Multistage crosslinking formaldehyde-free adhesive with three-dimensional network structure and preparation method thereof

Abstract

The invention discloses a multistage crosslinking formaldehyde-free adhesive with a three-dimensional network structure and a preparation method of the multistage crosslinking formaldehyde-free adhesive, and belongs to the technical field of high-molecular polymers. The environment-friendly formaldehyde-free adhesive is successfully prepared through the precise molecular structure design and the synergistic effect of the components, has high bonding strength, controllable viscosity, excellent water resistance, mildew resistance and flame retardance, is formaldehyde-free and environment-friendly, can effectively achieve bonding of various materials such as wood, paper and fiber products, and has the advantages of being high in adhesive strength, capable of controlling the viscosity and good in flame retardance. The application field is widened, the adhesive is endowed with good weather resistance and flame retardance due to unique composite polymer components and a cross-linked structure, the adhesive can still keep a stable bonding effect in a humid environment and under different weather conditions, the service life of a bonded product is remarkably prolonged, and the application range of the adhesive is widened. And meanwhile, powerful guarantee is provided for home safety by virtue of excellent flame retardant property.

Description

Technical Field

[0001] The invention belongs to the technical field of high molecular polymers, and particularly relates to a multi-stage cross-linked formaldehyde-free adhesive with a three-dimensional network structure and a preparation method thereof. Background Art

[0002] With rising global environmental awareness and increasingly stringent indoor air quality standards, the health risks and environmental pollution risks associated with formaldehyde emissions from traditional petroleum-based and urea-formaldehyde resin adhesives have become increasingly prominent, making them less adaptable to the current trend of green home furnishing. While bio-based adhesives, such as starch and cellulose, have achieved significant progress in achieving zero formaldehyde emissions, their industrialization still faces numerous obstacles.

[0003] On the one hand, to achieve the bonding strength required for industrial production, these bio-based adhesives often require complex modification processes, which undoubtedly increases production costs, making it difficult to strike a balance between cost and efficiency, and thus affecting their competitiveness in the market. On the other hand, existing bio-based adhesive systems have significant deficiencies in overall performance, making it difficult to simultaneously achieve the synergistic optimization of multiple properties such as water resistance, flame retardancy, and mildew resistance. This has significantly restricted the application and promotion of bio-based adhesives. Summary of the Invention

[0004] The purpose of this section is to summarize some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the abstract and title of this application to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions should not be used to limit the scope of the present invention.

[0005] In view of the problems of formaldehyde in the above-mentioned traditional adhesives and the existing bio-based adhesives, such as complex process, uncontrollable viscosity, difficulty in synergistically improving water resistance, flame retardancy and mildew resistance, the present invention innovatively proposes and constructs a phytic acid-based multi-stage cross-linking system. First, citric acid is extracted from the fermentation process of orange peels, and synergized with chitosan from shrimp and crab shells. Through the interaction between the two, a three-dimensional dynamic cross-linked network framework is initially constructed. This system lays a solid foundation for the subsequent performance improvement of adhesives. Then, natural phytic acid extracted from common agricultural waste such as rice bran and wheat husks is used, which is modified and introduced into the system. This step is intended to give the adhesive good adhesion, flame retardancy and mildew resistance, and to cross-link and synergize with chitosan to achieve a self-extinguishing flame retardant effect of the adhesive, greatly improving the fire resistance of the adhesive. Finally, through cross-linking reaction with tannic acid, the entire system is further improved to form a stable and dense three-dimensional network structure. The natural polyphenol structure of tannic acid also provides strong support for the improvement of its anti-mildew properties, making the final product perform well in anti-mildew.

[0006] During the research process, this study used advanced characterization techniques such as Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) to deeply explore the chemical bonding mechanism within the adhesive and the inherent principle of improved water resistance. Compared with traditional adhesives, this study successfully developed a pure plant-based wood adhesive that is formaldehyde-free and environmentally friendly, has controllable viscosity, high bonding strength, excellent water resistance, outstanding mildew resistance, and high-efficiency flame retardant properties. This innovative achievement not only provides a solid theoretical support for the green development of the home furnishing industry, but also contributes new innovative ideas and practical technical solutions to the realization of the country's "dual carbon" goals. It is expected to be widely used in future fields such as wood processing and furniture manufacturing, promote the sustainable development of the entire industry, and create a healthier and more environmentally friendly home environment for people.

[0007] Therefore, the purpose of the present invention is to address the core pain point of formaldehyde pollution and fire hazards in the field of wooden home furnishings, provide a theoretical basis for the green development of the home furnishing industry, and contribute innovative ideas and technical solutions to achieve the "dual carbon" goals. It overcomes the shortcomings of the existing technology and provides a formaldehyde-free, environmentally friendly, viscosity-controllable, and multifunctional adhesive with high bonding strength, water resistance, mildew resistance, and self-extinguishing flame retardant properties, aiming to break through the bottleneck of traditional technology.

[0008] In order to solve the above technical problems, the present invention provides the following technical solution, including: citric acid as the cross-linking hub, chitosan as the dynamic network skeleton, glycidyl methacrylate (GMA)-modified phytic acid (PA-GMA) as the flame retardant-adhesive dual-functional component, and tannic acid as the densification enhancer; the preparation process includes three stages: phytic acid modification, gradient cross-linking, and network curing, specifically: Step 1: Under urea catalysis and para-hydroxyanisole inhibition, phytic acid is grafted with epoxy groups by GMA to generate a PA-GMA prepolymer with controllable reaction activity; Step 2: Citric acid and chitosan are preliminarily constructed into a dynamic cross-linked network through ionic bonding / hydrogen bonding; Step 3: PA-GMA and tannic acid are introduced to strengthen the three-dimensional network through phosphate-phenolic hydroxyl covalent bonds to achieve multifunctionality.

[0009] In view of the above problems and / or the problems existing in the prior art, the present invention is proposed.

[0010] Therefore, the object of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure.

[0011] In order to solve the above technical problems, the present invention provides the following technical solutions: comprising:

[0012] After stirring and dissolving 70% pure phytic acid, urea, and polymerization inhibitor, the mixture is heated to 20-50°C and stirred. Glycidyl methacrylate is added dropwise under nitrogen protection over a period of 30 minutes to 2 hours. After the addition is complete, the reaction is continued for 4-8 hours to obtain a phytic acid functional monomer, designated as PA-GMA.

[0013] Completely dissolving citric acid and chitosan in distilled water, heating and stirring at 70°C to 100°C for gradient cross-linking, to obtain a citric acid / chitosan solution;

[0014] PA-GMA and tannic acid were sequentially added to the citric acid / chitosan solution, and the mixture was heated and stirred for network curing to obtain a formaldehyde-free adhesive.

[0015] As a preferred embodiment of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to the present invention, the polymerization inhibitor comprises one or more of p-hydroxyanisole, hydroquinone, 2,5-di-tert-butylhydroquinone, and NCAT-YC01.

[0016] As a preferred solution of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive with a three-dimensional network structure of the present invention, the mass ratio of the phytic acid, urea and polymerization inhibitor is 141:1:0.1.

[0017] As a preferred embodiment of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to the present invention, the temperature does not exceed 40° C. during the dropwise addition process.

[0018] As a preferred embodiment of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure of the present invention, the molar ratio of glycidyl methacrylate to phytic acid is 0.2-0.5:0.12-0.19.

[0019] As a preferred embodiment of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure of the present invention, the mass ratio of citric acid, chitosan and distilled water is 21-30:2-8:160-270.

[0020] As a preferred embodiment of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to the present invention, the mass ratio of PA-GMA to tannic acid is 2-7:4-9.

[0021] As a preferred embodiment of the method for preparing the multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure of the present invention, the network is cured at a temperature of 70° C. to 100° C. for 2 to 3 hours.

[0022] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a multi-stage cross-linked formaldehyde-free adhesive with a three-dimensional network structure. The adhesive is an inositol hexaphosphate ester with cycloinositol as the core and all six hydroxyl groups replaced by phosphate groups.

[0023] As a preferred embodiment of the multi-stage cross-linked formaldehyde-free adhesive with a three-dimensional network structure described in the present invention, the adhesive has a bonding strength of 7.46 to 12.96 MPa, a flame retardant grade of UL-94 V-0, and a bonding strength of 3 to 4.1 MPa after immersion in water for 31 hours.

[0024] As a preferred embodiment of the multi-stage cross-linked formaldehyde-free adhesive with a three-dimensional network structure described in the present invention, the formaldehyde-free adhesive has a bonding strength of 12.96 MPa, a flame retardant grade of UL-94V-0, and a bonding strength of 4.1 MPa after immersion in water for 31 hours.

[0025] Beneficial effects of the present invention:

[0026] (1) By constructing a dynamic cross-linking network with citric acid and chitosan, phytic acid and chitosan are used to synergistically retard the flame retardancy and tannic acid polyphenols are used for antibacterial purposes, so that the viscosity of the adhesive can be controlled, the bonding strength reaches 12.96 MPa, and the flame retardancy grade is UL-94V-0; the strength remains at 4.1 MPa after 31 hours of water immersion.

[0027] (2) The preparation process of the present invention simplifies the traditional multi-step modification process and reduces production energy consumption.

[0028] (3) The present invention uses renewable resources as raw materials. The raw materials are cheap and easy to obtain, and the synthesis process is simple and green and environmentally friendly. It fundamentally avoids the use of harmful substances such as formaldehyde in traditional adhesives, greatly reduces pollution to the environment and potential harm to human health, fully conforms to the concept of green chemistry and sustainable development, and achieves zero VOCs emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be derived from these drawings without inventive efforts. Among them:

[0030] Figure 1 This is a diagram of the synthesis mechanism of the formaldehyde-free adhesive of the present invention.

[0031] Figure 2 These are the infrared spectra of citric acid / chitosan, citric acid / chitosan / PA-GMA, and formaldehyde-free adhesive.

[0032] Figure 3 SEM images of formaldehyde-free adhesive hot-pressed plywood before (a) and after (b) water soaking treatment.

[0033] Figure 4 This is the viscosity control chart of formaldehyde-free adhesive.

[0034] Figure 5 Flowchart for tensile strength testing of adhesive-bonded hot-pressed plywood in dry conditions.

[0035] Figure 6 Flowchart for tensile strength testing of adhesive-bonded hot-pressed plywood in a humid environment.

[0036] Figure 7 Stress-displacement curves (left) and tensile strength comparison (right) of hot-pressed plywood prepared with formaldehyde-free adhesive and traditional adhesive without water treatment.

[0037] Figure 8 Stress-displacement curves of hot-pressed plywood prepared with formaldehyde-free adhesive after water immersion (left) and a comparison of the tensile strength of hot-pressed plywood prepared with formaldehyde-free adhesive and traditional adhesive after water immersion (right).

[0038] Figure 9 Vertical burning test images of pure wood (a), wood coated with traditional adhesive (b), and wood coated with formaldehyde-free adhesive (c).

[0039] Figure 10 The limiting oxygen index of pure wood (a), wood coated with traditional adhesive (b), and wood coated with formaldehyde-free adhesive (c).

[0040] Figure 11 These are 60-day mildew resistance test images of pure wood (a), wood coated with traditional adhesives (b), and wood coated with formaldehyde-free adhesives (c) in a humid environment. DETAILED DESCRIPTION

[0041] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

[0042] In the following description, many specific details are set forth to facilitate a full understanding 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 may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0043] Secondly, the term "one embodiment" or "embodiment" 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 various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0044] Unless otherwise specified, the raw materials used in the present invention are all commercially available.

[0045] The preparation method of the phytic acid multi-arm functional monomer (PA-GMA) of the present invention is as follows:

[0046] Phytic acid modification: 0.15 mol of 70% phytic acid solution, 1% (by total system mass) of urea (solid) and 0.1% of p-hydroxyanisole were added to a four-necked round-bottom flask equipped with a thermometer, an electric stirrer, a constant pressure dropping funnel and a nitrogen conduit, and the mixture was stirred and dissolved. The mixture was then heated to 30°C in a water bath. Under nitrogen protection and stirring, 0.3 mol of glycidyl methacrylate was added dropwise through a constant pressure dropping funnel within 2 h. The addition rate was strictly controlled to ensure that the temperature did not exceed 40°C during the addition process. After all the addition was completed, the reaction was continued at a constant temperature of 45°C for 5 h. The reaction solution became homogeneous and the reaction was stopped to obtain PA-GMA.

[0047] The performance of the formaldehyde-free adhesive prepared in the present invention was tested by the following method:

[0048] The viscosity of the adhesive was tested using a digital rotational viscometer (NDJ-8S).

[0049] Referring to GB / T 17657-2022, the bond strength of conventional adhesives and the formaldehyde-free adhesive prepared in this invention to wood was determined through tensile testing. The adhesive was applied to the overlapping area (25 mm x 25 mm) of two pine veneer sheets (100 mm x 25 mm x 3 mm) with a brush. The sheets were then hot-pressed at 110°C, 1.5 MPa, and a holding period of 10 minutes. The mechanical properties of the hot-pressed plywood were measured in dry and humid environments using an MTS universal tensile testing machine. The humid environment was simulated by a water immersion test: the plywood samples were first immersed in 63°C water for 3 hours and then in boiling water for 4 hours. The boiling-water-treated samples were then placed in 20°C water for 24 hours before being used in the bond strength test.

[0050] Referring to GB / T2408, after the adhesive brush board is hot-pressed, a vertical combustion tester is used to simulate the hot pressing temperature of 110°C for pure wood and wood coated with traditional adhesives and formaldehyde-free adhesives. Then, a vertical combustion test is carried out in accordance with GB / T2408 standard. The sample size is 130mm*12.5mm*3mm.

[0051] According to ASTM D2863-97, pure wood and wood coated with traditional adhesives and wood coated with formaldehyde-free adhesives were dried at a simulated hot pressing temperature of 110°C using an oxygen index tester, and then subjected to a limiting oxygen index (LOI) test. The sample size was 100mm*10mm*4mm.

[0052] The mildew resistance test was carried out in accordance with GB / T 35469-2017.

[0053] Example 1

[0054] This embodiment provides a method for preparing a formaldehyde-free adhesive. Figure 1 , specifically:

[0055] 1) Weigh 24 g of citric acid and completely dissolve it in 160 g of distilled water to obtain a clear, transparent aqueous citric acid solution. Then, add 6 g of chitosan, so that the mass ratio of citric acid, chitosan, and distilled water is 24:6:160. Stir continuously in a 40°C water bath until the chitosan is completely dissolved, forming a uniform, stable, and clear citric acid / chitosan solution.

[0056] 2) Next, add 4g of PA-GMA to a water bath at 90°C. Stir to ensure that the PA-GMA mixes evenly with the citric acid / chitosan solution at this temperature. During stirring, closely monitor the changes in the solution's state. Over time, the PA-GMA will gradually become evenly dispersed in the solution. This process may take some time. Ensure adequate and stable stirring to avoid local concentration differences caused by uneven stirring.

[0057] 3) After the PA-GMA is evenly dispersed, add 6g of tannic acid (i.e., a mass ratio of 4:6 for PA-GMA to tannic acid). Stir at room temperature for 1 hour to allow the tannic acid to initially disperse and begin interacting with the other components in the solution. At this point, the solution's viscosity may change slightly, and the color may darken slightly. Next, raise the temperature to 100°C and continue stirring for 2 hours. The high temperature accelerates the dissolution of the tannic acid and allows for a more complete chemical reaction with the PA-GMA in the solution, forming a polymer network structure with specific properties—the formaldehyde-free adhesive (citric acid / chitosan / PA-GMA / tannic acid adhesive). Throughout the stirring process, maintain temperature stability and uniform stirring to prevent degradation of the solution due to localized overheating or clumping due to uneven stirring. This adhesive can be used in bonding a variety of materials and exhibits excellent adhesion and stability.

[0058] Figure 2 The Fourier transform infrared spectra (FTIR) of citric acid / chitosan [the product obtained in step 1 of Example 1], citric acid / chitosan / PA-GMA [the product obtained in step 2 of Example 1], and the formaldehyde-free adhesive are shown in Figure 1. For the citric acid / chitosan system, the FTIR spectrum at 1500 cm -1 to 1700cm -1 The carbonyl peak of citric acid is obvious at 1174 cm -1 The vibration of CN bond appeared, which indicated that amide bond was formed by the reaction of citric acid and chitosan. After adding PAGMA to citric acid / chitosan, the hydroxyl peak and amino peak became narrower, with the peak at 1178cm -1 (CN) and 1635cm -1 A distinct peak at (C=O) indicates an esterification reaction between PAGMA and chitosan. When mixed with tannic acid, the hydroxyl peak narrows significantly, indicating hydrogen bond cleavage, i.e., esterification, between the tannic acid and chitosan. This demonstrates the successful construction of the three-dimensional cross-linked network, which plays a crucial role in improving its mechanical properties.

[0059] Figure 3 The following are SEM images of hot-pressed plywood coated with formaldehyde-free adhesive before and after water immersion treatment. The surface morphology of the bonded area after hot pressing and water immersion treatment of the formaldehyde-free adhesive was studied by SEM measurement. The microstructure of the cross-section sample of the formaldehyde-free adhesive was shown to observe whether the surface coating penetrated into the wood material. Wood is a water-absorbent and porous material. Formaldehyde-free adhesives are water-based adhesives that can easily penetrate into the microstructure (fibers and pores) of wood and fill them, subsequently forming a "network cross-linking" ( Figure 3a). Formaldehyde-free adhesives contain abundant hydroxyl groups, which are conducive to forming van der Waals forces and hydrogen bonds with the hydroxyl groups of cellulose and lignin in wood ( Figure 3 a), Moreover, after water immersion treatment, the continuous glue layer remained intact without any dissolution in the wood cell wall ( Figure 3 b), which is sufficient to prove that the formaldehyde-free adhesive has good water resistance after heat treatment.

[0060] Example 2

[0061] The difference between this embodiment and embodiment 1 is that the amount of citric acid added is adjusted to 28 g and the amount of chitosan is adjusted to 2 g, that is, the mass ratio of citric acid, chitosan and distilled water is 28:2:160. The rest of the preparation process is the same as that of embodiment 1 to obtain a formaldehyde-free adhesive.

[0062] Example 3

[0063] The difference between this embodiment and embodiment 1 is that the amount of citric acid added is adjusted to 26 g and the amount of chitosan is adjusted to 4 g, that is, the mass ratio of citric acid, chitosan and distilled water is 26:4:160. The rest of the preparation process is the same as that of embodiment 1 to obtain a formaldehyde-free adhesive.

[0064] Example 4

[0065] The difference between this embodiment and embodiment 1 is that the amount of citric acid added is adjusted to 22 g and the amount of chitosan is adjusted to 8 g, that is, the mass ratio of citric acid, chitosan and distilled water is 22:8:160. The rest of the preparation process is the same as that of embodiment 1 to obtain a formaldehyde-free adhesive.

[0066] Comparative Example 1

[0067] The difference between this comparative example and Example 1 is that the amount of chitosan is adjusted to 0 g, and the rest of the preparation process is the same as that of Example 1 to prepare an adhesive.

[0068] The adhesives of the above examples and comparative examples were subjected to performance tests, and the results compared with those of Example 1 are shown in Table 1.

[0069] Table 1

[0070] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 <![CDATA[Viscosity (mPa · s)]]> 20914 1045 8412 73438 340 Tensile strength (MPa) 12.96 7.46 10.31 8.39 3.54 Limiting oxygen index (%) 48.9 42.7 45.2 50.3 37.8

[0071] Combined with Table 1 and Figure 4It can be seen that adjusting the amount of chitosan has a significant effect on the performance of the formaldehyde-free adhesive. This is due to the existence of hydrogen bonds and other interaction forces between chitosan molecular chains and between chitosan and other components (such as citric acid, PA-GMA, and tannic acid). Chitosan also plays a certain cross-linking role in the adhesive system, which helps to form a complete network structure. Therefore, an appropriate amount of chitosan can provide sufficient intermolecular forces to make the network structure uniform and continuous, thereby better dispersing stress and improving mechanical properties. When the amount of chitosan added is too much, it may cause the network structure to be too dense, resulting in local aggregation, causing stress concentration, thereby reducing tensile strength. If the amount of chitosan added is too low, the continuity of the network structure will deteriorate. The amount of chitosan used will not only affect the viscosity and bonding properties of the overall system, but also affect the synergistic flame retardant properties between it and PA-GMA. According to Table 1 and Figure 4 As a result, the best technical effect can be obtained when the mass ratio of citric acid, chitosan and distilled water is 24:6:160.

[0072] Comparative Example 2

[0073] Commercially available traditional adhesive (white latex)

[0074] Figure 5 、 Figure 6 Flowcharts showing the tensile strength test of hot-pressed plywood bonded with adhesives under dry and wet conditions. Figure 7 The stress-displacement curves of hot-pressed plywood prepared with formaldehyde-free adhesive and commercially available traditional adhesive without water treatment are shown in the figure. Figure 8 The stress-displacement curves of hot-pressed plywood prepared with formaldehyde-free adhesive and commercially available conventional adhesive after water immersion treatment are shown. The composite adhesive exhibits a bonding strength of 12.96 MPa, can bear a load of more than 17 kg, and has excellent water resistance (no peeling after 31 hours of immersion, a bonding strength of 4.1 MPa, and can bear a load of 9.1 kg). Figure 8 The test results show that the bio-based composite adhesive has a significant advantage in bonding strength.

[0075] Figure 9The vertical combustion test diagrams of pure wood (a), wood coated with traditional adhesives (b), and wood coated with formaldehyde-free adhesives (prepared in Example 1) (c) are shown. It can be seen that pure wood and traditional adhesives were ignited and no self-extinguishing phenomenon occurred until the combustion was completed, while the wood coated with formaldehyde-free adhesives only showed a small flame on the wood surface after 25 seconds of contact with the flame. When the fire source was removed, pure wood and wood coated with traditional adhesives continued to burn, while wood coated with formaldehyde-free adhesives showed self-extinguishing properties. The dense carbonized layer formed on the wood surface can adhere to the wood surface without falling off, and it still did not burn after two ignitions, showing excellent flame retardant properties. This shows that the carbon layer formed by the adhesive on the wood surface when heated can prevent the spread of fire and heat transfer, thereby greatly improving the flame retardancy of the wood and inhibiting the combustion behavior of the wood. In summary, pure wood and wood coated with traditional adhesives failed the UL-94 test, while wood coated with the formaldehyde-free adhesive prepared in Example 1 of the present invention passed the UL-94 test and achieved UL-94 V-0 rating.

[0076] Figure 10 The Limiting Oxygen Index (LOI) is the limiting oxygen index (LOI) of pure wood, wood coated with traditional adhesives, and wood coated with formaldehyde-free adhesives. It is generally believed that the higher the LOI value, the more difficult the material to burn. An LOI greater than 27% is generally considered fire-resistant, while an LOI less than 27% is considered combustible. The LOI of wood coated with formaldehyde-free adhesives ranges from 48.7% to 50.2%, while that of pure wood ranges from 21.7% to 25.3%, and that of wood coated with traditional adhesives ranges from 25.9% to 27.1%. This indicates that formaldehyde-free adhesives offer significantly better flame retardancy than both pure wood and traditional adhesives.

[0077] Figure 11 The following graphs show the mildew resistance test results for pure wood, wood coated with a traditional adhesive, and wood coated with a formaldehyde-free adhesive. As can be seen, after 60 days in a humid environment, the wood coated with the formaldehyde-free adhesive prepared in Example 1 showed no mildew growth, resulting in a mildew resistance rating of 0. In contrast, the surfaces of pure wood and wood coated with a traditional adhesive showed heavy mildew growth.

[0078] Example 5

[0079] The difference between this embodiment and embodiment 1 is that the mass ratio of PA-GMA to tannic acid is adjusted to 2:8, and the rest of the preparation process is the same as that of embodiment 1 to prepare a formaldehyde-free adhesive.

[0080] Example 6

[0081] The difference between this embodiment and embodiment 1 is that the mass ratio of PA-GMA to tannic acid is adjusted to 3:7, and the rest of the preparation process is the same as that of embodiment 1 to prepare a formaldehyde-free adhesive.

[0082] Example 7

[0083] The difference between this embodiment and embodiment 1 is that the mass ratio of PA-GMA and tannic acid is adjusted to 5:5, and the rest of the preparation process is the same as that of embodiment 1 to prepare a formaldehyde-free adhesive.

[0084] Example 8

[0085] The difference between this embodiment and embodiment 1 is that the mass ratio of PA-GMA to tannic acid is adjusted to 6:4, and the rest of the preparation process is the same as that of embodiment 1 to prepare a formaldehyde-free adhesive.

[0086] The performance of the materials prepared in the above embodiment was tested, and the comparison results with those in Example 1 are shown in Table 2.

[0087] Table 2

[0088] Example 1 Example 5 Example 6 Example 7 Example 8 Tensile strength (MPa) 12.96 7.69 10.72 9.15 7.83 Self-extinguishing time (s) 0 14 5 0 0

[0089] As shown in Table 2, adjusting the mass ratio of PA-GMA to tannic acid significantly impacts the performance of the formaldehyde-free adhesive. This is because the multi-arm structure and double bonds of PA-GMA provide active sites for crosslinking reactions, and the addition of tannic acid further strengthens the network structure. Excessive tannic acid can lead to insufficient crosslinking, a less compact network structure, and decreased mechanical properties. Based on the results in Table 2, the optimal technical performance is achieved when the mass ratio of PA-GMA to tannic acid is 4:6.

[0090] In summary, the present invention successfully prepares an environmentally friendly formaldehyde-free adhesive through precise molecular structure design and synergistic effects between components. The adhesive has high bonding strength, controllable viscosity, excellent water and mildew resistance and flame retardant properties. It is not only formaldehyde-free and environmentally friendly, but also can effectively achieve the bonding of various materials such as wood, paper, and fiber products, broadening the application field. Its unique composite polymer composition and cross-linked structure give the adhesive good weather resistance and flame retardancy, so that it can still maintain a stable bonding effect in humid environments and different climatic conditions, significantly extending the service life of the bonded products. At the same time, it provides a strong guarantee for home safety with its excellent flame retardant properties.

[0091] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure, characterized in that: include, After stirring and dissolving 70% pure phytic acid, urea, and polymerization inhibitor, the mixture is heated to 20-50°C and stirred. Glycidyl methacrylate is added dropwise under nitrogen protection over a period of 30 minutes to 2 hours. After the addition is complete, the reaction is continued for 4-8 hours to obtain a phytic acid functional monomer, designated as PA-GMA. Completely dissolving citric acid and chitosan in distilled water, heating and stirring at 70°C to 100°C for gradient cross-linking, to obtain a citric acid / chitosan solution; PA-GMA and tannic acid are sequentially added to the citric acid / chitosan solution, and the mixture is heated and stirred for network curing to obtain a multi-stage cross-linked formaldehyde-free adhesive with a three-dimensional network structure.

2. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The polymerization inhibitor includes one or more of p-hydroxyanisole, hydroquinone, 2,5-di-tert-butylhydroquinone, and NCAT-YC01.

3. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The mass ratio of the phytic acid, urea and polymerization inhibitor is 141:1:0.

1.

4. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The temperature during the dropwise addition process does not exceed 40°C.

5. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The molar ratio of glycidyl methacrylate to phytic acid is 0.2-0.5:0.12-0.

19.

6. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The mass ratio of the citric acid, chitosan and distilled water is 21-30:2-8:160-270.

7. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The mass ratio of the PA-GMA to tannic acid is 2-7:4-9.

8. The method for preparing a multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 1, wherein: The network curing is carried out at a temperature of 70° C. to 100° C. and for a time of 2 to 3 hours.

9. A multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The adhesive is inositol hexaphosphate with cycloinositol as the core and all six hydroxyl groups are esterified and replaced by phosphate groups.

10. The multi-stage cross-linked formaldehyde-free adhesive having a three-dimensional network structure according to claim 9, characterized in that: The adhesive has a bonding strength of 7.46 to 12.96 MPa and a flame retardant grade of UL-94 V-0. After being immersed in water for 31 hours, the bonding strength still remains at 3 to 4.1 MPa.

Images