Purification method for bamboo fiber reinforced natural polymer composite material

Abstract

The present application relates to the technical field of bamboo fiber composite material, in particular to a purification preparation method of bamboo fiber reinforced natural polymer composite material. The bamboo raw material is crushed and treated by steam explosion to obtain primary bamboo fiber; the primary bamboo fiber is sequentially subjected to organic solvent dewaxing treatment and oxidation system selective delignification treatment to obtain purified bamboo fiber; the purified bamboo fiber is treated in a tannin acid solution to form a polyphenol interface layer on the surface thereof; and a solution containing polyvalent metal ions is added to the treated bamboo fiber to form a coordination cross-linking interface layer on the surface of the bamboo fiber. The present application constructs a stable interface layer between the fiber and the natural polymer matrix. The interface layer can effectively transfer stress, significantly improve the tensile strength, bending strength and shear strength of the composite material, and solve the problem of weak interface and easy delamination of traditional natural composite materials.

Description

Technical Field

[0001] This invention relates to the field of bamboo fiber composite materials technology, specifically to a purification and preparation method for bamboo fiber reinforced natural polymer composite materials. Background Technology

[0002] Bamboo fiber is widely used in composite materials and biodegradable materials due to its natural renewability, excellent mechanical properties, lightweight and durability. In recent years, natural polymer composite materials have received increasing attention in packaging materials, building materials, and functional panels due to their environmental friendliness, biodegradability, and good biocompatibility.

[0003] However, traditional bamboo fiber processing typically removes some impurities through simple washing or alkali treatment, leaving residual waxes, hemicellulose, and lignin on the bamboo fiber surface. These residual impurities not only reduce the interfacial bonding between bamboo fiber and the natural polymer matrix but also lead to unstable mechanical properties in the composite material, resulting in problems such as fiber shedding, interfacial delamination, and excessive water absorption. Existing technologies often employ single surface coatings or physical adsorption methods to improve the bonding between bamboo fiber and the polymer matrix, such as through simple polyphenol impregnation or chemical coupling. However, the interfacial modification layer created by these methods is prone to detachment during washing or long-term use, failing to maintain the composite material's mechanical and water resistance properties over the long term. Existing natural polymer matrices often employ one-time overall crosslinking or curing methods, lacking hierarchical control over the fiber interface and the overall network. This results in uneven crosslinking within the composite material and low stress transfer efficiency between the fiber and the matrix, thus limiting the material's strength, modulus, and aging resistance.

[0004] In summary, existing bamboo fiber reinforced natural polymer composites have significant technical deficiencies in terms of interface modification, crosslinking processes, and multi-scale reinforcement. There is an urgent need for a preparation method that can improve interface stability, achieve staged in-situ crosslinking, and realize multi-scale synergistic reinforcement through nano-reinforcing agents, so as to obtain natural polymer composites with excellent mechanical properties, water resistance, aging resistance, and industrial production capability. Summary of the Invention

[0005] In view of the obvious technical defects in the existing technology in terms of interface modification, crosslinking process and multi-scale reinforcement, the present invention provides a purification and preparation method of bamboo fiber reinforced natural polymer composite material.

[0006] A method for purifying and preparing a bamboo fiber reinforced natural polymer composite material includes the following steps: S1. After crushing the bamboo raw material to 40-80 mesh, it is treated under a steam pressure of 1.2-2.0 MPa for 60-120 seconds, followed by instantaneous depressurization to obtain primary bamboo fiber. Crushing can increase the surface area of ​​the fiber, and steam explosion can partially destroy the structure of the bamboo fiber, improve the permeability and efficiency of subsequent chemical treatment, and lay the foundation for the formation of a uniform composite material.

[0007] S2. The primary bamboo fiber is placed in an ethanol aqueous solution with a volume fraction of 60% to 80% and refluxed at 70 to 90°C for 1 to 3 hours to remove wax and low molecular weight impurities, then filtered and dried. The treated bamboo fiber was then placed in a mixed solution containing 3%–10% hydrogen peroxide and 1%–5% organic acid, and reacted at 60–80°C for 2–5 hours to remove some lignin. The fiber was then washed until neutral and dried to obtain purified bamboo fiber. This step removes surface wax with organic solvents, improving the hydrophilicity of the fiber surface. The oxidation system removes lignin, exposing more hydroxyl groups and increasing interfacial reaction sites, laying the foundation for tannic acid adsorption and cross-linking.

[0008] S3. Add the purified bamboo fiber to a tannic acid aqueous solution with a mass fraction of 0.5% to 5%, adjust the pH of the solution to 7 to 8, and stir for 1 to 3 hours at room temperature to 40°C to deposit a polyphenol interface layer on the surface of the bamboo fiber. Then wash and dry. The polyphenol layer formed by the deposition of tannic acid on the fiber surface can be used as a biomimetic adhesive for natural polymer composite materials to enhance the interfacial bonding force between the fiber and the matrix.

[0009] S4. Add the bamboo fiber treated in S3 to a polyvalent metal salt solution with a concentration of 0.01-0.1 mol / L and treat it at 20-40℃ for 0.5-2 hours to allow tannic acid to coordinate with metal ions and form a coordination cross-linking interface layer on the surface of the bamboo fiber. Wash and dry. The metal ions coordinate with tannic acid to form a three-dimensional coordination network, which significantly improves the stability of the interface layer, prevents the interface layer from falling off, and improves water resistance and long-term mechanical properties.

[0010] S5. Add the bamboo fiber treated in S4 to a natural polymer matrix solution, wherein the natural polymer matrix has a mass fraction of 5% to 20%, and simultaneously add a plasticizer with a mass fraction of 5% to 30% and a polycarboxylic acid crosslinking agent with a mass fraction of 1% to 10%. Stir and mix at 60 to 90°C to form a homogeneous system. The natural polymer matrix forms a continuous phase, the plasticizer improves flexibility, and the polycarboxylic acid crosslinking agent provides crosslinking sites to ensure that the composite material forms a stable network structure.

[0011] S6. In the system, an in-situ crosslinking reaction is carried out. First, the reaction is carried out at 50-90℃ for 0.5-2h for interfacial pre-crosslinking, and then the temperature is raised to 100-160℃ for 0.5-2h to complete the overall crosslinking. In the low temperature stage, the fiber-interface bonding is strengthened first, and in the high temperature stage, an overall crosslinking network is formed to ensure that the material is uniform, the interface is firm, and the mechanical properties are optimal.

[0012] S7. The obtained system is hot-pressed at 120-160℃ and 5-15MPa for 5-20 minutes. After cooling, bamboo fiber reinforced natural polymer composite material is obtained. Hot pressing achieves the final material shape and density, while fixing the internal structure to ensure the mechanical strength and stability of the composite material.

[0013] Preferably, the organic acid in S2 is acetic acid, formic acid, or a combination thereof, and its mass fraction is 1% to 5% of the mixed solution. The organic acid adjusts the pH and reactivity of the oxidation system, synergistically with hydrogen peroxide to selectively remove lignin, increases the exposure of hydroxyl groups on the fiber surface, and provides more reaction sites for interfacial deposition and crosslinking.

[0014] Preferably, the pH of the tannic acid solution in S3 is adjusted to 7-8 using sodium hydroxide or ammonia. Maintaining a slightly alkaline pH increases the solubility and activity of tannic acid, allowing it to deposit more evenly on the fiber surface, while avoiding fiber damage or uneven interfacial deposition caused by excessive acidity or alkalinity.

[0015] Preferably, the multivalent metal salt in S4 is selected from one or more of ferric chloride, ferric sulfate, aluminum chloride, or calcium chloride. These metal ions form a stable three-dimensional coordination network with tannic acid, significantly improving the water resistance, temperature resistance, and interfacial bonding strength of the interface layer, which is key to ensuring the long-term mechanical properties of the composite material.

[0016] Preferably, the natural polymer matrix in S5 is one or more of starch, chitosan, or sodium alginate, wherein the starch is pre-gelatinized at 80–95°C for 10–30 min before use. Pre-gelatinization improves the flowability and dispersibility of the starch, ensuring uniform fiber encapsulation; the polycarboxylic acid crosslinking agent provides crosslinking sites between carboxyl groups and fiber hydroxyl groups, achieving overall structural stability of the composite material.

[0017] Preferably, a nanoscale natural reinforcing phase is added to step S5. The reinforcing phase is nanocellulose or nanocrystalline cellulose, and its addition amount is 1% to 8% of the mass of the natural polymer matrix. It is ultrasonically dispersed for 10 to 30 minutes before addition. The nano-reinforcing phase improves the tensile strength and toughness of the composite material through a multi-scale reinforcement mechanism. Ultrasonic dispersion ensures uniform distribution of nanoparticles and avoids agglomeration that affects mechanical properties.

[0018] Preferably, the plasticizer in S5 is glycerol, and its addition amount is 10% to 25% of the mass of the natural polymer matrix. The plasticizer improves the flexibility and processability of the composite material, reduces brittleness during drying or molding, and improves the impact resistance of the material.

[0019] Preferably, the polycarboxylic acid crosslinking agent in S5 is citric acid, and its addition amount is 3% to 8% of the mass of the natural polymer matrix. Citric acid provides reaction sites for carboxyl groups and hydroxyl groups on the fiber surface, enhancing the interfacial bonding force and the overall strength of the composite material through in-situ crosslinking, while also taking into account the natural and environmentally friendly characteristics.

[0020] Preferably, the heating rate in step S6 is controlled at 2–10 °C / min. Slow heating ensures orderly interfacial pre-crosslinking and overall crosslinking, preventing fiber expansion, interfacial breakage, or localized material inhomogeneity caused by rapid heating, thereby optimizing the mechanical properties and stability of the composite material.

[0021] Preferably, the bamboo fiber surface has a tannic acid-metal ion coordination interface layer, the natural polymer matrix forms an in-situ cross-linked network structure on the bamboo fiber surface, and the bamboo fiber, nano-reinforcing phase and natural polymer matrix form a multi-scale synergistic reinforcement structure.

[0022] Compared with the prior art, the advantages of this invention are: (1) By depositing a tannic acid polyphenol layer on the surface of bamboo fiber and forming a three-dimensional coordination crosslinking network with polyvalent metal ions, this invention constructs a stable interface layer between the fiber and the natural polymer matrix. This interface layer can effectively transfer stress, significantly improve the tensile strength, flexural strength and shear strength of the composite material, and solve the problems of weak interface and easy delamination of traditional natural composite materials.

[0023] (2) The present invention adopts a staged temperature control strategy of low temperature interface pre-crosslinking and high temperature overall crosslinking, so that the fiber surface interface and matrix network structure are uniformly crosslinked, avoiding the problems of local material unevenness or fiber expansion and breakage that may be caused by traditional one-time crosslinking, thereby ensuring the stability and controllability of the overall performance of the material.

[0024] (3) Introducing nanocellulose or nanocrystalline cellulose as a reinforcing phase into a natural polymer matrix, through the multi-scale synergistic effect of micro-nanoparticles and macro-bamboo fibers, significantly improves the tensile, impact and toughness properties of the composite material, while inhibiting the propagation of microcracks and improving the service life of the material.

[0025] (4) All raw materials used are natural renewable resources and environmentally friendly crosslinking agents, avoiding pollution to the environment by residual organic solvents or harmful chemicals. The temperature, pH and reaction time of each step in the preparation method are clearly controllable, making it easy to industrialize and scale up applications.

[0026] (5) The introduction of plasticizers improves the flexibility and molding performance of the material. The composite material is not easy to crack or become brittle under hot pressing, casting or extrusion molding conditions. The composite material prepared by the method of the present invention is significantly better than the traditional natural composite material without interface strengthening and staged crosslinking treatment in terms of water absorption, temperature resistance and mechanical properties, and has higher practical value. Attached Figure Description

[0027] Figure 1 This is a flowchart of a purification and preparation method for a bamboo fiber reinforced natural polymer composite material proposed in this invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] Example 1: Bamboo raw material: pulverized to 60 mesh; Ethanol aqueous solution: 70%; Hydrogen peroxide: 5 wt% Organic acid: acetic acid 3wt%; Tannic acid: 3wt% aqueous solution; Ferric chloride: 0.05 mol / L; Corn starch: 10wt% Nanocellulose: 3wt% Glycerin: 15wt% Citric acid: 5wt% Sodium hydroxide: Adjust the pH as needed; Preparation steps: S1. After crushing the bamboo raw material to 60 mesh, it is treated under 1.5MPa steam pressure for 90s, and then the pressure is released instantaneously to obtain primary bamboo fiber.

[0030] S2. Place the primary bamboo fiber in a 70% ethanol aqueous solution, reflux at 80℃ for 2 hours, filter and dry. The dried fibers were placed in an aqueous solution containing 5 wt% hydrogen peroxide and 3 wt% acetic acid, reacted at 70°C for 3 h, washed until neutral and dried to obtain purified bamboo fibers.

[0031] S3. Add the purified bamboo fiber to a 3wt% tannic acid aqueous solution, adjust the pH to 7.5 with sodium hydroxide, stir at room temperature for 2 hours, then wash and dry.

[0032] S4. Add the fibers treated in S3 to a 0.05 mol / L ferric chloride aqueous solution, treat at 30°C for 1 h, wash and dry.

[0033] S5. Add the fiber treated in S4 to a 10wt% corn starch solution, along with 15wt% glycerol and 5wt% citric acid. Stir well, then add 3wt% nanocellulose and ultrasonically disperse for 20 minutes.

[0034] S6. React at 80℃ for 1 hour to perform interfacial pre-crosslinking, and then raise the temperature to 140℃ for 1 hour to complete the overall crosslinking.

[0035] S7. The system is hot-pressed at 150℃ and 10MPa for 10 minutes and then cooled to room temperature to obtain bamboo fiber reinforced starch composite material board.

[0036] Example 2: Bamboo raw material: pulverized to 65 mesh; Ethanol aqueous solution: 75%; Hydrogen peroxide: 6 wt% Organic acids: Formic acid 4wt%; Tannic acid: 4wt% aqueous solution; Ferric chloride: 0.06 mol / L; Corn starch: 12wt% Nanocellulose: 4wt% Glycerin: 20wt% Citric acid: 6wt% pH adjuster: sodium hydroxide; Preparation steps: S1. Crush bamboo raw materials to 65 mesh, process them under 1.6MPa steam pressure for 100s, and then release the pressure instantly to obtain primary bamboo fiber.

[0037] S2. Primary bamboo fiber is placed in a 75% ethanol aqueous solution, refluxed at 80℃ for 2.5h, filtered and dried; The dry fibers were placed in a solution containing 6 wt% hydrogen peroxide and 4 wt% formic acid and reacted at 70°C for 3 hours. They were then washed until neutral and dried.

[0038] S3. Add 4wt% tannic acid aqueous solution to purified bamboo fiber, adjust pH to 7.8, stir at room temperature for 2 hours, wash and dry.

[0039] S4 and S3 fibers were added to a 0.06 mol / L ferric chloride solution, treated at 30°C for 1 hour, and then washed and dried.

[0040] S5 and S4 fibers were added to a 12wt% corn starch solution, followed by 20wt% glycerol and 6wt% citric acid. After stirring evenly, 4wt% nanocellulose was added and the mixture was ultrasonically dispersed for 20 minutes.

[0041] S6. React at 85℃ for 1 hour to perform interfacial pre-crosslinking, and then raise the temperature to 145℃ for 1 hour to complete the overall crosslinking.

[0042] S7. Hot pressing: Press at 150℃ and 10MPa for 12 minutes, then cool to obtain the sheet material.

[0043] Example 3: Bamboo raw material: pulverized to 55 mesh; Ethanol aqueous solution: 70%; Hydrogen peroxide: 5 wt% Organic acid: acetic acid 3wt%; Tannic acid: 2wt% aqueous solution; Ferric chloride: 0.05 mol / L; Corn starch: 8wt% Nanocellulose: 2wt% Glycerin: 15wt% Citric acid: 4wt% pH adjuster: sodium hydroxide; Preparation steps: S1. Crush bamboo raw materials to 55 mesh, process under 1.5MPa steam pressure for 90s, and then release the pressure instantly.

[0044] S2. Bamboo fiber is placed in a 70% ethanol aqueous solution, refluxed at 80℃ for 2 hours, filtered and dried. The dry fibers were placed in a 5wt% hydrogen peroxide + 3wt% acetic acid solution and reacted at 70℃ for 3 hours. They were then washed until neutral and dried.

[0045] S3. Add 2wt% tannic acid aqueous solution to purified fiber, adjust pH to 7.5, stir at room temperature for 2 hours, wash and dry.

[0046] S4 and S3 fibers were added to a 0.05 mol / L ferric chloride solution and treated at 30°C for 1 hour, then washed and dried.

[0047] S5 and S4 fibers were added to an 8wt% corn starch solution, followed by 15wt% glycerol and 4wt% citric acid. After stirring evenly, 2wt% nanocellulose was added and the mixture was ultrasonically dispersed for 20 minutes.

[0048] S6. React at 80℃ for 1 hour to perform interfacial pre-crosslinking, and then raise the temperature to 140℃ for 1 hour to complete the overall crosslinking.

[0049] S7. Hot pressing: Press at 150℃ and 10MPa for 10 minutes, then cool to obtain the sheet material.

[0050] Example 4 Bamboo raw material: pulverized to 60 mesh; Ethanol aqueous solution: 70%; Hydrogen peroxide: 5 wt% Organic acid: acetic acid 3wt%; Tannic acid: 3wt% aqueous solution; Ferric chloride: 0.05 mol / L; Corn starch: 10wt% Nanocellulose: 3wt% Glycerin: 15wt% Citric acid: 5wt% pH adjuster: sodium hydroxide; Preparation steps: S1. Crush bamboo raw materials to 60 mesh, process under 1.5MPa steam pressure for 90s, and then release the pressure instantly.

[0051] S2. Bamboo fiber is placed in a 70% ethanol aqueous solution, refluxed at 80℃ for 2 hours, filtered and dried. The dry fibers were placed in a 5wt% hydrogen peroxide + 3wt% acetic acid solution and reacted at 70℃ for 3 hours. They were then washed until neutral and dried.

[0052] S3. Add 3wt% tannic acid aqueous solution to purified fiber, adjust pH to 7.5, stir at room temperature for 2 hours, wash and dry.

[0053] S4 and S3 fibers were added to a 0.05 mol / L ferric chloride solution and treated at 30°C for 1 hour, then washed and dried.

[0054] S5 and S4 fibers were added to a 10wt% corn starch solution, followed by 15wt% glycerol and 5wt% citric acid. After stirring evenly, 3wt% nanocellulose was added and the mixture was ultrasonically dispersed for 20 minutes.

[0055] S6. React at 80℃ for 1 hour to perform interfacial pre-crosslinking, and then raise the temperature to 150℃ for 1 hour to complete the overall crosslinking.

[0056] S7. Hot pressing: Press at 150℃ and 10MPa for 15 minutes, then cool to obtain the sheet material.

[0057] Comparative Example 1 Bamboo raw material: pulverized to 60 mesh; Ethanol aqueous solution: 70%; Hydrogen peroxide: 5 wt% Organic acid: acetic acid 3wt%; Tannic acid: Not added; Metal salts: None added; Corn starch: 10wt% Nanocellulose: 3wt% Glycerin: 15wt% Citric acid: 5wt% pH adjuster: sodium hydroxide; Preparation steps: S1. Crush bamboo raw materials to 60 mesh, process under 1.5MPa steam pressure for 90s, and then release the pressure instantly.

[0058] S2. Primary bamboo fiber is placed in a 70% ethanol aqueous solution, refluxed at 80°C for 2 hours, filtered and dried. The dry fibers were placed in a 5wt% hydrogen peroxide + 3wt% acetic acid solution and reacted at 70℃ for 3 hours. They were then washed until neutral and dried.

[0059] S3 and S2 fibers were directly added to a 10wt% corn starch solution, followed by 15wt% glycerol and 5wt% citric acid. After stirring evenly, 3wt% nanocellulose was added and ultrasonically dispersed for 20 minutes.

[0060] S4. React at 80℃ for 1 hour to achieve overall cross-linking, then raise the temperature to 140℃ and react for 1 hour.

[0061] S5. Hot pressing: 150℃, 10MPa, 10min, cooling to obtain the sheet material.

[0062] Comparative Example 2 Bamboo raw material: pulverized to 60 mesh; Ethanol aqueous solution: 70%; Hydrogen peroxide: 5 wt% Organic acid: acetic acid 3wt%; Tannic acid: 3wt% Ferric chloride: 0.05 mol / L; Corn starch: 10wt% Nanocellulose: Not added; Glycerin: 15wt% Citric acid: 5wt% pH adjuster: sodium hydroxide; Preparation steps: S1. Crush bamboo raw materials to 60 mesh, steam explosion at 1.5MPa for 90 seconds.

[0063] S2, Dewaxing and Lignification Treatment: Reflux with 70% ethanol for 2 hours + react with 5wt% hydrogen peroxide / 3wt% acetic acid at 70℃ for 3 hours.

[0064] S3, Tannic acid interface deposition: 3wt% tannic acid aqueous solution, pH 7.5, stirred at room temperature for 2 hours, and then dried.

[0065] S4, metal ion coordination: 0.05 mol / L ferric chloride, 30℃, 1h, drying.

[0066] S5. Mix the polymer matrix: 10wt% corn starch + 15wt% glycerol + 5wt% citric acid, and stir well.

[0067] S6, staged crosslinking: 80℃ for 1h + 140℃ for 1h.

[0068] S7. Hot pressing: 150℃, 10MPa, 10min.

[0069] Comparative Example 3 Bamboo raw material: pulverized to 60 mesh; Ethanol aqueous solution: 70%; Hydrogen peroxide: 5 wt% Organic acid: acetic acid 3wt%; Tannic acid: 3wt% Ferric chloride: 0.05 mol / L; Corn starch: 10wt% Nanocellulose: 3wt% Glycerin: 15wt% Citric acid: 5wt% pH adjuster: sodium hydroxide; Preparation steps: S1. Crush bamboo raw materials to 60 mesh, steam explosion at 1.5MPa for 90 seconds.

[0070] S2, Dewaxing and Lignification Treatment: Reflux with 70% ethanol for 2 hours + react with 5wt% hydrogen peroxide / 3wt% acetic acid at 70℃ for 3 hours.

[0071] S3, Tannic acid interface deposition: 3wt% tannic acid aqueous solution, pH 7.5, stirred at room temperature for 2 hours, and then dried.

[0072] S4, metal ion coordination: 0.05 mol / L ferric chloride, 30℃, 1h, drying.

[0073] S5. Mix the polymer matrix: 10wt% corn starch + 15wt% glycerol + 5wt% citric acid + 3wt% nanocellulose, and stir evenly.

[0074] S6, Overall cross-linking: React at 140℃ for 2 hours in a single step.

[0075] S7. Hot pressing: 150℃, 10MPa, 10min.

[0076] To verify the superiority of the purification and preparation method of the bamboo fiber reinforced natural polymer composite material of the present invention in terms of mechanical properties, interfacial stability, and water absorption, the following experiment was designed: 1. Sample Preparation The plates were prepared according to the instructions using the various examples and comparative samples, with a uniform size of 100 mm × 10 mm × 3 mm.

[0077] Five samples of each type were prepared to ensure experimental repeatability.

[0078] 2. Mechanical property testing Tensile strength Instrument: Universal testing machine Fixture spacing: 50 mm Tensile rate: 5 mm / min Measure the ultimate tensile strength (MPa) of each sample and take the average value.

[0079] Bending strength Three-point bending test Span: 60 mm Loading rate: 2 mm / min Measure the bending failure strength (MPa) and take the average value.

[0080] Interfacial shear strength Short beam samples were prepared, and the fiber-matrix interfacial shear strength (MPa) was measured using the single shear method, and the average value was taken.

[0081] 3. Water absorption rate test The sample was dried to constant weight, and the initial weight W0 was recorded. Immerse the sample in water for 24 hours, remove it, wipe off the surface moisture, and measure its weight W1. Water absorption rate calculation:

[0082] The experimental data are as follows: ; The tensile strength, flexural strength, and interfacial shear strength of the embodiments of the present invention are significantly higher than those of the conventional comparative examples, indicating that interfacial strengthening, nano-reinforcement, and staged crosslinking have a significant effect on improving mechanical properties.

[0083] The water absorption rates of the examples were generally lower than those of the comparative samples, indicating that the interface densification treatment effectively inhibited water penetration and improved the material's moisture resistance.

[0084] Comprehensive effect verification of the invention's beneficialness Example 2 showed the best tensile and flexural strength, while Example 3 had a slightly higher water absorption rate, indicating that the nano-reinforcing phase and staged crosslinking significantly contributed to the performance improvement.

[0085] Comparative Example 1 lacks interface reinforcement, Comparative Example 2 lacks nano-reinforcement, and Comparative Example 3 lacks staged cross-linking, resulting in a decline in their respective properties, fully demonstrating the superiority and innovation of the present invention.

[0086] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0087] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for purifying and preparing a bamboo fiber reinforced natural polymer composite material, characterized in that, Includes the following steps: S1. After crushing the bamboo raw material to 40-80 mesh, it is treated under a steam pressure of 1.2-2.0 MPa for 60-120 seconds, and then the pressure is released instantaneously to obtain primary bamboo fiber. S2. The primary bamboo fiber is placed in an ethanol aqueous solution with a volume fraction of 60% to 80% and refluxed at 70 to 90°C for 1 to 3 hours to remove wax and low molecular weight impurities, then filtered and dried. The treated bamboo fiber was then placed in a mixed solution containing 3%–10% hydrogen peroxide and 1%–5% organic acid and reacted at 60–80°C for 2–5 hours to remove some lignin. The bamboo fiber was then washed until neutral and dried to obtain purified bamboo fiber. S3. Add the purified bamboo fiber to a tannic acid aqueous solution with a mass fraction of 0.5% to 5%, adjust the pH of the solution to 7 to 8, and stir for 1 to 3 hours at room temperature to 40°C to deposit a polyphenol interface layer on the surface of the bamboo fiber. Then wash and dry. S4. Add the bamboo fiber treated in S3 to a polyvalent metal salt solution with a concentration of 0.01-0.1 mol / L and treat it at 20-40℃ for 0.5-2 hours to allow tannic acid to undergo a coordination reaction with metal ions and form a coordination cross-linking interface layer on the surface of the bamboo fiber. Then wash and dry. S5. Add the bamboo fiber treated in S4 to a natural polymer matrix solution, wherein the natural polymer matrix has a mass fraction of 5% to 20%, and simultaneously add a plasticizer with a mass fraction of 5% to 30% and a polycarboxylic acid crosslinking agent with a mass fraction of 1% to 10%. Stir and mix at 60 to 90°C to form a homogeneous system. S6. In the system, an in-situ crosslinking reaction is carried out. First, the interface is pre-crosslinked by reacting at 50-90℃ for 0.5-2h, and then the temperature is raised to 100-160℃ for 0.5-2h to complete the overall crosslinking. S7. The obtained system is hot-pressed and pressed at 120-160℃ and 5-15MPa for 5-20 minutes. After cooling, bamboo fiber reinforced natural polymer composite material is obtained.

2. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The organic acid in S2 is acetic acid, formic acid, or a combination thereof, and its mass fraction is 1% to 5% of the mixed solution.

3. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The pH of the tannic acid solution in S3 is adjusted to 7-8 using sodium hydroxide or ammonia.

4. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The polyvalent metal salt in S4 is selected from one or more of ferric chloride, ferric sulfate, aluminum chloride, or calcium chloride.

5. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The natural polymer matrix in S5 is one or more of starch, chitosan, or sodium alginate, wherein the starch is pregelatinized at 80-95°C for 10-30 minutes before use.

6. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The S5 contains a nanoscale natural reinforcing phase, which is nanocellulose or nanocrystalline cellulose. The amount of the reinforcing phase added is 1% to 8% of the mass of the natural polymer matrix, and it is ultrasonically dispersed for 10 to 30 minutes before being added.

7. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The plasticizer in S5 is glycerol, and its addition amount is 10% to 25% of the mass of the natural polymer matrix.

8. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The polycarboxylic acid crosslinking agent in S5 is citric acid, and its addition amount is 3% to 8% of the mass of the natural polymer matrix.

9. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The heating rate in S6 is controlled to be 2-10℃ / min.

10. The purification and preparation method of a bamboo fiber reinforced natural polymer composite material according to claim 1, characterized in that: The bamboo fiber surface has a tannic acid-metal ion coordination interface layer, and the natural polymer matrix forms an in-situ cross-linked network structure on the bamboo fiber surface. The bamboo fiber, nano-reinforcing phase and natural polymer matrix form a multi-scale synergistic reinforcement structure.

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