Flame-retardant high-density fiberboard and method for producing the same

Abstract

The application discloses a kind of flame-retardant high-density fiberboard and preparation method thereof, comprising the following steps: S1, the preparation of pretreatment mixed fiber;S2, the preparation of modified mixed fiber;S3, the preparation of composite flame retardant;S4, sizing and the preparation of fiberboard.The flame-retardant high-density fiberboard is treated by special treatment to plant fiber, and specific composite flame retardant is added, and toluene diisocyanate adhesive, light calcium carbonate and sodium silicate are matched, so that the obtained fiberboard has good bonding performance, good weather resistance, durable and stable adhesion, and also has good flame retardance and mechanical properties, meets the requirements of the industry, and has good application prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of artificial board, and particularly relates to a high-density fiberboard with fire resistance and a preparation method thereof. BACKGROUND

[0002] In recent years, the medium and high density fiberboard industry in China has developed very rapidly, and the original hard fiberboard has been gradually replaced by the medium and high density fiberboard. The medium density fiberboard in China mainly uses agricultural three left-overs or small-diameter timber as the main raw material, and the main components are wood fiber and resin glue. After hot grinding, drying, gluing, laying, hot pressing, post-processing and sanding, the medium density fiberboard is formed as an environmentally friendly furniture building material substitute for replacing original wood. The main advantages of the medium density fiberboard are low price, easy processing, uniform material quality, good physical properties, water and moisture resistance after adding paraffin and the like. The high density fiberboard has all the advantages of the medium density fiberboard due to its excellent physical properties, and is widely used in indoor and outdoor decoration, furniture, office, high-grade furniture, sound, interior decoration of high-grade cars, and can also be used as a manufacturing material for computer room anti-static floor, wall protection board, security door, wall board and partition board.

[0003] Currently, the raw material used in high-density fiberboard is mainly wood fiber or straw fiber, which is flammable material. The large number of applications in indoor will increase the fire hazard. The natural fiber material has poor water resistance without treatment, which will cause the poor dimensional stability and weather resistance of the fiberboard, and will cause a series of problems such as poor mechanical properties of the fiberboard. The flame retardant used in the prior art high-density fiberboard is basically a chemical flame retardant. The main processing method for the flame retardant performance of the medium and high-density fiberboard is to: 1) prepare a solution, emulsion or suspension of the flame retardant, and spray the board surface or immerse the board in the solution for a certain time. This processing method will introduce excess water in the flame retardant solution into the board, which will affect the thickness uniformity of the board. After the board absorbs water and swells, although it can be dried and shrunk by heat, the swelling and shrinking may not be consistent. Wet spraying or soaking can damage the internal bonding strength and smoothness of the board surface, which will have a negative impact on subsequent processing such as painting and veneering. The board needs to be further dried after wet spraying or soaking, which wastes heat energy. The board may be deformed after wet processing and drying. 2) The flame retardant is prepared into a solution or emulsion or suspension and then added to the adhesive applied to the medium and high-density fiberboard. This method is prone to uneven mixing with the adhesive, which will affect the uniformity of the flame retardant and the adhesive, and will affect the flame retardant effect. At the same time, some chemical components of the flame retardant may react with the adhesive, which will have a negative impact on the performance of the adhesive. Chinese patent application No. 200910014777.2 discloses a method for manufacturing medium and high-density flame-retardant fiberboard, which belongs to the technical field of board manufacturing. The method is to soften the wood chips by steaming, and then quantitatively control the wood chips to enter the high-speed hot mill through the screw discharge control system. A flame retardant mixture composed of a mixed solution of urea, oligophosphate and borate is injected into the mill chamber of the hot mill at a pressure higher than that of the hot mill. The wood fibers are mixed with the flame retardant mixture, and then the mixed wood fibers are sprayed out. The wood fibers are sprayed with a melamine-modified urea-formaldehyde resin adhesive in the fiber spraying pipe of the hot mill. The sprayed mixed wood fibers are dried, laid and formed, and then pressed by a hot press. However, this method has high process operation requirements, and the prepared high-density fiberboard may have unstable flame retardant performance.

[0004] Therefore, it is of great significance to develop a method for improving the flame retardant performance of high-density fiberboard without affecting its dimensional stability and strength. SUMMARY

[0005] In view of the shortcomings of the prior art, the purpose of the present application is to provide a flame-retardant high-density fiberboard and a preparation method thereof. The high-density fiberboard has good flame-retardant performance, good dimensional stability, high strength and good weather resistance, can meet the requirements of the industry, and has good application prospect.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0007] A preparation method of a flame-retardant high-density fiberboard, comprising the following steps:

[0008] S1, preparation of pretreated mixed fibers: after washing and drying the straw, discarded branches and miscellaneous wood, crushing, then adding sodium hydroxide solution for immersion, after the end of the immersion treatment, filtering, drying, obtaining the pretreated mixed fibers;

[0009] S2, preparation of modified mixed fibers: adding the pretreated mixed fibers obtained in step S1 into deionized water, then adding tetrabutyl titanate and ethylenediamine for hydrothermal reaction, after the reaction is completed, adding vinyltriethoxysilane ethanol solution for stirring reaction, after the reaction is completed, filtering, washing and drying the solid product, then adding it into toluene, then adding DOPO for constant temperature reaction under nitrogen condition, after the reaction is completed, filtering, washing and drying until the water content is not more than 5%, obtaining the modified mixed fibers;

[0010] S3, preparation of composite flame retardant: adding carbon nanotubes into concentrated nitric acid, after ultrasonic dispersion, stirring and reacting at 70-80℃ for 1-2h, after the reaction is completed, filtering, washing and drying, carrying out low-temperature plasma treatment, obtaining pretreated carbon nanotubes, adding the pretreated carbon nanotubes into EDC / NHS aqueous solution, adjusting the pH to 5-7, then adding aminoguanidine sulfonic acid for heating reaction, after the reaction is completed, filtering and drying, obtaining the composite flame retardant;

[0011] S4, sizing and preparation of fiberboard: uniformly mixing the adhesive, light calcium carbonate, sodium silicate and the composite flame retardant in step S3, adding into the modified mixed fibers in step S2, uniformly mixing, then carrying out laying, pre-pressing and hot-pressing treatment on the hot press, after molding, obtaining the flame-retardant high-density fiberboard.

[0012] Preferably, in step S1, the straw is one or more of corn straw, sorghum straw, wheat straw and rice straw; the mass concentration of the sodium hydroxide solution is 3-6%; the mass ratio of the straw, discarded branches and miscellaneous wood is 60-70:20-30:10-20.

[0013] Preferably, in step S1, the immersion temperature is 70-90℃, and the immersion time is 1-2h.

[0014] Preferably, in step S2, the mass ratio of the pretreated mixed fibers, deionized water, tetrabutyl titanate, ethylenediamine and vinyltriethoxysilane ethanol solution is 100:700-900:20-30:30-40:80-120; the mass concentration of the vinyltriethoxysilane in the vinyltriethoxysilane ethanol solution is 5-8%; the mass ratio of the solid product, toluene and DOPO is 50:400-500:20-30.

[0015] Preferably, the temperature of the hydrothermal reaction in step S2 is 100-120℃, the time is 3-5h; the temperature of the stirring reaction is 60-80℃, the time is 1-2h; the temperature of the constant temperature reaction is 90-110℃, the time is 4-6h.

[0016] Preferably, the low-temperature plasma treatment process in step S3 is: air is introduced, and the treatment is carried out at 300W power for 5-10min.

[0017] Preferably, the concentration of EDC in the EDC / NHS aqueous solution in step S3 is 50mM / L, the concentration of NHS is 50mM / L; the mass ratio of the pretreated carbon nanotube and guanidine sulfamate is 20-30:10-20, the temperature of the heating reaction is 60-70℃, the time is 2-4h.

[0018] Preferably, the mass ratio of the adhesive, light calcium carbonate, sodium silicate, composite flame retardant, and modified mixed fiber in step S4 is 100-120:3-5:4-8:10-15:40-60; the adhesive is one or both of toluene diisocyanate and polyacrylic acid.

[0019] Preferably, the temperature of the pre-pressing in step S4 is 30-50℃, the pressure is 3-5MPa, the time is 30-50s; the temperature of the hot-pressing treatment is 120-140℃, the pressure is 4-6MPa, the time is 1-2min.

[0020] The application also protects a flame-retardant high-density fiberboard prepared by the method.

[0021] Compared with the prior art, the application has the following beneficial effects:

[0022] (1) The preparation method of the flame-retardant high-density fiberboard provided by the present application first performs alkali treatment on straw, waste branches and miscellaneous wood. On the one hand, the originally dense structure of plant fiber can be penetrated by adhesive, thereby improving the bonding strength between the fiber and the adhesive. On the other hand, the cellulose structure can be exposed, thereby increasing the number of active reaction sites and being beneficial to subsequent reactions. Subsequently, the mixed fiber surface is coated with titanium dioxide through a hydrothermal reaction, thereby improving the aging resistance and flame retardance of the fiber. Then, vinyltriethoxysilane is added, which can improve the hydrophobicity of the mixed fiber and also introduce double bonds onto the mixed fiber. Then, the mixed fiber is reacted with DOPO. The active P-H bond in DOPO is reacted with the double bond in the mixed fiber, thereby grafting DOPO onto the mixed fiber. The polyphosphoric acid or metaphosphoric acid derivative generated by the pyrolysis of DOPO can accelerate the formation of a carbon layer, effectively inhibit the transmission of oxygen and heat, thereby improving the flame retardance of the high-density fiberboard. Compared with direct mixing, the method of chemical grafting can improve the dispersity of DOPO in the fiber, thereby not affecting the mechanical properties of the high-density fiberboard.

[0023] (2) The preparation method of the flame-retardant high-density fiberboard provided by the present application adds a composite flame retardant with carbon nanotubes as a matrix. Carbon nanotubes have the characteristics of heat resistance, heat conduction and high strength under high-temperature environment, which can further improve the flame retardance of the high-density fiberboard. The surface of the carbon nanotubes is carboxylated through acid oxidation treatment and plasma treatment. Then, the carbon nanotubes are reacted with aminoguanidine sulfamate, so that the aminoguanidine sulfamate is introduced onto the surface of the carbon nanotubes. The aminoguanidine sulfamate molecule contains a large amount of nitrogen elements and a small amount of sulfur elements, which can further improve the flame retardant efficiency and play a synergistic flame-retardant effect with DOPO on the mixed fiber, thereby improving the flame retardance of the fiberboard.

[0024] (3) The flame-retardant high-density fiberboard provided by the present application is prepared by performing special treatment on plant fiber and adding a specific composite flame retardant. The obtained fiberboard has good bonding properties, good weather resistance, durable and stable bonding force, good flame retardance and mechanical properties, meets the requirements of the industry, and has good application prospect. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be described clearly and completely in combination with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0026] The length of the carbon nanotubes is 10 μm, and the tube diameter is 40-60 nm.

[0027] Example 1

[0028] A method for preparing flame-retardant high-density fiberboard includes the following steps:

[0029] S1. Preparation of pretreated mixed fibers: 65g corn stalks, 20g waste branches and 15g miscellaneous wood were washed, dried and crushed. Then they were added to a 5% sodium hydroxide solution for impregnation at a temperature of 80℃ for 1.5h. After impregnation, the mixture was filtered and dried to obtain pretreated mixed fibers.

[0030] S2. Preparation of modified mixed fibers: 100g of pretreated mixed fibers from step S1 were added to 800g of deionized water, followed by 25g of tetrabutyl titanate and 35g of ethylenediamine. The mixture was hydrothermally reacted at 150℃ for 4h. After the reaction was completed, 100g of a 6% (w / w) vinyltriethoxysilane ethanol solution was added, and the mixture was stirred at 70℃ for 1.5h. After the reaction was completed, the mixture was filtered, and the solid product was washed and dried. 50g of the solid product was added to 450g of toluene, followed by 25g of DOPO. The mixture was subjected to a constant temperature reaction under nitrogen atmosphere at 100℃ for 5h. After the reaction was completed, the mixture was filtered, washed, and dried until the moisture content was no more than 5%, thus obtaining the modified mixed fibers.

[0031] S3. Preparation of composite flame retardant: Carbon nanotubes were added to concentrated nitric acid, ultrasonically dispersed, and stirred at 75°C for 1.5 h. After the reaction was completed, the mixture was filtered, washed, and dried, and then subjected to low-temperature plasma treatment. The low-temperature plasma treatment process was as follows: air was introduced, and the mixture was treated at 300W power for 8 min to obtain pretreated carbon nanotubes. 25g of pretreated carbon nanotubes were added to 400mE DDC / NHS aqueous solution (EDC concentration was 50mM / L, NHS concentration was 50mM / L), the pH was adjusted to 5, and then 15g of guanidine aminosulfonate was added. The mixture was heated at 65°C for 3 h. After the reaction was completed, the mixture was filtered and dried to obtain the composite flame retardant.

[0032] S4. Adhesive application and fiberboard preparation: Mix 110g of toluene diisocyanate adhesive, 4g of light calcium carbonate, 6g of sodium silicate, and 15g of composite flame retardant from step S3 evenly, add them to 60g of modified mixed fiber from step S2, mix evenly, and then lay, pre-press, and hot-press on a hot press. The pre-pressing temperature is 40℃, the pressure is 4MPa, and the time is 40s. The hot-pressing temperature is 130℃, the pressure is 5MPa, and the time is 2min. After molding, the flame-retardant high-density fiberboard is obtained.

[0033] Example 2

[0034] A method for preparing flame-retardant high-density fiberboard includes the following steps:

[0035] S1. Preparation of pretreated mixed fibers: 60g of sorghum straw, 25g of waste branches and 15g of miscellaneous wood were washed, dried and crushed. Then they were added to a 3% sodium hydroxide solution for impregnation at a temperature of 70℃ for 2 hours. After impregnation, the mixture was filtered and dried to obtain pretreated mixed fibers.

[0036] S2. Preparation of modified mixed fibers: 100g of pretreated mixed fibers from step S1 were added to 700g of deionized water, followed by 20g of tetrabutyl titanate and 30g of ethylenediamine. The mixture was hydrothermally reacted at 100℃ for 5h. After the reaction was completed, 80g of 8% vinyltriethoxysilane ethanol solution was added, and the mixture was stirred at 60℃ for 2h. After the reaction was completed, the mixture was filtered, and the solid product was washed and dried. 50g of the solid product was added to 400g of toluene, followed by 20g of DOPO. The mixture was subjected to a constant temperature reaction under nitrogen atmosphere at 90℃ for 6h. After the reaction was completed, the mixture was filtered, washed, and dried until the moisture content was no more than 5%, thus obtaining the modified mixed fibers.

[0037] S3. Preparation of composite flame retardant: Carbon nanotubes were added to concentrated nitric acid, ultrasonically dispersed, and stirred at 70°C for 2 hours. After the reaction was completed, the mixture was filtered, washed, and dried, and then subjected to low-temperature plasma treatment. The low-temperature plasma treatment process was as follows: air was introduced, and the mixture was treated at 300W power for 5 minutes to obtain pretreated carbon nanotubes. 20g of the pretreated carbon nanotubes were added to 400mE DDC / NHS aqueous solution (EDC concentration was 50mM / L, NHS concentration was 50mM / L), and the pH was adjusted to 6. Then, 10g of guanidine aminosulfonate was added, and the mixture was heated at 60°C for 4 hours. After the reaction was completed, the mixture was filtered and dried to obtain the composite flame retardant.

[0038] S4. Adhesive application and fiberboard preparation: Mix 110g of toluene diisocyanate adhesive, 4g of light calcium carbonate, 6g of sodium silicate, and 15g of composite flame retardant from step S3 evenly, add them to 60g of modified mixed fiber from step S2, mix evenly, and then lay, pre-press, and hot-press on a hot press. The pre-pressing temperature is 30℃, the pressure is 5MPa, and the time is 50s. The hot-pressing temperature is 120℃, the pressure is 6MPa, and the time is 2min. After molding, the flame-retardant high-density fiberboard is obtained.

[0039] Example 3

[0040] A method for preparing flame-retardant high-density fiberboard includes the following steps:

[0041] S1, preparation of pretreated mixed fibers: 70 g of wheat straw, 20 g of waste branches, 10 g of miscellaneous wood were cleaned and dried, crushed, then immersed in a sodium hydroxide solution with a mass concentration of 6%, the immersion temperature was 90°C, the immersion time was 1 h, after the end of the immersion treatment, filtration, drying, to obtain pretreated mixed fibers;

[0042] S2, preparation of modified mixed fibers: 100 g of pretreated mixed fibers in step S1 were added to 900 g of deionized water, then 30 g of tetrabutyl titanate and 40 g of ethylenediamine were added, and hydrothermal reaction was carried out at 120°C for 3 h, then 120 g of a vinyltriethoxysilane ethanol solution with a mass concentration of 5% was added, and stirring reaction was carried out at 80°C for 1 h, after the reaction was completed, filtration was carried out, the solid product was washed and dried, then 50 g of the solid product was added to 500 g of toluene, then 30 g of DOPO was added, and constant temperature reaction was carried out under nitrogen, the reaction temperature was 110°C, and the reaction time was 4 h, after the reaction was completed, filtration, washing and drying were carried out until the water content was not more than 5%, to obtain modified mixed fibers;

[0043] S3, preparation of composite flame retardant: carbon nanotubes were added to concentrated nitric acid, ultrasonic dispersion was carried out, then stirring reaction was carried out at 80°C for 1 h, after the reaction was completed, filtration, washing and drying were carried out, low-temperature plasma treatment was carried out, the low-temperature plasma treatment process was as follows: air was introduced, and treatment was carried out at a power of 300 W for 10 min, to obtain pretreated carbon nanotubes, 30 g of the pretreated carbon nanotubes was added to 400 mL of an EDC / NHS aqueous solution (the concentration of EDC was 50 mM / L, and the concentration of NHS was 50 mM / L), the pH was adjusted to 6, then 20 g of aminoguanidine sulfonic acid was added, and heating reaction was carried out at 70°C for 2 h, after the reaction was completed, filtration and drying were carried out, to obtain the composite flame retardant;

[0044] S4, sizing and preparation of fiberboard: 110 g of toluene diisocyanate adhesive, 4 g of light calcium carbonate, 6 g of sodium silicate and 15 g of the composite flame retardant in step S3 were uniformly mixed, and then added to 60 g of the modified mixed fibers in step S2, to obtain a mixture, the mixture was uniformly mixed, and then laid, pre-pressed and hot-pressed on a hot press, the pre-pressing temperature was 50°C, the pressure was 3 MPa, and the time was 30 s, the hot-pressing temperature was 140°C, the pressure was 4 MPa, and the time was 1 min, and after molding, the flame-retardant high-density fiberboard was obtained.

[0045] Comparative Example 1

[0046] A preparation method of a flame-retardant high-density fiberboard, comprising the following steps:

[0047] S1, preparation of pretreated mixed fibers: 65 g of corn straw, 20 g of discarded branches, 15 g of miscellaneous wood were washed, dried, crushed, and then immersed in a 5% sodium hydroxide solution, the immersion temperature was 80℃, the immersion time was 1.5h, after the immersion treatment was completed, filtration, drying, to obtain pretreated mixed fibers;

[0048] S2, preparation of composite flame retardant: carbon nanotubes were added to concentrated nitric acid, ultrasonic dispersion, then stirred at 75℃ for 1.5h, after reaction, filtration, washing, drying, low temperature plasma treatment, low temperature plasma treatment process: air was introduced, treated at 300W power for 8min, to obtain pretreated carbon nanotubes, 25g pretreated carbon nanotubes were added to 400ml EDC / NHS aqueous solution (the concentration of EDC was 50mM / L, the concentration of NHS was 50mM / L), the pH was adjusted to 5, then 15g guanidinium sulfamate was added, heated at 65℃ for 3h, after reaction, filtration, drying, to obtain the composite flame retardant;

[0049] S3, sizing and fiberboard preparation: 110g toluene diisocyanate adhesive, 4g light calcium carbonate, 6g sodium silicate, 15g composite flame retardant in step S2 were mixed uniformly, added to 60g pretreated mixed fibers in step S1, mixed uniformly, laid, pre-pressed and hot-pressed on a hot press, the pre-pressing temperature was 40℃, the pressure was 4MPa, the time was 40s, the hot-pressing temperature was 130℃, the pressure was 5MPa, the time was 2min, after molding, the flame-retardant high-density fiberboard was obtained.

[0050] Comparative example 2

[0051] A preparation method of a flame-retardant high-density fiberboard, comprising the following steps:

[0052] S1, preparation of pretreated mixed fibers: 65 g of corn straw, 20 g of discarded branches, 15 g of miscellaneous wood were washed, dried, crushed, and then immersed in a 5% sodium hydroxide solution, the immersion temperature was 80℃, the immersion time was 1.5h, after the immersion treatment was completed, filtration, drying, to obtain pretreated mixed fibers;

[0053] S2, preparation of modified mixed fibers: 100 g of pretreated mixed fibers in step S1 is added to 800 g of deionized water, then 25 g of tetrabutyl titanate and 35 g of ethylenediamine are added, and hydrothermal reaction is carried out at 150 DEG C for 4 h, then 100 g of a vinyltriethoxysilane ethanol solution with a mass concentration of 6% is added, and stirring reaction is carried out at 70 DEG C for 1.5 h, after the reaction is completed, filtration is carried out, and after the solid product is washed and dried, 50 g of the solid product is added to 450 g of toluene, then 25 g of DOPO is added, and constant temperature reaction is carried out under the condition of nitrogen, the reaction temperature is 100 DEG C, and the reaction time is 5 h, after the reaction is completed, filtration, washing and drying are carried out until the water content is not more than 5%, and the modified mixed fibers are obtained;

[0054] S3, sizing and preparation of fiberboard: 110 g of toluene diisocyanate adhesive, 4 g of light calcium carbonate, 6 g of sodium silicate and 15 g of carbon nanotubes are uniformly mixed and added to 60 g of modified mixed fibers in step S2, and then uniformly mixed, and then laid, pre-pressed and hot-pressed on a hot press, the pre-pressing temperature is 40 DEG C, the pressure is 4 MPa, and the time is 40 s, the hot-pressing temperature is 130 DEG C, the pressure is 5 MPa, and the time is 2 min, and then the flame-retardant high-density fiberboard is obtained.

[0055] The environmentally friendly high-density fiberboard prepared in examples 1-3 and comparative examples 1-2 of the application is subjected to performance testing, wherein the internal bond strength, static bending strength and elastic modulus are tested according to the standard LY / T 1611-2011 "Fiberboard for floor base materials", the water absorption thickness expansion rate is tested according to the standard GB / T 17657-2022 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels" - water absorption thickness expansion rate test - method 1, and the limiting oxygen index is tested according to the standard GB / T 2406.2-2009; the test results are shown in Table 1 below:

[0056] Table 1

[0057]

[0058] As can be seen from Table 1, the environmentally friendly high-density fiberboard provided by the application has excellent internal bond strength, static bending strength and elastic modulus, good water resistance and flame retardancy, and good application prospect.

[0059] Although embodiments of the application have been shown and described, it is to be understood that the application is not limited to these embodiments. Since modifications, equivalents and alternatives to these embodiments can be apparent to one of ordinary skill in the art, it is intended that the application be limited only by the claims and equivalents thereof.

Claims

1. A method for preparing flame-retardant high-density fiberboard, characterized in that, Includes the following steps: S1. Preparation of pretreated mixed fibers: Straw, waste branches and miscellaneous wood are washed, dried and crushed, then impregnated in sodium hydroxide solution. After impregnation, the mixture is filtered and dried to obtain pretreated mixed fibers. S2. Preparation of modified mixed fibers: The pretreated mixed fibers obtained in step S1 are added to deionized water, followed by tetrabutyl titanate and ethylenediamine, and a hydrothermal reaction is carried out. After the reaction is completed, vinyltriethoxysilane ethanol solution is added and the reaction is stirred. After the reaction is completed, the mixture is filtered, the solid product is washed and dried, and then added to toluene. DOPO is then added, and the reaction is carried out at a constant temperature under nitrogen. After the reaction is completed, the mixture is filtered, washed, and dried until the moisture content is no more than 5%, thus obtaining the modified mixed fibers. S3. Preparation of composite flame retardant: Carbon nanotubes are added to concentrated nitric acid, ultrasonically dispersed, and stirred at 70-80℃ for 1-2 hours. After the reaction is completed, the mixture is filtered, washed, and dried, and then subjected to low-temperature plasma treatment to obtain pretreated carbon nanotubes. The pretreated carbon nanotubes are added to EDC / NHS aqueous solution, the pH is adjusted to 5-7, and then guanidine aminosulfonate is added. The mixture is heated and reacted. After the reaction is completed, the mixture is filtered and dried to obtain the composite flame retardant. S4. Adhesive application and fiberboard preparation: The adhesive, light calcium carbonate, sodium silicate, and composite flame retardant from step S3 are mixed evenly and added to the modified mixed fibers from step S2. After being mixed evenly, the mixture is laid, pre-pressed, and hot-pressed on a hot press to obtain the flame-retardant high-density fiberboard. In step S2, the mass ratio of the pretreated mixed fiber, deionized water, tetrabutyl titanate, ethylenediamine, and vinyltriethoxysilane ethanol solution is 100:700-900:20-30:30-40:80-120; the mass concentration of vinyltriethoxysilane in the vinyltriethoxysilane ethanol solution is 5-8%; and the mass ratio of the solid product, toluene, and DOPO is 50:400-500:20-30. In step S3, the concentration of EDC in the EDC / NHS aqueous solution is 50 mM, and the concentration of NHS is 50 mM; the mass ratio of the pretreated carbon nanotubes to guanidine aminosulfonate is 20-30:10-20; the heating reaction temperature is 60-70℃, and the time is 2-4 h.

2. The preparation method according to claim 1, characterized in that, The straw mentioned in step S1 is one or more of corn straw, sorghum straw, wheat straw, and rice straw; the mass concentration of the sodium hydroxide solution is 3-6%; and the mass ratio of straw, waste branches, and miscellaneous wood is 60-70:20-30:10-20.

3. The preparation method according to claim 1, characterized in that, The immersion temperature in step S1 is 70-90℃, and the immersion time is 1-2 hours.

4. The preparation method according to claim 1, characterized in that, In step S2, the hydrothermal reaction is carried out at a temperature of 100-120℃ for 3-5 hours; the stirring reaction is carried out at a temperature of 60-80℃ for 1-2 hours; and the isothermal reaction is carried out at a temperature of 90-110℃ for 4-6 hours.

5. The preparation method according to claim 1, characterized in that, The low-temperature plasma treatment process described in step S3 is as follows: air is introduced, and the treatment is carried out at a power of 300W for 5-10 minutes.

6. The preparation method according to claim 1, characterized in that, In step S4, the mass ratio of the adhesive, light calcium carbonate, sodium silicate, composite flame retardant, and modified mixed fiber is 100-120:3-5:4-8:10-15:40-60; the adhesive is one or two of toluene diisocyanate and polyacrylic acid.

7. The preparation method according to claim 1, characterized in that, In step S4, the pre-pressing temperature is 30-50℃, the pressure is 3-5MPa, and the time is 30-50s; the hot pressing temperature is 120-140℃, the pressure is 4-6MPa, and the time is 1-2min.

8. A flame-retardant high-density fiberboard prepared by the method according to any one of claims 1-7.

Images