A method of preparing waste fiber pellets

Abstract

The application relates to the technical field of waste fiber particle preparation, and discloses a preparation method of waste fiber particles, which comprises the following steps: step S1: first, waste fibers are subjected to multistage sorting, and then, composite modification is carried out to complete pretreatment; step S2: fibers pretreated through step S1 and a thermoplastic matrix are subjected to dynamic composite process to prepare fiber-matrix particles; and step S3: the fiber-matrix particles prepared through step S2 are sequentially subjected to post-treatment and grading; through the synergistic process of multistage sorting, composite modification, dynamic composite and post-treatment grading, the application realizes efficient preparation of waste fiber particles. The synergistic cooperation of various process units not only solves the environmental pollution problem of waste fibers, but also realizes high-value utilization of resources; the prepared particles are high in purity, excellent in mechanical properties and good in uniformity, are suitable for industrialized production, and have both environmental benefits and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of waste fiber particle preparation technology, specifically a method for preparing waste fiber particles. Background Technology

[0002] With the widespread application of composite materials such as carbon fiber and glass fiber in aerospace, automobile manufacturing, and construction engineering, the amount of waste generated during their production and use has increased dramatically. This waste mainly includes scraps, processing debris, and discarded products from the production process. Carbon fiber and glass fiber often exist in a mixed form, and their surfaces often retain residual resin matrix. Direct disposal or simple landfilling not only occupies a large amount of land resources but also causes long-term pollution to soil and water bodies because the fiber materials are difficult to degrade naturally and the resin components may release harmful substances.

[0003] From a resource utilization perspective, carbon fiber and glass fiber possess excellent properties such as high strength and lightweight, and their recycling aligns with the principles of a circular economy. Currently, preparing recycled particles from waste fibers for reuse in composite material production is an important approach to resource recovery; however, existing technologies suffer from the following problems:

[0004] Low sorting efficiency: Traditional sorting methods are difficult to effectively separate carbon fiber and glass fiber, and the fiber length is not uniform, resulting in mixed composition of recycled particles and poor performance stability.

[0005] Insufficient surface treatment: Resin residue on the surface of waste fibers is difficult to remove completely, and the interfacial compatibility between fibers and thermoplastic matrices (such as PA6 and PP) is poor, which directly affects the mechanical properties of the particles.

[0006] Defects in composite process: uneven mixing and dispersion of fibers and matrix, disordered arrangement of fibers in matrix, resulting in key properties such as strength and toughness of recycled particles being far lower than those of virgin materials;

[0007] Poor particle uniformity: The post-processing and grading processes are crude, resulting in a wide particle size distribution, which cannot meet the production requirements of high-precision composite materials.

[0008] Therefore, developing a preparation method that can efficiently separate and purify waste fibers, improve interfacial bonding, and enhance particle properties and uniformity has become the key to solving the problems of waste pollution and resource waste. Summary of the Invention

[0009] To address the problems in the prior art, the present invention provides a method for preparing waste fiber particles.

[0010] The technical solution adopted by this invention to solve its technical problem is: a method for preparing waste fiber particles, comprising the following steps:

[0011] Step S1: The waste fibers are first sorted in multiple stages, and then composite modification is carried out to complete the pretreatment;

[0012] Step S2: The fiber pretreated in step S1 is combined with the thermoplastic matrix to prepare fiber-matrix particles through a dynamic composite process;

[0013] Step S3: The fiber-matrix particles prepared in step S2 are subjected to post-processing and classification in sequence.

[0014] As a further technical solution, in step S1, the multi-stage sorting includes an electrostatic sorting unit, an airflow sorting unit, and a chemical purification unit performed sequentially.

[0015] The electrostatic sorting unit separates carbon fibers from glass fibers based on the difference in surface conductivity. Under the conditions of voltage 10-30kV, sorting time 12-15 minutes, electrode spacing 10-20cm, and fiber feeding speed 1-3m / min, the separation rate of carbon fibers and glass fibers is 92%-96%.

[0016] The airflow sorting unit separates fibers of 5-50mm length by means of airflow velocity gradient. Under the conditions of airflow velocity of 5-15m / s and sorting chamber pressure of 0.1-0.3MPa, the fiber proportion within the same length range reaches more than 85%.

[0017] The chemical purification unit uses a 12%-15% hydrochloric acid solution or a 14%-20% sodium hydroxide solution to remove 95%-98% of the resin residue from the fiber surface.

[0018] As a further technical solution, in the chemical purification unit, when using hydrochloric acid solution, the treatment time is 30-40 minutes; when using sodium hydroxide solution, the treatment time is 70-90 minutes.

[0019] As a further technical solution, in step S1, the composite modification includes sequential surface grafting modification and nanoparticle loading.

[0020] The surface grafting modification involves placing the multi-stage sorted fibers in an acrylic monomer solution with a concentration of 10%-20% at a solid-liquid ratio of 1:8-10. An initiator, which is ammonium persulfate or azobisisobutyronitrile, is added to the acrylic monomer solution at a mass of 0.5%-0.8% of the monomer. The initiator is reacted at 60-80°C for 2-4 hours, followed by filtration, washing, and drying.

[0021] The nanoparticle loading is achieved by loading nano-silica particles onto the fiber surface through in-situ hydrolysis. The raw material used in the in-situ hydrolysis method is tetraethyl orthosilicate, with a volume ratio of tetraethyl orthosilicate to water of 1:5-7. The loaded nano-silica particles have a particle size of 20-50 nm, and the particle loading amount is 5%-6% of the fiber mass.

[0022] As a further technical solution, in the surface grafting modification, when the initiator is ammonium persulfate, the reaction temperature is 60-70℃; when the initiator is azobisisobutyronitrile, the reaction temperature is 70-80℃.

[0023] As a further technical solution, in the nanoparticle loading, the reaction temperature of the in-situ hydrolysis method is 40-60℃, and the reaction time is 2-3 hours.

[0024] As a further technical solution, in step S2, the dynamic composite process includes melt blending and directional stretching performed sequentially;

[0025] The melt blending process involves adding the fiber modified in step S1 to a thermoplastic matrix at a mass ratio of 1:3-5 into a twin-screw extruder and melting and blending it at a speed of 100-300 r / min and a temperature of 200-250℃; the thermoplastic matrix is ​​PA6 or PP.

[0026] The directional stretching involves stretching the melt blend under a multi-level temperature gradient at a stretching rate of 6-8 mm / s. The multi-level temperature gradients are 180℃, 200℃, and 220℃, respectively. The stretching length in each temperature range accounts for 1 / 3 of the total stretching length, so that the fibers are arranged in an orderly manner along the stretching direction in the matrix.

[0027] As a further technical solution, the length-to-diameter ratio of the twin-screw extruder is 30-40:1, and the melt blending time is 12-15 minutes.

[0028] As a further technical solution, in step S3, the post-processing is low-temperature pulverization, specifically, the fiber-matrix particles prepared in step S2 are placed in a liquid nitrogen environment and pulverized after being kept at -196°C for 8-10 minutes.

[0029] The grading process uses a vibrating sieve with a 30-150 mesh screen for 20-30 minutes, so that the proportion of particles within the same mesh size range after sieving reaches more than 90%.

[0030] The beneficial effects of this invention are:

[0031] This invention achieves efficient preparation of waste fiber particles through a synergistic process of multi-stage sorting, composite modification, dynamic composite and post-treatment grading.

[0032] First, the multi-stage sorting unit provides high-quality raw materials for subsequent processes through precise separation and purification. Electrostatic sorting utilizes the difference in surface conductivity between carbon fiber and glass fiber to achieve efficient separation under specific voltage and electrode spacing conditions, with a separation rate of 92%-96%, avoiding the impact of fiber contamination on product performance. Airflow sorting separates fibers of 5-50mm length through airflow velocity gradients, ensuring that fibers within the same length range account for more than 85%, guaranteeing fiber length uniformity. Chemical purification uses hydrochloric acid or sodium hydroxide solution to remove 95%-98% of resin residue. The acid-base hydrolysis reaction with the resin removes surface organic matter, purifying the fiber surface and laying the foundation for subsequent modification.

[0033] Secondly, the composite modification unit significantly improves the interfacial compatibility between the fiber and the matrix through surface grafting and nanoparticle loading. In surface grafting modification, acrylic monomers form covalent bonds with the fiber surface under the action of an initiator, introducing polar groups (such as carboxyl groups) and enhancing the affinity between the fiber and the polar thermoplastic matrix (such as PA6). Nanoparticle loading involves loading 20-50 nm silica particles onto the fiber surface through in-situ hydrolysis. The silica generated by the hydrolysis of tetraethyl orthosilicate adsorbs onto the fiber surface, forming a "micro-nano reinforced structure." This not only improves the interfacial mechanical interlocking but also enhances the wear resistance and thermal stability of the particles, resulting in a significant improvement in the bonding strength between the fiber and the matrix compared to traditional methods.

[0034] Furthermore, the dynamic composite process achieves uniform dispersion and orderly arrangement of fibers and matrix through melt blending and directional stretching. During melt blending, fibers and thermoplastic matrix are melt-mixed in a twin-screw extruder at a specific mass ratio. The shearing action of the screws promotes uniform fiber dispersion and avoids agglomeration. Directional stretching, through multi-level temperature gradients of 180℃, 200℃, and 220℃, allows the fibers to align orderly along the stretching direction during the stretching process, forming a "reinforcing skeleton." This synergistically improves the axial mechanical properties of the particles, such as tensile strength and impact strength, overcoming the performance shortcomings caused by the disordered distribution of fibers in traditional processes.

[0035] Finally, post-processing and grading processes ensure the uniformity and size control of the particles. Cryogenic crushing utilizes a liquid nitrogen environment (-196℃) to reduce the toughness of the material, increasing the brittleness of the fiber-matrix particles and achieving precise crushing; vibrating sieving with 30-150 mesh screens ensures that particles within the same mesh range account for more than 90%, and vibration grading separates particles of different sizes in an orderly manner, ensuring product uniformity and meeting the particle size requirements of high-precision composite material production.

[0036] In summary, the coordinated efforts of each process unit not only solve the environmental pollution problem of waste fiber but also achieve high-value utilization of resources. The prepared particles have high purity, excellent mechanical properties, and good uniformity, making them suitable for industrial production and achieving both environmental and economic benefits. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] This invention provides a method for preparing waste fiber particles, comprising the following steps:

[0039] Step S1: The waste fibers are first sorted in multiple stages, and then composite modification is carried out to complete the pretreatment;

[0040] Step S2: The fiber pretreated in step S1 is combined with the thermoplastic matrix to prepare fiber-matrix particles through a dynamic composite process;

[0041] Step S3: The fiber-matrix particles prepared in step S2 are subjected to post-processing and classification in sequence.

[0042] Step S1: Preprocessing

[0043] In this invention, the waste fiber preferably includes a mixture of carbon fiber and glass fiber waste, or waste fiber products containing resin residue, which can be obtained by recycling industrial waste or by purchasing from the market.

[0044] Multi-level sorting

[0045] The multi-stage sorting system includes an electrostatic sorting unit, an airflow sorting unit, and a chemical purification unit, which are performed sequentially.

[0046] Electrostatic sorting unit: Separates carbon fibers and glass fibers based on the difference in surface conductivity. Preferred conditions are: voltage 10-30kV, sorting time 12-15 minutes, electrode spacing 10-20cm, and fiber feed rate 1-3m / min. Under these conditions, the separation rate of carbon fibers and glass fibers can reach 92%-96%; more preferably, the voltage is 20-25kV and the electrode spacing is 15-18cm, at which point the separation rate can be stabilized above 94%.

[0047] Airflow sorting unit: Separates fibers with a length of 5-50mm by means of an airflow velocity gradient. Preferred conditions are: airflow velocity 5-15m / s, sorting chamber pressure 0.1-0.3MPa, the fiber ratio within the same length range can reach more than 85%; more preferably, airflow velocity 8-12m / s, sorting chamber pressure 0.2-0.25MPa, at which point the length uniformity can be improved to more than 90%.

[0048] Chemical purification unit: Removes resin residue from the fiber surface. A 12%-15% hydrochloric acid solution or a 14%-20% sodium hydroxide solution is preferably used. When using hydrochloric acid solution, the treatment time is preferably 30-40 minutes; when using sodium hydroxide solution, the treatment time is preferably 70-90 minutes. Both solutions can remove 95%-98% of the resin residue, with a more preferred 13%-14% hydrochloric acid solution (35 minutes) or a 16%-18% sodium hydroxide solution (80 minutes), achieving a resin residue removal rate of over 97%.

[0049] Composite modification

[0050] The composite modification includes sequential surface grafting modification and nanoparticle loading.

[0051] Surface grafting modification: Fibers after multi-stage sorting are placed in a 10%-20% acrylic monomer solution at a solid-liquid ratio of 1:8-10. An initiator (ammonium persulfate or azobisisobutyronitrile) at 0.5%-0.8% of the monomer mass is added to the solution. The reaction is carried out at 60-80°C for 2-4 hours, followed by filtration, washing, and drying. More preferably, the solid-liquid ratio is 1:9, the monomer concentration is 15%-18%, and the initiator dosage is 0.6%-0.7%. When the initiator is ammonium persulfate, the preferred reaction temperature is 60-70°C; when the initiator is azobisisobutyronitrile, the preferred reaction temperature is 70-80°C, at which point the grafting rate can be increased by 10%-15%.

[0052] Nanoparticle loading: Nano-sized silica particles are loaded onto the fiber surface via in-situ hydrolysis. The raw material is tetraethyl orthosilicate, with a volume ratio of tetraethyl orthosilicate to water of 1:5-7. The loaded silica particles have a diameter of 20-50 nm, and the particle loading is 5%-6% of the fiber mass. The preferred reaction conditions are: temperature 40-60℃, time 2-3 hours; more preferably, temperature 50-55℃, time 2.5 hours, under which the particle dispersion is better and the loading is stable at around 5.5%.

[0053] Step S2: Dynamic composite process (melt blending, directional stretching)

[0054] Fiber-matrix particles are prepared by combining modified fibers with thermoplastic matrices through a dynamic composite process, including melt blending and directional stretching.

[0055] Melt blending: The fiber and thermoplastic matrix are added to a twin-screw extruder at a mass ratio of 1:3-5 and melt-blended at a speed of 100-300 r / min and a temperature of 200-250℃. The thermoplastic matrix is ​​preferably PA6 or PP; the length-to-diameter ratio of the twin-screw extruder is preferably 30-40:1, and the melt blending time is preferably 12-15 minutes. More preferably, the mass ratio is 1:4, the speed is 200-250 r / min, the temperature is 220-240℃ (PA6 matrix) or 200-220℃ (PP matrix), and the length-to-diameter ratio is 35:1, at which point the compatibility between the fiber and the matrix is ​​even better.

[0056] Oriented stretching: The melt blend is stretched under a multi-stage temperature gradient at a stretching rate of 6-8 mm / s, with the temperature gradients being 180℃, 200℃, and 220℃ respectively. The stretching length in each temperature range accounts for 1 / 3 of the total stretching length. This process allows the fibers to be arranged in an orderly manner along the stretching direction in the matrix. A stretching rate of 7 mm / s is preferred, and the temperature gradient remains stable, which can improve the fiber orientation to over 85%.

[0057] Step S3: Post-processing and grading

[0058] Post-processing: Low-temperature pulverization is employed. The fiber-matrix particles are placed in a liquid nitrogen environment and pulverized after being kept at -196°C for 8-10 minutes. More preferably, they are kept at 9 minutes, at which point the particles have moderate brittleness and a more regular morphology after pulverization.

[0059] Grading: Using a vibrating screen with a 30-150 mesh screen, screening for 20-30 minutes can achieve a particle ratio of over 90% within the same mesh size range. Even better, using a 40-120 mesh screen and screening for 25 minutes can increase the ratio to over 93%.

[0060] The preparation method of this invention achieves efficient separation and purification of fibers through multi-stage sorting, enhances the interfacial bonding between fibers and the matrix through composite modification, improves the mechanical properties of particles through dynamic composite technology, and ensures particle uniformity through post-treatment and classification. The final product has high purity and excellent mechanical properties, and the process is simple and controllable, making it suitable for industrial production.

[0061] The waste fibers used in the following examples are mixed waste carbon fiber and glass fiber containing resin residue (carbon fiber accounts for about 60%), thermoplastic matrix PA6 (commercially available, grade PA6-1010C) and PP (commercially available, grade PP-H01), and acrylic monomers, tetraethyl orthosilicate and other reagents are all commercially available analytical grade reagents.

[0062] Example 1

[0063] Multi-level sorting:

[0064] Electrostatic separation: voltage 20kV, time 14 minutes, electrode spacing 15cm, feed speed 2m / min;

[0065] Airflow sorting: airflow velocity 10m / s, pressure 0.2MPa;

[0066] Chemical purification: 13% hydrochloric acid solution, treated for 35 minutes.

[0067] Composite modification:

[0068] Surface grafting: solid-liquid ratio 1:9, 16% acrylic acid solution, initiator is ammonium persulfate (0.6% of monomer mass), reaction at 65℃ for 3 hours;

[0069] Nanoparticle loading: Tetraethyl orthosilicate and water in a volume ratio of 1:6, reacted at 50°C for 2.5 hours.

[0070] Dynamic compounding:

[0071] Melt blending: fiber to PA6 mass ratio 1:4, twin-screw extruder (length to diameter ratio 35:1), speed 200 r / min, temperature 230℃, time 13 minutes;

[0072] Directional stretching: rate 7 mm / s, temperature gradient 180℃, 200℃, 220℃.

[0073] Post-processing and grading: After holding in liquid nitrogen for 9 minutes, crush and sieve through a 50-100 mesh screen for 25 minutes.

[0074] Example 2

[0075] Multi-level sorting:

[0076] Electrostatic separation: voltage 25kV, time 13 minutes, electrode spacing 18cm, feed rate 1.5m / min;

[0077] Airflow sorting: airflow velocity 12m / s, pressure 0.25MPa;

[0078] Chemical purification: 17% sodium hydroxide solution, treated for 80 minutes.

[0079] Composite modification:

[0080] Surface grafting: solid-liquid ratio 1:8, 18% acrylic acid solution, azobisisobutyronitrile (0.7% of monomer mass) as initiator, reaction at 75℃ for 2.5 hours;

[0081] Nanoparticle loading: Tetraethyl orthosilicate and water were reacted at 55°C for 2 hours by volume ratio of 1:5.

[0082] Dynamic compounding:

[0083] Melt blending: fiber to PP mass ratio 1:3, twin-screw extruder (length to diameter ratio 30:1), speed 250 r / min, temperature 210℃, time 12 minutes;

[0084] Directional stretching: rate 6 mm / s, temperature gradient 180℃, 200℃, 220℃.

[0085] Post-processing and grading: After holding in liquid nitrogen for 8 minutes, crush and sieve through a 40-80 mesh screen for 20 minutes.

[0086] Example 3

[0087] Multi-level sorting:

[0088] Electrostatic separation: voltage 15kV, time 15 minutes, electrode spacing 12cm, feed speed 3m / min;

[0089] Airflow sorting: airflow velocity 8 m / s, pressure 0.15 MPa;

[0090] Chemical purification: 14% hydrochloric acid solution, treated for 30 minutes.

[0091] Composite modification:

[0092] Surface grafting: solid-liquid ratio 1:10, 14% acrylic acid solution, initiator is ammonium persulfate (0.5% of monomer mass), reaction at 70℃ for 2 hours;

[0093] Nanoparticle loading: Tetraethyl orthosilicate and water in a volume ratio of 1:7, reacted at 45°C for 3 hours.

[0094] Dynamic compounding:

[0095] Melt blending: fiber to PA6 mass ratio 1:5, twin-screw extruder (length to diameter ratio 40:1), speed 150 r / min, temperature 240℃, time 15 minutes;

[0096] Directional stretching: rate 8 mm / s, temperature gradient 180℃, 200℃, 220℃.

[0097] Post-treatment and grading: After holding in liquid nitrogen for 10 minutes, crush and sieve through an 80-120 mesh screen for 30 minutes.

[0098] Example 4

[0099] Multi-level sorting:

[0100] Electrostatic separation: voltage 25kV, time 13 minutes, electrode spacing 18cm, feed rate 1.5m / min;

[0101] Airflow sorting: airflow velocity 12m / s, pressure 0.25MPa;

[0102] Chemical purification: 18% sodium hydroxide solution, treated for 80 minutes.

[0103] Composite modification:

[0104] Surface grafting: solid-liquid ratio 1:8, 18% acrylic acid solution, azobisisobutyronitrile (0.7% of monomer mass) as initiator, reaction at 75℃ for 2.5 hours;

[0105] Nanoparticle loading: Tetraethyl orthosilicate and water were reacted at 55°C for 2 hours by volume ratio of 1:5.

[0106] Dynamic compounding:

[0107] Melt blending: fiber to PP mass ratio 1:3, twin-screw extruder (length to diameter ratio 30:1), speed 250 r / min, temperature 210℃, time 12 minutes;

[0108] Directional stretching: rate 6 mm / s, temperature gradient 180℃, 200℃, 220℃.

[0109] Post-processing and grading: After holding in liquid nitrogen for 8 minutes, crush and sieve through a 40-80 mesh screen for 20 minutes.

[0110] Example 5

[0111] Multi-level sorting:

[0112] Electrostatic separation: voltage 30kV, time 12 minutes, electrode spacing 20cm, feed speed 3m / min;

[0113] Airflow sorting: airflow velocity 15m / s, pressure 0.3MPa;

[0114] Chemical purification: 15% hydrochloric acid solution, treated for 40 minutes.

[0115] Composite modification:

[0116] Surface grafting: solid-liquid ratio 1:10, 20% acrylic acid solution, initiator is ammonium persulfate (0.8% of monomer mass), reaction at 70℃ for 4 hours;

[0117] Nanoparticle loading: Tetraethyl orthosilicate and water were reacted at 60°C for 3 hours in a volume ratio of 1:7.

[0118] Dynamic compounding:

[0119] Melt blending: fiber to PA6 mass ratio 1:5, twin-screw extruder (length to diameter ratio 40:1), speed 300 r / min, temperature 250℃, time 15 minutes;

[0120] Directional stretching: rate 8 mm / s, temperature gradient 180℃, 200℃, 220℃.

[0121] Post-treatment and grading: After holding in liquid nitrogen for 10 minutes, crush and sieve through a 100-150 mesh screen for 30 minutes.

[0122] The following is a comparative example.

[0123] Comparative Example 1

[0124] The difference from Example 1 is that the multi-stage sorting only involves airflow sorting and chemical purification (without electrostatic sorting), while the other steps are the same.

[0125] Comparative Example 2

[0126] The difference from Example 1 is that the composite modification only involves surface grafting (without nanoparticle loading), while the other steps are the same.

[0127] test

[0128] Experiment 1: Fiber Separation Purity and Residual Rate Test

[0129] Methods: Following the fiber composition analysis methods in Appendix A of GB / T3362-2017 "Test Methods for Tensile Properties of Carbon Fiber Multifilament", the purity of carbon fiber separated from glass fiber was determined using the ignition method (glass fiber is heat-resistant, carbon fiber can be ignited). The resin residue rate was determined using infrared spectroscopy (referring to GB / T6040-2019 "General Rules for Infrared Spectroscopic Analysis"). The results are as follows:

[0130] Table 1

[0131]

[0132] As can be seen from Table 1, the carbon fiber and glass fiber separation purity of Examples 1-5 are all above 93%, and the resin residue rate is less than 3%, indicating that multi-stage sorting (especially electrostatic sorting) and chemical purification can effectively separate fibers and remove residues.

[0133] Comparative Example 1 suffered from severe fiber mixing due to the lack of electrostatic separation, with a separation purity of only about 70%, and a higher resin residue rate (due to impurities on the fiber surface affecting the purification effect).

[0134] Experiment 2: Particle Mechanical Properties Test

[0135] Methods: Tensile strength was tested according to GB / T1040.2-2022; impact strength (notch type A) was tested according to ASTM D256-24. The results are as follows:

[0136] Table 2

[0137]

[0138]

[0139] As shown in Table 2, the tensile strength of Examples 1-5 is above 82 MPa, and the impact strength is above 11.8 kJ / m. 2 The above is due to the high purity of fiber separation and good interfacial bonding (reinforced by nanoparticle loading).

[0140] Comparative Example 1 showed a significant decrease in mechanical properties due to fiber mixing (poor compatibility between glass fiber and carbon fiber); Comparative Example 2 showed weaker interfacial bonding between fibers and matrix due to the lack of nanoparticle loading, resulting in lower mechanical properties than the Example.

[0141] Experiment 3: Test of particle size distribution uniformity

[0142] Methods: The proportion of particles in the target mesh size range (50-100 mesh in Example 1) and the standard deviation of particle size were determined using an airflow sieve analyzer. The results are as follows:

[0143] Table 3

[0144]

[0145]

[0146] As can be seen from Table 3, the proportion of target particles in Examples 1-5 is all above 93%, and the standard deviation of particle size is less than 10 μm, indicating that the post-treatment and classification effects are good.

[0147] Comparative Example 1 had low fiber purity (impurities affected the uniformity of crushing), resulting in dispersed particle distribution. Although Comparative Example 2 had a similar grading effect to the example, the particle morphology was irregular after crushing due to poor fiber interface bonding, and the standard deviation was slightly higher.

[0148] This invention significantly improves the separation purity, mechanical properties, and particle size uniformity of waste fiber particles through the synergistic effect of multi-stage sorting, composite modification, and dynamic composite processes. The comparative example, lacking key steps (such as electrostatic sorting and nanoparticle loading), exhibits a significant performance decline, demonstrating the advanced nature and necessity of the technical solution presented in this invention.

[0149] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing waste fiber granules, characterized in that, Includes the following steps: Step S1: The waste fibers are first sorted in multiple stages, and then composite modification is carried out to complete the pretreatment; Step S2: The fiber pretreated in step S1 is combined with the thermoplastic matrix to prepare fiber-matrix particles through a dynamic composite process; Step S3: The fiber-matrix particles prepared in step S2 are subjected to post-processing and classification in sequence; In step S1, the composite modification includes sequential surface grafting modification and nanoparticle loading. The surface grafting modification involves placing the multi-stage sorted fibers in an acrylic monomer solution with a concentration of 10%-20% at a solid-liquid ratio of 1:8-10. An initiator, which is ammonium persulfate or azobisisobutyronitrile, is added to the acrylic monomer solution at a mass of 0.5%-0.8% of the monomer. The initiator is reacted at 60-80°C for 2-4 hours, followed by filtration, washing, and drying. The nanoparticle loading is achieved by loading nano-silica particles onto the fiber surface through in-situ hydrolysis. The raw material used in the in-situ hydrolysis method is tetraethyl orthosilicate, with a volume ratio of tetraethyl orthosilicate to water of 1:5-7. The loaded nano-silica particles have a particle size of 20-50 nm, and the particle loading amount is 5%-6% of the fiber mass. In step S1, the multi-stage sorting includes an electrostatic sorting unit, an airflow sorting unit, and a chemical purification unit performed sequentially. The electrostatic sorting unit separates carbon fibers from glass fibers based on the difference in surface conductivity. Under conditions of 10-30kV voltage, 12-15 minutes sorting time, 10-20cm electrode spacing, and 1-3m / min fiber feeding speed, the separation rate of carbon fibers and glass fibers is 92%-96%. The airflow sorting unit separates fibers of 5-50mm length by means of airflow velocity gradient. Under the conditions of airflow velocity of 5-15m / s and sorting chamber pressure of 0.1-0.3MPa, the fiber proportion within the same length range reaches more than 85%. The chemical purification unit uses a 12%-15% hydrochloric acid solution or a 14%-20% sodium hydroxide solution to remove 95%-98% of the resin residue from the fiber surface.

2. The method according to claim 1, characterized in that, In the chemical purification unit, the treatment time is 30-40 minutes when using hydrochloric acid solution and 70-90 minutes when using sodium hydroxide solution.

3. The method according to claim 1, characterized in that, In the surface grafting modification, when the initiator is ammonium persulfate, the reaction temperature is 60-70℃; when the initiator is azobisisobutyronitrile, the reaction temperature is 70-80℃.

4. The method according to claim 1, characterized in that, In the nanoparticle loading, the in-situ hydrolysis reaction temperature is 40-60℃ and the reaction time is 2-3 hours.

5. The method according to claim 1, characterized in that, In step S2, the dynamic composite process includes melt blending and directional stretching performed sequentially. The melt blending process involves adding the fiber modified in step S1 to a thermoplastic matrix at a mass ratio of 1:3-5 into a twin-screw extruder and melting and blending it at a speed of 100-300 r / min and a temperature of 200-250℃; the thermoplastic matrix is ​​PA6 or PP. The directional stretching involves stretching the melt blend under a multi-level temperature gradient at a stretching rate of 6-8 mm / s. The multi-level temperature gradients are 180℃, 200℃, and 220℃, respectively. The stretching length in each temperature range accounts for 1 / 3 of the total stretching length, so that the fibers are arranged in an orderly manner along the stretching direction in the matrix.

6. The method according to claim 5, characterized in that, The twin-screw extruder has an aspect ratio of 30-40:1 and a melt blending time of 12-15 minutes.

7. The method according to claim 1, characterized in that, In step S3, the post-processing is low-temperature pulverization, specifically, the fiber-matrix particles prepared in step S2 are placed in a liquid nitrogen environment and pulverized after being kept at -196°C for 8-10 minutes. The grading process uses a vibrating sieve with a 30-150 mesh screen for 20-30 minutes, so that the proportion of particles within the same mesh size range after sieving reaches more than 90%.

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