High-temperature-shearing-resistant epoxy structural adhesive and preparation method thereof

Abstract

The invention discloses a high-temperature-shear-resistant epoxy structural adhesive and a preparation method thereof, and belongs to the technical field of high-performance composite materials. The preparation method is suitable for rigid oxazolidinone modified epoxy resin, rigid oxazolidinone five-membered heterocycle is introduced and cooperates with bisphenol A type epoxy resin, bisphenol F type epoxy resin and core-shell modified epoxy resin, and other components are mutually cooperated, so that the problems that an epoxy resin matrix is reduced in modulus and insufficient in cohesion at high temperature are solved; the tensile shear strength at 80 DEG C is greater than or equal to 30MPa, the cohesive failure rate is greater than or equal to 95%, the normal-temperature impact strength is greater than or equal to 44N / mm, the heat and humidity resistance is excellent, and the adhesive can be suitable for bonding high-temperature key parts such as an automobile engine compartment and a three-electric system.

Description

Technical Field

[0001] This invention relates to the field of high-performance composite materials technology, and in particular to a high-temperature shear-resistant epoxy structural adhesive and its preparation method. Background Technology

[0002] With the rapid development of the automotive industry towards lightweighting, high performance, and new energy, the integration of vehicle body structures, the improvement of engine thermal efficiency, and the widespread application of the three-electric systems (battery, motor, and electronic control) in new energy vehicles have placed more stringent demands on the high-temperature resistance, mechanical reliability, and environmental safety of structural adhesives used in automobiles. Epoxy structural adhesives, due to their core advantages such as high bonding strength, excellent chemical corrosion resistance, and good compatibility with metals / composite materials, have become a core material for critical scenarios such as automotive body welding reinforcement, engine peripheral component fixing, chassis structure bonding, and sealing of the three-electric systems. Their performance directly affects the structural stability, service life, and safety redundancy of automobiles.

[0003] During actual automotive service, the operating temperature in core areas such as the engine compartment and exhaust pipe perimeter often reaches 80℃ or higher, and even approaches 120℃ under some extreme conditions. However, the molecular network of traditional epoxy structural adhesives is mainly composed of flexible ether bonds (COC) and methylene chains, resulting in a relatively low glass transition temperature (Tg). Under high-temperature environments, these adhesives are prone to entering a highly elastic state, leading to increased molecular chain segment movement, decreased cohesive energy density, and consequently, a sharp decline in shear strength and adhesive interface failure. This has become a core bottleneck restricting their large-scale application in critical high-temperature automotive components.

[0004] To improve the high-temperature resistance of epoxy structural adhesives, the industry has explored various technologies. Common improvement paths in existing technologies include: optimizing the epoxy resin matrix (such as using multifunctional epoxy, bisphenol F / S type epoxy, etc.) to enhance thermal stability by increasing the crosslinking density; adding rigid fillers (such as fumed silica, modified ceramic powder) or heat-resistant additives (such as polyimide micro powder) to inhibit molecular chain movement through physical filling; using elastomer modification (such as CTBN rubber, core-shell rubber) to balance high-temperature rigidity and room-temperature toughness; and optimizing the curing system (such as using latent curing agents, composite curing accelerators) to improve the thermal stability of the crosslinking network.

[0005] However, existing technologies still have significant limitations: First, relying solely on multifunctional epoxy or rigid filler modification can easily lead to increased brittleness of the adhesive layer and decreased impact resistance at room temperature; Second, although elastomer modification can improve toughness, the elastomer itself has limited heat resistance and is prone to softening at high temperatures, which can weaken the cohesive strength of the adhesive layer and result in insignificant improvement in high-temperature shear performance; Third, existing modification schemes are mostly physical blending or simple chemical grafting, which cannot fundamentally solve the core problems of molecular chain segment slippage and insufficient cohesive force at high temperatures.

[0006] Furthermore, existing solutions for automotive-grade epoxy structural adhesives primarily focus on wide-temperature-range bonding performance, but lack sufficient optimization for shear strength at temperatures of 80°C and above. Therefore, developing an automotive-grade epoxy structural adhesive that balances high-temperature shear strength with comprehensive mechanical properties has become a pressing technical challenge for the industry. Summary of the Invention

[0007] To address the aforementioned technical problems in the prior art, this invention aims to provide a high-temperature shear-resistant epoxy structural adhesive and its preparation method.

[0008] One objective of this invention is to provide a high-temperature shear-resistant epoxy structural adhesive, which, by weight, comprises the following components: The following components are included: 10-20 parts of bisphenol A type epoxy resin, 10-15 parts of bisphenol F type epoxy resin, 10-20 parts of core-shell modified epoxy resin, 15-25 parts of oxazolidinone modified epoxy resin, 1-5 parts of curing agent, 0.1-0.5 parts of curing accelerator, 10-30 parts of filler, 10-15 parts of toughening agent, 3-5 parts of desiccant, 2-5 parts of thixotropic agent, and 0.1-0.5 parts of silane coupling agent. The oxazolidinone-modified epoxy resin is prepared by a cycloaddition reaction between an isocyanate compound and the epoxy group of the epoxy resin, wherein the five-membered ring of the oxazolidinone is chemically bonded and embedded in the main chain of the epoxy resin.

[0009] Preferably, the number-average molecular weight Mn of the oxazolidinone modified epoxy resin is 700~900 Da, and the molecular weight distribution Mw / Mn is 1.6~2.5.

[0010] Preferably, the preparation method of the oxazolidinone-modified epoxy resin includes: S1: Add 4,4'-diaminodiphenylmethane and diethyl carbonate in a molar ratio of 1:12, add a catalyst accounting for 0.5~1.0% of the mass of 4,4'-diaminodiphenylmethane, and under nitrogen protection, use a gradient heating method: first heat to 110℃ and hold for 2h, then heat to 125~130℃ and stir vigorously for 5h, then cool to room temperature, and then remove excess diethyl carbonate to obtain the carbamate intermediate; S2: The carbamate intermediate was subjected to staged pyrolysis under vacuum of -0.098 MPa and 180~230℃: it was first initially pyrolyzed by holding at 180℃ for 1 h, and then further pyrolyzed by heating to 220~230℃ and holding for 2 h. The crude product was then collected by condensing through a 10~15℃ condenser. The crude product was then subjected to vacuum distillation at -0.098 MPa and 120℃, and after recrystallization and drying, 4,4'-diphenylmethane diisocyanate was obtained. S3: Add 4,4'-diphenylmethane diisocyanate and epoxy resin at a molar ratio of isocyanate group to epoxy group of 0.9:1.0, along with 0.3%~0.5% catalyst (by mass of epoxy resin) and 2%~3% dehydrating agent (by mass of total system). Dehydrate and dry the mixture at 100℃ and -0.09MPa vacuum for 1 hour. Then, react the mixture at 80~140℃ for 4~6 hours under nitrogen protection. After the reaction, wash the mixture with ethyl acetate and remove the solvent under vacuum to obtain oxazolidinone modified epoxy resin.

[0011] Preferably, the core-shell modified epoxy resin is an epoxy resin modified with core-shell particles, wherein the core-shell particles have a polybutadiene rubber core and a methyl methacrylate shell, and the average particle size of the core-shell particles is 100~200nm.

[0012] Preferably, the toughening agent comprises a polyurethane toughening agent, and the mass ratio of the polyurethane toughening agent to the oxazolidinone-modified epoxy resin is 1:1.25~2.

[0013] Preferably, the filler comprises heavy calcium carbonate and fibrous wollastonite in a mass ratio of 2 to 4:1.

[0014] Preferably, the heavy calcium carbonate has a particle size of 1~10μm and a purity of ≥98%.

[0015] Preferably, the fibrous wollastonite has an aspect ratio of 10-15:1 and a particle size of 600-1250 mesh.

[0016] Preferably, the thixotropic agent comprises fumed silica and quaternary ammonium salt modified bentonite, wherein the mass ratio of fumed silica to quaternary ammonium salt modified bentonite is 1.5 to 3:1.

[0017] Preferably, the specific surface area of ​​the fumed silica is 140~220 m² / g. 2 / g.

[0018] Preferably, the quaternary ammonium salt modified bentonite is an organobentonite modified with hexadecyltrimethylammonium chloride, octadecyltrimethylammonium chloride or dioctadecyldimethylammonium chloride, with an activation degree ≥95% and a particle size of 1~8μm.

[0019] Preferably, the silane coupling agent includes KH560 and KH590, wherein the amount of KH560 is 0.05~0.3 parts and the amount of KH590 is 0.05~0.2 parts.

[0020] Preferably, the epoxy equivalent of the bisphenol F type epoxy resin is 150~190 g / eq.

[0021] Preferably, the epoxy equivalent of the bisphenol A type epoxy resin is 180~190 g / eq.

[0022] Preferably, the curing accelerator includes an organic urea accelerator.

[0023] Preferably, the curing agent includes one or more of dicyandiamide, sebacic acid dihydrazide or adipic acid dihydrazide, with dicyandiamide being the most preferred; Preferably, the silane coupling agent includes KH560 and / or KH590.

[0024] Preferably, the epoxy structural adhesive further includes 0.01 to 0.05 parts by weight of pigment, wherein the pigment includes carbon black.

[0025] Preferably, the desiccant comprises calcium oxide desiccant.

[0026] A second objective of this invention is to provide a method for preparing the epoxy structural adhesive as described above, the method comprising: a: Raw material pretreatment: Place the desiccant, filler, and thixotropic agent in a vacuum drying oven at 110~120℃ and dry for 2~3 hours. Cool to room temperature before use. The pigment is pre-dispersed with 0.5-1 parts by weight of bisphenol A epoxy resin to prepare a pigment masterbatch for later use, in order to avoid pigment agglomeration; b: Matrix mixing: Add the remaining bisphenol A type epoxy resin, bisphenol F type epoxy resin, core-shell modified epoxy resin, and oxazolidinone modified epoxy resin to the planetary stirrer. Purge the air in the system with nitrogen three times. Control the stirring speed at 500~600 r / min and the temperature at 40~50℃. Stir for 20~30 min until the system is uniform and transparent. Then, while maintaining nitrogen protection and the above temperature and speed, slowly add toughening agent and silane coupling agent, and continue stirring for 30-40 minutes; Then, add the pretreated thixotropic agent, filler and desiccant to the system in sequence, adjust the stirring speed to 800~1000r / min and the temperature to 50~60℃, disperse at high speed for 40~60min, and stop the machine to scrape the wall once every 15min during the process; c: Vacuum degassing Switch the planetary stirrer to vacuum mode, control the vacuum degree to -0.09~-0.095MPa and the temperature to 45~55℃, and maintain the speed at 300~400r / min for degassing for 30~40min; d: Mixing of the curing system: Turn off the vacuum, cool down to 30~40℃, add curing agent, curing accelerator and pretreated pigment masterbatch, adjust the stirring speed to 500~600r / min, and stir for 20~30min; e: Post-processing: Under a vacuum of -0.09 to -0.095 MPa, the system is degassed at room temperature for 10 to 15 minutes, and the viscosity is measured. At 25°C, the viscosity is controlled at 8000 to 20000 mPa·s, thus obtaining the high-temperature shear-resistant epoxy structural adhesive.

[0027] The beneficial effects of this invention include: Significantly improved high-temperature shear properties: Through the rigidity enhancement of oxazolidinone heterocycles and the strengthening of cross-linking networks, the shear strength at 80℃ is ≥30MPa and the cohesive failure rate is ≥95%, which solves the technical problem of low high-temperature shear strength of traditional epoxy adhesives. Excellent balance of rigidity and toughness: Through the synergistic effect between the components, this invention ensures high-temperature rigidity while maintaining room-temperature impact strength ≥4.4N / mm and tensile shear strength ≥39MPa, thus meeting the comprehensive mechanical requirements of automotive working conditions. High temperature stability: The composite filler and the optimized curing system work together to achieve a strength reduction rate of ≤12.5% ​​after 480 hours of damp heat aging at 50℃ and 95% relative humidity. The weather resistance and creep resistance are suitable for the long-term service requirements of automobiles. Process adapted for industrialization: The preparation process requires no special equipment, and the temperature, rotation speed and degassing parameters are precisely controllable. The raw materials are all commercially available conventional products, and the viscosity and workability are adapted to the coating and bonding processes on the automotive production line. It can be used for bonding in high-temperature critical parts such as automotive engine compartments and electric drive systems. Detailed Implementation

[0028] The following description includes certain specific details to provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented without employing one or more of these specific details, but using other methods, components, materials, etc.

[0029] Unless otherwise required by the present invention, throughout the specification and the following claims, the words “comprising” and “including” shall be interpreted in an open-ended, inclusive sense, meaning “including but not limited to”.

[0030] Throughout this specification, the terms "an embodiment," "an embodiment," "a preferred embodiment," or "some embodiments" refer to including, in at least one embodiment, a specific reference element, structure, or feature associated with that embodiment. Therefore, the phrases "in an embodiment," "in a preferred embodiment," or "in some embodiments" appearing in different places throughout the specification do not necessarily all refer to the same embodiment. Furthermore, specific elements, structures, or features may be combined in one or more embodiments in any suitable manner.

[0031] According to a first aspect of the present invention, a high-temperature shear-resistant epoxy structural adhesive is provided, wherein the epoxy structural adhesive comprises the following components in parts by weight: The following components are included: 10-20 parts of bisphenol A type epoxy resin, 10-15 parts of bisphenol F type epoxy resin, 10-20 parts of core-shell modified epoxy resin, 15-25 parts of oxazolidinone modified epoxy resin, 1-5 parts of curing agent, 0.1-0.5 parts of curing accelerator, 10-30 parts of filler, 10-15 parts of toughening agent, 3-5 parts of desiccant, 2-5 parts of thixotropic agent, and 0.1-0.5 parts of silane coupling agent. The oxazolidinone-modified epoxy resin is prepared by a cycloaddition reaction between an isocyanate compound and the epoxy group of the epoxy resin, wherein the five-membered ring of the oxazolidinone is chemically bonded and embedded in the main chain of the epoxy resin.

[0032] In this invention, the glass transition temperature (Tg) of the resin can be effectively increased by chemically embedding the oxazolidinone five-membered heterocycle into the epoxy backbone. This is because the oxazolidinone ring is a rigid, planar five-membered heterocycle with high rotational steric hindrance. Inserting it as a rigid node into the epoxy resin network, which is originally dominated by flexible ether bonds (COC) and methylene chains, can greatly enhance the rigidity of the molecular chain.

[0033] When the ambient temperature reaches 80℃, ordinary epoxy resins may be close to or exceed their glass transition temperature Tg, and are in a highly elastic state with a sharp drop in modulus. However, resins containing oxazolidinones, due to their higher Tg, can still remain in a glassy or leathery state at 80℃, thus maintaining higher modulus and stiffness, which is the basis for resisting shear deformation.

[0034] Furthermore, the oxazolidinone ring contains highly polar carbonyl groups (C=O) and ether bonds (COC), and has a regular structure. This can significantly increase the dipole-dipole interactions between molecular chains, and if proton-donating groups are present, it can also increase hydrogen bonding. This, in turn, can greatly increase the cohesive energy density of the entire polymer network. At temperatures around 80°C, although the thermal motion within the gel layer intensifies, the stronger intermolecular forces can still effectively bind the chain segment movement, thus resisting the disentanglement and slippage caused by thermal activation, thereby maintaining high cohesive strength.

[0035] In this invention, bisphenol A type epoxy resin, bisphenol F type epoxy resin, oxazolidinone modified epoxy resin and core-shell modified epoxy resin constitute the resin matrix of the epoxy structural adhesive.

[0036] The bisphenol A type epoxy resin provides basic adhesion and processability, with moderate molecular chain flexibility, making it easily compatible with other resins. As a base matrix, it can be compounded with bisphenol F type epoxy resin to reduce system viscosity and improve workability; it also provides a dispersion carrier for oxazolidinone-modified epoxy resins and core-shell modified epoxy resins. The amount of the bisphenol A type epoxy resin used is, for example, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts, 16 parts, 17 parts, 18 parts, 19 parts, or 20 parts, and any value between these two.

[0037] The bisphenol F type epoxy resin has a slightly lower viscosity than the bisphenol A type epoxy resin. It exhibits good compatibility in the resin matrix and synergistically forms a low-viscosity, high-compatibility base system with the bisphenol A type epoxy resin, thus mitigating the construction difficulties caused by the high viscosity of oxazolidinone-modified epoxy resin. The amount of the bisphenol F type epoxy resin used is, for example, 10 parts, 10.5 parts, 11 parts, 11.5 parts, 12 parts, 12.5 parts, 13 parts, 13.5 parts, 14 parts, 14.5 parts, or 15 parts, as well as any values ​​between these two.

[0038] The epoxy equivalent of the bisphenol A type epoxy resin is preferably 180~190 g / eq.

[0039] The epoxy equivalent of the bisphenol F type epoxy resin is preferably 150~190 g / eq.

[0040] The oxazolidinone-modified epoxy resin, with its five-membered heterocyclic oxazolidinone chemically bonded into the main chain, significantly improves the resin's heat capacity (Tg) and cohesive energy density. Its molecular chains do not slip at 80°C, making it a core heat-resistant component of the epoxy structural adhesive. Combined with core-shell epoxy, it addresses the problem of increased brittleness resulting from increased rigidity. The amount of oxazolidinone-modified epoxy resin used is, for example, 15 parts, 16 parts, 17 parts, 18 parts, 19 parts, 20 parts, 21 parts, 22 parts, 23 parts, 24 parts, or 25 parts, or any value between these two ranges.

[0041] The core-shell modified epoxy resin is the core toughening component of the epoxy structural adhesive. The core-shell particles in the core-shell modified epoxy resin can absorb impact energy, undergo plastic deformation under stress, and prevent crack propagation; without weakening the rigid skeleton of the oxazolidinone epoxy. The amount of the core-shell modified epoxy resin is, for example, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts, 16 parts, 17 parts, 18 parts, 19 parts, or 20 parts, and any value between these two.

[0042] The rigid skeleton of oxazolidinone-modified epoxy resin and the island-type toughening structure of core-shell epoxy work together to form an interpenetrating network, which can not only ensure the shear resistance of epoxy structural adhesive at 80℃, but also improve its impact resistance at room temperature, thus breaking through the bottleneck of ordinary epoxy adhesives being brittle when heated and not resistant to high temperatures when toughened.

[0043] In this invention, the curing agent and the curing accelerator constitute the curing system. The design of the curing system can control the density of the cross-linked network of the adhesive layer, which directly affects the cohesive strength and heat resistance.

[0044] The curing agent is preferably a latent curing agent, which has extremely low reactivity with epoxy groups at room temperature and good storage stability of the adhesive. Upon reaching the activation temperature, it can undergo an addition reaction with the epoxy resin to form a dense cross-linked network. When the amount of the curing agent is less than 1 part, the cross-linking density is insufficient, and the cohesive strength of the adhesive layer is low. When the amount of the curing agent is greater than 5 parts, the excessive curing agent will lead to increased brittleness of the adhesive layer and decreased impact resistance at room temperature. The amount of the curing agent is, for example, 1 part, 1.5 parts, 2 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, or 5 parts, and any value between these two.

[0045] The curing accelerator can lower the activation temperature of the curing agent. For example, when using dicyandiamide as the curing agent and an organic urea accelerator, the activation temperature of the curing agent can be reduced from 180°C to 120-130°C, thereby shortening the curing time and improving the uniformity of the crosslinked network. The amount of the curing accelerator is, for example, 0.1 parts, 0.2 parts, 0.3 parts, 0.4 parts, or 0.5 parts, and any value between any two of the above.

[0046] Preferably, when the ratio of curing agent to accelerator is controlled at 10~20:1, it ensures both latency and facilitates control of crosslinking density.

[0047] In this invention, toughening agents, fillers, moisture absorbers, thixotropic agents, and silane coupling agents are functional additives that can directly affect the dispersibility, workability, and interfacial properties of the adhesive layer.

[0048] The toughening agent is preferably a polyurethane-modified epoxy resin, in which flexible polyurethane segments are embedded in its molecular chain. This allows it to form an interpenetrating polymer network (IPN) or island structure with the epoxy resin matrix, and has little impact on the heat resistance of the cured product. The amount of the toughening agent is, for example, 10 parts, 10.5 parts, 11 parts, 11.5 parts, 12 parts, 12.5 parts, 13 parts, 13.5 parts, 14 parts, 14.5 parts, or 15 parts, or any value between these two ranges.

[0049] The filler preferably comprises heavy calcium carbonate and / or fibrous wollastonite. Calcium carbonate can improve the compressive strength of the adhesive layer, while fibrous wollastonite can form a rigid supporting skeleton, enhancing the shear resistance of the epoxy structural adhesive. When the filler dosage is <10 parts, its reinforcing effect is not significant; when the filler dosage is >30 parts, the viscosity of the system increases rapidly, the workability decreases, and stress concentration within the adhesive layer is easily caused. The filler dosage is, for example, 10 parts, 12 parts, 14 parts, 16 parts, 18 parts, 20 parts, 22 parts, 24 parts, 26 parts, 28 parts, or 30 parts, and any value between these two.

[0050] The desiccant is used to control the moisture content of the epoxy structural adhesive system, preventing moisture from reacting with residual isocyanate groups in oxazolidinone-modified epoxy resins to generate CO2 bubbles, which could lead to pinholes and defects in the adhesive layer, thus ensuring stable high-temperature shear strength. The amount of the desiccant used is, for example, 3 parts, 3.2 parts, 3.4 parts, 3.6 parts, 3.8 parts, 4 parts, 4.2 parts, 4.6 parts, 4.8 parts, or 5 parts, or any value between these two.

[0051] The thixotropic agent is used to improve the thixotropic properties of the adhesive compound, prevent flow during application, and ensure uniform adhesive layer thickness. The thixotropic agent preferably includes fumed silica and / or quaternary ammonium salt-modified bentonite. The dosage of the thixotropic agent is, for example, 2 parts, 2.2 parts, 2.4 parts, 2.6 parts, 2.8 parts, 3 parts, 3.2 parts, 3.4 parts, 3.6 parts, 3.8 parts, 4 parts, 4.2 parts, 4.6 parts, 4.8 parts, or 5 parts, and any point values ​​between these two.

[0052] The silane coupling agent is used to form chemical bridges between the adhesive layer and the metal, and between the organic and inorganic components within the adhesive layer, thereby improving interfacial adhesion. The silane coupling agent preferably includes KH560 and / or KH590. The amount of the silane coupling agent used is, for example, 0.1 parts, 0.2 parts, 0.3 parts, 0.4 parts, or 0.5 parts, and any value between any two of the above.

[0053] In a preferred embodiment of the present invention, the number-average molecular weight Mn of the oxazolidinone modified epoxy resin is 700~900 Da, and the molecular weight distribution Mw / Mn is 1.6~2.5.

[0054] In this invention, if the number average molecular weight Mn of the oxazolidinone-modified epoxy resin is <700 Da, it indicates that the oxazolidinone ring incorporation rate is <85%, the proportion of flexible ether bonds in the main chain is too high, and the molecular chain is prone to slippage. The viscosity of the oxazolidinone-modified epoxy resin increases exponentially with the increase of molecular weight. If the number average molecular weight Mn of the oxazolidinone-modified epoxy resin is >900 Da, the viscosity of the resin at 25°C will exceed 10000 mPa·s, which will lead to a significant decrease in its compatibility with bisphenol A type epoxy resin, bisphenol F type epoxy resin, and core-shell modified epoxy resin in the formulation, and it is easy to cause stratification and agglomeration after mixing.

[0055] Meanwhile, high-viscosity resins can lead to difficulties in dispersion during planetary mixing, making it difficult to evenly disperse solid fillers, causing stress concentration points to easily form inside the adhesive layer, and making it prone to breakage at defects during high-temperature shearing.

[0056] When Mn = 700~900 Da, the oxazolidinone ring incorporation rate can be stabilized at over 90%, the main chain rigid skeleton is intact, and the shear deformation resistance of the adhesive layer can be significantly improved. At the same time, when Mn = 700~900 Da, the viscosity of the resin at 25℃ is controlled at 4000~6000 mPa·s, it has good compatibility with other epoxy components, and the mixed system is uniform and transparent, which can be more suitable for subsequent high-speed dispersion, vacuum degassing and other processes.

[0057] In this invention, if the molecular weight distribution Mw / Mn < 1.6, it indicates a very narrow molecular weight distribution, requiring a stringent living polymerization process with extremely high precision in controlling reaction temperature, stirring rate, and catalyst dosage. While this can be achieved in the laboratory, large batch-to-batch fluctuations during industrial production lead to a significant increase in costs. If the molecular weight distribution Mw / Mn > 2.5, the resin will contain a large number of both low-molecular-weight and high-molecular-weight components. Low-molecular-weight components are prone to migration and precipitation at high temperatures, leading to a decrease in local cohesive strength of the adhesive layer and interfacial damage upon shear failure. High-molecular-weight components tend to agglomerate to form rigid particles, which become stress concentration points within the adhesive layer, resulting in a decrease in room-temperature impact resistance.

[0058] In a preferred embodiment of the present invention, the method for preparing the oxazolidinone-modified epoxy resin includes: S1: Add 4,4'-diaminodiphenylmethane and diethyl carbonate in a molar ratio of 1:12, add a catalyst accounting for 0.5~1.0% of the mass of 4,4'-diaminodiphenylmethane, and under nitrogen protection, use a gradient heating method: first heat to 110℃ and hold for 2h, then heat to 125~130℃ and stir vigorously for 5h, then cool to room temperature, and then remove excess diethyl carbonate to obtain the carbamate intermediate; In step S1, the catalyst is sodium ethoxide or dibutyltin dilaurate. After the reaction is complete and cooled to room temperature, the method for removing excess diethyl carbonate is as follows: first, pre-concentrate by distillation at 80℃ and atmospheric pressure for 30 min, then distill under reduced pressure at 60℃ and -0.095MPa for 2 h to remove excess diethyl carbonate, and then wash with anhydrous ethanol, filter, and vacuum dry to obtain a carbamate intermediate with a purity ≥99%.

[0059] S2: The carbamate intermediate was subjected to staged pyrolysis under vacuum of -0.098 MPa and 180~230℃: it was first initially pyrolyzed by holding at 180℃ for 1 h, and then further pyrolyzed by heating to 220~230℃ and holding for 2 h. The crude product was then collected by condensing through a 10~15℃ condenser. The crude product was then subjected to vacuum distillation at -0.098 MPa and 120℃, and after recrystallization and drying, 4,4'-diphenylmethane diisocyanate was obtained. In step S2, anhydrous toluene is used for recrystallization. After recrystallization with anhydrous toluene, filtration and drying, 4,4'-diphenylmethane diisocyanate with a purity ≥99.5% can be obtained. S3: 4,4'-diphenylmethane diisocyanate and epoxy resin were added at a molar ratio of isocyanate group (-NCO) to epoxy group of 0.9:1.0. A catalyst of 0.3%~0.5% by mass of epoxy resin and a dehydrating agent of 2%~3% by mass of the total system were added. The mixture was dehydrated and dried at 100℃ and -0.09MPa vacuum for 1 hour. The reaction was carried out at 80~140℃ for 4~6 hours under nitrogen protection. After the reaction was completed, the mixture was washed with ethyl acetate and the solvent was removed under vacuum to obtain oxazolidinone modified epoxy resin.

[0060] In step S3, the catalyst is triphenylphosphine or 2-ethyl-4-methylimidazole, the dehydrating agent is a 4Å molecular sieve, and the epoxy resin is bisphenol A epoxy resin, bisphenol F epoxy resin, or a blend thereof.

[0061] In a preferred embodiment of the present invention, the core-shell modified epoxy resin is an epoxy resin modified with core-shell particles, wherein the core-shell particles have a polybutadiene rubber core and a methyl methacrylate shell, and the average particle size of the core-shell particles is 100~200nm.

[0062] In this invention, the core-shell particles have a polybutadiene rubber soft core and a methyl methacrylate hard shell. When the adhesive layer is subjected to external impact, the soft core can absorb energy through plastic deformation, preventing crack propagation and solving the problem of high rigidity and brittleness caused by oxazolidinone-modified epoxy resin. Simultaneously, the chemical structure of polybutadiene is stable and will not melt or degrade at 80°C, avoiding the high-temperature migration and precipitation of toughening components. The methyl methacrylate hard shell has good compatibility with the epoxy resin matrix in this invention and can effectively transfer stress to the epoxy matrix; the rigidity of the hard shell also mitigates the negative impact of high temperatures on the shear strength of the adhesive layer.

[0063] The core-shell particles, with a particle size of 100-200 nm, are beneficial for improving toughening efficiency while also ensuring good dispersibility. These core-shell particles can be uniformly dispersed in the epoxy matrix, forming an island-type toughening structure. If the particle size is <100 nm, the particle specific surface area is too large, making aggregation easy and reducing toughening efficiency. If the particle size is >200 nm, the particles will become stress concentration points within the adhesive layer, reducing the cohesive strength of the adhesive layer and making it prone to fracture at the particle interface during high-temperature shearing. The 100-200 nm particle size forms a gradient with the size of the filler in this invention, avoiding the aggregation and superposition of different solid phase components and ensuring the processing flowability of the adhesive.

[0064] The core-shell modified epoxy resin includes one or more of Tongzhou New Material 154A, Zhongyuan MX-154, or Jinghan New Material YT-3154A.

[0065] In a preferred embodiment of the present invention, the toughening agent includes a polyurethane toughening agent, which can react with dicyandiamide at medium to high temperatures to form an epoxy resin-polyurethane interpenetrating polymer network or island structure after curing, and has little effect on the glass transition temperature of the cured product.

[0066] In this invention, the toughening mechanism of the polyurethane toughening agent lies in the following: polyurethane macromolecular segments form an interpenetrating polymer network (IPN) or island structure with the epoxy resin matrix. During curing, phase separation occurs, forming toughening microregions in the continuous epoxy phase. When the adhesive layer is subjected to external impact, the flexible polyurethane segments can absorb energy through plastic deformation, preventing crack propagation. Simultaneously, the active groups at the ends of the polyurethane segments can chemically bond with the epoxy resin or curing agent, preventing the toughening agent from migrating and precipitating at high temperatures, thereby improving toughness while maintaining high-temperature shear strength.

[0067] The toughening efficiency of a toughening agent is closely related to its molecular chain length and the content of active groups. If the toughening agent's molecular chain is too short or the content of active groups is insufficient, the toughening efficiency will be inadequate, requiring an increase in the amount added. However, excessive addition will lead to an excessively high proportion of flexible components in the adhesive layer, and the molecular chain slippage will intensify at high temperatures. If the toughening agent's molecular chain is too long or the viscosity is too high, the compatibility with the epoxy matrix will decrease, and agglomeration and stratification will easily occur during planetary stirring, weakening the synergistic toughening effect with the core-shell modified epoxy resin. Therefore, the preferred toughening agent has a viscosity of 5000~50000 mPa·s at 25°C and an epoxy equivalent of 200~300 g / eq, or the ability to react with dicyandiamide to form an IPN structure.

[0068] In a preferred embodiment of the present invention, the polyurethane toughening agent is Hengchuang 102D-1 and / or 102D-1H. 102D-1 reacts with dicyandiamide to form a microscopic island-like toughening structure within the epoxy matrix; 102D-1H reacts with dicyandiamide to form a microscopic interpenetrating network (IPN) toughening structure within the epoxy matrix. Both have minimal impact on the heat resistance of the cured product and lack the distinctive pungent odor of terminal carboxyl-terminated nitrile rubber, making them suitable for applications with high odor requirements, such as automotive engine compartments and electric drive systems.

[0069] In a preferred embodiment of the present invention, the mass ratio of the polyurethane toughening agent to the oxazolidinone modified epoxy resin is 1:1.25~2, preferably 1:1.7.

[0070] In this invention, polyurethane, oxazolidinone-modified epoxy resin, and core-shell modified epoxy resin form a synergistic toughening network. The flexible segments of the polyurethane fill the voids in the cross-linked network, the island structure of the core-shell particles absorbs impact energy, and the rigid skeleton of the oxazolidinone maintains high-temperature cohesive strength. When the mass ratio of the polyurethane toughening agent to the oxazolidinone-modified epoxy resin is controlled at 1:1.25~2, the flexible segments of the polyurethane precisely fill the voids in the cross-linked network between the rigid nodes of the oxazolidinone, neither excessively diluting the density of the rigid skeleton nor insufficiently dissipating impact energy. If the ratio is lower than 1:2 (i.e., the polyurethane proportion is too small), the flexible phase is insufficient to buffer the stress concentration at the rigid nodes, resulting in brittle fracture of the adhesive layer; if the ratio is higher than 1:1.25 (i.e., the polyurethane proportion is too large), the flexible segments will disrupt the continuity of the rigid nodes of the oxazolidinone, leading to increased molecular chain slippage at high temperatures and a decrease in shear strength.

[0071] In a preferred embodiment of the present invention, the filler comprises heavy calcium carbonate and fibrous wollastonite in a mass ratio of 2 to 4:1.

[0072] Preferably, the heavy calcium carbonate has a particle size of 1~10μm and a purity of ≥98%.

[0073] Preferably, the fibrous wollastonite has an aspect ratio of 10-15:1 and a particle size of 600-1250 mesh.

[0074] In this invention, heavy calcium carbonate serves as a granular filler, and fibrous wollastonite serves as a one-dimensional needle-like filler. The two are combined to form a point-line interwoven filling structure. Heavy calcium carbonate acts as a filler base, filling the voids in the epoxy matrix and increasing the packing density and compressive strength of the adhesive layer; simultaneously, it reduces the cost of the adhesive, avoiding the excessive cost associated with using wollastonite alone. The fibrous wollastonite acts as a rigid framework, with fibers randomly dispersed in the adhesive layer to form an interwoven support network, significantly improving the shear deformation resistance and creep resistance of the adhesive layer. At 80°C, the wollastonite fibers effectively inhibit the slippage of epoxy molecular chains. Granular calcium carbonate fills the gaps between the fibers, making the internal structure of the adhesive layer denser and further enhancing cohesive strength; simultaneously, the fibrous framework support offsets the increased brittleness caused by calcium carbonate, achieving reinforcement without compromising toughness.

[0075] If the mass ratio of heavy calcium carbonate to fibrous wollastonite is less than 2:1, the higher proportion of wollastonite makes the fibers prone to entanglement and agglomeration, leading to increased viscosity of the adhesive and difficulty in uniform dispersion during planetary mixing, potentially even causing fiber agglomeration defects. It may also increase the brittleness of the adhesive layer and decrease its impact strength at room temperature. If the mass ratio of heavy calcium carbonate to fibrous wollastonite is greater than 4:1, the higher proportion of calcium carbonate results in insufficient support from the fiber skeleton, leading to a decrease in the adhesive layer's resistance to shear deformation at high temperatures. Excessive calcium carbonate may also worsen the adhesive layer's resistance to damp heat.

[0076] In a preferred embodiment of the present invention, the thixotropic agent comprises fumed silica and quaternary ammonium salt modified bentonite, wherein the mass ratio of fumed silica to quaternary ammonium salt modified bentonite is 1.5 to 3:1.

[0077] In this invention, the fumed silica is nano-sized amorphous silica with a surface rich in silanol groups. It forms a three-dimensional network structure in the epoxy resin matrix through intermolecular hydrogen bonds. When left to stand, it can significantly increase the viscosity of the adhesive and prevent it from flowing after application. When subjected to shear force (such as stirring or application extrusion), the hydrogen bond network is destroyed, the viscosity drops rapidly, and the fluidity for application is restored. After the shear force disappears, the hydrogen bond network is reconstructed, and the viscosity recovers.

[0078] Meanwhile, nano-sized fumed silica can fill the microscopic voids in epoxy groups, playing a reinforcing role and slightly improving the cohesive strength of the adhesive layer.

[0079] In quaternary ammonium salt modified bentonite, long-chain quaternary ammonium salt ions are inserted into the bentonite interlayer, changing it from hydrophilic to oleophilic, thus significantly improving its compatibility with epoxy groups. Quaternary ammonium salt modified bentonite is dispersed in epoxy groups in a lamellar manner, with the lamellars interacting through van der Waals forces to form a weakly flocculated structure, further enhancing the thixotropic effect. Its layered structure also inhibits shrinkage during the curing process of the adhesive, improving the dimensional stability of the adhesive layer.

[0080] The strong hydrogen bond network of fumed silica provides the basic thixotropic strength, preventing the compound from flowing when applied to a vertical plane; the weak flocculation structure of modified bentonite regulates the thixotropic index, avoiding excessive thixotropy that would make the compound difficult to extrude.

[0081] If the mass ratio of fumed silica to quaternary ammonium salt modified bentonite is less than 1.5:1, the extrusion resistance during sizing is high, and even stringing may occur, which is not conducive to the mass sizing of automotive parts. Excessive bentonite will also weaken the reinforcing effect of fumed silica, reducing the cohesive strength of the adhesive layer. If the mass ratio of fumed silica to quaternary ammonium salt modified bentonite is greater than 3:1, the nanoparticles of fumed silica are prone to agglomeration, forming micron-sized agglomerates. This not only leads to unstable thixotropic effects (large fluctuations in static viscosity) but also becomes a stress concentration point within the adhesive layer, resulting in a decrease in room temperature impact resistance.

[0082] Preferably, the specific surface area of ​​the fumed silica is 140~220 m² / g. 2 / g.

[0083] Preferably, the quaternary ammonium salt modified bentonite is an organobentonite modified with hexadecyltrimethylammonium chloride, octadecyltrimethylammonium chloride or dioctadecyldimethylammonium chloride, with an activation degree ≥95% and a particle size of 1~8μm.

[0084] The fumed silica includes one or more of Shandong Hongruitong New Materials HD172, Hubei Huifu Nanomaterials HB-139, or Dalian Fuchang Chemical FUSIL 215.

[0085] The quaternary ammonium salt modified bentonite includes Elementis Specialties BENTON 38 and / or Zhejiang Huatai New Materials BP922.

[0086] In a preferred embodiment of the present invention, the silane coupling agent includes KH560 and KH590, wherein the amount of KH560 is 0.05 to 0.3 parts and the amount of KH590 is 0.05 to 0.2 parts.

[0087] In this invention, KH560 is γ-glycidyl etheroxypropyltrimethoxysilane and KH590 is γ-mercaptopropyltrimethoxysilane. When used together, they can form a dual-bonded structure at the inorganic-organic interface, consisting of an epoxy-anchored matrix and a thiol-enhanced crosslinking structure.

[0088] Preferably, the epoxy equivalent of the bisphenol F type epoxy resin is 150~190 g / eq.

[0089] Preferably, the epoxy equivalent of the bisphenol A type epoxy resin is 180~190 g / eq.

[0090] Preferably, the curing accelerator includes an organic urea accelerator, specifically including complexed high-tech HUA5050 and / or PN50.

[0091] Preferably, the curing agent includes one or more of dicyandiamide, sebacic acid dihydrazide or adipic acid dihydrazide, with dicyandiamide being the most preferred.

[0092] Preferably, the epoxy structural adhesive further includes 0.01 to 0.05 parts by weight of pigment, wherein the pigment includes carbon black.

[0093] Preferably, the desiccant includes calcium oxide desiccant, specifically including Hangxin CaO and / or CGX-3.

[0094] A second objective of this invention is to provide a method for preparing the epoxy structural adhesive as described above, wherein the preparation method includes: a: Raw material pretreatment: Place the desiccant, filler, and thixotropic agent in a vacuum drying oven at 110~120℃ and dry for 2~3 hours. Cool to room temperature before use. The pigment is pre-dispersed with 0.5-1 parts by weight of bisphenol A epoxy resin to prepare a pigment masterbatch for later use, in order to avoid pigment agglomeration; b: Matrix mixing: Add the remaining bisphenol A type epoxy resin, bisphenol F type epoxy resin, core-shell modified epoxy resin, and oxazolidinone modified epoxy resin to the planetary stirrer. Purge the air in the system with nitrogen three times. Control the stirring speed at 500~600 r / min and the temperature at 40~50℃. Stir for 20~30 min until the system is uniform and transparent. Then, while maintaining nitrogen protection and the above temperature and speed, slowly add toughening agent and silane coupling agent, and continue stirring for 30-40 minutes; Then, add the pretreated thixotropic agent, filler and desiccant to the system in sequence, adjust the stirring speed to 800~1000r / min and the temperature to 50~60℃, disperse at high speed for 40~60min, and stop the machine to scrape the wall once every 15min during the process; c: Vacuum degassing Switch the planetary stirrer to vacuum mode, control the vacuum degree to -0.09~-0.095MPa and the temperature to 45~55℃, and maintain the speed at 300~400r / min for degassing for 30~40min; d: Mixing of the curing system: Turn off the vacuum, cool to 30~40℃, add curing agent, curing accelerator and pretreated pigment, adjust the stirring speed to 500~600r / min, stir for 20~30min; the pigment is added according to the application requirements. e: Post-processing: Under a vacuum of -0.09 to -0.095 MPa, the system is degassed at room temperature for 10 to 15 minutes, and the viscosity is measured. At 25°C, the viscosity is controlled at 8000 to 20000 mPa·s, thus obtaining the high-temperature shear-resistant epoxy structural adhesive.

[0095] The present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0096] In the following embodiments, unless otherwise specified, all raw material components are commercially available products.

[0097] Example 1 A high-temperature shear-resistant epoxy structural adhesive, with the following raw material composition by weight: Bisphenol A type epoxy resin: E-51, 15 parts; Bisphenol F type epoxy resin: Dow DER 354, 12 parts; Core-shell modified epoxy resin: Zhongyuan MX-154, 15 parts; Curing agent: Dicyandiamide, 3 parts; Curing accelerator: Complexed high-tech HUA5050, 0.3 parts; Filler: Heavy calcium carbonate (particle size 1~10μm, purity ≥98%), 15 parts; fibrous wollastonite (aspect ratio 10~15:1, 600~1250 mesh), 5 parts; Toughening agent: Hengchuang 102D-1H, 12 parts; Desiccant: CGX-3, 4 parts; Thixotropic agents: Fumed silica, Shandong Hongruitong New Materials HD172, 2 parts; Quaternary ammonium salt modified bentonite, Zhejiang Huatai New Materials BP922, 1 part; Silane coupling agents: KH560, 0.15 parts; KH590, 0.15 parts; Pigment: Carbon black, 0.03 parts; Oxazolidinone-modified epoxy resin: 20 parts, prepared by the following method: S1: Add 4,4'-diaminodiphenylmethane and diethyl carbonate in a molar ratio of 1:12, and add dibutyltin dilaurate catalyst at 0.8% of the mass of 4,4'-diaminodiphenylmethane. Under nitrogen protection, use a gradient heating method: first heat to 110℃ and hold for 2 hours, then heat to 125~130℃ and stir vigorously for 5 hours. Then cool to room temperature, pre-concentrate by distillation at 80℃ and atmospheric pressure for 30 minutes, and then distill under reduced pressure at 60℃ and -0.095MPa for 2 hours to remove excess diethyl carbonate. After washing with anhydrous ethanol, filtering, and vacuum drying, a carbamate intermediate with a purity ≥99% is obtained.

[0098] S2: The carbamate intermediate was subjected to staged pyrolysis under vacuum of -0.098 MPa and 180~230℃: it was first initially pyrolyzed by holding at 180℃ for 1 h, and then further pyrolyzed by heating to 220~230℃ and holding for 2 h. The crude product was then collected by condensing in a 10~15℃ condenser. The crude product was then subjected to vacuum distillation at -0.098 MPa and 120℃, recrystallized with anhydrous toluene, filtered and dried to obtain 4,4'-diphenylmethane diisocyanate with a purity ≥99.5%. S3: With a molar ratio of n(NCO):n(epoxy group) = 0.9:1.0, 4,4'-diphenylmethane diisocyanate and bisphenol A type epoxy resin E-51 were added, along with 0.4% of the epoxy resin mass of triphenylphosphine catalyst and 2.5% of the total system mass of 4Å molecular sieve dehydrating agent. The mixture was dehydrated and dried at 100℃ and -0.09MPa vacuum for 1 hour. After cooling to 60℃, the mixture was kept at this temperature and stirred. Under nitrogen protection, a gradient heating reaction was carried out: 80℃ for 1 hour, 120~130℃ for 3 hours, and 140℃ for 1 hour, for a total reaction time of 5 hours. After the reaction, the mixture was washed with ethyl acetate and the solvent was removed under vacuum to obtain oxazolidinone modified epoxy resin.

[0099] The number-average molecular weight and molecular weight distribution of oxazolidinone-modified epoxy resin were determined by gel permeation chromatography (GPC). Tetrahydrofuran (THF) was used as the solvent to prepare sample solutions of 0.5–1.0 mg / mL. The GPC test methods specified in GB / T 21863-2008 and QJ 1870-1990 were adopted. A calibration curve (Ri) was established using narrow-distribution polystyrene standards (compliant with GB / T 21864-2008). 2 =0.999), the number average molecular weight of the oxazolidinone-modified epoxy resin was determined to be 780 Da, and the molecular weight distribution PDI was 1.85.

[0100] Preparation method of epoxy structural adhesive: a: Raw material pretreatment: Place the desiccant, filler, and thixotropic agent in a vacuum drying oven at 110~120℃ and dry for 2~3 hours. Cool to room temperature before use. Pigment carbon black is pre-dispersed with 1 part by weight of bisphenol A epoxy resin to prepare carbon black masterbatch for later use, in order to avoid carbon black agglomeration; b: Matrix mixing: Add the remaining bisphenol A type epoxy resin, bisphenol F type epoxy resin, core-shell modified epoxy resin, and oxazolidinone modified epoxy resin to the planetary stirrer. Purge the air in the system with nitrogen three times. Control the stirring speed at 550 r / min and the temperature at 45℃. Stir for 20~30 min until the system is uniform and transparent. Then, while maintaining nitrogen protection and the above temperature and speed, slowly add toughening agent and silane coupling agent, and continue stirring for 30-40 minutes; Then, the pretreated thixotropic agent, filler and desiccant were added to the system in sequence, the stirring speed was adjusted to 900 r / min and the temperature to 55℃, and the mixture was dispersed at high speed for 50 min, with the machine stopped and the wall scraped once every 15 min during the process. c: Vacuum degassing Switch the planetary stirrer to vacuum mode, control the vacuum degree to -0.09~-0.095MPa and the temperature to 50℃, and maintain the speed at 350r / min for degassing for 30~40min; d: Mixing of the curing system: Turn off the vacuum, cool down to 35°C, add curing agent, curing accelerator and pretreated carbon black masterbatch, adjust the stirring speed to 550 r / min, and stir for 20~30 min; e: Post-processing: Under a vacuum of -0.09 to -0.095 MPa, the system is degassed at room temperature for 10 to 15 minutes, and the viscosity is measured. At 25°C, the viscosity is controlled at 8000 to 20000 mPa·s, thus obtaining the high-temperature shear-resistant epoxy structural adhesive.

[0101] Example 2 A high-temperature shear-resistant epoxy structural adhesive, with the following raw material composition by weight: Bisphenol A type epoxy resin: E-51, 11 parts; Bisphenol F type epoxy resin: Dow DER 354, 15 parts; Core-shell modified epoxy resin: YT-3154, a new material with refined enthalpy, 20 parts; Curing agent: Dicyandiamide, 4 parts; Curing accelerator: PN50, 0.4 parts; Filler: Heavy calcium carbonate (particle size 1~10μm, purity ≥98%), 15 parts; fibrous wollastonite (aspect ratio 10~15:1, 600~1250 mesh), 5 parts; Toughening agent: Hengchuang 102D-1H, 12 parts; Desiccant: Hangxin CaO, 4 parts; Thixotropic agents: Fumed silica, Shandong Hongruitong New Materials HD172, 2 parts; Quaternary ammonium salt modified bentonite, Zhejiang Huatai New Materials BP922, 1 part; Silane coupling agents: KH560, 0.15 parts; KH590, 0.15 parts; Pigment: Carbon black, 0.03 parts; Oxazolidinone-modified epoxy resin: 15 parts, prepared in the same way as in Example 1.

[0102] The preparation method of the epoxy structural adhesive is the same as that in Example 1.

[0103] Example 3 A high-temperature shear-resistant epoxy structural adhesive, with the following raw material composition by weight: Bisphenol A type epoxy resin: E-51, 20 parts; Bisphenol F type epoxy resin: Dow DER 354, 10 parts; Core-shell modified epoxy resin: Zhongyuan MX-154, 10 parts; Curing agent: Dicyandiamide, 5 parts; Curing accelerator: Complexed high-tech HUA5050, 0.5 parts; Filler: Heavy calcium carbonate (particle size 1~10μm, purity ≥98%), 15 parts; fibrous wollastonite (aspect ratio 10~15:1, 600~1250 mesh), 5 parts; Toughening agent: Hengchuang 102D-1H, 12 parts; Desiccant: CGX-3, 4 parts; Thixotropic agents: Fumed silica, Shandong Hongruitong New Materials HD172, 2 parts; Quaternary ammonium salt modified bentonite, Zhejiang Huatai New Materials BP922, 1 part; Silane coupling agents: KH560, 0.15 parts; KH590, 0.15 parts; Pigment: Carbon black, 0.03 parts; Oxazolidinone-modified epoxy resin: 24 parts, prepared by the same method as in Example 1.

[0104] The preparation method of the epoxy structural adhesive is the same as that in Example 1.

[0105] Example 4 The difference from Example 1 is that the filler is 20 parts of calcium bicarbonate, otherwise it is the same as Example 1.

[0106] Example 5 The difference from Example 1 is that the filler is 13 parts calcium bicarbonate and 7 parts fibrous wollastonite, otherwise it is the same as Example 1.

[0107] Example 6 The difference from Example 1 is that the filler is 16 parts calcium bicarbonate and 4 parts fibrous wollastonite, otherwise it is the same as Example 1.

[0108] Example 7 The difference from Example 1 is that the thixotropic agent is 1.8 parts of fumed silica and 1.2 parts of quaternary ammonium salt modified bentonite, while the rest is the same as in Example 1.

[0109] Example 8 The difference from Example 1 is that the thixotropic agent is 2.2 parts of fumed silica and 0.8 parts of quaternary ammonium salt modified bentonite, while the rest is the same as in Example 1.

[0110] Example 9 The difference from Example 1 is that the thixotropic agent is 3 parts of fumed silica, otherwise it is the same as Example 1.

[0111] Example 10 The difference from Example 1 is that the silane coupling agent is 0.3 parts KH560, otherwise it is the same as Example 1.

[0112] Comparative Example 1 The difference between the raw material formulation and Example 1 is that it does not contain oxazolidinone-modified epoxy resin, while the other raw materials and their amounts remain unchanged, and the preparation method is the same as in Example 1.

[0113] Performance testing (1) Tensile shear strength at 80℃ (MPa): According to GB / T 7124-2008, the substrate is Q235 cold-rolled steel plate, the adhesive layer thickness is 0.2mm, cured at 170℃ for 20min, kept at 80℃ for 4h, and then tested in an environment of 80℃. (2) Tensile shear strength at 25℃ (MPa): According to GB / T 7124-2008, the substrate is Q235 cold-rolled steel plate, the adhesive layer thickness is 0.2mm, cured at 170℃ for 20min, and tested after being placed at room temperature for 24h. (3) Cohesion failure rate (%): According to GB / T 16997, the proportion of the residual area of ​​the adhesive layer to the total bonded area is calculated after the shear test; (4) Impact strength at room temperature (N / mm): According to ISO 11343, the base material is Q235 cold-rolled steel sheet, and the test is conducted at room temperature; (5) Humidity and heat reduction rate (%): According to GB / T 14074-2017, the tensile shear strength was reduced by 480h at 50℃ and 95% relative humidity.

[0114] The test results of Examples 1-10 and Comparative Example 1 are shown in Table 1.

[0115] Table 1 Performance Test Results

[0116] As shown in Table 1, the tensile shear strength at 80°C of Examples 1-10 exceeded 30 MPa and the cohesive failure rate reached 95%, indicating that the introduction of the heterocyclic structure of oxazolidinone enhanced the rigid skeleton, increased the cohesive energy density, and strengthened the crosslinking network.

[0117] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, nor does it mean that the present invention must rely on the above process steps for implementation. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A high-temperature shear-resistant epoxy structural adhesive, characterized in that, The epoxy structural adhesive comprises the following components in parts by weight: The following components are included: 10-20 parts of bisphenol A type epoxy resin, 10-15 parts of bisphenol F type epoxy resin, 10-20 parts of core-shell modified epoxy resin, 15-25 parts of oxazolidinone modified epoxy resin, 1-5 parts of curing agent, 0.1-0.5 parts of curing accelerator, 10-30 parts of filler, 10-15 parts of toughening agent, 3-5 parts of desiccant, 2-5 parts of thixotropic agent, and 0.1-0.5 parts of silane coupling agent. The oxazolidinone-modified epoxy resin is prepared by a cycloaddition reaction between an isocyanate compound and the epoxy group of an epoxy resin, wherein the five-membered ring of the oxazolidinone is chemically bonded and embedded in the main chain of the epoxy resin. The number-average molecular weight (Mn) of the oxazolidinone-modified epoxy resin is 700-900 Da, and the molecular weight distribution (Mw / Mn) is 1.6-2.

5. The toughening agent includes a polyurethane toughening agent, and the mass ratio of the polyurethane toughening agent to the oxazolidinone-modified epoxy resin is 1:1.25~2. The filler comprises heavy calcium carbonate and fibrous wollastonite in a mass ratio of 2 to 4:

1.

2. The epoxy structural adhesive as described in claim 1, characterized in that, The preparation method of the oxazolidinone-modified epoxy resin includes: S1: Add 4,4'-diaminodiphenylmethane and diethyl carbonate in a molar ratio of 1:12, add a catalyst accounting for 0.5~1.0% of the mass of 4,4'-diaminodiphenylmethane, and under nitrogen protection, use a gradient heating method: first heat to 110℃ and hold for 2h, then heat to 125~130℃ and stir vigorously for 5h, then cool to room temperature, and then remove excess diethyl carbonate to obtain the carbamate intermediate; S2: The carbamate intermediate was subjected to staged pyrolysis under vacuum of -0.098 MPa and 180~230℃: it was first initially pyrolyzed by holding at 180℃ for 1 h, and then further pyrolyzed by heating to 220~230℃ and holding for 2 h. The crude product was then collected by condensing through a 10~15℃ condenser. The crude product was then subjected to vacuum distillation at -0.098 MPa and 120℃, and after recrystallization and drying, 4,4'-diphenylmethane diisocyanate was obtained. S3: Add 4,4'-diphenylmethane diisocyanate and epoxy resin at a molar ratio of isocyanate group to epoxy group of 0.9:1.0, along with 0.3-0.5% catalyst (by mass of epoxy resin) and 2-3% dehydrating agent (by mass of total system). Dehydrate and dry the mixture at 100℃ and -0.09MPa vacuum for 1 hour. Then, react the mixture at 80-140℃ for 4-6 hours under nitrogen protection. After the reaction, wash the mixture with ethyl acetate and remove the solvent under vacuum to obtain oxazolidinone-modified epoxy resin.

3. The epoxy structural adhesive as described in claim 1, characterized in that, The core-shell modified epoxy resin is an epoxy resin modified with core-shell particles. The core-shell particles have a polybutadiene rubber core and a methyl methacrylate shell, and the average particle size of the core-shell particles is 100~200nm.

4. The epoxy structural adhesive as described in claim 1, characterized in that, The heavy calcium carbonate has a particle size of 1~10μm and a purity of ≥98%; The fibrous wollastonite has an aspect ratio of 10-15:1 and a particle size of 600-1250 mesh.

5. The epoxy structural adhesive as described in claim 1, characterized in that, The thixotropic agent comprises fumed silica and quaternary ammonium salt modified bentonite, wherein the mass ratio of fumed silica to quaternary ammonium salt modified bentonite is 1.5~3:

1. The specific surface area of ​​the fumed silica is 140~220 m². 2 / g; The quaternary ammonium salt modified bentonite is an organobentonite modified with hexadecyltrimethylammonium chloride, octadecyltrimethylammonium chloride or dioctadecyldimethylammonium chloride, with an activation degree ≥95% and a particle size of 1~8μm.

6. The epoxy structural adhesive as described in claim 1, characterized in that, The silane coupling agent includes KH560 and KH590, wherein the amount of KH560 is 0.05~0.3 parts and the amount of KH590 is 0.05~0.2 parts; The curing accelerator includes an organic urea accelerator; The curing agent includes one or more of dicyandiamide, sebacic acid dihydrazide, or adipic acid dihydrazide.

7. The epoxy structural adhesive according to any one of claims 1-6, characterized in that, The epoxy structural adhesive also includes 0.01 to 0.05 parts by weight of pigment, including carbon black; The desiccant includes calcium oxide desiccant.

8. The method for preparing the epoxy structural adhesive according to any one of claims 1-7, characterized in that, The preparation method includes: a: Raw material pretreatment: Place the desiccant, filler, and thixotropic agent in a vacuum drying oven at 110~120℃ and dry for 2~3 hours. Cool to room temperature before use. The pigment is pre-dispersed with 0.5-1 parts by weight of bisphenol A epoxy resin to prepare a pigment masterbatch for later use, in order to avoid pigment agglomeration; b: Matrix mixing: Add the remaining bisphenol A type epoxy resin, bisphenol F type epoxy resin, core-shell modified epoxy resin, and oxazolidinone modified epoxy resin to the planetary stirrer. Purge the air in the system with nitrogen three times. Control the stirring speed at 500~600 r / min and the temperature at 40~50℃. Stir for 20~30 min until the system is uniform and transparent. Then, while maintaining nitrogen protection and the above temperature and speed, slowly add toughening agent and silane coupling agent, and continue stirring for 30-40 minutes; Then, add the pretreated thixotropic agent, filler and desiccant to the system in sequence, adjust the stirring speed to 800~1000r / min and the temperature to 50~60℃, disperse at high speed for 40~60min, and stop the machine to scrape the wall once every 15min during the process; c: Vacuum degassing Switch the planetary stirrer to vacuum mode, control the vacuum degree to -0.09~-0.095MPa and the temperature to 45~55℃, and maintain the speed at 300~400r / min for degassing for 30~40min; d: Mixing of the curing system: Turn off the vacuum, cool down to 30~40℃, add curing agent, curing accelerator and pretreated pigment masterbatch, adjust the stirring speed to 500~600r / min, and stir for 20~30min; e: Post-processing: Under a vacuum of -0.09 to -0.095 MPa, the system is degassed at room temperature for 10 to 15 minutes, and the viscosity is measured. At 25°C, the viscosity is controlled at 8000 to 20000 mPa·s, thus obtaining the high-temperature shear-resistant epoxy structural adhesive.

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