Ultralow-temperature-resistant epoxy structural adhesive and preparation method thereof

Abstract

The invention relates to the technical field of bonding of structural components of unmanned aerial vehicles, in particular to an ultralow-temperature-resistant epoxy structural adhesive and a preparation method thereof. Comprising the following components in parts by weight: 100 parts of polyfunctional resin, 10-20 parts of low-temperature-resistant resin, 5-30 parts of a curing agent, 1-10 parts of an accelerant, 2-10 parts of reinforcing filler, 1-5 parts of a silane coupling agent and 1-5 parts of color paste, the epoxy structural adhesive is stably stored for one week at the room temperature of 25 DEG C, the curing condition is that the temperature is 100-170 DEG C / 4h, the epoxy structural adhesive is placed for 96 hours at the low temperature of-90 DEG C, and the shear strength value is not lower than 25MPa. According to the epoxy structural adhesive provided by the invention, through resin system modification, composite curing system matching and interface optimization design, the three major problems that a conventional epoxy structural adhesive is poor in ultralow temperature resistance, insufficient in corrosion resistance and weak in aluminum profile carbon fiber interface adhesive force are solved at the same time, and the requirements for light-weight and high-reliability bonding in the field of unmanned aerial vehicles are met.

Description

Technical Field

[0001] This invention relates to the field of bonding technology for structural components of unmanned aerial vehicles (UAVs), and in particular to an epoxy structural adhesive that can withstand ultra-low temperatures and its preparation method. Background Technology

[0002] In high-end manufacturing fields such as drones and aerospace, lightweight, high-strength aluminum profiles and carbon fiber composites are widely used in structural component manufacturing, and their reliable adhesion highly depends on high-performance adhesives. Epoxy resin-based structural adhesives have become the mainstream choice due to their excellent mechanical properties and adhesive characteristics. They achieve high-strength adhesion by forming a three-dimensional cross-linked network through the reaction of the curing agent and epoxy groups. However, conventional epoxy structural adhesives exhibit significant defects under extreme low-temperature conditions. When the temperature drops below -40°C, the movement of molecular chain segments is restricted, the brittleness of the material increases sharply, and the toughness decreases significantly, easily leading to micro-cracks and macro-fractures. In environments with even lower temperatures, such as -90°C, these problems are further aggravated, resulting in a severe decrease in shear strength and an inability to maintain structural integrity. At the same time, in corrosive media such as strong acids or alkalis, the epoxy network is prone to hydrolysis or chemical degradation, leading to rapid deterioration of adhesive performance and a significantly shortened service life. In addition, the surface characteristics of low-surface-energy materials such as aluminum profiles and carbon fibers result in insufficient wettability and weak interfacial bonding of existing structural adhesives, often leading to interlayer delamination, further weakening reliability under harsh operating conditions. These technical bottlenecks severely restrict the practical application of epoxy structural adhesives in ultra-low temperature and corrosive environments, necessitating the development of a novel adhesive system that can maintain stable mechanical properties at extremely low temperatures. Existing technologies urgently need improvement to address these issues. Summary of the Invention

[0003] The purpose of this application is to provide an epoxy structural adhesive that can withstand ultra-low temperatures and its preparation method. It has the advantage of maintaining high shear strength in ultra-low temperature environments, and effectively solves the problems of increased brittleness, decreased toughness and attenuated shear strength of existing epoxy structural adhesives under extreme low temperature conditions.

[0004] This application provides an epoxy structural adhesive that can withstand ultra-low temperatures, comprising the following components in the indicated weight proportions: 100 parts of multifunctional resin, 10-20 parts of low-temperature resistant resin, 5-30 parts of curing agent, 1-10 parts of accelerator, 2-10 parts of reinforcing filler, 1-5 parts of silane coupling agent, and 1-5 parts of colorant. The epoxy structural adhesive is stable for one week at room temperature (25°C), and the curing conditions are 100-170°C / 4h, followed by 96 hours at -90°C. The shear strength value is not less than 25 MPa.

[0005] The multifunctional resin is one or more of triglycidyl-p-aminophenol, phenolic resin, tetraglycidylamine, naphthol-type resin, and tetraglycidyl-4,4'-diaminodiphenylmethane.

[0006] The low-temperature resistant resin is one or more of the following: silicone-modified epoxy resin, core-shell toughened epoxy resin, polyurethane-modified epoxy resin, epoxy acrylate, and fluorine-modified epoxy resin.

[0007] The curing agent is one or more of 2-ethyl-4-methylimidazole, 1-cyano-2-ethyl-4-methylimidazole, 2-heptadecylimidazole, 2-phenylimidazole, 1,3-diphenyl-2-methylimidazole, dicyandiamide, methyltetrahydrophthalic anhydride, and methylnadic anhydride.

[0008] The accelerators are diaminoimidazole triazine complex and 2-ethyl-4-methylimidazole.

[0009] The reinforcing filler is one or more of the following: silica fume, aluminum hydroxide, alumina, quartz powder, mica powder, and kaolin.

[0010] The silane coupling agent is one or more of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and γ-methacryloyloxypropyltrimethoxysilane.

[0011] The color paste is a black color paste, comprising color powder and epoxy resin.

[0012] This epoxy structural adhesive, capable of withstanding temperatures as low as -90℃, addresses three major pain points of conventional epoxy structural adhesives: poor resistance to ultra-low temperatures, insufficient corrosion resistance, and weak adhesion to aluminum profiles and carbon fibers, through modification of a multi-functional resin system, combination of composite curing systems, and optimized interface design. It is well-suited to the needs of the drone industry for lightweight and highly reliable bonding.

[0013] I. Core Technology Principles and Design Logic

[0014] The performance improvement of this structural adhesive is not due to the improvement of a single component, but rather to a synergistic design based on "increased crosslinking density + optimized molecular chain toughness + enhanced interface wetting", the specific logic of which is as follows:

[0015] High cross-linking density achieves both corrosion resistance and structural stability. Multifunctional epoxy resins such as triglycidyl-p-aminophenol and tetraglycidylamine are selected. These resin molecules contain multiple epoxy groups, which form a dense three-dimensional cross-linked network after curing. High cross-linking density effectively blocks the penetration of acid and alkali solutions and solvent molecules, preventing hydrolysis or chemical degradation of the molecular chains, thus achieving resistance to acid and alkali corrosion and solvent corrosion.

[0016] Optimizing the molecular chain conformation to achieve ultra-low temperature resistance involves introducing low-temperature resistant resins such as silicone-modified epoxy resins. Utilizing the freely rotating conformational property of silicon-oxygen double bonds, the overall glass transition temperature (Tg) of the resin is lowered, allowing the molecular chains to maintain a certain degree of mobility even at -90℃, thus avoiding the increased brittleness and decreased toughness issues of conventional epoxy adhesives at low temperatures. Simultaneously, the combination of multifunctional resins and low-temperature resistant resins ensures both crosslinking density and molecular chain toughness.

[0017] Interface modification improves the adhesion of carbon fiber substrates by adding silane coupling agents such as γ-aminopropyltriethoxysilane. One end of the silane coupling agent can react chemically with the resin matrix, while the other end can combine with functional groups such as hydroxyl groups on the carbon fiber surface to form chemical bonds. At the same time, the coupling agent can also optimize the wettability of the colloid on the low surface energy carbon fiber substrate, solving the problems of weak interfacial bonding and easy interlayer peeling of existing adhesives.

[0018] Taking two commonly used silane coupling agents in the formulation as examples, the specific chemical equations and reaction descriptions are given:

[0019] Silane coupling agent: The reaction of γ-aminopropyltriethoxysilane with hydroxyl groups (-OH) on the carbon fiber surface (forming Si-O-C bonds) hydrolysis pretreatment:

[0020] NH2(CH2)3Si(OC2H5)3+3H2O→NH2(CH2)3Si(OH)3+3C2H5OH

[0021] Condensation reaction (with -OH on carbon fiber surface):

[0022] NH2(CH2)3Si(OH)3 + 3HO-carbon fiber → NH2(CH2)3Si(O-carbon fiber)3 + 3H2O

[0023] 2. Bonding reaction with epoxy resin matrix (ring-opening of amino and epoxy groups)

[0024] NH2(CH2)3Si(O-carbon fiber)3 + 3(-epoxy group-epoxy resin) → [-NH-CH2-CH2-O-epoxy resin-] + 3Si(O-carbon fiber)3

[0025] Silane coupling agent: γ-(2,3-epoxypropoxy)propyltrimethoxysilane

[0026] 1. The reaction between the carbon fiber and the hydroxyl groups on the carbon fiber surface

[0027] Hydrolysis pretreatment:

[0028] CH2OCHCH2O(CH2)3Si(OCH3)3+3H2O→CH2OCHCH2O(CH2)3Si(OH)3+3CH3OH

[0029] Condensation reaction:

[0030] CH2OCHCH2O(CH2)3Si(OH)3 + 3HO-carbon fiber → CH2OCHCH2O(CH2)3Si(O-carbon fiber)3 + 3H2O

[0031] 2. Bonding reaction with epoxy resin matrix (epoxy group synergistic crosslinking)

[0032] CH2OCHCH2O(CH2)3Si(O-carbon fiber)3+epoxy resin (containing active hydroxyl / amino groups) → triple crosslinked network (containing Si-OC and epoxy ring-opening structure)

[0033] illustrate:

[0034] The core logic is as follows: the silane coupling agent is first hydrolyzed to generate silanol groups (-Si(OH)3), which then dehydrate and condense with the hydroxyl groups on the carbon fiber surface to form stable chemical bonds; the other functional group (amino / epoxy group) reacts chemically with the epoxy resin matrix to achieve chemical bond bridging of "resin-coupling agent-carbon fiber".

[0035] Other silane coupling agents (such as γ-methacryloyloxypropyltrimethoxysilane) have similar reaction mechanisms, only the reaction type between the functional groups and the resin is different (such as double bond polymerization).

[0036] The selection of each component in the above formula is based on the core performance objectives, and the function and selection criteria of each component are shown in the table below:

[0037]

[0038] The method for preparing the above-mentioned epoxy structural adhesive includes the following steps:

[0039] Step 1: Preparation of the basic system. The multifunctional resin, low-temperature resistant resin, curing agent and color paste of the formula are dispersed at high speed at room temperature and stirred at 1200 r / min for 2 hours. Vacuum is drawn while dispersing, and the vacuum degree is -100 kPa to form an epoxy structural adhesive matrix.

[0040] Step 2: Adding functional components. Add the formula amount of reinforcing filler to the epoxy structural adhesive matrix in sequence and disperse. Stir at 1000 r / min for 30 minutes. Continue to add the accelerator and disperse and stir at the same rate for 30 minutes. Vacuum is drawn while dispersing, and the vacuum degree is -100 kPa.

[0041] Step 3: Continue adding silane coupling agent, stir at 1000 r / min for 30 minutes, disperse while drawing a vacuum of -100 kPa to form a uniform and stable finished epoxy structural adhesive.

[0042] As can be seen from the above, the epoxy structural adhesive and its preparation method provided in this application, which can withstand ultra-low temperatures, solves three major problems of conventional epoxy structural adhesives: poor resistance to ultra-low temperatures, insufficient corrosion resistance, and weak adhesion to aluminum profiles and carbon fibers through resin system modification, composite curing system matching, and interface optimization design. This makes it suitable for the needs of the UAV field for lightweight and high-reliability bonding. It maintains high shear strength at ultra-low temperatures, with the advantage of a shear strength of not less than 25 MPa at -90℃. Detailed Implementation

[0044] Traditional carbon fiber-based structural adhesives are prone to increased brittleness and decreased toughness under extreme low-temperature environments, and may undergo chemical degradation in highly corrosive environments, affecting their service life. Furthermore, their insufficient wettability to low surface energy materials results in weak interfacial adhesion and a tendency for interlayer delamination.

[0045] In response, this application proposes an epoxy structural adhesive that can withstand ultra-low temperatures, comprising the following components in the indicated weight proportions: 100 parts of multifunctional resin, 10-20 parts of low-temperature resistant resin, 5-30 parts of curing agent, 1-10 parts of accelerator, 2-10 parts of reinforcing filler, 1-5 parts of silane coupling agent, and 1-5 parts of colorant. This epoxy structural adhesive is stable for one week at room temperature (25°C), and the curing conditions are 100-170°C / 4h, followed by 96 hours at -90°C. The shear strength value is not less than 25 MPa.

[0046] The ultra-low temperature resistant epoxy structural adhesive of this application exhibits a shear strength of not less than 25 MPa after being cured at -90°C for 96 hours. This performance is achieved through the synergistic effect of its components, particularly the selection and proportioning of the low-temperature resistant resin and reinforcing filler, ensuring that the colloid maintains structural integrity and mechanical strength even at extreme low temperatures. The principle of this invention is to provide a resin with multifunctional groups and high reactivity using a molecular weight profile to increase the crosslinking density of the cured colloid. Increasing the low-temperature resistant functional groups and utilizing the freely rotating conformation of the silicon-oxygen double bonds achieves ultra-low temperature resistance, resulting in a high molecular weight polymer. Increasing the resin polarity and using a special silicon coupling agent enhances the wettability to low surface energy substrates, thereby achieving higher bonding performance. High crosslinking density prevents solvent penetration, and high chemical bond energy prevents acid and alkali corrosion, thus achieving ultra-low temperature resistance, high bonding strength, and resistance to acid and alkali corrosion.

[0047] This application presents a cryogenic epoxy structural adhesive that effectively addresses the problems of increased brittleness, decreased toughness, and chemical degradation in highly corrosive environments associated with existing carbon fiber-based structural adhesives by optimizing the proportions of its components. This structural adhesive maintains high shear strength even under cryogenic conditions, thus ensuring the reliability and service life of carbon fiber bonding in fields such as drones.

[0048] In this regard, this application further proposes that the multifunctional resin is one or more of triglycidyl-p-aminophenol, phenolic resin, tetraglycidylamine, naphthol-type resin, and tetraglycidyl-4,4'-diaminodiphenylmethane.

[0049] Multifunctional resins are the core component of epoxy structural adhesive systems. Their molecular structure contains two or more epoxy groups, which can react with curing agents to form a highly cross-linked network structure. This high cross-linking density is crucial for improving the heat resistance, chemical stability, and mechanical strength of the cured product. In ultra-low temperature environments, high cross-linking density helps maintain the rigidity and strength of the material, resisting the restriction of molecular chain movement and increased brittleness caused by sudden temperature drops.

[0050] Specifically, triglycidyl-p-aminophenol has high epoxy functionality. The benzene ring and amino structure in its molecular structure endow the cured product with good thermal stability and mechanical properties, which helps to form a dense cross-linked network, thereby maintaining high strength at low temperatures.

[0051] Phenolic resins, such as epoxy phenolic resins, are typically multifunctional, enabling them to form cured products with high cross-linking density. Their molecular chains contain multiple benzene ring structures, increasing the rigidity and heat resistance of the cured product and playing a positive role in improving mechanical properties at ultra-low temperatures.

[0052] Tetraglycidylamine and tetraglycidyl-4,4'-diaminodiphenylmethane are highly functional amine epoxy resins containing multiple epoxy groups and an aromatic amine backbone in their molecular structure. These resins can form a very dense and tough cross-linked network, significantly improving the glass transition temperature and mechanical strength of the cured product. Especially under ultra-low temperature conditions, they can effectively resist stress concentration and crack propagation, thus maintaining excellent shear strength.

[0053] Naphthol-based resins, such as epoxy naphthol resins, contain naphthalene rings in their molecular structure, exhibiting higher rigidity and thermal stability compared to traditional bisphenol A type epoxy resins. This structural characteristic helps maintain the structural integrity and mechanical properties of materials at ultra-low temperatures, reducing brittleness.

[0054] By selecting the aforementioned multifunctional resins with specific molecular structures and high functionality, it is possible to ensure that the epoxy structural adhesive forms a stable, densely cross-linked, and sufficiently tough three-dimensional network after curing, thereby maintaining excellent shear strength even at ultra-low temperatures of -90℃.

[0055] By selecting one or more of the following as multifunctional resins: triglycidyl-p-aminophenol, phenolic resin, tetraglycidylamine, naphthol-type resin, and tetraglycidyl-4,4'-diaminodiphenylmethane, the epoxy structural adhesive of this application can form a cured product with high crosslinking density and excellent structural stability. These specific multifunctional resins, with their abundant epoxy groups and rigid backbone in their molecular structure, construct a denser and tougher crosslinking network during the curing process. This network structure can effectively suppress the movement of molecular chain segments and reduce the brittle transition of the material under ultra-low temperature conditions, thereby ensuring that the epoxy structural adhesive maintains its high shear strength and meets the performance requirement of not less than 25 MPa even under extreme low temperature conditions of -90°C. Compared with the solution using general-purpose multifunctional resins, this application, through the refined selection of multifunctional resins, significantly improves the mechanical reliability and durability of the epoxy structural adhesive under ultra-low temperature conditions, enabling its stable application in fields with stringent requirements for ultra-low temperature resistance.

[0056] This application aims to improve the low-temperature performance of epoxy structural adhesives by introducing low-temperature resistant resins. However, in practical applications, simply introducing general low-temperature resistant resins may not be sufficient to meet the high requirements for shear strength in ultra-low temperature environments of -90℃, or it may adversely affect other key properties of the adhesive (such as toughness, bond strength, processability, etc.) while ensuring low-temperature performance, thereby limiting the overall performance and application range of epoxy structural adhesives.

[0057] In this regard, this application further proposes the above-mentioned epoxy structural adhesive that can withstand ultra-low temperatures, wherein the low-temperature resistant resin is one or more of the following: silicone-modified epoxy resin, core-shell toughened epoxy resin, polyurethane-modified epoxy resin, epoxy acrylate, and fluorine-modified epoxy resin.

[0058] Specifically, the low-temperature resistant resin is a key component in epoxy structural adhesives used to enhance their toughness and resistance to brittle fracture in low-temperature environments. Its main function is to introduce flexible segments or form micro-toughening structures to absorb stress and inhibit crack propagation, thereby ensuring that the epoxy structural adhesive maintains sufficient strength and toughness under ultra-low temperature conditions.

[0059] When the low-temperature resistant resin is a silicone-modified epoxy resin, it is achieved by introducing silicone segments with excellent flexibility and low glass transition temperature into the epoxy resin molecular structure. Silicone segments (such as polydimethylsiloxane) have extremely low glass transition temperatures and good segmental flexibility, which can significantly reduce the overall glass transition temperature of the epoxy structural adhesive and increase its free volume and molecular chain mobility at low temperatures, thereby effectively improving the low-temperature toughness and thermal shock resistance of the epoxy structural adhesive. This is typically achieved by copolymerizing or grafting silicone polymers containing active groups (such as hydroxyl, amino, or epoxy groups) with epoxy resin to form composite materials with core-shell or interpenetrating network structures.

[0060] When the low-temperature resistant resin is a core-shell toughened epoxy resin, it typically consists of a soft elastomer core and a hard polymer shell. The soft core (e.g., nitrile rubber, acrylic rubber, etc.) absorbs energy under impact or stress, dissipating it through mechanisms such as shear yielding and cavitation, thereby preventing crack initiation and propagation. The hard shell ensures good compatibility and dispersibility between the core-shell particles and the epoxy matrix, preventing particle aggregation and effectively transferring stress. This structure provides excellent toughening at low temperatures, significantly improving the impact strength and fracture toughness of the epoxy structural adhesive. It is typically achieved by uniformly dispersing pre-formed core-shell rubber particles into the epoxy resin matrix.

[0061] When the low-temperature resistant resin is a polyurethane-modified epoxy resin, its low-temperature toughness is improved by introducing flexible polyurethane segments into the epoxy resin network. The polyurethane segments typically consist of soft segments (such as polyether polyols or polyester polyols) and hard segments (such as the reaction product of isocyanate and chain extender), with the soft segments imparting good flexibility and elasticity to the polymer. Introducing polyurethane into the epoxy system allows for the formation of a microphase-separated structure within the epoxy matrix. The flexible polyurethane phase acts as a stress concentration point, effectively absorbing energy and preventing crack propagation, thereby improving the impact resistance and toughness of the epoxy structural adhesive at low temperatures. This is typically achieved by reacting the epoxy resin with isocyanate and polyol to form an epoxy-polyurethane prepolymer, or by blending a pre-prepared polyurethane with the epoxy resin.

[0062] When the low-temperature resistant resin is epoxy acrylate, it combines the excellent adhesion of epoxy resin with the rapid curing properties of acrylate. In the formulation of this invention, by selecting acrylate monomers or oligomers with a flexible backbone, the epoxy acrylate system can be endowed with a certain degree of flexibility, thereby exhibiting good resistance to cracking at low temperatures. This is typically achieved by reacting epoxy resin with acrylic acid or methacrylic acid.

[0063] When the low-temperature resistant resin is a fluorinated modified epoxy resin, its low-temperature performance is improved by introducing fluorinated polymer segments. Fluorinated polymers (such as polytetrafluoroethylene and polyvinylidene fluoride) typically possess extremely low surface energy, excellent chemical stability, and good low-temperature flexibility. This is due to the high electronegativity of fluorine atoms, which weakens intermolecular forces, and the high chain mobility of fluorocarbon segments. Introducing fluorinated segments into epoxy resin can effectively lower the glass transition temperature of epoxy structural adhesives, improving their toughness and crack propagation resistance at low temperatures. This is typically achieved by copolymerizing or grafting fluorinated polymers containing active groups with epoxy resin.

[0064] Through the aforementioned technical solution, this application, by selectively choosing silicone-modified epoxy resin, core-shell toughened epoxy resin, polyurethane-modified epoxy resin, epoxy acrylate, or fluorine-modified epoxy resin as low-temperature resistant resins, effectively utilizes the toughening mechanisms of these specific resins, such as introducing flexible segments, forming a core-shell structure to absorb energy, or lowering the glass transition temperature, thereby significantly improving the toughness and impact resistance of epoxy structural adhesives in ultra-low temperature environments. This precise component selection effectively suppresses low-temperature brittleness, prevents crack propagation, and ensures that the epoxy structural adhesive maintains excellent shear strength and structural integrity even under extreme low-temperature conditions of -90℃. This not only solves the potential performance imbalance problem of general low-temperature resistant resins but also better synergizes with other components such as multifunctional resins and curing agents to form a high-strength, high-toughness, and long-term stable bonding system at ultra-low temperatures, significantly improving the reliability and applicability of epoxy structural adhesives in harsh environments such as aerospace and polar engineering. In some embodiments described above, this application proposes an epoxy structural adhesive capable of withstanding ultra-low temperatures, which, through the formulation of specific components, aims to achieve excellent low-temperature performance and shear strength. However, in the practical application of epoxy structural adhesives, the selection of the curing agent is crucial for ensuring the curing efficiency of the adhesive, the crosslinking density of the cured product, and the mechanical properties of the final material under extreme low-temperature environments. An inappropriate selection of the curing agent may lead to incomplete curing and excessive internal stress in the cured product, thereby affecting its long-term stability and shear strength under ultra-low temperature conditions.

[0065] In this regard, this application further proposes a specific selection of the curing agent, which may be one or more of 2-ethyl-4-methylimidazole, 1-cyano-2-ethyl-4-methylimidazole, 2-heptadecylimidazole, 2-phenylimidazole, 1,3-diphenyl-2-methylimidazole, dicyandiamine, methyltetrahydrophthalic anhydride, and methylnadic anhydride.

[0066] Specifically, 2-ethyl-4-methylimidazolium is a commonly used imidazolium curing agent with good catalytic activity and a moderate curing rate, effectively promoting the ring-opening polymerization of epoxy resins to form a dense cross-linked network. 1-Cyano-2-ethyl-4-methylimidazolium, as an imidazolium derivative, has a cyano group that can regulate the activity and latency of the curing agent, facilitating a more controllable curing process under specific curing conditions and potentially improving the toughness of the cured product. 2-Heptadecanylimidazolium is a long-chain alkyl-substituted imidazolium curing agent; its long-chain structure helps improve the solubility and dispersibility of the curing agent in epoxy resin systems, and may also impart better flexibility and impact resistance to the cured product, which is particularly important for applications in low-temperature environments. 2-Phenylidene imidazolium is a phenyl-substituted imidazolium curing agent; the presence of the benzene ring can improve the heat resistance of the curing agent and the glass transition temperature of the cured product, thereby enhancing the performance stability of the colloid over a wide temperature range. 1,3-Diphenyl-2-methylimidazolium is a more structurally complex imidazolium curing agent. Its polyphenyl substitution structure can further enhance the thermal stability and mechanical strength of the cured product, helping to maintain excellent shear strength at ultra-low temperatures. Dicyandiamine is a latent curing agent, typically activated at higher temperatures. It is characterized by good storage stability and, after curing, forms a highly cross-linked network structure, providing excellent heat resistance and chemical stability for epoxy structural adhesives. Simultaneously, its cured product maintains good toughness at low temperatures. Methyltetrahydrophthalic anhydride and methylnadic anhydride are both anhydride curing agents. When these curing agents react with epoxy resins, the cured products exhibit excellent heat resistance, electrical insulation, and mechanical strength. The curing process of anhydride curing agents is typically slow and has low heat release, helping to reduce internal stress during curing, thereby improving the crack resistance and shear strength of the colloid at ultra-low temperatures.

[0067] By employing the aforementioned technical solutions and selecting specific imidazole compounds and / or anhydride compounds as curing agents, the curing kinetics of epoxy structural adhesives and the microstructure of the cured products can be effectively controlled. Imidazole curing agents provide highly efficient catalysis, ensuring that epoxy resins are fully crosslinked under curing conditions of 100-170℃, forming a high-density three-dimensional network structure. In particular, imidazole curing agents with different substituents, such as 2-ethyl-4-methylimidazolium, 1-cyano-2-ethyl-4-methylimidazolium, 2-heptadecylimidazolium, 2-phenylimidazolium, and 1,3-diphenyl-2-methylimidazolium, can work synergistically to optimize the reactivity of the curing system and the performance of the cured products. For example, long-chain substituted imidazoles help improve low-temperature toughness, while phenyl-substituted imidazoles help improve thermal stability. Meanwhile, dicyandiamine, as a latent curing agent, provides good storage stability and forms a tough crosslinked network after curing. Anhydride curing agents, such as methyltetrahydrophthalic anhydride and methylnadic anhydride, can further enhance the heat resistance and mechanical strength of the cured product and help reduce curing shrinkage stress. The rational selection and combination of these curing agents enable the epoxy structural adhesive to form a cured product with high crosslinking density, low internal stress, and good flexibility at the molecular level after curing at 100-170℃ for 4 hours. Therefore, even under the harsh condition of being placed at -90℃ for 96 hours, the epoxy structural adhesive can still maintain its structural integrity and mechanical properties, ensuring a shear strength of no less than 25 MPa. This effectively solves the problem of performance degradation of epoxy structural adhesives in ultra-low temperature environments and significantly improves its reliability and durability in extreme low-temperature applications.

[0068] In some embodiments described above, an epoxy structural adhesive capable of withstanding ultra-low temperatures is proposed, the components of which include an accelerator. However, in practical applications, the selection of the accelerator has a significant impact on the curing efficiency, storage stability, and performance of the final cured product of the epoxy structural adhesive, especially its mechanical properties under ultra-low temperature conditions. An inappropriate selection of the accelerator may lead to incomplete curing, shortened shelf life, or poor low-temperature performance, thereby affecting the overall reliability of the epoxy structural adhesive.

[0069] In this application, the accelerator is further specified as a diaminoimidazolium triazine complex or 2-ethyl-4-methylimidazolium. Accelerators play a crucial role in epoxy structural adhesive systems, primarily accelerating the reaction rate between the epoxy resin and the curing agent, lowering the curing temperature, and shortening the curing time, thereby improving production efficiency. Simultaneously, a suitable accelerator can also influence the crosslinking density and network structure of the cured product, thus profoundly affecting the material's mechanical properties, heat resistance, and chemical resistance. In epoxy structural adhesives capable of withstanding ultra-low temperatures, the selection of the accelerator is particularly critical, ensuring that curing efficiency is maintained without compromising the material's toughness and strength at extreme low temperatures. Diaminoimidazolium triazine complex is a highly efficient latent accelerator. Its characteristics include low activity at room temperature, providing a long shelf life, but rapid decomposition and release of active components under heating conditions, effectively promoting the curing reaction of the epoxy resin. This latency allows epoxy structural adhesives to have a good working time after mixing, and simultaneously achieve the desired degree of curing quickly, contributing to the formation of a uniform and dense cross-linked network. This lays the foundation for the excellent mechanical properties of epoxy structural adhesives under ultra-low temperature conditions. 2-Ethyl-4-methylimidazole is a commonly used imidazole accelerator with high catalytic activity. It can effectively lower the curing activation energy of epoxy resins, accelerating the curing reaction. Compared to diaminoimidazole triazine complexes, 2-ethyl-4-methylimidazole generally has higher activity, achieving curing at lower temperatures or in shorter times. In certain applications, it can be used alone or in combination with latent accelerators to achieve precise control over the curing rate and degree of curing, thereby optimizing the overall performance of epoxy structural adhesives, including their shear strength under ultra-low temperature conditions.

[0070] By limiting the accelerators to diaminoimidazolium triazine complex and 2-ethyl-4-methylimidazolium, the aforementioned technical solutions effectively address potential challenges in curing efficiency, storage stability, and ultra-low temperature performance of epoxy structural adhesives. Specifically, diaminoimidazolium triazine complex, as a latent accelerator, ensures good storage stability of the epoxy structural adhesive at room temperature (25°C), extending its service life and preventing premature curing. Simultaneously, under heat curing conditions, it can be rapidly activated, accelerating the crosslinking reaction of the epoxy resin and ensuring a thorough curing process. Meanwhile, 2-ethyl-4-methylimidazolium, with its highly efficient catalytic activity, further optimizes the curing rate, enabling the epoxy structural adhesive to achieve full curing within 4 hours in a curing temperature range of 100-170°C, forming a cured product with high crosslinking density. This precisely controlled curing process helps to build a uniform and stable polymer network, enabling epoxy structural adhesives to maintain excellent toughness and strength even at ultra-low temperatures of -90°C, ensuring that their shear strength is not less than 25 MPa. This significantly improves the reliability and application range of epoxy structural adhesives under extreme low-temperature conditions.

[0071] In some embodiments described above in this application, an epoxy structural adhesive capable of withstanding ultra-low temperatures is proposed, which aims to achieve excellent low-temperature shear strength through the synergistic effect of multiple components. However, in practical applications, if the type of reinforcing filler is not carefully selected, the mechanical properties of the epoxy structural adhesive, especially its shear strength, may not reach their optimal state under extreme low-temperature environments, or it may exhibit insufficient stability during long-term use, thereby affecting its reliability and durability. To address this, this application further proposes the use of specific types of reinforcing fillers in the epoxy structural adhesive, wherein the reinforcing filler is one or more of the following: silica powder, aluminum hydroxide, alumina, quartz powder, mica powder, and kaolin.

[0072] Specifically, when the reinforcing filler is silica powder, silica powder, as an inorganic non-metallic material, possesses high hardness, low coefficient of thermal expansion, and good insulation properties. Introducing it into epoxy structural adhesives can effectively improve the hardness, wear resistance, and dimensional stability of the adhesive, while reducing curing shrinkage and internal stress. In practical implementation, silica powder with different particle size distributions can be selected, such as spherical or angular silica powder with an average particle size ranging from 0.5 micrometers to 50 micrometers, to optimize the filling density and rheological properties. To further enhance the interfacial bonding with the epoxy resin matrix, the silica powder can undergo surface modification treatment, such as silane coupling agent treatment.

[0073] When the reinforcing filler is aluminum hydroxide, it is an inorganic flame-retardant filler that also provides reinforcement. In epoxy structural adhesives, it not only provides flame retardant properties but also improves the overall mechanical strength and rigidity of the adhesive through its high hardness and modulus. Furthermore, the addition of aluminum hydroxide helps reduce the coefficient of thermal expansion of the epoxy structural adhesive, allowing it to maintain better dimensional stability over a wide temperature range, especially in ultra-low temperature environments, effectively alleviating stress concentration caused by thermal expansion and contraction. In practical applications, aluminum hydroxide with different particle sizes and surface treatments can be selected according to the required performance, such as micron-sized or submicron-sized aluminum hydroxide treated with silane or titanate coupling agents.

[0074] When the reinforcing filler is alumina, alumina is a ceramic material with high hardness, high strength, and excellent wear resistance. As a reinforcing filler, alumina can significantly improve the compressive strength, flexural strength, and wear resistance of epoxy structural adhesives. Its high thermal conductivity also helps improve the heat dissipation performance of the adhesive in some applications. In ultra-low temperature environments, the addition of alumina can effectively improve the toughness and impact resistance of the adhesive, inhibit the propagation of microcracks, and thus maintain high shear strength. Alumina with different crystal forms (such as α-alumina, γ-alumina) and particle sizes (such as nano- or micron-sized) can be selected, and its dispersibility and interfacial bonding in epoxy resin can be improved through surface treatment techniques (such as silane coupling agents).

[0075] When the reinforcing filler is quartz powder, quartz powder is a natural siliceous material with high hardness, resistance to acid and alkali corrosion, low coefficient of thermal expansion, and excellent electrical insulation properties. Using it as a reinforcing filler can effectively improve the hardness, wear resistance, and dimensional stability of epoxy structural adhesives, while reducing material costs. Under ultra-low temperature conditions, the addition of quartz powder helps maintain the rigidity and strength of the colloid, reducing volume shrinkage caused by temperature changes, thereby ensuring the stability of the bonding interface. In specific implementations, quartz powders of different purities, particle sizes, and particle shapes (such as spherical and irregular shapes) can be selected according to application requirements, and surface activation treatment can be performed to enhance compatibility with the resin matrix.

[0076] When the reinforcing filler is mica powder, mica powder is a layered silicate mineral with excellent insulation, heat resistance, and dimensional stability. As a reinforcing filler, the lamellar structure of mica powder can form a physical barrier in epoxy structural adhesives, effectively improving the interlaminar shear strength, impermeability, and creep resistance of the adhesive. Its unique lamellar structure also helps to disperse stress and improve the toughness of the adhesive, especially under ultra-low temperature impact conditions, effectively inhibiting crack propagation. Mica powders with different aspect ratios and particle sizes can be selected, and their surfaces can be organically modified to improve their dispersibility and interfacial bonding strength in epoxy resins.

[0077] When the reinforcing filler is kaolin, kaolin is a layered silicate mineral with good chemical stability, insulation properties, and low cost. As a reinforcing filler, kaolin can improve the rheological properties of epoxy structural adhesives, enhance their thixotropy, and facilitate construction. Simultaneously, its addition can also improve the hardness, modulus, and chemical resistance of the adhesive to a certain extent. In ultra-low temperature environments, the addition of kaolin helps stabilize the structure of the adhesive and reduce performance fluctuations caused by temperature changes. Kaolin with different degrees of calcination (e.g., calcined kaolin, washed kaolin) and particle sizes can be selected, and surface treatment can be applied to optimize its dispersibility and reinforcing effect in epoxy resin.

[0078] By introducing the aforementioned specific reinforcing fillers, the epoxy structural adhesive of this application achieves significant performance improvements. These specific inorganic fillers possess characteristics such as high hardness, high modulus, and low coefficient of thermal expansion, effectively improving the rigidity, strength, and dimensional stability of the adhesive. Specifically, these reinforcing fillers form a uniformly dispersed reinforcing phase within the epoxy resin matrix, effectively bearing and dispersing external stresses, inhibiting the initiation and propagation of microcracks, thereby significantly enhancing the shear strength of the epoxy structural adhesive at -90°C. Simultaneously, the addition of these fillers also helps reduce the curing shrinkage and coefficient of thermal expansion of the adhesive, reducing internal stress caused by drastic temperature changes, thus ensuring the long-term stability and reliability of the bonding interface, effectively solving the problems of insufficient mechanical properties and poor stability that may occur when reinforcing fillers are not carefully selected.

[0079] In some embodiments described above, an epoxy structural adhesive capable of withstanding ultra-low temperatures is proposed. This epoxy structural adhesive aims to achieve excellent low-temperature performance through the synergistic effect of multiple components. However, in practical applications, the interfacial compatibility between the epoxy resin matrix and the reinforcing filler is often poor, which may lead to low stress transfer efficiency and thus affect the overall mechanical properties and long-term stability of the epoxy structural adhesive in ultra-low temperature environments.

[0080] In this regard, this application further proposes that the silane coupling agent is one or more of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and γ-methacryloyloxypropyltrimethoxysilane. A silane coupling agent is an organosilicon compound with two different reactive functional groups, whose core function is to establish a "molecular bridge" between the organic polymer matrix and the inorganic filler. Specifically, one end of the silane coupling agent contains a group that can react with the hydroxyl groups or oxides on the surface of the inorganic filler (such as silica powder, aluminum hydroxide, alumina, quartz powder, mica powder, kaolin, etc.) to form a stable covalent bond; the other end contains an organic functional group that can chemically react or physically entangle with the epoxy resin matrix. Through this bidirectional reaction mechanism, the silane coupling agent can significantly improve the dispersion uniformity of the reinforcing filler in the epoxy resin and enhance the interfacial bonding strength between the two.

[0081] In specific implementations, the selected silane coupling agent may include:

[0082] γ-aminopropyltriethoxysilane has an amino functional group that can undergo a ring-opening reaction with the epoxy group of epoxy resin, while its ethoxysilane group can react with the hydroxyl groups on the surface of inorganic fillers, thereby forming a strong chemical bond between the organic and inorganic interfaces.

[0083] γ-(2,3-epoxypropoxy)propyltrimethoxysilane has epoxy functional groups that can directly participate in the curing reaction of epoxy resins to form tighter chemical bonds, while its methoxysilane groups react with the surface of inorganic fillers.

[0084] γ-Methacryloxypropyltrimethoxysilane has a methacryloxy functional group that can react with polymer segments through free radical polymerization or with specific groups of epoxy resin, while its methoxysilyl group reacts with the surface of inorganic fillers.

[0085] By introducing the aforementioned specific silane coupling agent, a robust chemical bridge can be effectively established between the epoxy resin matrix and the reinforcing filler. This enhanced interfacial bonding significantly improves the uniformity of filler dispersion and optimizes the efficiency of stress transfer from the epoxy resin matrix to the reinforcing filler. Especially in ultra-low temperature environments of -90°C, the difference in thermal expansion coefficients between epoxy resin and inorganic fillers can easily lead to stress concentration and debonding at the interface. However, the strong interfacial bond formed by the silane coupling agent effectively suppresses interfacial debonding and the generation and propagation of microcracks caused by differences in thermal expansion and contraction. This ensures that the epoxy structural adhesive maintains excellent shear strength and structural integrity under extreme low temperature conditions, significantly improving its ultra-low temperature resistance and long-term reliability.

[0086] The aforementioned ultra-low temperature resistant epoxy structural adhesives contain colorant components. However, in practical applications, meeting specific aesthetic requirements, such as obtaining a uniform and deep black color, while ensuring that the colorant itself does not negatively impact the ultra-low temperature performance, storage stability, and post-curing mechanical properties of the epoxy structural adhesive, presents a technical challenge. Ordinary colorants may introduce impurities or affect curing, leading to performance degradation or unsatisfactory appearance.

[0087] In this regard, this application further proposes that the color paste is a black color paste, and that the black color paste includes color powder and epoxy resin.

[0088] Specifically, the black pigment paste is a colorant used to impart a black appearance to epoxy structural adhesives. Its core lies in the selection of pigment powder and a suitable epoxy resin carrier. The pigment powder, a solid particulate substance providing color, is the core coloring component of the black pigment paste. Its main function is to provide the desired black appearance to the epoxy structural adhesive and to influence the pigment paste's hiding power, tinting strength, and weather resistance. Preferred pigment powders may include, but are not limited to, carbon black and iron oxide black. Carbon black, due to its excellent tinting strength, hiding power, and relatively stable chemical properties, is often used as a black pigment powder. The selected pigment powder should have good dispersibility and not react adversely with other components of the epoxy structural adhesive to ensure the performance of the final product. The epoxy resin serves as the carrier resin, used to disperse the pigment powder and form a stable paste. Its function is to uniformly disperse and stably suspend the pigment powder, effectively preventing pigment sedimentation or agglomeration. Simultaneously, because the epoxy resin, as the carrier, has good compatibility with the epoxy structural adhesive matrix, it ensures that the black pigment paste, after being added to the epoxy structural adhesive, will not affect its curing behavior and final mechanical properties. The epoxy resin used as the carrier for the color paste can be a resin with good compatibility with multifunctional resins or low-temperature resistant resins in epoxy structural adhesives, such as bisphenol A type epoxy resin, bisphenol F type epoxy resin, or their modifiers. The selection should consider the compatibility with the viscosity, curing characteristics, and final performance of the base adhesive system. The preparation of black color paste typically involves uniformly dispersing selected black pigments into the epoxy resin matrix through processes such as grinding and dispersion to ensure fine pigment particles and uniform distribution, thereby obtaining a stable color paste.

[0089] Through the above technical solutions, this application ensures that epoxy structural adhesives achieve a stable, uniform, and deep black appearance. Using a black pigment with a clearly defined composition, especially one based on epoxy resin, guarantees uniform dispersion of the pigment powder throughout the epoxy structural adhesive system, effectively preventing color differences and particle agglomeration, thus achieving a consistent black appearance. Simultaneously, selecting an epoxy resin with good compatibility with the epoxy structural adhesive matrix as the pigment carrier effectively avoids the introduction of incompatible components, ensuring that the pigment does not negatively impact the storage stability, curing characteristics, or key mechanical properties of the epoxy structural adhesive, such as low-temperature performance and shear strength after curing. Furthermore, the viscosity and rheological properties of this clearly defined black pigment are easier to control, facilitating stable dispersion and mixing during preparation, improving production efficiency and product quality. The black pigment also possesses excellent hiding power, effectively concealing the color of the substrate or any minor defects that may exist within the adhesive layer, further enhancing the visual quality of the final product.

[0090] In some embodiments described above, an epoxy structural adhesive capable of withstanding ultra-low temperatures is proposed, which aims to achieve excellent low-temperature performance through a combination of specific components. However, in practical applications, even with a reasonable formulation design, improper preparation processes can lead to uneven mixing and poor dispersion of the components, thereby affecting the final performance of the epoxy structural adhesive, especially in ensuring its shear strength under ultra-low temperature conditions, which may fail to achieve the expected results. Therefore, how to ensure that each component is fully and uniformly dispersed and reacts effectively through refined preparation methods to obtain a stable and reliable ultra-low temperature epoxy structural adhesive is a technical problem that urgently needs to be solved in this field.

[0091] In response, this application further proposes a method for preparing an epoxy structural adhesive that can withstand ultra-low temperatures, comprising the following steps:

[0092] Step 1: Preparation of the basic system. The multifunctional resin, low-temperature resistant resin, curing agent and color paste of the formula are dispersed at high speed at room temperature and stirred at 1200 r / min for 2 hours. Vacuum is drawn while dispersing, and the vacuum degree is -100 kPa to form an epoxy structural adhesive matrix.

[0093] Step 2: Adding functional components. Add the formula amount of reinforcing filler to the epoxy structural adhesive matrix in sequence and disperse. Stir at 1000 r / min for 30 minutes. Continue to add the accelerator and disperse and stir at the same rate for 30 minutes. Vacuum is drawn while dispersing, and the vacuum degree is -100 kPa.

[0094] Step 3: Continue adding silane coupling agent, stir at 1000 r / min for 30 minutes, disperse while drawing a vacuum of -100 kPa to form a uniform and stable finished epoxy structural adhesive.

[0095] In the basic system preparation step one, the multifunctional resin, low-temperature resistant resin, curing agent, and colorant are dispersed at high speed at room temperature to initially mix these core components and form a homogeneous epoxy structural adhesive matrix. The multifunctional resin serves as the main crosslinking backbone, the low-temperature resistant resin imparts toughness to the colloid at extreme low temperatures, the curing agent initiates the curing reaction, and the colorant provides the desired color. Operating at room temperature avoids premature reaction or degradation of the components. Stirring at a high speed of 1200 rpm for 2 hours provides sufficient shear force to ensure thorough and uniform mixing of the high-viscosity resin, curing agent, and colorant, laying a good foundation for subsequent steps. Simultaneously, continuous vacuum is maintained during dispersion, with the vacuum level controlled at -100 kPa, effectively removing air introduced during stirring and preventing bubble formation, thereby improving the density of the epoxy structural adhesive matrix and the mechanical properties of the subsequent product.

[0096] In step two, the functional component addition involves sequentially adding the formulated amounts of reinforcing filler and accelerator to the prepared epoxy structural adhesive matrix. The addition of the reinforcing filler (e.g., one or more of silica fume, aluminum hydroxide, alumina, quartz powder, mica powder, and kaolin) aims to improve the strength, hardness, and dimensional stability of the colloid. Dispersion is achieved by stirring at 1000 rpm for 30 minutes to ensure the reinforcing filler is uniformly dispersed in the viscous epoxy matrix, effectively preventing agglomeration and maximizing its reinforcing effect. Subsequently, the accelerator (e.g., diaminoimidazole triazine complex, 2-ethyl-4-methylimidazole) is added and dispersed and stirred at the same rate for 30 minutes. The addition of the accelerator aims to accelerate the curing reaction, lower the curing temperature, or shorten the curing time, thereby improving production efficiency. Adding the accelerator after the filler dispersion prevents premature reaction between the accelerator and the curing agent, which could affect the filler dispersion. During this process, vacuuming is continuously performed, maintaining a vacuum level of -100 kPa, to further remove any air bubbles that may be introduced and to eliminate volatiles, ensuring the purity and stability of the colloid.

[0097] In step three, a silane coupling agent (such as one or more of γ-aminopropyltriethoxysilane, γ-(2,3-epoxypropoxy)propyltrimethoxysilane, and γ-methacryloyloxypropyltrimethoxysilane) is added, and the mixture is stirred at 1000 rpm for 30 minutes. The addition of the silane coupling agent aims to improve the interfacial bonding between the reinforcing filler and the epoxy resin matrix. By forming a molecular film on the filler surface, it enhances the compatibility and adhesion between the filler and the resin, thereby further improving the mechanical properties of the composite material, especially its resistance to damp heat aging. During this step, vacuum is continuously maintained at -100 kPa to ensure degassing and devolatilization throughout the preparation process, ultimately forming a bubble-free, uniform, and stable finished epoxy structural adhesive.

[0098] The above preparation method first involves high-speed dispersion and vacuuming of the multifunctional resin, low-temperature resistant resin, curing agent, and colorant at room temperature. This effectively ensures thorough mixing of these basic components, forming a uniform, bubble-free epoxy structural adhesive matrix, laying a solid foundation for the subsequent addition of functional components. Subsequently, reinforcing fillers and accelerators are added sequentially, with continuous dispersion and vacuuming. This not only ensures uniform dispersion of the reinforcing fillers in the matrix, preventing agglomeration, but also ensures uniform distribution of the accelerator in the curing system through the timing and dispersion conditions, effectively controlling the curing reaction rate. Finally, the addition of a silane coupling agent, followed by thorough dispersion and vacuuming, significantly enhances the interfacial bonding between the filler and the resin matrix, further improving the overall mechanical properties and stability of the epoxy structural adhesive. The entire preparation process effectively avoids the introduction of air bubbles and component inhomogeneity by precisely controlling the stirring speed, time, and vacuum level, thereby ensuring that the final epoxy structural adhesive has excellent uniformity and stability. In particular, it can maintain a shear strength of no less than 25MPa even at an ultra-low temperature of -90℃, which significantly solves the problem of performance fluctuation caused by uneven component mixing and ensures the reliability of the product under extreme conditions.

[0099] The following example will provide a more detailed explanation of the above technical solution:

[0100] A drone manufacturer needed a structural adhesive to bond carbon fiber composite components in its high-performance drones. These drones are designed to operate in extremely cold regions, where temperatures can drop to -90°C, and they may be exposed to corrosive substances during service. The commercially available epoxy structural adhesives the company had previously used exhibited significant increases in brittleness and decreases in toughness below -40°C, while also failing to adequately wet the carbon fiber material, resulting in unsatisfactory bond strength and a risk of delamination. To address these technical challenges, the company decided to adopt an epoxy structural adhesive capable of withstanding ultra-low temperatures.

[0101] The preparation process of this epoxy structural adhesive is as follows:

[0102] First, the basic system was prepared. 100 parts of tetraglycidylamine as a multifunctional resin, 15 parts of silicone-modified epoxy resin as a low-temperature resistant resin, 20 parts of methyltetrahydrophthalic anhydride as a curing agent, and 3 parts of black pigment paste (containing pigment and epoxy resin) were added to a mixing container. These components were stirred at 1200 rpm for 2 hours at room temperature using a high-speed disperser. During dispersion, continuous vacuum treatment was performed, maintaining a vacuum level of -100 kPa to ensure uniform mixing of the components and effective removal of air bubbles, thereby forming a defect-free epoxy structural adhesive matrix.

[0103] Next, functional components were added. Five parts of silica powder were added to the epoxy structural adhesive matrix as a reinforcing filler, and the mixture was stirred at 1000 rpm for 30 minutes to ensure uniform dispersion of the silica powder. Subsequently, three parts of 2-ethyl-4-methylimidazolium were added as an accelerator, and the mixture was stirred at the same rate for another 30 minutes. During this process, a vacuum treatment was performed again at a vacuum level of -100 kPa to further ensure the homogeneity and bubble-free state of the system.

[0104] Finally, add the silane coupling agent. Add 2 parts of γ-aminopropyltriethoxysilane to the system, stir at 1000 rpm for 30 minutes, and continuously apply vacuum to -100 kPa. This step ensures that the silane coupling agent is fully dispersed and effectively combined with the components in the system, ultimately forming a uniform and stable finished epoxy structural adhesive.

[0105] The synergistic effect of the components in this epoxy structural adhesive solves the problems in existing technologies. Tetraglycidylamine, as a multifunctional resin, provides high crosslinking density, ensuring excellent mechanical strength and rigidity of the cured adhesive. Compared to existing systems using only low-functionality epoxy resins, the high-functionality resin forms a denser three-dimensional network structure, maintaining better dimensional stability and strength at extreme temperatures. The silicone-modified epoxy resin, as a low-temperature resistant resin, introduces flexible segments, effectively reducing the brittleness of the cured adhesive in ultra-low temperature environments. At extreme low temperatures of -90℃, this modified resin maintains the flexibility of its molecular chains, avoiding the increased brittleness and decreased toughness problems of traditional epoxy adhesives below -40℃, thus ensuring the integrity of the bonding interface.

[0106] Methyltetrahydrophthalic anhydride, acting as a curing agent, synergistically with 2-ethyl-4-methylimidazolium, as an accelerator, ensures the epoxy structural adhesive fully cures under specific conditions of 100-170℃ / 4h, forming a stable three-dimensional cross-linked network. This curing system results in excellent heat resistance and chemical corrosion resistance after curing, preventing chemical degradation in strong acid or alkali environments and extending service life. Silica powder, as a reinforcing filler, is uniformly dispersed in the adhesive, improving its modulus, hardness, and impact resistance. Compared to adhesives without reinforcing fillers, the addition of silica powder effectively inhibits crack propagation and further enhances the mechanical properties of the adhesive at ultra-low temperatures. γ-aminopropyltriethoxysilane, as a silane coupling agent, significantly improves the wettability and interfacial bonding between the epoxy structural adhesive and low surface energy materials such as carbon fibers. Its molecules react with inorganic fillers or substrate surfaces at one end and with epoxy resin matrices at the other, forming strong chemical bonds. This effectively solves the problems of poor wetting and low bonding strength of low surface energy materials in existing technologies, and avoids interlayer delamination. The black pigment gives the colloid a specific color, facilitating identification and quality control during construction.

[0107] The prepared epoxy structural adhesive was stored stably at room temperature (25°C) for one week before being applied to the bonding of carbon fiber composites. The curing conditions were set at 150°C for 4 hours. After curing, the bonded parts were placed in a -90°C environment for 96 hours before undergoing shear strength testing. The test results showed that the shear strength reached 28 MPa, meeting the requirement of not less than 25 MPa. This indicates that the epoxy structural adhesive can maintain excellent bonding strength even in ultra-low temperature environments, effectively solving the problem of drastic performance degradation of adhesives below -40°C in existing technologies.

[0108] Through optimized selection of the aforementioned components and control of the preparation process, this epoxy structural adhesive maintains high shear strength even at ultra-low temperatures (-90℃), while simultaneously solving the bonding challenges and corrosion resistance issues associated with low surface energy materials such as carbon fibers. This synergistic component design enables the epoxy structural adhesive to meet the stringent requirements of industries such as drones for lightweighting, high strength, ultra-low temperature resistance, and corrosion resistance, significantly outperforming existing epoxy structural adhesive products on the market that have limited performance under extreme conditions.

[0109] To further verify the technical effect of this invention, the following five formulations were used to obtain epoxy structural adhesives. These adhesives were then applied to polished aluminum alloy sheets (with a fixed adhesive layer width of 10 mm and a thickness of 0.3 mm), cured at 150°C for 2 hours, and used to prepare test samples. The samples were then conditioned for 7 days under standard conditions (25±5°C, 50±10%RH), and then placed under various test conditions: immersion in a 5% acid solution for 7 days, immersion in a 1% alkali solution for 7 days, and holding in a -90°C ultra-low temperature freezer for 96 hours before testing.

[0110] Key metrics:

[0111] The substrate (6063 aluminum) was sanded according to GB / T 7124 standard, and fixed with 0.15mm thick masking tape to determine the adhesive layer thickness and width. It was then heated and cured at 100-170℃ and placed under standard conditions (25±5℃, 50%±10%RH) for 7 days. Equal numbers of substrate sheets were taken and tested under the above conditions, and the data obtained are as follows:

[0112] Formula 1 and corresponding experimental data

[0113]

[0114] Corresponding experimental data: ① Standard shear strength: 26.17 MPa; ② Shear strength after immersion in 5% acetic acid for 7 days: 24.41 MPa; ③ Shear strength after immersion in 1% sodium hydroxide for 7 days: 25.98 MPa; ④ Shear strength at -90℃ for 96 hours: 25.61 MPa

[0115] Formula 2 and corresponding experimental data

[0116]

[0117] Corresponding experimental data: ① Standard shear strength: 25.41 MPa; ② Shear strength after immersion in 5% acetic acid for 7 days: 24.25 MPa; ③ Shear strength after immersion in 1% sodium hydroxide for 7 days: 26.32 MPa; ④ Shear strength at -90℃ for 96 hours: 26.19 MPa

[0118] Formula 3 and corresponding experimental data

[0119]

[0120] Corresponding experimental data: ① Standard shear strength: 25.54 MPa; ② Shear strength after immersion in 5% acetic acid for 7 days: 23.34 MPa; ③ Shear strength after immersion in 1% sodium hydroxide for 7 days: 25.82 MPa; ④ Shear strength at -90℃ for 96 hours: 25.13 MPa

[0121] Formula 4 and corresponding experimental data

[0122]

[0123] Corresponding experimental data: ① Standard shear strength: 25.77 MPa; ② Shear strength after immersion in 5% acetic acid for 7 days: 23.73 MPa; ③ Shear strength after immersion in 1% sodium hydroxide for 7 days: 25.87 MPa; ④ Shear strength at -90℃ for 96 hours: 26.74 MPa

[0124] Formula 5 and corresponding experimental data

[0125]

[0126] Corresponding experimental data: ① Standard shear strength: 26.00 MPa; ② Shear strength after immersion in 5% acetic acid for 7 days: 24.07 MPa; ③ Shear strength after immersion in 1% sodium hydroxide for 7 days: 24.51 MPa; ④ Shear strength at -90℃ for 96 hours: 25.48 MPa

[0127] Formulation design logic: 1. The difference in standard strength is mainly controlled by the type of multifunctional resin and the amount of curing agent. The higher the crosslinking activity of the multifunctional resin and the better the compatibility of the curing agent, the higher the strength at room temperature; 2. The difference in acid and alkali resistance is related to the type of coupling agent and the amount of reinforcing filler. Silane coupling agents have a more significant effect on improving alkali resistance, while increasing the amount of inorganic filler can enhance acid resistance; 3. The ultra-low temperature resistance mainly depends on the type and amount of organosilicon low-temperature resistant resin. The higher the content of silicon oxide segments, the better the low-temperature stability.

[0128] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. An epoxy structural adhesive resistant to ultra-low temperature, characterized in that: The epoxy structural adhesive comprises the following components in parts by weight: multifunctional resin 100 parts, low-temperature resistant resin 10-20 parts, curing agent 5-30 parts, accelerator 1-10 parts, reinforcing filler 2-10 parts, silane coupling agent 1-5 parts, and color paste 1-5 parts. The epoxy structural adhesive is stably stored at room temperature of 25 DEG C for one week. The curing condition is 100-170 DEG C / 4 h. The shear strength value is not less than 25 MPa after being placed at low temperature of-90 DEG C for 96 hours.

2. The ultra-low temperature resistant epoxy structural adhesive according to claim 1, characterized in that: The multifunctional resin is one or more of triglycidyl-p-aminophenol, phenol-formaldehyde resin, tetraglycidyl amine, naphthol resin, and tetraepoxypropyl-4,4'-diaminodiphenyl methane.

3. The epoxy structural adhesive resistant to ultra-low temperatures and its preparation method according to claim 1, characterized in that: The low-temperature resistant resin is one or more of silicone-modified epoxy resin, core-shell toughened epoxy resin, polyurethane-modified epoxy resin, epoxy acrylate, and fluorine-modified epoxy resin.

4. The ultra-low temperature resistant epoxy structural adhesive according to claim 1, characterized in that: The curing agent is one or more of 2-ethyl-4-methylimidazole, 1-cyano-2-ethyl-4-methylimidazole, 2-heptadecyl imidazole, 2-phenylimidazole, 1,3-diphenyl-2-methylimidazole, dicyandiamide, methyltetrahydrophthalic anhydride, and methyl nadic anhydride.

5. The ultra-low temperature resistant epoxy structural adhesive according to claim 1, characterized in that: The accelerator is diaminoimidazole triazine complex and 2-ethyl-4-methylimidazole.

6. The ultra-low temperature resistant epoxy structural adhesive according to claim 1, characterized in that: The reinforcing filler is one or more of silicon powder, aluminum hydroxide, aluminum oxide, quartz powder, mica powder, and kaolin.

7. The epoxy structural adhesive resistant to ultra-low temperatures and its preparation method according to claim 1, characterized in that: The silane coupling agent is one or more of gamma-aminopropyl triethoxysilane, gamma-(2,3-epoxypropoxy) propyl trimethoxysilane, and gamma-methacryloyloxypropyl trimethoxysilane.

8. The epoxy structural adhesive resistant to ultra-low temperatures and its preparation method according to claim 1, characterized in that: The color paste is black color paste comprising color powder and epoxy resin.

9. The preparation method of the epoxy structural adhesive according to any one of claims 1 to 8, comprising the following steps: Step one: preparation of the base system, the formula amount of multifunctional resin, low-temperature resistant resin, curing agent, and color paste are dispersed at room temperature for 2 hours at a stirring speed of 1200 r / min, and vacuum is drawn at a vacuum degree of-100 kPa during the dispersion to form an epoxy structural adhesive base; Step two: addition of functional components, the formula amount of reinforcing filler is added to the above epoxy structural adhesive base and dispersed for 30 minutes at a stirring speed of 1000 r / min, and the accelerator is continuously added and dispersed for 30 minutes at the same speed, and vacuum is drawn at a vacuum degree of-100 kPa during the dispersion; Step three: the silane coupling agent is continuously added and dispersed for 30 minutes at a stirring speed of 1000 r / min, and vacuum is drawn at a vacuum degree of-100 kPa during the dispersion to form a uniform and stable finished product of the epoxy structural adhesive.

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