CN111330453B - Polytetrafluoroethylene hollow fiber composite membrane and preparation method thereof - Google Patents

Polytetrafluoroethylene hollow fiber composite membrane and preparation method thereof Download PDF

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CN111330453B
CN111330453B CN202010149389.1A CN202010149389A CN111330453B CN 111330453 B CN111330453 B CN 111330453B CN 202010149389 A CN202010149389 A CN 202010149389A CN 111330453 B CN111330453 B CN 111330453B
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hollow fiber
ptfe
ptfe hollow
membrane
fiber membrane
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CN111330453A (en
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刘国昌
陈江荣
郭春刚
李�浩
车振宁
吕经烈
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Tianjin Institute of Seawater Desalination and Multipurpose Utilization MNR
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Tianjin Institute of Seawater Desalination and Multipurpose Utilization MNR
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/36—Pervaporation; Membrane distillation; Liquid permeation
    • B01D61/364—Membrane distillation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0002—Organic membrane manufacture
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/08—Hollow fibre membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B01D71/32—Polyalkenyl halides containing fluorine atoms
    • B01D71/36—Polytetrafluoroethene
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/38—Hydrophobic membranes

Abstract

The invention belongs to the technical field of separation membranes, and particularly relates to a polytetrafluoroethylene hollow fiber composite membrane and a preparation method thereof, wherein the composite membrane is of a double-layer structure and comprises a hydrophilic ultrafiltration skin layer on the outer side and a hydrophobic PTFE support layer on the inner side, the initial contact angle of the hydrophilic ultrafiltration skin layer is 20-85 degrees, the thickness of the hydrophilic ultrafiltration skin layer is 2-50 microns, the surface microporous structure is circular and uniformly distributed, the average pore diameter of micropores is 5-100 nm, the section of the hydrophilic ultrafiltration skin layer is a spongy pore, the initial contact angle of the hydrophobic PTFE support layer is 108-158 degrees, the microporous structure is a dotted line-shaped separation pore formed by gaps among fine fibers, the average pore diameter of the separation pore is 750-4000 nm, and the porosity is 65-85 percent. The invention has the beneficial effects that: the polytetrafluoroethylene hollow fiber membrane with the composite structure has lasting hydrophobicity and high-efficiency separation performance. Meanwhile, the preparation method is simple, convenient, efficient, economical and environment-friendly, and the prepared composite membrane has good stability.

Description

Polytetrafluoroethylene hollow fiber composite membrane and preparation method thereof
Technical Field
The invention belongs to the technical field of separation membranes, particularly relates to a microporous separation membrane and a preparation technology thereof, and particularly relates to a polytetrafluoroethylene hollow fiber composite membrane with an asymmetric microporous structure and a preparation method thereof.
Background
The multilayer composite mode is adopted to construct the asymmetric microporous structure of the PTFE hollow fiber membrane, and the method is an effective method for solving the problems of wide pore size distribution range and low porosity of products in the prior art. The PTFE hollow fiber membrane prepared by a stretching method is taken as a basis, the surface of the PTFE hollow fiber membrane is further modified to obtain a separation layer, so that the microporous structure and performance of the membrane can be accurately regulated and controlled, and the aim of accurate and efficient separation can be further fulfilled. At present, there are known techniques such as coating method, vapor deposition method, wrapping method and polymerization method.
Compared with the coating method and the wrapping method, the coating method and the wrapping method are more suitable for the practical industrial production. Vapor deposition methods have been studied, but are limited by strict process environment conditions and small PTFE membrane processing dimensions, and have not been suitable for large-scale industrial production of PTFE separation membranes for a while. The polymerization method has more related researches, mainly focuses on the hydrophilization modification of the PTFE microporous membrane and almost has no pore size regulation effect, such as patents CN 103191654A, CN 103394297A and CN 110394068A.
The coating method is that a layer of polymer solution or particles is uniformly attached to the surface of the prepared PTFE hollow fiber membrane, and the membrane is formed after solidification, so that the microporous structure of the membrane is adjusted or modified. For example, chinese patent CN 1566204a discloses a method for preparing a PTFE composite flat membrane, which comprises continuously and uniformly coating a polyurethane or block polyether ester solution on the surface of a biaxially oriented PTFE microporous membrane, and drying at 80-190 ℃ to obtain a moisture permeable flat membrane. The membrane surface is a moisture-permeable compact sealing layer, is suitable for manufacturing breathable waterproof clothing fabrics, but has low air flux and can not meet the requirements of the filtration and separation processes. Chinese patent CN108144459A discloses a three-layer polytetrafluoroethylene hollow fiber composite membrane, which consists of a surface separation layer, a transition layer and a PTFE support layer, wherein the surface separation layer is a polymer membrane, and the transition layer has a bonding effect. The implementation of the transition layer increases the complexity of the film preparation process, and the process time is long (for example, the boric acid modifier needs to be insulated and dried), which is not favorable for industrial production.
Different from the coating method, the wrapping method is to cut the prepared PTFE flat sheet membrane with smaller pore diameter into specific width and wrap the PTFE flat sheet membrane on the outer surface of the hollow fiber membrane, and then to tightly compound the PTFE flat sheet membrane and the hollow fiber membrane into a whole through sintering treatment, so as to achieve the effects of reducing the pore diameter of the membrane and improving the filtration precision, such as Chinese patents CN 103386256A and CN 110394068A. Although the wrapping method is mature, the final membrane separation precision is limited by the aperture of a flat membrane, and a microporous structure composite membrane below 50nm is difficult to prepare; secondly, it is necessary to consider increasing the bonding strength between the wrapping film and the base film to avoid falling off.
For this reason, the present invention is further extended based on the work of the applicant's patents CN 102961976B, CN 105521716B and CN 105854639 a.
Disclosure of Invention
The invention provides a polytetrafluoroethylene hollow fiber composite membrane and a preparation method thereof, aiming at the technical problems that the skin layer of the product in the prior art is compact or the aperture can not be regulated to the ultrafiltration range, the bonding strength between the skin layer and a supporting layer is poor, the preparation process is complex and the like. The purpose is to improve the bonding strength of a cortex layer and a supporting layer, reduce the pore diameter of a membrane surface layer, improve the porosity, endow the PTFE hollow fiber membrane with dual performances of ultrafiltration interception and hydrophobic membrane separation, improve the filtration and separation efficiency of the PTFE hollow fiber membrane in membrane contactor processes such as membrane distillation, membrane absorption and the like, improve the pressure resistance and water quality impact capacity, reduce the leakage risk and prolong the service life of the PTFE hollow fiber membrane.
The invention also aims to simplify the preparation process of the PTFE hollow fiber composite membrane with the asymmetric microporous structure and improve the production efficiency and the product quality.
In order to achieve the purpose, the invention provides the following technical scheme:
the polytetrafluoroethylene hollow fiber composite membrane is characterized by being of a double-layer structure and comprising a hydrophilic ultrafiltration epidermal layer on the outer side and a hydrophobic PTFE supporting layer on the inner side, wherein the initial contact angle of the ultrafiltration epidermal layer is 20-85 degrees, the thickness of the ultrafiltration epidermal layer is 2-50 microns, micropores on the surface of the ultrafiltration epidermal layer are circular and uniformly distributed, the average pore diameter is 5-100 nm, and the cross section of the ultrafiltration epidermal layer is of a spongy pore structure.
The initial contact angle of the hydrophobic PTFE support layer is 108-158 degrees, the microporous structure of the hydrophobic PTFE support layer is a dotted line-shaped separation hole formed by gaps among micro fibers, the average pore diameter of the separation hole is 750-4000 nm, and the porosity is 65-85%.
Preferably, the hydrophobic PTFE support layer has an initial contact angle of 120 ° to 158 °, thereby increasing its instantaneous water penetration pressure resistance. The average pore diameter of the micropores of the hydrophobic PTFE support layer is 1000nm-2000nm, which is beneficial to synchronously improving the porosity of the PTFE membrane, increasing the permeability of the PTFE membrane and improving the membrane separation efficiency. The increase of the aperture of the hydrophobic PTFE membrane supporting layer is based on the existence of the epidermal layer and the improvement of the contact angle of the supporting layer, and leakage can be prevented when the aperture is larger.
Preferably, the initial contact angle of the hydrophilic ultrafiltration epidermal layer is 40-80 degrees, the proper surface tension performance is kept, the buffer effect is exerted, the adsorption or interception of the hydrophilic ultrafiltration epidermal layer on organic matters in water is realized, and the wetting of the hydrophilic ultrafiltration epidermal layer on the composite membrane is slowed or avoided; the thickness of the hydrophilic ultrafiltration epidermal layer is 2-20 μm, the average pore diameter of the micropores is 10-30 nm, so that the hydrophilic ultrafiltration epidermal layer is endowed with smaller filtration resistance and higher ultrafiltration retention rate performance, and the high adaptability of the hydrophilic ultrafiltration epidermal layer to a feed liquid is improved.
Preferably, the hydrophilic ultrafiltration skin layer material is a flexible polymer, and the flexible polymer is more firmly and stably combined with the base film due to the flexibility and the adhesion performance of polyurethane, so the flexible polymer of the invention is further preferably thermoplastic polyurethane or a blend of the thermoplastic polyurethane and polysulfone, and the adhesion stability of the flexible polymer on the supporting layer is improved and the service life is prolonged by virtue of the excellent flexibility, aging resistance and high adhesion of the skin layer material.
The preparation method of the polytetrafluoroethylene hollow fiber composite membrane is characterized by comprising the following steps:
(1) preparation of support layer PTFE hollow fiber membrane
Sequentially carrying out heat treatment, stretching, sintering and shaping, cooling and rolling on the PTFE hollow fiber prepared by push forming to obtain a PTFE hollow fiber membrane with the stretching multiple of 300-600%;
(2) plasma activation treatment
Placing the PTFE hollow fiber membrane in a plasma filament treatment device, and carrying out dynamic plasma activation treatment on the stretched PTFE hollow fiber membrane to obtain an activated PTFE hollow fiber membrane;
(3) phase inversion method for preparing epidermal coating
And uniformly coating a flexible polymer solution on the surface of the activated PTFE hollow fiber membrane, and curing by a gel bath to obtain the PTFE hollow fiber composite membrane with the outer side being a hydrophilic ultrafiltration epidermal layer.
In the step (1), the pushing and forming process comprises the following steps: mixing PTFE dispersion resin and auxiliary oil according to the mass ratio of 3: 1-6: 1, performing, and then performing extrusion molding by paste extrusion equipment;
the heat treatment process comprises the following steps: carrying out heat treatment on the PTFE hollow fiber subjected to push forming at 300-350 ℃ to remove auxiliary oil;
the stretching process is as follows: stretching the PTFE hollow fiber after heat treatment at the temperature of between 20 and 95 ℃ with the stretching multiple of between 300 and 600 percent;
the sintering and shaping process comprises the following steps: sintering the stretched PTFE hollow fiber membrane at the temperature of 329-400 ℃ for 0.2-2 min to obtain the PTFE hollow fiber membrane with a microporous structure.
The invention improves the stretching multiple to more than 300 percent, can prepare the PTFE hollow fiber membrane with larger aperture and higher porosity, improves the surface roughness and the specific surface area thereof, and provides larger contact area for the attachment of the epidermal layer.
In the step (2), the electrode for plasma activation treatment is a pipeline electrode, the treatment speed is 3m/min-30m/min, the treatment power is 5kW-10kW, and the discharge distance is 3 mm-10 mm.
Through plasma activation treatment, the activity and roughness of the surface of the PTFE hollow fiber membrane can be improved, and the adhesion strength of the PTFE hollow fiber membrane and a skin layer is increased. The treatment method is simple, convenient, efficient, economic and pollution-free, and is suitable for large-scale production.
Preferably, in the step (2), during the plasma activation treatment, compressed air or a mixed gas of compressed air and oxygen is introduced into the conventional air atmosphere electrode, so that the treatment efficiency and the treatment effect can be improved.
In the step (3), the soft polymer solution is prepared by uniformly dissolving 10 to 20 weight percent of soft polymer in a solvent and simultaneously adding 10 to 30 weight percent of diluent, 0.5 to 3 weight percent of swelling agent and 0.2 to 2 weight percent of non-solvent; the diluent, the swelling agent and the non-solvent added in the invention are used for adjusting the structure and the performance of the micropores of the hydrophilic ultrafiltration epidermal layer and improving the affinity between the polymer solution and the solid epidermal layer and the supporting layer.
The specific process of coating the flexible polymer solution on the surface of the PTFE hollow fiber membrane comprises the following steps: and (3) stably injecting the flexible polymer solution into the die, controlling the thickness of the coating through the die, and uniformly coating the flexible polymer solution on the surface of the PTFE hollow fiber membrane continuously passing through the die at the coating speed synchronous with the processing speed of the plasma activation processing process.
The gel bath curing process is as follows: and the coated PTFE hollow fiber membrane enters a gel bath for phase separation and solidification after passing through an air stroke of 50-200 mm to prepare the required PTFE hollow fiber composite membrane with the microporous structure. The gel bath is a liquid phase gel bath containing a non-solvent, can be a non-solvent gel bath, and can also be a mixed solution gel bath of the non-solvent and the solvent, and the non-solvent and the solvent can be the same as the non-solvent and the solvent in the polymer solution.
As a preferred choice of the invention, the flexible polymer selected by the invention is thermoplastic polyurethane or a blend of thermoplastic polyurethane and polysulfone; the solvent is dimethylformamide or dimethylacetamide; the diluent is polar micromolecular organic matter such as butanone, toluene, tetrahydrofuran, chloroform, ethyl acetate and the like; the swelling agent is inorganic salt such as zinc chloride, lithium chloride, sodium chloride, magnesium chloride, etc.; the non-solvent is water, acetic acid, ethanol, glycerol, polyethylene glycol, etc.
In order to increase the contact angle of the PTFE hollow fiber membrane, in the step (1), the preparation of the support layer PTFE hollow fiber membrane further includes a hydrophobic finishing step, specifically: soaking the sintered and shaped PTFE hollow fiber membrane in a fluorine-silicon hydrophobic agent, drying at 40-80 ℃ for 4-10 s, cooling and rolling to obtain the PTFE hollow fiber membrane.
Preferably, the fluorine-silicon hydrophobic agent is perfluorinated long-chain chlorosilane or oxysilane. When in use, the fluorine-silicon hydrophobic agent is diluted by normal hexane to the concentration of 0.3 wt% -5 wt%.
The coating of the invention is that the coating thickness is controlled by a mould, the outer surface of the PTFE hollow fiber membrane continuously passing through the mould is uniformly coated with polymer solution, and the outer surface is solidified by gel bath phase separation to form an epidermal layer, thus realizing the continuous and stable coating preparation of the composite membrane. Such coating techniques are well known to those skilled in the art and the present invention is not described in excess.
In addition, the coating in the preparation method is only limited to adding the epidermal layer on the outer surface of the PTFE hollow fiber membrane, so that the defect of macropores on the surface layer of the PTFE hollow fiber membrane in the process of stretching and pore-forming can be overcome to a certain extent, and the internal micropore structure of the PTFE hollow fiber membrane is not modified or changed.
Compared with the prior art, the invention has the beneficial effects that: the surface layer of the PTFE hollow fiber composite membrane is of a hydrophilic ultrafiltration structure, has uniform micropores and smaller pore diameter, and can effectively intercept particulate matters, colloids and macromolecular organic matters and prevent the hydrophobic layer of the body from being polluted and wetted and penetrated; the PTFE supporting layer has larger aperture and high porosity, is beneficial to the high-efficiency permeation of gas and has high separation and filtration efficiency. The composite membrane has high pressure impact resistance and water impact risk capability in the application process. Therefore, the PTFE hollow fiber membrane with the composite structure has lasting hydrophobicity and high-efficiency separation performance. Meanwhile, the preparation method is simple, convenient, efficient, economical and environment-friendly, and the prepared composite membrane has good stability.
Drawings
FIG. 1 is a flow chart of a method for preparing a PTFE hollow fiber composite membrane according to the present invention;
FIG. 2 is a scanning electron microscope image of the longitudinal section, inner surface and outer surface of the PTFE hollow fiber composite membrane prepared in example 1 of the present invention;
FIG. 3 is a scanning electron microscope image of the whole cross section and a part of the cross section of a PTFE hollow fiber composite membrane prepared in example 1 of the present invention;
FIGS. 4a and 4b are scanning electron micrographs of the skin section and the outer surface of the PTFE hollow fiber composite membrane prepared in example 2 of the present invention, respectively;
FIGS. 5a and 5b are scanning electron micrographs of the skin section and the outer surface of the PTFE hollow fiber composite membrane prepared in example 3 of the present invention, respectively;
FIGS. 6a and 6b are scanning electron micrographs of a skin section and an outer surface of a PTFE hollow fiber composite membrane prepared in comparative example 1 of the present invention, respectively;
FIG. 7 is a scanning electron microscope image of a longitudinal section of a bonding portion between a skin layer and a support layer of a PTFE hollow fiber composite membrane prepared in comparative example 2 of the present invention;
FIGS. 8a and 8b are SEM images of the skin section and the outer surface of the PTFE hollow fiber composite membrane prepared in comparative example 3 of the present invention, respectively.
Detailed Description
The invention is further illustrated by the following examples. It should be understood that these examples are only for illustrating the contents of the present invention and are not intended to limit the scope of the present invention. Furthermore, it should be understood that various changes and modifications of equivalent forms to the present invention by those skilled in the art after reading the contents of the present invention also fall within the scope of the present invention defined by the claims.
Before processing, the PTFE dispersion resin is subjected to push forming and heat treatment to remove the auxiliary agent, and then the obtained PTFE hollow fiber is subjected to stretching operation, wherein the processes of push forming, heat treatment and agent removal are as follows: selecting PTFE dispersion resin with the molecular weight of 100-500 ten thousand, the compression ratio of 100-3000 times and the crystallinity of more than 98 percent to be mixed with auxiliary oil, preforming, and then pressing and molding by paste extrusion equipment to prepare nascent PTFE hollow fiber; the primary PTFE hollow fiber is thermally treated at 300-350 ℃ to remove the auxiliary oil.
In the present invention, the PTFE dispersion resin and the auxiliary oil are preferably mixed in the following ratio by weight: 3: 1-6: 1.
Then stretching the heat-treated nascent PTFE hollow fiber at the temperature of between 20 and 95 ℃ with the stretching multiple of between 300 and 600 percent; sintering and shaping the stretched PTFE hollow fiber with the microporous structure under the action of tension, wherein the sintering temperature is as follows: and sintering at the temperature of 329-400 ℃ for 0.2-2 min to obtain the PTFE hollow fiber with the microporous structure.
The above treatment method is disclosed in patent CN 102961976B by the inventor, based on which the present invention prepares PTFE hollow fiber membrane with stretching ratio of 300% -600%, the conventional method has stretching ratio of more than 300% and can not be used in membrane contactor process, and the present invention makes the membrane obtained after treating PTFE hollow fiber more favorable for skin layer adhesion and fit for use when the stretching ratio is more than 300%. Realizes the effective control of the microporous structure and obtains the PTFE hollow fiber membrane with the required pore size, higher porosity and high mechanical strength.
In the embodiment of the present invention, the processes of press molding, heat treatment and removal of the auxiliary agent for the PTFE hollow fiber are as follows: selecting PTFE F205 dispersion resin from Dajin company and Isopar G auxiliary agent from Exxon Mobil company according to the mass ratio of 4:1, curing for 24 hours at 60 ℃, and storing at room temperature for later use; filling the mixed raw materials into a pre-forming material cylinder to prepare a phi 30/16mm parison; filling the parison into a cylinder of a push-forming device, controlling the temperature of a die orifice to be 50 ℃ and the filament outlet speed to be 20m/min, and preparing the primary PTFE hollow fiber with the diameter of phi 1mm/0.7 mm; finally, the as-spun PTFE hollow fiber was subjected to heat treatment in a volatilization furnace to remove the auxiliary oil and strengthen the mechanical strength, to thereby obtain a PTFE hollow fiber, and the obtained PTFE hollow fiber was used in the following examples and comparative examples.
The process and specific parameters of the press molding, heat treatment and removal of the auxiliary are only limited for convenience of description of the present embodiment and comparison of the results of the embodiment, and the process and parameters of the press molding, heat treatment and removal of the auxiliary according to the present invention are not limited to the above description.
Before introducing the embodiments, first, a brief description will be given of each parameter measurement method involved in the embodiments of the present invention.
The initial contact angle is measured by a hanging drop type contact angle measuring instrument and is the contact angle measured when a membrane wire is in contact with a water drop for 2s, and the test method refers to the standard HY/T266-2018. The characteristics of the microporous structure (aperture and form) of the membrane are obtained by analyzing a field emission scanning electron microscope. The pore diameter of the PTFE hollow fiber membrane is measured by a capillary flow pore diameter analyzer, and the test method refers to CN 201510569827.9. The porosity of the PTFE hollow fiber membrane is measured and calculated by a gravimetric method, the thickness of each part of the composite membrane is obtained by testing through an optical microscope, and the testing method refers to the standard HY/T110-2008. The membrane water-resistant penetration pressure test adopts an external pressure mode, namely a water pump is used for slowly pressurizing the outer side of the membrane wire in a gradient of 10kPa, the pressure is kept for 1min after each pressure rise, the membrane wire is observed by naked eyes to have a leakage phenomenon, and the corresponding pressure value when the membrane leaks is the leakage pressure. The peeling strength test of the membrane epidermal layer and the supporting layer is obtained by adopting a test of a 0.1N electronic universal tensile machine, and the method refers to GB/T34444-.
Example 1:
the process of the PTFE hollow fiber composite membrane preparation of this example was performed according to fig. 1, but the present example does not involve a hydrophobic finishing step, and specifically the following operations were performed:
step 1, stretching PTFE hollow fibers prepared by push forming and heat treatment after removing auxiliaries, wherein the stretching ratio is 300 percent, and the stretching temperature is 30 ℃; and sintering and shaping at 360 ℃ for 2min after stretching, and rolling after cooling to obtain the PTFE hollow fiber membrane.
And 2, carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane prepared by stretching, wherein a pipeline type electrode is used, the treatment speed is 6m/min, the treatment power is 10kW, the treatment distance is 3mm, and the electrode is positioned in the air atmosphere.
Step 3, first, a polymer solution is prepared: dissolving thermoplastic polyurethane in a dimethylformamide solvent to prepare a solution with the polymer concentration of 15 wt%, adding chloroform with the total mass of 15 wt% of the polymer solution, 0.5wt% of lithium chloride and 1 wt% of acetic acid in the mixing process, mechanically stirring at 50 ℃ to prepare a uniform solution, and defoaming for later use; next, coating a polymer solution: the double-layer nested mechanical die is adopted for coating, and the process is as follows: continuously passing a PTFE hollow fiber membrane from the inside to serve as an inner layer, continuously and uniformly injecting a polymer solution into an outer layer, coating the surface of the PTFE hollow fiber membrane subjected to plasma activation treatment, wherein the coating thickness is 50 mu m, and the coating speed is 6 m/min; finally, the gel is cured to form a film: and (3) allowing the PTFE hollow fiber membrane after coating to pass through an air path of 150mm, then allowing the PTFE hollow fiber membrane to enter a pure water gel bath at 15 ℃, allowing the PTFE hollow fiber membrane to dynamically stay for 15s, curing to form a membrane, further allowing the PTFE hollow fiber membrane to pass through a pure water tank at 30 ℃ and washing for 200s to obtain the PTFE hollow fiber composite membrane with the asymmetric microporous structure, wherein the number of the PTFE hollow fiber composite membrane is 1 #.
The microporous structure of the prepared 1# PTFE hollow fiber composite membrane was analyzed, and the results are shown in FIGS. 2 and 3. As can be seen from both the longitudinal section of FIG. 2 and the cross section of FIG. 3, the surface of the PTFE hollow fiber membrane is compounded with an ultrafiltration skin layer with a thickness of approximately 50 μm, and the skin layer is tightly combined with the support layer without obvious limit. The section of the surface layer is of a spongy pore structure, micropores on the surface are round and uniformly distributed, and the pore diameter is 40 nm; the supporting layer is a dotted line-shaped hole, the average pore diameter is 750nm, and the porosity is 65%. Further, the initial contact angle of the composite film was tested, the initial contact angle of the inner surface was 112 ° and the initial contact angle of the outer surface was 83 °.
Example 2:
the preparation process of the PTFE hollow fiber composite membrane of this example is shown in fig. 1, and the specific operations are as follows:
step 1, adopting PTFE hollow fiber which is the same as that in the step 1 of the embodiment 1 to stretch, wherein the stretching ratio is 600 percent, and the stretching temperature is 30 ℃; after stretching, sintering and shaping at 400 ℃ for 1min to obtain the PTFE hollow fiber membrane; further, the PTFE hollow fiber membrane is soaked for 10 seconds at 40 ℃ by a perfluorodecyl trichlorosilane hydrophobic agent diluted by normal hexane to a concentration of 3wt%, and is cooled and rolled after being dried, so that the PTFE hollow fiber membrane with stronger hydrophobicity is prepared.
And 2, carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane, wherein a pipeline type electrode is used, the treatment speed is 3m/min, the treatment power is 5kW, the treatment distance is 5mm, and the electrode is positioned in the air atmosphere.
Step 3, first, a polymer solution is prepared: dissolving polysulfone in a dimethylacetamide solvent to prepare a solution with a polymer concentration of 18 wt%, adding butanone 10wt%, sodium chloride 1 wt% and glycerol 0.5wt% of the total mass of the polymer solution during mixing, mechanically stirring at 50 ℃ to prepare a uniform solution, and defoaming for later use; next, coating a polymer solution: continuously and uniformly coating the polymer solution on the surface of the PTFE hollow fiber membrane subjected to plasma activation treatment in the same way as in example 1, wherein the coating thickness is 20 mu m, and the coating speed is 3 m/min; finally, the gel is cured to form a film: and (3) allowing the PTFE hollow fiber membrane after coating to pass through an air path of 150mm, then allowing the PTFE hollow fiber membrane to enter a pure water gel bath at 15 ℃, allowing the PTFE hollow fiber membrane to dynamically stay for 30s, curing to form a membrane, and further allowing the PTFE hollow fiber membrane to pass through a pure water tank at 30 ℃ and be cleaned for 360s to obtain the PTFE hollow fiber composite membrane with the asymmetric microporous structure, wherein the serial number is # 2.
The microporous structure of the prepared 2# PTFE hollow fiber composite membrane was analyzed, and the results are shown in FIG. 4a and FIG. 4 b. As can be seen from the figure, the thickness of the ultrafiltration epidermal layer is 20 μm, the section spongy pores are dense, the surface micropores are loose and uniform, the overall permeability is good, and the pore diameter is 100 nm. The average pore diameter of the supporting layer is 2000nm, and the porosity is 85%. The initial contact angles of the inner surface and the outer surface of the composite film are 158 degrees and 62 degrees respectively.
Example 3:
the preparation process of the PTFE hollow fiber composite membrane of this example is shown in fig. 1, and the specific operations are as follows:
step 1, stretching PTFE hollow fiber which is the same as that in the step 1 of the embodiment 1 by the stretching multiple of 500 percent at the stretching temperature of 30 ℃; sintering and shaping for 1min at 380 ℃ after stretching to obtain the PTFE hollow fiber membrane; further soaking the PTFE hollow fiber membrane in perfluorodecyl triethoxysilane which is diluted to 0.5wt% by normal hexane, soaking for 10s at 40 ℃, drying, cooling and rolling to obtain the PTFE hollow fiber membrane with stronger hydrophobicity.
And 2, carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane, wherein the used electrode is a pipeline type electrode, the treatment speed is 30m/min, the treatment power is 8kW, the treatment distance is 10mm, and continuous mixed gas of oxygen with the oxygen concentration of 10 v/v% and compressed air is introduced into the electrode.
Step 3, first, a polymer solution is prepared: dissolving thermoplastic polyurethane and polysulfone in a dimethylformamide solvent according to a mass ratio of 4:1 to prepare a solution with a polymer concentration of 15 wt%, adding ethyl acetate, magnesium chloride and polyethylene glycol in an amount of 20wt% and 1 wt% of the total mass of the polymer solution in the preparation process, mechanically stirring at 50 ℃ to prepare a uniform solution, and defoaming for later use; next, coating a polymer solution: continuously and uniformly coating the polymer solution on the surface of the PTFE hollow fiber membrane subjected to plasma activation treatment by adopting a mechanical die and a mode which are the same as those of the embodiment 2, wherein the coating speed is 30 m/min; finally, the gel is cured to form a film: and (3) allowing the PTFE hollow fiber membrane after coating to pass through a 100mm air path and then enter a 15 ℃ pure water gel bath, allowing the PTFE hollow fiber membrane to dynamically stay for 8s, curing to form a membrane, and then allowing the membrane to pass through a 30 ℃ pure water tank and washing for 100s to obtain the PTFE hollow fiber composite membrane with the asymmetric microporous structure, wherein the serial number is 3 #.
The microporous structure of the prepared 3# PTFE hollow fiber composite membrane was analyzed, and the results are shown in fig. 5a and 5 b. As can be seen from the figure, the thickness of the ultrafiltration skin layer is 20 μm, the cross section of the sponge-like dense structure is between the No. 1 and No. 2 composite membranes, and the surface pore size is 50 nm. The average pore diameter of the support layer is 1600nm, and the porosity is 78%. The initial contact angles of the inner surface and the outer surface of the composite film are 152 degrees and 20 degrees respectively.
Example 4:
the preparation process of the PTFE hollow fiber composite membrane of this example specifically operates as follows:
step 1, the PTFE hollow fiber used in step 1 of example 1 was stretched at a stretch ratio of 500% and a stretching temperature of 30 ℃, under the same conditions as in step 1 of example 1.
And 2, carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane, using a pipeline type electrode, wherein the treatment speed is 15m/min, the treatment power is 10kW, the treatment distance is 10mm, and continuous compressed air is introduced into the electrode.
Step 3, first, a polymer solution is prepared: dissolving thermoplastic polyurethane in a dimethylformamide solvent, preparing a solution with an organic matter concentration of 15 wt%, adding tetrahydrofuran, zinc chloride and ethanol, wherein the tetrahydrofuran, the zinc chloride and the ethanol account for 30wt% of the total mass of a polymer solution, 0.5wt% and 1 wt% of the total mass of the polymer solution, mechanically stirring at 50 ℃ to prepare a uniform solution, and defoaming for later use; next, coating a polymer solution: continuously and uniformly coating the polymer solution on the surface of the PTFE hollow fiber membrane subjected to plasma activation treatment in the same manner as in example 1, wherein the coating thickness is 10 mu m, and the coating speed is 15 m/min; finally, the gel is cured to form a film: and (3) allowing the PTFE hollow fiber membrane after coating to pass through an air path of 150mm, then allowing the PTFE hollow fiber membrane to enter a pure water gel bath at 15 ℃, allowing the PTFE hollow fiber membrane to dynamically stay for 15s, curing to form a membrane, and further allowing the PTFE hollow fiber membrane to pass through a pure water tank at 30 ℃ and washing for 150s to obtain the PTFE hollow fiber composite membrane with the asymmetric microporous structure, wherein the number of the PTFE hollow fiber composite membrane is 4 #.
The microporous structure of the prepared 4# PTFE hollow fiber composite membrane is analyzed, the thickness of the ultrafiltration skin layer is 30 microns, the surface aperture is 10nm, and the characteristics of the section microporous structure are the same as the skin layer of the 1# PTFE hollow fiber composite membrane, and the membrane is in a spongy structure. The average pore diameter of the support layer is 1600nm, and the porosity is 78%. The initial contact angles of the inner surface and the outer surface of the composite film are respectively 120 degrees and 58 degrees.
Example 5:
the preparation process of the PTFE hollow fiber composite membrane of this example specifically operates as follows:
step 1, stretching was performed using the same PTFE hollow fiber as in step 1 of example 2, with a stretch ratio of 400% and a stretching temperature of 90 ℃, and the other conditions were the same as in step 1 of example 2.
And 2, carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane, wherein a pipeline type electrode is used, the treatment speed is 20m/min, the treatment power is 5kW, the treatment distance is 10mm, and the electrode is positioned in the air atmosphere.
Step 3, first, a polymer solution is prepared: dissolving thermoplastic polyurethane and polysulfone in a dimethylformamide solvent according to a mass ratio of 4:1 to prepare a solution with an organic matter concentration of 10wt%, adding ethyl acetate, zinc chloride and acetic acid, wherein the total mass of the polymer solution is 30wt%, 3wt% and 0.2wt% in the preparation process, mechanically stirring at 50 ℃ to prepare a uniform solution, and defoaming for later use; next, coating a polymer solution: the polymer solution is continuously and uniformly coated on the surface of the PTFE hollow fiber membrane subjected to plasma activation treatment in the same way as in example 1, the coating thickness specification is 5 mu m, and the coating speed is 20 m/min; finally, the gel is cured to form a film: and (3) allowing the coated PTFE hollow fiber membrane to pass through a 50mm air path and then enter a 15 ℃ pure water gel bath, allowing the PTFE hollow fiber membrane to dynamically stay for 8s, curing to form a membrane, and then allowing the membrane to pass through a 30 ℃ pure water tank and washing for 60s to obtain the PTFE hollow fiber composite membrane with the asymmetric microporous structure, wherein the serial number of the PTFE hollow fiber composite membrane is 5 #.
The microporous structure of the prepared 5# PTFE hollow fiber composite membrane is analyzed, the thickness of an ultrafiltration skin layer is 5 microns, the surface aperture is 95nm, and the characteristics of a section microporous structure are the same as those of a skin layer of a 1# PTFE hollow fiber composite membrane, and the structure is in a sponge structure. The average pore diameter of the support layer is 3870nm, and the porosity is 72%. The initial contact angles of the inner surface and the outer surface of the composite film are respectively 156 degrees and 76 degrees.
Comparative example 1:
step 1: a PTFE hollow fiber membrane was produced according to the same method and conditions as in step 1 of example 1.
Step 2: preparation of the polymer solution: thermoplastic polyurethane is dissolved in a dimethylformamide solvent to prepare a solution with the polymer concentration of 15 wt%, butanone accounting for 30wt% of the total mass of the polymer solution is added in the preparation process, and the solution is mechanically stirred at 50 ℃ to prepare a uniform solution for later use after defoaming.
And step 3: the polymer solution is continuously and uniformly coated on the surface of the PTFE hollow fiber membrane by adopting the method and the conditions which are the same as the steps 3 in the embodiment 1, and the PTFE hollow fiber composite membrane with the serial number of 6 is prepared after the curing and the drying at the temperature of 80 ℃ in an oven.
The microporous structure of the prepared # 6 PTFE hollow fiber composite membrane was analyzed, and the results are shown in FIGS. 6a and 6 b. As can be seen, the thickness of the hydrophilic ultrafiltration skin layer is 50 μm, and the cross section and the surface are dense and non-porous.
Comparative example 2:
step 1: a PTFE hollow fiber membrane was produced according to the same method and conditions as in step 1 of example 1.
Step 2: a composite membrane No. 7 was directly coated on the surface of the PTFE hollow fiber membrane in the same manner and under the same conditions as in step 3 of example 4.
When the microporous structure of the prepared 7# PTFE hollow fiber composite membrane is analyzed, the hydrophilic ultrafiltration epidermal layer and the supporting layer of the microporous structure are not obviously different from the microporous structure of the 4# PTFE hollow fiber composite membrane. However, the bond strength between the ultrafiltration skin layer and the support layer was poor and evidence of skin layer exfoliation was observed, as shown in fig. 7.
Comparative example 3:
step 1, a PTFE hollow fiber membrane was produced according to the same method and conditions as in step 1 of example 1.
Step 2, a dynamic plasma activation treatment was performed on the PTFE hollow fiber membrane in the same manner as in step 2 in example 1.
Step 3, first, a polymer solution is prepared: dissolving thermoplastic polyurethane in a dimethylformamide solvent, preparing a solution with the organic matter concentration of 18 wt%, mechanically stirring at 50 ℃ to prepare a uniform solution, and defoaming for later use; next, coating a polymer solution: a double-layer nested mechanical die is adopted, the coating thickness specification is 40 mu m, the polymer solution is continuously and uniformly coated on the surface of the PTFE hollow fiber membrane subjected to plasma activation treatment in the same way as in the embodiment 1, and the speed is 6 m/min; finally, the gel is cured to form a film: after the PTFE hollow fiber membrane coated passes through an air path of 100mm, the gel curing film forming method which is the same as that in the step 3 of the embodiment 1 is adopted to prepare the PTFE hollow fiber composite membrane with the asymmetric micropore structure, and the serial number is 8 #.
The microporous structure of the prepared No. 8 PTFE hollow fiber composite membrane was analyzed, and the results are shown in FIGS. 8a and 8 b. The thickness of the hydrophilic ultrafiltration epidermal layer is 40 μm, the section is finger-shaped holes, a more compact cortex exists on the outer surface of the hydrophilic ultrafiltration epidermal layer, and the surface aperture is 5 nm. The initial contact angles of the inner surface and the outer surface of the composite film are 115 degrees and 78 degrees respectively.
The peel strength between the skin layer and the support layer of the obtained PTFE hollow fiber composite membrane and the water penetration pressure (external pressure type) were measured, and the results are shown in table 1.
TABLE 1 test data for 8 samples in examples and comparative examples
Figure BDA0002401881570000111
The peel strengths of the 8 sample skin layers and the support layer prepared in the comparative examples 1 to 5 and the comparative examples 1 to 3 show that the adhesion of the skin layer on the surface of the PTFE hollow fiber membrane is effectively improved and the problem that the skin layer on the surface of the PTFE membrane is easy to fall off is solved by improving the stretching ratio, performing plasma activation on the PTFE hollow fiber membrane and optimizing the formula and process of the polymer solution. Meanwhile, compared with samples 1# and 7# and by adopting the same PTFE hollow fiber membrane supporting layer, the sample 1# shows stronger epidermal layer adhesion after being subjected to plasma activation treatment, and the water penetration pressure is also obviously improved.
In step, the multi-process preparation method of the PTFE hollow fiber composite membrane also realizes the regulation and control of the microporous structure of the epidermal layer of the composite membrane, such as the graph shown in figures 3-8, and endows the membrane with more excellent physical and chemical properties.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims (21)

1. The polytetrafluoroethylene hollow fiber composite membrane is characterized by being of a double-layer structure and comprising a hydrophilic ultrafiltration epidermal layer on the outer side and a hydrophobic PTFE support layer on the inner side, wherein the initial contact angle of the hydrophilic ultrafiltration epidermal layer is 58-85 degrees, the thickness of the hydrophilic ultrafiltration epidermal layer is 2-50 microns, and the average pore diameter of micropores is 5-100 nm; the initial contact angle of the hydrophobic PTFE support layer is 108-158 degrees;
the preparation method of the composite membrane comprises the following steps: (1) support layer PTFE hollow fiber membrane preparation: preparing PTFE hollow fibers into a PTFE hollow fiber membrane with the stretching multiple of 300-600%; (2) plasma activation treatment: carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane to obtain an activated PTFE hollow fiber membrane; wherein the plasma activation treatment is carried out in a plasma wire treatment device, the electrode is a pipeline type electrode, the treatment speed is 3-30 m/min, the treatment power is 5-10 kW, and the discharge distance is 3-10 mm; (3) phase inversion method skin layer coating preparation: and uniformly coating a flexible polymer solution on the surface of the activated PTFE hollow fiber membrane, and curing by a gel bath to obtain the PTFE hollow fiber composite membrane with the outer side being a hydrophilic ultrafiltration epidermal layer.
2. The composite membrane of claim 1 wherein the hydrophobic PTFE support layer has an average pore size of 750nm to 4000nm and a porosity of 65% to 85%.
3. The composite membrane according to claim 1, wherein the hydrophobic PTFE support layer has an initial contact angle of 120 ° to 158 °, and the microporous structure of the hydrophobic PTFE support layer is a dotted line of discrete pores formed by gaps between the microfine fibers, and the average pore diameter of the micropores is 1000nm to 2000 nm.
4. The composite membrane according to claim 1, wherein the initial contact angle of the hydrophilic ultrafiltration skin layer is 58 ° to 80 °, the micropores on the surface of the hydrophilic ultrafiltration skin layer are circular and uniformly distributed, the average pore size is 10nm to 30nm, the thickness is 2 μm to 20 μm, and the cross section of the hydrophilic ultrafiltration skin layer is a spongy pore.
5. The composite membrane of claim 1 wherein the hydrophilic ultrafiltration skin layer material is a flexible polymer.
6. The composite membrane of claim 5 wherein the hydrophilic ultrafiltration skin layer is a thermoplastic polyurethane or a blend of a thermoplastic polyurethane and polysulfone.
7. A preparation method of a polytetrafluoroethylene hollow fiber composite membrane is characterized by comprising the following steps:
(1) preparation of support layer PTFE hollow fiber membrane
Preparing PTFE hollow fibers into a PTFE hollow fiber membrane with the stretching multiple of 300-600%;
(2) plasma activation treatment
Carrying out dynamic plasma activation treatment on the PTFE hollow fiber membrane to obtain an activated PTFE hollow fiber membrane; wherein the plasma activation treatment is carried out in a plasma wire treatment device, the electrode is a pipeline type electrode, the treatment speed is 3-30 m/min, the treatment power is 5-10 kW, and the discharge distance is 3-10 mm;
(3) phase inversion method for preparing epidermal coating
Uniformly coating a flexible polymer solution on the surface of the activated PTFE hollow fiber membrane, and curing the PTFE hollow fiber membrane through a gel bath to obtain the PTFE hollow fiber composite membrane with the outer side being a hydrophilic ultrafiltration epidermal layer; wherein the average pore diameter of micropores of the hydrophilic ultrafiltration skin layer is 5nm-100nm, and the average pore diameter of separation pores of the PTFE support layer is 750nm-4000 nm.
8. The preparation method according to claim 7, wherein in the step (1), the PTFE hollow fiber is prepared by extrusion molding of PTFE dispersion resin, and then the PTFE hollow fiber membrane with the stretching ratio of 300-600% is obtained after heat treatment, stretching, sintering and shaping are carried out in sequence, cooling and rolling.
9. The production method according to claim 8, wherein in the step (1), the press molding is performed by: mixing and preforming PTFE dispersion resin and auxiliary oil according to the mass ratio of 3: 1-6: 1, and further performing extrusion molding by paste extrusion equipment.
10. The method according to claim 8, wherein the heat treatment process is: and (3) carrying out heat treatment on the PTFE hollow fiber subjected to push forming at 300-350 ℃ to remove the auxiliary oil.
11. The method of claim 8, wherein the stretching process comprises: and (3) stretching the heat-treated PTFE hollow fiber at the temperature of 20-95 ℃, wherein the stretching ratio is 300-600%.
12. The method according to claim 8, wherein the sintering and shaping process comprises: sintering the stretched PTFE hollow fiber membrane at the temperature of 329-400 ℃ for 0.2-2 min to obtain the PTFE hollow fiber membrane with a microporous structure.
13. The method according to claim 7, wherein in the step (2), compressed air or a mixed gas of compressed air and oxygen is introduced into the conventional air atmosphere electrode during the plasma activation treatment.
14. The method according to claim 7, wherein in the step (3), the soft polymer solution is prepared by: uniformly dissolving 10-20 wt% of flexible polymer in a solvent, and simultaneously adding 10-30 wt% of diluent, 0.5-3 wt% of swelling agent and 0.2-2 wt% of non-solvent.
15. The method according to claim 7, wherein in the step (3), the step of applying the soft polymer solution comprises: and (3) stably injecting the flexible polymer solution into the die, controlling the thickness of the coating through the die, and uniformly coating the polymer solution on the surface of the PTFE hollow fiber membrane continuously passing through the die at the coating speed synchronous with the processing speed of the plasma activation processing process.
16. The preparation method according to claim 7, wherein in the step (3), the gel bath curing is carried out by: and (3) making the coated PTFE hollow fiber membrane pass through an air stroke of 50-200 mm, and then entering a gel bath for phase separation and solidification to prepare the required PTFE hollow fiber composite membrane with the microporous structure.
17. The method of claim 14, wherein the soft polymer is a thermoplastic polyurethane or a blend of a thermoplastic polyurethane and a polysulfone; the solvent is dimethylformamide or dimethylacetamide; the diluent is at least one of butanone, toluene, tetrahydrofuran, chloroform and ethyl acetate; the swelling agent is at least one of zinc chloride, lithium chloride, sodium chloride and magnesium chloride; the non-solvent is at least one of water, acetic acid, ethanol, glycerol and polyethylene glycol.
18. The preparation method according to claim 8, wherein in the step (1), the preparation of the support layer PTFE hollow fiber membrane further comprises a hydrophobic finishing step, specifically: and (3) soaking the sintered and shaped PTFE hollow fiber membrane in a fluorine-silicon hydrophobic agent, drying and rolling.
19. The method according to claim 18, wherein the fluorine-silicon hydrophobizing agent is an n-hexane solution of a fluorine-silicon hydrophobizing agent, and the content of the fluorine-silicon hydrophobizing agent is 0.3 wt% to 5 wt%.
20. The method according to claim 18, wherein the fluorine-silicon hydrophobic agent is a perfluoro-long-chain chlorosilane or an oxysilane.
21. The method of claim 18, wherein the drying temperature is 40 ℃ to 80 ℃ and the drying time is 4s to 10 s.
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