A closed-loop recyclable solvent-free bio-based adhesive and its preparation method and application

Abstract

This invention discloses a closed-loop, recyclable, solvent-free bio-based adhesive, its preparation method, and its applications. Based on lipoic acid and tris(2-aminoethyl)amine, a covalently cross-linked, reusable, solvent-resistant bio-based adhesive was successfully synthesized via a condensation reaction between carboxyl and amino groups, using tris(2-aminoethyl)amine as a cross-linking agent. Specifically, lipoic acid and tris(2-aminoethyl)amine are mixed and then stirred at 100-150°C for 2 hours, utilizing reversible covalent amide bonds to synthesize a closed-loop, recyclable, solvent-free bio-based adhesive. The molar ratio of tris(2-aminoethyl)amine to lipoic acid is 0.2-1:1. The adhesive of this invention exhibits excellent shear strength, solvent resistance, and reusability, and can be chemically recycled in a closed-loop manner, demonstrating its potential for various practical applications.

Description

Technical Field

[0001] This invention belongs to the field of bio-based adhesive technology, specifically relating to a closed-loop, recyclable, solvent-free bio-based adhesive, its preparation method, and its application. Background Technology

[0002] Sustainability is becoming increasingly important in the chemical industry and human society due to factors such as dwindling oil resources, environmental issues, and the need for customized long-term solutions. A promising approach is to find alternative, renewable raw materials, reduce waste and emissions, and adopt environmentally friendly manufacturing processes. The adhesive materials sector is shifting from fossil fuel-based to bio-based, drawing from vegetable oils, proteins, sugars, and other renewable resources. In ancient times, early humans used natural substances extracted from plants and animals, such as plant resins, tar, and beeswax, to bind objects together. In modern society, the Industrial Revolution brought significant advancements in adhesive technology, facilitating the development of synthetic adhesives with versatility and high performance, such as epoxy resins and acrylics. While modern adhesives are widely used and effective, they also bring environmental impacts and health problems, such as resource depletion, volatile organic compounds (VOCs), toxicity, dependence on non-renewable resources, and long-term degradation.

[0003] Bio-based adhesives represent an innovative adhesive technology by combining renewable natural materials with the unique properties of molecular interactions. Despite progress, the bonding strength of bio-based adhesives remains relatively weaker than that of traditional synthetic adhesives due to the inherent weaknesses of non-covalent interactions. Current research focuses on improving the adhesive properties of adhesives, particularly by enhancing their adhesion and cohesive strength. Adhesion refers to the interfacial bonding strength between the adhesive and the adherend, and is typically enhanced through chemical modification of specific functional groups. Cohesive strength, on the other hand, refers to the intrinsic mechanical strength of the adhesive itself, which is usually fine-tuned by altering non-covalent interactions, such as hydrogen bonding, metal ligand coordination, and host-guest interactions. Since non-covalent interactions tend to result in soft polymer networks, covalent interactions are often introduced to enhance the network stiffness of the polymer. It is well known that the crosslinking density of a polymer has a profound impact on the mechanical strength of the material.

[0004] The highly cross-linked network structure gives it excellent mechanical properties and a stable structure. This highly cross-linked network is difficult to dissolve in common solvents and is also difficult to melt or break at high temperatures, making the recycling of highly cross-linked polymers virtually impractical. Therefore, most highly cross-linked polymer materials are discarded after use, posing a significant problem for a sustainable green economy and the environment. Chemical recycling is a plastic recycling method that uses chemical means to transform waste plastics into usable components. Its principle is not new; research on the chemical decomposition and depolymerization of polymers has been conducted for a long time. For example, studies have been conducted on the thermal depolymerization of polymethyl methacrylate (PMMA) and polyoxymethylene (CPOM), as well as the depolymerization reactions of condensation polymers such as nylon (PA) and polyethylene terephthalate (PET). Some of these plastics have already been used in factories as an in-process recycling method using chemical methods. Furthermore, chemical recycling at room temperature makes it easier to depolymerize plastic waste into soluble oligomers or even monomers to regenerate new polymers. Therefore, closed-loop chemical recycling, which converts highly cross-linked polymers into monomer raw materials under certain environmental conditions, has great development potential. It can fully restore or even improve the mechanical properties of recycled polymers, which will play an important role in the circular polymer economy.

[0005] Lipoic acid (TA), as a bio-based molecule, contains dynamic disulfide bonds and an inherent carboxyl group, giving it inherent adhesive properties. Its ability to participate in various non-covalent interactions and its dynamic reversibility have made it widely recognized as a preferred choice for bio-based adhesive formulations. Tris(2-aminoethyl)amine (TREN), with its three amino groups linked by a nitrogen atom to form a trifurcation structure, provides an excellent raw material for constructing highly cross-linked three-dimensional network structures. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to provide a closed-loop, recyclable, solvent-free bio-based adhesive, its preparation method, and its application.

[0007] Based on thioctic acid and tri(2-aminoethyl)amine, this invention successfully synthesizes a covalently cross-linked, reusable, solvent-resistant bio-based adhesive through a condensation reaction between carboxyl and amino groups, using triamine as a cross-linking agent.

[0008] The specific technical solution is as follows:

[0009] A method for preparing a closed-loop recyclable solvent-free bio-based adhesive includes the following steps: mixing lipoic acid and tris(2-aminoethyl)amine, and then stirring at 100-150°C for 2 hours to synthesize a closed-loop recyclable solvent-free bio-based adhesive using reversible covalent amide bonds, wherein the molar ratio of tris(2-aminoethyl)amine to lipoic acid is 0.2-1:1.

[0010] Preferably, the molar ratio of tris(2-aminoethyl)amine to thioctic acid is 0.6:1.

[0011] A closed-loop, recyclable, solvent-free bio-based adhesive prepared using the above-described method.

[0012] The application of a closed-loop, recyclable, solvent-free bio-based adhesive, which is coated onto a substrate to achieve bonding between the substrates, allows the bonded substrates to be placed in PE, MeCN, DCM, THF, EA, acidic solutions with pH=1, alkaline solutions with pH=14, aqueous solutions, or seawater, while maintaining a shear strength greater than 7 MPa at the overlap shear.

[0013] Further, the bonded substrate is heated at 100°C to recover the adhesive on the substrate. DMSO and TEA are added and stirred until completely dissolved to form a clear solution. Then, deionized water is added, and thioctic acid is precipitated. The turbid liquid is centrifuged and dried to obtain thioctic acid monomer powder and protonated tris(2-aminoethyl)amine.

[0014] The beneficial effects of this invention are as follows:

[0015] 1) The adhesive of the present invention has excellent shear strength, with CBA0.6 having a shear strength of up to 16.1 MPa;

[0016] 2) The adhesive of the present invention has excellent solvent resistance, especially in the most polar MeCN, where CBA0.6 can achieve an ultra-high shear strength of 14.5 MPa, which undoubtedly shows that CBA0.6 has good resistance to organic solvents, especially highly polar ones;

[0017] 3) The adhesive of the present invention can be reused and can be chemically recycled in a closed loop. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of lipoic acid;

[0019] Figure 2 This is a schematic diagram of the structure of tris(2-aminoethyl)amine;

[0020] Figure 3 This is a schematic diagram of the overlap of solvent-free bio-based adhesive (CBA);

[0021] Figure 4 For solvent-free bio-based adhesives (CBAs) with good bonding properties;

[0022] Figure 5 The solvent resistance and reusability of CBA0.6;

[0023] Figure 6The 1H NMR spectra of the recovered monomers and the unreacted monomers are shown. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto.

[0025] Characterization methods:

[0026] Overlap shear test: such as Figure 3 As shown, an appropriate amount of solvent-free bio-based adhesive (CBA) was applied to the substrate, covering an area of ​​10 mm × 20 mm. Test samples were prepared by hot pressing, then cooled to room temperature, and the bonding performance was evaluated using a universal tensile testing machine. The structure of thioctic acid is shown below. Figure 1 As shown, the structure of tris(2-aminoethyl)amine is as follows: Figure 2 As shown, the shear bond strength is determined by dividing the maximum force at failure by the overlapping contact area. Each sample dataset is tested at least 10 times, and the results are averaged to ensure accuracy.

[0027] Tensile test: An appropriate amount of bio-based adhesive was placed in a hot press mold and hot-pressed into a film (10mm*20mm). After cooling to room temperature, the tensile properties were evaluated using a universal tensile testing machine. Each sample dataset was tested at least 10 times, and the average value of the results was taken to ensure accuracy.

[0028] Solvent resistance test: Substrate samples bonded with solvent-free bio-based adhesive (CBA) were immersed in different solvents at room temperature for 24 hours for solvent resistance test. Immediately after removal, an overlap shear test was performed without any treatment. Each sample dataset was tested at least 10 times, and the results were averaged to ensure accuracy.

[0029] Example 1

[0030] A closed-loop, recyclable, solvent-free bio-based adhesive (CBAx) is prepared, where x represents the molar ratio of TREN to TA.

[0031] TA and TREN (with molar ratios of TREN to TA of 0.2, 0.4, 0.6, 0.8, and 1.0, respectively) were weighed into a reaction flask. The mixture was stirred at 150°C for 2 hours to obtain closed-loop recyclable solvent-free bio-based adhesives, labeled as CBA0.2, CBA0.4, CBA0.6, CBA0.8, and CBA1.0, respectively. No further purification or separation was required, and they could be directly applied to adhesion and other experiments.

[0032] To investigate the effect of TREN content in the adhesive on the adhesive bond strength, lap shear tests were conducted on iron plates with different proportions of CBAx. Figure 4As shown in (a) and (b) ((a) represents the relationship between adhesive displacement and lap shear strength, and the result of (b) is derived from (a)), when the TREN content increases to 0.6, the shear strength of CBA increases to 16.1 MPa. This increase may be due to the addition of TREN, which increases the degree of cross-linking of the cross-linking network, thereby enhancing the cohesive force of the adhesive. However, when the TREN content continues to increase from 0.6 to 1.0, the shear strength of CBA decreases to 7.8 MPa. This may be because the excess amino groups completely react all the carboxyl groups of thioctic acid, reducing the non-covalent interaction between the adhesive and the substrate, thereby reducing the adhesive strength.

[0033] Tensile tests were conducted on iron plates with different proportions of CBAx, such as... Figure 4 As shown in (c) and (d), the tensile strength gradually increases when the TREN content increases from 0.2 to 1.0, and the Young's modulus also increases from 60 MPa to 698 MPa. This indicates that the mechanical properties of the material are gradually enhanced as the crosslinking agent content increases. This also corroborates the previous conjecture that the improvement of CBA adhesive performance is due to the increase in the crosslinking agent TREN content, which enhances the strength of the material.

[0034] Example 2

[0035] The ability to operate in diverse environments is a necessary requirement for high-performance adhesives. Most bio-based and supramolecular adhesives do not resist water and organic solvents well; in fact, excessive immersion can cause swelling, affecting their adhesive properties. This study evaluated the solvent resistance of CBA0.6 combined with iron sheets by performing lap shear tests after immersion in different solvents, including organic solvent environments and underwater environments (PE, MeCN, DCM, THF, EA, acidic solutions at pH 1, alkaline solutions at pH 14, aqueous solutions, or seawater). After immersion in these solvents for 24 hours, the results were as follows... Figure 5As shown in (a) and (b) ((a) represents the relationship between adhesive displacement and lap shear strength, and the result of (b) is derived from (a)), it can be observed that the resistance of CBA0.6 gradually increases with increasing polarity. For example, it can reach 11.3 MPa, 11.7 MPa, 11.8 MPa, and 11.9 MPa in PE, DCM, THF, and EA, respectively. In the most polar MeCN, CBA0.6 can reach an ultra-high shear strength of 14.5 MPa. This undoubtedly shows that CBA0.6 has good resistance to organic solvents, especially highly polar ones. In addition, the strength in acidic solution (pH=1), aqueous solution, artificial seawater, and alkaline solution (pH=14) also reaches 13.6 MPa, 12.7 MPa, 9.6 MPa, and 7.4 MPa, respectively. Therefore, the results prove that CBA0.6 has excellent solvent resistance performance, which also shows broad development prospects for the selection of adhesive solvent resistance performance.

[0036] Example 3

[0037] Repeatable experiments

[0038] After the standard lap shear test, the used adhesive CBA0.6 was recycled, and samples were prepared again by hot pressing. After standing at room temperature for 10 minutes until the samples were completely cured, the second shear strength of CBA0.6 was recorded by lap shear test. The above adhesive test steps were repeated 8 times to obtain 10 repeated test results for CBA0.6. At least 10 tests were performed for each sample dataset, and the results were averaged to ensure accuracy.

[0039] like Figure 5 As shown in (c) and (d) ((c) represents the relationship between adhesive displacement and lap shear strength, and the result of (d) is derived from (c)), even after ten repeated tests, the shear strength of CBA0.6 remained at 12.1 MPa, retaining 75.2% of its original strength, which undoubtedly demonstrates the excellent reusability of CBA0.6.

[0040] Example 4

[0041] Recycling Experiment

[0042] Based on the properties of the monomers used, TA is poorly soluble in water but readily soluble in some organic solvents, while TREN is soluble in both water and some organic solvents. This difference in solubility between TA and TREN provides a good idea for the recycling design of CBA0.6 adhesive: the used CBA0.6 sample is recycled into a reaction flask, DMSO and a small amount of TFA are added, and the mixture is stirred until the solution is completely clear. Deionized water is then added to precipitate TA, while the protonated TREN remains in the solution. After centrifugation and drying of the turbid liquid, thioctic acid monomer powder and protonated tris(2-aminoethyl)amine monomer are obtained.

[0043] The purity of the recovered monomers was assessed using proton nuclear magnetic resonance spectroscopy. Figure 6 As can be seen, the 1H NMR spectra of the recovered monomers are basically consistent with those of the unreacted monomers. It is worth noting that the recovery rate of thioctic acid monomers reached 86%, and the recovery rate of tris(2-aminoethyl)amine monomers was 90%.

Claims

1. A method for preparing a closed-loop, recyclable, solvent-free bio-based adhesive, characterized in that, The preparation method of the solvent-free bio-based adhesive is as follows: thioctic acid and tris(2-aminoethyl)amine are mixed and then stirred at 100-150°C to synthesize a closed-ring recyclable solvent-free bio-based adhesive using reversible covalent amide bonds. The molar ratio of tris(2-aminoethyl)amine to thioctic acid is 0.2-1:

1. It is coated onto a substrate to achieve an overlap between the substrates, and the shear strength of the overlap shear is greater than 7 MPa. The substrate is an iron plate.

2. The method for preparing a closed-loop, recyclable, solvent-free bio-based adhesive as described in claim 1, characterized in that, The molar ratio of tris(2-aminoethyl)amine to thioctic acid is 0.6:

1.

3. A closed-loop recyclable solvent-free bio-based adhesive prepared by the preparation method described in claim 1 or 2.

4. An application of the closed-loop recyclable solvent-free bio-based adhesive as described in claim 3, characterized in that, It is coated onto the substrate to achieve bonding between the substrates. The bonded substrate can be placed in PE, MeCN, DCM, THF, EA, acidic solution with pH=1, alkaline solution with pH=14, aqueous solution or seawater, and the overlap shear performance still maintains a shear strength greater than 7MPa.

5. The application of the closed-loop, recyclable, solvent-free bio-based adhesive as described in claim 4, characterized in that, The bonded substrate was heated at 100°C to recover the adhesive on the substrate. DMSO and TEA were added and stirred until completely dissolved to form a clear solution. Deionized water was then added, and the precipitate was thioctic acid. The turbid liquid was centrifuged and dried to obtain thioctic acid monomer powder and protonated tris(2-aminoethyl)amine.

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