Soy protein-based nano-composite flame-retardant adhesive containing phosphorus, nitrogen and sulfur and preparation method thereof

Abstract

This invention relates to adhesive technology, aiming to provide a phosphorus-nitrogen-sulfur-containing soybean protein-based nanocomposite flame-retardant adhesive and its preparation method. This composite flame-retardant adhesive is prepared by mixing soybean protein, aqueous epoxy resin emulsion, phosphorus-nitrogen-sulfur-containing polyelectrolyte complex, montmorillonite, and deionized water; wherein, soybean protein is the matrix material, aqueous epoxy resin emulsion, phosphorus-nitrogen-sulfur-containing polyelectrolyte complex, and montmorillonite are the modifying materials, and deionized water is the dispersion medium. This invention utilizes the synergistic effects of montmorillonite nano-reinforcement, phosphorus-nitrogen-sulfur synergistic flame retardancy, and chemical cross-linking to simultaneously improve the adhesive's bonding strength, water resistance, and flame-retardant properties; effectively solving the technical problems of insufficient flame-retardant performance in existing waterborne polyurethane-modified soybean protein adhesives, the tendency of adding flame retardants alone to cause decreased system compatibility, and significant loss of water resistance and bonding strength; the raw materials used are green and environmentally friendly, the preparation process is mild and simple, and it can be directly applied on a large scale.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of adhesives, in particular to a soy protein-based nano-composite flame-retardant adhesive containing phosphorus, nitrogen and sulfur, a preparation method of the adhesive and application of the adhesive in the preparation of plywood. BACKGROUND

[0002] As a typical bio-based adhesive, soy protein adhesive uses agricultural by-products such as soybean meal and soybean powder as raw materials, has the core advantages of wide raw material sources, low production cost, strong renewability, excellent biodegradability and no formaldehyde release, and meets the green and low-carbon development trend of the wood processing industry, showing a broad application prospect in the production of plywood, particle board and other wood-based panels. However, the soy protein molecule structure is rich in hydrophilic groups such as amino and hydroxyl groups, and the intermolecular physical crosslinking network is mainly formed by weak hydrogen bonds, which leads to the key defects of poor water resistance and insufficient bonding strength. In a humid environment, it is easy to swell and dissociate molecular chains, which makes the interfacial bonding performance of wood-based panels decrease sharply, and it is difficult to meet the actual use requirements of water resistance and structural strength of indoor and outdoor wood-based panels, which seriously limits its large-scale popularization and application.

[0003] In order to improve the above-mentioned performance shortcomings of soy protein adhesive, a large number of modification researches have been carried out by researchers, among which chemical crosslinking modification has become the mainstream technology direction because of its stable and long-lasting modification effect. Waterborne epoxy resin, which is stably dispersed in water in the form of microparticles or droplets, has the characteristics of low volatile organic compound (VOC) release, environmental protection and safety, excellent mechanical strength and high crosslinking reaction activity, and has become one of the ideal modifiers of soy protein adhesive. Existing researches show that waterborne epoxy resin can react with the amino and hydroxyl groups in the soy protein molecule through the ring-opening addition reaction of the epoxy groups in its molecular chain, forming a dense and stable chemical crosslinking network, which can not only effectively block the hydrophilic groups on the surface of the soy protein matrix and reduce the water swelling phenomenon of the adhesive layer, but also enhance the intermolecular force, thereby significantly improving the water resistance and bonding strength of the adhesive. Related modification technologies have made stage-by-stage progress in laboratory research.

[0004] But need to note that the water-based epoxy resin itself belongs to the flammable polymer, the heat release rate is high when burning, the smoke release amount is big, only the soybean protein adhesive which is modified by the water-based epoxy resin single, its flame retardant performance has not been substantially improved, even because of the flammable characteristic of the epoxy resin, further increases the fire hazard of the artificial board. With the increasingly strict requirements of the building, furniture and other fields on the flame retardant performance of the artificial board, the existing soybean protein adhesive which is only modified by the water-based epoxy resin, because the flame retardant performance does not reach the standard, still difficult to meet the market application demand of the artificial board. In order to solve this problem, researchers explore a variety of types of flame retardants for composite modification, among which the phosphorus-containing flame retardant becomes the research hotspot in this field because of its halogen-free, low smoke, high efficiency, environmental friendly and other obvious advantages, and has good development prospect.

[0005] But need to note that the phosphorus-containing flame retardant (especially the ionic phosphorus-containing flame retardant) contains strong polar groups and negative charges in the molecular structure, and has poor compatibility with the water-based epoxy resin and the soybean protein adhesive, and direct introduction will cause serious system instability problems: on the one hand, the ionic phosphorus-containing flame retardant will greatly increase the ionic strength of the system after being dissolved in water, and through the action of polarity competition and charge neutralization, the double electric layer of the water-based epoxy resin latex particle surface is compressed, and the surface hydration layer is destroyed, so that the latex particle is quickly destabilized, flocculated and demulsified, and loses the modification function; on the other hand, the strong negative groups of the phosphorus-containing flame retardant will have strong ionic complexation with the protonated amino groups on the soybean protein molecular chain, and form a large number of intermolecular hydrogen bonds with the hydroxyl groups and amino groups in the protein molecules, so that the soybean protein molecular chain is instantaneously crosslinked and entangled, and the soybean protein adhesive is quickly gelled, loses the flowability and adhesive property, and cannot meet the actual production requirements such as gluing and hot pressing of plywood.

[0006] Therefore, how to effectively improve the flame retardant performance of the system on the basis of effectively improving the water resistance and bonding strength of the soybean protein adhesive by using the water-based epoxy resin, and at the same time avoid the problems of demulsification of the water-based epoxy resin and gelation of the soybean protein adhesive caused by the direct introduction of the phosphorus-containing flame retardant, so as to protect the stability and use performance of the system, has become a key technical problem to be solved urgently in the field of soybean protein-based environment-friendly adhesive technology. SUMMARY

[0007] In order to overcome the problems in the background art, the present application provides a soybean protein-based nano-composite flame-retardant adhesive containing phosphorus, nitrogen and sulfur, a preparation method and applications thereof.

[0008] In order to achieve the above-mentioned purposes, the technical scheme adopted by the present application to solve its technical problems is:

[0009] Firstly, a soybean protein-based nanocomposite flame-retardant adhesive containing phosphorus, nitrogen and sulfur is provided, which is prepared by mixing soybean protein, water-based epoxy resin emulsion, phosphorus, nitrogen and sulfur-containing polyelectrolyte complex, montmorillonite and deionized water; wherein the soybean protein is the base material, the water-based epoxy resin emulsion, the phosphorus, nitrogen and sulfur-containing polyelectrolyte complex and the montmorillonite are the modified materials, and the deionized water is the dispersion medium.

[0010] As a preferred scheme of the present application, in each raw material component of the adhesive, the mass ratio relationship of the soybean protein, the solid content in the water-based epoxy resin emulsion, the phosphorus, nitrogen and sulfur-containing polyelectrolyte complex and the montmorillonite is 40-93:5-40:1-10:1-10.

[0011] As a preferred scheme of the present application, the solid content in the water-based epoxy resin emulsion is 15-55 wt%. The solid content in the water-based epoxy resin emulsion refers to the substance left after the water-based epoxy emulsion is dried or cured at room temperature, which is mainly epoxy resin and also includes non-volatile emulsifiers, additives and curing agents (determined according to the emulsion formula).

[0012] As a preferred scheme of the present application, the water-based epoxy resin emulsion contains repeating epoxy groups in the molecular structure of the water-based epoxy resin and contains one of carboxyl and sulfonic acid hydrophilic groups.

[0013] As a preferred scheme of the present application, the phosphorus, nitrogen and sulfur-containing polyelectrolyte complex is prepared by complexation reaction of tetramethylammonium phosphate and chitosan, and the preparation process is as follows: tetramethylammonium phosphate and chitosan are weighed according to a mass ratio of 1:0.5-5; tetramethylammonium phosphate is added to a proper amount of deionized water, mechanically stirred until uniform and stable, to obtain a tetramethylammonium phosphate solution (the concentration is 1-5 wt% in examples); chitosan is added to a proper amount of glacial acetic acid solution (the concentration is 1-5 wt% in examples), mechanically stirred until uniform and stable, to obtain a chitosan solution (the concentration is 1-5 wt% in examples); the chitosan solution is added dropwise to the tetramethylammonium phosphate solution, to obtain a gelatinous complex; vacuum drying and grinding treatment are performed, to obtain a powder-like phosphorus, nitrogen and sulfur-containing polyelectrolyte complex.

[0014] Secondly, the present application also provides a preparation method of the soybean protein-based nanocomposite flame-retardant adhesive, which comprises the following steps:

[0015] (1) soybean protein is added to a proper amount of deionized water, stirred uniformly under heating and adjusted to alkaline, to obtain a soybean protein adhesive;

[0016] (2) water-based epoxy resin emulsion is added to the soybean protein adhesive, stirred uniformly, to obtain a composite adhesive;

[0017] (3) montmorillonite is added to a proper amount of deionized water, ultrasonically dispersed until uniform and stable, to obtain a montmorillonite suspension;

[0018] (4) adding the phosphorus-nitrogen-sulfur-containing polyelectrolyte complex and the montmorillonite suspension into the composite adhesive, stirring uniformly to obtain the soy protein-based nanocomposite flame-retardant adhesive.

[0019] As a preferred scheme of the present application, in the step (1) of preparing the soy protein adhesive, first stirring at 40-50 ℃ for 1-2 h, adjusting the pH value to 9-11 and the mass concentration to 15-40 wt%, and then continuing to stir for 1-2 h to obtain the soy protein adhesive with a mass concentration of 15-40 wt%.

[0020] Thirdly, the present application further provides an application method of the soy protein-based nanocomposite flame-retardant adhesive in preparing plywood, which specifically comprises: uniformly coating the soy protein-based nanocomposite flame-retardant adhesive on a single side surface of a wood board, and stacking and laying at least 3 wood boards according to the principle that the interlayer textures are perpendicular to each other; after heat pressing and cold pressing treatment, drying, and pretreatment in a temperature and humidity controlled environment, the product is ready for use.

[0021] As a preferred scheme of the present application, the single-side glue coating amount of the adhesive is 100-250 g / m 2 , and the product is left to stand for 5-20 min; the heat pressing treatment parameters are: temperature 100-125 ℃, pressure 1.0-1.5 MPa, and holding time 3-10 min; the cold pressing treatment refers to maintaining the pressure unchanged while naturally cooling, and the time is 3-10 min; the drying temperature is 50-70 ℃, and the time is 24-48 h; the pretreatment temperature is 20-25 ℃, the relative humidity is 50-60%, and the time is 24-48 h.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] 1. In the present application, the nitrogen-containing polyelectrolyte (chitosan) and the phosphorus-nitrogen-sulfur-containing flame-retardant component (tetrahydroxymethyl phosphoric acid sulfate) are pre-complexed to form a stable polyelectrolyte complex. This method can effectively neutralize the strong polarity and negative charge of tetrahydroxymethyl phosphoric acid sulfate, avoid its compression of the double electric layer of water-based epoxy resin latex particles and destruction of the hydration layer to cause demulsification. At the same time, it prevents strong electrostatic complexation and excessive entanglement of hydrogen bonds between tetrahydroxymethyl phosphoric acid sulfate and soy protein molecules, thereby avoiding instability problems such as gelation and loss of adhesion from the root.

[0024] 2. In the present application, the complexation reaction of tetrahydroxymethyl phosphoric acid sulfate and chitosan simultaneously introduces phosphorus, nitrogen and sulfur elements into the complex; the S element can release inert gas containing S during the combustion process of the adhesive, and the N element can release inert gas to dilute combustible volatile matter, thereby promoting flame retardation.

[0025] 3、The present application realizes the synchronous improvement of the adhesive bonding strength, water resistance and flame retardant performance by means of the multi-element synergistic effect of montmorillonite nano-enhancement, phosphorus-nitrogen-sulfur synergistic flame retardant and chemical crosslinking. The technical problems of insufficient flame retardant performance of the existing waterborne polyurethane modified soy protein adhesive, the decline of the system compatibility, the significant damage of the water resistance and the adhesive bonding strength caused by the addition of the flame retardant alone are effectively solved.

[0026] 4、The raw materials used in the present application are green and environmentally friendly, and the preparation process is mild and simple. The process does not require special equipment during preparation and use, and is highly compatible with the existing plywood industrial production process, and can be directly applied on a large scale. Therefore, it has environmental protection and practicality, and has a wide application prospect. DETAILED DESCRIPTION

[0027] The present application will be further described in detail below in combination with specific examples.

[0028] For the detection of the limiting oxygen index of plywood, the provisions in GB / T 2406.2-2009 “Plastics-Determination of the burning behavior in terms of flame spread-thickness-extration-Part 2: Horizontal and vertical method of test” are referred to. For the detection of the adhesive bonding strength of plywood, the provisions in GB / T 9846-2015 “General plywood” and GB / T 17657-2013 “Test methods of physical and chemical properties of wood-based panels and veneered wood-based panels” are referred to.

[0029] Example 1

[0030] (1) Soybean protein powder was added to 50 ml of deionized water, mechanically stirred at 40 ℃ for 1 h, then sodium hydroxide solution was added dropwise to adjust the pH value to 9.0, and mechanical stirring was continued for 1 h to prepare a soybean protein adhesive with a mass concentration of 15 wt%.

[0031] (2) The waterborne epoxy resin emulsion with a solid content of 15% was added to the soybean protein adhesive obtained in step (1), and mechanical stirring was continued until the system was uniform and stable to obtain a composite adhesive of soybean protein and waterborne epoxy resin emulsion.

[0032] (3) Tetramethylphosphonium sulfate and chitosan were weighed according to the mass ratio of 1:0.5; tetramethylphosphonium sulfate was added to deionized water and mechanically stirred until it was uniform and stable, and the concentration was diluted to 1 wt% to obtain a tetramethylphosphonium sulfate solution; chitosan powder was added to a 2 wt% glacial acetic acid solution and mechanically stirred until it was completely dissolved and uniform and stable to prepare a chitosan solution with a concentration of 3 wt%; under continuous mechanical stirring, the chitosan solution was added dropwise to the tetramethylphosphonium sulfate solution to obtain a gel-like complex, which was vacuum dried and ground to obtain a phosphorus-nitrogen-sulfur polyelectrolyte complex.

[0033] (4) 5 g of montmorillonite was added into 95 mL of deionized water, and ultrasonic dispersion was performed until a uniform and stable suspension was obtained, to obtain a montmorillonite suspension.

[0034] (5) The phosphorus-nitrogen-sulfur-containing polyelectrolyte complex obtained in step (3) and the montmorillonite suspension obtained in step (4) were added into the composite adhesive of soybean protein and water-based epoxy resin emulsion obtained in step (2), and mechanical stirring was performed until the system was uniform and stable, to obtain a soybean protein-based nano-composite flame-retardant adhesive. The mass ratio of the components was controlled as follows: soybean protein, water-based epoxy resin emulsion, solid content, phosphorus-nitrogen-sulfur-containing polyelectrolyte complex, and montmorillonite, 40:5:10:10.

[0035] (6) The soybean protein-based nano-composite flame-retardant adhesive obtained in step (5) was uniformly coated on the surface of a wood board at a single-side glue application amount of 100 g / m 2 After 5 min, three layers of wood boards were laid according to the principle that the interlayer textures were perpendicular to each other. Then, hot pressing was performed (hot pressing parameters: temperature 125℃, pressure 1.0 MPa, and holding time 10 min). After hot pressing, cold pressing was performed at a pressure of 1.0 MPa for 10 min. Then, drying was performed at 50℃ for 48 h. Then, the wood board was placed in an environment with a temperature of 23℃ and a relative humidity of 55% for 48 h of pretreatment, and was ready for use.

[0036] Performance test results: the limiting oxygen index of the plywood was 29.5, and the dry shear strength of the plywood was 1.05 MPa.

[0037] Example 2:

[0038] (1) Soybean protein powder was added into 50 mL of deionized water, and mechanical stirring was performed at 45℃ for 1.5 h. Then, sodium hydroxide solution was added dropwise to adjust the pH value to 10.0, and mechanical stirring was continued for 1.5 h, to obtain a soybean protein adhesive with a mass concentration of 30wt%.

[0039] (2) A water-based epoxy resin emulsion with a solid content of 55% was added into the soybean protein adhesive obtained in step (1), and mechanical stirring was performed until the system was uniform and stable, to obtain a composite adhesive of soybean protein and water-based epoxy resin emulsion.

[0040] (3) Tetramethylphosphonium sulfate and chitosan were weighed according to a mass ratio of 1:3. Tetramethylphosphonium sulfate was added into deionized water, and mechanical stirring was performed until the system was uniform and stable. The concentration was diluted to 3wt% to obtain a tetramethylphosphonium sulfate solution. Chitosan powder was added into a 5wt% glacial acetic acid solution, and mechanical stirring was performed until the chitosan powder was completely dissolved and the system was uniform and stable, to obtain a chitosan solution with a concentration of 1wt%. Under continuous mechanical stirring, the chitosan solution was added dropwise into the tetramethylphosphonium sulfate solution, to obtain a gelatinous complex. Vacuum drying and grinding were performed, to obtain a phosphorus-nitrogen-sulfur-containing polyelectrolyte complex.

[0041] (4) Put 5 g of montmorillonite into 95 mL of deionized water, and ultrasonically disperse to a uniform and stable suspension to obtain a montmorillonite suspension.

[0042] (5) Put the phosphorus-nitrogen-sulfur-containing polyelectrolyte complex obtained in step (3) and the montmorillonite suspension obtained in step (4) into the composite adhesive of soybean protein and water-based epoxy resin emulsion obtained in step (2), and mechanically stir until uniform and stable to obtain a soybean protein-based nano-composite flame-retardant adhesive. The mass ratio of each raw material component is controlled, and the mass ratio of soybean protein, water-based epoxy resin emulsion, phosphorus-nitrogen-sulfur-containing polyelectrolyte complex, and montmorillonite is 55:27:1:5.

[0043] (6) The soybean protein-based nano-composite flame-retardant adhesive obtained in step (5) is uniformly coated on the surface of the wood board at a single-side glue application amount of 250 g / m 2 After 20 min, 3 layers of wood boards are laid according to the principle of interlaminar texture perpendicular to each other; then hot pressing treatment is carried out (hot pressing parameters: temperature 100°C, pressure 1.2 MPa, holding time 3 min), and after hot pressing, cold pressing is carried out for 3 min at a pressure of 1.2 MPa; then drying is carried out at 70°C for 36 h; then pre-treatment is carried out at a temperature of 20°C and a relative humidity of 50% for 36 h, and the product is ready for use.

[0044] Performance test results: the limiting oxygen index of the plywood is 28.5, and the dry shear strength of the plywood is 1.50 MPa.

[0045] Example 3:

[0046] (1) Put soybean protein powder into 50 mL of deionized water, and mechanically stir at 50°C for 2 h, then add sodium hydroxide solution dropwise to adjust the pH value to 11.0, and continue to mechanically stir for 2 h to obtain a soybean protein adhesive with a mass concentration of 40wt%.

[0047] (2) Put the water-based epoxy resin emulsion with a solid content of 50% into the soybean protein adhesive obtained in step (1), and mechanically stir until the system is uniform and stable to obtain a composite adhesive of soybean protein and water-based epoxy resin emulsion.

[0048] (3) Take tetramethylphosphonium methyl sulfate and chitosan by mass ratio of 1:5; add tetramethylphosphonium methyl sulfate into deionized water, mechanically stir until uniform and stable, and dilute the concentration to 5wt% to obtain a tetramethylphosphonium methyl sulfate solution; add chitosan powder into a 1wt% glacial acetic acid solution, mechanically stir until completely dissolved and uniformly stable, and prepare a 5wt% chitosan solution; under continuous mechanical stirring, add the chitosan solution dropwise into the tetramethylphosphonium methyl sulfate solution to obtain a gel-like complex, vacuum dry, and grind to obtain a phosphorus-nitrogen-sulfur-containing polyelectrolyte complex.

[0049] (4) Add 5g of montmorillonite into 95mL of deionized water, ultrasonically disperse until a uniform and stable suspension is obtained, and obtain a montmorillonite suspension.

[0050] (5) Add the phosphorus-nitrogen-sulfur-containing polyelectrolyte complex obtained in step (3) and the montmorillonite suspension obtained in step (4) into the soybean protein and water-based epoxy resin emulsion composite adhesive obtained in step (2), and mechanically stir until uniform and stable to obtain a soybean protein-based nanocomposite flame-retardant adhesive. Control the feeding ratio of each raw material component, and the mass ratio of soybean protein, water-based epoxy resin emulsion solid, phosphorus-nitrogen-sulfur-containing polyelectrolyte complex, and montmorillonite is 93:40:5:1.

[0051] (6) Uniformly coat the soybean protein-based nanocomposite flame-retardant adhesive obtained in step (5) on the surface of the wood board at a single-sided glue coating amount of 180g / m 2 After 15min, complete the laying of 3 layers of wood boards according to the principle of perpendicular interlayer texture; then perform hot pressing treatment (hot pressing parameters: temperature 120℃, pressure 1.5MPa, holding time 5min), and after hot pressing, cold pressing for 5min at a pressure of 1.5MPa; then dry at 60℃ for 24h; then place in an environment with a temperature of 25℃ and a relative humidity of 60% for 24h of pretreatment, and stand by.

[0052] Performance test results: the limiting oxygen index of the plywood is 27.5, and the dry shear strength of the plywood is 1.25MPa.

[0053] Comparative Example 1:

[0054] (1) Add soybean protein powder into 50mL of deionized water, mechanically stir at 50℃ for 1h, then add sodium hydroxide solution dropwise to adjust the pH value to 10.0, and adjust the soybean protein concentration to 40wt%, continue to mechanically stir for 1h, and prepare a uniform soybean protein adhesive.

[0055] (2) Add the waterborne epoxy resin emulsion with a solid content of 50% into the soybean protein adhesive obtained in step (1), and mechanically stir until the system is uniform and stable to obtain a composite adhesive of soybean protein and waterborne epoxy resin emulsion. The feeding ratio of each raw material component is controlled, and the mass ratio of soybean protein to solid in the waterborne epoxy resin emulsion is 93:40.

[0056] (3) Uniformly coat the composite adhesive of soybean protein and waterborne epoxy resin emulsion obtained in step (2) on the surface of the wood board at a single-side glue coating amount of 180 g / m 2 After 15 min, complete the laying of 3 layers of wood boards according to the principle that the interlayer textures are perpendicular to each other; then perform hot pressing treatment (hot pressing parameters: temperature 120℃, pressure 1.5 MPa, holding time 5 min), and after the hot pressing is completed, cold pressing is performed for 5 min at a pressure of 1.5 MPa; then dry at 60℃ for 24 h; then place in an environment with a temperature of 25℃ and a relative humidity of 60% for 24 h of pretreatment, and stand by.

[0057] Performance test results: the limiting oxygen index of the plywood is 25.3, and the dry shear strength of the plywood is 1.42 MPa.

[0058] Comparative Example 2:

[0059] (1) Add soybean protein powder into 50 mL of deionized water, mechanically stir at 50℃ for 1 h, then add sodium hydroxide solution dropwise to adjust the pH value to 10.0, and adjust the soybean protein concentration to 40wt%, continue to mechanically stir for 1 h to prepare a uniform soybean protein adhesive.

[0060] (2) Add the waterborne epoxy resin emulsion with a solid content of 50% to the soybean protein adhesive obtained in step (1), and mechanically stir until the system is uniform, to obtain a composite adhesive of soybean protein and waterborne epoxy resin emulsion.

[0061] (3) Add tetramethylthiuram monosulfide into deionized water, mechanically stir until uniform and stable, and dilute the concentration to 1wt% to obtain a tetramethylthiuram monosulfide solution.

[0062] (4) Add 5 g of montmorillonite into 95 mL of deionized water, and ultrasonically disperse to obtain a uniform and stable suspension to obtain a montmorillonite suspension.

[0063] (5) Add the tetramethylthiuram monosulfide solution obtained in step (3) and the montmorillonite suspension obtained in step (4) into the composite adhesive of soybean protein and waterborne epoxy resin emulsion obtained in step (2), and mechanically stir until uniform to obtain a soybean protein-based nanocomposite flame-retardant adhesive. The feeding ratio of each raw material component is controlled, and the mass ratio of soybean proteins, solids in the waterborne epoxy resin emulsion, tetramethylthiuram monosulfide, and montmorillonite is 93:40:5:1.

[0064] (6) The soybean protein-based nanocomposite flame-retardant adhesive obtained in step (5) is coated on the surface of the wood board at a single-side coating amount of 180 g / m 2 The wood board is uniformly coated with the adhesive, and after 15 min, three layers of wood board are laid according to the principle that the interlayer texture is perpendicular to each other. Then, hot pressing is performed (hot pressing parameters: temperature 120℃, pressure 1.0-1.5 MPa, holding time 5 min), and after hot pressing, cold pressing is performed for 3 min at a pressure of 1.5 MPa. Then, drying is performed at 60℃ for 24 h. Then, the wood board is placed in an environment with a temperature of 25℃ and a relative humidity of 60% for 24 h of pretreatment, and is ready for use.

[0065] Performance test results: the three layers of wood board of the plywood are separated, the soybean protein-based flame-retardant adhesive is delaminated, the limiting oxygen index of the plywood cannot be tested, and the dry shear strength of the plywood is 0 MPa.

[0066] The differences between the examples and the comparative examples are as follows:

[0067] Comparative example 1 only adds water-based epoxy resin to the soybean protein adhesive, and comparative example 2 only adds tetramethylammonium phosphate to the composite adhesive of soybean protein and water-based epoxy resin. Examples 1-3 of the present application add a phosphorus-nitrogen polyelectrolyte complex to the composite adhesive containing soybean protein, water-based epoxy resin, and montmorillonite.

[0068] The test results of the examples and the comparative examples show that:

[0069] Firstly, comparative example 2 cannot prepare plywood because the addition of tetramethylammonium phosphate destroys the viscosity of the composite adhesive. In examples 1-3, the dry shear strength of the plywood is higher because of the addition of the phosphorus-nitrogen polyelectrolyte complex formed by the complexation of tetramethylammonium phosphate and chitosan. Secondly, the limiting oxygen index of examples 1-3 is significantly higher than that of comparative example 1, and the limiting oxygen index increases with the increase of the amount of phosphorus-nitrogen-sulfur polyelectrolyte complex and montmorillonite. Comparative example 2 cannot test the limiting oxygen index because it cannot prepare plywood. The above test data fully demonstrate the effectiveness of the technical solutions of the present application.

[0070] The above examples are only used to explain the present application, but not to limit the protection scope of the present application. Any equivalent changes or modifications made within the spirit and scope of the present application and the claims are considered to be within the protection scope of the present application.

Claims

1. A soybean protein-based nanocomposite flame-retardant adhesive containing phosphorus, nitrogen, and sulfur, characterized in that, It is prepared by mixing soybean protein, aqueous epoxy resin emulsion, phosphorus-nitrogen-sulfur polyelectrolyte complex, montmorillonite and deionized water; wherein, soybean protein is the matrix material, aqueous epoxy resin emulsion, phosphorus-nitrogen-sulfur polyelectrolyte complex and montmorillonite are the modifying materials, and deionized water is the dispersion medium.

2. The flame-retardant adhesive according to claim 1, characterized in that, The mass ratio of soybean protein, solids in waterborne epoxy resin emulsion, phosphorus-nitrogen-sulfur polyelectrolyte complex, and montmorillonite in the adhesive is 40–93:5–40:1–10:1–10.

3. The flame-retardant adhesive according to claim 1, characterized in that, The solid content of the aqueous epoxy resin emulsion is 15-55 wt%.

4. The flame-retardant adhesive according to claim 1, characterized in that, The aqueous epoxy resin emulsion contains repeating epoxy groups in its molecular structure, and also contains one of carboxyl groups and sulfonic acid hydrophilic groups.

5. The flame-retardant adhesive according to claim 1, characterized in that, The phosphorus-nitrogen-sulfur polyelectrolyte complex is prepared by a complexation reaction of tetrahydroxymethylphosphonic acid and chitosan, as detailed below: Weigh tetramethylol phosphate and chitosan at a mass ratio of 1:0.5 to 5; add tetramethylol phosphate to an appropriate amount of deionized water and mechanically stir until uniform and stable to obtain a tetramethylol phosphate solution; add chitosan to an appropriate amount of glacial acetic acid solution and mechanically stir until uniform and stable to obtain a chitosan solution. Chitosan solution was added dropwise to tetrahydroxymethylphosphoric acid solution to obtain a gel-like complex; after vacuum drying and grinding, a powdered phosphorus-nitrogen-sulfur polyelectrolyte complex was obtained.

6. The method for preparing the soybean protein-based nanocomposite flame-retardant adhesive according to any one of claims 1 to 5, characterized in that, Includes the following steps: (1) Add soybean protein to an appropriate amount of deionized water, stir evenly under heating conditions and adjust to alkalinity to obtain soybean protein adhesive; (2) Add waterborne epoxy resin emulsion to soybean protein adhesive and stir evenly to obtain composite adhesive; (3) Add montmorillonite to an appropriate amount of deionized water and ultrasonically disperse until uniform and stable to obtain a montmorillonite suspension; (4) Add the phosphorus-nitrogen-sulfur polyelectrolyte complex and montmorillonite suspension to the composite adhesive and stir evenly to obtain soybean protein-based nanocomposite flame retardant adhesive.

7. The preparation method according to claim 5, characterized in that, In step (1), when preparing the soybean protein adhesive, the mixture is first stirred at 40-50 °C for 1-2 hours, the pH value is adjusted to 9-11, and stirring is continued for 1-2 hours to obtain a soybean protein adhesive with a mass concentration of 15-40 wt%.

8. The method of applying the soybean protein-based nanocomposite flame-retardant adhesive of claim 1 in the preparation of plywood, characterized in that, Specifically, it includes: Soybean protein-based nanocomposite flame-retardant adhesive is evenly coated onto one side of the wood board. After drying, at least three layers of wood boards are stacked and laid according to the principle that the textures between the layers are perpendicular to each other. After hot pressing and cold pressing, the boards are dried and placed in a temperature and humidity controlled environment for pretreatment for later use.

9. The method according to claim 8, characterized in that, The adhesive has a single-sided coating weight of 100-250 g / m². 2 The air-drying process involves air-drying for 5–20 minutes; the hot-pressing parameters are: temperature 100–125℃, pressure 1.0–1.5MPa, and holding time 3–10 minutes; the cold-pressing process refers to maintaining constant pressure while naturally cooling for 3–10 minutes; the drying temperature is 50–70℃, and the time is 24–48 hours; the pretreatment temperature is 20–25℃, the relative humidity is 50–60%, and the time is 24–48 hours.