Low-stress type heat-conducting epoxy resin structural adhesive for ultra-low temperature environment and preparation method thereof

Abstract

The application relates to the technical field of adhesives, and particularly discloses a low-stress type heat-conducting epoxy resin structural adhesive for an ultralow-temperature environment and a preparation method thereof. The low-stress type heat-conducting epoxy resin structural adhesive for an ultralow-temperature environment comprises a component A and a component B which are separately stored and mixed in a weight ratio of 100: (22-28) when used; the component A contains the following raw materials in parts by weight: 100-120 parts of epoxy resin, 10-20 parts of solvent, 100-200 parts of heat-conducting filler, 10-25 parts of stress buffering microspheres and 0.5-3 parts of wetting dispersant; the heat-conducting filler is composed of boron nitride nanosheets and spherical alumina; the component B contains the following raw materials in parts by weight: 20-30 parts of a curing agent and 0.1-0.5 parts of an accelerator. The low-stress type heat-conducting epoxy resin structural adhesive for an ultralow-temperature environment not only is suitable for an ultralow-temperature environment, but also can simultaneously realize the improvement of low internal stress, high toughness retention rate and high thermal conductivity, thereby meeting the strict requirements of high-end manufacturing on low-temperature reliability.

Description

Technical Field

[0001] This application relates to the field of adhesive technology, and more specifically, to a low-stress thermally conductive epoxy resin structural adhesive for use in ultra-low temperature environments and its preparation method. Background Technology

[0002] Epoxy structural adhesives are widely used in structural bonding and encapsulation in high-tech fields such as microelectronics packaging, spacecraft manufacturing, and cryogenic Dewar assembly due to their excellent adhesive strength, good processability, and chemical stability. However, traditional epoxy structural adhesives face severe challenges in ultra-low temperature operating environments such as liquid nitrogen (-196℃).

[0003] Epoxy resin is a thermosetting plastic that undergoes a glass transition at low temperatures, resulting in a sharp increase in modulus and a significant decrease in toughness, making it hard and brittle. Simultaneously, traditional epoxy resins form a highly cross-linked, rigid three-dimensional network after curing. During the cooling process from room temperature to liquid nitrogen temperature, the resin itself undergoes severe shrinkage, generating enormous thermal mismatch stress. Therefore, epoxy resin experiences significant internal stress due to the glass transition and massive thermal shrinkage, making it highly susceptible to embrittlement, cracking, or debonding from the substrate. This can lead to device sealing failure, structural damage, and severely impact the long-term reliability of devices and systems operating in ultra-low temperature environments.

[0004] Moreover, in applications such as microelectronics and high-power low-temperature devices, the adhesive layer not only serves as a structural connection but also needs to function as a heat conduction channel, which requires the structural adhesive to have a high thermal conductivity. The conventional method is to fill the epoxy resin matrix with a high proportion of thermally conductive fillers. However, high filler content usually further increases the modulus and brittleness of the composite material system, exacerbating stress problems at low temperatures and creating a contradiction between "improved thermal conductivity" and "deterioration of stress / toughness".

[0005] Therefore, in order to make epoxy resin structural adhesives possess excellent ultra-low temperature toughness, low internal stress, and high thermal conductivity, making them particularly suitable for harsh low-temperature working environments such as microelectronic packaging and spacecraft, it is urgent to propose a solution to address the aforementioned technical problems. Summary of the Invention

[0006] In order to develop an epoxy resin structural adhesive specifically suitable for ultra-low temperature environments, which can simultaneously achieve low internal stress, high toughness retention and high thermal conductivity, so as to meet the stringent requirements of high-end manufacturing industry for low-temperature reliability, this application provides a low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments and its preparation method.

[0007] In a first aspect, this application provides a low-stress, thermally conductive epoxy resin structural adhesive for ultra-low temperature environments, employing the following technical solution: Low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments, comprising component A and component B, which are stored separately and mixed by weight in a ratio of 100:(22-28) when used; Component A comprises the following raw materials in parts by weight: 100-120 parts epoxy resin; Solvent 10-20 parts; 100-200 parts of thermally conductive filler; 10-25 parts of stress-buffering microspheres; 0.5-3 parts of wetting and dispersing agent; The stress-buffering microspheres were prepared by the following steps: S1. Take elastic polymer microspheres and disperse them in a mixed solvent composed of ethanol, water and ammonia. Then add silane coupling agent and stir to react. After centrifugation, washing and drying, pretreated elastic polymer microspheres are obtained. S2. Disperse the elastic polymer microspheres obtained in step S1 in a mixed solvent of ethanol and water, add ammonia and stir, then add an ethanol solution of tetraethyl orthosilicate dropwise while stirring, and continue stirring after the addition is complete to obtain a microsphere dispersion. S3. Add silane coupling agent to the microsphere dispersion obtained in step S3, and carry out a heating reflux reaction under nitrogen protection. After the reaction is completed, cool, centrifuge, wash and dry to obtain stress buffer microspheres. The thermally conductive filler is composed of boron nitride nanosheets and spherical alumina. Component B comprises the following raw materials in parts by weight: 20-30 parts of curing agent; Accelerator 0.1-0.5 parts.

[0008] By adopting the above technical solution, in component A, epoxy resin is the main body for forming a three-dimensional cross-linked network, serving as the continuous phase and matrix of the structural adhesive, providing basic adhesive and mechanical properties. The solvent mainly aims to adjust the viscosity, ensuring that the system still has suitable flowability and workability after adding a large amount of thermally conductive filler and stress-buffering microspheres, facilitating mixing, degassing, and potting / dispensing. The wetting and dispersing agent helps the thermally conductive filler and stress-buffering microspheres to be uniformly and stably dispersed in the epoxy resin matrix, preventing agglomeration. In component B, the curing agent mainly reacts chemically with the epoxy resin in component A, causing it to cross-link from a linear or branched structure into a three-dimensional network solid. The accelerator accelerates the reaction between the epoxy resin and the curing agent, lowering the curing temperature or shortening the curing time, thereby improving production efficiency.

[0009] Meanwhile, for the thermally conductive filler composed of boron nitride nanosheets and spherical alumina, the sheet-like boron nitride nanosheets can overlap in the system to form efficient in-plane thermal conductivity pathways. A large number of spherical alumina fill the gaps between the boron nitride nanosheets, acting as a thermal "bridge" connecting areas separated by the sheet-like filler, and synergistically constructing a stable, isotropic three-dimensional thermally conductive network with the sheet-like filler, thus forming a dense and efficient thermal conductivity pathway. For stress-buffered microspheres, during preparation, a silane coupling agent is first introduced onto the surface of the elastic polymer microspheres to initially improve its interface with the subsequent inorganic layer. Then, a layer of silica is coated onto the surface of the elastic microspheres using the sol-gel method. The silica shell gives the microspheres higher strength, thermal stability, and compatibility with subsequent silane treatment. The surface is then treated again with a silane coupling agent to give it active groups that can react with epoxy resin, ultimately obtaining "core-shell" structured microspheres. At ultra-low temperatures, when the resin matrix generates stress due to shrinkage, the core of the stress-buffering microspheres can undergo elastic compression or deformation, absorbing and dissipating strain energy like a "micro-spring," thereby actively releasing stress and preventing the initiation and propagation of microcracks. Meanwhile, the silica shell and its surface reactive functional groups can form strong chemical bonds with the epoxy resin matrix, ensuring effective stress transfer.

[0010] When thermally conductive fillers and stress-buffered microspheres are used together, they exhibit excellent synergistic effects. The fillers, primarily composed of boron nitride and alumina, form a high-strength, high-modulus rigid thermally conductive framework. Meanwhile, the stress-buffered microspheres, acting as flexible islands dispersed within the framework, effectively create numerous "energy dissipation points" along the crack propagation path. Together, they form a multi-layered crack pinning and deflection network, enabling the material to transition from brittle fracture to quasi-ductile fracture at low temperatures, thus effectively preventing low-temperature brittle fracture. Furthermore, they constitute a multi-scale stress-buffered system, jointly ensuring the integrity of the bonding interface at extremely low temperatures.

[0011] Therefore, the low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments obtained above is not only suitable for ultra-low temperature environments, but can also simultaneously achieve a balance between low internal stress, high toughness retention rate and high thermal conductivity, thereby meeting the stringent requirements of high-end manufacturing industries for low-temperature reliability.

[0012] Preferably, the stress-buffering microspheres have a particle size of 10-20 μm.

[0013] By adopting the above technical solutions, stress-buffering microspheres can effectively induce plastic deformation of the matrix, buffer stress, and bridge microcracks, achieving the best toughening and stress relaxation effects. At the same time, they can also achieve better synergistic effects with thermally conductive fillers. Moreover, the size of 10-20μm is much smaller than the thickness of typical structural adhesive layers or encapsulations, and can be uniformly dispersed as micro-functional units without destroying the macroscopic continuity and uniformity of the material.

[0014] Preferably, in the preparation of the stress-buffering microspheres, the concentration of ammonia after addition is 0.5-1.0 mol / L; and the concentration of tetraethyl orthosilicate is 0.3-0.5 mol / L after the ethanol solution of tetraethyl orthosilicate is added dropwise.

[0015] By employing the above technical solution, the concentration of ammonia water after addition is 0.5-1.0 mol / L. Under these conditions, the hydrolysis and condensation reaction rates of tetraethyl orthosilicate are mildly and effectively controlled, allowing silica to be deposited on the surface of microspheres in a uniform and dense film form, forming a complete, smooth, and controllable thickness nanoscale silica shell. A tetraethyl orthosilicate concentration of 0.3-0.5 mol / L can form a silica shell of moderate thickness on the microsphere surface. This thickness provides sufficient mechanical strength and chemical stability while maintaining the overall "rigid outside, flexible inside" characteristics of the microspheres, maximizing their buffering function.

[0016] Preferably, the thermally conductive filler is composed of boron nitride nanosheets and spherical alumina in a weight ratio of 1:(3-10).

[0017] By adopting the above technical solution, a high proportion of sheet-like nanofillers can severely degrade toughness due to stress concentration. At this ratio, a high proportion of spherical alumina is used as the main component, supplemented by only a small amount of boron nitride nanosheets. This combination avoids the dramatic increase in modulus and sharp decrease in toughness caused by the extensive use of sheet-like fillers, maintaining a certain degree of deformability in the matrix. This allows for a stable and better synergistic effect between the filler and the stress-buffering microspheres. Furthermore, since boron nitride nanosheets are relatively expensive, the above ratio also offers a significant cost advantage.

[0018] Preferably, the boron nitride nanosheets have a lateral dimension of 10-20 μm and a thickness of 60-100 nm; the spherical alumina particles have a diameter D50 of 5-10 μm.

[0019] By adopting the above technical solution, the boron nitride nanosheets of the above specifications interweave and support each other in space, forming a more stable microstructure; the spherical alumina of the above specifications is easily and uniformly dispersed under the action of shear and dispersant; at the same time, the combination of the two not only achieves optimal spatial stacking in physics, but also forms deep synergy in function, and forms a more stable combination with the stress buffer microspheres, thereby constructing an optimal thermally conductive network and stress management structure, and finally obtaining a high-quality low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments.

[0020] Preferably, the boron nitride nanosheets are modified before use, and the modification steps are as follows: A1. Boron nitride nanosheets were subjected to strong oxidation treatment to obtain activated boron nitride nanosheets. A2. The activated boron nitride nanosheets obtained in step A1 are dispersed in a mixed solvent composed of ethanol, water and ammonia, and then a silane coupling agent is added and stirred. After centrifugation, washing and drying, pretreated boron nitride nanosheets are obtained. A3. Disperse the pretreated boron nitride nanosheets obtained in step A2 in dichloromethane, add the polymerization initiator dropwise under an inert gas, continue stirring the reaction after the addition is complete, and after the reaction is completed, centrifuge, wash and dry to obtain boron nitride nanosheets with initiation sites. A4. Add the boron nitride nanosheets with initiation sites obtained in step A3, the catalyst, 2,2-dimethylolpropionic acid monomer, and solvent to a polymerization container, heat and stir the reaction under an inert atmosphere, and after the reaction is completed, cool, centrifuge, wash and dry to obtain modified boron nitride nanosheets.

[0021] By adopting the above technical solution, in the modification process, firstly, active oxygen-containing functional groups are introduced into the inert surface of boron nitride nanosheets through strong oxidation treatment; then, silane coupling agent treatment is performed to graft onto the activated surface of boron nitride nanosheets, providing subsequent reaction sites; next, initiator molecules capable of initiating subsequent polymerization reactions are immobilized through chemical reaction, ensuring that subsequent polymer chains can grow directly from the surface of boron nitride nanosheets; finally, through surface-initiated polymerization, a flexible polymer interface layer with controllable thickness and high grafting density is formed on the surface of boron nitride nanosheets, ultimately obtaining modified boron nitride nanosheets. During the use of modified boron nitride nanosheets, the aforementioned flexible polymer interface layer exhibits a lower glass transition temperature and better low-temperature elasticity. It can efficiently absorb and dissipate energy generated by thermal stress or impact loads through conformational changes and reversible deformation of its own chain segments. Moreover, the strong node bonding between the flexible polymer interface layer and the epoxy resin and curing agent not only reduces the interfacial thermal resistance between the boron nitride nanosheets and the epoxy matrix, enabling more efficient heat transfer, but also provides excellent dispersibility to ensure the efficient construction of the thermally conductive network. Thus, modifying boron nitride nanosheets before use can further improve and enhance low internal stress, high toughness retention, and high thermal conductivity at ultra-low temperatures.

[0022] Preferably, in step A4, the ratio of boron nitride nanosheets with initiation sites obtained in step A3 to catalyst, 2,2-dimethylolpropionic acid monomer, and solvent is 1g:(10-30)g:(0.1-0.3)g:(100-300)mL.

[0023] By adopting the above technical solution, it performs better in balancing the supply of reactants, the reaction rate and the reaction microenvironment, and thus can generate a flexible polymer interface layer with high graft density, moderate chain length and uniform coating on the surface of boron nitride nanosheets, thereby exerting a stable and better corresponding effect.

[0024] Preferably, the epoxy resin is composed of the following components in parts by weight: 60-80 parts of alicyclic epoxy resin; 20-40 parts of flexible epoxy resin.

[0025] By adopting the above technical solutions, alicyclic epoxy resins have the characteristics of low viscosity, high activity, low shrinkage and intrinsic low temperature resistance; the introduction of flexible segments into flexible epoxy resins can further reduce the crosslinking density of the cured network and improve the low temperature deformation capability; the combination of the two establishes an intrinsically stable polymer matrix that combines "rigidity" and "flexibility" for the entire material system, and lays the foundation for the synergistic effect of subsequent thermally conductive fillers and stress-buffering microspheres.

[0026] Secondly, this application provides a method for preparing a low-stress, thermally conductive epoxy resin structural adhesive for ultra-low temperature environments, employing the following technical solution: A method for preparing a low-stress, thermally conductive epoxy resin structural adhesive for ultra-low temperature environments includes the following steps: (1) Prepare raw materials containing waste component A and component B according to the formula; (2) After the epoxy resin, solvent and wetting and dispersing agent are mixed evenly, thermally conductive filler is added during the stirring process and dispersed at high speed. Then, stress buffer microspheres are added under low speed stirring. After mixing evenly, vacuum degassing treatment is performed to obtain component A. After the curing agent and accelerator are mixed, vacuum degassing treatment is performed to obtain component B.

[0027] By adopting the above technical solution, high-speed dispersion of thermally conductive filler ensures maximum functionality, while low-speed mixing of stress-buffering microspheres ensures structural stability. This ensures the full utilization of the functions of each component. Furthermore, components A and B are vacuum degassed separately, controlling the density of the final adhesive layer from the source and improving the reliability of the product.

[0028] In summary, this application has the following beneficial effects: 1. This application uses specially prepared stress-buffering microspheres combined with thermally conductive fillers composed of boron nitride nanosheets and spherical alumina. Through the synergy between the two, they not only jointly form a multi-layered crack pinning and deflection network, enabling the material to change from brittle fracture to quasi-ductile fracture at low temperatures, thereby effectively preventing low-temperature brittle fracture, but also form a multi-scale stress-buffering system. This makes the low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments not only suitable for ultra-low temperature environments, but also able to simultaneously achieve a balance of low internal stress, high toughness retention rate and high thermal conductivity, thereby meeting the stringent requirements of high-end manufacturing industries for low-temperature reliability. 2. This application modifies boron nitride nanosheets before use to form a flexible polymer interface layer on their surface. This not only efficiently absorbs and dissipates energy generated by thermal stress or impact loads through conformational changes and reversible deformation of its own chain segments, but also enables more efficient heat transfer and ensures the efficient construction of the heat conduction network. This leads to further improvement and enhancement of low internal stress, high toughness retention rate and high thermal conductivity at ultra-low temperatures. Detailed Implementation

[0029] The present application will be further described in detail below with reference to preparation examples, embodiments and comparative examples.

[0030] Unless otherwise specified, all raw materials used in the preparation examples, embodiments and comparative examples of this application are commercially available.

[0031] The alicyclic epoxy resin, TTA3150, with a purity of 99%, was purchased from Wuhan Huaxiang Kejie Biotechnology Co., Ltd. The flexible epoxy resin was purchased from Dow Chemical, specifically low-viscosity flexible epoxy resin XZ 92466. The wetting and dispersing agent was purchased from Shanghai Huiyi Materials Technology Co., Ltd. as wetting and dispersing agent BYK-LPX22550. The curing agent is BASF Baxxodur ECX2221 modified alicyclic amine epoxy curing agent from Germany; The accelerator was purchased as DMP-30 epoxy accelerator.

[0032] Preparation examples of raw materials and / or intermediates Preparation Example 1 A stress-buffering microsphere is prepared by the following steps: S1. Take elastic polymer microspheres and disperse them in a mixed solvent composed of ethanol, water and ammonia in a volume ratio of 80:15:5. Then add 5% of the silane coupling agent by mass of the elastic polymer microspheres and stir to react. Stir at 45°C for 5 hours. After centrifugation, washing and drying, pretreated elastic polymer microspheres are obtained. S2. The elastic polymer microspheres obtained in step S1 are dispersed in a mixed solvent of ethanol and water at a ratio of 1g:200mL, with a volume ratio of ethanol to water of 7:3. After adding ammonia, the mixture is stirred. Then, an ethanol solution of tetraethyl orthosilicate is added dropwise while stirring at 300rpm. After the addition is completed in 3 hours, the mixture is stirred and reacted for another 16 hours to obtain a microsphere dispersion. S3. Add silane coupling agent to the microsphere dispersion obtained in step S3. The proportion of silane coupling agent after addition is 3.5 wt%. Under nitrogen protection, heat to 105°C and reflux for 12 h. After the reaction is completed, cool, centrifuge, wash and dry to obtain stress buffer microspheres.

[0033] Note: In the above operations, the silane coupling agent used was KH-550, and the elastic polymer microspheres were purchased from Beijing Zhongke Keyou Technology Co., Ltd. as monodisperse PMMA microspheres. The stress buffer microspheres had a particle size of 15 μm. In step S2, the concentration of ammonia water after addition was 0.75 mol / L; after the ethanol solution of tetraethyl orthosilicate was added dropwise, the concentration of tetraethyl orthosilicate was 0.4 mol / L.

[0034] Preparation Example 2 A stress-buffering microsphere, which differs from the preparation example 1 in that the stress-buffering microsphere has a particle size of 10 μm.

[0035] Preparation Example 3 A stress-buffering microsphere, which differs from the preparation example 1 in that the stress-buffering microsphere has a particle size of 20 μm.

[0036] Preparation Example 4 A stress-buffering microsphere differs from Preparation Example 1 in that, in step S2, the concentration of ammonia after addition is 0.5 mol / L; and the concentration of tetraethyl orthosilicate is 0.3 mol / L after the ethanol solution of tetraethyl orthosilicate is added dropwise.

[0037] Preparation Example 5 A stress-buffering microsphere differs from Preparation Example 1 in that, in step S2, the concentration of ammonia after addition is 1.0 mol / L; and the concentration of tetraethyl orthosilicate is 0.5 mol / L after the ethanol solution of tetraethyl orthosilicate is added dropwise.

[0038] Preparation Example 6 A modified boron nitride nanosheet was prepared by the following modification steps: A1. Boron nitride nanosheets were subjected to strong oxidation treatment with concentrated nitric acid to obtain activated boron nitride nanosheets. A2. The activated boron nitride nanosheets obtained in step A1 are dispersed in a mixed solvent composed of ethanol, water and ammonia in a volume ratio of 80:15:5. Then, 6% of the mass of the activated boron nitride nanosheets is added and stirred for reaction. The reaction is carried out at 45°C for 5 hours. After centrifugation, washing and drying, pretreated boron nitride nanosheets are obtained. A3. The pretreated boron nitride nanosheets obtained in step A2 were dispersed in dichloromethane at a ratio of 1g:50mL, cooled to 5°C in an ice-water bath, and the polymerization initiator 2-bromoisobutyryl bromide was added dropwise under the protection of inert nitrogen gas. The mass ratio of 2-bromoisobutyryl bromide to pretreated boron nitride nanosheets was controlled at 1.2:1. After the addition was completed, the reaction was stirred at 25°C for 20h. After the reaction was completed, the boron nitride nanosheets with initiation sites were obtained by centrifugation, washing and drying. A4. Add the boron nitride nanosheets with initiation sites obtained in step A3, the catalyst, 2,2-dimethylolpropionic acid monomer, and solvent to a polymerization container, heat to 80°C under an inert nitrogen atmosphere and stir for 2 hours. After the reaction is completed, cool, centrifuge, wash and dry to obtain modified boron nitride nanosheets.

[0039] Note: In the above operations, the silane coupling agent used was KH-550, the catalyst was CuBr / bipyridine ligand, and the solvent was N-methylpyrrolidone. In step A4, the ratio of the boron nitride nanosheets with initiation sites obtained in step A3 to the catalyst, 2,2-dimethylolpropionic acid monomer, and solvent was 1 g: 20 g: 0.2 g: 200 mL. The boron nitride nanosheets used in this preparation example are the same as those in Example 1.

[0040] Preparation Example 7 A modified boron nitride nanosheet differs from preparation example 6 in that, in step A4, the ratio of the boron nitride nanosheet with initiation sites obtained in step A3 to the catalyst, 2,2-dimethylolpropionic acid monomer, and solvent is 1g:10g:0.1g:100mL.

[0041] Preparation Example 8 A modified boron nitride nanosheet differs from preparation example 6 in that, in step A4, the ratio of the boron nitride nanosheet with initiation sites obtained in step A3 to the catalyst, 2,2-dimethylolpropionic acid monomer, and solvent is 1 g:30 g:0.3 g:300 mL.

[0042] Example Example 1

[0043] A low-stress, thermally conductive epoxy resin structural adhesive for ultra-low temperature environments comprises component A and component B, which are stored separately and mixed at a weight ratio of 100:25 before use. The raw materials and corresponding weight parts of component A are shown in Table 1, and the raw materials and corresponding weight parts of component B are shown in Table 2. The adhesive is prepared through the following steps: (1) Prepare raw materials containing waste component A and component B according to the formula; (2) After the epoxy resin, solvent and wetting and dispersing agent are mixed evenly, thermally conductive filler is added during the stirring process, and high-speed dispersion is carried out at 2500 rpm for 40 min. Then stress buffer microspheres are added under low-speed stirring at 200 rpm. After mixing evenly for 25 min, vacuum degassing treatment is carried out to obtain component A; after mixing the curing agent and accelerator, vacuum degassing treatment is carried out to obtain component B.

[0044] Note: In the above operations, the solvent is acetone; the composition of the epoxy resin and the corresponding weight parts are shown in Table 3; the thermally conductive filler is composed of boron nitride nanosheets and spherical alumina in a weight ratio of 1:6.5; the transverse dimension of the boron nitride nanosheets is 15 μm and the thickness is 80 nm; the particle size D50 of the spherical alumina is 7.5 μm; the stress buffer microspheres were obtained from Preparation Example 1. Example 2-3

[0045] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the raw materials and corresponding weight parts of component A are shown in Table 1.

[0046] Table 1. Composition of Component A in Examples 1-3 and corresponding parts by weight (parts / kg) Examples 4-5

[0047] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the raw materials and corresponding weight parts of component B are shown in Table 2.

[0048] Table 2. Composition of Component B and corresponding weight parts (parts / kg) in Examples 1, 4-5 Examples 6-7

[0049] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the raw materials and corresponding weight parts of the epoxy resin are shown in Table 3.

[0050] Table 3. Composition of epoxy resin raw materials and corresponding parts by weight (parts / kg) in Examples 1 and 6-7 Example 8

[0051] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that it includes components A and B, which are stored separately and mixed at a weight ratio of 100:22 when used. Example 9

[0052] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that it includes components A and B, which are stored separately and mixed at a weight ratio of 100:28 when used. Example 10

[0053] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the thermally conductive filler is composed of boron nitride nanosheets and spherical alumina in a weight ratio of 1:3. Example 11

[0054] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the thermally conductive filler is composed of boron nitride nanosheets and spherical alumina in a weight ratio of 1:10. Example 12

[0055] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from that in Example 1 in that the boron nitride nanosheets have a lateral dimension of 10 μm and a thickness of 60 nm; and the spherical alumina particles have a diameter D50 of 5 μm. Example 13

[0056] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from that in Example 1 in that the boron nitride nanosheets have a lateral dimension of 20 μm and a thickness of 100 nm; and the spherical alumina particles have a particle size D50 of 10 μm. Example 14

[0057] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the stress-buffered microspheres are obtained from Preparation Example 2. Example 15

[0058] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the stress-buffered microspheres are obtained from Preparation Example 3. Example 16

[0059] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the stress-buffered microspheres are obtained from Preparation Example 4. Example 17

[0060] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that the stress-buffered microspheres are obtained from Preparation Example 5. Example 18

[0061] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from that in Example 1 in that the boron nitride nanosheets are modified before use, and the modified boron nitride nanosheets obtained by the modification treatment are obtained from Preparation Example 6. Example 19

[0062] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from that in Example 1 in that the boron nitride nanosheets are modified before use, and the modified boron nitride nanosheets obtained by the modification treatment are obtained from Preparation Example 7. Example 20

[0063] The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from that in Example 1 in that the boron nitride nanosheets are modified before use, and the modified boron nitride nanosheets obtained by the modification treatment are obtained from Preparation Example 8.

[0064] Comparative Example Comparative Example 1 The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that stress-buffering microspheres are not used in the raw materials of component A.

[0065] Comparative Example 2 The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that no thermally conductive filler is used in the raw materials of component A.

[0066] Comparative Example 3 The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that stress-buffering microspheres and thermally conductive fillers are not used in the raw materials of component A.

[0067] Comparative Example 4 The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that boron nitride nanosheets are not used in the thermally conductive filler.

[0068] Comparative Example 5 The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments differs from Example 1 in that spherical alumina is not used in the thermally conductive filler.

[0069] Performance testing Test samples: The low-stress thermally conductive epoxy resin structural adhesives for ultra-low temperature environments obtained in Examples 1-20 were selected as test samples 1-20, and the low-stress thermally conductive epoxy resin structural adhesives for ultra-low temperature environments obtained in Comparative Examples 1-5 were selected as control samples 1-5.

[0070] Test methods: (1) Thermal mismatch stress test at ultra-low temperature. Prepare a thermomechanical analyzer (TMA) equipped with a liquid nitrogen cooling system that can achieve precise temperature control from -196℃ to room temperature, as well as a precision balance (accuracy 0.1mg), a high-precision digital caliper (accuracy 0.001mm), and a constant temperature and humidity chamber (for sample condition adjustment); the laboratory environment is (23±2)℃ and relative humidity (50±10)%.

[0071] A cylindrical specimen with a diameter of 6 mm and a height of 5 mm was prepared using a low-stress, thermally conductive epoxy resin structural adhesive for ultra-low temperature environments. The specimen was to be fully cured under the specified curing conditions of "120℃ / 2h + 150℃ / 4h". Before testing, the cured specimen was placed in an environment of (23±2)℃ and (50±10)%RH for 24 hours.

[0072] Replace the sample holder with a planar quartz probe suitable for compression mode; precisely place the cylindrical sample in the center of the sample stage to ensure good contact with the upper and lower quartz plates; apply a constant initial force of 0.5 N to maintain contact without causing plastic deformation of the sample at low temperatures.

[0073] First, equilibrate at 30℃ for 5 minutes, then decrease the temperature from 30℃ to -196℃ at a rate of 5℃ / min, and hold at -196℃ for 10 minutes to ensure complete temperature uniformity. Next, increase the temperature from -196℃ to 30℃ at a rate of 5℃ / min. Continuously record the temperature (T) and sample height change (ΔL), with a sampling frequency of no less than 1 point / second.

[0074] The formula for calculating CTE, specifically the formula for calculating the linear thermal expansion coefficient α, is: α = dL / dT·L0 -1 L0 represents the initial height of the sample at the initial temperature (30℃), and dL / dT represents the slope of the linear fit of the length change-temperature curve within the selected temperature range. The CTE value is calculated for a temperature range of -196℃ to 100℃.

[0075] (2) Thermal conductivity test: The thermal conductivity of low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environment at -196℃ was tested by steady-state thermal resistance method (ASTM D5470).

[0076] (3) Strength retention rate test after thermal cycling: The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environment was tested according to GB / T 7124 (Determination of tensile shear strength of adhesives), and the obtained value was recorded as the initial shear strength.

[0077] A sample of low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments was placed in a high-low temperature alternating test chamber. The temperature was lowered from 25℃ to -196℃ (at a specified rate of 5℃ / min), held at -196℃ for 30 min, and then raised from -196℃ to 25℃ (at a specified rate of 10℃ / min), held at 25℃ for 15 min. This was recorded as one cycle. After completing 100 thermal cycles, the above test was performed in the same way, and the obtained value was recorded as the shear strength after the cycle.

[0078] Finally, calculate the strength retention rate (%) after cycling: shear strength after cycling / initial shear strength × 100%.

[0079] After performing the above tests on test samples 1-20 and control samples 1-5, the test results are recorded in Table 4.

[0080] Table 4. Test results of test samples 1-20 and control samples 1-5 As can be seen from Examples 1-17 and Comparative Examples 1-3, and Table 4, the specially prepared stress-buffering microspheres used in this application, combined with thermally conductive fillers composed of boron nitride nanosheets and spherical alumina, enable the low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments to not only be suitable for ultra-low temperature environments, but also simultaneously achieve a balance of low internal stress, high toughness retention, and high thermal conductivity. The test results obtained from the above tests all show significant improvement. Meanwhile, if either the stress-buffering microspheres or the thermally conductive filler is used alone, it is found that their improvement in reducing the CTE(α) value and improving the strength retention after cycling is limited, and the sum of the improvement effects brought by using either one alone is far less than that of the combination. In terms of thermal conductivity at -196℃, the thermally conductive filler plays a decisive role, but the lack of stress-buffering microspheres still causes a decrease in thermal conductivity, indicating that the use of stress-buffering microspheres has an improving effect on the distribution of thermally conductive fillers. Therefore, it can be seen that the stress-buffering microspheres and thermally conductive fillers can bring about a synergistic improvement effect of 1+1>2. Combined with Comparative Examples 4-5 and Table 4, it can be seen that the combination of boron nitride nanosheets and spherical alumina as thermally conductive fillers is far more effective than using only one of them. This indicates that the excellent performance of the framework structure formed between boron nitride nanosheets and spherical alumina, as well as the excellent synergistic effect between them and the stress-buffering microspheres, are essential.

[0081] As can be seen from Examples 1 and 18-20 and Table 4, the modification treatment of boron nitride nanosheets before use in this application can further improve and enhance the low internal stress, high toughness retention rate and high thermal conductivity at ultra-low temperatures, thereby obtaining a low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments with better application quality.

[0082] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A low-stress, thermally conductive epoxy resin structural adhesive for ultra-low temperature environments, characterized in that, This includes components A and B, which are stored separately and mixed by weight at a ratio of 100:(22-28) when used; Component A comprises the following raw materials in parts by weight: 100-120 parts epoxy resin; Solvent 10-20 parts; 100-200 parts of thermally conductive filler; 10-25 parts of stress-buffering microspheres; 0.5-3 parts of wetting and dispersing agent; The stress-buffering microspheres were prepared by the following steps: S1. Take elastic polymer microspheres and disperse them in a mixed solvent composed of ethanol, water and ammonia. Then add silane coupling agent and stir to react. After centrifugation, washing and drying, pretreated elastic polymer microspheres are obtained. S2. Disperse the elastic polymer microspheres obtained in step S1 in a mixed solvent of ethanol and water, add ammonia and stir, then add an ethanol solution of tetraethyl orthosilicate dropwise while stirring, and continue stirring after the addition is complete to obtain a microsphere dispersion. S3. Add silane coupling agent to the microsphere dispersion obtained in step S3, and carry out a heating reflux reaction under nitrogen protection. After the reaction is completed, cool, centrifuge, wash and dry to obtain stress buffer microspheres. The thermally conductive filler is composed of boron nitride nanosheets and spherical alumina. Component B comprises the following raw materials in parts by weight: 20-30 parts of curing agent; Accelerator 0.1-0.5 parts.

2. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: The stress-buffered microspheres have a particle size of 10-20 μm.

3. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: In the preparation of the stress-buffered microspheres, the concentration of ammonia after addition is 0.5-1.0 mol / L; after the ethanol solution of tetraethyl orthosilicate is added dropwise, the concentration of tetraethyl orthosilicate is 0.3-0.5 mol / L.

4. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: The thermally conductive filler is composed of boron nitride nanosheets and spherical alumina in a weight ratio of 1:(3-10).

5. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: The boron nitride nanosheets have a lateral dimension of 10-20 μm and a thickness of 60-100 nm; the spherical alumina particles have a diameter D50 of 5-10 μm.

6. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: The boron nitride nanosheets are modified before use. The modification steps are as follows: A1. Boron nitride nanosheets were subjected to strong oxidation treatment to obtain activated boron nitride nanosheets. A2. The activated boron nitride nanosheets obtained in step A1 are dispersed in a mixed solvent composed of ethanol, water and ammonia, and then a silane coupling agent is added and stirred. After centrifugation, washing and drying, pretreated boron nitride nanosheets are obtained. A3. Disperse the pretreated boron nitride nanosheets obtained in step A2 in dichloromethane, add the polymerization initiator dropwise under an inert gas, continue stirring the reaction after the addition is complete, and after the reaction is completed, centrifuge, wash and dry to obtain boron nitride nanosheets with initiation sites. A4. Add the boron nitride nanosheets with initiation sites obtained in step A3, the catalyst, 2,2-dimethylolpropionic acid monomer, and solvent to a polymerization container, heat and stir the reaction under an inert atmosphere, and after the reaction is completed, cool, centrifuge, wash and dry to obtain modified boron nitride nanosheets.

7. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: In step A4, the ratio of boron nitride nanosheets with initiation sites obtained in step A3 to catalyst, 2,2-dimethylolpropionic acid monomer, and solvent is 1g:(10-30)g:(0.1-0.3)g:(100-300)mL.

8. The low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: The epoxy resin is composed of the following components in parts by weight: 60-80 parts of alicyclic epoxy resin; 20-40 parts of flexible epoxy resin.

9. The preparation method of the low-stress thermally conductive epoxy resin structural adhesive for ultra-low temperature environments according to claim 1, characterized in that: Includes the following steps: (1) Prepare raw materials containing waste component A and component B according to the formula; (2) After the epoxy resin, solvent and wetting and dispersing agent are mixed evenly, thermally conductive filler is added during the stirring process and dispersed at high speed. Then, stress buffer microspheres are added under low speed stirring. After mixing evenly, vacuum degassing treatment is performed to obtain component A. After the curing agent and accelerator are mixed, vacuum degassing treatment is performed to obtain component B.

Images