Preparation method of polypropylene-based composite material regenerated after consumption and plastic part for vehicle

Abstract

The invention provides a preparation method of a polypropylene-based composite material regenerated after consumption and a plastic part for a vehicle. The preparation method comprises the following steps: carrying out graft modification on post-consumption regenerated polypropylene, and introducing an epoxy functional group and a carboxyl functional group to obtain modified regenerated polypropylene with double functional groups; the preparation method comprises the following steps: carrying out melt blending on modified regenerated polypropylene, a cross-linking agent, a Lewis acid catalyst and a filler, carrying out ester exchange reaction on the modified regenerated polypropylene under a heating condition to construct a dynamic ester bond network with reversible cross-linking capability, and inducing the filler to be directionally arranged under a melt shearing condition by the dynamic ester bond network, the filler comprises surface modified graphene and mica powder subjected to surface silanization treatment, and the cross-linking agent is aliphatic polybasic acid ester; and granulating the blend, and carrying out hot pressing treatment to obtain the post-consumer regenerated polypropylene-based composite material. The composite material disclosed by the invention has the characteristics of good dimensional stability, high impact toughness and heat-induced self-repairing property.

Description

Technical Field

[0001] This application relates to the technical field of recycling of renewable plastics, and particularly to a preparation method of a post-consumer recycled polypropylene-based composite material and a plastic part for vehicles. Background Art

[0002] Post-Consumer Recycled Polypropylene (PCR-PP), as an important recycled resource, can be applied to the fields of automotive interior and exterior trim. However, during its service life and the recycling process, PCR-PP undergoes multiple thermo-mechanical processes and long-term exposure to light, which easily leads to molecular chain breakage and oxidative degradation. This structural deterioration not only causes a decrease in its molecular weight and an increase in crystallinity, but also leads to mechanical property degradation problems such as an increase in melt flow rate and a decrease in elongation at break, seriously affecting the dimensional stability and impact toughness of the material.

[0003] To address the above problems, the prior art usually adds auxiliary components such as ultraviolet stabilizers, antioxidants, or carbon black to the blend system to delay the degradation process and maintain its basic mechanical properties. However, these additives have problems such as thermal volatilization, migration, and reaction deactivation, and it is difficult to continuously play a stabilizing role throughout the life cycle of the material, resulting in easy performance degradation of PCR-PP after recycling processing, especially the dimensional stability and impact resistance in high-temperature or outdoor environments are difficult to guarantee. Summary of the Invention

[0004] To solve the above problems, according to the first aspect of the present application, a preparation method of a post-consumer recycled polypropylene-based composite material is provided, including the following steps: Perform graft modification on post-consumer recycled polypropylene to introduce epoxy functional groups and carboxyl functional groups to obtain modified recycled polypropylene with dual functional groups; Melt-blend the modified recycled polypropylene, a cross-linking agent, a Lewis acid catalyst, and a filler, and under heating conditions, enable the modified recycled polypropylene to construct a dynamic ester bond network with reversible cross-linking ability through transesterification reaction. The dynamic ester bond network induces the filler to be oriented under melt shear conditions, thereby synergistically forming a blend with a three-dimensional guiding skeleton structure. The filler includes surface-modified graphene and surface-silanized mica powder, the cross-linking agent is an aliphatic polyacid ester, and the surface-modified graphene is graphene modified with chitosan-carboxyl end-capping; Granulate the blend and then perform hot pressing treatment to obtain the post-consumer recycled polypropylene-based composite material.

[0005] According to a second aspect of the present application, there is provided a plastic part for a vehicle, and the material of the plastic part for a vehicle includes virgin polypropylene and the post-consumer recycled polypropylene-based composite material obtained by the aforementioned preparation method.

[0006] According to the solution of the present application, by constructing a three-dimensional guiding skeleton structure, the problems of poor dimensional stability and low impact toughness caused by molecular chain degradation and structural disorder during the recycling process of PCR-PP are effectively alleviated. At the same time, the dynamic ester bond network constructed in the present application has the ability of reversible topological rearrangement under thermal triggering, and can achieve local de-crosslinking and topological rearrangement of polymer segments under mild heating conditions, endowing the material with good reprocessing performance and thermally induced self-healing ability. Compared with the traditional solution of delaying degradation with additives, the structure regulation mechanism proposed in the present application does not rely on easily inactivated components, has stronger stability and sustainability, and significantly improves the dimensional stability, impact absorption ability and service durability of the PCR-PP recycled material in engineering plastic application scenarios such as automotive interior and exterior trims.

[0007] The above description is only an overview of the technical solution of the present application. In order to be able to understand the technical means of the present application more clearly and implement it according to the content of the specification, the following describes in detail with preferred embodiments of the present application as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 A schematic flow chart showing a preparation method of a post-consumer recycled polypropylene-based composite material according to an embodiment of the present application; Figure 2 A schematic flow chart showing a preparation method of a modified recycled polypropylene with a bifunctional group according to an embodiment of the present invention; Figure 3 A schematic flow chart showing a preparation method of graphene modified with chitosan-carboxyl end groups according to an embodiment of the present invention; Figure 4 A Fourier transform infrared spectrum diagram of the epoxidized recycled polypropylene obtained in step S131 in the first embodiment of the present invention; Figure 5 An acid titration curve of the epoxidized recycled polypropylene sample obtained in step S131 in the first embodiment of the present invention; Figure 6 A Fourier transform infrared spectrum diagram of the modified recycled polypropylene with a bifunctional group obtained in step S141 in the first embodiment of the present invention; Figure 7 An acid-base titration curve of the modified recycled polypropylene sample with a bifunctional group obtained in step S141 in the first embodiment of the present invention; Figure 8Shows the scanning electron microscope image of the blend with a three-dimensional guiding skeleton structure in the first embodiment of the present invention; Figure 9 Shows the Fourier transform infrared spectroscopy spectrum of the chitosan-carboxyl-terminated modified graphene material in the first embodiment of the present invention. Detailed implementation manners

[0009] The technical solutions of the present application will be further described below in conjunction with specific embodiments.

[0010] Figure 1 Shows a schematic flow chart of a method for preparing a post-consumer recycled polypropylene-based composite material according to an embodiment of the present application. As Figure 1 shown, the preparation method includes: Step S100, graft-modify the post-consumer recycled polypropylene to introduce epoxy functional groups and carboxyl functional groups to obtain modified recycled polypropylene with dual functional groups; Step S200, melt-blend the modified recycled polypropylene, crosslinking agent, Lewis acid catalyst and filler, and under heating conditions, enable the modified recycled polypropylene to construct a dynamic ester bond network with reversible crosslinking ability through transesterification reaction. The dynamic ester bond network induces the filler to be oriented under the condition of melt shearing, thereby synergistically forming a blend with a three-dimensional guiding skeleton structure. The filler includes surface-modified graphene and surface-silanized mica powder, the crosslinking agent is an aliphatic polyacid ester, and the surface-modified graphene is graphene modified with chitosan-carboxyl end-capping; Step S300, granulate the blend and then perform hot pressing treatment to obtain a post-consumer recycled polypropylene-based composite material.

[0011] It should be explained that the "dynamic ester bond network" here refers to a reversible crosslinked structure introduced between polymer chains through transesterification reaction. Under specific temperature conditions, especially in the presence of a Lewis acid catalyst, the ester bond can break and recombine, so that the entire network exhibits reversible dynamic topological reconstruction ability. Different from the completely linear flow of traditional thermoplastic polymers, the dynamic ester bond network has a certain fluidity macroscopically, but still maintains the topological constraints of the crosslinked network at the microscopic level, that is, the movement of molecular segments is restricted by the network structure, showing typical dynamic covalent network characteristics.

[0012] During the melt shearing process, the dynamic ester bond network exhibits typical topological fluidity, that is, while maintaining the dynamic cross-linked structure, the polymer segments have a limited ability of ordered flow and spatial rearrangement. Under the action of melt shear stress, driven by the shear stress, the polymer segments migrate directionally along the stress direction. During the movement of the segments, they can form directional interfacial coupling forces through hydrogen bonding, weak coordination or interfacial interpenetration with polar functional groups such as hydroxyl groups, carboxyl groups and silyl groups existing on the filler surface. The interfacial coupling force generates adhesion traction and interfacial slip effects in the polymer flow direction, promoting the axial stacking orientation or ordered arrangement of the filler along the shear direction under the guidance of the polymer segments. With the end of shearing and the decrease in temperature, the dynamic ester bond network quickly restores its cross-linked structure, effectively solidifying and locking the arranged filler structure in the established orientation. Finally, the topological guidance of the polymer segments and the directional reinforcement of the filler synergistically construct a stable three-dimensional guiding skeleton structure in three-dimensional space.

[0013] According to the solution of this application, by constructing a three-dimensional guiding skeleton structure, the problems of poor dimensional stability and low impact toughness caused by molecular chain degradation and structural disorder during the recycling process of PCR-PP are effectively alleviated. At the microscopic level, the three-dimensional guiding skeleton structure improves the directional distribution ability of the filler and the efficiency of interfacial synergistic action, which helps to alleviate the structural instability problem caused by degraded segments. At the macroscopic level, the post-consumer recycled polypropylene-based composite material exhibits excellent dimensional retention ability and mechanical response ability, and can continuously maintain structural stability under high temperature, high load or complex outdoor environments. At the same time, the dynamic ester bond network constructed in this application has the ability of reversible topological rearrangement under thermal triggering, and can achieve local de-crosslinking and topological rearrangement of polymer segments under mild heating conditions, endowing the material with good reprocessing performance and thermal-induced self-healing ability. Compared with the traditional solution of using additives to delay degradation, the structural regulation mechanism proposed in this application does not rely on easily inactivated components, has stronger stability and sustainability, and significantly improves the dimensional stability, impact absorption ability and service durability of the PCR-PP recycled material in engineering plastic application scenarios such as automotive interior and exterior trims.

[0014] In this step S100, the post-consumer recycled polypropylene can be recycled polypropylene from fields such as automotive bumpers and packaging containers, and is preferably a commercial PCR-PP material with a stable source and batch consistency. For example, PCR-PP particles of grades such as rPP-W120A, rPP-W160A, and rPP-W115TA can be selected, which have the following representative performance parameters: the melt index is 3 g / 10 min to 8 g / 10 min (measurement conditions: 230 °C, 2.16 kg), the tensile strength is not less than 25 MPa, and the ash content is less than 1.5%. Such commercial PCR-PP particles have good fluidity, mechanical strength, and thermal stability, and are suitable for the performance balance requirements in the injection molding process, and can be used as the polymer matrix raw material in the recycled modification system.

[0015] Introducing the bifunctional structure of epoxy functional groups and carboxyl functional groups is the key basis for constructing a dynamic ester bond network. Among them, the epoxy group can undergo a ring-opening reaction with structures containing active hydrogen or polar groups (such as hydroxyl, carboxyl, amino, etc.) in the polymer chain segment or on the filler surface at the initial stage, thereby achieving effective interfacial bonding and the construction of a preliminary network structure. The carboxyl functional group undergoes a reversible transesterification reaction with the cross-linking agent under the catalysis of Lewis acid, and then forms an ester bond network with the ability of dynamic reconstruction. This bifunctional system achieves a synergistic effect in terms of structural function, that is, the epoxy group endows the network with good initial construction efficiency and interfacial bonding ability, while the carboxyl group provides reversible cross-linking ability under thermal triggering, enabling the material to have excellent fluidity, self-healing property, and deformation adaptability while maintaining structural stability, thereby significantly improving the dimensional stability, impact resistance, and service durability of the composite material. If only the epoxy group is introduced, the system mainly forms an irreversible permanent bonding structure, making it difficult to achieve subsequent network reconstruction and mechanical adjustment. If only the carboxyl functional group is contained, the initial cross-linking efficiency is low, the number of active sites is insufficient, and the interfacial reaction selectivity is poor, which is not conducive to constructing a uniform and controllable network structure. Therefore, it is necessary to introduce both epoxy and carboxyl functional groups simultaneously to construct a bifunctional structure to achieve the efficient formation and dynamic reorganization of the ester bond network at different stages.

[0016] Figure 2 The schematic flow chart of the preparation method of the bifunctional modified recycled polypropylene according to an embodiment of the present invention is shown. As Figure 2 shown, this preparation method includes: Step S110, cleaning the post-consumer recycled polypropylene; Step S120, dispersing the cleaned post-consumer recycled polypropylene in a preset solvent to obtain a post-consumer recycled polypropylene solution; Step S130: Mix the post-consumer recycled polypropylene solution with an epoxidizing reagent at a mass ratio of (5 - 10):1, then add a first catalyst and react to obtain epoxidized recycled polypropylene. Step S140: Mix the epoxidized recycled polypropylene with a carboxylating reagent at a mass ratio of (5 - 10):1, then add a second catalyst and react to obtain modified recycled polypropylene with dual functional groups.

[0017] In step S110, ensure the surface of the particles is clean and dry by solvent cleaning, hot air drying, or vacuum drying. When using solvent cleaning, solvents such as toluene, dichloromethane, acetone, etc. can be selected. Ultrasonic cleaning or the immersion method can be used to efficiently remove oils or other soluble substances on the particle surface. Through cleaning, impurities, residual chemicals, unreacted reagents, solvents, etc. that may exist on the surface of post-consumer recycled polypropylene (PCR-PP) particles can be removed, thus avoiding the influence of these impurities on the reaction efficiency and final performance.

[0018] In step S120, the preset solvent can be, for example, dichloromethane, toluene, dimethyl sulfoxamide, etc. The preset solvent can also be other solvents with good solubility for PCR-PP, as long as it can completely dissolve PCR-PP and has good dissolution stability for other reactants and catalysts in the reaction system.

[0019] In step S130, the mass ratio of the PCR-PP solution to the epoxidizing reagent can be, for example, 5:1, 6:1, 8:1, or 10:1, or any other value within (5 - 10):1. The design of this mass ratio range is to introduce an appropriate number and density of epoxy groups. If the proportion of the epoxidizing reagent is too low, incomplete reaction of the epoxy groups will occur. Conversely, if the proportion of the epoxidizing reagent is too high, overgrafting or crosslinking will occur, leading to unnecessary side reactions and affecting the controllability of the reaction and the uniformity of the final grafted structure. The epoxidizing reagent needs to have strong reactivity to ensure the effective introduction of epoxy groups and be compatible with the chemical structure of post-consumer recycled polypropylene. In some embodiments, the epoxidizing reagent is epichlorohydrin, styrene oxide, epoxy resin, epoxidized alcohol, epoxy acrylate, chloropropenyl epoxide, or epoxy silane.

[0020] The first catalyst can be selected, for example, as a Lewis acid catalyst, an organic acid catalyst or an inorganic acid catalyst. The Lewis acid catalyst can be, for example, boron trifluoride or aluminum chloride. The organic acid catalyst can be, for example, p-toluenesulfonic acid. The inorganic acid catalyst can be, for example, zinc chloride. The first catalyst is preferably boron trifluoride. The addition amount of the first catalyst is 0.5%-2% of the mass of the epoxidizing agent, and can be, for example, 0.5%, 1%, 1.5% or 2%. The reaction temperature in the step S130 is 80°C - 120°C, and can be, for example, 80°C, 90°C, 100°C, 110°C or 120°C. The reaction time is 2h - 4h, and can be, for example, 2h, 3h or 4h.

[0021] The grafting density of the epoxy group prepared in the embodiment of the present invention is 0.1%-5% (mass percentage), and the grafting density refers to the percentage of the mass of the epoxy group in the modified recycled polypropylene in the total mass of the polymer. The grafting density of the epoxy group directly affects the reaction activity of the modified recycled polypropylene. An appropriate grafting density (0.1%-5%) ensures sufficient reaction sites for subsequent chemical reactions (such as reactions with crosslinking agents). Too low a grafting density may lead to insufficient reaction sites, affecting the reaction efficiency, while too high a grafting density may lead to overcrosslinking or uneven grafting, affecting the performance of the final product.

[0022] In the step S140, the epoxidized recycled polypropylene and the carboxylating agent are in a mass percentage of, for example, 5:1, 6:1, 8:1 or 10:1, or can also be any other value in (5-10):1. The carboxylating agent is selected as a carboxyl source with a non-nucleophilic structure and an activatable double bond, and reagents with free carboxylic acid functional groups such as acetic acid, benzoic acid, maleic acid, etc. are avoided. In some embodiments, the carboxylating agent is selected as maleic anhydride or an acrylate reagent with a double bond structure. The acrylate reagent can be, for example, butyl acrylate, methyl acrylate, ethyl acrylate, hydroxyethyl acrylate or an acrylate oligomer such as polybutyl acrylate, or a copolymer of them and maleic anhydride, etc.

[0023] The second catalyst is a free radical catalyst or a metal complex catalyst, preferably a free radical catalyst. Examples of the free radical catalyst may include dicumyl peroxide, benzoyl peroxide, azobisisobutyronitrile, tert-butyl peroxide, di-tert-butyl peroxide, etc. Such catalysts decompose to generate free radicals at an appropriate temperature, inducing the activation of the C=C double bond in the acrylate or maleic anhydride molecule and undergoing a grafting reaction with the polypropylene backbone. Examples of the metal complex catalyst may include iron(III) acetylacetonate, cobalt(II) acetylacetonate, transition metal salts such as CuCl, FeCl3, etc. Such catalysts can form activatable intermediates with olefin or carboxylate groups in the reaction system, promoting the grafting efficiency and controlling the chain degree of the polymerization reaction, thereby obtaining a more regular graft structure. The second catalyst is preferably dicumyl peroxide, which has high thermal stability and decomposition temperature, and can slowly release free radicals under controlled conditions, ensuring both the controllability of the reaction and effectively avoiding the destruction of epoxy groups or the occurrence of side reactions.

[0024] In some embodiments, the addition amount of the second catalyst is 0.1% - 3% of the mass of the carboxylation reagent, and may be, for example, 0.1%, 0.5%, 1%, 2% or 3%. The reaction temperature is preferably controlled between 90°C and 120°C, such as 90°C, 110°C or 120°C, and the reaction time is 1 h to 3 h to ensure the full progress of the grafting reaction while suppressing the occurrence of side reactions.

[0025] The carboxyl functional group grafting density prepared in the embodiments of the present invention is 0.5% - 2% (mass percentage). The grafting density refers to the percentage of the mass of the carboxyl functional group in the total mass of the modified recycled polypropylene. The grafting density has a significant impact on the reaction efficiency of subsequent construction of the reversible crosslinking network and the performance of the final composite material. Specifically, too low a grafting density will result in a lack of sufficient functionalized sites in the polymer molecules, restricting the transesterification reaction with the multifunctional crosslinking agent and affecting the formation and stability of the dynamic network structure. Too high a grafting density may lead to an overly dense distribution of functional groups, easily inducing side reactions between chains or uneven grafting, thereby causing a decrease in the thermal stability of the polymer, deterioration of the melt processing performance, and even embrittlement. Therefore, controlling the grafting density of the carboxyl functional group within the above range can balance the crosslinkability, processability of the system, and the mechanical and thermal properties of the final composite material while ensuring sufficient chemical reactivity, improving its comprehensive application performance and repeated processing ability.

[0026] In step S140, the type of carboxylation reagent, the selection of the second catalyst, as well as conditions such as reaction temperature and reaction time have a crucial impact on effectively grafting carboxyl functional groups onto the backbone of epoxidized recycled polypropylene while avoiding side reactions between the carboxylation reagent and epoxy groups. Specifically, the selected carboxylation reagent should have low nucleophilicity and controllable reaction activity to avoid nucleophilic ring-opening reactions with epoxy groups. For this purpose, the carboxylation reagent used in this step is preferably a compound without free carboxylic acid functional groups and contains a C=C double bond structure in its molecule that can be initiated by free radicals. For example, maleic anhydride or various acrylate monomers can achieve grafting onto the backbone of epoxidized recycled polypropylene under the action of free radicals, thereby introducing stable carboxyl functional groups on the side of the polymer molecular chain. In terms of the catalytic system, the second catalyst is preferably a free radical initiator that can decompose and release free radicals under mild conditions, such as dicumyl peroxide. This type of catalyst has a relatively high decomposition starting temperature and can slowly release free radicals within the reaction temperature range of 90°C to 120°C. It can not only effectively induce the free radical addition of the double bond structure in the carboxylation reagent but also significantly reduce the risk of interference or damage to epoxy functional groups, thereby achieving controllable regulation of grafting position and grafting density. The reaction temperature in this step can not only ensure the initiation efficiency of the catalyst but also inhibit the thermal degradation of epoxy groups or the possibility of participating in side reactions. At the same time, the reaction time is controlled within 1 hour to 3 hours, which can ensure the full progress of the carboxylation grafting reaction while limiting the accumulation of side reactions and the generation of by-products.

[0027] In step S200, the mass ratio of the modified recycled polypropylene, crosslinking agent, Lewis acid catalyst, surface-modified graphene, and surface-silanized mica powder is 100:(1 - 5):(0.1 - 0.5):(2 - 5):(13 - 20). For example, it can be 100:1:0.1:2:13, 100:3:0.2:3:15, or 100:5:0.5:5:20, or any other ratio within 100:(1 - 5):(0.1 - 0.5):(2 - 5):(13 - 20).

[0028] The crosslinking agent is an aliphatic polyacid ester. The aliphatic polyacid ester is selected from tributyl citrate, triethyl citrate, dibutyl malonate, dioctyl adipate, diisooctyl sebacate, or diisooctyl succinate. The aliphatic polyacid ester contains multiple ester groups or potential carboxyl groups and can undergo transesterification reactions with carboxyl functional groups or epoxy functional groups on the molecular chain of the modified recycled polypropylene under the catalysis of Lewis acid, effectively constructing an ester bond crosslinking network. Moreover, the generated ester bonds are reversible under heat, catalyst, or shear conditions, endowing the blend system with dynamic characteristics such as remolding, repair, and reprocessing.

[0029] During multiple experiments, the inventors attempted to use various crosslinking agents that might react with the functional groups in the modified recycled polypropylene and were expected to form an ester bond network. In addition to the aliphatic polyacid esters used in the embodiments of the present application, they also included polyfunctional epoxides such as 1,4-butanediol diglycidyl ether and ER-331, as well as polyfunctional acids such as tripolyphosphoric acid and citric acid. However, the experimental results showed that none of the above crosslinking agents could achieve the goal of constructing a dynamic ester bond network with reversible characteristics. Specifically, although polyfunctional epoxides can react with carboxyl functional groups in the material to form an ester bond structure, their crosslinking process is an irreversible addition reaction and cannot undergo controllable de-crosslinking behavior under thermal induction conditions, so they do not have the ability of dynamic transesterification. Although polyfunctional acids can undergo esterification reactions with hydroxyl functional groups in the polymer, the equilibrium constant of such reactions is low, the reaction rate is slow, and they are sensitive to temperature and reaction conditions, making it difficult to achieve precise structural regulation, and the stability and reversibility of the generated ester bond network are insufficient. In addition, some polyacid compounds are prone to side reactions at high temperatures, further affecting the material properties. Therefore, except for aliphatic polyacid esters, other crosslinking agents have obvious deficiencies in terms of reaction activity, dynamic performance, thermal stability, and controllability, and it is difficult to meet the technical purpose of constructing a dynamic ester bond network with reversible crosslinking ability in this application.

[0030] The Lewis acid catalyst is selected as Zn(OAc)2, SnCl2 or AlCl3. When selected as Zn(OAc)2 and SnCl2, their concentrations are both 0.5wt%-2wt%, for example, they can be 0.5wt%, 1wt% or 2wt%. Among them, Zn(OAc)2 and SnCl2 are medium-strength Lewis acids and can effectively catalyze the transesterification reaction within the above concentration range, which can not only promote the reaction rate but also prevent side reactions such as polymerization, degradation or excessive crosslinking. In addition, Zn 2+ and Sn 2+ have mild coordination ability, which is beneficial to achieving equilibrium control in the formation of the dynamic ester bond network. This concentration range can provide a sufficient density of catalytic centers to meet the requirements of reaction active sites. When selected as AlCl3, its concentration is 1.5wt%-3wt%, for example, it can be 1.5wt%, 2wt% or 3wt%. AlCl3 is a strong Lewis acid catalyst, and its catalytic activity is much higher than that of Zn 2+ and Sn 2+ . If the concentration is too low, it is difficult to effectively construct a dynamic network in a short time. However, if the concentration is too high, it is easy to cause unnecessary side reactions such as main chain degradation and excessive crosslinking. Therefore, it is necessary to appropriately increase the dosage to 1.5wt%-3wt% to maintain sufficient catalytic efficiency while controlling side reactions.

[0031] The transesterification reaction is carried out at a reaction temperature of 170°C - 200°C under heating conditions. For example, it can be 170°C, 180°C, 190°C or 200°C. This reaction condition can ensure the reaction efficiency while suppressing the occurrence of side reactions, and effectively construct a dynamic ester bond cross-linking network with reversible performance. In the embodiments of the present application, melt blending provides a uniform reaction environment for the transesterification reaction, and the transesterification reaction gradually occurs under the heating conditions after melt blending or at the end of blending.

[0032] After systematic verification, the embodiments of the present application select surface-modified graphene and surface-silanized mica powder as co-fillers, which have key structural and performance advantages. The main reasons are as follows: First, surface-modified graphene can introduce polar functional groups such as carboxyl and hydroxyl groups, which can form hydrogen bonds, π–π stacking or electrostatic interactions with ester groups, carboxyl groups, hydroxyl groups, etc. existing in the dynamic ester bond network, realizing the co-response and interfacial adhesion of the filler during the migration process of polymer segments. After the surface of mica powder treated by surface silanization is treated with surface functional groups, such as introducing -Si(OR)3, -NH2, -COOH and other groups, it can produce weak coordination bonding or interfacial interpenetration with the dynamic polymerization network, effectively enhancing the ability of polymer segments to orient and traction the filler, and promoting the orderly construction and locking of the filler structure during the shear induction process. Second, surface-modified graphene has a high aspect ratio and flexible bending characteristics, and is easy to form continuous conductive channels and stress conduction chains along the flow path in the melt shear field. As a rigid flaky inorganic filler, surface-silanized mica powder has excellent organic compatibility and space support ability, and plays a role of directional damping and skeleton support in the network. After the two are combined, a three-dimensional guiding skeleton structure with soft-hard coordination and soft-rigid complementarity is formed, jointly enhancing the directivity and structural rigidity. Third, surface-modified graphene has excellent electrical conductivity and thermal stability. After introduction, it can improve the antistatic, heat conduction and thermal stability of the material. Surface-silanized mica powder has good electrical insulation, lamellar dispersibility and dimensional stability. The regular arrangement formed under shear guidance can significantly improve the dimensional stability and creep resistance. The synergistic effect of the two makes the final blend system have high strength, high toughness, high thermal conductivity and excellent stability while realizing the orderly construction of the structure.

[0033] Figure 3 Shows a schematic flow chart of a preparation method of graphene modified by chitosan-carboxyl capping according to an embodiment of the present invention. As Figure 3 shown, the preparation method includes: Step S210, oxidizing graphene to obtain graphene oxide containing carboxyl, hydroxyl and epoxy groups; Step S220, mixing graphene oxide and chitosan, and carrying out an amidation reaction in the presence of a coupling agent to obtain a covalent grafting product of graphene oxide-chitosan; Step S230: React the covalent grafting product with maleic anhydride or monoesters of fumaric acid under catalytic conditions to introduce carboxyl end groups, obtaining a chitosan-carboxyl end group modified graphene precursor; Step S240: Dry the chitosan-carboxyl end group modified graphene precursor to obtain powdered chitosan-carboxyl end group modified graphene.

[0034] In this step S210, this graphene oxide can be prepared by methods such as the Hummers method, the Tour method, etc.

[0035] In this step S220, the mixed solvent for mixing the graphene oxide and chitosan is water or a polar mixed solvent. This polar mixed solvent can be, for example, a DMF / DMSO solvent. In this step S220, the graphene oxide needs to be ultrasonicated in water or a polar solvent for 30 min - 1 h first to obtain a homogeneous dispersion, avoiding aggregation and ensuring the exposure of surface active sites. Also, chitosan needs to be fully stirred and dissolved in dilute acid (usually an aqueous acetic acid solution with pH < 6) for more than 2 h first to obtain a clear and particle-free chitosan solution. Then, the chitosan solution is added dropwise to the graphene oxide dispersion, and they are fully mixed under magnetic or mechanical stirring. The mass ratio of graphene oxide to chitosan is 1:(1 - 5), and can be, for example, 1:1, 1:2, 1:3, 1:4, or 1:5. Finally, a coupling agent is added to promote the condensation reaction between carboxyl and amino groups. The mass ratio of the coupling agent to graphene oxide is (0.2 - 1.5):1, and can be, for example, 0.2:1, 0.5:1, 1:1, 1.2:1, or 1.5:1. The selection of this coupling agent needs to consider being stable or available in water or a polar solvent, being able to activate carboxyl groups under mild conditions, having by-products that are easy to remove, having minimal impact on the dispersibility and stability of the target material, and not introducing toxic residues or impurities that are difficult to remove. Based on these considerations, the coupling agent is finally determined to be selected from 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholine hydrochloride, or a combination of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide.

[0036] The mass ratio of graphene oxide to chitosan is preferably within the above range, which can ensure the grafting efficiency and maintain the stability of the reaction system. The mass ratio of the coupling agent to graphene oxide is controlled between 0.2:1 and 1.5:1, which can balance the reaction activity and the control of side reactions. The pH value of the amidation reaction is controlled at 4.5 - 6, for example, it can be 4.5, 5 or 6. The reaction temperature of the amidation reaction is 25°C - 50°C, for example, it can be 25°C, 35°C, 45°C or 50°C. The reaction time of the amidation reaction is 6h - 12h, for example, it can be 6h, 8h, 10h or 12h. The pH value of this reaction is the optimal range set to balance the carboxyl activation efficiency and the nucleophilic activity of the chitosan amino group. The setting of the reaction temperature and reaction time can effectively increase the reaction rate while avoiding structural degradation, ensure the full formation of amide bonds, reach a stable level of the grafting reaction, and inhibit the occurrence of side reactions.

[0037] In this step S230, the catalytic conditions include using pyridine, triethylamine or 4-dimethylaminopyridine as a catalyst or promoter, and the reaction is carried out in a polar organic solvent or a mixed system thereof with water. The pH value is 6 - 8, for example, it can be 6, 7 or 8. The reaction temperature is 50°C - 90°C, for example, it can be 50°C, 60°C, 70°C, 80°C or 90°C. The reaction time is 4h - 10h, for example, it can be 4h, 5h, 6h, 8h or 10h.

[0038] In this step S240, the graphene precursor modified with chitosan-carboxyl capping is dried under vacuum conditions at 40°C - 60°C, preferably dried at a constant temperature of 50°C for 12h - 24h in a vacuum environment of -0.08MPa, to fully remove the residual solvent and moisture in the system and obtain a powdery chitosan-carboxyl capping modified graphene material.

[0039] The graphene modified with chitosan-carboxyl end groups obtained by the above preparation method can form multiple hydrogen bonds and electrostatic coupling interactions with the dynamic ester bond network due to the introduction of chitosan segments and carboxyl terminals. Compared with unmodified graphene, this modified structure significantly improves its dispersion stability and directional migration efficiency in the dynamic ester bond network. After synergistic action with rigid mica powder treated by surface silanization, the two achieve soft-rigid synergistic orientation arrangement under melt shear induction, and jointly construct a three-dimensional oriented framework structure with continuity, stress responsiveness and structural support. In addition, the introduced carboxyl terminals can have multiple interactions with the polymer matrix, which helps to improve the interfacial thermal stability and interfacial adhesion, and avoid interfacial instability of the three-dimensional oriented framework structure under the action of thermal cycling or shear stress. At the same time, the steric hindrance effect of chitosan segments helps to limit the aggregation and overlap of graphene sheets, enabling it to maintain excellent sheet flattening and structural integrity during the synergistic arrangement process of multi-component fillers. The three-dimensional oriented framework structure formed by the synergistic combination of this surface-modified graphene and silanized mica powder finally realizes the multiple enhancements of structural orientation, conductivity, dimensional stability, thermal conductivity and mechanical properties.

[0040] This step S300 includes the following steps: extruding the blend through the head of a twin-screw extruder, and preparing columnar or spherical particles by a pelletizer after the discharged material is cooled; drying the particles at 80°C - 120°C for 2h - 6h; placing the dried particles in a hot pressing mold, preheating the mold to 190°C - 230°C, hot pressing at 5MPa - 15MPa for 3min - 10min, and then demolding by means of cold mold shaping to obtain a post-consumer recycled polypropylene-based composite material.

[0041] In one embodiment, a benzotriazole-based ultraviolet absorber and a block-structured antioxidant are further added during the melt blending process. This block-structured antioxidant is formed by block copolymerization or graft modification of a capped receptor-structured antioxidant and a polypropylene segment.

[0042] It should be noted that the block-structured antioxidant refers to a structure formed by chemically block copolymerizing or grafting a capped receptor-structured antioxidant and a polypropylene segment. One end of this block-structured antioxidant contains functional groups that can stabilize free radicals or capture free radicals, such as hindered amine types (HALS, Hindered Amine Light Stabilizers), hindered phenol types, etc., endowing it with light and heat stabilization effects; the other end is covalently connected to the polypropylene segment, enhancing its compatibility and interfacial bonding ability with the recycled polypropylene matrix. Such block-structured antioxidants effectively limit the problems of easy migration and volatility of traditional antioxidants, can achieve more uniform dispersion in the polypropylene matrix, and greatly improve its processing stability and long-term weather resistance. The capped receptor-structured antioxidant can include hindered amine light stabilizers capped by acyl, alkyl, epoxy, etc., such as 2,2,6,6-tetramethylpiperidine structures, hindered phenol antioxidants or their derivatives such as capped block polymers of 2,6-Di-tert-butyl-4-methylphenol (BHT). Therefore, the block-structured antioxidant can be, for example, a block copolymer constructed by graft copolymerization of a capped hindered amine light stabilizer such as a 2,2,6,6-tetramethylpiperidine structure and a polypropylene segment, for example, a structural antioxidant (g-PP-HALS) obtained by introducing a polypropylene segment such as tetramethylpiperidine-based HALS into polypropylene by grafting or copolymerization, a polyethylene-amide-piperidine block copolymer, or a block polymer of polypropylene-acyl-capped HALS. The block-structured antioxidant can also be, for example, a block antioxidant formed by copolymerizing a hindered phenol antioxidant (such as 2,6-Di-tert-butyl-4-methylphenol, BHT) with a polyolefin segment by polymerization or grafting, for example, a structural antioxidant (g-PP-BHT) obtained by introducing BHT into the polypropylene segment by grafting or copolymerization, a block polyolefin antioxidant containing a BHT-capped structure, or a block copolymer containing phenolic hydroxyl groups.

[0043] The benzotriazole ultraviolet absorbers can be, for example, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-5'-tert-butylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3',5'-di-tert-butylphenyl)benzotriazole, 2-(2'-hydroxy-4'-octoxyphenyl)benzotriazole, or 2-(2'-hydroxy-4'-isononyloxyphenyl)benzotriazole.

[0044] In some embodiments, the mass ratio of the modified recycled polypropylene, benzotriazole ultraviolet absorber, and block-structured antioxidant is 100:(0.1 - 3):(0.5 - 5), and can be, for example, 100:0.1:0.5, 100:1:3, 100:2:4, or 100:3:5.

[0045] By selecting benzotriazole ultraviolet absorbers and block-structured antioxidants and setting the above mass ratios, while inhibiting the photo-degradation induced by ultraviolet light, it is also possible to inhibit the thermal oxidative degradation induced by free radicals, thereby enhancing the stability of the material throughout its life cycle.

[0046] In particular, the embodiments of the present application also provide a plastic part for a vehicle. The material of the plastic part for a vehicle includes virgin polypropylene and the post-consumer recycled polypropylene-based composite material obtained by the foregoing preparation method. The plastic part for a vehicle includes an instrument panel, an interior door panel, a center console assembly, a door trim panel, etc.

[0047] In one embodiment, the mass ratio of the virgin polypropylene to the post-consumer recycled polypropylene-based composite material is 100:(20 - 90), and for example, it can be 100:20, 100:30, 100:50, 100:70, or 100:90.

[0048] The key and importance of the process steps of the present application are elaborated below through specific examples and comparative examples.

[0049] Example 1: The embodiments of the present application provide a preparation method of a post-consumer recycled polypropylene-based composite material, and the preparation method includes: Step S111: Select PCR-PP of the rPP-W120A grade, and place the PCR-PP in dichloromethane for cleaning by ultrasonic cleaning method; Step S121: Disperse the cleaned PCR-PP in dichloromethane to form a PCR-PP solution under stirring conditions; Step S131: Mix the PCR-PP solution and epichlorohydrin according to a mass percentage of 6:1, then add boron trifluoride-ether complex which is 1% of the mass of the epoxidizing agent, react at 90 °C for 3 h, and remove the solvent by rotary evaporation after cooling to obtain epoxidized recycled polypropylene; Step S141: Mix the epoxidized recycled polypropylene and maleic anhydride according to a mass percentage of 5:1, add diisopropylbenzene peroxide which is 0.5% of the mass of maleic anhydride, react at 90 °C for 1.5 h, pour the product into excess methanol after the reaction to precipitate, filter and dry to obtain modified recycled polypropylene with dual functional groups; Step S211: Uniformly mix the modified recycled polypropylene, a crosslinking agent, a Lewis acid catalyst, surface-modified graphene, and surface-silanized mica powder in a mass ratio of 100:3:0.2:3:15 in a high-speed mixer to obtain a mixture, wherein the crosslinking agent is dioctyl adipate, the Lewis acid catalyst is Zn(OAc)2 with a concentration of 1 wt%, and the surface-modified graphene is graphene modified by chitosan-carboxyl end-capping; Step S221: Feed the mixture into a twin-screw extruder, maintain the screw rotation speed at 150 rpm, and react at 180 °C to obtain a blend with a three-dimensional guiding framework structure; Step S311: Extrude the blend through the die head of the twin-screw extruder into a strip shape, and cool and solidify it in a cooling water tank; Step S332: Remove moisture through an air dryer and prepare columnar or spherical particles through a pelletizer; Step S333: Dry the particles at 90 °C for 3 h; Step S334: Place the dried particles in a hot pressing mold, preheat the mold to 200 °C, hot press at 8 MPa for 4 min, and then demold using a cold mold setting method to obtain a post-consumer recycled polypropylene-based composite material.

[0050] Among them, the preparation method of graphene modified with chitosan-carboxyl end groups in step S211 includes the following steps: Step S201: Oxidize graphene using the Tour method to obtain graphene oxide containing carboxyl, hydroxyl, and epoxy groups; Step S202: Ultrasonic the graphene oxide in DMF / DMSO solvent for 30 min to obtain a homogeneous dispersion, and stir the chitosan in an acetic acid aqueous solution for 3 h to obtain a clear and particle-free chitosan solution; Step S203: Dropwise add the chitosan solution into the graphene oxide dispersion, mix well under magnetic stirring, and then add 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholine hydrochloride coupling agent to obtain a mixed solution, where the mass ratio of the coupling agent, graphene oxide, and chitosan is 0.5:1:3; Step S204: Control the pH value of the mixed solution at 5, and heat the mixed solution to 45 °C for amidation reaction, and maintain the reaction for 7 h to obtain a covalent grafting product of graphene oxide-chitosan; Step S205: React the covalent grafting product with maleic anhydride under a triethylamine catalyst, control the pH value of the reaction system at 6, the reaction temperature at 60 °C, and the reaction time at 5 h, so as to introduce a carboxyl end group to obtain a chitosan-carboxyl end group modified graphene precursor; Step S206: Dry the chitosan-carboxyl end group modified graphene precursor in a vacuum environment of -0.08 MPa at 50 °C for 12 h to obtain a powdery chitosan-carboxyl end group modified graphene material.

[0051] Figure 4 Shows the Fourier transform infrared spectrum of the epoxidized recycled polypropylene obtained in step S131 of the first embodiment of the present invention. From Figure 4 It can be seen that at 2950 cm-1 The stretching vibration absorption peak of the C-H bond appears at 1460 cm -1 The bending vibration absorption peak of CH2 appears at 910 cm. These two are the typical characteristic absorption peaks of the main chain structure of polypropylene. At 910 cm -1 The ring vibration absorption peak of the epoxy group appears, and the deformation vibration absorption peak of the epoxy group appears at 840 cm -1 Both are the typical characteristic peaks of the epoxy group, indicating that the epoxy group has been successfully grafted onto the molecular chain of PCR-PP.

[0052] Figure 5 Figure 11 shows the acid titration curve of the epoxidized recycled polypropylene sample obtained in step S131 of Example 1 of the present invention. After the sample was pretreated with hydrochloric acid, the epoxy group was ring-opened to form a structure containing hydroxyl groups and reacted with the excess hydrochloric acid, and then back-titrated with a standard alkali solution (such as 0.1 mol / L NaOH). From Figure 5 it can be seen that the equivalence point appears at about 5.0 mL, and the pH value rises rapidly, indicating that the remaining acid in the system has been neutralized by the alkali, indirectly indicating the presence of a titratable epoxy group structure in the sample. According to the volume and concentration of NaOH consumed in the back-titration, the molar number of epoxy groups in the sample can be calculated. Combining with the sample mass, the grafting density of epoxy groups is further calculated, and the result is about 2 wt%.

[0053] Figure 6 Figure 17 shows the Fourier transform infrared spectrum of the modified recycled polypropylene with bifunctional groups obtained in step S141 of Example 1 of the present invention. From Figure 6 it can be seen that the stretching vibration absorption peak of the C-H bond appears at 2950 cm -1 The stretching vibration absorption peak of the C-H bond appears at 1460 cm -1 The bending vibration absorption peak of CH2 appears at 1460 cm. These two are the typical characteristic absorption peaks of the main chain structure of polypropylene. At 3210 cm -1 The stretching vibration absorption peak of the O-H bond of the carboxyl group appears, and the stretching vibration absorption peak of the C=O bond of the carboxyl group appears at 1710 cm -1 Both are the typical characteristic peaks of the carboxyl group. At 910 cm -1 The ring vibration absorption peak of the epoxy group appears, and the deformation vibration absorption peak of the epoxy group appears at 840 cm -1 Both are the typical characteristic peaks of the epoxy group. This indicates that the epoxy functional group and the carboxyl functional group have been successfully grafted onto the molecular chain of PCR-PP.

[0054] Figure 7The acid-base titration curve of the modified regenerated polypropylene sample with bifunctional groups obtained in step S141 of Embodiment 1 of the present invention is shown. The standard sodium hydroxide solution is used to directly titrate the free carboxyl groups in the sample. By comparing with the blank control sample solution and combining the volume and concentration of the titrant, the content of carboxyl groups in the sample can be quantitatively analyzed. From Figure 7 it can be seen that the equivalence point appears at about 6.5 mL, and the pH value rises rapidly near this point, indicating that the titrant has completely neutralized the carboxyl functional groups in the sample, reflecting the presence of titratable acidic groups in the system. According to the concentration of NaOH of 0.06 mol / L and the sample mass of 3 g, the grafting density of the carboxyl functional group is calculated to be about 0.6 wt%.

[0055] Figure 8 The scanning electron microscope image of the blend with a three-dimensional guiding skeleton structure in Embodiment 1 of the present invention is shown. From Figure 8 it can be observed that the graphene sheets and silanized mica powder are arranged in the same direction, indicating that under the induction of melt shearing, obvious axial stacking and ordered orientation of the fillers occur. In addition, there is a clear interface between the sheets, and a porous structure is presented around it, indicating that under the topological action of the dynamic ester bond network, the polymer segments effectively guide and limit the spatial distribution of the fillers, realizing the ordered construction of the fillers. The sheet structure further exhibits multi-level spatial interpenetration and directional arrangement characteristics, indicating that the constructed skeleton network is not limited to two-dimensional planar distribution, but extends along the shearing direction to form a three-dimensional guiding skeleton network with a deep structure.

[0056] Figure 9 The Fourier transform infrared spectroscopy spectrum of the graphene material modified with chitosan-carboxyl end groups in Embodiment 1 of the present invention is shown. As Figure 9 can be seen, typical amide I band and amide II band appear at 1650 cm -1 and 1550 cm -1 , indicating that chitosan has been successfully grafted onto the surface of graphene oxide through amidation reaction. The enhancement of the carboxyl C=O absorption peak at 1720 cm -1 , as well as the C-O absorption peak in the region of 1240 cm -1 , indicates that the carboxyl end group structure introduced by maleic anhydride has been formed. In addition, the stretching vibration absorption peak of the graphene aromatic six-membered ring skeleton C=C is still retained at 1620 cm -1 , proving that the basic skeleton structure of graphene oxide has not been destroyed. The enhancement of the broad O-H / N-H peak at 3300 cm -1 reflects the enrichment of hydrophilic functional groups such as hydroxyl, amino, and carboxyl groups in the material. These characteristics jointly verify the covalent grafting of chitosan and carboxyl end group modification on the surface of graphene.

[0057] Comparative Example 1: The difference between this comparative example and Example 1 is that it does not include step S141. That is, during the functionalization modification of post-consumer recycled polypropylene, only epoxy functional groups are introduced, and carboxyl functional groups are not further introduced.

[0058] Comparative Example 2: The difference between this comparative example and Example 1 is that it does not include step S131, that is, epoxy functional groups are not introduced into post-consumer recycled polypropylene. Correspondingly, in step S141, recycled polypropylene and maleic anhydride are directly mixed at a mass ratio of 5:1, and 0.5% of diisopropylbenzene peroxide based on the mass of maleic anhydride is added as a radical initiator, and the reaction is carried out at 90 °C for 1.5 hours. After the reaction, the product is poured into excess methanol for precipitation, filtered and dried to obtain modified recycled polypropylene containing only carboxyl functional groups.

[0059] Comparative Example 3: The way of introducing functional groups in steps S131 and S141 of this comparative example is different from that in Example 1, and the hydroxyl and amino bifunctional groups are used. Step S131 of this Comparative Example 3 is to mix the PCR-PP solution and 2-hydroxyethyl methacrylate at a mass percentage of 6:1, add an appropriate amount of azobisisobutyronitrile as an initiator, and react at 90 °C for 3 hours. After cooling, the solvent is removed by rotary evaporation to obtain hydroxylated recycled polypropylene. Step S141 is to mix the above hydroxylated recycled polypropylene with a grafting reagent containing vinylamine at a mass percentage of 5:1, add 1 wt% of benzoyl peroxide, and react at 90 °C for 1.5 hours. After the reaction, the product is poured into excess methanol for precipitation, filtered and dried to obtain modified recycled polypropylene with hydroxyl and amino bifunctional groups.

[0060] Comparative Example 4: The difference between this comparative example and Example 1 is that the filler used in step S211 only includes mica powder with surface silanization treatment and does not include surface-modified graphene. Therefore, this comparative example does not perform all the operations regarding graphene modification in steps S201 to S206, and only retains the addition and dispersion process of mica powder.

[0061] Comparative Example 5: The difference between this comparative example and Example 1 is that the filler used in step S211 only includes surface-modified graphene and does not include mica powder with surface silanization treatment. The modification method of graphene is the same as that in steps S201 to S206 in Example 1.

[0062] Comparative Example 6: The difference between this comparative example and Example 1 is that surface-modified graphene is replaced with carbon black in step S211. Specifically, the graphene modification steps S201 to S206 are not carried out, but carbon black and mica powder with surface silanization treatment are directly added to the composite system at the same mass ratio.

[0063] Comparative Example VII: The difference between this comparative example and Example 1 is that in step S211, the graphene modified by chitosan-carboxyl capping is replaced by graphene oxide without chitosan grafting or carboxyl capping modification.

[0064] Comparative Example VIII: The difference between this comparative example and Example 1 is that in step S211, the crosslinking agent used is replaced by citric acid from dioctyl adipate. Other steps remain the same.

[0065] The following Table 1 shows the comparison results of the post-consumer recycled polypropylene-based composites prepared in the examples and each comparative example.

[0066]

[0067] For the mechanical, thermal and durability properties of the post-consumer recycled polypropylene-based composites, the following test methods and conditions were used for characterization and evaluation. The heat distortion temperature (HDT) test was carried out according to the ISO 75-2:2013 standard, under a load of 1.8 MPa, in a Vicat softening point tester (Vicat-50N type), with a heating rate of 120 °C / h, and the temperature at which the sample deformation reached the standard specified deformation was recorded. The coefficient of thermal expansion (CTE) test was carried out using a thermomechanical analyzer (TMA Q400, TA Instruments), with a heating rate of 5 °C / min, a test temperature range of 25 - 150 °C, and a sampling size of 10 mm × 10 mm × 2 mm.

[0068] The CTE value was calculated according to the ASTM E831-2014 standard, based on the relationship between the change in sample length and the change in temperature. The Izod impact strength was carried out according to the ISO 180:2000 standard, using a cantilever beam impact tester (Zwick / Roell HIT5.5P), with a sample size of 80 mm × 10 mm × 4 mm, a notch depth of 2 mm, and an impact speed of 3.5 m / s. The test temperature was 23 ± 2 °C, and the relative humidity was 50 ± 5%. The tensile strength was carried out according to the ISO 527-2:2012 standard, using an electronic universal material testing machine (Instron 5967), with a loading rate of 5 mm / min, the test sample was a standard dumbbell shape, with a thickness of 2 mm, a width of 10 mm, and a parallel part length of 60 mm, and the test environment was 23 ± 2 °C, 50 ± 5% RH.

[0069] The flexural strength was tested in accordance with ISO 178:2010 standard using an electronic universal material testing machine (Instron 5967) equipped with a three-point bending test fixture. The sample size was 80 mm in length, 10 mm in width, and 4 mm in thickness, and the support span was set to 64 mm (16 times the thickness). The loading rate was 2 mm / min, the environmental conditions were 23 ± 2 °C and relative humidity 50 ± 5%. The flexural load-displacement curve was recorded, and the maximum flexural stress was taken as the flexural strength (unit: MPa). Each group of samples was tested with no less than 5 pieces, and the results were averaged and the standard deviation was given.

[0070] The storage modulus E’ was obtained by dynamic mechanical property (DMA) testing using a DMA Q800 dynamic mechanical analyzer in accordance with ISO 6721-1:2019 standard. The sample size was 35 mm in length, 10 mm in width, and 2 mm in thickness. The single cantilever beam mode was used for the test, the frequency was 1 Hz, the heating rate was 3 °C / min, and the temperature range was -50 °C to 150 °C. The storage modulus E’, loss modulus E’’, and tanδ curves were measured, and the glass transition temperature (Tg) corresponding to the peak position of tanδ was recorded.

[0071] As can be seen from Table 1, in Comparative Example 1, only epoxy functional groups were introduced, lacking carboxyl functional groups to construct a dynamic network, resulting in a decrease in HDT to 110 °C, an increase in CTE to 7.8×10 -5 / °C, and a decrease in Izod impact strength to 35.2 kJ / m 2 , a decrease in tensile strength to 27.4 MPa, a decrease in flexural strength to 74.5, a decrease in E’(25 °C) to 1100 MPa, the comprehensive performance deteriorated, and both dimensional stability and mechanical strength decreased significantly. In Comparative Example 2, only carboxyl functional groups were introduced, lacking epoxy groups to provide initial interfacial bonding, resulting in insufficient interfacial bonding force, a decrease in HDT to 112 °C, an increase in CTE to 7.5×10 -5 / °C, and decreases in Izod impact strength, tensile strength, flexural strength, and E’(25 °C). The overall mechanical properties and thermal stability were inferior to those of Example 1. In Comparative Example 3, hydroxy and amino hetero-bifunctional groups were used for modification. Although it had certain reactivity, the efficiency of constructing the dynamic network was low, resulting in a decrease in HDT to 115 °C and an increase in CTE to 7.0×10 -5 / °C, and decreases in Izod impact strength, tensile strength, flexural strength, and E’(25 °C). Although the performance was better than that of Comparative Example 1 and Comparative Example 2 with single functional groups, it was still lower than that of the bifunctional group system of this application. In Comparative Example 4, only silanized mica powder was used, lacking flexible lamellar guiding fillers, with a loose framework, an HDT of only 113 °C, and an impact strength of 36.0 kJ / m 2, the storage modulus is 1200 MPa, indicating that the rigid filler alone cannot effectively construct a dense guiding skeleton. In Comparative Example Five, only graphene modified by chitosan was used, lacking the spatial support of mica powder, resulting in insufficient overall rigidity, with an HDT of 117 °C and an impact strength of 39.2 kJ / m 2 , the storage modulus is 1300 MPa, and the skeleton stability is still inferior to that of the flexible-rigid synergistic system. In Comparative Example Six, the flexible lamellar graphene was replaced with carbon black. Carbon black has no lamellar guiding structure and serious agglomeration, resulting in a decrease in HDT to 108 °C and an increase in CTE to 8.1×10 -5 / °C, and the impact strength is only 30.5 kJ / m 2 , the storage modulus drops to 950 MPa, and the mechanical and thermal properties deteriorate significantly. In Comparative Example Seven, unmodified graphene oxide was used, lacking chitosan grafting and carboxyl capping modification, resulting in insufficient interfacial bonding force. Although the storage modulus was increased to 1350 MPa, the overall heat distortion temperature and impact strength were lower than those of Example 1, indicating that the modification process plays an important role in improving interfacial compatibility and skeleton stability. In Comparative Example Eight, the crosslinking agent was replaced from dioctyl adipate to citric acid. Due to the low crosslinking reaction efficiency and insufficient dynamic network stability, the HDT dropped to 120 °C and the impact strength decreased to 42.0 kJ / m 2 , the storage modulus drops to 1450 MPa, indicating that the aliphatic polyacid ester crosslinking agent has more advantages in realizing the construction of a dynamic ester bond network.

[0072] The above comparative data fully prove that through the reasonable design of the bifunctional modification system, flexible-rigid synergistic filler structure and dynamic ester bond network, the present invention has successfully constructed a post-consumer recycled polypropylene-based composite material system with high dimensional stability, high strength and toughness, and excellent thermodynamic properties, and its performance is superior to various comparative examples.

Claims

1. A preparation method of a post-consumer recycled polypropylene-based composite material, characterized in that, It includes the following steps: Graft-modify the post-consumer recycled polypropylene to introduce epoxy functional groups and carboxyl functional groups to obtain modified recycled polypropylene with dual functional groups; Melt-blend the modified recycled polypropylene, cross-linking agent, Lewis acid catalyst and filler, and under heating conditions, enable the modified recycled polypropylene to construct a dynamic ester bond network with reversible cross-linking ability through transesterification reaction. The dynamic ester bond network induces the filler to be oriented under the condition of melt shearing, thereby synergistically forming a blend with a three-dimensional guiding skeleton structure. The filler includes surface-modified graphene and surface-silanized mica powder. The cross-linking agent is an aliphatic polyacid ester, and the surface-modified graphene is graphene modified by chitosan-carboxyl capping; Granulate the blend and then perform hot pressing treatment to obtain the post-consumer recycled polypropylene-based composite material.

2. The preparation method according to claim 1, wherein, The mass ratio of the modified recycled polypropylene, cross-linking agent, Lewis acid catalyst, surface-modified graphene and surface-silanized mica powder is 100:(1-5):(0.1-0.5):(2-5):(13-20).

3. The preparation method according to claim 2, characterized in that, The Lewis acid catalyst is selected from Zn(OAc)2, SnCl2 or AlCl3. The concentrations of Zn(OAc)2 and SnCl2 are both 0.5wt%-2wt%, and the concentration of AlCl3 is 1.5wt%-3wt%.

4. The preparation method according to claim 3, characterized in that, The reaction temperature of the transesterification reaction is 170°C-200°C, and the reaction time is 0.5h-2h.

5. The preparation method according to claim 4, characterized in that, The preparation method of the graphene modified by chitosan-carboxyl capping includes the following steps: Oxidize the graphene to obtain graphene oxide containing carboxyl, hydroxyl and epoxy groups; Mix the graphene oxide and chitosan, and carry out amidation reaction in the presence of a coupling agent to obtain a covalent grafting product of graphene oxide-chitosan; React the covalent grafting product with maleic anhydride or monofumarate ester under catalytic conditions to introduce a carboxyl capping group to obtain a chitosan-carboxyl capping modified graphene precursor; Dry the chitosan-carboxyl capping modified graphene precursor to obtain powdered chitosan-carboxyl capping modified graphene.

6. The preparation method according to claim 5, characterized in that, In the step of mixing the graphene oxide and chitosan and carrying out amidation reaction in the presence of a coupling agent to obtain a covalent grafting product of graphene oxide-chitosan, the mixed solvent for mixing the graphene oxide and chitosan is water or a polar mixed solvent, and the mass ratio of the graphene oxide and chitosan is 1:(1-5); [[ID= ​ 7. The preparation method according to claim 6, characterized in that In the step of reacting the covalent grafting product with maleic anhydride or monoesters of fumaric acid under catalytic conditions to introduce a carboxyl end group and obtain a graphene precursor modified with chitosan-carboxyl end groups, the catalytic conditions include using pyridine, triethylamine or 4-dimethylaminopyridine as a catalyst or promoter, the reaction is carried out in a polar organic solvent or a mixed system thereof with water, the pH value is 6-8, the temperature is 50°C-90°C, and the reaction time is 4h-10h.

8. The preparation method according to any one of claims 1-7, characterized in that, In the melt blending process, a benzotriazole ultraviolet absorber and a block structure antioxidant are further added, and the block structure antioxidant is formed by block copolymerization or graft modification of a capped receptor structure antioxidant with a polypropylene chain segment; The mass ratio of the modified recycled polypropylene, the benzotriazole ultraviolet absorber and the block structure antioxidant is 100:(0.1-3):(0.5-5).

9. A plastic part for a vehicle, characterized in that, The material of the vehicle plastic part includes virgin polypropylene and a post-consumer recycled polypropylene-based composite material obtained by the preparation method according to any one of claims 1-8.

10. The plastic part for a vehicle according to claim 9, characterized in that, The mass ratio of the virgin polypropylene and the post-consumer recycled polypropylene-based composite material is 100:(20-90).

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