Density gradient wood-plastic composite material and preparation method thereof

Abstract

The invention provides a density gradient wood-plastic composite material and a preparation method thereof, and belongs to the technical field of environment-friendly composite materials. According to the method, wood cell walls are damaged through ultrasonic pretreatment to form a microporous structure, the growth rate of ice crystals is regulated and controlled in combination with a gradient freezing process, and accurate control over pore distribution and the material density gradient is achieved; meanwhile, a dynamic gradient extrusion technology is adopted, and through multi-hopper feeding and segmented temperature control, gradient distribution that the content of wood flour is linearly and progressively decreased in the extrusion direction is formed. According to the method, the pore structure is optimized through ultrasonic and freezing processes, and the combination of the wood flour and the plastic matrix is enhanced through interface modification of the silane coupling agent and the nano clay, so that the wood-plastic composite material with high bending strength, stable gradient distribution and excellent weather resistance is finally obtained. Compared with a traditional process, the interface shear strength and the mechanical property are remarkably improved, meanwhile, controllable pore distribution and function partition design are achieved, and the method is suitable for the fields of building decoration, outdoor engineering and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of environmentally friendly composite materials, and particularly relates to a density gradient wood-plastic composite material and a preparation method thereof. Background Art

[0002] Wood-plastic composites (WPCs) are environmentally friendly building materials that combine the natural texture of wood with the durability of plastic by combining wood fibers with thermoplastics. They are widely used in architectural decoration, outdoor furniture, and other fields. However, with the diversification of application requirements, the performance limitations of traditional homogeneous WPC structures have gradually emerged. These include weak interfacial bonding between wood and plastic, resulting in insufficient strength, uneven pore distribution affecting weather resistance, and a single-density structure that is unable to meet functional zoning requirements. Therefore, the development of new wood-plastic composites with gradient structures, controlled pore distribution, and enhanced interfacial bonding has become a research hotspot.

[0003] In the existing technology, the preparation of wood-plastic composite materials mainly relies on mechanical mixing, foaming or simple extrusion processes. For example, although the stiffness of the material can be improved by physically filling wood particles with a plastic matrix, the weak interface bonding leads to insufficient bending strength; some studies use coupling agent modification or foaming processes to control porosity, but excessive coupling agents can easily cause matrix phase separation, and uneven distribution of foamed pores can easily lead to local structural failure. In addition, the realization of density gradient structures often relies on multi-layer pressing or layered injection molding, which has problems such as complex process, discontinuous gradient transition, and difficulty in coordinated control of porosity and wood powder content. For example, although the gradient freezing process can form a porous structure, a single freezing rate can easily lead to ice crystal coarsening or pore collapse; the problem of wood powder migration in dynamic extrusion limits the stability of the gradient distribution. Summary of the Invention

[0004] The present invention overcomes the deficiencies of the prior art and provides a density gradient wood-plastic composite material and a preparation method thereof.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a method for preparing a density gradient wood-plastic composite material, comprising the following steps:

[0006] S1, crushing the wood raw material into sawdust, mixing the sawdust with water and then performing ultrasonic treatment;

[0007] S2, freezing the ultrasonically treated sawdust in stages and freeze-drying them under vacuum conditions;

[0008] S3, further crushing the dried sawdust into wood powder;

[0009] S4, mixing wood flour with a thermoplastic matrix and an interfacial enhancer in a gradient ratio, and performing dynamic gradient extrusion through a twin-screw extruder under segmented temperature control and variable screw speed to form a gradient distribution in which the wood flour content decreases along the extrusion direction;

[0010] S5. Hot-pressing the extruded composite material and performing surface treatment.

[0011] Furthermore, in step S1, the moisture content of the wood raw material is ≤15%, the particle size of the wood chips after crushing is 2-5 mm, and the solid-liquid ratio of the wood chips to water is 1:1-1:5.

[0012] Furthermore, in step S1, the frequency of ultrasonic treatment is 25-35 kHz, and the treatment time is 5-25 min.

[0013] Furthermore, in step S2, the staged gradient freezing includes:

[0014] The first stage is to cool down to -5℃ to -20℃ and pre-freeze for 1-4h.

[0015] The second stage is cooling to -15℃ to -30℃ and maintaining for 0.5-3h.

[0016] The third stage is cooling to -30℃ to -50℃ and deep freezing for 2-6 hours.

[0017] Furthermore, in step S2, the vacuum degree of vacuum freeze drying is 0.05-0.5 kPa, the heating rate is 0.5-4°C / h, and the final temperature is raised to -20°C to 10°C.

[0018] Furthermore, in step S3, the particle size of the wood powder is 3-60 μm, and particles of 20-60 μm are retained by sieving.

[0019] Furthermore, in step S4, the thermoplastic plastic matrix is selected from polypropylene, polyethylene, or a biodegradable material, and has a particle size of 2-6 mm. The interface enhancer includes a silane coupling agent and nanoclay, wherein the silane coupling agent accounts for 0.8%-1.6% of the mass of the thermoplastic plastic matrix, and the nanoclay accounts for 1%-6% of the mass of the thermoplastic plastic matrix.

[0020] Furthermore, in step S4, the wood powder content is linearly reduced from 20%-40% at the inlet to 0%-20% at the outlet in the extrusion direction. The temperature control range of the twin-screw extruder is 140-200°C, and the screw speed is 80-300rpm.

[0021] Furthermore, in step S5, the hot pressing pressure is 3-15 MPa, the temperature is 130-170°C, and the holding time is 2-10 min. The surface treatment includes polishing to a surface roughness Ra ≤ 8 μm and applying a UV curing coating or a film.

[0022] Another technical solution provided by the present invention is a density gradient wood-plastic composite material prepared based on the above preparation method.

[0023] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0024] (1) The present invention provides a density gradient wood-plastic composite material and a preparation method thereof, which optimizes the distribution of wood powder content through ultrasonic pretreatment, gradient freezing to control the pore structure, and dynamic gradient extrusion, significantly improving the interfacial bonding strength and mechanical properties of the wood-plastic composite material, while achieving controllable pore distribution and functional zoning.

[0025] (2) The present invention utilizes a silane coupling agent and nanoclay for synergistic modification. The silane coupling agent strengthens the interface between wood powder and plastic through chemical bonding and physical entanglement, while the nanoclay is dispersed in the matrix to form a physical cross-linked network, effectively inhibiting the expansion of interfacial defects. Compared with traditional single coupling agent or non-interface treatment processes, the present invention significantly improves interfacial shear strength while avoiding the matrix phase separation problem caused by excessive addition, achieving a balance between interfacial bonding strength and overall material continuity.

[0026] (3) The present invention regulates the ice crystal growth rate by combining ultrasonic treatment with gradient freezing, forming a gradient pore structure with a hierarchical distribution of macropores and micropores. Compared with conventional freeze-drying or foaming processes, the present invention avoids the problems of uneven pore distribution or excessive porosity through staged freezing and vacuum sublimation, ensuring the gas permeability of the material while maintaining the integrity of the matrix structure, significantly improving the flexural strength and impact resistance.

[0027] (4) The chemical bonding formed by the silane coupling agent and the physical cross-linking network of the nanoclay provide stable support for the pore structure formed by gradient freezing, inhibiting the collapse of the pore wall during extrusion molding; at the same time, the graded pore structure provides a directional channel for the melt flow, promoting the uniform dispersion of the interface enhancer, and ultimately achieving the simultaneous improvement of mechanical properties and functional zoning, breaking through the technical bottleneck of the traditional process in which it is difficult to balance interface strength and pore control. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0029] Figure 1 The present invention is a flow chart of a density gradient wood plastic composite material and a preparation method thereof;

[0030] Figure 2 is a flow chart of gradient freezing. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. Unless otherwise specified in the following examples, all raw materials are commercially available or prepared by conventional methods in the art.

[0032] Application Overview:

[0033] Based on an in-depth exploration of the preparation process of wood-plastic composite materials, it was unexpectedly discovered that the comprehensive performance of wood-plastic composite materials can be significantly improved by combining ultrasonic pretreatment and vacuum freeze-drying technology with a gradient freezing process and dynamic extrusion parameter control. Traditional processes have deficiencies in interface bonding, pore distribution, and gradient control, resulting in limited mechanical properties and functional zoning of the material. The present invention utilizes the cavitation effect to destroy the wood cell wall to form a directional microporous structure, and controls the ice crystal growth rate in stages to regulate the pore distribution. It also dynamically matches the wood powder content gradient with the melt flow to inhibit wood powder migration, ultimately forming a composite material with strengthened interface bonding, uniform pore distribution, and stable density gradient. This breakthrough in the technical route maximizes the synergistic effect of wood fiber and plastic matrix, solving the problem that a single method in traditional processes is difficult to take into account pore structure, interface bonding, and gradient distribution, providing an innovative solution for the large-scale production of high-performance wood-plastic composite materials.

[0034] Exemplary methods:

[0035] A method for preparing a density gradient wood-plastic composite material comprises the following steps:

[0036] S1, crushing the wood raw material into sawdust, mixing the sawdust with water and then performing ultrasonic treatment;

[0037] S2, freezing the ultrasonically treated sawdust in stages and freeze-drying them under vacuum conditions;

[0038] S3, further crushing the dried sawdust into wood powder;

[0039] S4, mixing wood flour with a thermoplastic matrix and an interfacial enhancer in a gradient ratio, and performing dynamic gradient extrusion through a twin-screw extruder under segmented temperature control and variable screw speed to form a gradient distribution in which the wood flour content decreases along the extrusion direction;

[0040] S5. Hot-pressing the extruded composite material and performing surface treatment.

[0041] The above method destroys the wood cell wall through ultrasonic treatment to form a microporous structure, controls the growth of ice crystals through gradient freezing to adjust the pore gradient distribution, realizes the wood powder content gradient corresponding to different functional zones through dynamic extrusion, and optimizes the material density through hot pressing.

[0042] Below, each step will be described in detail.

[0043] In step S1, the moisture content of the wood raw material is ≤15% to prevent excessive ice crystals from damaging the pore structure while ensuring drying efficiency; the particle size of the crushed wood chips is 2-5 mm, taking into account both ultrasonic treatment penetration and micropore uniformity; the solid-liquid ratio of the wood chips to water is 1:1-1:5 to adapt to the differences in water absorption of different wood species and balance water penetration.

[0044] Furthermore, the ultrasonic treatment frequency is 25-35kHz, utilizing the cavitation effect to generate microjets and shock waves. The treatment time is 5-25 minutes, ensuring the formation of micropores and preventing carbonization of the wood. Ultrasonic pretreatment is used before freeze-drying. The heat energy generated by the interaction between ultrasound and the medium and the ultrasonic cavitation effect destroy the cell walls of the sawdust, forming open pores and reducing the resistance of internal ice crystals to sublimation.

[0045] In step S2, the staged gradient freezing includes:

[0046] The first stage is to cool down to -5℃ to -20℃ and pre-freeze for 1-4h to ensure that the free water is completely frozen;

[0047] The second stage is cooling to -15℃ to -30℃ and maintaining for 0.5-3h;

[0048] The third stage is cooling to -30℃ to -50℃ and deep freezing for 2-6 hours to avoid residual liquid water affecting the drying efficiency.

[0049] The staged gradient freezing from -5℃ to -50℃ controls the growth rate of ice crystals. The initial ice nuclei are formed in the first stage, the coarsening of ice crystals is inhibited in the second stage, and the ice crystals are completely frozen in the third stage, forming a gradient structure combining macropores and micropores.

[0050] Furthermore, the vacuum degree of vacuum freeze drying is 0.05-0.5kPa, which reduces the vapor partial pressure, accelerates sublimation, and can avoid material collapse; the heating rate is 0.5-4℃ / h, balancing the ice crystal sublimation rate and the thermal stress of the material; the final temperature is raised to -20℃ to 10℃, and the bound water is removed in stages.

[0051] In step S3, the wood powder particle size is 3-60 μm. Too small a particle size causes agglomeration, while too large a particle size affects melt penetration. By crushing the wood powder to 3-60 μm, the specific surface area is increased. The wood powder is sieved to retain particles of 20-60 μm, balancing dispersibility and pore-filling efficiency, and adapting to the viscosity of the plastic melt.

[0052] In step S4, the thermoplastic matrix is selected from polypropylene, polyethylene, or a biodegradable material, with a particle size of 2-6 mm to ensure melt uniformity. The interfacial enhancers include a silane coupling agent and nanoclay, with the silane coupling agent accounting for 0.8%-1.6% of the thermoplastic matrix mass and the nanoclay accounting for 1%-6% of the plastic mass to avoid excessive amounts that could reduce melt fluidity. The silane coupling agent (KH-550) bonds with the wood powder surface hydroxyl groups, while the nanoclay (montmorillonite) is dispersed within the plastic matrix to enhance interfacial bonding strength.

[0053] Furthermore, the gradient ratio is that the wood powder content decreases linearly from 20%-40% at the inlet to 0%-20% at the outlet in the extrusion direction; the segmented temperature control range of the twin-screw extruder is 140-200°C, and the screw speed is 80-300rpm.

[0054] Dynamic gradient extrusion includes 20%-40% wood powder in the inlet layer and 0%-20% in the outlet layer, corresponding to different functional zones. Through multi-hopper feeding and segmented temperature control of 140-200°C, a balance between melt fluidity and pore filling is achieved. The screw speed is 80-300rpm, which is adapted to the shear requirements under different ratios.

[0055] In step S5, the pressure of hot pressing is 3-15MPa, the temperature is 130-170℃, and the holding time is 2-10min to eliminate internal bubbles; the surface treatment includes polishing to a surface roughness Ra≤8μm to improve the smoothness, and coating with UV curing paint or film to enhance weather resistance.

[0056] Example Materials:

[0057] A density gradient wood-plastic composite material is prepared based on the above preparation method.

[0058] Example 1:

[0059] S1. Grind the wood into sawdust with an average particle size of 4 mm and mix with water at a solid-to-liquid ratio of 1:3. Treat in an ultrasonic device at a frequency of 25 kHz and a power of 250 W for 15 minutes. Centrifuge and dehydrate to a moisture content of ≤5%.

[0060] S2. Spread the sawdust treated in S1 onto a freezing plate with a thickness of 10 mm and perform a gradient freezing process. The first stage is pre-freezing at -8°C for 2 hours, the second stage is cooling to -18°C and maintaining freezing for 1 hour, and the third stage is cooling to -35°C and deep freezing for 4 hours. The sawdust treated in S1 is then heated at a rate of 2°C per hour to -20°C under a vacuum of 0.2 kPa and maintained for 4 hours. The temperature is then raised to 0°C and maintained for 2 hours to complete ice crystal sublimation dehydration.

[0061] S3. The dried sawdust is crushed into a particle size of 50 μm by a jet mill, and wood powder particles with a particle size of 50 μm are retained by vibration screening.

[0062] S4. PP granules and 0.8% silane coupling agent (by weight) were stirred in a high-speed mixer at 800 rpm for 5 minutes. Nano-montmorillonite (by weight) was added and mixing continued for 3 minutes to obtain a mixture. The mixture was fed into a twin-screw extruder, where a multi-hopper feeding system fed different mixtures: a mixture containing 30% wood flour was added to the inlet hopper, a mixture containing 10% wood flour was added to the outlet hopper, and a mixture containing 20% wood flour was added to the middle hopper in a linear manner. The feed rate was controlled to form a linear gradient of wood flour content decreasing from 30% at the inlet to 10% at the outlet. The twin-screw extruder was temperature-controlled in sections: 160°C in the feed section, 180°C in the melt section, and 170°C in the discharge section. The screw speed was 200 rpm in the feed section and 150 rpm in the discharge section.

[0063] S5. The extruded material is hot-pressed into a sheet with dimensions of 100 mm × 100 mm × 5 mm using a mold, maintaining the pressure at 10 MPa and 150°C for 8 minutes. The sheet surface is polished to a roughness of Ra = 3 μm and coated with a UV curing coating with a film thickness of 20 μm.

[0064] Example 2:

[0065] Different from Example 1, in this Example S2, the first stage of gradient freezing is pre-freezing at -10°C for 2 hours, the second stage is cooled to -25°C and maintained frozen for 1 hour, and the third stage is cooled to -45°C and deep-frozen for 4 hours; in S4, the silane coupling agent accounts for 1.2% of the PP mass.

[0066] Example 3:

[0067] Different from Example 1, in this Example S2, the first stage of gradient freezing is pre-freezing at -12°C for 2 hours, the second stage is cooled to -30°C and maintained frozen for 1 hour, and the third stage is cooled to -50°C and deep-frozen for 4 hours; in S4, the silane coupling agent accounts for 1.6% of the PP mass.

[0068] Example 4:

[0069] Different from Example 1, the ultrasonic frequency in S1 of this example is 28 kHz; and the silane coupling agent in S4 accounts for 1.2% of the mass of PP.

[0070] Example 5:

[0071] Different from Example 4, in Example S2, the first stage of gradient freezing is pre-freezing at -10°C for 2 hours, the second stage is cooled to -25°C and maintained frozen for 1 hour, and the third stage is cooled to -45°C and deep frozen for 4 hours.

[0072] Example 6:

[0073] Different from Example 4, in this Example S2, the first stage of gradient freezing is pre-freezing at -12°C for 2 hours, the second stage is cooled to -30°C and maintained frozen for 1 hour, and the third stage is cooled to -50°C and deep-frozen for 4 hours; in S4, the silane coupling agent accounts for 0.8% of the PP mass.

[0074] Example 7:

[0075] Different from Example 1, the ultrasonic frequency in S1 of this Example is 35 kHz; and the silane coupling agent in S4 accounts for 1.6% of the mass of PP.

[0076] Example 8:

[0077] Different from Example 7, in this Example S2, the first stage of gradient freezing is pre-freezing at -10°C for 2 hours, the second stage is cooled to -25°C and maintained frozen for 1 hour, and the third stage is cooled to -45°C and deep-frozen for 4 hours; in S4, the silane coupling agent accounts for 0.8% of the PP mass.

[0078] Example 9:

[0079] Different from Example 7, in this Example S2, the first stage of gradient freezing is pre-freezing at -12°C for 2 hours, the second stage is cooled to -30°C and maintained frozen for 1 hour, and the third stage is cooled to -50°C and deep-frozen for 4 hours; in S4, the silane coupling agent accounts for 1.2% of the PP mass.

[0080] Experimental Example 1:

[0081] In this experimental example, the wood raw material is pine sawdust (moisture content ≤ 12%, Linan Wood Industry), the thermoplastic plastic matrix is polypropylene PP (T30S, Sinopec, melt index 12g / 10min); the interfacial enhancers are silane coupling agent KH-550 (Aladdin Reagent) and nanoclay (montmorillonite, Zhejiang Fenghong).

[0082] According to the preparation methods provided in Examples 1-9, boards with a size of 100 mm × 100 mm × 5 mm were prepared respectively. The micropore structure on the surface of the wood powder was observed by scanning electron microscopy, and the number of micropores per unit area (number / μm) was calculated using image analysis software.2 ); porosity and pore size distribution were determined according to GB / T 35463-2017; interfacial shear strength between wood flour and PP matrix was tested with reference to ASTM D3165 (single fiber pull-out test) (unit: J / m 2 ); refer to GB / T 1447-2005 (three-point bending test) to test the flexural strength (unit: MPa); refer to GB / T35463-2017 to determine the density of the outer and inner layers (unit: g / cm 3 ), and the results are shown in the following table:

[0083] Table 1 Key performance parameters

[0084]

[0085]

[0086]

[0087] The flexural strength increased significantly as the ultrasonic frequency increased from 25kHz to 28kHz, reaching a peak of 68.9MPa. However, the performance decreased when the ultrasonic frequency was further increased to 35kHz, indicating that there is an optimal threshold for ultrasonic frequency. The effect of silane concentration on interfacial shear strength reached a peak of 2.3J / m at 1.2%. 2 . When the concentration is lower than 0.8% or higher than 1.6%, the performance is significantly reduced. Under low temperature difference conditions (-8°C to -35°C), the porosity of gradient freezing is maintained at 17.3% to 19.2%, while high temperature difference conditions (-12°C to -50°C) cause the porosity to surge to 24.8% to 26.5%. However, excessive porosity weakens the mechanical properties. For example, the bending strength of Example 3 is only 39.1MPa. In addition, the micropore density is positively correlated with the ultrasonic frequency. At 35kHz, the micropore density reaches 20.5 / μm. 2 , but when high frequency and high temperature difference are superimposed, the pore distribution is uneven, resulting in performance deterioration.

[0088] In Example 5, the best performance was achieved with ultrasonic 28kHz, gradient freezing from -10℃ to -25℃ to -45℃, and silane concentration of 1.2%. The flexural strength reached a peak value of 68.9MPa, and the interfacial shear strength reached an optimal value of 2.3J / m 2, the porosity remains at an equilibrium value of 21%. This is because the 28kHz ultrasonic frequency matches the inherent vibration frequency of the wood cellulose molecular chain, and the microjets generated by the collapse of cavitation bubbles directionally destroy the hemicellulose-lignin composite structure, providing directional nucleation sites for subsequent gradient freezing. Ice crystals grow along the micropore walls to avoid disordered coarsening. Large-pore ice crystals are formed in the -10℃ pre-freezing stage and a through-pore network is formed in the -45℃ deep freezing stage, which enhances the capillary permeability of the melt. In dynamic extrusion, the screw speed is dynamically matched with the melt viscosity to ensure the stability of the wood powder gradient distribution. In addition, when the silane concentration is 1.2%, the silanol groups generated by the hydrolysis of KH-550 react with the hydroxyl groups of the wood powder through a condensation reaction to form Si-OC covalent bonds. At the same time, the amino groups and PP chain segments are entangled through van der Waals forces, which jointly suppress pore defects and improve the interfacial shear strength.

[0089] Example 3, with ultrasound at 25kHz, gradient cooling from -12°C to -30°C to -50°C, and a silane concentration of 1.6%, performed the worst, with a flexural strength of only 39.1 MPa and an interfacial shear strength of 0.9 J / m 2 , porosity 24.8%. This is because the ice crystal nucleation rate is too fast during the -12℃ pre-freezing stage, and the deep freezing at -50℃ causes the internal stress of the ice crystals to exceed the fracture toughness of the wood, resulting in cracking of the micropore wall. The crack network destroys the pore gradient, the melt penetration is blocked, and the inner layer density increases abnormally. When the silane concentration is 1.6%, the silane molecules form reverse micelles through hydrophobic interaction, occupy the wood powder-PP interface, hinder the melt flow, and cause the wood powder gradient distribution to weaken. In addition, the 25kHz ultrasonic cavitation energy is insufficient, only destroying the surface cellulose, failing to form a through micropore network, further aggravating the coarsening of ice crystals during the freezing stage.

[0090] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. A method for preparing a density gradient wood-plastic composite material, characterized in that: The following steps are involved: S1, crushing the wood raw material into sawdust, mixing the sawdust with water and then performing ultrasonic treatment; S2, freezing the ultrasonically treated sawdust in stages and freeze-drying them under vacuum conditions; S3, further crushing the dried sawdust into wood powder; S4, mixing the wood flour with the thermoplastic matrix and the interfacial enhancer in a gradient ratio, and performing dynamic gradient extrusion through a twin-screw extruder under segmented temperature control and variable screw speed to form a gradient distribution in which the wood flour content decreases along the extrusion direction; S5. Hot-pressing the extruded composite material and performing surface treatment.

2. The preparation method according to claim 1, characterized in that In step S1, the moisture content of the wood raw material is ≤15%, the particle size of the pulverized wood chips is 2-5 mm, and the solid-liquid ratio of the mixture of the wood chips and water is 1:1-1:

5.

3. The preparation method according to claim 1, characterized in that In step S1, the frequency of the ultrasonic treatment is 25-35 kHz, and the treatment time is 5-25 min.

4. The preparation method according to claim 1, characterized in that In step S2, the staged gradient freezing includes: The first stage is cooling to -5℃ to -20℃ and pre-freezing for 1-4h; The second stage is cooling to -15℃ to -30℃ and maintaining for 0.5-3h; The third stage is cooling to -30℃ to -50℃ and deep freezing for 2-6 hours.

5. The preparation method according to claim 4, characterized in that In step S2, the vacuum degree of the vacuum freeze drying is 0.05-0.5 kPa, the heating rate is 0.5-4°C / h, and the final temperature is raised to -20°C to 10°C.

6. The preparation method according to claim 1, characterized in that In step S3, the particle size of the wood powder is 3-60 μm, and particles of 20-60 μm are retained by sieving.

7. The preparation method according to claim 1, characterized in that In step S4, the thermoplastic plastic matrix is selected from polypropylene, polyethylene or biodegradable material, and has a particle size of 2-6 mm; the interface enhancer includes a silane coupling agent and nanoclay, wherein the silane coupling agent accounts for 0.8%-1.6% of the mass of the thermoplastic plastic matrix, and the nanoclay accounts for 1%-6% of the mass of the thermoplastic plastic matrix.

8. The preparation method according to claim 7, characterized in that In step S4, the gradient ratio is that the wood powder content decreases linearly from 20%-40% at the inlet to 0%-20% at the outlet in the extrusion direction; the segmented temperature control range of the twin-screw extruder is 140-200°C, and the screw speed is 80-300rpm.

9. The preparation method according to claim 1, characterized in that In step S5, the pressure of the hot pressing molding is 3-15 MPa, the temperature is 130-170° C., and the holding time is 2-10 min; the surface treatment includes polishing to a surface roughness Ra ≤ 8 μm and coating with UV curing paint or film.

10. A density gradient wood-plastic composite material, prepared based on the preparation method according to any one of claims 1 to 9.

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