A method for preparing highly water-resistant, glue-free, synergistically cross-linked bamboo-based boards

Abstract

This invention discloses a method for preparing a highly water-resistant, glue-free, synergistically cross-linked bamboo-based board, comprising the following steps: preparing polyphenol-modified bamboo powder; adding glucuronic acid solution dropwise to the polyphenol-modified bamboo powder, reacting at 50℃~65℃, then adding citric acid solution, and reacting again to obtain cross-linked modified bamboo powder; adjusting the moisture content of the cross-linked modified bamboo powder to 8wt%~12wt%, and pre-pressing it into a blank; covering the upper and lower surfaces of the blank with a release film and a steel plate respectively, then placing it in a flatbed hot press, and performing segmented pressing molding at 155℃~165℃ to obtain the highly water-resistant, glue-free, synergistically cross-linked bamboo-based board. The resulting board requires no traditional adhesives, possesses high mechanical strength, excellent water resistance, and dimensional stability, and the process is mild with green raw materials, making it suitable for the field of green building materials and providing a new approach for the efficient utilization of bamboo resources.

Description

Technical Field

[0001] This invention relates to a method for preparing bamboo-based panels, belonging to the technical field of composite materials and bamboo-based engineered wood products. Background Technology

[0002] With the efficient utilization of bamboo resources and the rapid development of the green building materials industry, bamboo-based engineered wood products are gradually becoming an important material to replace wood-based panels due to their advantages such as short growth cycle, high renewability, and excellent mechanical properties. However, most existing bamboo-based panels rely on urea-formaldehyde resin, phenolic resin, or isocyanate-based synthetic resins as adhesives for bonding and molding. Although this achieves high mechanical strength, it generally suffers from problems such as formaldehyde release, high energy consumption in the production process, and a heavy environmental burden, making it difficult to meet the current development needs of green environmental protection and low-carbon manufacturing. Especially in the field of interior decoration and furniture, higher requirements are placed on low volatile organic compound (VOC) emissions and environmental performance. Therefore, the development of glue-free or low-glue bamboo-based panel preparation technologies is of great significance.

[0003] Currently, so-called "glue-free boards" mostly achieve self-bonding by increasing the hot-pressing temperature and pressure, utilizing the thermoplastic flow of natural lignin in bamboo powder or bamboo fiber. However, this method often suffers from problems such as weak bonding interfaces, high internal porosity, and poor water resistance. Bamboo is rich in cellulose and hemicellulose, which have numerous hydrophilic hydroxyl groups, making them prone to moisture absorption and swelling. This leads to a decrease in strength and an increase in thickness expansion rate under water immersion conditions, severely limiting their application in humid environments. Furthermore, single physical compaction or single chemical modification methods cannot simultaneously improve strength and water resistance; existing technologies still have shortcomings in the controllability of cross-linking structure construction and multi-level synergy.

[0004] In recent years, polyphenolic natural substances have attracted attention in the field of wood material modification. Polyphenolic compounds such as tannins can generate quinone-based active intermediates under oxidative conditions, which then undergo cross-linking reactions with the substrate surface, thereby improving interfacial bonding strength. Simultaneously, aldehydes or polycarboxylic acids can undergo condensation or esterification reactions with hydroxyl groups to construct covalent cross-linked networks, improving the material's water resistance. However, existing related technologies mostly employ single cross-linking systems, failing to fully utilize the synergistic effects of multiple cross-linking mechanisms, and lacking systematic control over the reaction process and post-curing behavior, resulting in limited modification effects.

[0005] Therefore, developing a method for preparing glue-free bamboo-based boards based on multi-level reaction regulation and synergistic cross-linking to construct a dense structure, in order to achieve a balance between high water resistance and high mechanical properties, has become an urgent technical problem to be solved in this field. Summary of the Invention The purpose of this invention is to provide a method for preparing highly water-resistant, glue-free, synergistically cross-linked bamboo-based boards, so as to solve the above-mentioned problems existing in the prior art.

[0006] This invention is achieved through the following technical solution: A method for preparing a highly water-resistant, glue-free, synergistically cross-linked bamboo-based board includes the following steps: Preparation of polyphenol-modified bamboo powder; Glucuronium solution was added dropwise to the polyphenol-modified bamboo powder, and after reacting at 50℃~65℃, citric acid solution was added. After the reaction, cross-linked modified bamboo powder was obtained. The moisture content of the cross-linked modified bamboo powder is adjusted to 8wt%~12wt%, and it is pre-pressed into a green body; After covering the upper and lower surfaces of the blank with release film and steel plate respectively, it is placed in a flat hot press and subjected to segmented pressure molding at 155℃~165℃ to obtain the high water-resistant glue-free synergistic cross-linked bamboo-based board.

[0007] As a preferred embodiment, the preparation method of the polyphenol-modified bamboo powder includes the following steps: Tannic acid was added to 0.05 mol / L disodium hydrogen phosphate buffer to obtain a 2% tannic acid solution. Bamboo powder is preheated at 50℃~60℃ for 5min~10min, then added to the tannic acid solution. After stirring and soaking at 55℃~65℃, laccase solution is added in two portions according to the total enzyme amount. The first portion is 60% of the total enzyme amount, and the reaction is carried out for 15min. The second portion is 40% of the total enzyme amount, and the reaction is carried out for another 15min to obtain the polyphenol-modified bamboo powder.

[0008] As a preferred embodiment, the bamboo powder has a size of 40-80 mesh.

[0009] As a preferred embodiment, the solid-liquid ratio of the bamboo powder to the tannic acid solution is 1:10 to 1:20 (g:mL).

[0010] As a preferred embodiment, the total enzyme content of the laccase solution is 100 U / g to 150 U / g based on the dry weight of the bamboo powder.

[0011] As a preferred embodiment, the amount of glucuronid added is 0.8wt%~1.5wt% of the dry weight of bamboo powder (based on the amount of pure glucuronidid), and the amount of citric acid solution added is 0.5wt%~1wt% of the dry weight of bamboo powder (based on the amount of pure citric acid).

[0012] As a preferred embodiment, in the segmented pressure molding process, the pressure is first pre-pressed at 5MPa for 2 minutes, and then increased to 15MPa and held for 6 minutes.

[0013] A highly water-resistant, glue-free, synergistically cross-linked bamboo-based board obtained by the aforementioned preparation method.

[0014] The basic principle of this invention is as follows: 1. This invention involves the staged addition of laccase to catalyze a controlled oxidation reaction of tannic acid, generating quinones and free radical active intermediates on the surface of bamboo powder, constructing a uniformly distributed polyphenol active network, providing reaction sites for subsequent cross-linking reactions, and improving interfacial binding capacity.

[0015] 2. Glucuronium undergoes a condensation reaction with polyphenol oxidation products and hydroxyl groups on the surface of bamboo powder to form a primary covalent cross-linked structure. At the same time, citric acid is introduced and undergoes an esterification reaction with hydroxyl groups during hot pressing to construct a secondary cross-linked network. This reduces the content of hydrophilic groups through synergistic cross-linking, thereby improving structural stability and water resistance.

[0016] 3. During the hot pressing process, the natural lignin in the bamboo powder undergoes thermoplastic flow and fills the pores. After cooling, it solidifies and forms a dense structure together with the covalent cross-linked network mentioned above. This achieves the synergistic effect of physical compaction and chemical cross-linking, thereby improving the mechanical strength and dimensional stability of the board.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention constructs a multi-level covalent network structure through controlled enzymatic oxidation and synergistic crosslinking with aldehydes and carboxylic acids, which significantly improves the bonding strength and crosslinking density between bamboo powder particles. High-strength boards can be obtained without adding traditional synthetic resin adhesives, taking into account both mechanical and environmental performance.

[0018] 2. This invention introduces a dual synergistic mechanism of glucuronidation condensation crosslinking and citric acid thermal esterification followed by curing, which effectively reduces the exposure of hydrophilic hydroxyl groups on the surface of bamboo powder and further densifies the structure during hot pressing, thereby significantly reducing the water absorption rate and thickness expansion rate of the board, and improving water resistance and dimensional stability.

[0019] 3. The process conditions of this invention are mild, the raw materials are widely available, the reaction process is controllable, and it is easy to scale up production. This not only increases the added value of bamboo resources, but also meets the development needs of green and low-carbon building materials, and has good prospects for industrial application. Attached Figure Description

[0020] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 The flowchart of the preparation process of the high water-resistant glue-free synergistic cross-linked bamboo-based board of the present invention. Detailed Implementation

[0021] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0022] Example 1 This embodiment provides a method for preparing highly water-resistant, glue-free, synergistically cross-linked bamboo-based boards, such as... Figure 1 As shown, it includes the following steps: 1. Preheat wood flour in a 55℃ oven for 5 minutes. Dissolve tannic acid in a 0.05mol / L disodium hydrogen phosphate buffer solution to obtain a tannic acid mass fraction of 2%. Add bamboo flour to the tannic acid solution, with a solid-liquid ratio of 1:10 (g:mL). Stir and soak at 60℃ for 20 minutes to allow tannic acid to be fully adsorbed onto the bamboo flour surface. Then, proceed to the controlled enzymatic oxidation stage: Add laccase solution in two portions, based on a total enzyme dosage of 100U / g (dry basis) of bamboo flour. Add 60% of the total enzyme dosage in the first portion and stir at 60℃ for 15 minutes to partially oxidize tannic acid into quinone active intermediates and form primary free radical active sites on the bamboo flour surface. Then, add the remaining 40% of the laccase solution and continue the reaction for 15 minutes to regulate the degree of polyphenol oxidation and free radical density, constructing a uniform and controllable polyphenol oxidation crosslinking precursor network.

[0023] 2. After the controlled oxidation reaction described above is completed, a glucuronidate solution diluted with disodium hydrogen phosphate buffer (0.5 wt% pure glucuronidate based on dry bamboo powder) is slowly added dropwise to the reaction system. The mixture is stirred at 60°C for 20 min to allow glucuronidate to undergo condensation crosslinking with the polyphenol oxidation products and the hydroxyl groups on the bamboo powder surface, forming a primary covalent crosslinking network. Citric acid (0.5 wt% citric acid based on dry bamboo powder) is then added, and the mixture is stirred at 60°C for 10 min to ensure uniform dispersion and hydrogen bonding / complexation pre-bonding with the bamboo powder surface, thereby introducing an esterification crosslinking precursor that can be triggered in the subsequent hot pressing stage. After the reaction is complete, the solid is separated by filtration, and the crosslinked modified bamboo powder material is obtained by filtration, washing, and drying.

[0024] 3. Adjust the moisture content of the modified bamboo powder to 10 wt%. Then, evenly spread the moistened modified bamboo powder onto a metal mold frame, and form a uniform preform through vibration and light pre-pressing. A polytetrafluoroethylene (PTFE) release film and a steel plate are respectively laid on the top and bottom of the preform (i.e., PTFE release film on the upper surface and steel plate on the lower surface). The preform is placed in a flatbed hot press and hot-pressed in sections at 160℃: first, a pre-press of 5 MPa is applied for 2 minutes, then the pressure is increased to 15 MPa and held for 6 minutes. After cooling to below 60℃, the bamboo-based board is demolded. The experimental preparation process is as follows: Figure 1 As shown in Table 1.

[0025] Example 2 This embodiment provides a method for preparing highly water-resistant, glue-free, synergistically cross-linked bamboo-based boards, such as... Figure 1 As shown, it includes the following steps: 1. Preheat wood flour in a 55℃ oven for 5 minutes. Dissolve tannic acid in a 0.05mol / L disodium hydrogen phosphate buffer solution to obtain a tannic acid mass fraction of 2%. Add bamboo flour to the tannic acid solution, with a solid-liquid ratio of 1:15 (g:mL). Stir and soak at 65℃ for 30 minutes to allow tannic acid to be fully adsorbed onto the bamboo flour surface. Then, proceed to the controlled enzymatic oxidation stage: Add laccase solution in two portions, based on a total enzyme dosage of 150U / g (dry basis) of bamboo flour. Add 60% of the total enzyme dosage in the first portion and stir at 60℃ for 15 minutes to partially oxidize tannic acid into quinone active intermediates and form primary free radical active sites on the bamboo flour surface. Then, add the remaining 40% of the laccase solution and continue the reaction for 15 minutes to regulate the degree of polyphenol oxidation and free radical density, constructing a uniform and controllable polyphenol oxidation crosslinking precursor network.

[0026] 2. After the controlled oxidation reaction is completed, a glucuronic acid solution diluted with disodium hydrogen phosphate buffer (1 wt% of glucuronic acid based on dry bamboo powder) is slowly added dropwise to the reaction system, and the mixture is stirred at 60°C for 30 min to allow glucuronic acid to undergo condensation crosslinking with the polyphenol oxidation product and the hydroxyl groups on the bamboo powder surface, forming a primary covalent crosslinking network. Citric acid (1 wt% of citric acid based on dry bamboo powder) is then added, and the mixture is stirred at 60°C for 10 min to allow the citric acid to be uniformly dispersed and form hydrogen bonds / complexes with the bamboo powder surface, thus introducing an esterification crosslinking precursor that can be triggered in the subsequent hot pressing stage. After the reaction is complete, the solid is separated by filtration, and the crosslinked modified bamboo powder material is obtained by filtration, washing, and drying.

[0027] 3. Adjust the moisture content of the modified bamboo powder to 12wt%, then evenly spread the conditioned modified bamboo powder into a metal mold frame. Form a uniform preform through vibration and light pre-pressing. Cover the preform with PTFE release film and steel plate (i.e., PTFE release film on the upper surface and steel plate on the lower surface). Place the preform into a flatbed hot press and hot-press it in sections at 155℃: first apply 5MPa pre-pressing for 2 minutes, then increase the pressure to 15MPa and hold for 6 minutes. Cool to below 60℃ and demold to obtain bamboo-based boards. Performance tests are shown in Table 1.

[0028] Example 3 This embodiment provides a method for preparing highly water-resistant, glue-free, synergistically cross-linked bamboo-based boards, such as... Figure 1 As shown, it includes the following steps: 1. Preheat wood flour in a 55℃ oven for 5 minutes. Dissolve tannic acid in a 0.05mol / L disodium hydrogen phosphate buffer solution to obtain a tannic acid mass fraction of 2%. Add bamboo flour to the tannic acid solution, with a solid-liquid ratio of 1:12 (g:mL). Stir and soak at 60℃ for 25 minutes to allow tannic acid to be fully adsorbed onto the bamboo flour surface. Then, proceed to the controlled enzymatic oxidation stage: Add laccase solution in two portions, based on a total enzyme dosage of 120U / g (dry basis) of bamboo flour. Add 60% of the total enzyme dosage in the first portion and stir at 60℃ for 15 minutes to partially oxidize tannic acid into quinone active intermediates and form primary free radical active sites on the bamboo flour surface. Then, add the remaining 40% of the laccase solution and continue the reaction for 15 minutes to regulate the degree of polyphenol oxidation and free radical density, constructing a uniform and controllable polyphenol oxidation crosslinking precursor network.

[0029] 2. After the controlled oxidation reaction described above is completed, a glucuronic acid solution diluted with disodium hydrogen phosphate buffer (1.5 wt% of glucuronic acid based on dry bamboo powder) is slowly added dropwise to the reaction system, and the mixture is stirred at 60°C for 30 min to allow glucuronic acid to undergo condensation crosslinking with the polyphenol oxidation product and the hydroxyl groups on the bamboo powder surface, forming a primary covalent crosslinking network. Citric acid (0.5 wt% of citric acid based on dry bamboo powder) is then added, and the mixture is stirred at 60°C for 10 min to allow the citric acid to be uniformly dispersed and form hydrogen bonds / complexes with the bamboo powder surface, thus introducing an esterification crosslinking precursor that can be triggered in the subsequent hot pressing stage. After the reaction is complete, the solid is separated by filtration, and the crosslinked modified bamboo powder material is obtained by filtration, washing, and drying.

[0030] 3. Adjust the moisture content of the modified bamboo powder to 10 wt%. Then, evenly spread the moistened modified bamboo powder onto a metal mold frame, and form a uniform preform through vibration and light pre-pressing. Cover the preform with PTFE release film and steel plate (i.e., PTFE release film on the upper surface and steel plate on the lower surface). Place the preform into a flatbed hot press and hot-press it in sections at 165℃: first apply a pre-press of 5 MPa for 2 minutes, then increase the pressure to 15 MPa and hold for 6 minutes. Cool to below 60℃ and demold to obtain bamboo-based boards. Performance tests are shown in Table 1.

[0031] Comparative Example 1: The difference between this comparative example and Example 1 is that the amount of laccase added in step 1) is 0, while the other conditions remain unchanged, resulting in bamboo-based boards. The performance tests are shown in Table 1.

[0032] Comparative Example 2: The difference between this comparative example and Example 1 is that the amount of citric acid added in step 2) is 0, while the other conditions remain unchanged, resulting in bamboo-based boards. The performance tests are shown in Table 1.

[0033] Comparative Example 3: The difference between this comparative example and Example 1 is that the amount of glucuronic acid added in step 2) is 0, while the other conditions remain unchanged, resulting in bamboo-based boards. The performance tests are shown in Table 1.

[0034] Table 1. Physical and chemical properties of bamboo-based boards

[0035] As shown in Table 1, the density of the bamboo-based board prepared in Example 1 is 0.97 g·cm³. -3 The static bending strength and internal bond strength were 42.6 MPa and 0.82 MPa, respectively, and the 24-hour water absorption rate and thickness expansion rate were 18.5% and 6.2%, respectively. Compared with Comparative Example 1 without laccase, this invention significantly improved the internal bond strength and static bending strength of the board through controlled enzymatic oxidation. Compared with Comparative Examples 2 and 3, which used only a single crosslinking agent, this invention constructed a double network structure through "glucuronidation condensation crosslinking - citric acid thermal esterification followed by curing," which reduced the 24-hour water absorption rate of the board to 18.5%, the thickness expansion rate to 6.2%, and significantly improved the water resistance and wet bond strength.

[0036] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a highly water-resistant, glue-free, synergistically cross-linked bamboo-based board, characterized in that, Includes the following steps: Preparation of polyphenol-modified bamboo powder; A glucuronic acid solution diluted with disodium hydrogen phosphate buffer was added dropwise to the polyphenol-modified bamboo powder. After reacting at 50℃~65℃, a citric acid solution was added. After the reaction, cross-linked modified bamboo powder was obtained. The moisture content of the cross-linked modified bamboo powder is adjusted to 8wt%~12wt%, and it is pre-pressed into a green body; After covering the upper and lower surfaces of the blank with release film and steel plate respectively, it is placed in a flat hot press and subjected to segmented pressure molding at 155℃~165℃ to obtain the high water-resistant glue-free synergistic cross-linked bamboo-based board.

2. The method for preparing high water-resistant, glue-free, synergistically cross-linked bamboo-based panels according to claim 1, characterized in that, The preparation method of the polyphenol-modified bamboo powder includes the following steps: Tannic acid was added to 0.05 mol / L disodium hydrogen phosphate buffer to obtain a 2% tannic acid solution. Bamboo powder is preheated at 50℃~60℃ for 5min~10min, then added to the tannic acid solution. After stirring and soaking at 55℃~65℃, laccase solution is added in two portions according to the total enzyme amount. The first portion is 60% of the total enzyme amount, and the reaction is carried out for 15min. The second portion is 40% of the total enzyme amount, and the reaction is carried out for another 15min to obtain the polyphenol-modified bamboo powder.

3. The method for preparing high water-resistant, glue-free, synergistically cross-linked bamboo-based boards according to claim 2, characterized in that, The bamboo powder has a size of 40-80 mesh.

4. The method for preparing high water-resistant, glue-free, synergistically cross-linked bamboo-based panels according to claim 2, characterized in that, The solid-liquid ratio of the bamboo powder to the tannic acid solution is 1:10 to 1:20 (g:mL).

5. The method for preparing high water-resistant, glue-free, synergistically cross-linked bamboo-based panels as described in claim 2, characterized in that, The total enzyme content of the laccase solution is 100 U / g to 150 U / g, calculated based on the dry weight of the bamboo powder.

6. The method for preparing high water-resistant, glue-free, synergistically cross-linked bamboo-based panels as described in claim 1, characterized in that, The amount of glucuronide added is 0.8wt%~1.5wt% of the dry weight of bamboo powder, and the amount of citric acid solution added is 0.5wt%~1wt% of the dry weight of bamboo powder.

7. The method for preparing high water-resistant, glue-free, synergistically cross-linked bamboo-based panels as described in claim 1, characterized in that, In the segmented pressure molding process, the pressure is first pre-pressed at 5MPa for 2 minutes, and then increased to 15MPa and held for 6 minutes.

8. A highly water-resistant, glue-free, synergistically cross-linked bamboo-based board obtained by the preparation method according to any one of claims 1 to 7.

Images