High-strength fire-retardant plywood and preparation method thereof

Abstract

This application discloses a high-strength flame-retardant plywood and its preparation method. The plywood comprises flame-retardant veneers stacked sequentially with adhesive resin. The flame-retardant veneers are obtained by impregnating the veneers with a flame-retardant liquid. The flame-retardant liquid comprises the following components in mass concentrations: 8-15 wt% silicone-modified urea-formaldehyde resin, 5-10 wt% modified DOPO, and 0.5-2 wt% additives. The modified DOPO is prepared from DOPO, 4,4'-diaminobenzophenone, and 4-carboxybenzaldehyde in a molar ratio of 1.1-1.3:1:2-2.2. The DOPO and 4,4'-diaminobenzophenone undergo nucleophilic addition to obtain diaminoDOPO, which is then reacted with 4-carboxybenzaldehyde via a Schiff base reaction to obtain modified DOPO. This application effectively alleviates the problem that flame retardants easily deposit and crystallize in wood during the impregnation flame-retardant treatment process, causing deterioration of the impact toughness of the plywood.

Description

Technical Field

[0001] This application relates to the field of flame-retardant wood, and in particular to a high-strength flame-retardant plywood and its preparation method. Background Technology

[0002] Plywood is widely used in construction, furniture, and interior decoration due to its excellent overall performance. However, its flammability limits its application in situations with high fire safety requirements. To improve the flame retardant properties of plywood, the commonly used method is flame-retardant impregnation of the veneer. This involves immersing the wood veneer in a flame retardant solution, allowing the flame retardant to penetrate into the pores and cell cavities of the wood through liquid penetration, thus achieving a certain flame-retardant effect. This method is simple and low-cost, and can impart good flame-retardant properties to plywood to a certain extent. However, in the impregnation process, the flame retardant mainly fills the wood pores in a physical deposition and crystallization form, without forming stable chemical bonds with cellulose, lignin, etc., leading to easy precipitation and loss during subsequent processing or use. This not only reduces the flame-retardant performance but may also cause metal corrosion problems. In addition, to meet the requirements of flame retardant rating, it is usually necessary to increase the flame retardant loading in the board. Although this can improve the board density and static mechanical properties such as compressive strength, rigid flame retardants will fill the deformable space of the board, deteriorate the impact toughness of the board, and make the board easy to crack when it is impacted, making it difficult to meet the needs of structural components, load-bearing parts and other scenarios. Summary of the Invention

[0003] To address the problem that flame retardants deposit and crystallize extensively in wood during the impregnation flame retardant treatment process, leading to a deterioration in the impact toughness of the board, this application provides a high-strength flame-retardant plywood and its preparation method.

[0004] In a first aspect, this application provides a high-strength flame-retardant plywood comprising flame-retardant veneers stacked sequentially by adhesive resin. The flame-retardant veneers are obtained by impregnating the veneers with a flame-retardant liquid. The flame-retardant liquid comprises the following components in mass concentrations: 8-15 wt% silicone-modified urea-formaldehyde resin, 5-10 wt% modified DOPO, and 0.5-2 wt% additives. The modified DOPO is derived from DOPO, 4,4'-diaminobenzophenone, and 4-carboxybenzaldehyde in a molar ratio of 1.1-1.3:1:2-2.2. The DOPO and 4,4'-diaminobenzophenone undergo nucleophilic addition to obtain diaminoDOPO, which is then reacted with 4-carboxybenzaldehyde via a Schiff base reaction to obtain modified DOPO.

[0005] In any of the above technical solutions, the method for preparing modified DOPO is as follows: 4,4'-Diaminobenzophenone was dissolved in a solvent by stirring, and the temperature was raised to 80-90°C. DOPO solvent was added dropwise under nitrogen protection. After the addition was complete, an acid-binding agent was added, and the reaction was carried out at 100-110°C for 4-6 hours. After the reaction was completed, the reaction solution was cooled to room temperature, deionized water was added to precipitate the solid product, and the product was washed with water and dried to obtain diaminoDOPO. DiaminoDOPO and 4-carboxybenzaldehyde were added to a solvent and stirred until homogeneous. The pH of the system was adjusted to 4.5–5.5, an acidic catalyst was added, and the temperature was raised to 120–140 °C. The reaction was carried out for 6–8 hours. After the reaction was completed, the reaction solution was distilled under reduced pressure to remove part of the solvent. The remaining reaction solution was mixed with ice water and stirred to precipitate a solid. After filtration, washing, and drying, modified DOPO was obtained.

[0006] This application utilizes modified DOPO synthesized from DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide), 4,4'-diaminobenzophenone, and 4-carboxybenzaldehyde. The preparation process first involves a nucleophilic addition reaction between the pH bond of DOPO and the carbonyl group of 4,4'-diaminobenzophenone, generating a diamino DOPO intermediate containing two amino groups. This intermediate further reacts with the aldehyde group of 4-carboxybenzaldehyde via a Schiff base reaction to form a modified DOPO product containing two carboxyl groups (pH control inhibits the amidation side reaction between the amino and carboxyl groups). This modified product not only retains the original high-efficiency gas-phase flame retardant effect of DOPO but also achieves a phosphorus-nitrogen synergistic flame retardant effect by introducing nitrogen, significantly improving the flame retardant efficiency. More importantly, the modified DOPO molecule has two carboxyl functional groups at its ends. These active groups can undergo esterification reactions with hydroxyl groups in the wood cell wall to form stable chemical bonds, thereby effectively inhibiting the migration and loss of flame retardants during use and improving flame retardant durability. In addition, the dicarboxyl structure enables it to form a moderately cross-linked network with wood polysaccharide components. This cross-linked structure imparts higher compressive strength to the material, and because the bonding method differs from the physical filling and rigid deposition of traditional flame retardants, it does not significantly damage the tough skeleton structure of the wood itself. Therefore, it can better maintain the deformation capacity of the wood and significantly reduce the adverse effects on impact toughness.

[0007] In any of the above technical solutions, the organosilicon-modified urea-formaldehyde resin is prepared by hydrosilylation of an alkenyl urea-formaldehyde resin prepolymer and a hydrogen-containing silicone oil in a mass ratio of 12 to 20:1.

[0008] In any of the above technical solutions, the hydrosilylation is carried out under the catalysis of a platinum catalyst, and the amount of the platinum catalyst is 10 to 30 ppm of the total amount of alkenyl urea-formaldehyde resin prepolymer and hydrogen-containing silicone oil.

[0009] In any of the above technical solutions, the alkenyl urea-formaldehyde resin prepolymer is obtained by polycondensation of urea, formaldehyde, and 2-aminoethyl acrylate in a molar ratio of 1:1.2-1.8:0.3-0.5.

[0010] In any of the above technical solutions, the preparation method of the alkenyl urea-formaldehyde resin prepolymer is as follows: Formaldehyde is dissolved in water by stirring to prepare a formaldehyde aqueous solution. The solution is heated to 40–50°C, and the pH is adjusted to 8.0–9.0. 50%–60% of the total urea is added, and the solution is stirred and heated to 80–90°C for further reaction. The pH is adjusted to 4–5, and 20%–40% of the total urea and part of 2-aminoethyl acrylate are added. The solution is then kept at 80–90°C for further reaction. The remaining urea and 2-aminoethyl acrylate are added, and the solution is cooled to 65–70°C for 20–30 minutes. After the reaction is complete, the solution is cooled to below 50°C, and the pH is adjusted to 7.0–8.0. The solution is then cooled and discharged to obtain the final product.

[0011] In any of the above technical solutions, the 2-aminoethyl acrylate accounts for 80-90 wt% of the total amount.

[0012] In any of the above technical solutions, the hydrogen content of the hydrogen-containing silicone oil is 0.1% to 1.2%.

[0013] This application introduces organosilicon-modified urea-formaldehyde resin into a flame retardant liquid. This resin is prepared by hydrosilylation of an alkenyl-containing urea-formaldehyde resin prepolymer with a hydrogen-containing silicone oil. While traditional urea-formaldehyde resin impregnation can enhance the rigidity and mechanical strength of wood, its dense cross-linked network and excessively rigid molecular chains typically lead to increased brittleness and decreased impact performance. This application effectively improves the toughness and elasticity of the resin by introducing acrylate segments and organosilicon flexible segments into the urea-formaldehyde resin molecule. The acrylate component can improve brittleness by adjusting the polymer's plastic behavior, while the introduction of organosilicon segments not only further reduces the resin's cross-linking density and enhances the flexibility of the molecular chains, but also significantly compensates for the increase in rigidity caused by resin curing, thus helping to maintain good impact toughness of the board. Simultaneously, the inherent hydrophobic properties of the organosilicon component can effectively reduce the hygroscopicity of the plywood, thereby improving the stability of the board in humid environments.

[0014] It is worth noting that adding 2-aminoethyl acrylate during the second urea addition helps achieve a good alkenyl grafting rate while ensuring the mechanical properties of the urea-formaldehyde resin. Adding it too early is detrimental to ensuring its mechanical properties, while adding it too late is detrimental to the participation of 2-aminoethyl acrylate in the reaction. In addition, the small amount of 2-aminoethyl acrylate added in the second step has a capping effect, which is beneficial to improving the reactivity of the modified urea-formaldehyde resin product with hydrogen-containing silicone oil.

[0015] In any of the above technical solutions, the impregnation treatment involves placing the veneer under a vacuum condition of -0.08 to -0.1 MPa for 1 to 2 hours, then injecting flame retardant liquid to immerse the veneer, pressurizing it to 1.1 to 1.3 MPa for 0.5 to 1 hour, depressurizing, and removing and drying it to constant weight.

[0016] In any of the above technical solutions, the auxiliary agent is an organosilicon-modified polyether surfactant.

[0017] The main function of silicone-modified polyether surfactants is to improve the permeability and dispersibility of flame retardant liquids, which helps to improve the uniformity of impregnation treatment.

[0018] Secondly, this application provides a method for preparing high-strength flame-retardant plywood. The method involves uniformly applying a resin adhesive to modified veneers according to the raw material composition of the high-strength flame-retardant plywood described in any of the first aspects. The adhesive-coated veneers are then stacked together to form a plywood blank, which is subsequently subjected to cold pressing and hot pressing. The cold pressing temperature is 20–30°C, the pressure is 0.6–1.0 MPa, and the time is 1–2 h. The hot pressing temperature is 100–120°C, the pressure is 0.8–1.5 MPa, and the time is 5–15 min.

[0019] In any of the above technical solutions, the resin adhesive is urea-formaldehyde resin, and 1 to 1.5 wt% NH4Cl curing agent may be added to the urea-formaldehyde resin.

[0020] The number of veneers in the plywood of this application can be stacked and compounded according to actual needs. Generally, 3-7 veneers are used to form a board blank. The surface of the plywood can be laminated with decorative materials such as high-temperature decorative film, engineered wood, and calcium silicate board.

[0021] In summary, this application has the following beneficial effects: This application employs a composite flame-retardant liquid system of reactive modified DOPO flame retardant and silicone-modified urea-formaldehyde resin, combined with a vacuum-pressure impregnation process. This allows the flame-retardant components to form stable chemical bonds and cross-linked structures with the wood components, significantly improving the flame-retardant performance of plywood while effectively avoiding the problems of easy flame retardant loss and poor durability in traditional physical impregnation methods. Furthermore, this solution, while imparting higher static mechanical properties (compressive strength) to the board, significantly reduces the loss of impact toughness by improving resin toughness and reducing rigid deposition, thus achieving a good balance between flame-retardant performance and overall mechanical properties. Detailed Implementation

[0022] Preparation Example

[0023] Preparation Example 1-1, modified DOPO, was prepared by following these steps: 1 mol of 4,4'-diaminobenzophenone was dissolved in 800 mL of N,N-dimethylformamide with stirring. The temperature was raised to 85 °C, and 400 mL of N,N-dimethylformamide solution containing 1.2 mol of DOPO was slowly added dropwise under nitrogen protection. After the addition was complete, 3.0 g of triethylamine was added as an acid-binding agent, and the reaction was carried out at 105 °C for 5 h. After the reaction was completed, the reaction solution was cooled to room temperature, poured into 2000 mL of deionized water to precipitate the solid product, filtered, washed three times with deionized water, and dried under vacuum at 80 °C for 12 h to obtain the diaminoDOPO intermediate.

[0024] All the prepared diaminoDOPO and 2.1 mol of 4-carboxybenzaldehyde were added to 1000 mL of toluene, stirred until homogeneous, and the pH of the system was adjusted to 4.5–5.5 (using dilute formic acid). 1.0 g of p-toluenesulfonic acid was added as a catalyst, and the mixture was heated to 130 °C and reacted for 7 h. After the reaction was complete, the reaction solution was distilled under reduced pressure to remove some of the solvent. The remaining concentrated reaction solution was poured into 2000 g of ice water, stirred vigorously to precipitate a solid, filtered, washed twice with ethanol, and then washed twice with deionized water. The solid was then dried under vacuum at 70 °C for 24 h to obtain the modified DOPO product.

[0025] Preparation Examples 1-2: Modified DOPO was prepared by following these steps: 1 mol of 4,4'-diaminobenzophenone was dissolved in 750 mL of N-methylpyrrolidone under stirring. The temperature was raised to 80 °C, and 350 mL of N-methylpyrrolidone solution containing 1.1 mol of DOPO was slowly added dropwise under nitrogen protection. After the addition was complete, 2.5 g of pyridine was added as an acid-binding agent, and the reaction was carried out at 100 °C for 6 h. After the reaction was completed, the reaction solution was cooled to room temperature and poured into 1800 mL of deionized water to precipitate the solid product. After filtration, the product was washed three times with deionized water and dried under vacuum at 75 °C for 15 h to obtain the diaminoDOPO intermediate.

[0026] All the prepared diaminoDOPO and 2.0 mol of 4-carboxybenzaldehyde were added to 900 mL of xylene, stirred until homogeneous, and the pH of the system was adjusted to 4.5–5.5 (using dilute formic acid). 0.8 g of p-toluenesulfonic acid was added as a catalyst, and the mixture was heated to 120 °C and reacted for 8 h. After the reaction was complete, the reaction solution was distilled under reduced pressure to remove some of the solvent. The remaining concentrated reaction solution was poured into 1500 g of ice water, and the solid was precipitated by vigorous stirring. After filtration, the solid was washed twice with ethanol and then twice with deionized water, and dried under vacuum at 65 °C for 26 h to obtain the modified DOPO product.

[0027] Preparation Examples 1-3: Modified DOPO was prepared by following these steps: 1 mol of 4,4'-diaminobenzophenone was dissolved in 850 mL of N,N-dimethylformamide with stirring. The temperature was raised to 90 °C, and 450 mL of N,N-dimethylformamide solution containing 1.3 mol of DOPO was slowly added dropwise under nitrogen protection. After the addition was complete, 3.5 g of triethylamine was added as an acid-binding agent, and the reaction was carried out at 110 °C for 4 h. After the reaction was completed, the reaction solution was cooled to room temperature and poured into 2200 mL of deionized water to precipitate the solid product. After filtration, the product was washed four times with deionized water and dried under vacuum at 85 °C for 10 h to obtain the diaminoDOPO intermediate.

[0028] All the prepared diaminoDOPO and 2.2 mol of 4-carboxybenzaldehyde were added to 1100 mL of toluene, stirred until homogeneous, and the pH of the system was adjusted to 4.5–5.5 (using dilute formic acid). 1.2 g of p-toluenesulfonic acid was added as a catalyst, and the temperature was raised to 140 °C for 6 h. After the reaction was completed, the reaction solution was distilled under reduced pressure to remove part of the solvent. The remaining concentrated reaction solution was poured into 2500 g of ice water, stirred vigorously to precipitate a solid, filtered, washed three times with ethanol, and then washed three times with deionized water. The solid was dried under vacuum at 75 °C for 22 h to obtain the modified DOPO product.

[0029] Preparation Examples 1-4: Modified DOPO was prepared by following these steps: 1 mol of 4,4'-diaminobenzophenone was dissolved in 800 mL of N,N-dimethylformamide with stirring. The temperature was raised to 85 °C, and 400 mL of N,N-dimethylformamide solution containing 1.2 mol of DOPO was slowly added dropwise under nitrogen protection. After the addition was complete, 3.0 g of triethylamine was added as an acid-binding agent, and the reaction was carried out at 105 °C for 5 h. After the reaction was completed, the reaction solution was cooled to room temperature and poured into 2000 mL of deionized water to precipitate the solid product. After filtration, the product was washed three times with deionized water and dried under vacuum at 80 °C for 12 h. The resulting diaminoDOPO intermediate was the modified DOPO.

[0030] Preparation Example 2-1: Modified urea-formaldehyde resin was prepared according to the following method: 7.5 mol of formaldehyde was dissolved in an appropriate amount of water to prepare a 37% (w / w) formaldehyde aqueous solution, which was then transferred to a three-necked flask equipped with a condenser, stirrer, and thermometer. The temperature was raised to 45°C, and the pH was adjusted to 8.5 with a 10% sodium hydroxide aqueous solution. 2.5 mol of urea was added, and after stirring to dissolve, the temperature was raised to 85°C. The reaction was maintained at this temperature for 30 minutes until the reaction solution became milky white and turbid. The pH was then adjusted to 4.5 with a 20% ammonium chloride aqueous solution, and 2.0 mol of urea and 2.25 mol of 2-aminoethyl acrylate (90% of the total amount) were added. The reaction was continued at 85°C until the viscosity reached the required level (150-200 mPa·s). The remaining 0.5 mol of urea and 0.25 mol of 2-aminoethyl acrylate were added, and the temperature was lowered to 68°C for 25 minutes. After the reaction is complete, the temperature is lowered to 40°C, and the pH is adjusted to approximately 7.5 using a 10% sodium hydroxide aqueous solution. The material is then discharged after cooling to below 35°C to obtain an alkenyl urea-formaldehyde resin prepolymer.

[0031] 1600g of the alkenyl urea-formaldehyde resin prepolymer prepared above was mixed with 100g of hydrogen-containing silicone oil (RH-LHC-3) with a hydrogen content of 0.8% in a reactor. After stirring evenly, a chloroplatinic isopropanol solution at 25 ppm relative to the total mass was added as a catalyst. Under nitrogen protection, the temperature was slowly raised to 80℃ and the reaction was maintained for 5 hours. After the reaction was completed, the temperature was lowered to below 40℃ and the product was discharged to obtain the modified urea-formaldehyde resin.

[0032] Preparation Example 2-2: Modified urea-formaldehyde resin was prepared according to the following method: 6.0 mol of formaldehyde was dissolved in an appropriate amount of water to prepare a 37% (w / w) formaldehyde aqueous solution, which was then transferred to a reactor. The temperature was raised to 40°C, and the pH was adjusted to 8.0 with a 10% sodium hydroxide aqueous solution. 3.0 mol of urea was added, and after stirring to dissolve, the temperature was raised to 80°C and maintained until the reaction solution became slightly turbid. The pH was adjusted to 4.0 with a 20% ammonium chloride aqueous solution, and then 1.5 mol of urea and 1.35 mol of 2-aminoethyl acrylate (90% of the total amount) were added. The reaction was continued at 80°C until the viscosity reached the required level (150-200 mPa·s). The remaining 0.5 mol of urea and 0.15 mol of 2-aminoethyl acrylate were added, and the temperature was lowered to 65°C and reacted for 30 min. After the reaction was completed, the temperature was lowered to 45°C, and the pH was adjusted to 7.0 with a 10% sodium hydroxide aqueous solution. The product was then cooled and discharged to obtain an alkenyl urea-formaldehyde resin prepolymer.

[0033] 1200g of the alkenyl urea-formaldehyde resin prepolymer prepared above was mixed with 100g of hydrogen-containing silicone oil (RH-H57) with a hydrogen content of 0.13%. After stirring evenly, 10ppm of platinum catalyst relative to the total mass was added. Under nitrogen protection, the temperature was slowly raised to 75℃ and the reaction was maintained for 4 hours. After the reaction was completed, the material was cooled and discharged to obtain the modified urea-formaldehyde resin.

[0034] Preparation Examples 2-3: Modified urea-formaldehyde resin was prepared according to the following method: 9.0 mol of formaldehyde was dissolved in an appropriate amount of water to prepare a 37% (w / w) formaldehyde aqueous solution, which was then transferred to a reactor. The temperature was raised to 50°C, and the pH was adjusted to 9.0 with a 10% sodium hydroxide aqueous solution. 3.0 mol of urea was added, and after stirring to dissolve, the temperature was raised to 90°C and maintained until the reaction solution became noticeably turbid. The pH was adjusted to 5.0 with a 20% ammonium chloride aqueous solution, and then 1.0 mol of urea and 2.0 mol of 2-aminoethyl acrylate (80% of the total amount) were added. The reaction was continued at 90°C until the viscosity reached the required level (200-250 mPa·s). The remaining 1.0 mol of urea and 0.5 mol of 2-aminoethyl acrylate were added, and the temperature was lowered to 70°C and reacted for 20 min. After the reaction was completed, the temperature was lowered to 35°C, and the pH was adjusted to 8.0 with a 10% sodium hydroxide aqueous solution. The product was then cooled and discharged to obtain an alkenyl urea-formaldehyde resin prepolymer.

[0035] 2000g of the alkenyl urea-formaldehyde resin prepolymer prepared above was mixed with 100g of hydrogen-containing silicone oil (RH-H512) with a hydrogen content of 1.2%. After stirring evenly, 30ppm of platinum catalyst relative to the total mass was added. Under nitrogen protection, the temperature was slowly raised to 85℃ and the reaction was maintained for 6 hours. After the reaction was completed, the material was cooled and discharged to obtain the modified urea-formaldehyde resin.

[0036] Preparation Example 2-4, modified urea-formaldehyde resin, differs from Preparation Example 2-1 in that the full amount of 2-aminoethyl acrylate is added during the second addition of urea.

[0037] Preparation Example 2-5, modified urea-formaldehyde resin, differs from Preparation Example 2-1 in that the hydrosilylation step is not performed, that is, the alkenyl urea-formaldehyde resin prepolymer is used as the final modified urea-formaldehyde resin product.

[0038] Preparation Example 2-6, urea-formaldehyde resin, differs from Preparation Example 2-1 in that the hydrosilylation step is not performed, and 2-aminoethyl acrylate is not added during the synthesis of the urea-formaldehyde resin. The specific preparation method is as follows: 7.5 mol of formaldehyde was dissolved in an appropriate amount of water to prepare a 37% (w / w) formaldehyde aqueous solution, which was then transferred to a three-necked flask equipped with a condenser, stirrer, and thermometer. The temperature was raised to 45°C, and the pH was adjusted to 8.5 using a 10% sodium hydroxide aqueous solution. 2.5 mol of urea was added, stirred until dissolved, and then the temperature was raised to 85°C. The reaction was maintained at this temperature for 30 minutes, until the reaction solution became milky white and turbid. The pH was then adjusted to 4.5 using a 20% ammonium chloride aqueous solution, and another 2.0 mol of urea was added. The reaction was continued at 85°C until the viscosity reached the required level (150-200 mPa·s). The remaining 0.5 mol of urea was added, and the temperature was lowered to 68°C for 25 minutes. After the reaction was completed, the temperature was lowered to 40°C, and the pH was adjusted to approximately 7.5 using a 10% sodium hydroxide aqueous solution. The product was then discharged after cooling to below 35°C. Example

[0039] Example 1: A high-strength flame-retardant plywood was prepared by the following steps: Weigh 1000g of the silicone-modified urea-formaldehyde resin from Preparation Example 2-1, 65g of the modified DOPO from Preparation Example 1-1, and 12g of the silicone-modified polyether surfactant (BYK-346). Add these to an appropriate amount of deionized water while stirring, mix thoroughly, and bring the volume to a total of 10kg. Stir until homogeneous to prepare a flame-retardant liquid. Take a eucalyptus veneer measuring 400mm × 400mm × 3.0mm and place it in an impregnation tank. Evacuate the tank to a pressure of -0.09MPa and maintain this pressure for 1.5 hours. While maintaining the vacuum, inject the flame-retardant liquid into the tank until the veneer is completely submerged. Then, pressurize the tank with compressed air to 1.2MPa and maintain the pressure for 45 minutes. After depressurization, remove the veneer, drain excess liquid from the surface, and dry it in a 70℃ oven until constant weight to obtain the flame-retardant veneer.

[0040] Prepare E0 grade urea-formaldehyde resin adhesive (52% solids content), add 1.2% ammonium chloride (by weight of urea-formaldehyde resin) as a curing agent, and stir evenly to obtain the adhesive. Use a roller coater to evenly apply the adhesive to the surface of the flame-retardant veneer (double-sided application rate: 230g / m²). 2 The veneers are stacked after being coated with adhesive, with five veneers forming one board blank. The board blanks are pre-cured by cold pressing at room temperature (25℃) and 0.8MPa pressure for 1.5 hours. Then, the board blanks are transferred to a hot press and hot-pressed at 110℃ and 1.2MPa pressure for 10 minutes. After hot pressing, the boards are unloaded, cooled, trimmed, and sanded to obtain the high-strength flame-retardant plywood.

[0041] Example 2: A high-strength flame-retardant plywood was prepared by the following steps: Weigh 800g of the silicone-modified urea-formaldehyde resin from Preparation Example 2-2, 50g of the modified DOPO from Preparation Example 1-2, and 5g of the silicone-modified polyether surfactant (BYK-348). Add these to an appropriate amount of deionized water while stirring, mix thoroughly, and bring the volume to a total of 10kg. Stir until homogeneous to prepare a flame-retardant liquid. Take a eucalyptus veneer measuring 400mm × 400mm × 3.0mm and place it in an impregnation tank. Evacuate the tank to a pressure of -0.08MPa and maintain this pressure for 2 hours. While maintaining the vacuum, inject the flame-retardant liquid into the tank until the veneer is completely submerged. Then, pressurize the tank with compressed air to 1.1MPa and maintain the pressure for 60 minutes. After depressurization, remove the veneer, drain excess liquid from the surface, and dry it in a 70℃ oven until constant weight to obtain the flame-retardant veneer.

[0042] Prepare E0 grade urea-formaldehyde resin adhesive (52% solids content), add 1.0% ammonium chloride (by weight of urea-formaldehyde resin) as a curing agent, and stir evenly to obtain the adhesive. Use a roller coater to evenly apply the adhesive to the surface of the flame-retardant veneer (double-sided application rate: 230 g / m²). 2 The veneers are stacked after being coated with adhesive, with five flame-retardant veneers forming one board blank. The board blanks are then cold-pressed for 2 hours at room temperature (25℃) and 0.6MPa pressure for pre-curing. The board blanks are then transferred to a hot press and hot-pressed at 105℃ and 1.0MPa pressure for 15 minutes. After hot pressing, the boards are removed, cooled, trimmed, and sanded to obtain the high-strength flame-retardant plywood.

[0043] Example 3: A high-strength flame-retardant plywood was prepared by the following steps: Weigh 1500g of the silicone-modified urea-formaldehyde resin from Preparation Examples 2-3, 100g of the modified DOPO from Preparation Examples 1-3, and 20g of the silicone-modified polyether surfactant (BYK-346). Add these to an appropriate amount of deionized water while stirring, mix thoroughly, and bring the volume to a total of 10kg. Stir until homogeneous to prepare a flame-retardant liquid. Take a eucalyptus veneer measuring 400mm × 400mm × 3.0mm and place it in an impregnation tank. Evacuate the tank to a pressure of -0.1MPa and maintain this pressure for 1 hour. While maintaining the vacuum, inject the flame-retardant liquid into the tank until the veneer is completely submerged. Then, pressurize the tank with compressed air to 1.3MPa and maintain the pressure for 35 minutes. After depressurization, remove the veneer, drain excess liquid from the surface, and dry it in a 75℃ oven until constant weight to obtain the flame-retardant veneer.

[0044] Prepare E0 grade urea-formaldehyde resin adhesive (52% solids content), add 1.2% ammonium chloride (by weight of urea-formaldehyde resin) as a curing agent, and stir evenly to obtain the adhesive. Use a roller coater to evenly apply the adhesive to the surface of the flame-retardant veneer (double-sided application rate: 230g / m²). 2The veneers are stacked after being coated with adhesive, with five veneers forming one board blank. The board blanks are pre-cured by cold pressing at room temperature (25℃) and 0.8MPa pressure for 1.5 hours. Then, the board blanks are transferred to a hot press and hot-pressed at 110℃ and 1.2MPa pressure for 10 minutes. After hot pressing, the boards are unloaded, cooled, trimmed, and sanded to obtain the high-strength flame-retardant plywood.

[0045] Example 4, a high-strength flame-retardant plywood, differs from Example 1 in that an equal amount of the modified urea-formaldehyde resin used in Preparation Examples 2-4 is used to replace the modified urea-formaldehyde resin used in Preparation Example 2-1.

[0046] Example 5, a high-strength flame-retardant plywood, differs from Example 1 in that an equal amount of the modified urea-formaldehyde resin used in Preparation Examples 2-5 is used to replace the modified urea-formaldehyde resin used in Preparation Example 2-1.

[0047] Example 6, a high-strength flame-retardant plywood, differs from Example 1 in that an equal amount of urea-formaldehyde resin from Preparation Examples 2-6 is used to replace the modified urea-formaldehyde resin from Preparation Example 2-1. Comparative Example

[0048] Comparative Example 1, a high-strength flame-retardant plywood, differs from Example 6 in that the modified DOPO of Preparation Example 1-1 is replaced with an equal amount of the modified DOPO of Preparation Example 1-4.

[0049] Comparative Example 2, a high-strength flame-retardant plywood, differs from Example 6 in that an equal amount of unmodified DOPO is used to replace the modified DOPO in Preparation Example 1-1.

[0050] Performance testing

[0051] Experiment 1: Flame retardant performance test The test was conducted according to GB / T 2406.2-2009, "Determination of Combustion Behavior by Oxygen Index Method for Plastics - Part 2: Room Temperature Test". 100mm × 10mm specimens (thickness equal to the plywood thickness) were cut from the plywood of each example and comparative example. The specimen was vertically fixed in the combustion chamber of the oxygen indexer, and a nitrogen-oxygen mixture (flow rate 10 L / min) was introduced. The top of the specimen was ignited, and the minimum oxygen concentration (accurate to 0.1%) required for a combustion damage length of 50mm was recorded. The test was repeated until a stable LOI value was obtained, which was recorded as the initial LOI. Five parallel samples were tested in each group, and the average value was taken.

[0052] Test 2: Flame Retardant Durability Test Several specimens with dimensions of (20±1) mm × (20±1) mm × (plywood thickness) mm were cut from the plywood obtained in the examples and comparative examples. Five parallel specimens were prepared for each group. Before testing, the specimens were dried to constant weight in an oven at (50±2)℃, and their initial oxygen index (LOI) was tested (method as in Experiment 1). The specimens were completely immersed in a beaker containing deionized water, with the water volume being at least 20 times the total volume of the specimens. The beakers were placed in a constant temperature water bath and heated to (65±2)℃, maintaining this temperature for (24±0.5) hours. The specimens were removed, their surface moisture was absorbed with filter paper, and then they were dried again to constant weight in an oven at (50±2)℃. The oxygen index (LOI) of the specimens after boiling in water was tested again. The flame retardant retention rate (%) was calculated as (boiling oxygen index (LOI) / initial oxygen index (LOI)) × 100%.

[0053] Experiment 3: Compressive strength test along the grain The test was conducted according to the specifications in Section 4.25, "Determination of Compressive Strength Along the Grain," of GB / T 17657-2022, "Determination of Impact Toughness of Wood-based Panels and Decorative Wood-based Panels." A specimen with dimensions of length l = (23±1) mm, width b = (15±0.5) mm, and thickness t = (15±0.5) mm was cut from the plywood, with the length direction aligned with the grain. The specimen was placed on a support, and a constant loading rate was applied to induce complete failure within (60±30) s. The maximum load F at specimen failure was recorded. max The accuracy is to 1% of the load value. The test longitudinal compressive strength is calculated using the following formula: Compressive strength = F max / (b×t)

[0054] Experiment 4: Impact Toughness Test The test was conducted according to Section 4.22, "Determination of Impact Toughness," of GB / T 17657-2022, "Determination of Impact Toughness of Wood-based Panels and Decorative Wood-based Panels." A specimen with dimensions of length l = (300±2) mm, width b = (20±1) mm, and thickness t = (15±1) mm was cut from the plywood. The specimen was placed stably and symmetrically on the testing machine support. The radius of curvature of the support and the pendulum punch end was 15 mm, the distance between the two supports was 240 mm, and the support height should be greater than 20 mm. The test surface of the specimen was positioned facing the direction of the impact force, and the impact force acted on the middle of the specimen. The specimen broke in one strike. The energy Q consumed when the specimen broke in one strike was recorded, accurate to 1 J.

[0055] Experiment 5: Hygroscopicity Test Several specimens with dimensions of 20mm × 20mm × 2.6mm (veneer thickness) were cut from the flame-retardant veneers obtained in the examples or comparative examples, with 5 parallel specimens in each group. The specimens were first dried to constant weight in an oven at (50±2)℃, and their initial mass M0 was recorded. The dried specimens were then placed in an SDH low-temperature humidity chamber. After being stored at (26.7±0.1)℃ and (92.5±2.5)% relative humidity for 66 hours, the humidity decreased to (80.5±2.5)%. After 114 hours of testing, the humidity increased to (85.5±2.5)% until the end of the experiment (total duration 282 hours). The specimens were removed, and their mass M1 was quickly measured using a balance with an accuracy of 0.001g. The hygroscopicity of the wood was assessed based on the percentage increase in mass before and after the change.

[0056] Quality improvement rate = (M1-M0) / M0×100%.

[0057] Table 1. Test Results

[0058] Analysis of experimental results: Compared to Example 1, Example 4 (with one-time addition of 2-aminoethyl acrylate) showed poorer performance in impact toughness and hygroscopicity. This indicates that the stepwise addition process of 2-aminoethyl acrylate has a positive impact on ensuring the toughness of the board and the hydrophobic effect of the final resin. This may be because the secondary addition of 2-aminoethyl acrylate has a sealing effect, which helps to improve the grafting rate of the modified urea-formaldehyde resin and the hydrogen-containing silicone oil, thereby affecting the subsequent grafting rate of organosilicon segments and the deformability of the final cured network.

[0059] Furthermore, Example 5 (without hydrosilylation modification) showed significantly poorer performance in impact toughness and hygroscopicity. This indicates that the introduction of organosilicon segments is crucial for improving toughness and reducing hygroscopicity. This is because the lack of the toughening effect of flexible organosilicon segments and their inherent hydrophobic properties causes the resin system to revert to a rigid traditional urea-formaldehyde resin network, leading to increased brittleness; simultaneously, it fails to effectively improve hydrophobicity.

[0060] Example 6 (using conventional urea-formaldehyde resin) performed the worst in terms of impact toughness and hygroscopicity. This indicates that both the introduction of 2-aminoethyl acrylate and organosilicon modification play a decisive role in improving the overall performance. Conventional UF resin is too rigid and brittle, and its strong hydrophilicity results in poor impact resistance and poor dimensional stability of the veneers treated with it in humid and hot environments.

[0061] Compared to Example 6, Comparative Example 1 (using diamino DOPO that has not reacted with 4-carboxybenzaldehyde) showed extremely poor performance in flame retardant durability, while its compressive strength, impact toughness, and hygroscopicity showed no significant improvement. This indicates that the carboxylation modification of DOPO plays a crucial role in improving the durability (resistance to leaching) of flame retardants and also has a significant effect on improving the mechanical properties of wood. This is because diamino DOPO lacks carboxyl functional groups that react with the hydroxyl groups of wood and still mainly exists in the pores of wood through physical processes. Therefore, it is largely lost during boiling, leading to a sharp decline in flame retardant performance. Furthermore, it cannot cross-link with the active hydroxyl groups of wood, which is detrimental to its ability to improve wood strength and toughness.

[0062] Comparative Example 2 (using unmodified DOPO) performed the worst in terms of flame retardancy, compressive strength, and impact toughness, which also proves the necessity of carboxyl grafting. Pure DOPO has a small molecular weight, which makes it easy to deposit and crystallize in the pores of wood, resulting in uneven distribution. It also cannot form effective chemical bonds with wood and is more prone to loss.

[0063] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high-strength fire-retardant plywood comprising fire-retardant veneers sequentially stacked by a bonding resin, characterized in that, The flame-retardant veneer is obtained by impregnating the veneer with a flame-retardant liquid. The flame-retardant liquid comprises the following components in mass concentrations: 8-15 wt% silicone-modified urea-formaldehyde resin, 5-10 wt% modified DOPO, and 0.5-2 wt% additives. The raw materials for the modified DOPO include DOPO, 4,4'-diaminobenzophenone, and 4-carboxybenzaldehyde in a molar ratio of 1.1-1.3:1:2-2.

2. The DOPO and 4,4'-diaminobenzophenone undergo nucleophilic addition to obtain diaminoDOPO, and the diaminoDOPO is then reacted with 4-carboxybenzaldehyde via a Schiff base reaction to obtain the modified DOPO. The organosilicon-modified urea-formaldehyde resin is prepared by hydrosilylation of an alkenyl urea-formaldehyde resin prepolymer and a hydrogen-containing silicone oil in a mass ratio of 10–20:1; the alkenyl urea-formaldehyde resin prepolymer is prepared by polycondensation of urea, formaldehyde, and 2-aminoethyl acrylate in a molar ratio of 1:1.2–1.8:0.3–0.5; the preparation method of the alkenyl urea-formaldehyde resin prepolymer is as follows: Formaldehyde is dissolved in water by stirring to prepare a formaldehyde aqueous solution. The solution is heated to 40–50°C, and the pH is adjusted to 8.0–9.

0. 50%–60% of the total urea is added, and the solution is stirred and heated to 80–90°C for further reaction. The pH is adjusted to 4–5, and 20%–40% of the total urea and part of 2-aminoethyl acrylate are added. The solution is then kept at 80–90°C for further reaction. The remaining urea and 2-aminoethyl acrylate are added, and the solution is cooled to 65–70°C for 20–30 minutes. After the reaction is complete, the solution is cooled to below 50°C, and the pH is adjusted to 7.0–8.

0. The solution is then cooled and discharged to obtain the final product.

2. The high-strength fire-retardant plywood according to claim 1, characterized by, The method for preparing the modified DOPO is as follows: 4,4'-Diaminobenzophenone was dissolved in a solvent by stirring, and the temperature was raised to 80-90°C. DOPO solvent was added dropwise under nitrogen protection. After the addition was complete, an acid-binding agent was added, and the reaction was carried out at 100-110°C for 4-6 hours. After the reaction was completed, the reaction solution was cooled to room temperature, deionized water was added to precipitate the solid product, and the product was washed with water and dried to obtain diaminoDOPO. DiaminoDOPO and 4-carboxybenzaldehyde were added to a solvent and stirred until homogeneous. An acidic catalyst was added, and the temperature was raised to 120–140 °C. The reaction was carried out for 6–8 h. After the reaction was completed, the reaction solution was distilled under reduced pressure to remove part of the solvent. The remaining reaction solution was mixed with ice water and stirred to precipitate a solid. After filtration, washing, and drying, modified DOPO was obtained.

3. The high-strength fire-retardant plywood according to claim 1, wherein The 2-aminoethyl acrylate portion comprises 80-90 wt% of the total.

4. The high-strength fire-retardant plywood according to claim 1, wherein The hydrogen content of the hydrogen-containing silicone oil is 0.1% to 1.2%.

5. The high-strength flame-retardant plywood according to claim 1, characterized in that, The impregnation process involves placing the veneer under a vacuum of -0.08 to -0.1 MPa for 1 to 2 hours, then injecting flame retardant liquid to immerse the veneer, pressurizing it to 1.1 to 1.3 MPa for 0.5 to 1 hour, depressurizing, and drying it to constant weight.

6. The high-strength flame-retardant plywood according to claim 1, characterized in that, The additive is an organosilicon-modified polyether surfactant.

7. The method for preparing high-strength flame-retardant plywood according to any one of claims 1 to 6, characterized in that, The modified veneer is uniformly glued with resin adhesive, and the glued veneers are stacked together to form a slab blank, which is then subjected to cold pressing and hot pressing in sequence. The cold pressing temperature is 20-30℃, the pressure is 0.6-1.0MPa, and the time is 1-2h, while the hot pressing temperature is 100-120℃, the pressure is 0.8-1.5MPa, and the time is 5-15min.

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