A recycled polypropylene composite material and a method for preparing the same

Abstract

This invention discloses a recycled polypropylene composite material and its preparation method, relating to the field of composite materials, comprising the following steps: S1, providing waste rubber powder with a first predetermined mesh size and waste natural fibers with a second predetermined mesh size; S2, weighing raw materials according to mass parts, wherein the raw materials include 70-85 parts of recycled polypropylene, 5-25 parts of the waste rubber powder, and 5-20 parts of the waste natural fibers, and mixing them to obtain a premix; S3, performing melt blending and extrusion granulation on the premix to obtain composite material granules; S4, processing the composite material granules through injection molding or compression molding to obtain the recycled polypropylene composite material. This invention improves the impact toughness and rigidity of recycled polypropylene by adding waste rubber powder and waste natural fibers, realizing the utilization of solid waste resources.

Description

Technical Field

[0001] This invention relates to the field of composite materials, and more specifically to a recycled polypropylene composite material and its preparation method. Background Technology

[0002] As one of the most produced general-purpose plastics, the efficient recycling and high-value utilization of polypropylene waste is of great significance for resource recycling and environmental protection. After undergoing use, recycling, and reprocessing, recycled polypropylene often experiences problems such as a decrease in molecular weight and breakage of molecular chains, leading to a significant deterioration in its mechanical properties, especially impact toughness. This severely restricts its direct application in demanding structural components or engineering fields.

[0003] In existing technologies, methods to improve the performance of recycled polypropylene mainly rely on adding virgin resin, elastomer toughening agents, or rigid inorganic fillers. However, adding virgin resin weakens the environmental and economic value of recycling; adding elastomer toughening agents, while improving toughness, is costly and leads to a significant decrease in rigidity and strength; adding rigid inorganic fillers, while enhancing rigidity, easily induces brittle fracture. In recent years, some studies have attempted to use solid wastes such as waste tire rubber powder, waste natural fibers, or fly ash as additives to fill polypropylene. However, when waste tire rubber powder is added alone, the material's rigidity and strength decrease significantly; when waste natural fibers are added alone, the material lacks toughness and is prone to brittle fracture; simply mixing multiple solid wastes, due to the lack of synergistic design and interface control of their respective properties, makes it difficult to simultaneously improve the material's rigidity and toughness.

[0004] Therefore, how to utilize multi-source solid waste to synergistically enhance recycled polypropylene and simultaneously improve its rigidity and toughness is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention aims to address one of the technical problems in related technologies to a certain extent. To this end, this invention provides a recycled polypropylene composite material and its preparation method. By adding waste rubber powder and waste natural fibers, the impact toughness and rigidity of the recycled polypropylene are improved, thus realizing the utilization of solid waste resources.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a recycled polypropylene composite material includes the following steps: S1, providing waste rubber powder with a first set mesh size and waste natural fibers with a second set mesh size; S2, weigh the raw materials according to the mass fractions, the raw materials include 70-85 parts of recycled polypropylene, 5-25 parts of the waste rubber powder and 5-20 parts of waste natural fiber, and mix them to obtain a premix; S3, the premixed material is melt-blended and extruded and granulated to obtain composite material granules; S4, the recycled polypropylene composite material is obtained by injection molding or compression molding of the composite material granules.

[0007] This application addresses the inherent technical problems of insufficient impact toughness and difficulty in balancing rigidity and toughness in recycled polypropylene by synergistically utilizing two solid wastes, waste rubber powder and waste natural fibers, as reinforcing and toughening components. Compared to existing technologies that rely on expensive virgin resins or elastomer toughening agents, this application significantly reduces raw material costs and enhances the environmental value of the resulting composite material. Specifically, the introduction of waste tire rubber powder significantly improves the impact toughness of the composite material, contrasting with the increased brittleness resulting from the addition of rigid fillers in existing technologies. Simultaneously, the addition of waste natural fibers compensates for the potential decrease in rigidity caused by the addition of rubber powder, ensuring that the composite material maintains sufficient strength and rigidity while retaining toughness, overcoming the limitations of existing technologies that struggle to achieve a balance between rigidity and toughness through single solid waste modification. Furthermore, this application ensures uniform dispersion and synergistic effects of each component in the recycled polypropylene matrix through controlled component proportions and melt blending processes, resulting in excellent comprehensive mechanical properties. Compared to existing technologies that simply mix multiple solid wastes without synergistic design, this approach is superior.

[0008] Optionally, in step S2, the mass ratio of the waste rubber powder to the waste natural fiber is (1.5:1) to (2:1).

[0009] Optionally, in step S2, the raw material further includes 5-20 parts of modified fly ash, which is obtained by modifying raw fly ash. The raw fly ash is one or more of coal-fired power plant fly ash or waste incineration fly ash. The modification process includes: drying the raw fly ash at 100-110°C for 3-5 hours, then adding 1.0%-2.5% of an aluminate coupling agent by mass of the raw fly ash, and stirring at 800-1200 rpm at 80-100°C for 20-40 minutes.

[0010] Optionally, in step S2, the raw materials further include 0-5 parts of a compatibilizer, wherein the compatibilizer is maleic anhydride-grafted polypropylene; the mixing is carried out in a high-speed mixer for 10-20 minutes, at a speed of 500-800 rpm, and at room temperature.

[0011] Optionally, in step S1, providing waste natural fibers with a second set mesh size specifically includes: taking waste natural fiber sources, crushing them, rinsing the crushed material with hot water at 60~80℃ for 10~30 minutes, and then drying and sieving it to the second set mesh size.

[0012] Optionally, in step S2, before weighing the waste natural fibers, the waste natural fibers are subjected to alkali treatment and coupling modification treatment in sequence; the alkali treatment includes: using a sodium hydroxide solution with a concentration of 1-3 mol / L, a treatment temperature of 60-80℃, and a treatment time of 1-2 hours; the coupling modification treatment includes: using a titanate coupling agent or a silane coupling agent accounting for 1.0%-2.0% of the mass of the waste natural fibers.

[0013] Optionally, step S3 specifically includes: feeding the premixed material into a twin-screw extruder for melt blending and extrusion granulation; the twin-screw extruder has a screw length-to-diameter ratio of (40:1) to (48:1) and a screw speed of 200 to 350 rpm; the twin-screw extruder is divided into a first heating zone, a second heating zone, a third heating zone, a fourth heating zone, and a die head zone along the material conveying direction from the feed port to the die head, wherein the temperature of the first heating zone is 170 to 180°C, the temperature of the second heating zone is 180 to 190°C, the temperature of the third heating zone is 190 to 200°C, the temperature of the fourth heating zone is 195 to 205°C, and the temperature of the die head zone is 190 to 200°C.

[0014] Optionally, step S3 further includes drying the composite material granules at a temperature of 80-90°C for 2-4 hours; in step S4, the injection molding process conditions are: injection temperature of 180-200°C, injection pressure of 40-80 MPa, and mold temperature of 40-60°C; the compression molding process conditions are: compression molding temperature of 180-200°C, compression molding pressure of 10-20 MPa, and holding time of 5-10 minutes.

[0015] Optionally, in step S1, the first set mesh size is 80~120 mesh, and the second set mesh size is 80 mesh; the waste rubber powder is one or more of waste tire rubber powder, waste household rubber powder, and industrial waste rubber powder; the waste natural fiber is one or more of sugarcane bagasse, straw fiber, hemp fiber, bamboo fiber, or coconut shell fiber.

[0016] Furthermore, the present invention also provides a recycled polypropylene composite material, obtained by the aforementioned method for preparing recycled polypropylene composite materials. The reasoning process for the beneficial effects of the recycled polypropylene composite material provided by the present invention and the aforementioned method for preparing recycled polypropylene composite materials is similar, and will not be repeated here.

[0017] These features and advantages of the present invention will be disclosed in detail in the following specific embodiments and accompanying drawings. The preferred embodiments or means of the present invention will be shown in detail in conjunction with the accompanying drawings, but are not intended to limit the technical solutions of the present invention. In addition, each of these features, elements and components appearing in the following text and drawings is a plurality of, and different symbols or numbers are used for convenience of representation, but all represent parts with the same or similar construction or function. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings: Figure 1 This is a schematic flowchart of the preparation method of the recycled polypropylene composite material in this invention. Detailed Implementation

[0019] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described are intended to explain the present invention and should not be construed as limiting the invention.

[0020] The terms "an embodiment," "example," or "trademark" used in this specification refer to a particular feature, structure, or characteristic described in connection with the embodiment itself that may be included in at least one embodiment disclosed in this patent. The phrase "in an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0021] The applicant's research revealed that molecular chain breakage during the recycling and reprocessing of recycled polypropylene leads to a deterioration in the material's toughness. Specifically, when modifying recycled polypropylene using a single solid waste, the introduction of waste rubber powder, while improving toughness, reduces the rigidity and strength of the recycled polypropylene. Conversely, the addition of waste natural fibers, while enhancing rigidity, easily triggers brittle fracture. Furthermore, simple mixing of multiple solid wastes, due to insufficient interfacial compatibility, fails to simultaneously optimize the rigidity and toughness of the resulting composite material, resulting in overall mechanical properties that do not meet engineering application requirements. For example, in the production of automotive interior structural components, recycled polypropylene composites are used to manufacture dashboard brackets. When only waste tire rubber powder is added, the material's impact strength improves but its flexural modulus decreases, making the bracket prone to deformation during assembly. When only bagasse fiber is added, flexural strength increases but impact strength is insufficient, causing microcracks to form in the bracket under vehicle vibration loads. Thus, this imbalance between rigidity and toughness directly affects the structural integrity and service reliability of the components, leading to a lower product qualification rate.

[0022] If the above problems are not addressed, recycled polypropylene composites will be unsuitable for structural components requiring high mechanical properties. Poor interfacial bonding will accelerate performance degradation during long-term service, increasing the risk of premature product failure. Furthermore, the low efficiency of solid waste resource utilization fails to effectively leverage the synergistic value of solid waste from various sources.

[0023] Example: This example provides a method for preparing recycled polypropylene composite material, such as... Figure 1 As shown, it includes the following steps: S1, providing waste rubber powder with a first set mesh size and waste natural fibers with a second set mesh size; S2, weigh the raw materials according to the mass parts, the raw materials include 70-85 parts of recycled polypropylene, 5-25 parts of waste rubber powder and 5-20 parts of waste natural fiber, and mix them to obtain a premix; S3, the premix is ​​melt-blended and extruded to obtain composite material granules; S4, Recycled polypropylene composite material is obtained by injection molding or compression molding of composite material granules.

[0024] In this embodiment, the recycled polypropylene composite material refers to a composite material with improved performance obtained by modifying recycled polypropylene as the matrix by adding other components. This type of composite material can realize the high-value utilization of waste plastics and also meet the performance requirements of specific application scenarios.

[0025] In step S1, waste rubber powder with a first predetermined mesh size and waste natural fibers with a second predetermined mesh size are provided. Waste rubber powder refers to granular or powdered rubber material obtained by crushing waste rubber products (such as waste tires, waste rubber hoses, etc.). Waste rubber powder can be used as a toughening component in composite materials. Waste rubber powder can be obtained in various ways; for example, recycled waste tires can be mechanically crushed and then sieved using screening equipment to obtain waste rubber powder that meets the first predetermined mesh size requirement. Waste natural fibers refer to fibrous materials obtained by processing agricultural waste (such as bagasse, straw, hemp, etc.) or plant waste. This material has high rigidity and strength and can be used as a reinforcing component in composite materials. Waste natural fibers can be obtained from agricultural waste; for example, bagasse, straw, etc., can be collected, then preliminarily washed, dried, and mechanically crushed, and then sieved using screening equipment to obtain waste natural fibers that meet the second predetermined mesh size requirement. By pretreating these two types of solid waste and controlling their particle size or fiber length, their dispersibility and reinforcing and toughening effects in composite materials can be affected.

[0026] In step S2, raw materials are weighed according to their mass proportions. These raw materials include 70-85 parts recycled polypropylene, 5-25 parts waste rubber powder, and 5-20 parts waste natural fibers. After mixing, a premix is ​​obtained. The premix refers to the mixture obtained by initially mixing various raw materials according to a preset ratio during the composite material preparation process. The premix ensures that the components are uniformly dispersed during the subsequent melt blending process. It is understood that the weighing of raw materials can be done using a high-precision electronic scale to ensure that each component is proportioned according to the preset mass proportions. The mixing of raw materials can be carried out using various mixing equipment, such as a drum mixer, a V-type mixer, or a simple mechanical stirring device, to initially mix the weighed recycled polypropylene, waste rubber powder, and waste natural fibers to ensure that the components are initially uniformly dispersed and to avoid localized agglomeration.

[0027] In step S3, the premixed material is melt-blended and extruded granulated to obtain composite material granules. Composite material granules refer to granular composite materials obtained after melt blending and extrusion granulation, and these granules are intermediate products before the final product molding. Melt blending refers to the process of mixing multiple polymers or polymers and fillers in a molten state under heating. This process uses mechanical shearing to fully disperse and mix the components, forming a homogeneous melt. Extrusion granulation refers to the process of extruding the melt-blended material into strips through an extruder die, followed by cooling and pelletizing to prepare granular composite materials. Granulation facilitates subsequent molding and processing. The granulation process can be carried out using a single-screw extruder or a twin-screw extruder.

[0028] Specifically, after the premix is ​​fed into the extruder, the polypropylene matrix melts under heating and the shearing action of the screw. Waste rubber powder and waste natural fibers are fully dispersed and mixed in the molten polypropylene, forming a homogeneous melt. Subsequently, the melt is extruded into strips through the extruder die, cooled and solidified in a cooling water tank, and then cut into uniform composite material pellets by a pelletizer. This step ensures uniform dispersion of each component at the microscopic level, thereby further optimizing the material's properties.

[0029] In step S4, the composite material granules are processed into recycled polypropylene composite materials using injection molding or compression molding. The composite material granules can be appropriately dried before molding to remove any moisture. Specifically, for further processing and molding, the granules can be fed into an injection molding machine, where they are heated and melted to form a melt. This melt is then injected under high pressure into a pre-designed mold cavity, cooled, solidified, and demolded to obtain a recycled polypropylene composite material product with a predetermined shape and size. Alternatively, the granules can be placed in a preheated mold, where pressure and heat are applied to melt and fill the mold cavity. After cooling and solidification, the product is demolded to obtain the desired recycled polypropylene composite material product. Both molding processes can process the composite material granules into final products to meet different application requirements. It should be noted that injection molding is a molding method in which molten plastic material is injected under pressure into a mold cavity, and after cooling and solidification, a product of the desired shape is obtained. This process is suitable for producing plastic products with complex shapes and precise dimensions. Compression molding is a molding process in which plastic material is placed in a preheated mold, and pressure and heat are applied to melt it and fill the mold cavity. After cooling and solidification, the desired shape of the product is obtained. This process is suitable for producing large-sized, thick-walled or flat plastic products.

[0030] In step S2, the mass ratio of waste rubber powder to waste natural fiber is 1.5:1 to 2:1.

[0031] In this embodiment, waste rubber powder, as a recyclable filler, is added to recycled polypropylene material to improve the toughness and impact resistance of the composite material, and also helps reduce production costs. Waste rubber powder has a wide range of sources, including waste tire rubber powder, waste household rubber powder, and industrial waste rubber powder. In the composite material system, the particle size distribution and surface characteristics of the waste rubber powder have a significant impact on its compatibility with the polymer matrix. Waste natural fibers are environmentally friendly biomass reinforcing materials that can improve the stiffness, tensile strength, and other mechanical properties of the composite material, and have certain biodegradability potential. Waste natural fibers come from various sources, such as bagasse, straw fiber, hemp fiber, bamboo fiber, or coconut shell fiber. Furthermore, the length, diameter, and whether or not the waste natural fibers have undergone pretreatment all affect their reinforcing effect and dispersion state in the composite material. The mass ratio of waste rubber powder to waste natural fibers limits the relative content of these two fillers in the premix. By controlling this ratio, the synergistic effect of these two fillers with significantly different properties in the recycled polypropylene matrix can be optimized. This improves the stiffness of the composite material without sacrificing its toughness and enhances its overall processing performance. Specifically, in this embodiment, the mass ratio of waste rubber powder to waste natural fibers is controlled within the range of 1.5:1 to 2:1. Within this range, the elastomeric properties of waste rubber powder help alleviate the brittleness that natural fibers may cause in the polypropylene matrix. At the same time, the reinforcing effect of natural fibers can compensate for the insufficient strength of pure rubber powder filler. In other words, by optimizing the relative content of the two fillers, not only can the mechanical properties of the composite material be balanced, but the rheological behavior of the melt can also be improved, processing difficulty reduced, and the quality of the composite granules and the performance stability of the final product ensured.

[0032] In step S2, the raw materials also include 5 to 20 parts of modified fly ash, which is obtained by modifying the raw fly ash. The raw fly ash is one or more of coal-fired power plant fly ash or waste incineration fly ash. The modification process includes: drying the raw fly ash at 100 to 110°C for 3 to 5 hours, then adding 1.0% to 2.5% of an aluminate coupling agent by mass of the raw fly ash, and stirring at 800 to 1200 rpm at 80 to 100°C for 20 to 40 minutes.

[0033] In this embodiment, modified fly ash refers to surface-treated fly ash. Fly ash is an industrial waste, mainly composed of oxides such as silicon, aluminum, and calcium, and possesses a certain degree of activity. However, untreated fly ash surfaces are typically hydrophilic, resulting in poor compatibility with hydrophobic polymer matrices such as polypropylene. Direct addition may lead to a decrease in the mechanical properties of the composite material. By modifying the fly ash, its surface properties can be improved, allowing it to better bond with the polymer matrix, thus becoming a low-cost reinforcing filler, enhancing the rigidity, hardness, and other mechanical properties of the composite material, and reducing material costs. The raw fly ash can originate from coal-fired power plant fly ash or waste incineration fly ash. These two types of fly ash are common industrial wastes, with slight differences in composition and properties, but both contain usable mineral components. The specific choice can be adjusted according to the actual waste source and material performance requirements. The drying step in the modification process removes moisture from the fly ash surface. Fly ash typically contains a certain amount of adsorbed water or crystal water, which can affect the reaction effect of subsequent coupling agents and may generate bubbles during composite material processing, affecting material performance. Moisture can be removed by drying at 100-110℃ for 3-5 hours. Aluminate coupling agent is a surface modifier that reacts with hydroxyl groups or oxides on the fly ash surface to form an organic film, thereby improving the interfacial compatibility between fly ash and the polypropylene matrix. Specifically, the mechanism of action of the aluminate coupling agent is that one end of the coupling agent molecule reacts with the surface of the inorganic filler, while the other end entangles or reacts with the organic polymer matrix, acting as a "bridge" to enhance interfacial bonding. The amount of aluminate coupling agent added should be controlled within the range of 1.0%-2.5% of the raw fly ash mass to ensure that the coupling agent can fully cover the fly ash surface while avoiding the negative effects that may result from excessive addition. The stirring process should be carried out at 80-100℃ and a speed of 800-1200 rpm for 20-40 minutes to ensure that the aluminate coupling agent is uniformly dispersed and fully reacts with the fly ash surface. Stirring at a certain temperature can accelerate the reaction rate between the coupling agent and the fly ash surface and improve the modification efficiency. In addition, high-speed stirring helps to break up the agglomeration of coupling agent, allowing it to cover the surface of fly ash particles more evenly and ensuring the modification effect, while sufficient stirring time ensures that the reaction proceeds fully.

[0034] In summary, by introducing modified fly ash into the raw materials in step S2 and subjecting the fly ash to specific modification treatment, the performance of the composite material can be significantly improved. First, the raw fly ash is dried at 100-110°C for 3-5 hours to thoroughly remove the moisture adsorbed on the surface of the fly ash particles, preventing the generation of steam during subsequent melt blending, which could lead to porosity or poor interfacial bonding in the composite material. Then, an appropriate amount (1.0%-2.5% of the raw fly ash mass) of aluminate coupling agent is added, and the mixture is stirred at 800-1200 rpm for 20-40 minutes at 80-100°C. This modification treatment allows the aluminate coupling agent to fully and uniformly react chemically or physically with the surface of the fly ash particles, forming an organic coating layer on the fly ash surface. This coating layer reduces the surface energy of the fly ash, improves its oleophilicity, and thus enhances the interfacial compatibility and bonding strength between the fly ash and the recycled polypropylene matrix, as well as waste rubber powder and waste natural fibers. When modified fly ash is mixed with recycled polypropylene, waste rubber powder and waste natural fibers, it can be more evenly dispersed in the polymer matrix during subsequent melt blending and extrusion granulation processes, avoiding the problem of easy agglomeration of unmodified fly ash. At the same time, the use of industrial waste also reduces the production cost of composite materials.

[0035] In step S2, the raw materials also include 0-5 parts of a compatibilizer, which is maleic anhydride-grafted polypropylene; the mixing process is carried out in a high-speed mixer, the mixing time is 10-20 min, the mixing speed is 500-800 rpm, and the mixing temperature is room temperature.

[0036] In this embodiment, a compatibilizer is introduced into the raw materials. The compatibilizer can improve the interfacial compatibility between two or more incompatible polymers or components, reducing interfacial tension and enhancing interfacial adhesion, thereby promoting uniform dispersion of each component during the mixing process. Maleic anhydride-grafted polypropylene (PP-g-MAH) serves as a compatibilizer. Its polypropylene backbone has good compatibility with the recycled polypropylene matrix. The grafted maleic anhydride groups can chemically react with the hydroxyl groups on the surface of waste natural fibers to form chemical bonds, thus establishing a "molecular bridge" between the recycled polypropylene and the waste natural fibers. This improves the interfacial bonding strength, increases stress transfer efficiency, and ultimately enhances the comprehensive mechanical properties of the composite material. Furthermore, this application discloses that the mixing process is carried out in a high-speed mixer. The high-speed mixer uses a high-speed rotating impeller to subject the materials to intense shearing, friction, and impact, thereby achieving rapid and uniform mixing. For example, the high-speed mixer can be a vertical or horizontal high-speed mixer, with a mixing time of 10–20 minutes to ensure thorough mixing of the materials. Appropriate mixing time ensures the compatibilizer functions effectively, allowing waste rubber powder and waste natural fibers to achieve good dispersion in the recycled polypropylene matrix, avoiding localized unevenness caused by insufficient mixing. Simultaneously, excessive mixing time can lead to material degradation or overheating. Furthermore, the mixing speed of the high-speed mixer is 500-800 rpm. This speed range provides sufficient shear force to promote depolymerization and dispersion of the components, especially for fibrous or granular waste natural fibers and waste rubber powder, where high shear force facilitates their uniform distribution in the polymer matrix. However, the speed must also be controlled within a reasonable range to prevent excessive shear degradation or equipment wear due to excessive speed. The mixing temperature is room temperature, i.e., the mixing process is carried out at ambient temperature to avoid premature softening, melting, or thermal degradation of the recycled polypropylene, waste rubber powder, or waste natural fibers due to high temperatures, thus preserving the original properties of the raw materials.

[0037] The steps for providing waste natural fibers with a second set mesh number include: taking waste natural fiber sources and crushing them, rinsing the crushed material with hot water at 60~80℃ for 10~30 minutes, and then drying and sieving it to the second set mesh number.

[0038] In this embodiment, the pulverization process reduces the particle size of waste natural fibers and increases their specific surface area, thereby facilitating subsequent cleaning and modification processes and promoting their uniform dispersion in the polymer matrix. Specifically, the pulverization method can employ mechanical shearing, grinding, or impact, such as using a hammer mill, disc mill, or air jet mill. Alternatively, the fibers can be pre-treated by cutting or tearing to achieve the desired particle size range. Hot water rinsing aims to remove dust, silt, water-soluble impurities, some organic matter, and microorganisms that may be present in the waste natural fibers, improving fiber cleanliness. Hot water treatment helps dissolve and remove impurities adhering to the fiber surface, while appropriate temperature and time ranges ensure cleaning effectiveness without causing excessive damage to the fiber structure. In addition to hot water rinsing, cold water rinsing combined with mechanical stirring, or pretreatment with a weak acid / weak alkali solution can be used to remove specific types of impurities. The drying process removes residual moisture from the fibers, preventing moisture from generating bubbles or undergoing hydrolysis reactions during subsequent melt blending, which could affect the performance of the composite material. The sieving process can control the particle size distribution of waste natural fibers, ensuring that they reach the preset second mesh size, thereby guaranteeing the uniform dispersion of fibers in the polypropylene matrix and improving the mechanical properties of the composite material.

[0039] Specifically, drying can be carried out using methods such as hot air circulating oven, vacuum drying oven, or microwave drying. Sieving can be achieved using equipment such as vibrating screen, drum screen, or air classifier to obtain fibers with uniform particle size.

[0040] In summary, pretreatment of waste natural fibers improved their cleanliness, surface activity, and particle size uniformity. When these optimized waste natural fibers are melt-blended with recycled polypropylene and other raw materials, they achieve more uniform dispersion, reduce agglomeration, and promote better interfacial bonding between the fibers and the polymer matrix. Therefore, the mechanical properties, processing performance, and stability of the final recycled polypropylene composite material are improved, overcoming the performance degradation or instability issues that may result from untreated natural fibers.

[0041] Before weighing the waste natural fibers, the waste natural fibers are subjected to alkali treatment and coupling modification treatment in sequence.

[0042] In this embodiment, alkaline treatment is a method of chemically modifying natural fibers using an alkaline solution. Its function is to remove non-cellulose components such as hemicellulose, lignin, and pectin from the surface of the natural fibers, thereby exposing more cellulose microfibers and increasing the specific surface area and surface roughness of the fibers. Therefore, alkaline treatment not only improves the mechanical interlocking between the fibers and the polymer matrix but also exposes more hydroxyl groups, providing more reaction sites for subsequent coupling modification. For example, sodium hydroxide solution or other alkaline substances such as potassium hydroxide solution can be used for treatment. Coupling modification refers to surface modification of natural fibers using coupling agents. Coupling agents are molecules with two or more different reactive functional groups. One end can react with active groups such as hydroxyl groups on the surface of the natural fibers, while the other end can chemically bond or physically entangle with the polymer matrix (such as recycled polypropylene), thereby establishing a "molecular bridge" between the fibers and the matrix, thus improving the interfacial compatibility and bonding strength between the hydrophilic natural fibers and the hydrophobic polymer matrix. Commonly used coupling agents, besides titanate or silane coupling agents, can also be aluminate coupling agents, etc. Waste natural fibers undergo alkali treatment and coupling modification treatment before weighing. First, alkali treatment pre-activates and purifies the fiber surface, removing impurities and increasing active sites, creating conditions for the coupling agent to function. The subsequent coupling modification treatment constructs an interfacial layer on the pre-treated fiber surface, ensuring the coupling agent can fully exert its bridging effect, thereby achieving a stronger interfacial bond between the fiber and the matrix.

[0043] In summary, natural fibers are typically hydrophilic, while recycled polypropylene is hydrophobic, resulting in poor interfacial compatibility between the two. This leads to the formation of weak interfaces in composite materials, affecting their mechanical properties and overall stability. The solution presented in this application addresses this problem by subjecting the waste natural fibers to alkali treatment and coupling modification treatments sequentially before mixing them with recycled polypropylene. Specifically, the alkali treatment first cleans and activates the surface of the waste natural fibers, increasing surface roughness and the number of active hydroxyl groups by removing non-cellulose components, making the fiber surface more readily bonded to the coupling agent. Subsequently, the coupling modification treatment uses a coupling agent (such as a titanate coupling agent or a silane coupling agent) to form a molecular bridge on the fiber surface. One end of the coupling agent reacts with the cellulose hydroxyl groups exposed after alkali treatment, while the other end exhibits good compatibility or reactivity with the recycled polypropylene matrix. During subsequent melt blending, the coupling agent establishes a stable bond between the fibers and polypropylene, thereby enhancing the interfacial adhesion between them.

[0044] The premixed material is fed into a twin-screw extruder for melt blending and extrusion granulation. The twin-screw extruder has a specific screw length-to-diameter ratio and screw speed, and is divided into multiple precisely temperature-controlled heating zones along the material conveying direction.

[0045] In this embodiment, the screw length-to-diameter ratio is 40:1 to 48:1, ensuring sufficient residence time for the material within the extruder for thorough melting, plasticizing, and mixing. The screw speed is 200 to 350 rpm, providing suitable shear strength, which facilitates the uniform dispersion of waste rubber powder and waste natural fibers in the recycled polypropylene melt and promotes interfacial bonding between components, while preventing material degradation due to excessive shear. Furthermore, the twin-screw extruder is sequentially divided into a first heating zone, a second heating zone, a third heating zone, a fourth heating zone, and a die head zone along the material conveying direction from the feed port to the die head, with each heating zone having a specific temperature range. The first heating zone has a temperature of 170~180℃, used for initial preheating and softening of the premix; the second heating zone has a temperature of 180~190℃, allowing the recycled polypropylene to begin melting; the third heating zone has a temperature of 190~200℃, ensuring the recycled polypropylene is fully melted, and achieving deep dispersion and mixing of waste rubber powder and waste natural fibers in this area; the fourth heating zone has a temperature of 195~205℃, used for further homogenizing the melt and providing stable melt pressure for extrusion; the die head zone has a temperature of 190~200℃, maintaining a stable melt temperature and ensuring smooth extrusion molding. By segmented temperature control and optimized extrusion parameters, the problem of melt blending of multi-component materials in premixes can be solved, ensuring uniform dispersion of each component, avoiding material degradation, and thus obtaining high-performance composite material granules.

[0046] Specifically, a twin-screw extruder is an extrusion device with two parallel or conical screws. Through the rotation and meshing of the screws, it conveys, shears, mixes, plasticizes, and extrudes materials, providing highly efficient mixing and plasticizing capabilities. It is suitable for the preparation of multi-component composite materials. Examples include co-rotating twin-screw extruders and counter-rotating twin-screw extruders. The screw length-to-diameter ratio (L / D ratio) is a key parameter affecting the material's residence time, shear history, and mixing effect within the extruder. A longer L / D ratio provides a longer melt-blending zone, which is beneficial for thorough plasticization and uniform mixing; a shorter L / D ratio may be suitable for materials that are shear-sensitive or do not require long residence times. The screw speed directly affects the material's shear rate, mixing intensity, and yield. Higher speeds enhance shearing action, improving mixing and production efficiency, but may increase the risk of material degradation; lower speeds provide relatively mild shearing action and are suitable for heat-sensitive or shear-sensitive materials. Furthermore, the twin-screw extruder is divided into multiple heating zones along the material conveying direction from the feed port to the die head. This segmented heating allows for temperature control of the material at different stages, ensuring gradual melting and thorough mixing without degradation. Understandably, the temperature of each heating zone can be regulated using electric heaters and cooling systems, and can also be monitored in real time by temperature sensors. The temperature settings of each heating zone optimize the melting of polypropylene and the dispersion of waste rubber powder and waste natural fibers, while preventing overheating and degradation, thus ensuring the performance of the composite material.

[0047] In summary, by employing a twin-screw extruder and controlling its screw length-to-diameter ratio, screw speed, and the temperature of each heating zone, it is possible to achieve efficient and uniform melt blending and extrusion granulation of premixes containing multiple components such as recycled polypropylene, waste rubber powder, and waste natural fibers. This solves the problems of uneven mixing and material degradation that may occur during the extrusion process in existing technologies, improves the dispersion uniformity of waste rubber powder and waste natural fibers in the recycled polypropylene matrix, and enhances the interfacial bonding force between the components.

[0048] In step S3 above, the composite material granules are dried at a temperature of 80-90°C for 2-4 hours. In step S4, the injection molding process conditions are: injection temperature of 180-200°C, injection pressure of 40-80 MPa, and mold temperature of 40-60°C. The compression molding process conditions are: compression molding temperature of 180-200°C, compression molding pressure of 10-20 MPa, and holding time of 5-10 minutes.

[0049] In this embodiment, drying the composite material granules removes any residual moisture or other volatile substances that may be present. The presence of moisture can affect subsequent molding processes; for example, it can vaporize at high temperatures, generating bubbles, leading to surface defects or internal pores in the product, and even causing hydrolytic degradation of the polymer, thereby reducing the material's mechanical properties. Drying ensures granule quality and product performance. Drying methods can include, but are not limited to, hot air circulation drying, vacuum drying, or dehumidification drying. The drying temperature is 80-90°C, and the drying time is 2-4 hours. It is understood that the drying temperature needs to be high enough to promote moisture evaporation, but not too high to avoid material degradation or adhesion. The drying time needs to ensure sufficient moisture escape while avoiding over-drying that would waste energy or alter material properties. Injection molding process conditions directly affect the surface quality, mechanical properties, and production efficiency of the product. The injection molding temperature is 180-200°C, which refers to the temperature of the molten material before injection. A suitable injection temperature ensures good material flowability, allowing it to fully fill the mold cavity while preventing material degradation due to excessive heat or insufficient flowability, resulting in defects such as underfilling or weld lines due to insufficient temperature. Injection pressure is 40-80 MPa, which propels the molten material into the mold cavity and maintains pressure. Appropriate injection pressure ensures full filling of the mold cavity, overcomes melt flow resistance, effectively compacts the product, and reduces shrinkage. Mold temperature is 40-60℃, referring to the temperature of the mold cavity. Mold temperature significantly affects the cooling rate and internal stress distribution of the product. Compression molding is suitable for products with large dimensions, complex shapes, or high mechanical property requirements. Compression molding temperature is 180-200℃, the temperature at which the mold is heated. A suitable compression molding temperature softens the material sufficiently and ensures good flowability, allowing it to fill the mold cavity under pressure while preventing overheating and degradation. Compression molding pressure is 10-20 MPa, the pressure applied to the mold. Appropriate molding pressure ensures sufficient material compaction, eliminates internal voids, and results in a product with uniform density and good mechanical properties. The holding time is 5–10 minutes; this refers to the duration of pressure maintenance during the molding process. During the holding phase, the material continues to solidify and cool under pressure to ensure dimensional stability, uniform density, and minimize shrinkage and warping.

[0050] In summary, by adding a drying step after extrusion granulation of the composite material granules and controlling the subsequent injection molding or compression molding process conditions, the preparation process of recycled polypropylene composite materials was optimized. Drying at a temperature of 80-90℃ for 2-4 hours ensures that the granules reach the ideal moisture content before entering the molding equipment, laying the foundation for producing high-quality products. For injection molding, controlling the injection temperature at 180-200℃ ensures that the recycled polypropylene composite material has suitable melt flowability during injection, fully filling the mold cavity while avoiding material degradation due to excessive temperature. Setting the injection pressure at 40-80 MPa ensures that the melt can overcome flow resistance, achieving complete mold filling and effective compaction of the product. Controlling the mold temperature at 40-60℃ helps regulate the cooling rate of the product and optimize the crystal structure. For compression molding, setting the molding temperature at 180-200℃ allows the material to soften sufficiently before being compressed, facilitating flow and molding. A molding pressure of 10-20 MPa ensures sufficient material compaction. Holding time of 5 to 10 minutes maintains pressure during the material's cooling and solidification process, inhibiting shrinkage and warping of the product and ensuring dimensional stability.

[0051] In one specific implementation, after the premixed material is fed into a twin-screw extruder for melt blending and extrusion granulation, the resulting composite material granules can be transferred to a hot air circulating drying oven for drying. The temperature of the drying oven can be set to 85°C, and the granules are kept there for 3 hours to fully remove internal moisture. After drying, the granules are removed and immediately sent to molding equipment. If injection molding is used, the dried composite material granules can be added to a hydraulic injection molding machine. The barrel temperature of the injection molding machine can be set in stages; for example, the temperature gradient from the feed inlet to the nozzle can be set to 180°C, 185°C, and 190°C, and the nozzle temperature can be set to 195°C. The injection pressure can be set to 65 MPa to ensure that the melt can quickly and uniformly fill the mold cavity. The mold cooling water circulation system can adjust the mold temperature to 50°C to control the cooling rate of the product. If compression molding is used, the dried composite material granules can be pre-weighed and placed in a flat vulcanizing machine or hydraulic press. The heating plate temperature of the mold can be set to 190°C. After the mold is closed, apply a molding pressure of 15 MPa and maintain the pressure for 7 minutes to ensure that the material is fully cured and obtains stable dimensions.

[0052] In step S1, the first set mesh size is 80~120 mesh, and the second set mesh size is 80 mesh; the waste rubber powder is one or more of waste tire rubber powder, waste household rubber powder, and industrial waste rubber powder; the waste natural fiber is one or more of sugarcane bagasse, straw fiber, hemp fiber, bamboo fiber, or coconut shell fiber.

[0053] In this embodiment, the first set mesh size refers to the particle size range of the waste rubber powder. Limiting the first set mesh size to 80-120 mesh ensures that the waste rubber powder has suitable fineness and uniformity. If the particle size is too large, it may lead to uneven dispersion in the polymer matrix, forming stress concentration points; if the particle size is too small, it may increase the tendency to agglomerate, similarly affecting the dispersion effect. A particle size range of 80-120 mesh is beneficial for achieving good dispersion of waste rubber powder in recycled polypropylene, thereby enabling it to exert its toughening or filling effect. Specifically, the mesh size can be achieved through fine grinding combined with sieving technology, or obtained through air classification, etc.

[0054] The second set mesh size refers to the particle size of the waste natural fibers. Setting the second set mesh size to 80 mesh ensures the uniformity of the waste natural fibers' dimensions. Uniform fiber distribution can transfer stress and improve the mechanical properties of the composite material. Uneven fiber size may lead to areas with excessively high or low fiber content, creating weak points. An 80-mesh particle size is beneficial for maintaining a certain aspect ratio during melt blending, while avoiding excessive entanglement or agglomeration. This mesh size can be obtained through precise sieving after mechanical crushing, or by controlling the grinding parameters.

[0055] Furthermore, the source of waste rubber powder is limited to one or more of waste tire rubber powder, waste household rubber powder, and industrial waste rubber powder. Waste rubber powder from different sources may vary in crosslinking density, component purity, and surface activity. Clearly defining the source is beneficial for selecting rubber powder with stable properties and good processing adaptability, ensuring the predictability of composite material performance. For example, waste tire rubber powder typically has a high degree of crosslinking, providing good elasticity to the composite material; while waste household or industrial waste rubber powder may have different chemical compositions, allowing for selection based on specific needs. The source of waste natural fibers is limited to one or more of bagasse, straw fiber, hemp fiber, bamboo fiber, or coconut shell fiber. These natural fibers have different chemical compositions, crystallinity, aspect ratio, and surface properties. Clearly defining their source is beneficial for selecting the appropriate fiber type based on the performance requirements of the target composite material. For example, bagasse and straw fibers are widely available and inexpensive; hemp and bamboo fibers typically have high strength and stiffness; and coconut shell fibers have good toughness and abrasion resistance. By selecting specific types of natural fibers, the mechanical properties, density, and cost of the composite material can be controlled.

[0056] This embodiment also provides a recycled polypropylene composite material, which is prepared by the above-described method for preparing recycled polypropylene composite material.

[0057] In this embodiment, the recycled polypropylene composite material uses recycled polypropylene as the matrix and is formed by introducing components such as waste rubber powder and waste natural fibers, and through a specific processing technology. It is essentially a multiphase composite system. Through the synergistic effect between the components, it can achieve performance optimization and resource utilization of waste.

[0058] Composite materials can exist in various forms. For example, after extrusion granulation, they can be in the form of composite material granules, which are the basis for subsequent molding and processing. After injection molding or compression molding, composite materials can be presented as final products with preset shapes and functions, such as sheets, profiles or various structural components.

[0059] In summary, this application obtains a recycled polypropylene composite material by mixing raw materials such as recycled polypropylene, waste rubber powder, and waste natural fibers in a specified mass ratio, followed by melt blending, extrusion granulation, and subsequent injection molding or compression molding. Throughout the preparation process, waste rubber powder and waste natural fibers are uniformly dispersed in the recycled polypropylene matrix as reinforcing or modifying components. The addition of waste rubber powder effectively improves the impact toughness and crack resistance of the material, while waste natural fibers enhance its stiffness and strength. Alkali treatment and coupling modification of the waste natural fibers improve their interfacial compatibility with the polypropylene matrix, enhancing the bonding force between the two phases and ensuring the overall performance of the composite material. Furthermore, optimization of parameters such as temperature control, screw speed, and aspect ratio during melt blending and extrusion granulation ensures thorough mixing and uniform dispersion of each component in the polypropylene melt, avoiding agglomeration and resulting in a composite material with a uniform structure and stable performance. The final injection molding or compression molding process transforms the composite material granules into a final product with a preset shape and size, enabling it to be directly applied in various fields.

[0060] Preparation Example 1: This preparation example provides a method for preparing a recycled polypropylene composite material, including the following steps: (1) Raw material pretreatment Fly ash pretreatment: Take fly ash from a coal-fired power plant and dry it at 105℃ for 4 hours; weigh 100g of dried fly ash, heat it to 90℃, then add 2.0g of aluminate coupling agent (HYA2), and stir it in a high-speed mixer at 1000rpm for 30 minutes to obtain modified fly ash; Pretreatment of waste tire rubber powder: Take waste tire rubber powder, pass it through an 80-mesh sieve and set it aside for later use; Sugarcane bagasse pretreatment: Sugarcane bagasse fiber was crushed and passed through an 80-mesh sieve. 100g of the sieve residue was weighed and 500 mL of approximately 2.2 mol / L sodium hydroxide solution (calculated from 8% sodium hydroxide solution) was added to the residue. The residue was then treated in a water bath at 70℃ for 1.5h. The sugarcane bagasse was filtered and washed with water until neutral, and dried at 80℃ until its weight no longer changed. The dried sugarcane bagasse was heated to 90℃, and 1.5g of titanate coupling agent (HY311) was added. The mixture was stirred at 1000 rpm for 30min in a high-speed mixer to obtain modified sugarcane bagasse. (2) Mixing Weigh out 70 parts of recycled polypropylene, 10 parts of modified fly ash, 10 parts of waste tire rubber powder, 10 parts of modified bagasse, and 3 parts of maleic anhydride-grafted polypropylene by mass. Place them in a high-speed mixer and mix at 600 rpm for 15 minutes at room temperature to obtain a premix. (3) Melt granulation The premixed material was fed into a twin-screw extruder (length-to-diameter ratio 44:1). The temperatures of the first heating zone, the second heating zone, the third heating zone, the fourth heating zone and the die head zone were set to 175℃, 185℃, 195℃, 200℃ and 195℃ respectively. The screw speed was 250 rpm, and the material was extruded and granulated. (4) Molding The granules were dried at 80℃ for 3 hours and then injection molded at an injection temperature of 190℃ and an injection pressure of 60 MPa using an injection molding machine. The mold temperature was 50℃, resulting in dumbbell-shaped samples.

[0061] Preparation Example 2: The preparation steps of this preparation example are the same as those of preparation example 1, except that the proportions of each group in step (2) are as follows: 70 parts of recycled polypropylene, 15 parts of modified fly ash, 10 parts of waste tire rubber powder, 5 parts of modified sugarcane bagasse, and 3 parts of maleic anhydride grafted polypropylene.

[0062] Preparation Example 3: The preparation steps of this preparation example are the same as those of preparation example 1, except that the proportions of each group in step (2) are as follows: 70 parts of recycled polypropylene, 5 parts of modified fly ash, 15 parts of waste tire rubber powder, 10 parts of modified sugarcane bagasse, and 3 parts of maleic anhydride grafted polypropylene.

[0063] Preparation Example 4: The preparation steps of this preparation example are the same as those of preparation example 1, except that the proportions of each group in step (2) are as follows: 70 parts of recycled polypropylene, 10 parts of modified fly ash, 5 parts of waste tire rubber powder, 15 parts of modified sugarcane bagasse, and 3 parts of maleic anhydride grafted polypropylene.

[0064] Preparation Example 5: The preparation steps of this preparation example are the same as those of preparation example 1, except that the proportions of each group in step (2) are as follows: 60 parts of recycled polypropylene, 15 parts of modified fly ash, 15 parts of waste tire rubber powder, 10 parts of modified sugarcane bagasse, and 3 parts of maleic anhydride grafted polypropylene.

[0065] Preparation Example 6: The preparation steps of this preparation example are the same as those of preparation example 1, except that maleic anhydride grafted polypropylene is not added in step (2), and the ratio is: 80 parts of recycled polypropylene, 10 parts of modified fly ash, 10 parts of waste tire rubber powder, and 10 parts of modified sugarcane bagasse.

[0066] Comparative Example 1: The preparation steps of this comparative example are the same as those of Preparation Example 1, except that: waste tire rubber powder is not added in step (2), and the ratio is: 80 parts of recycled polypropylene, 10 parts of modified fly ash, 10 parts of modified bagasse, and 3 parts of maleic anhydride grafted polypropylene.

[0067] Comparative Example 2: The preparation steps of this comparative example are the same as those of Preparation Example 1, except that: no modified bagasse is added in step (2), and the ratio is: 80 parts of recycled polypropylene, 10 parts of modified fly ash, 10 parts of waste tire rubber powder, and 3 parts of maleic anhydride grafted polypropylene.

[0068] Comparative Example 3: The preparation steps of this comparative example are the same as those of Preparation Example 1, except that: no modified fly ash is added in step (2), and the ratio is: 80 parts of recycled polypropylene, 10 parts of waste tire rubber powder, 10 parts of modified sugarcane bagasse, and 3 parts of maleic anhydride grafted polypropylene.

[0069] Comparative Example 4: The preparation steps of this comparative example are the same as those of Preparation Example 1, except that in step (2), only recycled polypropylene and maleic anhydride-grafted polypropylene are added, with a ratio of 97 parts recycled polypropylene and 3 parts maleic anhydride-grafted polypropylene.

[0070] Comparative Example 5: The preparation steps of this comparative example are the same as those of Preparation Example 1, except that the fly ash and bagasse were not modified by coupling agent in step (1).

[0071] The composite material granules prepared in Examples 1 to 6 and Comparative Examples 1 to 5 were dried and then injection molded into standard specimens. The mechanical properties were tested in accordance with national standards (GB / T 1040.1-2025 Determination of tensile properties of plastics - Part 1: General, GB / T 9341-2008 Test method for flexural properties of plastics, GB / T 1843-2008 Determination of impact strength of plastic cantilever beams). The results are shown in Table 1.

[0072] Table 1: Test results of mechanical properties of composite materials from preparation examples and comparative examples

[0073] The test results in Table 1 show that: Comparing Preparation Example 1 with Comparative Examples 1 to 4, it can be seen that the present invention achieves a balanced improvement in the rigidity and toughness of the composite material through the synergistic addition of three solid wastes: waste tire rubber powder, bagasse fiber, and fly ash. Comparative Example 1 (without waste tire rubber powder) showed a significant decrease in impact strength, only 4.2 kJ / m²; Comparative Example 2 (without bagasse) showed a significant decrease in tensile and flexural strength; Comparative Example 3 (without fly ash) had lower overall performance than Preparation Example 1; and Comparative Example 4 (pure recycled polypropylene) had the lowest impact strength, only 3.5 kJ / m². This indicates that all three solid waste components are indispensable, and the synergistic effect of the three is superior to any two combinations.

[0074] Comparing Preparation Example 1 with Comparative Example 5, it can be seen that after coupling modification, the tensile strength, flexural strength and impact strength of the composite material of fly ash and bagasse are significantly improved, indicating that coupling modification improves the interfacial bonding between the filler and the matrix and improves the stress transfer efficiency.

[0075] Comparing Preparation Example 1 and Preparation Example 6, it can be seen that, without the addition of a compatibilizer (maleic anhydride-grafted polypropylene), the overall mechanical properties of the composite material are slightly lower than those of Preparation Example 1 with the addition of a compatibilizer, but still significantly better than the unmodified and the comparative examples lacking a certain solid waste. This indicates that, based on coupling modification, even without the use of a compatibilizer, the method of the present invention can still effectively achieve synergistic reinforcement and toughening of multi-source solid waste, and has good feasibility.

[0076] Comparing preparation examples 1 to 5, it can be seen that the performance of the composite material can be controlled within a certain range by adjusting the ratio of the three types of solid waste: increasing the content of waste tire rubber powder (preparation example 3) can improve impact toughness, but will sacrifice some rigidity; increasing the content of sugarcane bagasse (preparation example 4) can improve rigidity and strength, but the impact toughness will decrease; an appropriate amount of fly ash (preparation example 1) helps to balance the overall performance. In practical applications, an appropriate ratio can be selected according to specific needs.

[0077] The raw materials and reagents used in the preparation example were sourced as follows: Recycled polypropylene (rPP) was obtained by washing and drying recycled polypropylene crushed material; waste tire rubber powder (GTR) was purchased from a rubber recycling plant with a particle size of 80 mesh; fly ash was obtained from a waste incineration power plant; bagasse was obtained from a sugar factory; aluminate coupling agent (HYA2) and titanate coupling agent (HY311) were both commercially available; sodium hydroxide and maleic anhydride grafted polypropylene were commercially available.

[0078] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments above. Any modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A method for preparing a recycled polypropylene composite material, characterized in that, Includes the following steps: S1, providing waste rubber powder with a first set mesh size and waste natural fibers with a second set mesh size; S2, weigh the raw materials according to the mass fractions, the raw materials include 70-85 parts of recycled polypropylene, 5-25 parts of the waste rubber powder and 5-20 parts of waste natural fiber, and mix them to obtain a premix; S3, the premixed material is melt-blended and extruded and granulated to obtain composite material granules; S4, the recycled polypropylene composite material is obtained by injection molding or compression molding of the composite material granules.

2. The preparation method according to claim 1, characterized in that, In step S2, the mass ratio of the waste rubber powder to the waste natural fiber is (1.5:1) to (2:1).

3. The preparation method according to claim 1 or 2, characterized in that, In step S2, the raw material further includes 5-20 parts of modified fly ash, which is obtained by modifying raw fly ash. The raw fly ash is one or more of coal-fired power plant fly ash or waste incineration fly ash. The modification process includes: drying the raw fly ash at 100-110°C for 3-5 hours, then adding 1.0%-2.5% of an aluminate coupling agent by mass of the raw fly ash, and stirring at 800-1200 rpm at 80-100°C for 20-40 minutes.

4. The preparation method according to claim 1, characterized in that, In step S2, the raw materials also include 0-5 parts of a compatibilizer, which is maleic anhydride-grafted polypropylene; the mixing is carried out in a high-speed mixer for 10-20 minutes, at a speed of 500-800 rpm, and at room temperature.

5. The preparation method according to claim 1, characterized in that, In step S1, providing waste natural fibers with a second set mesh size specifically includes: taking waste natural fiber sources, crushing them, rinsing the crushed material with hot water at 60~80℃ for 10~30 minutes, and then drying and sieving it to the second set mesh size.

6. The preparation method according to claim 1 or 5, characterized in that, In step S2, before weighing the waste natural fibers, the waste natural fibers are subjected to alkali treatment and coupling modification treatment in sequence; the alkali treatment includes: using a sodium hydroxide solution with a concentration of 1-3 mol / L, a treatment temperature of 60-80℃, and a treatment time of 1-2 hours; the coupling modification treatment includes: using a titanate coupling agent or a silane coupling agent accounting for 1.0%-2.0% of the mass of the waste natural fibers.

7. The preparation method according to claim 1, characterized in that, Step S3 specifically includes: feeding the premixed material into a twin-screw extruder for melt blending and extrusion granulation; the twin-screw extruder has a screw length-to-diameter ratio of (40:1) to (48:1) and a screw speed of 200 to 350 rpm; the twin-screw extruder is divided into a first heating zone, a second heating zone, a third heating zone, a fourth heating zone, and a die head zone along the material conveying direction from the feed port to the die head, wherein the temperature of the first heating zone is 170 to 180°C, the temperature of the second heating zone is 180 to 190°C, the temperature of the third heating zone is 190 to 200°C, the temperature of the fourth heating zone is 195 to 205°C, and the temperature of the die head zone is 190 to 200°C.

8. The preparation method according to claim 7, characterized in that, In step S3, the composite material granules are dried at a temperature of 80-90°C for 2-4 hours. In step S4, the injection molding process conditions are: injection temperature of 180-200°C, injection pressure of 40-80 MPa, and mold temperature of 40-60°C. The compression molding process conditions are: compression molding temperature of 180-200°C, compression molding pressure of 10-20 MPa, and holding time of 5-10 minutes.

9. The preparation method according to claim 1, characterized in that, In step S1, the first set mesh size is 80-120 mesh, and the second set mesh size is 80 mesh; the waste rubber powder is one or more of waste tire rubber powder, waste household rubber powder, and industrial waste rubber powder; the waste natural fiber is one or more of sugarcane bagasse, straw fiber, hemp fiber, bamboo fiber, or coconut shell fiber.

10. A recycled polypropylene composite material, characterized in that, The recycled polypropylene composite material is prepared by the method described in any one of claims 1 to 9.

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