Modified pMDI adhesive, preparation method and application of modified pMDI adhesive in curing forming of wood board

Abstract

The invention relates to the technical field of adhesives, in particular to a modified pMDI adhesive, a preparation method and application of the modified pMDI adhesive in curing forming of wood planes.The preparation method comprises the following steps that pMDI is heated in the nitrogen atmosphere, a modified catalyst is dropwise added into the heated pMDI, a reaction system is obtained after dropwise adding is completed, the reaction system is kept at the constant temperature, and the modified pMDI adhesive is obtained; adding the modified lignin into the cooled reaction system in batches, then adding phosphoric acid and a nano silicon dioxide dispersion liquid, and stirring to obtain the modified pMDI adhesive. According to the method, curing of the modified pMDI adhesive is accelerated through steam preheating, the activity of the modified catalyst can be activated in the high-temperature and high-humidity environment of steam preheating, the cross-linking reaction rate of the modified pMDI adhesive is increased, the production efficiency is improved, and the situation that the plate is too high in brittleness and buckling deformation is generated is avoided; due to the excellent water resistance of the modified pMDI adhesive, the influence of short-time moisture of steam preheating can be remarkably reduced, and the interface bonding force between the adhesive and the wood fibers is prevented from being reduced.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, specifically to a modified pMDI adhesive, its preparation method, and its application in the curing and molding of wood-based panels. Background Technology

[0002] The production of engineered wood products (fiberboard, particleboard, and oriented strand board) mainly includes the following processes: wood unit preparation, drying, gluing, assembly, hot pressing, and post-processing. Among all these processes, hot pressing is the process of fully curing the adhesive under certain time, pressure, and temperature conditions to firmly bond smaller wood units together. It is also the process of realizing the "assembly" of engineered wood products from "separate" to "assembled". The hot pressing process is the most energy-intensive process in engineered wood product production and is the key and core factor determining the production efficiency of engineered wood products. The efficiency of hot pressing mainly depends on the curing rate of the adhesive used. Therefore, the choice of adhesive is also one of the factors affecting the hot pressing process.

[0003] Steam preheating technology for accelerating the curing and molding of wood-based panels is a temperature and humidity coupled preheating technology developed based on the heat and moisture transfer characteristics of wood materials and the curing reaction mechanism of adhesives. It addresses the efficiency and quality bottlenecks of traditional panel curing and molding processes. The core of this technology is to achieve rapid and uniform temperature and humidity control inside the panel through steam preheating, providing suitable thermodynamic conditions for the cross-linking and curing of adhesives. At the same time, it optimizes the interfacial bonding state of wood fibers, ultimately achieving the goal of shortening the curing cycle and improving the molding quality.

[0004] However, the short-term moisture contact caused by steam preheating can still easily reduce the interfacial bonding force between the adhesive and the wood fiber. In addition, the conventional pMDI curing rate is relatively slow and cannot make full use of the thermodynamic conditions of steam preheating to achieve rapid curing. As a result, it is difficult to balance the board forming efficiency and the interfacial bonding quality. Therefore, in response to the problems mentioned in the background art, those skilled in the art have proposed a modified pMDI adhesive, a preparation method and its application in the curing and forming of wood-based panels. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a modified pMDI adhesive, a preparation method thereof, and its application in the curing and molding of wood-based panels, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a modified pMDI adhesive includes the following steps: S1. Under a nitrogen atmosphere, heat pMDI to 40-45℃, start stirring, and drop the modified catalyst into the heated pMDI. After the addition is complete, keep the temperature at 40-45℃ and stir for 1-1.5h to obtain the reaction system. S2. Keep the reaction system at a constant temperature of 40-45℃, stir for 1-1.5h, and then lower the temperature of the reaction system to 25-30℃. S3. Add modified lignin to the cooled reaction system in 3-4 portions, with an interval of 10-15 minutes between each addition. Then add phosphoric acid and nano silica dispersion and stir for 20-30 minutes to obtain modified pMDI adhesive. The modified catalyst in step S1 is prepared through the following steps; S11. Add dibutyltin dilaurate to anhydrous ethanol and stir until homogeneous to obtain a mixed system. Add glacial acetic acid dropwise to adjust the pH of the mixed system to 5-6. S12. Heat the adjusted mixture to 40-45℃, add the coupling agent modification liquid and stir at a constant temperature for 1-1.5h to obtain the precursor catalyst. S13. The precursor catalyst is vacuum dried at 60-65℃ for 1-1.5h to remove anhydrous ethanol, and then cooled to room temperature to obtain the modified catalyst.

[0007] Furthermore, the modified lignin in step S3 is obtained through the following steps: S21. Add lignin to anhydrous ethanol and disperse evenly, then add sodium hydroxide and stir until completely dissolved. React at room temperature for 1-2 hours to obtain the lignin system. S22. Heat the lignin system to 50-60℃ and add propylene oxide dropwise to obtain the precursor lignin. S23. Dry the precursor lignin for 2-3 hours to remove anhydrous ethanol and excess moisture, grind it and pass it through a 100-200 mesh filter to obtain modified lignin.

[0008] Furthermore, the coupling agent modified liquid in step S12 is prepared through the following steps: Anhydrous ethanol and ethylene glycol were stirred at 20-25℃ for 15-20 min to obtain a mixed solvent. γ-aminopropyltriethoxysilane coupling agent and titanate coupling agent were added, and the mixture was ultrasonically dispersed for 15-20 min to obtain a coupling agent modified solution.

[0009] Furthermore, in steps S1 and S3, the mass ratio of pMDI to the modified catalyst, modified lignin, phosphoric acid, and nano-silica is 100:(1-3):(5-10):(0.5-1):(2-5).

[0010] Furthermore, in steps S11 and S12, the mass ratio of dibutyltin dilaurate to anhydrous ethanol, glacial acetic acid, and coupling agent modified solution is 1:(10-20):(0.05-0.1):(2-4).

[0011] Furthermore, in steps S21 and S22, the mass ratio of lignin to anhydrous ethanol, sodium hydroxide, and propylene oxide is 1:(15-25):(0.05-0.1):(0.1-0.2).

[0012] Furthermore, in the preparation of the coupling agent modified liquid, the mass ratio of anhydrous ethanol to ethylene glycol is (1.5-2.5):1, the mass ratio of γ-aminopropyltriethoxysilane to titanate coupling agent is (1.5-2.5):1, and the mass ratio of the mixed solvent to the total mass of the two coupling agents is 1:(0.04-0.08).

[0013] Furthermore, in step S1, the dropping rate of the modified catalyst is 0.5-2.0 g / min, in step S11 the dropping rate of glacial acetic acid is 0.1-0.3 g / min, and in step S22 the dropping rate of propylene oxide is 0.3-1.0 g / min.

[0014] Furthermore, the modified pMDI adhesive is prepared according to the preparation method described above.

[0015] Furthermore, the modified pMDI adhesive is used in the accelerated curing and molding of wood-based panels through steam pretreatment.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention accelerates the curing of modified pMDI adhesive through steam preheating. The high temperature and humidity environment of steam preheating can activate the activity of the modified catalyst, increase the crosslinking reaction rate of the modified pMDI adhesive, improve production efficiency, soften wood fibers, alleviate the internal stress of the pMDI cured layer, and prevent excessive brittleness and warping deformation of the board. The excellent water resistance of the modified pMDI adhesive can significantly reduce the short-term moisture impact of steam preheating, and prevent the decrease in the interfacial bonding force between the adhesive and wood fibers. At the same time, steam preheating can reduce the difference in moisture content between the surface layer and the core layer of the board, and avoid the concentration of internal stress induced by the difference in drying shrinkage during subsequent hot pressing, thereby significantly improving the dimensional stability and interfacial bonding quality of the board.

[0017] 2. The synergistic crosslinking effect of modified lignin and pMDI in this invention effectively reduces the brittleness of the cured layer, improves the flexibility and crack resistance of the adhesive layer, and, combined with the softening effect of steam preheating on wood fibers, further mitigates the internal stress concentration during the board forming process, avoids defects such as warping and deformation due to excessive brittleness, and significantly improves the structural flatness and mechanical stability of wood boards. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the preparation process of the modified pMDI adhesive of the present invention; Figure 2 This is a schematic diagram of the preparation process of the modified catalyst of the present invention; Figure 3 This is a schematic diagram of the preparation process of the modified lignin of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Please see Figures 1-3 The present invention provides a technical solution: Example 1: A method for preparing a modified pMDI adhesive includes the following steps: Preparation of coupling agent modified solution: 15g of anhydrous ethanol and 10g of ethylene glycol were stirred at 20℃ for 15min, then 0.6g of γ-aminopropyltriethoxysilane coupling agent and 0.4g of titanate coupling agent were added, and the mixture was ultrasonically dispersed for 15min to obtain the coupling agent modified solution.

[0021] Preparation of modified catalysts: Add 1g of dibutyltin dilaurate to 10g of anhydrous ethanol and stir until homogeneous. Add 0.05g of glacial acetic acid dropwise at 0.1g / min to adjust the pH to 5. Heat to 40℃, add 2g of coupling agent modification solution and stir at constant temperature for 1h. Dry under vacuum at 60℃ for 1h and cool to obtain the modified catalyst.

[0022] Preparation of modified lignin: 10g of lignin was dispersed in 150g of anhydrous ethanol, 0.5g of sodium hydroxide was added and stirred to dissolve, and the reaction was carried out at room temperature for 1h. The temperature was raised to 50℃, and 1g of propylene oxide was added dropwise at 0.3g / min. The mixture was stirred for 2h, dried at 105℃ for 2h, and ground through a 100-mesh filter to obtain modified lignin.

[0023] Preparation of modified pMDI adhesive: 100g of pMDI was heated to 40℃ under a nitrogen atmosphere, and 1g of modified catalyst was added dropwise at 0.5g / min. The mixture was stirred at a constant temperature for 1h, and then stirred at 40℃ for another 1h. The temperature was then lowered to 25℃, and 5g of modified lignin was added in three portions at 10min intervals. 0.5g of phosphoric acid and 2g of nano-silica dispersion were added, and the mixture was stirred slowly and evenly for 20min to obtain the modified pMDI adhesive.

[0024] Example 2: A method for preparing a modified pMDI adhesive includes the following steps: Preparation of coupling agent modified solution: 18g of anhydrous ethanol and 10g of ethylene glycol were stirred at 22℃ for 17 min, then 0.75g of γ-aminopropyltriethoxysilane coupling agent and 0.75g of titanate coupling agent were added, and the mixture was ultrasonically dispersed for 17 min to obtain the coupling agent modified solution.

[0025] Preparation of modified catalysts: Add 1g of dibutyltin dilaurate to 15g of anhydrous ethanol and stir until homogeneous. Add 0.07g of glacial acetic acid dropwise at 0.2g / min to adjust the pH to 5.4. Heat to 42℃, add 3g of coupling agent modification solution and stir at constant temperature for 1.2h. Dry under vacuum at 62℃ for 1.2h and cool to obtain the modified catalyst.

[0026] Preparation of modified lignin: 10g of lignin was dispersed in 200g of anhydrous ethanol, and 0.7g of sodium hydroxide was added and stirred to dissolve. The reaction was carried out at room temperature for 1.5h, then heated to 55℃, and 1.5g of propylene oxide was added dropwise at 0.6g / min. The mixture was stirred for 2.5h, dried at 105℃ for 2.5h, and ground through a 150-mesh filter to obtain modified lignin.

[0027] Preparation of modified pMDI adhesive: 100g of pMDI was heated to 42℃ under a nitrogen atmosphere, and 2g of modified catalyst was added dropwise at 1.2g / min. The mixture was stirred at a constant temperature for 1.2h, and then stirred at 42℃ for another 1.2h. The mixture was then cooled to 27℃, and 7g of modified lignin was added in three portions at 12min intervals. 0.7g of phosphoric acid and 3.5g of nano-silica dispersion were added, and the mixture was stirred slowly and evenly for 25min to obtain the modified pMDI adhesive.

[0028] Example 3: A method for preparing a modified pMDI adhesive includes the following steps: Preparation of coupling agent modified solution: 22g of anhydrous ethanol and 10g of ethylene glycol were stirred at 24℃ for 19 min, 1g of γ-aminopropyltriethoxysilane coupling agent and 1g of titanate coupling agent were added, and the mixture was ultrasonically dispersed for 19 min to obtain a coupling agent modified solution.

[0029] Preparation of modified catalysts: Add 1g of dibutyltin dilaurate to 18g of anhydrous ethanol and stir until homogeneous. Add 0.09g of glacial acetic acid dropwise at 0.25g / min to adjust the pH to 5.7. Heat to 44℃, add 3.5g of coupling agent modification solution and stir at constant temperature for 1.4h. Dry under vacuum at 64℃ for 1.4h and cool to obtain the modified catalyst.

[0030] Preparation of modified lignin: 10g of lignin was dispersed in 230g of anhydrous ethanol, and 0.9g of sodium hydroxide was added and stirred to dissolve. The reaction was carried out at room temperature for 1.8h, then heated to 58℃, and 1.8g of propylene oxide was added dropwise at 0.8g / min. The mixture was stirred for 2.8h, dried at 105℃ for 2.8h, and ground through a 180-mesh filter to obtain modified lignin.

[0031] Preparation of modified pMDI adhesive: 100g of pMDI was heated to 44℃ under a nitrogen atmosphere, and 2.5g of modified catalyst was added dropwise at 1.8g / min. The mixture was stirred at a constant temperature for 1.4h, and then stirred at 44℃ for another 1.4h. The mixture was then cooled to 29℃, and 9g of modified lignin was added in four portions at 14min intervals. 0.9g of phosphoric acid and 4.5g of nano-silica dispersion were added, and the mixture was stirred slowly and evenly for 28min to obtain the modified pMDI adhesive.

[0032] Example 4: A method for preparing a modified pMDI adhesive includes the following steps: Preparation of coupling agent modified solution: 25g of anhydrous ethanol and 10g of ethylene glycol were stirred at 25℃ for 20min. Then, 1.2g of γ-aminopropyltriethoxysilane coupling agent and 1.2g of titanate coupling agent were added and ultrasonically dispersed for 20min to obtain the coupling agent modified solution.

[0033] Preparation of modified catalysts: Add 1g of dibutyltin dilaurate to 20g of anhydrous ethanol and stir until homogeneous. Add 0.1g of glacial acetic acid dropwise at 0.3g / min to adjust the pH to 6. Heat to 45℃, add 4g of coupling agent modification solution and stir at constant temperature for 1.5h. Dry under vacuum at 65℃ for 1.5h and cool to obtain the modified catalyst.

[0034] Preparation of modified lignin: 10g of lignin was dispersed in 250g of anhydrous ethanol, 1g of sodium hydroxide was added and stirred to dissolve, and the reaction was carried out at room temperature for 2h. The temperature was raised to 60℃, and 2g of propylene oxide was added dropwise at 1.0g / min. The mixture was stirred for 3h, dried at 105℃ for 3h, and ground through a 200-mesh filter to obtain modified lignin.

[0035] Preparation of modified pMDI adhesive: 100g of pMDI was heated to 45℃ under a nitrogen atmosphere, and 3g of modified catalyst was added dropwise at 2.0g / min. The mixture was stirred at a constant temperature for 1.5h, and then stirred at 45℃ for another 1.5h. The mixture was then cooled to 30℃, and 10g of modified lignin was added in four portions at 15min intervals. 1g of phosphoric acid and 5g of nano-silica dispersion were added, and the mixture was stirred slowly and evenly for 30min to obtain the modified pMDI adhesive.

[0036] Comparative Example 1 Compared to Example 1, Comparative Example 1 replaced the modified catalyst with dibutyltin dilaurate catalyst, and the remaining steps were exactly the same as in Example 1.

[0037] Comparative Example 2 Comparative Example 2 differs from Example 1 in that the modified lignin is replaced with conventional lignin, while the remaining steps are exactly the same as in Example 1.

[0038] Comparative Example 3 Compared to Example 1, Comparative Example 3 replaced the modified pMDI adhesive with a polymeric diphenylmethane diisocyanate adhesive.

[0039] The dried eucalyptus shavings for both the surface and core layers were placed on a drum-type glue applicator for gluing. Modified pMDI adhesives prepared in Examples 1-4 and Comparative Examples 1-3 were applied using an atomization method. The application amounts of modified pMDI adhesive for the surface and core layers were 6% and 5% respectively (based on the mass percentage of oven-dried shavings). The glued surface and core shavings were then laid out in three layers with a directional orientation, using a three-layer structure. The board density was controlled at 650 kg / m³. 3 The pre-compression pressure is 0.5MPa and the pre-compression time is 30s to obtain a slab. The slab is then preheated using a steam penetration method. Specifically, the steam generator produces saturated or superheated steam, which is directed vertically upwards towards the slab, penetrating it from top to bottom for effective preheating. This allows the saturated or superheated steam to pass rapidly through the slab. During this steam-penetration preheating process, the steam generator pressure is controlled at 3 bar, the preheating time is 4 seconds, the steam flow rate is 85 kg / h, and the steam temperature is controlled at 120°C. The steam generator uses a mixture of steam and air, making it easy to control the temperature between the core and surface layers of the slab. A gradient hot-pressing process of "pressurization-depressurization-holding-depressurization" is employed, with a hot-pressing temperature of 180°C, a hot-pressing time of 3-6 minutes, and a hot-pressing pressure of 2 MPa. The preheated board blank was quickly fed into a multi-functional hot press for hot pressing. In the first stage, high pressure compressed the board blank to the target thickness within 5 seconds. In the second stage, the pressure was appropriately reduced to allow internal steam to escape smoothly. In the third stage, pressure was maintained to complete the final curing and shaping. In the fourth stage, the pressure was slowly released, and the board was removed and allowed to cool naturally to room temperature, resulting in a wood-based panel with dimensions of 500mm × 500mm × 18mm. According to the national standard GB / T17657-2022 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels", the static bending strength, modulus of elasticity, and internal bond strength of seven wood-based panels prepared with different modified pMDI adhesives were tested. The specific test results are shown in Table 1 below. Table 1 Mechanical properties of wood-based panels prepared in different embodiments and comparative examples As can be seen from the data in Table 1, there is a significant difference in the hot-pressing curing time between modified and unmodified pMDI adhesives. The improvement in curing rate is particularly noticeable. Compared with the conventional unmodified pMDI adhesive in Example 3, under the same hot-pressing temperature and pressure conditions, the mechanical properties of the board are relatively low at hot-pressing times of 3 min and 4 min, with static bending strength, elastic modulus, and internal bond strength being the lowest at each time point. The properties reach their optimal levels at a hot-pressing time of 5 min, indicating that its curing rate is insufficient during short hot-pressing times, and a longer hot-pressing time is required to achieve the desired performance. The modified pMDI adhesives of Examples 1-4 and the modified systems of Comparative Examples 1-2 all exhibited high mechanical properties at 3 min and 4 min hot pressing, representing their optimal values ​​for their respective applications. These properties were significantly better than those of Comparative Example 3 at the same time point, indicating that the curing rate of the modified adhesive was significantly improved, allowing for full curing in a short time. Furthermore, the mechanical properties of the molded boards were excellent. At the same time, the performance of the modified systems decreased slightly with prolonged hot pressing time, further confirming that the modified adhesives do not require long-term hot pressing and can achieve full curing in a short time, meeting the needs of efficient and low-carbon production in the wood-based panel industry.

[0040] The adhesive was tested using Example 1 and Comparative Example 3: Curing peak temperature test: Differential scanning calorimeter (DSC) was used to place the adhesive sample in the test crucible and perform thermal scanning at a constant heating rate to record the heat flow changes during the curing reaction of the adhesive and determine the temperature (°C) at which the curing reaction reaches its peak. Curing rate test: Under the same temperature and humidity environment as the board molding (120℃ steam preheating + 180℃ hot pressing), the time from the start of cross-linking to complete curing of the adhesive was tested. (min), combined with the thickness of the cured layer (mm), calculated according to the formula: in, Curing rate (mm / min) The hot-pressing efficiency coefficient is taken as 0.8; Strength retention test: Standard adhesive samples coated with adhesive and bonded together are placed in a boiling water bath for 24 hours. After boiling, they are removed and air-dried to constant weight, and their adhesive strength is tested. At the same time, the bonding strength of blank bonded samples that had not undergone boiling water treatment was tested. Calculate according to the formula: in, The strength retention rate (%) was used to obtain the strength retention rate related to the water resistance of the adhesive. The specific test results are shown in Table 2 below: Table 2. Performance comparison between the modified pMDI adhesive in Example 1 and the pMDI adhesive in Comparative Example 3. As can be seen from the data in Table 2, the modified pMDI adhesive of Example 1 is superior to the conventional pMDI adhesive of Comparative Example 3 in all core performance aspects. The modified adhesive has a lower peak curing temperature, indicating that it requires a lower temperature to start the curing reaction and is more likely to undergo cross-linking reaction in a warm and humid environment with steam preheating. The curing rate is nearly doubled, which significantly shortens the curing cycle of the adhesive and can effectively improve the production efficiency of wood-based panels. The bond strength retention rate after 24 hours of boiling in water is 27.7% higher, which shows that the water resistance of the modified adhesive is significantly improved. The improvement of these three properties confirms the effectiveness of the design concept of this invention, which activates the curing reaction by modifying the catalyst and optimizes the adhesive layer structure by modifying lignin. The modified pMDI adhesive is not only suitable for the process conditions of steam preheating, but also solves the problem of slow curing rate of conventional pMDI adhesive, providing material support for the efficient and high-quality molding of wood-based panels.

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a modified pMDI adhesive, characterized in that, Includes the following steps: S1. Under a nitrogen atmosphere, heat pMDI to 40-45℃, start stirring, and drop the modified catalyst into the heated pMDI. After the addition is complete, keep the temperature at 40-45℃ and stir for 1-1.5h to obtain the reaction system. S2. Keep the reaction system at a constant temperature of 40-45℃, stir for 1-1.5h, and then lower the temperature of the reaction system to 25-30℃. S3. Add modified lignin to the cooled reaction system in 3-4 portions, with an interval of 10-15 minutes between each addition. Then add phosphoric acid and nano silica dispersion and stir for 20-30 minutes to obtain modified pMDI adhesive. The modified catalyst in step S1 is prepared through the following steps; S11. Add dibutyltin dilaurate to anhydrous ethanol and stir until homogeneous to obtain a mixed system. Add glacial acetic acid dropwise to adjust the pH of the mixed system to 5-6. S12. Heat the adjusted mixture to 40-45℃, add the coupling agent modification liquid and stir at a constant temperature for 1-1.5h to obtain the precursor catalyst. S13. The precursor catalyst is vacuum dried at 60-65℃ for 1-1.5h to remove anhydrous ethanol, and then cooled to room temperature to obtain the modified catalyst.

2. The method for preparing the modified pMDI adhesive according to claim 1, characterized in that, The modified lignin in step S3 is obtained through the following steps: S21. Add lignin to anhydrous ethanol and disperse evenly, then add sodium hydroxide and stir until completely dissolved. React at room temperature for 1-2 hours to obtain the lignin system. S22. Heat the lignin system to 50-60℃ and add propylene oxide dropwise to obtain the precursor lignin. S23. Dry the precursor lignin for 2-3 hours to remove anhydrous ethanol and excess moisture, grind it and pass it through a 100-200 mesh filter to obtain modified lignin.

3. The method for preparing the modified pMDI adhesive according to claim 1, characterized in that, The coupling agent modified liquid in step S12 is prepared through the following steps: Anhydrous ethanol and ethylene glycol were stirred at 20-25℃ for 15-20 min to obtain a mixed solvent. γ-aminopropyltriethoxysilane coupling agent and titanate coupling agent were added, and the mixture was ultrasonically dispersed for 15-20 min to obtain a coupling agent modified solution.

4. The method for preparing the modified pMDI adhesive according to claim 1, characterized in that, In steps S1 and S3, the mass ratio of pMDI to modified catalyst, modified lignin, phosphoric acid and nano silica is 100:(1-3):(5-10):(0.5-1):(2-5).

5. The method for preparing the modified pMDI adhesive according to claim 1, characterized in that, In steps S11 and S12, the mass ratio of dibutyltin dilaurate to anhydrous ethanol, glacial acetic acid and coupling agent modified solution is 1:(10-20):(0.05-0.1):(2-4).

6. The method for preparing the modified pMDI adhesive according to claim 2, characterized in that, In steps S21 and S22, the mass ratio of lignin to anhydrous ethanol, sodium hydroxide, and propylene oxide is 1:(15-25):(0.05-0.1):(0.1-0.2).

7. The method for preparing the modified pMDI adhesive according to claim 3, characterized in that, In the preparation of the coupling agent modified solution, the mass ratio of anhydrous ethanol to ethylene glycol is (1.5-2.5):1, the mass ratio of γ-aminopropyltriethoxysilane to titanate coupling agent is (1.5-2.5):1, and the mass ratio of the mixed solvent to the total mass of the two coupling agents is 1:(0.04-0.08).

8. The method for preparing the modified pMDI adhesive according to claim 2, characterized in that, The dropping rate of the modified catalyst in step S1 is 0.5-2.0 g / min, the dropping rate of glacial acetic acid in step S11 is 0.1-0.3 g / min, and the dropping rate of propylene oxide in step S22 is 0.3-1.0 g / min.

9. A modified pMDI adhesive, characterized in that, It is prepared according to the preparation method according to any one of claims 1-8 above.

10. The application of the modified pMDI adhesive according to claim 9 in the accelerated curing and molding of wood-based panels through steam pretreatment.

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