Water resistant, high-tack, bio-based adhesive

Abstract

This invention discloses a water-resistant, high-adhesion bio-based adhesive. It utilizes the unique catechol structure of tea polyphenols by epoxy modification of tea polyphenols to prepare a bio-based crosslinking agent—tea polyphenol glycidyl ether (TPGE)—possessing both epoxy groups and interfacial activity. TPGE is then reacted with malic acid (MAL), ricinoleic acid (RA), and tannic acid (TAN) to obtain the bio-based adhesive. The resulting bio-based adhesive exhibits excellent water resistance and high bonding strength. Therefore, this invention provides a new approach for developing high-performance bio-based adhesives and expands the application of natural tea polyphenol derivatives in the field of high-performance functional polymers, demonstrating significant industrial application prospects.

Description

Technical Field

[0001] This invention belongs to the field of adhesive materials, and relates to a water-resistant and highly adhesive bio-based adhesive, particularly a bio-based adhesive prepared based on tea polyphenol glycidyl ether. Background Technology

[0002] Adhesives are indispensable functional materials in modern industrial systems, playing a crucial role in product manufacturing, structural assembly, and material repair. Their applications cover almost all areas of bonding and structural integration of related materials, such as structural bonding and lightweight component integration in automobile manufacturing, thermal insulation materials and wood structure assembly in the construction industry, precision packaging and interface protection in the electronics industry, and rapid bonding solutions widely used in the packaging industry. It can be said that adhesives have become a vital support for driving technological upgrades and improving product performance in the manufacturing industry.

[0003] Currently, the dominant adhesive systems in industry are primarily derived from petrochemical raw materials. The synthesis of these products often involves high energy consumption and emits large amounts of volatile organic compounds (VOCs) and greenhouse gases, exacerbating environmental pollution and potentially posing health risks to producers and end-users. With increasingly scarce fossil resources, petroleum-based adhesives, characterized by high emissions and significant environmental impact, face severe sustainability challenges. Therefore, exploring bio-based adhesives that utilize renewable resources, possess high adhesive performance, and are environmentally friendly has become a current research hotspot in the materials science field.

[0004] Compared to traditional systems, bio-based adhesives are greener in terms of sourcing, and their preparation process is expected to reduce energy consumption and pollution emissions. Furthermore, through rational molecular structure design, they can achieve excellent water resistance and mechanical properties. In the context of the deepening adoption of green manufacturing concepts, developing high-performance, low-environmental-impact bio-based adhesives not only helps alleviate resource and ecological pressures but also plays a crucial role in promoting sustainable industrial transformation. Against this backdrop, the design of green monomers based on natural biomass derivatives has become a research hotspot in recent years.

[0005] Tea polyphenols, a natural polyphenol abundant in tea, are compounds with a unique natural catechol structure. Their molecules have a benzene ring backbone and are rich in phenolic hydroxyl groups, which endow them with excellent antioxidant properties, adhesion, and solvent resistance. Although tea polyphenol derivatives have been explored in the food packaging field, research on their application in adhesives is still in the exploratory stage. Based on this, this invention uses tea polyphenols as raw materials to synthesize a bio-based crosslinking agent, tea polyphenol glycidyl ether (TPGE), with bicyclic epoxy functional groups through a nucleophilic substitution reaction. TPGE is then reacted with malic acid (MAL), ricinoleic acid (RA), and tannic acid (TAN). Through esterification and etherification reactions between the epoxy groups of TPGE and the carboxylic acid groups of MAL and RA, as well as ring-opening reactions with the phenolic hydroxyl groups of TAN, a three-dimensional dynamic crosslinking network is constructed, resulting in a novel bio-based adhesive that is heat-resistant, water-resistant, highly adhesive, and reprocessable. Summary of the Invention

[0006] This invention addresses the technical limitations of existing bio-based adhesives, particularly those based on common plant phenol glycidyl ethers, which struggle to balance bonding strength and water resistance in humid environments, by providing a water-resistant, highly adhesive bio-based adhesive.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A water-resistant, highly adhesive bio-based adhesive is prepared from the following raw materials by weight percentage through an in-situ crosslinking reaction: Tannic acid content: 4.69-5.77% Tea polyphenol glycidyl ethers: 51.96%~60.98% Malic acid content: 3.75%~4.62%. Ricinoleic acid 0.94~1.15%, Anhydrous ethanol 29.64~36.49%; The sum of the weight percentages of the above raw materials is 100%.

[0008] Furthermore, the tea polyphenol glycidyl ether is prepared by epichlorohydrin epoxidation modification of tea polyphenols, and its molecular structure contains both epoxy groups and catechol hydroxyl groups.

[0009] Furthermore, the preparation of the bio-based adhesive includes the following steps: (1) Mix and dissolve tannic acid and tea polyphenol glycidyl ether in anhydrous ethanol and stir at 70 °C for 30 min; (2) Add malic acid and ricinoleic acid to the mixed solution obtained in step (1), and stir at 70 °C for 1 h to obtain the final product.

[0010] This invention creatively selects tea polyphenols with a catechol structure, epoxidizes them to obtain tea polyphenol glycidyl ether (TPGE), and then synergistically constructs a multi-layer crosslinked network with tannic acid (TAN), malic acid (MAL), and ricinoleic acid (RA). TPGE plays a dual role in this system: on the one hand, the epoxy groups on its side chains can undergo esterification and etherification reactions with the carboxyl groups of malic acid (MAL) and ricinoleic acid (RA) to construct the main framework of the crosslinked network, where the long-chain alkyl structure of RA provides the necessary toughness for the network; on the other hand, the catechol hydroxyl groups on the TPGE backbone and the abundant phenolic hydroxyl groups of tannic acid (TAN) can not only undergo ring-opening reactions to form covalent bonds, but also form a large number of intermolecular hydrogen bonds and π-π stacking interactions, constituting a reversible physical crosslinked network, which greatly increases the crosslinking density and structural stability of the network. This multi-linking mechanism, which integrates covalent bonds (ester bonds, ether bonds, open ring structures) and physical interactions (hydrogen bonds, π-π stacking), enables the adhesive system obtained by this invention to construct a dense multi-linking network, thereby significantly improving the adhesive strength and water resistance of the adhesive.

[0011] The significant advantages of this invention are as follows: 1. The bio-based adhesive of this invention constructs a multi-linked network, enabling the adhesive to exhibit both high cohesive strength (up to 13.05 MPa) and excellent hydrolysis resistance (strength retention rate ≥55% after immersion in water for 6 hours), achieving a balance between high strength and high water resistance, and meeting the growing demand of the adhesive market for high-performance adhesives.

[0012] 2. This invention uses tea polyphenols as the main raw material to prepare tea polyphenol glycidyl ether through epoxidation modification, and further produces a high-performance adhesive. It significantly improves the utilization efficiency of biomass resources, reduces the dependence of existing adhesives on petroleum-based raw materials, conforms to the development direction of green chemistry and low-carbon manufacturing, and has good environmental value.

[0013] 3. The adhesive of the present invention can maintain high bonding strength even under prolonged water immersion conditions, indicating that its cross-linked structure has good hydrolysis resistance and can meet the practical application requirements in humid or water environments. Attached Figure Description

[0014] Figure 1 The Fourier transform infrared spectrum of the tea polyphenol glycidyl ether prepared in the examples is shown.

[0015] Figure 2 The Fourier transform infrared spectrum of the adhesive prepared in Example 3.

[0016] Figure 3 The images show the DSC test results of the adhesives prepared in Examples 1-6.

[0017] Figure 4 The TGA test results are for the adhesives prepared in Examples 1-6.

[0018] Figure 5 The image shows the water resistance test results of the adhesive prepared in Example 3. Detailed Implementation

[0019] A water-resistant, highly adhesive bio-based adhesive is prepared by mixing and dissolving 4.69–5.77% tannic acid and 51.96–60.98% tea polyphenol glycidyl ether in 29.64–36.49% anhydrous ethanol by weight percentage, and stirring at 70 °C for 30 min; then adding 3.75–4.62% malic acid and 0.94–1.15% ricinoleic acid, and stirring at 70 °C for 1 h to obtain adhesive TMTR.

[0020] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto.

[0021] The preparation method of tea polyphenol glycidyl ether used in the examples is as follows: In a 250 mL four-necked flask equipped with a thermometer, magnetic stirrer, condenser, and constant pressure dropping funnel, 10 g of tea polyphenols, 32.5 g of epichlorohydrin, 125 mL of deionized water, and 6 g of sodium hydroxide were added sequentially. The mixture was stirred until the solids were completely dissolved. The reaction system was then heated to 90 °C and reacted under stirring for 5 h to ensure complete reaction. The mixture was then cooled to room temperature and the crude product was left at 0 °C overnight. Finally, the crude product was washed with deionized water, filtered, and dried to obtain tea polyphenol glycidyl ether (TPGE), whose chemical structural formula is: .

[0022] Figure 1 The Fourier transform infrared spectrum of the prepared tea polyphenol glycidyl ether is shown. Infrared spectroscopy analysis confirmed that the tea polyphenol glycidyl ether is at 2925 cm⁻¹. -1 and 2854 cm -1 The presence of characteristic absorption peaks for asymmetric and symmetric stretching vibrations of aliphatic -CH2- at indicates that the methylene structure of the glycidyl group has been successfully introduced; simultaneously, at 940 cm⁻¹... -1 The presence of a characteristic vibrational absorption peak of the epoxide COC structure nearby further confirms the presence of the epoxide group; while the original tea polyphenols are located at 1242 cm⁻¹. -1 and 1042cm -1The absorption peak intensity at the CO structure of the phenolic hydroxyl group was significantly reduced, proving that some phenolic hydroxyl groups had been converted into ether bonds through epoxidation. The changes in the above characteristic peaks together prove that the epoxidation reaction selectively occurs at the phenolic hydroxyl site of the tea polyphenol molecule and successfully incorporates the glycidyl group structure.

[0023] All other raw materials used in the examples were commercially available products; all tests were conducted according to conventional testing methods, and the parameter settings of the testing instruments were performed in accordance with the equipment manufacturer's recommended conditions. Example 1

[0024] In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 4.5 g of tea polyphenol glycidyl ether. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and the mixture was stirred at a constant temperature for 1 h. The resulting adhesive was labeled TMTR1. Example 2

[0025] In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 5.0 g of tea polyphenol glycidyl ether. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and the mixture was stirred at a constant temperature for 1 h. The resulting adhesive was labeled TMTR2. Example 3

[0026] In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 5.5 g of tea polyphenol glycidyl ether. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and the mixture was stirred at a constant temperature for 1 h. The resulting adhesive was labeled TMTR3. Example 4

[0027] In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 6.0 g of tea polyphenol glycidyl ether. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and the mixture was stirred at a constant temperature for 1 h. The resulting adhesive was labeled TMTR4. Example 5

[0028] In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 6.5 g of tea polyphenol glycidyl ether. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and the mixture was stirred at a constant temperature for 1 h. The resulting adhesive was labeled TMTR5. Example 6

[0029] In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 7.0 g of tea polyphenol glycidyl ether. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and the mixture was stirred at a constant temperature for 1 h. The resulting adhesive was labeled TMTR6.

[0030] Typical plant phenolic derivatives with the same catechol or polyphenol structure—caffeic acid epoxy resin (CA) and catechol epoxy resin (CT)—were selected as comparative monomers. (The caffeic acid epoxy resin was epoxidized according to patent CN116283837A, and the catechol epoxy resin was epoxidized according to the literature "Aouf C, Le Guernevé, Christine, Caillol S, et al. Study of the O-glycidylation of natural phenolic compounds. The relationship between the phenolic structure and the reaction mechanism[J].Tetrahedron, 2013, 69(4):1345-1353.", with epoxy values ​​of 0.47 mol / 100 g and 0.60 mol / 100 g, respectively.) The epoxy value of 5.5 g of tea polyphenol glycidyl ether used in Example 3 was 0.37 mol / 100 g. g) Convert the amount of monomer used to an equivalent amount.

[0031] Comparative Example 1 In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 4.3 g of caffeic acid epoxy resin. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and stirring was continued at a constant temperature for 1 h.

[0032] Comparative Example 2 In a flask equipped with a stirrer, 0.5 g of tannic acid was dissolved in 4 mL of anhydrous ethanol, followed by the addition of 3.4 g of catechol epoxy resin. The mixture was stirred at 70 °C for 30 min. Subsequently, 0.4 g of malic acid and 0.1 g of ricinoleic acid were added to the system, and stirring was continued at a constant temperature for 1 h.

[0033] Performance testing methods: Infrared spectroscopy analysis was performed using a Nicolet iS50 infrared spectrometer with a wavelength range of 4000-500 cm⁻¹.-1 Spectral resolution 4 cm -1 .

[0034] The adhesive bonding performance was tested using an ETM104B universal testing machine according to the national standard GB / T 7124-2008. The procedure involved first cleaning the substrate surface with ethyl acetate and acetone. Then, 0.20 g of adhesive was evenly applied to a 12.5 mm × 25 mm area on each substrate. The substrates were then transferred to a 180 °C oven for 1 min to ensure complete solvent evaporation. Dovetail clips were used to secure the bonding area to maintain tight contact between the adhesive and the substrate, and the mixture was allowed to cure for 8 h. Tensile shear strength tests were performed after the samples cooled to room temperature for 1 h.

[0035] Glass transition temperature of adhesives ( T g The test was performed using a Q2000 differential scanning calorimeter. The sample was cooled from room temperature to -10 °C at a rate of 10 °C / min and held for 5 min, then heated from -80 °C to 250 °C at a rate of 10 °C / min. The extrapolated onset temperature in the DSC curve was used as... T g .

[0036] The thermal stability of the adhesive was tested using a TG209F3 thermogravimetric analyzer (NETZSCH, Germany). The procedure involved placing the sample (~10 mg) in an alumina ceramic crucible and heating it from 30 °C to 600 °C at a rate of 10 °C / min under a nitrogen atmosphere. The temperature at which the adhesive mass loss reached 5% (T5%) was obtained.

[0037] The water resistance of the adhesive was tested using an ETM104B universal testing machine. The procedure involved immersing the adhesive sample in deionized water at room temperature for different durations, followed by testing the tensile shear strength of the adhesive sample.

[0038] Figure 2 Fourier transform infrared spectrum of the adhesive prepared in Example 3. The infrared spectrum shows that TMTR is at 1716 cm⁻¹. -1 A more pronounced C=O stretching vibration absorption peak appears at 1200 cm⁻¹ than that of TPGE. -1 The CO absorption peak near the 876 cm⁻¹ is enhanced. -1 and 758 cm -1 The weakening of the characteristic peak of the epoxy group indicates that the epoxy group has undergone a ring-opening reaction and formed covalently linked structures such as ester and ether bonds. This result confirms that a stable covalent cross-linked network has been established in the system.

[0039] More importantly, the catechol structural units retained in the molecular skeleton of the tea polyphenol glycidyl ether (TPGE) can not only participate in covalent reactions during the curing process, but also form a large number of intermolecular hydrogen bonds with the phenolic hydroxyl groups in the tannic acid (TAN) molecule, and may generate π–π stacking interactions, thereby further constructing a high-density physical cross-linking network on the basis of the covalent network.

[0040] Table 1 Performance data of the embodiments and comparative examples

[0041] Experimental results show that the Tg values ​​(~97 °C) of the samples obtained in each embodiment are similar, but the strengths differ significantly. Among them, the sample obtained in Example 3 exhibits the best tensile shear strength, which confirms the contribution of physical interactions. That is, Tg mainly reflects the chain segment motion, while the strength also depends on the interfacial interaction and energy dissipation mechanism. Figure 3 , 4 Meanwhile, under similar epoxy values, the bonding strength of the samples obtained in Comparative Examples 1 and 2 was significantly reduced. This is because although caffeic acid and catechol also possess catechol or polyphenol structures, their molecular structures are relatively simple, lacking the multi-categoryl structural units and multi-site reactive capabilities of tea polyphenols, making it difficult to form a high-density multi-linked network in the system. This demonstrates that the use of tea polyphenol glycidyl ether as a crosslinking monomer in this invention has significant structural advantages and is irreplaceable.

[0042] Figure 5 The figure shows the water resistance test results of the adhesive prepared in Example 3. As can be seen from the figure, after immersion for 6 hours, the shear strength of the adhesive decreases rapidly with the extension of immersion time, but the change tends to stabilize after 6 hours. This further confirms its dual network structure, namely, the dense covalent cross-linked network effectively hinders the penetration of water molecules, and even if a small amount of water molecules invade and destroy some hydrogen bonds (physical cross-linking), the chemical cross-linked network, which serves as the skeleton, remains intact, thus ensuring the basic mechanical properties of the material.

[0043] It should be noted that the above description is merely a preferred embodiment of the present invention, intended to illustrate the principles of the invention, and does not constitute a limitation on the scope of protection of the present invention. For those skilled in the art, appropriate modifications or equivalent substitutions can be made to the technical solutions in the above embodiments without departing from the core ideas of the present invention, and all such modifications should be covered within the scope of protection of the present invention.

Claims

1. A water-resistant, highly adhesive bio-based adhesive, characterized in that, The bio-based adhesive is prepared from the following raw materials by weight percentage through an in-situ crosslinking reaction: Tannic acid content: 4.69-5.77% Tea polyphenol glycidyl ethers: 51.96%~60.98% Malic acid content: 3.75%~4.62%. Ricinoleic acid 0.94~1.15%, Anhydrous ethanol 29.64~36.49%; The sum of the weight percentages of the above raw materials is 100%.

2. The bio-based adhesive according to claim 1, characterized in that, The tea polyphenol glycidyl ether is prepared by epichlorohydrin epoxidation modification of tea polyphenols, and its molecular structure contains both epoxy groups and catechol hydroxyl groups.

3. The bio-based adhesive according to claim 1, characterized in that, Its preparation includes the following steps: (1) Mix and dissolve tannic acid and tea polyphenol glycidyl ether in anhydrous ethanol and stir at 70 °C for 30 min; (2) Add malic acid and ricinoleic acid to the mixed solution obtained in step (1), and stir at 70 °C for 1 h to obtain the final product.

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