A lignin-phenolic resin adhesive and its preparation method

Abstract

A lignin-phenolic resin adhesive and its preparation method are disclosed. The preparation method of the lignin-phenolic resin adhesive includes the following steps: an activation step, in which lignin, an alkaline catalyst, and water are mixed in a reactor, diglycidyl ether (a glycol) and a tertiary amine are added, the mixture is heated to 60-70°C and held at that temperature for 60-90 minutes to obtain modified lignin; and a gelling step, in which phenol and formaldehyde aqueous solution are added to the reactor and reacted under heating conditions, and the reaction is terminated by cooling when the viscosity reaches 280-320 mPa·s. The mass ratio of lignin to phenol is greater than or equal to 40%. The lignin-phenolic resin adhesive of the present invention can achieve a high lignin substitution rate while maintaining good mechanical properties, high storage stability, good workability, and low curing requirements.

Description

Technical Field

[0001] This application relates to the field of adhesive technology, and more specifically to a lignin phenolic resin adhesive and its preparation method. Background Technology

[0002] Phenolic resins are widely used in structural timber, outdoor panels, and container flooring due to their excellent water resistance, heat resistance, and bonding strength. However, phenol is derived from petrochemical resources, resulting in fluctuating prices and high costs. Furthermore, traditional phenolic resin adhesives have the problem of high formaldehyde emissions.

[0003] Lignin is a naturally abundant renewable aromatic compound with a molecular structure similar to phenol. It is an ideal green raw material to replace phenol in the preparation of phenolic resins and has some applications. However, existing industrial applications generally have many problems, such as difficulty in achieving high lignin substitution and high curing conditions.

[0004] Specifically, unmodified lignin is difficult to replace at a high ratio: under conventional processes, when alkali lignin replaces more than 30% of phenol, the resin viscosity increases sharply, it is prone to gelation, curing is incomplete, and the bond strength decreases significantly. When the replacement rate reaches more than 40%, the dry bond strength is <1.3 MPa and the wet bond strength is <0.7 MPa, which is far below the requirements of GB / T 9846-2015 for Class I plywood (dry strength ≥2.0 MPa, wet bond strength ≥1.5 MPa), and the storage stability is less than 15 days, making industrial application impossible. Traditional lignin-based phenolic resin adhesives have limited curing systems, resulting in high curing temperatures and energy consumption. Traditional strong-base catalytic systems require curing temperatures ≥150℃ and have long curing times. Single nano-oxide catalysts have insufficient catalytic efficiency, while imidazole catalysts are costly and have poor storage stability, making it difficult to balance cost and performance.

[0005] Therefore, there is a need for a lignin-phenolic resin adhesive and its preparation method to at least partially solve the above problems. Summary of the Invention

[0006] The summary section introduces a series of simplified concepts, which will be further explained in detail in the detailed description section. This summary section is not intended to limit the key and essential technical features of the claimed technical solution, nor is it intended to determine the scope of protection of the claimed technical solution.

[0007] To at least partially solve the above problems, a first aspect of this application provides a method for preparing a lignin-phenolic resin adhesive, the method comprising the following steps: The activation step involves mixing lignin, an alkaline catalyst, and water in a reactor, adding diglycidyl ether (a diol) and tertiary amines, heating to 60-70°C, and maintaining the temperature for 60-90 minutes to obtain modified lignin. In the gelation step, phenol and formaldehyde aqueous solution are added to the reactor and reacted under heating conditions. When the viscosity reaches 280~320mPa·s, the reaction is terminated by cooling. The mass ratio of lignin to phenol is greater than or equal to 40%.

[0008] The lignin-phenolic resin adhesive according to the present invention can achieve a high lignin substitution rate while maintaining good mechanical properties, high storage stability, good workability, and low curing conditions.

[0009] Optionally, the preparation method of the lignin phenolic resin adhesive further includes a synergistic catalytic step, which is performed after the gelation step. The synergistic catalytic step includes: cooling the reactor to 60-70°C, adding an inorganic acid catalyst and stirring for 15-20 min, then adding a metal oxide catalyst and stirring for 25-35 min to obtain the lignin phenolic resin adhesive.

[0010] Optionally, the diol-based diglycidyl ether is polyethylene glycol diglycidyl ether.

[0011] Optionally, the tertiary amine is triethanolamine.

[0012] Optionally, the inorganic acid catalyst is boric acid.

[0013] Optionally, the metal oxide catalyst is nano-zinc oxide.

[0014] Optionally, the alkaline catalyst is selected from at least one of sodium hydroxide and potassium hydroxide; and / or The formaldehyde aqueous solution is an aqueous solution with a formaldehyde mass fraction of 36-38%.

[0015] Optionally, the amount of each component is as follows: Phenol, 95-105 parts by weight Formaldehyde aqueous solution, 180-220 parts by weight. Lignin, 40-60 parts by weight Diol diglycidyl ether, 7-8 parts by weight Tertiary amines, 3-4 parts by weight, Alkaline catalyst, 8-12 parts by weight Inorganic acid catalyst, 1-2 parts by weight Metal oxide catalyst, 0.5~1 parts by weight, The amount of water used is 15-30% of the total amount of the phenol, the formaldehyde aqueous solution, the lignin, the tertiary amine, the alkaline catalyst, the inorganic acid catalyst, and the metal oxide catalyst.

[0016] Optionally, the gelation step includes: In the co-condensation step, 55-65% of the formaldehyde aqueous solution and the phenol are added to the reactor and the temperature is raised to 90-95°C and reacted for 55-65 minutes. In the viscosity control step, the remaining formaldehyde aqueous solution is added to the reactor or added to the reactor in 2 to 3 portions every 15 to 20 minutes, and the reaction is continued at 90 to 95°C. When the viscosity reaches 280 to 320 mPa·s, the temperature is lowered to terminate the reaction.

[0017] A second aspect of the present invention provides a lignin-phenolic resin adhesive, wherein the lignin-phenolic resin adhesive is prepared by the preparation method of the lignin-phenolic resin adhesive described in the first aspect above.

[0018] The lignin-phenolic resin adhesive according to this application has similar technical effects to the preparation method of the lignin-phenolic resin adhesive of the first aspect described above. Detailed Implementation

[0019] The following description provides numerous specific details to offer a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described to avoid confusion with this application.

[0020] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of the stated features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or combinations thereof.

[0021] Ordinal numbers such as “first” and “second” used in this application are merely identifiers and have no other meaning, such as a specific order. Moreover, for example, the term “first component” does not in itself imply the existence of a “second component,” and the term “second component” does not in itself imply the existence of a “first component.”

[0022] It should be noted that the terms "upper," "lower," "front," "back," "left," "right," "inner," "outer," and similar expressions used herein are for illustrative purposes only and are not intended to be limiting. The terms "parallel" / "perpendicular," and similar expressions used in this application encompass both absolutely parallel / perpendicular relationships and approximately parallel / perpendicular relationships (e.g., relationships differing from absolutely parallel / perpendicular relationships by a range of -5° to +5°), and have equivalent effects.

[0023] This application provides a lignin-phenolic resin adhesive and its preparation method. The preparation method of the lignin-phenolic resin adhesive includes an activation step and a gelation step.

[0024] The activation step is configured as follows: lignin, alkaline catalyst and water are mixed in a reactor, diglycidyl ether and tertiary amine are added, the temperature is raised to 60~70℃ and the reaction is maintained for 60~90 min to obtain modified lignin.

[0025] The lignin is alkali lignin with a purity ≥85% and ash content ≤5%, preferably derived from black liquor extracts from poplar or pine sulfate pulping. The alkaline catalyst is selected from at least one of sodium hydroxide and potassium hydroxide, preferably sodium hydroxide. The formaldehyde aqueous solution is an aqueous solution with a formaldehyde mass fraction of 36-38%, preferably 37%. The diglycidyl ether is polyethylene glycol diglycidyl ether. It is preferably a product with a number-average molecular weight (Mn) of 200-600 and an epoxy value between 0.5-0.8 eq / 100g. The tertiary amine is triethanolamine.

[0026] The gelling step involves adding an aqueous solution of phenol and formaldehyde to a reactor and reacting it under heating conditions. The reaction is terminated by cooling when the viscosity reaches 280–320 mPa·s. The mass ratio of lignin to phenol is greater than or equal to 40%, preferably 40–60%.

[0027] The lignin-phenolic resin adhesive of the present invention can achieve a high lignin substitution rate while maintaining good mechanical properties, high storage stability, good workability, and low curing requirements.

[0028] Preferably, the preparation method of the lignin-phenolic resin adhesive further includes a synergistic catalysis step, which is performed after the gelation step. The synergistic catalysis step is configured as follows: after the reactor temperature is lowered to 60-70°C, an inorganic acid catalyst is added and stirred for 15-20 minutes, followed by the addition of a metal oxide catalyst and stirring for 25-35 minutes to obtain the lignin-phenolic resin adhesive. The inorganic acid catalyst is boric acid. The metal oxide catalyst is nano-zinc oxide with an average particle size of 10-50 nm, preferably 20-30 nm. More preferably, the obtained lignin-phenolic resin adhesive is filtered before discharge.

[0029] The gelation step preferably employs stepwise polymerization. It includes a co-condensation step and a viscosity control step. The co-condensation step is configured as follows: 55-65% of the formaldehyde aqueous solution and phenol are added to the reactor and the temperature is raised to 90-95°C for 55-65 minutes. The viscosity control step is configured as follows: the remaining formaldehyde aqueous solution is added to the reactor, or added in 2-3 portions every 15-20 minutes at a replenishment rate of 2-5 parts by weight per minute, and the reaction continues at 90-95°C. When the viscosity reaches 280-320 mPa·s, the temperature is lowered to terminate the reaction.

[0030] In the process of forming the lignin-phenolic resin adhesive, the amounts of each component are as follows: 95-105 parts by weight of phenol, 180-220 parts by weight of formaldehyde aqueous solution, 40-60 parts by weight of lignin, 7-8 parts by weight of diglycidyl ether (a glycol), 3-4 parts by weight of tertiary amine, 8-12 parts by weight of alkaline catalyst, 1-2 parts by weight of inorganic acid catalyst, 0.5-1 part by weight of metal oxide catalyst, and an appropriate amount of water. The amount of water is 15-30% of the total amount of phenol, formaldehyde aqueous solution, lignin, tertiary amine, alkaline catalyst, inorganic acid catalyst, and metal oxide catalyst, preferably 20-25%.

[0031] More specifically, the detailed preparation method of lignin phenolic resin adhesive can be found in the following process.

[0032] Mix 40-60 parts by weight of lignin, 8-12 parts by weight of alkaline catalyst and all the water in a reactor, add 7-8 parts by weight of diglycidyl ether and 3-4 parts by weight of tertiary amine, heat to 60-70℃ and keep the temperature for 60-90 min. Add 95-105 parts by weight of phenol and 108-132 parts by weight of formaldehyde aqueous solution to the reactor and heat to 90-95℃ and react for 55-65 minutes. Add the remaining formaldehyde aqueous solution to the reactor or add it to the reactor in 2 to 3 portions every 15 to 20 minutes, and continue the reaction at 90 to 95°C. When the viscosity reaches 280 to 320 mPa·s, cool down to terminate the reaction. After the reactor is cooled to 60-70°C, 1-2 parts by weight of inorganic acid catalyst are added and stirred for 15-20 minutes. Then, 0.5-1 parts by weight of metal oxide catalyst are added and stirred for 25-35 minutes. After cooling to 35-45°C, the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0033] The present application will now be described in more detail with reference to embodiments and comparative examples.

[0034] Example 1.

[0035] The lignin substitution rate (lignin mass divided by phenol mass) is 40%.

[0036] 42 parts by weight of lignin, 8 parts by weight of sodium hydroxide, and 15% by weight of water of other components were mixed in a reactor. 7 parts by weight of polyethylene glycol diglycidyl ether and 3 parts by weight of triethanolamine were added. The mixture was heated to 60°C and kept at that temperature for 60 minutes.

[0037] 105 parts by weight of phenol and 108 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 90°C and reacted for 60 minutes.

[0038] The remaining 72 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 15-minute intervals (the addition rate was 2 parts by weight per minute), and the reaction was continued at 90°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction. After the reactor is cooled to 60°C, 1 part by weight of boric acid is added and stirred for 15 minutes, followed by 0.5 parts by weight of nano zinc oxide and stirred for 30 minutes. The temperature is then lowered to 40°C, and the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0039] Example 2.

[0040] The lignin substitution rate (lignin mass divided by phenol mass) is 40%.

[0041] 40.8 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0042] 102 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0043] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 3 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0044] After the reactor temperature is reduced to 65°C, 1.5 parts by weight of boric acid are added and stirred for 18 minutes. Then, 0.75 parts by weight of nano zinc oxide are added and stirred for 30 minutes. The temperature is reduced to 40°C, and then the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0045] Example 3 The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0046] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0047] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0048] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 3 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0049] After the reactor temperature is reduced to 65°C, 1.5 parts by weight of boric acid are added and stirred for 18 minutes. Then, 0.75 parts by weight of nano zinc oxide are added and stirred for 30 minutes. The temperature is reduced to 40°C, and then the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0050] Example 4.

[0051] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0052] 47.5 parts by weight of lignin, 12 parts by weight of sodium hydroxide, and 25% by weight of water of other components were mixed in a reactor. 8 parts by weight of polyethylene glycol diglycidyl ether and 4 parts by weight of triethanolamine were added, and the mixture was heated to 70°C and kept at that temperature for 90 minutes.

[0053] Add 95 parts by weight of phenol and 132 parts by weight of 37% formaldehyde aqueous solution to the reactor and heat to 95°C and react for 60 min.

[0054] The remaining 88 parts by weight of formaldehyde aqueous solution were added to the reactor at one time (the addition rate was 5 parts by weight per minute), and the reaction was continued at 95°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0055] After the reactor temperature is reduced to 70°C, 2 parts by weight of boric acid are added and stirred for 20 minutes. Then, 1.0 part by weight of nano zinc oxide is added and stirred for 30 minutes. The temperature is reduced to 40°C, and then the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0056] Example 5.

[0057] The lignin substitution rate (lignin mass divided by phenol mass) is 60%.

[0058] 60 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0059] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0060] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 3 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0061] After the reactor temperature is reduced to 65°C, 1.5 parts by weight of boric acid are added and stirred for 18 minutes. Then, 0.75 parts by weight of nano zinc oxide are added and stirred for 30 minutes. The temperature is reduced to 40°C, and then the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0062] Example 6.

[0063] The lignin substitution rate (lignin mass divided by phenol mass) is 60%.

[0064] 57 parts by weight of lignin, 12 parts by weight of sodium hydroxide, and 30% by weight of water of other components were mixed in a reactor. 8 parts by weight of polyethylene glycol diglycidyl ether and 4 parts by weight of triethanolamine were added, and the mixture was heated to 70°C and kept at that temperature for 90 minutes.

[0065] Add 95 parts by weight of phenol and 132 parts by weight of 37% formaldehyde aqueous solution to the reactor and heat to 95°C and react for 60 min.

[0066] The remaining 88 parts by weight of formaldehyde aqueous solution were added to the reactor at one time (the addition rate was 5 parts by weight per minute), and the reaction was continued at 95°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0067] After the reactor temperature is reduced to 70°C, 2 parts by weight of boric acid are added and stirred for 20 minutes. Then, 1.0 part by weight of nano zinc oxide is added and stirred for 30 minutes. The temperature is reduced to 40°C, and then the mixture is filtered through a 100-mesh stainless steel filter to obtain lignin phenolic resin adhesive.

[0068] Comparative Example 1.

[0069] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0070] Mix 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide and 20% by weight of water of other components in a reactor, add 3.5 parts by weight of triethanolamine, heat to 65°C and keep the temperature for 75 min. 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0071] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0072] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0073] Comparative Example 2.

[0074] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0075] Mix 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components in a reactor. Add 7.5 parts by weight of polyethylene glycol diglycidyl ether, heat to 65°C, and maintain the temperature for 75 minutes.

[0076] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0077] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0078] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0079] Comparative Example 3.

[0080] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0081] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0082] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0083] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0084] After the reactor is cooled to 60°C, it is stirred for 30 minutes, then cooled to 40°C, and finally filtered through a 100-mesh stainless steel filter before being discharged.

[0085] Comparative Example 4.

[0086] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0087] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor, heated to 65°C, and kept at that temperature for 75 minutes.

[0088] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0089] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0090] After the reactor is cooled to 60°C, it is stirred for 30 minutes, then cooled to 40°C, and finally filtered through a 100-mesh stainless steel filter before being discharged.

[0091] Comparative Example 5.

[0092] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0093] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0094] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0095] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute). The reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0096] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0097] Comparative Example 6 The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0098] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 10 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added. The mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0099] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0100] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0101] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0102] Comparative Example 7.

[0103] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0104] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 1.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0105] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0106] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0107] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0108] Comparative Example 8.

[0109] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0110] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 5.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0111] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0112] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0113] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0114] Comparative Example 9.

[0115] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0116] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0117] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0118] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0119] After the reactor temperature drops to 65°C, add 0.5 parts by weight of boric acid and stir for 18 minutes, then add 0.75 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0120] Comparative Example 10.

[0121] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0122] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0123] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0124] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0125] After the reactor temperature is reduced to 65°C, 3 parts by weight of boric acid are added and stirred for 18 minutes. Then, 0.75 parts by weight of nano zinc oxide are added and stirred for 30 minutes. The temperature is then reduced to 40°C, and the mixture is then filtered through a 100-mesh stainless steel filter before being discharged.

[0126] Comparative Example 11.

[0127] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0128] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0129] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0130] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0131] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 0.2 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0132] Comparative Example 12.

[0133] The lignin substitution rate (lignin mass divided by phenol mass) is 50%.

[0134] 50 parts by weight of lignin, 10 parts by weight of sodium hydroxide, and 20% by weight of water of other components were mixed in a reactor. 7.5 parts by weight of polyethylene glycol diglycidyl ether and 3.5 parts by weight of triethanolamine were added, and the mixture was heated to 65°C and kept at that temperature for 75 minutes.

[0135] 100 parts by weight of phenol and 120 parts by weight of 37% formaldehyde aqueous solution were added to the reactor and the temperature was raised to 92°C and reacted for 60 minutes.

[0136] The remaining 80 parts by weight of formaldehyde aqueous solution were added to the reactor in two portions at 18-minute intervals (the addition rate was 5 parts by weight per minute), and the reaction was continued at 92°C. When the viscosity reached 320 mPa·s, the temperature was lowered to terminate the reaction.

[0137] After the reactor temperature drops to 65°C, add 1.5 parts by weight of boric acid and stir for 18 minutes, then add 1.5 parts by weight of nano zinc oxide and stir for 30 minutes. After cooling to 40°C, filter the material through a 100-mesh stainless steel filter screen.

[0138] The components of Examples 1-6 and Comparative Examples 1-12 are summarized in Table 1 below.

[0139] Table 1 .

[0140] The following tests were performed on Examples 1-6 and Comparative Examples 1-12, and the results are shown in Table 2.

[0141] Viscosity test: According to GB / T 2794-2013, the viscosity was measured at 20 rpm using a Brookfield DV-II+ rotational viscometer at 25°C.

[0142] Free formaldehyde content: Tested according to GB / T 14074-2017 using the hydroxylamine hydrochloride method.

[0143] Bond strength (dry / wet): Bond strength was tested using an electronic universal testing machine in accordance with GB / T 17657-2013 and GB / T 9846-2015. The wet strength test conditions were to boil the sample in water at 63±3℃ for 3 hours.

[0144] Curing conditions: The curing temperature and curing time were measured using a temperature-time curve recorder of a hot press. The criteria for judgment were that the adhesive layer was completely cured, no longer sticky, and no longer stringy.

[0145] Storage stability: The sample was placed in an environment of 25±2℃, and samples were taken every 30 days to observe the layering, gelation and precipitation phenomena, and the viscosity change was measured. The viscosity change rate was calculated as ((detected viscosity - initial viscosity) / initial viscosity × 100%).

[0146] Table 2 .

[0147] Among them, the lignin phenolic resin adhesives obtained in Examples 1-6 have moderate viscosity, good workability and coating properties, good storage stability and bonding strength higher than the standard requirements (both dry and wet strengths need to reach 1.8 MPa).

[0148] Furthermore, Examples 1-6 were characterized by infrared spectroscopy using a Nicolet iS10 Fourier transform infrared spectrometer with KBr pellets and a scanning range of 4000–400 cm⁻¹. -1 The results showed that the modified lignin was at 1630 cm⁻¹ -1 The peak shape remains unchanged near (aromatic ring skeletal vibration), while the peak shape at 3400 cm⁻¹ remains unchanged. -1 The stretching vibration peak of the phenolic hydroxyl group (OH) was significantly weakened at 1100 cm⁻¹. -1 The presence of a characteristic absorption peak of COC ether bond nearby indicates that polyethylene glycol diglycidyl ether has chemically bonded to the phenolic hydroxyl group of lignin.

[0149] For Comparative Example 1, the viscosity was too high and the bonding strength was substandard. It delaminated after 7 days of storage and completely gelled after 15 days. Infrared spectroscopy characterization showed that the modified sample had a viscosity of 1100–1150 cm⁻¹. -1 The COC stretching vibration peak was not enhanced at 1600–1580 cm⁻¹. -1 The absorption peak of the phenolic hydroxyl group did not decrease significantly, confirming that no ring-opening reaction occurred between polyethylene glycol diglycidyl ether and lignin. Therefore, without polyethylene glycol diglycidyl ether modification, the phenolic hydroxyl groups of lignin were not fully activated, leaving a large amount of residual phenolic hydroxyl groups. The addition of boric acid catalyzed rapid polymerization, resulting in excessive final viscosity and poor water resistance and storage stability.

[0150] For Comparative Example 2, without triethanolamine, lignin could not be effectively dispersed in the system, polyethylene glycol diglycidyl ether was difficult to react fully with lignin, and the compatibility was extremely poor. After the addition of boric acid, the acidic environment accelerated the precipitation of aggregates, and physical thickening led to an increase in final viscosity. Both strength and stability were seriously substandard.

[0151] For Comparative Example 3, in the absence of boric acid / ZnO synergistic catalytic system, although the basic viscosity was controllable and the final viscosity was normal, the curing activation energy did not decrease, and curing required 26 minutes at 156°C, which was much higher than the 10-12 minutes at 130°C in Examples 1-6. Furthermore, DSC differential scanning calorimetry of Comparative Example 3 showed a significantly broadened curing exothermic peak and insufficient crosslinking density, leading to a decrease in wet strength to 1.33 MPa, failing to meet the Class I plate standard.

[0152] For Comparative Example 4, without composite modification or synergistic catalysis, the high substitution rate of lignin resulted in almost no reaction between lignin and the phenolic system, extremely slow polycondensation, and irreversible aggregation and pre-crosslinking occurred in the system when the viscosity control node was barely reached, making the final product completely unusable.

[0153] For Comparative Example 5, when the amount of polyethylene glycol diglycidyl ether was insufficient, only some of the lignin phenolic hydroxyl groups were activated, the hydrogen bonds within the lignin molecule were not sufficiently broken, and the exposed active sites were insufficient, thus prolonging the polycondensation time and failing to meet the standards for water resistance and storage stability.

[0154] For Comparative Example 6, infrared spectroscopy characterization showed that the COC peak was abnormally enhanced and broadened. Excessive polyethylene glycol diglycidyl ether on the surface caused epoxy self-polymerization side reaction in the system, which resulted in increased viscosity and decreased adhesive strength.

[0155] For Comparative Example 7, when the amount of triethanolamine added was too low, it could not provide sufficient alkaline sites and dispersion effect, resulting in severe aggregation of lignin in the system. This led to a decrease in the modification efficiency of polyethylene glycol diglycidyl ether. After the addition of boric acid, the acidic environment further aggravated the precipitation of lignin, ultimately resulting in excessive viscosity and a decrease in storage stability and bonding strength.

[0156] For Comparative Example 8, infrared characterization was performed, and the results were observed at 1550–1580 cm⁻¹. -1 The presence of a distinct new peak indicating an amine-epoxy side reaction (a characteristic peak of primary amine addition products) proves that excess triethanolamine induced the side reaction crosslinking. This suggests that while the viscosity and strength remain acceptable due to the side reaction between excess amine and epoxy groups, the excessive triethanolamine leads to a significant increase in viscosity during storage due to the continued occurrence of the side reaction, resulting in insufficient storage stability.

[0157] For Comparative Example 9, the low amount of boric acid added reduced the synergistic catalytic effect, resulting in a limited reduction in the curing activation energy and excessively high curing conditions. Furthermore, DSC analysis showed insufficient crosslinking density, leading to a significant decrease in bond strength.

[0158] For Comparative Example 10, excessive boric acid caused the system to become too acidic, which promoted the premature condensation of hydroxymethyl groups. This not only caused the viscosity to exceed the standard, but the uncontrollable reaction that occurred prematurely also produced a large amount of flocculation during storage and led to the release of a large amount of free formaldehyde.

[0159] For Comparative Example 11, when the amount of nano-zinc oxide added is too small, the number of catalytically active sites is insufficient, which cannot provide enough surface catalytic activity to reduce the curing energy barrier, resulting in increased curing conditions. Furthermore, insufficient catalysis also reduces the crosslinking density, leading to a decrease in bond strength.

[0160] For Comparative Example 12, excessive nano zinc oxide leads to difficulty in dispersion and severe agglomeration, which not only significantly increases viscosity but also generates a large number of defects, resulting in a decrease in bond strength.

[0161] A second aspect of the present invention provides a lignin-phenolic resin adhesive, which is prepared by the method for preparing the lignin-phenolic resin adhesive of the first aspect described above.

[0162] The lignin-phenolic resin adhesive according to this application has similar technical effects to the preparation method of the lignin-phenolic resin adhesive of the first aspect described above.

[0163] The processes and steps described in all the preferred embodiments above are merely examples. Unless adverse effects occur, various processing operations can be performed in a different order than the above-described processes. The order of steps in the above processes can also be added, combined, or deleted according to actual needs.

[0164] Unless otherwise defined, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for descriptive purposes only and is not intended to limit the scope of this application. Features described in one embodiment may be applied, alone or in combination with other features, to another embodiment, unless that feature is not applicable in that other embodiment or is otherwise stated.

[0165] This application has been described through the above embodiments. However, it should be understood that the above embodiments are only for illustrative purposes. This application is not limited to the above embodiments. Many variations and modifications can be made based on the teachings of this application, and all such variations and modifications fall within the scope of protection claimed in this application.

Claims

1. A method for preparing a lignin-phenolic resin adhesive, characterized in that, The preparation method of the lignin phenolic resin adhesive includes the following steps: The activation step involves mixing lignin, an alkaline catalyst, and water in a reactor, adding diglycidyl ether (a diol) and tertiary amines, heating to 60-70°C, and maintaining the temperature for 60-90 minutes to obtain modified lignin. In the gelation step, phenol and formaldehyde aqueous solution are added to the reactor and reacted under heating conditions. When the viscosity reaches 280~320mPa·s, the reaction is terminated by cooling. The mass ratio of lignin to phenol is greater than or equal to 40%.

2. The method for preparing the lignin-phenolic resin adhesive according to claim 1, characterized in that, The preparation method of the lignin phenolic resin adhesive further includes a synergistic catalytic step, which is performed after the gelation step. The synergistic catalytic step includes: cooling the reactor to 60~70℃, adding an inorganic acid catalyst and stirring for 15~20min, then adding a metal oxide catalyst and stirring for 25~35min to obtain the lignin phenolic resin adhesive.

3. The method for preparing the lignin-phenolic resin adhesive according to claim 1, characterized in that, The diglycidyl ether is polyethylene glycol diglycidyl ether.

4. The method for preparing the lignin-phenolic resin adhesive according to claim 1, characterized in that, The tertiary amine is triethanolamine.

5. The method for preparing the lignin-phenolic resin adhesive according to claim 2, characterized in that, The inorganic acid catalyst is boric acid.

6. The method for preparing lignin-phenolic resin adhesive according to claim 2, wherein the metal oxide catalyst is nano-zinc oxide.

7. The method for preparing the lignin-phenolic resin adhesive according to claim 1, characterized in that, The alkaline catalyst is selected from at least one of sodium hydroxide and potassium hydroxide; and / or The formaldehyde aqueous solution is an aqueous solution with a formaldehyde mass fraction of 36-38%.

8. The method for preparing the lignin-phenolic resin adhesive according to claim 2, characterized in that, The dosage of each component is as follows: Phenol, 95-105 parts by weight Formaldehyde aqueous solution, 180-220 parts by weight. Lignin, 40-60 parts by weight Diol diglycidyl ether, 7-8 parts by weight Tertiary amines, 3-4 parts by weight, Alkaline catalyst, 8-12 parts by weight Inorganic acid catalyst, 1-2 parts by weight Metal oxide catalyst, 0.5~1 parts by weight, The amount of water used is 15-30% of the total amount of the phenol, the formaldehyde aqueous solution, the lignin, the tertiary amine, the alkaline catalyst, the inorganic acid catalyst, and the metal oxide catalyst.

9. The method for preparing the lignin-phenolic resin adhesive according to claim 1, characterized in that, The gelation step includes: In the co-condensation step, 55-65% of the formaldehyde aqueous solution and the phenol are added to the reactor and the temperature is raised to 90-95°C and reacted for 55-65 minutes. In the viscosity control step, the remaining formaldehyde aqueous solution is added to the reactor or added to the reactor in 2 to 3 portions every 15 to 20 minutes, and the reaction is continued at 90 to 95°C. When the viscosity reaches 280 to 320 mPa·s, the temperature is lowered to terminate the reaction.

10. A lignin-phenolic resin adhesive, characterized in that, The lignin phenolic resin adhesive is prepared by the method for preparing lignin phenolic resin adhesive according to any one of claims 1-9.