Lignin-based formaldehyde-free adhesive, lignin-based plywood and preparation method and application of lignin-based formaldehyde-free adhesive

Abstract

The invention relates to the field of refining and application of wood fiber biomass, and discloses a lignin-based formaldehyde-free adhesive, a lignin-based plywood and a preparation method and application of the lignin-based formaldehyde-free adhesive and the lignin-based plywood. The lignin-based formaldehyde-free adhesive disclosed by the invention comprises the following components: L-cysteine lignin, and the solid content of the lignin-based formaldehyde-free adhesive is 10-45wt%. The method comprises the following steps: mixing an aqueous solution of L-cysteine hydrochloride and lactic acid with raw materials for pretreatment; performing extraction and vacuum filtration on the obtained suspension to obtain dissolved lignin and hemicellulose filtrate, and performing extraction and purification on the filtrate to obtain L-cysteine lignin; the lignin-based formaldehyde-free adhesive is prepared by uniformly dispersing the L-cysteine lignin in water. According to the invention, the used solvent is green and nontoxic in source, the hydrated lignin-based adhesive with good fluidity is prepared with low solid content, and the prepared plywood shows excellent dry and wet bonding strength, solvent resistance and weather resistance, and meets the preparation requirements of national I-class plywood.

Description

Technical Field

[0001] This invention relates to the field of refining and application of lignocellulosic biomass, and more specifically, to a lignin-based formaldehyde-free adhesive and lignin-based plywood, as well as their preparation methods and applications. Background Technology

[0002] In recent years, my country's wood processing industry has developed rapidly, and the consumption of wood adhesives has increased significantly. With people's growing awareness of environmental protection and health, the environmental value requirements for wood products are becoming increasingly stringent, making the preparation of low-toxicity adhesives an important direction for the development of plywood adhesives.

[0003] The widespread use of phenolic adhesives and the difficulty in substituting formaldehyde-assisted polymerization have prompted researchers to seek green alternatives. Lignin, with its phenolic phenylpropane-like structure, can replace phenol in addition / condensation polymerization with monomers at the ortho-position of the phenolic hydroxyl groups to prepare lignin-based polymers. However, due to the structural heterogeneity of lignin after separation, it often requires further modification to increase active sites and promote full reaction between formaldehyde and lignin. The abundant presence of the β–O–4 structure leads to the significant blocking of phenolic hydroxyl groups in natural lignin. Therefore, lignin-like structures, represented by organic solvent lignin, need to be protected by hydrolysis or reductive degradation into phenolic oligomers / monomers to ensure product homogeneity, low condensation, and high activity (high phenolic hydroxyl content). Industrial lignins, represented by sulfate lignin and lignin sulfonate, often have low purity, wide molecular weight distribution, and high condensation degree, requiring demethylation, hydroxymethylation, and phenolation to increase reaction sites and improve the performance of the cured prepolymer.

[0004] Although the introduction of modified lignin can, to some extent, replace phenol in the production of aldehyde-based crosslinking agents with higher bonding strength, lignin-based aldehyde adhesives also have drawbacks such as darker color, higher viscosity (unfavorable for large-scale sizing), and more severe curing conditions. Considering the lack of mature, large-scale, sustainable production technology, the relatively low cost-effectiveness of lignin-based aldehyde adhesives makes their large-scale use in plywood processing difficult. Due to the carcinogenicity of formaldehyde and the defects of lignin-based adhesives in terms of color, viscosity, and curing conditions, their promotion is limited. Currently, existing lignin-based formaldehyde-free adhesives include lignin-furfural and lignin-polyurethane composite systems, but these materials struggle to balance environmental friendliness, bonding performance (water resistance, weather resistance, solvent resistance), and cost.

[0005] From the perspective of adhesive working principle, adhesive force mainly originates from the attraction between molecules at the adhesive interface and the cohesive force of the adhesive itself. Designing high-performance lignin-based adhesives requires improving both cohesive force and compatibility and diffusivity with the wood panel interface. To address these issues, Shuai Li et al. developed a low-condensation lignin aqueous dispersion. This material undergoes self-crosslinking via acid catalysis during hot pressing, significantly improving cohesive force and bonding strength. However, the high viscosity of such adhesives still poses a challenge for large-scale industrial application. This study used a formaldehyde system as a stabilizing group during lignin separation and required additional acid catalysis to achieve the self-crosslinking process of lignin during hot pressing. The high viscosity of the hydrated adhesive still needs further optimization to adapt to industrial application. Lignin has a relatively large molecular weight and lacks hydrophilic groups. Therefore, developing a novel high-strength lignin hydrated adhesive for plywood is beneficial for achieving high-value utilization of lignin and further expanding the application areas of plywood. Summary of the Invention

[0006] To overcome the defects of the prior art, such as formaldehyde dependence, difficulty in application, and low bonding performance, this invention provides a lignin-based formaldehyde-free adhesive.

[0007] Another object of the present invention is to provide an application of a lignin-based formaldehyde-free adhesive;

[0008] Another object of the present invention is to provide a lignin-based plywood;

[0009] Another object of the present invention is to provide a method for preparing lignin-based plywood.

[0010] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0011] A lignin-based formaldehyde-free adhesive with a solid content of 10-45 wt% and comprising L-cysteine ​​lignin.

[0012] Preferably, the solid content is 10–33 wt%.

[0013] Preferably, the solid content is 20–33 wt%.

[0014] Furthermore, the β–O–4 content in the L-cysteine ​​lignin is calculated to be 38.4–42.7% based on the aromatic ring.

[0015] Furthermore, in the lignin-based formaldehyde-free adhesive, the mass ratio of L-cysteine ​​lignin to water is 1:1.2 to 9 (g / g).

[0016] Preferably, in the lignin-based formaldehyde-free adhesive, the mass ratio of L-cysteine ​​lignin to water is 1:2 to 9 (g / g).

[0017] Preferably, in the lignin-based formaldehyde-free adhesive, the mass ratio of L-cysteine ​​lignin to water is 1:2 to 4 (g / g).

[0018] When the mass ratio of L-cysteine ​​lignin to water is 1:1.2 (g / g), the solid content is 45 wt%; when the mass ratio of L-cysteine ​​lignin to water is 1:2 (g / g), the solid content is 33 wt%; when the mass ratio of L-cysteine ​​lignin to water is 1:4 (g / g), the solid content is 20% wt%; and when the mass ratio of L-cysteine ​​lignin to water is 1:9 (g / g), the solid content is 10% wt%.

[0019] Furthermore, the lignin-based formaldehyde-free adhesive is prepared by the following method: dispersing L-cysteine ​​lignin in water, stirring for 1 to 4 hours, and stirring at a speed of 250 to 1000 rpm / min.

[0020] Preferably, the stirring time is 3 to 4 hours and the stirring speed is 250 to 500 rpm / min.

[0021] Preferably, the stirring temperature is 25–30°C.

[0022] Furthermore, the L-cysteine ​​lignin is prepared by the following method:

[0023] S1: Mix the raw materials with the pretreatment solution for pretreatment;

[0024] S2: The suspension obtained from the pretreatment is extracted and vacuum filtered to obtain filtrate and filter residue;

[0025] S3: The filtrate was extracted and purified to obtain L-cysteine ​​lignin.

[0026] Furthermore, the pretreatment solution is obtained by mixing L-cysteine ​​hydrochloride, lactic acid and water in a mass-volume ratio of 1g:(5-15)mL:(2-5)mL.

[0027] More preferably, the mass-to-volume ratio of L-cysteine ​​hydrochloride, lactic acid, and water is 1 g: 10 mL: 5 mL, and the concentration of lactic acid is 90 wt%.

[0028] Preferably, the raw material is a grass (Poaceae).

[0029] Preferably, the raw materials are sugarcane bagasse, bamboo, or straw from grasses (such as wheat straw or rice straw).

[0030] The above raw materials do not require complicated and energy-intensive extraction and fine grinding processes.

[0031] Preferably, the solid-liquid ratio of the sugarcane bagasse to the pretreatment liquid is 1:12-15 (g / mL).

[0032] More preferably, the solid-liquid ratio of the sugarcane bagasse to the pretreatment liquid is 1:12 (g / mL).

[0033] Preferably, the pretreatment temperature is 80–120°C and the pretreatment time is 1.5–6 hours.

[0034] More preferably, the pretreatment process is carried out at 90°C for 6 hours under magnetic stirring at 350 rpm / min.

[0035] The bonding structure is crucial for the adhesive strength and cohesion of adhesives. When the pretreatment temperature is too high, the chemical structure of LCL is prone to degradation, especially key bonding structures such as β-O-4 bonds, which break. The further condensed lignin after degradation loses many active sites and cannot form an effective cross-linking network, affecting its bonding ability with wood and thus impacting adhesive performance. Excessive temperature can also cause LCL molecules to undergo over-oxidation or aggregation reactions, forming larger molecular byproducts, resulting in an excessively dark color. Dark colors are usually associated with a high degree of lignin condensation. Lignin with excessive condensation structures tends to form a rigid cross-linking network, increasing the brittleness of the adhesive layer and consequently affecting the durability and flexibility of the plywood.

[0036] Preferably, the extraction is performed by adding an organic solvent.

[0037] More preferably, the organic solvent is a mixture of acetone and water; the volume ratio of the organic solvent to the oven-dry weight of the wood fiber raw material is 20-30:1 (g / g).

[0038] More preferably, the volume ratio of the organic solvent to the oven-dry mass of the wood fiber raw material is 20:1 (g / g).

[0039] More preferably, the volume ratio of acetone to water in the acetone-water mixed solvent is 1:1 (V / V).

[0040] Preferably, the stirring time for the extraction is 2 hours and the stirring speed is 500 rpm / min.

[0041] Preferably, the extraction and purification steps are as follows: the obtained filtrate is subjected to rotary evaporation and pH adjustment to obtain a lignin suspension; the lignin suspension is refrigerated and allowed to stand, diluted, centrifuged, washed, and freeze-dried to obtain L-cysteine ​​lignin (LCL).

[0042] The yield of LCL obtained by the above method is 65-70%, the purity is >90% (based on NREL lignin), and the β-O-4 content is 38.4-42.7% (based on aromatic rings).

[0043] An application of the lignin-based formaldehyde-free adhesive for gluing wood materials.

[0044] Preferably, the wood material includes logs, boards, engineered wood products, wood fiber materials, wood-plastic composites, wood chips or shavings.

[0045] Furthermore, the wood materials include poplar and paulownia.

[0046] A method for preparing lignin-based plywood, which is prepared using the lignin-based formaldehyde-free adhesive.

[0047] Preferably, the application rate of the lignin-based formaldehyde-free adhesive is not less than 80 g / m³. 2 .

[0048] Preferably, the application rate of the lignin-based formaldehyde-free adhesive is not less than 100 g / m³. 2 .

[0049] Furthermore, the bonding conditions of the lignin-based formaldehyde-free adhesive are as follows: hot pressing temperature of 80–190°C, hot pressing time of 2–8 min, and hot pressing pressure of 1–1.5 MPa.

[0050] Preferably, the hot-pressing temperature is 120–190°C and the hot-pressing time is 5–8 min.

[0051] Preferably, the hot-pressing temperature is 90°C, the hot-pressing time is 8 min, and the hot-pressing pressure is 1 MPa.

[0052] A lignin-based plywood is prepared by the method for preparing lignin-based plywood.

[0053] L-cysteine ​​is the only amino acid among the more than 20 amino acids that make up proteins that possesses a reducing sulfhydryl group, exhibiting antioxidant properties, preventing food browning, and preventing pigmentation. This invention utilizes a protected β-O-4 structure of light-colored, low-condensation L-cysteine ​​lignin via an L-cysteine ​​hydrochloride / lactic acid system to prepare a lignin-based formaldehyde-free adhesive. The L-cysteine ​​lignin of this invention exhibits stronger hydrophilicity, resulting in better diffusion of the prepared lignin-based formaldehyde-free adhesive onto wood panels. Based on the characteristics of lignin separation using a lactic acid system, no additional acid is required; the free lactic acid present in the lignin facilitates acid-catalyzed self-crosslinking, promoting lignin curing during hot pressing.

[0054] The lignin-based formaldehyde-free adhesive of this invention requires no additional modification process for the separated lignin. Utilizing a green separation system of L-cysteine ​​hydrochloride / lactic acid, lignin is efficiently separated from plant cell walls while preserving as much of the β-O-4 structure as possible in the separated lignin molecules. This allows the separated lignin to fully self-crosslink and form C-C bonds during hot-pressing curing, increasing the cohesive force of the lignin adhesive. Simultaneously, the slightly acidic nature of the lignin-based formaldehyde-free adhesive promotes the curing process of lignin. Utilizing the excellent diffusion and compatibility of the lignin-based formaldehyde-free adhesive on wood panels, as well as the adsorption of water molecules on the bonding surface by its amino and carboxyl groups, plywood (LCL-based plywood) prepared with a low lignin solids content exhibits excellent dry and wet bonding strength, solvent resistance, and weather resistance, meeting the national Class I plywood preparation requirements. Furthermore, this adhesive is environmentally friendly and non-toxic, low-viscosity (good flowability), has a simple preparation process, and is low-cost, making it suitable for large-scale coating and production of green, high-performance formaldehyde-free plywood.

[0055] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0056] 1. Significantly increased bonding strength. The lignin-based formaldehyde-free adhesive of this invention exhibits better diffusion and compatibility, forming a smaller contact angle on the wood surface, thus enhancing interfacial forces; it also forms C-C bonds for self-crosslinking, significantly improving the adhesive's cohesiveness and water resistance. The prepared lignin-based plywood achieves a dry strength of 4.0±0.23 MPa, which is 5.7 times the national standard (0.7 MPa). The wet strength is 3.35±0.56 MPa, meeting the requirements of Class I plywood in China and far exceeding those of traditional adhesives.

[0057] 2. Improved viscosity stability and flowability, suitable for industrial applications. Within a solid content range of 10–45 wt%, the lignin-based formaldehyde-free adhesive of this invention maintains good flowability at different concentrations, avoiding the problems of difficult delivery and uniform coating associated with traditional high-viscosity adhesives. It exhibits good performance under hot-pressing conditions of 80–190°C, and high dry and wet strength can be achieved even at lower lignin concentrations.

[0058] 3. Excellent weather resistance and solvent resistance. Lignin-based plywood prepared using the lignin-based formaldehyde-free adhesive of this invention maintains high bonding strength even after being immersed in different solvents (ice water, acid, alkali, ethanol, salt water, etc.) for 12 hours. The strength in alkaline solution is 2.72±0.34 MPa, and the strength in salt water is 2.49±0.44 MPa. After repeated high-temperature boiling, freezing, and drying cycles, the strength of the plywood only decreases slightly and remains higher than the national standard. For example, the bonding strength after four aging cycles is still 2.44±0.11 MPa.

[0059] 4. Environmental Protection and Safety. This invention uses an L-cysteine ​​hydrochloride / lactic acid system to replace formaldehyde as the catalytic and separation system, achieving an environmentally friendly and non-toxic adhesive formulation. Compared to traditional formaldehyde-based adhesives, this invention avoids the volatile hazards of formaldehyde, meeting market demands for health and environmental protection. Attached Figure Description

[0060] Figure 1 This is a schematic diagram of the preparation process for LCL-based plywood.

[0061] Figure 2 This is an example of the dimensions of a two-layer plywood specimen.

[0062] Figure 3 These are scanning electron microscope images of the bonded surfaces of LCL-based plywood from Examples 3 and 17.

[0063] Figure 4 Fourier transform infrared spectra of the LCL-based plywood adhesive surface and a blank wood board in Example 17.

[0064] Figure 5 The contact angles of DL-based and LCL-based adhesives on wood panels at different dilution ratios. Detailed Implementation

[0065] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in this technical field.

[0066] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0067] Examples 1-17

[0068] refer to Figure 1 Preparation process for preparing lignin-based plywood (LCL-based plywood).

[0069] 1. Preparation of LCL

[0070] L-cysteine ​​hydrochloride, 90 wt% lactic acid, and water were mixed at a mass-to-volume ratio of 1 g:10 mL:5 mL. Sugarcane bagasse (solid-to-liquid ratio of 1 g / 12 mL) was added, and the mixture was pretreated at 90 °C for 6 h with magnetic stirring at 350 rpm / min. An acetone / water solution (1:1, V / V) at a volume-to-mass ratio of 20:1 (mL:g) relative to the oven-dry raw material was added to the pretreated solid-liquid mixture, and the mixture was stirred at 500 rpm / min at room temperature for 2 h. Vacuum filtration was performed to separate the filtrate and residue. The filtrate was rotary evaporated at 50 °C, and the pH was adjusted to 5–7 to obtain a lignin suspension. The lignin suspension was refrigerated at 4 °C for 12 h, diluted, centrifuged at 9000 rpm / min for 20 min, washed, and freeze-dried to obtain L-cysteine ​​lignin (LCL). The yield of LCL was 65-70%, the purity was >90% (based on NREL lignin), and the β-O-4 content was 38.4-42.7% (based on aromatic rings).

[0071] 2. Preparation of lignin-based formaldehyde-free adhesives

[0072] L-cysteine ​​lignin is added to water at a mass ratio of 1:2 to 4 (g / g) and stirred to disperse, thus obtaining a lignin-based formaldehyde-free adhesive.

[0073] 3. Preparation of lignin-based plywood

[0074] The plywood is made of poplar wood, with dimensions of 200mm × 100mm × 3mm, and an adhesive application rate of 100g / m². 2 The plywood test specimens were prepared by hot pressing using a vulcanizing machine and then cut using a tabletop cutter. The plywood test specimens in this invention were prepared according to the national standard (GB / T 17657-2013), and their style and dimensions are as follows: Figure 2 As shown, the sawing depth continues until the glued surface completely disappears.

[0075] The stirring conditions for preparing lignin-based formaldehyde-free adhesives and the hot-pressing conditions for preparing lignin-based plywood in Examples 1-17 are shown in Table 1.

[0076] Table 1. Stirring and hot pressing conditions for Examples 1-17

[0077]

[0078] Example 18

[0079] refer to Figure 1 Preparation process for preparing lignin-based plywood (LCL-based plywood).

[0080] 1. Preparation of LCL

[0081] L-cysteine ​​hydrochloride, 90 wt% lactic acid, and water were mixed at a mass-to-volume ratio of 1 g: 5 mL: 2 mL. Sugarcane bagasse (solid-to-liquid ratio of 1 g / 12 mL) was added, and the mixture was pretreated at 80 °C for 6 h with magnetic stirring at 350 rpm / min. An acetone / water solution (1:1, V / V) at a volume-to-mass ratio of 30:1 (mL:g) relative to the oven-dry raw material was added to the pretreated solid-liquid mixture, and the mixture was stirred at 500 rpm / min at room temperature for 2 h. Vacuum filtration was performed to separate the filtrate and residue. The filtrate was rotary evaporated at 50 °C, and the pH was adjusted to 5–7 to obtain a lignin suspension. The lignin suspension was refrigerated at 4 °C for 12 h, diluted, centrifuged at 9000 rpm / min for 20 min, washed, and freeze-dried to obtain L-cysteine ​​lignin (LCL).

[0082] 2. Preparation of lignin-based formaldehyde-free adhesives

[0083] L-cysteine ​​lignin was added to water at a mass ratio of 1:9 (g / g) and dispersed at 250 rpm / min for 2 hours at 30°C to obtain a lignin-based formaldehyde-free adhesive with a solid content of 10 wt%.

[0084] 3. Preparation of lignin-based plywood

[0085] The plywood boards are made of paulownia wood, with dimensions of 200mm × 100mm × 3mm, and an adhesive application rate of 80g / m². 2 The plywood was hot-pressed using a vulcanizing machine (190℃, 8 min, 1 MPa), and then cut into samples using a desktop cutter. The plywood test samples in this invention were prepared according to the national standard (GB / T 17657-2013), and their style and dimensions are as follows: Figure 2 As shown, the sawing depth continues until the glued surface completely disappears.

[0086] Example 19

[0087] refer to Figure 1 Preparation process for preparing lignin-based plywood (LCL-based plywood).

[0088] 1. Preparation of LCL

[0089] L-cysteine ​​hydrochloride, 90 wt% lactic acid, and water were mixed at a mass-to-volume ratio of 1 g:15 mL:5 mL. Wheat straw (solid-to-liquid ratio 1 g / 15 mL) was added, and the mixture was pretreated at 120 °C for 1.5 h with magnetic stirring at 350 rpm / min. An acetone / water solution (1:1, V / V) at a volume-to-mass ratio of 20:1 (mL:g) relative to the oven-dry raw material was added to the pretreated solid-liquid mixture, and the mixture was stirred at 500 rpm / min at room temperature for 2 h. Vacuum filtration was performed to separate the filtrate and residue. The filtrate was rotary evaporated at 50 °C, and the pH was adjusted to 5–7 to obtain a lignin suspension. The lignin suspension was refrigerated at 4 °C for 12 h, diluted, centrifuged at 9000 rpm / min for 20 min, washed, and freeze-dried to obtain L-cysteine ​​lignin (LCL).

[0090] 2. Preparation of lignin-based formaldehyde-free adhesives

[0091] L-cysteine ​​lignin was added to water at a mass ratio of 1:1.2 (g / g), and the mixture was stirred and dispersed at 1000 rpm / min for 1 h at 25 °C to obtain a lignin-based formaldehyde-free adhesive with a solid content of 45 wt%.

[0092] 3. Preparation of lignin-based plywood

[0093] The plywood boards are made of paulownia wood, with dimensions of 200mm × 100mm × 3mm, and an adhesive application rate of 110g / m². 2 The plywood was hot-pressed using a vulcanizing machine (150℃, 8 min, 1 MPa), and then cut into samples using a desktop cutter. The plywood test samples in this invention were prepared according to the national standard (GB / T 17657-2013), and their style and dimensions are as follows: Figure 2 As shown, the sawing depth continues until the glued surface completely disappears.

[0094] Comparative Example 1

[0095] 1. Mild acid hydrolysis of lignin by dioxane

[0096] Prepare 30g of oven-dried sugarcane bagasse (40-60 mesh). Mix 180mL of hydrochloric acid solution with 820mL of dioxane solution to achieve a final acid concentration of 0.1mol / L. Mix the sugarcane bagasse with the acid solution and react at 88℃ for 2 hours. Filter the mixture, and wash the residue three times with 200mL of dioxane / water (82 / 12), then wash with 1.5L of water until neutral. Collect all the filtrates and rotary evaporate at 40℃ until the filtrate volume is not less than 500mL. Pour the filtrate into 2L of ice water to precipitate lignin and let it stand overnight. Centrifuge and wash the precipitate until the supernatant is near the center. Freeze-dry the precipitate to obtain dioxane lignin (DL). The yield of DL was 35.38%.

[0097] 2. Preparation of lignin-based formaldehyde-free adhesives

[0098] Add DL to water, stir and disperse to obtain DL adhesive.

[0099] 3. Preparation of lignin-based plywood

[0100] The technical solution is consistent with that of Example 17.

[0101] Comparative Example 2

[0102] 1. Sulfate lignin

[0103] A certain volume of concentrated sugarcane bagasse sulfate black liquor was diluted, centrifuged to remove the precipitate, and the supernatant was collected. While continuously stirring, 4M hydrochloric acid was slowly added using a separatory funnel until the pH was <3. The solution was then placed in an ice-water bath and allowed to stand overnight. The precipitate was collected by centrifugation, washed with deionized water, and then freeze-dried to obtain crude lignin (KL). The β–O–4 content in the obtained KL was 3.5%.

[0104] 2. Preparation of lignin-based formaldehyde-free adhesives

[0105] Add KL to water, stir and disperse to obtain KL adhesive.

[0106] 3. Preparation of lignin-based plywood

[0107] The technical solution is consistent with that of Example 17.

[0108] Comparative Examples 3-8

[0109] The technical solutions of Comparative Examples 3 to 8 are similar to those of Example 11, except that the stirring conditions for preparing the lignin-based formaldehyde-free adhesive and the hot-pressing conditions for preparing the lignin-based plywood are shown in Table 2.

[0110] Table 2. Stirring and hot pressing conditions for Comparative Examples 3–8

[0111]

[0112] Detection methods

[0113] 1. Bond strength

[0114] The bonding strength was uniformly calculated by testing the maximum shear force of the specimen using a universal testing machine (CMT5504, China Meters Industrial Systems Co., Ltd.). The tensile force direction was parallel to the plywood grain direction, the tensile speed was 10 mm / min, and the distance between the clamps was 50 mm.

[0115] (1) Dry strength test method: For samples with hot pressing temperature of 120℃ and below, the dry strength is tested directly after being placed at room temperature for 24 hours after hot pressing. For LCL-based plywood prepared at 150-190℃, due to its high bonding strength and the increased brittleness of the thin wood board after hot pressing due to pore shrinkage, direct testing results in a high breakage rate of the wood after tensile testing, which cannot reflect the true bonding strength of LCL-based plywood. Therefore, all samples are tested after boiling in water at 40℃ for 3 hours to swell the plywood, improve the strength of the plywood itself, and increase the accuracy of the corresponding bonding strength test.

[0116] (2) Wet strength test method: After hot pressing, all specimens were placed at room temperature for 24 hours, then placed in hot water at 63℃ for 3 hours and taken out. After equilibration at room temperature for 10 minutes, the wet strength of the tensile test was carried out.

[0117] Bond strength (MPa) = Maximum shear force (N) / Actual bonded area (mm²) 2 )

[0118] 2. Solvent resistance test method: The prepared plywood specimens were immersed in water at -20℃, hydrochloric acid solution, sodium hydroxide alkaline solution, salt water (simulated seawater) and ethanol for 12 hours respectively. After being taken out and left to stand at room temperature for 10 minutes, tensile tests were performed. Three specimens were selected for parallel tests under different conditions. The calculation formula is the same as that for wet and dry strength.

[0119] 3. Aging performance (extreme weather resistance) test method: The prepared plywood specimens were boiled in water at 100℃ for 3 hours, frozen at -20℃ for 3 hours, and dried at 40℃ for 12 hours, which was recorded as the first aging. The above process was repeated for the second, third and fourth aging, and the bonding strength was tested. Three specimens were taken for parallel sample testing for different aging times. The calculation formula is the same as that for wet and dry strength.

[0120] 3. Test methods for lignin-hydrated adhesives and plywood:

[0121] ①FE-SEM Scanning Electron Microscopy Test: The adhesive surfaces of different samples were cut into thin slices using a desktop cutter. After being sprayed with platinum for 30 seconds, the surfaces were observed using a field emission scanning electron microscope (SU5000, HITACHI) at an accelerating voltage of 3kV.

[0122] ② Fourier Transform Infrared (FTIR) Measurement: The FTIR spectra of the original glued surface were recorded using a Nicolet IS50 (Thermo Fisher, USA) instrument. The captured spectral range was 4000-400 cm⁻¹. -1 Each spectrum was scanned 32 times, with a resolution of 4 cm⁻¹. -1 .

[0123] ③ Test of the contact angle between the adhesive and the wood board: Dilute the hydrated adhesive 10 times and 20 times and test the contact angle. Select a sanded thin wood board and measure the contact angle using a surface tensiometer (DCAT 21, Germany).

[0124] ④ pH test: The pH of the hydrated adhesive is tested using precision pH test paper.

[0125] Analysis and Explanation

[0126] Table 1 shows that as temperature and pressure increase, and hot-pressing time increases over a short period, both dry and wet bond strengths show an increasing trend. For porous wood panels, sufficient contact between the adhesive and the wood panel helps reduce interfacial intermolecular distance and effectively improves intermolecular dispersive attraction and hydrogen bonding. When the hot-pressing temperature increases from 80℃ to 190℃, both dry and wet strengths gradually increase, especially wet strength, which only begins to appear above 150℃. This is because high temperatures can more effectively activate chemical reactions in lignin adhesives (such as acid-catalyzed C / C bond self-crosslinking), increasing the cohesive force of the adhesive and the hydrogen bonding between it and the wood. As the hot-pressing pressure increases from 1MPa to 1.5MPa, the bond strength gradually increases. Higher pressure allows the adhesive to penetrate more easily into the pores of the wood, enhancing mechanical anchoring, reducing the distance between interfacial molecules, and thus increasing attraction and bond strength. Appropriately extending the hot-pressing time can generate a more stable crosslinked structure, which has a positive effect on bond strength. In Example 17, a lignin concentration of 20 wt% achieved a dry strength of 4.0 MPa and a wet strength of 3.35 MPa under appropriate hot-pressing conditions (e.g., 190°C, 8 minutes, 1 MPa), demonstrating that even a low concentration of lignin can achieve excellent bonding results under appropriate conditions.

[0127] Table 1. Bond strength of lignin-based plywood

[0128]

[0129]

[0130] In Comparative Example 3, high-speed stirring easily introduces air into the adhesive, forming bubbles that affect the uniformity of the adhesive layer during coating and reduce the bonding strength of the plywood. Excessive stirring speed also damages the structure of lignin, affecting the utilization rate of its active sites. Reducing the stirring time also results in insufficient exposure and uniform dispersion of lignin's active sites, affecting bonding strength. In Comparative Example 4, excessively long stirring times may cause self-polymerization reactions of the active sites in lignin, leading to increased system viscosity. High-viscosity adhesives are difficult to coat uniformly, reducing their permeability in the wood pores and thus weakening mechanical anchoring force; it may also damage the structure of lignin molecules, reducing their reactivity with other molecules and affecting bonding strength.

[0131] In Comparative Example 5, although the high lignin concentration enhanced intermolecular attraction, the excessively high adhesive viscosity resulted in poor flowability during application, making uniform coating difficult. High concentration also hindered adhesive diffusion and penetration depth, reducing mechanical anchoring with wood pores and affecting overall bonding, leading to limited improvement in dry strength. In Comparative Example 6, the low lignin concentration resulted in weak C-C crosslinking and hydrogen bond formation, leading to decreased cohesion. In Comparative Example 7, the excessively low hot-pressing temperature slowed the acid-catalyzed self-crosslinking reaction rate in the lignin-based adhesive, weakening the adhesive's curing effect; it also limited the adhesive's diffusion rate, making it difficult to penetrate deep into the wood pores; and it was insufficient to activate the active sites in lignin molecules, resulting in an insufficiently dense crosslinking network. Even extending the hot-pressing time did not alter the impact on bonding strength caused by the excessively low hot-pressing temperature. When the hot-pressing temperature is too high, the wood is prone to pyrolysis, damaging the cellulose and hemicellulose structure and weakening its strength. High temperatures also cause rapid evaporation of moisture from the wood, shrinking its pore structure and making it more brittle, thus reducing the fit between the bonded surfaces. Plywood prepared under excessive hot-pressing pressure is more prone to developing greater internal stress after cooling, leading to product deformation or cracking. In Comparative Example 8, insufficient pressure reduces the penetration depth of the adhesive, worsens the adhesion between the wood and the adhesive, weakens intermolecular forces, and leaves residual air bubbles or uneven adhesive layers, reducing the overall strength of the plywood.

[0132] From the electron micrograph of the glued surface ( Figure 3 As can be seen, due to the difference in the compatibility between lignin and the wood panel interface at different temperatures, the surface adhesive of 80-8 (Example 3) exhibits a more pronounced grainy texture than 190-8 (Example 17) under the same pressure. This is evident from the infrared spectrum (…). Figure 4 It can be seen that after the wood board is coated with LCL-based formaldehyde-free adhesive (Example 17), the thickness is 3693-3010 cm. -1 The peak width and intensity of polar peaks such as hydroxyl groups on the adhesive surface are significantly increased, indicating enhanced hydrogen bonding at the adhesive surface.

[0133] It is worth noting that in the examples provided in Table 1, the LCL-based plywood only achieved wet strength after 8 minutes at 120°C. This indicates that LCL-based hydrated adhesives require sufficient time and temperature control to promote the curing process and fully form a CC self-crosslinking network, thus promoting the formation of hydrophobic hydrocarbon structures containing aryl groups. In general, the LCL-based plywoods in Examples 1-11, prepared under mild hot-pressing conditions, can achieve 1-4 times the national dry strength standard, meeting the requirements of National Class III plywood. Examples 12-17 all meet the national Class II and above strength standards; Examples 13-17 meet the requirements of National Class I plywood, with Example 17 exhibiting a dry strength 5.7 times the national standard and a wet strength 4.8 times the national standard.

[0134] Table 2 Comparison of dry and wet strength of different types of lignin

[0135] Lignin Dry strength (MPa) Wet strength (MPa) Comparative Example 1 DL 2.16±0.47 1.07±0.19 Comparative Example 2 KL 1.11±0.26 0 Example 17 LCL 4.0±0.23 3.35±0.56

[0136] Table 2 shows that the bonding strength of lignin-hydrated adhesives is directly related to the degree of lignin condensation, but is also affected by other factors. LCL exhibits a greater degree of condensation than DL, which is closer to the original lignin in sugarcane bagasse, yet its bonding strength is stronger, contrary to previously reported trends. This is because LCL molecules contain more polar groups (amino, carboxyl, and hydroxyl groups), increasing intermolecular forces. Contact angle data indicate ( Figure 5 LCL-based adhesives exhibit a smaller contact angle with wood panels at the same viscosity, resulting in stronger diffusion and better compatibility, which facilitates sufficient contact between adhesive molecules and the wood panel interface. Furthermore, at the same lignin concentration (20 wt%), the pH of LCL-based adhesives is approximately 3.5, while that of DL-based adhesives is approximately 6.0. Acid catalysis is more conducive to the formation of C-C bonds during curing, thereby increasing the adhesive's cohesive strength. KL, with its large molecular weight and strong structural heterogeneity, is less effective in cross-linking and exhibits lower dry strength.

[0137] Table 3 Solvent resistance properties of LCL-prepared plywood

[0138] solvent Bond strength (MPa) -20℃ water 2.29±0.11 pH=3.4 acidic solution 2.02±0.23 pH=10.7 alkaline solution 2.72±0.34 saline solution (3.5%) 2.49±0.44 Anhydrous ethanol 2.59±0.11

[0139] The results in Table 3 show that LCL-based plywood (Example 17) can maintain strong bonding strength (3 times the national standard) in various solvent environments that may be encountered in daily life. It has the strongest resistance to alkali and the relatively weaker resistance to acid. It can also maintain a high bonding strength of 2.49±0.44MPa in salt water with a salt concentration similar to that of seawater, which demonstrates the wide applicability of LCL-based plywood.

[0140] Table 4 Extreme Weather Resistance of LCL-Based Plywood

[0141] Aging times Bond strength (MPa) 1 2.85±0.17 2 2.78±0.21 3 2.61±0.08 4 2.44±.0.11

[0142] Table 4 shows the extreme weather resistance test results of LCL-based plywood (Example 17). After repeated high-temperature boiling and low-temperature freezing cycles, the plywood strength decreased by 2-10% compared to its original wet strength, but the bonding strength consistently far exceeded the national standard, demonstrating the strength stability of LCL-based plywood. Class I plywood in the national standard has excellent weather resistance and boiling water resistance; therefore, LCL-based plywood can be adjusted to simultaneously meet the requirements of Class I, II, and III plywood in the national standard, resulting in a wide range of applications.

[0143] The lignin-based formaldehyde-free adhesive prepared by this invention has super strong bonding strength. Its dry and wet strength is 2 to 3 times the theoretical value of lignin hydrated adhesives reported in existing research [Bonding wood with uncondensed lignins as adhesives[J].Nature,2023,621(7979):511-515], and 5 to 6 times the national standard (0.7MPa). Even under extreme environments, it can still reach 4 times the national standard.

[0144] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A lignin-based formaldehyde-free adhesive, characterized in that, The solid content is 10-45 wt%, and the components include L-cysteine ​​lignin.

2. The lignin-based formaldehyde-free adhesive according to claim 1, characterized in that, The lignin-based formaldehyde-free adhesive is prepared by the following method: L-cysteine ​​lignin is dispersed in water, and the stirring time is 1 to 4 hours and the stirring speed is 250 to 1000 rpm / min.

3. The lignin-based formaldehyde-free adhesive according to claim 1, characterized in that, The β–O–4 content in the L-cysteine ​​lignin is calculated to be 38.4–42.7% based on the aromatic ring.

4. The lignin-based formaldehyde-free adhesive according to claim 1, characterized in that, The L-cysteine ​​lignin was prepared by the following method: S1: Mix the raw materials with the pretreatment solution for pretreatment; S2: The suspension obtained from the pretreatment is extracted and vacuum filtered to obtain filtrate and filter residue; S3: The filtrate was extracted and purified to obtain L-cysteine ​​lignin.

5. The lignin-based formaldehyde-free adhesive according to claim 3, characterized in that, The pretreatment solution is obtained by mixing L-cysteine ​​hydrochloride, lactic acid and water in a mass-volume ratio of 1g:(5-15)mL:(2-5)mL.

6. The lignin-based formaldehyde-free adhesive according to claim 3, characterized in that, During pretreatment, the solid-liquid ratio of the raw material to the pretreatment solution is 1g:12-15mL, the pretreatment temperature is 80-120℃, and the pretreatment time is 1.5-6h.

7. The application of the lignin-based formaldehyde-free adhesive according to any one of claims 1 to 6, characterized in that, Used for gluing wood materials.

8. A method for preparing lignin-based plywood, characterized in that, It is prepared using the lignin-based formaldehyde-free adhesive according to any one of claims 1 to 5.

9. The method for preparing lignin-based plywood according to claim 8, characterized in that, The bonding conditions for the lignin-based formaldehyde-free adhesive are: hot pressing temperature of 80–190°C, hot pressing time of 2–8 min, and hot pressing pressure of 1–1.5 MPa.

10. A lignin-based plywood, characterized in that, It is prepared by the method for preparing lignin-based plywood according to any one of claims 8 to 9.

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