Polytetrafluoroethylene composite filter membrane and preparation method thereof

Abstract

The invention relates to the technical field of membrane preparation, in particular to a polytetrafluoroethylene composite filtering membrane and a preparation method thereof.In order to prolong the service life of the filtering membrane, the filtering membrane material is reinforced in the mode that a high-strength polytetrafluoroethylene filtering membrane and a polytetrafluoroethylene porous membrane are stacked and combined to form a composite membrane; in the process of preparing the high-strength polytetrafluoroethylene filtering membrane, the carbon nano tube is used as a reinforcing material and is modified, a long carbon chain and a carbon-carbon double bond are introduced into the surface of the carbon nano tube, and then the carbon nano tube reacts with vinyl fluoride, so that a tetrafluoroethylene-like structure is generated on the surface of the carbon nano tube; the binding property and the dispersity between the carbon nano tube and the polytetrafluoroethylene are improved; moreover, in order to improve the defect that the tensile property is reduced after the carbon nanotubes are introduced into the polytetrafluoroethylene, a membrane stretching process is limited, and the strength and the membrane flux of the polytetrafluoroethylene composite filter membrane are effectively ensured.

Description

Technical Field

[0001] The invention relates to the technical field of membrane preparation, in particular to a polytetrafluoroethylene composite filtering membrane and a preparation method thereof. Background Art

[0002] At present, membrane bioreactors are often used in industrial wastewater treatment to recycle water resources. Compared with the traditional AO method, membrane bioreactors mainly use filter membranes to purify sewage. They have high treatment efficiency and short purification cycle, and can achieve rapid sewage treatment. However, the loss of filter membranes in membrane bioreactors is relatively serious. After a long period of use, the filter membranes are often contaminated or damaged, making it impossible to purify sewage. Therefore, it is necessary to improve this phenomenon. Summary of the invention

[0003] The object of the present invention is to provide a polytetrafluoroethylene composite filter membrane and a preparation method thereof, so as to solve the problems raised in the prior art.

[0004] To achieve the above object, the present invention provides the following technical solution: a method for preparing a polytetrafluoroethylene composite filter membrane, comprising the following steps:

[0005] S1. Preparation of modified carbon nanotubes;

[0006] 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;

[0007] 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;

[0008] 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;

[0009] 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;

[0010] S2. Preparation of high-strength polytetrafluoroethylene filter membrane;

[0011] 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;

[0012] S3. Preparation of polytetrafluoroethylene porous membrane;

[0013] 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;

[0014] S4. Preparation of polytetrafluoroethylene composite filter membrane;

[0015] 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.

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

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

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

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

[0020] Furthermore, 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] Furthermore, 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] Furthermore, in step S2, the mass ratio of the fluorinated carbon nanotubes, polytetrafluoroethylene, and n-butanol is (0.5-2.5):40:(6-10).

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

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

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] In order to improve the service life of the filter membrane, the present invention uses a high-strength polytetrafluoroethylene filter membrane and a polytetrafluoroethylene porous membrane to stack and combine to form a composite membrane to strengthen the filter membrane material; in the process of preparing the high-strength polytetrafluoroethylene filter membrane, the present invention uses carbon nanotubes as a reinforcing material, and on this basis, the carbon nanotubes are also surface-modified;

[0028] The present invention first uses octadecenedioic acid as a raw material. The octadecenedioic acid is a dicarboxylic acid substance containing a long carbon chain. The octadecenedioic acid is reacted with an excess of 1,3-propylenediamine containing a diamine group, so that an amino group can be introduced into the long carbon chain, and then reacts with the carboxyl group generated on the surface of the carbon nanotube after being treated with nitric acid to graft, so that a long carbon chain and an amino group are introduced on the surface of the carbon nanotube. Then, the octadecenedioic acid is reacted with p-vinylbenzoic acid containing a vinyl group, so that an olefin bond is further introduced on the surface of the carbon nanotube. Under the action of an initiator, the carbon-carbon double bond in the olefin structure reacts with styrene, so that a tetrafluoroethylene-like structure is generated on the surface of the carbon nanotube, so that the binding and dispersibility between the carbon nanotube and the polytetrafluoroethylene are improved, and the agglomeration of the carbon nanotube is avoided, which causes the defect of uneven strength of the high-strength polytetrafluoroethylene filter membrane.

[0029] Furthermore, in order to improve the defects of decreased tensile properties and uneven pore distribution caused by the introduction of carbon nanotubes into polytetrafluoroethylene, the present application also adds n-butanol as a volatile substance to the raw materials when preparing the high-strength polytetrafluoroethylene filter membrane. At high temperatures, the n-butanol will gradually evaporate, thereby forming tiny gaps on the surface of the membrane. On this basis, the present invention further heats the membrane and 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 DESCRIPTION

[0030] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making any creative work shall fall within the scope of protection of the present invention.

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

[0032] Embodiment 1. A method for preparing a polytetrafluoroethylene composite filter membrane comprises the following steps:

[0033] S1. Preparation of modified carbon nanotubes;

[0034] S11. Disperse 1 part of carbon nanotubes into 1000 parts of 68wt% concentrated nitric acid by weight, heat to 84°C, stir and react for 4h, centrifuge and separate the precipitate, wash the precipitate with deionized water until neutral, and vacuum dry to obtain activated carbon nanotubes;

[0035] S12. By weight, under nitrogen atmosphere protection, nitrogen was passed through DMF for 2.5 hours, and then 1 part of 1,3-propylenediamine and 0.05 parts of triethylamine were added thereto. After mixing evenly, a dilution of octadecenedioic acid consisting of octadecenedioic acid and DMF was added dropwise within 3 hours, wherein the dilution contained 1.5 parts of octadecenedioic acid. During the addition, the temperature of the reaction system was controlled to be 70° C. After the addition was completed, the temperature was continued to rise to 110° C., and the reaction was stirred for 8 hours. After the excess solvent was removed by rotary evaporation, a long-chain diamine intermediate was obtained;

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

[0037] S14. Disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene by weight, and after ultrasonic dispersion for 1 hour, add 1 part of p-vinylbenzoic acid thereto, heat to 107°C, stir and react for 4 hours, centrifuge and separate the precipitate, wash the precipitate twice with toluene, disperse the precipitate again in deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure to 2MPa, heat to 85°C, react for 8 hours, release the pressure and discharge, and vacuum evaporate the product to constant weight to obtain fluorinated carbon nanotubes;

[0038] S2. Preparation of high-strength polytetrafluoroethylene filter membrane;

[0039] By weight, 0.5 parts of the fluorinated carbon nanotubes prepared in step S1 were mixed with 40 parts of polytetrafluoroethylene, and after blending for 25 minutes, they were mixed again with 6 parts of n-butanol, heated to 78°C, mixed evenly, and then kept warm for 24 hours. After the mixture was extruded and calendered into a film, the film was placed in an environment of 117°C, the air pressure was controlled to be 800Kpa, and after standing for 15 minutes, it was returned to normal temperature and pressure, and stood for 24 hours. The film was heated to 155°C again, and the film was biaxially stretched in the extrusion direction and in a direction perpendicular to the extrusion direction, wherein the stretching endorsement in the extrusion direction was 7.1 times, and the stretching multiple in the perpendicular extrusion direction was 9.5 times, and finally a high-strength polytetrafluoroethylene filter membrane with a thickness of 10 μm was obtained;

[0040] S3. Preparation of polytetrafluoroethylene porous membrane;

[0041] 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, wherein the stretching multiple in the extrusion direction is 5.1 times, and the stretching multiple in the perpendicular extrusion direction is 8.2 times, to obtain a polytetrafluoroethylene porous film with a thickness of 20 μm;

[0042] S4. Preparation of polytetrafluoroethylene composite filter membrane;

[0043] The high-strength polytetrafluoroethylene filter membrane prepared in step S2 and the polytetrafluoroethylene porous membrane prepared in step S3 are stacked and hot-pressed together at a temperature of 135° C. and a pressure of 1 MPa. After the hot-pressing is completed, the polytetrafluoroethylene composite filter membrane is obtained.

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

[0045] Compared with Example 1, this example increases the amount of octadecenedioic acid added in step S1;

[0046] S1. Preparation of modified carbon nanotubes;

[0047] S11. Disperse 1 part of carbon nanotubes into 1000 parts of 68wt% concentrated nitric acid by weight, heat to 84°C, stir and react for 4h, centrifuge and separate the precipitate, wash the precipitate with deionized water until neutral, and vacuum dry to obtain activated carbon nanotubes;

[0048] S12. By weight, under nitrogen atmosphere protection, nitrogen was passed through DMF for 2.5 hours, and then 1 part of 1,3-propylenediamine and 0.05 parts of triethylamine were added thereto. After mixing evenly, a dilution of octadecenedioic acid consisting of octadecenedioic acid and DMF was added dropwise within 3 hours, wherein the dilution contained 2.2 parts of octadecenedioic acid. During the addition, the temperature of the reaction system was controlled to be 70° C. After the addition was completed, the temperature was continued to rise to 110° C., and the reaction was stirred for 8 hours. After the excess solvent was removed by rotary evaporation, a long-chain diamine intermediate was obtained;

[0049] S13. Disperse 5 parts of the long-chain diamine intermediate in DMF by weight under a nitrogen atmosphere, heat to 62°C, stir and mix for 20 min, add 1 part of activated carbon nanotubes and 0.05 parts of triethylamine, react by ultrasonic vibration for 8 h, centrifuge and separate the precipitate, wash the product 4 times with anhydrous ethanol, and vacuum dry to constant weight to obtain amino-modified carbon nanotubes;

[0050] S14. Disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene by weight, and after ultrasonic dispersion for 1 hour, add 1 part of p-vinylbenzoic acid thereto, heat to 107°C, stir and react for 4 hours, separate the precipitate by centrifugation and wash the precipitate twice with toluene, disperse the precipitate again in deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure to 2MPa, heat to 85°C, react for 8 hours, release the pressure and discharge the material, and evaporate the product in vacuum to constant weight to obtain fluorinated carbon nanotubes.

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

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

[0053] S1. Preparation of modified carbon nanotubes;

[0054] S11. Disperse 1 part of carbon nanotubes into 1000 parts of 68wt% concentrated nitric acid by weight, heat to 84°C, stir and react for 4h, centrifuge and separate the precipitate, wash the precipitate with deionized water until neutral, and vacuum dry to obtain activated carbon nanotubes;

[0055] S12. By weight, under nitrogen atmosphere protection, nitrogen was passed through DMF for 2.5 hours, and then 1 part of 1,3-propylenediamine and 0.05 parts of triethylamine were added thereto. After mixing evenly, a dilution of octadecenedioic acid consisting of octadecenedioic acid and DMF was added dropwise within 3 hours, wherein the dilution contained 2.2 parts of octadecenedioic acid. During the addition, the temperature of the reaction system was controlled to be 70° C. After the addition was completed, the temperature was continued to rise to 110° C., and the reaction was stirred for 8 hours. After the excess solvent was removed by rotary evaporation, a long-chain diamine intermediate was obtained;

[0056] S13. In a nitrogen atmosphere, 50 parts of the long-chain diamine intermediate were dispersed in DMF, heated to 62°C, stirred and mixed for 20 minutes, and then 1 part of activated carbon nanotubes and 0.05 parts of triethylamine were added thereto. After ultrasonic shock reaction for 8 hours, the precipitate was separated by centrifugation, and the product was washed with anhydrous ethanol 4 times and vacuum dried to constant weight to obtain amino-modified carbon nanotubes;

[0057] S14. Disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene by weight, and after ultrasonic dispersion for 1 hour, add 1 part of p-vinylbenzoic acid thereto, heat to 107°C, stir and react for 4 hours, separate the precipitate by centrifugation and wash the precipitate twice with toluene, disperse the precipitate again in deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure to 2MPa, heat to 85°C, react for 8 hours, release the pressure and discharge the material, and evaporate the product in vacuum to constant weight to obtain fluorinated carbon nanotubes.

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

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

[0060] S1. Preparation of modified carbon nanotubes;

[0061] S11. Disperse 1 part of carbon nanotubes into 1000 parts of 68wt% concentrated nitric acid by weight, heat to 84°C, stir and react for 4h, centrifuge and separate the precipitate, wash the precipitate with deionized water until neutral, and vacuum dry to obtain activated carbon nanotubes;

[0062] S12. By weight, under nitrogen atmosphere protection, nitrogen was passed through DMF for 2.5 hours, and then 1 part of 1,3-propylenediamine and 0.05 parts of triethylamine were added thereto. After mixing evenly, a dilution of octadecenedioic acid consisting of octadecenedioic acid and DMF was added dropwise within 3 hours, wherein the dilution contained 2.2 parts of octadecenedioic acid. During the addition, the temperature of the reaction system was controlled to be 70° C. After the addition was completed, the temperature was continued to rise to 110° C., and the reaction was stirred for 8 hours. After the excess solvent was removed by rotary evaporation, a long-chain diamine intermediate was obtained;

[0063] S13. In a nitrogen atmosphere, 50 parts of the long-chain diamine intermediate were dispersed in DMF, heated to 62°C, stirred and mixed for 20 minutes, and then 1 part of activated carbon nanotubes and 0.05 parts of triethylamine were added thereto. After ultrasonic shock reaction for 8 hours, the precipitate was separated by centrifugation, and the product was washed with anhydrous ethanol 4 times and vacuum dried to constant weight to obtain amino-modified carbon nanotubes;

[0064] S14. Disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene by weight, and after ultrasonic dispersion for 1 hour, add 50 parts of p-vinylbenzoic acid thereto, heat to 107°C, stir and react for 4 hours, centrifuge and separate the precipitate, wash the precipitate twice with toluene, disperse the precipitate again in deionized water, add 0.1 parts of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure to 2MPa, heat to 85°C, react for 8 hours, release the pressure and discharge the material, and vacuum evaporate the product to constant weight to obtain fluorinated carbon nanotubes.

[0065] Embodiment 5. A method for preparing a polytetrafluoroethylene composite filter membrane, comprising the following steps:

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

[0067] S2. Preparation of high-strength polytetrafluoroethylene filter membrane;

[0068] By weight, 0.5 parts of the fluorinated carbon nanotubes prepared in step S1 were mixed with 40 parts of polytetrafluoroethylene, blended for 25 minutes, and then mixed with 6 parts of n-butanol again, heated to 78°C, mixed evenly, and kept warm for 24 hours. After the mixture was extruded and calendered into a film, the film was placed in an environment of 117°C, the air pressure was controlled to be 800Kpa, and after standing for 15 minutes, it was returned to normal temperature and pressure, and stood for 24 hours. The film was heated to 155°C again, and the film was biaxially stretched in the extrusion direction and in a direction perpendicular to the extrusion direction, wherein the stretching endorsement in the extrusion direction was 7.1 times, and the stretching multiple in the perpendicular extrusion direction was 9.5 times, and finally a high-strength polytetrafluoroethylene filter membrane with a thickness of 10 μm was obtained.

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

[0070] Compared with Example 1, this example does not prepare a high-strength polytetrafluoroethylene filter membrane;

[0071] S1. Preparation of polytetrafluoroethylene porous membrane;

[0072] 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, and the film is biaxially stretched in the extrusion direction and in a direction perpendicular to the extrusion direction, wherein the stretching multiple in the extrusion direction is 5.1 times, and the stretching multiple in the perpendicular extrusion direction is 8.2 times, to obtain a polytetrafluoroethylene porous film with a thickness of 30 μm;

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

[0074] Compared with Example 1, in the step S2 of preparing the high-strength polytetrafluoroethylene filter membrane, the stretching treatment was performed without heating to 155° C.;

[0075] S1. Preparation of modified carbon nanotubes;

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

[0077] S12. By weight, under nitrogen atmosphere protection, nitrogen was passed through DMF for 2.5 hours, and then 1 part of 1,3-propylenediamine and 0.05 parts of triethylamine were added thereto. After mixing evenly, a dilution of octadecenedioic acid consisting of octadecenedioic acid and DMF was added dropwise within 3 hours, wherein the dilution contained 1.5 parts of octadecenedioic acid. During the addition, the temperature of the reaction system was controlled to be 70° C. After the addition was completed, the temperature was continued to rise to 110° C., and the reaction was stirred for 8 hours. After the excess solvent was removed by rotary evaporation, a long-chain diamine intermediate was obtained;

[0078] S13. Disperse 5 parts of the long-chain diamine intermediate in DMF by weight under a nitrogen atmosphere, heat to 62°C, stir and mix for 20 min, add 1 part of activated carbon nanotubes and 0.05 parts of triethylamine, react by ultrasonic vibration for 8 h, centrifuge and separate the precipitate, wash the product 4 times with anhydrous ethanol, and vacuum dry to constant weight to obtain amino-modified carbon nanotubes;

[0079] S14. Disperse 1 part of the amino-modified carbon nanotubes prepared in step S13 into toluene by weight, and after ultrasonic dispersion for 1 hour, add 1 part of p-vinylbenzoic acid thereto, heat to 107°C, stir and react for 4 hours, centrifuge and separate the precipitate, wash the precipitate twice with toluene, disperse the precipitate again in deionized water, add 0.1 part of potassium persulfate initiator, and introduce vinyl fluoride, control the vinyl fluoride gas pressure to 2MPa, heat to 85°C, react for 8 hours, release the pressure and discharge, and vacuum evaporate the product to constant weight to obtain fluorinated carbon nanotubes;

[0080] S2. Preparation of high-strength polytetrafluoroethylene filter membrane;

[0081] By weight, 0.5 parts of the fluorinated carbon nanotubes prepared in step S1 were mixed with 40 parts of polytetrafluoroethylene, and after blending for 25 minutes, they were mixed again with 6 parts of n-butanol, heated to 78°C, mixed evenly, and then kept warm for 24 hours. After the mixture was extruded and calendered into a film, the film was placed in an environment of 117°C, the air pressure was controlled to be 800Kpa, and after standing for 15 minutes, it was returned to normal temperature and pressure, and stood for 24 hours. The film was biaxially stretched in the extrusion direction and in a direction perpendicular to the extrusion direction, wherein the stretching endorsement in the extrusion direction was 7.1 times, and the stretching multiple in the perpendicular extrusion direction was 9.5 times, and finally a high-strength polytetrafluoroethylene filter membrane with a thickness of 10 μm was obtained;

[0082] S3. Preparation of polytetrafluoroethylene porous membrane;

[0083] 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, wherein the stretching multiple in the extrusion direction is 5.1 times, and the stretching multiple in the perpendicular extrusion direction is 8.2 times, to obtain a polytetrafluoroethylene porous film with a thickness of 20 μm;

[0084] S4. Preparation of polytetrafluoroethylene composite filter membrane;

[0085] The high-strength polytetrafluoroethylene filter membrane prepared in step S2 and the polytetrafluoroethylene porous membrane prepared in step S3 are stacked and hot-pressed together at a temperature of 135° C. and a pressure of 1 MPa. After the hot-pressing is completed, the polytetrafluoroethylene composite filter membrane is obtained.

[0086] Detection: The composite filtration membranes prepared in Examples 1-5 and Comparative Examples 1-2 were prepared as samples and tested:

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

[0088] Retention rate test: prepare a sodium sulfate solution with a concentration of 1g / L, use a composite filter membrane to filter the sodium sulfate solution, the filtration pressure is 0.2MPa, the running time is 30min, the test temperature is 25±2℃, test the concentration of the feed liquid before and after filtration, and calculate the retention rate;

[0089] The interception rate calculation formula is: R = (1-c 1 / c2) × 100%;

[0090] Wherein, R is the retention rate; C1 is the concentration of the feed solution after filtration; C2 is the concentration of the feed solution before filtration;

[0091] The water flux test was performed on the samples prepared in Examples 1-5 and Comparative Examples 1-2 according to HY / T 051-1999;

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

[0093]

[0094] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in 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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