Method for softening bamboo wood through cooperation of metal ion-based eutectic solvent and high-temperature steam

Abstract

The invention belongs to the technical field of bamboo processing and modification, and particularly relates to a method for softening bamboo through cooperation of a metal ion-based eutectic solvent and high-temperature steam based on an in-situ modification mechanism. The method comprises the following steps: preparing a metal ion-based eutectic solvent (MDES) taking metal salt as a hydrogen bond acceptor; the bamboo wood is placed in the solvent to be soaked, and the solvent permeates into cell walls; and after the bamboo is taken out and surface floating liquid is wiped off, high-temperature steam treatment is directly carried out without water washing. According to the method, a crystalline region is destroyed by virtue of strong coordination of metal ions and cellulose, and meanwhile, lignin is induced to generate in-situ depolymerization by virtue of MDES in a high-temperature water vapor environment, so that a low-molecular-weight lignin segment is generated. The method solves the problems of low bamboo yield and damaged mechanical strength caused by the fact that a large amount of lignin is removed in an existing chemical softening technology, through an in-situ plasticizing mechanism, deep softening is achieved while the high yield of the bamboo is kept, the anti-bending elastic modulus is greatly reduced, and the deformation machining performance of the bamboo is improved.

Description

Technical Field

[0001] This invention belongs to the field of bamboo processing and modification technology, specifically relating to a method for synergistic softening of bamboo by a metal ion-based eutectic solvent and high-temperature steam based on an in-situ modification mechanism. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Bamboo, as a biomass material with rapid growth, strong regeneration capacity, and excellent mechanical properties, is widely used in building decoration, furniture manufacturing, and vehicle interiors under the global trend of "replacing plastics with bamboo." However, bamboo's natural characteristics, such as thin-walled hollow structure, hierarchical fiber structure, and high crystallinity, along with its gradient density distribution, make it prone to stress concentration during deformation processing such as flattening, large-curvature bending, and compression. This can lead to problems such as cracking and large rebound, making it difficult to meet the processing requirements of high-value-added irregularly shaped bamboo products. Therefore, to solve these problems, effective softening treatment of bamboo is necessary.

[0004] Currently, commonly used methods for softening bamboo mainly include physical methods (hydrothermal treatment, microwave treatment) and chemical methods (alkali impregnation, ammonia fumigation). Traditional physical methods, such as high-temperature steam treatment, while simple and environmentally friendly, primarily act on non-crystalline regions and have negligible ability to damage the highly resistant cellulose crystalline regions of bamboo, leaving the treated bamboo with a still relatively high modulus of elasticity. Chemical methods, while significantly reducing hardness, often come with serious environmental pollution.

[0005] In recent years, eutectic solvents (DES) have attracted attention as a green solvent. However, existing DES research mainly focuses on using organic ammonium salts such as choline chloride and betaine as hydrogen bond acceptors. A published patent application (CN202510026614.5) reports a method for preparing softened wood, the resulting softened wood, and its applications. However, this patent uses traditional organic eutectic solvents (such as the choline chloride / lactic acid system), which typically follow a "subtractive manufacturing" approach, creating pores by removing a large amount of lignin, thereby reducing hardness. Although this method can soften materials, it leads to severe mass loss (typically >20%), resulting in low material yield, significantly reduced mechanical strength, and the need for subsequent large-scale water washing to remove dissolved lignin, causing water waste and environmental pollution.

[0006] To date, no research has proposed a new technology that can both deeply disrupt the crystalline region and retain the main components of bamboo to achieve "in-situ plasticizing and softening." Therefore, developing a high-yield, high-efficiency bamboo synergistic softening method has significant practical application value. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of low bamboo yield, damaged strength, and environmental pollution caused by the large-scale removal of lignin in the prior art, and to provide a method for softening bamboo by synergistic use of metal ion-based eutectic solvent and high-temperature steam based on in-situ modification mechanism.

[0008] To achieve the above objectives, this application adopts the following technical solution: In the first aspect, this application proposes a method for softening bamboo by synergistic use of metal ion-based eutectic solvent and high-temperature steam based on in-situ modification mechanism, comprising the following steps: (1) Preparation of metal ion-based eutectic solvent (MDES): using Lewis acidic metal salt as hydrogen bond acceptor (HBA), mixing with hydrogen bond donor (HBD), heating and stirring to form a uniform and transparent eutectic solvent; (2) MDES impregnation pretreatment: placing the bamboo to be treated in the eutectic solvent or its aqueous solution prepared in step (1) for impregnation treatment, so that the solvent penetrates into the interior of the bamboo cell wall; (3) Solvent retention and synergistic steam treatment: taking out the bamboo treated in step (2), wiping off the surface liquid without washing with water, retaining the eutectic solvent that has penetrated into the interior of the bamboo, and then placing it in a high-temperature and high-humidity environment for saturated steam softening treatment.

[0009] Further, the metal salt mentioned in step (1) is selected from one or more of zinc chloride, aluminum chloride, magnesium chloride, ferric chloride, and calcium chloride; the hydrogen bond donor is selected from one or more of lactic acid, citric acid, urea, ethylene glycol, and glycerol; the molar ratio of the two is 1:5 to 5:1.

[0010] Furthermore, the metal ion-based eutectic solvent system does not contain quaternary ammonium salt hydrogen bond acceptors.

[0011] Further, the impregnation treatment in step (2) adopts the vacuum pressure impregnation method: first, it is treated for 10 to 60 min under a vacuum of -0.05 to -0.1 MPa, then the eutectic solvent is introduced, and then it is treated for 30 to 180 min under a pressure of 0.5 to 2.0 MPa.

[0012] Furthermore, between the impregnation treatment in step (2) and the steam treatment in step (3), there is an aging and diffusion step: the bamboo material after wiping off the surface liquid is sealed and aged at room temperature for 12 to 72 hours.

[0013] Furthermore, the temperature of the high-temperature steam treatment in step (3) is 100℃~160℃, and the treatment time is 10min~240min.

[0014] Secondly, this application proposes a softened bamboo material prepared using the above method.

[0015] The beneficial effects of this invention are as follows: This invention achieves a unique "crystal deconstruction" mechanism through the synergy of metal ion-based DES and high-temperature steam: compared to traditional methods that can only act on amorphous regions, the metal ions in this invention (such as Zn²⁺) can... + Al³ + It has a high charge density and empty orbitals, which can form stable coordination bonds with hydroxyl groups on cellulose chains, competitively destroying the original intermolecular hydrogen bond network, thereby directly leading to a decrease in the crystallinity of cellulose and a relaxation of microfibril orientation.

[0016] Experimental results show that the bamboo prepared by the method of this invention has a significantly better softening effect than ordinary DES treatment without metal ions. The short-term compression rate of the treated bamboo can reach over 57%, and the bending elastic modulus is reduced to 9.15 GPa, which greatly improves the plastic processing performance of bamboo and provides technical support for the manufacture of complex-shaped bamboo products. Attached Figure Description

[0017] Figure 1 This is a schematic diagram comparing the X-ray diffraction (XRD) patterns of Comparative Examples 1 and 2 (untreated material and material treated with steam only) with Example 1 (metal ion-based MDES-treated material). The figure shows that the metal ion system can significantly reduce the crystallinity of cellulose, which is impossible with traditional organic DES.

[0018] Figure 2 This is a schematic diagram comparing the dynamic thermomechanical analysis (DMA) loss modulus curves of Comparative Examples 1 and 2 (untreated material and material treated with steam only) with Example 1 (metal ion-based MDES treated material). The figure shows a significant reduction in the glass transition temperature (Tg) of the bamboo after treatment, confirming the excellent plasticizing and softening effect. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The embodiments provide detailed implementation methods and specific operation processes, but the scope of protection of the present invention is not limited to the following embodiments.

[0020] In the following examples, the metal ion-based eutectic solvent was prepared by heating and stirring.

[0021] Example 1 Preparation of metal ion-based eutectic solvent: Using 2 parts D-lactic acid (LA) as a hydrogen bond donor and 1 part zinc chloride (ZnCl2) as a hydrogen bond acceptor (molar ratio), the mixture was heated and stirred in a 90°C water bath for 60 min until a homogeneous and transparent liquid was formed. After cooling, deionized water was added by mass percentage to prepare a 70% LA / ZnCl2 modified solution.

[0022] Bamboo processing and impregnation: Three-year-old moso bamboo was selected and processed into test materials with dimensions of 50mm×20mm×5mm. The bamboo was placed in a pressure vessel and treated under a vacuum of -0.09 MPa for 45 min, and then the modified liquid was drawn in and impregnated under pressure of 0.8 MPa for 90 min.

[0023] In-situ modification treatment: After impregnation, the bamboo was removed, and excess liquid was wiped off the surface with a dry cloth (without washing). It was then wrapped in plastic wrap and aged at room temperature for 24 hours to promote solvent diffusion into the micropores of the cell walls. Subsequently, the bamboo was placed in a high-temperature steam generator and treated in a saturated steam environment at 130°C for 1 hour. After cooling, softened bamboo was obtained.

[0024] Example 2 Preparation of metal ion-based eutectic solvent: Using 1 part citric acid (CA) and 1 part zinc chloride (ZnCl2) as raw materials (molar ratio), the solutions were stirred and dissolved at 95°C to prepare a 60% CA / ZnCl2 modified solution.

[0025] Processing and impregnation of bamboo: The process is the same as in Example 1, but the impregnation conditions are adjusted as follows: vacuum degree -0.1 MPa for 60 min, pressure 1.0 MPa for 120 min.

[0026] In-situ modification treatment: The bamboo was removed, the surface liquid was wiped off, and it was aged at room temperature for 48 hours. Then it was placed in a steam environment at 120℃ for 1.5 hours to obtain softened bamboo.

[0027] Example 3 Preparation of metal ion-based eutectic solvent: MDES was prepared by heating and melting 2 parts urea and 1 part aluminum chloride hexahydrate (AlCl3·6H2O) as raw materials, and then prepared into an 80% concentration aqueous solution.

[0028] Bamboo processing and impregnation: Vacuum pressure impregnation is used, with a pressure of 1.5 MPa maintained for 60 min.

[0029] In-situ modification treatment: The bamboo was removed, its surface was dried, and it was aged for 12 hours. Subsequently, it was treated with steam at 140℃ for 40 minutes to obtain softened bamboo.

[0030] Comparative Example 1 Untreated bamboo material, only subjected to humidity conditioning, served as a blank control.

[0031] Comparative Example 2 Simple high-temperature steam treatment. Bamboo of the same specifications was placed in saturated steam at 130℃ for 1 hour (without DES impregnation).

[0032] Comparative Example 3 Organic DES treatment Solvent preparation: Using 2 parts D-lactic acid (LA) and 1 part choline chloride (ChCl) as raw materials (molar ratio), 1 wt% ferric chloride was added as a catalyst.

[0033] Processing procedure: The impregnation process is the same as in Example 1. After impregnation, the bamboo is reacted at 90°C for 2 hours, then rinsed repeatedly with a large amount of deionized water until neutral to remove the dissolved lignin, and finally dried.

[0034] Effect testing and data analysis The bamboo materials prepared in the above embodiments and comparative examples were subjected to performance tests to examine the mass loss rate, lignin retention rate (Klason method), crystallinity (XRD) and mechanical properties. The results are shown in Table 1.

[0035] Table 1 Comparison of physical and mechanical properties and component changes of bamboo materials in each group

[0036] As shown in Table 1, Comparative Example 3 exhibits a mass loss rate as high as 24.5% and a lignin retention rate of only 18.2%, indicating that it primarily achieves softening by removing lignin to create pores. Although the compression ratio is increased, material loss is severe. In contrast, Example 1 of this invention has a mass loss rate of only 4.20%, a lignin retention rate as high as 92.50%, a compression ratio of 57.92%, and the lowest flexural modulus of elasticity. This demonstrates that this invention successfully achieves deep softening without significant component loss.

[0037] Figure 1 These are the XRD patterns of Comparative Examples 1, 2, and Example 1. From... Figure 1 As can be seen, the crystallinity of Example 1 decreased from 33.90% to 30.30%, indicating a significant change in crystal structure; while the crystallinity of Comparative Example 4 (organic DES) changed only slightly (approximately 34.20%). This directly corroborates the innovative mechanism of "metal ion coordination disrupting hydrogen bonds in the crystalline region" proposed in this invention.

[0038] Figure 2 These are the DMA loss modulus curves for Comparative Examples 1, 2, and Example 1. From... Figure 2As can be seen, the glass transition temperature (Tg) of untreated bamboo (Comparative Example 1) is as high as 213.01℃, indicating restricted molecular chain movement. However, after treatment with the present invention (Example 1), the Tg of the bamboo is significantly reduced to 180.14℃, a decrease of 32.87℃. This shows that MDES small molecules successfully penetrate into the non-crystalline and partially crystalline regions of the cell wall, significantly increasing the free volume between molecular chains and playing a highly efficient plasticizing and lubricating role, thereby endowing bamboo with excellent macroscopic deformation capabilities.

[0039] In summary, by selecting a metal ion-based eutectic solvent, this invention successfully overcomes the bottleneck of softening efficiency in traditional methods, and is a highly efficient and controllable pretreatment technology for deep processing of bamboo.

Claims

1. A method for synergistically softening bamboo using a metal ion-based eutectic solvent and high-temperature steam, characterized in that, Includes the following steps: (1) Preparation of metal ion-based eutectic solvent (MDES): Lewis acidic metal salts are used as hydrogen bond acceptors (HBA) and mixed with hydrogen bond donors (HBD) at a molar ratio of 1:5 to 5:

1. The mixture is heated and stirred to form a uniform and transparent eutectic solvent. The metal ion-based eutectic solvent system does not contain quaternary ammonium salt hydrogen bond acceptors. (2) MDES impregnation pretreatment: The bamboo to be treated is placed in the eutectic solvent or its aqueous solution prepared in step (1) for impregnation treatment, so that the solvent penetrates into the cell wall of the bamboo. (3) Solvent retention and synergistic steam treatment: The bamboo treated in step (2) is taken out, the surface liquid is wiped off and it is not washed with water. The eutectic solvent that has penetrated into the bamboo is retained. Then, it is placed in a high temperature and high humidity environment for saturated steam softening treatment, so that the internal components of the bamboo are modified in situ, and thus softened bamboo is obtained.

2. The method according to claim 1, characterized in that, The metal salt is selected from one or more of zinc chloride, aluminum chloride, magnesium chloride, ferric chloride, and calcium chloride; the hydrogen bond donor (HBD) is selected from one or more of lactic acid, citric acid, oxalic acid, acetic acid, urea, ethylene glycol, and glycerol.

3. The method according to claim 2, characterized in that, The metal ion-based eutectic solvent is preferably composed of zinc chloride or aluminum chloride as hydrogen bond acceptors and lactic acid or citric acid as hydrogen bond donors.

4. The method according to claim 1, characterized in that, In step (2), the metal ion-based eutectic solvent is used in the form of an aqueous solution, and the MDES concentration is 10% to 90% by mass percentage, preferably 50% to 80%.

5. The method according to claim 1, characterized in that, The impregnation treatment in step (2) includes atmospheric pressure impregnation, vacuum pressure impregnation, or ultrasonic-assisted impregnation; wherein the conditions for vacuum pressure impregnation are: first, maintain a vacuum of -0.05 to -0.1 MPa for 10 to 60 min, and then maintain a pressure of 0.5 to 2.0 MPa for 30 to 180 min.

6. The method according to claim 1, characterized in that, Between the impregnation treatment in step (2) and the steam treatment in step (3), there is also an aging and diffusion step: the bamboo material after wiping off the surface liquid is sealed and aged at room temperature for 12h to 72h to promote the diffusion of the solvent into the cellulose crystallization area and the gaps between the microfibrils of the bamboo material.

7. The method according to claim 1, characterized in that, The temperature of the high-temperature steam treatment in step (3) is 100℃~160℃ and the treatment time is 10 min~240 min; the preferred temperature is 120℃~140℃ and the preferred time is 30 min~90 min.

8. The method according to claim 1, characterized in that, During the high-temperature steam treatment, the metal ion-based eutectic solvent that penetrates into the bamboo induces in-situ depolymerization of lignin, generating low molecular weight lignin fragments. These low molecular weight lignin fragments remain in the gaps between bamboo fibers and act as plasticizers.

9. A softened bamboo material prepared by the method according to any one of claims 1-8, characterized in that, The softened bamboo has a lignin retention rate of more than 80%, a short-time compression rate of more than 55%, and a bending modulus of elasticity of less than 10 GPa.

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