Bonding method for low sizing amount and high strength plywood based on lignin formaldehyde-free adhesive

Abstract

The present invention discloses a bonding method for a high-strength plywood with a low sizing amount based on a lignin formaldehyde-free adhesive, belonging to the technical field of adhesives. In the present invention, a lignin-based formaldehyde-free adhesive is used to bond veneers, and the single-sided sizing amount is not more than 80 g / m<supgt;2< / supgt>. After hot pressing and curing, the bonding strength of the obtained high-strength plywood is not less than 0.7 MPa; the lignin-based formaldehyde-free adhesive is prepared by separating and treating lignocellulosic raw materials or kraft lignin with a mixed solution. The adhesive of the present invention can reach a dry strength of 0.9 - 1.3 MPa and a class II wet strength of 0.8 - 1.0 MPa after bonding three-layer poplar veneers; the elastic modulus and static bending strength after bonding five-layer plywood are significantly higher than 5500 MPa and 32 MPa; the plywood manufactured by the method of the present invention far exceeds the plywood bonded with industrial urea-formaldehyde resin glue, and the bonding strength is equivalent to that of industrial-grade phenolic-bonded plywood.

Description

Technical Field

[0001] The present invention belongs to the technical field of adhesives, and more specifically, relates to a bonding method for a low-sizing high-strength plywood based on lignin-free formaldehyde adhesives. Background Art

[0002] The adhesives used in plywood mainly include phenolic, urea-formaldehyde, and melamine resins. These adhesives mainly use formaldehyde as the main raw material for preparation, and there is a problem of free formaldehyde release. The development of formaldehyde-free adhesives can effectively solve the problem of free formaldehyde release and has broad market prospects. Lignin, as the binder in plant cells, endows the plant body with rigidity and the ability to resist external erosion. Conventional lignin separation causes great damage to the structure of lignin, and most of the obtained lignin has a series of problems such as low purity, low activity, and large molecular weight distribution, and it needs to be modified to meet the preparation requirements of subsequent adhesives. Problems such as low lignin yield, high cost, and high toxicity caused by the processes of lignin screening, purification, and modification seriously limit the development of lignin-free formaldehyde adhesives.

[0003] The inventors of the present invention have previously found that the system composed of polyols, maleic acid, and choline chloride has a good pretreatment effect on lignocellulosic raw materials, can remove most of the lignin in lignocellulosic raw materials, and the separated lignin directly exists as a flowable viscous adhesive, and this adhesive can achieve bonding of different materials at room temperature (Chinese Patent: CN115029104A A method for preparing lignin-free formaldehyde adhesives and its products), but the bonding strength is small, the bonding mechanism is unclear, and no industrial utilization method has been found yet.

[0004] To solve the above problems, the present application uses this lignin-free formaldehyde adhesive for bonding plywood and elaborates and optimizes its bonding mechanism and curing process, hoping to open up a new market for the use of this product in plywood bonding and completely solve the problem of free formaldehyde release in plywood. Summary of the Invention

[0005] Aiming at the above problems existing in the prior art, the technical problem to be solved by the present invention is to provide a bonding method for a low-sizing high-strength plywood based on lignin-free formaldehyde adhesives, which can achieve high-strength bonding of wood veneers.

[0006] To solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] A bonding method for a low-sizing high-strength plywood based on lignin-free formaldehyde adhesives, using a lignin-based formaldehyde-free adhesive to bond veneers, and the single-sided sizing amount is not more than 80 g / m 2, after hot pressing and curing, the plywood with high strength has a bonding strength of not less than 0.7 MPa; the lignin-based formaldehyde-free adhesive is prepared by separating and treating wood fiber raw materials or kraft lignin; wherein, the mixed solution is composed of choline chloride, long-chain diol and maleic acid.

[0008] Preferably, the veneer is made of poplar.

[0009] Preferably, the hot pressing temperature is 150 - 190 °C, the pressure is 1 MPa, and the hot pressing time is 1.1 - 4.4 min / mm.

[0010] The preparation process of the lignin formaldehyde-free adhesive is as follows:

[0011] 1) Mix choline chloride, long-chain diol and maleic acid in a molar ratio of 1:0.5 - 2:1 - 2, and heat at 90 °C until a homogeneous and transparent DES solution is formed;

[0012] 2) Mix the wood fiber raw material or kraft lignin with the DES solution prepared in step 1) in a mass ratio of 1:10, react at 100 - 140 °C for 6 - 12 h. After the reaction, add a solvent 3 - 5 times the volume of the mixed solution, stir and perform solid-liquid separation. Rotate evaporate the obtained solution to remove the solvent, wash with water and dry to obtain the lignin formaldehyde-free adhesive.

[0013] The bonding method of the high-strength plywood with low sizing amount based on the lignin formaldehyde-free adhesive specifically includes the following steps:

[0014] 1) Prepare the lignin formaldehyde-free adhesive

[0015] Mix choline chloride, long-chain diol and maleic acid in a molar ratio of 1:0.5 - 2:1 - 2, and heat at 90 °C until a homogeneous and transparent DES solution is formed;

[0016] Mix the wood fiber raw material or kraft lignin with the prepared DES solution in a mass ratio of 1:10, react at 100 - 140 °C for 6 - 12 h. After the reaction, add a solvent 3 - 5 times the volume of the mixed solution, stir and perform solid-liquid separation. Rotate evaporate the obtained solution to remove the solvent, wash with water and dry to obtain the lignin formaldehyde-free adhesive;

[0017] 2) Prepare the high-strength plywood

[0018] Directly and evenly apply the lignin formaldehyde-free adhesive prepared in step 1) on the surface of the veneer or evenly apply it on the surface of the veneer after heating. The single-sided sizing amount is 50 - 75 g / m 2, veneers are assembled with adjacent veneer fiber directions perpendicular to each other, and then hot-pressed and cured. The hot-pressing temperature is 150 - 190 °C, the pressure is 1.0 MPa, and the hot-pressing time is 1.1 - 4.4 min / mm to obtain high-strength plywood.

[0019] Preferably, the bonding method of the low-sizing high-strength plywood based on lignin-free adhesive specifically includes the following steps:

[0020] 1) Prepare lignin-free adhesive

[0021] Mix choline chloride, 1,4-butanediol and maleic acid in a molar ratio of 1:2:1, heat and continuously stir at 90 °C until a homogeneous and transparent DES solution is formed;

[0022] Mix kraft lignin with the prepared DES solution in a mass ratio of 1:10, react at 110 °C for 12 h. After the reaction, add hot water with a volume three times that of the mixed solution, stir and then separate the solid and liquid. After rotary evaporation, washing and drying of the obtained lignin-rich pretreatment solution, lignin-free adhesive is obtained;

[0023] 2) Prepare high-strength plywood

[0024] Evenly apply the lignin-free adhesive prepared in step 1) on the surface of the veneer, and the single-sided sizing amount is 50 g / m 2 , veneers are assembled with adjacent veneer fiber directions perpendicular to each other to obtain a three-layer plywood blank, and then hot-pressed and cured. The hot-pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot-pressing time is 2.2 min / mm to obtain high-strength plywood.

[0025] Preferably, the bonding method of the low-sizing high-strength plywood based on lignin-free adhesive specifically includes the following steps:

[0026] 1) Prepare lignin-free adhesive

[0027] Mix choline chloride, 1,4-butanediol and maleic acid in a molar ratio of 1:2:1, heat and continuously stir at 90 °C until a homogeneous and transparent DES solution is formed;

[0028] Mix bamboo powder with the prepared DES solution in a mass ratio of 1:10, react at 110 °C for 6 h. After the reaction, add an ethanol aqueous solution with a volume five times that of the mixed solution, stir and then separate the solid and liquid. After rotary evaporation, washing and drying of the obtained lignin-rich pretreatment solution, lignin-free adhesive is obtained;

[0029] 2) Prepare high-strength plywood

[0030] Apply the lignin - free formaldehyde adhesive prepared in step 1) evenly on the surface of the veneer, with a single - side sizing amount of 50 g / m 2 , stack the veneers with adjacent veneer fiber directions perpendicular to each other to obtain a three - layer plywood blank, and carry out hot - pressing and curing. The hot - pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot - pressing time is 2.2 min / mm to obtain high - strength plywood.

[0031] Preferably, the bonding method of the low - sizing - amount high - strength plywood based on the lignin - free formaldehyde adhesive specifically includes the following steps:

[0032] 1) Prepare the lignin - free formaldehyde adhesive

[0033] Mix choline chloride, 1,4 - butanediol and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and stir continuously until a homogeneous and transparent DES solution is formed;

[0034] Mix kraft lignin with the prepared DES solution in a mass ratio of 1:10, react at 100 °C for 12 h. After the reaction, add hot water with a volume 3 times that of the mixed solution, stir and then carry out solid - liquid separation. After rotary evaporation, washing and drying of the obtained lignin - rich pretreatment solution, the lignin - free formaldehyde adhesive is obtained;

[0035] 2) Prepare high - strength plywood

[0036] Apply the lignin - free formaldehyde adhesive prepared in step 1) evenly on the surface of the veneer, with a single - side sizing amount of 50 g / m 2 , stack the veneers with adjacent veneer fiber directions perpendicular to each other to obtain a three - layer plywood blank, and carry out hot - pressing and curing. The hot - pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot - pressing time is 2.2 min / mm to obtain high - strength plywood.

[0037] Preferably, the bonding method of the low - sizing - amount high - strength plywood based on the lignin - free formaldehyde adhesive specifically includes the following steps:

[0038] 1) Prepare the lignin - free formaldehyde adhesive

[0039] Mix choline chloride, 1,4 - butanediol and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and stir continuously until a homogeneous and transparent DES solution is formed;

[0040] Mix bamboo powder with the prepared DES solution in a mass ratio of 1:10, react at 100 °C for 6 h. After the reaction, add an ethanol - water solution with a volume 5 times that of the mixed solution, stir and then carry out solid - liquid separation. After rotary evaporation, washing and drying of the obtained lignin - rich pretreatment solution, the lignin - free formaldehyde adhesive is obtained;

[0041] 2) Prepare high - strength plywood

[0042] Apply the lignin - free formaldehyde adhesive prepared in step 1) evenly on the surface of the veneer, and the single - side sizing amount is 50 g / m 2 , and the adjacent veneers are assembled with their fiber directions perpendicular to each other to obtain a three - layer plywood blank. After hot - pressing and curing, the hot - pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot - pressing time is 2.2 min / mm, thus obtaining high - strength plywood.

[0043] Preferably, the bonding method of the low - sizing - amount high - strength plywood based on the lignin - free formaldehyde adhesive specifically includes the following steps:

[0044] 1) Prepare the lignin - free formaldehyde adhesive

[0045] Mix choline chloride, 1,4 - butanediol and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and stir continuously until a homogeneous and transparent DES solution is formed;

[0046] Mix kraft lignin and the prepared DES solution in a mass ratio of 1:10, react at 110 °C for 12 h. After the reaction, add hot water with a volume three times that of the mixed solution, stir and then separate the solid and liquid. After rotary evaporation, washing and drying of the obtained lignin - rich pretreatment solution, the lignin - free formaldehyde adhesive is obtained;

[0047] 2) Prepare high - strength plywood

[0048] Apply the lignin - free formaldehyde adhesive prepared in step 1) evenly on the surface of the veneer, and the single - side sizing amount is 50 g / m 2 , and the adjacent veneers are assembled with their fiber directions perpendicular to each other to obtain a three - layer plywood blank. After hot - pressing and curing, the hot - pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot - pressing time is 2.2 min / mm, thus obtaining high - strength plywood.

[0049] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0050] 1) The lignin - free formaldehyde adhesive adopted in the present invention is a brownish - brown fluid with a certain viscosity. At room temperature, the viscosity of this adhesive is 109 - 10709 Pa·s, and as the temperature rises from room temperature to 100 °C, the viscosity of this adhesive can significantly decrease to 0.2 - 11 Pa·s, indicating that this adhesive can meet the sizing requirements under different conditions through simple temperature control;

[0051] 2) The present invention can achieve high-strength bonding of wood veneers. The dry strength of poplar three-layer veneers after bonding can reach 0.9 - 1.3 MPa, and the wet strength of Class II can reach 0.8 - 1.0 MPa, meeting the requirements of the national standard of 0.7 MPa (GB / T9846 - 2015); the elastic modulus and static bending strength after bonding five-layer plywood are 9375 - 9678 MPa and 74 - 91 MPa respectively, significantly higher than the national standard requirements of 5500 MPa and 32 MPa, far exceeding the bonding of plywood with industrial urea-formaldehyde resin glue, and the bonding strength is equivalent to that of industrial-grade phenolic-bonded plywood;

[0052] 3) The lignin-based formaldehyde-free adhesive prepared by the present invention has a sizing amount of only 50 g / m 2 when bonding wooden boards, the sizing amount is significantly lower than that of traditional phenolic and urea-formaldehyde adhesives. The hot pressing temperature, time, and pressure have a wide range, and it can be applied to most woods and hot pressing equipment, having good industrial prospects. Description of the Drawings

[0053] Figure 1 It is a graph of the lignin removal rate and adhesive recovery rate of the lignin formaldehyde-free adhesives prepared in Examples 1 - 13; among them, Figure a is a graph of the lignin removal rate and adhesive recovery rate of the lignin formaldehyde-free adhesives prepared in Examples 1 - 4; Figure b is a graph of the lignin removal rate and adhesive recovery rate of the lignin formaldehyde-free adhesives prepared in Examples 3, 5 - 7; Figure c is a graph of the lignin removal rate and adhesive recovery rate of the lignin formaldehyde-free adhesives prepared in Examples 3, 8 - 11; Figure d is a graph of the lignin removal rate and adhesive recovery rate of the lignin formaldehyde-free adhesives prepared in Examples 3, 12 - 13;

[0054] Figure 2 It is a graph of the bonding strength of the lignin formaldehyde-free adhesive prepared in Example 3 for bonding three-layer plywood under different hot pressing conditions and sizing amounts; among them, Figure a is a graph of the bonding strength under different hot pressing conditions when the sizing amount is 50 g / m 2 ; Figure b is a graph of the bonding strength under different sizing amounts when the hot pressing temperature and time are 180 °C and 10 min respectively;

[0055] Figure 3 It is a graph of the bonding strength after bonding three-layer plywood with the lignin formaldehyde-free adhesives prepared in Examples 1 - 13 and the enzymatically hydrolyzed groundwood lignin (CEL) without DES solvent treatment; among them, Figure a is a graph of the bonding strength after bonding three-layer plywood with the lignin formaldehyde-free adhesives prepared in Examples 1 - 4; Figure b is a graph of the bonding strength after bonding three-layer plywood with the lignin formaldehyde-free adhesives prepared in Examples 3, 8 - 11; Figure c is a graph of the bonding strength after bonding three-layer plywood with the lignin formaldehyde-free adhesives prepared in Examples 3, 5 - 7; Figure d is a graph of the bonding strength after bonding three-layer plywood with the lignin formaldehyde-free adhesives prepared in Examples 3, 12 - 13;

[0056] Figure 4 Viscosity diagrams of the lignin formaldehyde-free adhesives prepared in Examples 3, 5 - 11;

[0057] Figure 5 Diagrams of the elastic modulus and static bending strength after bonding five-layer plywood with the adhesives of Examples 3, 7, 10, phenolic resin, and urea-formaldehyde resin;

[0058] Figure 6 Two-dimensional NMR characterization results diagrams of the lignin formaldehyde-free adhesives prepared in Examples 3, 5, 7 and the enzymatically hydrolyzed groundwood lignin (CEL) without DES solvent treatment; wherein, Figure a is the fatty region of the two-dimensional NMR spectrum, and Figure b is the aromatic region of the two-dimensional NMR spectrum;

[0059] Figure 7 Infrared spectra diagrams of the bonding surfaces of the lignin formaldehyde-free adhesive prepared in Example 3, poplar veneer, and plywood after hot pressing;

[0060] Figure 8 Diagrams of the adhesive recovery rates of the lignin formaldehyde-free adhesives prepared in Examples 19 - 26; wherein, Figure a is the diagram of the adhesive recovery rates of the lignin formaldehyde-free adhesives prepared in Examples 19 - 22, and Figure b is the diagram of the adhesive recovery rates of the lignin formaldehyde-free adhesives prepared in Examples 19, 23 - 26;

[0061] Figure 9 Diagrams of the bonding strengths after bonding poplar three-layer boards with the lignin formaldehyde-free adhesives prepared in Examples 19 - 26; wherein, Figure a is the diagram of the bonding strengths after bonding poplar three-layer boards with the lignin formaldehyde-free adhesives prepared in Examples 19 - 22, and Figure b is the diagram of the bonding strengths after bonding poplar three-layer boards with the lignin formaldehyde-free adhesives prepared in Examples 19, 23 - 26. Detailed implementation manners

[0062] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described below in conjunction with specific embodiments. In the following embodiments, unless otherwise specified, the technical means used are all conventional means well known to those skilled in the art. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained by purchasing in the market.

[0063] Example 1

[0064] A preparation method of a lignin formaldehyde-free adhesive, comprising the following steps:

[0065] 1) Mix choline chloride, 1,2-pentanediol, and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and continuously stir until a homogeneous and transparent DES solution is formed;

[0066] 2) Mix the bamboo powder with the DES solution obtained in step 1) at a mass ratio of 1:10, react at 110 °C for 6 h. After the reaction, add an ethanol aqueous solution with a volume 5 times that of the mixed solution, where the volume concentration of the ethanol aqueous solution is 50%. After stirring for 1 h, perform solid-liquid separation to obtain a pretreatment solution rich in lignin and pretreatment materials;

[0067] 3) Rotate and evaporate the pretreatment solution rich in lignin at 70 °C to remove the ethanol therein, and supplement appropriate deionized water to precipitate the lignin. Pour off the supernatant and add deionized water for stirring and washing, and dry in an oven at 70 °C to obtain a lignin formaldehyde-free adhesive.

[0068] Example 2

[0069] When preparing the lignin formaldehyde-free adhesive, mix choline chloride, 1,2-butanediol and maleic acid at a molar ratio of 1:1:1, and the remaining preparation methods and parameters are the same as those in Example 1.

[0070] Example 3

[0071] When preparing the lignin formaldehyde-free adhesive, mix choline chloride, 1,4-butanediol and maleic acid at a molar ratio of 1:1:1, and the remaining preparation methods and parameters are the same as those in Example 1.

[0072] Example 4

[0073] When preparing the lignin formaldehyde-free adhesive, mix choline chloride, 1,5-pentanediol and maleic acid at a molar ratio of 1:1:1, and the remaining preparation methods and parameters are the same as those in Example 1.

[0074] After neutralizing the pretreatment materials obtained in Examples 1-4 with distilled water, analyze the main component content thereof, and calculate the lignin removal rate; weigh the lignin formaldehyde-free adhesive dried in an oven at 70 °C to calculate the yield, and the yield is calculated based on the mass of lignin added to the reaction system. The results are as Figure 1 shown in Figure a and Table 1.

[0075] Table 1 Lignin removal rate and adhesive recovery yield of lignin-based adhesives prepared in Examples 1-4

[0076] Example 1 Example 2 Example 3 Example 4 Lignin removal rate (%) 62.7 71.1 78.8 81.0 Recovery rate of adhesive (%) 315.4 389.5 426.7 521.5

[0077] As Figure 1 shown in Figure a and Table 1, for the moso bamboo raw materials treated with different choline chloride / polyol / maleic acid systems, the lignin removal rate is 61.6% - 81.0%, indicating that the DESs with different polyol groups have different dissociation effects on lignocellulosic raw materials; the lignins obtained after pretreatment with DESs with different long-chain polyol groups are all fluid adhesives with a certain viscosity, and the yield is 315.4% - 521.5%.

[0078] Example 5

[0079] When preparing the lignin-free formaldehyde adhesive, choline chloride, 1,4-butanediol and maleic acid were mixed at a molar ratio of 1:2:1, and the remaining preparation methods and parameters were the same as those in Example 3.

[0080] Example 6

[0081] When preparing the lignin-free formaldehyde adhesive, choline chloride, 1,4-butanediol and maleic acid were mixed at a molar ratio of 1:0.5:1, and the remaining preparation methods and parameters were the same as those in Example 3.

[0082] Example 7

[0083] When preparing the lignin-free formaldehyde adhesive, choline chloride, 1,4-butanediol and maleic acid were mixed at a molar ratio of 1:1:2, and the remaining preparation methods and parameters were the same as those in Example 3.

[0084] After neutralizing the pretreated materials obtained in Example 3 and Examples 5-7 with distilled water, the main component contents were analyzed, and the lignin removal rate was calculated; the lignin-free formaldehyde adhesive dried in an oven at 70 °C was weighed to calculate the yield, and the yield was calculated based on the mass of lignin added to the reaction system. The results are as Figure 1 shown in Table 2.

[0085] Table 2 Lignin removal rate and adhesive recovery yield of lignin-free formaldehyde adhesives prepared in Examples 3, 5-7

[0086] Example 3 Example 5 Example 6 Example 7 Lignin removal rate (%) 78.8 76.9 78.9 76.1 Recovery rate of adhesive (%) 426.7 252.8 426.7 801.4

[0087] From Figure 1 Table 2, it can be seen that after pretreatment with different molar ratios of choline chloride / 1,4-butanediol / maleic acid, the lignin removal rate changed little (76.1% - 78.9%), indicating that the dissociation efficiency of lignocellulosic raw materials was similar under different molar ratios in this system. However, the recovery yields of the lignin-free formaldehyde adhesives differed significantly, and they increased significantly with the increase in the amount of maleic acid used (252.8% - 801.4%), indicating that the increase in the amount of maleic acid could significantly promote the cross-linking reaction between lignin and the DES solvent in this pretreatment system. The esterification reaction between maleic acid and 1,4-butanediol and the graft cross-linking reaction between maleic acid and the lignin side chain are important reaction processes for the formation of lignin-free formaldehyde adhesives.

[0088] Example 8

[0089] When preparing the lignin-free formaldehyde adhesive, bamboo powder was mixed with the DES solution prepared in step 1) at a mass ratio of 1:10 and reacted at 100 °C for 6 h, and the remaining preparation methods and parameters were the same as those in Example 3.

[0090] Example 9

[0091] When preparing the lignin - free formaldehyde adhesive, bamboo powder is mixed with the DES solution obtained in step 1) at a mass ratio of 1:10, and the reaction is carried out at 120 °C for 6 h. The remaining preparation methods and parameters are the same as those in Example 3.

[0092] Example 10

[0093] When preparing the lignin - free formaldehyde adhesive, bamboo powder is mixed with the DES solution obtained in step 1) at a mass ratio of 1:10, and the reaction is carried out at 130 °C for 6 h. The remaining preparation methods and parameters are the same as those in Example 3.

[0094] Example 11

[0095] When preparing the lignin - free formaldehyde adhesive, bamboo powder is mixed with the DES solution obtained in step 1) at a mass ratio of 1:10, and the reaction is carried out at 140 °C for 6 h. The remaining preparation methods and parameters are the same as those in Example 3.

[0096] After neutralizing the pretreated materials obtained in Example 3 and Examples 8 - 11 with distilled water, analyze the main component content and calculate the lignin removal rate; weigh the lignin - free formaldehyde adhesive dried in an oven at 70 °C to calculate the yield, and the yield is calculated based on the mass of lignin added to the reaction system. The results are as Figure 1 shown in Figure c and Table 3.

[0097] Table 3 Lignin removal rate and adhesive recovery yield of lignin - free formaldehyde adhesives prepared in Examples 3, 8 - 11

[0098] Example 3 Example 8 Example 9 Example 10 Example 11 Lignin removal rate (%) 78.8 72.0 85.0 82.4 74.5 Recovery rate of adhesive (%) 426.7 237.8 589.7 667.2 942.1

[0099] From Figure 1 Figure c and Table 3, it can be seen that the lignin removal rate gradually increases from 72.0% (100 °C) to 85.0% (120 °C) as the temperature rises. As the temperature continues to increase, the lignin condensation reaction intensifies, and the lignin removal rate decreases. At the same time, the recovery rate of the lignin - free formaldehyde adhesive increases significantly from 237.8% to 942.1%, indicating that increasing the temperature can significantly improve the esterification cross - linking reaction between lignin and the DES solvent. However, the increase in the cross - linking reaction between the solvent and lignin will form a greater steric hindrance, thereby affecting the reaction of the active sites on the lignin benzene ring with the components in the lignocellulosic raw material.

[0100] Example 12

[0101] When preparing the lignin - free formaldehyde adhesive, poplar is selected as the lignocellulosic raw material, and the remaining preparation methods and parameters are the same as those in Example 3.

[0102] Example 13

[0103] When preparing the lignin-free formaldehyde adhesive, masson pine was selected as the lignocellulosic raw material, and the remaining preparation methods and parameters were the same as those in Example 3.

[0104] After neutralizing the pretreated materials obtained in Examples 3 and 12 - 13 with distilled water, the main component contents were analyzed, and the lignin removal rate was calculated; the lignin-free formaldehyde adhesive dried in an oven at 70 °C was weighed to calculate the yield, and the yield was calculated based on the mass of lignin added to the reaction system. The results are as Figure 1 shown in Figure d and Table 4.

[0105] Table 4 Lignin removal rate and adhesive recovery yield of the lignin-free formaldehyde adhesives prepared in Examples 3 and 12 - 13

[0106] Example 3 Example 12 Example 13 Lignin yield (%) 78.8 70.4 48.5 Recovery rate of adhesive (%) 426.7 769.5 960.3

[0107] As Figure 1 shown in Figure d and Table 4, when the molar ratio of choline chloride / 1,4-butanediol / maleic acid was 1:1:1 and 110 °C for 6 h was used as the optimal condition for pretreating different lignocellulosic raw materials, compared with gramineous bamboo, the lignin removal rate was lower after pretreating wood with this system. The lignin removal rate of masson pine was only 48.5%. This result was due to the relatively high degree of lignification and denser structure of wood. The lignin glue recovery rate (relative to the lignin removed from the system) was opposite to the lignin removal rate. The recovery yield of the lignin-free formaldehyde adhesive was significantly higher after pretreating poplar and masson pine than that of moso bamboo. This result indicates that the structure of lignin in wood is more likely to crosslink with the DES solvent.

[0108] Example 14

[0109] A bonding method for a low sizing amount and high strength plywood based on a lignin-free formaldehyde adhesive, comprising the following steps:

[0110] 1) Bond three-layer plywood with the lignin-free formaldehyde adhesive prepared in Example 3. The plywood was made of poplar veneer with a thickness of 1.5 mm, and the poplar veneer was cut into 140 mm in length and 120 mm in width.

[0111] 2) The single-sided sizing amount was 50 - 100 g / m 2 , and the lignin glue was evenly applied to the surface of the poplar board by weighing.

[0112] 3) After placing the adjacent layers of poplar veneers with their fiber directions perpendicular to each other, they were hot-pressed and cured by a hot press. The hot-pressing temperature was 150 - 190 °C, the pressure was 1.0 MPa, and the hot-pressing time was 1.1 - 4.4 min / mm.

[0113] After cutting the formed plywood into standard test pieces, the dry strength and Class II wet strength were measured by a universal tensile testing machine (SUNS UTM6503). The results are as Figure 2 shown.

[0114] As Figure 2 shown in a, when the sizing amount is 50 g / m 2 , at a relatively low hot-pressing temperature of 170 °C, the required hot-pressing time is 20 min, and the hot-pressing time significantly decreases to 10 min as the temperature increases (180 - 190 °C). However, too high a hot-pressing temperature is still difficult to meet the actual industrial production requirements. By introducing a small amount of sulfuric acid as a catalyst (0.1 wt%) into the lignin adhesive obtained in Example 3, the present invention can significantly reduce the hot-pressing temperature to 150 °C / 10 min. Under this condition, the dry and wet bonding strengths are 1.0 and 0.8 MPa, respectively, both exceeding the minimum bonding strength of 0.7 MPa required by the national standard.

[0115] As Figure 2 shown in b, at a hot-pressing temperature and time of 180 °C and 10 min, respectively, the present invention selected sizing amounts of 50, 75, and 100 g / m 2 for single-sided bonding. The results showed that the bonding strength of the plywood could exceed the national standard requirements at a sizing amount of 50 g / m 2 . As the sizing amount increased from 50 g / m 2 to 75 g / m 2 , the dry and wet bonding strengths of the plywood increased from 1.2 / 1.0 MPa to 1.4 / 1.1 MPa. Continuing to increase the sizing amount to 100 g / m 2 , the strength of the plywood hardly increased any further, indicating that a sizing amount of 50 - 75 g / m 2 was sufficient to meet the bonding strength requirements of the plywood. In summary, the sizing amount of this lignin formaldehyde-free adhesive is only 50 g / m 2 , far lower than that of traditional phenolic and urea-formaldehyde wood adhesives (150 - 300 g / m 2 ); the hot-pressing curing conditions of this lignin formaldehyde-free adhesive have a wide range, can meet different plywood production equipment, and have great potential for industrial production.

[0116] Example 15

[0117] Bond three-layer plywood with the lignin obtained in Examples 1 - 13 and the enzyme-hydrolyzed groundwood lignin (CEL) without DES solvent treatment. The specific bonding process is as follows: The plywood uses poplar veneer with a thickness of 1.5 mm, and the single-sided sizing amount is 50 g / m 2, which is formed by hot-pressing and curing the veneers of adjacent layers with their fiber directions perpendicular to each other. The hot-pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot-pressing time is 2.2 min / mm. After the formed plywood is cut into standard test pieces, the dry strength and Class II wet strength are measured by a universal tensile testing machine (SUNS UTM6503). The results are as Figure 3 shown and can be seen from Table 5.

[0118] Table 5 Gluing strength of three-ply plywood bonded with lignin-based formaldehyde-free adhesives prepared by DES pretreatment with different polyol groups and enzymatically hydrolyzed milled lignin (CEL) without DES solvent treatment

[0119]

[0120]

[0121] As can be seen from Figure 3 a, the dry and wet bonding strengths of the lignin-based formaldehyde-free adhesives prepared by DES pretreatment with different polyol groups for poplar plywood are both higher than 0.7 MPa, indicating that the adhesives prepared by this system can be used for the preparation of plywood.

[0122] As can be seen from Figure 3 b, the dry strength and wet strength of the plywood bonded with enzymatically hydrolyzed milled lignin (CEL) without DES solvent treatment are 1.01 and 0.77 MPa respectively, meeting the minimum national standard requirement of 0.7 MPa. However, this lignin is in powder form and is difficult to match the current plywood sizing process. Moreover, the preparation process of CEL is complex, costly, and has a low yield, making it difficult to achieve large-scale production. After DES solvent pretreatment, the lignin is in a viscous fluid state and can be thermally cured, which is very similar to the phenolic resin used in the plywood industry. The mechanical properties of the lignin-based formaldehyde-free adhesives prepared under different conditions in this system for bonding three-ply boards are mostly better than those of the CEL-bonded specimens, indicating that this DES solvent can effectively activate lignin and increase the bonding opportunity between it and the main components in the wood fiber raw material during the hot-pressing process, thus improving the bonding strength.

[0123] As can be seen from Figure 3 c, as the temperature in the DES solvent system increases ( Figure 3 b) and the amount of maleic acid used increases ( Figure 3 c), the dry and wet bonding strengths of the plywood decrease from 1.32 and 1.03 MPa to 0.94 and 0.78 MPa, and from 1.46 and 0.80 MPa to 1.08 and 0.73 MPa respectively, but still meet the national standard requirements. The decrease in this bonding strength is due to the increased condensation reaction of lignin under the pretreatment conditions of stronger acidity and higher temperature and the excessive cross-linking reaction between the DES solvent and lignin, resulting in an increase in steric hindrance on the benzene ring of lignin, which reduces the bonding opportunity between this adhesive and the components in the wood fiber raw material during the hot-pressing process.

[0124] As can be seen from Figure 3 d, the lignin formaldehyde-free adhesives obtained from three raw materials with different wood properties, namely bamboo, poplar, and masson pine, after pretreatment have similar bonding performance to poplar three-ply plywood. The dry strength and wet strength are 1.02 - 1.15 MPa and 0.83 - 0.96 MPa respectively, both exceeding the national standard gluing standard of 0.7 MPa, indicating that this system can use various different lignocellulosic raw materials to prepare lignin formaldehyde-free adhesives with high bonding performance.

[0125] As can be seen from Figure 4 it, the lignin formaldehyde-free adhesives obtained under different pretreatment conditions (Examples 3, 5 - 11) have a viscosity of 109 - 10709 Pa·s at room temperature. As the temperature increases from room temperature to 100 °C, the viscosity of this adhesive can significantly decrease to 0.2 - 11 Pa·s, indicating that this lignin formaldehyde-free adhesive can meet the sizing requirements under different conditions through simple temperature control. The specific viscosity data are shown in Table 6 below.

[0126] Table 6 Viscosity of lignin formaldehyde-free adhesives prepared in Examples 3, 5 - 11 at different temperatures

[0127]

[0128] Example 16

[0129] The lignin formaldehyde-free adhesives of Example 3, Example 7, Example 10, phenolic resin adhesive, and urea-formaldehyde resin adhesive were used to bond five-ply plywood. The specific bonding process is as follows:

[0130] 1) The prepared lignin formaldehyde-free adhesive was used to bond five-ply plywood. The plywood was made of poplar veneer with a thickness of 1.5 mm, and the poplar veneer was cut into lengths and widths of 200 and 50 mm respectively;

[0131] 2) The single-sided sizing amount of the lignin formaldehyde-free adhesive was 50 g / m 2 , and the single-sided sizing amounts of phenolic and urea-formaldehyde adhesives were 100 g / m 2 . The lignin adhesive was evenly applied to the surface of the poplar board by weighing.

[0132] 3) The fiber directions of adjacent layers of poplar veneer were placed perpendicular to each other and then hot-pressed and cured by a hot press. The hot-pressing temperature was 180 °C, the pressure was 1.0 MPa, and the hot-pressing time was 2.7 min / mm.

[0133] The elastic modulus and static bending strength of the plywood were measured by an electronic universal tensile testing machine (SUNS UTM304SLXY, Shenzhen Sansi Zongheng Technology Co., Ltd.), and the results are as Figure 5 shown in Table 7.

[0134] Table 7 Elastic modulus and static bending strength of five-layer plywood bonded with phenolic resin, urea-formaldehyde resin, and adhesives prepared in Examples 3, 7, and 10

[0135] Elastic modulus MPa Flexural strength MPa Example 3 9375.3 91.3 Example 7 9678.7 87.7 Example 10 9417.7 73.8 Phenolic resin adhesive 8943.0 88.2 Urea-formaldehyde resin adhesive 8151.3 69.7

[0136] As can be seen from Figure 5 Table 7, the elastic modulus and static bending strength of the adhesives prepared under different conditions are 9375 - 9679 MPa and 73.8 - 91.3 MPa, respectively. The static bending strength decreases significantly with the increase in the amount of maleic acid and the rise in the pretreatment temperature. This result is similar to the trend of the bonding strength of three-layer plywood. At high amounts of maleic acid and reaction temperatures, the condensation reaction of lignin increases, leading to an excessive cross-linking reaction of the lignin adhesive. This causes a reduction in the active sites of the lignin benzene ring and side chains, an increase in steric hindrance, and limits the reaction of the lignin adhesive with lignin and other components in poplar wood, thus resulting in a decrease in the bonding effect. However, the elastic modulus and static bending strength of the lignin adhesives prepared under the above different conditions are significantly higher than those of urea-formaldehyde resin adhesives and higher than or close to those of phenolic resin. This result indicates that the adhesives prepared from this system for bonding multi-layer boards have properties close to or better than those of traditional industrial-grade phenolic or urea-formaldehyde adhesives and can replace fossil-based phenolic and urea-formaldehyde adhesives to become a new type of green bio-based formaldehyde-free plywood adhesive.

[0137] Example 17

[0138] The lignin formaldehyde-free adhesives prepared in Examples 3, 5, and 7 and the untreated enzymatic hydrolysis groundwood lignin (CEL) were dissolved in deuterated dimethyl sulfoxide and subjected to two-dimensional NMR characterization. The results are as Figure 6 shown in Table 8.

[0139] Table 8 Two-dimensional NMR characterization results of the lignin formaldehyde-free adhesives prepared in Examples 3, 5, and 7 and the untreated enzymatic hydrolysis groundwood lignin (CEL)

[0140]

[0141] As Figure 6As shown in Figure a, the aliphatic region of CEL lignin contains abundant β-O-4 aryl ether bonds, β-β, and β-5 derivative peaks, with contents of 59 / 100Ar, 4 / 100Ar, and 5 / 100Ar, respectively. After pretreatment with different molar ratios of choline chloride / 1,4-butanediol / maleic acid system, the aryl ether bonds decreased significantly to 9 / 100Ar (L121), 5 / 100Ar (L111), and 0 / 100Ar (L112). A new derivative peak A”α appeared at 80.05 / 4.49 ppm, and this peak was considered to be the peak of butanediol or its derivative grafted at the α-position of the lignin side chain. In addition, the peak of the oxidation of the γ-position of the lignin side chain in CEL was small (A’γ, 63.0 / 4.36 ppm), while this peak (green round frame) in the prepared adhesive was significantly enhanced. The enhancement of this peak was due to the esterification cross-linking reaction between maleic acid and its derivatives and the γ-position hydroxyl group of the lignin side chain. With the increase of the pretreatment intensity (the increase of maleic acid dosage), the butanediol or its derivative grafted at the α-position of the lignin side chain decreased significantly, and the content of aryl ether bonds decreased from 55 / 100Ar (L121) to 53 / 100Ar (L111) and 0 (L112). This result was consistent with its bonding effect, indicating that the departure of the α-position group of the lignin side chain and the cleavage of aryl ether bonds would reduce the esterification cross-linking between the adhesive and cellulose and hemicellulose in poplar veneer during the hot pressing process, thereby weakening the bonding strength.

[0142] The spectrum of the aromatic region of lignin is as Figure 6 shown in Figure b. The main constituent units S, G, H in CEL lignin and the lignin-carbohydrate related peak (PCE) are clearly visible. After the pretreatment, the lignin undergoes different degrees of polycondensation reaction, and with the increase of maleic acid dosage, the polycondensation reaction gradually increases from 4 / 100Ar (L121) to 10 / 100Ar (L111) and 28 / 100Ar (L112). The increase of the polycondensation reaction is consistent with the decrease of the bonding strength of the plywood. Different from the CEL spectrum, the appearance of new derivative peaks at δC / δH 129.0 / 6.2 and 132.9 / 6.7 ppm is due to the esterification reaction between maleic acid or its derivatives and the γ-position of the lignin side chain. This reaction effectively extends the length of the lignin side chain and increases the carboxyl content of the lignin side chain.

[0143] Example 18

[0144] The formation of the connecting bonds before and after bonding of the lignin formaldehyde-free adhesive prepared in Example 3 was characterized by infrared, and the results are as Figure 7 shown. The bonding and curing process is as follows:

[0145] 1) Bond the lignin formaldehyde-free adhesive prepared in Example 3 to a three-layer plywood. The plywood uses poplar veneer with a thickness of 1.5 mm, and the poplar veneer is cut into 140 and 120 mm in length and width, respectively.

[0146] 2) The sizing amount on one side is 50 g / m 2 , and the lignin glue is evenly applied to the surface of poplar wood boards by means of metering and weighing;

[0147] 3) After the fiber directions of adjacent layers of poplar veneers are placed perpendicular to each other, they are hot-pressed and cured by a hot press. The hot-pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot-pressing time is 2.2 min / mm;

[0148] As can be seen from Figure 7, the carbonyl absorption peak representing hemicellulose in poplar veneers is located at 1730 cm -1 , and the relevant absorption peaks of the lignin aromatic ring are located at 1602 and 1510 cm -1 . Compared with poplar veneers, the carbonyl absorption peak in the prepared lignin formaldehyde-free adhesive shows a blue shift (1730 - 1713 cm -1 ), indicating that the lignin formaldehyde-free adhesive contains more ester groups. This result is consistent with the esterification reaction of maleic acid and its derivatives with the γ-position of the lignin side chain in the NMR spectrum.

[0149] The absorption peaks of the lignin aromatic ring (1602 cm -1 and 1510 cm -1 ) are significantly enhanced, indicating that the adhesive retains the aromatic ring skeleton structure of lignin. After the adhesive is hot-pressed and bonded with poplar veneers, the bonding surface of the plywood contains rich ester groups (1713 cm -1 ) and the lignin aromatic ring structure (1602 cm -1 and 1510 cm -1 ), indicating that a large amount of esterification crosslinking has occurred between the adhesive and poplar veneers. This crosslinking reaction endows the plywood with bonding strength.

[0150] The grafting reaction of the lignin side chain with butanediol, maleic acid and its derivatives is the main reason for the viscous fluid state of the lignin formaldehyde-free adhesive. During the hot-pressing and curing process of the adhesive, the esterification crosslinking with cellulose, hemicellulose and lignin in the wood boards provides part of the structural strength of the plywood, and the more stable C-C bonds formed by the condensation reaction of the less condensed lignin with the uncondensed lignin in the wood boards provide the other part of the structural strength of the plywood.

[0151] Example 19

[0152] A preparation method of a lignin formaldehyde-free adhesive, comprising the following steps:

[0153] 1) Mix choline chloride, 1,4-butanediol and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and stir continuously until a homogeneous and transparent DES solution is formed;

[0154] 2) Mix the sulfate lignin with the DES solution prepared in step 1) at a mass ratio of 1:10, react at 110 °C for 12 h. After the reaction, add hot water (95 °C) with a volume three times that of the mixed solution, stir for 1 h, and then let it settle naturally for 5 h;

[0155] 3) After the liquid is layered, pour out the upper liquid rich in DES solvent, and rotary evaporate it at 70 °C to recover the unreacted DES solvent. The lignin adhesive precipitated in the lower layer is stirred and washed with deionized water until the washing liquid is neutral, and then dried in an oven at 70 °C to obtain the lignin formaldehyde-free adhesive and calculate the yield, where the yield is based on the mass of the added lignin.

[0156] Example 20

[0157] When preparing the lignin formaldehyde-free adhesive, mix choline chloride, 1,4-butanediol, and maleic acid in a molar ratio of 1:2:1, and the remaining preparation methods and parameters are the same as those in Example 19.

[0158] Example 21

[0159] When preparing the lignin formaldehyde-free adhesive, mix choline chloride, 1,4-butanediol, and maleic acid in a molar ratio of 1:1:2, and the remaining preparation methods and parameters are the same as those in Example 19.

[0160] Example 22

[0161] When preparing the lignin formaldehyde-free adhesive, mix choline chloride, 1,4-butanediol, and maleic acid in a molar ratio of 1:0.5:1, and the remaining preparation methods and parameters are the same as those in Example 19.

[0162] Example 23

[0163] When preparing the lignin formaldehyde-free adhesive, mix the sulfate lignin with the DES solution prepared in step 1) at a mass ratio of 1:10, react at 100 °C for 12 h, and the remaining preparation methods and parameters are the same as those in Example 19.

[0164] Example 24

[0165] When preparing the lignin formaldehyde-free adhesive, mix the sulfate lignin with the DES solution prepared in step 1) at a mass ratio of 1:10, react at 120 °C for 12 h, and the remaining preparation methods and parameters are the same as those in Example 19.

[0166] Example 25

[0167] When preparing the lignin formaldehyde-free adhesive, mix the sulfate lignin with the DES solution prepared in step 1) at a mass ratio of 1:10, react at 130 °C for 12 h, and the remaining preparation methods and parameters are the same as those in Example 19.

[0168] Example 26

[0169] When preparing the lignin formaldehyde-free adhesive, kraft lignin and the DES solution prepared in step 1) are mixed at a mass ratio of 1:10, and reacted at 140 °C for 12 h. The remaining preparation methods and parameters are the same as those in Example 19.

[0170] After the pretreated materials obtained in Examples 19-26 are neutralized with distilled water, they are dried in an oven at 70 °C, weighed, and the yield is calculated. The yield is calculated based on the mass of lignin added to the reaction system ( Figure 8 is the yield of the adhesive, and the yield is the mass ratio of the separated lignin adhesive after drying to the added kraft lignin). The results are as Figure 8 shown in Table 9.

[0171] Table 9 Yield of the lignin formaldehyde-free adhesive prepared in Examples 19-26.

[0172]

[0173]

[0174] As can be seen from Figure 8 Table 9, the yield of the kraft-based lignin formaldehyde-free adhesive increases from 150.3% to 430.0% and from 181.8% to 456.9% respectively with the increase of the amount of maleic acid and the pretreatment temperature, indicating that the increase of the amount of maleic acid and the temperature during the pretreatment process can significantly increase the cross-linking reaction of the DES solvent with kraft lignin, thereby increasing the yield of the lignin adhesive.

[0175] Example 27

[0176] The lignin formaldehyde-free adhesive prepared in Examples 19-26 is used to bond three-layer plywood. The specific bonding process is the same as that in Example 18. After the formed plywood is cut into standard test pieces, the dry strength and Class II wet strength are measured by a universal tensile testing machine (SUNS UTM6503) at a tensile speed of 5 mm / min and a 5 kN force sensor. The results are as Figure 9 shown in Table 10.

[0177] Table 10 Gluing strength of the lignin formaldehyde-free adhesive prepared in Examples 19-26 for bonding three-layer plywood.

[0178] Bonding dry strength MPa Bonding wet strength MPa Example 19 1.56 1.35 Example 20 1.23 1.0 Example 21 0.99 0.90 Example 22 1.0 0.93 Example 23 1.44 1.28 Example 24 1.11 1.04 Example 25 1.02 0.88 Example 26 0.89 0.88

[0179] As can be seen from Figure 9 Table 10, the differences in the lignin adhesives prepared at different molar ratios are relatively large. When the amount of maleic acid is small, the bonding effect of the adhesive on poplar veneer is better, and the dry and wet strengths can reach 1.23 / 1.0 MPa (L 1:2:1, i.e., Example 20) and 1.56 / 1.35 MPa (L 1:1:1 , i.e., Example 19), as the amount of maleic acid increases, the bonding strength decreases to 0.99 / 0.90 MPa (L 1:1:2 , i.e., Example 21) and 1.0 / 0.93 MPa (L 1:0.5:1 , i.e., Example 22). Similar to the results of the bonding strength of the adhesive with the increase in the amount of maleic acid, after raising the pretreatment temperature, the dry and wet bonding strengths of the adhesive decreased from 1.56 / 1.35 MPa to 0.99 / 0.9 MPa respectively, but still remained higher than the minimum standard of 0.7 MPa. More stringent pretreatment conditions correspond to a higher yield of the lignin-free formaldehyde adhesive, but the bonding strength of the lignin-free formaldehyde adhesive decreases accordingly. Through condition optimization, the present invention can provide technical and theoretical guidance for the production of lignin-free formaldehyde adhesives with different bonding strengths.

[0180] Comparative Example 1

[0181] 1) Respectively mix choline chloride / 1,4-butanediol / citric acid, choline chloride / 1,4-butanediol / malic acid, choline chloride / 1,3-propanediol / maleic acid, and choline chloride / ethylene glycol / maleic acid in a molar ratio of 1:1:1, heat at 80 °C, and continuously stir until a homogeneous and clear liquid is formed to obtain a deep eutectic solvent;

[0182] 2) Mix bamboo powder with the deep eutectic solvent obtained in step 1) at a mass ratio of 1:10, react at 110 °C for 6 h. After the pretreatment is completed, add an ethanol aqueous solution with a volume 5 times that of the mixed solution, where the volume concentration of the ethanol aqueous solution is 50%, and stir for 1 h; perform solid-liquid separation to obtain a pretreatment liquid rich in lignin and a pretreatment material;

[0183] 3) Wash the pretreatment material with distilled water until it is neutral, analyze the content of its main components, and calculate the lignin removal rate; rotate and evaporate the pretreatment liquid rich in lignin at 60 °C to remove the ethanol therein, and supplement an appropriate amount of deionized water to precipitate the lignin. Centrifuge the pretreatment liquid after removing the ethanol to obtain lignin.

[0184] Although systems such as choline chloride / 1,4-butanediol / citric acid and choline chloride / 1,4-butanediol / oxalic acid can achieve the removal of lignin in bamboo, the removed lignin mainly exists in the form of powder, and it is impossible to obtain a lignin glue similar to the liquid fluid obtained in the present invention.

[0185] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A bonding method for a high-strength plywood with low sizing amount based on lignin formaldehyde-free adhesive, characterized in that, Specifically, it includes the following steps: 1) Prepare lignin-free formaldehyde adhesive Mix choline chloride, 1,4-butanediol, and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and continuously stir until a homogeneous and transparent DES solution is formed; Mix sulfate lignin with the prepared DES solution in a mass ratio of 1:10, react at 110 °C for 12 h. After the reaction, add hot water with a volume three times that of the mixed solution, stir and then separate the solid and liquid. After rotary evaporation, washing, and drying of the obtained lignin-rich pretreatment solution, lignin-free formaldehyde adhesive is obtained; 2) Prepare high-strength plywood Apply the lignin - free formaldehyde adhesive prepared in step 1) evenly on the surface of the veneer, and the single - side sizing amount is 50 g / m 2 , assemble the veneers with adjacent veneer fiber directions perpendicular to each other to obtain a three - layer plywood blank, and carry out hot - pressing curing. The hot - pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot - pressing time is 2.2 min / mm to obtain high - strength plywood.

2. A bonding method for a high-strength plywood with low sizing amount based on lignin formaldehyde-free adhesive, characterized in that, Specifically, it includes the following steps: 1) Prepare lignin-free formaldehyde adhesive Mix choline chloride, 1,4-butanediol, and maleic acid in a molar ratio of 1:1:1, heat at 90 °C and continuously stir until a homogeneous and transparent DES solution is formed; Mix sulfate lignin with the prepared DES solution in a mass ratio of 1:10, react at 100 °C for 12 h. After the reaction, add hot water with a volume three times that of the mixed solution, stir and then separate the solid and liquid. After rotary evaporation, washing, and drying of the obtained lignin-rich pretreatment solution, lignin-free formaldehyde adhesive is obtained; 2) Prepare high-strength plywood Apply the lignin - free formaldehyde adhesive prepared in step 1) evenly on the surface of the veneer, and the single - side sizing amount is 50 g / m 2 , stack the veneers with adjacent veneer fiber directions perpendicular to each other to obtain a three - layer plywood blank. After hot - pressing and curing, the hot - pressing temperature is 180 °C, the pressure is 1.0 MPa, and the hot - pressing time is 2.2 min / mm to produce high - strength plywood.

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