A method for preparing regenerated aramid composite fibers

Abstract

This invention belongs to the field of aramid recycling technology, specifically relating to a method for preparing recycled aramid composite fibers. Waste aramid fibers are pulverized and mixed with an ionic liquid [C4mim]Cl, graphene oxide, and nanocellulose. The mixture is then subjected to intermittent microwave irradiation under a protective atmosphere to obtain a depolymerization mixture. Terephthaloyl chloride and p-phenylenediamine are added to the depolymerization mixture, and after stirring and reaction, the mixture is extruded through a spinneret into an ethanol-water solution containing silicon carbide nanoparticles. After solidification, the mixture is removed and subjected to gradient drying to obtain recycled aramid composite fibers. This invention successfully solves the core pain points of waste aramid recycling—difficult depolymerization, poor performance, and low value—through the synergistic effect of graphene oxide, nanocellulose, silicon carbide nanoparticles, and a stepped drying process, achieving molecular-level closed-loop recycling and restoring the material's intrinsic properties.

Description

Technical Field

[0001] This invention belongs to the field of aramid recycling technology, specifically relating to a method for preparing recycled aramid composite fibers. Background Technology

[0002] Aramid fibers, due to their excellent high strength, high modulus, heat resistance, and chemical corrosion resistance, are widely used in key fields such as national defense, aerospace, security, and electronic information. However, the amount of scrap, test materials, and end-of-life waste products such as bulletproof vests generated during their production process is increasing year by year. Because of aramid's extremely high chemical stability and bioinertness, it is difficult to degrade in the natural environment. Incineration or landfill disposal not only wastes strategic resources but also causes serious environmental problems. Therefore, achieving efficient and high-value recycling of waste aramid has become a major issue that the industry urgently needs to address.

[0003] Currently, research on the recycling of waste aramid fibers mainly focuses on physical and chemical recycling methods, but both have significant limitations:

[0004] Physical recycling typically involves mechanically crushing waste aramid fibers into aramid pulp, which is then used as a reinforcing filler in rubber materials. While this method is simple, it severely damages the fiber structure and molecular chain integrity of the aramid, leading to a sharp decline in the mechanical properties of the recycled material, resulting in low product value. Furthermore, it cannot achieve closed-loop recycling and is essentially a form of downgrading.

[0005] Chemical recycling aims to depolymerize aramid polymers into monomers or oligomers, which can then be repolymerized and spun. This is an ideal path to achieving a circular economy. Existing chemical methods mainly include high-temperature and high-pressure hydrolysis, supercritical fluid methods, and ionic liquid dissolution methods. However, high-temperature and high-pressure hydrolysis suffers from harsh reaction conditions, severe equipment corrosion, and huge energy consumption, and generates large amounts of saline wastewater or waste acid, resulting in heavy environmental pollution. Supercritical fluid methods utilize the special solubility of supercritical carbon dioxide to treat aramid, but this method has extremely high equipment requirements, high investment costs, and mainly yields short fibers or micropowders, which are difficult to use for regenerating continuous filaments. Although ionic liquid dissolution avoids strong acids and alkalis, the current recycling efficiency has not reached an ideal level. Some aramid fibers, due to their complex structure or tight binding with other substances, are difficult to completely dissolve or separate, which affects the purity and quality of the recycled aramid. The dissolution process is slow, and the dissolved fibers are usually directly molded into materials, failing to achieve complete depolymerization and reconstruction of the molecular chains, resulting in poor product performance.

[0006] Chinese patent document CN 119686107 A discloses a method for preparing flame-retardant and shielding military tent fabric from recycled waste aramid fabric. The steps include: Step 1: ionic liquid treatment of waste aramid fabric; Step 2: opening of aramid fabric and recycling of aramid fibers; Step 3: preparation of recycled flame-retardant pre-filaments; Step 4: preparation of tent base fabric; and Step 5: preparation of heat-insulating and shielding military tent fabric. However, this method does not truly recover the inherent high performance of aramid; it merely alters its physical form and combines it with recycled materials of lower performance, resulting in a low overall product performance ceiling. The mechanical opening process damages the fibers, leading to a significant decrease in tensile strength. Furthermore, the process is cumbersome, energy-intensive, and has many control difficulties, increasing industrialization costs and complexity. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing recycled aramid composite fibers. By leveraging the synergistic effect of graphene oxide, nanocellulose, silicon carbide nanoparticles, and a step-drying process, this method addresses the core pain points of waste aramid recycling, such as difficulty in depolymerization, poor performance, and low value, achieving molecular-level closed-loop recycling and restoring the material's intrinsic properties.

[0008] To achieve this objective, the technical solution of the present invention provides a method for preparing recycled aramid composite fibers, characterized by comprising the following steps:

[0009] (1) The waste aramid fiber is crushed and then mixed with ionic liquid [C4mim]Cl, graphene oxide and nanocellulose to obtain a mixed liquid;

[0010] (2) Under a protective atmosphere, the mixed liquid is subjected to intermittent microwave irradiation to obtain a depolymerized mixture;

[0011] (3) Add terephthaloyl chloride and p-phenylenediamine to the depolymerization mixture, stir and react to obtain the polymerization solution;

[0012] (4) The polymer solution is extruded through a spinneret into an ethanol aqueous solution (coagulation solution) containing silicon carbide nanoparticles. After coagulation, it is taken out and then dried in a gradient to obtain regenerated aramid composite fiber.

[0013] Preferably, in step (1), the mass ratio of waste aramid fiber, ionic liquid [C4mim]Cl, graphene oxide and nanocellulose is 1:5~10:0.005~0.03:0.001~0.005.

[0014] Preferably, in step (1), the stirring temperature is 55~65℃, the stirring time is 100~140min, and the stirring speed is 180~200rpm.

[0015] Preferably, in step (2), the protective atmosphere is nitrogen or argon, the microwave radiation temperature is 120~150℃, the total microwave radiation time is 10~30min, the microwave radiation power is 500~800W, and the intermittent microwave radiation mode is radiation for 9~11s with an interval of 4~6s.

[0016] Preferably, in step (3), the mass ratio of the depolymerization mixture, terephthaloyl chloride, and p-phenylenediamine is 100:8~9:3~4.

[0017] Preferably, in step (3), the stirring temperature is 55~65℃, the stirring time is 2~4h, and the stirring speed is 150~250rpm.

[0018] Preferably, in step (4), the mass ratio of ethanol, water and silicon carbide nanoparticles in the ethanol aqueous solution containing silicon carbide nanoparticles is 1:0.25~0.5:0.001~0.005.

[0019] Preferably, in step (4), the spinneret orifice diameter is 0.08~0.1mm, the extrusion rate is 7~9m / min, and the draw ratio is 1.5~3.

[0020] Preferably, in step (4), the drying conditions are as follows: drying at 70~90℃ for 15~25min, then drying at 110~130℃ for 35~45min, and then drying at 145~155℃ for 10~15min.

[0021] Compared with the prior art, the beneficial effects of this invention are as follows:

[0022] (1) This invention introduces graphene oxide (GO) during the depolymerization process. The carboxyl groups (-COOH) and hydroxyl groups (-OH) abundant at the edges of GO sheets can strongly insert into the aramid molecular chains, disrupting their regular hydrogen bond network and greatly reducing the depolymerization energy barrier in the crystalline region. Simultaneously, GO is an excellent microwave absorber, capable of converting microwave energy into heat energy, achieving instantaneous local heating at the molecular level, greatly improving depolymerization efficiency and selectivity. During repolymerization and spinning, GO, as a nano-reinforcement, forms a strong interfacial interaction with the repolymerized aramid molecular chains due to its large specific surface area, effectively transferring loads and reducing stress concentration, thereby significantly improving the mechanical properties of the final fiber. The method of this invention achieves a depolymerization rate of 90.3-92.5% for waste aramid, and the resulting recycled aramid composite fiber has a tensile strength of 3.58-3.82 GPa and a modulus of 110-118 GPa. Compared to commercially available aramid fibers (with tensile strength of 3.8~4.0 GPa and modulus of 120~130 GPa), the regenerated fibers of this invention have achieved more than 85% of the mechanical properties of virgin aramid fibers. Aramid fabrics used in soft bulletproof vests typically require a monofilament strength of no less than 3.2 GPa; aramid reinforcing cores in optical cables typically require a modulus >100 GPa and a strength >3.0 GPa. The regenerated aramid fibers prepared by this invention fully meet these performance standards, and the raw materials are derived from waste aramid, resulting in a cost far lower than that of fresh aramid synthesized from petrochemical monomers. Due to the introduction of GO and SiC, the regenerated fibers exhibit superior thermal and electrical conductivity compared to fresh aramid, providing a possibility for the development of multifunctional composite materials.

[0023] (2) In this invention, nanocellulose (CNC) is added during the depolymerization process. CNC has a high aspect ratio and abundant surface functional groups. Its main functions are: first, as a dispersant, it prevents the hydrophobic GO sheets from agglomerating in the ionic liquid and ensures that they play an effective role; second, its surface hydroxyl groups can adsorb or coat impurities (such as carbonized particles and oligomers) in waste aramid through hydrogen bonding, preventing these impurities from interfering with the subsequent depolymerization and repolymerization reactions.

[0024] (3) By adding silicon carbide nanoparticles (SiC) to the coagulation bath, the SiC particles are fixed in situ on the surface and inside of the fiber during the coagulation process. These high-hardness and high-modulus nanoparticles themselves play a reinforcing role; and they roughen the fiber surface. When the regenerated fiber is used as a composite material reinforcement, it can significantly increase the mechanical interlocking force with the resin matrix, thereby greatly improving the interfacial shear strength.

[0025] (4) This invention utilizes gradient drying. Since aramid is a rigid-chain polymer, its high performance stems from its high orientation and crystalline structure. The programmed temperature-step drying method employed in this invention is crucial: in the first stage at 70-90℃, most of the solvent and water are gently removed to prevent rapid surface crusting; in the second stage at 110-130℃, the molecular chains gain sufficient mobility, begin structural rearrangement and hydrogen bond reformation, promoting crystallization; in the third stage at 140-160℃, heat setting is performed to release internal stress, stabilize the supramolecular structure, and further enhance crystallinity and orientation. Fixed-temperature drying cannot achieve this optimized process. The initial high temperature may cause the solvent to evaporate violently, forming pores and defects, and the molecular chains do not undergo an ordered structural relaxation process, ultimately resulting in a loose fiber structure, numerous defects, and poor performance.

[0026] (5) This invention achieves molecular-level closed-loop recycling by breaking and rebuilding the chemical bonds of waste aramid fibers, restoring the material's intrinsic properties and representing an upgraded recycling method. Compared to traditional chemical recycling methods, the aramid composite fibers regenerated by this invention exhibit orders-of-magnitude performance improvements. This invention successfully addresses the core pain points of waste aramid recycling—difficult depolymerization, poor performance, and low value—through the synergistic effect of graphene oxide, nanocellulose, silicon carbide nanoparticles, and a stepped drying process.

[0027] (6) The present invention provides a buffer period during the depolymerization process through intermittent microwave radiation. Within 5 seconds after the microwave is turned off, the temperature inside the system is balanced through heat conduction and convection, avoiding the rapid accumulation of heat. At the same time, this provides time for the diffusion and stabilization of the active intermediates or monomers generated by depolymerization, preventing them from over-reacting or decomposing near the reaction hotspot. Detailed Implementation

[0028] In the description of this invention, it should be noted that unless specific conditions are specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. The microwave radiation instrument used in this invention is the Discover SP closed-container microwave synthesizer manufactured by CEM Corporation, USA, with a power of 0~300W and a temperature of 0~250℃. Other reagents or instruments whose manufacturers are not specified are all commercially available conventional products; the ionic liquid [C4mim]Cl refers to 1-butyl-3-methylimidazolium chloride ionic liquid.

[0029] The technical solutions provided by the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. 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. Example 1

[0030] 20g of crushed waste aramid fiber was mixed with 150g of ionic liquid [C4mim]Cl, 0.3g of graphene oxide and 0.06g of nanocellulose at 60℃ and 190rpm for 120min to obtain 170.24g of mixed liquid.

[0031] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 135℃, the radiation power at 650W, and the radiation mode at intermittent (10s per working interval, 5s interval) for a total radiation reaction time of 20min, yielding 170.05g of depolymerization mixture.

[0032] Add 14.45g terephthaloyl chloride and 5.95g p-phenylenediamine to 170.05g of depolymerization mixture, and stir at 60℃ and 200rpm for 3h to obtain polymerization solution;

[0033] The polymer solution was extruded through a 0.09 mm spinneret at a rate of 8 m / min and a draw ratio of 2 into a coagulation solution composed of 1000 g ethanol, 400 g water, and 3 g silicon carbide nanoparticles. The solution was then removed and dried at 80 °C for 20 min, 120 °C for 40 min, and 150 °C for 12.5 min to obtain regenerated aramid composite fibers. Example 2

[0034] 20g of crushed waste aramid fiber was mixed with 200g of ionic liquid [C4mim]Cl, 0.6g of graphene oxide and 0.1g of nanocellulose at 65℃ and 200rpm for 100min to obtain 220.54g of mixed liquid.

[0035] Under nitrogen protection, the mixed liquid was subjected to microwave radiation at a temperature of 150℃, a radiation power of 800W, and an intermittent radiation mode (9s working, 4s interval), with a total radiation reaction time of 10min, yielding 220.20g of depolymerized mixed liquid.

[0036] Add 19.82g of terephthaloyl chloride and 8.81g of p-phenylenediamine to 220.20g of depolymerization mixture, and stir at 55℃ and 150rpm for 4h to obtain polymerization solution;

[0037] The polymer solution was extruded through a 0.08 mm spinneret at a rate of 7 m / min and a draw ratio of 1.5 into a coagulation solution composed of 1000 g ethanol, 250 g water, and 1 g silicon carbide nanoparticles. The solution was then removed and dried at 70 °C for 25 min, 110 °C for 45 min, and 145 °C for 15 min to obtain regenerated aramid composite fibers. Example 3

[0038] 20g of crushed waste aramid fiber was mixed with 100g of ionic liquid [C4mim]Cl, 0.1g of graphene oxide and 0.02g of nanocellulose at 55℃ and 180rpm for 140min to obtain 120.08g of mixed liquid.

[0039] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 120℃, the radiation power at 500W, and the radiation mode at intermittent radiation (11s working, 6s interval), for a total radiation reaction time of 30min, yielding 120.00g of depolymerization mixture.

[0040] Add 9.6g terephthaloyl chloride and 3.6g p-phenylenediamine to 120.00g of depolymerization mixture, and stir at 65℃ and 250rpm for 2h to obtain polymerization solution;

[0041] The polymer solution was extruded through a 0.1 mm spinneret at a rate of 9 m / min and a draw ratio of 3 into a coagulation solution composed of 1000 g ethanol, 500 g water, and 5 g silicon carbide nanoparticles. The solution was then removed and dried at 90 °C for 15 min, 130 °C for 35 min, and 155 °C for 10 min to obtain regenerated aramid composite fibers. Comparative Example 1

[0042] Oxide-free graphene: 20g of crushed waste aramid fiber, 150g of ionic liquid [C4mim]Cl, and 0.06g of nanocellulose were stirred and mixed at 60℃ and 190rpm for 120min to obtain 169.98g of mixed liquid;

[0043] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 135℃, the radiation power at 650W, and the radiation mode at intermittent radiation (10s working, 5s interval), and the reaction was carried out for 20min to obtain 169.88g of depolymerization mixture.

[0044] Add 14.44 g of terephthaloyl chloride and 5.94 g of p-phenylenediamine to 169.88 g of depolymerization mixture, and stir at 60 °C and 200 rpm for 3 h to obtain polymerization solution;

[0045] The polymer solution was extruded through a 0.09 mm spinneret at a rate of 8 m / min and a draw ratio of 2 into a coagulation solution composed of 1000 g ethanol, 400 g water, and 3 g silicon carbide nanoparticles. The solution was then removed and dried at 80 °C for 20 min, 120 °C for 40 min, and 150 °C for 12.5 min to obtain regenerated aramid composite fibers. Comparative Example 2

[0046] Nanocellulose-free: 20g of crushed waste aramid fiber was mixed with 150g of ionic liquid [C4mim]Cl and 0.3g of graphene oxide at 60℃ and 190rpm for 120min to obtain 170.18g of mixed liquid;

[0047] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 135℃, the radiation power at 650W, and the radiation mode at intermittent radiation (10s working, 5s interval), and the reaction was carried out for 20min to obtain 169.99g of depolymerization mixture.

[0048] Add 14.45g of terephthaloyl chloride and 5.95g of p-phenylenediamine to 169.99g of depolymerization mixture, and stir at 60℃ and 200rpm for 3h to obtain polymerization solution;

[0049] The polymer solution was extruded through a 0.09 mm spinneret at a rate of 8 m / min and a draw ratio of 2 into a coagulation solution composed of 1000 g ethanol, 400 g water, and 3 g silicon carbide nanoparticles. The solution was then removed and dried at 80 °C for 20 min, 120 °C for 40 min, and 150 °C for 12.5 min to obtain regenerated aramid composite fibers. Comparative Example 3

[0050] Silicon carbide-free nanoparticles: 20g of crushed waste aramid fiber was mixed with 150g of ionic liquid [C4mim]Cl, 0.3g of graphene oxide and 0.06g of nanocellulose at 60℃ and 190rpm for 120min to obtain 170.24g of mixed liquid.

[0051] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 135℃, the radiation power at 650W, and the radiation mode at intermittent radiation (10s working, 5s interval), and the reaction was carried out for 20min to obtain 170.05g of depolymerization mixture.

[0052] Add 14.45g terephthaloyl chloride and 5.95g p-phenylenediamine to 170.05g of depolymerization mixture, and stir at 60℃ and 200rpm for 3h to obtain polymerization solution;

[0053] The polymer solution was extruded through a 0.09 mm spinneret at a rate of 8 m / min and a draw ratio of 2 into a coagulation solution composed of 1000 g ethanol and 400 g water. The solution was then removed and dried at 80 °C for 20 min, at 120 °C for 40 min, and at 150 °C for 12.5 min to obtain regenerated aramid composite fibers. Comparative Example 4

[0054] Fixed drying time: 20g of crushed waste aramid fiber was mixed with 150g of ionic liquid [C4mim]Cl, 0.3g of graphene oxide and 0.06g of nanocellulose at 60℃ and 190rpm for 120min to obtain 170.24g of mixed liquid;

[0055] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 135℃, the radiation power at 650W, and the radiation mode at intermittent radiation (10s working, 5s interval), and the reaction was carried out for 20min to obtain 170.05g of depolymerization mixture.

[0056] Add 14.45g terephthaloyl chloride and 5.95g p-phenylenediamine to 170.05g of depolymerization mixture, and stir at 60℃ and 200rpm for 3h to obtain polymerization solution;

[0057] The polymer solution was extruded through a 0.09 mm spinneret at a rate of 8 m / min and a draw ratio of 2 into a coagulation solution composed of 1000 g ethanol, 400 g water, and 3 g silicon carbide nanoparticles. The solution was then removed and dried at 120 °C for 72.5 min to obtain regenerated aramid composite fibers. Comparative Example 5

[0058] The specific steps for obtaining recycled aramid fibers using only traditional physical crushing are as follows:

[0059] Waste aramid fabrics or scraps are manually sorted to remove large metal, plastic and other foreign objects.

[0060] Then, the large pieces of material are initially crushed into fragments of 30-50mm;

[0061] The fragments were placed into a shear crusher, the speed was controlled at 5000 rpm, and sheared for 20 minutes under nitrogen protection to obtain 1~3 mm aramid short fibers. Comparative Example 6

[0062] 20g of crushed waste aramid fiber was mixed with 150g of ionic liquid [C4mim]Cl, 0.3g of graphene oxide and 0.06g of nanocellulose at 60℃ and 190rpm for 120min to obtain 170.24g of mixed liquid.

[0063] The mixed liquid was subjected to microwave radiation under nitrogen protection, with the temperature controlled at 135℃, the radiation power at 650W, and the radiation mode at continuous radiation, for 20 minutes to obtain 170.05g of depolymerization mixture.

[0064] Add 14.45g terephthaloyl chloride and 5.95g p-phenylenediamine to 170.05g of depolymerization mixture, and stir at 60℃ and 200rpm for 3h to obtain polymerization solution;

[0065] The polymer solution was extruded through a 0.09 mm spinneret at a rate of 8 m / min and a draw ratio of 2 into a coagulation solution composed of 1000 g ethanol, 400 g water, and 3 g silicon carbide nanoparticles. The solution was then removed and dried at 80 °C for 20 min, 120 °C for 40 min, and 150 °C for 12.5 min to obtain regenerated aramid composite fibers.

[0066] Experimental Example 1

[0067] Depolymerization rate test of waste aramid fibers: Slowly pour the extracted depolymerization mixture sample (denoted as M1) into a large amount of deionized water; perform vacuum filtration using a dried and accurately weighed microporous membrane (denoted as M2); repeatedly wash the filter cake with a large amount of deionized water until the washing liquid is neutral; place the filter membrane with the filter cake in a vacuum drying oven and dry to constant weight; remove the dried filter membrane + filter cake, cool to room temperature, and weigh accurately (denoted as M3). Depolymerization rate (%) = [1-(M3-M2) / (M1*w)]*100%

[0068] In the formula, M1 is the mass of the depolymerization mixture sample taken, M2 is the initial mass of the dried filter membrane, M3 is the total mass of the dried filter membrane and insoluble matter, and w is the initial mass fraction of aramid in the depolymerization mixture.

[0069] Experimental Example 2

[0070] Mechanical property testing of recycled aramid composite fibers: Tensile strength was tested using an Instron 3365 electronic single yarn tensile testing machine in accordance with standard GB / T 3916-2013.

[0071]

[0072] As shown in the table, Example 1 showed a 17.1 percentage point increase in depolymerization rate compared to Comparative Example 1 (which did not include graphene oxide for depolymerization). This is because the carboxyl groups (-COOH) and hydroxyl groups (-OH) abundant at the edges of the GO sheets can strongly insert into the aramid molecular chains, disrupting their regular hydrogen bond network and significantly reducing the depolymerization energy barrier in the crystalline region. Simultaneously, GO is an excellent microwave absorber, capable of converting microwave energy into heat energy, achieving instantaneous localized heating at the molecular level, greatly improving depolymerization efficiency and selectivity. Its tensile strength increased by 34.0%, and its modulus increased by 28.3%. This is because during repolymerization and spinning, GO, as a nano-reinforcing material, forms a strong interfacial interaction with the repolymerized aramid molecular chains due to its large specific surface area, effectively transferring loads and reducing stress concentration, thereby significantly improving the mechanical properties of the final fiber. Without GO, depolymerization is incomplete, and the recycled material contains more defects, affecting the mechanical properties of the fiber.

[0073] Compared to Comparative Example 2 (which did not include nanocellulose depolymerization), Example 1 showed an 8.9 percentage point increase in depolymerization rate, a 23.2% increase in tensile strength, and a 20.4% increase in modulus. This is because CNC has a high aspect ratio and abundant surface functional groups. Its main functions are: first, to act as a dispersant, preventing the hydrophobic GO sheets from agglomerating in the ionic liquid and ensuring its effective function; and second, its surface hydroxyl groups can adsorb or coat impurities (such as carbonized particles and oligomers) in waste aramid through hydrogen bonding, preventing these impurities from interfering with subsequent depolymerization and repolymerization reactions. Without CNC, GO dispersion is uneven, impurity removal efficiency is low, leading to hindered depolymerization reactions, increased byproducts, and decreased monomer purity and yield.

[0074] Compared to Comparative Example 3 without silicon carbide nanoparticles, Example 1 showed a 17.5% increase in tensile strength and a 12.4% increase in modulus. This is because the addition of SiC nanoparticles in the coagulation bath allowed them to be fixed in situ on the fiber surface and inside during the fiber formation process. These high-hardness, high-modulus nanoparticles themselves acted as reinforcements. Furthermore, they roughened the fiber surface, which significantly increased the mechanical interlocking force with the resin matrix when the regenerated fiber was used as a composite material reinforcement, thereby greatly improving the interfacial shear strength.

[0075] Compared to Comparative Example 4, which did not undergo gradient drying, Example 1 showed a 25.2% increase in tensile strength and a 24.2% increase in modulus. This is because aramid is a rigid-chain polymer, and its high performance stems from its high orientation and crystalline structure. The programmed temperature gradient drying method used in this invention is crucial: in the first stage (70-90°C), most of the solvent and moisture are gently removed, preventing rapid surface crusting; in the second stage (110-130°C), the molecular chains gain sufficient mobility, initiating structural rearrangement and hydrogen bond reformation, promoting crystallization; in the third stage (140-160°C), heat setting is performed to release internal stress, stabilize the supramolecular structure, and further improve crystallinity and orientation. Fixed-temperature drying (such as 120°C in Comparative Example 4) cannot achieve this optimized process. The initial high temperature may cause violent solvent evaporation, forming pores and defects, and the molecular chains do not undergo an ordered structural relaxation process, ultimately resulting in a loose fiber structure, numerous defects, and poor performance.

[0076] Compared to Comparative Example 5, which uses traditional physical recycling methods, Example 1 shows a 218.3% increase in tensile strength and a 162.2% increase in modulus. This is because physical methods only alter the physical form, severely damaging the fiber's continuity and the integrity of the molecular chain, resulting in downgraded recycling. In contrast, the chemical method of this invention involves the breaking and reconstruction of chemical bonds, achieving closed-loop recycling at the molecular level and restoring the material's intrinsic properties, thus representing upgraded recycling. Therefore, the performance is improved by orders of magnitude. This invention successfully addresses the core pain points of waste aramid recycling—difficult depolymerization, poor performance, and low value—through the synergistic effect of graphene oxide, nanocellulose, silicon carbide nanoparticles, and a stepped drying process.

[0077] Compared to Comparative Example 6 with continuous microwave radiation, Example 1 showed a 7.5 percentage point increase in depolymerization rate. This is because intermittent microwave radiation provides a buffer period; within 5 seconds of microwave shutdown, the system achieves temperature equilibrium through heat conduction and convection, avoiding rapid heat accumulation. Simultaneously, this provides time for the diffusion and stabilization of reactive intermediates or monomers generated during depolymerization, preventing excessive reaction or decomposition near reaction hotspots. In contrast, continuous microwave radiation causes the system, especially for GO and ionic liquids with strong microwave absorption, to absorb far more energy than it dissipates, easily leading to localized overheating. The temperature in these localized areas can trigger a series of side reactions, such as partial carbonization of the benzene ring structure of aramid at excessively high temperatures, generating black carbonaceous particles, and the recovered parabens... Monomers such as diamines are unstable at high temperatures and may be oxidized or further decomposed, leading to a decrease in monomer recovery rate. Furthermore, under continuous microwave field action, GO sheets may vibrate, curl, or even break due to the absorption of large amounts of energy, reducing their aspect ratio and compromising structural integrity. This weakens their ability to disrupt the aramid hydrogen bond network. Tensile strength can be increased by 21.3%, and modulus by 20.4%. This is because the smaller size and incomplete structure of GO reduces its effectiveness as a nano-reinforcing agent in regenerated fibers. Excessive thermal stress not only affects small molecules and GO but may also cause random breakage or cross-linking of depolymerized aramid oligomer chains, affecting the orderly growth of molecular chains during subsequent repolymerization and leading to a wider molecular weight distribution of the regenerated polymer, thus impacting the final fiber performance. The above description uses Example 1 as an example to illustrate its performance and advantages compared to existing technologies and comparative examples. Other examples also have equivalent advantages and will not be compared here.

Claims

1. A method for preparing recycled aramid composite fiber, characterized in that, Includes the following steps: (1) After crushing the waste aramid fiber, it is mixed with ionic liquid [C4mim]Cl, graphene oxide and nanocellulose to obtain a mixed liquid; (2) Under a protective atmosphere, the mixed liquid is subjected to intermittent microwave radiation to obtain a depolymerized mixed liquid; the protective atmosphere is nitrogen or argon, the microwave radiation temperature is 120~150℃, the total microwave radiation time is 10~30min, the microwave radiation power is 500~800W, and the intermittent microwave radiation mode is radiation for 9~11s with an interval of 4~6s. (3) Add terephthaloyl chloride and p-phenylenediamine to the depolymerization mixture, stir and react to obtain the polymerization solution; (4) The polymer solution is extruded through a spinneret into an ethanol aqueous solution containing silicon carbide nanoparticles. After solidification, it is taken out and then dried in a gradient to obtain regenerated aramid composite fiber.

2. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (1), the mass ratio of waste aramid fiber, ionic liquid [C4mim]Cl, graphene oxide, and nanocellulose is 1:5~10:0.005~0.03:0.001~0.

005.

3. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (1), the stirring temperature is 55~65℃, the stirring time is 100~140min, and the stirring speed is 180~200rpm.

4. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (3), the mass ratio of the depolymerization mixture, terephthaloyl chloride, and p-phenylenediamine is 100:8~9:3~4.

5. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (3), the stirring temperature is 55~65℃, the stirring time is 2~4h, and the stirring speed is 150~250rpm.

6. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (4), the mass ratio of ethanol, water and silicon carbide nanoparticles in the ethanol aqueous solution containing silicon carbide nanoparticles is 1:0.25~0.5:0.001~0.

005.

7. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (4), the spinneret aperture is 0.08~0.1mm, the extrusion rate is 7~9m / min, and the draw ratio is 1.5~3.

8. The method for preparing a recycled aramid composite fiber according to claim 1, characterized in that, In step (4), the drying conditions are as follows: drying at 70~90℃ for 15~25min, then drying at 110~130℃ for 35~45min, and then drying at 145~155℃ for 10~15min.

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