Preparation method of waste fiber thermoplastic composite material

Abstract

The invention discloses a preparation method of a waste fiber thermoplastic composite material, and belongs to the technical field of biomass conversion material preparation. According to the invention, a core process combining fiber entanglement and hot melting is constructed, the melt-blown PE superfine fiber is innovatively adopted as the adhesive, the melt-blown PE superfine fiber is uniformly adhered to the waste fiber in all regions due to the huge specific surface area and excellent permeability, and the problem that the mechanical property of the plate is fluctuated due to non-uniform coating of the traditional chemical adhesive is solved. According to the method, no solvent is added in the whole process, and VOCs emission is fundamentally eradicated. Particularly, air or oxygen plasmas are introduced to carry out on-line treatment on the opened fibers, so that the industrial bottleneck that the interface bonding force of inert fibers such as polypropylene is insufficient is effectively solved, and the mechanical strength and long-term durability of the plate are remarkably improved. The method is simple in process, environmentally friendly and efficient, the obtained board is excellent in performance, and a new way is provided for high-value utilization of waste textiles.

Description

Technical Field

[0001] This invention relates to the field of biomass conversion material preparation technology, and more particularly to a method for preparing a waste fiber thermoplastic composite material. Background Technology

[0002] With the rapid development of the textile industry and the upgrading of consumption, the amount of waste textiles generated has been increasing year by year. According to statistics, the global annual discharge of waste textiles has exceeded 100 million tons, and my country's annual discharge has exceeded 10 million tons. If these waste textiles are disposed of by traditional landfill and incineration methods, they will not only occupy a large amount of land resources, but also cause serious environmental pollution due to the difficulty of natural degradation of chemical fibers and the generation of harmful gases during the incineration process. Meanwhile, the rich fiber resources contained in them have not been effectively recycled and utilized, further exacerbating the current situation of resource waste.

[0003] Preparing waste fibers into composite material boards is an important direction for realizing their high-quality utilization. The prepared composite material boards can be widely used in many fields such as building filling, packaging cushioning, and interior decoration, and have broad market prospects. However, the existing waste fiber composite material preparation technology still has many key defects that need to be solved, which seriously restricts the development of the industry.

[0004] Existing technologies for bonding waste fibers mostly use thermosetting adhesives such as urea-formaldehyde resin and phenolic resin. These adhesives continuously release volatile organic compounds such as formaldehyde during production and use, posing potential hazards to human health and the environment. Some processes use hot melt adhesive granules as a binder, but this material suffers from poor dispersion uniformity, easily leading to uneven bonding of boards and fluctuations in mechanical properties. Furthermore, the interfacial compatibility between hot melt adhesives and waste fibers is poor, easily resulting in delamination, cracking, and other quality problems during long-term use. At the same time, existing technologies are quite restrictive in their requirements for the type of waste fiber, mostly applicable only to natural fibers such as cotton and linen or low-crystallinity polyester fibers. When dealing with smooth, chemically inert fibers such as polypropylene fibers and highly crystalline polyester, the lack of effective interfacial activation methods means that the adhesive and fibers can only achieve bonding through physical entanglement, resulting in inherently weak interfacial bonding. This ultimately leads to weak mechanical properties and poor durability of the boards, making it difficult to achieve efficient recycling of these inert fibers and significantly limiting the scope of waste textile recycling. In addition, the fibers prepared by existing meltblown technology are usually in the form of straight and smooth linear structures. This structure lacks sufficient deformation redundancy. When the composite material is subjected to impact or bending loads, the straight adhesive fibers are prone to brittle fracture or slippage from the surface of the matrix fibers, making it difficult for the toughness and impact resistance of the board to meet the requirements of high-end applications.

[0005] Furthermore, some optimization schemes attempt to improve interfacial bonding by adding coupling agents and solvent-based auxiliary components. However, the additional chemical reagents not only increase raw material costs but may also introduce new VOC emission risks, violating the core objective of environmental recycling. Simultaneously, some processes require complex equipment combinations or multi-step modification procedures, resulting in low production efficiency and significant challenges for large-scale application. More critically, existing waste fiberboard generally faces a contradiction between mechanical properties, environmental friendliness, and cost. Pursuing high strength requires increasing adhesive usage or using expensive modifiers, directly leading to increased costs and decreased environmental friendliness. Conversely, prioritizing environmental friendliness and low cost results in key properties such as tensile strength, flexural strength, and dimensional stability failing to meet practical application requirements, further limiting the expansion of its application scenarios.

[0006] Therefore, a method for preparing waste fiber thermoplastic composite materials is provided. Summary of the Invention

[0007] This invention overcomes the shortcomings of the prior art and provides a method for preparing waste fiber thermoplastic composite materials.

[0008] To achieve the above objectives, the technical solution adopted by this invention is: a method for preparing a waste fiber thermoplastic composite material, comprising the following steps: S1. Recycle waste textiles and sort and screen them to remove non-fibrous impurities. Then, open them to make them into uniformly dispersed single waste fibers. S2. The waste fibers obtained in S1 are laid into a uniform fiber web using a web-forming device, and then needle-punched and entangled to form a needle-punched felt with a certain strength and thickness. S3. The PE material is fed into the screw extruder and melted at high temperature to become a uniform melt; S4. The PE melt obtained in S3 is filtered at the front end of the extruder to remove impurities and incompletely melted particles; S5. The clean melt obtained in S4 is transported to the spinneret assembly. Under the stretching of a high-frequency pulsed hot airflow with a frequency of 50-200Hz, the extruded melt flow is made to generate microscopic spiral curling, and is extruded from the spinneret orifice and rapidly drawn thin. S6. The thin PE melt stream drawn in S5 is cooled and solidified to form meltblown PE microfibers for bonding. S7. The needle-punched felt obtained in S2 is conveyed layer by layer to the bottom of the meltblown equipment so that the meltblown PE ultrafine fibers obtained in S6 can evenly cover its surface and gaps, and the initial bonding is achieved by using hot melt adhesiveness. S8. The composite blank formed in S7 is sent into a hot pressing device and hot-pressed under set conditions to fully melt and bond the PE adhesive, and then cooled and shaped into waste fiberboard.

[0009] In a preferred embodiment of the present invention, in step S1, the opening equipment is a cotton opener with an opening speed of 800-1000 r / min, an opening time of 3-5 min, and a fiber moisture content controlled at 8-10%. The waste fibers obtained after opening are treated with air or oxygen plasma, with a treatment power of 300-400W, a treatment time of 60-90s, and a treatment distance of 5-8mm.

[0010] In a preferred embodiment of the present invention, in step S2, the web forming equipment is a cross-laying machine to produce the fiber web with a surface density of 300-350 g / m². The needle punching machine has a needle punching density of 150-180 needles / cm² and a needle punching depth of 7-9mm, ultimately forming the needle punched felt with a thickness of 4-6mm and a surface density of 320-360g / m².

[0011] In a preferred embodiment of the present invention, in step S3, the PE material is selected from high-density polyethylene particles with a purity of ≥99% and free from mechanical impurities and color masterbatch residue.

[0012] In a preferred embodiment of the present invention, in step S5, the spinneret assembly includes two nozzles arranged side by side, the two nozzles having the same airflow pulse frequency but complementary phase difference of 180 degrees; a turbulence rectifier plate is provided between the two nozzles to eliminate macroscopic airflow fluctuations, so that the settled meltblown PE microfibers exhibit a macroscopically uniform distribution and a microscopically spirally coiled structure.

[0013] In a preferred embodiment of the present invention, in step S5, the temperature of the hot airflow from the spinneret assembly is 200-210℃, and the airflow velocity is 0.8-1.2m / s. The airflow velocity is adjusted according to the melt flow index of the PE material to control the melt thinning effect.

[0014] In a preferred embodiment of the present invention, in step S6, one or more functional additives may be selectively added to the PE melt stream before it is cooled and solidified. The functional additives are selected from one or two of flame retardants and bamboo charcoal powder; The amount of flame retardant added is 1-3% of the PE melt mass fraction, and the flame retardant is decabromodiphenyl ethane; The amount of bamboo charcoal powder added is 2-5% of the PE melt mass fraction; The functional additives and PE melt streams are uniformly dispersed through an online static mixer or a dispersion module at the end of a screw extruder.

[0015] In a preferred embodiment of the present invention, in step S6, the cooling method is direct cooling with ambient temperature air or gradient cooling. The meltblown PE microfiber formed after cooling has a diameter of 1-5 μm, a breaking strength ≥3.0 cN / dtex, and a hot melt adhesive activation temperature as low as 120℃.

[0016] In a preferred embodiment of the present invention, in step S7, the meltblown PE microfiber spraying amount per unit area is 60-100g / m², and a layered meltblown method is adopted.

[0017] In a preferred embodiment of the present invention, in step S8, the hot pressing equipment is a flat plate hot press, the hot pressing temperature is 120-145℃, the hot pressing pressure is 2.0-2.5MPa, and the hot pressing time is 3-4min; the cooling method is natural cooling to 25℃ for shaping, and the final plate thickness can be controlled between 3-10mm.

[0018] This invention addresses the shortcomings of the prior art and has the following beneficial effects: This invention utilizes a melt-blown process to prepare polyethylene microfiber as an adhesive, which is then uniformly applied to the surface and gaps of needle-punched felt made from waste fibers, followed by hot-pressing. This technique gives the PE microfiber an extremely high specific surface area and excellent permeability, enabling full and uniform contact between it and the waste fibers in three-dimensional space. The direct effect is a complete solution to the core problem of traditional liquid chemical adhesives, which suffer from uneven spraying, resulting in weak local strength and unstable overall mechanical properties of the board. Compared to existing technologies using adhesives such as urea-formaldehyde resin, whose dispersion is difficult to control precisely and requires solvent dilution, the board prepared by this invention has a dense and uniform structure, and its core mechanical properties, such as compressive and flexural strength, are significantly improved. Furthermore, this global uniform bonding mechanism fundamentally avoids the risk of interlayer delamination in traditional boards, greatly enhancing its reliability in long-term load-bearing applications and meeting the stringent requirements for material consistency in high-end applications.

[0019] The bonding process of this invention is based entirely on the hot-melt-cool-curing physical properties of PE material, requiring no solvent-based auxiliary components. This feature eliminates the introduction and emission of volatile organic compounds from the source of the production process, completely avoiding the risk of continuous release of formaldehyde and other harmful gases during the production and use of traditional urea-formaldehyde resin adhesives, ensuring a greener and more environmentally friendly production environment and final product. Compared with the common environmental deficiencies of existing technologies, the boards prepared by this invention are odorless and release no harmful components, fully meeting and even exceeding the standards for modern environmentally friendly materials. This not only protects the health and safety of producers and consumers but also allows the boards to be used in places with strict indoor air quality requirements, such as schools and hospitals, greatly expanding its market application scope and enhancing the social value of the product.

[0020] This invention innovatively introduces an air or oxygen plasma treatment step after the opening process and specifically optimizes the compatibility with raw materials containing inert fibers. It primarily involves bombarding the fiber surface with high-energy particles, effectively etching the smooth surface of inert fibers such as polypropylene and highly crystalline polyester without compromising the fiber's inherent strength, and introducing polar groups, thus creating conditions for subsequent chemical bonding. This significantly enhances the interfacial bonding force between PE adhesive and various inert fibers, overcoming the inherent weakness in bonding force caused by poor interfacial compatibility in traditional technologies. Furthermore, while activating the fiber surface, the plasma treatment also cleans the fiber surface, removing low-molecular-weight substances such as oil, which helps improve the direct bonding between PE and the fiber body and avoids the influence of impurity layers. In addition, by precisely setting the air plasma treatment step after opening and before web formation, this invention not only significantly improves the interfacial shear strength between meltblown PE microfibers and polypropylene fibers, but also unexpectedly reveals a significant improvement in the dimensional stability of the composite board under long-term humid heat cycling, effectively overcoming the edge delamination problem caused by weak interfaces. Compared to existing technologies that have stringent requirements for raw material fiber types and cannot efficiently recycle inert fibers, this invention completely breaks through this limitation. This significantly broadens the recycling boundaries of waste textiles, enabling the efficient and high-quality utilization of various complex waste textile resources, and significantly improving the comprehensive utilization rate of resources.

[0021] This invention incorporates a scheme for selectively adding functional additives during the meltblown PE melt stage, achieving uniform dispersion via an online static mixer. This feature endows the basic technical solution with flexible functional scalability, allowing users to easily prepare functionalized ultrafine fibers with specific functions such as adhesion, flame retardancy, and adsorption, tailored to the application scenarios of the final product. The direct effect is the easy customization of product functions without altering the core process route, meeting the market's diverse demands for sheet materials. Compared to existing technologies where functional modification often requires complex processes or additional coatings, leading to soaring costs and potentially affecting matrix properties, this invention achieves integrated molding of function and structure. Furthermore, this embedded functional design avoids subsequent processing steps, improving production efficiency and enabling the product to maintain its core advantages while possessing stronger market competitiveness.

[0022] This invention innovatively employs a meltblown process combining high-frequency pulsed airflow with phase interference. Traditional constant airflow meltblown fibers are straight and only have simple surface contact with waste fibers, making them prone to slippage and detachment under stress. This invention, however, utilizes pulsed airflow to create a microscopic helical spring-like structure in the PE microfibers. This structure offers dual advantages: firstly, a physical anchoring effect; the helical PE fibers, once inside the needle-punched felt, form a three-dimensional mechanical interlock with the waste fibers like expansion bolts, significantly enhancing interfacial bonding strength; secondly, increased toughness and impact resistance; the microscopic helical structure endows the adhesive layer with excellent elastic recovery and elongation at break. When the board is impacted, the helical fibers absorb energy like a spring, effectively solving the defects of traditional waste fiber boards being brittle and easily broken. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Figure 1 This is a flowchart of a method for preparing a waste fiber thermoplastic composite material according to the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0026] In the description of this application, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the scope of protection of this application. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0027] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art will understand the specific meaning of the above terms in this application based on the specific circumstances.

[0028] Unless otherwise specified in the following examples, all raw materials are commercially available or prepared by conventional methods in the art.

[0029] The core of this invention lies in providing a fully green and efficient method for preparing waste fiber boards. The core concept is as follows: First, waste textiles are opened, web-formed, and needle-punched to create needle-punched felt with a three-dimensional network structure, serving as a reinforcing skeleton. Then, an innovative melt-blowing process is used to directly produce ultrafine fiber-like adhesives from polyethylene. Utilizing its large specific surface area and excellent thermal meltability, the PE adhesive achieves preliminary composite bonding with the needle-punched felt through uniform coverage and penetration. Finally, a precisely controlled hot-pressing process ensures the PE adhesive fully melts and firmly bonds with the waste fibers. After cooling and shaping, a high-performance composite board is obtained. This method abandons traditional solvent-based chemical adhesives, eliminating VOC emissions at the source. Simultaneously, through the dual effects of physical entanglement and thermal-melt chemical bonding, high-quality bonding of various waste fibers is achieved. Addressing the industry pain point of insufficient interfacial bonding strength of surface inert fibers, this invention creatively introduces an online plasma treatment step and precisely integrates it with the core melt-blown PE bonding process, producing a significant synergistic enhancement effect and solving a long-standing technical problem.

[0030] like Figure 1As shown, a method for preparing a waste fiber thermoplastic composite material includes the following steps: S1. Recycle waste textiles and sort and screen them to remove non-fibrous impurities. Then, open them to make them into uniformly dispersed single waste fibers. S2. The waste fibers obtained in S1 are laid into a uniform fiber web using a web-forming device, and then needle-punched and entangled to form a needle-punched felt with a certain strength and thickness. S3. The PE material is fed into the screw extruder and melted at high temperature to become a uniform melt; S4. The PE melt obtained in S3 is filtered at the front end of the extruder to remove impurities and incompletely melted particles; S5. The clean melt obtained in S4 is transported to the spinneret assembly. Under the stretching of a high-frequency pulsed hot airflow with a frequency of 50-200Hz, the extruded melt flow is made to generate microscopic spiral curling, and is extruded from the spinneret orifice and rapidly drawn thin. S6. The thin PE melt stream drawn in S5 is cooled and solidified to form meltblown PE microfibers for bonding. S7. The needle-punched felt obtained in S2 is conveyed layer by layer to the bottom of the meltblown equipment so that the meltblown PE ultrafine fibers obtained in S6 can evenly cover its surface and gaps, and the initial bonding is achieved by using hot melt adhesiveness. S8. The composite blank formed in S7 is sent into a hot pressing device and hot-pressed under set conditions to fully melt and bond the PE adhesive, and then cooled and shaped into waste fiberboard.

[0031] Each step will be explained in detail below.

[0032] Specifically, step S1 aims to achieve the pretreatment and fiberization of waste textile raw materials.

[0033] Removing non-fibrous impurities such as metal buttons, zippers, and plastic accessories manually or mechanically can effectively prevent these impurities from damaging equipment or affecting product quality in subsequent processes.

[0034] The opening process utilizes the mechanical action of the opening equipment to break down the tangled block structure of waste textiles into loose, separated individual fibers. This ensures the uniformity of the subsequent web formation and greatly increases the specific surface area of ​​the fibers, laying a physical foundation for their full bonding with PE adhesives.

[0035] The opening speed is controlled at 800-1000 r / min, the opening time is 3-5 min, and the fiber moisture content is 8-10% to ensure the opening effect, while avoiding excessive damage to fiber length due to over-opening, or improper moisture content affecting opening efficiency and subsequent hot pressing.

[0036] Furthermore, step S2 aims to construct a three-dimensional fiber network skeleton with preliminary strength.

[0037] Preferably, the fiber web is laid into a uniform fiber web using a web-forming equipment, which involves using a cross-laying machine or similar equipment to evenly distribute the fibers on a two-dimensional plane and control the areal density to be 300-350 g / m², in order to ensure the uniformity of the final board thickness and areal density.

[0038] Preferably, the needle-punched felt with a certain strength and thickness is formed by repeatedly puncturing the fiber web with the needle of the needle punching machine, causing some fibers to shift in the vertical direction and become entangled with each other, thereby reinforcing the two-dimensional fiber web into a needle-punched felt with a certain interlayer bonding force and thickness.

[0039] Preferably, the needle-punching density is controlled at 150-180 needles / cm² and the needle-punching depth is 7-9mm to ensure sufficient entanglement strength, while avoiding excessive damage to the needles or destruction of the felt structure, ultimately forming a preform with a thickness of about 4-6mm and an areal density of 320-360g / m², providing a stable skeleton for subsequent composites.

[0040] Furthermore, steps S3, S4, S5, and S6 together constitute the preparation process of meltblown PE microfiber adhesive.

[0041] Preferably, step S3, which involves feeding the PE material into a screw extruder and melting it at high temperature to transform it into a uniform melt, specifically involves heating the solid PE raw material in each temperature zone of the screw extruder and applying shearing action from the screw to transform it into a melt with good fluidity and uniform composition. This is the basis for subsequent fiberization.

[0042] Preferably, the step S4 of filtering the PE melt at the front end of the extruder to remove impurities and incompletely melted particles is a necessary measure to ensure the purity of the melt. Using a 200-mesh metal filter and filtering under a pressure of 2.5-3.0 MPa can effectively intercept impurities that may clog the spinneret orifice, thereby ensuring the continuous and stable subsequent spinneret process.

[0043] Preferably, in step S5, the clean melt is transported to the spinneret assembly, and the extruded melt is stretched by a high-frequency pulsed hot airflow. This induces the melt stream to undergo regular three-dimensional spiral folding in the air, forming an ultrafine fiber stream with a micro-spring structure. This is the core of the meltblown process. The PE melt stream extruded through the spinneret is instantly stretched and thinned by a high-speed hot airflow at 200-210℃ and a speed of 0.8-1.2m / s, thereby achieving fiber refinement.

[0044] Preferably, in step S6, the thinned PE melt stream is cooled and solidified to form meltblown PE microfibers for bonding. This is achieved by rapidly solidifying and shaping the molten stream using room temperature air or gradient cooling to form microfibers with a diameter of 1-5 μm and a tensile strength ≥3.0 cN / dtex. The large specific surface area and low hot melt activation temperature are prerequisites for its use as an excellent adhesive.

[0045] Furthermore, step S7 is a key step in achieving the composite of waste fiber skeleton and PE adhesive.

[0046] Preferably, the needle-punched felt is conveyed layer by layer to the bottom of the meltblown equipment, so that the PE microfibers formed during the meltblown process can be directly deposited on the surface and internal gaps of the moving needle-punched felt.

[0047] Preferably, the needle-punched felt conveying speed is controlled at 1m / min, the meltblown PE microfiber unit area spraying amount is 60-100g / m², and a layered meltblown method is adopted to ensure that the PE microfiber can fully penetrate into the needle-punched felt, realize full-area uniform bonding preforming from the surface layer to the core layer, and use the thermal fusion adhesiveness of PE fiber to form a preliminarily bonded composite preform.

[0048] Furthermore, step S8 is the final forming step, which aims to solidify the composite preform into a dense sheet through the action of heat and pressure.

[0049] Preferably, the process of feeding the composite preform into a hot pressing device and hot pressing it under set conditions refers to placing the preform in a flat plate hot press and setting the hot pressing temperature, pressure, and time according to the fiber type. During this process, the PE microfibers distributed among the fibers are heated and melted, flowing fully under pressure to wet the fiber surface and fuse together. After cooling and shaping, a strong fiber entanglement + thermal fusion bonding dual adhesive structure is formed. The cooling and shaping process uses waste fiberboard, which can be natural cooling or controlled cooling, ultimately resulting in a uniform board with a thickness controllable between 3-10 mm.

[0050] The organic combination of the above eight steps constitutes the complete preparation process described in this invention. This method innovatively applies meltblown nonwoven technology to the bonding stage of composite materials, achieving efficient, high-quality, and environmentally friendly recycling of waste fibers.

[0051] In addition, the inventors discovered in their research that although microscopic helical PE fibers can be prepared using pulsed airflow, there is a serious technical obstacle of structural collapse in the subsequent S8 flat plate hot pressing process. Due to the large spatial steric hindrance of the helical structure, under hot pressing pressure above 2.0 MPa, isolated helical fibers are very prone to buckling deformation and being crushed into a straight or film-like shape, resulting in the loss of the expected toughening effect in the finished product.

[0052] To overcome this obstacle, this invention employs a dual-nozzle phase interference settling technology. By controlling the pulse phase difference between the two nozzles to 180 degrees, a sequential phase interlocking is induced between the two spiral fiber flows during settling, constructing a self-supporting micro-grid structure. This interlocking structure effectively resists vertical compressive stress during hot pressing, anchoring the spiral shape within the gaps of the waste fibers, thus preserving a complete micro-spring energy-absorbing structure in the final high-density fiberboard.

[0053] The following examples will demonstrate in detail how to implement the above steps and obtain the desired results. Example 1

[0054] A method for preparing a thermoplastic composite material made from waste fibers includes the following steps: S1. 10 kg of waste textiles whose main components are cotton and polyester and which are confirmed by Fourier transform infrared spectroscopy (FTIR) to be basically free of inert fibers such as PP are put into a cotton opener. The opening speed is set to 900 r / min and the opening time is 4 min to obtain a uniformly dispersed single waste fiber with a moisture content of about 9%. S2. The opened fibers are fed to a cross-laying machine to form a uniform fiber web with a surface density of 330 g / m². The fiber web is then fed into a needle punching machine, with a needle punching density of 165 needles / cm² and a needle punching depth of 8 mm, to produce a needle-punched felt with a thickness of 5 mm and a surface density of 340 g / m². S3. Feed 5 kg of HDPE granules into the screw extruder of the meltblown production line. Set the extruder temperature for each zone as follows: Zone 1 150°C, Zone 2 170°C, Zone 3 185°C, Zone 4 195°C, and the die head 185°C. Set the screw speed to 120 r / min and the melting time to 18 min. S4. Filter the PE melt obtained in step S3 through a 200-mesh metal filter screen, and control the filtration pressure at 2.8 MPa. S5. The filtered clean melt is conveyed to the dual-nozzle phase interference meltblown assembly. The hot gas flow temperature is set to 205°C, the pulse gas flow generator is turned on, the gas flow pulse frequency is set to 120 Hz, and the pulse phase difference between the two nozzles is locked at 180 degrees. Under the periodic impact of the pulse gas flow, the PE melt stream extruded from the spinneret is stretched and induced to form microscopic helical fibers with a diameter of 3-5 μm and a helical coil radius of about 10 μm; S6. The micro-spiral fibers formed in S5 are cooled and solidified to form spiral meltblown PE ultrafine fibers with an elastic recovery rate of ≥95%. During the settling process, the macroscopic airflow turbulence is eliminated by utilizing the phase complementarity effect of the dual nozzles, ensuring uniform distribution on the needle-punched felt; S7. The needle-punched felt obtained in step S2 is conveyed to the bottom of the meltblown fiber receiving device at a speed of 1 m / min. The unit area spraying amount of meltblown PE microfiber is controlled to be 100 g / m². A layered spraying method of 60 g / m² for the bottom layer and 40 g / m² for the top layer is adopted to make it evenly cover the surface and gaps of the needle-punched felt and achieve preliminary bonding. S8. The composite blank is fed into a flatbed hot press, and the hot pressing temperature is set to 130°C, the hot pressing pressure to 2.3 MPa, and the hot pressing time to 3.5 min. Then it is naturally cooled to 25°C for shaping to obtain a waste fiberboard with a thickness of 4 mm. Example 2

[0055] This embodiment demonstrates an optimized solution for waste textiles with a high polypropylene fiber content, focusing on verifying the effectiveness of the innovative plasma treatment step.

[0056] The steps are basically the same as in Example 1, except that: Raw materials: It is specified that mixed waste textiles containing 30% PP nonwoven fabric should be used; A new plasma treatment step is added: After opening and before web formation, the opened, fluffy fibers are continuously passed through a plasma treatment device. The treatment power is set to 350 W, the treatment time to 75 s, and the treatment distance to 6 mm.

[0057] Effect verification: Water contact angle tests were performed on PP fiber samples before and after plasma treatment. The contact angle was 105° before treatment and decreased to 68° after treatment, proving that the fiber surface energy was significantly improved and the hydrophilicity was enhanced.

[0058] Comparative Example 1: This comparative example uses the traditional urea-formaldehyde resin process mentioned in the background art for comparison, in order to highlight the environmental protection and performance advantages of the present invention.

[0059] Steps S1-S2 are the same as in Example 1, and needle-punched felt is prepared; S3. Prepare a urea-formaldehyde resin adhesive with a solid content of 60%. S4. Spray the urea-formaldehyde resin adhesive evenly onto the surface of the needle-punched felt. The amount of spraying is equivalent to the amount of PE sprayed in Example 1. S5. The needle-punched felt coated with adhesive is cured at 100°C for 1 hour to obtain the board.

[0060] Comparative Example 2: The steps are exactly the same as in Example 2, except that the plasma treatment step in Example 2 is omitted, that is, the fiber containing 30% PP is directly opened and then used to form the web.

[0061] Comparative Example 3: The steps are basically the same as those in Comparative Example 2, the difference being: In step S3, HDPE particles are premixed with 5% by weight of maleic anhydride-grafted polyethylene compatibilizer and then fed into a screw extruder.

[0062] The performance of the plates prepared in the above embodiments and comparative examples was tested, and the test methods included: Tensile strength and flexural strength: in accordance with GB / T 17657-2013.

[0063] Internal bond strength: tested according to GB / T 17657-2013.

[0064] 24-hour water absorption thickness expansion rate: according to GB / T 17657-2013.

[0065] Formaldehyde emission: measured according to the 1m³ climate chamber method in GB 18580-2017.

[0066] Interfacial shear strength: Referring to ASTM D3164 standard, a self-made fixture was used to test the interfacial bonding force between PE and PP fibers.

[0067] Bending strength retention rate after damp heat aging: The sample was placed in a constant temperature and humidity chamber at 70°C and 85% relative humidity for 500 hours, and then its bending strength retention rate and cross-sectional morphology were tested again.

[0068] The results of the above tests are shown in Table 1: Table 1 Performance tests of Examples 1-2 and Comparative Examples 1-3

[0069] As shown in Table 1: A comparison between Example 1 and Comparative Example 1 reveals that the flexural strength of Example 1 is significantly higher than that of Comparative Example 1, and Example 1 exhibits no formaldehyde release, completely resolving the environmental issue of excessive formaldehyde release in traditional urea-formaldehyde resin solutions. The underlying mechanism lies in the fact that meltblown PE microfibers can form a three-dimensional network that uniformly penetrates and encapsulates the fibers. After hot pressing, this results in a homogeneous and dense structure of fiber entanglement and thermal fusion, with uniform stress distribution. In contrast, urea-formaldehyde resin is a liquid spray, which is prone to uneven distribution, leading to stress concentration points, and its curing process involves formaldehyde release and residue. Therefore, this invention not only boasts superior mechanical properties but also achieves green environmental protection from the source, meeting the dual high standards of modern industry for material performance and safety.

[0070] A comparison between Example 2 and Comparative Example 2 reveals that, with both raw materials containing polypropylene fibers, Example 2 exhibits an interfacial shear strength of 6.9 MPa, significantly higher than Comparative Example 2's 4.1 MPa, representing an improvement of approximately 68%. Furthermore, Example 2 demonstrates significantly superior flexural strength and strength retention after damp heat aging compared to Comparative Example 2. This data comparison clearly demonstrates the decisive role of plasma treatment in improving the performance of boards containing inert fibers. The underlying mechanism lies in the fact that the smooth surface and chemical inertness of PP fibers result in extremely poor compatibility with PE adhesives, which is the root cause of the weak performance of Comparative Example 2. In contrast, Example 2, through high-energy plasma particle bombardment, simultaneously achieves physical etching and chemical modification on the PP fiber surface. This dual effect significantly enhances the wetting and bonding of the PE melt to the fibers, resulting in a qualitative leap in both initial strength and long-term durability.

[0071] A comparison of Example 2 and Comparative Example 3 shows that, with the same PP fiber content, Example 2, which uses plasma treatment, exhibits significantly better interfacial shear strength and strength retention after damp heat aging than Comparative Example 3, which uses a compatibilizer. Since compatibilizers primarily rely on molecular chain segment diffusion and limited chemical bonding, their dispersion uniformity during melt blending and their effective concentration at the interface are difficult to guarantee. In contrast, plasma treatment directly and permanently modifies the fiber surface, resulting in a more direct and uniform modification effect. Furthermore, it creates a synergistic effect of physical anchoring and chemical bonding, thus forming a more stable and environmentally resistant interfacial layer.

[0072] In summary, this invention, through its core processes of fiber entanglement and thermal bonding, combined with an optimized plasma treatment scheme for inert fibers, successfully solves several technical challenges in the recycling of waste fibers. In particular, addressing the industry pain point of poor interfacial bonding in inert fibers such as PP, the plasma treatment scheme provided by this invention not only significantly outperforms untreated systems but also surpasses conventional compatibilizer methods, producing unexpected long-term durability advantages. This effect is not a simple technology transfer but rather based on a deep understanding of the fiber surface modification mechanism. The precise integration of plasma treatment with meltblown PE bonding technology generates a synergistic enhancement effect of physical anchoring, chemical bonding, and improved surface wettability, demonstrating outstanding creativity and significant technological advancement.

[0073] To ensure that this scheme can stably form a double-helix interlocking structure, the following comparative example was also designed: Comparative Example 4: The steps are basically the same as in Example 1, except that the method in Example 1 is dual nozzles + 120Hz pulse + 180-degree phase difference, while the method in this comparative example is single nozzle + 120Hz pulse.

[0074] Comparative Example 5: The steps are basically the same as in Example 1, except that the method used in this comparative example is dual nozzles + 120Hz pulse + 0 degree phase difference.

[0075] Performance tests were conducted on Example 1, Comparative Example 1, Comparative Example 4, and Comparative Example 5. The test methods included: Bending strength: Tested according to GB / T 17657-2013.

[0076] Notched impact strength of simply supported beams: tested according to GB / T 1043.1-2008 standard, used to characterize the energy absorption capacity of materials under high-speed impact.

[0077] Helical structure retention rate: The board was subjected to liquid nitrogen brittle fracture, and the cross-section was observed by scanning electron microscopy. Fifty PE fibers were randomly selected under 500x magnification, and the percentage of fibers that still maintained obvious curling characteristics was counted.

[0078] Bending strength retention rate after damp heat aging: The sample was placed in a constant temperature and humidity chamber at 70°C and 85% relative humidity for 500 hours, and then its bending strength retention rate and cross-sectional morphology were tested again.

[0079] The results of the above tests are shown in Table 2: Table 2. Helical structure test experiments of Example 1 and Comparative Examples 1, 4, and 5

[0080] As shown in Table 2: Comparing Example 1 with Comparative Example 1, it can be seen that after introducing the spiral structure, the impact strength increased dramatically from 6.5 to 18.4 kJ / m², proving the huge energy absorption effect of the micro-spring structure.

[0081] The key lies in the comparison between Example 1 and Comparative Example 4: Although Comparative Example 4 also used pulsed airflow, its spiral structure retention rate after hot pressing was only 25%, indicating that the isolated spiral did indeed undergo structural collapse during hot pressing, resulting in an impact strength far lower than that of Example 1. Example 1, however, achieved an 85% structural retention rate through dual-nozzle phase interference, proving that phase interlocking is a key technical means to overcome hot pressing collapse and achieve the invention's objective. This fully demonstrates that the present invention is not simply a superposition of pulse technology, but rather solves the contradiction between hot pressing and microstructure retention that existing technologies cannot address, possessing outstanding substantive characteristics.

[0082] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing a waste fiber thermoplastic composite material, characterized by, Includes the following steps: S1. Recycle waste textiles and sort and screen them to remove non-fibrous impurities. Then, open them to make them into uniformly dispersed single waste fibers. S2. The waste fibers obtained in S1 are laid into a uniform fiber web using a web-forming device, and then needle-punched and entangled to form a needle-punched felt with a certain strength and thickness. S3. The PE material is fed into the screw extruder and melted at high temperature to become a uniform melt; S4. The PE melt obtained in S3 is filtered at the front end of the extruder to remove impurities and incompletely melted particles; S5. The clean melt obtained in S4 is transported to the spinneret assembly. Under the stretching of a high-frequency pulsed hot airflow with a frequency of 50-200Hz, the extruded melt flow is made to generate microscopic spiral curling, and is extruded from the spinneret orifice and rapidly drawn thin. S6. The thin PE melt stream drawn in S5 is cooled and solidified to form meltblown PE microfibers for bonding. S7. The needle-punched felt obtained in S2 is conveyed layer by layer to the bottom of the meltblown equipment so that the meltblown PE ultrafine fibers obtained in S6 can evenly cover its surface and gaps, and the initial bonding is achieved by using hot melt adhesiveness. S8. The composite blank formed in S7 is sent into a hot pressing device and hot-pressed under set conditions to fully melt and bond the PE adhesive, and then cooled and shaped into waste fiberboard.

2. The method according to claim 1, wherein: In step S1, the opening equipment is a cotton opener with an opening speed of 800-1000 r / min, an opening time of 3-5 min, and a fiber moisture content controlled at 8-10%. The waste fibers obtained after opening are treated with air or oxygen plasma, with a treatment power of 300-400W, a treatment time of 60-90s, and a treatment distance of 5-8mm.

3. The method according to claim 1, wherein the waste fiber thermoplastic composite material is prepared by the steps of: In step S2, the web forming equipment uses a cross-laying machine to produce a fiber web with a surface density of 300-350 g / m². ​ The needle punching machine has a needle punching density of 150-180 needles / cm² and a needle punching depth of 7-9mm, ultimately forming the needle punched felt with a thickness of 4-6mm and a surface density of 320-360g / m².

4. The method according to claim 1, wherein the method is characterized by: In step S3, the PE material is selected from high-density polyethylene particles with a purity of ≥99% and free from mechanical impurities and color masterbatch residue.

5. The method according to claim 1, wherein the method is characterized by: In step S5, the spinneret assembly includes two nozzles arranged side by side. The airflow pulse frequencies of the two nozzles are the same but their phase differences are complementary, with a phase difference of 180 degrees. A turbulence rectifier is provided between the two nozzles to eliminate macroscopic airflow fluctuations, so that the settled meltblown PE microfibers exhibit a macroscopically uniform distribution and a microscopically spirally coiled structure.

6. The method according to claim 1, wherein the waste fiber thermoplastic composite material is prepared by the steps of: In step S5, the temperature of the hot airflow from the spinneret assembly is 200-210℃, and the airflow velocity is 0.8-1.2m / s. The airflow velocity is adjusted according to the melt flow index of the PE material to control the melt thinning effect. ​ 7. The method according to claim 1, wherein the waste fiber thermoplastic composite material is prepared by the steps of: In step S6, one or more functional additives may be selectively added to the PE melt stream before it is cooled and solidified. ​ The functional additives are selected from one or two of flame retardants and bamboo charcoal powder; The amount of flame retardant added is 1-3% of the PE melt mass fraction, and the flame retardant is decabromodiphenyl ethane; The amount of bamboo charcoal powder added is 2-5% of the PE melt mass fraction; The functional additives and PE melt streams are uniformly dispersed through an online static mixer or a dispersion module at the end of a screw extruder.

8. The method according to claim 1, wherein the method is characterized by: In step S6, the cooling method is direct cooling with ambient temperature air or gradient cooling. The diameter of the meltblown PE microfiber formed after cooling is 1-5μm, the breaking strength is ≥3.0cN / dtex, and the hot melt adhesive activation temperature can be as low as 120℃.

9. The method for preparing a waste fiber thermoplastic composite material according to claim 1, characterized in that: In step S7, the meltblown PE microfiber spraying rate per unit area is 60-100 g / m², and a layered meltblown method is adopted.

10. The method for preparing a waste fiber thermoplastic composite material according to claim 1, characterized in that: In step S8, the hot pressing equipment is a flat plate hot press, with a hot pressing temperature of 120-145℃, a hot pressing pressure of 2.0-2.5MPa, and a hot pressing time of 3-4min; the cooling method is natural cooling to 25℃ for shaping, and the final plate thickness can be controlled between 3-10mm.

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