A bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol and a preparation method thereof

Abstract

The present application relates to a kind of bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol and its preparation method.Resveratrol-based epoxy resin RSEP is prepared by the reaction of resveratrol and epichlorohydrin;Aldehyde compound and glutamic acid are first condensed to form an intermediate containing imine structure by Schiff base, then addition reaction occurs with phosphorus-containing hydrogen bond compound, one-pot method to prepare reactive flame-retardant curing agent FVD;Then RSEP is co-cured with furfurylamine and FVD at active hydrogen and epoxy group equivalent ratio of 1:1 to obtain intrinsic flame-retardant bio-based epoxy adhesive.The system introduces rigid aromatic structure of resveratrol, bio-based Schiff base structure and phosphorus-containing flame-retardant unit into the crosslinked network, realizes the integration design of resin matrix, curing component and flame-retardant structure.The obtained adhesive has high storage modulus, good solvent resistance and temperature adaptability, excellent carbon formation barrier and fire suppression capacity, and is suitable for metal bonding, wood flame-retardant coating, electronic packaging bonding or composite interface bonding and other fields.

Description

Technical Field

[0001] This invention belongs to the field of bio-based polymer materials and functional adhesives, specifically relating to a bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive that uses resveratrol as an epoxy resin precursor and an aldehyde-based compound / glutamic acid / phosphorus-containing hydrogen-bonded compound-derived reactive curing agent as the flame-retardant structure source, as well as a method for preparing the adhesive. Background Technology

[0002] Epoxy adhesives are widely used in electronic packaging, coatings, protective composite materials, and structural bonding due to their high bonding strength, good dimensional stability, chemical resistance, and excellent electrical insulation properties. However, existing commercial epoxy systems still heavily rely on petroleum-based raw materials, with bisphenol A epoxy resins having long dominated the market. Meanwhile, the epoxy network itself is flammable, releasing high heat and producing fumes and harmful decomposition products during combustion, limiting its application in high-end manufacturing and safety-sensitive scenarios. Public reviews indicate that while bio-based thermosetting resins have become an important research direction, their industrialization still faces challenges such as insufficient sustainability of sources, inadequate overall performance, and insufficient functional synergy.

[0003] In recent years, research on constructing bio-based epoxy resins from natural molecules such as vegetable oils, lignin, rosin, tannic acid, eugenol, vanillin, and resveratrol has been increasing (Journal of Polymer Science, 2021, 59, 1474). Resveratrol, in particular, possesses both triphenolic hydroxyl groups and a rigid stilbene backbone, which is beneficial for introducing multifunctional epoxy groups, increasing crosslinking density, and imparting high thermal and mechanical properties to the material. Previous studies have reported that resveratrol-based epoxy systems can achieve high Tg, good mechanical properties, and certain low flammability (Chemical Engineering Journal, 2020, 383, 123124); domestic literature also shows that natural aromatic platform molecules such as resveratrol or vanillin have good application prospects in recyclable, bio-based epoxy systems. However, in general, existing publicly available technologies focus more on the resin itself or its recyclability, and are still relatively insufficient in the integrated design of "high bio-based content + intrinsic flame retardancy + adhesive application" (Polymer Chemistry, 2026, 17, 393).

[0004] To address the insufficient flame retardancy of epoxy materials, existing technologies typically employ methods such as excessive inorganic filler loading, addition of halogenated flame retardants, or introduction of DOPO-based phosphorus-containing flame retardant structures. The first two approaches often lead to increased viscosity, decreased interfacial compatibility, impaired mechanical properties, or combustion toxicity issues. Therefore, reactive DOPO derivatives have become a key focus of epoxy flame retardancy research in recent years. Publicly available information indicates that reactive DOPO derivatives can be grafted into the epoxy network through monomer pre-reaction, reactive curing agents, or hybrid structures, thereby reducing migration and precipitation, and improving flame retardancy through a dual mechanism of gas-phase free radical quenching and condensed-phase char promotion (Polymer Degradation and Stability, 2022, 202, 110020). Furthermore, phosphorus / nitrogen-containing synergistic systems, due to their combined acid source, char-forming, and foaming / dilution effects, are considered more suitable for constructing efficient halogen-free flame-retardant epoxy systems (Composites Part B: Engineering, 2024, 276, 111362; Progress in OrganicCoatings, 2023, 182, 107631). However, existing technologies still lack a high-performance flame-retardant adhesive solution that simultaneously achieves the synergistic coupling of resveratrol-based epoxy resin, bio-based nitrogen-containing Schiff base structure, and DOPO reactive flame-retardant unit. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing petroleum-based epoxy adhesives, such as poor sustainability, reliance on externally added flame retardants for flame retardant modification, and difficulty in simultaneously achieving flame retardant and adhesive properties. This invention provides a resveratrol-based bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive and its preparation method. This method involves co-curing resveratrol-based epoxy resin (RSEP) with a reactive flame-retardant curing agent (FVD) containing an aldehyde / glutamic acid / phosphorus hydrogen-bonded compound structure. This allows the flame-retardant structure to be chemically bonded into the cross-linked network, significantly improving both flame retardant and char-blocking properties while maintaining the adhesive's application performance.

[0006] The present invention achieves the above objectives by adopting the following technical solution: A bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol, the adhesive comprising resveratrol-based epoxy resin RSEP and a curing system; the curing system comprising furfurylamine FA and reactive flame-retardant curing agent FVD, wherein the total equivalent of active hydrogen and the total equivalent of epoxy groups are controlled at 1:1, wherein FVD is prepared by a one-pot reaction of an aldehyde compound, glutamic acid and a phosphorus-containing hydrogen-bonded compound.

[0007] First, an aldehyde compound and glutamic acid were subjected to Schiff base condensation in anhydrous ethanol to form an intermediate containing imine bonds and carboxyl / amino reaction sites. Then, an ethanol solution containing a phosphorus-hydrogen bond compound was added to the system for an addition reaction, resulting in a one-pot preparation of the reactive flame-retardant curing agent FVD. This FVD simultaneously possesses a bio-based aromatic structure, a nitrogen-containing Schiff base structure, a phosphorus-containing structure, and secondary amine groups, enabling it to participate in epoxy curing and exert a synergistic phosphorus-nitrogen flame-retardant effect during combustion. Next, RSEP was mixed with furfurylamine (FA) and FVD, controlling the total equivalent of active hydrogen to the total equivalent of epoxy groups at a 1:1 ratio, and adjusting the proportion of FVD in the active hydrogen of the curing agent. After degassing and curing, a series of RSEP / FVD-X bio-based flame-retardant epoxy adhesives were obtained.

[0008] Furthermore, the phosphorus-containing hydrogen-bonded compound is one or more of diphenylphosphine oxide, diethyl phosphite and diisopropyl phosphite, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO).

[0009] Furthermore, the aldehyde compound is one or more of the following: octanal, nonanal, decanal, lauraldehyde, benzaldehyde, phenylacetaldehyde, cinnamaldehyde, vanillin, privet aldehyde, β-methylphenylpropanal, 3,6-dimethyl-3-cyclohexene-1-carboxaldehyde, citric acid, lily aldehyde, citral, citronellol, hydroxycitronellol, and perillaldehyde.

[0010] Furthermore, the RSEP is prepared by reacting resveratrol with epichlorohydrin under the catalysis of tetrabutylammonium bromide, followed by alkali treatment to close the ring. The resveratrol-based epoxy resin RSEP is prepared by reacting resveratrol with excess epichlorohydrin under the catalysis of tetrabutylammonium bromide, followed by alkali treatment to close the ring. This resin contains a multifunctional epoxy structure and a rigid aromatic skeleton, which can provide a high crosslinking density and a good thermomechanical basis for the curing system.

[0011] Furthermore, the molar ratio of resveratrol to epichlorohydrin is 1:(18-22).

[0012] Furthermore, the FVD accounts for 0.4 to 1.0% of the total equivalent of active hydrogen in the curing agent.

[0013] Furthermore, the proportion of FVD in the total equivalent of active hydrogen in the curing agent is 0.4, 0.6, 0.8, or 1.0.

[0014] Furthermore, the FVD accounts for 0.6 to 1.0% of the total equivalent of active hydrogen in the curing agent.

[0015] Furthermore, the preparation method of the FVD includes: adding an aldehyde compound and glutamic acid in anhydrous ethanol at a molar ratio of 1:1, reacting at 70-90°C for 6-10 h to form a Schiff base intermediate, then adding an ethanol solution containing a phosphorus hydrogen bond compound in an equal molar amount to the aldehyde compound, continuing the reaction at 70-90°C for 20-28 h, and obtaining the FVD after filtration, washing and vacuum drying.

[0016] A method for preparing a bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol includes the following steps: mixing RSEP, FA and FVD in a set equivalent ratio, degassing under vacuum, curing at 100℃ for 2 h, and then curing at 140℃ for 2 h to obtain a cured adhesive.

[0017] Furthermore, the adhesive, after curing, has a limiting oxygen index of not less than 28.8%, preferably not less than 29.1%, and a UL-94 vertical flammability rating of V-1 or V-0.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: Compared to previous methods that relied on large amounts of inorganic fillers, halogenated flame retardants, or simple physical blends of phosphorus-containing flame retardant additives, this invention directly embeds the flame-retardant structure into the epoxy crosslinking network through a reactive curing agent. This significantly reduces the risk of migration and precipitation, while avoiding problems such as a sharp increase in system viscosity, decreased processability, and weakened interfacial adhesion caused by high filler content. More importantly, this invention is not a simple combination of "bio-based resin + externally added flame retardant," but rather utilizes natural molecules such as resveratrol, aldehyde compounds, and glutamic acid to construct the resin and curing agent framework, and then endows the reactive phosphorus-containing flame-retardant units with phosphorus-containing hydride compounds. This forms a unified molecular design route that integrates bio-based, reactive, and phosphorus-nitrogen synergy, a technical approach that is significantly different from existing publicly available technologies.

[0019] In terms of results, the system of this invention achieves a significant improvement in flame retardancy while maintaining the application properties of the adhesive: compared with the control group without FVD, the LOI increased from 26.6% to a maximum of 33.9%, the UL-94 rating improved from unrated to V-0, and the V-0 rating was maintained after coating the wood board; at the same time, PHRR and THR decreased by 72.3% and 60.2% respectively, the char residue increased to 37.5 wt%, and the degree of graphitization of the char residue was significantly improved. Experiments also show that the RSEP / FVD-0.4 sample exhibited minimal fluctuations in lap shear strength when immersed in various solvents or tested under conditions of 0–100℃, indicating that the system combines flame retardancy, thermomechanical properties, and environmental adaptability. Among them, RSEP / FVD-0.6 performs particularly well in balancing flame retardancy rating and application, making it suitable as a preferred embodiment. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 Synthetic routes and structural characterization diagrams of RSEP and FVD; Figure 2 Schematic diagram of the mechanical properties, DMA, surface energy, and load-bearing capacity of the RSEP / FVD-X system; Figure 3 UL-94 vertical burning test diagram and flame retardant coating test diagram for wood panels; Figure 4 The HRR, THR, SPR, TSP, COP, and CO2P curves obtained from cone calorimetry; Figure 5 The appearance morphology and SEM image of the carbon residue after cone calorimetry; Figure 6 The image shows the Raman spectrum of the carbon residue. Figure 7 XPS image of residual carbon; Figure 8 This is a schematic diagram of the flame-retardant mechanism of the system of the present invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] It should be noted that, in specific embodiments of the present invention, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the use of phrases such as "comprising one" to define an element does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Those skilled in the art will understand the specific meaning of the above terms in the present invention through the specific circumstances.

[0024] Further analysis: The raw materials used in the experiments of the embodiments and comparative examples of the present invention are as follows, but are not limited to the following raw materials. The present invention only uses the following raw materials as specific examples to further illustrate the effect of the bio-based phosphorus and nitrogen synergistic flame retardant epoxy adhesive based on resveratrol described in the present invention.

[0025] The raw materials used in the experiments of the embodiments and comparative examples of the present invention are as follows, but are not limited to the following raw materials. The present invention only uses the following raw materials as specific examples to further illustrate the effect of the bio-based phosphorus and nitrogen synergistic flame retardant epoxy adhesive based on resveratrol described in the present invention.

[0026] Raw materials: Resveratrol, tetrabutylammonium bromide, vanillin, glutamic acid, DOPO, furfurylamine, methanol, ethanol, acetone and epichlorohydrin can all be commercially available analytical grade reagents.

[0027] Synthesis of RSEP. In a 500 mL four-necked flask equipped with a stirrer, thermometer, and reflux condenser, under nitrogen protection, 22.8 g (0.1 mol) of resveratrol and 184 g (approximately 2 mol) of epichlorohydrin were added. The mixture was heated to 90 °C, and then 2.28 g of tetrabutylammonium bromide was added. The mixture was stirred and refluxed for 5 h. After heating was stopped, a 40 wt% sodium hydroxide aqueous solution was added dropwise to bring the actual amount of NaOH added to 12 g (0.3 mol). The reaction was continued overnight. After the reaction was complete, the white solid was removed by filtration. The resulting organic phase was washed twice with water, and excess epichlorohydrin was removed by vacuum distillation to obtain a pale yellow viscous liquid, RSEP, with an epoxy equivalent of approximately 146 g / mol.

[0028] Synthesis of FVD: 10 g (0.066 mol) of vanillin, 9.17 g (0.066 mol) of glutamic acid, and 100 mL of anhydrous ethanol were added to a 250 mL four-necked flask, and the mixture was heated to 85 °C and reacted for 8 h. Then, an ethanol solution of DOPO (14.25 g (0.066 mol)) was added, and the reaction was continued at 85 °C for 24 h. After the reaction was complete, the mixture was filtered, ultrasonically washed three times with ethanol, and dried under vacuum at 60 °C for 12 h to obtain a pale yellow powder, FVD.

[0029] The present invention includes Examples 1-4 and Comparative Example 1, the formulations of which are shown in Table 1.

[0030]

[0031] After the above samples were mixed evenly, they were degassed under vacuum and then cured by bulk curing at 100℃ / 2 h + 140℃ / 2 h and by shear curing of the sample.

[0032] Performance testing methods FTIR was performed using a Bruker ALPHA II; ^1H NMR was performed using a Bruker Avance 400 MHz; DMA was performed in double cantilever mode, with a heating rate of 3℃ / min, a frequency of 2 Hz, and a temperature range of -80 to 250℃; the lap shear specimens were made of steel plates with dimensions of 100 mm × 25 mm × 1.5 mm and an overlap area of ​​25 mm × 12.5 mm; LOI was performed according to ISO 4589, cone calorimetry according to ISO 5660, and SEM, XPS, and Raman spectroscopy were used for characterizing the morphology and structure of the carbon residue.

[0033] The performance of Examples 1-4 and Comparative Example 1 is shown in Table 2.

[0034] Furthermore, in the coating test on the wooden board surface in Example 2, the coating self-extinguished within 3 seconds after being ignited twice, still achieving the V-0 rating.

[0035] (1) Thermomechanical and operational stability Comparative Example 1 exhibits high tensile strength and lap shear strength; however, with the introduction of FVD, the storage modulus of the system continues to increase. The storage modulus of Example 4 reaches 3176 MPa at room temperature, which is 64.7% higher than that of Comparative Example 1 (1928 MPa). After being immersed in acetone, brine, n-hexane, ethanol, and water for 24 h, the lap shear strength of Example 1 fluctuates by no more than 1 MPa compared to air conditions; when tested within the range of 0–100 °C, the strength fluctuation also does not exceed 1 MPa, indicating that it has good environmental adaptability.

[0036] (2) Residual char structure and flame retardant mechanism Raman results showed that the ID / IG ratio decreased from 4.10 in Comparative Example 1 to 2.33 in Example 4, indicating that the graphitization degree of the residual char increased after the introduction of FVD. XPS detected P–O–C characteristics, indicating that the DOPO structure was chemically fixed in the network. Combined data from SEM, cone calorimetry, and gaseous products show that the system of this invention mainly exerts its flame-retardant effect through a synergistic effect of "gas-phase free radical quenching + condensed phase dense char shielding".

[0037] Discussion of the effects of the examples: Technical support for inventiveness and novelty From a technical perspective, Comparative Example 1 is only an RSEP / FA curing system and does not possess a reactive phosphorus-containing flame-retardant structure. Examples 1-4, on the other hand, integrate the vanillin-glutamic acid Schiff base structure with the DOPO phosphorus-containing structure into the curing agent via FVD, and further covalently embed it into the epoxy crosslinking network. Compared with existing mass-filled or simply additive DOPO flame-retardant epoxy resins, the key difference of this invention is that the flame-retardant unit does not exist as an inert additive, but as a network building unit that can participate in curing, thus taking into account flame-retardant efficiency, char stability, and long-term reliability.

[0038] From a technical perspective, Examples 2-4 demonstrate significant improvements over Comparative Example 1: on the one hand, key flame retardant indicators such as LOI, UL-94, PHRR, THR, and char residue are comprehensively optimized; on the other hand, the energy storage modulus is significantly improved, while Example 1 maintains good solvent resistance and temperature stability. Example 2 strikes a balance between V-0 flame retardant rating and practical application, while Example 4 exhibits the strongest flame retardant capability. This demonstrates that the present invention does not simply replace raw materials, but rather achieves a balance between "high bio-based content—high flame retardancy—applicable adhesives" through the synergistic design of molecular structure and curing network, exhibiting outstanding substantive features and significant progress.

[0039] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered within the scope of protection of the present invention.

Claims

1. A bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol, characterized in that: The adhesive is composed of resveratrol-based epoxy resin RSEP and a curing system; the curing system includes furfurylamine FA and reactive flame retardant curing agent FVD, and the total equivalent of active hydrogen and the total equivalent of epoxy groups are controlled at 1:1, wherein FVD is prepared by one-pot reaction of aldehyde compound, glutamic acid and phosphorus-containing hydrogen bond compound.

2. The resveratrol-based bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive according to claim 1, characterized in that: The phosphorus-containing hydrogen-bonded compound is one or a mixture of diphenylphosphine oxide, diethyl phosphite and diisopropyl phosphite, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO).

3. The resveratrol-based bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive according to claim 1, characterized in that: The aldehyde compound is one or more of the following: octanal, nonanal, decanal, lauraldehyde, benzaldehyde, phenylacetaldehyde, cinnamaldehyde, vanillin, privet aldehyde, β-methylphenylpropanal, 3,6-dimethyl-3-cyclohexene-1-carboxaldehyde, citric acid, lily aldehyde, citral, citronellol, hydroxycitronellol, and perillaldehyde.

4. The resveratrol-based bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive according to claim 1, characterized in that: The RSEP was prepared by reacting resveratrol and epichlorohydrin under the catalysis of tetrabutylammonium bromide, followed by cyclization through alkali treatment.

5. The resveratrol-based bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive according to claim 4, characterized in that: The molar ratio of resveratrol to epichlorohydrin is 1:(18-22).

6. The bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol according to claim 1, characterized in that: The FVD accounts for 0.4 to 1.0% of the total equivalent of active hydrogen in the curing agent.

7. The resveratrol-based bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive according to claim 1, characterized in that, The preparation method of FVD includes: adding an aldehyde compound and glutamic acid in anhydrous ethanol at a molar ratio of 1:1, reacting at 70-90°C for 6-10 h to form a Schiff base intermediate, then adding an ethanol solution of a phosphorus-hydrogen bonded compound in an equal molar amount to the aldehyde compound, continuing the reaction at 70-90°C for 20-28 h, and obtaining FVD after filtration, washing and vacuum drying.

8. A method for preparing a bio-based phosphorus-nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol as described in any one of claims 1-7, characterized in that, The process includes the following steps: mixing RSEP, FA and FVD in a set equivalent ratio, degassing under vacuum, curing at 100°C for 2 hours, and then curing at 140°C for 2 hours to obtain a cured adhesive.

9. The preparation method of the bio-based phosphorus and nitrogen synergistic flame-retardant epoxy adhesive based on resveratrol according to claim 8, characterized in that, The adhesive, after curing, has a limiting oxygen index of not less than 28.8% and a UL-94 vertical flammability rating of V-1 or V-0.

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