A method for producing a hollow fiber microporous membrane and a hollow fiber microporous membrane

Abstract

The application relates to the separation field and particularly discloses a preparation method of a hollow fiber microporous membrane and the hollow fiber microporous membrane. The preparation method of the hollow fiber microporous membrane comprises the following steps: preparing a casting solution, wherein the casting solution comprises the following raw materials in parts by weight: polyvinylidene fluoride 30-40 parts, a diluent 70-75 parts, and Span 80 0.4-0.6 parts; the raw materials are uniformly mixed and vacuum degassed to prepare the casting solution; the weight-average molecular weight of the polyvinylidene fluoride is 800,000; the diluent comprises 1-ethyl-3-methyl imidazole bis(trifluoromethylsulfonyl) imidazole salt and choline acetate in a weight ratio of 17:3; and extrusion molding, wherein the casting solution is extruded through a spinning nozzle with the diluent as a core liquid, and the hollow fiber microporous membrane is obtained after cooling, winding, extraction and drying. The hollow fiber microporous membrane disclosed by the application can maintain a high flux while reducing VOCs emission.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of separation, more particularly, it relates to a method for preparing a hollow fiber microporous membrane and a hollow fiber microporous membrane. BACKGROUND

[0002] VOCs, volatile organic compounds, are one of the important sources of environmental pollution. Gas separation membrane technology is a simple operation, energy saving and environmental protection method for VOCs treatment, which can efficiently separate and recover VOCs by using gas separation membrane, and can significantly improve the effect of VOCs treatment.

[0003] Polyvinylidene fluoride (PVDF) has excellent thermal stability, chemical resistance and easy processing into films, and has been widely used in microfiltration, ultrafiltration, membrane bioreactor, membrane distillation, gas separation, removal of pollutants in water, biofuel collection, lithium ion battery separator and ion exchange membrane, etc. It is also a common membrane material for VOCs gas separation membrane.

[0004] Thermally induced phase separation (TIPS) is a new type of microporous membrane preparation technology, its principle is to form a homogeneous solution of some thermoplastic, semi-crystalline polymer and some high-boiling point small molecule diluent at high temperature, and then induce liquid-liquid (L-L) phase separation or solid-liquid (S-L) phase separation due to solubility and polymer crystallization, etc. After extracting the diluent, a microporous structure is formed in the space occupied by the diluent. Because the TIPS method has good repeatability in the membrane forming process and high accuracy in the control of membrane pore structure, it is now used to prepare PVDF hollow fiber microporous membrane.

[0005] When preparing PVDF hollow fiber microporous membrane by TIPS method, the selection of diluent is crucial, which can determine the polymer crystallization process and the morphology of the final membrane, thereby affecting the membrane properties such as pore size, strength, flux, etc. Dibutyl phthalate (DBP) is widely used in the preparation of PVDF membrane, but it belongs to high VOCs material, which is easy to release in the environment, causing potential harm to human health and environment, resulting in that the preparation process of PVDF hollow fiber microporous membrane is not low VOCs, and the VOCs treatment using it is greatly limited. SUMMARY

[0006] In order to maintain the high flux of the hollow fiber microporous membrane while reducing the VOCs emission in the preparation process of the hollow fiber microporous membrane, the present application provides a method for preparing a hollow fiber microporous membrane and a hollow fiber microporous membrane.

[0007] In a first aspect, the present application provides a method for preparing a hollow fiber microporous membrane, which adopts the following technical scheme:

[0008] A method for preparing a hollow fiber microporous membrane, comprising the following steps:

[0009] Preparation of casting solution: the casting solution comprises the following raw materials by weight: polyvinylidene fluoride 30-40 parts, diluent 70-75 parts, and Span 80 0.4-0.6 parts. The raw materials are mixed and vacuum degassed to obtain the casting solution. The weight average molecular weight of the polyvinylidene fluoride is 800,000. The diluent comprises 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt and choline acetate in a weight ratio of 17:3.

[0010] Extrusion molding: the casting solution is extruded through a spinneret with the diluent as the core liquid, and then cooled, wound, extracted, and dried to obtain the hollow fiber microporous membrane.

[0011] By adopting the above technical solution, the use of polyvinylidene fluoride with a weight average molecular weight of 800,000 makes the membrane have high breaking strength and can withstand high pressure impact and organic solvent corrosion in VOCs treatment. The diluent uses a mixed ionic liquid diluent, a combination of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt and choline acetate. The 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt (containing TFSI- anion) can provide high thermal stability to ensure the stability of the membrane structure in high-temperature waste gas treatment, and also dominate PVDF dissolution, inhibit hydrolysis, and improve solvent stability. Choline acetate can enhance biodegradability, reduce toxicity, reduce the viscosity of the mixed system, accelerate phase separation, and form a uniform sponge layer. The membrane structure formed by the combination of the two as a diluent has through sponge-like pores, enhancing the mass transfer path of VOCs during subsequent membrane application, significantly reducing mass transfer resistance, and improving flux, thereby improving separation efficiency. Moreover, the mechanical strength of the membrane is significantly improved, and there is no VOCs emission in the preparation process, truly achieving green and efficient production. Span 80 can eliminate surface pinholes, reduce membrane pore defects, and further improve the treatment efficiency of the membrane. Therefore, the hollow fiber microporous membrane maintains high flux while reducing VOCs emissions during the preparation process.

[0012] Optionally, the casting solution further comprises 2-4 parts by weight of polyethylene glycol 6000 and 5-7 parts by weight of nano-clay.

[0013] By adopting the above technical solution, the nano-clay enters the polyvinylidene fluoride matrix with high molecular weight, and the combination of the two can improve the strength of the membrane. Polyethylene glycol 6000 can promote the formation of open pore structure in the membrane, improve the porosity, and nano-clay can compensate for the decrease in membrane strength caused by the increase in porosity. The combination of the two can optimize the pore structure of the membrane and improve the uniformity of pore formation. In addition, oil or particulate matter in VOCs is prone to block the membrane pores, and polyethylene glycol 6000 can reduce the pollution caused by hydrophobic adsorption.

[0014] Optionally, the casting solution further comprises 1-3 parts by weight of polyvinylpyrrolidone and 2-4 parts by weight of lithium chloride.

[0015] By adopting the above technical solutions, the two are compounded to synergistically improve the hydrophilicity of the membrane, enhance the anti-pollution property, reduce the grease adsorption, thereby improving the flux and improving the separation efficiency.

[0016] Optionally, the casting solution further comprises 4-6 parts by weight of ZIF-8.

[0017] By adopting the above technical solutions, ZIF-8 has a microporous structure, and doping ZIF-8 can improve the screening ability of the membrane to cope with the separation of complex components. At the same time, PVP can effectively inhibit the agglomeration of ZIF-8, and nanoclay can compensate for the strength loss caused by the addition of ZIF-8. Multiple raw materials are compounded to form a high-strength and high-flux green hollow fiber microporous membrane.

[0018] Optionally, the extrusion flow rate of the casting solution is 10 mL / min; and the core liquid flow rate is 4 mL / min.

[0019] Optionally, the cooling temperature is 15°C, the cooling liquid is deionized water, and the cooling rate is 25°C / min.

[0020] Optionally, the extraction includes ethanol primary extraction and supercritical CO2 secondary extraction.

[0021] By adopting the above technical solutions, the residual diluent can be completely removed.

[0022] In a second aspect, the application provides a hollow fiber microporous membrane, which adopts the following technical solutions:

[0023] A hollow fiber microporous membrane is prepared by the above-mentioned method for preparing a hollow fiber microporous membrane.

[0024] By adopting the above technical solutions, the prepared hollow fiber microporous membrane can maintain high flux while reducing VOCs emissions during the preparation process.

[0025] In summary, the application has the following beneficial effects:

[0026] 1. Because this application uses polyvinylidene fluoride with a weight average molecular weight of 800,000, the membrane has high tensile strength and can withstand the high-pressure impact and organic solvent corrosion during VOCs treatment. The diluent used is a mixed ionic liquid, forming a membrane structure with interconnected sponge-like channels, enhancing the mass transfer path of VOCs during subsequent membrane applications, significantly reducing mass transfer resistance, increasing flux, and thus improving separation efficiency. It also significantly improves the mechanical strength of the membrane, and the preparation process has no VOCs emissions, truly achieving green and efficient operation. Span 80 can eliminate surface pinholes, reduce membrane pore defects, and further improve membrane processing efficiency. Therefore, it achieves the effect of maintaining the high flux of hollow fiber microporous membranes while reducing VOCs emissions during the hollow fiber microporous membrane preparation process.

[0027] 2. In this application, PEG6000 and nano-clay are preferred. The nano-clay enters the high molecular weight polyvinylidene fluoride body, and the combination of the two can improve the membrane strength. PEG6000 can promote the formation of an open-pore structure in the membrane and increase the porosity. The nano-clay can compensate for the decrease in membrane strength caused by the increase in porosity. The combination of the two can optimize the membrane pore structure and improve the uniformity of pore formation.

[0028] 3. The method of this application produces hollow fiber microporous membranes that can maintain high flux while reducing VOC emissions during the preparation process. Detailed Implementation

[0029] The present application will be further described in detail below with reference to the embodiments. It should be noted that: unless otherwise specified, the conditions in the following embodiments shall be performed under conventional conditions or conditions recommended by the manufacturer, and the raw materials used in the following embodiments shall be commercially available unless otherwise specified.

[0030] 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imine salt, [EMIM]TFSI, CAS:174899-82-2.

[0031] Nano clay, purchased from Beishi Zongheng Scientific Co., Ltd., surface modified, manufacturer: Sigma, item number 682632, MDL number: MFCD00147658.

[0032] Choline acetate, CAS No.: 14586-35-7.

[0033] ZIF-8, CAS No. 59061-53-9, was purchased from Xi'an Qiyue Biotechnology.

[0034] Example

[0035] Example 1

[0036] A method for preparing a hollow fiber microporous membrane includes the following steps:

[0037] Preparation of casting solution:

[0038] The casting solution comprises the following raw materials: polyvinylidene fluoride, diluent, and Span 80. The amounts of the raw materials are shown in Table 1.

[0039] The polyvinylidene fluoride has a weight average molecular weight of 800,000 and is dried at 80°C under vacuum for 12 hours before use to remove moisture.

[0040] The diluent comprises 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and choline acetate in a weight ratio of 17:3, which are mixed uniformly. The diluent is dehydrated at 120°C under vacuum for 48 hours before use to reduce the moisture content to ≤50 ppm.

[0041] The polyvinylidene fluoride is dissolved in the diluent under an inert atmosphere. The polyvinylidene fluoride is completely dissolved by stirring at 180°C for 3 hours. Then, Span 80 is added and mixed uniformly. The casting solution is degassed under vacuum at -0.1 MPa.

[0042] Extrusion molding:

[0043] The casting solution is extruded through a spinneret with an inner diameter of 1.2 mm and an outer diameter of 2.5 mm, using the diluent as a core liquid. The spinning temperature of the barrel is 175°C, and the temperature of the spinneret is 180°C. The extrusion flow rate of the casting solution is 10 mL / min, and the flow rate of the core liquid is 4 mL / min. Then, the hollow fiber is cooled in a water bath at 15°C with deionized water at a cooling rate of 25°C / min. After that, the hollow fiber is wound up with an air gap height of 20 cm and a winding speed of 10 m / min. Then, the hollow fiber is extracted. First, the primary extraction is performed by immersing the hollow fiber in anhydrous ethanol at 70°C for 12 hours. Then, the secondary extraction is performed by cleaning the hollow fiber with supercritical CO2 at 40°C and 20 MPa. Finally, the hollow fiber is dried by critical point drying (CPD) to obtain a hollow fiber microporous membrane.

[0044] A hollow fiber microporous membrane is prepared by the above method.

[0045] Example 2

[0046] The difference between this example and Example 1 is that the amounts of the raw materials are different, as shown in Table 1.

[0047] Example 3

[0048] The difference between this example and Example 1 is that the amounts of the raw materials are different, as shown in Table 1.

[0049] Example 4

[0050] The difference between this example and Example 2 is that the casting solution further comprises polyethylene glycol 6000 (PEG6000) and nano-clay, and the amounts of the raw materials are shown in Table 1. The PEG6000 is dried at 80°C for 6 hours before use. The nano-clay is a nano-clay with a silane-modified surface.

[0051] The preparation method of the casting solution is as follows:

[0052] Under an inert atmosphere, polyvinylidene fluoride is dissolved in a diluent, and stirred at 180°C for 3h to completely dissolve the polyvinylidene fluoride, and then polyethylene glycol 6000, nanoclay and Span 80 are sequentially added, and stirred at a high speed (2000 rpm) for 30 minutes to mix uniformly. Vacuum degassing is performed at -0.1 MPa to prepare the casting solution.

[0053] Example 5

[0054] The difference between this example and Example 4 is that the amounts of polyethylene glycol 6000 and nanoclay are different, as shown in Table 1.

[0055] Example 6

[0056] The difference between this example and Example 4 is that the amounts of polyethylene glycol 6000 and nanoclay are different, as shown in Table 1.

[0057] Example 7

[0058] The difference between this example and Example 5 is that polyethylene glycol 6000 is not used in this example.

[0059] Example 8

[0060] The difference between this example and Example 5 is that nanoclay is not used in this example.

[0061] Example 9

[0062] The difference between this example and Example 5 is that the casting solution further comprises polyvinylpyrrolidone (PVP, K30) and lithium chloride, and the amounts are shown in Table 1. The polyvinylpyrrolidone and lithium chloride are dried at 80°C for 6h before use.

[0063] The preparation method of the casting solution is as follows:

[0064] Under an inert atmosphere, polyvinylidene fluoride is dissolved in a diluent, and stirred at 180°C for 3h to completely dissolve the polyvinylidene fluoride, and then polyethylene glycol 6000, polyvinylpyrrolidone, lithium chloride, nanoclay and Span 80 are sequentially added, and stirred at a high speed (2000 rpm) for 30 minutes to mix uniformly. Vacuum degassing is performed at -0.1 MPa to prepare the casting solution.

[0065] Example 10

[0066] The difference between this example and Example 9 is that the amounts of polyvinylpyrrolidone and lithium chloride are different, as shown in Table 1.

[0067] Example 11

[0068] The difference between this embodiment and embodiment 9 is that the amount of polyvinylpyrrolidone and lithium chloride is different, and the details are shown in Table 1.

[0069] Example 12

[0070] The difference between this embodiment and embodiment 10 is that this embodiment does not contain polyvinylpyrrolidone.

[0071] Example 13

[0072] The difference between this embodiment and embodiment 10 is that this embodiment does not contain lithium chloride.

[0073] Example 14

[0074] The difference between this embodiment and embodiment 10 is that the casting solution also contains ZIF-8, and the amount is shown in Table 1.

[0075] The preparation method of the casting solution is as follows:

[0076] Under an inert atmosphere, polyvinylidene fluoride is dissolved in a diluent, and stirred at 180℃ for 3h to completely dissolve the polyvinylidene fluoride. Then polyethylene glycol 6000, polyvinylpyrrolidone, lithium chloride, and nanoclay are added in sequence, and ZIF-8 and Span 80 are added below 60℃. High-speed shearing (2000 rpm) is used to stir for 10 minutes to preliminarily disperse the ZIF-8, and then ultrasonic treatment (200 W, 5 min) is used to mix uniformly. Vacuum degassing is performed at -0.1 MPa to obtain the casting solution.

[0077] Example 15

[0078] The difference between this embodiment and embodiment 14 is that the amount of ZIF-8 is different, and the details are shown in Table 1.

[0079] Example 16

[0080] The difference between this embodiment and embodiment 14 is that the amount of ZIF-8 is different, and the details are shown in Table 1.

[0081] Comparative example

[0082] Comparative example 1

[0083] A preparation method of a polyvinylidene fluoride hollow fiber membrane is as follows:

[0084] (1) Ordinary molecular weight polyvinylidene fluoride (weight average molecular weight 330,000) / diluent (dibutyl phthalate) is added to a reaction kettle at a mass ratio of 26 / 74, and mixed and stirred at 160℃ for 3h, and then vacuum degassing is performed for 3h to obtain a uniform casting solution.

[0085] (2) The casting solution was continuously extruded from the hollow fiber spinneret at a flow rate of 0.05 L / min after passing through a spinning pump for metering, and then immersed in a coagulation bath (deionized water) at 25°C to cool and form, and then the membrane filaments were immersed in anhydrous ethanol for 48 h to extract the residual diluent in the membrane, and then immersed in deionized water for 24 h to remove residual ethanol, and finally the membrane filaments were naturally dried to obtain a polyvinylidene fluoride hollow fiber membrane.

[0086] A polyvinylidene fluoride hollow fiber membrane prepared by the above preparation method.

[0087] Comparative Example 2

[0088] The difference between this comparative example and Example 2 is that the weight average molecular weight of the polyvinylidene fluoride in this comparative example is 330,000.

[0089] Comparative Example 3

[0090] The difference between this comparative example and Example 2 is that the diluent in this comparative example is dibutyl phthalate.

[0091] Comparative Example 4

[0092] The difference between this comparative example and Example 2 is that the diluent in this comparative example is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt.

[0093] Comparative Example 5

[0094] The difference between this comparative example and Example 2 is that the diluent in this comparative example is choline acetate.

[0095] Comparative Example 6

[0096] The difference between this comparative example and Example 2 is that there is no Span 80 in this comparative example.

[0097] Table 1: Types and amounts of raw materials of the casting solution in each example and comparative example

[0098]

[0099] Performance detection test

[0100] Detection method

[0101] 1. Determination of porosity: The average pore size and porosity of each of the above examples and comparative examples were measured by mercury intrusion method, and the average value was taken from five tests for each membrane, and the results are shown in Table 2.

[0102] 2. Pure water flux test: Take 300 mm membrane filament sealed end casting with 25℃ pure water at 0.1 MPa pressure for 2h, then calculate the pure water flux under the condition of unit area and time, test five times and take the average value, the results are shown in Table 2.

[0103] 3. Breaking strength test: Take 100 mm fiber membrane, stretch the experiment on the universal testing machine at a linear speed of 10 mm / s, record the maximum force when the membrane filament breaks, measure the breaking strength, test five times and take the average value, the results are shown in Table 2.

[0104] 4. Determination of contact angle: The contact angle of the membrane was determined by contact angle tester (DSA100), 1 drop of water (1um) was dropped on the membrane with a needle tube, the experiment was measured 3 times, and the average value was taken as the contact angle, the results are shown in Table 2. The lower the contact angle, the stronger the anti-pollution property of the surface membrane.

[0105] Table 2 Test results

[0106]

[0107] In combination with Example 2 and Comparative Examples 1-6, the preparation method of the hollow fiber microporous membrane in the application does not use high VOCs raw materials in the preparation process, uses mixed ionic liquid as a diluent, and has no VOCs emission in the preparation process, achieving green production. As can be seen from Table 2, the prepared hollow fiber microporous membrane has high flux, high strength, and pore structure suitable for VOCs treatment.

[0108] Analysis principle: In the preparation process of the membrane, the casting solution adopts polyvinylidene fluoride with a weight average molecular weight of 800,000, so that the membrane has high breaking strength and can withstand the high pressure impact and organic solvent corrosion in VOCs treatment. The diluent adopts a mixed ionic liquid diluent, a combination of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt and choline acetate. The 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide salt (containing TFSI⁻ anion) can provide high thermal stability to ensure the stability of the membrane structure in high-temperature waste gas treatment, and also dominate PVDF dissolution, inhibit hydrolysis, improve solvent stability, choline acetate can enhance biodegradability, reduce toxicity, reduce the viscosity of the mixed system, accelerate phase separation, and form a uniform sponge layer. The membrane structure formed after the two are compounded as a diluent has through sponge-like pores, enhances the mass transfer path of VOCs in subsequent application of the membrane, significantly reduces the mass transfer resistance, improves the flux, thereby improving the separation efficiency, and significantly improves the mechanical strength of the membrane. There is no VOCs emission in the preparation process, which truly achieves green and efficient production. Span 80 can eliminate surface pinholes, reduce membrane pore defects, and further improve the treatment efficiency of the membrane. Therefore, the high flux of the hollow fiber microporous membrane is maintained while reducing the VOCs emission in the preparation process of the hollow fiber microporous membrane.

[0109] As can be seen from the combination of Embodiment 2 and Embodiment 5 and Table 2, the addition of PEG6000 and nanoclay in the casting solution, the nanoclay enters the polyvinylidene fluoride matrix with high molecular weight, and the combination of the two can bring the improvement of the membrane strength; polyethylene glycol 6000 can promote the formation of open pore structure, improve the porosity, and the nanoclay can make up for the decline of the membrane strength caused by the improvement of the porosity, and the combination of the two can optimize the membrane pore structure and improve the pore uniformity. In addition, the oil or particulate matter in VOCs is easy to block the membrane pores, and polyethylene glycol 6000 can reduce the pollution caused by hydrophobic adsorption. In combination with Embodiments 7 and 8 and Table 2, it can be seen that the combination of PEG6000 and nanoclay has the best effect.

[0110] As can be seen from the combination of Embodiment 5, Embodiment 10 and Table 2, PVP and lithium chloride are also added in the casting solution, and the combination of the two can synergistically improve the hydrophilicity of the membrane, enhance the anti-pollution property, reduce the oil adsorption, thereby improve the flux and increase the separation efficiency. In combination with Embodiments 12 and 13 and Table 2, it can be seen that the combination of PVP and lithium chloride has the best effect.

[0111] As can be seen from the combination of Embodiment 10, Embodiment 16 and Table 2, after the addition of ZIF-8 in the casting solution, ZIF-8 has a nano-porous structure, which can effectively separate complex components such as toluene, so that the membrane can be more suitable for VOCs treatment. The separation efficiency of the membrane is also effectively improved.

[0112] The specific embodiments are only an explanation of the present application, and are not a limitation of the present application. Those skilled in the art can make modifications to the embodiments without creative contribution after reading the present specification, and the modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A method for producing a hollow fiber microporous membrane, characterized by, The method comprises the following steps: Preparation of casting solution: the casting solution comprises the following raw materials in parts by weight: polyvinylidene fluoride 30-40 parts, diluent 70-75 parts, and Span 80 0.4-0.6 parts; the raw materials are mixed and vacuum degassed to obtain the casting solution; the weight average molecular weight of the polyvinylidene fluoride is 800,000; the diluent comprises 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide salt and choline acetate in a weight ratio of 17:3; Extrusion molding: the casting solution is extruded through a spinner with the diluent as a core liquid, and then the hollow fiber microporous membrane is obtained after cooling, winding, extraction, and drying.

2. The method of claim 1, wherein: The casting solution further comprises 2-4 parts by weight of polyethylene glycol 6000 and 5-7 parts by weight of nano-clay.

3. The method of claim 2, wherein: The casting solution further comprises 1-3 parts by weight of polyvinylpyrrolidone and 2-4 parts by weight of lithium chloride.

4. The method of claim 3, wherein: The casting solution further comprises 4-6 parts by weight of ZIF-8.

5. The method of claim 1, wherein: The extrusion flow rate of the casting solution is 10 mL / min; and the core liquid flow rate is 4 mL / min.

6. The method of claim 1, wherein: The cooling temperature is 15℃, the cooling liquid is deionized water, and the cooling rate is 25℃ / min.

7. The method of claim 1, wherein: The extraction comprises primary extraction with ethanol and secondary extraction with supercritical CO2.

8. A hollow fiber microporous membrane prepared by the method of any one of claims 1-7.

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