A formaldehyde-free modified guar gum wood adhesive, its preparation method and application

Abstract

This invention discloses a formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application, belonging to the field of wood adhesive technology. It aims to solve the problems of environmental pollution caused by traditional formaldehyde-containing adhesives and the poor water resistance and insufficient bonding strength of guar gum itself. The method includes: dissolving guar gum and then oxidizing and degrading it with hydrogen peroxide, followed by activation with a silane coupling agent, reinforcement with polyvinyl alcohol, and modification by butyl acrylate graft copolymerization. Finally, a cross-linking reaction is carried out simultaneously with the addition of a wood interface modifier composed of sodium lignosulfonate, quaternized chitosan, nano-cellulose whiskers, and nano-zirconia supported zirconium phosphate. The reaction environment is optimized by adjusting the pH of the system to 6.0-6.5. The adhesive obtained by this invention is environmentally friendly and formaldehyde-free, suitable for high-strength bonding of eucalyptus plywood.

Description

Technical Field

[0001] This invention belongs to the field of wood adhesive technology. Specifically, this invention relates to a formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application. Background Technology

[0002] Wood adhesives are a core material in the manufacture of engineered wood products, and their performance directly affects the mechanical strength and environmental friendliness of these products. Developing high-performance formaldehyde-free adhesives has become an important research direction, but several technical bottlenecks remain.

[0003] Traditional formaldehyde-based adhesives require formaldehyde as a crosslinking agent, leading to the continuous release of free formaldehyde during the use of engineered wood products, posing environmental and health risks. There is an urgent market demand for formaldehyde-free adhesives, but existing products cannot completely replace traditional formaldehyde-containing resins in terms of performance and cost. Biomass-based formaldehyde-free adhesives, such as those based on soybean protein and starch, are environmentally friendly, but generally suffer from insufficient dry strength and poor water resistance, with wet strength often below the national standard of 0.7 MPa. This is mainly due to the weak polarity of biomass molecules, limited interfacial bonding, and the tendency of hydrophilic groups to degrade the adhesive layer upon contact with water. Guar gum, a natural polysaccharide, is a potential environmentally friendly base material, but its unmodified products have inherent defects: excessively high aqueous solution viscosity, typically ≥5000 mPa·s or even higher, resulting in poor flowability, and a lack of effective crosslinking sites in the molecular chains, leading to brittle adhesive layers and poor heat resistance. Existing modification methods such as etherification and esterification often involve toxic reagents and are not environmentally friendly or safe enough.

[0004] Eucalyptus is a major economic timber species in Guangxi Zhuang Autonomous Region, and its rotary-cut veneers are widely used in plywood production. However, existing adhesives are not well-suited to the interfacial characteristics of high-density, high-extractable-content woods like eucalyptus. Phenolic and oily extracts in eucalyptus easily migrate to the interface, forming a weak interfacial layer, and acidic components may catalyze the degradation of the adhesive layer. Although researchers have attempted to improve interfacial properties by adding single components such as lignin sulfonates or common silane coupling agents, it remains difficult to achieve a synergistic effect of multiple functions including penetration, reinforcement, and acid neutralization. Nano-reinforcing components, such as nanocellulose whiskers, tend to agglomerate in practical applications, making it difficult to disperse uniformly in the colloid and effectively act on the interface, thus limiting their reinforcing effect.

[0005] The aforementioned issues collectively restrict the application of formaldehyde-free adhesives in high-performance eucalyptus plywood, making the development of an adhesive that can simultaneously meet the requirements of formaldehyde-free environmental protection, high dry and wet strength, good process adaptability, and excellent interface compatibility still a challenge. Summary of the Invention

[0006] One objective of this invention is to address the common problems of insufficient dry / wet bonding strength and poor water resistance in traditional formaldehyde-free wood adhesives, especially those based on guar gum. Simultaneously, considering the interfacial characteristics of high-density, high-extractable woods such as eucalyptus, this invention aims to solve the technical challenges of poor adhesive permeability and weak interfacial bonding, providing a method for preparing a formaldehyde-free adhesive with excellent overall performance suitable for industrial production.

[0007] Another objective of this invention is to address the problem that concentrated exothermic reactions during the crosslinking process can easily lead to explosive polymerization, resulting in uncontrolled gel or product viscosity and thus affecting production stability and product consistency. By optimizing the process control of the crosslinking process, the reaction is ensured to proceed smoothly, guaranteeing product quality.

[0008] Another objective of this invention is to address the problems of easy aggregation of nano-components in wood interface modifiers, uneven dispersion in colloids, and poor compatibility, so as to avoid the decrease in interface modification effect and possible system instability caused by their aggregation, thereby improving the effective efficiency of the modifier at the interface.

[0009] Another objective of this invention is to further address the problems of nanodiamonds being difficult to disperse stably due to their high surface energy, and the tendency of silane coupling agent KH560 to fail due to self-condensation when added in a single step, in order to improve the nano-reinforcement effect and interfacial coupling efficiency, and further enhance the bonding strength and durability of the adhesive interface.

[0010] To achieve the above objectives, the present invention provides a method for preparing formaldehyde-free modified guar gum wood adhesive, comprising the following steps:

[0011] Dissolve guar gum in distilled water at 55-65℃ for 20-40 min; add hydrogen peroxide at 2-5% of the mass of guar gum and react for 20-40 min; add silane coupling agent at 1-3% of the mass of guar gum and react for 20-40 min.

[0012] Add 15-20% polyvinyl alcohol (by weight of guar gum), heat to 75-85℃ and react for 20-40 min, then cool to 55-65℃; add initiator and react for 20-40 min; pre-emulsify butyl acrylate with emulsifier, the mass of butyl acrylate being 1.5-2.5 times the mass of guar gum; add the pre-emulsion to the reaction system at 55-60℃ for grafting reaction for 1.5-2.5 h.

[0013] After the grafting reaction is completed, a wood interface modifier is added and a cross-linking reaction is carried out. The wood interface modifier comprises the following components by mass percentage: 50-55% sodium lignosulfonate, 20-30% quaternized chitosan, 25-35% nano-cellulose whiskers, and 5-8% nano-zinc oxide-supported zirconium phosphate. The amount of the wood interface modifier added is calculated and added according to 0.9-1.5% of the sum of the solid mass of all the raw materials. After adding the wood interface modifier, the pH of the system is adjusted to 6.0-6.5 with triethanolamine. After the cross-linking reaction is completed, a formaldehyde-free modified guar gum wood adhesive is obtained.

[0014] Preferably, the crosslinking reaction of the present invention is carried out in stages. In the first stage, the temperature is raised to 65-75°C, and nitrogen-containing crosslinking monomers and wood interface modifiers are added simultaneously, and the reaction is carried out for 10-20 min. In the second stage, the temperature is raised to 75-85°C, epoxy crosslinking monomers are added, and the reaction is carried out for 20-40 min. During the reaction, the viscosity of the system is monitored. When the viscosity rise rate exceeds 5 mPa·s / min, a polymerization inhibitor is added immediately.

[0015] Preferably, the wood interface modifier of the present invention undergoes in-situ activation treatment before being added, and the specific steps are as follows:

[0016] a. Sodium lignosulfonate and nano zinc oxide-supported zirconium phosphate were premixed at a mass ratio of (7~10):1, and then ultrasonically treated at 55-65℃ to obtain a mixture;

[0017] b. Add quaternized chitosan and nanocellulose whiskers to the mixture obtained in step a, and disperse them at high speed under nitrogen protection to form a homogeneous slurry;

[0018] c. Inject the homogeneous slurry obtained in step b into the crosslinking reaction system at a rate of 2-3 mL / min, and carry out the reaction under a negative pressure of 25-35 kPa.

[0019] Preferably, during the ultrasonic treatment in step a of the present invention, nanodiamonds accounting for 0.3-0.5% of the total mass of sodium lignosulfonate and nano zinc oxide-supported zirconium phosphate are added simultaneously. The nanodiamonds have a particle size of 5-10 nm and undergo the following pretreatment: soaking in a 0.5-1 wt% hexadecyltrimethylammonium bromide ethanol solution, centrifuging, and then vacuum drying at 55-65°C to obtain nanodiamonds with cation-modified surfaces.

[0020] When performing high-speed dispersion in step b, add 0.05-0.08% of the total mass of the slurry system of silane coupling agent KH560; add silane coupling agent KH560 in two parts: first, add 60% of its total amount at the initial stage of dispersion, and second, add the remaining 40% 2 minutes before the end of dispersion.

[0021] Preferably, in the staged crosslinking reaction of the present invention, the nitrogen-containing crosslinking monomer is diacetone acrylamide (DAAM), and the amount used is 1-3% of the mass of butyl acrylate; the epoxy crosslinking monomer is glycidyl methacrylate (GMA), and the amount used is 1-3% of the mass of butyl acrylate.

[0022] Preferably, the polymerization inhibitor of the present invention is tetrachlorobenzoquinone, and the amount used is 0.03-0.05% of the mass of butyl acrylate.

[0023] Preferably, the grafting reaction of the present invention is carried out at 60°C for 2 hours, and the amount of butyl acrylate is twice the mass of guar gum.

[0024] Preferably, the silane coupling agent of the present invention is KH550, KH560 or KH570; and the emulsifier is sodium dodecyl sulfate (SDS).

[0025] This invention provides a formaldehyde-free modified guar gum wood adhesive prepared by the aforementioned preparation method.

[0026] The application of the formaldehyde-free modified guar gum wood adhesive prepared by the preparation method provided by the present invention in eucalyptus plywood includes:

[0027] S1. Cut eucalyptus veneers with a moisture content of 8-12% to the preset size; this moisture content is sufficient for effective wetting and penetration of the adhesive.

[0028] S2. Apply glue to the eucalyptus veneer using a double-sided gluing method, with a glue application rate of 120-220 g / m². 2 Based on the density and extractive properties of eucalyptus veneer, after applying the adhesive, it should be aged for 10-15 minutes in an environment of 20-25℃ and 50-60% relative humidity. The gentle aging process allows the moisture in the adhesive to evaporate moderately, initially thickening it, while giving the adhesive sufficient time to wet the wood surface and begin to penetrate into the pores.

[0029] S3. Assemble the blanks perpendicularly along the grain direction of adjacent veneers and pre-press them for 30-40 minutes under a pressure of 0.8-1.2 MPa; this will allow the blanks to be initially shaped, expel some air, and allow the veneers to initially bond together.

[0030] S4. Perform two-stage hot-press curing. Stage 1 involves hot-pressing at 105-110℃ and 1.0-1.5 MPa for 2-3 minutes. This relatively low temperature and pressure allows the adhesive to flow further, penetrate, and fully wet the wood, while avoiding excessive overflow of eucalyptus extract or bursting due to a sudden increase in steam pressure under high temperature and pressure.

[0031] In the second stage, the temperature is raised to 130-135℃ and hot-pressed at 2.0-2.2 MPa for 8-10 minutes. The sufficient heat and pressure promote the full cross-linking and curing reaction between the epoxy groups of cross-linking monomers such as GMA in the adhesive and the wood components and the adhesive itself, forming a water-resistant, high-strength, dense adhesive layer.

[0032] S5. After the board is depressurized, cool it to 55℃-60℃ with hot air within 20 minutes; quickly terminate the thermochemical reaction, fix the adhesive layer structure, prevent over-curing that leads to increased brittleness, and also help reduce residual stress caused by excessive temperature difference.

[0033] Then, it is cured at room temperature of 20-25℃ for at least 48 hours to allow the internal stress of the board to be fully released, the moisture distribution to become more even, and finally the bonding strength to reach and maintain its optimal state.

[0034] The present invention has at least the following beneficial effects:

[0035] 1. This invention comprehensively improves the performance of adhesives through multi-step chemical modification and wood interface modifiers. The developed composite wood interface modifier plays a crucial role in addressing the problems of poor penetration, weak interface layer, and acid degradation caused by the high density and high extractive content of eucalyptus wood. Sodium lignosulfonate, due to its wood-like structure, effectively improves adhesive permeability and carries other components into the wood pores; quaternized chitosan neutralizes acidic extractives, eliminating their damage to the adhesive layer; nano-cellulose whiskers form a rigid reinforcing network at the interface, greatly enhancing mechanical bonding strength; and nano-zinc oxide loaded with zirconium phosphate enhances durability. Simultaneously, the pH of the entire system is adjusted to 6.0-6.5, creating an optimal weakly acidic environment for epoxy group crosslinking and wood bonding. Under the premise of being completely formaldehyde-free and using only non-toxic additives, the original high viscosity of guar gum is completely overcome, achieving excellent dry and wet bonding strength. The resulting adhesive fully meets the national standard requirements for water-resistant plywood (Class II), and formaldehyde release is undetectable, complying with ENF environmental standards.

[0036] 2. This invention also optimizes the staged crosslinking reaction by adding crosslinking monomers with different functions in stages to achieve synergistic effects between DAAM and GMA. DAAM preferentially penetrates into the pores of eucalyptus wood and reacts initially, reducing the system polarity during subsequent GMA crosslinking, thereby creating a uniform reaction environment and simultaneously improving the toughness and water resistance of the adhesive layer; while GMA forms a high-density crosslinking network, ensuring the final strength. Real-time monitoring of viscosity changes and immediate addition of a polymerization inhibitor when the rate of increase exceeds the critical point can effectively prevent burst polymerization or gelation caused by concentrated exothermic reaction. This proactive reaction blocking approach greatly improves the stability and safety of the production process, ensuring that the final product has consistently excellent performance and good flowability, making it suitable for continuous industrial production.

[0037] 3. This invention also involves in-situ activation of the wood interface modifier, significantly improving its final performance and solving the problems of easy agglomeration and uneven dispersion of nano-components. Ultrasonic pretreatment effectively disrupts the initial agglomeration of components such as nano-zinc oxide loaded with zirconium phosphate, laying the foundation for subsequent uniform dispersion. High-speed dispersion under nitrogen protection prevents the oxidative degradation of active components such as quaternized chitosan and nano-cellulose during the process, ensuring the stability and activity of the slurry. Finally, the homogeneous slurry is slowly injected into the reaction system under negative pressure, which not only helps to eliminate air bubbles but also promotes the fusion of the modifier and the adhesive matrix. This allows the functional nano-components to migrate more efficiently to the wood interface and fully exert their penetration, strengthening, and neutralizing effects, thereby significantly improving the strength and durability of the final bonded structure.

[0038] 4. This invention significantly improves performance by introducing pretreated nanodiamonds and optimizing the addition method of silane coupling agents, effectively solving the problems of nanomaterial dispersion and coupling efficiency. CTAB-modified nanodiamonds have reduced surface energy, and their cationic properties enhance electrostatic adsorption with negatively charged wood fiber surfaces, acting as a super-rigid reinforcing phase and effectively improving the mechanical properties of the adhesive layer. The silane coupling agent KH560 is added in two stages. The first addition anchors the nanomaterial surface during the initial dispersion stage, preventing re-agglomeration; the second addition, before the end of dispersion, provides sufficient active groups to react with the polymer matrix, thereby maximizing its bridging and coupling effect and synergistically improving interfacial bonding strength and toughness.

[0039] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0040] Figure 1 The images show actual photos of the modified guar gum wood adhesives prepared in Examples 1-3 of this invention.

[0041] Figure 2 The images show actual photos of the modified guar gum wood adhesives prepared in Examples 4-6 of this invention.

[0042] Figure 3 The images shown are actual pictures of the adhesives prepared in Comparative Examples 1-4 of the present invention; wherein D1 is Comparative Example 1, D2 is Comparative Example 2, D3 is Comparative Example 3, and D4 is Comparative Example 4.

[0043] Figure 4The images shown are actual pictures of the adhesives prepared in Comparative Examples 5-8 of the present invention; wherein D5 is Comparative Example 5, D6 is Comparative Example 6, D7 is Comparative Example 7, and D8 is Comparative Example 8.

[0044] Figure 5 The images shown are actual pictures of the adhesives prepared in Comparative Examples 9-11 of this invention; D9 is Comparative Example 9, D10 is Comparative Example 10, and D11 is Comparative Example 11.

[0045] Figure 6 Viscosity diagrams of the adhesives prepared in the embodiments and comparative examples of the present invention;

[0046] Figure 7 The diagram shows the dry and wet bonding strength of the adhesives prepared in the embodiments and comparative examples of this invention. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to examples, so that those skilled in the art can implement it based on the description.

[0048] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0049] Example 1

[0050] This embodiment describes a formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application in the preparation of eucalyptus plywood.

[0051] 1. Preparation of modified guar gum wood adhesive

[0052] The main ingredients are as follows:

[0053] Guar gum 10.0 g (Guar gum GUARAN-6 from Beijing Guarrun Technology Co., Ltd., food grade, with a 1% aqueous solution viscosity of 6000 mPa•s);

[0054] 185.0 g of distilled water;

[0055] 0.3 g of hydrogen peroxide (H2O2, 30%) (3% of the mass of guar gum);

[0056] Silane coupling agent (KH560) 0.2 g (2% of the mass of guar gum);

[0057] Polyvinyl alcohol (PVA-1788) 1.7 g (17% of the mass of guar gum);

[0058] Initiator (potassium persulfate, K2S2O8) 0.4 g;

[0059] Butyl acrylate (BA) 20.0 g (twice the weight of guar gum);

[0060] Emulsifier (sodium dodecyl sulfate, SDS) 0.3 g;

[0061] 0.4 g of nitrogen-containing crosslinking monomer (diacetone acrylamide, DAAM) (2% of the mass of BA);

[0062] 0.4 g of epoxy crosslinking monomer (glycidyl methacrylate, GMA) (2% of the mass of BA);

[0063] Polymerization inhibitor (tetrachlorobenzoquinone) 0.008 g (0.04% of BA mass).

[0064] In the wood interface modifier, sodium lignosulfonate 0.172 g, accounting for 46.7% of the modifier's mass; quaternized chitosan with a degree of substitution >0.5 0.079 g, accounting for 21.5% of the modifier's mass; nano-cellulose whiskers with an aspect ratio of 30-50 0.095 g, accounting for 25.8% of the modifier's mass; and nano-zinc oxide-supported zirconium phosphate 0.022 g, accounting for 6.0% of the modifier's mass. The total amount of wood interface modifier is 0.368 g, accounting for 1.09% of the sum of the solid mass of all input raw materials.

[0065] The preparation method of nano-zirconia-supported zirconium phosphate is as follows: 5.0 g of zirconium phosphate (Zr(HPO4)2·H2O) powder was placed in a 250 mL beaker, and 100 mL of 0.1 mol / L Zn(NO3)2 solution was added. Then, 4.2 g of sodium citrate was added. The beaker was placed in an ultrasonic cleaner and ultrasonically dispersed at 25 °C for 30 min. After dispersion, the mixture was transferred to a 200 mL high-pressure reactor, sealed, and placed in a forced-air drying oven at 160 °C for 6 h. After the reaction, the mixture was naturally cooled to room temperature, and the product was centrifuged at 8000 rpm for 10 min, and washed three times alternately with deionized water and anhydrous ethanol. The washed product was placed in a vacuum drying oven and dried at 60 °C for 12 h. Finally, the dried block product was ground and passed through a 300-mesh sieve to obtain nano-zirconia-supported zirconium phosphate composite powder for later use.

[0066] The adhesive preparation steps are as follows:

[0067] Guar gum and distilled water were added to a four-necked flask equipped with a condenser, stirrer, and thermometer. The mixture was stirred at 300 rpm for 30 min in a 60°C water bath to dissolve the gum. Hydrogen peroxide was then slowly added dropwise, and the reaction continued at 60°C for another 30 min. Silane coupling agent KH560 was added, and the reaction continued at 60°C for 30 min. Polyvinyl alcohol (PVA) was added, the temperature was raised to 80°C, and the reaction continued for 30 min. The system was then cooled to 60°C. Potassium persulfate was added as an initiator, and the reaction continued at 60°C for 30 min. Butyl acrylate was pre-emulsified with emulsifier SDS for 15 min to form an emulsion. This pre-emulsion was slowly added dropwise to the reaction system over 2 hours at 60°C. After the addition was complete, the reaction continued at 60°C for 2 h to complete the grafting reaction.

[0068] Four components—sodium lignosulfonate, quaternized chitosan, nano-cellulose whiskers, and nano-zirconia-supported zirconium phosphate—were placed in a centrifuge tube, and the tube was rotated to mix the materials evenly, thus obtaining a wood interface modifier.

[0069] After the grafting reaction was completed, the system temperature was adjusted to 60℃. The mixed wood interface modifier was added and stirred at 300 rpm for 15 min to ensure thorough dispersion. The pH of the system was adjusted to 6.2 by slow titration with triethanolamine aqueous solution. 0.4 g of the nitrogen-containing crosslinking monomer DAAM was added, and the temperature was raised to 70℃ for 15 min. 0.4 g of the epoxy crosslinking monomer GMA was added, and the temperature was raised to 80℃ for 30 min. After the reaction was completed, the mixture was cooled to room temperature and discharged, yielding a milky white, homogeneous modified guar gum wood adhesive product with a solid content of 20.8%.

[0070] 2. The modified guar gum wood adhesive was applied to eucalyptus plywood, and its bonding performance was tested.

[0071] (1) Apply the prepared modified guar gum wood adhesive evenly to the eucalyptus veneer. The veneer should be 18cm × 18cm thick and 1.5mm thick. One veneer should be glued on both sides, and the other two veneers should be glued on one side only, covering the entire surface. Each side should require 6.5g (approximately 200g / m²). 2 Guar gum wood adhesive.

[0072] (2) After the veneer is glued, it is placed in an environment of 25℃ and 55% relative humidity (RH) for 12 min to age. The three layers of veneer are assembled vertically with the wood grain of the adjacent veneers and pre-pressed at 1.0 MPa for 35 min to complete the pre-pressing treatment. Then, it is hot-pressed at 108℃ and 1.5 MPa for 2 min; then hot-pressed at 130℃ and 2MPa for 10 min. After depressurization, the board is cooled to below 60℃ with hot air at 58℃ for 20 minutes, and then cured at room temperature for 48 h to obtain the three-layer plywood.

[0073] (3) The prepared three-layer plywood was sawn to form a 100 mm × 25 mm specimen with a groove depth of 2 / 3 of the core board thickness. The specimen was clamped on a universal testing machine to determine the bond strength. The bond strength was calculated according to the bond strength formula in GB / T 17657-2022. Formaldehyde release was also measured using the 1m value specified in the national standard GB / T 17657-2022. 3 Climate chamber method for testing.

[0074] Example 2

[0075] This embodiment of the formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application differ from Example 1 in that the amount of wood interface modifier added is adjusted to 1.0% of the theoretical total solid mass of the adhesive, that is, the total mass of the wood interface modifier is about 0.317 g, including 0.148 g of sodium lignin sulfonate; 0.068 g of quaternized chitosan; 0.082 g of nanocellulose whiskers; and 0.019 g of nano zinc oxide-supported zirconium phosphate.

[0076] The preparation process, cross-linking reaction, and subsequent application testing steps are the same as in Example 1.

[0077] Example 3

[0078] This embodiment presents a formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application. The difference between this embodiment and Example 1 is that the amount of wood interface modifier added is adjusted to 1.5% of the theoretical total solid mass of the adhesive, i.e., the total mass of the wood interface modifier is approximately 0.476 g. This includes 0.222 g of sodium lignin sulfonate, 0.102 g of quaternized chitosan, 0.123 g of nano-cellulose whiskers, and 0.029 g of nano-zinc oxide-supported zirconium phosphate.

[0079] The preparation process, cross-linking reaction, and subsequent application testing steps are the same as in Example 1.

[0080] Example 4

[0081] The formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application in this embodiment differ from those in Example 1 in that:

[0082] After the grafting reaction was completed, the system temperature was adjusted to 60°C. The pH of the system was pre-adjusted to 6.2 by slow titration with triethanolamine aqueous solution.

[0083] The crosslinking reaction was carried out in stages, specifically as follows: In the first stage, the system was heated to 70°C. While stirring, 0.4 g of the nitrogen-containing crosslinking monomer DAAM and 0.368 g of the pre-mixed wood interface modifier were added simultaneously. The stirring rate was maintained at 300 rpm, and the reaction was carried out at this temperature for 15 min. In the second stage, the system was heated to 80°C. 0.4 g of the epoxy crosslinking monomer GMA was added, and the reaction was initiated while the system viscosity was monitored in real time using a rotational viscometer, recording data every minute. When the viscosity increase rate reached over 5 mPa·s / min after approximately 25 min of reaction, 0.008 g of the polymerization inhibitor tetrachlorobenzoquinone was immediately added. The reaction was continued for 5 min, and then heating was stopped. After the reaction was completed, the product was cooled to room temperature and discharged, yielding a milky white, homogeneous modified guar gum wood adhesive product with a solid content of 20.5%.

[0084] Other steps and parameters are the same as in Example 1. This includes testing on eucalyptus plywood.

[0085] Example 5

[0086] The formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application in this embodiment differ from those in Example 4 in that:

[0087] After the grafting reaction was completed, the system temperature was adjusted to 60°C. The pH of the system was pre-adjusted to 6.2 by slow titration with an aqueous triethanolamine solution.

[0088] The wood interface modifier undergoes further in-situ activation treatment before being added, as detailed below:

[0089] a. 0.172 g of sodium lignosulfonate and 0.022 g of nano zinc oxide-supported zirconium phosphate were placed in a 50 mL beaker at a mass ratio of 10:1. 10 mL of distilled water was added, and the mixture was treated in a 60℃ water bath and under 200W ultrasonic power for 20 min to obtain a preliminarily dispersed mixture.

[0090] b. Add 0.079 g of quaternized chitosan and 0.095 g of nanocellulose whiskers to the mixture from step a. Transfer the mixture to a container fitted with a high-speed disperser, purge with nitrogen for protection, and then disperse at 5000 rpm for 10 min to form a uniform activated slurry free of visible particles.

[0091] c. Transfer the reactants to a 250 mL glass reactor. Connect the reactor, equipped with a stirrer and condenser, to a vacuum system, evacuate the system, and maintain the system pressure at 30 kPa. Under this negative pressure and stirring (300 rpm), inject the activated slurry obtained in step b and 0.4 g of the nitrogen-containing crosslinking monomer DAAM into the reaction system at a rate of 2.5 mL / min using a constant flow pump to carry out the first stage of the reaction. This injection and reaction process lasts for 15 min.

[0092] In the second stage, the vacuum was removed, and the reaction system was restored to normal pressure. The temperature was raised to 80℃, and 0.4 g of the epoxy crosslinking monomer GMA was added to start the reaction, with the viscosity of the system monitored in real time. When the viscosity increase rate reached more than 5 mPa·s / min after about 22 minutes of reaction, 0.008 g of the polymerization inhibitor tetrachlorobenzoquinone was immediately added. The reaction was continued for another 8 minutes (the total reaction time for the second stage was 30 minutes), and then heating was stopped. After the reaction was completed, the product was cooled to room temperature and discharged, yielding a milky white, homogeneous, and fine modified guar gum wood adhesive product with a solid content of 20.7%.

[0093] Other steps and parameters are the same as in Example 4. This includes testing on eucalyptus plywood.

[0094] Example 6

[0095] The formaldehyde-free modified guar gum wood adhesive, its preparation method, and its application in this embodiment differ from those in Example 5 in that:

[0096] After the grafting reaction was completed, the system temperature was adjusted to 60°C. The pH of the system was pre-adjusted to 6.2 by slow titration with triethanolamine aqueous solution.

[0097] Similarly, the wood interface modifier was activated, and nanodiamonds were added, as detailed below:

[0098] a. Weigh 20.0 mg of nanodiamond powder and add it to 100 mL of 0.75 wt% CTAB ethanol solution. Disperse the solution ultrasonically for 30 min to obtain a suspension with a concentration of 0.2 mg / mL. Take 3.88 mL of this suspension (containing 0.776 mg of nanodiamond), centrifuge, discard the supernatant, and dry the precipitate in a vacuum drying oven at 60℃ to obtain cation-modified nanodiamonds for later use. Reference: Gennadii A. Badun, Maria G. Chernysheva, et al. Adsorption of alkyltrimethylammonium bromides on nanodiamonds. Fullerens, Nanotubes and Carbon Nanostructures, 2020, VOL. 28, NO. 5, 361–367.

[0099] 0.172 g of sodium lignosulfonate and 0.022 g of zinc oxide nano-supported zirconium phosphate were placed in a 50 mL beaker, and 10 mL of distilled water was added. Simultaneously, 0.776 mg of the pretreated nanodiamond was added. The mixture was treated in a 60℃ water bath under 200W ultrasonic power for 20 min to obtain a preliminarily dispersed mixture.

[0100] b. High-speed dispersion and stepwise addition of KH560: Add 0.079 g of quaternized chitosan and 0.095 g of nanocellulose whiskers to the mixture from step a. Transfer the mixture to a container supplied with a high-speed disperser, purge with nitrogen for protection, and then start the high-speed disperser (5000 rpm).

[0101] For the first addition, immediately add a solution accounting for 60% of the total KH560 volume during the initial dispersion stage (the total mass of KH560 is 0.065% of the expected total mass of the slurry, which is 0.426 g, or 0.277 mg. The initial addition of 0.166 mg can be achieved by diluting it to a certain concentration and then transferring the corresponding volume for further addition). Continue dispersion for 8 min.

[0102] For the second addition, 0.111 mg of the remaining 40% KH560 solution was added 2 minutes before the end of dispersion. Dispersion continued until the total time was 10 minutes, resulting in a homogeneous activated slurry free of visible particles.

[0103] c. The first stage of negative pressure injection and crosslinking reaction is exactly the same as in Example 3. The reactants are transferred to a 250 mL glass reactor, a vacuum is drawn, and the system pressure is maintained at 30 kPa. The activated slurry obtained in step b and 0.4 g of nitrogen-containing crosslinking monomer DAAM are injected into the reaction system together at a rate of 2.5 mL / min using a constant flow pump to carry out the first stage reaction for 15 min.

[0104] The second stage of crosslinking was the same as in Example 5, involving the removal of the vacuum and restoration of the system to normal pressure. The temperature was raised to 80°C, and 0.4 g of the epoxy crosslinking monomer GMA was added to initiate the reaction, with the system viscosity monitored in real time. After approximately 20 minutes of reaction, when the viscosity increase rate reached over 5 mPa·s / min, 0.008 g of the polymerization inhibitor tetrachlorobenzoquinone was immediately added. The reaction was continued for another 10 minutes (the total reaction time for the second stage was 30 minutes), and then heating was stopped. After the reaction was complete, the product was cooled to room temperature and discharged, yielding a milky white, homogeneous, and fine modified guar gum wood adhesive product with a solid content of 20.9%.

[0105] Other steps and parameters are the same as in Example 5. This includes testing on eucalyptus plywood.

[0106] Comparative Example 1 (D1) - No interface modifiers

[0107] The preparation process was basically the same as in Example 1, except that no wood interface modifier was added. After the grafting reaction was completed, the system temperature was adjusted to 60°C, and the crosslinking reaction was carried out directly. 0.4 g of DAAM was added, and the temperature was raised to 70°C for 15 min; then 0.4 g of GMA was added, and the temperature was raised to 80°C for 30 min. After the reaction was completed, the mixture was cooled to room temperature and discharged to obtain the product, which had a solid content of 20.5%.

[0108] Other steps and parameters are the same as in Example 1, including testing on eucalyptus plywood.

[0109] Comparative Example 2 (D2) - Sodium lignosulfonate added as a single component

[0110] The preparation process is basically the same as in Example 1, except that only 0.368 g of sodium lignosulfonate is added, and quaternized chitosan, nanocellulose whiskers and nano zinc oxide supported zirconium phosphate are not added.

[0111] Specifically, after the grafting reaction was completed, the system temperature was adjusted to 60℃. 0.368 g of sodium lignosulfonate was added, and the mixture was stirred at 300 rpm for 15 min to ensure thorough dispersion. The pH of the system was adjusted to 6.2 using triethanolamine. Subsequently, a cross-linking reaction was carried out: 0.4 g of DAAM was added, and the temperature was raised to 70℃ for 15 min; then 0.4 g of GMA was added, and the temperature was raised to 80℃ for 30 min. After the reaction was completed, the mixture was cooled to room temperature and the product was discharged, with a solid content of 20.6%.

[0112] Other steps and parameters are the same as in Example 1, including testing on eucalyptus plywood.

[0113] Comparative Example 3 (D3) - using conventional interface treatment agent silane coupling agent KH560

[0114] The preparation process is basically the same as in Example 1, except that instead of adding a wood interface modifier, silane coupling agent KH560 is added at the same stage.

[0115] Specifically, after the grafting reaction was completed, the system temperature was adjusted to 60℃. 0.368 g of silane coupling agent KH560 was added, and the mixture was stirred at 300 rpm for 15 min to ensure thorough reaction and dispersion. The pH of the system was adjusted to 6.2 using triethanolamine. Subsequently, a cross-linking reaction was carried out: 0.4 g of DAAM was added, and the temperature was raised to 70℃ for 15 min; then 0.4 g of GMA was added, and the temperature was raised to 80℃ for 30 min. After the reaction was completed, the mixture was cooled to room temperature and the product was discharged, with a solid content of 20.5%.

[0116] Other steps and parameters are the same as in Example 1, including testing on eucalyptus plywood.

[0117] Comparative Example 4 (D4) - Modifier added after crosslinking

[0118] The preparation process was basically the same as in Example 1, except that the order of adding the wood interface modifier was changed. First, all cross-linking reactions of DAAM and GMA were completed. After the system cooled to 50°C, the wood interface modifier was added, and the mixture was stirred for 15 minutes. The resulting product had a solid content of 20.7%.

[0119] Other steps and parameters are the same as in Example 1, including testing on eucalyptus plywood.

[0120] Comparative Example 5 (D5) - Sodium Lignosulfonate-Free

[0121] The preparation process was basically the same as in Example 1, except that sodium lignosulfonate was not added to the wood interface modifier. To maintain a relatively constant total amount of modifier, the mass of sodium lignosulfonate was proportionally distributed to the other three components: 0.106 g of quaternized chitosan, 0.127 g of nanocellulose whiskers, and 0.135 g of nano-zinc oxide-supported zirconium phosphate. These three components were dry-mixed and added during the crosslinking reaction. The resulting product had a solid content of 20.8%.

[0122] Other preparation steps are the same as in Example 1. This includes testing on eucalyptus plywood.

[0123] Comparative Example 6 (D6) - Quaternized Chitosan

[0124] The preparation process was basically the same as in Example 1, except that quaternized chitosan was not added to the wood interface modifier portion. Instead, the mass of quaternized chitosan was proportionally distributed to the other three components, keeping the total amount of modifier essentially unchanged. The components were: sodium lignosulfonate 0.216 g; nanocellulose whiskers 0.119 g; and nano-zirconia-supported zirconium phosphate 0.033 g. These three components were dry-mixed and added during the crosslinking reaction, followed by subsequent operations. The resulting product had a solid content of 20.6%.

[0125] Other preparation steps are the same as in Example 1. This includes testing on eucalyptus plywood.

[0126] Comparative Example 7 (D7) - No nanocellulose whiskers

[0127] The preparation process was basically the same as in Example 1, except that nanocellulose whiskers were not added to the wood interface modifier. Instead, the mass of the nanocellulose whiskers was proportionally distributed to the other three components, keeping the total amount of modifier basically unchanged. The components included: sodium lignosulfonate 0.229 g; quaternized chitosan 0.105 g; and nano-zirconia-supported zirconium phosphate 0.034 g. These three components were dry-mixed and added during the crosslinking reaction, followed by subsequent operations. The resulting product had a solid content of 20.5%.

[0128] Other preparation steps are the same as in Example 1. This includes testing on eucalyptus plywood.

[0129] Comparative Example 8 (D8) - Single crosslinking monomer DAAM

[0130] The preparation process is basically the same as in Example 1, except that the second stage of the crosslinking reaction is omitted and only a single crosslinking monomer, DAAM, is used for the reaction.

[0131] Specifically, after the grafting reaction was completed, the system temperature was adjusted to 60°C. A wood interface modifier was added, dispersed, and the pH was adjusted to 6.2 (the steps were the same as in Example 1). Only the first stage of crosslinking was performed: 0.4 g of DAAM was added, and the temperature was raised to 70°C for 15 min. The second stage was omitted; GMA was not added, and the product was directly cooled and discharged after the reaction. The obtained product had a solid content of 20.4%.

[0132] Other preparation steps are the same as in Example 1. This includes testing on eucalyptus plywood.

[0133] Comparative Example 9 (D9) – Single crosslinking monomer GMA

[0134] The preparation process is basically the same as in Example 1, except that the first stage of the crosslinking reaction is omitted and only the single crosslinking monomer GMA is reacted.

[0135] Specifically, after the grafting reaction was completed, the system temperature was adjusted to 60°C. A wood interface modifier was added, dispersed, and the pH was adjusted to 6.2 (the steps were the same as in Example 1). Only the second stage of crosslinking was performed; DAAM was not added, and the temperature was directly raised to 80°C. 0.4 g of GMA was added, and the reaction was allowed to proceed for 30 min. After the reaction, the product was cooled and discharged. The obtained product had a solid content of 20.7%.

[0136] Other preparation steps are the same as in Example 1. This includes testing on eucalyptus plywood.

[0137] Comparative Example 10 (D10) - Two crosslinking monomers were mixed and added without stepwise steps.

[0138] The preparation process is basically the same as in Example 1, except that the two crosslinking monomers DAAM and GMA are added at the same time during the crosslinking reaction.

[0139] Specifically, after the grafting reaction was completed, the system temperature was adjusted to 60°C. A wood interface modifier was added, dispersed, and the pH was adjusted to 6.2 (the steps were the same as in Example 1). The staged crosslinking was cancelled, and the system temperature was raised to 80°C. 0.4 g of DAAM and 0.4 g of GMA were added simultaneously at once, and the reaction was allowed to proceed for 40 min. After the reaction was completed, the material was cooled and discharged, yielding an adhesive product with a solid content of 20.5%.

[0140] Other preparation steps are the same as in Example 1. This includes testing on eucalyptus plywood.

[0141] Comparative Example 11 (D11) - Add GMA first, then DAAM

[0142] The preparation process is exactly the same as in Example 1, except that the order of adding the crosslinking monomers is reversed in the crosslinking reaction.

[0143] Specifically, after the grafting reaction was completed, the system temperature was adjusted to 60°C. A wood interface modifier was added, dispersed, and the pH was adjusted to 6.2 (the steps were the same as in Example 1). Staged crosslinking was then performed, but in reverse order: Stage 1: Temperature was raised to 70°C, 0.4 g of GMA was added, and the reaction was allowed to proceed for 15 min. Stage 2: Temperature was raised to 80°C, 0.4 g of DAAM was added, and the reaction was allowed to proceed for 30 min. After the reaction was completed, the material was cooled and discharged. The resulting adhesive product had a solid content of 20.6%.

[0144] The other preparation steps are exactly the same as in Example 1, including application to eucalyptus plywood for testing.

[0145] Figure 1 The images show actual photos of the modified guar gum wood adhesives prepared in Examples 1-3 of this invention. Figure 2 The images show actual photos of the modified guar gum wood adhesives prepared in Examples 4-6 of this invention. Figure 3 The images show actual pictures of the modified guar gum wood adhesives prepared in Comparative Examples 1-4 of the present invention; wherein D1 is Comparative Example 1, D2 is Comparative Example 2, D3 is Comparative Example 3, and D4 is Comparative Example 4. Figure 4 The images shown are actual pictures of the modified guar gum wood adhesives prepared in Comparative Examples 5-8 of the present invention; wherein D5 is Comparative Example 5, D6 is Comparative Example 6, D7 is Comparative Example 7, and D8 is Comparative Example 8. Figure 5 The images show the actual products of the modified guar gum wood adhesives prepared in Comparative Examples 9-11 of this invention; D9 is Comparative Example 9, D10 is Comparative Example 10, and D11 is Comparative Example 11. Figure 6 Viscosity diagrams of the modified guar gum wood adhesives prepared in the embodiments and comparative examples of the present invention; Figure 7 The images show the dry and wet bonding strength of the modified guar gum wood adhesives prepared in the embodiments and comparative examples of this invention.

[0146] Effect Analysis

[0147] Table 1. Performance of products obtained in the examples and comparative examples

[0148]

[0149] from Figure 6-7 As can be seen from Table 1 above, Example 1, by introducing a composite wood interface modifier and combining it with pH adjustment and cross-linking reaction, significantly improved the dry strength of the adhesive to 2.35 MPa and the wet strength to 1.18 MPa, far exceeding the national standard requirement of 0.7 MPa, thus solving the problems of poor permeability, interference from acidic extracts, and insufficient water resistance in eucalyptus interface bonding.

[0150] Example 2 used less wood interface modifier than Example 1, resulting in slightly lower performance. Example 3 used 1.5% wood interface modifier; although its dry strength was slightly higher than Example 1, its wet strength of 1.16 MPa was slightly lower than Example 1's 1.18 MPa. This is because excessive wood interface modifier increases the risk of local agglomeration, which slightly affects the interface's density and water resistance. Considering both cost and performance, the wood interface modifier addition ratio in Example 1 is the preferred option.

[0151] Example 4, based on Example 1, further optimized the crosslinking process by employing a staged crosslinking reaction and real-time monitoring of viscosity changes, with timely addition of polymerization inhibitors. This optimization improved the dry strength to 2.40 MPa and the wet strength to 1.22 MPa, while also reducing the viscosity to 978.45 mPa·s. This demonstrates that staged crosslinking and reaction control effectively avoided the risk of explosive polymerization, improved reaction uniformity and product consistency, and further enhanced the toughness and water resistance of the adhesive layer.

[0152] Example 5, based on Example 4, further activates the wood interface modifier in situ, including ultrasonic pretreatment, high-speed dispersion under nitrogen protection, and negative pressure injection. These treatments significantly improve the dispersibility and activity of the nano-components, further increasing the dry strength to 2.52 MPa, the wet strength to 1.28 MPa, and the viscosity to 888.93 mPa·s. This indicates that the in-situ activation treatment effectively solves the problems of easy agglomeration and uneven dispersion of the nano-components, enhancing their functional performance at the interface.

[0153] Example 6, based on Example 5, further introduced pretreated nanodiamonds and optimized the addition of silane coupling agent KH560 by adding it in two stages. This significantly improved the dry strength to 2.78 MPa and the wet strength to 1.45 MPa. Although the viscosity increased slightly to 1248.67 mPa·s, it was still within an acceptable range. This indicates that the introduction of nanodiamonds enhanced the rigidity of the adhesive layer, while the stepwise addition of KH560 maximized its coupling effect, further strengthening the interfacial bonding strength and durability.

[0154] Compared to Example 1, the performance of D1 decreased significantly, with dry strength decreasing by approximately 31% and wet strength decreasing by approximately 30%. Comparative Example 1, lacking a wood interface modifier, even with the same matrix synthesis and crosslinking process, only achieved conventional bonding performance, particularly the crucial wet strength, failing to effectively solve the problem of eucalyptus interface bonding.

[0155] Compared to Example 1, D2 exhibits 24% lower dry strength and 26% lower wet strength, resulting in significantly lower performance. Although sodium lignosulfonate alone provides slightly better performance than D1 due to improved permeability, its performance is still poor because it cannot provide nano-reinforcement, neutralize acidic extracts, or synergistically construct a robust and water-resistant complete interface layer.

[0156] The performance of D3 is not outstanding, only comparable to D1 without modification. The reason is that although the conventional silane coupling agent KH560 can provide a certain coupling effect, it cannot achieve the multiple synergistic effects of the composite modifier of this invention, such as penetration, neutralization of acidic extracts, and nano-reinforcement. Therefore, its dry and wet strengths are both poor.

[0157] Compared to Example 1, the performance of D4 was significantly reduced, with dry strength down by 20% and wet strength down by 21%. This is because the modifier was added only after the crosslinking reaction was completed. Its components could not participate in the formation of the adhesive network through chemical bonding, but were merely physically mixed in the system. Therefore, its reinforcing and penetrating functions could not be fully utilized, especially the improvement in water resistance was limited.

[0158] Compared to Example 1, D5 exhibited a significant decrease in performance, with dry strength down 21% and wet strength down 24%. This is due to the lack of sodium lignosulfonate, which reduced the overall permeability and compatibility of the modifier system with wood. Reinforcing components such as nanocellulose whiskers could not be effectively carried into the depths of the wood pores, remaining mostly within the adhesive layer itself, resulting in a severe loss of their interfacial reinforcing effect and a substantial decrease in wet strength.

[0159] Compared to Example 1, the dry strength of D6 decreased by approximately 17%, and the wet strength decreased sharply by approximately 29%, reaching only 0.84 MPa, even lower than the level of D1 without the modifier. This is because, lacking the neutralizing effect of quaternized chitosan, the acidic extracts in eucalyptus wood damage the adhesive interface during hot pressing and use (especially in humid and hot environments), significantly reducing the wet strength of the adhesive layer. Therefore, quaternized chitosan is crucial for ensuring long-term water resistance.

[0160] Compared to Example 1, the performance of D7 decreased significantly, with dry strength down 27% and wet strength down 25%. Its dry strength value is very close to that of D1 without the modifier, which is 1.62 MPa. This demonstrates that nanocellulose whiskers can provide good interfacial mechanical strength and rigid support. Without nanocellulose whiskers, the modifier is only left with the functions of lignin penetration and chitosan neutralization, losing its ability to form a nano-reinforcing network at the interface and thus failing to effectively improve the dry and wet bonding strength.

[0161] Compared to Example 1, D8's performance is severely inadequate, with a wet strength of 0.62 MPa, lower than the national standard of ≥0.7 MPa. This is because DAAM primarily provides toughness and hydrogen bonding, but cannot form a stable, water-resistant covalent crosslinking network. Its hydrophilic groups cause the adhesive layer to weaken significantly upon contact with water; DAAM alone cannot solve the core problem of poor wet strength and water resistance in formaldehyde-free adhesives.

[0162] Compared to Example 1, D9 exhibits low dry strength with significant data fluctuations, and its wet strength barely meets the minimum requirement. This is because while GMA can form a rigid network that is beneficial for water resistance, it lacks the toughening and preferential penetration effects of DAAM, resulting in a brittle adhesive layer and insufficient interfacial bonding with the wood. Therefore, GMA alone cannot simultaneously achieve high strength and good toughness, resulting in poor dry and wet strength performance.

[0163] Compared to Example 1, the performance of D10 decreased significantly, with dry strength decreasing by approximately 31% and wet strength decreasing by approximately 36%. This is because when DAAM and GMA are added simultaneously, a competitive reaction occurs. The highly active GMA reacts preferentially, occupying most of the reaction sites, preventing DAAM from being effectively functionalized. Consequently, its toughening and permeability-improving effects cannot be realized, resulting in significantly lower performance than the stepwise addition scheme in Example 1.

[0164] Compared with Example 1, the performance of D11 decreased significantly, indicating that the order of addition has a great impact on the performance of the adhesive. This is because adding GMA first will quickly form a dense cross-linked network, which will prevent the subsequent penetration and functionalization of DAAM, thus preventing it from playing a pre-penetration role. This results in poor interfacial bonding, defects in the network structure, and ultimately affects the dry and wet strength performance.

[0165] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for the present invention. Further modifications can be readily implemented by those skilled in the art.

Claims

1. A process for the preparation of a non-formaldehyde modified guar gum wood adhesive characterized in that, The method comprises the following steps: The guar gum is dissolved in distilled water at 55-65℃ for 20-40 min; hydrogen peroxide with a mass of 2-5% of the mass of the guar gum is added dropwise, and the reaction is carried out for 20-40 min; a silane coupling agent with a mass of 1-3% of the mass of the guar gum is added, and the reaction is carried out for 20-40 min; polyvinyl alcohol with a mass of 15-20% of the mass of the guar gum is added, the temperature is raised to 75-85℃, and the reaction is carried out for 20-40 min, and then the temperature is lowered to 55-65℃; an initiator is added, and the reaction is carried out for 20-40 min; butyl acrylate and an emulsifier are pre-emulsified to obtain a pre-emulsion, and the mass of the butyl acrylate is 1.5-2.5 times the mass of the guar gum; the pre-emulsion is added to the reaction system at 55-60℃, and the grafting reaction is carried out for 1.5-2.5 h; wherein, after the grafting reaction is completed, the pH of the system is adjusted to 6.0-6.5 by using triethanolamine, and then a staged crosslinking reaction is carried out, in the first stage, the temperature is raised to 65-75℃, and a nitrogen-containing crosslinking monomer, diacetone acrylamide and a wood interface modifier are synchronously added, and the reaction is carried out for 10-20 min; in the second stage, the temperature is continuously raised to 75-85℃, and an epoxy type crosslinking monomer, glycidyl methacrylate, is added, and the reaction is carried out for 20-40 min; the wood interface modifier comprises the following components in percentage by mass: sodium lignosulfonate 46.7%, quaternary ammonium chitosan 20-21.5%, nanocellulose whiskers 25-25.8%, and nano-zinc oxide loaded zirconium phosphate 6-8%; the addition amount of the wood interface modifier is 1-1.5% of the total mass of the solids of the adhesive; After the crosslinking reaction is completed, the formaldehyde-free modified guar gum wood adhesive is obtained.

2. The production method according to claim 1, characterized by, During the crosslinking reaction, the viscosity of the system is monitored, and when the viscosity rising rate exceeds 5 mPa·s / min, a polymerization inhibitor is immediately added.

3. The preparation method according to claim 1, characterized in that, The wood interface modifier is subjected to in-situ activation treatment before being added, and the specific steps are as follows: a. The sodium lignosulfonate and the nano-zinc oxide loaded zirconium phosphate are premixed, and ultrasonic treatment is carried out at 55-65℃ to obtain a mixture; b. The quaternary ammonium chitosan and the nanocellulose whiskers are added to the mixture obtained in step a, and dispersion is carried out under nitrogen protection to form a homogeneous slurry; c. The homogeneous slurry obtained in step b is injected into the crosslinking reaction system at a rate of 2-3 mL / min, and the reaction is carried out under a negative pressure of 25-35 kPa.

4. The preparation method according to claim 3, characterized in that, During the ultrasonic treatment in step a, nano-diamonds with a particle size of 5-10 nm are synchronously added, and the nano-diamonds are subjected to the following pretreatment: immersion treatment with a 0.5-1 wt% hexadecyltrimethylammonium bromide ethanol solution, centrifugal separation, and vacuum drying at 55-65℃ to obtain nano-diamonds with a cation-modified surface; During the dispersion in step b, 0.05-0.08% of the silane coupling agent KH560 is added based on the total mass of the slurry system; the silane coupling agent KH560 is added in two times: 60% of the total amount is added at the initial stage of dispersion, and the remaining 40% is added 2 min before the end of the dispersion.

5. The preparation method according to claim 2, characterized in that, The amount of nitrogen-containing crosslinking monomer, diacetone acrylamide, is 1-3% of the mass of butyl acrylate; the amount of epoxy crosslinking monomer, glycidyl methacrylate, is 1-3% of the mass of butyl acrylate.

6. The preparation method according to claim 2, characterized in that, The polymerization inhibitor is tetrachloroquinone, and the amount is 0.03-0.05% of the mass of butyl acrylate.

7. The preparation method according to claim 1, characterized in that, The grafting reaction is carried out at 60℃ for 2 h, and the amount of butyl acrylate is 2 times the mass of guar gum.

8. The preparation method according to claim 1, characterized in that, The silane coupling agent is KH550, KH560 or KH570; and the emulsifier is sodium dodecyl sulfate.

9. A formaldehyde-free modified guar gum wood adhesive prepared by the preparation method of any one of claims 1-8.

10. Use of the formaldehyde-free modified guar gum wood adhesive prepared by the method according to any one of claims 1 to 8 in eucalyptus plywood, characterized in that, Comprising: S1, cutting eucalyptus veneer with a water content of 8-12% to a predetermined size; S2, the eucalyptus wood veneer is glued by double-sided gluing method, the gluing amount is 120-220 g / m 2 ; after gluing, the veneer is aged in an environment with temperature of 20-25℃ and relative humidity of 50-60% for 10-15 min; S3, assembling the veneers perpendicularly to the adjacent veneer grain direction, and pre-pressing at a pressure of 0.8-1.2 MPa for 30-40 min; S4, two-stage hot pressing and curing, stage one at 105-110℃, 1.0-1.5 MPa for 2-3 min; stage two at 130-135℃, 2.0-2.2 MPa for 8-10 min; S5, after the pressure is released, the board is cooled to 55℃-60℃ within 20 min by hot air at 55-60℃; and then, the board is cured at room temperature of 20-25℃ for at least 48 h.

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