining spinning parameters of the spinning solution, wherein the spinning parameters include bubble content in the spinning solution, spinning solution viscosity and spinning solution density;

[0019] Step S42, determining a spinning coefficient based on the spinning parameters;

[0020] Step S43, determining the degassing vacuum degree based on the spinning coefficient and the preset vacuum degree.

[0021] Furthermore, in the step S4, it includes:

[0022] Determine the spinning rate and spinning distance according to the spinning coefficient;

[0023] The vacuum degassed spinning solution is injected into the water coagulation bath through a spinneret based on the spinning rate and spinning distance.

[0024] Furthermore, in step S5, the performance test step includes:

[0025] Mark a first point and a second point on a preset length of the hollow fiber membrane sample to be tested, and clamp both ends of the hollow fiber membrane sample to be tested for stretching;

[0026] Obtaining a stress-strain curve between a first point and a second point during the stretching process of the hollow fiber membrane sample under test until the hollow fiber membrane sample under test breaks;

[0027] The tensile strength and elongation at break of the hollow fiber membrane were determined based on the stress-strain curve.

[0028] Further, in the step S2, determining the stirring parameter according to the first parameter includes:

[0029] A first coefficient is determined based on the first parameter and a preset parameter threshold, and a stirring parameter is determined based on the first coefficient.

[0030] Furthermore, in step S1, the pore-forming agent includes any one of polyethylene glycol, ethylene glycol, and polyvinyl pyrrolidone, or a mixture of any two or more thereof, and the first solvent includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, or a mixture of any two or more thereof.

[0031] Further, in the step S31, the ratio of the first preset mass to the second preset mass ranges from (0.5 to 2):1.

[0032] Furthermore, in step S3, the coagulation bath is a two-stage coagulation bath, the water coagulation bath is the first stage, the slot length of the water coagulation bath accounts for 2 / 3 of the total length of the coagulation bath, and the key coagulation bath is the second stage, the slot length of the key coagulation bath accounts for 1 / 3 of the total length of the coagulation bath.

[0033] Further, in the step S32, the pH adjustment includes:

[0034] Add tris(hydroxymethyl)aminomethane) buffer solution to the first mixed solution after uniform stirring to adjust the pH to 8-10.

[0035] Compared with the prior art, the beneficial effect of the present invention is that the present invention determines the stirring parameters based on the first parameter of the first mixed solution, can ensure the uniformity of the spinning solution, thereby improving the quality of the finished product, and can also improve production efficiency and stability, and avoid excessive stirring. A two-stage coagulation bath tank design of a water coagulation bath and a key coagulation bath is adopted. The water coagulation bath first performs preliminary shaping on the spinning solution, so that the spinning solution has a certain structural basis before entering the key coagulation bath, which helps to control the basic form of the membrane, and then enters the key coagulation bath. The graphene oxide has good antibacterial and mechanical properties, and the polydopamine formed by the self-polymerization of dopamine hydrochloride has good hydrophilicity and biocompatibility. The components in the key coagulation bath further interact with the membrane to adjust the microstructure of the membrane, thereby optimizing the separation performance and permeability of the membrane. The hollow fiber membrane is formed in the key coagulation bath, and the components in the key coagulation bath will adhere to the surface and internal pores of the membrane, making the membrane surface more hydrophilic, reducing the adsorption of biological molecules, thereby improving the anti-biological contamination ability of the membrane. Vacuum degassing the spinning solution based on the degassing vacuum degree can effectively remove bubbles in the spinning solution. The presence of bubbles will affect the molding quality and cause holes and breaks in the membrane. By accurately controlling the degassing vacuum degree, the spinning solution can be ensured to be uniform and bubble-free, improving the quality stability of the spinning solution, and thus ensuring the quality and stability of the finished product. By performing performance tests on the hollow fiber membrane and determining whether the immersion is complete based on the test results, the performance stability of the finished product can be ensured.

[0036] Furthermore, the present invention can ensure the stability and consistency of the key coagulation bath by precisely controlling the ratio of graphene oxide and dopamine hydrochloride. By adjusting the pH, the key coagulation bath can reach the most suitable pH environment, promote the self-polymerization reaction of dopamine hydrochloride and the interaction between graphene oxide and the membrane, thereby optimizing the modification effect of the coagulation bath on the membrane and improving the anti-biological fouling performance of the membrane.

[0037] Furthermore, the present invention determines the spinning coefficient and then determines the degassing vacuum degree based on key parameters such as the bubble content, viscosity and density in the spinning solution. It can more accurately treat spinning solutions with different characteristics and more effectively remove bubbles in the spinning solution, avoiding problems such as clogging of the spinning solution at the spinneret and unstable fine flow, thereby improving the spinning efficiency and quality.

[0038] Furthermore, the present invention determines the spinning rate and spinning distance according to the spinning coefficient, so that the spinning solution can enter the water coagulation bath at a suitable speed and trajectory, thereby improving the stability and continuity of the spinning process and thus improving the quality of the finished product.

[0039] Furthermore, the present invention sets a longer water coagulation bath position, which can allow the spinning solution to have more sufficient time to exchange solvents with water and achieve sufficient initial coagulation. The shorter key coagulation bath position reduces the usage of the key coagulation bath, thereby reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a flow chart of a method for preparing a polyvinylidene fluoride composite hollow fiber membrane according to an embodiment of the present invention;

[0041] Figure 2 This is a schematic diagram of the structure of a polyvinylidene fluoride composite hollow fiber membrane according to an embodiment of the present invention;

[0042] Figure 3 This is a schematic structural diagram of a coagulation bath according to an embodiment of the present invention;

[0043] Figure 4 A schematic diagram of a process for determining the degassing vacuum degree according to an embodiment of the present invention;

[0044] In the figure: 1. Graphene; 2. Dopamine; 3. Water coagulation bath; 4. Key coagulation bath; 5. Traction pulley; 6. Hollow fiber membrane. DETAILED DESCRIPTION

[0045] In order to make the objects and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with embodiments; it should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0046] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention.

[0047] It should be noted that, in the description of the present invention, terms such as "up", "down", "left", "right", "inside" and "outside" indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings. This is merely for the convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation on the present invention.

[0048] In addition, it should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0049] See also Figure 1-Figure 3 As shown, Figure 1 This is a flow chart of a method for preparing a polyvinylidene fluoride composite hollow fiber membrane according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of a polyvinylidene fluoride composite hollow fiber membrane according to an embodiment of the present invention; Figure 3 Schematic diagram of the structure of a coagulation bath according to an embodiment of the present invention; in the figure: graphene 1, dopamine 2, water coagulation bath 3, key coagulation bath 4, traction pulley 5, hollow fiber membrane 6.

[0050] The embodiment of the present invention provides a method for preparing a polyvinylidene fluoride composite hollow fiber membrane, comprising:

[0051] Step S1, mixing polyvinylidene fluoride powder, a pore-forming agent and a first solvent according to a preset ratio to obtain a first mixed solution, and obtaining a first parameter of the first mixed solution, wherein the first parameter is a mass fraction of the mass of the solute in the first mixed solution to the total mass of the first mixed solution;

[0052] Specifically, in step S1, the pore-forming agent includes any one of polyethylene glycol, ethylene glycol, and polyvinyl pyrrolidone, or a mixture of any two or more thereof, and the first solvent includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, or a mixture of any two or more thereof.

[0053] In implementation, the actual implementer can determine the preset ratio according to the actual situation or the ratio of polyvinylidene fluoride powder, pore-forming agent and the first solvent in the spinning solution preparation process that has passed the qualification test based on historical data, or determine the preset ratio based on the first parameter, and control the value range of the first parameter to be 15% to 30%, preferably 20% to 25% through the preset ratio.

[0054] Step S2, determining a stirring parameter according to the first parameter, and stirring the first mixed solution based on the stirring parameter to obtain a spinning solution, wherein the stirring parameter includes a stirring time, a stirring speed, and a stirring temperature;

[0055] Specifically, in step S2, determining the stirring parameter according to the first parameter includes:

[0056] A first coefficient is determined based on the first parameter and a preset parameter threshold, and a stirring parameter is determined based on the first coefficient.

[0057] In implementation, the first coefficient is determined according to the ratio of the first parameter to the preset parameter threshold, the stirring time is determined according to the product of the first coefficient and the standard stirring time, the stirring speed is determined according to the product of the first coefficient and the standard stirring speed, and the stirring temperature is determined according to the product of the first coefficient and the standard stirring temperature.

[0058] It can be understood that the actual implementers can set the preset parameter threshold according to the actual situation or based on the mean of the first parameter in the spinning solution preparation process that has passed the qualification test in historical data. Preferably, the value range of the preset parameter threshold is set to 20%~25%, and the standard stirring parameters (standard stirring time, standard stirring speed, standard stirring temperature) are set according to the corresponding stirring parameter mean when the preset parameter threshold is used as the first parameter. Preferably, the standard stirring time value range is set to 6h~7h, the standard stirring speed value range is set to 500r / min~600r / min, and the standard stirring temperature value range is set to 60℃~70℃.

[0059] Step S3, preparing a water coagulation bath and a key coagulation bath, and filling the key coagulation bath and the water coagulation bath into corresponding slots of the coagulation bath tank, wherein the key coagulation bath is prepared based on graphene oxide and dopamine hydrochloride;

[0060] It is understandable that graphene oxide has good antibacterial and mechanical properties, and polydopamine formed by the self-polymerization of dopamine hydrochloride has good hydrophilicity and biocompatibility. A two-stage coagulation bath design is adopted, which includes a water coagulation bath and a key coagulation bath. The water coagulation bath first preliminarily shapes the spinning solution so that the spinning solution has a certain structural basis before entering the key coagulation bath, which helps to control the basic morphology of the membrane. Then it enters the key coagulation bath, and the components in the key coagulation bath further interact with the membrane to adjust the microstructure of the membrane, such as pore size and distribution, thereby optimizing the separation performance and permeability of the membrane.

[0061] Specifically, in step S3, the steps of preparing the key coagulation bath include:

[0062] Step S31, dissolving a first preset mass of graphene oxide 1 in water, and adding a second preset mass of dopamine hydrochloride to obtain a first mixed solution;

[0063] Step S32, stirring the first mixed solution uniformly and adjusting the pH value to obtain the key coagulation bath.

[0064] Specifically, in step S31, the ratio of the first preset mass to the second preset mass ranges from (0.5 to 2):1.

[0065] Preferably, the ratio of the first preset mass to the second preset mass is 1:1.

[0066] Specifically, in step S32, the pH adjustment includes:

[0067] Add tris(hydroxymethyl)aminomethane) buffer solution to the first mixed solution after stirring to adjust the pH to 8 to 10. Preferably, the pH is 8.5 to 9.5.

[0068] In the implementation, the first mixed solution contains dopamine hydrochloride. In a weakly alkaline environment of pH 8 to 10, dopamine hydrochloride will be deprotonated to form dopamine 2. At this time, the catechol group in dopamine 2 will be oxidized, and then self-polymerization will occur to form polydopamine (PDA). Polydopamine has good adhesion, hydrophilicity and biocompatibility, and can be attached to the surface of polyvinylidene fluoride fiber to improve the hydrophilicity and anti-biological contamination performance of the fiber membrane. Graphene oxide exists in the first mixed solution. In an environment of pH 8 to 10, the oxygen-containing functional groups (such as carboxyl, hydroxyl, etc.) on its surface will dissociate, making the surface of graphene oxide negatively charged. This negatively charged state can increase the stability of graphene oxide in the solution, prevent its agglomeration and precipitation, and is conducive to being evenly distributed in polyvinylidene fluoride fibers in subsequent processes, so as to better exert its enhanced fiber mechanical properties and anti-biological contamination performance.

[0069] In practice, graphene oxide is prepared by a modified Hummers method.

[0070] It is understood that, in general, the concentration of graphene oxide 1 in the key coagulation bath is 0.005 g / L to 0.1 g / L, preferably, the concentration of graphene oxide 1 is 0.01 g / L, too low a concentration of graphene oxide 1 will lead to uneven coating on the surface of the hollow fiber membrane, and too high a concentration of graphene oxide 1 will lead to the membrane pores of the hollow fiber membrane being covered by dopamine, affecting the permeation flux. In practice, the key coagulation bath needs to be prepared as needed.

[0071] The present invention can ensure the stability and consistency of the key coagulation bath by precisely controlling the ratio of graphene oxide 1 and dopamine hydrochloride, and can make the key coagulation bath reach the most suitable pH environment by pH adjustment, promote the self-polymerization reaction of dopamine hydrochloride and the interaction between graphene oxide 1 and the membrane, thereby optimizing the modification effect of the coagulation bath on the membrane and improving the anti-biological contamination performance of the membrane.

[0072] Specifically, in step S3, the coagulation bath is a two-stage coagulation bath, the water coagulation bath 3 is the first stage, and the slot length of the water coagulation bath 3 accounts for 2 / 3 of the total length of the coagulation bath, and the key coagulation bath 4 is the second stage, and the slot length of the key coagulation bath 4 accounts for 1 / 3 of the total length of the coagulation bath.

[0073] In practice, the water coagulation bath 3 is mainly used to phase separate the spinning solution and initially solidify it into a shape. In addition to water as the main component, other substances may be added according to different needs. The specific components are not limited and will not be described here.

[0074] The present invention sets a longer water coagulation bath 3 slot, which can give the spinning solution more sufficient time to exchange solvent with water and achieve sufficient initial coagulation. The shorter key coagulation bath 4 slot reduces the usage of the key coagulation bath 4, thereby reducing production costs.

[0075] Step S4, vacuum degassing the spinning solution based on the degassing vacuum degree, injecting the vacuum degassing spinning solution into the water coagulation bath 3 through a spinneret, and then pulling it into the key coagulation bath 4 to obtain a hollow fiber membrane 6;

[0076] See also Figure 4 As shown, it is a schematic diagram of the process of determining the degassing vacuum degree according to an embodiment of the present invention; specifically, in step S4, the degassing vacuum degree is determined by the following steps:

[0077] Step S41, obtaining spinning parameters of the spinning solution, wherein the spinning parameters include bubble content in the spinning solution, spinning solution viscosity and spinning solution density;

[0078] Step S42, determining a spinning coefficient based on the spinning parameters;

[0079] Step S43, determining the degassing vacuum degree based on the spinning coefficient and the preset vacuum degree.

[0080] In implementation, data fitting is performed on the bubble content, spinning solution viscosity and spinning solution density in the spinning solution to construct a mathematical model between the spinning coefficient and the bubble content, spinning solution viscosity and spinning solution density in the spinning solution, so as to determine the spinning coefficient based on the spinning parameters, and determine the degassing vacuum degree according to the product of the spinning coefficient and the preset vacuum degree.

[0081] It is understandable that the actual implementer can set the preset vacuum degree according to the actual situation or based on the average value of the degassing vacuum degree that has passed the qualification test in the historical data. Preferably, the preset vacuum degree value range is set to -0.08 MPa to -0.1 MPa.

[0082] The present invention determines the spinning coefficient and then determines the degassing vacuum degree based on key parameters such as the bubble content, viscosity and density in the spinning solution. It can more accurately treat spinning solutions with different characteristics, more effectively remove bubbles in the spinning solution, and avoid problems such as clogging of the spinning solution at the spinneret and unstable fine flow, thereby improving the spinning efficiency and quality.

[0083] Specifically, in step S4, it includes:

[0084] Determine the spinning rate and spinning distance according to the spinning coefficient;

[0085] The vacuum degassed spinning solution is injected into the water coagulation bath 3 through a spinneret based on the spinning rate and spinning distance.

[0086] In implementation, the spinning rate is determined according to the product of the spinning coefficient and the standard spinning rate, and the spinning distance is determined according to the product of the spinning coefficient and the standard spinning distance.

[0087] It can be understood that the actual implementers can set the standard spin rate according to the actual situation or based on the average of the spin rates that have passed the qualification test in the historical data. Preferably, the standard spin rate value range is set to 30mL / min~50mL / min; the actual implementers can set the standard spin distance according to the actual situation or based on the average of the spin distances that have passed the qualification test in the historical data. Preferably, the standard spin distance value range is set to 9cm~12cm.

[0088] The present invention determines the spinning rate and spinning distance according to the spinning coefficient, so that the spinning solution can enter the water coagulation bath 3 at a suitable speed and trajectory, thereby improving the stability and continuity of the spinning process and thus improving the quality of the finished product.

[0089] In the implementation, the temperature of the key coagulation bath 4 is maintained at 20°C to 40°C, and the film is wound on the traction pulley 5 and immersed in the key coagulation bath 4 for 10h to 20h.

[0090] Step S5, immersing the hollow fiber membrane 6 in the key coagulation bath 4, and performing a performance test on the hollow fiber membrane 6, and determining whether the immersion of the hollow fiber membrane 6 is completed based on the performance test result;

[0091] Specifically, in step S5, the performance test step includes:

[0092] Mark a first point and a second point on a preset length of the hollow fiber membrane sample to be tested, and clamp both ends of the hollow fiber membrane sample to be tested for stretching;

[0093] Obtaining a stress-strain curve between a first point and a second point during the stretching process of the hollow fiber membrane sample under test until the hollow fiber membrane sample under test breaks;

[0094] The tensile strength and elongation at break of the hollow fiber membrane were determined based on the stress-strain curve.

[0095] In practice, the tensile force (i.e. stress) and the elongation (i.e. strain) of the hollow fiber membrane sample under test between the first point and the second point are detected and data fitting is performed to obtain the stress-strain curve between the first point and the second point. The tensile strength refers to the maximum stress that the material can withstand before tensile fracture. The stress value corresponding to the highest point on the stress-strain curve is found, and the stress value is the tensile strength of the hollow fiber membrane. If there are multiple peaks in the stress-strain curve, the last peak is selected as the tensile strength. The strain value corresponding to the fracture point is found on the stress-strain curve, and the strain value is the fracture elongation of the hollow fiber membrane. The fracture elongation refers to the ratio of the elongation of the material at fracture to the original length.

[0096] It is understood that the tensile strength is compared with the standard tensile strength, and the elongation at break is compared with the standard elongation at break. If the tensile strength is greater than the standard tensile strength and the elongation at break is greater than the standard elongation at break, the hollow fiber membrane is judged to be soaked. The actual implementation personnel can set the standard tensile strength according to the actual situation or based on the tensile strength of the soaked hollow fiber membrane that has passed the qualification test in the historical data. Preferably, the standard tensile strength value range is set to 20MPa~30MPa. The actual implementation personnel can set the standard elongation at break according to the actual situation or based on the elongation at break of the soaked hollow fiber membrane that has passed the qualification test in the historical data. Preferably, the standard elongation at break value range is set to 0.5~0.6.

[0097] In practice, the hollow fiber membrane is not wound and collected immediately after entering the critical coagulation bath, but needs to be wound and collected after the immersion is completed.

[0098] Step S6: if the soaking is completed, the hollow fiber membrane is rinsed and dried to obtain a polyvinylidene fluoride composite hollow fiber membrane.

[0099] In practice, the hollow fiber membrane was taken out, the surface solution was washed off with clean water, and then dried at 60° C. to obtain a polyvinylidene fluoride composite hollow fiber membrane.

[0100] The present invention determines stirring parameters based on the first parameter of the first mixed solution, can ensure the uniformity of the spinning solution, thereby improving the quality of the finished product, and can also improve production efficiency and stability, and avoid excessive stirring. A two-stage coagulation bath tank design of a water coagulation bath and a key coagulation bath is adopted, the water coagulation bath first performs preliminary shaping on the spinning solution, so that the spinning solution has a certain structural basis before entering the key coagulation bath, which helps to control the basic form of the membrane, and then enters the key coagulation bath, the graphene oxide has good antibacterial and mechanical properties, the polydopamine formed by the self-polymerization of dopamine hydrochloride has good hydrophilicity and biocompatibility, and the components in the key coagulation bath further interact with the membrane to adjust the microstructure of the membrane, thereby optimizing the separation performance and permeability of the membrane. The hollow fiber membrane is formed in the key coagulation bath, and the components in the key coagulation bath will adhere to the surface and internal pores of the membrane, making the membrane surface more hydrophilic, reducing the adsorption of biomolecules, thereby improving the anti-biological contamination ability of the membrane. Vacuum degassing the spinning solution based on the degassing vacuum degree can effectively remove bubbles in the spinning solution. The presence of bubbles will affect the molding quality and cause holes and breaks in the membrane. By accurately controlling the degassing vacuum degree, the spinning solution can be ensured to be uniform and bubble-free, improving the quality stability of the spinning solution, and thus ensuring the quality and stability of the finished product. By performing performance tests on the hollow fiber membrane and determining whether the immersion is complete based on the test results, the performance stability of the finished product can be ensured.

[0101] Specifically, the present invention improves the traditional polyvinylidene fluoride wet spinning process, adopts a two-stage coagulation bath design for the first time, and realizes the stable adhesion of graphene on the surface of the hollow fiber membrane by sequentially injecting the spinning solution into the first stage water coagulation bath and the second stage key coagulation bath of graphene oxide and dopamine. The present invention utilizes the antibacterial properties of graphene on the membrane surface, which can significantly improve the anti-biological contamination ability of the separation membrane, and has important research significance and practical value.

[0102] Example 1

[0103] Adding polyvinylidene fluoride powder and a pore-forming agent to N,N-dimethylformamide, controlling the mass ratio of polyvinylidene fluoride to the pore-forming agent to be 2, controlling the first parameter to be 20%, stirring at 60° C. for 6 hours to completely dissolve the polyvinylidene fluoride and the pore-forming agent, and obtaining a spinning solution;

[0104] Dissolve graphene oxide in water, add dopamine hydrochloride, control the mass ratio of graphene oxide to dopamine hydrochloride to be 0.5, the concentration of graphene oxide in the coagulation bath to be 0.01 g / L, and adjust the pH of the graphene oxide-dopamine mixed solution to 8.5 with tris(hydroxymethyl)aminomethane buffer solution as a key coagulation bath for standby use;

[0105] The polyvinylidene fluoride spinning solution is used as the shell solution after vacuum degassing, and the speed of the spinning solution sampling pump and the speed of the winding roller pulling the lining are controlled. The spinning solution and the lining are injected into the spinning solution through the spinneret. Under the stretching action of the pulling device, the spinning solution is first shaped in a 60°C water coagulation bath at a stretching speed of 10m / min, and then enters the key coagulation bath. The temperature of the key coagulation bath is maintained at 20-40°C. After the membrane is immersed in the key coagulation bath for 10 hours, the hollow fiber membrane is taken out, the surface solution is rinsed off with clean water, and then it is dried at 60°C to obtain a polyvinylidene fluoride hollow fiber membrane with anti-biological contamination function.

[0106] The prepared polyvinylidene fluoride composite hollow fiber membrane has a pore size range of 100nm to 500nm, and a permeation flux greater than 1000LMH. The adhesion and reproduction of microorganisms such as Escherichia coli on the graphene-modified membrane surface were significantly slowed down. After 48 hours of bacterial culture, no complete biofilm was observed on the membrane surface. The results of membrane filtration experiments showed that the polyvinylidene fluoride hollow fiber membrane with anti-biological fouling function can reduce the water flux by about 20%, reduce the extracellular polymer content on the membrane surface by about 15%, and effectively improve the anti-biological fouling performance.

[0107] Example 2

[0108] Adding polyvinylidene fluoride powder and a pore-forming agent to N,N-dimethylacetamide, controlling the mass ratio of polyvinylidene fluoride to the pore-forming agent to be 5, controlling the mass fraction of the solute to be 25%, stirring at high speed at 60° C. for 6 hours to completely dissolve the polyvinylidene fluoride and the pore-forming agent, and obtaining a spinning solution;

[0109] (2) dissolving graphene oxide in water, adding dopamine hydrochloride, controlling the mass ratio of graphene oxide to dopamine hydrochloride to be 1, the concentration of graphene oxide in the coagulation bath to be 0.05 g / L, and adjusting the pH of the graphene oxide-dopamine mixed solution to 8.5 with tris(hydroxymethyl)aminomethane buffer solution as a key coagulation bath for standby use;

[0110] (3) The polyvinylidene fluoride spinning solution is vacuum degassed as the shell liquid and water as the core liquid. The spinning solution is simultaneously passed through the spinneret of a spinning machine, and the flow rates of the shell liquid and the core liquid are controlled to be 1:1 before being spun into a 60°C water coagulation bath for final shaping. The hollow fiber membrane is then pulled into the key coagulation bath at a speed of 5 m / min, and the temperature of the key coagulation bath is maintained at 20°C to 40°C. After the membrane is immersed in the key coagulation bath for 20 hours, the hollow fiber membrane is taken out, the surface solution is rinsed off with clean water, and then dried at 60°C to obtain a polyvinylidene fluoride hollow fiber membrane with anti-biological contamination function.

[0111] The average pore size of the prepared polyvinylidene fluoride hollow fiber membrane is 10nm~50nm, and the permeation flux is 50~500LMH. The experimental results show that the number of bacteria adhering to the membrane surface is significantly reduced by more than 30%, and can be removed by backwashing, proving that bacteria and other microorganisms are killed on the membrane surface and can be easily removed by backwashing. Backwashing after the 4h filtration test can effectively restore the membrane flux to about 95% of the initial membrane flux, and the flux recovery rate is 20%~30% higher than that of the blank polyvinylidene fluoride hollow fiber membrane.

[0112] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A method for preparing a polyvinylidene fluoride composite hollow fiber membrane, characterized in that: include: Step S1, mixing polyvinylidene fluoride powder, a pore-forming agent and a first solvent according to a preset ratio to obtain a first mixed solution, and obtaining a first parameter of the first mixed solution, wherein the first parameter is a mass fraction of the mass of the solute in the first mixed solution to the total mass of the first mixed solution; Step S2, determining a stirring parameter according to the first parameter, and stirring the first mixed solution based on the stirring parameter to obtain a spinning solution, wherein the stirring parameter includes a stirring time, a stirring speed, and a stirring temperature; Step S3, preparing a water coagulation bath and a key coagulation bath, and filling the key coagulation bath and the water coagulation bath into corresponding slots of the coagulation bath tank, wherein the key coagulation bath is prepared based on graphene oxide and dopamine hydrochloride; Step S4, vacuum degassing the spinning solution based on the degassing vacuum degree, injecting the vacuum degassing spinning solution into the water coagulation bath through a spinneret, and then pulling it into the key coagulation bath to obtain a hollow fiber membrane; Step S5, immersing the hollow fiber membrane in the key coagulation bath, and performing a performance test on the hollow fiber membrane, and determining whether the immersion of the hollow fiber membrane is complete based on the performance test result; Step S6, if the soaking is completed, the hollow fiber membrane is rinsed and dried to obtain a polyvinylidene fluoride composite hollow fiber membrane; The steps of preparing the key coagulation bath include: Step S31, dissolving a first preset mass of graphene oxide in water, and adding a second preset mass of dopamine hydrochloride to obtain a first mixed solution; Step S32, stirring the first mixed solution uniformly and adjusting the pH value to obtain the key coagulation bath; In step S5, the performance test step includes: Mark a first point and a second point on a preset length of the hollow fiber membrane sample to be tested, and clamp both ends of the hollow fiber membrane sample to be tested for stretching; Obtaining a stress-strain curve between a first point and a second point of the hollow fiber membrane sample under test during the stretching process until the hollow fiber membrane sample under test breaks; Determining the tensile strength and elongation at break of the hollow fiber membrane based on the stress-strain curve; If the tensile strength is greater than the standard tensile strength and the elongation at break is greater than the standard elongation at break, it is determined that the soaking of the hollow fiber membrane is complete.

2. The method for preparing a polyvinylidene fluoride composite hollow fiber membrane according to claim 1, characterized in that: In step S4, the degassing vacuum degree is determined by the following steps: Step S41, obtaining spinning parameters of the spinning solution, wherein the spinning parameters include bubble content in the spinning solution, spinning solution viscosity and spinning solution density; Step S42, determining a spinning coefficient based on the spinning parameters; Step S43, determining the degassing vacuum degree based on the spinning coefficient and the preset vacuum degree.

3. The method for preparing the polyvinylidene fluoride composite hollow fiber membrane according to claim 2, characterized in that: In the step S4, it includes: Determine the spinning rate and spinning distance according to the spinning coefficient; The vacuum degassed spinning solution is injected into the water coagulation bath through a spinneret based on the spinning rate and spinning distance.

4. The method for preparing the polyvinylidene fluoride composite hollow fiber membrane according to claim 3, characterized in that: In the step S2, determining the stirring parameter according to the first parameter includes: A first coefficient is determined based on the first parameter and a preset parameter threshold, and a stirring parameter is determined based on the first coefficient.

5. The method for preparing a polyvinylidene fluoride composite hollow fiber membrane according to claim 4, characterized in that: In step S1, the pore-forming agent includes any one of polyethylene glycol, ethylene glycol, and polyvinyl pyrrolidone, or a mixture of any two or more thereof, and the first solvent includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, or a mixture of any two or more thereof.

6. The method for preparing the polyvinylidene fluoride composite hollow fiber membrane according to claim 5, characterized in that: In the step S31, the ratio of the first preset mass to the second preset mass ranges from (0.5 to 2):

1.

7. The method for preparing a polyvinylidene fluoride composite hollow fiber membrane according to claim 6, characterized in that: In step S3, the coagulation bath is a two-stage coagulation bath, the water coagulation bath is the first stage, the slot length of the water coagulation bath accounts for 2 / 3 of the total length of the coagulation bath, and the key coagulation bath is the second stage, the slot length of the key coagulation bath accounts for 1 / 3 of the total length of the coagulation bath.

8. The method for preparing a polyvinylidene fluoride composite hollow fiber membrane according to claim 7, characterized in that: In the step S32, the pH adjustment includes: Add tris(hydroxymethyl)aminomethane) buffer solution to the first mixed solution after uniform stirring to adjust the pH to 8-10.

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