A light-heat dual-curing structural adhesive composition resistant to extreme environments

Abstract

The application relates to the technical field of adhesives, and discloses a light-heat dual-curing structural adhesive composition resistant to extreme environments, which solves the problem that, in the prior art, due to the difference in the curing time sequence of light first and heat later, the light-cured network formed in advance physically limits the later heat-curing reaction, thereby causing incomplete curing and stress concentration, and comprises a latent heat curing agent coated in a photolysis microcapsule, and a latent heat releasing agent with a decomposition temperature lower than the activation temperature of the curing agent is also blended in the microcapsule wall material. The application has a unified curing mechanism and an internally self-triggered synchronous curing starting point, thereby avoiding the problem of physical limitation and stress concentration of the light-cured network formed in advance on the later heat-curing reaction in the traditional light-heat dual-curing adhesive, and improving the long-term reliability of the adhesive.

Description

Technical Field

[0001] This invention relates to a photothermal dual-curing adhesive composition resistant to extreme environments, belonging to the field of adhesive technology. Background Technology

[0002] Currently, to balance production efficiency and final bonding performance, photothermal dual-curing adhesives have become a common technical choice. They achieve complementary advantages through rapid positioning in the photocuring stage and deep bonding in the thermal curing stage. The technical approach commonly used in the industry is to construct a curing relay system. First, ultraviolet light is used to initiate a chain polymerization reaction on the surface of the material, forming a skeleton network with initial strength to fix the workpiece in a short time. Then, overall heating is used to initiate a gradual polymerization reaction of another thermal curing component in the system to complete the overall curing, including shaded areas that cannot be reached by light.

[0003] However, when applied to scenarios with higher requirements for long-term service reliability, the inherent physical constraints in its curing sequence lead to non-uniformity in the microstructure and stress state of the final cured product. The chain polymerization reaction mechanism of the skeleton network formed by photocuring determines that it will rapidly increase the glass transition temperature of the system, causing the adhesive layer to rapidly enter a high-viscosity gel state or even a glassy state locally. While completing the positioning, this pre-cured network also constitutes a physical barrier to the subsequent thermosetting molecular chain segments that require sufficient diffusion and migration to react, directly leading to the following technical problems: 1. Incomplete thermosetting reaction and crosslinking 1. Low density restricts the molecular diffusion of thermosetting components in the pre-formed rigid network, making it difficult for the chemical reaction in the shaded area and deep layers to reach the theoretically complete level. The final cured product has structural defects of uneven cross-linking network at the microscopic level. 2. The generation and curing of internal stress: Thermosetting reaction is usually accompanied by volume shrinkage, but in a space fixed by a photocured skeleton, this shrinkage cannot proceed freely, thus generating and curing internal stress at the molecular level. These internal stresses, as stress concentration points in the structure, will become the root cause of inducing microcracks and leading to adhesive failure when subjected to long-term fatigue or temperature cycling.

[0004] To address this issue, the industry has attempted to improve compatibility by adjusting the chemical structures of the photocuring and thermal curing systems. However, these improvements have consistently failed to overcome the fundamental constraint of the curing sequence of photocuring followed by thermal curing. As long as a pre-formed curing network exists, its physical suppression of subsequent reactions is unavoidable. Meanwhile, to address the moisture absorption and adhesion issues of low-temperature curing systems during open-air storage, some technical solutions exist, such as the low-temperature heating-curing structural adhesive composition disclosed in Chinese invention patent CN110461981B. This composition enhances the environmental stability and final corrosion resistance of the adhesive by adding hygroscopic agents such as calcium oxide to an epoxy resin containing a microencapsulated curing agent. However, these technical solutions primarily focus on counteracting the effects of external environments such as moisture through formulation improvements, without addressing the fundamental problem of physical limitations and internal stress concentration caused by the difference in curing sequence (photocuring followed by thermal curing) in photocuring dual-curing systems. Therefore, the technical problem to be solved by this invention is to provide a method that enables rapid positioning while ensuring that the entire adhesive layer, including the light-illuminated area and the shadow-illuminated area, can be cured within a synchronous time window through a uniform chemical mechanism, so as to structurally avoid physical limitations and internal stress caused by differences in curing time. Summary of the Invention

[0005] This invention provides a photothermal dual-curing structural adhesive composition resistant to extreme environments. Its main purpose is to solve the problem in existing photothermal dual-curing technologies where the difference in curing sequence (photocuring first, then heating) leads to the physical limitation of the later thermal curing reaction by the photocuring network formed earlier, resulting in incomplete curing and internal stress concentration.

[0006] To achieve the above objectives, the present invention provides a photothermal dual-curing structural adhesive composition resistant to extreme environments, the composition comprising: a thermosetting resin;

[0007] A latent thermosetting agent having an activation temperature T_a;

[0008] A photolytic microcapsule is used to encapsulate a latent thermosetting agent to achieve physical isolation from the thermosetting resin. The wall material of the photolytic microcapsule has a thermal response point, and the wall material of the photolytic microcapsule breaks down due to chain rupture when irradiated with light of a specific wavelength.

[0009] A latent exothermic agent is incorporated into the wall material of photolytic microcapsules. The latent exothermic agent has a decomposition temperature T_d, which is lower than the activation temperature T_a. The action of light of a specific wavelength is defined as an unlocking mechanism that disrupts the physical isolation of the photolytic microcapsules. The decomposition of the latent exothermic agent is configured as a triggering mechanism that generates a self-propagating heat wave at the decomposition temperature T_d. The self-propagating heat wave ensures that even in non-light-illuminated areas, the photolytic microcapsules rupture, and the latent thermosetting agent is simultaneously activated at the activation temperature T_a.

[0010] Preferably, the thermosetting resin is epoxy resin; the latent thermosetting agent is an amine curing agent or an acid anhydride curing agent.

[0011] Preferably, the wall material of the photolytic microcapsule is composed of a polymer containing o-nitrobenzyl ester groups or coumarin groups.

[0012] Preferably, the latent exothermic agent is an azo compound.

[0013] Preferably, the following energy triggering relationship is satisfied between the activation of the latent exothermic agent and the latent thermosetting agent and the overall heat capacity of the composition, based on a total weight of 100 parts: η=(m_D·ΔH_D) / (m_total·C_p·(T_a-T_d))>1, where η is the energy triggering factor, m_D is the weight of the latent exothermic agent, ΔH_D is the decomposition enthalpy of the latent exothermic agent, m_total is the total weight of the composition, C_p is the average specific heat capacity of the composition, T_a is the activation temperature of the latent thermosetting agent, and T_d is the decomposition temperature of the latent exothermic agent.

[0014] Preferably, the latent thermosetting agent itself is a synergistic curing system, which includes a primary curing agent B1 and a dormant synergist B2; wherein, the curing reactivity of the dormant synergist B2 is catalyzed and enhanced by the hydroxyl intermediate generated by the reaction of the thermosetting resin and the primary curing agent B1, so as to construct an intrinsic positive feedback loop of product catalysis-accelerated reaction-heat replenishment after the curing reaction is initiated, so as to resist the heat dissipation during the curing process.

[0015] Preferably, the wall material of the photodegradable microcapsule is composed of a block copolymer, which comprises at least one photosensitive biodegradable rigid block and at least one flexible elastomer block.

[0016] Preferably, the block copolymer further includes functional groups that can react with the thermosetting resin, so as to achieve interfacial chemical bonding and anchoring between the structural units of the block copolymer and the matrix formed by the thermosetting resin after curing.

[0017] Preferably, the photosensitive biodegradable rigid block, after degradation, forms oligomer fragments that migrate as plasticizers to the flexible elastomer block region and lower the glass transition temperature of the flexible elastomer block, so as to ensure that the adhesive layer formed after curing retains its toughening properties at low temperatures.

[0018] Preferably, the oligomer fragments formed after degradation contain functional groups that can react with thermosetting resins to chemically bond the oligomer fragments to the final cured network to inhibit their migration.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1. A novel curing process control method was established by encapsulating a latent thermosetting agent within a photodegradable microcapsule sensitive to a specific wavelength of light. In this method, the effect of the specific wavelength of light is limited to a physical, non-contact unlocking tool used to disrupt the physical isolation of the microcapsule in a selected area, releasing the curing agent for initial positioning. Subsequent overall heating of the entire component utilizes the thermal instability or increased permeability of the microcapsule wall material itself to release the curing agent in the shaded area. Since both the illuminated and non-illuminated areas ultimately rely on the same thermosetting reaction mechanism, the entire adhesive layer can undergo stress distribution and relaxation within a unified time window during curing shrinkage. This avoids the physical limitations and stress confinement problems of the previously formed photocuring network on the subsequent thermosetting reaction in traditional photothermal dual-curing methods.

[0021] 2. By introducing a latent exothermic agent with a decomposition temperature lower than the activation temperature of the latent thermosetting agent into the wall material of the photolytic microcapsules, an internal energy relay mechanism is introduced to initiate the curing process. When the adhesive system faces uneven heat transfer conditions such as large-sized or high thermal conductivity substrates, as long as the temperature at any point in the adhesive layer first reaches the decomposition temperature of the exothermic agent, the instantaneously released chemical energy will trigger the rupture of surrounding microcapsules and activate the main curing reaction. The main curing reaction itself is also exothermic, thus forming a self-propagating heat wave from the inside out. This changes the curing initiation process from relying on the uniform conduction of external heat to being dominated by the spontaneous propagation of internal chemical reactions, so that the synchronicity of curing initiation is no longer subject to the physical properties and size of the substrate.

[0022] 3. When the latent thermosetting agent itself consists of a main curing agent and a dormant synergist, and the reactivity of the dormant synergist can be catalyzed and enhanced by the intermediate products generated by the reaction between the main curing agent and the resin, the entire curing system has the ability to sustain and amplify the process. After the exothermic agent completes the synchronous ignition, even under extreme heat dissipation conditions, the intermediate products generated by the main curing reaction will act as a catalyst to awaken the synergist that was originally in a dormant state, opening up a second efficient curing path. The new heat generated by its violent reaction will, in turn, maintain and accelerate the reaction process of the entire system, thus constructing an intrinsic positive feedback loop of product catalysis-accelerated reaction-replenished heat, avoiding the problem of curing stall or insufficient crosslinking density that may be caused by local temperature drop.

[0023] 4. When the wall material of the photodegradable microcapsule is composed of a block copolymer containing photosensitive biodegradable rigid blocks and flexible elastomer blocks, after fulfilling its initial function as a physical barrier for the curing agent, the structural units dispersed in the resin matrix after curing also play the role of an in-situ toughening phase. Furthermore, if the rigid blocks are designed to be oligomer fragments formed after degradation, which can migrate as plasticizers to the regions of the covalently linked flexible elastomer blocks and reduce the glass transition temperature of the latter, then the system actively improves the physical properties of the other component at low temperatures by using the sacrificial decomposition products of one component during the curing process. This results in the final adhesive layer not only achieving toughness at room temperature but also avoiding the problem of its toughened structure failing due to material glass transition in extreme environments such as deep space and low temperatures. Attached Figure Description

[0024] Figure 1 This is a flow chart of the curing process based on internal energy relay and positive feedback closed loop of the present invention;

[0025] Figure 2 This is a comparison chart of the curing rates of the synergistic curing system of the present invention at different temperatures. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] This invention provides a photothermal dual-curing structural adhesive composition resistant to extreme environments. It comprises a curing system consisting of a thermosetting resin, a latent thermosetting agent encapsulated in photolytic microcapsules, and a latent exothermic agent blended in the microcapsule wall material. In this system, the action of specific wavelength light is defined as a physical unlocking step, while the decomposition of the latent exothermic agent is set as the internal triggering step for the synchronous curing reaction. Ultimately, through a unified thermosetting chemical mechanism, the synchronous curing of the entire adhesive layer is achieved. The thermosetting resin is the main material constituting the three-dimensional network structure of the cured adhesive layer, providing the system with basic mechanical strength, heat resistance, and chemical stability. In a specific engineering practice, the thermosetting resin can be selected from bisphenol A type epoxy resin, phenolic epoxy resin, or alicyclic epoxy resin. Due to its high adhesive strength and low curing shrinkage after curing, it is widely used in structural bonding applications. To address the objective requirement of maintaining the fluidity of the adhesive composition for a long time during the application and component positioning processes, this solution includes a latent thermosetting agent. Here, "latent" means that the curing agent has low reactivity with thermosetting resins at room temperature or slightly higher operating temperatures, and the viscosity of the system remains essentially unchanged. Only when the temperature rises to the preset activation temperature T_a is its reactivity activated, initiating the crosslinking and curing reaction. The latent thermosetting agent can be a modified amine curing agent, an imidazole derivative, or an anhydride curing agent. For example, dicyandiamide or its modified derivatives with an activation temperature T_a in the range of 120°C to 150°C can be selected to have a pot life of not less than 8 hours under standard operating conditions.

[0028] To achieve controlled release of latent thermosetting agents in both space and time, this solution employs a photolytic microcapsule for physical isolation and encapsulation. The wall material of this photolytic microcapsule is sensitive to light of a specific wavelength. When irradiated with ultraviolet light, for example, at a center wavelength of 365 nm, the photosensitive groups on the polymer chains of the wall material break chemical bonds, causing the microcapsule structure to rupture and release the encapsulated latent thermosetting agent at a specific point in the irradiated area. This process serves only as a physical unlocking step, used to increase the viscosity of the adhesive locally at room temperature, thus achieving initial positioning of the workpiece. To achieve this photolytic function, the wall material of the photolytic microcapsule can be made of materials containing o-nitrobenzyl ester groups or fragrances. The polymer is composed of genistein derivative groups, which can undergo photolysis under light excitation at a specific wavelength. Furthermore, to ensure that the latent thermosetting agent can be released simultaneously in the subsequent heating and curing stage in the shaded areas where light cannot reach, the wall material of the photolytic microcapsules also has a thermal response point. By adjusting the molecular weight of the wall material polymer and the ratio of comonomers, its glass transition temperature or softening point is set to be slightly lower than or equal to the activation temperature T_a of the curing agent. When the whole is heated, the microcapsules in the shaded areas soften or become more permeable due to heat, and can also release the curing agent. Thus, the curing reaction is started simultaneously when T_a is reached in both the illuminated and shaded areas.

[0029] When bonding large-size or high-thermal-conductivity substrates, external heat is difficult to transfer evenly and rapidly to the adhesive layer, resulting in different temperature rise rates in different areas. This poses a technical challenge to synchronous release triggered by external heat sources. To address this issue, this solution incorporates a latent exothermic agent into the wall material of the photolytic microcapsules. This latent exothermic agent has a decomposition temperature T_d, which is set below the activation temperature T_a of the latent thermosetting agent. The latent exothermic agent can be an azo compound, such as azobisisobutyronitrile or azobisisoheptanenitrile, which releases heat upon reaching its decomposition temperature. Decomposition reaction; thus, during the overall heating process, as long as the temperature at any point in the adhesive layer reaches T_d first, the exothermic agent at that point will decompose, and the heat energy released will form a high-temperature point on the spot. The heat generated by this high-temperature point will cause the local temperature within the surrounding micrometer range to exceed T_a, thereby not only detonating the surrounding microcapsules, but also directly activating the newly released curing agent. Since the main curing reaction itself is also exothermic, a heat wave that propagates from the inside out is formed, changing the curing initiation process from relying on external heat conduction to being dominated by the spontaneous propagation of internal chemical reactions, so as to achieve the synchronicity of curing initiation under non-uniform heating conditions.

[0030] To ensure the reliable triggering of the aforementioned internal energy relay step, the amount of latent exothermic agent added is configured to satisfy an energy triggering relationship with the enthalpy of decomposition and the heat capacity of the entire composition. This relationship can be quantified by an energy triggering factor η, calculated as η = (m_D·ΔH_D) / (m_total·C_p·(T_a-T_d))>1; where m_D is the weight fraction of the latent exothermic agent, ΔH_D is the enthalpy of decomposition per unit weight of the latent exothermic agent, m_total is the total weight fraction of the composition, C_p is the average specific heat capacity of the composition, T_a is the activation temperature of the latent thermosetting agent, and T_d is the decomposition temperature of the latent exothermic agent. The significance of this relationship is to ensure that the heat released during the decomposition of the exothermic agent... The total heat generated by the exothermic agent is greater than the energy required to heat the entire composition system from the decomposition temperature T_d of the exothermic agent to the activation temperature T_a of the curing agent. When η>1, the heat generated by the exothermic agent is sufficient to drive the subsequent curing reaction. In one embodiment, if an azo compound with T_d=110°C is selected as the exothermic agent and a modified amine with T_a=120°C is selected as the curing agent, the average specific heat capacity C_p of the composition is about 1.5 J / (g·K), and the decomposition enthalpy ΔH_D of the exothermic agent is about 200 J / g, then in order to satisfy the condition η>1, based on a total weight of 100 parts of the composition, the weight parts m_D of the exothermic agent must satisfy m_D>(100·1.5·(120-110)) / 200, that is, m_D>7.5 parts.

[0031] Under extreme heat dissipation conditions, such as when bonding thick metal parts, the initial heat wave generated by the exothermic agent may be absorbed by the substrate, causing the local temperature to drop after the reaction starts, resulting in incomplete curing. To establish a feedback mechanism that can resist heat dissipation during the process, this solution further configures the latent thermosetting agent into a synergistic curing system, which includes a primary curing agent B1 and a dormant synergist B2. The dormant synergist B2 itself has low reactivity at the activation temperature T_a, but its activity can be catalyzed and enhanced by the hydroxyl intermediates generated during the reaction of the primary curing agent B1 and the thermosetting resin. When the curing reaction is initiated by the aforementioned heat wave, the primary curing agent B1 reacts first to generate hydroxyl groups. These hydroxyl products then act as in-situ catalysts, activating the synergist B2 and opening up a second curing pathway. The reaction of the synergist B2 replenishes the heat, which in turn maintains and accelerates the reaction process of the entire system, thereby constructing a product catalytic-accelerated reaction. The process involves supplementing heat to ensure the curing reaction can be self-sustaining and proceeding under heat dissipation conditions, resulting in a uniform cross-linked network. Specifically, to construct the intrinsic positive feedback loop of product catalysis-accelerated reaction-heat supplementation, a specific, non-limiting component composition of the synergistic curing system B is as follows: the main curing agent B1, which serves as the main reactant, is a modified aliphatic amine that reacts with the epoxy groups of the thermosetting resin at the activation temperature T_a to rapidly generate secondary hydroxyl intermediates; the dormant synergist B2 is 2-ethyl-4-methylimidazolium. This imidazolium compound has limited reactivity with epoxy groups alone at T_a, but the catalytic activity of the tertiary nitrogen atom on its imidazolium ring can be enhanced by the hydroxyl intermediate generated in situ by the main curing agent B1 through hydrogen bonding. This effect significantly reduces the energy barrier of its ring-opening polymerization, causing its reaction rate to increase sharply. Thus, after curing starts, the curing pathway is efficiently supplemented to replenish heat to resist heat dissipation.

[0032] To enhance the toughness of the cured adhesive layer to resist impact and ensure its effectiveness at low temperatures, the wall material of the photodegradable microcapsules can be composed of a block copolymer. This block copolymer comprises at least one photosensitive, biodegradable rigid block as the outer shell and at least one flexible, elastomer block as the core. After curing, these ruptured microcapsule walls are dispersed as microphases within the resin matrix, where the flexible elastomer block regions absorb energy, thus achieving a toughening effect. To address the issue of elastomers losing toughness due to glass transition at low temperatures, the rigid blocks are designed so that, after photodegradation, the resulting oligomer fragments can act as plasticizers, migrating to the flexible elastomer block regions and lowering the glass transition temperature of the flexible blocks, thereby maintaining the toughening properties of the adhesive layer at low temperatures. To inhibit the migration of these plasticizer oligomer fragments during long-term service, both the degraded oligomer fragments and the block copolymer contain functional groups capable of reacting with thermosetting resins during the curing process. The structural units of the block copolymer and the oligomer fragments acting as plasticizers are chemically bonded to the final cured network. To obtain an adhesive layer that retains its toughening properties even at low temperatures, the wall material of the photodegradable microcapsules is composed of a block copolymer synthesized by atom transfer radical polymerization (ATRP). A specific structure of this copolymer is as follows: the photosensitive, biodegradable rigid block is formed by random copolymerization of o-nitrobenzyl acrylate and glycidyl methacrylate, wherein the o-nitrobenzyl acrylate group serves as the photosensitive unit, and the epoxy functional group contained in the glycidyl methacrylate group is used to achieve interfacial chemical bonding and anchoring between the structural units of the block copolymer and the oligomer fragments formed after photodegradation and the matrix formed by the thermosetting resin after curing; the flexible elastomer block is polymerized from butyl acrylate, whose low glass transition temperature is used to absorb impact energy, and the physical properties of this flexible block can be modulated by the rigid block oligomer fragments, which act as plasticizers and migrate to its region after photodegradation.

[0033] In a specific embodiment of a photothermal dual-curing structural adhesive composition resistant to extreme environments, in order to achieve stable reproduction of the technical effect and define the technical contribution of each core component in the system, the weight ratio of each component is limited as follows: based on a total weight of 100 parts of the composition, the weight of the thermosetting resin is 40-70 parts; the weight of the latent thermosetting agent is 10-30 parts; the weight of the polymer constituting the photolytic microcapsule wall material is 5-20 parts; and the weight of the latent exothermic agent blended in the wall material is 8-20 parts. The determination of the above ratio range is based on the following technical fact: when the weight of the thermosetting resin is less than 40 parts, its content as the main material of the network structure after curing is insufficient, resulting in low crosslinking density of the cured product, and the cohesive strength and mechanical modulus of the adhesive layer do not meet the structural bonding requirements.When the weight percentage of the latent thermosetting agent exceeds 70 parts, the excess resin in the system leads to insufficient relative content of the curing agent and other functional components, incomplete curing reaction, and a decrease in the heat resistance and chemical resistance of the cured product. Simultaneously, the excessively high viscosity of the system affects the application process. When the weight percentage of the latent thermosetting agent is less than 10 parts, the number of functional groups of the curing agent relative to the reactive groups of resin A is insufficient, resulting in an imbalance in the stoichiometry. This prevents the curing reaction from reaching the predetermined crosslinking density, severely affecting the final bond strength. When the weight percentage exceeds 30 parts, the excess curing agent results in unreacted small molecules remaining in the curing network, acting as plasticizers and causing the adhesive layer to vitrify. The transition temperature decreases, high-temperature mechanical properties deteriorate, and there is a risk of small molecule migration and precipitation during long-term service. When the weight percentage of the polymer constituting the photolytic microcapsule wall material is less than 5 parts, the wall material thickness is insufficient to form structurally complete microcapsules, making them prone to physical damage during adhesive storage or stirring, causing premature release of the curing agent and significantly shortening the pot life of the composition. When its weight percentage is greater than 20 parts, excessive non-reactive wall material polymers are dispersed as impurities in the cured matrix, reducing the crosslinking density of the matrix itself and forming stress concentration points at the interface, thereby reducing the limiting mechanical properties of the adhesive layer. When the weight percentage of the latent exothermic agent is less than 8 parts, the energy of the system... With a trigger factor η value close to 1, under conditions of rapid heat dissipation such as high thermal conductivity substrates, the heat generated by its decomposition is insufficient to compensate for heat loss and drive the curing reaction to achieve self-propagation throughout the adhesive layer, making it impossible to guarantee the synchronicity of curing initiation. When its weight percentage exceeds 20 parts, the decomposition process of the exothermic agent is too violent, and the large amount of gas generated in a short time, such as nitrogen gas generated by the decomposition of azo compounds, cannot escape in time, forming voids and defects inside the adhesive layer, destroying the compactness of the adhesive interface, and causing a sharp decrease in adhesive strength. In the composition, there is a synergistic technical effect among the components, and the latent thermosetting agent is physically isolated by photolytic microcapsules, providing the composition with a pot life. The latent exothermic agent is blended into the wall material of microcapsule C, providing an internal heat source for the curing process. Without the physical isolation of microcapsule C, if the curing agent and the exothermic agent are directly mixed in resin A, the system will not have latent properties and cannot be stored or used. If the exothermic agent is missing and heating is only carried out by an external heat source, the temperature difference in different areas will cause asynchronous curing processes when facing the heat sink effect, resulting in internal stress. The combination of photolytic microcapsules and latent exothermic agents uses light to unlock and locate the curing reaction in space and uses internal heat waves to synchronously trigger the curing reaction in time. This spatial unlocking-time synchronization coupling mechanism cannot be achieved by a single component or a simple combination of any two components.

[0034] Example 1: In the bonding and assembly of a large aerospace main load-bearing structural component, a large aluminum alloy reinforcing rib with a thickness of 20mm needs to be bonded to the inner surface of a curved composite material skin with an area of ​​2m². This adhesive layer needs to withstand high stress loads and temperature cycles from -60°C to +150°C during service. Under such conditions, conventional photocurable and thermocurable adhesives present two technical challenges: first, the initial photocuring network restricts the reaction of the subsequent thermocuring components, generating internal stress; second, the aluminum alloy reinforcing rib, acting as a heat sink, causes different temperature rise rates in different areas of the adhesive layer during oven heating. To address the asynchrony in the curing process, an adhesive composition is used in this application. First, the adhesive composition is applied to the bonding surface of the aluminum alloy reinforcing rib. Then, the reinforcing rib is aligned and attached to a predetermined position on the inner surface of the composite skin, and uniform contact pressure is applied to ensure a tight, gapless bonding interface. A handheld 365nm ultraviolet LED light source is used to briefly irradiate the edge area of ​​the reinforcing rib. The photolytic microcapsule wall material in the irradiated area ruptures, releasing the latent thermosetting agent inside, causing the local adhesive viscosity to rise and gel within 90 seconds, initially fixing the reinforcing rib to the predetermined position on the curved skin.

[0035] Subsequently, the pre-positioned assembly was moved into a hot air circulating oven and heated at a rate of 5°C / min. When the oven temperature reached 110°C, the adhesive layer area closest to the heat source and located on the skin side was the first to reach the decomposition temperature T_d of the latent exothermic agent, which decomposed and released heat energy. This process formed an internal heat wave propagating from the skin side to the aluminum alloy reinforcing rib side. The energy of this heat wave caused the microcapsules C at the leading edge to rupture due to a sudden increase in local temperature, and activated the released latent thermosetting agent by exceeding its activation temperature T_a (120°C). This chain-triggered reaction driven by internal chemical energy caused the curing reaction of the entire adhesive layer, including the slowly heated deep part near the aluminum alloy side, to be initiated synchronously within a short time window. During the curing reaction, due to the continuous absorption of heat by the aluminum alloy substrate, the temperature of the latent thermosetting agent was increased. The temperature of the adhesive layer on the aluminum alloy side tends to drop from T_a. At this point, the hydroxyl intermediate product generated by the reaction of the main curing agent B1 and the thermosetting resin catalyzes and enhances the reactivity of the synergist B2. The curing reaction of the synergist B2 replenishes the heat absorbed by the substrate, forming a self-sustaining reaction process and offsetting the incomplete curing that may be caused by the heat sink effect. After curing, non-destructive testing of the bonded component showed that there were no voids or debonding defects at the entire bonding interface. Differential scanning calorimetry tests conducted on samples from different areas showed that the glass transition temperature was consistent throughout the adhesive layer. The component was placed in a high and low temperature shock chamber and subjected to 500 cycles between -60°C and +150°C. Microscopic observation after the test revealed no microcracks at the bonding interface or inside the adhesive layer, indicating that the bonding performance of the component meets the design requirements.

[0036] Example 2: To objectively verify the curing synchronicity and final bonding performance of the composition of the present invention under uneven heat transfer conditions, the following comparative test was conducted. The substrate used in the test was a glass fiber epoxy composite board with dimensions of 100mm x 25mm x 2mm and a 6061 aluminum alloy block with dimensions of 100mm x 25mm x 10mm, to simulate a bonding scenario with differences in thermal conductivity. During the test, a K-type thermocouple with an accuracy of ±0.5°C was used to monitor the temperature in real time at three locations near the center of the adhesive layer: the composite material side and the aluminum alloy side. The data recording frequency was 1Hz. The curing equipment was a hot air circulating oven with a programmable temperature control function and a temperature control accuracy of ±1°C.

[0037] The experiment included the sample group of this invention and two control groups. The sample group of this invention used a composite formulation. Control group A used a commercially available photothermal dual-curing epoxy adhesive, whose photocuring part was an acrylate system initiated by ultraviolet light, and the thermal curing part was a latent amine curing agent. Control group B used the same formulation as the sample group of this invention, but without the addition of a latent exothermic agent. All sample groups were prepared by uniformly coating the adhesive onto the surface of an aluminum alloy block to form a 0.5 mm thick adhesive layer, which was then bonded to the composite material plate with a 25 mm x 12.5 mm overlap. The edges of the overlap area were irradiated with 365 nm ultraviolet light for initial positioning. Finally, all test pieces were placed horizontally in an oven and heated to 130°C at a rate of 5°C / min and held for 60 min. During the curing process, differences in temperature change curves were observed among the different groups. In control groups A and B, the temperature rise rate of the monitoring point near the aluminum alloy side was lower than that of the other two monitoring points. When the oven reached 130°C, the actual temperature of this point lagged behind by more than 15°C. The sample group of this invention showed similar temperature differences in the early stage of heating, but when the temperature of the monitoring point near the composite material side reached around 110°C, the temperature rise rate of the monitoring point near the aluminum alloy side accelerated, and within 20 seconds, the temperature of the monitoring point became consistent with that of the other monitoring points and stabilized together at the curing temperature. This phenomenon indicates that the decomposition of the latent exothermic agent generated heat inside the adhesive layer, compensating for the temperature lag caused by the difference in thermal conductivity of the substrate. After curing, the lap shear strength of each sample group was tested, and the data are shown in Table 1.

[0038] Sample group name Monitoring point location Time (min) required to reach activation temperature T_a (120°C) Average shear strength after curing (MPa) Control group A Near composite material side 20.1 18.5 center of adhesive layer 21.5 Near aluminum alloy side 28.8 Control group B Near composite material side 20.2 22.4 center of adhesive layer 21.6 Near aluminum alloy side 29.5 Sample of the present invention Near composite material side 20.3 35.2 center of adhesive layer 20.7 Near aluminum alloy side 20.8

[0039] Experimental data show that control group A has lower mechanical properties due to inconsistent curing times of the photocurable and thermocurable networks and uneven heating; control group B, although using a unified thermocuring mechanism, cannot overcome the heat sink effect in the absence of a latent exothermic agent, resulting in asynchronous curing and limited performance improvement; the sample group of this invention, through the internal exothermic process of the latent exothermic agent, makes the curing initiation process unaffected by uneven external heat conduction, thus achieving a synchronous curing process and forming a uniform cross-linked network structure.

[0040] To further elucidate the key role of the internal energy relay mechanism of this invention in overcoming the heat sink effect and ensuring curing synchronization, and to verify its influence on the uniformity of the microstructure of the final cured product, the following comparative example 1 is established.

[0041] Comparative Example 1: To verify the performance of the composition of the present invention in dealing with the heat sink effect when the internal energy relay mechanism constructed in the absence of a latent exothermic agent is not included, the following comparative experiment was conducted; The adhesive composition used in this comparative example, except for the latent exothermic agent (azo compound) not added to the photolytic microcapsule wall material, the other components, including thermosetting resin (bisphenol A type epoxy resin), latent thermosetting agent (modified amine curing agent with activation temperature T_a of 120°C) encapsulated by photolytic microcapsules, and the polymer of photolytic microcapsule wall material, are strictly consistent with the sample group of the present invention in Example 2.

[0042] The experimental procedure and test conditions were exactly the same as in Example 2. A 100mm×25mm×10mm 6061 aluminum alloy block (simulating a heat sink) was overlapped and bonded to a 100mm×25mm×2mm glass fiber epoxy composite board. K-type thermocouples were used to monitor the temperature in real time at three locations: near the composite material side, the center of the adhesive layer, and near the aluminum alloy side. After the specimen was positioned by 365nm ultraviolet light irradiation at the edge of the overlap area, it was placed in a hot air circulating oven and heated to 130°C at a programmed rate of 5°C / min, and held at that temperature for 60 minutes. The test results were recorded and analyzed. Analysis: During the curing process, temperature monitoring data showed that the temperature rise rate of the monitoring point near the aluminum alloy side was significantly slower than that of the other two monitoring points. The monitoring point near the composite material side reached the curing agent activation temperature T_a (120°C) at 20.2 minutes, while the monitoring point near the aluminum alloy side did not reach this temperature until 29.5 minutes. The temperature difference between the two resulted in a curing start time difference of up to 9.3 minutes. This phenomenon indicates that, in the absence of an internal heat release mechanism, the heat sink effect of the aluminum alloy substrate significantly disrupted the synchronicity of the curing process. After curing, the performance of each sample group was tested, and the results are summarized in Table 2.

[0043] Table 2: Curing process data and performance of Comparative Example 1 under simulated heat sink conditions.

[0044] Test Project Monitoring point location Test data Technical Analysis Time required to reach activation temperature T_a (120°C) Near composite material side 20.2min - center of adhesive layer 21.6min - Near aluminum alloy side 29.5min The curing start-up times are not synchronized, with a time difference exceeding 9 minutes. Average shear strength after curing (MPa) - 22.4 Due to internal stress concentration, the strength is only 63.6% of that of the sample group of this invention. Glass transition temperature after curing (T_g) Near composite material side 154.2°C - Near aluminum alloy side 135.8°C The significantly low T_g value and broadened transition peak indicate incomplete curing.

[0045] The experimental results show that, although the same unified thermosetting mechanism as the present invention is used, in the absence of a latent exothermic agent, the adhesive system cannot overcome the heat sink effect caused by uneven thermal conductivity of the substrate through internal energy compensation. The severe asynchrony of the curing process leads to significant internal stress in the adhesive layer due to differential shrinkage, and causes incomplete curing in the near-heat sink area. Ultimately, this manifests as a significant reduction in adhesive strength and uneven microstructure of the cured product.

[0046] Example 3: This example combines Figures 1 to 2 This describes a photothermal dual-curing structural adhesive composition resistant to extreme environments, such as... Figure 1 As shown, in the initial state, the curing agent is physically isolated by photolytic microcapsules. After receiving light of a specific wavelength, it enters the photo-unlocking and positioning stage in step one. At this time, the photolytic microcapsules rupture and release the curing agent, achieving initial fixation of the workpiece. Then, it enters the overall heating triggering stage in step two, providing initial energy for internal energy relay. When the decomposition temperature of the latent exothermic agent reaches T_d, it generates a self-propagating heat wave. This process constitutes the internal energy relay mechanism of key module one. This heat wave causes all microcapsules to rupture, synchronously releasing and activating the curing agent, and then enters step three. The thermosetting resin completes the cross-linking reaction under a unified thermosetting mechanism. In this process, the reaction of the main curing agent B1 generates a hydroxyl intermediate product, which catalyzes the reaction of the synergist B2 and replenishes the heat. This process constitutes the internal positive feedback closed loop of the product catalytic acceleration reaction and heat replenishment in key module two. This closed loop ensures complete curing by resisting heat dissipation, and finally obtains a uniform, stress-free, high-performance adhesive layer.

[0047] like Figure 2 As shown, the horizontal axis represents curing time (min), and the vertical axis represents conversion rate (%). The three curves in the figure represent the curing process under different conditions: the solid line represents the conversion rate curve when the main curing agent is activated at 120°C, the dashed line represents the conversion rate curve of the synergistic system containing the main curing agent and the dormant synergist at 125°C, and the dotted line represents the conversion rate curve when the synergistic system is fully activated at 130°C, showing that the synergistic system has a faster curing rate and a higher final conversion rate at higher temperatures.

[0048] Example 4: To apply the composition of the present invention to specific engineering practice, it is necessary to establish a reproducible procedure to calibrate the thermodynamic parameters of key components and guide the preparation of photolytic microcapsules, thereby verifying the energy triggering relationship η>1. The technical content of this work is to transform the design criterion η=(m_D·ΔH_D) / (m_total·C_p·(T_a-T_d))>1 into a deterministic process supported by experimental data.

[0049] The first step of this procedure is to analyze the thermal properties of the raw materials. A differential scanning calorimeter (DSC) with a controllable heating rate ranging from 0.1°C / min to 20°C / min and a temperature measurement accuracy of no less than 0.1°C is used to perform dynamic temperature rise tests on both latent exothermic agent and latent thermosetting agent samples. The exothermic agent sample is heated at a heating rate of 10°C / min under a nitrogen atmosphere, and its DSC curve is recorded. The temperature at the intersection of the initial tangent of the exothermic peak and the baseline is defined as its decomposition temperature T_d. The peak area obtained by integrating this exothermic peak is its decomposition enthalpy ΔH_D per unit weight. The curing process is then performed under the same test conditions. The onset temperature of the exothermic curing peak of a mixture of curing agent and thermosetting resin is defined as the activation temperature T_a of the curing agent. Through this step, a set of thermodynamic parameters of the material is obtained. In this embodiment, the T_d of a certain azo compound exothermic agent is measured to be 112.5°C, and the ΔH_D is 210.3 J / g. The T_a of a certain modified amine curing agent in epoxy resin is 125.0°C. The second step of this procedure is to calculate the formulation parameters. All components except the latent exothermic agent are uniformly mixed according to a preset ratio, and the average specific heat capacity C_p of the base mixture in the temperature range from T_d to T_a is measured using DSC. Subsequently, the measured T_d... Substituting the values ​​of T_a, ΔH_D, and C_p into the energy triggering relationship, the minimum weight fraction of the latent exothermic agent required to make η>1 is calculated. In this embodiment, the C_p of the base mixture is measured to be 1.6 J / (g·K), and the theoretical minimum weight fraction of the exothermic agent required to meet the conditions is calculated to be (100·1.6·(125.0-112.5)) / 210.3, which is approximately 9.5 parts. In the formulation design, to leave a process margin, the amount of latent exothermic agent added, m_D, is set to be 10 to 12 parts. The third step of this procedure is the preparation of photolytic microcapsules. Using the solvent evaporation method, the block copolymer used as the wall material is combined with the material selected in the previous step. The latent exothermic agent is dissolved in dichloromethane to form an oil phase; the micro powder of the latent thermosetting agent selected in the previous step is dispersed in deionized water containing surfactant to form an aqueous phase; the oil phase is dropwise added to the aqueous phase under high-speed shear stirring at 6000 rpm to form a water-in-oil emulsion. This shear rate is used to control the emulsion droplet size within the target range; subsequently, dichloromethane is evaporated and removed under negative pressure, allowing the wall material and exothermic agent to co-deposit on the surface of the curing agent particles, forming core-shell structured microcapsules; the prepared microcapsule samples are observed under a scanning electron microscope to confirm their regular morphology, and thermogravimetric analysis confirms that the actual loading rate of each component is consistent with the design value.

[0050] Example 5: To scale up the production of the composition of this invention, a set of quality control and process parameter adaptation procedures needs to be established to address the performance differences of different batches of raw materials and the changes in thermal environment under different bonding conditions. This procedure aims to ensure that the curing behavior of the adhesive product in specific applications remains consistent with the design specifications. This procedure includes an incoming raw material inspection step. For each new batch of latent exothermic agent and latent thermosetting agent, samples must be taken and tested using differential scanning calorimetry to obtain the actual decomposition temperature T_d and activation temperature T_a of the batch of material. The measured values ​​are compared with the standard values ​​in the material's technical specifications. If the deviation is within the acceptable range, i.e., the change in T_d is no greater than ±2.0°C, then the batch of raw materials is deemed qualified. If it exceeds this range, the energy triggering factor η needs to be recalculated based on the new T_d and T_a values, and the amount of latent exothermic agent added to the composition, m_D, needs to be adjusted so that the adjusted formulation still satisfies the condition η>1.

[0051] The procedure further includes a process verification step for a specific application. Before bonding a new substrate combination, at least three standard lap-shear samples must be prepared, with the substrate of the samples being the same as the actual workpiece. A thermocouple with an accuracy of ±0.5°C is pre-embedded in the adhesive layer of the sample, and the sample is positioned and heated for curing according to the standard process document. During the curing process, the temperature curve inside the adhesive layer is recorded, and it is verified whether the temperature rise curve near the heat sink can be synchronized with other monitoring points after the latent exothermic agent is triggered. If there is a lag in the temperature rise, the oven holding time is increased in 10-minute increments until the temperature curve of the test sample meets the requirements. This verified holding time is used as the standardized curing process parameter for this specific substrate combination.

[0052] Example 6: This example provides an experimental procedure for determining the ratio of rigid blocks to flexible elastomer blocks within a block copolymer in a photolytic microcapsule wall material. This addresses the technical problem of the mutual constraint between room-temperature toughness and low-temperature toughness after adhesive curing. In low-temperature bonding applications, the chemical structure of the block copolymer needs to be quantitatively designed so that its structural units retain their toughening effect in the cured matrix at low temperatures after fulfilling their function as a microcapsule wall material. To this end, a systematic experimental procedure needs to be established to calibrate the ratio of rigid to flexible blocks in the block copolymer and its influence on the mechanical properties of the final adhesive layer at different temperatures.

[0053] The procedure first involves preparing a set of adhesive composition samples with identical components except for the block copolymer component. The block copolymers used are a series of photosensitive biodegradable rigid and flexible elastomer block copolymers, with the weight ratio of rigid to flexible blocks set at four gradients: 30:70, 40:60, 50:50, and 60:40. Using these four different block copolymers and following the above procedure, four corresponding photodegradable microcapsules are prepared, which are then formulated into four final adhesive compositions. The second step of the procedure is a comparative test of mechanical properties. Using the four adhesive compositions, standard-sized impact strength test strips are prepared. Each set of strips is divided into two batches: one batch undergoes a cantilever beam impact test at a standard environment of 25°C to determine its room-temperature impact strength; the other batch is first kept at a low-temperature environment of -100°C for 2 hours, followed by a cantilever beam impact test. Impact tests were conducted to determine the low-temperature impact strength. The test results showed that as the proportion of rigid blocks increased from 30% to 60%, the room-temperature impact strength of the samples increased monotonically, while the low-temperature impact strength peaked at a rigid-to-flexible ratio of 40:60, and decreased at ratios of 50:50 and 60:40. The third step of the procedure was to determine the ratio range based on the test data. Data analysis showed that while a rigid block ratio of 30:70 was beneficial for flexibility at low temperatures, the density of crosslinking points after curing was insufficient, limiting the overall toughness. A rigid block ratio of 50:50 or higher resulted in oligomer fragments after degradation that were insufficient to fully plasticize the remaining flexible blocks, or increased brittleness, leading to deterioration in low-temperature toughness. Therefore, to balance the comprehensive mechanical properties under room and low-temperature conditions, the weight ratio of rigid blocks to flexible elastomer blocks in the block copolymer was determined to be between 35:65 and 45:55.

[0054] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A photothermal dual-curing structural adhesive composition resistant to extreme environments, characterized in that, The composition comprises: A thermosetting resin; A latent thermosetting agent having an activation temperature T a ; A photolytic microcapsule is used to encapsulate a latent thermosetting agent. The wall material of the photolytic microcapsule has a thermal response point, and the wall material of the photolytic microcapsule breaks down due to chain rupture when irradiated with light of a specific wavelength. A latent exothermic agent is incorporated into the wall material of photolytic microcapsules. The latent exothermic agent has a decomposition temperature T. d And the decomposition temperature T d Below activation temperature T a Specifically, the effect of light of a specific wavelength is defined as disrupting the unlocking mechanism of the physical isolation of the photolytic microcapsules, while the decomposition of the latent exothermic agent is set at a decomposition temperature T. d The triggering mechanism generates a self-propagating heat wave, which ensures that even in non-light-exposed areas, photolytic microcapsules rupture and latent thermosetting agents activate at temperature T. a The next one is activated synchronously; In addition, the thermosetting resin is epoxy resin, and the latent thermosetting agent is an amine curing agent or an anhydride curing agent; The wall material of the photolytic microcapsules is composed of polymers containing o-nitrobenzyl ester groups or coumarin groups; The latent exothermic agent is an azo compound; Based on a total weight of 100 parts of the composition, the activation of the latent exothermic agent and the latent thermosetting agent, and the overall heat capacity of the composition, satisfy the following energy triggering relationship: η=(m D ⋅ΔH D ) / (m total ⋅C p ⋅(T a -T d ))>1, where η is the energy triggering factor, m D ΔH represents the weight fraction of the latent exothermic agent. D The enthalpy of decomposition of the latent exothermic agent, m total C represents the total weight parts of the composition. p T represents the average specific heat capacity of the composition. a T is the activation temperature of the latent thermosetting agent. d This refers to the decomposition temperature of the latent exothermic agent. Based on a total weight of 100 parts of the composition, the thermosetting resin comprises 40-70 parts by weight; the latent thermosetting agent comprises 10-30 parts by weight; the polymer constituting the photolytic microcapsule wall material comprises 5-20 parts by weight; and the latent exothermic agent blended in the wall material comprises 8-20 parts by weight.

2. The photothermal dual-curing structural adhesive composition resistant to extreme environments according to claim 1, characterized in that, The latent thermosetting agent itself is a synergistic curing system, which includes a main curing agent B1 and a dormant synergist B2. The curing reactivity of the dormant synergist B2 is catalyzed and enhanced by the hydroxyl intermediate generated by the reaction between the thermosetting resin and the main curing agent B1, so as to construct an intrinsic positive feedback loop of product catalysis-accelerated reaction-heat replenishment after the curing reaction is initiated.

3. The photothermal dual-curing structural adhesive composition resistant to extreme environments according to claim 1, characterized in that, The wall material of the photodegradable microcapsule is composed of a block copolymer, which includes at least one photosensitive biodegradable rigid block and at least one flexible elastomer block.

4. The photothermal dual-curing structural adhesive composition resistant to extreme environments according to claim 3, characterized in that, The block copolymer also contains functional groups that can react with the thermosetting resin, so that after curing, the structural units of the block copolymer and the matrix formed by the thermosetting resin can interact.

5. The photothermal dual-curing structural adhesive composition resistant to extreme environments according to claim 3, characterized in that, The photosensitive biodegradable rigid block, after degradation, forms oligomer fragments that migrate as plasticizers to the flexible elastomer block region and lower the glass transition temperature of the flexible elastomer block.

6. The photothermal dual-curing structural adhesive composition resistant to extreme environments according to claim 5, characterized in that, The oligomer fragments formed after degradation contain functional groups that can react with thermosetting resins.

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