A bio-based non-isocyanate polyurethane wood adhesive, method of making and use

Abstract

The present application belongs to the technical field of wood adhesive, and particularly relates to a bio-based non-isocyanate polyurethane wood adhesive, a preparation method and an application. The adhesive is obtained by modifying epoxy soybean oil to obtain a soybean oil-based cyclic carbonate, and then ring-opening addition with furan-2,5-diformyl hydrazine to prepare a bio-based non-isocyanate polyurethane water dispersion. The non-isocyanate polyurethane water dispersion can be further polymerized with acrylamide, an initiator and a crosslinking agent to form an adhesive containing bio-based non-isocyanate polyurethane and polyacrylamide, which has a porous structure or a knitted chain entanglement structure, so that the tearing energy is greatly increased, the acid and alkali resistance is good, and the dry and wet shear strength is improved.

Description

Technical Field

[0001] This invention belongs to the field of wood adhesive technology, specifically relating to a bio-based non-isocyanate polyurethane wood adhesive, its preparation method, and its uses. Background Technology

[0002] Wood, as a renewable and environmentally friendly natural material, is widely used in construction, decoration, furniture manufacturing, packaging, and transportation. Adhesives are the core material for bonding substrates in wood processing. Traditional wood adhesives are mainly "three-aldehyde" adhesives, including phenolic resin, urea-formaldehyde resin, and melamine-formaldehyde resin. These adhesives dominate the market due to their high bonding strength, low cost, and good process compatibility. However, they pose a potential formaldehyde release hazard during production and use. Currently, polyurethane adhesives have become a research focus due to their high bonding strength, strong water resistance, and lack of formaldehyde addition and release. However, the isocyanate monomers required for polyurethane preparation are toxic, and long-term exposure can cause serious harm to human health and the production environment. Furthermore, the raw materials are derived from non-renewable petrochemical resources, which does not align with the green development concept under the "dual carbon" goal.

[0003] Epoxidized soybean oil, as a renewable and environmentally friendly material, has begun to be used in adhesives. However, epoxidized soybean oil-based adhesives generally suffer from low bond strength (see CN118085816A). Regarding the toxicity of isocyanates, the field currently employs bio-based raw materials to prepare bio-based cyclic carbonates, and then generates non-isocyanate polyurethanes through ring-opening addition reactions of cyclic carbonates with amine compounds. This technology helps to achieve a non-toxic alternative to the phosgene method for preparing isocyanates. At present, bio-based non-isocyanate polyurethanes have been preliminarily studied as wood adhesives, but problems such as complex synthesis processes, low utilization of bio-based raw materials, high brittleness of the adhesive layer, and weak interfacial forces with wood limit their further promotion and application in the wood industry (e.g., the non-isocyanate polyurethane adhesive in CN119144265A).

[0004] Therefore, there is an urgent need to develop a bio-based non-isocyanate polyurethane wood adhesive that combines high cohesion and high interfacial toughness, prepared from biomass resources, in order to promote the green and low-carbon transformation of the wood processing industry. Summary of the Invention

[0005] To address the above problems, the purpose of this invention is to provide a bio-based non-isocyanate polyurethane wood adhesive, its preparation method, and its uses. Specifically, the technical solution of this invention includes:

[0006] A bio-based non-isocyanate polyurethane wood adhesive, said wood adhesive comprising a bio-based non-isocyanate polyurethane of formula (I) and polyacrylamide:

[0007] ( )

[0008] In formula I, R1 is: ;

[0009] In equation I, R2 is: .

[0010] Preferably, the wood adhesive exhibits a porous structure or a knitted chain entanglement structure in its scanning electron microscope (SEM) image. Wood adhesives exhibiting a knitted chain entanglement structure in their SEM images show the best performance.

[0011] Preferably, the atomic force microscopy image of the wood adhesive shows a microphase separation morphology.

[0012] A method for preparing a bio-based non-isocyanate polyurethane wood adhesive, the method comprising the following steps:

[0013] Preparation of S1 soybean oil-based cyclic carbonate: Epoxidized soybean oil and CO2 were reacted under Lewis base catalysis to obtain soybean oil-based cyclic carbonate;

[0014] Preparation of S2 bio-based non-isocyanate polyurethane: Furan-2,5-dicarboxyhydrazide and soybean oil-based cyclic carbonate were subjected to ring-opening addition to obtain a bio-based non-isocyanate polyurethane aqueous dispersion.

[0015] Preferably, the reaction heating temperature in step S1 is 110 °C ~ 140 °C, the CO2 pressure is 1.0 ~ 3 MPa, and the reaction time is 4 ~ 8 h.

[0016] Preferably, in step S2, the mass ratio of soybean oil-based cyclic carbonate to furan-2,5-dicarboxyhydrazide is (1.5-2.5):1.

[0017] Preferably, the Lewis base is selected from any one of tetrabutylammonium bromide, triethylamine, tetraethylammonium bromide, dimethyl sulfide, and acrylamide.

[0018] In an improved embodiment, the preparation method further includes the following steps:

[0019] Preparation of S3 polyacrylamide-bio-based non-isocyanate polyurethane: Acrylamide, initiator, crosslinking agent and bio-based non-isocyanate polyurethane aqueous dispersion are mixed and polymerized by heating.

[0020] Preferably, in step S3, the initiator is ammonium persulfate and the crosslinking agent is N,N'-methylenebisacrylamide; the mass ratio of acrylamide, N,N'-methylenebisacrylamide, non-isocyanate polyurethane dispersion, and ammonium persulfate in step S3 is (7-20):(0.00001-0.00003):11.3:0.0000069.

[0021] The present invention further provides the use of any of the aforementioned bio-based non-isocyanate polyurethane wood adhesives as wood adhesives.

[0022] Beneficial effects:

[0023] (1) This invention provides a bio-based non-isocyanate polyurethane wood adhesive with high cohesive strength and high interfacial toughness. This adhesive uses polyacrylamide as a matrix (especially polyacrylamide with entangled polymer chains). Hard-phase bio-based non-isocyanate polyurethane clusters are uniformly embedded in the soft-phase polyacrylamide skeleton through hydrogen bonds, resulting in significantly improved tear energy, good acid and alkali resistance, and enhanced dry and wet shear strength. In particular, adhesives with microphase separation and chain entanglement structures benefit from the synergistic effect of the entangled polyacrylamide chains and bio-based non-isocyanate polyurethane clusters, which contribute to improved ductility and stiffness. The hydrogen bonds between the bio-based non-isocyanate polyurethane clusters and polyacrylamide chains act as dynamic sacrificial bonds, effectively dissipating energy through breakage and recombination. The soft phase of the polyacrylamide chains can elongate along the stress direction, and the synergistic effect of hydrogen bonds and polyacrylamide chains imparts excellent tensile properties. The hydrogen bonds in the bio-based non-isocyanate polyurethane clusters are oriented, and the cluster structure can withstand greater stress dispersion, enabling the polymer to enhance stiffness. Therefore, this microphase separation structure endows the adhesive with high cohesive strength.

[0024] (2) Bio-based non-isocyanate polyurethane adhesives with microphase separation structure can promote the formation of dense hydrogen bonds and covalent bonds at the wood interface through urethane bonds and amide bonds in polyacrylamide. At the same time, some acrylamide forms a strong topological entanglement with the wood through penetration, diffusion and polymerization at the wood interface, thereby improving the interfacial force between the adhesive and the wood.

[0025] (3) This invention uses bio-based raw materials to replace fossil raw materials, effectively achieving the renewability and sustainability of raw materials. No organic solvents are used in the preparation process, resulting in low energy consumption and facilitating industrial production and promotion. Attached Figure Description

[0026] Figure 1 The NMR spectrum of the synthesis of epoxidized soybean oil and soybean oil-based cyclic carbonate;

[0027] Figure 2 The infrared spectrum of the synthesis of epoxidized soybean oil and soybean oil-based cyclic carbonate;

[0028] Figure 3 The infrared spectrum of the synthesis of non-isocyanate polyurethane;

[0029] Figure 4 ,in Figure 4a is a scanning electron microscope image of the bio-based non-isocyanate polyurethane wood adhesive of Example 1. Figure 4 b is a scanning electron microscope image of the bio-based non-isocyanate polyurethane wood adhesive of Example 2;

[0030] Figure 5 Scanning electron microscope (a) and atomic force microscope (b) images of the bio-based non-isocyanate polyurethane wood adhesive of Example 3;

[0031] Figure 6 Tensile properties of the bio-based non-isocyanate polyurethane wood adhesive used in this example;

[0032] Figure 7 This example illustrates the damage to wood bonded with a bio-based non-isocyanate polyurethane wood adhesive.

[0033] Figure 8 The following are the results of the bonding performance and dry and wet shear strength of the bio-based non-isocyanate polyurethane wood adhesive in the examples.

[0034] Figure 9 This is a comparison diagram of the tearing energy of samples 1-3.

[0035] Figure 10 This is a comparison graph showing the tear energy of samples from Example 3, Comparative Example 4, and Comparative Example 5.

[0036] Figure 11 Comparison of dry and wet shear strength of plywood prepared with adhesives from Examples 1-5. Detailed Implementation

[0037] To make the above-mentioned objects, embodiments, features and advantages of the present invention more apparent and understandable, the present invention will be described in detail below with reference to embodiments.

[0038] The following detailed embodiments provide numerous specific technical details to facilitate a thorough understanding and implementation of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions, substitutions, or transformations without departing from the essence and basic principles of the present invention, such as equivalent substitutions of similar catalysts, solvents, etc. Therefore, the present invention is not limited to the following specific embodiments.

[0039] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0040] The epoxidized soybean oil (analytical grade, epoxy value = 0.52~0.63), tetrabutylammonium bromide (analytical grade, 99.0%), and furan-2,5-dicarboxyhydrazide (analytical grade, 99.0%) mentioned in the following specific embodiments are from Aladdin Biochemical Technology Co., Ltd.; N,N'-methylenebisacrylamide (analytical grade, >99.0%), ammonium persulfate (analytical grade, >99.0%), and acrylamide (analytical grade, >99.0%) are from Shanghai Maclean Biochemical Technology Co., Ltd.; and methanol is from Nanjing Chemical Reagent Co., Ltd.

[0041] Example 1: A bio-based non-isocyanate polyurethane adhesive and its preparation method

[0042] This embodiment provides a method for preparing a bio-based non-isocyanate polyurethane adhesive.

[0043] The main steps of the preparation method of this bio-based non-isocyanate polyurethane adhesive are as follows:

[0044] (1) 100.0 g of epoxidized soybean oil and 10.0 g of tetrabutylammonium bromide catalyst were added to a 250 mL high-pressure reactor, mixed and stirred, and heated to 130 °C. CO2 was introduced into the high-pressure reactor to 1.5 MPa, and the reaction was carried out for 6 h until the pressure no longer changed, at which point the reaction was complete, yielding a golden viscous product, which was soybean oil-based carbonate. Soybean oil-based epoxidized carbonate... 1 The H-NMR (CDCl3) and infrared analysis spectra are shown below. Figure 1 and Figure 2 .Depend on Figure 1 It can be seen that it is similar to epoxidized soybean oil. 1 Compared to ¹H NMR, soybean oil-based cyclic carbonates exhibit a cyclic carbonate peak in the range of 4.45–4.90 ppm. Infrared analysis shows that, compared to epoxidized soybean oil, soybean oil-based cyclic carbonates have a peak at 822.01 cm⁻¹. -1 The characteristic absorption peak of the epoxy group at 1800.21 cm⁻¹ disappears. -1 Characteristic peaks of carbonates appear at [location].

[0045] (2) 10.12 g of soybean oil-based cyclic carbonate and 5.26 g of furan-2,5-dicarboxyhydrazide were thoroughly mixed (the mass ratio of soybean oil-based cyclic carbonate to furan-2,5-dicarboxyhydrazide was 1.92395:1), and stirred at 85 °C for 4 h to obtain a non-isocyanate polyurethane dispersion with urethane structural units. Figure 3 Infrared spectroscopy reveals that, compared to soybean oil-based cyclic carbonates, non-isocyanate polyurethanes exhibit higher activity at 3288 cm⁻¹. -1 The amino peak at 1800.21 cm⁻¹ disappears. -1 The characteristic peak of cyclic carbonate at 1298.34 cm⁻¹ weakens, while the peak at 1298.34 cm⁻¹ weakens.-1 1740.44 cm -1 and 3299.61 cm -1 The infrared absorption peaks are attributed to COC, C=O, and NH in the urethane bond, respectively. Therefore, furan-2,5-dicarboxyhydrazide reacts with soybean oil-based cyclic carbonate to produce a non-isocyanate polyurethane.

[0046] like Figure 4 As shown in Figure a, the dense structure of the non-isocyanate polyurethane adhesive without polyacrylamide prepared in this embodiment, which exhibits uneven soft and hard phases, is mainly attributed to the excessive stacking of the hard phase region.

[0047] Example 2: A bio-based non-isocyanate polyurethane adhesive and its preparation method

[0048] This embodiment provides a method for preparing a bio-based non-isocyanate polyurethane adhesive, differing from Example 1 only in the addition of step (3): 7.5 g of acrylamide, 1350 μL of N,N'-methylenebisacrylamide aqueous solution (N,N'-methylenebisacrylamide mass concentration of 1.5 wt%), and 1.66 g of water are added to 11.3 g of non-isocyanate polyurethane dispersion and mixed evenly. Then, 300 μL of ammonium persulfate aqueous solution (ammonium persulfate mass concentration of 2.3 wt%) is added at room temperature and poured into a glass mold. After degassing and defoaming, the mixture is polymerized at 60 °C for 3 hours to form a bio-based non-isocyanate polyurethane gel adhesive. The calculated mass ratio of acrylamide, N,N'-methylenebisacrylamide, non-isocyanate polyurethane dispersion, and ammonium persulfate in this process is 7.5:0.00002:11.3:0.0000069.

[0049] like Figure 4 As shown in b, the non-isocyanate polyurethane adhesive containing polyacrylamide prepared in this embodiment exhibits a large number of porous structures. With the introduction of polyacrylamide, a cross-linked adhesive structure is prepared, but the porous structure becomes smaller.

[0050] Example 3: A bio-based non-isocyanate polyurethane adhesive and its preparation method

[0051] This embodiment provides a method for preparing a bio-based non-isocyanate polyurethane adhesive, differing from Example 1 only in the addition of step (3): 15.0 g of acrylamide and 1000 μL of N,N'-methylenebisacrylamide aqueous solution (N,N'-methylenebisacrylamide mass concentration is 1.5 wt%) are added to 11.3 g of non-isocyanate polyurethane aqueous dispersion and mixed evenly. Then, 300 μL of ammonium persulfate aqueous solution (ammonium persulfate mass concentration is 2.3 wt%) is added at room temperature and poured into a glass mold. After degassing and defoaming, the mixture is polymerized at 60 °C for 3 h to form a bio-based non-isocyanate polyurethane gel adhesive. The calculated mass ratio of acrylamide, N,N'-methylenebisacrylamide, non-isocyanate polyurethane dispersion, and ammonium persulfate in this process is 15:0.000015:11.3:0.0000069.

[0052] like Figure 5 As shown, when the acrylamide content is further increased, the water content is decreased. This embodiment prepares a bio-based non-isocyanate polyurethane adhesive with a knitted chain entanglement structure. Figure 5 a) This structure is significantly different from the porous structures of Examples 1 and 2. Atomic force microscopy revealed that the adhesive exhibits a uniform microphase separation morphology. Figure 5 b).

[0053] Example 4: Tensile property analysis of bio-based non-isocyanate polyurethane adhesive

[0054] The tensile properties of bio-based non-isocyanate polyurethane adhesives were evaluated by the tensile stress-strain behavior of polymer gel samples.

[0055] The adhesive samples prepared in Examples 1, 2, and 3 were cut into dumbbell-shaped strips with a length of 40 mm, a width of 4 mm, and a thickness of 2 mm. These strips were then subjected to uniaxial tensile testing using a universal tensile testing machine to determine tensile fracture. All uniaxial tensile tests were conducted at 50 mm / min. -1 The process was carried out. Young's modulus, tensile fracture stress, and fracture strain were obtained from the stress-strain curves. A 1 mm notch was cut into one side of the sample to create a notched specimen. Each group of samples was tested in at least three parallel experiments. The tear energy was calculated from the tensile force-displacement curves of the unnotched and notched specimens.

[0056] Figure 6The tensile properties of the bio-based non-isocyanate polyurethane wood adhesive are shown in the diagram. In this diagram, a represents the tensile stress-strain diagram of the adhesives in different embodiments, b and c represent the tear energy comparison diagrams of the adhesives in different embodiments, and d represents the Young's modulus, fracture stress, fracture strain, and tear energy diagrams of the adhesives in different embodiments. Comparison of Examples 1, 2, and 3 shows that with the introduction of polyacrylamide, the fracture strain increased from 101.35% to 562.61%, the Young's modulus increased from 245.84 kPa to 841.20 kPa, and the tear energy increased from 597.09 J / m². -2 Increased to 17195.08 J m -2 By introducing chain-entangled polyacrylamide (the adhesive in Example 3), the cohesive strength of the adhesive was significantly improved.

[0057] Example 5: Analysis of the interfacial adhesion performance between bio-based non-isocyanate polyurethane adhesive and wood

[0058] The interfacial bonding properties of bio-based non-isocyanate polyurethane adhesives were tested by peeling and tensile properties of adhesive samples at a 90° angle to the wood interface.

[0059] The adhesives prepared in Examples 1, 2, and 3 were used to prepare a gel adhesive on eucalyptus veneer measuring 2.5 cm wide, 8 cm long, and 2 mm thick using a conventional in-situ polymerization method (e.g., reference: Advanced Materials 2018 paper: Topological Adhesion of Wet Materials; https: / / doi.org / 10.1002 / adma.201800671). A universal tensile testing machine was used at a speed of 10 mm / min. -1 A 90° peel test was conducted at a constant peel speed to test the interfacial toughness of the adhesive.

[0060] The adhesives prepared in Examples 1, 2, and 3 were used to prepare a gel adhesive between two layers of eucalyptus veneer, each 5 cm wide, 5 cm long, and 2 mm thick, using a conventional in-situ polymerization method (e.g., reference: Advanced Materials 2018 paper: Topological Adhesion of Wet Materials; https: / / doi.org / 10.1002 / adma.201800671). A universal tensile testing machine was used at a speed of 10 mm / min. -1 Tensile tests were conducted at a constant tensile speed to test the tensile strength between the adhesive and the wood.

[0061] The interfacial toughness of the adhesives in Examples 1-3 increased from 424.53 J / m with increasing polyacrylamide content (increased acrylamide dosage).-2 Increased to 1581.56 J m -2 The tensile breaking force increased from 592.33 N to 3057.79 N. The interfacial toughness between the adhesive and the wood was improved by introducing a chain entanglement structure (the adhesive of Example 3).

[0062] Example 6: Bonding performance of bio-based non-isocyanate polyurethane adhesive for eucalyptus veneer

[0063] Using the bio-based non-isocyanate polyurethane adhesives prepared in Examples 1, 2, and 3 as the base adhesives, three-layer eucalyptus plywood was obtained by coating, pre-pressing, and hot-pressing eucalyptus veneers.

[0064] The manufacturing process of the three-layer eucalyptus plywood is as follows: the three-layer eucalyptus veneer consists of an upper layer, a lower layer, and a core layer. A bio-based non-isocyanate polyurethane adhesive is applied to both sides of the core layer, while the upper and lower layers are applied to only one side, with an adhesive application rate of 180 g / m². 2 Then, the coated upper layer, core layer, and lower layer are assembled along the direction perpendicular to the wood grain. The mixture is then subjected to room temperature cold pressing (1.0 MPa, 2 h) on a flat vulcanizing machine; followed by hot pressing at 120 °C (1.0 MPa, 405 s); finally, it is cooled to room temperature to obtain eucalyptus plywood.

[0065] The dry and wet bonding strength and other mechanical properties of the three types of plywood were tested according to the test methods for Class II plywood in GB / T 9846-2015 "Ordinary Plywood".

[0066] Depend on Figure 7 and Figure 8 It can be seen that the boards pressed with pure non-isocyanate adhesive (adhesive of Example 1) exhibit weak dry / wet shear strength (0.93 / 0.53 MPa) due to their brittleness and large number of hydrophilic groups. The wood failure rate after lap shearing is almost negligible. After the introduction of polyacrylamide (adhesive of Example 2), the shear strength is improved (dry strength 2.64 MPa, wet strength 0.92 MPa). When chain-entangled polyacrylamide is further introduced (adhesive of Example 3), the pressed boards have a dry / wet strength of 4.21 / 3.69 MPa, and the wood failure rate is as high as 90%. Force-distance curves were obtained from the dry and wet shear strength tests of the boards to evaluate the bonding work. Pure isocyanate polyurethane adhesive showed poor bonding performance (dry bonding work 0.23 J, wet bonding work 0.09 J), while the introduction of polyacrylamide increased the dry bonding work to 2.03 J and the wet bonding work to 0.14 J. In contrast, the introduction of chain-entangled polyacrylamide resulted in significantly higher adhesion work (2.92 J for dry bonding and 1.87 J for wet bonding), further demonstrating its excellent adhesive performance.

[0067] Example 7: Analysis of the bonding performance of bio-based non-isocyanate polyurethane adhesives in extreme environments.

[0068] The bonded wood samples prepared in Example 6 were immersed in water with different pH values, including acidic (pH=3-5), alkaline (pH=9-11), and neutral (pH=6-8.5), for 24 hours. They were then completely dried before the lap shear test. The bond strength before and after immersion was tested using standard methods, and all samples retained more than 90% of their original bond strength. Furthermore, after immersion in solvents such as ethanol, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, acetonitrile, ethyl acetate, dichloromethane, and methanol for 24 hours, the bond strength of the bonded wood samples remained above 3.1 MPa.

[0069] Example 8: Preparation and performance testing of exploratory wood adhesives

[0070] This embodiment provides the preparation and performance test results of several other exploratory examples (as comparative examples) of wood adhesives tested in the study.

[0071] The difference between the preparation method of the wood adhesive in Comparative Example 1 and Example 3 is that the ammonium persulfate aqueous solution (ammonium persulfate mass concentration of 2.3 wt%) in step (3) is 100 μL.

[0072] The difference between the preparation method of the wood adhesive in Comparative Example 2 and Example 3 is that the ammonium persulfate aqueous solution (ammonium persulfate mass concentration of 2.3 wt%) in step (3) is 800 μL.

[0073] The difference between the preparation method of the wood adhesive in Comparative Example 3 and Example 3 is that the ammonium persulfate aqueous solution (ammonium persulfate mass concentration of 2.3 wt%) in step (3) is 2000 μL.

[0074] The preparation method of the wood adhesive in Comparative Example 4 is as follows:

[0075] (1) 10.12g of unmodified epoxidized soybean oil (without carbonate modification) and 5.26g of furan-2,5-dicarboxyhydrazide were thoroughly mixed and stirred at 85 °C for 4h to obtain a dispersion.

[0076] (2) Add 15.0 g of acrylamide and 1000 μL of N,N'-methylenebisacrylamide aqueous solution (N,N'-methylenebisacrylamide mass concentration is 1.5 wt%) to 11.3 g of the dispersion in step (1) and mix well. Add 2000 μL of ammonium persulfate aqueous solution (ammonium persulfate mass concentration is 2.3 wt%) at room temperature and pour into a glass mold. After degassing and defoaming, polymerize the mixture at 60 °C for 3 h to form a bio-based non-isocyanate polyurethane gel adhesive.

[0077] Comparative Example 5 (This adhesive was prepared using a method previously obtained by the inventors from another exploratory example, see Chemical Engineering Journal 2025 paper: A bio-based non-isocyanate polyurethane adhesive with microphase-separated structure and topological entanglement to simultaneously realize enhanced cohesion and interfacial adhesion; https: / / doi.org / 10.1016 / j.cej.2025.171421)

[0078] (1) 100.0 g of sorbitol glycidyl ether and 10.0 g of tetrabutylammonium bromide catalyst were added to a 250 mL high-pressure reactor, mixed and stirred and heated to 130 °C. CO2 was introduced into the high-pressure reactor to 1.5 MPa and the reaction was carried out for 6 hours until the pressure no longer changed and the reaction was completed. The sorbitol cyclic carbonate obtained was a golden sticky product.

[0079] (2) Dissolve 58.2 g of sorbitol cyclic carbonate and 19.7 g of maleic anhydride in 30 mL of acetone. Stir the mixture at 130 °C for 3 hours under nitrogen protection. After the reaction is complete, remove the solvent by vacuum distillation, add triethanolamine to adjust the degree of neutralization to 90%, and obtain a white viscous aqueous sorbitol cyclic carbonate.

[0080] (3) 20.0 g of soy protein isolate was dispersed in 150 mL of deionized water and stirred at 85 °C for 4 hours to obtain a uniform soy protein isolate dispersion. At pH=10, 1.3 g of aqueous sorbitan cyclic carbonate was added to 10.0 g of soy protein dispersion and reacted at 85 °C for 30 min to obtain a bio-based non-isocyanate polyurethane dispersion.

[0081] (4) 7.5 g acrylamide, 1350 μL N,N-methylenebisacrylamide (1.5 wt%) and 1.66 g water were added to 11.3 g of bio-based non-isocyanate polyurethane dispersion and mixed evenly. 300 μL ammonium persulfate (2.3 wt%) was added at room temperature and poured into a glass mold. After degassing and defoaming, the mixture was polymerized at 60 °C for 3 hours to form a bio-based non-isocyanate polyurethane adhesive gel.

[0082] The bio-based non-isocyanate polyurethane gel adhesive samples prepared in Comparative Examples 1-5 were cut into dumbbell-shaped strips with a length of 40 mm, a width of 4 mm, and a thickness of 2 mm. Tensile fracture tests were then conducted using a universal tensile testing machine with a uniaxial tensile strength of 50 mm / min. -1 The process was carried out by cutting a 1 mm notch in the middle of the sample to create a notched specimen. Each group of samples was tested in at least three parallel experiments, and the tear energy was calculated from the tensile force-displacement curves of the unnotched and notched specimens.

[0083] like Figure 9 As shown in Examples 1, 2, and 3, it can be seen that when the mass ratio of acrylamide to ammonium persulfate is not properly controlled, the tear energy of the prepared polymer is very small, far lower than that of the adhesives in Examples 2 and 3. Figure 6 (Using the same methods and test conditions as the comparative example).

[0084] like Figure 10 As shown, compared with Comparative Examples 4 and 5, the non-isocyanate polyurethane adhesive with microphase separation structure (sample of Example 3) has a tear energy that is much higher than that of the adhesive prepared from unmodified epoxidized soybean oil and the reported sorbitol-soybean protein-based non-isocyanate polyurethane adhesive.

[0085] Using the bio-based non-isocyanate polyurethane adhesives prepared in Comparative Examples 1-5 as the base adhesives, three-layer eucalyptus plywood was obtained by coating, pre-pressing, and hot-pressing eucalyptus veneers.

[0086] The manufacturing process of the three-layer eucalyptus plywood is as follows: the three-layer eucalyptus veneer consists of an upper layer, a lower layer, and a core layer. A bio-based non-isocyanate polyurethane adhesive prepolymer is coated on both sides of the core layer, while the upper and lower layers are coated on only one side, with an adhesive application rate of 180 g / m². 2 Then, the coated upper layer, core layer, and lower layer are assembled along the direction perpendicular to the wood grain. The mixture is then subjected to room temperature cold pressing (1.0 MPa, 2 h) on a flat vulcanizing machine; followed by hot pressing at 120 °C (1.0 MPa, 405 s); finally, it is cooled to room temperature to obtain eucalyptus plywood.

[0087] The dry and wet bond strengths and other mechanical properties of three types of plywood were tested according to the test methods for Class II plywood in GB / T 9846-2015 "Ordinary Plywood" (the test methods and conditions were the same as in Example 6). Figure 8 , Figure 11 Test results show that the dry and wet bond strengths of plywood prepared with adhesives in Comparative Examples 1-4 are significantly lower than those of plywood prepared with the bio-based non-isocyanate polyurethane adhesive in Example 3. Compared to the adhesive prepared by the method of another previously obtained exploratory example (Comparative Example 5), the dry / wet shear strength of plywood prepared in Example 3 is also further improved.

Claims

1. A bio-based non-isocyanate polyurethane wood adhesive, characterized in that, The wood adhesive contains the bio-based non-isocyanate polyurethane and polyacrylamide of formula (I): （I）； In equation (I), R1 is: ; In equation (I), R2 is: .

2. The bio-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, The scanning electron microscope images of the wood adhesive show a porous structure or a knitted chain entanglement structure.

3. The bio-based non-isocyanate polyurethane wood adhesive according to claim 2, characterized in that, The atomic force microscopy image of the wood adhesive shows a microphase separation morphology.

4. A method for preparing a bio-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, The preparation method of the bio-based non-isocyanate polyurethane includes the following steps: Preparation of S1 soybean oil-based cyclic carbonate: Epoxidized soybean oil and CO2 were reacted under Lewis base catalysis to obtain soybean oil-based cyclic carbonate; Preparation of S2 bio-based non-isocyanate polyurethane: Furan-2,5-dicarboxyhydrazide and soybean oil-based cyclic carbonate were subjected to ring-opening addition to obtain a bio-based non-isocyanate polyurethane aqueous dispersion.

5. The method for preparing a bio-based non-isocyanate polyurethane wood adhesive according to claim 4, characterized in that, The reaction heating temperature in step S1 is 110 °C ~ 140 °C, the CO2 pressure is 1.0 ~ 3 MPa, and the reaction time is 4 ~ 8 h.

6. The method for preparing a bio-based non-isocyanate polyurethane wood adhesive according to claim 4, characterized in that, In step S2, the mass ratio of soybean oil-based cyclic carbonate to furan-2,5-dicarboxyhydrazide is (1.5–2.5):

1.

7. The method for preparing a bio-based non-isocyanate polyurethane wood adhesive according to claim 4, characterized in that, The Lewis base is selected from any one of tetrabutylammonium bromide, triethylamine, tetraethylammonium bromide, dimethyl sulfide, and acrylamide.

8. The method for preparing a bio-based non-isocyanate polyurethane wood adhesive according to claim 4, characterized in that, The preparation method further includes the following steps: Preparation of S3 polyacrylamide-bio-based non-isocyanate polyurethane: Acrylamide, initiator, crosslinking agent and bio-based non-isocyanate polyurethane aqueous dispersion are mixed and polymerized by heating.

9. The method for preparing a bio-based non-isocyanate polyurethane wood adhesive according to claim 8, characterized in that, In step S3, the crosslinking agent is N,N'-methylenebisacrylamide and the initiator is ammonium persulfate; the mass ratio of acrylamide, N,N'-methylenebisacrylamide, non-isocyanate polyurethane dispersion and ammonium persulfate in step S3 is (7-20):(0.00001-0.00003):11.3:0.0000069.

10. Use of a bio-based non-isocyanate polyurethane wood adhesive according to any one of claims 1 to 3, or a bio-based non-isocyanate polyurethane wood adhesive prepared by any one of claims 4 to 9, as a wood adhesive.

Images