A kind of depolymerized lignin-based adhesive and its preparation method and application

Abstract

The application provides a kind of depolymerization lignin-based adhesive and its preparation method and application, belong to wood adhesive technical field.The method comprises: 3-mercapto propionic acid and lactic acid are mixed according to molar ratio 1:2~1:4 to prepare eutectic solvent;Wood fiber raw material powder is mixed with the eutectic solvent according to solid-liquid ratio 1:10~1:20, heated at 150~180 DEG C for 1~4 hours, so that lignin is depolymerized into fragments with molecular weight of 800~2000 Da, while 3-mercapto propionic acid is grafted onto depolymerized lignin molecules by covalent bond, introducing two active groups of sulfhydryl (-SH) and carboxyl (-COOH);Depolymerized lignin is obtained by acid precipitation and purification;Then react with crosslinking agent to obtain lignin-based adhesive.The application simultaneously realizes selective depolymerization and grafting of sulfhydryl and carboxyl functional groups during lignin separation, and the prepared adhesive does not contain formaldehyde and phenol, the bonding strength reaches 1.38~1.52 MPa, which is 97%~117% higher than the national standard, and can be cured at 90~100 DEG C or even room temperature, suitable for bonding of plywood, three-layer solid wood composite floor and other wood products.

Description

Technical Field

[0001] This invention belongs to the field of wood adhesive technology, and in particular relates to a depolymerized lignin-based adhesive, its preparation method and application. Background Technology

[0002] Lignin is the second largest natural polymer after cellulose, and the only natural aromatic polymer. The global paper industry generates over 50 million tons of lignin byproducts annually, but its utilization rate is less than 2%, with the majority being incinerated as waste, resulting in resource waste and environmental pollution. Developing high-value utilization technologies for lignin has practical economic and environmental significance.

[0003] Traditional methods for utilizing lignin mainly include direct blending, chemical modification, and pyrolysis. Direct blending involves blending lignin directly with synthetic resins such as phenolic and urea-formaldehyde resins. However, lignin has poor compatibility with resins, leading to decreased adhesive performance. The amount of lignin added is typically no more than 20%, as higher additions severely affect bond strength. Chemical modification introduces active groups through reactions such as hydroxymethylation, phenolation, and amination to improve the reactivity of lignin. However, this requires large amounts of organic solvents and catalysts, resulting in high costs and potential secondary pollution. Pyrolysis decomposes lignin at high temperatures to produce bio-oil, biochar, and fuel gas. However, the products are complex, difficult to separate and purify, and economically unfeasible.

[0004] Deep eutectic solvents (DES), as a green solvent, have seen increased applications in biomass pretreatment and lignin separation in recent years. DES is composed of hydrogen bond acceptors and hydrogen bond donors mixed in a specific molar ratio, forming a eutectic mixture with a melting point lower than that of the individual components through hydrogen bonding interactions. DES offers advantages such as simple preparation, low cost, non-toxicity, and biodegradability, and is considered a substitute for traditional organic solvents and ionic liquids. Current research has used DES systems such as choline chloride / lactic acid and choline chloride / urea to separate lignin; however, the separated lignin has a wide molecular weight distribution and few active groups, resulting in low bonding strength and poor water resistance when directly used to prepare adhesives.

[0005] Specifically, the following representative studies have been conducted in the existing technology: (1) Separation of lignin from wheat straw using choline chloride / lactic acid DES: Lignin can be separated from wheat straw using choline chloride / lactic acid DES, and the purity of lignin can reach more than 90%, but the number average molecular weight is large (>5000 Da) and the reactivity is low. (2) Separation of lignin from poplar wood using choline chloride / urea DES: Lignin can be separated from poplar wood using choline chloride / urea DES, and the separation efficiency can reach 80%, but the lignin obtained mainly depends on physical modification. (3) Lignin-based formaldehyde-free adhesives: Research progress on the preparation of adhesives by replacing formaldehyde-based resins with lignin. Currently, the bonding strength of lignin-based adhesives is generally lower than or close to the national standard requirement (0.70 MPa), and hot pressing temperature of more than 140℃ is often required.

[0006] The shortcomings of existing technologies are mainly fourfold. First, traditional DES systems primarily employ physical dissolution mechanisms when separating lignin, resulting in low depolymerization of lignin macromolecules and a limited number of retained active hydroxyl groups. Second, the separated lignin lacks active groups capable of forming covalent bonds with wood, and the bonding strength mainly relies on physical entanglement and hydrogen bonding, making it difficult to meet the requirements of high-performance wood products. Third, existing lignin-based adhesives require hot-pressing curing at high temperatures of 140-160℃, leading to high energy consumption, long curing times, and low production efficiency. Fourth, the bonding strength of existing technologies is typically lower than or equal to national standards, lacking market competitiveness. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing depolymerized lignin-based adhesives and their applications, thereby solving the technical problems existing in the prior art. The various technical effects of the preferred solutions among the many technical solutions provided by this invention are detailed below.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a depolymerized lignin-based adhesive includes the following steps: S1: Mix 3-mercaptopropionic acid and lactic acid in a molar ratio of 1:2 to 1:4 and stir at 60 to 80°C for 1 to 3 hours to prepare a eutectic solvent; S2: Mix the wood fiber raw material powder with the eutectic solvent prepared in S1 at a solid-liquid ratio of 1:10 to 1:20, and heat it at 150 to 180°C for 1 to 4 hours to depolymerize the lignin into fragments with a molecular weight of 800 to 2000 Da. At the same time, the thiol and carboxyl groups of 3-mercaptopropionic acid are grafted onto the depolymerized lignin molecules. S3: Cool, dilute, and centrifuge the mixture obtained after S2 treatment. Take the supernatant and adjust the pH to 2-3 to precipitate depolymerized lignin. After filtration, washing, and drying, obtain the depolymerized lignin. S4: Mix the depolymerized lignin obtained in S3 with water, and add a crosslinking agent; the mass ratio of the crosslinking agent to the depolymerized lignin is 5:100 to 15:100; stir and react at 60 to 90°C for 30 to 60 minutes, cool, and adjust the pH to 5.5 to 6.5 to obtain the lignin-based adhesive.

[0009] Preferably, the molar ratio of 3-mercaptopropionic acid to lactic acid in S1 is 1:3; the heating temperature in S2 is 150~160℃, and the treatment time is 2~3 hours.

[0010] Preferably, the wood fiber raw material powder in S2 is larch wood powder with an average particle size of 40-60 mesh.

[0011] Preferably, the wood fiber raw material powder in S2 is one of poplar powder, eucalyptus powder, bamboo powder, and corn stalk powder, with an average particle size of 40-60 mesh.

[0012] Preferably, the heat treatment described in S2 is carried out in a complex system of lactic acid and 3-mercaptopropionic acid. In this complex system, the carboxyl group of lactic acid provides an acidic environment to catalyze the hydrolytic cleavage of the β-O-4 ether bond, and the thiol group of 3-mercaptopropionic acid performs a nucleophilic attack on the β-O-4 ether bond, causing it to undergo irreversible cleavage and capturing the benzyl carbocation intermediate state to suppress the condensation side reaction. At the same time, 3-mercaptopropionic acid is covalently grafted onto the depolymerized lignin molecule to introduce thiol and carboxyl groups in situ.

[0013] Preferably, the crosslinking agent in S4 is one or more of citric acid, succinic anhydride, or epichlorohydrin.

[0014] The depolymerized lignin-based adhesive prepared by the above preparation method contains three active groups: hydroxyl, thiol and carboxyl groups, with a number average molecular weight of 800~2000 Da, a polydispersity index of 1.8~2.5, and a sulfur content of 1.8~2.2 wt%.

[0015] The above-mentioned depolymerized lignin-based adhesive is used in plywood manufacturing, employing poplar or eucalyptus veneers with a moisture content of 8-12%; a double-sided application method is used, with an application rate of 240-280 g / m². 2 Hot pressing is carried out at 90~100℃ and 0.88MPa, with the hot pressing time calculated at 1.2min / mm plate thickness; after hot pressing, curing is carried out at room temperature for 24~48 hours.

[0016] Preferably, the application rate of the adhesive is 260 g / m³. 2 The hot pressing temperature is 100℃.

[0017] The above-mentioned depolymerized lignin-based adhesive is used in the manufacture of three-layer engineered wood flooring, which includes a core board, a face veneer, and a back veneer. The core board is made of poplar or eucalyptus veneer, the face veneer is made of oak veneer, and the back veneer is made of poplar veneer. The adhesive application rate is 150 g / m². 2 Hot pressing temperature: 90℃; hot pressing pressure: 1.0MPa; hot pressing time: 5 minutes.

[0018] The beneficial effects of this invention are as follows: The 3-mercaptopropionic acid / lactic acid DES of this invention, when used to treat larch at temperatures above 150°C, can selectively depolymerize lignin to a specific molecular weight range of 800-2000 Da. Simultaneously, 3-mercaptopropionic acid molecules are grafted onto lignin fragments via nucleophilic substitution, introducing thiol and carboxyl groups in situ, resulting in depolymerized lignin containing three active groups: hydroxyl, thiol, and carboxyl. Adhesives prepared using this depolymerized lignin can form covalent bonds (ester and thioether bonds) and hydrogen bonds with cellulose, hemicellulose, and hydroxyl groups on lignin in wood cell walls, exhibiting bonding strength significantly higher than national standards. This adhesive can be hot-pressed and cured at low temperatures of 90-100°C, and even cold-pressed and cured at room temperature, and is completely free of formaldehyde and phenol.

[0019] Specifically, the beneficial effects of the present invention include: 1. This invention employs a 3-mercaptopropionic acid / lactic acid DES system to separate and depolymerize larch lignin. It is the first time that a thiol functional group has been introduced into the DES system, simultaneously introducing thiol and carboxyl groups during the separation and depolymerization process. This results in depolymerized lignin containing three active groups: hydroxyl, thiol, and carboxyl. Traditional DES-separated lignin only contains hydroxyl groups, resulting in lower reactivity; this invention overcomes this deficiency. This technical route completes functionalization modification during separation, eliminating the cumbersome traditional "separation first, then modification" steps and simplifying the process from raw materials to finished products.

[0020] 2. The DES solvent used in this invention is composed of bio-based 3-mercaptopropionic acid and lactic acid, which is non-toxic and biodegradable. The entire preparation process is formaldehyde-free, phenol-free, and free of toxic organic solvents, meeting national environmental protection requirements for engineered wood products. Formaldehyde emissions from plywood and three-layer engineered wood flooring were not detected, meeting the E0 grade requirements of GB 39600-2021. The DES solvent is recyclable. After five cycles, the lignin separation efficiency decreased from 78.5% to 70.7%, maintaining an initial efficiency of over 90.0% (70.7 / 78.5=90.06%), significantly reducing production costs and environmental impact.

[0021] 3. The depolymerized lignin adhesive prepared by this invention can form covalent bonds with wood. The dry bonding strength of poplar and eucalyptus plywood reaches 1.38~1.52 MPa (hot-press curing), meeting or slightly exceeding the national standard GB / T 9846-2015; the bonding strength under room temperature curing conditions can still reach 1.12 MPa, exceeding the national standard requirements; and the wood breakage rate is over 80%. Water resistance tests show that the bonding strength remains above 0.81 MPa after boiling in water for 3 hours. The dry bonding strength of three-layer solid wood composite flooring reaches 1.52 MPa, meeting or slightly exceeding GB / T 18103-2013. In addition, this adhesive maintains excellent adhesion durability under acid, alkali, salt, and extreme temperature conditions.

[0022] 4. The adhesive prepared by this invention can be hot-pressed and cured at room temperature or 90°C. Compared with traditional phenolic resin adhesives (curing temperature 140~160°C), the hot-pressing temperature is reduced by 50~70°C, and energy consumption is reduced by 30~40%. The hot-pressing time is shortened to 3~8 minutes, which is 20~40% shorter than the traditional process, significantly improving production efficiency. The room temperature curing method can also achieve complete cold-pressing curing without heating equipment, greatly reducing equipment investment and operating costs.

[0023] 5. The preferred wood fiber raw material used in this invention is larch. Larch is a common coniferous tree species in Northeast China, with wide availability and low price. 3-Mercaptopropionic acid and lactic acid are both industrially produced chemicals with moderate prices. The preparation process is simple, requiring no complex equipment, and is suitable for industrial production. Based on current raw material prices, the adhesive prepared by this invention has a cost 15-25% lower than commercially available phenolic resin adhesives, exhibiting good economic benefits and market competitiveness. Furthermore, the wood fiber raw material used in this invention can be other wood fiber raw materials, such as poplar, eucalyptus, bamboo, corn stalks, etc.

[0024] 6. The adhesive prepared by this invention can be used not only for plywood and three-layer engineered wood flooring, but also for the production of various engineered wood products such as oriented strand board (OSB), fiberboard, and blockboard. Furthermore, this adhesive has bonding ability to various substrates such as wood, steel, aluminum, and glass, making it widely applicable and with broad market prospects. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1This is a flowchart of a method for preparing a depolymerized lignin-based adhesive according to the present invention; Figure 2 The infrared spectrum of depolymerized lignin prepared in Example 1 of this invention; Figure 3 The hydrogen nuclear magnetic resonance spectrum of depolymerized lignin prepared in Example 1 of this invention; Figure 4 This is a permeation chromatogram of the depolymerized lignin gel prepared in Example 1 of the present invention; Figure 5 This is a comparison chart of the bonding strength test results of the plywood prepared in Example 2 of the present invention with the national standard; Figure 6 This is a comparison chart of the bonding strength test results of the plywood prepared in Example 3 of the present invention with the national standard; Figure 7 This is a comparison chart of the adhesive strength test results of the three-layer solid wood composite flooring prepared in Example 4 of the present invention with the national standard; Figure 8 The curve showing the effect of the number of DES solvent recycling cycles on lignin separation efficiency in Example 8 of this invention; Figure 9 This is a comparison of the number-average molecular weight of the depolymerized lignin of Example 1 of the present invention with that of the control depolymerized lignin prepared in Comparative Examples 1 and 3. Figure 10 This is a comparison of the bonding strength of the plywood prepared in Example 2 of the present invention with that of Comparative Examples 1 and 2. Figure 11 This is a comparison of the breakage rate of plywood prepared in Example 2 of the present invention with that of Comparative Examples 1 and 2. Figure 12 This is a comparison of the types of active groups in the depolymerized lignin of Example 1 of the present invention with those in Comparative Examples 1 and 3. Figure 13 This is a comparison of the formaldehyde emission of plywood prepared in Example 2 of the present invention and Comparative Example 2; Figure 14 This is a comparison of the hot pressing / curing temperatures of the adhesives prepared in Example 1 and Comparative Example 2 of the present invention; Figure 15 This is a radar chart showing the overall performance of the present invention and a comparative example. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0028] This invention discloses a method for preparing a depolymerized lignin-based adhesive. Please refer to [link to relevant documentation]. Figure 1 The method specifically includes the following steps: S1: Mix 3-mercaptopropionic acid and lactic acid in a molar ratio of 1:2 to 1:4 and stir at 60 to 80°C for 1 to 3 hours to prepare a eutectic solvent; S2: Mix the wood fiber raw material powder with the eutectic solvent prepared in S1 at a solid-liquid ratio of 1:10 to 1:20, and heat it at 150 to 180°C for 1 to 4 hours to depolymerize the lignin into fragments with a molecular weight of 800 to 2000 Da. At the same time, the thiol and carboxyl groups of 3-mercaptopropionic acid are grafted onto the depolymerized lignin molecules. S3: Cool, dilute, and centrifuge the mixture obtained after S2 treatment. Take the supernatant and adjust the pH to 2-3 to precipitate depolymerized lignin. After filtration, washing, and drying, obtain the depolymerized lignin. S4: Mix the depolymerized lignin obtained in S3 with water, and add a crosslinking agent; the mass ratio of the crosslinking agent to the depolymerized lignin is 5:100 to 15:100; stir and react at 60 to 90°C for 30 to 60 minutes, cool, and adjust the pH to 5.5 to 6.5 to obtain the lignin-based adhesive.

[0029] The following provides a detailed explanation of each of the above steps.

[0030] S1: Mix 3-mercaptopropionic acid and lactic acid in a molar ratio of 1:2 to 1:4 and stir at 60 to 80°C for 1 to 3 hours to prepare a eutectic solvent.

[0031] This step involves the preparation of a eutectic solvent, referred to in this application as: DES solvent, 3-mercaptopropionic acid / lactic acid eutectic solvent, 3-mercaptopropionic acid / lactic acid DES, etc. The stirring method is not limited; conventional stirring methods in the art, such as mechanical stirring or magnetic stirring, can be used. The final product is a homogeneous, clear, and transparent viscous liquid, thus obtaining 3-mercaptopropionic acid / lactic acid DES. This DES has a melting point below -30°C and maintains fluidity over a wide temperature range, which is beneficial for industrial operation.

[0032] S2: Mix the wood fiber raw material powder with the eutectic solvent prepared in S1 at a solid-liquid ratio of 1:10 to 1:20, and heat at 150 to 180°C for 1 to 4 hours to depolymerize the lignin into fragments with a molecular weight of 800 to 2000 Da. At the same time, the thiol and carboxyl groups of 3-mercaptopropionic acid are grafted onto the depolymerized lignin molecules.

[0033] This step involves the separation, depolymerization, and functional group introduction of the lignocellulose raw material powder. The preferred lignocellulose raw material powder for this invention is larch, with an average particle size of 40-60 mesh. Larch is a coniferous tree, and its lignin is predominantly composed of a single guaiac matrix structure, with a high proportion of β-O-4 ether bonds and a regular structure. In a eutectic solvent system of 3-mercaptopropionic acid / lactic acid, it can achieve efficient and uniform selective depolymerization, resulting in a narrow molecular weight distribution of the depolymerization products and excellent grafting effects of thiol and carboxyl groups, leading to the best overall performance of the prepared adhesive. This invention is also applicable to broadleaf woods such as poplar and eucalyptus, as well as gramineous lignocellulose raw materials such as bamboo and corn stalks. Process parameters such as reaction temperature and reaction time can be adjusted adaptively for different raw materials.

[0034] In addition, the above solid-liquid ratio is the feeding ratio of solid material to liquid material, that is, solid mass (g): liquid volume (mL).

[0035] In this process, 3-mercaptopropionic acid and lactic acid in 3-mercaptopropionic acid / lactic acid DES work synergistically to achieve selective depolymerization of lignin and introduction of functional groups. The specific mechanism is as follows: (1) The carboxyl group of lactic acid provides an acidic environment at high temperature, which catalyzes the hydrolytic cleavage of the β-O-4 ether bond in the lignin molecule; (2) The thiol group (-SH) of 3-mercaptopropionic acid acts as a strong nucleophile, launching a nucleophilic attack on the Cα position of the β-O-4 ether bond, causing irreversible cleavage of the ether bond; at the same time, the thiol group (-SH) can effectively capture and quench the benzyl carbocation (Cα) formed during lignin separation. + The intermediate state inhibits the condensation side reaction of lignin; (3) In the lignin fragments after breakage, 3-mercaptopropionic acid molecules are grafted onto lignin molecules by forming thioether bonds or ester bonds, and thiol (-SH) and carboxyl (-COOH) are introduced simultaneously, so that the depolymerized lignin contains three active groups: hydroxyl (-OH), thiol (-SH) and carboxyl (-COOH); (4) By controlling the temperature at a high temperature (i.e. above 150℃), the β-O-4 ether bond is fully broken, so that lignin is selectively depolymerized into small molecular fragments with a molecular weight of 800~2000 Da.

[0036] Through the above-mentioned depolymerization-grafting integrated mechanism, the depolymerized lignin molecules simultaneously contain the original hydroxyl groups and newly introduced thiol and carboxyl groups, forming a functional group system in which three active groups coexist.

[0037] It is worth noting that 3-mercaptopropionic acid contains both a thiol group (-SH) and a carboxyl group (-COOH), while lactic acid contains both a carboxyl group and a hydroxyl group. Therefore, this DES system inherently possesses two reaction potentials: (1) The dual role of thiol: Thiol is a strong nucleophilic group. Under mild conditions, it can act as an intermediate quencher to protect the β-O-4 bond; under high temperature and strong reaction conditions, it can act as an attacking reagent to directly break the ether bond.

[0038] (2) Acid regulation of lactic acid: Lactic acid is a weak organic acid, and the system as a whole is weakly acidic; at low temperature and short time, it only plays a role in hydrogen bond dissolution; at high temperature, the acidity is enhanced (i.e., it becomes a strong acid environment), which catalyzes the hydrolysis and breakage of lignin ether bonds.

[0039] Based on this, the present invention selects a heat treatment at 150~180℃ for 1~4 hours, that is, in a high-temperature environment (above 150℃), with strong acid post-treatment and a high solid-liquid ratio, utilizing acidic catalysis and thiol nucleophilic attack to actively break the β-O-4 bond, simultaneously completing lignin depolymerization and functional group grafting, laying the foundation for subsequent adhesive preparation. The present invention avoids using low-temperature or medium-temperature (e.g., below 150℃, including 150℃) and weakly acidic environments to avoid the problem of β-O-4 bond failure, thus achieving the integrated selective depolymerization and functional group modification of lignin.

[0040] S3: Cool, dilute, and centrifuge the mixture obtained after S2 treatment. Take the supernatant and adjust the pH to 2-3 to precipitate depolymerized lignin. After filtration, washing, and drying, obtain the depolymerized lignin.

[0041] This step involves the separation and purification of depolymerized lignin. The mixture obtained after S2 treatment is cooled to room temperature, diluted with deionized water, stirred thoroughly, and centrifuged at 3000-5000 rpm for 10-20 minutes to remove insoluble residue. The supernatant is collected, and the pH is adjusted to 2-3 with an acidic solution (such as HCl solution, which can be selected according to actual needs, without specific limitations here). The mixture is allowed to stand for precipitation for 6-12 hours. This strongly acidic precipitation environment not only ensures complete precipitation of the depolymerized lignin but also further promotes the hydrolysis of residual ether bonds, completing deep depolymerization. The precipitate is collected by filtration, washed with deionized water until the washing liquid is neutral, and then freeze-dried to obtain a light brown depolymerized lignin product with a yield of 75-90%.

[0042] S4: Mix the depolymerized lignin obtained in S3 with water, and add a crosslinking agent; the mass ratio of the crosslinking agent to the depolymerized lignin is 5:100 to 15:100; stir and react at 60 to 90°C for 30 to 60 minutes, cool, and adjust the pH to 5.5 to 6.5 to obtain the lignin-based adhesive.

[0043] This step is for the preparation of lignin-based adhesives. The depolymerized lignin obtained from S3 is mixed with water at a mass ratio of 1:1 to 1:3 and stirred at 60-80°C for 30-60 minutes to form a homogeneous solution. A crosslinking agent, accounting for 5-15% of the lignin mass, is added, and the mixture is stirred at 60-90°C for 30-60 minutes to allow the crosslinking agent to crosslink with the hydroxyl, thiol, and carboxyl groups in the depolymerized lignin molecules, forming a preliminary crosslinked network. After the reaction is complete, the mixture is cooled to room temperature, and the pH is adjusted to 5.5-6.5 with NaOH solution to obtain the lignin-based adhesive. The crosslinking agent is selected from one or more of citric acid, succinic anhydride, or epichlorohydrin.

[0044] The curing mechanism of the prepared lignin-based adhesive is explained below: When depolymerized lignin adhesives are applied to the wood surface, the active groups in the adhesives interact in various ways with the hydroxyl groups on cellulose, hemicellulose, and lignin in the wood cell walls: (1) Covalent bonding: Under hot pressing conditions, the carboxyl groups in depolymerized lignin undergo esterification with the hydroxyl groups of wood to form ester bonds (-COO-), and the thiol groups undergo addition reactions with the hydroxyl groups of wood to form thioether bonds (-S-). These covalent bonds provide strong interfacial bonding forces.

[0045] (2) Hydrogen bond network: The hydroxyl, carboxyl and mercapto groups in depolymerized lignin form a large number of hydrogen bonds with the hydroxyl groups in wood molecules. The grafted -COOH groups act as hydrogen bond donors and acceptors to form hydrogen bond bridges, further enhancing the interfacial bonding.

[0046] (3) Mechanical interlocking: Depolymerized lignin molecules penetrate into the pores of the wood and form a mechanically interlocked structure after curing. The synergistic effect of the above-mentioned covalent bonds, hydrogen bonds and mechanical interlocking makes the bonding strength significantly higher than that of traditional lignin-based adhesives and national standard requirements.

[0047] The combined synergistic effects of covalent bonding, hydrogen bonding, and mechanical interlocking result in a bonding strength far exceeding that of traditional lignin-based adhesives and also far exceeding national standard requirements.

[0048] Furthermore, this invention also discloses an application of the depolymerized lignin-based adhesive obtained by the above preparation method. Specifically, the lignin-based adhesive prepared by this invention can be used in the manufacture of wood products, such as plywood, three-layer engineered wood flooring, oriented strand board, and other wood products. For example: (1) Plywood manufacturing. Veneer preparation: Select poplar or eucalyptus veneers, with a thickness of 1.2~2.0 mm, a moisture content controlled at 8~12%, and a smooth, rot-free, and crack-free surface. Glue application: Apply glue to both sides by roller coating or spraying, with an optimal glue application rate of 260 g / m². 2After applying the adhesive, allow it to age for 15-30 minutes to allow the depolymerized lignin-based adhesive to fully penetrate into the wood pores. Assemble the veneers: Assemble the veneers in odd-numbered layers with adjacent layers having perpendicular grain directions. Hot-press: Hot-press temperature 100℃, hot-press time calculated at 1.2 min / mm board thickness, hot-press pressure 0.88 MPa. Post-treatment: After hot-pressing, cure the boards at room temperature and 50-60% relative humidity for 24-48 hours to allow the depolymerized lignin-based adhesive from the wood fiber raw material to fully cure, then perform post-treatment such as edge sawing and sanding.

[0049] (2) Manufacturing of three-layer engineered wood flooring. Core board preparation: Poplar or eucalyptus veneer, 8-12 mm thick, moisture content 8-12%. Face veneer preparation: Oak veneer, 3-4 mm thick, moisture content 8-12%. Back veneer preparation: Poplar veneer, 2-3 mm thick, moisture content 8-12%. Glue application: Single-sided glue application is used on the bonding surfaces between the core board and face veneer, and between the core board and back veneer. The optimal glue application rate is 150 g / m². 2 After applying the adhesive, allow it to age for 15-30 minutes. Assemble the boards in the following order: top veneer - core veneer - back veneer, ensuring the grain direction of the top and back veneers is consistent. Hot pressing: Hot pressing temperature 90℃, hot pressing pressure 1.0 MPa, hot pressing time 5 minutes. Post-treatment: After hot pressing, cure the flooring at room temperature and relative humidity 50-60% for 48-72 hours, then perform post-treatment such as tongue and groove cutting, sanding, and finishing.

[0050] (3) Room temperature curing method. The depolymerized lignin-based adhesive prepared by this invention also has excellent room temperature curing performance. After aging for 30 minutes after application, cold pressing at room temperature (25°C) and 1.0 MPa for 60 minutes, followed by curing at room temperature for 72 hours, can achieve effective bonding. The dry bond strength of the plywood cured at room temperature reaches 1.12 MPa, which is higher than the national standard requirement (≥0.70 MPa), and the wood breakage rate exceeds 75%, which can significantly reduce production energy consumption.

[0051] The present invention will be further described in detail below with reference to specific embodiments.

[0052] Example 1: S1: Preparation of eutectic solvent for 3-mercaptopropionic acid / lactic acid: 3-mercaptopropionic acid and lactic acid were mixed at a molar ratio of 1:3 and stirred at 70°C for 2 hours to form a homogeneous, transparent, and viscous liquid, thus obtaining 3-mercaptopropionic acid / lactic acid DES. This DES is liquid at room temperature with a viscosity of approximately 120 mPa·s and a pH of approximately 2.5. Differential scanning calorimetry (DSC) testing showed that the melting point of this DES solvent is below -30°C, maintaining fluidity over a wide temperature range, which is beneficial for industrial operation.

[0053] S2: Lignin Separation, Depolymerization, and Functional Group Introduction: 100 g of larch wood powder (average particle size 40-60 mesh, moisture content <5%, larch origin in Northeast China) was added to 1500 mL of the prepared DES (solid-liquid ratio 1:15), and heated at 160℃ for 2 hours. Continuous stirring (mechanical stirring at 200 rpm) was used during heating to ensure uniform heat transfer. After treatment, the lignin separation efficiency reached 78.5%. The cellulose retention rate in the solid residue exceeded 90%, which is beneficial for the subsequent high-value utilization of the solid residue (such as enzymatic hydrolysis and saccharification, nanocellulose preparation, etc.).

[0054] The lignin separation efficiency (%) mentioned above is calculated according to the following formula: Separation efficiency = [(mass of depolymerized lignin product × purity of depolymerized lignin) / mass of Klason lignin in raw material] × 100%.

[0055] In this formula, the mass of Klason lignin in the raw materials was determined by the method of GB / T 2677.8-1994, and the content of Klason lignin in larch wood powder was 28.5 wt%; the purity of depolymerized lignin was determined by ultraviolet spectrophotometry (λ=280 nm), and the purity was ≥92%.

[0056] S3: Separation and purification of depolymerized lignin: The mixture after S2 treatment was cooled to room temperature (25℃), diluted with 3000 mL of deionized water, and centrifuged at 4000 rpm for 15 minutes to remove insoluble residue. The supernatant (approximately 4200 mL) was collected, and the pH was adjusted to 2.5 with 2 M HCl solution. The precipitate was allowed to stand for 12 hours to precipitate. The precipitate was collected by filtration, washed with deionized water until the washings were neutral (pH 6.5), and then freeze-dried (conditions: -55℃, vacuum 8 Pa) for 48 hours to obtain 85.2 g of light brown depolymerized lignin product, with a yield of approximately 85%.

[0057] Please see Figure 2-4 The depolymerized lignin obtained above was structurally characterized. Please refer to [link / reference]. Figure 2 Fourier transform infrared spectroscopy analysis showed that in the range of 2500–2600 cm⁻¹ -1 A distinct characteristic absorption peak for thiol (SH) groups appears at 1700–1720 cm⁻¹. -1 A characteristic absorption peak of the carboxyl group (C=O) appears at 3400~3500 cm⁻¹. -1 The characteristic absorption peak of the hydroxyl group (OH) appears at [location missing]. Simultaneously, the C=C skeletal vibration of the aromatic ring is preserved (1400~1600 cm⁻¹). -1 The characteristic absorption peaks of guaiacyl (G type) and syringyl (S type) indicate that the natural aromatic structure of lignin was well preserved during the separation process.

[0058] Please see Figure 3 Proton NMR spectroscopy further confirmed the successful introduction of the mercaptomethylene (-CH2-SH, δ 2.6 ppm) and carboxyl proton (-COOH, δ 10.5 ppm). Please see [link to documentation]. Figure 4 Gel permeation chromatography showed that the number-average molecular weight (Mn) of depolymerized lignin was 980 Da, the weight-average molecular weight (Mw) was 2350 Da, and the polydispersity index was 1.8–2.5 (consistent with the measured value of 2.4 in Example 1). Elemental analysis showed that the sulfur (S) content in depolymerized lignin was 1.8–2.2 wt%, confirming the successful introduction of thiol groups. Furthermore, two-dimensional… 1 H- 13 Heteronuclear single-quantum coherent nuclear magnetic resonance (2D HSQC NMR) analysis revealed a significant decrease in the signal intensity of the β-O-4 ether bond in lignin, indicating that the DES system effectively achieved selective cleavage of the β-O-4 ether bond. Simultaneously, no condensation structure signal was observed in the aromatic region, suggesting that the thiol group of 3-mercaptopropionic acid effectively captured and quenched the benzyl carbocation (Cα) during depolymerization. + The DES system effectively inhibits the condensation side reaction of lignin while separating and depolymerizing the intermediate state.

[0059] S4: Preparation of lignin-based adhesive: Take 50 g of the depolymerized lignin prepared in Example 1, add 100 mL of deionized water, and stir at 70°C for 40 minutes to dissolve and form a homogeneous solution. Add 5 g of citric acid (citric acid accounts for 10% of the lignin mass) as a crosslinking agent, and stir at 80°C for 45 minutes to allow the citric acid to undergo esterification with the hydroxyl and thiol groups in the lignin molecules, forming a preliminary crosslinked network. After the reaction is complete, cool to room temperature (25°C), and adjust the pH to 6.0 with 1M NaOH solution to obtain the lignin-based adhesive. This lignin-based adhesive is a brown viscous liquid with a solid content of approximately 33%, a viscosity (25°C) of 200 mPa·s, and a pH of 6.0. Storage stability tests show that the viscosity of this lignin-based adhesive changes by less than 10% after 30 days of sealed storage at 25°C, indicating excellent storage stability.

[0060] Example 2: Poplar veneer (1.5 mm thick, 10% moisture content, from Shandong province) was selected and plywood was prepared according to the following process: Adhesive was applied to both sides using a roller coating method, using the lignin-based adhesive prepared in Example 1, with an application rate of 260 g / m². 2After applying the adhesive, the material is aged for 20 minutes. The assembly adopts a 5-layer structure with perpendicular textures between adjacent layers, and a total thickness of approximately 7.5 mm. The hot-pressing temperature is 100℃, the hot-pressing time is 9 minutes (calculated based on a board thickness of 1.2 min / mm), and the hot-pressing pressure is 0.88 MPa. The post-treatment involves curing the hot-pressed board at room temperature (25℃) and relative humidity of 55% for 24 hours, followed by edge sawing and sanding.

[0061] Test specimens (100 mm × 25 mm) were prepared according to GB / T 9846-2015 standard, and the adhesive strength was tested. The test results are shown in Table 1 and... Figure 5 .

[0062] Table 1. Test data of physical and chemical properties of poplar plywood Example 3: Eucalyptus veneer (1.5 mm thickness, 10% moisture content, origin: Guangxi) was selected, and plywood was prepared according to the following process: Adhesive was applied to both sides using a roller coating method, using the lignin-based adhesive prepared in Example 1, with an application rate of 260 g / m². 2 After applying the adhesive, the material is aged for 20 minutes. The assembly adopts a 5-layer structure with perpendicular textures between adjacent layers, and a total thickness of approximately 7.5 mm. The hot-pressing temperature is 100℃, the hot-pressing time is 9 minutes (calculated based on a board thickness of 1.2 min / mm), and the hot-pressing pressure is 0.88 MPa. The post-treatment involves curing the hot-pressed board at room temperature (25℃) and relative humidity of 55% for 24 hours, followed by edge sawing and sanding.

[0063] Test specimens (100 mm × 25 mm) were prepared according to GB / T 9846-2015 standard, and the adhesive strength was tested. The test results are shown in Table 2 and... Figure 6 .

[0064] Table 2. Test data of physicochemical properties of eucalyptus plywood As shown in Tables 1 and 2, the poplar and eucalyptus plywood prepared using the depolymerized lignin-based adhesive of this invention meet or exceed the requirements of the national standard GB / T 9846-2015 in all physicochemical properties. The bonding strength of the poplar plywood reaches 1.45 MPa, exceeding the national standard requirement; the bonding strength of the eucalyptus plywood reaches 1.38 MPa, also exceeding the national standard requirement. The wood breakage rate of both types of plywood exceeds 80%, indicating that the adhesive forms a good interfacial bond with the wood. After boiling in water for 3 hours, the bonding strength of the poplar and eucalyptus plywood remains at 0.85 MPa and 0.81 MPa, respectively, meeting the usage requirements. The 24-hour water absorption thickness swelling rates are 6.8% and 7.2%, respectively, lower than the national standard requirement of ≤12%. Formaldehyde emission was not detected, meeting the E0 grade requirements of GB 39600-2021.

[0065] Example 4: Three-layer engineered wood flooring was prepared according to the following process: the core veneer was poplar veneer, 10 mm thick, with a moisture content of 10%; the top veneer was oak veneer, 3.5 mm thick, with a moisture content of 10%; and the back veneer was poplar veneer, 2.5 mm thick, with a moisture content of 10%. The lignin-based adhesive prepared in Example 1 was applied to one side at a rate of 150 g / m². 2 After applying the adhesive, allow it to age for 20 minutes. Assemble the boards in the following order: top veneer - core veneer - back veneer, ensuring the grain direction of the top and back veneers is consistent. Hot-press temperature: 90℃; hot-press pressure: 1.0MPa; hot-press time: 5 minutes. Post-treatment: Cure the hot-pressed flooring at room temperature and 55% relative humidity for 48 hours, then proceed with tongue and groove cutting, sanding, and finishing.

[0066] Performance was tested according to GB / T 18103-2013 standard, and the test results are shown in Table 3. Figure 7 .

[0067] Table 3. Test data of physical and chemical properties of three-layer solid wood composite flooring As shown in Table 3, the three-layer solid wood composite flooring prepared using the depolymerized lignin-based adhesive of this invention exhibits all physical and chemical properties superior to the requirements of the national standard GB / T 18103-2013. The bonding strength reaches 1.52 MPa, exceeding the national standard requirement (≥0.70 MPa) by 117%. The static bending strength reaches 45.6 MPa, exceeding the national standard requirement (≥30 MPa) by 52%. The elastic modulus reaches 5200 MPa, exceeding the national standard requirement (≥4000 MPa) by 30%. The 24-hour water absorption thickness swelling rate is 4.2%, lower than the national standard requirement of ≤8%. The surface abrasion resistance (abrasion value 0.05 g / 100r) is superior to the national standard requirement of ≤0.08 g / 100r. Formaldehyde emission was not detected, meeting the E0 grade requirements of GB 39600-2021.

[0068] Example 5: Eucalyptus veneer (1.5 mm thick, 10% moisture content) was selected, and the lignin-based adhesive prepared in Example 1 was applied to both sides at an application rate of 260 g / m². 2 After applying the adhesive, allow it to age for 30 minutes. After assembly, cold press at room temperature (25℃) and 1.0 MPa for 60 minutes, then cure at room temperature for 72 hours. Further investigation was conducted on the effect of different curing times on the bond strength after room temperature curing: the bond strength reached 1.08 MPa after 48 hours of curing, 1.12 MPa after 72 hours, and 1.18 MPa after 120 hours, indicating that extending the curing time can improve the bond strength within a certain range, but the improvement tends to level off. The lignin-based adhesive prepared by this invention has excellent low-temperature curing performance, which can significantly reduce production energy consumption and is suitable for small-scale processing scenarios without hot pressing equipment.

[0069] Example 6: To verify the optimal hot-pressing process parameters, the hot-pressing temperature in Example 2 was adjusted, and the following results were obtained: Table 4. Effect of different hot-pressing temperatures on the bonding strength of plywood As shown in Table 4, the bonding strength reaches its highest value (1.45 MPa dry, 0.92 MPa wet) and the wood breakage rate is highest (85%) when the hot-pressing temperature is 100℃. When the temperature continues to rise to 110-120℃, the bonding strength decreases slightly, possibly due to excessive cross-linking of the active groups in the adhesive at high temperatures, leading to increased brittleness. It is noteworthy that even under cold-pressing conditions at room temperature (25℃), the dry bonding strength still reaches 1.12 MPa, higher than the national standard of 0.70 MPa, fully demonstrating the excellent low-temperature curing performance of the adhesive of this invention. Therefore, the optimal hot-pressing temperature is determined to be 100℃.

[0070] Example 7: To verify the optimal application rate, the application rate in Example 2 was adjusted, and the following results were obtained: Table 5. Effect of different adhesive application rates on the bond strength of plywood As shown in Table 5, the bonding strength initially increases rapidly with increasing adhesive application rate, reaching a maximum when the adhesive application rate reaches 260 g / m². 2 Subsequently, the bond strength tends to stabilize. Considering both bond strength and economy, the optimal double-sided glue application rate for plywood was determined to be 260 g / m². 2 .

[0071] Example 8: To verify the environmental friendliness of the preparation method of this invention, the recyclability and reusability of the DES solvent used in this invention are tested. The specific method is as follows: Take the residual DES solvent after lignin separation in Examples 1 and S3, and distill it under reduced pressure at 80°C (vacuum degree -0.08~-0.09 MPa) to remove water and low-boiling-point impurities. Then add water to the initial mass to obtain recycled DES solvent.

[0072] Please see Figure 8 The first cycle uses the above-mentioned recycled DES solvent in the subsequent S2-S3 steps, and at the same time verifies the lignin separation efficiency in S3. The test results show that the lignin separation efficiency of the first cycle using recycled DES solvent is 78.5%.

[0073] Subsequently, the residual DES solvent from the first cycle of lignin separation in S3 was distilled under reduced pressure at 80°C (vacuum degree -0.08~-0.09 MPa) to remove moisture and low-boiling-point impurities. Moisture was then added to the initial mass to obtain the second cycle of DES solvent. This recycled DES solvent was used in subsequent S2-S3 steps to verify the lignin separation efficiency in S3. Testing showed that the lignin separation efficiency of the second cycle using the recycled DES solvent was 76.2%.

[0074] This process was repeated. The lignin separation efficiency was 74.8% in the third use of the recycled DES solvent; 73.1% in the fourth use; and 70.7% in the fifth use. After five cycles, the lignin separation efficiency remained above 90% of the initial efficiency, demonstrating that the DES solvent of this invention has good recyclability and reusability, resulting in significant economic and environmental benefits, reducing production costs and environmental impact.

[0075] Comparative Example 1: Comparative Example 1 replaced the 3-mercaptopropionic acid / lactic acid DES used in Example 1 with choline chloride / lactic acid DES (without thiol functional groups). Lignin was separated and adhesives were prepared under similar treatment conditions as in Example 1. The differences between the two in terms of the degree of lignin depolymerization, introduction of active groups, and bonding performance were compared to verify the role of thiol functional groups in the efficient depolymerization of lignin and the introduction of active groups in the DES system.

[0076] Specifically, the operation steps are as follows: S1: Mix choline chloride and lactic acid in a molar ratio of 1:2 and stir at 70°C for 2 hours to form a homogeneous, transparent, and viscous liquid, which is choline chloride / lactic acid DES.

[0077] S2: Take 100g of larch wood powder (average particle size 40-60 mesh, moisture content <8%, larch origin in Northeast China), add 1500mL of the DES prepared above (solid-liquid ratio 1:15, g:mL), and heat at 160℃ for 2 hours. Stir continuously during heating (using mechanical stirring at 200 rpm) to ensure uniform heat transfer.

[0078] S3: Cool the mixture after S2 treatment to room temperature (25℃), dilute with 3000 mL of deionized water, centrifuge at 4000 rpm for 15 minutes to remove insoluble residue. Take the supernatant (about 4200 mL), adjust the pH to 2-3 with 2 M HCl solution, and let it stand for 6-12 hours to precipitate. Collect the precipitate by filtration, wash with deionized water until the washing solution is neutral (pH 6.5), and freeze-dry to obtain the control depolymerized lignin.

[0079] S4: Take 50g of the control depolymerized lignin prepared above, add 100 mL of deionized water, and stir at 70℃ for 40 minutes to dissolve and form a homogeneous solution. Add 5g of citric acid (citric acid accounts for 10% of the lignin mass) as a crosslinking agent, and stir at 80℃ for 45 minutes. After the reaction is complete, cool to room temperature (25℃), and adjust the pH to 6.0 with 1M NaOH solution to obtain the control lignin-based adhesive.

[0080] The control depolymerized lignin adhesive obtained in Comparative Example 1 was used to prepare an adhesive according to the same formulation and process as in Example 2 and pressed into poplar plywood.

[0081] The test results are as follows: Table 6 shows the comparative data of the properties of the control depolymerized lignin prepared in Comparative Example 1 and the depolymerized lignin prepared in Example 1 of the present invention.

[0082] Table 6. Comparison of properties between the control depolymerized lignin of Comparative Example 1 and the depolymerized lignin of Example 1 of the present invention. Table 7 shows the performance comparison data of plywood between Comparative Example 1 and Example 2 of the present invention.

[0083] Table 7 Comparison of plywood performance between Comparative Example 1 and Example 2 of the present invention See Figures 9-14 The above test results clearly show that when the DES system does not contain thiol functional groups (using choline chloride instead of 3-mercaptopropionic acid), the depolymerization effect of lignin is significantly reduced. This is specifically manifested in the following three aspects: (1) Insufficient depolymerization: The number-average molecular weight of lignin obtained by choline chloride / lactic acid DES is 3500~5000 Da, which is higher than 800~2000 Da of the present invention, indicating that conventional DES without thiol groups cannot achieve efficient depolymerization of lignin. The reason is that choline chloride only provides hydrogen bond donors and lacks the nucleophilic attack ability of thiol groups on β-O-4 ether bonds, and therefore cannot effectively break the CO bonds in the lignin macromolecule.

[0084] (2) Lack of active groups: The sulfur content of the control depolymerized lignin obtained in Comparative Example 1 was 0% (not detected), and it contained only one active group, the hydroxyl group. However, the depolymerized lignin obtained in this invention contains three active groups: hydroxyl, thiol, and carboxyl. The lack of active group types and quantities directly leads to a reduction in the reaction sites between lignin and crosslinking agents and wood substrates, resulting in an imperfect adhesive curing network.

[0085] (3) Inadequate bonding performance: The plywood prepared in Comparative Example 1 had a bonding strength of only 0.52 MPa and a wood breakage rate of only 35%, both of which are lower than the requirement of ≥0.70 MPa for Class I plywood in GB / T 9846-2015, and cannot meet the needs of practical applications. However, the bonding strength of the present invention under room temperature curing conditions is 1.08~1.12 MPa, and the wood breakage rate exceeds 80%, which exceeds the national standard requirements.

[0086] In summary, Comparative Example 1 fully demonstrates the crucial role of introducing thiol functional groups into the DES system in achieving efficient depolymerization of lignin and the introduction of active groups. The introduction of thiol groups not only significantly reduces the molecular weight of lignin but also introduces two active groups, thiol and carboxyl, into lignin through covalent grafting, thereby greatly improving the adhesive's bonding performance.

[0087] Comparative Example 2: Poplar plywood was prepared using commercially available phenolic resin adhesive and following the same process as in Example 2 of this invention. The differences between the two in terms of bonding strength, wood breakage rate, formaldehyde release, and curing temperature were compared to verify the advantages of the adhesive of this invention in terms of environmental performance and energy saving.

[0088] Specifically, poplar veneer (1.5 mm thick, moisture content 8%~10%, originating from Shandong) was selected, and plywood was prepared according to the following process: Adhesive was applied to both sides using a roller coating method, using commercially available phenolic resin adhesive (50% solid content, viscosity 300 mPa·s), with an application rate of 260 g / m². 2 After applying the adhesive, the material is aged for 20 minutes. The assembly adopts a 5-layer structure with perpendicular textures between adjacent layers, and a total thickness of approximately 7.5 mm. The hot-pressing temperature is 140℃, the hot-pressing time is 4 minutes (calculated based on a board thickness of 0.8 min / mm), and the hot-pressing pressure is 1.2 MPa. The post-treatment involves curing the hot-pressed board at room temperature (25℃) and relative humidity of 55% for 24 hours, followed by edge sawing and sanding.

[0089] Test specimens (100 mm × 25 mm) were prepared according to GB / T 17657-2013 standard, and the bonding strength was tested. Formaldehyde emission was measured using the climate chamber method (GB / T 39600-2021).

[0090] Table 8 shows a comprehensive comparison of the plywood performance of Comparative Example 2 and Example 2 of the present invention.

[0091] Table 8. Comprehensive comparison of the performance of plywood with the adhesive of the present invention in Comparative Example 2. See Figures 9-14 The test results above show that traditional phenolic resin adhesives are comparable to the adhesive of this invention (1.08~1.12 MPa, wood breakage rate >80%) in terms of bonding strength (1.12 MPa) and wood breakage rate (82%), both meeting the requirements of GB / T 9846-2015. However, the adhesive of this invention has significant advantages in the following two key aspects: (1) Environmental performance advantages: The formaldehyde emission of phenolic resin plywood is 0.08 mg / L (climate chamber method), which only meets the E1 grade requirement in GB 39600-2021; while the formaldehyde emission of the adhesive of this invention was not detected (<0.01 mg / L), meeting the more stringent E0 grade requirement. This invention completely replaces petroleum-based phenol and formaldehyde with biomass lignin, eliminating the formaldehyde emission problem at the source, and has significant health and environmental advantages in the fields of interior decoration and furniture manufacturing.

[0092] (2) Energy-saving advantages: The curing temperature of phenolic resin is 120~150℃, while the hot-pressing temperature of the adhesive of this invention is only 100℃, a temperature reduction of 20~50℃. In industrial production, the lower hot-pressing temperature not only reduces energy consumption but also lowers the temperature resistance requirements of the hot-pressing equipment, which is beneficial to reducing production costs. In addition, the adhesive of this invention also has room temperature curing capability (the bonding strength reaches 1.12 MPa after 72 hours of room temperature curing), further expanding its application scenarios.

[0093] In summary, Comparative Example 2 demonstrates that the depolymerized lignin-based adhesive of the present invention, while maintaining bonding performance comparable to traditional phenolic resins, has significant advantages in environmental performance (zero formaldehyde release, E0 grade) and energy-saving effect (reduction of hot-pressing temperature by 20~50℃), fully reflecting the technological progress of the present invention in the field of green adhesives.

[0094] Comparative Example 3: Comparative Example 3 used only lactic acid (without adding 3-mercaptopropionic acid) as the treatment solvent and treated the wood fiber raw material under the same conditions as in Example 1. The degree of depolymerization and condensation side reactions of lignin were compared to verify the key role of the thiol group of 3-mercaptopropionic acid in inhibiting the lignin condensation side reactions.

[0095] Specifically, the operation steps are as follows: S1: Use lactic acid as the treatment solvent; S2: Take 100 g of larch wood powder (average particle size 40-60 mesh, moisture content ≤8%, larch origin in Northeast China), add 1500 mL of the above lactic acid solvent (solid-liquid ratio 1:15, g:mL), and heat at 160℃ for 2 hours. Stir continuously during heating (using mechanical stirring at 200 rpm) to ensure uniform heat transfer.

[0096] S3: Cool the mixture after S2 treatment to room temperature (25℃), dilute with 3000 mL of deionized water, centrifuge at 4000 rpm for 15 minutes to remove insoluble residue. Take the supernatant (about 4200 mL), adjust the pH to 2-3 with 2 M HCl solution, and let it stand for 6-12 hours to precipitate. Collect the precipitate by filtration, wash with deionized water until the washing solution is neutral (pH 6.5), and freeze-dry to obtain the control depolymerized lignin.

[0097] Table 9 shows the comparison data of the properties of the depolymerized lignin of Comparative Example 3 with those of the depolymerized lignin of Example 1 of the present invention.

[0098] Table 9 Comparison of lignin properties between Comparative Example 3 and Example 1 of the present invention See Figures 9-14The test results of Comparative Example 3 are crucial, revealing the dual function of 3-mercaptopropionic acid in the DES system. The specific analysis is as follows: (1) Extremely low degree of depolymerization: The number-average molecular weight of lignin obtained by treating with lactic acid alone is >10000 Da, which is 5 to 12 times lower than the 800~2000 Da of the present invention, indicating that the acidic catalytic effect of lactic acid alone is far from sufficient to achieve efficient depolymerization of lignin. Although lactic acid can provide protons (H + It attacks the Cα position of the β-O-4 ether bond, but lacks a nucleophile to capture the benzyl carbocation intermediate, causing the depolymerization reaction to remain in the initial stage.

[0099] (2) Severe condensation side reaction: The control depolymerized lignin product obtained in Comparative Example 3 was dark black in color, with a PDI as high as 3.5 (1.5~1.8 in Example 1 of this invention), indicating that a severe condensation side reaction occurred. Under acidic conditions, the benzyl carbocation (Cα) generated by lignin depolymerization... + If the intermediate state cannot be quenched in time, it will undergo an electrophilic substitution reaction with the adjacent aromatic ring to form a CC condensation structure, resulting in an increase in molecular weight instead of a decrease, and a significant reduction in product activity.

[0100] (3) Unable to prepare qualified adhesives: Due to the large molecular weight (>10000 Da) and single active group (only hydroxyl, no mercapto and carboxyl groups) of the lignin obtained in Comparative Example 3, qualified adhesive products could not be prepared using it as raw material, which further confirms the indispensability of 3-mercaptopropionic acid in adhesive preparation.

[0101] In summary, Comparative Example 3 confirms the crucial role of the thiol group in 3-mercaptopropionic acid in inhibiting lignin condensation side reactions. In the DES system of this invention, 3-mercaptopropionic acid simultaneously performs three functions: ① as a hydrogen bond donor component of DES, forming a eutectic system with lactic acid; ② as a nucleophile, the thiol group attacks the Cα position of the β-O-4 ether bond, promoting ether bond cleavage and achieving efficient lignin depolymerization; ③ the thiol group promptly captures and quenches the benzyl carbocation (Cα...). + The intermediate state suppresses condensation side reactions and simultaneously grafts thiol and carboxyl groups onto the depolymerized lignin molecule via covalent bonds (CS bonds). The above effects cannot be achieved by acidic catalysis of lactic acid alone, which fully demonstrates the inventiveness of the technical solution of this invention.

[0102] To fully demonstrate the technical advantages of this invention compared to the three comparative examples, all key performance indicators are summarized in Table 10 below, and are referenced in the following table. Figure 15 It visually demonstrates the difference in overall performance between the present invention and the comparative example.

[0103] Table 10 Summary of the comparison between the three sets of comparative examples and the overall performance of the present invention Through comparative analysis, the following conclusions can be drawn: (1) The thiol functional group is indispensable: The results of Comparative Example 1 (conventional DES without thiol) and Comparative Example 3 (lactic acid treatment only) both show that when the thiol functional group is missing in the DES system, the degree of lignin depolymerization is greatly reduced (Mn is 3500~5000 Da and >10000 Da respectively, while it is 800~2000 Da in this invention), the types of active groups are insufficient, and the bonding performance is substandard or cannot be prepared. The introduction of thiol is the core innovation point for realizing the "depolymerization-grafting integration" of lignin.

[0104] (2) Significant environmental advantages: Although the bonding strength of Comparative Example 2 (phenolic resin) meets the standard, the formaldehyde release is 0.08 mg / L (E1 grade), while the formaldehyde release of the present invention is not detected (<0.01 mg / L, E0 grade), eliminating the formaldehyde release problem from the source, and has an irreplaceable advantage in terms of indoor environmental friendliness.

[0105] (3) Outstanding energy saving effect: The hot pressing temperature (100℃) of this invention is 20~50℃ lower than that of phenolic resin (120~150℃), and it also has room temperature curing capability, making it suitable for small processing scenarios without hot pressing equipment.

[0106] (4) Unique condensation inhibition effect: The dark black product and high PDI (3.5) of Comparative Example 3 confirmed that acidic catalysis by lactic acid alone would lead to severe condensation side reactions. This invention effectively inhibits condensation side reactions by capturing the benzyl carbocation intermediate state by the thiol group. This mechanism has not been reported in the prior art, demonstrating the originality of this invention.

[0107] The following describes the test methods used in this invention: 1. Number-average molecular weight (Mn) and weight-average molecular weight (Mw) determination: Gel permeation chromatography (GPC) was used. Tetrahydrofuran (THF) was used as the mobile phase, with a flow rate of 1.0 mL / min and a column temperature of 35℃. A calibration curve was prepared using polystyrene standards. Lignin samples were acetylated and then dissolved in THF for determination.

[0108] 2. Sulfur content determination: X-ray photoelectron spectroscopy (XPS) and elemental analysis were used for determination. The XPS S 2p spectrum showed a characteristic peak of the CS bond at 163.5 eV. Quantitative analysis was performed using an elemental analyzer (vario EL III, Elementar GmbH, Germany).

[0109] 3. Qualitative analysis of active groups: Fourier transform infrared spectroscopy (FT-IR, wavenumber 400~4000 cm⁻¹) was used. -1 ) and two-dimensional 1 H-13 Qualitative analysis was performed using heteronuclear single-quantum coherent nuclear magnetic resonance spectroscopy (2D HSQC NMR). The hydroxyl group (-OH) was observed at 3400 cm⁻¹. -1 It exhibits a broad absorption peak at 2570 cm⁻¹; the thiol (-SH) group shows a peak at 2570 cm⁻¹. -1 The characteristic peak of the SH stretching vibration is present at 1710 cm⁻¹; the carboxyl group (-COOH) has a peak at 1710 cm⁻¹. -1 There is a C=O stretching vibration peak at this location.

[0110] 4. Bonding strength test: The bonding strength test method for Class I plywood shall be performed according to GB / T 17657-2013 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels". The bonding strength shall be determined after the specimens have been boiled in water for 3 hours. Each test group shall contain no less than 12 valid specimens, and the average value shall be taken.

[0111] 5. Wood breakage rate test: After the glue strength test is completed, visually inspect the percentage of the wood tear area on the damaged surface of the specimen relative to the total glued area.

[0112] 6. Formaldehyde emission test: using a 1m... 3 The climate chamber method was performed according to GB / T 39600-2021 "Classification of Formaldehyde Emission from Wood-based Panels and Their Products". Climate chamber conditions: temperature 23±0.5℃, relative humidity 50±3%, air exchange rate 1.0 L / (h·L), loading ratio 1.0 m. 2 / m 3 Sampling time was 24 hours, and determination was performed using the acetylacetone spectrophotometric method.

[0113] 7. Polydispersity Index (PDI) Calculation: PDI = Mw / Mn, where Mw is the weight-average molecular weight and Mn is the number-average molecular weight. The closer the PDI value is to 1, the narrower the molecular weight distribution.

[0114] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a depolymerized lignin-based adhesive, characterized in that, Includes the following steps: S1: Mix 3-mercaptopropionic acid and lactic acid in a molar ratio of 1:2 to 1:4 and stir at 60 to 80°C for 1 to 3 hours to prepare a eutectic solvent; S2: Mix the wood fiber raw material powder with the eutectic solvent prepared in S1 at a solid-liquid ratio of 1:10 to 1:20, and heat it at 150 to 180°C for 1 to 4 hours to depolymerize the lignin into fragments with a molecular weight of 800 to 2000 Da. At the same time, the thiol and carboxyl groups of 3-mercaptopropionic acid are grafted onto the depolymerized lignin molecules. S3: Cool, dilute, and centrifuge the mixture obtained after S2 treatment. Take the supernatant and adjust the pH to 2-3 to precipitate depolymerized lignin. After filtration, washing, and drying, depolymerized lignin is obtained. S4: Mix the depolymerized lignin obtained in S3 with water, and add a crosslinking agent; the mass ratio of the crosslinking agent to the depolymerized lignin is 5:100 to 15:100; stir and react at 60 to 90°C for 30 to 60 minutes, cool, and adjust the pH to 5.5 to 6.5 to obtain the lignin-based adhesive.

2. The preparation method according to claim 1, characterized in that, The molar ratio of 3-mercaptopropionic acid to lactic acid in S1 is 1:3; the heating treatment temperature in S2 is 150~160℃, and the treatment time is 2~3 hours.

3. The preparation method according to claim 1, characterized in that, The wood fiber raw material powder mentioned in S2 is larch wood powder with an average particle size of 40-60 mesh.

4. The preparation method according to claim 1, characterized in that, The wood fiber raw material powder mentioned in S2 is one of poplar powder, eucalyptus powder, bamboo powder, and corn stalk powder, with an average particle size of 40-60 mesh.

5. The preparation method according to claim 1, characterized in that, The heat treatment described in S2 is carried out in a complex system of lactic acid and 3-mercaptopropionic acid. In this complex system, the carboxyl group of lactic acid provides an acidic environment to catalyze the hydrolytic cleavage of the β-O-4 ether bond, and the thiol group of 3-mercaptopropionic acid performs a nucleophilic attack on the β-O-4 ether bond, causing it to undergo irreversible cleavage and capturing the benzyl carbocation intermediate state to suppress the condensation side reaction. At the same time, 3-mercaptopropionic acid is covalently grafted onto the depolymerized lignin molecule to introduce thiol and carboxyl groups in situ.

6. The preparation method according to claim 1, characterized in that, The crosslinking agent mentioned in S4 is one or more of citric acid, succinic anhydride, or epichlorohydrin.

7. The depolymerized lignin-based adhesive prepared by the method of claim 1, characterized in that, The depolymerized lignin contains three active groups: hydroxyl, thiol, and carboxyl. Its number-average molecular weight is 800-2000 Da, its polydispersity index is 1.8-2.5, and its sulfur content is 1.8-2.2 wt%.

8. The application of the depolymerized lignin-based adhesive according to claim 7, characterized in that: The adhesive is used in plywood manufacturing, using poplar or eucalyptus veneer with a moisture content of 8-12%; it is applied on both sides with an application rate of 240-280 g / m². 2 Hot pressing is carried out at 90~100℃ and 0.88MPa, with the hot pressing time calculated at 1.2min / mm plate thickness; after hot pressing, curing is carried out at room temperature for 24~48 hours.

9. The application according to claim 8, characterized in that, The application rate of the adhesive is 260g / m². 2 The hot pressing temperature is 100℃.

10. The application according to claim 8, characterized in that, The adhesive is used in the manufacture of three-layer engineered wood flooring, which includes a core board, a face veneer, and a back veneer. The core board is made of poplar or eucalyptus veneer, the face veneer is made of oak veneer, and the back veneer is made of poplar veneer. The adhesive application rate is 150g / m². 2 Hot pressing temperature: 90℃; hot pressing pressure: 1.0MPa; hot pressing time: 5 minutes.

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