A high-strength dense wood composite material based on pore regulation and flexible polymer network construction and a preparation method thereof

Abstract

This invention relates to a high-strength, dense wood composite material based on pore regulation and flexible polymer network construction, and its preparation method, belonging to the field of wood modification and composite material technology. The method first involves controlled pre-extraction of hemicellulose from wood using low-concentration NaOH to open cell wall pores and expose active sites. Then, the pretreated wood is impregnated in an aqueous solution containing acrylamide and ammonium persulfate, followed by in-situ polymerization with heating to form a flexible polyacrylamide network. Subsequently, a citric acid / itaconic acid crosslinking system is introduced, and polycarboxylic acid crosslinking anchoring and structural densification are achieved during hot pressing. Finally, the composite material is obtained through cooling and drying. This invention, through a synergistic pathway of "pore regulation – network construction – chemical anchoring – hot pressing locking," balances strength, toughness, and dimensional stability while preserving the wood skeleton, making it suitable for low-cost, green, and large-scale preparation.

Description

Technical Field

[0001] This invention belongs to the field of wood modification and composite materials technology, specifically relating to a high-strength dense wood composite material and its preparation method, which controls cell wall porosity through hemicellulose pre-extraction, constructs a flexible polyacrylamide network in situ inside the wood, and achieves synergistic reinforcement through polycarboxylic acid crosslinking and hot pressing. Background Technology

[0002] Natural wood boasts advantages such as renewable resources and high specific strength, but its hydrogen-bonded structure limits its mechanical properties, and it is prone to dimensional changes and brittle cracking when exposed to moisture. Densified wood, through methods like deconsolidation and hot pressing, can significantly improve strength and modulus, becoming a research hotspot in recent years (e.g., delignification or chemical / physical densification methods). However, these methods have drawbacks: on the one hand, while introducing hydrophobic polymers into the pores can significantly improve mechanical properties, the densified wood is still prone to springback and cracking under humid and hot cycles; on the other hand, traditional fillers often only improve stiffness without providing energy-efficient toughness, and the processing is costly, complex (requiring large amounts of organic solvents or high liquid volumes), and the appearance is difficult to maintain.

[0003] While wood can be reinforced using nanofibers (CNC / CNF) or high-content resin impregnation, this method is typically costly, complex, or can negatively impact the appearance and dimensional stability of the logs. Furthermore, nanofiber systems often exist as low-solids hydrogels, requiring significant moisture removal before application. Their surface is also rich in polar hydroxyl groups, which can limit compatibility and dispersion uniformity when blended with hydrophobic polymers. Directly filling wood cell walls with polymer resins or nanofibers, limited by permeability, often requires high liquid-to-liquid ratios or complex pretreatment, resulting in high costs and significant variations in appearance and dimensions.

[0004] Itaconic acid (IA) is a small molecule compound with both dicarboxyl groups and unsaturated double bonds, which can serve as a potential reaction bridge between the active hydroxyl groups of wood cell walls and polymer networks. Compared with single conventional polycarboxylic acids, it has the potential to improve the density of interfacial reaction sites, enhance network anchoring, and improve the stability of wet structures.

[0005] While there are reports of using flexible polymers for wood reinforcement in existing technologies, most of these techniques do not involve constructing a flexible network in situ within the cell wall pores while preserving the main wood skeleton, and then further achieving stable fixation through chemical cross-linking. Simultaneously, there are few technical solutions that utilize controlled pre-extraction of hemicellulose to regulate the cell wall pore structure and expose reaction sites, thereby increasing the entry of low-cost small molecules or aqueous polymers into the cell wall to form a stable network. For example, CN111978490A and CN113954191A are based on prior delignification treatment, followed by the introduction of an acrylamide system to prepare elastic or ultra-flexible wood. The former emphasizes the elasticity of the material, while the latter mainly emphasizes ultra-flexibility, flame retardancy, and anti-fouling properties. Both techniques aim to achieve flexibility by weakening the original wood structure. WO2011042609A1, on the other hand, impregnates carboxyl-containing cross-linking agents and low-molecular-weight oligomers into the wood cell wall, improving dimensional stability through esterification. Its focus is on the cross-linking fixation itself. CN101954662A uses organic acid anhydrides to swell grafted cell walls and fills cell cavities with polymerizable monomers, belonging to the organic monomer modification route. US20060216538A1 belongs to a wood treatment method of impregnation with reactive monomers followed by polymerization and curing, focusing on improving wood stability through monomer penetration and polymerization. Unlike the above technologies, this application does not use a delignification route, nor does it rely on organic monomers or organic solvents to swell grafted cell walls. Instead, while retaining the main wood skeleton, it first controls cell wall porosity and improves the accessibility of reaction sites through controlled pre-extraction of hemicellulose, then constructs a PAM flexible network in situ inside the wood, and subsequently combines CA / IA composite crosslinking and hot-press densification to achieve fixation and reinforcement, thereby taking into account the strength, toughness, and dimensional stability of the wood. Summary of the Invention

[0006] The purpose of this invention is to address the problem in existing technologies that it is difficult to maintain the main framework of wood while simultaneously ensuring strength, toughness, and dimensional stability, by providing a high-strength, dense wood composite material and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: S1. Hemicellulose pre-extraction: The wood is treated with NaOH aqueous solution; S2, PAM precursor impregnation: Prepare an aqueous solution of acrylamide and add ammonium persulfate (APS) as an initiator. Immerse the wood treated in step S1 in this impregnation solution; S3, In-situ polymerization / network formation: The impregnated wood obtained in step S2 is heated to activate APS to initiate the in-situ polymerization of AM monomers inside the cell wall to form a polyacrylamide (PAM) network. S4. Crosslinking and hot pressing: The wood sample obtained in step S3 is immersed in an aqueous solution containing a mixture of citric acid and itaconic acid. After being removed, the surface residue is wiped dry. After drying, it is placed in a mold for hot pressing treatment, so that citric acid and itaconic acid crosslink and anchor with the wood hydroxyl groups and / or PAM network, thereby achieving wood densification. S5. Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain the dense wood composite material.

[0008] Furthermore, in step S1, the concentration of the NaOH solution is 2 wt%, the liquid to wood mass ratio is 20:1, and it is heated in an 80°C water bath for 2 h.

[0009] Furthermore, in step S2, the concentration of the acrylamide aqueous solution is 1.0 mol / L, the amount of ammonium persulfate added is 0.5 wt% of the mass of the acrylamide monomer, and the amount of acrylamide aqueous solution used is based on completely submerging the wood sample.

[0010] Furthermore, in step S2, the impregnation method is vacuum impregnation, the vacuum degree is -0.08 MPa, and the impregnation time is 30 min.

[0011] Furthermore, in step S3, the in-situ polymerization temperature is 70 °C and the polymerization time is 6 h.

[0012] Furthermore, in step S4, the total mass fraction of CA and IA in the crosslinking solution is 10 wt%, the molar ratio of citric acid to itaconic acid is 1:1, and the amount of crosslinking solution used is based on completely submerging the wood sample.

[0013] Furthermore, in step S4, the impregnation includes vacuum impregnation for 30 min and room temperature and pressure impregnation for 6 h.

[0014] Furthermore, in step S4, the hot pressing temperature is 180 ℃, the hot pressing pressure is 2 MPa, and the hot pressing time is 20 min.

[0015] Furthermore, in step S4, during the hot pressing process, the wood sample is placed in a mold, covered with aluminum foil on both sides, and then hot pressed.

[0016] A second aspect of the present invention is to provide a high-strength, dense wood composite material based on pore regulation and a flexible polymer network structure, prepared by the above method.

[0017] Compared with the prior art, the beneficial effects of the technical solution provided by the present invention are as follows: 1. This invention regulates the pore structure of wood cell walls through controlled pre-extraction of hemicellulose, thereby improving the penetration ability of acrylamide precursors and cross-linking components into the cell walls while preserving the main wood skeleton as much as possible, and increasing the accessible sites for subsequent reactions.

[0018] 2. The present invention constructs a flexible polyacrylamide network in situ inside the wood, which helps to alleviate local stress concentration, improve the deformation coordination of the material under load, thereby reducing the tendency of dense wood to fracture brittlely and improving its toughness.

[0019] 3. This invention uses a CA / IA composite crosslinking system and combines it with a hot-press densification process to fix the wood cell walls and flexible network, which helps to reduce water absorption swelling and thickness rebound, and improve the dimensional stability of the material and its ability to maintain structure under humid and hot conditions.

[0020] 4. This invention uses an aqueous phase treatment system. The NaOH, AM, APS, CA and IA used are all common chemicals. It does not rely on deep lignin removal treatment, nor does it require expensive nanofibers or high liquid ratio resin impregnation. The process is relatively simple and has the feasibility of scale-up implementation.

[0021] 5. This invention combines controllable pre-extraction of hemicellulose, in-situ construction of PAM flexible network, CA / IA composite crosslinking and hot-press densification, achieving synergistic improvement in strength, toughness and dimensional stability while preserving the main skeleton of wood. Detailed Implementation

[0022] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0023] It should be noted that the poplar strips used in the following embodiments and comparative examples are 60×10×5 mm in size, mainly used for impregnation, hot pressing and subsequent physical and mechanical property testing under laboratory conditions, and do not constitute a limitation on the size and shape of the raw materials applicable to this invention; in practical applications, this invention is also applicable to wood substrates such as wood boards, wood chips or veneers of different sizes.

[0024] Example 1: A method for preparing high-strength, dense wood composite materials based on pore regulation and flexible polymer network construction, the steps of which are as follows: (1) Pre-extraction of hemicellulose: The wood was treated in a 2% NaOH aqueous solution at a liquid-to-wood mass ratio of 20:1 and heated at 80 °C for 2 h. This step selectively removes some hemicellulose and opens the nanopores of the cell wall, significantly increasing the amount of active hydroxyl groups on the cellulose surface, creating conditions for subsequent impregnation, penetration and cross-linking. After treatment, the wood was ultrasonically rinsed with deionized water until neutral (pH ~7) and the surface moisture was wiped dry.

[0025] (2) PAM precursor impregnation: Prepare a 1.0 M acrylamide aqueous solution and add 0.5 wt% ammonium persulfate (APS) as an initiator. Immerse the pretreated wood in the impregnation solution and impregnate for 30 minutes under vacuum (about -0.08 MPa) to ensure that the PAM precursor and APS fully penetrate into the cell wall pores.

[0026] (3) In-situ polymerization / network formation: The impregnated wood samples were transferred to a 70 °C oven for 6 h of impregnation, and APS was activated to initiate the in-situ polymerization of AM monomers into a polyacrylamide (PAM) network inside the cell wall. Then, the samples were naturally cooled to room temperature.

[0027] (4) CA / IA Impregnation: Prepare a CA / IA crosslinking solution, wherein the total mass fraction of CA and IA is 10 wt%, and the molar ratio of CA to IA is 1:1. The amount of crosslinking solution used should be sufficient to completely submerge the wood sample. Place the polymerized wood strips in the crosslinking solution, first impregnate them under vacuum for 30 minutes, and then impregnate them at room temperature and pressure for 6 hours to allow the crosslinking agent to fully penetrate into the wood. After removal, wipe the surface with filter paper to remove any residual liquid.

[0028] (5) Hot pressing densification: After drying, the wood strips are placed in a mold, covered with aluminum foil on both sides, and placed in a hot press. They are then pressed at 180°C and 2 MPa for 20 minutes. At this time, citric acid / itaconic acid can undergo esterification with the hydroxyl groups in the wood components under hot pressing conditions, and play a role in fixing and anchoring the PAM network.

[0029] (6) Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain a high-strength, high-toughness, and dimensionally stable dense wood composite material.

[0030] In this embodiment, orthogonal experiments were conducted using alkali concentration, acrylamide concentration, and CA / IA ratio. Alkali concentrations of 2%, 4%, and 6% were set; acrylamide concentrations of 0.5M, 1M, and 1.5M were set; and CA / IA ratios of 3:1, 1:1, and 1:3 were used for the experiments. The results are as follows: Alkali concentration (%) AM concentration (mol / L) CA:IA Elastic modulus (MPa) Bending strength (MPa) 2 0.5 3:1 25588.54 282.57 2 1 1:1 32151.94 381.75 2 1.5 1:3 29664.37 333.65 4 0.5 1:3 25000.72 289.80 4 1 3:1 27209.73 301.48 4 1.5 1:1 26471.14 277.46 6 0.5 1:1 26850.17 270.63 6 1 1:3 23174.93 273.52 6 1.5 3:1 19818.62 253.15 Table 1 Orthogonal Experiment Because the samples in this study were modified dense wood strips, and their dimensions differed from the standard flawless small samples, a non-standard span scheme was adopted for the three-point bending test. The test principle and mechanical parameter calculations were based on GB / T 1927.9-2021 and GB / T1927.10-2021. The actual test conditions were set as a span of 40 mm and a loading speed of 2 mm / min. The measured results showed that the elastic modulus of the wood reached 32151.94 MPa, which is about 9.8 times higher than that of the log (3260.63 MPa); the bending strength reached 381.75 MPa, which is about 5.7 times higher than that of the log (66.99 MPa).

[0031] Although embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the scope of the invention is not limited to the contents disclosed in the embodiments.

[0032] Comparative Example 1 The experimental steps are as follows: (1) Pre-extraction of hemicellulose: The wood was treated in deionized water at a liquid-to-wood mass ratio of 20:1 and heated at 80°C for 2 h. After treatment, the wood was ultrasonically rinsed with deionized water until neutral (pH ~ 7) and the surface moisture was wiped dry.

[0033] (2) PAM precursor impregnation: Prepare a 1.0 M acrylamide aqueous solution and add 0.5 wt% ammonium persulfate (APS) as an initiator. Place the pretreated wood into the impregnation solution and impregnate it under vacuum (about -0.08 MPa) for 30 minutes to ensure that the PAM precursor and APS fully penetrate into the cell wall pores.

[0034] (3) In-situ polymerization / network formation: The impregnated wood sample was transferred to a 70 °C oven for 6 h of impregnation, and APS was activated to initiate the in-situ polymerization of AM monomers into a polyacrylamide (PAM) network inside the cell wall. Then it was naturally cooled to room temperature.

[0035] (4) CA / IA Impregnation: Prepare a CA / IA crosslinking solution, wherein the total mass fraction of CA and IA is 10 wt%, and the molar ratio of CA to IA is 1:1. The amount of crosslinking solution used should be sufficient to completely submerge the wood sample. Place the polymerized wood strips in the crosslinking solution, first impregnate them under vacuum for 30 minutes, and then impregnate them at room temperature and pressure for 6 hours to allow the crosslinking agent to fully penetrate into the wood. After removal, wipe the surface with filter paper to remove any residual liquid.

[0036] (5) Hot pressing densification: After drying, the wood strips are placed in a mold, covered with aluminum foil on both sides, and placed in a hot press. They are then pressed at 180°C and 2 MPa for 20 minutes. At this time, citric acid / itaconic acid covalently crosslinks with PAM and cellulose / lignin hydroxyl groups.

[0037] (6) Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain dense wood composite material.

[0038] The wood dimensions in this comparative example differ from those of the standard flawless small sample; therefore, a non-standard span scheme was used for the three-point bending test. The test principle and mechanical parameter calculations were based on GB / T 1927.9-2021 and GB / T 1927.10-2021. The actual test conditions were set as a span of 40 mm and a loading speed of 2 mm / min. The measured results showed that the wood's elastic modulus reached 26432.51 MPa, approximately 8.1 times higher than that of the log (3260.63 MPa); the bending strength reached 138.09 MPa, approximately 2 times higher than that of the log (66.99 MPa).

[0039] Compared with Comparative Example 1, Embodiment 1 of this application has the following advantages: Compared to Comparative Example 1, Example 1 added NaOH pretreatment, which can open the pores of the wood, allowing the impregnation liquid to fully enter the cell pores and ultimately improve the mechanical properties.

[0040] Comparative Example 2 The experimental steps are as follows: (1) Pre-extraction of hemicellulose: The wood was treated in a 2% NaOH aqueous solution at a liquid-to-wood mass ratio of 20:1 and heated at 80 °C for 2 h. This step selectively removes some hemicellulose and opens the nanopores of the cell wall, significantly increasing the amount of active hydroxyl groups on the cellulose surface, creating conditions for subsequent impregnation, penetration and cross-linking. After treatment, the wood was ultrasonically rinsed with deionized water until neutral (pH ~7) and the surface moisture was wiped dry.

[0041] (2) CA / IA Impregnation: Prepare a CA / IA crosslinking solution, wherein the total mass fraction of CA and IA is 10 wt%, and the molar ratio of CA to IA is 1:1. The amount of crosslinking solution used should be sufficient to completely impregnate the wood sample. Place the pretreated wood strips in the crosslinking solution, first impregnate them under vacuum for 30 minutes, and then impregnate them at room temperature and pressure for 6 hours to allow the crosslinking agent to fully penetrate into the wood. After removal, wipe the surface with filter paper to remove any residual liquid.

[0042] (3) Hot pressing to densify: After drying, place the wooden strip in the mold, cover both sides with aluminum foil, place it in the hot press, and press for 20 minutes at 180°C and 2 MPa.

[0043] (4) Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain dense wood composite material.

[0044] The wood dimensions in this comparative example differ from those of the standard flawless small sample; therefore, a non-standard span scheme was used for the three-point bending test. The test principle and mechanical parameter calculations were based on GB / T 1927.9-2021 and GB / T 1927.10-2021. The actual test conditions were set as a span of 40 mm and a loading speed of 2 mm / min. The measured results showed that the wood's elastic modulus reached 33168.36 MPa, an increase of approximately 10.17 times compared to the elastic modulus of the log (3260.63 MPa); the bending strength reached 207.41 MPa, an increase of approximately 3.1 times compared to the bending strength of the log (66.99 MPa).

[0045] Compared with Comparative Example 2, Embodiment 1 of this application has the following advantages: Compared to Comparative Example 2, Example 1 adds PAM precursor impregnation and in-situ polymerization network formation. The flexible polyacrylamide network absorbs energy under load, inhibits crack propagation, significantly improves the fracture toughness and impact resistance of the composite material, and overcomes the brittleness of a simple hydrogen bond network.

[0046] Comparative Example 3 The experimental steps are as follows: (1) Pre-extraction of hemicellulose: The wood was treated in a 2% NaOH aqueous solution at a liquid-to-wood mass ratio of 20:1 and heated at 80 °C for 2 h. This step selectively removes some hemicellulose and opens the nanopores of the cell wall, significantly increasing the amount of active hydroxyl groups on the cellulose surface, creating conditions for subsequent impregnation, penetration and cross-linking. After treatment, the wood was ultrasonically rinsed with deionized water until neutral (pH ~7) and the surface moisture was wiped dry.

[0047] (2) PAM precursor impregnation: Prepare a 1.0 M acrylamide aqueous solution and add 0.5 wt% ammonium persulfate (APS) as an initiator. Immerse the pretreated wood in the impregnation solution and impregnate for 30 minutes under vacuum (about -0.08 MPa).

[0048] (3) In-situ polymerization / network formation: The impregnated wood sample was transferred to a 70 °C oven for 6 h to allow AM monomers to polymerize in situ inside the cell wall to form a PAM network. Then, it was directly dried and hot-pressed without further CA / IA crosslinking impregnation.

[0049] (4) Hot pressing to densify: After drying, place the wooden strip in the mold, cover both sides with aluminum foil, place it in the hot press, and press for 20 minutes at 180°C and 2 MPa.

[0050] (5) Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain dense wood composite material.

[0051] The wood dimensions in this comparative example differ from those of the standard flawless small sample; therefore, a non-standard span scheme was used for the three-point bending test. The test principle and mechanical parameter calculations were based on GB / T 1927.9-2021 and GB / T 1927.10-2021. The actual test conditions were set as a span of 40 mm and a loading speed of 2 mm / min. The measured results showed that the wood's elastic modulus reached 28045.78 MPa, an increase of approximately 8.60 times compared to the elastic modulus of the log (3260.63 MPa); the bending strength reached 189.75 MPa, an increase of approximately 2.83 times compared to the bending strength of the log (66.99 MPa).

[0052] Compared with Comparative Example 3, Embodiment 1 of this application has the following advantages: Compared to Comparative Example 3, Example 1 further introduces a CA / IA composite crosslinking system, which can more stably anchor the PAM flexible network inside the wood cell wall, reduce wet rebound and interface relaxation after hot pressing, thereby further improving bending strength and structural stability while maintaining high stiffness.

[0053] Comparative Example 4 The experimental steps are as follows: (1) Pre-extraction of hemicellulose: The wood was treated in a 2% NaOH aqueous solution at a liquid-to-wood mass ratio of 20:1 and heated at 80 °C for 2 h. This step selectively removes some hemicellulose and opens the nanopores of the cell wall, significantly increasing the amount of active hydroxyl groups on the cellulose surface, creating conditions for subsequent impregnation, penetration and cross-linking. After treatment, the wood was ultrasonically rinsed with deionized water until neutral (pH ~7) and the surface moisture was wiped dry.

[0054] (2) PAM precursor impregnation: Prepare a 1.0 M acrylamide aqueous solution and add 0.5 wt% ammonium persulfate (APS) as an initiator. Immerse the pretreated wood in the impregnation solution and impregnate for 30 minutes under vacuum (about -0.08 MPa).

[0055] (3) In-situ polymerization / network formation: The impregnated wood sample was transferred to a 70 °C oven for 6 h to allow AM monomers to polymerize in-situ inside the cell wall to form a PAM network.

[0056] (4) CA impregnation: Prepare a CA crosslinking solution with a CA mass fraction of 10 wt%. The amount of crosslinking solution should be sufficient to completely submerge the wood sample. Place the polymerized wood strips in the crosslinking solution and impregnate them under vacuum for 30 minutes, then under normal temperature and pressure for 6 hours to allow the crosslinking agent to fully penetrate the wood. After removal, wipe the surface with filter paper to remove any residual liquid.

[0057] (5) Hot pressing to densify: After drying, place the wooden strip in the mold, cover both sides with aluminum foil, place it in the hot press, and press for 20 minutes at 180°C and 2 MPa.

[0058] (6) Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain dense wood composite material.

[0059] The wood dimensions in this comparative example differ from those of the standard flawless small sample; therefore, a non-standard span scheme was used for the three-point bending test. The test principle and mechanical parameter calculations were based on GB / T 1927.9-2021 and GB / T 1927.10-2021. The actual test conditions were set as a span of 40 mm and a loading speed of 2 mm / min. The measured results showed that the wood's elastic modulus reached 30948.78 MPa, an increase of approximately 9.49 times compared to the elastic modulus of the log (3260.63 MPa); the bending strength reached 202.04 MPa, an increase of approximately 3.02 times compared to the bending strength of the log (66.99 MPa).

[0060] Compared with Comparative Example 4, Embodiment 1 of this application has the following advantages: Compared to Comparative Example 4, Example 1 further introduces itaconic acid into the citric acid base, which makes the composite crosslinking system have higher interfacial reactivity and richer structural regulation capabilities. This is beneficial to enhance the synergistic fixation of wood cell walls and PAM networks, and further improve bending strength and dimensional stability under humid and hot conditions.

Claims

1. A method for preparing a high-strength, dense wood composite material based on pore regulation and flexible polymer network construction, characterized in that, Includes the following steps: S1. Hemicellulose pre-extraction: The wood is treated with NaOH aqueous solution; S2, PAM precursor impregnation: Prepare an aqueous solution of acrylamide and add ammonium persulfate (APS) as an initiator. Immerse the wood treated in step S1 in this impregnation solution; S3, In-situ polymerization / network formation: The impregnated wood obtained in step S2 is heated to activate APS to initiate the in-situ polymerization of AM monomers inside the cell wall to form a polyacrylamide (PAM) network. S4. Crosslinking and hot pressing: The wood sample obtained in step S3 is immersed in an aqueous solution containing a mixture of citric acid and itaconic acid. After being removed, the surface residue is wiped dry. After drying, it is placed in a mold for hot pressing treatment, so that citric acid and itaconic acid crosslink and anchor with the wood hydroxyl groups and / or PAM network, thereby achieving wood densification. S5. Cooling and drying: After hot pressing, the sample is taken out, cooled to room temperature, and dried to constant weight to obtain the dense wood composite material.

2. The preparation method according to claim 1, characterized in that, In step S1, the concentration of the NaOH solution is 2wt%, the liquid to wood mass ratio is 20:1, and it is heated in an 80 °C water bath for 2 h.

3. The preparation method according to claim 1, characterized in that, In step S2, the concentration of the acrylamide aqueous solution is 1.0 mol / L, the amount of ammonium persulfate added is 0.5 wt% of the mass of acrylamide monomer, and the amount of acrylamide aqueous solution used is based on completely submerging the wood sample.

4. The preparation method according to claim 3, characterized in that, In step S2, the impregnation method is vacuum impregnation, the vacuum degree is -0.08 MPa, and the impregnation time is 30 min.

5. The preparation method according to claim 1, characterized in that, In step S3, the in-situ polymerization temperature is 70 °C and the polymerization time is 6 h.

6. The preparation method according to claim 1, characterized in that, In step S4, the total mass fraction of CA and IA in the crosslinking solution is 10 wt%, the molar ratio of citric acid to itaconic acid is 1:1, and the amount of crosslinking solution used is based on completely immersing the wood sample.

7. The preparation method according to claim 6, characterized in that, In step S4, the impregnation includes vacuum impregnation for 30 minutes and room temperature and pressure impregnation for 6 hours.

8. The preparation method according to claim 1, characterized in that, In step S4, the hot pressing temperature is 180 ℃, the hot pressing pressure is 2 MPa, and the hot pressing time is 20 min.

9. The preparation method according to claim 1, characterized in that, In step S4, during the hot pressing process, the wood sample is placed in a mold, covered with aluminum foil on both sides, and then hot pressed.

10. A high-strength, dense wood composite material based on pore regulation and a flexible polymer network, characterized in that, It is prepared by the preparation method described in any one of claims 1-9.