Preparation method of hollow fiber membrane for aircraft fuel tank inerting system

Abstract

The invention relates to the field of preparation of gas separation membranes, in particular to a preparation method of a hollow fiber membrane for an aircraft fuel tank inerting system. By optimizing the preparation method and process conditions in the spinning process of the dry-wet phase separation process, the hollow fiber membrane generates a compact skin layer with proper thickness and a support layer structure with proper density, so that the improvement and balance of multiple parameters such as gas permeability, gas selectivity, physical and mechanical properties, packaging strength and the like of the membrane silk are realized; the invention provides the preparation method of the hollow fiber membrane which can be used for an actual aircraft fuel tank inerting system.

Description

Technical Field

[0001] This invention relates to the field of gas separation membrane preparation, and specifically to a method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system. Background Technology

[0002] Fuel tank explosions are a major cause of aircraft accidents. Both domestic and international research has focused on reducing the flammability of fuel tanks to ensure aircraft fuel tank safety. Combustion requires an ignition source, fuel, and oxygen; fuel tank explosions can be prevented by removing any one of these three elements. Active explosion prevention technology, which involves introducing inert gas into the upper gas phase space of the fuel tank to control and maintain the oxygen volume fraction in the gas phase space, is the most widely used. Airborne nitrogen inerting technology, which uses hollow fiber membranes to produce nitrogen-rich gas, has been widely applied both domestically and internationally; therefore, the preparation methods of hollow fiber membranes are becoming increasingly important.

[0003] There are various methods for preparing hollow fibers, among which solution spinning is a relatively mature method. This method often uses a dry-wet spinning process. The general process involves preparing a spinning solution with the optimal casting solution composition, then degassing the prepared solution in a feed tank. After degassing, the solution continues to flow through a feed pipe into the spinning head and is sprayed out together with the prepared core solution. After passing through an air gap to allow some solvent to evaporate, the spinning solution enters a coagulation bath where it undergoes transformation and solidification to form a film. Finally, the nascent hollow fibers are wound onto the take-up roller by its traction.

[0004] Currently reported patents related to hollow fiber membrane preparation include improvements to the preparation process and optimizations through changes in the composition of the casting solution, core solution, or coagulation bath. The optimizations primarily aim to improve film quality and form a non-destructive skin layer. For example, Chinese patent CN106861447A discloses a method for preparing a polysulfone fiber membrane. By optimizing process conditions such as air gap distance, ambient air temperature and humidity, traction rate, water bath time, water washing time, and water washing composition, it prepares a polysulfone hollow fiber membrane suitable for use in the air separation component of an aircraft fuel tank inerting system. Chinese patent CN118698351A discloses a polyethersulfone / polysulfone hollow fiber membrane and its preparation method. This method increases the ultrafiltration coefficient of the dialysis membrane by adjusting the ratio of polysulfone to polyethersulfone, and adds hydrophilic materials to reduce the static contact angle of the membrane surface. This simplifies the manufacturing process while increasing the membrane's tensile strength and protein removal rate, and improving the effective pore size. Chinese patent CN113877443A discloses a spinning process for preparing a hollow fiber membrane with an asymmetric structure and a defect-free dense epidermis. By optimizing the ratio of added electrophilic reagents, a mixture of highly volatile and weakly volatile organic solvents, non-solvents, and electrolytes, a hollow fiber membrane without damage to the epidermis is prepared.

[0005] In summary, existing inventions often focus on a single core parameter in the preparation of hollow fiber membranes, enhancing the specific parameters of the membrane through optimization of process conditions and formulations. However, in practical applications, especially in aircraft fuel tank inerting systems, the performance requirements for hollow fiber membranes are multifaceted, demanding that all membrane parameters be appropriate and balanced. This is something not found in current literature. Furthermore, research on the physical and mechanical properties of hollow fiber membranes in practical applications and their bonding performance with encapsulation materials is also scarce. Summary of the Invention

[0006] Based on the problems summarized above, this invention provides a method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system. Its main feature is that, through innovative optimization of the preparation process, the prepared hollow fiber membrane meets the requirements of actual aircraft fuel tank inerting systems in several key performance aspects, including gas permeability, gas selectivity, physical and mechanical properties, and sealing tightness. All performance aspects are balanced and reasonable. The specific technical solution is as follows:

[0007] A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system includes the following steps:

[0008] Step 1: Mix polymer A, polymer additive B, and solvent C according to a certain mass fraction and stir until completely dissolved to obtain casting solution. Vacuum degas the casting solution for later use. Mix solvent and water evenly to prepare core solution. Vacuum degas the core solution for later use.

[0009] Step 2: The casting solution and core solution obtained in Step 1 are continuously and slowly sprayed from the spinning head at a specific flow rate, and pass through an air gap of a certain distance at a specific stretching speed to obtain nascent fibers;

[0010] Step 3: The nascent fibers obtained in Step 2 are subjected to sedimentation and stretching in multiple discontinuous coagulation baths and then drawn to a take-up spool to obtain a crude hollow fiber membrane.

[0011] Step 4: The crude hollow fiber membrane obtained in Step 3 is soaked in stages to achieve complete solvent exchange, and then processed by a ventilation and drying module to obtain a hollow fiber membrane for aircraft fuel tank inerting system.

[0012] Further, in step one, polymer A is polyimide, polymer additive B is polyvinylpyrrolidone, and solvent C is N-methylpyrrolidone;

[0013] The step one, which involves specifying a certain mass fraction, specifically includes: the mass fraction of polymer A is 28%, the mass fraction of polymer additive B is 7%, and the mass fraction of solvent C is 65%.

[0014] The complete dissolution described in step one includes specific parameters such as a dissolution time of 5 to 7 days and a dissolution temperature of 20 to 65°C.

[0015] The vacuum degassing described in step one has specific parameters including a degassing vacuum degree of 20-50 kPa and a degassing time of 18-24 hours.

[0016] The solvent and water described in step one are mixed evenly to prepare the core solution, wherein the solvent is N-methylpyrrolidone; and the water accounts for 80% by volume.

[0017] Further, the specific flow rate mentioned in step two, wherein the casting fluid flow rate is 7 ml / min and the core fluid flow rate is 5 ml / min;

[0018] The specific stretching speed mentioned in step two is 45 meters per minute;

[0019] The air gap mentioned in step two has a height of 5 centimeters.

[0020] Furthermore, in step three, a coagulation bath is provided in which a two-component mixture of water and isopropanol is added, wherein the isopropanol accounts for 30% by volume.

[0021] Furthermore, the discontinuous coagulation baths mentioned in step three are multiple coagulation baths with different contents temperatures.

[0022] Furthermore, the multiple coagulation baths with different contents temperatures include a first normal bath, a heating bath, and a second normal bath.

[0023] Furthermore, the length ratio of the first normal bath, the heated bath, and the second normal bath ranges from 3:1:10 to 3:5:10.

[0024] Furthermore, the internal liquid temperature of the first normal bath and the second normal bath is set to 15°C, and the internal liquid temperature range of the heated bath is set to 25-70°C.

[0025] Furthermore, polyethylene glycol or γ-butyrolactone is added to the contents of the heating bath.

[0026] Furthermore, the specific process of the staged soaking described in step four includes: first, soaking and rinsing in deionized water for 10 days, changing the deionized water every 12 hours, then soaking in ethanol for 1 hour, and then soaking in n-hexane for 30 minutes.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] This invention optimizes the hollow fiber membrane preparation process based on the actual performance requirements of hollow fiber membranes in aircraft fuel tank inerting systems. In particular, it employs multiple temperature-differential coagulation baths in conjunction with a protective film agent to control the coagulation bath environment of the polymer polyimide at different stages of film formation. This allows the membrane fibers to form a dense skin layer of suitable thickness and a support layer structure of suitable density, thereby improving and balancing multiple parameters of the membrane fibers, such as gas permeability, gas selectivity, physical and mechanical properties, and encapsulation strength. This provides a method for preparing hollow fiber membranes that can be used in actual aircraft fuel tank inerting systems. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the dry-wet phase separation spinning process used in this invention;

[0030] Figure 2 A statistical chart showing the loss of membrane fiber durability under different coagulation bath settings in Example 1;

[0031] Figure 3 This is a statistical chart showing the loss of membrane fiber durability after adding different protective agents in Example 3;

[0032] Figure 4 The morphology of the membrane fibers under the experimental conditions of PI-8 group in Example 3 is shown. Detailed Implementation

[0033] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0034] The reagents and instruments used in the examples can be purchased from the market, and the detection methods adopt conventional methods well known in the art. Based on the requirements of the application in the airborne fuel tank inerting system, the detection mainly focuses on several aspects, including gas permeability, mixed gas selectivity, membrane fiber durability, membrane fiber and epoxy encapsulation strength, and observation of the morphology of each layer of a single hollow fiber membrane.

[0035] 1. Sample preparation:

[0036] Take 100 hollow fiber membranes, each 450 mm long after drying, bundle them together, bend them into a U-shape, and cast the open end with epoxy resin to form a test sample.

[0037] 2. Preparation of samples for microscopic morphology scanning:

[0038] The single hollow fiber membrane was subjected to low-temperature brittle fracture in liquid nitrogen, and a thin layer of Pt was coated onto the membrane sample using a sputtering coating machine.

[0039] 3. Gas permeability testing methods:

[0040] (1) Turn on the temperature control system of the test device and set the test temperature to 20-30℃.

[0041] (2) Place the test sample into the test device, connect the test gas pipeline to the nitrogen (purity ≥99%) gas source, turn on the nitrogen gas source, and replace the test pipeline 2 to 8 times; adjust the gas test pressure to 0.1 to 0.5 MPa, stabilize for 10 to 15 minutes, and then use a soap bubble flow meter to measure the gas flow rate Q of the nitrogen passing through. N2 Repeat the test 3 to 5 times, take the average value, and turn off the nitrogen source after the test is completed.

[0042] (3) Connect the test gas pipeline to an oxygen (purity ≥99%) source, turn on the oxygen source, and replace the test pipeline 2 to 8 times; adjust the gas test pressure to 0.1 to 0.5 MPa, stabilize for 10 to 15 minutes, and then use a soap bubble flow meter to test the gas flow rate Q of the oxygen passing through. O2 Repeat the test 3 to 5 times, take the average value, and turn off the oxygen source after the test is completed.

[0043] (4) Calculation of permeation rate:

[0044]

[0045] J i Let i be the permeation rate of gas i through the hollow fiber membrane, in cm. 3 (STP) / cm 2 scmHg; Q i Let be the volumetric flow rate of gas i at standard temperature and pressure, in cm³. 3 (STP) / s; A is the total effective membrane area, cm² 2 ∆P represents the pressure difference across the membrane, in cmHg. (The units will be converted to GPU for easier comparison later; 1 GPU = 10^60 ... -6 cm 3 (STP) / cm 2 scmHg)

[0046] 4. Calculation of gas mixture selectivity:

[0047] The permeability selectivity of two different pure gases i and j is generally expressed by the separation coefficient α. i / j This is expressed as a calculation using the following formula:

[0048] 5. Membrane fiber durability test

[0049] Take 5-10 samples and test their gas permeability and separation performance at regular intervals (testing methods are the same as described above). The specific time intervals for testing are set as follows:

[0050] Table 1. Test Time Stages for Membrane Filament Durability Test

[0051] Storage time (daily) Test interval 0-300 Test every 30 days 300-900 Test every 150 days

[0052] 6. Strength of membrane fibers and epoxy encapsulation

[0053] Take 3 to 5 samples and use a universal testing machine to measure the mechanical strength parameters of the samples. Set the test temperature to 25°C and hold for 30 seconds, then stretch at a fixed speed until breakage.

[0054] Example 1

[0055] A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system is as follows:

[0056] (1) A certain amount of polyimide (PI) polymer is dissolved in N-methylpyrrolidone (NMP) and polyvinylpyrrolidone (PVP) is added. The ratio of the three is 28wt% / 65wt% / 7wt%, and a casting solution is prepared in a dissolving vessel.

[0057] (2) Stirring accelerates the dissolution of PI and PVP. After complete dissolution, the casting solution is subjected to vacuum degassing treatment. The dissolution time is 5 to 7 days, the stirring temperature is 20 to 65°C, the degassing vacuum degree is 20 to 50 kPa, and the degassing time is 18 to 24 hours.

[0058] (3) Prepare the core solution with H2O / NMP (80 / 20, v / v%), and then perform vacuum degassing treatment on it. The parameters are the same as the vacuum degree and time of the casting solution.

[0059] (4) With the core liquid flow rate fixed at 5 ml / min, the casting liquid is continuously sprayed from the spinneret at a flow rate of 7 ml / min under a pressure of 4-5 bar. The air gap height between the spinneret and the bath liquid is fixed at 5 cm, and the stretching speed is set to 45 m / min. The proportions of the three-stage coagulation bath are shown in Table 2.

[0060] Table 2. Different coagulation bath setup ratios (through length)

[0061] Group Name Coagulation bath setting ratio (through length) Temperature distribution in the coagulation bath PI-1 3:1:10 Normal - Heating - Normal PI-2 3:3:10 Normal - Heating - Normal PI-3 3:5:10 Normal - Heating - Normal

[0062] The normal temperature is set to 15℃, the heating temperature is set to 35℃, and the coagulation bath composition is isopropanol / water (IPA / H2O, 30 / 70, v / v%). After passing through the coagulation bath, the yarn is drawn to the take-up drum by the winding wheel.

[0063] (5) The collected hollow fiber membranes were first soaked and washed in deionized water treated by reverse osmosis for 10 days, with the deionized water being changed every 12 hours. Then, they were soaked in ethanol for 1 hour, followed by soaking in n-hexane for 30 minutes. After being removed from the solvent, they were placed in a ventilated place for 48 hours, and then placed in an oven at 50-70°C to dry to constant weight.

[0064] The prepared hollow fiber membrane was tested after sample preparation, and the results were as follows:

[0065] Table 3 Gas permeability at different coagulation bath setup ratios

[0066] Group Name Coagulation bath setting ratio (through length) Nitrogen permeability (GPU) Oxygen permeability (GPU) PI-1 3:1:10 4.72 26.95 PI-2 3:3:10 6.72 42.54 PI-3 3:5:10 11.34 51.26

[0067] Table 4 Gas selectivity for different coagulation bath setup ratios

[0068] Group Name Coagulation bath setting ratio (through length) <![CDATA[O2 / N2 selectivity]]> PI-1 3:1:10 5.71 PI-2 3:3:10 6.33 PI-3 3:5:10 4.52

[0069] The loss of membrane fiber durability under different coagulation bath settings is shown in the attached figure. Figure 2 It can be seen that different coagulation bath settings have little impact on the durability of the membrane fibers. The durability of the membrane fibers is more affected by the composition of the casting solution and the composition of the coagulation bath.

[0070] Table 5. Test results of membrane fiber strength under different coagulation bath settings.

[0071] Group Name Coagulation bath setting ratio (through length) Fractured parts Elongation at break (%) Tensile strength (MPa) Elastic modulus (MPa) PI-1 3:1:10 Middle of the membrane filament 18.23 17.32 720.0 PI-2 3:3:10 Middle of the membrane filament 18.15 17.15 813.65 PI-3 3:5:10 Middle of the membrane filament 16.82 18.73 827.25

[0072] Table 6. Test results of epoxy encapsulation strength with different coagulation bath ratios.

[0073] Group Name Coagulation bath setting ratio (through length) Fractured parts Elongation at break (%) Tensile strength (MPa) Elastic modulus (MPa) PI-1 3:1:10 middle of the sample 3.5 57.11 2340 PI-2 3:3:10 middle of the sample 3.2 52.41 2280 PI-3 3:5:10 middle of the sample 3.2 52.14 2122

[0074] Based on the above test results, the setting ratio of the coagulation bath at different stages mainly affects gas permeability and gas selectivity, while having a smaller impact on the durability and mechanical strength of the membrane fibers. Therefore, the parameters of the ideal coagulation bath setting ratio group PI-2 were selected, and the ratio was further subdivided and optimized for gas permeability and gas selectivity. The ratio range of 3:2:10 to 3:4:10 was selected and compared repeatedly with the same preparation and testing methods mentioned above. Finally, the optimal setting ratio of the coagulation bath was determined to be 3:2.7:10.

[0075] Example 2

[0076] (1) A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system, which is prepared by the method of Example 1, and the following parameters are set: PI / NMP / PVP (28wt% / 65wt% / 7wt%) is the mass ratio of casting liquid, H2O / NMP (80 / 20, v / v%) is the core liquid formulation, IPA / H2O (30 / 70, v / v%) is the coagulation bath, the core liquid flow rate is fixed at 5 ml / min, the casting liquid is continuously sprayed from the spinneret at a flow rate of 7 ml / min, the air gap height is 5 cm, and the stretching speed is set to 45 m / min.

[0077] (2) Three different coagulation bath heating temperatures were used for testing. The specific temperature configurations are shown in Table 7.

[0078] Table 7 Different coagulation bath heating temperatures

[0079] Group Name Temperature of the heating section of the coagulation bath (°C) Coagulation bath distribution ratio Temperature distribution in the coagulation bath PI-4 35 3:2.7:10 Normal - Heating - Normal PI-5 50 3:2.7:10 Normal - Heating - Normal PI-6 70 3:2.7:10 Normal - Heating - Normal

[0080] After sample preparation, the prepared hollow fiber membrane was tested for gas permeability and gas selectivity. The results are as follows:

[0081] Table 8 Gas permeability at different heating zone solidification bath temperatures

[0082] Group Name Temperature of the heating section of the coagulation bath (°C) Nitrogen permeability (GPU) Oxygen permeability (GPU) PI-4 35 8.13 58.13 PI-5 50 3.51 42.65 PI-6 70 23.12 69.59

[0083] Table 9 Gas selectivity at different heating zone solidification bath temperatures

[0084] Group Name Temperature of the heating section of the coagulation bath (°C) <![CDATA[O2 / N2 selectivity]]> PI-4 35 7.15 PI-5 50 12.13 PI-6 70 3.01

[0085] Based on the above test results, the temperature of the coagulation bath in the heating section has a certain impact on gas permeability and gas selectivity. This is mainly due to the increased elasticity of polyimide at higher temperatures, which helps in the formation of a denser structure, but also reduces gas permeability to some extent. However, when the temperature is too high, the exchange of components between phases accelerates, and this excessively rapid exchange can damage part of the skin structure, leading to a decrease in gas selectivity and an increase in permeability. Therefore, based on the test results, a more ideal temperature range of 25–45℃ was further refined and optimized, and the final determined temperature of the coagulation bath in the heating section was 38℃.

[0086] Example 3

[0087] (1) A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system, which is prepared by the method of Example 1, and the following parameters are set: PI / NMP / PVP (28wt% / 65wt% / 7wt%) is the mass ratio of casting liquid, H2O / NMP (80 / 20, v / v%) is the core liquid formulation, IPA / H2O (30 / 70, v / v%) is the coagulation bath component of the normal section, the core liquid flow rate is fixed at 5 ml / min, the casting liquid is continuously sprayed from the spinneret at a flow rate of 7 ml / min, the air gap height is 5 cm, and the stretching speed is set to 45 m / min.

[0088] (2) Different protective film agents were added to the heating section of the coagulation bath for testing. The specific component configurations are shown in Table 10.

[0089] Table 10. Addition of different protective film agents in the heating section of the coagulation bath

[0090] Group Name Types and proportions of protective film agents for the heating section of the coagulation bath Coagulation bath distribution ratio Temperature distribution in the coagulation bath PI-7 IPA / H2O (30 / 70, v / v%) 3:2.7:10 Normal - Heating (38℃) - Normal PI-8 IPA / H2O / Polyethylene Glycol (PEG) (30 / 65 / 5, v / v%) 3:2.7:10 Normal - Heating (38℃) - Normal PI-9 IPA / H2O / γ-Butyrolactone (GBL) (30 / 65 / 5, v / v%) 3:2.7:10 Normal - Heating (38℃) - Normal

[0091] The prepared hollow fiber membrane was tested after sample preparation, and the results were as follows:

[0092] Table 11 Gas permeability after adding different protective film agents

[0093] Group Name Types and proportions of protective film agents for the heating section of the coagulation bath Nitrogen permeability (GPU) Oxygen permeability (GPU) PI-7 IPA / H2O (30 / 70, v / v%) 9.23 65.35 PI-8 IPA / H2O / Polyethylene Glycol (PEG) (30 / 65 / 5, v / v%) 8.02 66.00 PI-9 IPA / H2O / γ-Butyrolactone (GBL) (30 / 65 / 5, v / v%) 7.80 61.23

[0094] Table 12 Gas selectivity after adding different protective film agents

[0095] Group Name Types and proportions of protective film agents for the heating section of the coagulation bath <![CDATA[O2 / N2 selectivity <!-- 6 -->]]> PI-7 IPA / H2O (30 / 70, v / v%) 7.08 PI-8 IPA / H2O / Polyethylene Glycol (PEG) (30 / 65 / 5, v / v%) 8.23 PI-9 IPA / H2O / γ-Butyrolactone (GBL) (30 / 65 / 5, v / v%) 7.85

[0096] The loss of membrane durability after adding different protective agents is shown in the attached figure. Figure 3 It can be seen that the addition of different protective agents has little effect on the durability of the membrane fibers.

[0097] Table 13 Test results of film fiber strength after adding different protective agents

[0098] Group Name Types and proportions of protective film agents for the heating section of the coagulation bath Fractured parts Elongation at break (%) Tensile strength (MPa) Elastic modulus (MPa) PI-7 <![CDATA[IPA / H2O(30 / 70,v / v%)]]> Middle of the membrane filament 17.15 18.41 821.25 PI-8 <![CDATA[IPA / H2O / Polyethylene glycol (PEG) (30 / 65 / 5, v / v%)]]> Middle of the membrane filament 15.05 21.95 841.45 PI-9 <![CDATA[IPA / H2O / γ-butyrolactone (GBL) (30 / 65 / 5, v / v%)]]> Middle of the membrane filament 16.36 20.81 837.33

[0099] Table 14 Test results of epoxy encapsulation strength after adding different protective film agents

[0100] Group Name Types and proportions of protective film agents for the heating section of the coagulation bath Fractured parts Elongation at break (%) Tensile strength (MPa) Elastic modulus (MPa) PI-7 <![CDATA[IPA / H2O(30 / 70,v / v%)]]> middle of the sample 3.1 52.11 2315 PI-8 <![CDATA[IPA / H2O / Polyethylene glycol (PEG) (30 / 65 / 5, v / v%)]]> middle of the sample 3.1 62.31 2420 PI-9 <![CDATA[IPA / H2O / γ-butyrolactone (GBL) (30 / 65 / 5, v / v%)]]> middle of the sample 3.5 60.04 2398

[0101] Based on the above test results, the addition of different protective agents has a positive effect on gas permeability and gas selectivity. The addition of protective agents plays a certain role in forming a non-damaging skin layer and a suitable support layer, thereby improving the overall quality of membrane formation. In addition, it also improves the mechanical strength of the membrane fibers to a certain extent. Through comparison, it was found that the selection of polyethylene glycol (PEG) as a protective agent makes the various parameters of the membrane fibers more balanced.

[0102] Comparative Example 1 (CG-1)

[0103] A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system is disclosed, with preparation conditions and methods similar to those in Example 1. The difference between Example 1 and Example 2 is that the coagulation bath is not segmented, but a continuous coagulation bath with the same total length as the three-segment coagulation bath in Example 1 is used, and the coagulation bath temperature is set to the normal temperature of 15°C.

[0104] Comparative Example 2 (CG-2)

[0105] A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system is disclosed, with preparation conditions and methods similar to those in Example 1. The difference between Example 1 and Example 2 is that the coagulation bath is not segmented, but a continuous coagulation bath with the same total length as the three-segment coagulation bath in Example 1 is used, and the coagulation bath temperature is set to the normal temperature of 38°C.

[0106] Comparative Example 3 (CG-3)

[0107] A method for preparing a hollow fiber membrane for an aircraft fuel tank inerting system is disclosed, with preparation conditions and methods similar to those in Example 1. The difference from Example 1 is that the coagulation bath is not segmented, but a continuous coagulation bath with the same total length as the three-segment coagulation bath in Example 1 is used. The coagulation bath temperature is set to the normal temperature of 38°C, and the composition of the coagulation bath is set to IPA / H2O / polyethylene glycol (PEG) (30 / 65 / 5, v / v%).

[0108] The test results of the three control groups were compared with the membrane fiber performance of the optimized PI-8 group in Example 3 above. The results are as follows:

[0109] Table 15 Gas permeability of the comparative and optimized groups

[0110] Group Name Differences in conditions between different groups Nitrogen permeability (GPU) Oxygen permeability (GPU) CG-1 15℃ Continuous coagulation bath 3.12 10.23 CG-2 38℃ Continuous coagulation bath 2.95 17.08 CG-3 38℃ continuous coagulation bath, with added polyethylene glycol (PEG). 4.81 29.00 PI-8 Segmented coagulation bath (normal - heating to 38℃ - normal), polyethylene glycol (PEG) added in the heating section. 8.02 66.00

[0111] Table 16 Gas selectivity of the control group and the optimized group

[0112] Group Name Differences in conditions between different groups <![CDATA[O2 / N2 selectivity]]> CG-1 15℃ Continuous coagulation bath 3.28 CG-2 38℃ Continuous coagulation bath 5.79 CG-3 38℃ continuous coagulation bath, with added polyethylene glycol (PEG). 6.03 PI-8 Segmented coagulation bath (normal - heating to 38℃ - normal), polyethylene glycol (PEG) added in the heating section. 8.23

[0113] Table 17. Membrane fiber strength test results for the control group and the optimized group

[0114] Group Name Differences in conditions between different groups Fractured parts Elongation at break (%) Tensile strength (MPa) Elastic modulus (MPa) CG-1 15℃ Continuous coagulation bath Random parts 26.14 12.02 786.12 CG-2 38℃ Continuous coagulation bath Middle of the membrane filament 19.26 16.35 807.15 CG-3 38℃ continuous coagulation bath, with added polyethylene glycol (PEG). Middle of the membrane filament 14.36 22.81 855.63 PI-8 Segmented coagulation bath (normal - heating to 38℃ - normal), polyethylene glycol (PEG) added in the heating section. Middle of the membrane filament 15.05 21.95 841.45

[0115] Table 18. Epoxy Encapsulation Strength Test Results for the Comparison Example and the Optimized Group

[0116] Group Name Differences in conditions between different groups Fractured parts Elongation at break (%) Tensile strength (MPa) Elastic modulus (MPa) CG-1 15℃ Continuous coagulation bath middle of the sample 3.2 46.12 1995 CG-2 38℃ Continuous coagulation bath middle of the sample 3.2 50.14 2403 CG-3 38℃ continuous coagulation bath, with added polyethylene glycol (PEG). middle of the sample 3.2 57.26 2512 PI-8 Segmented coagulation bath (normal - heating to 38℃ - normal), polyethylene glycol (PEG) added in the heating section. middle of the sample 3.1 62.31 2420

[0117] Based on the comparative test results above, it can be seen that the segmented coagulation bath treatment combined with mid-stage heating and the addition of a protective film agent can improve the overall quality of the membrane fibers, balancing the gas permeability and gas selectivity to achieve ideal applicable parameters. Heating treatment and the addition of the protective film agent can also improve the strength of the membrane fibers to a certain extent. The balanced improvement in various membrane fiber properties is attributed to the fact that after the membrane fibers are initially formed in the first stage of the coagulation bath, they enter the heating stage with the protective film agent, which enhances the elasticity of the polyimide (PI). At this stage, the structure of the skin and support layer is optimized, becoming increasingly dense. Then, the process transitions to a relatively long normal temperature stage, which slows down the formation rate of the dense structure, preventing the skin layer from becoming too thick and the support layer structure from becoming too dense, thus affecting the overall gas permeability of the membrane fibers. Scanning electron microscopy provides a more direct view of the structure of the prepared membrane fibers, as shown in the attached figure. Figure 4 It is evident that the membrane fibers under PI-8 conditions formed a dense, undamaged, and moderately thick skin layer. Its support layer did not exhibit finger-like or teardrop-shaped cavities, nor did it form an overly dense structure. This structure also demonstrates the balance and excellence of the membrane fibers in terms of gas permeability, gas selectivity, and physical and mechanical properties.

Claims

1. A method of making a hollow fiber membrane for use in an aircraft fuel tank inerting system, characterized by, Comprising the following steps: Step one, the polymer A, polymer additive B and solvent C according to a certain mass fraction mixed stirring until completely dissolved to get casting solution, the casting solution is vacuum degassing for standby; The solvent and water are mixed uniformly to configure the core liquid, and the core liquid is vacuum degassing for standby; Step two, the casting solution and the core liquid obtained in step one are continuously and slowly sprayed from the spinning head at a certain flow rate, and pass through a certain distance of air gap at a certain stretching speed to obtain the primary fiber; Step three, the primary fiber obtained in step two is precipitated and stretched by discontinuous multiple coagulation bath tanks to the yarn collecting cylinder, thereby obtaining a crude hollow fiber membrane; Step four, the crude hollow fiber membrane obtained in step three is immersed for complete solvent exchange, and after being treated by a ventilation drying treatment module, a hollow fiber membrane for an aircraft fuel tank inerting system is obtained.

2. The hollow fiber membrane for an aircraft fuel tank inerting system preparation method of claim 1, wherein the polymer A in step one is polyimide, the polymer additive B is polyvinylpyrrolidone, and the solvent C is N-methylpyrrolidone; The mass fraction of the polymer A in step one is 28%, the mass fraction of the polymer additive B is 7%, and the mass fraction of the solvent C is 65%; The complete dissolution in step one has specific parameters including a dissolution time of 5-7 days and a dissolution temperature of 20-65℃; The vacuum degassing in step one has specific parameters including a degassing vacuum degree of 20-50 kPa and a degassing time of 18-24 hours; The solvent in step one is N-methylpyrrolidone, and the water accounts for 80% by volume.

3. The hollow fiber membrane for an aircraft fuel tank inerting system preparation method of claim 1, wherein the specific flow rate in step two, wherein the casting solution flow rate is 7 ml / min, and the core liquid flow rate is 5 ml / min; The specific stretching speed in step two is 45 m / min; The air gap in step two has a height of 5 cm. The coagulation bath tank in step three has a two-component mixture of water and isopropyl alcohol, and the isopropyl alcohol accounts for 30% by volume. The discontinuous multiple coagulation bath tanks in step three are multiple coagulation bath tanks with different content liquid temperatures.

4. A method of making a hollow fiber membrane for use in an aircraft fuel tank inerting system as defined in claim 1, wherein, The multiple coagulation bath tanks with different content liquid temperatures include a first normal bath tank, a heating bath tank, and a second normal bath tank.

5. A method of making a hollow fiber membrane for use in an aircraft fuel tank inerting system as defined in claim 1, wherein, The length ratio of the first normal bath tank, the heating bath tank, and the second normal bath tank ranges from 3:1:10 to 3:5:

10.

6. A method of making a hollow fibre membrane for use in an aircraft fuel tank inerting system as claimed in claim 5, characterised in that, The content liquid temperature of the first normal bath tank and the second normal bath tank is set to 15℃, and the content liquid temperature of the heating bath tank ranges from 25-70℃.

7. A method of making a hollow fibre membrane for use in an aircraft fuel tank inerting system as claimed in claim 6, characterised in that, Polyethylene glycol or γ-butyrolactone is also added to the content liquid of the heating bath tank.

8. A method of producing a hollow fibre membrane for use in an aircraft fuel tank inerting system as claimed in claim 6 or 7, characterised in that, ​ 9. A method of making a hollow fibre membrane for use in an aircraft fuel tank inerting system as claimed in claim 6 or 7, characterised in that, ​ 10. A method of making a hollow fiber membrane for use in an aircraft fuel tank inerting system as defined in claim 1, wherein, The step four is to soak in stages, and the specific process includes: first, soaking in deionized water for 10 days, and changing the deionized water every 12 hours, then soaking in ethanol for 1 hour, and then soaking in n-hexane for 30 minutes.

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