A polytetrafluoroethylene composite filter membrane and its preparation method

Abstract

The present invention relates to the technical field of membrane preparation, specifically a polytetrafluoroethylene composite filtration membrane and a preparation method thereof. In order to improve the service life of the filtration membrane, the present invention enhances the filtration membrane material in the form of a composite membrane formed by stacking a high-strength polytetrafluoroethylene filtration membrane and a polytetrafluoroethylene porous membrane; in the process of preparing the high-strength polytetrafluoroethylene filtration membrane, the present invention uses carbon nanotubes as a reinforcing material and conducts a modification treatment on it. After introducing long carbon chains and carbon-carbon double bonds on its surface, it reacts with vinyl fluoride, thereby generating a tetrafluoroethylene-like structure on the surface of the carbon nanotubes, improving the binding property and dispersibility between the carbon nanotubes and polytetrafluoroethylene; moreover, in order to improve the defect of the decrease in tensile properties caused by the introduction of carbon nanotubes in polytetrafluoroethylene, the present application also defines the membrane stretching process, effectively ensuring the strength and membrane flux of the polytetrafluoroethylene composite filtration membrane.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane preparation, and specifically to a polytetrafluoroethylene composite filtration membrane and a preparation method thereof. Background Art

[0002] At present, in industrial wastewater treatment, the membrane bioreactor method is often used to recycle water resources. Compared with the traditional AO method, the membrane bioreactor mainly uses a filtration membrane to purify sewage. It has a fast treatment efficiency and a short purification cycle, and can achieve rapid sewage treatment. However, the loss of the filtration membrane in the membrane bioreactor is relatively serious. After long-term use, the filtration membrane is often contaminated or damaged, resulting in the inability to purify sewage. Therefore, it is necessary to improve this phenomenon. Summary of the Invention

[0003] The purpose of the present invention is to provide a polytetrafluoroethylene composite filtration membrane and a preparation method thereof to solve the problems raised in the prior art.

[0004] To achieve the above purpose, the present invention provides the following technical solution: A preparation method of a polytetrafluoroethylene composite filtration membrane, comprising the following steps:

[0005] S1. Prepare modified carbon nanotubes;

[0006] S11. Disperse carbon nanotubes into concentrated nitric acid, heat up to 78 - 90 °C, stir and react for 2 - 4 h, then centrifuge to separate the precipitate. After washing the precipitate with deionized water until neutral, vacuum dry it to obtain activated carbon nanotubes;

[0007] S12. Under the protection of a nitrogen atmosphere, pass nitrogen into DMF for 1 - 2.5 h, then add 1,3 - propanediamine and triethylamine into it. After mixing evenly, dropwise add an octadecenedioic acid dilution solution into it within 2 - 4.5 h. During the dropping process, control the reaction system temperature at 65 - 72 °C. After the dropping is completed, continue to heat up to 105 - 115 °C, stir and react for 2 - 12 h, then rotary evaporate to remove the excess solvent to obtain a long-chain diamine intermediate;

[0008] S13. Under a nitrogen atmosphere, disperse the long-chain diamine intermediate into DMF, heat up to 62 - 64 °C, stir and mix for 10 - 25 min, then add the activated carbon nanotubes and triethylamine into it. After ultrasonic oscillation reaction for 5 - 12 h, centrifuge to separate the precipitate, and wash the product with absolute ethanol 2 - 5 times, then vacuum dry it to constant weight to obtain amino-modified carbon nanotubes;

[0009] S14. Disperse the amino-modified carbon nanotubes prepared in step S13 in toluene. After ultrasonic dispersion for 0.5 - 1.5 h, add p-vinylbenzoic acid to it, heat up to 102 - 109 °C, stir and react for 4 - 5 h. Then, centrifuge to separate the precipitate and wash the precipitate with toluene 2 - 3 times. After that, redisperse the precipitate in deionized water, add an initiator, and introduce vinyl fluoride. Control the vinyl fluoride gas pressure to be 2 - 3 MPa, heat up to 85 - 95 °C, react for 4 - 8 h, release the pressure and discharge the material. Vacuum evaporate the product to constant weight to obtain fluorinated carbon nanotubes;

[0010] S2. Prepare a high-strength polytetrafluoroethylene filtration membrane;

[0011] Mix the fluorinated carbon nanotubes prepared in step S1 with polytetrafluoroethylene. After blending for 15 - 30 min, mix with n-butanol again, heat up to 55 - 80 °C, keep it uniform by insulation and statically stand for 12 - 24 h. After extruding and rolling the mixture into a film, place the film in an environment of 115 - 117 °C, control the air pressure to be 400 - 800 Kpa, statically stand for 10 - 15 min, then return to normal temperature and pressure, and statically stand for 12 - 24 h. Heat up the film to 115 - 165 °C again, and biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction to obtain a high-strength polytetrafluoroethylene filtration membrane;

[0012] S3. Prepare a polytetrafluoroethylene porous membrane;

[0013] After extruding the polytetrafluoroethylene into a film, biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction to obtain a polytetrafluoroethylene porous membrane;

[0014] S4. Prepare a polytetrafluoroethylene composite filtration membrane;

[0015] Stack the high-strength polytetrafluoroethylene filtration membrane prepared in step S2 and the polytetrafluoroethylene porous membrane prepared in step S3, and thermally press and bond them to obtain the polytetrafluoroethylene composite filtration membrane.

[0016] Further, in step S11, the concentration of the concentrated nitric acid is 65 - 70 wt%.

[0017] Further, in step S11, the mass ratio of the carbon nanotubes to the concentrated nitric acid is 1:(500 - 1500).

[0018] Further, in step S12, the diluent of the octadecenedioic acid is a mixed solution of octadecenedioic acid and DMF;

[0019] The mass ratio of the 1,3-propanediamine, triethylamine, and octadecenedioic acid is 1:(0.05 - 0.1):(1.5 - 2.2).

[0020] Further, in step S13, the mass ratio of the long-chain diamine intermediate, triethylamine, and activated carbon nanotubes is (5 - 50):(0.05 - 0.15):1.

[0021] Further, in step S14, the mass ratio of the amino-modified carbon nanotubes, p-vinylbenzoic acid, and initiator is 1:(1 - 50):(0.05 - 0.5);

[0022] The initiator is potassium persulfate.

[0023] Further, in step S2, the mass ratio of the fluorinated carbon nanotubes, polytetrafluoroethylene, and n-butanol is (0.5 - 2.5):40:(6 - 10).

[0024] Further, in step S2, the thickness of the high-strength polytetrafluoroethylene filtration membrane is 5 - 10 μm.

[0025] Further, in step S3, the thickness of the polytetrafluoroethylene porous membrane is 15 - 20 μm.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] In order to improve the service life of the filtration membrane, the present invention uses a form in which a high-strength polytetrafluoroethylene filtration membrane and a polytetrafluoroethylene porous membrane are stacked and combined to form a composite membrane to enhance the filtration membrane material; in the process of preparing the high-strength polytetrafluoroethylene filtration membrane, the present invention uses carbon nanotubes as a reinforcing material, and on this basis, the surface of the carbon nanotubes is also modified.

[0028] The present invention first uses 9,10-octadecenedioic acid as a raw material. 9,10-octadecenedioic acid is a dicarboxylic acid substance containing a long carbon chain. Reacting it with excessive 1,3-propanediamine containing a diamine group can introduce an amino group into the long carbon chain, and then react and graft with the carboxyl group generated on the surface of the carbon nanotubes after being treated with nitric acid, so as to introduce a long carbon chain and an amino group on the surface of the carbon nanotubes. Then reacting it with p-vinylbenzoic acid containing a vinyl group further introduces an olefin bond on the surface of the carbon nanotubes. Under the action of an initiator, the carbon-carbon double bond in the olefin structure reacts with styrene, so as to generate a polytetrafluoroethylene-like structure on the surface of the carbon nanotubes, thereby improving the binding property and dispersibility between the carbon nanotubes and polytetrafluoroethylene, and avoiding the defect that the agglomeration of carbon nanotubes causes uneven strength of the high-strength polytetrafluoroethylene filtration membrane.

[0029] Moreover, in order to improve the defects of decreased tensile properties and uneven pore distribution caused by the introduction of carbon nanotubes into polytetrafluoroethylene, when preparing the high-strength polytetrafluoroethylene filter membrane in this application, n-butanol is also added to the raw materials as a volatile substance. At high temperatures, n-butanol will gradually volatilize, thereby forming tiny voids on the surface of the membrane. On this basis, the present invention further heats up the membrane and then stretches it again, effectively avoiding the defect of decreased tensile properties after the introduction of carbon nanotubes, and effectively ensuring the strength and membrane flux of the polytetrafluoroethylene composite filter membrane. Detailed implementation manners

[0030] Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0031] The carbon nanotubes used in this application are TF220 type carbon nanotubes, with a tube diameter of 10 - 15 nm; and a tube length of 5 - 15 μm.

[0032] Example 1. A method for preparing a polytetrafluoroethylene composite filter membrane, comprising the following steps:

[0033] S1. Prepare modified carbon nanotubes;

[0034] S11. By weight, disperse 1 part of carbon nanotubes into 1000 parts of concentrated nitric acid with a concentration of 68 wt%, heat up to 84 °C, stir and react for 4 h, then after centrifuging and separating the precipitate, wash the precipitate with deionized water until neutral, and then dry it in vacuum to obtain activated carbon nanotubes;

[0035] S12. By weight, under the protection of a nitrogen atmosphere, pass nitrogen into DMF for 2.5 h, then add 1 part of 1,3-propanediamine and 0.05 part of triethylamine thereto, mix evenly, and then dropwise add a diluent of octadecenedioic acid composed of octadecenedioic acid and DMF, containing 1.5 parts of octadecenedioic acid, during the dropping process, control the reaction system temperature at 70 °C, after the dropping is completed, continue to heat up to 110 °C, stir and react for 8 h, then rotary evaporate to remove the excess solvent to obtain a long-chain diamine intermediate;

[0036] S13. By weight, under a nitrogen atmosphere, disperse 5 parts of the long-chain diamine intermediate into DMF, heat up to 62 °C, stir and mix for 20 min, then add 1 part of activated carbon nanotubes and 0.05 part of triethylamine thereto, ultrasonically oscillate and react for 8 h, then centrifuge and separate the precipitate, and wash the product 4 times with absolute ethanol, and then dry it in vacuum to constant weight to obtain amino-modified carbon nanotubes;

[0037] S14. By weight, disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene. After ultrasonic dispersion for 1 h, add 1 part of p-vinylbenzoic acid thereto, raise the temperature to 107 °C, stir and react for 4 h, then centrifuge to separate the precipitate and wash the precipitate with toluene twice. Then, redisperse the precipitate into deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride. Control the vinyl fluoride pressure to be 2 MPa, raise the temperature to 85 °C, react for 8 h, relieve the pressure and discharge the material, and evaporate the product in vacuum to constant weight to obtain fluorinated carbon nanotubes;

[0038] S2. Prepare a high-strength polytetrafluoroethylene filtration membrane;

[0039] By weight, mix 0.5 part of the fluorinated carbon nanotubes prepared in step S1 with 40 parts of polytetrafluoroethylene. After blending for 25 min, mix with 6 parts of n-butanol again, raise the temperature to 78 °C, keep the mixture uniform and stand still for 24 h. After extruding and calendering the mixture into a film, place the film in an environment of 117 °C, control the air pressure to be 800 Kpa, stand still for 15 min, then return to normal temperature and pressure, stand still for 24 h, raise the temperature of the film to 155 °C again, and biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction. Among them, the stretching multiple in the extrusion direction is 7.1 times, and the stretching multiple in the direction perpendicular to the extrusion direction is 9.5 times. Finally, obtain a high-strength polytetrafluoroethylene filtration membrane with a thickness of 10 μm;

[0040] S3. Prepare a polytetrafluoroethylene porous membrane;

[0041] After extruding the polytetrafluoroethylene into a film, biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction. Among them, the stretching multiple in the extrusion direction is 5.1 times, and the stretching multiple in the direction perpendicular to the extrusion direction is 8.2 times, to obtain a polytetrafluoroethylene porous membrane with a thickness of 20 μm;

[0042] S4. Prepare a polytetrafluoroethylene composite filtration membrane;

[0043] Stack the high-strength polytetrafluoroethylene filtration membrane prepared in step S2 and the polytetrafluoroethylene porous membrane prepared in step S3, and thermally press and bond them. The thermal pressing temperature is 135 °C, and the thermal pressing pressure is 1 MPa. After the thermal pressing is completed, obtain the polytetrafluoroethylene composite filtration membrane.

[0044] Example 2. A method for preparing a polytetrafluoroethylene composite filtration membrane, comprising the following steps:

[0045] Compared with Example 1, the addition amount of octadecenedioic acid in step S1 is increased in this example;

[0046] S1. Prepare modified carbon nanotubes;

[0047] S11. By weight, disperse 1 part of carbon nanotubes into 1000 parts of concentrated nitric acid with a concentration of 68 wt%, heat up to 84 °C, stir and react for 4 h, after centrifuging to separate the precipitate, wash the precipitate with deionized water until neutral, and then dry it in vacuum to obtain activated carbon nanotubes;

[0048] S12. By weight, under the protection of nitrogen atmosphere, after introducing nitrogen into DMF for 2.5 h, add 1 part of 1,3-propanediamine and 0.05 part of triethylamine into it, after mixing evenly, dropwise add an octadecenedioic acid dilution solution composed of octadecenedioic acid and DMF into it within 3 h, where the octadecenedioic acid content is 2.2 parts. During the dropping process, control the temperature of the reaction system at 70 °C. After the dropping is completed, continue to heat up to 110 °C, stir and react for 8 h, then remove the excess solvent by rotary evaporation to obtain a long-chain diamine intermediate;

[0049] S13. By weight, under nitrogen atmosphere, disperse 5 parts of the long-chain diamine intermediate into DMF, heat up to 62 °C, stir and mix for 20 min, then add 1 part of the activated carbon nanotubes and 0.05 part of triethylamine into it, after ultrasonic oscillation reaction for 8 h, centrifuge to separate the precipitate, and wash the product 4 times with absolute ethanol, then dry it in vacuum to constant weight to obtain amino-modified carbon nanotubes;

[0050] S14. By weight, disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene, ultrasonically disperse for 1 h, then add 1 part of p-vinylbenzoic acid into it, heat up to 107 °C, stir and react for 4 h, after centrifuging to separate the precipitate and washing the precipitate 2 times with toluene, redisperse the precipitate into deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure at 2 MPa, heat up to 85 °C, react for 8 h, then relieve the pressure and discharge the material, and evaporate the product in vacuum to constant weight to obtain fluorinated carbon nanotubes.

[0051] Example 3. A method for preparing a polytetrafluoroethylene composite filter membrane, comprising the following steps:

[0052] Compared with Example 2, in this example, the addition amount of the long-chain diamine intermediate in step S13 is increased;

[0053] S1. Prepare modified carbon nanotubes;

[0054] S11. By weight, disperse 1 part of carbon nanotubes into 1000 parts of concentrated nitric acid with a concentration of 68 wt%, heat up to 84 °C, stir and react for 4 h, after centrifuging to separate the precipitate, wash the precipitate with deionized water until neutral, and then dry it in vacuum to obtain activated carbon nanotubes;

[0055] S12. By weight, under the protection of a nitrogen atmosphere, after introducing nitrogen into DMF for 2.5 h, add 1 part of 1,3-propanediamine and 0.05 part of triethylamine thereto. After mixing evenly, dropwise add an octadecenedioic acid dilution solution composed of octadecenedioic acid and DMF thereto within 3 h, wherein the octadecenedioic acid content is 2.2 parts. During the dropping process, control the temperature of the reaction system at 70 °C. After the dropping is completed, continue to raise the temperature to 110 °C, stir and react for 8 h, then rotary evaporate to remove the excess solvent to obtain a long-chain diamine intermediate;

[0056] S13. By weight, under a nitrogen atmosphere, disperse 50 parts of the long-chain diamine intermediate in DMF, raise the temperature to 62 °C, stir and mix for 20 min, then add 1 part of activated carbon nanotubes and 0.05 part of triethylamine thereto. After ultrasonic oscillation reaction for 8 h, centrifuge to separate the precipitate, and wash the product 4 times with absolute ethanol, then vacuum dry to constant weight to obtain amino-modified carbon nanotubes;

[0057] S14. By weight, disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 in toluene, ultrasonically disperse for 1 h, then add 1 part of p-vinylbenzoic acid thereto, raise the temperature to 107 °C, stir and react for 4 h, then centrifuge to separate the precipitate and wash the precipitate 2 times with toluene. Then redisperse the precipitate in deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure at 2 MPa, raise the temperature to 85 °C, react for 8 h, then relieve the pressure and discharge the material, and vacuum evaporate the product to constant weight to obtain fluorinated carbon nanotubes.

[0058] Example 4. A method for preparing a polytetrafluoroethylene composite filtration membrane, comprising the following steps:

[0059] Compared with Example 3, the amount of p-vinylbenzoic acid added in step S14 is increased in this example;

[0060] S1. Prepare modified carbon nanotubes;

[0061] S11. By weight, disperse 1 part of carbon nanotubes in 1000 parts of concentrated nitric acid with a concentration of 68 wt%, raise the temperature to 84 °C, stir and react for 4 h, then centrifuge to separate the precipitate, wash the precipitate with deionized water until neutral, and then vacuum dry to obtain activated carbon nanotubes;

[0062] S12. By weight, under the protection of nitrogen atmosphere, after introducing nitrogen into DMF for 2.5 h, add 1 part of 1,3-propanediamine and 0.05 part of triethylamine thereto. After mixing evenly, dropwise add an octadecenoic acid dilution solution composed of octadecenoic acid and DMF thereto within 3 h, wherein the octadecenoic acid content is 2.2 parts. During the dropping process, control the temperature of the reaction system at 70 °C. After the dropping is completed, continue to heat up to 110 °C, stir and react for 8 h, then remove the excess solvent by rotary evaporation to obtain a long-chain diamine intermediate;

[0063] S13. By weight, under nitrogen atmosphere, disperse 50 parts of the long-chain diamine intermediate into DMF, heat up to 62 °C, stir and mix for 20 min, then add 1 part of activated carbon nanotubes and 0.05 part of triethylamine thereto. After ultrasonic oscillation reaction for 8 h, centrifuge to separate the precipitate, and wash the product 4 times with absolute ethanol, then vacuum dry to constant weight to obtain amino-modified carbon nanotubes;

[0064] S14. By weight, disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene, ultrasonically disperse for 1 h, then add 50 parts of p-vinylbenzoic acid thereto, heat up to 107 °C, stir and react for 4 h, then centrifuge to separate the precipitate and wash the precipitate 2 times with toluene. Then redisperse the precipitate into deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure at 2 MPa, heat up to 85 °C, react for 8 h, then relieve the pressure and discharge the material, and vacuum evaporate the product to constant weight to obtain fluorinated carbon nanotubes.

[0065] Example 5. A preparation method of a polytetrafluoroethylene composite filter membrane, comprising the following steps:

[0066] Compared with Example 4, the addition amount of n-butanol in step S2 is increased in this example;

[0067] S2. Prepare a high-strength polytetrafluoroethylene filter membrane;

[0068] By weight, mix 0.5 part of the fluorinated carbon nanotubes prepared in step S1 with 40 parts of polytetrafluoroethylene, blend for 25 min, then mix with 6 parts of n-butanol again, heat up to 78 °C, mix evenly and keep standing for 24 h. After extruding and calendering the mixture into a film, place the film in an environment of 117 °C, control the air pressure at 800 Kpa, keep standing for 15 min, then return to normal temperature and pressure, keep standing for 24 h, heat up the film to 155 °C again, and biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction, wherein the stretching multiple in the extrusion direction is 7.1 times and the stretching multiple in the direction perpendicular to the extrusion direction is 9.5 times, and finally obtain a high-strength polytetrafluoroethylene filter membrane with a thickness of 10 μm.

[0069] Comparative Example 1. A method for preparing a polytetrafluoroethylene composite porous membrane, comprising the following steps:

[0070] Compared with Example 1, a high-strength polytetrafluoroethylene filtration membrane was not prepared in this example;

[0071] S1. Prepare a polytetrafluoroethylene porous membrane;

[0072] After extruding the polytetrafluoroethylene into a film, the film was successively biaxially stretched in the extrusion direction and the direction perpendicular to the extrusion direction. The film was successively biaxially stretched in the extrusion direction and the direction perpendicular to the extrusion direction. Among them, the stretching multiple in the extrusion direction was 5.1 times, and the stretching multiple in the direction perpendicular to the extrusion direction was 8.2 times, to obtain a polytetrafluoroethylene porous membrane with a thickness of 30 μm;

[0073] Comparative Example 2. A method for preparing a polytetrafluoroethylene composite filtration membrane, comprising the following steps:

[0074] Compared with Example 1, in this comparative example, when preparing the high-strength polytetrafluoroethylene filtration membrane in step S2, the stretching treatment was carried out without heating to 155 °C;

[0075] S1. Prepare modified carbon nanotubes;

[0076] S11. By weight, disperse 1 part of carbon nanotubes into 1000 parts of concentrated nitric acid with a concentration of 68 wt%, heat to 84 °C, stir and react for 4 h, then centrifuge and separate the precipitate, wash the precipitate with deionized water until neutral, and then dry it in vacuum to obtain activated carbon nanotubes;

[0077] S12. By weight, under nitrogen atmosphere protection, after introducing nitrogen into DMF for 2.5 h, add 1 part of 1,3-propanediamine and 0.05 part of triethylamine to it, mix evenly, and then dropwise add an octadecenedioic acid dilution solution composed of octadecenedioic acid and DMF within 3 h, wherein it contains 1.5 parts of octadecenedioic acid. During the dropping process, control the temperature of the reaction system at 70 °C. After the dropping is completed, continue to heat to 110 °C, stir and react for 8 h, then rotary evaporate to remove the excess solvent to obtain a long-chain diamine intermediate;

[0078] S13. By weight, under nitrogen atmosphere, disperse 5 parts of the long-chain diamine intermediate into DMF, heat to 62 °C, stir and mix for 20 min, then add 1 part of activated carbon nanotubes and 0.05 part of triethylamine to it, carry out ultrasonic oscillation reaction for 8 h, then centrifuge and separate the precipitate, and wash the product 4 times with absolute ethanol, and then dry it in vacuum to constant weight to obtain amino-modified carbon nanotubes;

[0079] S14. By weight, disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene. After ultrasonic dispersion for 1 h, add 1 part of p-vinylbenzoic acid thereto, heat up to 107 °C, stir and react for 4 h. After centrifuging to separate the precipitate and washing the precipitate with toluene twice, redisperse the precipitate into deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride. Control the vinyl fluoride pressure to be 2 MPa, heat up to 85 °C, react for 8 h, relieve the pressure and discharge the material. Evaporate the product under vacuum to constant weight to obtain fluorinated carbon nanotubes;

[0080] S2. Prepare a high-strength polytetrafluoroethylene filtration membrane;

[0081] By weight, mix 0.5 part of the fluorinated carbon nanotubes prepared in step S1 with 40 parts of polytetrafluoroethylene. After blending for 25 min, mix with 6 parts of n-butanol again, heat up to 78 °C, keep the mixture uniform and stand still for 24 h. After extruding and rolling the mixture into a film, place the film in an environment of 117 °C, control the air pressure to be 800 Kpa, stand still for 15 min, then return to normal temperature and pressure, and stand still for 24 h. Biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction. Among them, the stretching multiple in the extrusion direction is 7.1 times, and the stretching multiple in the direction perpendicular to the extrusion direction is 9.5 times. Finally, obtain a high-strength polytetrafluoroethylene filtration membrane with a thickness of 10 μm;

[0082] S3. Prepare a polytetrafluoroethylene porous membrane;

[0083] After extruding the polytetrafluoroethylene into a film, biaxially stretch the film successively in the extrusion direction and the direction perpendicular to the extrusion direction. Among them, the stretching multiple in the extrusion direction is 5.1 times, and the stretching multiple in the direction perpendicular to the extrusion direction is 8.2 times, to obtain a polytetrafluoroethylene porous membrane with a thickness of 20 μm;

[0084] S4. Prepare a polytetrafluoroethylene composite filtration membrane;

[0085] Stack the high-strength polytetrafluoroethylene filtration membrane prepared in step S2 and the polytetrafluoroethylene porous membrane prepared in step S3, and thermally press and bond them. The thermal pressing temperature is 135 °C, and the thermal pressing pressure is 1 MPa. After the thermal pressing is completed, obtain the polytetrafluoroethylene composite filtration membrane.

[0086] Detection: Prepare the composite filtration membranes prepared in Examples 1-5 and Comparative Examples 1-2 into samples and test them:

[0087] Internal pressure breaking water pressure test: Under the environment of 25 ± 2 °C, apply a stable water pressure to one side of the polytetrafluoroethylene porous membrane of the samples prepared in Examples 1-5 and Comparative Examples 1-2, and observe the water pressure borne when a destructive water column appears on the surface of the composite filtration membrane;

[0088] Rejection rate detection: Prepare a sodium sulfate solution with a concentration of 1 g / L, filter the sodium sulfate solution using a composite filtration membrane, with a filtration pressure of 0.2 MPa, an operating time of 30 min, a test temperature of 25 ± 2 °C. Measure the concentrations of the feed liquid before and after filtration, and calculate the rejection rate;

[0089] The formula for calculating the rejection rate is: R = (1 - c1 / c2) × 100%;

[0090] Wherein, R - rejection rate; C1 - concentration of the feed liquid after filtration; C2 - concentration of the feed liquid before filtration;

[0091] According to HY / T 051-1999, perform water flux tests on the samples prepared in Examples 1-5 and Comparative Examples 1-2;

[0092] The test results are shown in the following table;

[0093]

[0094] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a polytetrafluoroethylene composite filter membrane, characterized in that: The following steps are involved: S1. Preparation of modified carbon nanotubes; S11. dispersing carbon nanotubes in concentrated nitric acid, heating to 78-90°C, stirring for 2-4h, centrifuging and separating the precipitate, washing the precipitate with deionized water until neutral, and vacuum drying to obtain activated carbon nanotubes; S12. Under nitrogen atmosphere protection, nitrogen was passed through DMF for 1-2.5 hours, 1,3-propylenediamine and triethylamine were added thereto, and after mixing evenly, the diluted octadecenedioic acid solution was added dropwise within 2-4.5 hours. During the addition, the temperature of the reaction system was controlled to be 65-72° C. After the addition was completed, the temperature was continued to rise to 105-115° C., and the reaction was stirred for 2-12 hours. After the excess solvent was removed by rotary evaporation, a long-chain diamine intermediate was obtained; S13. In a nitrogen atmosphere, the long-chain diamine intermediate is dispersed in DMF, the temperature is raised to 62-64° C., and the mixture is stirred for 10-25 min. Then, activated carbon nanotubes and triethylamine are added thereto. After ultrasonic vibration reaction for 5-12 h, the precipitation is separated by centrifugation, and the product is washed with anhydrous ethanol for 2-5 times, and then vacuum dried to constant weight to obtain amino-modified carbon nanotubes; S14. The amino-modified carbon nanotubes prepared in step S13 are dispersed in toluene, and after ultrasonic dispersion for 0.5-1.5 hours, p-vinylbenzoic acid is added thereto, and the temperature is raised to 102-109° C., and the reaction is stirred for 4-5 hours. The precipitate is separated by centrifugation and washed with toluene for 2-3 times, and the precipitate is dispersed again in deionized water, an initiator is added, and vinyl fluoride is introduced, and the vinyl fluoride gas pressure is controlled to be 2-3 MPa, and the temperature is raised to 85-95° C., and the reaction is carried out for 4-8 hours, and the pressure is released and the product is vacuum evaporated to constant weight to obtain fluorinated carbon nanotubes; S2. Preparation of high-strength polytetrafluoroethylene filter membrane; The fluorinated carbon nanotubes prepared in step S1 are mixed with polytetrafluoroethylene, and after blending for 15-30 minutes, they are mixed with n-butanol again, and the temperature is raised to 55-80°C. After mixing evenly, the mixture is kept warm and allowed to stand for 12-24 hours. After the mixture is extruded and rolled into a film, the film is placed in an environment of 115-117°C, and the air pressure is controlled to be 400-800Kpa. After standing for 10-15 minutes, the film is returned to normal temperature and pressure, and is allowed to stand for 12-24 hours. The film is heated to 115-165°C again, and the film is biaxially stretched in the extrusion direction and in a direction perpendicular to the extrusion direction to obtain a high-strength polytetrafluoroethylene filter membrane; S3. Preparation of polytetrafluoroethylene porous membrane; After the polytetrafluoroethylene is extruded into a film, the film is biaxially stretched in the extrusion direction and in a direction perpendicular to the extrusion direction to obtain a polytetrafluoroethylene porous film; S4. Preparation of polytetrafluoroethylene composite filter membrane; The high-strength polytetrafluoroethylene filter membrane prepared in step S2 and the polytetrafluoroethylene porous membrane prepared in step S3 are stacked and hot-pressed to obtain the polytetrafluoroethylene composite filter membrane.

2. The method for preparing a polytetrafluoroethylene composite filter membrane according to claim 1, characterized in that: In step S11, the concentration of concentrated nitric acid is 65-70wt%.

3. The method for preparing a polytetrafluoroethylene composite filter membrane according to claim 1, characterized in that: In step S11, the mass ratio of the carbon nanotubes to concentrated nitric acid is 1:(500-1500).

4. The method for preparing a polytetrafluoroethylene composite filter membrane according to claim 1, characterized in that: In step S12, the octadecenedioic acid dilution solution is a mixed solution of octadecenedioic acid and DMF; The mass ratio of the 1,3-propylenediamine, triethylamine and octadecenedioic acid is 1:(0.05-0.1):(1.5-2.2).

5. The method for preparing a polytetrafluoroethylene composite filtration membrane according to claim 1, characterized in that: In step S13, the mass ratio of the long-chain diamine intermediate, triethylamine and activated carbon nanotubes is (5-50):(0.05-0.15):

1.

6. The method for preparing a polytetrafluoroethylene composite filtration membrane according to claim 1, characterized in that: In step S14, the mass ratio of the amino-modified carbon nanotubes, p-vinylbenzoic acid, and initiator is 1:(1-50):(0.05-0.5); The initiator is potassium persulfate.

7. The method for preparing a polytetrafluoroethylene composite filtration membrane according to claim 1, characterized in that: In step S2, the mass ratio of the fluorinated carbon nanotubes, polytetrafluoroethylene, and n-butanol is (0.5-2.5):40:(6-10).

8. The method for preparing a polytetrafluoroethylene composite filtration membrane according to claim 1, characterized in that: In step S2, the thickness of the high-strength polytetrafluoroethylene filter membrane is 5-10 μm.

9. The method for preparing a polytetrafluoroethylene composite filtration membrane according to claim 1, characterized in that: In step S3, the thickness of the polytetrafluoroethylene porous membrane is 15-20 μm.

10. A polytetrafluoroethylene composite filtration membrane prepared by the preparation method according to any one of claims 1 to 9.

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