High-strength double-layer hollow fiber membrane and preparation method thereof

Abstract

The invention provides a high-strength double-layer hollow fiber membrane and a preparation method, and relates to the technical field of fiber materials, the high-strength double-layer hollow fiber membrane comprises a double-layer composite structure composed of an outer layer and an inner layer, the outer layer is of a compact structure, the inner layer is of a hollow porous structure, and interface fusion occurs at interlacing points of the outer layer and the inner layer. The double-layer composite structure with gradient modulus is constructed, so that the outer compact separation layer and the inner porous supporting layer form mechanical complementation, and the technical defects that a traditional single-layer hollow fiber membrane is prone to structural collapse and rapid in permeation flux attenuation under the operation conditions of high pressure and high mechanical strength are overcome; therefore, the defect that a single material is difficult to give consideration to both characteristic selective separation performance and high-strength mechanical supporting performance is overcome.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fiber materials, in particular to a high-strength double-layer hollow fiber membrane and a preparation method. BACKGROUND

[0002] Hollow fiber refers to a fiber-like separation medium with a continuous through-hole axial cavity, the cavity diameter accounts for 30%-70% of the fiber outer diameter, the wall layer is a porous structure (pore diameter 0.01-1 μm), and it has both fluid channel and separation functions; Hollow fiber membrane refers to a composite structure in which a selective separation layer is constructed on the wall layer of the hollow fiber, and the thickness of the separation layer is 10%-50% of the wall thickness. The innovation of focused weaving structure, especially the composite weaving based on different material hollow fibers, is applied in the fields of separation membranes, gas filtration, water treatment, etc. By optimizing the weaving parameters and weaving methods, the comprehensive performance of the membrane material is significantly improved, the combination of light weight and high strength of the membrane material is realized, the energy consumption is reduced, and the service life is prolonged.

[0003] The defects of traditional single-layer hollow fibers are usually insufficient mechanical strength, which leads to easy collapse. In the patent CN202510253209.7, the burst pressure of single-layer PMP fiber (polymethylpentene fiber) is about 2 bar under standard experimental conditions. In addition, the single-layer structure is prone to creep deformation under long-term pressure load, which causes the interface peeling between the dense layer and the porous support layer. Especially in the natural gas decarburization working condition containing trace plasticizing components (CO2 concentration > 20%), the traditional polyimide hollow fiber membrane will aggravate the structural instability due to the plasticizing effect, and the selective layer will be irreversibly damaged when the operating pressure exceeds 1.5 MPa. In the patent CN201610795980.8, the polyimide hollow fiber membrane has stability only under a pressure of 0.5 MPa. SUMMARY

[0004] Therefore, the present application provides a high-strength double-layer hollow fiber membrane and a method, so that the outer dense separation layer and the inner porous support layer form mechanical complementation, thereby solving the shortcomings that a single material cannot simultaneously consider the characteristic selective separation performance and the high-strength mechanical support performance.

[0005] To solve the above problems, the technical scheme adopted by the present application is as follows:

[0006] One of the objects of the present application is to provide a high-strength double-layer hollow fiber membrane, which comprises a double-layer composite structure composed of an outer layer and an inner layer, the outer layer is a dense structure, the inner layer is a porous structure in a hollow shape, and the outer layer and the inner layer are fused at the interlacing points.

[0007] In some embodiments, the inner layer is a high-strength engineering fiber material, and the engineering fiber material comprises at least one of PET, PBT, PA, aramid fiber, carbon fiber and glass fiber.

[0008] In some embodiments, the inner layer has an inner diameter of 1.8±0.05 mm and a porosity of 65-70%.

[0009] In some embodiments, the outer layer is a high separation selectivity fiber comprising at least one of PMP, PP, PVDF, PS, PES and CA.

[0010] In some embodiments, the outer layer has a thickness of 200±10 μm.

[0011] In some embodiments, at the interlaced points of the outer layer and the inner layer, the casting solution of the outer layer can penetrate into the pores of the inner layer and form interface fusion after solidification.

[0012] One of the purposes of the present application is to provide a preparation method of the high-strength double-layer hollow fiber membrane, comprising the following steps:

[0013] The braided tube of the inner layer and the casting solution of the outer layer are synchronously extruded by using a co-extrusion molding process to form a double-layer composite structure, and interface fusion occurs at the interlaced points of the outer layer and the inner layer.

[0014] In some embodiments, the temperature of the die cavity of the casting solution of the outer layer is controlled at 240±5°C to ensure the rheological properties of the casting solution melt, and the passage of the braided tube of the inner layer is maintained at a constant temperature of 60°C to transport the braided tube.

[0015] In some embodiments, the pressure of the die cavity of the casting solution of the outer layer is greater than the pressure of the passage of the braided tube of the inner layer.

[0016] In some embodiments, the ratio of the extrusion speed of the casting solution of the outer layer to the pulling speed of the braided tube of the inner layer is 1.2:1.

[0017] The present application adopts the above technical solution, and has the following beneficial effects:

[0018] The high-strength double-layer hollow fiber membrane and the method provided by the present application comprise a double-layer composite structure composed of an outer layer and an inner layer, the outer layer is a dense structure, the inner layer is a porous structure in a hollow shape, and interface fusion occurs at the interlaced points of the outer layer and the inner layer. The present application forms mechanical complementation of the dense separation layer of the outer layer and the porous support layer of the inner layer by constructing a double-layer composite structure with a gradient modulus, overcomes the technical defects that the traditional single-layer hollow fiber membrane is prone to structural collapse and rapid attenuation of permeation flux under high pressure and high mechanical strength operating conditions, and thus solves the shortcomings that a single material is difficult to balance the characteristic selective separation performance and high-strength mechanical support performance. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments of the present application or the prior art description. Obviously, the drawings described below only constitute some of the embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0020] Figure 1 The structure schematic diagram of the inner layer provided by the embodiments of the present application is shown in the left drawing and the surface is shown in the right drawing.

[0021] Figure 2 The actual schematic diagram of the braided tubular object provided by the embodiments of the present application is shown.

[0022] Figure 3 The spinneret pattern drawing provided by the embodiments of the present application is shown. DETAILED DESCRIPTION

[0023] The embodiments of the present application will be described in detail below, and examples of the embodiments are shown in the drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are intended to explain the present application, and cannot be understood as a limitation of the present application.

[0024] In the description of the present application, it should be understood that the terms "upper", "lower", "horizontal", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.

[0025] In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0026] In order to make the purpose, technical solutions and advantages of the present application more clear and obvious, the following will further describe the present application in combination with the drawings and embodiments.

[0027] The structure schematic diagram of the high-strength double-layer hollow fiber membrane provided by the embodiments of the present application includes a double-layer composite structure composed of an outer layer and an inner layer, the outer layer is a dense structure, the inner layer is a porous structure in a hollow shape, and interface fusion occurs at the interlacing points of the outer layer and the inner layer. The following will describe the technical solutions realized in detail.

[0028] It can be understood that the present application creates a composite hollow fiber membrane structure. It is not simply physically superimposed, but through the ingenious material selection and preparation process, the functions of each layer are clear and synergistic, and finally the advantages of "high strength, high separation, high flux" which are difficult to be simultaneously considered for traditional hollow fiber membranes are realized. The inner layer is like a skeleton, providing core mechanical support; the outer layer is responsible for precise separation as a separation layer; and the interface fusion layer between the two is firmly combined with the inner and outer layers.

[0029] The specific implementation mode between each layer is described below.

[0030] Please refer to Figure 1 , the inner layer is a high-strength woven tube lining, which uses high-strength engineering fiber materials. For example: PET (polyester), PBT (polybutylene terephthalate), PA (nylon), aramid fiber, carbon fiber, glass fiber, etc. These are all high-strength (high tensile strength, high modulus) engineering material fibers.

[0031] Specifically, the key structural parameters of the inner layer are: an inner diameter of 1.8±0.05mm, and a porosity of 65-70% (high porosity ensures low flow resistance and does not affect the flux).

[0032] It can be understood that the inner layer is the core mechanical support: like the steel frame of a building, it bears all the tensile, bending, and extrusion stresses during the operation of the membrane, preventing the membrane from breaking or collapsing; it provides shape stability, which can inhibit the shrinkage or deformation of the outer layer polymer during molding and use, ensuring the long-term stability of the membrane hole structure.

[0033] The outer layer is a high-separation-selectivity fiber, including at least one of PMP, PP, PVDF, PS, PES, and CA. The above materials all have excellent chemical stability, hydrophobicity, and gas permeability, and are commonly used to prepare membrane distillation or gas separation membranes.

[0034] Specifically, the thickness of the outer layer is 200±10μm, and it is a defect-free dense structure. The dense and pinhole-free outer skin can ensure that only substances (such as specific gas molecules or water vapor) that meet the design requirements can pass through, while other substances are retained.

[0035] It can be understood that in this embodiment, the inner layer is a through-type finger-shaped pore. This pore structure has high porosity and low mass transfer resistance, which is the core of realizing high flux. The finger-shaped pores directly pass through the inner woven layer, providing a fast passage for the separated substances; the outer layer has selective separation characteristics, relying on its dense skin to achieve precise screening or selective permeation, and relying on the low-resistance mass transfer path provided by the inner layer finger-shaped pore structure.

[0036] Further, the interface fusion occurs at the interlacing points of the outer layer and the inner layer.

[0037] Specifically, at the interlaced point of the outer layer and the inner layer, the casting solution of the outer layer can penetrate into the pores of the inner layer and form a three-dimensional interface of mechanical interlocking and physical and chemical combination after solidification.

[0038] It can be understood that under the action of the three-dimensional interface, the external load can be effectively transmitted from the outer layer with lower strength to the inner layer with high strength, avoiding interlayer peeling; mechanical integrity is ensured, and if the interface combination is not firm, the two layers will separate under pressure fluctuation, resulting in membrane failure.

[0039] It can be understood that the outer layer, the inner layer and the interface are not isolated, but have a synergistic effect, which solves the shortcomings of single material that cannot simultaneously consider characteristic selective separation performance and high strength mechanical support performance, which will be described in detail as follows:

[0040] Firstly, the inner layer solves the strength problem, but the pore is too large and has no separation function, and the outer layer solves the separation and flux problem, but the pure polymer membrane has poor mechanical strength and is easy to break, so the combination of the two can make up for each other's shortcomings and achieve the comprehensive performance that single material cannot achieve.

[0041] Secondly, the strong inner layer allows the outer layer (separation layer) to be thinner and the pore to be more optimized without worrying about the strength problem, thereby further improving the flux; the inner layer inhibits the creep and shrinkage of the outer layer, ensures that the finger-shaped pore does not collapse for a long time, and maintains the stability of high flux; at the same time, the unique process ensures the interface fusion quality, so that the composite membrane becomes a whole rather than two layers that are easy to peel off, and the whole structure is organically combined and the performance is doubled.

[0042] The high-strength double-layer hollow fiber membrane provided by the application forms mechanical complementation between the dense separation layer of the outer layer and the porous support layer of the inner layer by constructing a double-layer composite structure with gradient modulus, overcomes the technical defects of traditional single-layer hollow fiber membranes that are prone to structural collapse and rapid decay of permeation flux under high pressure and high mechanical strength operating conditions, and solves the shortcomings of single material that cannot simultaneously consider characteristic selective separation performance and high strength mechanical support performance.

[0043] The application also provides a preparation method of the high-strength double-layer hollow fiber membrane, which comprises the following steps: synchronously extruding the braided tube of the inner layer and the casting solution of the outer layer by using a co-extrusion molding process to form a double-layer composite structure, and interface fusion occurs at the interlaced point of the outer layer and the inner layer.

[0044] Specifically, the following detailed steps are included:

[0045] Step 1: Prepare the casting solution of the outer layer, and the following takes the casting solution of PMP as an example for detailed description.

[0046] 20-50wt% of polymethylpentene (PMP) particles are added to a closed container with heating and stirring functions together with a mixed diluent (one or more of dioctyl phthalate, dibutyl phthalate, dioctyl adipate, trioctyl citrate).

[0047] The system is heated to a high temperature of 240-260°C and stirring is continued for 4-8 hours until the PMP is completely dissolved, forming a uniform, viscous, transparent solution. At the same time, nitrogen (N2) is introduced into the container to replace the air during the entire heating process to prevent the PMP polymer from being oxidatively degraded at high temperature.

[0048] After the dissolution is complete, the vacuum system is turned on and the casting solution is degassed for 4-8 hours. This is done to remove the tiny bubbles generated during stirring and heating, which, if left in the liquid, will cause pinholes, defects, and serious damage to the dense separation layer of the final film when it is formed.

[0049] Step 2: Preparation of the inner layer of the braided tube

[0050] Using a 36-spindle braiding machine, high-strength synthetic monofilaments (such as 50μm diameter PET monofilaments) are braided into a tubular shape. Please refer to Figure 2 .

[0051] The braiding parameters are precisely controlled to ensure that the braided tube produced meets the following core specifications:

[0052] Inner diameter: 1.8±0.05mm (extremely high precision requirements)

[0053] Porosity: 65%-70%

[0054] This braided tube, as a prefabricated "skeleton", needs to be prepared in advance and installed on the pay-off stand.

[0055] Step 3: Co-extrusion molding

[0056] A customized double-layer spinneret is used, please refer to Figure 3 , which includes:

[0057] Inner layer channel: a smooth channel with an inner diameter of 2.0mm for pulling the braided tube. There is a very fine gap of 0.2mm between the channel and the braided tube. This gap is crucial: it must be small enough to prevent reverse leakage of the PMP solution, but large enough to allow the braided tube to pass smoothly.

[0058] Outer layer channel: instead of a simple circular ring, it uses a spiral distribution of micro-holes design. These micro-holes allow the PMP solution to flow out uniformly and stably, forming a ring-shaped wrapping, avoiding turbulence or uneven thickness.

[0059] The specific process parameters are as follows:

[0060] The temperature control includes:

[0061] The outer layer die cavity: the temperature is strictly controlled at 240±5℃. The purpose is to maintain the good fluidity and appropriate viscosity of PMP casting solution, so that it is easy to extrude and spread uniformly.

[0062] The inner layer channel: the temperature is maintained at 60℃. This temperature preheats the braided tube, on the one hand to avoid the cold braided tube from causing local rapid cooling and solidification when it meets the high-temperature PMP solution, and on the other hand to promote the preliminary fusion of the two at the interface.

[0063] The pressure control includes:

[0064] The extrusion pressure of the inner and outer layers is adjusted by a high-precision PID control system.

[0065] The outer layer PMP solution pressure: 0.25±0.02MPa (falls within the optimal range of 0.1-0.4MPa)

[0066] The inner layer braided tube tension / pressure: 0.08±0.01MPa

[0067] It can be understood that the outer pressure is significantly greater than the inner pressure, which ensures that the PMP solution actively and tightly wraps around the outer periphery of the braided tube, rather than being "expanded" or "pushed back" by the braided tube, which is the key to achieving uniform coating.

[0068] The speed control includes:

[0069] The ratio of the extrusion speed of the PMP solution to the pulling speed of the braided tube is 1.2:1.

[0070] It can be understood that a slight "stretching" effect is produced. The extrusion speed is slightly faster than the pulling speed, so that the nascent fiber is moderately stretched in the air, which helps to orient the PMP molecular chains, thereby improving the mechanical properties of the film and affecting the final pore size.

[0071] Specifically, the simultaneous extrusion is implemented by customizing the spinneret, the outer layer die cavity temperature is controlled at 240±5℃ to ensure the rheological properties of the PMP melt, the inner layer channel maintains a constant temperature of 60℃ to deliver the braided tube, the extrusion pressure is accurately adjusted by the PID control system (0.25±0.02MPa for the outer layer and 0.08±0.01MPa for the inner layer), a double-layer composite structure is formed, and interface fusion occurs at the interlacing point of the outer layer and the inner layer.

[0072] Further, it further includes the step 4 of the microstructure shaping treatment.

[0073] It is understood that after extrusion from the spinneret, the nascent composite fiber will pass through an air gap distance of 0.1-30 cm before entering the coagulation bath. During this short air travel, the solvent on the surface of the casting solution will partially evaporate, and the concentration will change, which is crucial for the instantaneous formation of a dense, defect-free skin layer when entering the coagulation bath. This skin is the basis for the high separation selectivity of the membrane.

[0074] Further, the fiber passes through two coagulation bath tanks of different temperatures and compositions in sequence to complete solidification.

[0075] The first coagulation zone (instantaneous quenching) uses a water bath with a medium temperature of 30°C, and the residence time is 2 seconds.

[0076] It is understood that water is a non-solvent for the diluent of PMP. After contact, rapid and intense non-solvent-induced phase separation occurs. This process instantaneously freezes and solidifies the fiber's shape, forming the outermost dense separation layer and the rudiment of the internal finger-like pores. The low temperature aims to accelerate the phase separation speed.

[0077] The second coagulation zone (slow extraction) uses an ethanol bath with a medium temperature of 50°C, and the residence time is 8 seconds.

[0078] It is understood that ethanol is miscible with both water and the diluent of PMP. Its role is to gently and slowly extract and exchange the residual diluent inside the fiber. The higher temperature increases the molecular motion, making the solidification process more gradual, which is beneficial for the formation of the internal sponge-like porous support structure and the full development of the finger-like pores.

[0079] Through the gradient changes from "water bath" to "ethanol bath" and from "low temperature" to "high temperature", the casting solution is successfully induced to undergo phase separation to different degrees and speeds from the outside to the inside, ultimately forming a gradient pore structure with a dense outer surface, through finger-like pores in the inner layer, and a sponge layer in the middle.

[0080] Further, it also includes step 5, the post-processing and collection steps.

[0081] The fully solidified composite hollow fiber membrane is guided out by the guide roller, washed thoroughly with deionized water to remove residual solvents and coagulants. Finally, the finished fiber membrane is collected into a roll by a winding machine, completing the entire preparation process.

[0082] Through the above precisely designed steps and parameter control, the composite hollow fiber membrane described above is ultimately obtained: an outer layer of 200±10 μm defect-free dense skin, an internal through finger-like pore, and a perfect fusion with the high-strength woven tube lining.

[0083] The application provides a preparation method of a high-strength double-layer hollow fiber membrane, which realizes accurate compounding of two different polymer materials through a spinning head co-extrusion technology. The application overcomes the technical defects of traditional single-layer hollow fiber membranes, such as easy structural collapse under high pressure and high mechanical strength operating conditions and fast permeation flux attenuation, provides super-high mechanical strength by using a woven fabric, solves the deformation problem of a traditional solution spinning lining, and is suitable for harsh separation scenes such as high-pressure reverse osmosis, gas separation and carbon capture, water treatment and resource recovery, and cell / tissue culture reactors.

[0084] The above technical solutions of the application are described in detail below in combination with specific embodiments.

[0085] Embodiment

[0086] The specific operation is to prepare the composite fiber by using a dry-wet process, control the PMP casting solution extrusion pressure to be 0.1-0.4 MPa, the air gap to be 0.1-30 cm, the extrusion speed to be 1.2:1 of the weaving fabric traction speed, and the coagulation bath to adopt a temperature gradient design, that is, the first zone is a 30℃ water bath (stopping for 2 s), and the second zone is a 50℃ ethanol bath (stopping for 8 s) to solidify the sponge layer, so as to form a gradient pore size to induce the fiber to form the required pore structure; the spinning head is additionally provided with a weaving fabric traction channel (an inner diameter of 2 mm and a gap of 0.2 mm with the PET woven tube), so as to ensure that the weaving fabric passes through at a uniform speed; and the outer PMP solution extrusion channel adopts a spiral distribution micropore, so as to uniformly coat the weaving fabric.

[0087] The preparation method of the high-strength double-layer hollow fiber membrane comprises the following steps:

[0088] (1) Preparing a (PMP) outer layer casting solution: 20-50 wt% of polymethylpentene (PMP) is heated (240-260℃) and dissolved in a diluent (di-n-octyl phthalate, di-n-butyl phthalate, di-n-octyl adipate, tri-n-octyl citrate), nitrogen gas is introduced to protect the heating atmosphere, the heating time is 4-8 h, and then vacuum degassing treatment is performed for 4-8 h;

[0089] (2) Preparing a woven tube lining: a 36-spindle weaving machine is used to weave a tubular structure with an inner diameter of 1.8±0.05 mm and a porosity of 65-70% by using 50μm diameter chemical fiber high-strength monofilaments (PET, PBT, PA, aramid fiber, carbon fiber and glass fiber);

[0090] (3) Co-extrusion molding: synchronous extrusion is implemented by customizing a spinning head, the outer layer mold cavity temperature is controlled to be 240±5℃ to ensure the rheological properties of the PMP melt, the inner layer channel maintains a constant temperature of 60℃ to convey the woven tube, and the extrusion pressure (0.25±0.02 MPa for the outer layer and 0.08±0.01 MPa for the inner layer) is accurately adjusted by a PID control system;

[0091] The scanning electron microscope characterization shows that the obtained fiber skin layer thickness is 200±10 μm and presents a defect-free dense structure, and the inner support layer has no obvious shrinkage, forming a through type of finger-shaped pore channel.

[0092] According to the above scheme, the performance parameters of the high-strength double-layer hollow fiber membrane obtained by the application are as follows:

[0093]

[0094] As can be seen from the above examples, the high-strength double-layer hollow fiber membrane provided by the application overcomes the technical defects of the conventional single-layer hollow fiber membrane that is prone to structural collapse and rapid attenuation of permeation flux under high pressure and high mechanical strength operating conditions, thereby solving the shortcomings that a single material is difficult to balance the characteristic selective separation performance and high-strength mechanical support performance.

[0095] The above is only a preferred embodiment of the application, and only the technical principle of the application is specifically described, and these descriptions are only for explaining the principle of the application, and cannot be explained as a limitation on the protection scope of the application in any way. Based on the explanation here, any modification, equivalent replacement and improvement made within the spirit and principle of the application, and other specific embodiments of the application that can be thought of by those skilled in the art without creative labor, should be included in the protection scope of the application.

Claims

1. A high-strength double-layer hollow fiber membrane, characterized in that, It includes a two-layer composite structure consisting of an outer layer and an inner layer. The outer layer is a dense structure, and the inner layer is a hollow porous structure. Interface fusion occurs at the intersection of the outer and inner layers.

2. The high-strength double-layer hollow fiber membrane as described in claim 1, characterized in that, The inner layer is a high-strength engineering fiber material, which includes at least one of PET, PBT, PA, aramid, carbon fiber and glass fiber.

3. The high-strength double-layer hollow fiber membrane as described in claim 1 or 2, characterized in that, The inner diameter of the inner layer is 1.8±0.05mm, and the porosity is 65-70%.

4. The high-strength double-layer hollow fiber membrane as described in claim 1, characterized in that, The outer layer is a fiber with high separation selectivity, which includes at least one of PMP, PP, PVDF, PS, PES and CA.

5. The high-strength double-layer hollow fiber membrane as described in claim 1 or 4, characterized in that, The thickness of the outer layer is 200±10μm.

6. The high-strength double-layer hollow fiber membrane as described in claim 1, characterized in that, At the intersection of the outer and inner layers, the casting liquid of the outer layer can penetrate into the pores of the inner layer and form an interface fusion after solidification.

7. A method for preparing a high-strength double-layer hollow fiber membrane as described in claim 1, characterized in that, Includes the following steps: The inner braided tube and the outer casting liquid are simultaneously extruded using a co-extrusion molding process to form a double-layer composite structure, and interface fusion occurs at the interlacing points of the outer and inner layers.

8. The method for preparing a high-strength double-layer hollow fiber membrane as described in claim 7, characterized in that, The temperature of the mold cavity of the outer casting liquid is controlled at 240±5℃ to ensure the rheological properties of the casting liquid melt, and the braided tube channel of the inner layer is maintained at a constant temperature of 60℃ to transport the braided tube.

9. The method for preparing a high-strength double-layer hollow fiber membrane as described in claim 7, characterized in that, The pressure of the casting liquid in the outer layer's mold cavity is greater than the pressure of the braided tube channel in the inner layer.

10. The method for preparing a high-strength double-layer hollow fiber membrane as described in claim 7, characterized in that, The ratio of the extrusion speed of the casting solution in the outer layer to the traction speed of the braided tube in the inner layer is 1.2:1.

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