A lignin-epoxidized carboxymethyl chitosan adhesive, its preparation method and application

Abstract

This invention relates to the field of biomass polymer materials and wood adhesives, specifically disclosing a lignin-epoxidized carboxymethyl chitosan adhesive, its preparation method, and its application. The preparation method includes the following steps: mixing a carboxymethyl chitosan solution with ethylene glycol diglycidyl ether and stirring until the solution turns milky white; then adding lignin and mixing to obtain the lignin-epoxidized carboxymethyl chitosan adhesive. The preparation method provided by this invention can be completed within 5 minutes at room temperature and pressure, and can be used immediately. The process is simple, solving the technical problem of poor storage stability of traditional bio-based adhesives, and also reducing the amount of crosslinking agent used in the preparation process. The adhesive provided by this invention is compatible with various substrates, is water-resistant, acid and alkali-resistant, and salt-resistant, and can adapt to various extreme environments; plywood made with this adhesive has excellent comprehensive performance and also has the advantages of being safe, non-toxic, and environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of biomass polymer materials and wood adhesives, specifically to a lignin-epoxidized carboxymethyl chitosan adhesive, its preparation method, and its application. Background Technology

[0002] With the continued growth in global demand for wood-based engineering materials, the demand for wood adhesives is also steadily increasing. Currently, the market is dominated by traditional aldehyde-based adhesives (such as urea-formaldehyde resin and phenolic resin), whose production relies on fossil resources and gradually releases formaldehyde during use, posing potential hazards to human health and the environment. To promote green and sustainable development, the development of formaldehyde-free wood adhesives based on biomass raw materials has become a research hotspot.

[0003] Lignin (L), as a natural adhesive component in plant cell walls, is abundant and has the potential to replace phenolic compounds in the preparation of bio-based adhesives. However, existing lignin-epoxy adhesive systems still have two main drawbacks: first, the compatibility between lignin alkaline solutions and glycidyl ether crosslinking agents is poor, often requiring high amounts of crosslinking agents or complex prepolymerization processes, leading to cumbersome preparation processes, increased costs, and environmental risks; second, the resulting adhesives have poor storage stability, as lignin and epoxy groups gradually react to form a gel during storage, resulting in a decrease or even complete loss of adhesive properties.

[0004] Therefore, developing an adhesive that is simple to prepare, requires no complex prepolymerization, can be prepared and used immediately, can effectively reduce the amount of crosslinking agent added, and has high bonding strength, is environmentally friendly and inexpensive is crucial for promoting the green development of wood adhesives. Summary of the Invention

[0005] Purpose of the invention: The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a lignin-epoxidized carboxymethyl chitosan adhesive, its preparation method and application.

[0006] Inventive Concept: This invention utilizes a strategy combining "chemical modification" and "physical action" to prepare lignin-epoxidized carboxymethyl chitosan (L-GCMCS) adhesive, enabling the adhesive to be used immediately after preparation. At the same time, it also combines a "hot-press crosslinking" strategy to prepare plywood, achieving high performance in plywood.

[0007] First, at room temperature, the primary amino groups on the carboxymethyl chitosan (CMCS) molecular chain dissolved in water undergo nucleophilic attack on the terminal epoxy groups of ethylene glycol diglycidyl ether (GDE) and a ring-opening reaction occurs. By controlling the amount of epoxy groups, CMCS is partially epoxidized to form an epoxidized carboxymethyl chitosan (GCMCS) prepolymer with active epoxy side groups. At this point, the solution turns milky white, forming a preliminary spatial framework structure. Subsequently, lignin (L) is added to the reaction system. The numerous phenolic and alcoholic hydroxyl groups on lignin form strong hydrogen bonds with the amino, hydroxyl, and carboxymethyl groups on the CMCS backbone, enabling lignin to be uniformly dispersed in the GCMCS prepolymer, resulting in a mixed adhesive solution. This mixed adhesive solution is the ready-to-use adhesive L-GCMCS, thus solving the problem of poor compatibility between GDE and lignin alkaline solutions and providing reaction sites for subsequent crosslinking, while avoiding the problem of poor storage stability of existing bio-based adhesives.

[0008] Secondly, during the hot-pressing process, the high temperature promotes the polymerization reaction of the L-GCMCS adhesive: on the one hand, the epoxy groups on GCMCS undergo ring-opening reactions with the phenolic hydroxyl groups of lignin to form new ether bonds; on the other hand, the epoxy groups on GCMCS can also further crosslink with the unreacted secondary amines of GCMCS itself. These two reactions work synergistically to form a dense and stable three-dimensional polymer network at the interface and within the wood and other materials, endowing the adhesive layer with excellent cohesive strength and interfacial adhesion. Simultaneously, under the influence of moisture, the L-GCMCS adhesive can penetrate into the porous structure of the wood, forming mechanical interlocking after curing, further enhancing the bonding strength.

[0009] To solve the above-mentioned technical problems, the present invention discloses the following technical solution:

[0010] In a first aspect, the present invention discloses a lignin-epoxidized carboxymethyl chitosan adhesive and its preparation method.

[0011] The preparation method includes the following steps: mixing a carboxymethyl chitosan (CMCS) solution with ethylene glycol diglycidyl ether (GDE) and stirring until the solution turns milky white to obtain epoxidized carboxymethyl chitosan (GCMCS); then adding lignin (L) and mixing thoroughly to obtain lignin-epoxidized carboxymethyl chitosan (L-GCMCS) adhesive.

[0012] The carboxymethyl chitosan solution is an aqueous solution of CMCS, and the mass concentration of CMCS in the solution is 10%-20%, such as 15%.

[0013] The ethylene glycol diglycidyl ether is added to the carboxymethyl chitosan solution under stirring, and the mass of the ethylene glycol diglycidyl ether is 5%-20% of the mass of the carboxymethyl chitosan solution, such as 5%, 10%, 15% and 20%.

[0014] The mass ratio of lignin to carboxymethyl chitosan is 1:0.5-1.5, such as 1:1.

[0015] Wherein, after the lignin is added and fully mixed, water is added to the reaction system to adjust the solid content of the reaction system to 15%-25%, such as 20%; the solid content is the total mass of lignin, carboxymethyl chitosan and ethylene glycol diglycidyl ether / the total mass of the adhesive; the total mass of the adhesive is the mass of lignin, carboxymethyl chitosan, ethylene glycol diglycidyl ether and water.

[0016] The total preparation time of the method is less than 5 minutes, enabling immediate use.

[0017] The lignin-epoxidized carboxymethyl chitosan adhesive prepared by the above method is also within the scope of protection of this invention.

[0018] In a second aspect, the present invention discloses the application of the lignin-epoxidized carboxymethyl chitosan adhesive described in the first aspect above in the preparation of plywood.

[0019] The application includes coating the lignin-epoxidized carboxymethyl chitosan adhesive onto the surface of a veneer, assembling the coated veneers, and hot-pressing them to obtain plywood.

[0020] The amount of adhesive applied is 180-220 g / m². 2 For example, 200 g / m 2 .

[0021] The veneer includes wood panels, which may include eucalyptus, poplar, bamboo, etc.

[0022] The hot pressing conditions are as follows: hot pressing at 150-190℃ and 0.5-1.5 MPa for 5-30 min, such as hot pressing at 170℃ and 1 MPa for 20 min.

[0023] The lignin-epoxidized carboxymethyl chitosan adhesive provided by this invention has a simple and mild preparation method that can be completed in 5 minutes, allowing for on-site preparation and use. At the same time, the adhesive (L-GCMCS) produced by this invention has excellent dry / wet bonding strength, low formaldehyde release rate, and can meet the E0 national standard.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) This invention proposes an "on-demand preparation" strategy, which avoids the time-consuming and energy-intensive traditional prepolymerization process. The entire adhesive preparation process can be completed within 5 minutes at room temperature and pressure, and can be put into use immediately. That is, the process provided by this invention is simple, fundamentally avoids the performance degradation problem of adhesives due to storage, solves the technical problem of poor storage stability of traditional bio-based adhesives, and promotes its practical application.

[0026] (2) This invention introduces CMCS as a "composting bridging agent," which solves the core problem of poor compatibility between lignin alkaline solution and glycidyl ether crosslinking agents through strong hydrogen bonding, thus significantly reducing the amount of crosslinking agent used. In the optimal formulation of this invention, the crosslinking agent GDE accounts for only 6.6% of the total mass of the adhesive, which is much lower than that of similar ready-mixed lignin adhesives (usually >30%), significantly improving safety and economy.

[0027] (3) The plywood prepared by the adhesive provided by the present invention has a dry bonding strength of up to 3.62 ± 0.19 MPa and a wet bonding strength of up to 1.58 ± 0.16 MPa, which far exceeds the national standard (≥0.7 MPa) requirement. In addition, the adhesive has good applicability to a variety of substrates and is resistant to extreme environments such as water, acids and alkalis, and salt water.

[0028] (4) The adhesive provided by the present invention is based on renewable lignin and carboxymethyl chitosan. It has low formaldehyde release during production and use, which meets the requirements of today's society for green and sustainable materials. Attached Figure Description

[0029] Figure 1 Infrared spectra of CMCS, GCMCS, L, L-GCMCS and CL-GCMCS.

[0030] Figure 2 The graphs show the TG and DSC test curves for CMCS.

[0031] Figure 3 The graphs show the TG and DSC test curves for GCMCS.

[0032] Figure 4 The graphs show the TG and DSC test curves for L.

[0033] Figure 5 The graphs show the TG and DSC test curves for L-GCMCS.

[0034] Figure 6 The images are SEM characterization images, a)-f) are CMCS, GCMCS, L, L-GCMCS, CL-GCMCS and locally magnified CL-GCMCS, respectively.

[0035] Figure 7 The results show the adhesive performance of L-GCMCS on different materials.

[0036] Figure 8 The results of performance comparison of plywood prepared by bonding L-GCMCS with conventional adhesives.

[0037] Figure 9 The adhesive properties of plywood prepared by L-GCMCS after immersion in different solvents are tested.

[0038] Figure 10 The formaldehyde release test results are for L-GCMCS and traditional adhesives.

[0039] Figure 11 The results show the price comparison between L-GCMCS and traditional adhesives. Detailed Implementation

[0040] The present invention can be better understood from the following embodiments. However, those skilled in the art will readily understand that the descriptions in the embodiments are for illustrative purposes only and should not, and will not, limit the invention as detailed in the claims.

[0041] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0042] In the following examples, the carboxymethyl chitosan had a weight-average molecular weight of 165073 Da and was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; the alkali lignin had a weight-average molecular weight of 3440 Da, a phenolic hydroxyl content of 1.58 mmol / g, and an ash content of 0.8% and was purchased from Jinan Yanghai Environmental Protection Materials Co., Ltd.; and the ethylene glycol diglycidyl ether had an epoxy value >0.7 eq / 100g and was purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0043] In the following examples, the solid content is a mass percentage; solid content = m1 / m2 × 100%.

[0044] Where: m1 is the total mass of non-volatile components in the reaction system; if the reaction system contains alkali lignin, ethylene glycol diglycidyl ether, and carboxymethyl chitosan, then m1 is the total mass of these three components; if it contains only some of these components, then m1 only includes the mass of non-volatile solids in the corresponding component. m2 is the total mass of the adhesive, which is the total mass of m1 and water in the reaction system.

[0045] In the following examples, the amount of crosslinking agent = mass of GDE / mass of adhesive × 100%, which is mass of GDE / m2 × 100%.

[0046] Example 1: GDE-5%

[0047] (1) Preparation of epoxidized carboxymethyl chitosan (GCMCS): Carboxymethyl chitosan (CMCS) was dissolved in water to prepare a 15 wt% CMCS solution. 5% ethylene glycol diglycidyl ether (GDE) was added to the CMCS solution with stirring until the solution turned milky white, thus obtaining GCMCS.

[0048] (2) Preparation of lignin-epoxidized carboxymethyl chitosan (L-GCMCS) adhesive: Add alkali lignin (the mass ratio of alkali lignin to CMCS is 1:1) to the system obtained in step (1), add deionized water to adjust the solid content of the system to 20%, mix thoroughly, and the L-GCMCS adhesive is obtained.

[0049] The entire process of preparing L-GCMCS adhesive in steps (1) and (2) takes less than 5 minutes.

[0050] (3) Preparation of plywood: The obtained L-GCMCS adhesive was uniformly coated on the surface of poplar veneer, with an adhesive application rate of 200 g / m². 2 The glued veneers were arranged in a crisscross pattern along the wood grain direction and then hot-pressed at 170℃ and 1 MPa for 15 minutes to prepare plywood. The resulting plywood was designated as sample E1.

[0051] Example 2: GDE-10%

[0052] The amount of GDE added in Example 1 was changed to 10%, while the other conditions remained unchanged. The resulting plywood was designated as Sample E2.

[0053] Example 3: GDE-15%

[0054] The amount of GDE added in Example 1 was changed to 15%, while the other conditions remained unchanged. The resulting plywood was designated as Sample E3.

[0055] In this embodiment, the concentration of the CMCS solution is 15wt%, the mass of GDE is 15% of the mass of the CMCS solution, the mass ratio of alkali lignin to CMCS is 1:1, and the solid content is 20%. Therefore, the mass ratio of CMCS, GDE and alkali lignin is 1:1:1, and the amount of crosslinking agent GDE is 1 / (3 / 20%) = 6.7%.

[0056] Example 4: GDE-20%

[0057] The amount of GDE added in Example 1 was changed to 20%, while the other conditions remained unchanged. The resulting plywood was designated as Sample E4.

[0058] Example 5: Hot pressing for 10 minutes

[0059] Take the adhesive formulation of Example 3 and prepare plywood with the same amount of adhesive. The hot pressing conditions are 170°C and 1 MPa for 10 minutes to prepare plywood. The plywood obtained is recorded as sample E5.

[0060] Example 6: Hot pressing for 20 minutes

[0061] The adhesive formulation of Example 3 was used to prepare plywood with the same amount of adhesive. The hot pressing conditions were 170°C and 1 MPa for 20 minutes. The resulting plywood was designated as sample E6.

[0062] Example 7: Hot pressing for 25 minutes

[0063] The adhesive formulation of Example 3 was used to prepare plywood with the same amount of adhesive. The plywood was prepared by hot pressing at 170°C and 1 MPa for 25 minutes. The plywood obtained was recorded as sample E7.

[0064] Comparative Example 1: Pure carboxymethyl chitosan adhesive

[0065] (1) Preparation of adhesive: Dissolve carboxymethyl chitosan (CMCS) in deionized water to prepare a CMCS solution with a solid content of 20%, stir evenly, and the adhesive is obtained.

[0066] (2) Plywood preparation: The above adhesive is applied at a ratio of 200 g / m 2 The glue was applied to the surface of poplar veneer. The glued veneers were then assembled in a crisscross pattern according to the wood grain direction. The plywood was prepared by hot pressing at 170℃ and 1 MPa for 15 minutes. The resulting plywood was designated as sample D1.

[0067] Comparative Example 2: Lignin-Carboxymethyl Chitosan Blend Adhesive

[0068] (1) Preparation of adhesive: Dissolve carboxymethyl chitosan (CMCS) in water to make a 15wt% CMCS solution. Add alkali lignin (the mass ratio of alkali lignin to CMCS is 1:1) directly to the CMCS solution and mix. Add deionized water to adjust the solid content of the system to 20% and mix thoroughly to obtain the adhesive.

[0069] (2) Plywood preparation: Same as Comparative Example 1, the obtained plywood is denoted as sample D2.

[0070] Comparative Example 3: Epoxidized Carboxymethyl Chitosan (GCMCS) Adhesive

[0071] (1) Preparation of adhesive: Dissolve carboxymethyl chitosan (CMCS) in deionized water to prepare a 15wt% CMCS solution. Add 15% by weight of ethylene glycol diglycidyl ether (GDE) to the CMCS solution while stirring. Stir until the solution turns milky white. Add deionized water to adjust the solid content of the system to 20%. Mix thoroughly to obtain the adhesive.

[0072] (2) Plywood preparation: Same as Comparative Example 1, the obtained plywood is denoted as sample D3.

[0073] Comparative Example 4: Lignin-ethylene glycol diglycidyl ether blend adhesive

[0074] (1) Preparation of adhesive: Dissolve alkali lignin in deionized water to prepare a 15wt% lignin alkali solution. Add 15% by weight of ethylene glycol diglycidyl ether (GDE) to the lignin alkali solution while stirring, stir evenly, add deionized water to adjust the solid content of the system to 20%, mix thoroughly, and the adhesive is obtained.

[0075] (2) Plywood preparation: Same as Comparative Example 1, the obtained plywood is denoted as sample D4.

[0076] Experimental Results and Analysis

[0077] 1. Adhesive properties

[0078] According to GB / T 17657-2022 standard, the plywood prepared in the above embodiments and comparative examples was sawn into standard specimens, and the dry / wet bonding strength was tested using a universal testing machine (wet bonding strength was tested after the plywood was soaked in boiling water for 3 hours). The results are shown in Table 1.

[0079] Table 1. Adhesive properties of adhesives under different process conditions

[0080]

[0081] Results Discussion

[0082] (1) Synergistic optimization of components and processes

[0083] The results from D1 to D4 show that: pure CMCS (D1) or a physical mixture of CMCS and lignin (D2) cannot provide sufficient adhesive strength and water resistance due to the absence of the crosslinking agent GDE. GCMCS without added lignin (D3) shows significantly improved dry adhesive properties, thanks to the increased degree of crosslinking; however, due to the poor water resistance of CMCS, it still does not exhibit good wet adhesive properties. A mixture of pure lignin and GDE (D4) does not show good adhesive properties because the poor compatibility between lignin alkali solution and GDE makes it difficult for them to undergo a crosslinking reaction.

[0084] The results from E1 to E4 show that the bond strength initially increases and then decreases with increasing GDE content. This indicates that the introduction of GDE is key to achieving high bond strength and water resistance, as GDE can chemically crosslink with CMCS and lignin, forming stable, insoluble polymers. When the GDE content is 15%, the crosslinking density and chain segment flexibility reach a balance, resulting in optimal performance.

[0085] (2) The effect of hot pressing process on performance

[0086] Comparing E5-E7, it can be seen that, at the optimal dosage of GDE, appropriately extending the hot pressing time (20 min) can significantly promote the curing and cross-linking reaction, allowing the adhesive performance to reach its peak. Further extending the time to 25 min no longer significantly improves performance, indicating that the curing reaction has nearly reached completion.

[0087] As can be seen, this invention, through a combination of chemical modification (CMCS epoxidation) and physical action (molecular hydrogen bonding between lignin and GCMCS), not only achieves "ready-to-use" operation of wood adhesives (the entire adhesive preparation process takes less than 5 minutes), but also reduces the amount of crosslinking agent to 6.6%, far lower than the amount reported in the literature for similar ready-to-use bio-based adhesives. Furthermore, the plywood made with the adhesive provided by this invention exhibits comprehensive bonding performance (up to 3.62 ± 0.19 MPa dry strength and 1.58 ± 0.16 MPa wet strength) far exceeding national standards and comparable to traditional phenolic resins, demonstrating excellent adhesive effects and significant advantages in terms of high performance and environmental friendliness.

[0088] 2. FTIR characterization

[0089] To elucidate the adhesion mechanism of L-GCMCS, Fourier transform infrared (FTIR) spectroscopy analysis was performed on CMCS, GCMCS, L, L-GCMCS, and epoxidized carboxymethyl chitosan (CL-GCMCS) cured by L-GCMCS adhesive during hot pressing of plywood to identify changes in functional groups during adhesive formation and curing.

[0090] like Figure 1 As shown, it is located at 1656 cm. -1 and 1554 cm -1 The characteristic peaks are attributed to the C=O and NH bonds in CMCS, respectively, at 1061 cm⁻¹. -1 The nearby peaks are attributed to ether bonds in CMCS, confirming the carboxymethylation of chitosan. After the introduction of GDE, GCMCS showed a peak at 906 cm⁻¹. -1 844 cm -1 and 755 cm -1 A new characteristic peak appeared at 1061 cm⁻¹, corresponding to an epoxy group; simultaneously, 1061 cm⁻¹ -1The increased intensity and significant broadening of the nearby absorption peaks indicate a substantial increase in the ether bond content of GCMCS, confirming that epoxy groups have been successfully grafted onto the CMCS molecular chain via chemical bonds. Lignin at 1602 cm⁻¹... -1 and 1512 cm -1 The region exhibits characteristic aromatic skeletal vibration peaks, while 1336 cm⁻¹... -1 and 1218 cm -1 The peaks at the specified locations are related to phenolic hydroxyl groups. When lignin is blended with GCMCS, the FTIR spectrum of L-GCMCS simultaneously displays characteristics of both lignin and GCMCS. After heat curing, the characteristic peaks of epoxy groups and NH bonds disappear. These results indicate that during high-temperature curing, epoxy groups react extensively with the NH groups of CMCS, thus forming a stable high-molecular-weight polymer.

[0091] 3. TG-DSC characterization

[0092] The effect of temperature on the thermal behavior of adhesives can be analyzed using TG-DSC. Figures 2-5 As shown, the mass loss in the 30-140℃ range is mainly due to the evaporation of residual moisture. In the 140-260℃ range, GCMCS experiences faster mass loss than CMCS, primarily due to the decomposition of epoxy groups, further confirming the successful introduction of epoxy groups into CMCS. Main-chain decomposition of GCMCS occurs in the 280-410℃ range, with a maximum decomposition temperature of 366.0℃ in the DTG curve. In contrast, main-chain decomposition of CMCS occurs in the 220-360℃ range, with a DTG peak of 303.1℃. The increased decomposition temperature stems from the crosslinking between CMCS and GDE, which increases molecular weight and structural stability, thereby enhancing the material's thermal stability. At 800℃, the char residue of GCMCS is 23.38%, lower than CMCS's 34.35%, mainly due to the decomposition of epoxy groups and ether bonds originating from GDE.

[0093] The main chain decomposition temperature of lignin is 299-405℃, and the char residue reaches 42.50%, both higher than that of CMCS. This is because the aromatic ring structure in L-GCMCS is more stable than the glycosidic bonds in CMCS. After being combined with lignin, the TG curve of L-GCMCS shows that the decomposition temperature increases to 338-425℃, and the DTG peak is 388.6℃, higher than that of GCMCS and L alone. This result indicates that at high temperatures, the phenolic hydroxyl groups in lignin react with the epoxy groups in GCMCS to form a dense and stable three-dimensional structure. Furthermore, the disappearance of the epoxy group decomposition peak in the 140-260℃ range further proves that the introduction of lignin promotes the reaction between the phenolic hydroxyl groups and epoxy groups, forming new stable ether bonds. Therefore, it can be concluded that the thermodynamic properties of L-GCMCS adhesive are significantly enhanced after heat curing.

[0094] 4. Scanning electron microscopy characterization

[0095] The morphological evolution of the adhesive was further examined using scanning electron microscopy (SEM). Figure 6 As shown, CMCS exhibits only a small number of micropores. After epoxidation, GCMCS displays a significant porous structure. This morphology originates from partial cross-linking during the epoxidation process, which generates a preliminary spatial framework. Subsequently, during freeze-drying, a honeycomb structure forms as moisture evaporates. When GCMCS is blended with lignin, the prominent honeycomb structure previously observed in GCMCS is no longer visible in the L-GCMCS system. This change can be attributed to the small particle size of lignin and its good compatibility with GCMCS, allowing lignin to fully penetrate the internal structure. After high-temperature curing, the resulting CL-GCMCS exhibits a smooth, non-porous surface, indicating that lignin and GCMCS have undergone sufficient cross-linking, forming a dense, high-strength polymer network. This structural evolution lays the foundation for improved adhesive bonding performance and weather resistance.

[0096] 5. Other performance tests of the adhesive

[0097] (1) Adhesive properties for different materials

[0098] Referring to Example 6, the poplar board was replaced with other materials, and the results were as follows: Figure 7 As shown in the figure, L-GCMCS adhesive has wide applicability and exhibits good bonding ability to eucalyptus, poplar, bamboo, glass, ceramics, and stainless steel.

[0099] (2) Comparison of adhesive properties with traditional adhesives

[0100] Referring to Example 6, the L-GCMCS adhesive was replaced with conventional adhesives (polyvinyl acetate adhesive PVAC, urea-formaldehyde resin adhesive UF, phenolic resin adhesive PF, and polyurethane adhesive PU), and the results were as follows. Figure 8 As shown in the figure, the plywood prepared by the L-GCMCS of this invention is far superior to the minimum industrial requirement (0.7 MPa), and its adhesive performance on poplar wood is better than most traditional adhesives, demonstrating its potential to replace traditional adhesives.

[0101] (3) Resistance test to extreme adhesive conditions

[0102] The plywood prepared in Example 6 was immersed in different solvents for 7 days. The solvents included water, seawater, an acidic solution (pH=1), an alkaline solution (pH=13), and the organic solvent dimethyl sulfoxide (DMSO). The adhesive properties of the plywood were tested after 7 days, and the results are as follows: Figure 9As shown in the figure, the test results indicate that the adhesive strength of the plywood is still significantly higher than the minimum industrial requirement of 0.7 MPa, demonstrating good adaptability to extreme environments.

[0103] (4) Formaldehyde content test of adhesives

[0104] According to national standard GB / T 17657-2022, the formaldehyde release of the adhesive L-GCMCS prepared in Example 6 and traditional adhesives (urea-formaldehyde resin adhesive UF and phenolic resin adhesive PF) was determined using the desiccator method. The results... Figure 10 As shown, the L-GCMCS adhesive prepared by the present invention based on bio-based carboxymethyl chitosan and lignin exhibits a formaldehyde release of 0.13 mg / L, which meets the national standard of E0 grade.

[0105] (5) Price comparison with traditional adhesives

[0106] from Figure 11 A price comparison shows that the price of the adhesive L-GCMCS prepared in Example 6 is higher than that of polyvinyl acetate adhesive PVAC and urea-formaldehyde resin adhesive UF, but lower than that of phenolic resin adhesive PF and polyurethane adhesive PU.

[0107] L-GCMCS adhesive exhibits superior adhesive performance compared to PVAC, UF, and PF adhesives, and is formaldehyde-free and widely applicable. L-GCMCS adhesive has the potential to replace traditional adhesives.

[0108] In summary, this invention proposes a novel one-pot, on-the-fly preparation strategy for lignin-epoxidized carboxymethyl chitosan adhesives. The introduction of carboxymethyl chitosan solves the problem of poor compatibility between lignin alkali solution and glycidyl ether, thereby reducing the amount of crosslinking agent required. Furthermore, the safe and convenient preparation process of L-GCMCS adhesives overcomes the environmental problems caused by traditional complex processes, as well as the challenge of performance degradation of bio-based adhesives during storage. The L-GCMCS adhesive obtained by this invention exhibits excellent bonding strength and broad environmental adaptability, surpassing most traditional petroleum-based adhesives. Simultaneously, the L-GCMCS adhesive also possesses biodegradability and scalable production characteristics, pointing to a promising path for the green and sustainable development of adhesives and the high-value utilization of lignin.

[0109] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. 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, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a lignin-epoxidized carboxymethyl chitosan adhesive, characterized in that, Includes the following steps: Carboxymethyl chitosan solution was mixed with ethylene glycol diglycidyl ether and stirred until the solution turned milky white; then lignin was added and mixed to obtain lignin-epoxidized carboxymethyl chitosan adhesive.

2. The preparation method according to claim 1, characterized in that, The carboxymethyl chitosan solution contains 10%-20% carboxymethyl chitosan by mass.

3. The preparation method according to claim 1, characterized in that, The mass of the ethylene glycol diglycidyl ether is 5%-20% of the mass of the carboxymethyl chitosan solution.

4. The preparation method according to claim 1, characterized in that, The mass ratio of lignin to carboxymethyl chitosan is 1:0.5-1.

5.

5. The preparation method according to claim 1, characterized in that, After adding lignin, water is added to the reaction system to adjust the solid content of the reaction system to 15%-25%.

6. The preparation method according to claim 1, characterized in that, The total preparation time of the method is less than 5 minutes.

7. A lignin-epoxidized carboxymethyl chitosan adhesive prepared by the method according to any one of claims 1-6.

8. The use of the lignin-epoxidized carboxymethyl chitosan adhesive according to claim 7 in the preparation of plywood.

9. The application according to claim 8, characterized in that, The lignin-epoxidized carboxymethyl chitosan adhesive is coated onto the surface of the veneer, and the coated veneers are assembled and hot-pressed to obtain plywood.

10. The application according to claim 9, characterized in that, The hot pressing conditions are 150-190℃ and 0.5-1.5 MPa for 5-30 min.

Images