Aerogel fiber prepared from waste aramid fiber under extremely cold conditions and method thereof

Abstract

The application relates to an aerogel fiber prepared from waste aramid fiber under extremely cold conditions and a preparation method thereof, and belongs to the technical field of textile materials. The method comprises the following steps: mechanically opening and chemically cleaning an outer layer fabric of waste fire-fighting clothes to obtain pure aramid short fibers; the aramid short fibers are dissolved in a mixed solvent of trifluoroacetic acid and methane sulfonic acid, and graphene, acrylamide monomers and a photoinitiator are added to prepare a photosensitive composite spinning solution; the photosensitive composite spinning solution is extruded into a coagulation bath through a wet spinning method and is synchronously crosslinked through ultraviolet irradiation; an axial temperature gradient is applied to nascent fibers to induce directional ice crystal growth, and a precursor with axial parallel pores is formed through freeze-drying; and finally, the aerogel fiber is obtained through liquid nitrogen cryogenic treatment and gradient temperature recovery shaping. Through control of the molecular structure and morphology in the fiber preparation process, the application realizes efficient recycling of waste aramid fiber, and the prepared aerogel fiber has excellent performances such as high porosity, high strength and low thermal conductivity.

Description

Technical Field

[0001] This invention belongs to the field of textile materials technology, specifically relating to an aerogel fiber prepared from waste aramid fibers under extremely cold conditions and the method thereof. Background Technology

[0002] As firefighting equipment reaches the end of its service life, more than 100,000 tons of used firefighting suits are generated globally each year. Some of these suits are made of composite materials such as aramid 1414, conductive fibers, and flame-retardant coatings for their outer fabric. Aramid fibers have excellent properties such as high temperature resistance and high strength. If they can be effectively recycled and reused, they will have significant economic and environmental benefits.

[0003] Currently, there are several main technical approaches for recycling waste aramid fibers:

[0004] One method is the traditional physical and mechanical recycling method, which uses simple mechanical crushing or opening to turn waste aramid fibers into short fibers or fiber bundles. However, the fiber properties are significantly reduced after this method, and its application range is limited.

[0005] The second method is chemical degradation, which uses strong acids or alkalis to decompose aramid fibers into small molecule compounds, which are then repolymerized. However, this method is complex, costly, and causes significant environmental pollution.

[0006] Third is the wet spinning re-forming method, which re-forms the dissolved aramid solution into fibers through wet spinning, but it has problems such as low porosity and poor thermal insulation performance.

[0007] In the existing wet spinning remolding route, the internal structure of the fiber cannot be effectively controlled, resulting in disordered pore distribution of the obtained fiber. This fails to meet the comprehensive requirements for high-performance thermal insulation and mechanical strength of the material. Furthermore, there are shortcomings in the treatment of conductive fibers and flame-retardant coatings in aramid fibers during the recycling process, making it difficult to achieve effective separation and full utilization of each component. This poses a huge challenge to the efficient recycling of waste fire suits.

[0008] To address these issues, this invention proposes a method for preparing aerogel fibers from waste aramid fibers under extremely cold conditions. Summary of the Invention

[0009] This invention overcomes the shortcomings of the prior art and provides an aerogel fiber prepared from waste aramid fiber under extremely cold conditions and a method thereof.

[0010] To achieve the above objectives, the technical solution adopted by this invention is as follows: a method for preparing aerogel fibers using waste aramid fibers under extremely cold conditions, comprising the following steps:

[0011] S1. Mechanically open and perform composite chemical cleaning on the outer fabric of waste fire suits to obtain pure aramid short fibers;

[0012] S2. Dissolve pure aramid short fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, add graphene, acrylamide and photoinitiator to prepare a photosensitive composite spinning solution;

[0013] S3. The photosensitive composite spinning solution is squeezed into the coagulation bath by wet spinning and cross-linked by ultraviolet irradiation to form nascent fibers.

[0014] S4. Apply an axial temperature gradient to the nascent fibers to induce directional ice crystal growth and obtain directional frozen fibers;

[0015] S5. Vacuum freeze-dry the oriented cryogenic fibers to obtain an aerogel fiber precursor with axially parallel channels.

[0016] S6. The aerogel fiber precursor is immersed in liquid nitrogen for cryogenic treatment and then subjected to gradient temperature recovery to obtain aerogel fibers.

[0017] Further, in step S1, mechanical opening includes processing waste fire suit fabric with an opening machine having a toothed roller speed of 800-1200 rpm and a tooth gap of 0.5-1 mm to obtain fiber bundles with a length of 10-20 mm; composite chemical cleaning includes immersing the fiber bundles in a composite cleaning solution and ultrasonically treating them at 65-75℃ for 4-6 hours; the cleaning solution includes 0.1-0.15 g of sodium carbonate, 0.05-0.08 g of sodium dodecyl sulfonate, and 50-60 ml of water per gram of fiber.

[0018] Further, in step S2, the volume ratio of trifluoroacetic acid to methanesulfonic acid in the mixed solvent is 1:1; the mass fraction of pure aramid short fibers in the mixed solvent is 10-15 wt%.

[0019] Further, in step S2, the mixed solution of pure aramid short fibers, graphene, acrylamide and photoinitiator includes 3-8 wt% graphene, 1-3 wt% acrylamide and 0.5-2 wt% photoinitiator by weight of aramid.

[0020] Furthermore, after ultrasonic dispersion of the mixed solution in an ice bath at 0-5℃ for 20-40 min, it is degassed under a vacuum of -0.09 to -0.1 MPa for 1-2 h.

[0021] Further, in step S3, the coagulation bath is a mixture of ethanol and water in a volume ratio of 7:3, and the temperature is 20-25℃; the ultraviolet irradiation conditions are a wavelength of 365nm and an intensity of 80-120mW / cm². 2 The irradiation time is 30-90 seconds, and the irradiation position is 10-20 mm below the surface of the coagulation bath.

[0022] Further, in step S4, the nascent fibers are vertically clamped onto a copper cold table. The axial temperature gradient of the copper cold table is 25-35℃ / m, with the contact end temperature being -40 to -30℃ and the opposite end temperature being -10 to -5℃. The freezing and cooling rate is 1-3℃ / min, the ambient humidity is 40-50%RH, and the freezing time is 120-180min.

[0023] Furthermore, in step S5, the freeze-drying conditions are: chamber temperature -38 to -32°C, vacuum degree 5-8 Pa, sublimation rate 0.2-0.5 mm / h, and drying time 24-48 h; the drying endpoint is determined when the fiber weight loss reaches 99.5%.

[0024] Furthermore, the cryogenic treatment in step S6 includes immersing the aerogel fiber precursor in liquid nitrogen at -196℃ at a speed of 10-20 cm / min for 3-5 min, transferring it to a -80℃ low-temperature chamber for equilibration for 120-180 min, raising the temperature to -20℃ at a rate of 0.5-1℃ / min, and then raising it to 25℃ at a rate of 2-3℃ / min.

[0025] Another technical solution provided by the present invention is an aerogel fiber prepared from waste aramid fiber under extremely cold conditions. Based on the above method, the aerogel fiber has a pore diameter of 1-20 μm, an aspect ratio of 50-100, an axial parallel through-hole ratio of more than 90%, and a porosity of 92-97%.

[0026] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0027] This invention recovers pure aramid short fibers from waste fire suits through mechanical opening and composite chemical cleaning. The fibers are dissolved in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, and then graphene, acrylamide, and a photoinitiator are added to form a photosensitive composite spinning solution. During wet spinning and extrusion into the coagulation bath, ultraviolet irradiation is simultaneously applied to trigger in-situ crosslinking. An axial temperature gradient is then applied to the nascent fibers to induce directional ice crystal growth. After freeze-drying, an axial pore precursor is formed. Finally, aerogel fibers are obtained through liquid nitrogen cryogenic treatment and gradient reheating to transform waste aramid fibers into aerogel fibers with high porosity, high strength, and low thermal conductivity. This achieves the goal of solid waste resource utilization and the preparation of materials adapted to extreme environments.

[0028] By adding acrylamide monomers and photoinitiators, wavelengths of 365 nm and intensities of 80 to 120 mW / cm² were simultaneously applied in the coagulation bath. 2UV irradiation triggers free radical polymerization to build a covalent cross-linked network between aramid molecular chains. This process significantly increases the fiber cross-linking density, directly enhancing the intrinsic strength of the material. Compared to traditional wet spinning processes without light cross-linking, tensile strength is significantly improved. The cross-linked network also provides a rigid framework for subsequent ice crystal growth, effectively suppressing microcrack defects caused by phase transformation stress.

[0029] An axial temperature gradient of 25 to 35 °C / m was applied using a copper cold stage, controlling the temperature at the contact end to be -40 °C to -30 °C and at the opposite end to -10 °C to -5 °C, with directional freezing at a rate of 1 to 3 °C / min. This gradient field drove the orderly growth of ice crystals along the long axis of the fibers, forming a columnar template that achieved an extremely high level of channel orientation. Compared to disordered channel structures, the axially parallel through-holes significantly extended the phonon transport path. The photocrosslinking network synergistically restricted the radial expansion of ice crystals, ensuring channel continuity and a high proportion, and significantly reducing the material's thermal conductivity.

[0030] The three-dimensional skeleton formed by photocrosslinking precisely constrains the growth dimension of ice crystals and suppresses disordered branching; the axial tensile stress generated by ice crystal growth pulls the molecular chains to oriented along the pore wall, solving the bottleneck of mutual exclusion between the strength and thermal insulation performance of high-porosity materials, and achieving a simultaneous leap in tensile strength and thermal insulation efficiency.

[0031] The aerogel precursor was immersed in liquid nitrogen at -196°C for 3 to 5 minutes at a speed of 10 to 20 cm / min to achieve ultra-rapid freezing of molecular chain motion. After equilibration at -80°C, it was gradually warmed to room temperature at a gradient of 0.5 to 1°C / min to eliminate residual internal stress from the drying process. Compared to untreated materials, the resilience was significantly improved. The ultra-rapid freezing of liquid nitrogen, combined with the topological morphology of the photocrosslinking network and the ice crystal template, ensured that the high-porosity structure remained intact during extreme temperature changes from -196°C to 25°C. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a flowchart of a method for preparing aerogel fibers from waste aramid fibers under extremely cold conditions.

[0034] Figure 2 This is an electron microscope image of aerogel fibers. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Many specific details are set forth in the following description to provide a full understanding of the invention. However, the invention may be practiced in other ways different from those described herein, and therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below. Unless otherwise specified in the following embodiments, all raw materials are commercially available or prepared by conventional methods in the art. Specifically, trifluoroacetic acid was purchased from Sinopharm, specification AR (Shanghai Test); methanesulfonic acid was purchased from Sinopharm, specification AR (Shanghai Test); acrylamide was purchased from Sinopharm, specification AR (Shanghai Test); the photoinitiator was phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, purchased from Aladdin, purity ≥97%; used fire suits were purchased from the local fire department, the outer fabric comprising aramid 1414 (≥85%), carbon fiber conductive yarn (antistatic, 5-8%), and a polytetrafluoroethylene (PTFE) flame-retardant coating (thickness 0.2-0.5mm).

[0037] Exemplary method:

[0038] like Figure 1 As shown, a method for preparing aerogel fibers using waste aramid fibers under extremely cold conditions includes the following steps:

[0039] S1. Mechanically open and perform composite chemical cleaning on the outer fabric of waste fire suits to obtain pure aramid short fibers;

[0040] S2. Dissolve pure aramid short fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, add graphene, acrylamide and photoinitiator to prepare a photosensitive composite spinning solution;

[0041] S3. The photosensitive composite spinning solution is squeezed into a coagulation bath of ethanol and water by wet spinning, and ultraviolet irradiation crosslinking is carried out simultaneously to form nascent fibers.

[0042] S4. Apply an axial temperature gradient to the nascent fibers to induce directional ice crystal growth and obtain directional frozen fibers;

[0043] S5. Vacuum freeze-dry the oriented cryogenic fibers to obtain an aerogel fiber precursor with axially parallel channels.

[0044] S6. The aerogel fiber precursor is immersed in liquid nitrogen for cryogenic treatment and then subjected to gradient temperature recovery to obtain aerogel fibers.

[0045] Below, each step will be explained in detail.

[0046] Step S1 aims to efficiently separate pure and soluble aramid staple fibers from the outer fabric of discarded fire suits. The outer layer of fire suits typically uses high-performance materials such as aramid 1313 or aramid 1414, and is composited with conductive fibers and a flame-retardant coating. These components need to be specifically separated during the recycling process.

[0047] In this step, the waste fire suit fabric undergoes mechanical opening treatment. An opening machine applies directional tearing force and combing action to the outer layer of the waste fire suit fabric. By adjusting the toothed roller speed within the range of 800 rpm to 1200 rpm, and with a tooth gap of 0.5 mm to 1 mm, the dense woven structure is broken down into discrete fiber bundles, making the fiber bundles fully fluffy, and controlling the output fiber length within the range of 10 mm to 20 mm. During the opening process, metal parts and non-fiber impurities must be removed simultaneously, ultimately obtaining short fiber bundles with a length concentrated between 10 mm and 20 mm. This avoids dust loss from excessively short fibers or reduced solvent penetration efficiency from excessively long fibers. At this point, flame-retardant coating residues still adhere to the fibers.

[0048] To thoroughly remove surface contaminants, a composite chemical cleaning system is required. The system consists of sodium carbonate, sodium dodecyl sulfate, and water, with the following ratio: 0.1g to 0.15g sodium carbonate, 0.05g to 0.08g sodium dodecyl sulfate, and 50ml to 60ml water per gram of fiber. Sodium carbonate decomposes oily contaminants through saponification, while sodium dodecyl sulfate removes flame-retardant coating particles through molecular emulsification. The opened fibers are immersed in the cleaning solution and ultrasonically treated at 65℃ to 75℃, with the ultrasonic frequency maintained in the range of 35kHz to 45kHz, for 4 to 6 hours.

[0049] The fibers are treated continuously at 100℃ to 110℃ for 1.5 to 2.5 hours, with the hot air flow rate controlled at 1 m / s to 1.5 m / s. Moisture is desorbed through molecular thermal motion, resulting in a final fiber moisture content of no more than 0.5 wt%. The dried fibers are then subjected to a second treatment using a precision shearing device with a blade gap set at 0.1 mm to 0.3 mm, reducing the length to below 5 mm and increasing the fiber specific surface area.

[0050] Step S2 aims to construct a composite spinning solution system with spinnability and photocuring capabilities. This step uses the chopped aramid fibers obtained in step S1 as the core solute, achieving molecular-level dissociation through a specific acid solvent, while simultaneously introducing a graphene reinforcing phase and a photosensitive component to endow the material with multifunctional properties.

[0051] In this step, the chopped aramid fibers obtained in S1 are added to a mixed solvent system of trifluoroacetic acid and methanesulfonic acid, with the volume ratio of trifluoroacetic acid to methanesulfonic acid controlled at 1:1. The dissolution process is carried out in a closed reactor, with the temperature maintained between 15°C and 25°C to prevent solvent evaporation. The mass fraction of aramid fibers is controlled between 10 wt% and 15 wt% to balance the solution viscosity and spinnability. The solution is continuously treated with mechanical stirring at a rate of 200 rpm to 300 rpm for 5 to 7 hours until a homogeneous, transparent, viscous solution is formed, which is an aramid solution with a dynamic viscosity of 1500 cP to 5000 cP within the spinnability window. The strong fluorination effect of trifluoroacetic acid breaks the hydrogen bonds between aramid molecules, while methanesulfonic acid provides a protonated environment to promote molecular chain dissociation, achieving complete dissolution of the aramid to form a viscous colloidal solution.

[0052] Graphene powder, acrylamide reactive monomer, and photoinitiator were sequentially added to the dissolved aramid solution to obtain a mixed solution. The amount of graphene powder added was 3 wt% to 8 wt% of the aramid mass. The solution was treated using an ice-bath ultrasonic dispersion device at 0°C to 5°C for 20 to 40 minutes, with the ultrasonic power controlled within the range of 300 W to 500 W, to ensure uniform dispersion of the graphene in a monolayer form. Simultaneously, the acrylamide functional monomer was added at 1 wt% to 3 wt% of the aramid mass. The carbon-carbon double bonds in the acrylamide functional monomer will provide crosslinking sites for subsequent photocuring. Finally, 0.5 wt% to 2 wt% of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide was added as a photoinitiator. The solution was stirred at a low speed of 50 rpm to 100 rpm for 30 to 60 minutes under light-protected conditions to ensure molecular-level mixing of all components.

[0053] The mixed solution is transferred to a vacuum degassing device and left to stand for 1 to 2 hours under a negative pressure of -0.09 MPa to -0.1 MPa to eliminate bubbles introduced by stirring, thereby increasing the transmittance of the solution to over 90% and narrowing the viscosity fluctuation range to ±5%.

[0054] Step S3 aims to achieve fiber morphology shaping through wet spinning and to form a three-dimensional network structure by triggering an in-situ cross-linking reaction using ultraviolet irradiation. This step uses the photosensitive spinning solution prepared in step S2 as raw material, and performs photocuring to lock the molecular conformation while completing the phase separation process in the coagulation bath, thereby improving the mechanical integrity of the nascent fiber.

[0055] In this step, the deaerated spinning solution is injected into the storage tank of the wet spinning unit and delivered to the spinneret at a stable flow rate of 0.5 ml / min to 1 ml / min using a metering pump. The spinneret orifice diameter is in the range of 0.1 mm to 0.2 mm to balance the extrusion pressure and fiber diameter control requirements. After the spinning solution is extruded from the spinneret orifice, it immediately enters a coagulation bath system composed of ethanol and water, with a volume ratio of ethanol to water of 7:3, and the temperature is maintained at 20°C to 25°C. In the coagulation bath, the acid solvent and the alcohol-water medium undergo bidirectional diffusion, causing the aramid molecular chains to separate and re-aggregate. This process forms the cortex structure of the nascent fiber within 5 to 10 seconds.

[0056] The key to this step is the simultaneous implementation of ultraviolet (UV) irradiation crosslinking when the spinning solution enters the coagulation bath. Specifically, when the nascent fibers are immersed to a depth of 10 mm to 20 mm below the surface of the coagulation bath, a 365 nm UV light source is activated to vertically irradiate the fiber surface. The irradiation intensity is controlled at 80 mW / cm². 2 Up to 120mW / cm 2 The irradiation range and time were set to 30 to 90 seconds. Ultraviolet photons excited the phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide photoinitiator in the spinning solution, causing it to cleave and generate benzoyl free radicals. These active free radicals attacked the carbon-carbon double bonds of acrylamide, initiating a chain polymerization reaction, and simultaneously forming covalent cross-linking points with the amino groups on the aramid molecular chain through hydrogen extraction.

[0057] The cured fibers are pulled away from the coagulation bath by guide rollers at a speed between 2 m / min and 4 m / min. The fiber diameter shrinks to between 100 μm and 150 μm, and a uniform and dense layer is formed on the surface. The fibers are then rinsed with a 20 vol% to 30 vol% ethanol aqueous solution to remove residual solvent, followed by ultrasonic cleaning with water for 5 to 10 minutes to remove ionic impurities.

[0058] Step S4 aims to precisely control the ice crystal growth direction through an axial temperature gradient, constructing an axially parallel pore template for aerogel fibers. This step uses the nascent cross-linked fibers obtained in step S3 as the substrate, employing a high thermal conductivity material to establish a unidirectional heat transfer path, inducing water molecules to crystallize oriented along the fiber's long axis, forming a regularly arranged array of ice crystals.

[0059] In this step, the newly cross-linked fibers are vertically fixed to the surface of the copper cold stage. The length of a single fiber is controlled within the range of 20cm to 50cm, and the fiber spacing is maintained at 5mm to 10mm to prevent them from sticking together. The copper cold stage is made of 99.9% pure electrolytic copper plate with a thermal conductivity of 398W / (m·K), ensuring linear temperature gradient transfer. During clamping, low thermal resistance silicone grease is used to fill the contact surface between the fiber and the cold stage to reduce interfacial heat loss.

[0060] The fiber-laden cooling stage is placed in a programmed temperature control device, with the contact end temperature set to -30°C to -40°C and the opposite end maintained at -5°C to -10°C, creating an axial temperature gradient of 25°C / m to 35°C / m. Freezing is performed at a cooling rate of 1°C / min to 3°C / min, with the ambient humidity controlled within the range of 40%RH to 50%RH throughout the process. When the internal temperature of the fiber drops to -15°C to -20°C, ice crystals preferentially nucleate at the low-temperature end and grow along the temperature gradient direction. This process lasts for 120 to 180 minutes until complete freezing.

[0061] Under the influence of an axial temperature gradient, water molecules inside the fiber undergo nucleation and growth phase transitions. The lower end first reaches a supercooled state, where water molecules bind together through hydrogen bonds to form ice crystal nuclei. The temperature gradient drives the ice crystal front to extend directionally along the fiber's long axis, and water molecules arrange themselves layer by layer at the solid-liquid interface, forming a columnar crystal template. This process is essentially a result of the balance between the thermodynamic driving force (temperature gradient) and the kinetic conditions (molecular diffusion rate). Insufficient temperature gradient leads to random branching of the ice crystals, while excessively high gradients result in overly rapid growth and defects.

[0062] The ice crystal growth behavior during freezing is regulated by multiple parameters. The selection of a temperature gradient of 25℃ / m to 35℃ / m is based on ice crystal kinetics research. This range stabilizes the ice crystal growth rate within the range of 5μm / s to 10μm / s, ensuring the formation of columnar crystals with diameters of 1μm to 20μm. A cooling rate of 1℃ / min to 3℃ / min is set to avoid supercooling exceeding 10℃, preventing the formation of random dendrites. The final ice crystals have an aspect ratio of 50 to 100, and their axial parallelism deviation is less than 5°, laying the structural foundation for subsequent sublimation and the formation of directional channels.

[0063] Step S5 aims to achieve directional sublimation of ice crystals through vacuum freeze-drying, transforming the ice crystal template constructed in step S4 into an axially parallel channel structure. This step uses pre-frozen fibers as the processing object, controlling the phase transition process in a simulated extremely cold vacuum environment to ensure accurate replication and high-fidelity shaping of the structure.

[0064] The fiber, having undergone directional freezing, is transferred along with the copper cold stage to a freeze-drying chamber. The chamber is pre-cooled to -38°C to -32°C, with the temperature strictly controlled below the ice crystal eutectic point to suppress recrystallization. The vacuum system is then activated to reduce the chamber pressure to below 10 Pa, with the optimal operating range being 5 Pa to 8 Pa. The vacuum environment reduces the partial pressure of water vapor, causing the ice crystals to undergo a solid-gas phase transition and directly sublimate, avoiding the risk of pore collapse due to liquid phase formation.

[0065] The chamber temperature was maintained at a constant -35℃±3℃, and the vacuum fluctuation range was controlled within ±1Pa. Under these conditions, the ice crystal sublimation rate remained stable at 0.2mm / h to 0.5mm / h, and the entire drying process lasted 24h to 48h. Mass loss was monitored in real time, and drying was terminated when the weight loss rate approached 99.5%. In this step, the ice crystals sublimated orderly along the axial direction, and the remaining channels perfectly replicated the original ice crystal morphology, forming parallel through-holes with diameters of 1μm to 20μm and aspect ratios of 50 to 100.

[0066] Step S6 aims to lock the microstructure of the aerogel fibers obtained in step S5 under extremely cold conditions, eliminate residual internal stress from the drying process, and improve the stability of the pore morphology. It should be noted that "extremely cold conditions" does not refer to a general low-temperature environment, but specifically to the process centered on liquid nitrogen cryogenic treatment (-196℃) and its applicable scenarios. Specifically, this step uses freeze-dried aerogel fibers as the treatment object, utilizing the extreme low temperature of liquid nitrogen to instantly freeze the polymer chain movement, combined with a gradient warming strategy to avoid thermal shock damage.

[0067] In this step, the dried aerogel fiber bundles are immersed in a liquid nitrogen tank at a uniform speed of 10 cm / min to 20 cm / min, ensuring that the fibers are completely submerged in the liquid nitrogen medium at -196°C. The immersion time is strictly controlled within the range of 3 to 5 minutes. This time period allows the fiber core temperature to drop from 25°C to below -150°C within 60 seconds, achieving a sudden halt in molecular chain motion. Titanium alloy clamps are used during the operation, with a thermal conductivity as low as 7 W / (m·K), which reduces cold loss. When the temperature drops sharply below the transition point, the molecular chain motion freezes instantly, and the pore wall structure is locked in an expanded state. This process eliminates drying stress by suppressing the thermal motion of the molecular chains, preventing pore collapse during rewarming.

[0068] After the fibers were removed from liquid nitrogen, they were transferred within 3 seconds to a cryogenic chamber pre-cooled to -80°C and maintained for 120 to 180 minutes to homogenize the temperature. Then, the temperature was increased to -20°C at a rate of 0.5 to 1°C / min, and then to 25°C at a rate of 2 to 3°C / min. This segmented reheating allowed the polymer chains to gradually relax, preventing the pore walls from cracking due to thermal stress.

[0069] Example product:

[0070] like Figure 2 As shown, an aerogel fiber prepared from waste aramid fibers under extremely cold conditions, based on the above exemplary method, has a pore diameter of 1-20 μm, an aspect ratio of 50-100, an axial parallel through-hole ratio of more than 90%, and a porosity of 92-97%.

[0071] Example 1:

[0072] Step S1: Use an opening machine to process waste fire suit fabric, setting the toothed roller speed to 1000 r / min and the tooth gap to 0.8 mm to obtain short fiber bundles with a length of 15 mm. Prepare a cleaning solution containing 0.12 g / g sodium carbonate and 0.06 g / g sodium dodecyl sulfonate using the obtained fiber bundles, and ultrasonically treat it at 70℃ and 40 kHz for 5 h. After treatment, dry it in a hot air drying oven at 105℃ with a wind speed of 1.2 m / s for 2 h to obtain pure aramid short fibers.

[0073] Step S2: The aramid short fibers obtained in S1 were added to a mixed solvent of trifluoroacetic acid and methanesulfonic acid in a volume ratio of 1:1, controlling the aramid mass fraction to be 12%. The mixture was stirred in a reactor at 20°C at 250 r / min for 6 h to form a solution with a viscosity of 3500 cP. 3% graphene, 2% acrylamide, and 1% photoinitiator were added to this solution. The mixture was ultrasonically dispersed at 3°C ​​with a power of 400 W for 30 min, followed by stirring at 80 r / min in the dark for 45 min. Finally, the mixture was placed under vacuum degassing at a negative pressure of 0.095 MPa for 1.5 h.

[0074] Step S3: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22℃. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 80 mW / cm². 2 Irradiate for 40 seconds. The cured fibers are extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0075] Step S4: Vertically fix the nascent fibers obtained in S3 onto a copper cold table, with each fiber being 300mm in length and spaced 8mm apart. Set the temperature at the contact end of the cold table to -35℃ and the temperature at the opposite end to -10℃, creating an axial temperature gradient of 25℃ / m. Freeze at a cooling rate of 2℃ / min for 150min at an ambient humidity of 45%RH to obtain frozen fibers.

[0076] Step S5: Place the frozen fiber obtained in S4 in a freeze-drying chamber at a temperature of -35°C, reduce the chamber pressure to 6Pa, and maintain this pressure for 36 hours to allow the ice crystals to sublimate, thus obtaining dried fiber.

[0077] Step S6: Immerse the dried fibers obtained in S5 into liquid nitrogen at a speed of 15 cm / min, and remove them after 3 minutes of complete immersion. Transfer them to a low-temperature chamber at -80℃ and let them stand for 150 minutes. Increase the temperature to -20℃ at a rate of 0.8℃ / min, and then increase it to 25℃ at a rate of 2.5℃ / min to obtain aerogel fibers.

[0078] Example 2:

[0079] In this embodiment, steps S3 and S6 differ from those in Embodiment 1.

[0080] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 100 mW / cm². 2 Irradiation for 50 seconds (photocrosslinking dose 5.0 J / cm²) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0081] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min for 4 minutes until completely submerged, then removed. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0082] Example 3:

[0083] In this embodiment, steps S3 and S6 differ from those in Embodiment 1.

[0084] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 120 mW / cm². 2 Irradiation for 60 seconds (photocrosslinking dose 7.2 J / cm) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0085] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min, and then removed after being completely submerged for 5 minutes. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0086] Example 4:

[0087] In this embodiment, steps S4 and S6 differ from those in Embodiment 1.

[0088] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed onto a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 30°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0089] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min for 4 minutes until completely submerged, then removed. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0090] Example 5:

[0091] In this embodiment, steps S3, S4, and S6 differ from those in Embodiment 1.

[0092] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 100 mW / cm². 2 Irradiation for 50 seconds (photocrosslinking dose 5.0 J / cm²) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0093] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed onto a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 30°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0094] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min for 4 minutes until completely submerged, then removed. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0095] Example 6:

[0096] In this embodiment, steps S3, S4, and S6 differ from those in Embodiment 1.

[0097] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 120 mW / cm². 2 Irradiation for 60 seconds (photocrosslinking dose 7.2 J / cm) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0098] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed onto a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 30°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0099] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min, and then removed after being completely submerged for 3 minutes. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0100] Example 7:

[0101] In this embodiment, steps S3, S4, and S6 differ from those in Embodiment 1.

[0102] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 80 mW / cm². 2 Irradiation for 40 seconds (photocrosslinking dose 3.2 J / cm) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0103] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed on a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 35°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0104] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min, and then removed after being completely submerged for 5 minutes. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0105] Example 8:

[0106] In this embodiment, steps S3, S4, and S6 differ from those in Embodiment 1.

[0107] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 100 mW / cm². 2 Irradiation for 50 seconds (photocrosslinking dose 5.0 J / cm²) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0108] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed on a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 35°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0109] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min, and then removed after being completely submerged for 3 minutes. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0110] Example 9:

[0111] In this embodiment, steps S3, S4, and S6 differ from those in Embodiment 1.

[0112] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are then guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source is turned on, and the intensity is controlled at 120 mW / cm². 2Irradiation for 60 seconds (photocrosslinking dose 7.2 J / cm) 2 The cured fibers were extracted at a traction speed of 3 m / min, and then rinsed with 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0113] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed on a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 35°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0114] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min for 4 minutes until completely submerged, then removed. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0115] Example 10:

[0116] In this embodiment, steps S4 and S6 differ from those in Embodiment 1.

[0117] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed onto a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. The temperature at the contact end of the cold table is set to -35°C, and the temperature at the opposite end is set to -10°C, forming an axial temperature gradient of 30°C / m. The fibers are then frozen at a cooling rate of 2°C / min for 150 min at an ambient humidity of 45%RH to obtain frozen fibers.

[0118] In step S6 of this embodiment: The dried fibers obtained in S5 are immersed in liquid nitrogen at a speed of 15 cm / min, and then removed after being completely submerged for 5 minutes. They are then transferred to a low-temperature chamber at -80°C and left to stand for 150 minutes. The temperature is then increased to -20°C at a rate of 0.8°C / min, and then increased to 25°C at a rate of 2.5°C / min to obtain aerogel fibers.

[0119] Example 11:

[0120] In this embodiment, step S3 differs from that in embodiment 1, and step S6 is omitted.

[0121] In step S3 of this embodiment: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22°C. When the fibers are immersed 15 mm below the liquid surface, no ultraviolet irradiation treatment is performed. The solidified fibers are pulled out at a traction speed of 3 m / min and then rinsed with a 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0122] Example 12:

[0123] In this embodiment, step S4 differs from that in embodiment 1, and step S6 is omitted.

[0124] In step S4 of this embodiment, the nascent fibers obtained in S3 are vertically fixed on a copper cold table, with each fiber being 300 mm in length and spaced 8 mm apart. Without setting a temperature gradient (the temperature at both the contact end and the opposite end of the cold table is -35°C), the fibers are frozen for 150 minutes at a cooling rate of 2°C / min under an ambient humidity of 45%RH to obtain frozen fibers.

[0125] Comparative Example 1:

[0126] Step S1: Use an opening machine to process waste fire suit fabric, setting the toothed roller speed to 1000 r / min and the tooth gap to 0.8 mm to obtain short fiber bundles with a length of 15 mm. Prepare a cleaning solution containing 0.12 g / g sodium carbonate and 0.06 g / g sodium dodecyl sulfonate using the obtained fiber bundles, and ultrasonically treat it at 70℃ and 40 kHz for 5 h. After treatment, dry it in a hot air drying oven at 105℃ with a wind speed of 1.2 m / s for 2 h to obtain pure aramid short fibers.

[0127] Step S2: The aramid short fibers obtained in S1 were added to a mixed solvent of trifluoroacetic acid and methanesulfonic acid in a volume ratio of 1:1, controlling the aramid mass fraction to be 12%. The mixture was stirred in a reactor at 20°C at 250 r / min for 6 h to form a solution with a viscosity of 3500 cP. 3% graphene was added to this solution. The mixture was ultrasonically dispersed at 3°C ​​with a power of 400 W for 30 min, followed by stirring at 80 r / min in the dark for 45 min. Finally, the mixture was placed under vacuum degassing at a negative pressure of 0.095 MPa for 1.5 h.

[0128] Step S3: The spinning solution obtained in S2 is extruded through a spinneret with an orifice diameter of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers are guided into a coagulation bath with an ethanol to water volume ratio of 7:3 and a bath temperature of 22℃. The fibers are pulled out at a traction speed of 3 m / min and then rinsed with a 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min.

[0129] Step S4: Immerse the fibers in anhydrous ethanol, replacing the solution every 30 minutes, and continue this process for 24 hours. Then transfer the fibers to liquid carbon dioxide and purify them at 15°C and 5 MPa pressure for 8 hours.

[0130] Step S5: Place the fiber in an autoclave and inject liquid carbon dioxide until the volume is 80% full. Increase the temperature to 31.1°C at a rate of 1°C / min, while simultaneously pressurizing to 7.39 MPa. Maintain the supercritical state for 6 hours. Slowly reduce the pressure to atmospheric pressure at a rate of 0.1 MPa / min.

[0131] Step S6: Take out the fiber and let it stand in a constant temperature room at 25℃ for 24 hours with an ambient humidity of 40%RH.

[0132] Experimental Example 1:

[0133] Aerogel fibers were prepared according to the preparation methods provided in Examples 1-12 and Comparative Example 1, respectively. The comparison table of preparation methods is shown below:

[0134] Table 1 Comparison of Preparation Methods

[0135] Group Photocrosslinking dose Axial temperature gradient Cryo treatment time Comparative Example 1 - - - Example 1 <![CDATA[3.2J / cm 2 (80mW / cm 2 ×40s)]]> 25℃ / m 3min Example 2 <![CDATA[5.0J / cm 2 (100mW / cm 2 ×50s)]]> 25℃ / m 4min Example 3 <![CDATA[7.2J / cm 2 (120mW / cm 2 ×60s)]]> 25℃ / m 5min Example 4 <![CDATA[3.2J / cm 2 (80mW / cm 2 ×40s)]]> 30℃ / m 4min Example 5 <![CDATA[5.0J / cm 2 (100mW / cm 2 ×50s)]]> 30℃ / m 4min Example 6 <![CDATA[7.2J / cm 2 (120mW / cm 2 ×60s)]]> 30℃ / m 3min Example 7 <![CDATA[3.2J / cm 2 (80mW / cm 2 ×40s)]]> 35℃ / m 5min Example 8 <![CDATA[5.0J / cm 2 (100mW / cm 2 ×50s)]]> 35℃ / m 3min Example 9 <![CDATA[7.2J / cm 2 (120mW / cm 2 ×60s)]]> 35℃ / m 4min Example 10 <![CDATA[5.0J / cm 2 (100mW / cm 2 ×50s)]]> 30℃ / m 5min Example 11 <![CDATA[5.0J / cm 2 (100mW / cm 2 ×50s)]]> - - Example 12 - 30℃ / m -

[0136] Wherein, photocrosslinking dose = intensity × time.

[0137] Five groups of samples were prepared from the aerogel fibers prepared according to the preparation methods provided in Examples 1-12 and Comparative Example 1, and the following tests were performed:

[0138] 1. Porosity was determined using the liquid impregnation method: Aerogel fiber samples were immersed in anhydrous ethanol under a vacuum of 0.095 MPa for 30 minutes until fully saturated, and then suspended to drain for 30 seconds to remove free liquid from the surface; the wet weight (m) of the saturated samples was weighed using an analytical balance with an accuracy of 0.01 mg. w buoyancy in water m f and the dry weight m after drying at 105℃ for 4 hours d Through formula Calculate the bulk density (ρ) e Take 0.789 g / cm 3 (where is the density of ethanol), final porosity Where the aramid skeleton density ρ s Take 1.44 g / cm 3 ;

[0139] 2. Tensile strength testing: Using an electronic universal testing machine, a single 100mm long aerogel fiber was clamped in a pneumatic fixture with a pretension of 0.05cN / dtex. The clamping distance was set to 50mm, and the tensile speed to 10mm / min until fracture. The maximum breaking strength F (converted to MPa) was recorded. The tensile strength σ was calculated as follows: The fiber cross-sectional area A is calculated using the fiber diameter measurement (at least 30 valid samples per group);

[0140] 3. Orientation degree was determined using SEM-FFT: The fiber sample was quenched in liquid nitrogen and then fractured to obtain a flat cross-section, which was then coated with a 5nm gold film. Five regions (2048×1536 pixels resolution) were randomly acquired along the axial direction using a field emission scanning electron microscope at 5kV and 5000x magnification. The grayscale images were then subjected to a fast Fourier transform using ImageJ software to generate a spectrum. An ellipse was fitted to the spectrum pattern, and the formula was applied... Calculate the orientation degree (a and b are the lengths of the major and minor axes of the ellipse, respectively), and finally take the average of 5 regions; the closer the orientation degree f is to 100%, the higher the axial parallelism of the channel;

[0141] 4. Thermal conductivity was measured using the transient planar heat source method: Three parallel aerogel fibers were placed on a -20℃ constant temperature platform. A 1mm diameter nickel-chromium alloy thin-film probe was used to press vertically against the fiber bundle surface under a contact pressure of 20kPa. A constant heating power of 15mW was applied for 8 seconds, and the temperature response curve was recorded. The measurement was then performed using the mathematical model λ=k·ρ·C. p Calculate thermal conductivity (k is the thermal diffusivity, ρ is the bulk density, C). p The specific heat capacity is taken as 1.2 J / g·K, and the test temperature range covers -40℃ to 80℃;

[0142] 5. Resilience test: Using a spherical indenter (12.7 mm in diameter), the fiber sample is vertically compressed at a rate of 1 mm / s until 50% of the initial thickness is deformed. The load is removed after 30 seconds. The residual deformation δ is recorded 60 seconds after unloading. r With the maximum compressive displacement δ m According to the formula Calculate the rebound rate by repeating the test 10 times under the conditions of ambient temperature 25℃±1℃ and relative humidity 65%±3%.

[0143] The experimental results for the samples are shown in the table below:

[0144] Table 2 Comparison of Experimental Results

[0145]

[0146]

[0147] When the axial temperature gradient is fixed at 30℃ / m, the photocrosslinking dose is 5.0 J / cm. 2 Example 5 yielded the highest tensile strength of 46.8 MPa. The dosage was reduced to 3.2 J / cm. 2In Example 4, the strength decreased to 43.6 MPa, and although the porosity of 94.7% was high, the orientation degree of 96.8% was lower than that of Example 5 (98.1%). The dosage was increased to 7.2 J / cm². 2 In Example 6, the strength dropped to 44.3 MPa, and the thermal conductivity increased to 0.026 W / m·K.

[0148] Fixed photocrosslinking dose 5.0 J / cm 2 Under these conditions, Example 5, with a gradient of 30℃ / m, achieved an orientation degree of 98.1% and a thermal conductivity as low as 0.020 W / m·K. Example 2, with a gradient of 25℃ / m, saw its orientation degree decrease to 94.3% and its thermal conductivity increase to 0.036 W / m·K. Example 8, with a gradient of 35℃ / m, further reduced its orientation degree to 92.1% and its tensile strength decreased to 40.3 MPa.

[0149] At the same photocrosslinking dose of 5.0 J / cm 2 With an axial temperature gradient of 30°C / m, Example 5, treated for 4 minutes, achieved a resilience of 94%. Example 6, treated for 3 minutes, saw its resilience decrease to 89%, while its thermal conductivity increased to 0.026 W / m·K. Example 10, treated for 5 minutes, achieved a resilience of 91%, and although its porosity reached 96.2%, its strength decreased to 45.7 MPa.

[0150] Comparative Example 1, using a conventional supercritical drying process, exhibited a porosity of 88.2%, a tensile strength of 36.8 MPa, a thermal conductivity of 0.045 W / m·K, and a resilience of 62%, reflecting the intrinsic defects of the disordered pore structure. This process relies on physical solvent displacement and supercritical phase transition, resulting in the random aggregation of molecular chains to form isotropic channels, which limits the mechanical and thermal insulation properties.

[0151] Example 5 uses a photocrosslinking dose of 5.0 J / cm 2 Combined with an axial temperature gradient of 30℃ / m, it achieves a comprehensive breakthrough in porosity of 95.6%, tensile strength of 46.8MPa, orientation degree of 98.1%, thermal conductivity of 0.020W / m·K, and resilience of 94%.

[0152] Specifically, during the photocrosslinking stage, ultraviolet radiation excites the homolytic cleavage of phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide molecules. The resulting highly reactive free radicals attack the carbon-carbon double bonds of acrylamide, forming a long-chain network of polyacrylamide. Simultaneously, the free radicals abstract amino hydrogen atoms from the aramid molecular chains, establishing covalent crosslinking bridges between aramid segments. The ultraviolet radiation dose is 5.0 J / cm². 2 Precise control of the reaction rate forms a moderately cross-linked network that provides mechanical support with a tensile strength of 46.8 MPa. The cross-linked network fixes the molecular conformation through covalent bonds, providing a stable framework for subsequent phase transitions.

[0153] Furthermore, an axial temperature gradient of 30℃ / m establishes a thermodynamic driving force, inducing water molecules to align orderly along the fiber's long axis. At the low-temperature end, water molecules bond through hydrogen bonds to form ice crystal nuclei, while at the solid-liquid interface, water molecules grow epitaxially layer by layer to form columnar crystals. This phase transition process is strictly controlled by the axial temperature gradient; a suitable gradient ensures that the ice crystal growth direction aligns with the fiber axis, forming parallel channels with an orientation degree of 98.1%. The channels remaining after ice crystal sublimation effectively extend the phonon transport path, reducing the thermal conductivity to 0.020 W / m·K.

[0154] Furthermore, the three-dimensional network formed by photocrosslinking provides confined space for ice crystal growth, inhibiting radial expansion of ice crystals and ensuring pore continuity. The tensile stress generated by ice crystal growth acts inversely on the crosslinking network, forcing the aramid molecular chains to align parallel to the pore walls, enhancing load-bearing efficiency. This molecular-level interlocking increases tensile strength by 27.2% compared to Comparative Example 1, while simultaneously increasing porosity by 7.4 percentage points. Liquid nitrogen treatment, by transforming the molecular chain locking structure, instantly freezes the molecular chain movement, eliminating drying stress and achieving a resilience rate of 94%, ensuring long-term stability.

[0155] Example 3 used a photocrosslinking dose of 7.2 J / cm. 2 An axial temperature gradient of 25°C / m resulted in a tensile strength of 36.5 MPa, porosity of 89.3%, orientation degree of 88.1%, thermal conductivity of 0.043 W / m·K, and resilience of 65%, significantly lower than in Example 5. This is due to the ultraviolet dose of 7.2 J / cm². 2 The photoinitiator degradation rate was doubled, and the high concentration of free radicals led to excessively high acrylamide polymerization. Differential scanning calorimetry (DSC) showed a crosslinking density of 85%, with the free volume of the molecular chains compressed to below 0.5 nm. The rigid network hindered chain segment movement, reducing the elongation at break by 35%. More seriously, the excessive polyacrylamide chains covered the active sites on the aramid surface, hindering interfacial interactions during subsequent phase transitions. A low-temperature gradient of 25 °C / m reduced the ice crystal growth rate to 5 μm / s, prolonging the solid-liquid interface duration. The rigid crosslinking network could not release stress through chain segment relaxation, leading to microcracks at the ice crystal interface. Scanning electron microscopy showed a crack density of 300 cracks / mm. 2 This becomes a weak point in the mechanical structure. Simultaneously, the excessive adsorption of water molecules by the hydrophilic segments of polyacrylamide disrupts the continuity of the axial temperature gradient, inducing random branching of ice crystals and forming disordered channels. Excessive cross-linking creates a high-rigidity network, inhibiting the molecular chain flexibility required for the epitaxial growth of ice crystals. Slow-growing ice crystals accumulate stress at the interface, triggering microcracks that damage structural integrity. Local solvent enrichment at the cracks forms liquid pools, further disrupting the temperature field and generating dendritic crystals. This vicious cycle ultimately leads to a decrease in orientation degree to 88.1%, and a 22% reduction in tensile strength compared to Example 5. While liquid nitrogen treatment partially repairs the damage, it cannot reverse the molecular-level defects; the rebound rate of only 65% ​​confirms the loss of structural stability.

[0156] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing aerogel fibers from waste aramid fibers under extremely cold conditions, characterized in that, Includes the following steps: S1. Mechanically open and perform composite chemical cleaning on the outer fabric of waste fire suits to obtain pure aramid short fibers; S2. Dissolve the pure aramid short fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, and add graphene, acrylamide and photoinitiator to prepare a photosensitive composite spinning solution; S3. The photosensitive composite spinning solution is extruded into a coagulation bath by wet spinning, and ultraviolet irradiation crosslinking is carried out simultaneously to form nascent fibers. S4. Apply an axial temperature gradient to the nascent fibers to induce directional ice crystal growth, thereby obtaining directional frozen fibers; S5. The directional freeze-dried fiber is subjected to vacuum freeze-drying to obtain an aerogel fiber precursor with axially parallel channels. S6. The aerogel fiber precursor is immersed in liquid nitrogen for cryogenic treatment and then subjected to gradient temperature recovery to obtain aerogel fiber. In step S3, the coagulation bath is a mixture of ethanol and water in a volume ratio of 7:3, and the temperature is 20-25℃; the ultraviolet irradiation conditions are a wavelength of 365nm, an intensity of 80-120mW / cm², a time of 40-60s, and the irradiation position is 10-20mm below the surface of the coagulation bath. In step S4, the nascent fibers are vertically clamped onto a copper cold table. The axial temperature gradient of the copper cold table is 30-35℃ / m, with the contact end temperature being -40 to -30℃ and the opposite end temperature being -10 to -5℃. The freezing and cooling rate is 1-3℃ / min, the ambient humidity is 40-50%RH, and the freezing time is 120-180min. The cryogenic treatment described in step S6 includes immersing the aerogel fiber precursor in liquid nitrogen at -196℃ at a speed of 10-20 cm / min for 3-5 min, transferring it to a -80℃ low-temperature chamber for equilibration for 120-180 min, raising the temperature to -20℃ at a rate of 0.5-1℃ / min, and then raising it to 25℃ at a rate of 2-3℃ / min.

2. The method according to claim 1, characterized in that, In step S1, the mechanical opening includes processing waste fire suit fabric with an opening machine having a toothed roller speed of 800-1200 rpm and a tooth gap of 0.5-1 mm to obtain fiber bundles with a length of 10-20 mm; the composite chemical cleaning includes immersing the fiber bundles in a composite cleaning solution and ultrasonically treating them at 65-75℃ for 4-6 hours; the composite cleaning solution includes 0.1-0.15 g of sodium carbonate, 0.05-0.08 g of sodium dodecyl sulfonate and 50-60 ml of water per gram of fiber.

3. The method according to claim 1, characterized in that, In step S2, the volume ratio of trifluoroacetic acid to methanesulfonic acid in the mixed solvent is 1:1; the mass fraction of the pure aramid short fiber in the mixed solvent is 10-15 wt%.

4. The method according to claim 1, characterized in that, Step S2 includes a mixed solution of the pure aramid short fiber, graphene, acrylamide and photoinitiator, comprising 3-8 wt% graphene, 1-3 wt% acrylamide and 0.5-2 wt% photoinitiator by weight of aramid.

5. The method according to claim 4, characterized in that, The mixed solution was ultrasonically dispersed in an ice bath at 0-5℃ for 20-40 minutes, and then degassed under a vacuum of -0.09 to -0.1 MPa for 1-2 hours.

6. The method according to claim 1, characterized in that, In step S5, the freeze-drying conditions are: chamber temperature -38 to -32℃, vacuum degree 5-8Pa, sublimation rate 0.2-0.5mm / h, and drying time 24-48h; the drying endpoint is determined when the fiber weight loss rate reaches 99.5%.

7. An aerogel fiber prepared from waste aramid fibers under extremely cold conditions, prepared according to the method of any one of claims 1-6, characterized in that, The aerogel fiber has a pore diameter of 1-20 μm, an aspect ratio of 50-100, an axial parallel through-hole ratio of more than 90%, and a porosity of 92-97%.

Images