Nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber and preparation method thereof

Abstract

The invention discloses a nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste-cotton-based regenerated cellulose aerogel fiber and a preparation method thereof.The aerogel fiber is a composite fiber formed by taking a three-dimensional porous network framework formed by regenerated cellulose as a matrix and taking nitrogen-phosphorus-silicon co-doped carbon dots which are uniformly dispersed and immobilized in the framework as a reinforcing phase, wherein the co-doped carbon dots are prepared from citric acid, piperazine pyrophosphate and a silane coupling agent through a hydrothermal method, and the addition amount of the co-doped carbon dots is 2.5-12.5% of the mass of the cellulose. By introducing the nitrogen-phosphorus-silicon co-doped carbon dots, the mechanical property of the regenerated cellulose aerogel fiber is enhanced in a breakthrough manner, the regenerated cellulose aerogel fiber is endowed with excellent flame-retardant property, and the defects that the existing regenerated cellulose fiber is poor in mechanical property and single in function are overcome; and an effective way is provided for high-valued and multifunctional recycling of the waste cotton fabric.

Description

Technical Field

[0001] This invention belongs to the field of regenerated cellulose functional materials and their preparation technology. It relates to the preparation of high-performance regenerated cellulose aerogel fibers using waste cotton fabrics as raw materials through a resource-based recycling process. In particular, by introducing nitrogen, phosphorus and silicon co-doped carbon dots as reinforcing phases and functional units, waste cotton fabrics are upgraded and regenerated into regenerated cellulose aerogel fibers with excellent mechanical properties. Background Technology

[0003] Currently, the recycling technologies for waste cotton fabrics are mainly divided into two categories: physical recycling and chemical recycling.

[0004] Physical recycling involves mechanically opening and carding waste cotton fabrics to produce fibers, which are then respun. Its advantages include simple processing and relatively low costs. However, this method also has significant limitations. After repeated use and washing, cotton fibers undergo irreversible degradation, such as decreased polymerization, shortened length, and weakened strength. The harsh mechanical processing of physical recycling further exacerbates this damage, leading to a significant decline in the length uniformity, strength, and spinnability of the recycled cotton fibers. Therefore, recycled cotton fibers produced through physical recycling cannot meet the stringent quality requirements of high-end textiles and are typically only used for downgraded applications such as rags, fillings, or low-grade textiles, resulting in limited economic value and environmental benefits.

[0005] For example, Wang et al. (Preparation and characterization of cotton fiberfragments from model textile waste via mechanical milling and enzymedegradation. Cellulose (2023, 30(17), 10879-10904.) reported that mechanical grinding can convert waste cotton fabrics into cotton fiber fragments, but it also significantly shortens the fiber length. To compensate for the limitations of mechanical recycling, some studies have focused on optimizing recycling processes to improve the quality of regenerated fibers, such as Wanassi et al. (Value-added waste cottonyarn: Optimization of recycling process and spinning of reclaimed fibers). Industrial Crops and Products (2016, 87, 27-32.) Improve the performance of recycled fibers by optimizing the shear length and number of passes during the recycling process.

[0006] Chemical recycling involves dissolving cellulose in waste cotton fabrics using chemical methods and then reshaping it into regenerated cellulose fibers through spinning. Theoretically, this can achieve the "upgrading and remanufacturing" of cellulose and is currently the most effective way to achieve high-value recycling of waste cotton fabrics.

[0007] The mainstream process for preparing regenerated cellulose fibers in industry is still the traditional viscose process, which uses a combination of carbon disulfide and sodium hydroxide to dissolve cellulose. Although the technology is mature and easy to implement on a large scale, its fatal flaw is that it requires the use of a large amount of toxic carbon disulfide as a solvent. The production process generates harmful gases such as hydrogen sulfide and carbon dioxide, as well as wastewater, which pose a serious threat to the environment and human health. In addition, the process is lengthy, energy-intensive, and complex to control.

[0008] In recent years, novel green solvent systems, such as ionic liquids, alkali / urea aqueous solutions, and zinc chloride aqueous solutions, have provided more environmentally friendly options for the dissolution and spinning of cellulose. Among them, the ionic liquid method has attracted widespread attention because it can efficiently dissolve cellulose, the process does not involve chemical derivatization, and the solvent can be recycled.

[0009] Wu et al. (Efficient recycle of waste poly-cotton and preparation of cellulose and polyester fibers using the system of ionic liquid and dimethylsulfoxide. Journal of Molecular Liquids (2023, 388: 122757.) ionic liquids 1-allyl-3-methylimidazolium chloride ([Amim]Cl) and 1-ethyl-3-methylimidazolium diethyl phosphate ([Emim]DEP) were screened out and mixed with dimethyl sulfoxide (DMSO) as a solvent to selectively dissolve cellulose from waste cotton fabrics, thereby achieving the separation of cotton and polyester.

[0010] Shirin Asaadi et al. (Renewable High-Performance Fibers from the ChemicalRecycling of Cotton Waste Utilizing an Ionic Liquid. ChemSusChem , 2016, 9(22):3250-3258.)Using the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-ene acetate ([DBNH]OAc) to completely dissolve waste cotton textiles, high-quality recycled textile fibers were successfully prepared, with performance significantly better than mechanically recycled products.

[0011] However, ionic liquids currently face numerous challenges, including extremely high costs, high viscosity leading to difficulties in defoaming and spinning, and complex recycling and purification processes, which severely restrict their large-scale industrial application.

[0012] In addition, inorganic salt solvent systems such as zinc chloride aqueous solution have also attracted much attention due to their unique advantages. This method is low-cost, has strong dissolving power, and the solvent is easy to recycle. Existing technologies have proven that zinc chloride solution can efficiently dissolve waste cotton fabrics, and through subsequent regeneration and spinning processes, successfully prepare regenerated cellulose gels, fibers, and even porous aerogel materials. These "dissolve-regenerate" technologies based on green solvents are driving the chemical recycling of waste cotton fabrics towards a more environmentally friendly and higher-value direction, gradually becoming a potential alternative to traditional viscose methods.

[0013] However, even with this advanced technological approach, the resulting regenerated cellulose fibers still have significant limitations, restricting their high-end applications.

[0014] First, the mechanical properties of the material are a major weakness. Due to the reduced degree of polymerization of cellulose during the recycling process and the difficulty in perfectly controlling the regenerated three-dimensional network structure, the resulting regenerated cellulose aerogel fibers generally have low strength, high brittleness, and poor flexibility, making it difficult to meet the mechanical performance requirements of functional textiles, composite material reinforcements, and other fields.

[0015] Secondly, the materials have limited functionality and added value. Existing research mainly focuses on the porosity and biocompatibility of regenerated cellulose aerogel fibers, and the products are mostly used in basic fields such as thermal insulation and adsorption. There is a lack of effective functional integration methods, making it difficult to endow them with high-value-added properties such as flame retardancy, antibacterial properties, and conductivity, which cannot meet the diverse needs of emerging industries such as smart textiles and flexible electronics.

[0016] Lu et al. (Preparation and properties of waste cotton based regeneratedcellulose aerogel fibers. Journal of Physics: Conference Series , 2025, 3093(1): 012001.)The process of using zinc chloride dissolution-regeneration to recycle waste cotton fabrics to prepare regenerated cellulose aerogel fibers has effectively controlled the microstructure of regenerated cellulose fibers and achieved certain performance improvements by systematically optimizing process parameters such as dissolution, spinning, coagulation and post-treatment. However, the improvement effect of this "process-structure-performance" optimization based on the pure cellulose system has reached its limit. Due to the short molecular chain and destruction of the crystalline region of regenerated cellulose itself, its intrinsic mechanical strength is limited, and it is difficult to achieve breakthrough progress by simply adjusting the process.

[0017] Therefore, there is an urgent technical need and broad application prospects for developing a regenerated cellulose aerogel fiber from waste cotton fabric that is based on green solvent technology, introduces a novel, multi-scale reinforcing phase and mechanism, and simultaneously achieves significant enhancement of mechanical properties and multifunctional integration. Summary of the Invention

[0018] The purpose of this invention is to provide a nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber and its preparation method. By introducing nitrogen-phosphorus-silicon co-doped carbon dot, the mechanical properties of waste cotton-based regenerated cellulose aerogel fiber are significantly enhanced, and it is endowed with good flame retardant function.

[0019] To achieve the above-mentioned objectives, this invention first provides a nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber, which is a composite aerogel fiber composed of the following components: Regenerated cellulose aerogel fibers, serving as the matrix phase, constitute a continuous three-dimensional porous network framework; and Nitrogen-phosphorus-silicon co-doped carbon dots, which serve as a reinforcing phase, are uniformly dispersed at the nanoscale and immobilized within the regenerated cellulose aerogel fibers. The nitrogen-phosphorus-silicon co-doped carbon dots are prepared by hydrothermal reaction in an aqueous solution system using citric acid as the carbon source, piperazine pyrophosphate as the nitrogen and phosphorus source, and silane coupling agent as the silicon source.

[0020] Furthermore, in the composite aerogel fiber of the present invention, the amount of nitrogen-phosphorus-silicon co-doped carbon dots added is 2.5 to 12.5% ​​of the mass of the regenerated cellulose aerogel fiber.

[0021] The nitrogen-phosphorus-silicon co-doped carbon dots of the present invention significantly enhance the mechanical properties of waste cotton-based regenerated cellulose aerogel fibers and endow them with certain flame-retardant properties through the introduction of nitrogen-phosphorus-silicon co-doped carbon dots.

[0022] Furthermore, the present invention also provides a method for preparing the nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber. The method involves using waste cotton fabric as raw material, obtaining pretreated cotton cellulose through chemical treatment, dissolving it in zinc chloride solution to obtain regenerated cellulose spinning solution, adding a certain amount of nitrogen-phosphorus-silicon co-doped carbon dot for blending, and then using it as a blending spinning solution for wet spinning. The resulting regenerated cellulose nascent gel fiber is formed in an anhydrous ethanol coagulation bath, and the post-treatment yields reinforced regenerated cellulose aerogel fiber with a porous structure.

[0023] Furthermore, the more specific preparation method of the nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fiber of the present invention includes: S1. First, use sodium hydroxide solution to pretreat the washed and dried waste cotton fabric to remove impurities such as pectin, and then use sulfuric acid solution for pretreatment to reduce its degree of polymerization, thus obtaining pretreated cotton cellulose. S2. Add the pretreated cotton cellulose to a 65-75 wt% zinc chloride aqueous solution and heat to dissolve to obtain a regenerated cellulose spinning solution; S3. Add nitrogen, phosphorus and silicon co-doped carbon dots to the regenerated cellulose spinning solution, so that they account for 2.5 to 12.5 wt% of the cellulose in the spinning solution, and disperse them evenly to obtain a blended spinning solution. S4. After the blended spinning solution is degassed, it is wet spun and extruded through a spinneret into an anhydrous ethanol coagulation bath to form and be drawn and oriented to form regenerated cellulose primary gel fibers. S5. The regenerated cellulose nascent gel fiber is subjected to static aging treatment with 50wt% ethanol aging solution and then freeze-dried to obtain nitrogen, phosphorus and silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber.

[0024] More specifically, in the sodium hydroxide solution pretreatment, the preferred sodium hydroxide solution concentration is 10-20 wt%, and the treatment is carried out at 65-85°C for 1.5-4 hours. In the sulfuric acid solution pretreatment, the preferred sulfuric acid solution concentration is 20-25 wt%, and the treatment is carried out at 60-70°C for 20-40 minutes.

[0025] After the above chemical treatment and washing to neutrality, the degree of polymerization (DP) of waste cotton fabrics is reduced to 380-500.

[0026] Furthermore, in this invention, the pretreated cotton cellulose is added to an aqueous solution of zinc chloride and heated to 80-90°C for 120-150 minutes to dissolve and obtain a regenerated cellulose spinning solution.

[0027] Furthermore, the concentration of the regenerated cellulose spinning solution is preferably 2 to 7 wt%.

[0028] The blended spinning solution needs to be degassed before wet spinning. This invention does not limit the degassing method of the blended spinning solution; any conventional degassing method can be used, including but not limited to any one or a combination of ultrasonic degassing, centrifugal degassing, or static degassing.

[0029] More specifically, the wet spinning of the present invention is carried out at room temperature, preferably using a spinneret of 18-22G, and the blended spinning solution is extruded into a coagulation bath to solidify and form at a spinning speed of 0.3-0.65m / min.

[0030] Furthermore, the draw ratio of the draw orientation is preferably 1 to 1.2 times.

[0031] Furthermore, the aging treatment of the regenerated cellulose nascent gel fibers is carried out at room temperature, with a preferred aging time of 20–80 min.

[0032] Furthermore, the preparation method of the present invention may further include placing the aged regenerated cellulose nascent gel fibers in a tert-butanol solution for solvent replacement, so as to replace the solvent that is not easy to dry in the regenerated cellulose nascent gel fibers with an easy-to-dry solvent, which facilitates the subsequent freeze-drying of the regenerated cellulose aerogel fibers.

[0033] Furthermore, the present invention involves winding the regenerated cellulose nascent gel fibers and then freeze-drying them for 24–36 hours to obtain final nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fibers.

[0034] Furthermore, the specific preparation method of the nitrogen-phosphorus-silicon co-doped carbon dots is to disperse the carbon source citric acid, the nitrogen-phosphorus source piperazine pyrophosphate and the silicon source silane coupling agent in an aqueous solution, carry out a hydrothermal reaction at 180-220°C for 6-12 hours, and then freeze-dry the reaction product after purification to obtain the nitrogen-phosphorus-silicon co-doped carbon dots.

[0035] Furthermore, the preferred mass ratio of the carbon source, nitrogen and phosphorus source to the silicon source is 1:2 to 5:5 to 11.

[0036] More specifically, the silane coupling agent is preferably silane coupling agent KH792.

[0037] This invention addresses the shortcomings of current recycled cellulose aerogel fibers based on waste cotton fabrics, such as insufficient mechanical properties and limited functionality. It innovatively constructs specific nitrogen-phosphorus-silicon co-doped carbon dots and introduces them into the waste cotton-based recycled cellulose aerogel fiber system, achieving a leap from single-material recycling to the preparation of high-performance composite materials. Specifically: Breakthrough enhancement in mechanical properties: Nitrogen-phosphorus-silicon co-doped carbon dots serve as a multifunctional nano-reinforcing phase. The abundant functional groups such as -COOH, -OH, and -NH2 on their surface can form a dense hydrogen bond network and physical cross-linking points with the regenerated cellulose molecular chains. When the fiber is under load, it effectively transfers and disperses stress and inhibits the propagation of microcracks. Compared with similar fibers without added carbon dots, the tensile strength, elongation at break, and toughness of the aerogel fiber of this invention are synergistically and significantly improved. The improvement is far greater than that that can be achieved by simply optimizing the process parameters, thus breaking through the performance bottleneck of pure regenerated cellulose aerogel fiber materials.

[0038] Synergistic optimization of structure and performance: The introduction of nitrogen, phosphorus and silicon co-doped carbon dots not only serves as a nanofiller, but may also regulate the nucleation and phase separation behavior of cellulose during coagulation and regeneration, which helps to form a more uniform and dense three-dimensional nanoporous network structure. This dual effect of "nano-reinforcement-structure guidance" enables aerogel fibers to achieve high strength while maintaining excellent flexibility and structural integrity.

[0039] Multifunctional integration and added value enhancement: The synergistic doping of multiple elements such as N, P, and Si also endows carbon dots with unique intrinsic flame-retardant properties. Therefore, the aerogel fibers prepared by this invention not only achieve mechanical reinforcement but also simultaneously possess excellent flame-retardant properties, thereby upgrading the product from a traditional heat insulation / adsorption material to an integrated "heat insulation-flame retardant" material that can be used in high-end safety protection fields. Its specific application areas include, but are not limited to, fireproof and heat-insulating layers for interiors of buildings and vehicles, lightweight heat insulation and flame-retardant coating materials for battery modules of new energy vehicles, heat insulation and flame-retardant layers for special protective clothing (such as fire suits and industrial thermal protective clothing), flame-retardant cushioning packaging and heat insulation layers for precision instruments and equipment or electronic components, and flame-retardant base materials for flexible wearable devices.

[0040] Green and high-value recycling: This invention enhances the functionality of optimized green processes such as zinc chloride dissolution and ethanol coagulation, successfully transforming low-value waste cotton textiles into high-performance, multi-functional, and high-value-added products. It opens up a new technical path for the resource utilization of waste textiles and achieves a balance between environmental and economic benefits. Attached Figure Description

[0041] Figure 1 This is a transmission electron microscope image and particle size distribution of nitrogen, phosphorus and silicon co-doped carbon dots.

[0042] Figure 2 This is a scanning electron microscope image of nitrogen, phosphorus, and silicon co-doped carbon dots reinforcing waste cotton-based regenerated cellulose aerogel fibers.

[0043] Figure 3 This is a comparison of the infrared spectra of waste cotton fabrics and regenerated cellulose aerogel fibers.

[0044] Figure 4 It is a comparison of the X-ray diffraction patterns of waste cotton fabrics and regenerated cellulose aerogel fibers.

[0045] Figure 5 This is a transmission electron microscope image and particle size distribution of nitrogen, phosphorus, and silicon co-doped carbon dots reinforced waste cotton-based regenerated cellulose aerogel fibers.

[0046] Figure 6 This is a demonstration of the mechanical properties of nitrogen, phosphorus, and silicon co-doped carbon dots reinforced waste cotton-based regenerated cellulose aerogel fibers suspending a 500g weight.

[0047] Figure 7 This study investigates the effect of different nitrogen, phosphorus, and silicon co-doped carbon dot contents on the stress-strain behavior of regenerated cellulose aerogel fibers.

[0048] Figure 8 This is a comparison chart of TG and DTG of pure regenerated cellulose aerogel fiber and nitrogen, phosphorus and silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber. Implementation

[0049] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings and specific examples. It should be noted that the embodiments are merely illustrative and are intended to provide a thorough understanding of the technical solutions of the present invention and to provide guidance for those skilled in the art to implement and apply the present invention. It should be understood that these descriptions do not constitute any limitation on the scope of protection of the present invention.

[0050] Unless otherwise expressly stated, the production processes, experiments, tests or analysis methods involved in the embodiments of the present invention are all considered to be conventional methods known to those skilled in the art, and only need to be implemented in accordance with conventional conditions or relevant product instructions. The steps and names involved are also generally clear and unambiguous in the art.

[0051] The instruments, equipment, raw materials, reagents, or samples used in the embodiments are not subject to any special restrictions on their source. They are all conventional products that can be purchased through regular commercial channels or prepared by known methods, and their source does not have a substantial impact on the implementation results of the present invention.

[0052] Unless otherwise expressly defined, the scientific and technical terms used in this invention have the meanings commonly understood by one of ordinary skill in the art. In case of any conflict, the definitions in this specification shall prevail.

[0053] The terms “comprising,” “including,” “having,” etc., used in this invention should be understood as open-ended, meaning “including but not limited to.” The term “and / or” includes any and all combinations of one or more of the associated listed items. Quantitative terms such as “a,” “one,” etc., do not exclude multiples; “multiple” or “a variety” refers to quantities greater than or equal to two.

[0054] The terms "preferred", "better", and "exemplary" used in this invention are only used to describe specific solutions or effects and are not intended to limit the necessary scope of the solution or the scope of protection.

[0055] This invention relates to the description of numerical parameters (such as quantity, concentration, temperature, time, etc.), and it should be understood that reasonable deviations naturally exist due to measuring instruments, operational errors, statistical fluctuations, etc. The range of such deviations should be within limits acceptable to those skilled in the art based on common sense. Example

[0056] Example 1

[0057] Take 1.5g of citric acid as a carbon source, 6g of piperazine pyrophosphate as a nitrogen and phosphorus source, and 21mL of silane coupling agent KH792 as a silicon source. Add them sequentially to 40mL of deionized water, sonicate to disperse, and stir for 30min to form a homogeneous precursor solution.

[0058] The precursor solution was transferred to a 100 mL polytetrafluoroethylene-lined reactor and hydrothermally reacted at 200 °C for 8 h. After cooling, the reaction solution was filtered through a 0.45 µm microporous membrane and then centrifuged at 9000 r / min for 15 min. The supernatant was collected and dialyzed against flowing deionized water for 48 h using a dialysis bag with a molecular weight cutoff of 1000 Da. The purified dispersion was freeze-dried to obtain a yellow nitrogen-phosphorus-silicon co-doped carbon dot powder.

[0059] from Figure 1 The TEM image shows that the prepared carbon dots exhibit good monodispersity within the field of view, with no obvious agglomeration. The particle size distribution in the lower left corner shows that the particle size is uniform, mainly distributed in the range of 2 to 4 nm. The high-resolution transmission electron microscope (HRTEM) image in the upper right corner clearly shows that the carbon dots have obvious lattice fringes with a lattice spacing of about 0.17 nm, indicating that the carbon dots have good graphitization structure and crystallinity.

[0060] Example 2

[0061] Take cleaned and dried waste cotton fabric, cut it into pieces with a side length of 5cm, stir it in a 12wt% sodium hydroxide solution at 65℃ for 2 hours, wash it with water until neutral and dry it; then place it in a 25wt% sulfuric acid solution, stir it at 70℃ for 20 minutes, wash it with water until neutral and dry it to obtain pretreated cotton cellulose.

[0062] Weigh 2.5g of pretreated cotton cellulose and add it to 47.5mL of 73wt% zinc chloride aqueous solution. Stir and dissolve thoroughly at 85℃ for 150min to prepare a 5wt% regenerated cellulose spinning solution.

[0063] Weigh 0.0625 g of the carbon dot powder prepared in Example 1 and add it to the above spinning solution. Stir continuously for 1 hour in a water bath at 85°C and supplement with intermittent ultrasonic treatment to obtain a uniform blended spinning solution. Let it stand to remove bubbles.

[0064] The wet spinning process involves adding the degassed blended spinning solution into the feed cylinder and extruding it through a 20G single-hole spinneret at a spinning linear speed of 0.65 m / min into an anhydrous ethanol coagulation bath for phase separation and molding. The nascent gel fibers are then obtained by stretching the fibers at a stretch ratio of 1.1 and then winding and collecting them.

[0065] The nascent gel fibers were introduced into a 50wt% ethanol aging solution and allowed to stand for 40 minutes. Then they were transferred to deionized water and soaked for 30 minutes before being washed until neutral. Subsequently, they were immersed in tert-butanol for solvent replacement. Each replacement lasted for 6 hours and was repeated 3 times.

[0066] The fibers after replacement were freeze-dried at -80℃ for 24 hours to prepare nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fibers. The carbon dot composite content in this regenerated cellulose aerogel fiber was 2.5 wt%.

[0067] Figure 2 The cross-sectional microstructure SEM images of the above-mentioned regenerated cellulose aerogel fibers are presented. The cross-sectional image on the left shows that the fiber has a relatively regular circular cross-section, while the magnified image on the right further reveals that the regenerated cellulose aerogel fiber has a rich and uniform porous network structure inside. This unique three-dimensional nano-network structure is beneficial to endowing the fiber with low density and high specific surface area characteristics.

[0068] Figure 3 FTIR comparison images of waste cotton fabrics and regenerated cellulose aerogel fibers are provided, showing an observation at 895 cm⁻¹. -1 The characteristic peaks are relatively stable; this absorption peak is caused by the vibration of β-glycosidic bonds; 1023 cm⁻¹ -1 For C and CO stretching vibrations, 1162 cm -1 For COC stretching vibration, 1335cm -1 This is a CH deformation vibration, 3420cm -1 The left and right sides represent the -OH stretching vibrations, indicating that both waste cotton fabric and regenerated cellulose aerogel fibers possess all the characteristic functional groups of cellulose. After dissolution and regeneration, compared with waste cotton fabric, the -OH stretching vibration peak of regenerated cellulose aerogel fibers decreased from 3330 cm⁻¹. -1 It was moved to 3420cm -1 And 1105cm -1 The small peak at this point disappears. This absorption peak is considered to be a characteristic peak of cellulose type I, indicating that the crystalline structure of cellulose changes from cellulose type I to cellulose type II during the dissolution and regeneration process.

[0069] By comparing the characteristic absorption peaks of the two, the reorganization of intramolecular and intermolecular hydrogen bonds of cellulose molecules during the dissolution and regeneration of waste cotton fabrics is clearly reflected, as well as the transformation of chemical structure from cellulose type I to cellulose type II, confirming the occurrence of the regeneration process.

[0070] Figure 4The figures show the XRD patterns of waste cotton fabrics and regenerated cellulose aerogel fibers. As can be seen from the figures, the diffraction peaks of cellulose in waste cotton fabrics are at 2θ=14.8° and 22.6°, which are typical sharp diffraction peaks of natural cellulose type I. However, the diffraction peaks of cellulose after dissolution and regeneration in regenerated cellulose aerogel fibers are at 2θ=20.3°. The diffraction peak positions have shifted and the peak shapes have broadened, exhibiting typical cellulose type II crystallization characteristics. This further indicates that cellulose transforms from type I to type II crystal structure during the dissolution process, further confirming from the crystal structure level that the crystal structure of waste cotton fabrics undergoes the expected transformation after dissolution and regeneration.

[0071] from Figure 5 TEM images of nitrogen, phosphorus, and silicon co-doped carbon dots reinforced waste cotton-based regenerated cellulose aerogel fibers show that the carbon dots were successfully introduced and uniformly distributed in the cellulose matrix without significant phase separation or agglomeration. The particle size distribution chart in the lower left corner shows that the particle size is uniform, mainly distributed in the range of 3–7 nm. The lattice fringes in the inset indicate that the composite carbon dots still maintain a good crystal structure, confirming the effectiveness of the composite process of this invention and the good compatibility between the carbon dot nanofiller and the regenerated cellulose aerogel fiber matrix.

[0072] A single nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fiber prepared above was weighed, and its mass was only 0.015g. For example... Figure 6 As shown, the fine fibers can stably suspend a 500g weight without breaking. This phenomenon directly demonstrates that the composite aerogel fiber possesses excellent macroscopic tensile strength and load-bearing capacity, capable of suspending loads exceeding 30,000 times its own weight.

[0073] Example 3

[0074] Weigh out 0.125 g, 0.1875 g, 0.25 g and 0.3125 g of the carbon dot powder prepared in Example 1, respectively, and add it to 50 mL of the 5 wt% regenerated cellulose spinning solution in Example 2. According to the wet spinning process in Example 2, prepare nitrogen, phosphorus and silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fibers with carbon dot composite contents of 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt%, respectively.

[0075] Example 4

[0076] Take cleaned and dried waste cotton fabric, cut it into pieces with a side length of 5cm, stir it in a 12wt% sodium hydroxide solution at 65℃ for 2 hours, wash it with water until neutral and dry it; then place it in a 25wt% sulfuric acid solution, stir it at 70℃ for 20 minutes, wash it with water until neutral and dry it to obtain pretreated cotton cellulose.

[0077] Weigh 2g of pretreated cotton cellulose and add it to 48mL of 73wt% zinc chloride aqueous solution. Stir and dissolve thoroughly at 85℃ for 150min to prepare 4wt% regenerated cellulose spinning solution.

[0078] Weigh 0.25g of the carbon dot powder prepared in Example 1 and add it to the above spinning solution. Stir continuously for 1 hour in a water bath at 85°C and supplement with intermittent ultrasonic treatment to obtain a uniform blended spinning solution. Let it stand to remove bubbles.

[0079] The wet spinning process involves adding the degassed blended spinning solution into the barrel and extruding it through a 20G single-hole spinneret at a spinning line speed of 0.55 m / min into an anhydrous ethanol coagulation bath for phase separation and shaping. The nascent gel fibers are then obtained by stretching the fibers at a stretch ratio of 1.2 and then winding and collecting them.

[0080] The nascent gel fibers were introduced into a 50wt% ethanol aging solution and allowed to stand for 60 minutes. They were then transferred to deionized water and soaked for 30 minutes before being washed until neutral. Subsequently, they were immersed in tert-butanol for solvent replacement. Each replacement lasted for 6 hours and was repeated 3 times.

[0081] After replacement, the fibers were freeze-dried at -80℃ for 24 hours to prepare nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fibers.

[0082] Example 5

[0083] Take cleaned and dried waste cotton fabric, cut it into pieces with a side length of 5cm, stir it in a 12wt% sodium hydroxide solution at 65℃ for 2 hours, wash it with water until neutral and dry it; then place it in a 25wt% sulfuric acid solution, stir it at 70℃ for 20 minutes, wash it with water until neutral and dry it to obtain pretreated cotton cellulose.

[0084] Weigh 1g of pretreated cotton cellulose and add it to 49mL of 73wt% zinc chloride aqueous solution. Stir and dissolve thoroughly at 85℃ for 150min to prepare 2wt% regenerated cellulose spinning solution.

[0085] Weigh 0.3125g of the carbon dot powder prepared in Example 1 and add it to the above spinning solution. Stir continuously for 1 hour in a water bath at 85°C and supplement with intermittent ultrasonic treatment to obtain a uniform blended spinning solution. Let it stand to remove bubbles.

[0086] The wet spinning process involves adding the degassed blended spinning solution into the barrel and extruding it through a 20G single-hole spinneret at a spinning line speed of 0.6 m / min into an anhydrous ethanol coagulation bath for phase separation and molding. The nascent gel fibers are then obtained by stretching the fibers at a stretch ratio of 1.05 and then winding and collecting them.

[0087] The nascent gel fibers were introduced into a 50wt% ethanol aging solution and allowed to stand for 80 minutes. Then they were transferred to deionized water and soaked for 30 minutes before being washed until neutral. Subsequently, they were immersed in tert-butanol for solvent replacement. Each replacement lasted for 6 hours and was repeated 3 times.

[0088] After replacement, the fibers were freeze-dried at -80℃ for 24 hours to prepare nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fibers.

[0089] Example 6

[0090] Take cleaned and dried waste cotton fabric, cut it into pieces with a side length of 5cm, stir it in a 12wt% sodium hydroxide solution at 65℃ for 2 hours, wash it with water until neutral and dry it; then place it in a 25wt% sulfuric acid solution, stir it at 70℃ for 20 minutes, wash it with water until neutral and dry it to obtain pretreated cotton cellulose.

[0091] Weigh 1.5g of pretreated cotton cellulose and add it to 48.5mL of 73wt% zinc chloride aqueous solution. Stir and dissolve thoroughly at 85℃ for 150min to prepare a 3wt% regenerated cellulose spinning solution.

[0092] Weigh 0.125g of the carbon dot powder prepared in Example 1 and add it to the above spinning solution. Stir continuously for 1 hour in a water bath at 85°C and supplement with intermittent ultrasonic treatment to obtain a uniform blended spinning solution. Let it stand to remove bubbles.

[0093] The wet spinning process involves adding the degassed blended spinning solution into the barrel and extruding it through a 20G single-hole spinneret at a spinning line speed of 0.5 m / min into an anhydrous ethanol coagulation bath for phase separation and shaping. The nascent gel fibers are then obtained by stretching the fibers at a stretch ratio of 1.15 and then winding and collecting them.

[0094] The nascent gel fibers were introduced into a 50wt% ethanol aging solution, allowed to stand for 20 minutes, then transferred to deionized water and soaked for 30 minutes before being washed until neutral. Subsequently, they were immersed in tert-butanol for solvent replacement, with each replacement lasting 6 hours and repeated 3 times.

[0095] After replacement, the fibers were freeze-dried at -80℃ for 24 hours to prepare nitrogen-phosphorus-silicon co-doped carbon dot-reinforced waste cotton-based regenerated cellulose aerogel fibers.

[0096] Comparative Example 1

[0097] Take cleaned and dried waste cotton fabric, cut it into pieces with a side length of 5cm, stir it in a 12wt% sodium hydroxide solution at 65℃ for 2 hours, wash it with water until neutral and dry it; then place it in a 25wt% sulfuric acid solution, stir it at 70℃ for 20 minutes, wash it with water until neutral and dry it to obtain pretreated cotton cellulose.

[0098] Weigh 2.5g of pretreated cotton cellulose and add it to 47.5mL of 73wt% zinc chloride aqueous solution. Stir and dissolve thoroughly at 85℃ for 150min to prepare 5wt% regenerated cellulose spinning solution. Let stand to remove bubbles.

[0099] The wet spinning process involves adding the degassed regenerated cellulose spinning solution into a feed cylinder and extruding it through a 20G single-hole spinneret at a spinning linear speed of 0.65 m / min into an anhydrous ethanol coagulation bath for phase separation and shaping. The nascent gel fibers are then obtained by stretching the fibers at a stretch ratio of 1.1 and then winding and collecting them.

[0100] The nascent gel fibers were introduced into a 50wt% ethanol aging solution and allowed to stand for 40 minutes. Then they were transferred to deionized water and soaked for 30 minutes before being washed until neutral. Subsequently, they were immersed in tert-butanol for solvent replacement. Each replacement lasted for 6 hours and was repeated 3 times.

[0101] After replacement, the fibers were freeze-dried at -80℃ for 24 hours to prepare waste cotton-based regenerated cellulose aerogel fibers.

[0102] Application Example 1

[0103] Using the pure regenerated cellulose aerogel fiber of Comparative Example 1 as a control, the mechanical properties of the composite aerogel fibers prepared by doping with different mass fractions (2.5-12.5 wt%) of nitrogen, phosphorus and silicon co-doped carbon dots in Examples 2 and 3 were tested.

[0104] The test was conducted using an HD021NS electronic single yarn tensile tester, following the method specified in GB / T 3916-2013 "Determination of breaking strength and elongation at break of single yarn in packaged textiles (CRE method)". The test was performed after equilibration for 24 hours under standard atmospheric conditions of (20±2)℃ and (65±4)% relative humidity. The yarn clamping length was 5cm, the stretching speed was 200mm / min, and each sample was tested 50 times. The arithmetic mean of the results was taken.

[0105] Through the Figure 7 Quantitative analysis of the stress-strain curves revealed that the introduction of nitrogen-phosphorus-silicon co-doped carbon dots significantly altered the mechanical behavior of the composite aerogel fibers. Compared to the pure regenerated cellulose aerogel fibers of Comparative Example 1, the tensile strength of all composite aerogel fibers was significantly improved, with the improvement initially increasing and then decreasing with the carbon dot content. At a carbon doping content of 7.5 wt%, the composite aerogel fibers exhibited the best overall mechanical properties, with a tensile strength reaching a maximum of 43 MPa, approximately 4.7 times that of pure regenerated cellulose aerogel fibers; simultaneously, the elongation at break remained around 7.1%, demonstrating good toughness.

[0106] The above conclusions indicate that the nitrogen-phosphorus-silicon co-doped carbon dots prepared in this invention construct an effective nano-reinforcement system in the regenerated cellulose matrix. The abundant amino, silanol, and phosphorus-containing groups on the surface of the carbon dots form a covalent bond and a high-density multiple hydrogen bond network with the cellulose molecular chains. This strong interfacial interaction can not only effectively inhibit molecular chain slippage (reinforcement), but also achieve efficient stress transfer and uniform dissipation (toughening) through the "break-reconstruction" mechanism of hydrogen bonds and the way of hindering the propagation of dimensional cracks. Thus, the composite material is endowed with excellent mechanical properties and plays a significant role in both reinforcement and toughening.

[0107] Application Example 2

[0108] Using the pure regenerated cellulose aerogel fiber of Comparative Example 1 as a control, the flame retardant properties of the nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber of Example 2 were tested.

[0109] The test was conducted according to GB / T 5454-1997 "Test for Burning Performance of Textiles - Oxygen Index Method", using an M606B digital oxygen index tester. Aerogel fibers prepared in Example 1 and Comparative Example 1 were woven into plain weave fabrics. Samples measuring 150mm × 58mm were taken and tested using a tip-ignition method to determine the limiting oxygen index (LOI). A mixture of high-purity oxygen and nitrogen was used as the flowing gas, and propane as the ignition gas. The minimum oxygen concentration required for the sample to burn continuously for 3 minutes after ignition or for the damaged length to reach 50mm was recorded; this concentration value was the LOI value. Each group of samples was tested in parallel 15 times, and the final result was the average value.

[0110] Test results show that the LOI value of the pure regenerated cellulose aerogel fiber in Comparative Example 1 is only 19.0%, which is a flammable material. However, the LOI value of the nitrogen, phosphorus and silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber prepared in Example 2 reaches 27.5%, which is a significant increase of about 41%, reaching the flame-retardant level.

[0111] Figure 8 Thermogravimetric analysis (TG) and micro-scale thermogravimetric analysis (DTG) curves of pure regenerated cellulose aerogel fiber in Comparative Example 1 and nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber in Example 2 are further compared.

[0112] Analysis results show that the introduction of nitrogen-phosphorus-silicon co-doped carbon dots significantly improves the thermal stability and char formation ability of composite aerogel fibers. Under a nitrogen atmosphere, the main thermal decomposition peak temperature of pure regenerated cellulose aerogel fibers (…) T max The temperature is approximately 305℃, while the nitrogen, phosphorus, and silicon co-doped carbon dots reinforce waste cotton-based regenerated cellulose aerogel fiber... T maxThe temperature was increased to around 330℃, indicating that the strong hydrogen bonding between the nitrogen-phosphorus-silicon co-doped carbon dots and the regenerated cellulose molecular chains effectively restricted molecular chain movement, delayed the initiation of thermal decomposition, and improved the thermal stability of the material. More importantly, the composite aerogel fiber exhibited a significantly higher char residue rate in the high-temperature region, increasing from approximately 33% in pure regenerated cellulose aerogel fiber to approximately 40%. This is attributed to the catalytic effect of the nitrogen-phosphorus-silicon co-doped carbon dots at high temperatures, promoting the dehydration and carbonization of cellulose and forming a more stable and denser char layer. This char layer can act as a physical barrier during combustion, effectively blocking the diffusion of heat and oxygen, inhibiting the release of combustible volatiles, and preventing the spread of combustion, thereby endowing the material with excellent flame-retardant properties at the condensed phase level.

[0113] Comparative Example 2

[0114] Zhou et al. (Nanocellulose aerogel-based porous coaxial fibers for thermalinsulation[J]. Nano Energy (2020, 68(C): 104305.) Porous aerogel fibers with a core-sheath structure were prepared using cellulose nanofibers and regenerated cellulose as precursors via coaxial wet spinning and freeze-drying. These fibers exhibited a low density (0.2 g / cm³). 3 It features high porosity (85%), stress of 23.5 MPa, strain of approximately 6.6%, and excellent thermal insulation performance.

[0115] Although the coaxial fiber effectively protects its fragile internal aerogel structure through a dense cellulose sheath, significantly improving its mechanical stability, the freeze-drying process used in its preparation is typically time-consuming and energy-intensive, limiting its potential for large-scale, high-speed, continuous production. Furthermore, cellulose materials are naturally hydrophilic; without hydrophobic modification, their porous structure is prone to moisture absorption and collapse, leading to a significant decrease in thermal insulation performance in humid environments, thus limiting their stability in complex outdoor environments.

[0116] Comparative Example 3

[0117] Mroszczok J et al. (Cellulose Aerogel Fibers for Thermal Encapsulation ofDiesel Hybrid Engines for Fuel Savings in Cars[J]. Materials Today: Proceedings(2017, 4: S244-S248.) High-performance cellulose aerogel fibers were prepared by dissolving microcrystalline cellulose in zinc chloride hydrate melt and using a wet spinning process with controlled diffusion rate in the coagulation bath and supercritical carbon dioxide drying technology. These fibers exhibit extremely low thermal conductivity (0.026 W / (m·K)), a stress value of approximately 1.6 MPa, and a strain value of approximately 3.9%.

[0118] Although the aerogel fiber exhibits excellent thermal insulation properties and clear prospects for automotive industry applications, its high hygroscopicity of the cellulose matrix may lead to shrinkage of the aerogel skeleton and a decrease in thermal insulation performance in humid environments due to the low efficiency and high energy consumption of wet spinning and special drying processes.

[0119] The above embodiments of the present invention do not describe all details exhaustively, nor do they limit the present invention to the embodiments described above. Various changes, modifications, substitutions, and variations made by those skilled in the art to these embodiments without departing from the principles and spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A nitrogen-phosphorus-silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber, which is a composite aerogel fiber composed of the following components: Regenerated cellulose aerogel fibers, serving as the matrix phase, constitute a continuous three-dimensional porous network framework; and Nitrogen-phosphorus-silicon co-doped carbon dots, which serve as a reinforcing phase, are uniformly dispersed at the nanoscale and immobilized within the regenerated cellulose aerogel fibers. in, The nitrogen-phosphorus-silicon co-doped carbon dots are prepared by hydrothermal reaction in an aqueous solution system using citric acid as the carbon source, piperazine pyrophosphate as the nitrogen and phosphorus source, and silane coupling agent as the silicon source.

2. The regenerated cellulose aerogel fiber according to claim 1, characterized in that... The amount of nitrogen, phosphorus, and silicon co-doped carbon dots added is 2.5 to 12.5% ​​of the mass of the regenerated cellulose aerogel fiber.

3. A method for preparing nitrogen-phosphorus-silicon co-doped carbon dots reinforced waste cotton-based regenerated cellulose aerogel fiber as described in claim 1, wherein waste cotton fabric is used as raw material, pretreated cotton cellulose is obtained through chemical treatment, and then regenerated cellulose is obtained by dissolving it in zinc chloride solution. A certain amount of nitrogen-phosphorus-silicon co-doped carbon dots is added and blended as a blending spinning solution for wet spinning. The regenerated cellulose nascent gel fiber is formed in an anhydrous ethanol coagulation bath, and the reinforced regenerated cellulose aerogel fiber with a porous structure is obtained through post-treatment.

4. The preparation method according to claim 3, comprising: S1. First, use sodium hydroxide solution to pretreat the washed and dried waste cotton fabric to remove impurities such as pectin, and then use sulfuric acid solution for pretreatment to reduce its degree of polymerization, thus obtaining pretreated cotton cellulose. S2. Add the pretreated cotton cellulose to a 65-75 wt% zinc chloride aqueous solution and heat to dissolve to obtain a regenerated cellulose spinning solution; S3. Add nitrogen, phosphorus and silicon co-doped carbon dots to the regenerated cellulose spinning solution, so that they account for 2.5 to 12.5 wt% of the cellulose in the spinning solution, and disperse them evenly to obtain a blended spinning solution. S4. After the blended spinning solution is degassed, it is wet spun and extruded through a spinneret into an anhydrous ethanol coagulation bath to form and be drawn and oriented to form regenerated cellulose primary gel fibers. S5. The regenerated cellulose nascent gel fiber is subjected to static aging treatment with 50wt% ethanol aging solution and then freeze-dried to obtain nitrogen, phosphorus and silicon co-doped carbon dot reinforced waste cotton-based regenerated cellulose aerogel fiber.

5. The preparation method according to claim 4, characterized in that: In the sodium hydroxide solution pretreatment, the sodium hydroxide solution concentration is 10-20 wt%, and the solution is heated to 65-85℃ for 1.5-4 hours; in the sulfuric acid solution pretreatment, the sulfuric acid solution concentration is 20-25 wt%, and the solution is heated to 60-70℃ for 20-40 minutes.

6. The preparation method according to claim 4, characterized in that: Pretreated cotton cellulose is added to an aqueous solution of zinc chloride and heated to 80–90°C for 120–150 min to dissolve, resulting in a regenerated cellulose spinning solution with a concentration of 2–7 wt%.

7. The preparation method according to claim 4, characterized in that: The wet spinning process is carried out at room temperature. The blended spinning solution is extruded into a coagulation bath at a spinning speed of 0.3 to 0.65 m / min using a spinneret with a specification of 18 to 22 G and then solidified. After being drawn and oriented at a draw ratio of 1 to 1.2, it is aged at room temperature for 20 to 80 min.

8. The preparation method according to claim 4, characterized in that: It also includes solvent replacement of aged regenerated cellulose nascent gel fibers in a tert-butanol solution.

9. The preparation method according to claim 3 or 4, characterized in that: The nitrogen-phosphorus-silicon co-doped carbon dots are obtained by dispersing carbon source citric acid, nitrogen-phosphorus source piperazine pyrophosphate and silicon source silane coupling agent in an aqueous solution, hydrothermally reacting at 180-220°C for 6-12 hours, purifying the reaction product and freeze-drying it.

10. The preparation method according to claim 9, characterized in that: The mass ratio of the carbon source, nitrogen and phosphorus source to the silicon source is 1:2 to 5:5 to 11.

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