Wood plastic material for outdoor display stands

Abstract

The present application relates to the field of wood-plastic materials, and particularly relates to a wood-plastic material for outdoor exhibition stands and a preparation method thereof. The wood-plastic material is made of wood fibers, polyethylene, a synergist and a lubricant. The synergist is nanosized bifunctional coupled lignin, which is prepared by the following steps: grafting maleic anhydride to lignin after purification and activation, then covalently grafting an ultraviolet inhibitor and an antioxidant in steps, and finally obtaining nanoparticles through nanometer precipitation. The synergist has the effects of interfacial compatibility of crosslinking wood fibers and polyethylene, ultraviolet light absorption and antioxidation. The tensile strength of the obtained wood-plastic material reaches 43.5 MPa, the color difference ΔE after xenon lamp aging is only 1.3, and the tensile strength retention rate is 98.5%. The present application solves the problems of easy migration of small molecule additives and poor dispersibility of lignin through chemical bonding and nanosizing synergy, realizes the unity of long-acting anti-aging and high mechanical properties, and has a wide application prospect in the field of outdoor exhibition stands.

Description

Technical Field

[0001] This invention relates to the field of wood-plastic composite materials, specifically to a wood-plastic composite material for outdoor exhibition racks and its preparation method. Background Technology

[0002] Wood-plastic composites (WPC) are a new type of environmentally friendly material made from biomass fibers (such as wood fiber and bamboo fiber) and thermoplastic plastics (such as polyethylene and polypropylene). WPC combines the texture of wood with the processing properties of plastics, and has advantages such as moisture resistance, insect resistance, and recyclability. It has been widely used in outdoor building materials, landscaping, and interior decoration. With the increasing global demand for sustainable green materials, the WPC industry has developed rapidly in recent years.

[0003] Existing wood-plastic composites suffer from interfacial compatibility issues in practical applications: wood fiber surfaces are rich in polar hydroxyl groups, while the polyethylene and other plastic matrices are non-polar hydrophobic materials. This polarity difference leads to uneven dispersion of wood fibers and low interfacial bonding strength, resulting in decreased mechanical properties and increased water absorption of the composite material. Current technologies use maleic anhydride-grafted polyolefins as compatibilizers, but this cannot solve the anti-aging problem, requiring the addition of various functional additives, leading to complex formulations and increased costs. Furthermore, lignin, a major byproduct of the papermaking industry, has a huge annual output, but is currently mostly processed as low-value fuel, resulting in low utilization rates for high-value applications. Although lignin contains phenolic hydroxyl groups, alcoholic hydroxyl groups, and aromatic rings, possessing UV absorption, free radical scavenging capabilities, and interfacial coupling potential, unmodified lignin exhibits poor dispersibility and is prone to aggregation in polymer matrices. Its intrinsic UV resistance and antioxidant capacity are limited, making it difficult to meet the requirements for long-term outdoor use. Moreover, existing lignin-based wood-plastic materials have limited functionality. Meanwhile, wood-plastic composites are prone to degradation when exposed to ultraviolet radiation and thermo-oxidative aging outdoors. Existing technologies use physical blending to add small-molecule UV stabilizers and antioxidants, but these additives lack chemical bonds with the polymer matrix and are prone to migration, precipitation, and volatilization during long-term use, leading to a decline in anti-aging performance and a shortened service life. In summary, there is currently no multifunctional integrated solution that can simultaneously address multiple issues such as interfacial compatibility, lignin dispersion, and long-term anti-aging effects.

[0004] In summary, existing wood-plastic composite technologies face multiple challenges, including interfacial compatibility, lignin dispersion, and long-term anti-aging effects. Currently, no single technology can simultaneously address all these issues. Some studies have attempted simple compounding of lignin with UV stabilizers or antioxidants, but this remains a physical mixing process and fails to achieve functional integration at the molecular level. Furthermore, the poor dispersibility of lignin itself persists. Therefore, developing a lignin-based multifunctional material that integrates interfacial coupling, UV absorption, and antioxidant functions with long-term stability, and applying it to wood-plastic composites, is of significant practical importance for improving the overall performance of wood-plastic composites, extending their outdoor lifespan, and realizing the high-value utilization of lignin. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention aims to provide a wood-plastic composite material for outdoor exhibition racks and its preparation method. This wood-plastic composite material has excellent interfacial bonding strength, long-lasting UV resistance and antioxidant properties, and can solve the problems of poor interfacial compatibility between wood fiber and polyethylene matrix, poor dispersion and single function of lignin in polymer matrix, and insufficient long-lasting anti-aging performance due to easy migration of small molecule UV stabilizers and antioxidants in existing wood-plastic composite materials.

[0006] One of the objectives of this invention is to provide a wood-plastic composite material for outdoor exhibition racks, comprising the following components by weight: 40-60 parts wood fiber powder, 30-50 parts polyethylene, 3-10 parts synergist, and 1-3 parts lubricant.

[0007] Furthermore, the preparation method of the synergist includes the following steps: (1) Activated lignin is obtained by reacting lignin, pyridine and epichlorohydrin at 50-70°C for 3-6 hours. (2) Under an inert atmosphere, the activated lignin, maleic anhydride and benzoyl peroxide described in step (1) are added to tetrahydrofuran and reacted at 60-70°C for 4-8 hours. After treatment, maleic anhydride grafted lignin is obtained. (3) Under an inert atmosphere, the maleic anhydride grafted lignin described in step (2) is added to anhydrous tetrahydrofuran, and an anti-ultraviolet agent and 4-dimethylaminopyridine are added. The mixture is reacted at 70-90°C for 6-10 hours to obtain anti-ultraviolet lignin. (4) The UV-resistant lignin, antioxidant, N,N'-dicyclohexylcarbodiimide and 4-dimethylaminopyridine described in step (3) are added to anhydrous tetrahydrofuran and reacted at 30~60℃ for 12~24h. After treatment, bifunctional coupled lignin is obtained. (5) Dissolve the bifunctional coupled lignin described in step (4) in tetrahydrofuran to prepare a 10-100 mg / mL bifunctional coupled lignin solution; add the bifunctional coupled lignin solution dropwise to 0.05wt%-0.5wt% Tween-80 deionized water, stir at room temperature for 15-60 min, remove tetrahydrofuran by rotary evaporation, and freeze-dry the nanoparticle dispersion to obtain the synergist.

[0008] Further, in step (1), the mass ratio of lignin, pyridine and epichlorohydrin is 1: (0.01~0.05):(0.1~0.3), the reaction temperature is 50~70℃, and the reaction time is 3~5 hours.

[0009] Further, the mass ratio of activated lignin, maleic anhydride and benzoyl peroxide in step (2) is 1:(0.5-2.0):(0.02-0.04).

[0010] Further, in step (3), the mass ratio of maleic anhydride grafted lignin, UV stabilizer, and 4-dimethylaminopyridine is 1: (0.2-0.6): (0.01-0.03); the UV stabilizer is at least one of 2-(2'-hydroxy-5'-methylphenyl)-benzotriazole (UV-P), 2-(2'-hydroxy-5'-tert-octylphenyl)-benzotriazole (UV-329), or 2,2'-methylenebis[6-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol] (UV-360).

[0011] Further, in step (4), the mass ratio of the UV-resistant lignin, antioxidant, N,N'-dicyclohexylcarbodiimide and 4-dimethylaminopyridine is 1: (0.2~0.5): (0.3~0.8): (0.01~0.03); the antioxidant is at least one of gallic acid, 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid or caffeic acid.

[0012] Further, the mass ratio of bifunctional coupled lignin to Tween in step (5) is 200~2000:1.

[0013] The second objective of this invention is to provide a method for preparing wood-plastic composite materials for outdoor exhibition racks, the preparation process including the following steps: S1. The wood fiber is pulverized to 80-200 mesh and dried until the moisture content is less than 2% to obtain wood cellulose powder; S2 Weigh out the wood fiber powder, polyethylene, synergist and lubricant according to the specified proportions, mix the components evenly, and then melt-blend and extrude at 150~230°C to obtain wood-plastic masterbatch; S3 The wood-plastic masterbatch described in step S2 is hot-pressed to obtain the wood-plastic material for the outdoor exhibition rack.

[0014] Compared with the prior art, the present invention has the following beneficial effects: This invention involves epoxy-activating lignin to increase its reactive sites, and then introducing maleic anhydride into the lignin framework via free radical grafting to form maleic anhydride-grafted lignin. This grafted product contains abundant anhydride groups, which can undergo esterification with the hydroxyl groups on the wood fiber surface to form strong chemical bonds. Simultaneously, the remaining phenolic hydroxyl groups and long-chain structures in the lignin molecule can undergo strong physical entanglement and partial chemical bonding with the polyethylene matrix, thus constructing a dual interfacial bonding mode of "chemical bridging + physical anchoring" between the wood fiber and polyethylene. Furthermore, this invention covalently grafts UV stabilizers and antioxidants onto the same lignin macromolecular framework via esterification, forming bifunctional coupled lignin. This chemical bonding method immobilizes the UV stabilizers and antioxidants on the lignin, completely avoiding the problem of anti-aging performance degradation caused by physical migration, precipitation, and volatilization of traditional small-molecule additives during long-term use. Compared to the complex physical blending schemes in existing technologies that involve adding compatibilizers, UV protectants, and antioxidants separately, this invention achieves "one agent, multiple functions" molecular integration, which not only simplifies the formulation and process, but also ensures long-term stability of the UV protection and antioxidant functions, maintaining excellent performance even after long-term outdoor exposure.

[0015] This invention relates to nanoparticles prepared from bifunctional coupled lignin via a nanoprecipitation method. These nanoparticles possess a large specific surface area and high surface activity, enabling them to be uniformly dispersed within a polyethylene matrix during melt blending, avoiding the agglomeration phenomenon commonly seen in micron-sized lignin. More importantly, these nanoparticles tend to accumulate at the interface between wood fibers and polyethylene, forming a "nano-bridging" structure: one end of the nanoparticle is tightly bound to the wood fiber via ester bonds, while the other end forms a continuous phase with the polyethylene matrix through chain entanglement. This nanoscale interface reinforcement mechanism makes stress transfer between wood fibers and polyethylene more efficient, effectively inhibiting fiber pull-out and interface debonding under external forces, thereby significantly improving tensile strength, flexural strength, and impact toughness. Furthermore, the nanoparticles themselves possess a certain degree of rigidity and can be used as reinforcing fillers to improve the modulus of the composite material. Simultaneously, due to the hydrophobic properties of lignin, the uniform dispersion after nano-sizing effectively reduces the water absorption and thickness expansion rate of the composite material, improving dimensional stability. Compared with the existing technology that directly uses micron-sized lignin as a filler, the nano-sized bifunctional synergist of the present invention can achieve better mechanical reinforcement and lower water absorption rate at the same addition amount.

[0016] The benzotriazole UV stabilizer grafted with the synergist of this invention can efficiently absorb high-energy ultraviolet light in the 280–400 nm wavelength band and harmlessly release the energy as heat through intramolecular proton transfer, thereby preventing photodegradation of wood fibers and breakage of polyethylene chains caused by ultraviolet light. On the other hand, the hindered phenolic antioxidant grafted with the synergist can capture reactive species such as peroxy radicals and alkoxy radicals generated during photo-oxidation and thermo-oxidative aging, terminating free radical chain reactions and delaying the aging process of materials. More importantly, the lignin framework itself also has a certain UV shielding ability and free radical quenching ability. The three work together to form a multi-layered protection network of "absorption-conversion-quenching", and its synergistic effect is far greater than the simple sum of single components. In addition, since the UV stabilizer and antioxidant are covalently fixed on the lignin nanoparticles, and the lignin nanoparticles are firmly anchored in the wood fiber-polyethylene interface region through chemical bonds and physical entanglement, the anti-aging function acts precisely on the most protected location (i.e., the interface region), achieving "precise protection". After prolonged outdoor use, the surface of the wood-plastic composite material of this invention is not prone to fading or powdering, maintains high mechanical properties, has a significantly extended service life, and greatly reduces maintenance costs. Therefore, this invention is particularly suitable for manufacturing products such as outdoor exhibition racks, garden fences, and building exterior wall panels that are exposed to the natural environment for extended periods, and has broad application prospects and market value. Attached Figure Description

[0017] Figure 1 Scanning electron microscope image of the synergist. Detailed Implementation

[0018] The present invention will be further described in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments. Specific conditions not specified in the embodiments are based on conventional conditions or product instructions. Unless otherwise specified, all reagents or instruments used are conventional products obtained through commercial channels.

[0019] Example 1 A wood-plastic composite material for outdoor exhibition racks comprises the following parts by weight: 50 parts poplar fiber powder, 40 parts high-density polyethylene, 6.5 parts synergist, and 2 parts stearic acid; its specific preparation method is as follows: S1. Poplar fiber is pulverized to 120 mesh and dried at 105℃ until the moisture content is less than 2% to obtain poplar fiber powder. S2 places all components into a high-speed mixer and mixes them at 1200 rpm for 8 minutes. The mixture is then fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1. The temperatures for each zone are set as follows: feeding zone 160℃, melting zone 185℃, homogenization zone 200℃, and die head 210℃. The screw speed is 25 rpm. After extrusion, stretching, cooling, and pelletizing, the pellets are dried at 80℃ for 4 hours to obtain wood-plastic masterbatch. S3 takes wood-plastic masterbatch and lays it into a flat hot press mold. It is pre-pressed at 190℃ and 10MPa for 4 minutes, then pressurized to 18MPa and held for 4 minutes. After holding the pressure and cooling to below 40℃, it is demolded to obtain the wood-plastic material for outdoor exhibition racks.

[0020] The preparation process of the synergist includes the following steps: (1) Take 100 parts of lignin, dissolve it in 1 L of 0.1 M sodium hydroxide solution, stir to dissolve and filter. Adjust the pH of the filtrate to 2.5 with dilute hydrochloric acid to precipitate the precipitate, filter, wash with deionized water until neutral, and vacuum dry at 60℃ for 24 h. Take 50 parts of the purified lignin, add 7.5 parts of epichlorohydrin and 1.5 parts of pyridine, reflux at 60℃ for 4.5 hours, cool, filter, wash and dry to obtain activated lignin.

[0021] (2) Take 40 parts of activated lignin, add 40 parts of maleic anhydride and 1.2 parts of benzoyl peroxide, dissolve in 400 mL of tetrahydrofuran, and stir for 6 hours under nitrogen protection at 65℃. After the reaction is completed, pour the mixture into 2 L of ice water to precipitate, filter, wash three times with a tetrahydrofuran / water (1:5) mixture, and dry under vacuum at 50℃ to obtain maleic anhydride grafted lignin.

[0022] (3) Take 30 parts of maleic anhydride grafted lignin, dissolve it in 300 mL of anhydrous tetrahydrofuran, add 12 parts of UV-P1 and 0.6 parts of 4-dimethylaminopyridine, react at 80℃ under nitrogen protection for 8 hours, pour the reaction solution into diethyl ether to precipitate, filter and wash to obtain UV-resistant lignin.

[0023] (4) Dissolve 20 parts of UV-resistant lignin in 200 mL of anhydrous tetrahydrofuran, add 8 parts of gallic acid, 12 parts of N,N'-dicyclohexylcarbodiimide and 0.4 parts of 4-dimethylaminopyridine, react at 40 °C for 18 hours, filter to remove dicyclohexylurea byproduct, concentrate the filtrate and pour it into diethyl ether to precipitate, filter, wash and dry to obtain bifunctional coupled lignin.

[0024] (5) Take 5 parts of bifunctional coupled lignin and dissolve them in 100 mL of tetrahydrofuran to prepare a 50 mg / mL solution. Under vigorous stirring, add the solution dropwise to 500 mL of deionized water containing 0.2% Tween-80 at a rate of 1.0 mL / min. After the addition is complete, continue stirring for 30 minutes. Remove the tetrahydrofuran by rotary evaporation to obtain an aqueous dispersion of nanoparticles. Freeze-dry the dispersion to obtain the synergist. The scanning electron microscopy results of the synergist are shown below. Figure 1 As shown.

[0025] Example 2 A wood-plastic composite material for outdoor exhibition racks comprises the following components by weight: 40 parts bamboo fiber powder, 30 parts low-density polyethylene, 3 parts synergist, and 1 part paraffin wax; its specific preparation method is as follows: S1. Bamboo fiber is pulverized to 80 mesh and dried until the moisture content is less than 2% to obtain bamboo fiber powder. S2 mixes all components evenly in a high-speed mixer and then feeds them into a co-rotating twin-screw extruder. The extruder's temperature settings are as follows: feeding zone 150℃, melting zone 170℃, homogenization zone 180℃, and die head 190℃; the screw speed is 20 rpm. After extrusion, stretching, cooling, and pelletizing, the pellets are dried at 80℃ for 4 hours to obtain wood-plastic masterbatch. S3 takes wood-plastic masterbatch and lays it into a flat hot press mold. It is pre-pressed at 190℃ and 10MPa for 4 minutes, then the pressure is increased to 18MPa and held for 4 minutes. After holding the pressure and cooling to below 40℃, it is demolded to obtain wood-plastic composite board.

[0026] The preparation process of the synergist includes the following steps: (1) Take 100 parts of lignin, dissolve it in 1 L of 0.08 M sodium hydroxide solution, stir to dissolve and filter. Adjust the pH of the filtrate to 2.0 with dilute hydrochloric acid to precipitate the precipitate, filter, wash with deionized water until neutral, and dry under vacuum at 60℃ for 24 h. Take 50 parts of the purified lignin, add 5 parts of epichlorohydrin and 0.5 parts of pyridine, reflux at 50℃ for 6 hours, cool, filter, wash and dry to obtain activated lignin.

[0027] (2) Take 40 parts of activated lignin, add 20 parts of maleic anhydride and 0.8 parts of benzoyl peroxide, dissolve in 400 mL of tetrahydrofuran, and stir for 8 hours under nitrogen protection at 60℃. After the reaction is completed, pour the mixture into 2 L of ice water to precipitate, filter, wash three times with a tetrahydrofuran / water (1:5) mixture, and dry under vacuum at 50℃ to obtain maleic anhydride grafted lignin.

[0028] (3) Take 30 parts of maleic anhydride grafted lignin, dissolve it in 300 mL of anhydrous tetrahydrofuran, add 6 parts of UV-329 and 0.3 parts of 4-dimethylaminopyridine, react at 70℃ under nitrogen protection for 10 hours, pour the reaction solution into diethyl ether to precipitate, filter and wash to obtain UV-resistant lignin.

[0029] (4) Dissolve 20 parts of UV-resistant lignin in 200 mL of anhydrous tetrahydrofuran, add 4 parts of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid, 6 parts of N,N'-dicyclohexylcarbodiimide and 0.2 parts of 4-dimethylaminopyridine, react at room temperature for 24 hours, filter to remove dicyclohexylurea byproduct, concentrate the filtrate and pour it into diethyl ether to precipitate, filter, wash and dry to obtain bifunctional coupled lignin.

[0030] (5) Take 5 parts of bifunctional coupled lignin and dissolve them in 500 mL of tetrahydrofuran to prepare a 10 mg / mL solution. Under vigorous stirring, add the solution dropwise to 500 mL of deionized water containing 0.05% Tween-80 at a rate of 0.2 mL / min. After the addition is complete, continue stirring for 60 minutes. Remove the tetrahydrofuran by rotary evaporation to obtain an aqueous dispersion of nanoparticles. After freeze-drying, obtain the synergist.

[0031] Example 3 A wood-plastic composite material for outdoor exhibition racks comprises the following parts by weight: 60 parts coconut shell fiber powder, 50 parts linear low-density polyethylene, 10 parts synergist, and 3 parts polyethylene wax; its specific preparation method is as follows: S1. Coconut shell fiber is crushed to 200 mesh and dried until the moisture content is less than 2% to obtain coconut shell fiber powder. S2 mixes all components evenly in a high-speed mixer and then feeds them into a co-rotating twin-screw extruder. The extruder's temperature settings are as follows: feeding zone 180℃, melting zone 210℃, homogenization zone 220℃, and die head 230℃; the screw speed is 50 rpm. After extrusion, stretching, cooling, and pelletizing, the pellets are dried at 80℃ for 4 hours to obtain wood-plastic masterbatch. S3 takes wood-plastic masterbatch and lays it into a flat hot press mold. It is pre-pressed at 190℃ and 10MPa for 4 minutes, then the pressure is increased to 18MPa and held for 4 minutes. After holding the pressure and cooling to below 40℃, it is demolded to obtain wood-plastic composite board.

[0032] The preparation process of the synergist includes the following steps: (1) Take 100 parts of lignin, dissolve it in 1 L of 0.18 M sodium hydroxide solution, stir to dissolve and filter. Adjust the pH of the filtrate to 3.0 with dilute hydrochloric acid to precipitate the precipitate, filter, wash with deionized water until neutral, and vacuum dry at 60℃ for 24 h. Take 50 parts of the purified lignin, add 15 parts of epichlorohydrin and 2.5 parts of pyridine, reflux at 70℃ for 3 hours, cool, filter, wash and dry to obtain activated lignin.

[0033] (2) Take 40 parts of activated lignin, add 80 parts of maleic anhydride and 1.5 parts of benzoyl peroxide, dissolve in 400 mL of tetrahydrofuran, and stir for 4 hours under nitrogen protection at 70℃. After the reaction is completed, pour the mixture into 2 L of ice water to precipitate, filter, wash three times with a tetrahydrofuran / water (1:5) mixture, and dry under vacuum at 50℃ to obtain maleic anhydride grafted lignin.

[0034] (3) Take 30 parts of maleic anhydride grafted lignin, dissolve it in 300 mL of anhydrous tetrahydrofuran, add 18 parts of UV-360 anti-ultraviolet agent and 0.9 parts of 4-dimethylaminopyridine, react at 90℃ under nitrogen protection for 6 hours, pour the reaction solution into diethyl ether to precipitate, filter and wash to obtain UV-resistant lignin.

[0035] (4) Dissolve 20 parts of UV-resistant lignin in 200 mL of anhydrous tetrahydrofuran, add 10 parts of caffeic acid, 16 parts of N,N'-dicyclohexylcarbodiimide and 0.6 parts of 4-dimethylaminopyridine, react at 60 °C for 12 hours, filter to remove dicyclohexylurea byproduct, concentrate the filtrate and pour it into diethyl ether to precipitate, filter, wash and dry to obtain bifunctional coupled lignin.

[0036] (5) Take 5 parts of bifunctional coupled lignin and dissolve them in 50 mL of tetrahydrofuran to prepare a 100 mg / mL solution. Under vigorous stirring, add the solution dropwise to 500 mL of deionized water containing 0.5% Tween-80 at a rate of 2.0 mL / min. After the addition is complete, continue stirring for 15 minutes. Remove the tetrahydrofuran by rotary evaporation to obtain an aqueous dispersion of nanoparticles. Freeze-dry the dispersion to obtain the synergist.

[0037] Comparative Example 1 A wood-plastic composite material, the specific preparation process of which is as follows: Take 50 parts poplar fiber, 40 parts high-density polyethylene, 6.5 parts of the above-mentioned synergist, and 2 parts stearic acid. Also, add 0.5 parts of physical mixing UV-P. Place all components in a high-speed mixer and mix at 1200 rpm for 8 minutes. Extruder temperature settings: feeding zone 180℃, melting zone 210℃, homogenization zone 220℃, die head 230℃, screw speed 50 rpm. After extrusion, drawing, cooling, and pelletizing, dry the pellets at 80℃ for 4 hours. Spread the dried pellets into a flat hot press mold, pre-press at 190℃ and 10 MPa for 4 minutes, then increase the pressure to 18 MPa and hold for 4 minutes. After holding the pressure and cooling to below 40℃, demold to obtain wood-plastic composite board.

[0038] The preparation process of the synergist differs from that of Example 1 in step (3). The specific preparation process of step (3) is as follows: Take 30 parts of maleic anhydride grafted lignin, dissolve it in 300 mL of anhydrous tetrahydrofuran, add 12 parts of gallic acid, 18 parts of N,N'-dicyclohexylcarbodiimide and 0.6 parts of 4-dimethylaminopyridine, react at 40°C for 18 hours, filter, precipitate, wash and dry to obtain monofunctional coupled lignin containing only antioxidant side chains; Comparative Example 2 A wood-plastic composite material, the specific preparation process of which is as follows: Take 50 parts of poplar fiber powder, 40 parts of high-density polyethylene, 6.5 parts of the above-mentioned synergist, 2 parts of stearic acid, and 0.5 parts of gallic acid, and put them into a high-speed mixer. Mix at 1200 rpm for 8 minutes. Extruder temperature settings: feeding zone 180℃, melting zone 210℃, homogenization zone 220℃, die head 230℃, screw speed 50 rpm. After extrusion, drawing, cooling, and pelletizing, the pellets are dried at 80℃ for 4 hours. The dried pellets are spread into a flat hot press mold and pre-pressed at 190℃ and 10 MPa for 4 minutes, then the pressure is increased to 18 MPa and held for 4 minutes. After holding the pressure and cooling to below 40℃, the product is demolded to obtain wood-plastic composite board.

[0039] The preparation process of the synergist differs from that of Example 1 in step (3). The specific preparation process of step (3) is as follows: Take 30 parts of maleic anhydride grafted lignin, dissolve it in 300 mL of anhydrous tetrahydrofuran, add 12 parts of UV-P anti-ultraviolet agent and 0.6 parts of 4-dimethylaminopyridine, react at 80°C under nitrogen protection for 8 hours, and after precipitation and washing, obtain monofunctional coupled lignin containing only the side chain of the UV-P anti-ultraviolet agent. Comparative Example 3 Take 50 parts of poplar fiber powder (crushed to 120 mesh and dried at 105℃ until the moisture content is less than 2%), 40 parts of high-density polyethylene, 2 parts of stearic acid, 6.5 parts of lignin, 0.5 parts of UV-P, and 0.5 parts of gallic acid, and put them into a high-speed mixer and mix at 1200 rpm for 8 minutes. Feed the mixture into a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1. The temperatures of each section are set as follows: feeding zone 160℃, melting zone 185℃, homogenization zone 200℃, and die head 210℃, with a screw speed of 25 rpm. After extrusion, stretching, cooling, and pelletizing, the pellets are dried at 80℃ for 4 hours. The dried pellets are spread into a flat hot press mold and pre-pressed at 190℃ and 10 MPa for 4 minutes, then pressurized to 18 MPa and held for 4 minutes. After holding the pressure and cooling to below 40℃, the product is demolded to obtain wood-plastic composite board.

[0040] Performance testing The performance of the wood-plastic composite boards prepared in Examples 1-3 and Comparative Examples 1-3 was tested, and the test results are shown in Table 1. I. Mechanical Performance Testing: According to GB / T 1040.1-2025, the DXLL-5000 from Shanghai Dengjie Machinery Equipment Co., Ltd. was used. The mechanical properties of the samples were tested using an electronic tensile testing machine at an ambient temperature of 25±1℃. The results are shown in Table 1. II. Weathering Performance Test: According to GB / T3512-2014, the prepared standard specimens were placed in a Jinghong drying oven for a 30-day indoor thermo-oxidative aging test at a temperature of 100℃. The tensile strength retention rate of the material after 30 days of aging was tested. The results are shown in Table 1. III. Xenon Lamp Accelerated Aging Test: According to GB / T 16422.2-1999, xenon lamps are used for accelerated aging tests. The radiation spectrum energy distribution of xenon lamps is close to that of sunlight, and its working state is less affected by changes in external conditions. Therefore, it can be used to test the aging resistance of wood-plastic composite materials.

[0041] Table 1 Performance Test Results Test results show that the technical solution of the present invention is significantly superior to the comparative example in four key indicators: dispersion stability, interfacial bonding, mechanical properties, and high temperature resistance.

[0042] As shown in Table 1, the tensile strengths of Examples 1 and 3 were 43.5 MPa and 42.9 MPa, respectively, which were higher than those of Comparative Examples 1 and 2, and significantly higher than that of Comparative Example 3. The tensile strength of Example 2 was 40.6 MPa, slightly lower than that of Comparative Example 2 but higher than that of Comparative Examples 1 and 3. Overall, Examples 1-3 of the present invention all exhibited good mechanical properties, with Example 1 being the best. Comparative Example 3, due to the use of unmodified lignin and the physical mixing of small molecule additives, had the worst interfacial bonding and the lowest tensile strength.

[0043] The tensile strength retention rates of Examples 1-3 were significantly higher than those of Comparative Examples 1-3. Specifically, Example 1 achieved a retention rate as high as 98.5%, Example 3 97.8%, and Example 2 95.4%, indicating that the wood-plastic composite material prepared by this invention can maintain excellent mechanical properties even after long-term UV aging. The retention rates of Comparative Examples 1-3 were 80.5%, 86.4%, and 78.6%, respectively, all significantly lower than those of the Examples. In particular, Comparative Example 3 had a retention rate of only 78.6%, indicating that the physically mixed small-molecule additives and unmodified lignin could not provide long-term anti-aging protection.

[0044] After accelerated aging under a xenon lamp, the surface color differences ΔE of Examples 1-3 were 1.3, 1.8, and 2.1, respectively, all within the "very slight to slight discoloration" level. Among them, Example 1 had the lowest ΔE at 1.3, exhibiting the best color stability. In contrast, Comparative Examples 1 and 2 showed significant discoloration, while Comparative Example 3 showed severe discoloration. The comprehensive mechanical and anti-aging performance test results show that the tensile strength of Examples 1-3 of this invention is at a high level. Examples 1 and 3 are significantly higher than Comparative Example 3 (which is a physical blend of unmodified lignin), and are roughly equivalent to or slightly better than Comparative Examples 1 and 2 (which are monofunctional couplings). More importantly, the tensile strength retention rate of Examples 1-3 after aging is as high as 95.4%–98.5%, while that of Comparative Examples 1-3 is only 80.5%, 86.4%, and 78.6%, respectively.

[0045] The above results fully demonstrate that only by simultaneously grafting UV stabilizers and antioxidants onto the same lignin framework via covalent bonds and then nano-sizing them can the migration and precipitation of small molecule additives be effectively inhibited, achieving long-term synergistic protection against UV and oxidation. At the same time, the interfacial bridging effect of nanoparticles ensures excellent mechanical properties, with Example 1 showing the best overall effect.

[0046] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A wood-plastic composite material for outdoor exhibition racks, characterized in that, It contains the following components by weight: 40-60 parts wood fiber powder, 30-50 parts polyethylene, 3-10 parts synergist, and 1-3 parts lubricant.

2. The wood-plastic composite material for outdoor exhibition racks according to claim 1, characterized in that, The preparation process of the synergist includes the following steps: (1) Activated lignin is obtained by reacting lignin, pyridine and epichlorohydrin at 50-70°C for 3-6 hours. (2) Under an inert atmosphere, the activated lignin, maleic anhydride and benzoyl peroxide described in step (1) are added to tetrahydrofuran and reacted at 60-70°C for 4-8 hours. After treatment, maleic anhydride grafted lignin is obtained. (3) Under an inert atmosphere, the maleic anhydride grafted lignin described in step (2) is added to anhydrous tetrahydrofuran, and an anti-ultraviolet agent and 4-dimethylaminopyridine are added. The mixture is reacted at 70-90°C for 6-10 hours to obtain anti-ultraviolet lignin. (4) The UV-resistant lignin, antioxidant, N,N'-dicyclohexylcarbodiimide and 4-dimethylaminopyridine described in step (3) are added to anhydrous tetrahydrofuran and reacted at 30~60℃ for 12~24h. After treatment, bifunctional coupled lignin is obtained. (5) Dissolve the bifunctional coupled lignin described in step (4) in tetrahydrofuran to prepare a 10-100 mg / mL bifunctional coupled lignin solution; add the bifunctional coupled lignin solution dropwise to 0.05wt%-0.5wt% Tween-80 deionized water, stir at room temperature for 15-60 min, remove tetrahydrofuran by rotary evaporation, and freeze-dry the nanoparticle dispersion to obtain the synergist.

3. The wood-plastic composite material for outdoor exhibition racks according to claim 1, characterized in that, The wood fiber powder is selected from at least one of poplar fiber powder, bamboo fiber powder, or coconut shell fiber powder; the polyethylene is selected from at least one of high-density polyethylene, low-density polyethylene, or linear low-density polyethylene; and the lubricant is selected from at least one of stearic acid, paraffin wax, or polyethylene wax.

4. The wood-plastic composite material for outdoor exhibition racks according to claim 2, characterized in that, The mass ratio of lignin, pyridine and epichlorohydrin in step (1) is 1: (0.01~0.05): (0.1~0.3).

5. The wood-plastic composite material for outdoor exhibition racks according to claim 2, characterized in that, The mass ratio of activated lignin, maleic anhydride and benzoyl peroxide in step (2) is 1:(0.5-2.0):(0.02-0.04).

6. The wood-plastic composite material for outdoor exhibition racks according to claim 2, characterized in that, In step (3), the mass ratio of maleic anhydride grafted lignin, UV stabilizer, and 4-dimethylaminopyridine is 1: (0.2-0.6): (0.01-0.03); the UV stabilizer is at least one of 2-(2'-hydroxy-5'-methylphenyl)-benzotriazole (UV-P), 2-(2'-hydroxy-5'-tert-octylphenyl)-benzotriazole (UV-329), or 2,2'-methylenebis[6-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol] (UV-360).

7. The wood-plastic composite material for outdoor exhibition racks according to claim 2, characterized in that, The mass ratio of the UV-resistant lignin, antioxidant, N,N'-dicyclohexylcarbodiimide and 4-dimethylaminopyridine in step (4) is 1: (0.2~0.5): (0.3~0.8): (0.01~0.03); the antioxidant is at least one of gallic acid, 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid or caffeic acid.

8. The wood-plastic composite material for outdoor exhibition racks according to claim 2, characterized in that, The mass ratio of bifunctional coupled lignin to Tween in step (5) is 200~2000:

1.

9. A method for preparing a wood-plastic composite material for outdoor exhibition racks according to any one of claims 1 to 8, characterized in that, The preparation process includes the following steps: S1. The wood fiber is pulverized to 80-200 mesh and dried until the moisture content is less than 2% to obtain wood cellulose powder; S2 Weigh out the wood fiber powder, polyethylene, synergist and lubricant according to the specified proportions, mix the components evenly, and then melt-blend and extrude at 150~230°C to obtain wood-plastic masterbatch; S3 The wood-plastic masterbatch described in step S2 is hot-pressed to obtain the wood-plastic material for the outdoor exhibition rack.

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