Dialdehyde lignin and dialdehyde protection lignin adhesive as well as preparation method and application thereof

Abstract

The invention relates to the field of adhesives, and discloses a dialdehyde lignin and dialdehyde protection lignin adhesive as well as a preparation method and application thereof. The method comprises the following steps: adding a dialdehyde solution and acid into the wood biomass and 1, 4-dioxane, and reacting at 75-95 DEG C for 3-7 hours; carrying out solid-liquid separation to obtain a liquid component, adjusting the pH value, and adding the liquid component into water; and carrying out solid-liquid separation to obtain a solid component, and cleaning and drying to obtain the dialdehyde lignin. The preparation method comprises the following steps: dissolving gelatin in an alkali solution, adding borate, nano-hydroxyapatite and dialdehyde lignin, and uniformly dispersing to obtain the dialdehyde protected lignin adhesive. The plywood prepared from the adhesive shows excellent mechanical properties, the wet strength of the plywood is larger than or equal to 1.0 MPa and the dry strength of the plywood is 2.2 MPa under the hot pressing condition of 120 DEG C, the flame retardant property, the heat stability and the moisture-proof stability of the plywood are remarkably improved, and the plywood further has the good mildew-proof characteristic.

Description

Technical Field

[0001] The present invention relates to the field of adhesives, and more specifically, to a dialdehyde lignin and a dialdehyde-protected lignin adhesive, a preparation method thereof, and an application thereof. Background Art

[0002] With the increasing demand for high-performance materials in various industries, the adhesive market shows a growth trend. Especially in the fields of electronics, medical care, and renewable materials, the market potential is huge. Traditional adhesives include phenolic, urea-formaldehyde, and melamine-formaldehyde resin adhesives, which are typical petroleum-based wood adhesives and are derived from petroleum. The synthetic raw materials of such adhesives mainly come from petrochemical products such as formaldehyde, phenol, and urea. Among them, phenolic resin (PF) adhesives are the most widely used due to their high bonding strength, good water resistance, and good heat resistance. The phenolic resin adhesive is formed by the addition and condensation reactions of phenol and formaldehyde under the action of an acidic or alkaline catalyst, and finally forms a thermosetting resin with a three-dimensional network structure. Phenol is also harmful to the eyes, skin, respiratory tract, central nervous system, and heart of humans. Formaldehyde has also been listed as a carcinogenic substance with genotoxicity by the International Agency for Research on Cancer of the World Health Organization.

[0003] To overcome the problems of formaldehyde release and non-renewability of petroleum, researchers have explored using biomass to replace phenol to prepare adhesives, such as CN118085812A, a preparation method of a lignin adhesive; CN114736652A, a method for extracting lignin from lignocellulose biomass as an adhesive; CN118027321A, a preparation method and application of a phenolic resin modified by lignocellulose components, as well as formaldehyde-free bio-based adhesives, such as CN111100581A, a preparation method of a fully bio-based adhesive; CN119371933A, a preparation method of a high-strength gelatin-based wood-based panel adhesive; and CN118620555A, a preparation method and application of a gelatin-based wood adhesive. However, in the process of using biomass to replace phenol in the above-mentioned existing technologies, formaldehyde is still released; and the crosslinking degree is low, high temperature is required to ensure fluidity in wood, and the water resistance, mildew resistance, and flame retardancy are poor.

[0004] There is currently no report on the preparation of a bio-based adhesive by mixing dialdehyde and gelatin. Summary of the Invention

[0005] The present invention aims to overcome the defects of low crosslinking degree, high temperature required to ensure fluidity in wood, poor water resistance, mildew resistance, and flame retardancy in the above-mentioned existing technologies, and provides a dialdehyde lignin;

[0006] Another object of the present invention is to provide a preparation method of the dialdehyde lignin;

[0007] Another object of the present invention is to provide an application of the dialdehyde lignin;

[0008] Another object of the present invention is to provide a dialdehyde-protected lignin adhesive;

[0009] Another object of the present invention is to provide a preparation method of a dialdehyde-protected lignin adhesive;

[0010] Another object of the present invention is to provide an application of a dialdehyde-protected lignin adhesive.

[0011] To solve the above technical problems, the technical solution of the present invention is as follows:

[0012] A preparation method of dialdehyde lignin, adding a dialdehyde solution and hydrochloric acid to lignocellulose and 1,4-dioxane, and reacting at a temperature of 75-95 °C for 3-7 h; after cooling, solid-liquid separation is carried out to obtain a liquid component, after adjusting the pH, adding it to water; solid-liquid separation is carried out to obtain a solid component, after washing and drying, dialdehyde lignin (GAL) is obtained.

[0013] Preferably, the lignocellulose is a powder with a mesh size of 40-60.

[0014] Preferably, the lignocellulose includes eucalyptus, poplar, pine and willow.

[0015] Preferably, the reaction is stirred at a speed of 300-600 rpm.

[0016] Preferably, the dialdehyde solution includes terephthalaldehyde, glyoxal, glutaraldehyde, malonaldehyde.

[0017] Preferably, the mass percentage of the dialdehyde solution is 40%-60%.

[0018] Furthermore, lignocellulose: 1,4-dioxane = 1:7-10 (m / v, g / mL); lignocellulose: dialdehyde solution = 1:0.25-3 (m / v, g / mL).

[0019] Preferably, lignocellulose: hydrochloric acid = 1:0.42 (m / v, g / mL).

[0020] Preferably, the concentration of the hydrochloric acid is 36.5%-37.5%.

[0021] Furthermore, the pH of the liquid component is adjusted to 2.5-3.5, and water with a volume of 8-15 times is added.

[0022] Preferably, solid-liquid separation is carried out by filtration with a Buchner funnel, and the solid is washed with water until the pH value of the filtrate is neutral, and then dried at room temperature. The obtained solid is dialdehyde lignin.

[0023] A dialdehyde lignin, prepared by the above preparation method.

[0024] An application of the dialdehyde lignin for preparing an adhesive.

[0025] A dialdehyde-protected lignin adhesive prepared by using the dialdehyde lignin.

[0026] A preparation method of the dialdehyde-protected lignin adhesive, which dissolves gelatin (Gel) in an alkaline solution, adds borate, nano-hydroxyapatite (nHA), and dialdehyde lignin (GAL), and after uniform dispersion, obtains the dialdehyde-protected lignin adhesive.

[0027] The present invention uses lignocellulose as a raw material, 1,4-dioxane as a solvent, dialdehyde as a protecting agent, and hydrochloric acid as a catalyst to isolate highly reactive lignin from biomass. The aldehyde group of the dialdehyde reacts with the amino group of gelatin to form a hydrophobic C=N through a Schiff base reaction, preparing an environmentally friendly and water-resistant biomass-based adhesive. At the same time, the borate ions of the added sodium borate form a cross-linked network with the aldehyde groups, benzene ring carbons, and polysaccharide carbons of the dialdehyde lignin and gelatin, preventing water molecules from entering. The formed adhesive penetrates into the wood board gaps and cures to form glue nails. Finally, the added nHA effectively improves the gel properties of Gel, and the prepared adhesive has good anti-mildew, antibacterial, and flame-retardant properties.

[0028] Preferably, the borate is a borate with a solubility of not less than 0.01 g / 100 mL under room temperature alkaline conditions.

[0029] Preferably, the borate includes sodium borate (STB), magnesium borate, and aluminum borate.

[0030] Preferably, the gelatin is biotechnological grade gelatin, and the particle size of the nano-hydroxyapatite is 80 - 200 nm.

[0031] Preferably, the particle size of the nano-hydroxyapatite is 200 nm.

[0032] Further, GAL:Gel = 3 - 5 (w / w, g / g), STB:(GAL + Gel) = 0.005 - 0.04 (w / w, g / g), Gel:nHA = 10 - 70 (w / w, g / g), alkaline solution:(GAL + Gel + STB + nHA) = 1.5 - 2 (v / w, mL / g).

[0033] Further, the concentration of the alkaline solution is 0.15 - 0.25 mol / L.

[0034] Preferably, at a temperature of 25 - 35 °C, the alkaline solution adds gelatin and stirs to dissolve; adds borate and stirs for 3 - 5 min; adds nHA and stirs for 20 - 40 min, and then adds GAL.

[0035] An application of the dialdehyde-protected lignin adhesive for the processing and manufacturing of raw wood boards or wood-based panels.

[0036] The lignin adhesive of the present invention uses an alkaline solution as the main solvent and gelatin, borate, nano-hydroxyapatite, and dialdehyde lignin as core components. The method of the present invention first extracts dialdehyde lignin from biomass. The 1,3-diol structure in the lignin structure reacts with the aldehyde group of dialdehyde lignin to form a stable dioxane structure, separating lignin from biomass without the release of toxic formaldehyde. Further, dialdehyde lignin and gelatin are mixed in an alkaline solution to prepare a dialdehyde lignin-gelatin adhesive (GAL-Gel). The aldehyde group of dialdehyde lignin reacts with the amino group of gelatin to form a Schiff base reaction, improving the bonding strength. Due to the gel properties of gelatin, the prepared GAL-Gel has a high degree of gelation, and the reaction between the adhesive and the veneer is not sufficient. Adding sodium borate not only improves the mildew-proof and flame-retardant properties of the adhesive, but also slightly alleviates the gelation degree of the GAL-Gel adhesive. The borate ions of sodium borate form a cross-linked structure with the benzene ring carbon of dialdehyde lignin and the polysaccharide carbon of gelatin, but the alleviating degree is limited. Therefore, adding nano-hydroxyapatite effectively improves the gelation degree of the adhesive and further enhances the mildew-proof and flame-retardant properties. The hydroxyl group and phosphate ions of nano-hydroxyapatite form hydrogen bonds or coordination bonds with borate ions, forming a stable cross-linked network in the adhesive.

[0037] Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

[0038] 1. Significantly improve the bonding strength and water resistance. The present invention forms a dense cross-linked network through the Schiff base reaction between dialdehyde lignin and gelatin, and further introduces borate and nano-hydroxyapatite to assist cross-linking, making the prepared plywood exhibit excellent mechanical properties, with a wet strength ≥ 1.0 MPa and a dry strength up to 2.2 MPa at most.

[0039] 2. The hot pressing temperature is significantly reduced, and the energy-saving effect is prominent. Compared with the prior art that requires a high temperature of 170 °C for hot pressing to barely reach the qualified strength, the present invention can obtain a wet strength of 1.0 MPa and a dry strength of 1.9 MPa under the hot pressing condition of 120 °C at the lowest through a synergistic cross-linking system, effectively reducing energy consumption and improving production efficiency.

[0040] 3. Improve the flame retardancy and thermal stability. The total heat release (THR) and mass loss rate (HRR) of the adhesive of the present invention are lower than those of the prior art. The addition of STB and nHA promotes an increase in cross-linking density, forming a fireproof layer, and significantly improving the flame retardant performance and thermal stability of the board.

[0041] 4. Reduce hygroscopicity and improve durability. The moisture absorption rate of the adhesive of the present invention is lower than that of the comparative example, and the residue rate is higher. This is attributed to the formation of a stable cross-linked network between borate ions and nHA and the colloidal molecules, reducing the hydrophilic groups and effectively improving the moisture resistance stability of the material.

[0042] 5. Endow good anti-mildew properties. STB and nHA are added to the adhesive of the present invention, and it remains mold-free within 15 days. However, mildew spots appear on the comparative example without the addition of additives on the 10th day, indicating that the adhesive of the present invention can effectively delay the growth of microorganisms and improve the anti-mildew performance of the material. Description of the Drawings

[0043] Figure 1 It is the two-dimensional NMR spectrum of dialdehyde lignin in Example 1;

[0044] Figure 2 It is the infrared spectrum of dialdehyde lignin in Example 1;

[0045] Figure 3 It is the XPS spectrum of dialdehyde lignin in Example 1;

[0046] Figure 4 It is the diffuse reflection spectrum of dialdehyde lignin in Example 1;

[0047] Figure 5 It is the infrared spectrum of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0048] Figure 6 It is the XPS C1s peak-fitting spectrum of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0049] Figure 7 It is the XPS O1s peak-fitting spectrum of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0050] Figure 8 It is the XPS N1s peak-fitting spectrum of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0051] Figure 9 It is the DSC spectrum of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0052] Figure 10 It is the SEM image of the adhesive prepared in Example 1;

[0053] Figure 11 It is the microscopic image of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0054] Figure 12 It is the cone calorimetry spectrum of the adhesives prepared in Example 1, Comparative Example 4 and Comparative Example 10;

[0055] Figure 13 Moisture absorption characteristics diagrams of the adhesives prepared in Example 1, Comparative Example 4, and Comparative Example 10;

[0056] Figure 14 Mildew-proof characteristics diagrams of the adhesives prepared in Example 1, Comparative Example 4, and Comparative Example 10;

[0057] Figure 15 State diagrams of the adhesives in Example 1, Comparative Example 4, and Comparative Example 10 after being placed for 10 minutes. Detailed implementation manners

[0058] The present invention will be further described below in conjunction with the accompanying drawings of the specification and specific embodiments. However, the embodiments do not limit the present invention in any form. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the technical field.

[0059] Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0060] The gelatin used in the following implementation schemes is biotechnology-grade gelatin, and the particle size of the nano-hydroxyapatite used is 80 - 200 nm.

[0061] Example 1

[0062] (1) Extraction of dialdehyde lignin

[0063] First, place 15 g of 40 - 60 mesh eucalyptus powder and 135 mL of 1,4-dioxane into a 350 mL thick-walled pressure-resistant bottle. After adding a magnetic stirrer to the thick-walled pressure-resistant bottle, place it in an 80 °C oil bath. Then, add 15 mL of 50% glutaraldehyde solution and 6.3 mL of 37% hydrochloric acid solution in sequence. Start timing for 5 h after adding the hydrochloric acid.

[0064] Then, cool the reaction solution to room temperature, filter the solid-liquid mixture with a Buchner funnel, collect the filtrate, neutralize it to slightly acidic (pH 2.5 - 3.5) with saturated sodium bicarbonate (0.84 g of sodium bicarbonate and 10.12 g of deionized water), slowly add the neutralized filtrate to 15 times the amount of deionized water, perform solid-liquid separation by filtration with a Buchner funnel, wash the solid with deionized water until the pH value of the filtrate is neutral, and then perform drying at room temperature to obtain GAL.

[0065] (2) Preparation of the adhesive

[0066] First, dissolve gelatin in 0.2 mol / L sodium hydroxide solution at a dissolution temperature of 30 °C. After adding sodium borate, stir for 5 min, then add nano-hydroxyapatite and stir for 30 min. Then add GAL and mix. The ratio of sodium hydroxide solution: (GAL + Gel + STB + nHA) = 1.5, GAL:Gel = 4, STB:(GAL + Gel) = 0.01, Gel:nHA = 50. After uniform dispersion, the GAL-Gel-STB-nHA adhesive is obtained.

[0067] (3) Preparation of plywood

[0068] Select eucalyptus veneers to prepare three-layer plywood. Among them, the moisture content of eucalyptus veneers is 10% - 15%, and the length, width and thickness are 140 mm, 100 mm and 1.5 mm respectively; select one of the veneers as the core board, apply the adhesive on both sides, and the application amount is 100 g / m 2 ; After the sizing is completed, adhere two eucalyptus veneers to the upper and lower surfaces of the core board, and place them in the dark at room temperature for closed aging for 5 - 30 min; perform hot pressing on the adhered three-layer board, the hot pressing pressure is 1 - 1.8 Mpa, the hot pressing temperature is 160 °C, and the hot pressing time is 4 - 16 min. After hot pressing is completed, place the plywood at room temperature for 48 - 72 h.

[0069] Example 2

[0070] (1) Extraction of dialdehyde lignin

[0071] First, place 15 g of poplar wood powder with a mesh size of 40 - 60 and 105 mL of 1,4-dioxane in a 350 mL thick-walled pressure-resistant bottle. After adding a magnetic stirrer to the thick-walled pressure-resistant bottle, place it in an oil bath at 75 °C. Then add 3.75 mL of 36.5% terephthalaldehyde solution and 6.3 mL of 37% hydrochloric acid solution in sequence. After adding hydrochloric acid, stir and react at a speed of 600 rpm for 3 h.

[0072] Then cool the reacted solution to room temperature, filter the solid-liquid mixture with a Buchner funnel, collect the filtrate, neutralize it to slightly acidic (pH 2.5 - 3.5) with saturated sodium bicarbonate (0.84 g of sodium bicarbonate and 10.12 g of deionized water), slowly add the neutralized filtrate to 8 times the amount of deionized water, perform solid-liquid separation by filtration with a Buchner funnel, wash the solid with deionized water until the pH value of the filtrate is neutral, and then dry it at room temperature to obtain GAL.

[0073] (2) Preparation of adhesive

[0074] First, dissolve gelatin in 0.25 mol / L sodium hydroxide solution at a dissolution temperature of 35 °C. After adding sodium borate, stir for 3 min, then add nano-hydroxyapatite and stir for 20 min. Then add GAL and mix. The ratio of sodium hydroxide solution : (GAL + Gel + STB + nHA) = 2, GAL : Gel = 5, STB : (GAL + Gel) = 0.005, Gel : nHA = 70. After uniform dispersion, the GAL-Gel-STB-nHA adhesive is obtained.

[0075] (3) Preparation of plywood

[0076] Select eucalyptus veneers to prepare three-layer plywood. Among them, the moisture content of eucalyptus veneers is 10% - 15%, and the length, width and thickness are 140 mm, 100 mm and 1.5 mm respectively; select one of the veneers as the core board, apply the adhesive on both sides, and the application amount is 140 g / m 2 ; After sizing, adhere two eucalyptus veneers to the upper and lower surfaces of the core board, and place them in the dark at room temperature for closed aging for 5 - 30 min; perform hot pressing on the adhered three-layer board, with a hot pressing pressure of 1 - 1.8 Mpa, a hot pressing temperature of 160 °C, and a hot pressing time of 4 - 16 min. After hot pressing, place the plywood at room temperature for 48 - 72 h.

[0077] Example 3

[0078] (1) Extraction of dialdehyde lignin

[0079] First, place 15 g of 40 - 60 mesh pine wood powder and 150 mL of 1,4-dioxane in a 350 mL thick-walled pressure-resistant bottle. After adding a magnetic stirrer to the thick-walled pressure-resistant bottle, place it in an oil bath at 95 °C, and successively add 45 mL of 60% glyoxal solution and 6.3 mL of 37.5% hydrochloric acid solution. After adding hydrochloric acid, stir and react at a speed of 300 rpm for 7 h.

[0080] Then cool the reaction solution to room temperature, filter the solid-liquid mixture with a Buchner funnel, collect the filtrate, neutralize it to slightly acidic (pH 2.5 - 3.5) with saturated sodium bicarbonate (0.84 g of sodium bicarbonate, 10.12 g of deionized water), slowly add the neutralized filtrate to 15 times deionized water, filter with a Buchner funnel for solid-liquid separation, and wash the solid with deionized water until the pH value of the filtrate is neutral, and then perform drying at room temperature to obtain GAL.

[0081] (2) Preparation of adhesive

[0082] First, dissolve gelatin in 0.15 mol / L sodium hydroxide solution at a dissolution temperature of 25 °C. After adding sodium borate, stir for 5 min, then add nano-hydroxyapatite and stir for 40 min. Then add GAL and mix. The ratio of sodium hydroxide solution:(GAL + Gel + STB + nHA) = 2, GAL:Gel = 3, STB:(GAL + Gel) = 0.01, Gel:nHA = 10. After uniform dispersion, the GAL-Gel-STB-nHA adhesive is obtained.

[0083] (3) Preparation of plywood

[0084] Select eucalyptus veneers to prepare three-layer plywood. Among them, the moisture content of eucalyptus veneers is 10% - 15%, and the length, width and thickness are 140 mm, 100 mm and 1.5 mm respectively; select one of the veneers as the core board, apply the adhesive on both sides, and the amount of adhesive applied is 140 g / m 2 ; After the adhesive application is completed, adhere two eucalyptus veneers to the upper and lower surfaces of the core board, and place them in the dark at room temperature for closed aging for 5 - 30 min; perform hot pressing on the adhered three-layer board, the hot pressing pressure is 1 - 1.8 Mpa, the hot pressing temperature is 120 °C, and the hot pressing time is 4 - 16 min. After the hot pressing is completed, place the plywood at room temperature for 48 - 72 h.

[0085] Examples 4 - 5

[0086] The technical solutions are similar to those of Example 1, except that in the preparation of the adhesive, sodium borate is replaced with magnesium borate and aluminum borate respectively.

[0087] Comparative Examples 1 - 2

[0088] The technical solutions are similar to those of Example 1, except that in the preparation of the adhesive, Gel:nHA are 10 and 70 respectively.

[0089] Comparative Example 3

[0090] (1) Extraction of dialdehyde lignin

[0091] The technical solution is the same as that of Example 1.

[0092] (2) Preparation of adhesive

[0093] Mix the dried GAL with 0.2 mol / L sodium hydroxide solution at a ratio of 1:2, and obtain the GAL adhesive after uniform dispersion.

[0094] (3) Preparation of plywood

[0095] The method is similar to that of Example 1, except that in the preparation of plywood, the hot pressing temperature is 170 °C.

[0096] Comparative Example 4

[0097] The technical solution is similar to Comparative Example 3, with the difference being the preparation of the adhesive:

[0098] First, dissolve gelatin in 0.2 mol / L sodium hydroxide solution at a dissolution temperature of 30 °C, then add GAL and mix. GAL:Gel = 4, and after uniform dispersion, the GAL-Gel adhesive is obtained.

[0099] Comparative Examples 5 - 6

[0100] The technical solution is similar to Comparative Example 4, with the difference being that in the preparation of the adhesive, the sodium hydroxide solution is replaced with 0.2 mol / L sulfuric acid and water respectively to obtain the GAL-Gel adhesive.

[0101] Comparative Examples 7 - 8

[0102] The technical solution is similar to Comparative Example 4, with the difference being that in the preparation of the adhesive, GAL:Gel is 1.5:1 and 3:1 respectively to obtain the GAL-Gel adhesive.

[0103] Comparative Example 9

[0104] The technical solution is similar to Comparative Example 4, with the difference being that in the preparation of the adhesive, sodium hydroxide solution:(GAL + Gel + STB + nHA) = 1.5:1 to obtain the GAL-Gel adhesive.

[0105] Comparative Example 10

[0106] The technical solution is similar to Comparative Example 3, with the difference being the preparation of the adhesive:

[0107] First, dissolve gelatin in 0.2 mol / L sodium hydroxide solution at a dissolution temperature of 30 °C, add sodium borate and stir for 5 min, then add GAL and mix. GAL:Gel = 4, STB:(GAL + Gel) = 0.01, and after uniform dispersion, the GAL-Gel-STB adhesive is obtained.

[0108] Comparative Examples 11 - 12

[0109] The technical solution is similar to Comparative Example 10, with the difference being that in the preparation of the adhesive, STB:(GAL + Gel) is 0.005 and 0.004 respectively;

[0110] Comparative Example 13

[0111] The technical solution is similar to Comparative Example 10, with the difference being that in the preparation of the adhesive, sodium borate is replaced with calcium borate, and the hot pressing temperature is 160 °C to prepare the GAL-Gel-STB adhesive.

[0112] Testing method

[0113] 1. Performance Testing of Adhesives

[0114] The plywood produced was tested according to the testing method of GB / T17657-1999 "Test Methods for Physical and Chemical Properties of Wood-Based Panels and Decorative Wood-Based Panels".

[0115] 2. Residual Rate and Moisture Absorption Rate

[0116] The adhesive was cured and dried in an oven at 120°C for 12 h for the residual rate test.

[0117] The dried adhesive was ground into powder for the moisture absorption rate test.

[0118] Residual Rate: The sample obtained after drying was completely immersed in water at 60°C for 6 h, and then dried at 105°C until it reached a constant weight. The residual rate was calculated using Equation 1-1.

[0119] Moisture Absorption Rate: The dried powder was placed in a container with a relative humidity (RH) of 80% (saturated KCl solution) at 50°C. The weight of the adhesive sample was recorded every 2 h until it reached a constant weight. The moisture absorption was calculated using Equation 1-2.

[0120]

[0121] Where m is the mass of the sample after immersion, g; M is the mass of the sample before immersion, g; m0 is the mass of the sample before moisture absorption, g; m1 is the mass of the sample after moisture absorption, g.

[0122] Analysis and Explanation

[0123] As can be seen from Table 1, when the addition amount of nano-hydroxyapatite (nHA) is too small (such as in Comparative Example 1), the degree of crosslinking is insufficient, resulting in a decrease in wet strength; when the addition amount is too large (such as in Comparative Example 2), the compactness of the colloid system decreases, which also weakens the bonding strength. When GAL is used alone (such as in Comparative Example 3) or the ratio of GAL to Gel is unreasonable (such as in Comparative Examples 7, 8, and 9), the crosslinking network is incomplete, resulting in varying degrees of decrease in wet strength and dry strength. When sulfuric acid or pure water is used instead of sodium hydroxide solution (such as in Comparative Examples 5 and 6), the alkaline condition is destroyed, which is not conducive to the Schiff base reaction, resulting in the inability to form an effective crosslinking structure and ultimately the loss of bonding performance. When the borate ratio is insufficient (such as in Comparative Examples 11 and 12) or calcium borate is used instead (such as in Comparative Example 13), the extremely low water solubility, lack of reactivity, and poor dispersibility lead to a decrease in crosslinking density, wet strength, and dry strength.

[0124] In addition, by adding STB and nHA, covalent crosslinking was formed with GAL and Gel, and the crosslinking between the aldehyde group of GAL and the amino group of Gel was promoted. This not only improved the bonding strength but also significantly reduced the hot pressing temperature to 120 °C (Example 3), and excellent wet strength and dry strength could still be obtained. When not added or added unreasonably in the comparative examples, almost all required hot pressing conditions of up to 170 °C to barely achieve certain properties, indicating the important role of STB and nHA in promoting low-temperature and rapid crosslinking, and further proving the superiority of the technical solution of the present invention in reducing energy consumption and optimizing process conditions.

[0125] Table 1 Adhesive Performance Test

[0126]

[0127] It can be seen from Figure 1 that the three stronger absorption signals marked in blue in the side chain region of the 2D HSQC NMR spectrum of GAL obtained in Example 1 originate from the dioxane structure formed by acetal protection in GAL, corresponding to the C-H crosslinking signals at the γ, α, and β positions of the GAL side chain from top to bottom. A large number of free active reaction sites were observed in the aromatic ring region of the 2D HSQC NMR spectrum of GAL, such as the C5 and C6 positions of guaiacyl (G 5,6 ), and the C2 and C6 positions of syringyl (S 2,6 ), which ensured the high crosslinking reaction performance of lignin.

[0128] A new C=O stretching vibration peak appeared at 1678 cm -1 in the GAL obtained in Example 1, proving the existence of a large number of carbonyl groups (aldehyde groups) in GAL. This may be because only one of the two aldehyde groups of GA underwent an acetal reaction with the hydroxyl group on lignin to form a dioxane structure, and the other aldehyde group did not participate in the reaction and remained intact ([[]] Figure 2 ). In addition, the influence of GAL mainly focused on the formation of C-O structures (phenolic hydroxyl groups and ether bonds) and C=O structures, improving the reactivity of lignin, and the ratio of C-C / C=C was still very high, indicating that glutaraldehyde could protect the main structure of lignin ([[]] Figure 3 ). GAL had better absorption ability for ultraviolet light and visible light. Since cellulose and hemicellulose did not absorb ultraviolet light and visible light under the measurement conditions, it indicated the deposition of lignin on the surface of GAL ([[]] Figure 4 ).

[0129] It can be seen from Figure 5 that in the GAL-Gel-STB adhesive obtained in Comparative Example 10, a peak at 1720 cm -1The absorption peak of the C=O bond weakens, indicating that the hydroxyl group of the STB borate ion undergoes an esterification reaction with the free C=O; 1592 cm -1 The absorption peak at is attributed to the stretching vibration of C=N, indicating that the aldehyde group of GAL reacts with the amino group of Gel to form Schiff base reaction to generate C=N. From Figure 6 It can be seen that with the addition of STB, the C-C / C=C content increases, the C-O content decreases, and the C=O content increases. This is because the borate ion of STB forms a covalent cross-linking network with the hydroxyl group under hot pressing conditions, resulting in a decrease in the relative content of C-O, indicating that a dehydration / dehydrogenation reaction may occur during hot pressing, and a stable C-C bond is formed, thus making the plywood exhibit better bonding strength. After further adding nHA, the C-C / C=C content increases, and the C-O and C=O contents decrease because the addition of nHA promotes the cross-linking between GAL and Gel, forming more C-C bonds. From Figure 7 It can be seen that with the addition of STB and nHA, the C-O content decreases and the C=O content increases because the addition of STB and nHA causes some C-O ether bonds to break.

[0130] From Figure 8 It can be seen that C=N linking bonds appear in GAL-Gel, GAL-Gel-STB, and GAL-Gel-STB-nHA, indicating that a Schiff base reaction occurs between GAL and Gel under hot pressing conditions to form C=N. Due to the addition of STB and nHA, the C-N linking bond content decreases and the C=N linking bond content increases. Under alkaline conditions, nHA uniformly disperses Gel, reduces agglomeration, exposes more amino groups, and promotes the reaction with the aldehyde group of GAL, resulting in an increase in the C=N content and a decrease in the C-N content.

[0131] From Figure 9 It can be seen that the glass transition temperature and melting temperature of GAL-Gel, GAL-Gel-STB, and GAL-Gel-STB-nHA increase in turn, indicating that the addition of STB and nHA enhances the hydrogen bond interaction. The glass transition temperature and melting temperature are both lower than the hot pressing temperature (120 °C). The lower glass transition temperature and melting temperature help the adhesive to flow and penetrate into the wood at low temperature, thus improving the bonding strength.

[0132] From Figure 10 An obvious bonding line can be seen, which is due to the mutual cross-linking between adhesives to form a complete and continuous adhesive layer; at the same time, it can be observed that the adhesive significantly penetrates into the complete cell cavity. The better fluidity of the adhesive in Example 1 enables it to penetrate more deeply into the porous structure of the wood, enhancing the cross-linking between the adhesive and the wood, thereby improving the bonding strength ( Figure 11 ).

[0133] From Figure 12It can be seen that the total heat release (THR) and mass loss (HRR) of GAL-Gel, GAL-Gel-STB, and GAL-Gel-STB-nHA decrease in turn, which are respectively, indicating that the addition of STB and nHA increases the crosslinking degree of the adhesive. The crosslinking network can improve the flame retardancy of the wooden board, and an effective fireproof layer is formed on the surface of the adhesive. The formation of the fireproof layer inhibits the diffusion of volatile gases and heat transfer during the combustion process.

[0134] It can be seen from Figure 13 that the moisture absorption rate and residue rate of GAL-Gel, GAL-Gel-STB, and GAL-Gel-STB-nHA decrease in turn. The moisture absorption rates are 3.5%, 3.3%, and 3.3% respectively, and the residue rates are 98.7%, 90.2%, and 81.8% respectively. This may be because the borate ions of STB form a crosslinking network with the hydroxyl and carboxyl groups of GAL and Gel, consuming hydrophilic groups and enhancing stability; the hydroxyl and phosphate ions on the surface of nHA form hydrogen bonds or coordination bonds with the hydroxyl, carboxyl, and amino groups in GAL and Gel, reducing free hydrophilic groups and increasing the crosslinking density.

[0135] It can be seen from Figure 14 that GAL-Gel showed mildew on the 10th day ( Figure 14 ), while GAL-Gel-STB and GAL-Gel-STB-nHA did not show mildew on the 15th day ( Figure 14 ), indicating that the addition of STB and nHA can inhibit the mildew of the adhesive to a certain extent. In addition, after adding STB and nHA, the fluidity of the adhesive can be effectively improved ( Figure 15 ).

[0136] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limiting the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A preparation method of dialdehyde lignin, characterized in that, A dialdehyde solution and hydrochloric acid are added to lignocellulose and 1,4-dioxane for reaction; solid-liquid separation is carried out to obtain a liquid component, which is adjusted to pH and then added to water; solid-liquid separation is carried out to obtain a solid component, which is washed and dried to obtain dialdehyde lignin; Among them, lignocellulose:1,4-dioxane = 1:7-10 (m / v); lignocellulose:dialdehyde solution = 1:0.25-3 (m / v).

2. The preparation method of dialdehyde lignin according to claim 1, wherein The reaction is carried out at a temperature of 75-95 °C for 3-7 h.

3. The preparation method of dialdehyde lignin according to claim 1, characterized in that, The pH of the liquid component is adjusted to 2.5-3.5, and it is added to water with a volume 8-15 times that of the liquid component.

4. A dialdehyde lignin, characterized in that, Prepared by the preparation method according to any one of claims 1-3.

5. Use of the dialdehyde lignin according to claim 4, characterized in that, For the preparation of adhesives.

6. A dialdehyde-protected lignin adhesive, characterized in that, Prepared using the dialdehyde lignin according to claim 4.

7. A preparation method of the dialdehyde-protected lignin adhesive according to claim 6, characterized in that, Gelatin is dissolved in an alkali solution, borate, nano-hydroxyapatite and dialdehyde lignin are added, and after being uniformly dispersed, a dialdehyde-protected lignin adhesive is obtained.

8. The preparation method of the dialdehyde-protected lignin adhesive according to claim 7, wherein, Dialdehyde lignin:gelatin = 3-5 (w / w), borate:(dialdehyde lignin + gelatin) = 0.005-0.04 (w / w), gelatin:nano-hydroxyapatite = 10-70 (w / w), alkali solution:(dialdehyde lignin + gelatin + borate + nano-hydroxyapatite) = 1.5-2 (v / w).

9. The preparation method of the dialdehyde-protected lignin adhesive according to claim 7, characterized in that, The concentration of the alkali solution is 0.15-0.25 mol / L.

10. Use of the dialdehyde-protected lignin adhesive according to claim 6, characterized in that, For the processing and manufacturing of raw wood boards or wood-based panels.

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