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

Abstract

The invention relates to aerogel fibers prepared from waste aramid fibers under an extremely cold condition and a method thereof, and belongs to the technical field of textile materials. The method comprises the following steps: carrying out mechanical opening and composite chemical cleaning on the outer-layer fabric of the waste firefighter uniform to obtain pure aramid short fibers; dissolving in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, and adding graphene, an acrylamide monomer and a photoinitiator to prepare a photosensitive composite spinning solution; extruding into a coagulating bath through wet spinning, and synchronously performing ultraviolet radiation crosslinking; applying an axial temperature gradient to the nascent fiber to induce directional ice crystal growth, and freeze-drying to form an axial parallel pore channel precursor; finally, the aerogel fiber is obtained through liquid nitrogen subzero treatment and gradient temperature returning shaping. By controlling the molecular structure and form in the fiber preparation process, efficient recycling of the waste aramid fiber is achieved, and the prepared aerogel fiber has the excellent performance of high porosity, high strength, low heat conductivity and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of textile materials, and in particular relates to an aerogel fiber prepared by using waste aramid fibers under extremely cold conditions and a method thereof. Background Art

[0002] As firefighting equipment reaches the end of their service lives, over 100,000 tons of used firefighting uniforms are generated globally each year. Some of these uniforms are made from composite outer fabrics of aramid 1414, conductive fibers, and flame-retardant coatings. Aramid fiber, with its excellent properties such as high temperature resistance and strength, has significant economic and environmental benefits if effectively recycled and reused.

[0003] At present, there are mainly the following technical routes for the recycling of waste aramid fibers:

[0004] The first is the traditional physical and mechanical recycling method, which converts waste aramid fibers into short fibers or fiber bundles through simple mechanical crushing or opening treatment. However, the fiber performance after treatment by this method is significantly reduced, and its application range is limited.

[0005] The second is chemical degradation, which uses strong acid or alkali to decompose aramid fibers into small molecular compounds and then repolymerize them. However, this method is complex, costly, and causes significant environmental pollution.

[0006] The third is the wet spinning and reshaping method, which reshapes 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 and reshaping route, the inability to effectively control the internal structure of the fiber results in disordered pore distribution in the resulting fiber, which cannot meet the comprehensive requirements of high-performance thermal insulation and mechanical strength of the material. In addition, there are deficiencies in the treatment of the conductive fiber and flame-retardant coating in the aramid fiber during the recycling process, making it difficult to effectively separate and fully utilize the various components, posing a huge challenge to the efficient recycling of waste fire suits.

[0008] To this end, the present invention proposes a method for preparing aerogel fibers using waste aramid fibers under extremely cold conditions to solve the above problems. Summary of the Invention

[0009] The present invention overcomes the deficiencies of the prior art and provides an aerogel fiber prepared by using waste aramid fibers under extremely cold conditions and a method thereof.

[0010] To achieve the above-mentioned object, the technical solution adopted by the present invention is: a method for preparing aerogel fibers using waste aramid fibers under extremely cold conditions, comprising the following steps:

[0011] S1. Mechanically loosen and perform composite chemical cleaning on the outer fabric of waste firefighting uniforms to obtain pure aramid staple fibers;

[0012] S2, dissolving pure aramid staple fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, adding graphene, acrylamide and a photoinitiator to prepare a photosensitive composite spinning solution;

[0013] S3, squeezing the photosensitive composite spinning solution into a coagulation bath by wet spinning, and simultaneously performing ultraviolet radiation cross-linking to form nascent fibers;

[0014] S4, applying an axial temperature gradient to the nascent fiber to induce directional ice crystal growth, thereby obtaining directional frozen fiber;

[0015] S5. vacuum freeze-drying the oriented frozen fibers to obtain an aerogel fiber precursor having axially parallel pores;

[0016] S6. Immersing the aerogel fiber precursor in liquid nitrogen for cryogenic treatment, and then performing gradient temperature recovery to shape the aerogel fiber.

[0017] Furthermore, in step S1, mechanical opening includes using an opener with a tooth roller speed of 800-1200 rpm and a tooth gap spacing of 0.5-1 mm to process the waste firefighting clothing fabric to obtain a fiber bundle with a length of 10-20 mm; the composite chemical cleaning includes immersing the fiber bundle in a composite cleaning solution and ultrasonically treating it at 65-75°C for 4-6 hours; the cleaning solution includes 0.1-0.15 g of sodium carbonate, 0.05-0.08 g of sodium dodecyl sulfate, and 50-60 ml of water per gram of fiber.

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

[0019] Furthermore, in step S2, the mixed solution of pure aramid staple fibers, graphene, acrylamide and photoinitiator comprises 3-8 wt% of graphene, 1-3 wt% of acrylamide and 0.5-2 wt% of photoinitiator based on the mass of the aramid.

[0020] Furthermore, the mixed solution was dispersed in an ice bath at 0-5° C. by ultrasonication for 20-40 min, and then degassed at a vacuum degree of -0.09 to -0.1 MPa for 1-2 h.

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

[0022] Furthermore, in step S4, the spun fiber is vertically clamped on a copper cold plate, the axial temperature gradient of the copper cold plate is 25-35°C / m, the contact end temperature is -40 to -30°C, and the opposite end temperature is -10 to -5°C; the freezing cooling rate is 1-3°C / min, the ambient humidity is 40-50% RH, and the freezing time is 120-180 min.

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

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

[0025] Another technical solution provided by the present invention is: an aerogel fiber prepared using 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 axially parallel through-hole ratio greater than 90%, and a porosity of 92-97%.

[0026] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0027] The present invention recovers pure aramid staple fibers from waste firefighting uniforms through mechanical opening and composite chemical cleaning, dissolves the fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, and then adds graphene, acrylamide and a photoinitiator to form a photosensitive composite spinning solution; when the fibers are wet-spun and squeezed into a coagulation bath, ultraviolet irradiation is simultaneously implemented to trigger in-situ crosslinking, and then an axial temperature gradient is applied to the nascent fibers to induce directional ice crystal growth, which is then freeze-dried to form an axial channel precursor, and finally aerogel fibers are obtained through liquid nitrogen deep freezing treatment and gradient temperature recovery shaping, thereby converting the waste aramid fibers into aerogel fibers with high porosity, high strength and low thermal conductivity, thereby achieving the goals of solid waste resource utilization and preparation of materials adaptable to extreme environments.

[0028] By adding acrylamide monomer and photoinitiator, the photopolymerization was carried out simultaneously in the coagulation bath at a wavelength of 365 nm and an intensity of 80 to 120 mW / cm 2Ultraviolet radiation triggers a free radical polymerization reaction, forming a covalent crosslink network between the aramid molecular chains. This process significantly increases the fiber crosslink density, directly enhancing the material's intrinsic strength. Compared to traditional wet spinning processes without light crosslinking, this achieves a significant increase in tensile strength. The crosslinked network also provides a rigid framework for subsequent ice crystal growth, effectively suppressing microcracks caused by phase transition stress.

[0029] A copper cold stage is used to apply an axial temperature gradient of 25 to 35°C / m, controlling the temperature of the contact end from -40°C to -30°C and the temperature of the opposite end from -10°C to -5°C, with directionally freezing at a rate of 1 to 3°C / min. This gradient field drives the ice crystals to grow in an orderly manner along the long axis of the fiber to form a columnar template, so that the pore orientation reaches an extremely high level. Compared with the disordered pore structure, the axially parallel through-holes significantly extend the phonon transmission path. The optically cross-linked network synergistically limits the radial expansion of ice crystals, ensuring the continuity and high proportion of the pores, and significantly reducing the thermal conductivity of the material.

[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 reversely pulls the molecular chains to align along the pore wall, solving the mutually exclusive bottleneck of strength and thermal insulation performance of high-porous materials, and achieving a simultaneous leap in tensile strength and thermal insulation efficiency.

[0031] The aerogel precursor is immersed in liquid nitrogen at -196°C for 3 to 5 minutes at a speed of 10 to 20 cm / min, completely submerging it. This achieves ultra-rapid freezing of the molecular chain motion. After equilibration at -80°C, the temperature is gradually returned to room temperature at a rate of 0.5 to 1°C / min to eliminate residual internal stresses from the drying process. Compared to the untreated material, the rebound rate is significantly improved. The ultra-rapid freezing of the liquid nitrogen, combined with the topological morphology of the photocrosslinked network and the ice crystal template, ensures that the highly porous structure remains intact during extreme temperature fluctuations from -196°C to 25°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0033] Figure 1 This is a flow chart of a method for preparing aerogel fibers using waste aramid fibers under extremely cold conditions;

[0034] Figure 2 This is an electron microscope image of aerogel fibers. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0036] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below. Unless otherwise specified in the following examples, all raw materials are commercially available or prepared by conventional methods in the art. Among them, trifluoroacetic acid was purchased from Sinopharm, with a specification of AR (Shanghai test); methanesulfonic acid was purchased from Sinopharm, with a specification of AR (Shanghai test); acrylamide was purchased from Sinopharm, with a specification of AR (Shanghai test); the photoinitiator is phenyl bis (2,4,6-trimethylbenzoyl) phosphine oxide, purchased from Aladdin, with a purity of ≥97%; used firefighting suits were purchased from the local fire department, and the outer fabric includes aramid 1414 (accounting for ≥85%), carbon fiber conductive yarn (antistatic, accounting for 5-8%), and polytetrafluoroethylene (PTFE) flame retardant coating (thickness 0.2-0.5mm).

[0037] Exemplary methods:

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

[0039] S1. Mechanically loosen and perform composite chemical cleaning on the outer fabric of waste firefighting uniforms to obtain pure aramid staple fibers;

[0040] S2, dissolving pure aramid staple fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, adding graphene, acrylamide and a photoinitiator to prepare a photosensitive composite spinning solution;

[0041] S3, squeezing the photosensitive composite spinning solution into a coagulation bath of ethanol and water by wet spinning, and simultaneously performing ultraviolet radiation cross-linking to form nascent fibers;

[0042] S4, applying an axial temperature gradient to the nascent fiber to induce directional ice crystal growth, thereby obtaining directional frozen fiber;

[0043] S5. vacuum freeze-drying the oriented frozen fibers to obtain an aerogel fiber precursor having axially parallel pores;

[0044] S6. Immersing the aerogel fiber precursor in liquid nitrogen for cryogenic treatment, and then performing gradient temperature recovery to shape the aerogel fiber.

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

[0046] Step S1 aims to efficiently separate pure and soluble aramid staple fibers from the outer fabric of discarded firefighter uniforms. The outer layers of firefighter uniforms are typically made of high-performance materials such as aramid 1313 or aramid 1414, combined with conductive fibers and flame-retardant coatings. These components require targeted separation during the recycling process.

[0047] In this step, the fabric of the used firefighting uniforms is mechanically opened, using an opener to apply directional tearing and combing forces to the outer fabric of the used firefighting uniforms. By adjusting the gear roller speed between 800 and 1200 rpm and maintaining a tooth gap spacing of 0.5 to 1 mm, the densely woven structure is dissociated into discrete fiber bundles, which are fully fluffed and controlled to a length of 10 to 20 mm. The opening process simultaneously removes metal fittings and non-fiber impurities, ultimately yielding short fiber bundles with a length of 10 to 20 mm. This avoids dust loss from fibers that are too short or reduced solvent penetration efficiency from fibers that are too long. At this point, the fibers still retain residue from the flame-retardant coating.

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

[0049] The treatment is continued at 100°C to 110°C for 1.5 to 2.5 hours, with a hot air flow rate of 1 to 1.5 m / s. Molecular thermal motion drives water diffusion and desorption, resulting in a final fiber moisture content of no more than 0.5wt%. The dried fibers are then subjected to a secondary treatment using precision shearing equipment with a blade gap set between 0.1mm and 0.3mm, reducing their length to less than 5mm and increasing their specific surface area.

[0050] Step S2 aims to construct a composite spinning solution system with both spinnability and photocuring capabilities. This step uses the chopped aramid fibers obtained in step S1 as the core solute, achieving molecular-level dissociation using a specific acid solvent. The graphene reinforcement phase and photosensitive components are then introduced simultaneously to impart multifunctional properties to the material.

[0051] In this step, the chopped aramid fibers obtained in S1 are put into a mixed solvent system of trifluoroacetic acid and methanesulfonic acid, and the volume ratio of trifluoroacetic acid to methanesulfonic acid is controlled to be 1 to 1. The dissolution process is carried out in a closed reactor, and the temperature is maintained in the range of 15°C to 25°C to avoid solvent volatilization. The mass fraction of the aramid fiber is controlled in the range of 10wt% to 15wt%, balancing the viscosity of the solution and the spinnability. The treatment is continued at a mechanical stirring rate of 200rpm to 300rpm for 5h to 7h until a uniform, transparent, viscous solution is formed, which is an aramid solution with a dynamic viscosity reaching a spinnable window of 1500cP to 5000cP. The strong fluorination effect of trifluoroacetic acid destroys the hydrogen bonds between aramid molecules, and methanesulfonic acid provides a protonated environment to promote molecular chain dissociation, thereby achieving complete dissolution of aramid to form a viscous colloidal solution.

[0052] Graphene powder, acrylamide reaction monomer and photoinitiator are added to the dissolved aramid solution in sequence to obtain a mixed solution. The amount of graphene powder added is 3wt% to 8wt% of the mass of the aramid. An ice bath ultrasonic dispersion device is used to treat it at 0℃ to 5℃ for 20min to 40min, and the ultrasonic power is controlled in the range of 300W to 500W to uniformly disperse the graphene in the form of a single layer. Acrylamide functional monomer is added simultaneously, with a mass of 1wt% to 3wt% of the aramid. The carbon-carbon double bond contained in the acrylamide functional monomer will provide cross-linking sites for subsequent photocuring. Finally, phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide is added as a photoinitiator at a mass of 0.5wt% to 2wt% of the aramid mass. Stir at a low speed of 50rpm to 100rpm for 30min to 60min under light-proof conditions to ensure molecular mixing of each component.

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

[0054] Step S3 aims to achieve fiber morphology through wet spinning and utilizes ultraviolet radiation to trigger an in-situ crosslinking reaction to form a three-dimensional network structure. This step uses the photosensitive spinning solution prepared in step S2 as raw material. Phase separation is completed in the coagulation bath, and photocuring is performed simultaneously to lock the molecular conformation, 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 device and transported to the spinneret by a metering pump at a stable flow rate of 0.5ml / min to 1ml / min. The spinneret aperture ranges from 0.1mm to 0.2mm, balancing the extrusion pressure and fiber diameter control requirements. After the spinning solution is extruded from the spinneret hole, it immediately enters a coagulation bath system composed of ethanol and water, with a volume ratio of ethanol to water equal to 7 to 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 reaggregate. This process forms the cortical structure of the primary fiber within 5s to 10s.

[0056] The key to this step is to implement UV crosslinking simultaneously with the spinning solution entering the coagulation bath. Specifically, when the raw fiber is immersed in the coagulation bath at a depth of 10mm to 20mm, a UV light source with a wavelength of 365nm is started to irradiate the fiber surface vertically. The irradiation intensity is controlled at 80mW / cm 2 Up to 120mW / cm 2 The irradiation time ranges from 30s to 90s. UV photons excite the phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide photoinitiator in the spinning solution, causing it to cleave and produce benzoyl radicals. These reactive radicals attack the carbon-carbon double bonds of acrylamide, initiating a chain polymerization reaction. Simultaneously, through hydrogen abstraction, they form covalent crosslinks with amino groups on the aramid molecular chains.

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

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

[0059] In this step, the newly cross-linked fibers are vertically fixed to the surface of a copper cold plate. Individual fibers are kept between 20 and 50 cm in length, with a spacing of 5 to 10 mm to prevent adhesion. The copper cold plate is constructed from 99.9% pure electrolytic copper with a thermal conductivity of 398 W / (m·K), ensuring a linear temperature gradient. During clamping, low-thermal-resistance silicone grease is applied to the interface between the fiber and the cold plate to minimize interfacial heat loss.

[0060] The fiber-clamping cold stage is placed in a programmable temperature control device. The temperature of the contact end of the cold stage is set to -30°C to -40°C, and the opposite end is maintained at -5°C to -10°C, forming 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, and the ambient humidity is controlled in 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. This process lasts for 120 to 180 minutes until complete freezing.

[0061] Under the action of the axial temperature gradient, the water molecules inside the fiber undergo a phase transition process of nucleation and growth. The low-temperature end first reaches a supercooled state, and the water molecules are orderly combined through hydrogen bonds to form ice crystal nuclei. The temperature gradient drives the ice crystal front to extend in a directional manner along the long axis of the fiber, and the water molecules are orderly arranged layer by layer at the solid-liquid interface to form a columnar crystal template. This process is essentially the result of a balance between the thermodynamic driving force (temperature gradient) and the kinetic conditions (molecular diffusion rate). When the temperature gradient is insufficient, the ice crystals branch randomly, and when the gradient is too high, the growth is too rapid and defects are generated.

[0062] The growth behavior of ice crystals during freezing is controlled by multiple parameters. The temperature gradient of 25°C / m to 35°C / m was chosen based on research on ice crystal dynamics. This range stabilizes the ice crystal growth rate within a range of 5μm / s to 10μm / s, ensuring the formation of columnar crystals with diameters of 1μm to 20μm. The cooling rate of 1°C / min to 3°C / min avoids supercooling exceeding 10°C, which prevents the formation of random dendrites. The resulting ice crystals have an aspect ratio of 50 to 100, with an axial parallelism deviation of less than 5°, laying the structural foundation for the formation of directional channels during subsequent sublimation.

[0063] Step S5 aims to achieve directional sublimation of ice crystals through vacuum freeze-drying, transforming the ice crystal template created in step S4 into an axially parallel pore structure. This step, using pre-frozen fibers, controls the phase transition process in a simulated extreme cold vacuum environment, ensuring precise replication and high-fidelity finalization of the structure.

[0064] The directionally frozen fibers, along with the copper cold plate, are transferred to a freeze-drying chamber. The chamber is pre-cooled to a temperature between -38°C and -32°C, strictly below the ice crystal eutectic point, to suppress recrystallization. The vacuum system is activated to reduce the chamber pressure to below 10 Pa, with an optimal operating range of 5 Pa to 8 Pa. This vacuum environment reduces the water vapor partial pressure, allowing the ice crystals to undergo a solid-gas phase transition and sublimate directly, thus avoiding the risk of pore collapse caused by the formation of a liquid phase.

[0065] The chamber temperature is maintained at a constant -35℃±3℃, and the vacuum fluctuation range is controlled within ±1Pa. Under this condition, the sublimation rate of ice crystals is stable at 0.2mm / h to 0.5mm / h, and the entire drying process lasts 24h to 48h. The mass loss is monitored in real time, and the drying is terminated when the weight loss rate approaches 99.5%. In this step, the ice crystals sublimate in an orderly manner along the axial direction, and the remaining pores perfectly replicate the original ice crystal morphology, forming parallel through holes with a diameter of 1μm to 20μm and an aspect ratio of 50 to 100.

[0066] Step S6 aims to lock the microstructure of the aerogel fiber obtained in step S5 through extreme cold conditions, eliminate the internal stress remaining in the drying process, and improve the stability of the pore morphology. It should be noted that extreme cold conditions do not refer to low-temperature environments in general, but specifically refer to process links centered on liquid nitrogen cryogenic treatment (-196°C) and its adapted application scenarios. Specifically, this step uses freeze-dried aerogel fibers as the processing object, uses the extreme low temperature of liquid nitrogen to achieve instant freezing of polymer chain movement, and combines a gradient warming strategy to avoid thermal shock damage.

[0067] In this step, the dried aerogel fiber bundle is immersed in a liquid nitrogen storage tank at a uniform speed of 10 cm / min to 20 cm / min to ensure that the fiber is completely immersed in the liquid nitrogen medium at -196°C. The immersion time is strictly controlled in the range of 3 minutes to 5 minutes. This time period can reduce the temperature of the fiber core layer from 25°C to below -150°C within 60 seconds, achieving a sudden stagnation of the molecular chain movement. The operation process uses a titanium alloy clamp with a thermal conductivity as low as 7W / (m·K), which can reduce the loss of cooling capacity. When the temperature drops sharply below the transition point, the molecular chain movement is instantly frozen, and the pore wall structure is locked in an expanded state. This process eliminates drying stress by suppressing the thermal motion of the molecular chain and avoids the collapse of the pores when the temperature returns to normal.

[0068] After removing the fiber from liquid nitrogen, transfer it to a cryogenic chamber pre-cooled to -80°C within 3 seconds and maintain the temperature for 120 to 180 minutes to achieve uniform temperature. The temperature is then raised to -20°C at a rate of 0.5°C / min to 1°C / min, and then to 25°C at a rate of 2°C / min to 3°C / min. This stepwise heating allows for gradual relaxation of the polymer chains, preventing cracking of the pore walls due to thermal stress.

[0069] Example products:

[0070] like Figure 2 As shown, an aerogel fiber is prepared using waste aramid fiber under extremely cold conditions. Based on the above exemplary method, the aerogel fiber has a pore diameter of 1-20 μm, an aspect ratio of 50-100, a proportion of axially parallel through holes greater than 90%, and a porosity of 92-97%.

[0071] Example 1:

[0072] Step S1: Use an opener to process discarded firefighting uniform fabric, setting the gear roller speed at 1000 rpm and the tooth gap spacing at 0.8 mm to obtain a 15 mm long staple fiber bundle. The resulting fiber bundle is then treated with a cleaning solution containing 0.12 g sodium carbonate / g fiber and 0.06 g sodium dodecyl sulfate / g fiber. The resulting fiber bundle is then ultrasonically treated at 70°C and 40 kHz for 5 hours. After treatment, the fiber bundle is dried in a hot air drying oven at 105°C at a wind speed of 1.2 m / s for 2 hours to obtain pure aramid staple fibers.

[0073] Step S2: The aramid staple fibers obtained in S1 were placed in a 1:1 (volume ratio) mixture of trifluoroacetic acid and methanesulfonic acid, with the aramid mass fraction controlled to 12%. The mixture was stirred in a reactor at 250 rpm for 6 hours at 20°C to form a solution with a viscosity of 3500 cP. To this solution were added 3% graphene, 2% acrylamide, and 1% photoinitiator. Ultrasonic dispersion was performed at 400 W at 3°C ​​for 30 minutes, followed by stirring at 80 rpm in the dark for 45 minutes. Finally, the mixture was vacuum degassing at a negative pressure of 0.095 MPa for 1.5 hours.

[0074] Step S3: The spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were then introduced into a coagulation bath with a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 80 mW / cm 2 The solidified fibers were extracted at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and ultrasonically cleaned with water for 8 min to obtain spun fibers.

[0075] Step S4: The spun fibers obtained in S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C on the contact end and -10°C on the opposite end, creating an axial temperature gradient of 25°C / m. The fibers were frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0076] Step S5: The frozen fiber obtained in S4 is placed in a freeze-drying chamber at a temperature of -35°C, and the chamber pressure is reduced to 6 Pa, and maintained for 36 hours to allow ice crystals to sublime, thereby obtaining dry fiber.

[0077] Step S6: Immerse the dried fiber obtained in S5 in liquid nitrogen at a speed of 15 cm / min, completely submerge for 3 minutes, and then remove it. Transfer it to a low-temperature chamber at -80°C and let it rest for 150 minutes. Raise the temperature to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min to obtain an aerogel fiber.

[0078] Example 2:

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

[0080] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were then introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 100 mW / cm 2 , irradiation 50s (photocrosslinking dose 5.0J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0081] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 4 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0082] Example 3:

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

[0084] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 120 mW / cm 2 , irradiation 60s (photocrosslinking dose 7.2J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0085] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 5 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0086] Example 4:

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

[0088] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C at the contact end and -10°C at the opposite end, creating an axial temperature gradient of 30°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0089] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 4 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0090] Example 5:

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

[0092] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were then introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 100 mW / cm 2 , irradiation 50s (photocrosslinking dose 5.0J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0093] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C at the contact end and -10°C at the opposite end, creating an axial temperature gradient of 30°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0094] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 4 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0095] Example 6:

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

[0097] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 120 mW / cm 2 , irradiation 60s (photocrosslinking dose 7.2J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0098] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C at the contact end and -10°C at the opposite end, creating an axial temperature gradient of 30°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0099] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 3 minutes, and then removed. The fiber was transferred to a low-temperature chamber at -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0100] Example 7:

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

[0102] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 80 mW / cm 2 , irradiation 40s (photocrosslinking dose 3.2J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0103] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C on the contact end and -10°C on the opposite end, creating an axial temperature gradient of 35°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0104] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 5 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0105] Example 8:

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

[0107] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were then introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 100 mW / cm 2 , irradiation 50s (photocrosslinking dose 5.0J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0108] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C on the contact end and -10°C on the opposite end, creating an axial temperature gradient of 35°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0109] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 3 minutes, and then removed. The fiber was transferred to a low-temperature chamber at -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0110] Example 9:

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

[0112] In step S3 of this embodiment, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form nascent fibers. The nascent fibers were introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. When the fibers were immersed 15 mm below the liquid surface, a 365 nm ultraviolet light source was turned on with an intensity of 120 mW / cm 2, irradiation 60s (photocrosslinking dose 7.2J / cm 2 The solidified fibers were drawn out at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned with water for 8 min to obtain nascent fibers.

[0113] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C on the contact end and -10°C on the opposite end, creating an axial temperature gradient of 35°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0114] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 4 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0115] Example 10:

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

[0117] In this example, step S4: The spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. The platen temperature was set at -35°C at the contact end and -10°C at the opposite end, creating an axial temperature gradient of 30°C / m. The fibers were then frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0118] In step S6 of this embodiment, the dried fiber obtained in step S5 was immersed in liquid nitrogen at a speed of 15 cm / min, completely immersed for 5 minutes, and then removed. The fiber was transferred to a low-temperature chamber at a temperature of -80°C and allowed to stand for 150 minutes. The temperature was then raised to -20°C at a rate of 0.8°C / min, and then to 25°C at a rate of 2.5°C / min, thereby obtaining an aerogel fiber.

[0119] Example 11:

[0120] In this embodiment, the difference from embodiment 1 is step S3, and step S6 is eliminated.

[0121] In step S3 of this example, the spinning solution obtained in step S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form spun fibers. The spun fibers were then introduced into a coagulation bath having a volume ratio of ethanol to water of 7:3 and a bath temperature of 22°C. No UV irradiation was performed when the fibers were immersed 15 mm below the liquid surface. The solidified fibers were then removed at a pulling speed of 3 m / min, rinsed with a 25 vol% ethanol aqueous solution, and then ultrasonically cleaned with water for 8 minutes to obtain spun fibers.

[0122] Example 12:

[0123] In this embodiment, the difference from Embodiment 1 is step S4, and step S6 is eliminated.

[0124] In step S4 of this embodiment, the spun fibers obtained in step S3 were vertically fixed to a copper cooling platen, each 300 mm long and 8 mm apart. No temperature gradient was set (the contact end and the opposing end of the cooling platen were both at -35°C), and the fibers were frozen at a cooling rate of 2°C / min for 150 min in an ambient humidity of 45% to obtain frozen fibers.

[0125] Comparative Example 1:

[0126] Step S1: Use an opener to process discarded firefighting uniform fabric, setting the gear roller speed at 1000 rpm and the tooth gap spacing at 0.8 mm to obtain a 15 mm long staple fiber bundle. The resulting fiber bundle is then treated with a cleaning solution containing 0.12 g sodium carbonate / g fiber and 0.06 g sodium dodecyl sulfate / g fiber. The resulting fiber bundle is then ultrasonically treated at 70°C and 40 kHz for 5 hours. After treatment, the fiber bundle is dried in a hot air drying oven at 105°C at a wind speed of 1.2 m / s for 2 hours to obtain pure aramid staple fibers.

[0127] Step S2: The aramid staple fibers obtained in S1 were placed in a 1:1 (volume ratio) mixture of trifluoroacetic acid and methanesulfonic acid, with the aramid mass fraction controlled to 12%. The mixture was stirred in a reactor at 250 rpm for 6 hours at 20°C to form a solution with a viscosity of 3500 cP. 3% graphene was added to this solution. Ultrasonic dispersion was performed at 400 W at 3°C ​​for 30 minutes, followed by stirring at 80 rpm in the dark for 45 minutes. Finally, the mixture was vacuum degassing at a negative pressure of 0.095 MPa for 1.5 hours.

[0128] Step S3: The spinning solution obtained in S2 was extruded through a spinneret with a pore size of 0.15 mm at a flow rate of 0.8 mL / min to form spun fibers. The spun fibers were then introduced into a coagulation bath with a volume ratio of ethanol to water of 7:3 at a temperature of 22°C. The fibers were then withdrawn at a pulling speed of 3 m / min and rinsed with a 25 vol% ethanol aqueous solution and then ultrasonically cleaned for 8 minutes.

[0129] Step S4: Immerse the fiber in anhydrous ethanol, replacing the solution every 30 minutes for 24 hours, and then transfer it to liquid carbon dioxide for replacement 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. Raise the temperature to 31.1°C at a rate of 1°C / min while simultaneously increasing the pressure to 7.39 MPa. Maintain the supercritical state for 6 hours. Slowly reduce the pressure to ambient pressure at a rate of 0.1 MPa / min.

[0131] Step S6: Take out the fiber and place it in a constant temperature room at 25° C. 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 preparation method comparison table is shown in the following table:

[0134] Table 1 Preparation method comparison table

[0135] Group Photocrosslinking dose Axial temperature gradient Cryogenic 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 using 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. Determination of porosity by liquid immersion method: Immerse the aerogel fiber sample in anhydrous ethanol for 30 minutes under a vacuum degree of 0.095 MPa until it is completely saturated, and then hang it to drain for 30 seconds to remove the free liquid on the surface; use an analytical balance with an accuracy of 0.01 mg to weigh the wet weight of the saturated sample m w , buoyancy in water m f And the dry weight after drying at 105℃ for 4 hours m d , through the formula Calculate the volume density (ρ e Take 0.789g / cm 3 , is the density of ethanol), the final porosity The aramid skeleton density ρ s Take 1.44g / cm 3 ;

[0139] 2. Tensile strength test: Using an electronic universal material testing machine, a single aerogel fiber with a length of 100 mm was clamped in a pneumatic clamp with a pre-tension of 0.05 cN / dtex. The clamping distance was set to 50 mm and the tensile speed was set to 10 mm / min until it broke. The maximum breaking strength F (unit converted to MPa) was recorded. The tensile strength σ is specifically The fiber cross-sectional area A is calculated by measuring the fiber diameter (at least 30 valid samples per group);

[0140] 3. Orientation was determined by SEM-FFT method: the fiber sample was quenched in liquid nitrogen and then fractured to obtain a flat cross section, and a 5 nm gold film was sprayed on it. Five random area images (resolution 2048 × 1536 pixels) were collected along the axial direction using a field emission scanning electron microscope at 5 kV and 5000 times magnification. The grayscale image was subjected to fast Fourier transform using ImageJ software to generate a spectrum. The spectrum pattern was fitted with an ellipse and the formula was used. 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 the five 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 plane heat source method: three parallel aerogel fibers were placed on a -20°C constant temperature platform. A 1mm diameter nickel-chromium alloy thin film probe was pressed 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 and analyzed using the mathematical model λ = k·ρ·C p Calculate thermal conductivity (k is the thermal diffusivity, ρ is the volume 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. Rebound rate test: Use a spherical indenter (diameter 12.7 mm) to vertically compress the fiber sample at a rate of 1 mm / s until the initial thickness is 50% deformed, hold for 30 seconds, and then unload the load; record the residual deformation δ at 60 seconds after unloading r and the maximum compression displacement δ m , according to the formula Calculate the rebound rate and repeat the test 10 times under the conditions of ambient temperature 25℃±1℃ and relative humidity 65%±3%.

[0143] The experimental results of the samples are shown in the following table:

[0144] Table 2 Experimental results comparison table

[0145]

[0146]

[0147] When the axial temperature gradient is fixed at 30℃ / m, the photocrosslinking dose is 5.0J / cm 2 The highest tensile strength of 46.8 MPa was obtained in Example 5. The dose was reduced to 3.2 J / cm 2The strength of Example 4 dropped to 43.6 MPa, and the porosity was 94.7%, but the orientation degree was 96.8%, which was lower than 98.1% of Example 5. The dose was increased to 7.2 J / cm 2 The strength of Example 6 dropped back 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 30°C / m gradient, achieved an orientation degree of 98.1% and a thermal conductivity as low as 0.020 W / m·K. Example 2, with a 25°C / m gradient, saw its orientation degree drop to 94.3%, while its thermal conductivity rose to 0.036 W / m·K. Example 8, with a 35°C / m gradient, saw its orientation degree further drop to 92.1%, while its tensile strength decayed to 40.3 MPa.

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

[0150] Comparative Example 1, using a traditional supercritical drying process, achieved a porosity of 88.2%, a tensile strength of 36.8 MPa, a thermal conductivity of 0.045 W / m·K, and a rebound of 62%, reflecting the inherent defects of the disordered pore structure. This process relies on physical solvent displacement and supercritical phase transition, causing the molecular chains to randomly aggregate to form isotropic pores, which limits 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°C / m, a comprehensive breakthrough was achieved in porosity of 95.6%, tensile strength of 46.8MPa, orientation degree of 98.1%, thermal conductivity of 0.020W / m·K, and rebound rate of 94%.

[0152] Specifically, during the photocrosslinking stage, UV radiation stimulates the homolytic cleavage of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide molecules, generating highly reactive free radicals that attack the carbon-carbon double bonds of acrylamide, forming a long-chain polyacrylamide network. Simultaneously, the free radicals capture amino hydrogen atoms from the aramid molecular chain, establishing covalent crosslinks between the aramid chain segments. The UV radiation dose was 5.0 J / cm 2 The reaction rate is precisely controlled to form a moderately cross-linked network, providing 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°C / m establishes a thermodynamic driving force, driving the orderly alignment of water molecules along the long axis of the fiber. Water molecules at the low-temperature end combine through hydrogen bonds to form ice crystal nuclei, and at the solid-liquid interface, water molecules grow epitaxially layer by layer to form columnar crystals. This phase transition is strictly regulated by the axial temperature gradient. A moderate gradient aligns the ice crystal growth direction with the fiber axis, forming parallel channels with an orientation degree of 98.1%. The channels left behind by the sublimated ice crystals effectively extend the phonon transmission path, reducing the thermal conductivity to 0.020 W / m·K.

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

[0155] In Example 3, the photocrosslinking dose was 7.2 J / cm 2 With an axial temperature gradient of 25°C / m, the tensile strength is 36.5 MPa, the porosity is 89.3%, the orientation is 88.1%, the thermal conductivity is 0.043 W / m·K, and the rebound is 65%, which are significantly lower than those in Example 5. This is due to the UV dose of 7.2 J / cm 2 The photoinitiator cracking rate is doubled, and the high concentration of free radicals leads to excessive acrylamide polymerization. Differential scanning calorimetry shows that the cross-linking density reaches 85%, and the free volume of the molecular chain is compressed to below 0.5nm. The rigid network hinders the movement of the chain segments, causing the material's elongation at break to drop by 35%. More seriously, the excessive polyacrylamide chains cover the active sites on the aramid surface, hindering the interfacial interaction during the subsequent phase change process. The low temperature gradient of 25℃ / m reduces the ice crystal growth rate to 5μm / s, prolonging the existence time of the solid-liquid interface. The rigid cross-linked network cannot release stress through chain segment relaxation, resulting in microcracks at the ice crystal interface. Scanning electron microscopy shows that the crack density is as high as 300 / mm 2 , becoming a mechanical weak point. At the same time, the hydrophilic chain segments of polyacrylamide excessively adsorb water molecules, destroying the continuity of the axial temperature gradient and inducing random branching of ice crystals to form disordered channels. Excessive cross-linking produces a high-rigidity network, which inhibits the flexibility of the molecular chains required for the epitaxial growth of ice crystals. Slow-growing ice crystals accumulate stress at the interface, causing microcracks to destroy the structural integrity. The solvent is locally enriched at the cracks to form a liquid pool, which further disrupts the temperature field to generate dendritic crystals. This vicious cycle ultimately causes the degree of orientation to drop to 88.1%, and the tensile strength is 22% lower than that of Example 5. Although 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] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. A method for preparing aerogel fibers using waste aramid fibers under extreme cold conditions, characterized in that: The following steps are involved: S1. Mechanically loosen and perform composite chemical cleaning on the outer fabric of waste firefighting uniforms to obtain pure aramid staple fibers; S2, dissolving the pure aramid staple fibers in a mixed solvent of trifluoroacetic acid and methanesulfonic acid, adding graphene, acrylamide and a photoinitiator to prepare a photosensitive composite spinning solution; S3, squeezing the photosensitive composite spinning solution into a coagulation bath by wet spinning, and simultaneously performing ultraviolet radiation cross-linking to form nascent fibers; S4, applying an axial temperature gradient to the as-spun fiber to induce directional ice crystal growth, thereby obtaining directional frozen fiber; S5. vacuum freeze-drying the oriented frozen fibers to obtain an aerogel fiber precursor having axially parallel pores; S6. Immersing the aerogel fiber precursor in liquid nitrogen for cryogenic treatment, and then subjecting it to gradient temperature recovery for shaping, thereby obtaining aerogel fibers.

2. The method according to claim 1, characterized in that In step S1, the mechanical opening includes using an opening machine with a tooth roller speed of 800-1200 rpm and a tooth gap spacing of 0.5-1 mm to process the waste firefighting clothing fabric to obtain a fiber bundle with a length of 10-20 mm; the composite chemical cleaning includes immersing the fiber bundle in a composite cleaning solution and ultrasonically treating it at 65-75°C for 4-6 hours; the cleaning solution includes 0.1-0.15g of sodium carbonate, 0.05-0.08g of sodium dodecyl sulfate, and 50-60ml 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; and the mass fraction of the pure aramid staple fibers in the mixed solvent is 10-15 wt%.

4. The method according to claim 1, wherein In step S2, the mixed solution of the pure aramid staple fibers, graphene, acrylamide and photoinitiator comprises 3-8 wt% of graphene, 1-3 wt% of acrylamide and 0.5-2 wt% of photoinitiator based on the mass of the aramid.

5. The method according to claim 4, characterized in that The mixed solution was dispersed by ultrasonic in an ice bath at 0-5° C. for 20-40 min, and then degassed at a vacuum degree of -0.09 to -0.1 MPa for 1-2 h.

6. The method according to claim 1, characterized in that In step S3, the coagulation bath is a mixture of ethanol and water in a volume ratio of 7:3, the temperature is 20-25°C; the ultraviolet irradiation conditions are a wavelength of 365nm and an intensity of 80-120mW / cm 2 , time 30-90s, the irradiation position is 10-20mm below the surface of the coagulation bath.

7. The method according to claim 1, characterized in that In step S4, the as-spun fiber is vertically clamped on a copper cold plate, the axial temperature gradient of the copper cold plate is 25-35°C / m, the contact end temperature is -40 to -30°C, and the opposite end temperature is -10 to -5°C; the freezing cooling rate is 1-3°C / min, the ambient humidity is 40-50% RH, and the freezing time is 120-180 min.

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

9. The method according to claim 1, characterized in that The cryogenic treatment in step S6 includes immersing the aerogel fiber precursor in -196°C liquid nitrogen at a speed of 10-20 cm / min, completely immersing it for 3-5 minutes, transferring it to a -80°C low-temperature chamber for equilibration for 120-180 minutes, heating it to -20°C at a rate of 0.5-1°C / min, and then heating it to 25°C at a rate of 2-3°C / min.

10. An aerogel fiber prepared from waste aramid fibers under extreme cold conditions, based on the method according to any one of claims 1 to 9, characterized in that: The aerogel fiber has a pore diameter of 1-20 μm, an aspect ratio of 50-100, an axially parallel through-hole ratio greater than 90%, and a porosity of 92-97%.

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