Temperature latent curing agent and preparation method thereof

Abstract

The invention relates to the technical field of high polymer materials, and discloses a temperature latent curing agent and a preparation method thereof.The curing agent is prepared from a polyamine compound and a polythiol compound through a proton transfer reaction at the temperature of 15-30 DEG C. In the reaction, the molar ratio of active amine hydrogen to thiol groups is 1: (0.9-1.1), and a stable ion pair structure is formed. The invention also discloses a single-component epoxy curing system containing the curing agent, epoxy resin and a dielectric constant jump additive. The system is endowed with excellent storage stability by utilizing an ion pair structure; meanwhile, after heating, the compatibility of the dielectric constant jump additive and the epoxy resin is changed, and the dielectric constant of the system is triggered to rise, so that ion pair constraint is weakened, and the curing agent is dissociated and rapidly cured at a moderate temperature. According to the technology, the contradiction between latency and curing conditions is solved, and a cured product has good thermal and mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a temperature-latent curing agent and its preparation method. Background Technology

[0002] Epoxy resins are widely used in coatings, adhesives, composite materials, and electronic packaging due to their excellent bonding properties, mechanical strength, chemical resistance, and electrical insulation. Epoxy resins are thermoplastic prepolymers and must react with a curing agent to form a three-dimensional cross-linked network structure to exhibit their final properties. Depending on the application method, epoxy curing systems are mainly divided into two-component and one-component forms. Two-component systems require precise mixing of epoxy resin and curing agent before use, which is inconvenient and results in a limited pot life, restricting their application in automated production lines. To overcome these inconveniences, one-component epoxy curing systems, which pre-mix epoxy resin with a latent curing agent, have emerged. These systems eliminate the need for on-site weighing and mixing, simplifying the process and significantly improving production efficiency.

[0003] However, the design of the core component of a single-component system—the latent curing agent—always faces a fundamental contradiction. On the one hand, to ensure a sufficiently long shelf life, the curing agent must remain chemically inert at room temperature or normal transport temperatures, i.e., high latency. On the other hand, to meet the demands of efficient and energy-saving production, it is desirable for the curing agent to be rapidly activated at a relatively mild temperature upon heating, triggering a fast and complete curing reaction.

[0004] Existing technologies struggle to perfectly balance this contradiction. For example, traditional curing agents, such as dicyandiamide (DICY), while exhibiting excellent storage stability, typically have curing temperatures as high as 170°C. This not only results in high energy consumption but also makes them unsuitable for heat-sensitive electronic components or substrates. To lower the curing temperature, the industry has attempted to develop novel curing agents or introduce accelerators into the system. However, this often comes at the cost of sacrificing storage stability, leading to a significant increase in viscosity or even gelation of the single-component system during storage, drastically shortening the product's shelf life. Furthermore, while physical isolation methods such as microencapsulation can address this contradiction to some extent, the preparation process is relatively complex and costly. Additionally, the introduced capsule wall material may become a weak point in the mechanical properties of the cured material, affecting the reliability of the final product.

[0005] Therefore, developing a latent curing agent that is simple to prepare, can ensure long-term storage stability, can achieve rapid curing at moderate temperatures, and can give the cured product excellent comprehensive properties remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The technical problem to be solved by this invention is that in existing single-component epoxy systems, latent curing agents often have the contradiction of excessively high curing temperature or insufficient storage stability.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution: In a first aspect, this application provides a temperature-latent curing agent, employing the following technical solution: A temperature-latent curing agent is prepared by reacting raw materials containing component A and component B under preset conditions; wherein component A is a polyamine compound containing at least one primary or secondary amine group, and component B is a polythiol compound; the molar ratio of the active amine hydrogen in component A to the thiol group in component B is 1:(0.9-1.1).

[0008] By adopting the above technical solution, this technical solution provides a product formed through a pre-reaction as a curing agent. Chemically, the polyamine compound, as component A, has an amine group (-NH2 or -NHR) in its molecular structure that is basic and can act as a proton acceptor; the polythiol compound, as component B, has a thiol group (-SH) in its molecular structure that is acidic and can act as a proton donor. Under the aforementioned preset reaction conditions, a proton transfer reaction occurs between the amine group and the thiol group: the active hydrogen proton (H2) on the thiol group... + The ion transfers to the nitrogen atom of the amine group, forming a quaternary ammonium ion (-NH3). + or -NH2R + ) and thiolate anions (-S - These two ions with opposite charges combine through electrostatic attraction to form a macroscopically neutral ion pair structure.

[0009] At room temperature or normal storage temperatures, this ion pair structure is stable, binding highly reactive amine and thiolate functional groups within the ionic bond structure, preventing them from moving freely and undergoing ring-opening reactions with the epoxy groups of the epoxy resin. Therefore, when this curing agent is mixed with epoxy resin to form a one-component system, the system does not undergo significant curing reactions during storage, exhibiting a low viscosity increase over long-term storage, thus achieving excellent storage stability.

[0010] Preferably, the polyamine compound is isophorone diamine, m-phenylenediamine, or polyetheramine D-230.

[0011] By employing the above-mentioned technical solutions and selecting these specific polyamine compounds, the ion pairs formed between them and polythiol compounds can possess specific spatial configurations and dissociation barriers. This directly affects the temperature and rate of thermal dissociation of the curing agent, and the molecular structure of these amines determines the basic framework of the crosslinking network after curing, thereby enabling the final cured product to obtain the glass transition temperature, thermal stability, and mechanical strength required for specific applications.

[0012] Preferably, the polythiol compound is pentaerythritol tetra(3-mercaptopropionate) or trimethylolpropane tri(3-mercaptopropionate).

[0013] By employing the above technical solution and selecting these polythiol compounds with specific functionalities (4 and 3, respectively), the multiple thiol groups in their structure can form multi-point ion-pair structures with polyamine compounds, enhancing the stability of individual curing agent molecules. Simultaneously, as chain growth and crosslinking points in the curing reaction, their functionality and molecular chain flexibility directly affect the control of the crosslinking density and toughness of the final cured product, contributing to obtaining mechanical properties that combine high tensile strength and moderate elongation at break.

[0014] Preferably, the molar ratio of the active amine hydrogen to the thiol group is 1:1.

[0015] By employing the above technical solution, controlling the molar ratio of active functional groups within a stoichiometric ratio maximizes proton transfer between amine and thiol groups, forming ion-pair structures. This minimizes residual free amine or thiol groups that do not participate in ion-pair formation. This directly enhances the latent properties of the curing agent, forming the basis for excellent storage stability. Simultaneously, it ensures a stoichiometric balance of the reactive functional group ratio during thermally activated curing, facilitating the formation of a fully cross-linked, low-defect-density three-dimensional network structure, thus ensuring the cured product possesses excellent final properties.

[0016] Preferably, the curing agent is an ion-pair compound formed by a proton transfer reaction between the amine group of the polyamine compound and the thiol group of the polythiol compound.

[0017] By employing the aforementioned technical solution, the structure of the curing agent is chemically defined, clarifying that its latent properties originate from the ion-pair structure, rather than physical adsorption or coating. This structure is stable at room temperature, but at a specific threshold temperature, thermal input overcomes the binding energy of the ion pairs, causing them to dissociate and release highly reactive amine and thiolate anions, thereby initiating a rapid and concentrated curing reaction. This mechanism determines that the curing agent possesses both low-temperature storage stability and rapid curing at medium temperatures.

[0018] Secondly, this application provides a method for preparing a temperature-latent curing agent, employing the following technical solution: A method for preparing a temperature-latent curing agent as described in the first aspect, comprising the following steps: The polythiol compound is added dropwise to the polyamine compound at a temperature of 15–30°C, and the reaction is continued for 60–120 minutes after the addition is completed to obtain the temperature-latent curing agent.

[0019] By employing the above technical solution, this preparation method provides process conditions for precise control of the reaction process. Its purpose is to selectively promote the amine-thiol proton transfer reaction and inhibit other possible side reactions that could lead to non-latent products. Specifically, its effects are as follows: First, the reaction temperature is controlled within the lower range of 15–30 °C. This temperature range is set based on reaction kinetics. On the one hand, this temperature is sufficient to maintain adequate fluidity of the raw material components (especially high-viscosity polythiols), thereby achieving homogeneous mixing at the molecular level under mechanical stirring. On the other hand, this temperature condition significantly reduces the reaction rate of possible covalent bond formation reactions between amine and epoxy or thiol groups (such as Michael addition reactions). Proton transfer reactions, as acid-base neutralization reactions, have low activation energies and fast reaction rates, and are absolutely dominant at this temperature. Therefore, by controlling the temperature, this method achieves high kinetic selectivity for the target reaction (ion pair formation).

[0020] Secondly, the polythiol compound is added to the polyamine compound dropwise. Since the proton transfer reaction is exothermic, slowly adding one reactant to another effectively controls the reaction rate and heat release rate of the entire system. This avoids a sudden increase in local temperature exceeding the preset 15–30°C control range caused by mixing reactants all at once, thus preventing side reactions caused by local overheating and ensuring the uniformity and stability of the temperature throughout the reaction system.

[0021] Finally, after the dropwise addition is complete, continue stirring for 60–120 minutes. This step ensures the completeness of the reaction. After macroscopic homogenization, continuous mechanical stirring provides sufficient contact time and collision opportunities for the reactant molecules, allowing most of the amine and thiol groups in the system to complete proton transfer and form stable ion-pair structures. This minimizes the content of free, highly reactive functional groups in the final product, thus ensuring the high latency performance of the curing agent as a homogeneous liquid.

[0022] Preferably, the temperature condition is 20–25°C.

[0023] By adopting the above technical solution, the reaction temperature range is further narrowed to 20–25°C, providing a more optimized process window for the preparation process. This temperature range has been verified as an equilibrium point that can minimize the occurrence of side reactions while ensuring that the raw material system has a good operating viscosity, thereby obtaining a curing agent product with a purer structure and more stable latent properties.

[0024] Preferably, the preparation method is carried out under solvent-free conditions.

[0025] By adopting the above technical solution, no non-reactive solvents are introduced into the entire preparation process. The technical advantages are: first, it simplifies the production process, eliminating the need for subsequent solvent removal steps, thus reducing energy consumption and equipment requirements; second, it avoids the potential adverse effects of solvent residue on the subsequent curing performance of the epoxy system; and third, the resulting curing agent product is 100% active, which can be directly used in downstream formulations, improving the effective substance content and ease of application of the product.

[0026] Thirdly, this application provides a one-component epoxy curing system, which adopts the following technical solution: A one-component epoxy curing system comprising the following components in parts by weight: epoxy resin: 100 parts; temperature-latent curing agent as described in any one aspect: 40-50 parts; dielectric constant jump additive: 10-30 parts.

[0027] By adopting the above technical solution, this technical solution constructs a thermally triggered response system by compounding the ion-pair type latent curing agent described in the first aspect with a dielectric constant jump additive, thus solving the problem that latent curing agents usually require high thermal energy to activate. Its mechanism of action is as follows: At room temperature or normal storage temperatures, the dielectric constant-jumping additive exhibits low compatibility with the epoxy resin matrix, existing as a physically discontinuous micro-dispersed phase within the system. The ion-pair type latent curing agent is dispersed within this two-phase system. In this case, the overall macroscopic polarity of the system is determined by the dominant, low-polarity epoxy resin matrix, which has a low dielectric constant. According to physicochemical principles, a lower dielectric constant environment helps maintain strong electrostatic attraction between ion pairs, ensuring structural stability and thus guaranteeing the latent performance of the entire cured system during storage.

[0028] When the system is heated, after reaching a specific temperature range, the compatibility between the dielectric constant jump additive and the epoxy resin matrix undergoes a significant change, transforming from incompatibility or partial compatibility to complete compatibility. This process causes the original micro-dispersed phase structure to disappear, and the additive molecules diffuse uniformly into the epoxy resin matrix. This results in a rapid increase in the polarity of the curing agent ions with respect to the surrounding microscopic chemical environment within a short period of time, i.e., a jump increase in the local dielectric constant of the system.

[0029] An increase in dielectric constant significantly weakens the electrostatic attraction between positive and negative ions bound in the ion pairs. As a result, within a narrow temperature range below its pure thermal decomposition temperature, a large number of ion pairs undergo concentrated, avalanche-like dissociation, instantaneously releasing high concentrations of highly reactive amine and thiolate anion functional groups. These released reactive functional groups immediately undergo efficient ring-opening polymerization with the epoxy groups in the system, initiating rapid and complete curing. This approach achieves precise control over the initiation of the curing reaction through changes in the physicochemical behavior of the chemical components, ensuring that curing only occurs when a specific temperature threshold is reached, manifested as a single, sharp, and concentrated curing exothermic peak on the differential scanning calorimetry (DSC) curve.

[0030] Preferably, the dielectric constant jump additive is selected from propylene carbonate or sulfolane.

[0031] The above technical solution employs propylene carbonate or sulfolane as dielectric constant-changing additives because these two highly polar compounds exhibit a significant change in compatibility with industrially commonly used bisphenol A type epoxy resins and other matrices within a specific and practical temperature range (e.g., 100–150°C). This allows the "trigger" temperature of the curing reaction to be precisely designed within the desired application process window. By selecting specific additives, the curing temperature can be controlled to meet the requirements of different application scenarios for curing process conditions, thereby improving the versatility and process controllability of the technical solution.

[0032] In summary, the present invention has at least one of the following beneficial technical effects: 1. The temperature-latent curing agent provided by this invention exhibits excellent storage stability when mixed with epoxy resin to form a one-component epoxy curing system. This is because the curing agent of this invention undergoes a proton transfer reaction with a polyamine compound and a polythiol compound beforehand, forming an ion-pair structure stable at room temperature. This structure binds highly reactive amine and thiol functional groups to ionic bonds, effectively inhibiting premature reactions with epoxy groups at storage temperature, thereby significantly extending the pot life of the one-component system and reducing the rate of viscosity increase over time. 2. This invention achieves rapid, controllable, and complete curing at a moderate temperature (128–139°C) by combining an ion-pair type latent curing agent with a dielectric constant jump additive. The mechanism lies in the fact that when the system is heated to a specific temperature, the compatibility between the dielectric constant jump additive and the epoxy matrix increases, leading to a sharp increase in the polarity of the microenvironment of the curing agent. This causes concentrated dissociation of the ion-pair structure, instantly releasing a large number of active functional groups. This effect allows the curing reaction to proceed efficiently at temperatures far lower than those of traditional curing agents (such as DICY), and achieves a higher curing enthalpy, indicating a more complete curing reaction. 3. The single-component epoxy curing system prepared by this invention, after curing at a relatively low curing temperature, exhibits excellent thermal and mechanical properties in its cured product. Thanks to the controllable and complete curing process described above, the system forms a highly dense and uniform three-dimensional network structure. This structure results in a high glass transition temperature (Tg) and thermal decomposition temperature (Td5) in the cured product, demonstrating good heat resistance and thermal stability. Simultaneously, this network structure mechanically combines high tensile strength with moderate elongation at break, exhibiting superior toughness while maintaining strength compared to traditional high-temperature curing systems. Attached Figure Description

[0033] Figure 1 The infrared spectrum comparison diagram of the temperature-latent curing agent and its reaction raw materials prepared in Example 1 of the present invention. Detailed Implementation

[0034] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0035] The epoxy resin matrix includes bisphenol A diglycidyl ether (DGEBA), industrial grade, with an epoxy equivalent (EEW) of 184–190 g / eq; and bisphenol F diglycidyl ether (DGEBF), industrial grade, with an EEW of 165–175 g / eq.

[0036] The basic amine components used to prepare ion-pair latent curing agents include: Isophorone diamine (IPDA), CAS No. 2855-13-2, industrial grade, active hydrogen equivalent (AHEW) of 42.6 g / eq; m-Phenylenediamine (MXDA), CAS No. 1477-55-0, industrial grade, AHEW 34.1 g / eq; Polyetheramine D-230, whose main component is polypropylene glycol di(2-aminopropyl) ether, has the CAS number 9046-10-0. Its molecular structure is characterized by aminopropyl groups at both ends of the polyoxypropylene chain. It is an industrial grade product with an average molecular weight of approximately 230 g / mol and an AHEW of approximately 58–60 g / eq.

[0037] The acidic thiol component used to prepare ion-pair latent curing agents includes: Pentaerythritol tetra(3-mercaptopropionate) (PETMP), CAS No. 7575-23-7, industrial grade, with a thiol functionality of 4 and a molecular weight of 488.66 g / mol. Trimethylolpropane tris(3-mercaptopropionate) (TMPMP), CAS No. 33007-83-9, industrial grade, with a thiol functionality of 3 and a molecular weight of 398.55 g / mol.

[0038] Example 1: This example provides a method for preparing latent curing agent-1 (IPDA-PETMP ion pair), the steps of which are as follows: In a 500 mL four-necked flask equipped with a mechanical stirrer, dropping funnel, thermometer, and nitrogen protection device, 170.3 g (1.0 mol) of isophorone diamine (IPDA) was added. Nitrogen protection was activated, and mechanical stirring was started at 150 RPM. The reaction flask was placed in a water bath, and the temperature of the material inside the vessel was maintained at 20–25 °C. 488.7 g (1.0 mol) of pentaerythritol tetra(3-mercaptopropionate) (PETMP) was added dropwise over 60 minutes using the dropping funnel. During the addition, the temperature inside the vessel was controlled to not exceed 25 °C by adjusting the water bath temperature. After the addition was complete, the reaction was continued at 20–25 °C for 90 minutes with stirring. After the reaction was complete, stirring was stopped, yielding a colorless, transparent, viscous, homogeneous liquid, designated as latent curing agent-1.

[0039] In this preparation, the molar ratio of the active amine hydrogen of IPDA to the active thiol group of PETMP is 1:1.

[0040] Example 2: This example provides a method for preparing latent curing agent-2 (MXDA-TMPMP ion pair), the steps of which are as follows: In a reaction apparatus identical to that of Example 1, 102.1 g (0.75 mol) of m-phenylenediamine (MXDA) was added. Nitrogen protection was activated, and mechanical stirring was started at 200 RPM. The reaction flask was placed in a water bath, and the temperature of the material inside the vessel was maintained at 25–30°C. 398.6 g (1.0 mol) of trimethylolpropane tris(3-mercaptopropionate) (TMPMP) was added dropwise over 45 minutes using a dropping funnel. During the addition, the temperature inside the vessel was controlled to not exceed 30°C. After the addition was complete, the reaction was continued at 25–30°C for 60 minutes with stirring. After the reaction was complete, stirring was stopped, yielding a pale yellow, transparent, viscous, homogeneous liquid, designated as latent curing agent-2. In this preparation, the molar ratio of the active amine hydrogen of MXDA to the active thiol group of TMPMP was 1:1.

[0041] Example 3: This example provides a method for preparing latent curing agent-3 (D-230-PETMP ion pair), the steps of which are as follows: In a reaction apparatus identical to that of Example 1, 230.0 g (approximately 1.0 mol) of polyetheramine D-230 was added. Nitrogen protection was activated, and mechanical stirring was started at 100 RPM. The reaction flask was placed in a water bath, and the temperature of the material inside the vessel was maintained at 15–20°C. 488.7 g (1.0 mol) of pentaerythritol tetra(3-mercaptopropionate) (PETMP) was added dropwise over 90 minutes using a dropping funnel. During the addition, the temperature inside the vessel was controlled to not exceed 20°C. After the addition was complete, the reaction was continued with stirring at 15–20°C for 120 minutes. After the reaction was complete, stirring was stopped, yielding a colorless, transparent, high-viscosity homogeneous liquid, designated as latent curing agent-3. In this preparation, the molar ratio of the active amine hydrogens of D-230 to the active thiol groups of PETMP was approximately 1:1.

[0042] Comparative Example 1: This comparative example provides a method for preparing comparative composition-1 (a physical mixture of IPDA and PETMP), as follows: Compared with Example 1, the difference is that 170.3 g (1.0 mol) of isophorone diamine (IPDA) and 488.7 g (1.0 mol) of pentaerythritol tetra(3-mercaptopropionate) (PETMP) were simply physically mixed at 20-25°C for 10 minutes and then discharged immediately without subsequent heat preservation and stirring reaction steps, so as to obtain a comparative physical mixture that did not form stable ion pairs, which is designated as comparative composition-1.

[0043] Comparative Example 2: This comparative example provides a method for preparing comparative composition-2 (a single-component amine comparative compound), as follows: This comparative example does not involve reaction preparation. For comparison, isophorone diamine (IPDA) from the starting material is used directly as a single-component amine comparative.

[0044] Comparative Example 3: This comparative example provides a method for preparing comparative composition-3 (a single-component thiol comparative compound), as follows: This comparative example does not involve reaction preparation. For comparison, pentaerythritol tetra(3-mercaptopropionate) (PETMP) from the starting material is used directly as a single-component thiol comparative.

[0045] Comparative Example 4: This comparative example provides a method for preparing comparative curing agent-4 (conventional technology comparative material), as follows: This comparative example does not involve reaction preparation. For comparison, commercially available micronized dicyandiamide (DICY) is used directly as a comparison material for traditional latent curing agent technology.

[0046] To further illustrate the application effect of the latent curing agent of the present invention prepared in the above embodiments, and to compare its performance with the comparative material in the comparative examples, the present invention provides the following application examples and application comparative examples for preparing a single-component epoxy curing system.

[0047] Application Example A: This application example provides a method for preparing a one-component epoxy curing system, as detailed below: In a vacuum mixing apparatus equipped with a planetary stirrer, 100.0 parts by weight of bisphenol A diglycidyl ether (DGEBA) and 20.0 parts by weight of propylene carbonate (PC) were added. The mixture was stirred at 100 RPM for 15 minutes at 25°C to allow the PC to form a uniform micro-dispersion phase in the DGEBA. Subsequently, 44.5 parts by weight of the latent curing agent-1 prepared in Example 1 were added. The mixing apparatus was sealed, and the vacuum level in the system was evacuated to below 100 Pa. The mixture was stirred at 30°C and a low speed of 50 RPM for 45 minutes until a homogeneous, bubble-free liquid system was obtained.

[0048] Application Example B: Compared with Application Example A, the difference is that 45.1 parts by weight of latent curing agent-2 prepared in Example 2 is used instead of latent curing agent-1. All other components and preparation processes are the same.

[0049] Application Example C: The difference from Application Example A is that 20.0 parts by weight of sulfolane (SL) is used instead of propylene carbonate (PC). All other components and preparation processes are the same.

[0050] Application Comparative Example D: The difference compared to Application Example A is that propylene carbonate (PC) is not added to the formulation. All other components and preparation processes are the same.

[0051] Application Comparative Example E: Compared with Application Example A, the difference is that 44.5 parts by weight of the comparative composition-1 prepared in Comparative Example 1 is used instead of latent curing agent-1. All other components and preparation processes are the same.

[0052] Comparative Example F: This application provides a comparative method for preparing a comparative system based on existing technology, as detailed below: In a vacuum mixer equipped with a planetary stirrer, 100.0 parts by weight of bisphenol A diglycidyl ether (DGEBA) and 11.4 parts by weight of the comparative curing agent-4 (DICY) from Comparative Example 4 were added. The mixture was vigorously stirred at 200 RPM for 60 minutes at 40°C to form a uniform dispersion of DICY powder in the epoxy resin. Using the same vacuum degassing process as in Application Example A, a milky white liquid system containing solid dispersed particles was finally obtained.

[0053] Test Example 1: Verification of Ion Pair Formation (FTIR Infrared Spectroscopy) The purpose of this test is to obtain and compare the changes of specific chemical functional groups before and after the preparation process through infrared spectroscopy analysis, so as to verify that a chemical interaction occurs between basic amines and acidic thiols.

[0054] Test sample: Sample a: Latent curing agent-1 prepared in Example 1; Sample b: Raw material isophorone diamine (IPDA); Sample c: Raw material pentaerythritol tetra(3-mercaptopropionate) (PETMP); Sample d: Comparative composition-1 prepared in Comparative Example 1.

[0055] Test method: Fourier transform infrared spectroscopy was used, and the samples were placed between potassium bromide salt plates using the liquid film method for testing. The spectral scanning range was set to 4000–4000 cm⁻¹. -1 The resolution is 4cm. -1 The infrared absorption spectra of each sample were obtained by scanning 32 times.

[0056] The infrared absorption spectrum generated based on the obtained infrared spectral data is as follows: Figure 1 As shown. The analysis results of the spectrum are as follows: The spectrum of raw material PETMP (sample c) at 2570 cm⁻¹-1 The spectrum exhibits a sharp, characteristic absorption peak at 3360 cm⁻¹, attributed to the stretching vibration of the thiol group (-SH). The spectrum of the raw material IPDA (sample b) shows a peak at 3360 cm⁻¹. -1 and 3290cm -1 The peak exhibits a dual characteristic absorption peak attributable to the antisymmetric and symmetric stretching vibrations of the NH bond in the primary amine group (-NH2).

[0057] The spectrum of the comparative composition-1 (sample d) prepared in Comparative Example 1, which is a physical mixture of IPDA and PETMP, shows characteristic absorption peaks of thiol groups (-SH) and primary amine groups (-NH2) at the corresponding positions mentioned above. The position and shape of each absorption peak are not substantially different from those of the single raw material.

[0058] The spectrum of the latent curing agent-1 (sample a) prepared in Example 1 shows that it is located at 2570 cm⁻¹. -1 The characteristic absorption peak of the thiol group (-SH) at 3360 cm⁻¹ has completely disappeared. Meanwhile, the peak originally belonging to the primary amine group (-NH₂) at 3360 cm⁻¹ has disappeared. -1 and 3290cm -1 The double absorption peak at 3200–2800 cm⁻¹ has also disappeared, replaced by a peak at 3200–2800 cm⁻¹. -1 A new, broad, strong absorption band appeared in the region, which is attributed to the quaternary ammonium cation (-NH3). + The stretching vibration of the NH bond in ).

[0059] The changes in the spectral data above indicate that the preparation process in Example 1 caused a proton transfer between the primary amine group of IPDA and the thiol group of PETMP, forming a quaternary ammonium salt-thiol salt ion pair structure. This structure differs from the physical mixture in Comparative Example 1.

[0060] Test Example 2: Basic Verification of Latency and Thermally Activated Curing Ability The purpose of this test is to evaluate the room temperature storage capacity and thermally initiated curing capacity of different systems by periodically observing the state changes of the samples at room temperature and measuring their gelation time at a specified high temperature.

[0061] Test sample: Sample A: The single-component epoxy curing system prepared in Example A; Sample B: The single-component epoxy curing system prepared in Comparative Example E.

[0062] The testing method is as follows: 1) Observation of storage conditions at room temperature: Step 1: Take 10 g of sample A and sample B respectively, place them in a clean glass bottle with an inner diameter of 20 mm, and seal the bottle opening; Step 2: Store the sample vials in a constant temperature environment of 25±2℃; Step 3: At the designated time points of 24 hours, 48 ​​hours and 72 hours after storage, tilt the sample vial to 90 degrees, observe and record the fluidity of the sample to determine whether it remains in a liquid state or has gelled.

[0063] 2) Determination of high-temperature gelation time: Step 1: Set and stabilize the surface temperature of a digital display constant temperature heating stage at 120±1℃; Step 2: Use a dropper to take about 0.5 g of the sample to be tested (sample A and freshly prepared sample B), and quickly drop it onto the aluminum foil placed in the center of the heating stage, while starting the timer at the same time; Step 3: Use a clean glass rod to periodically touch and tease the droplet. When the glass rod can pull out a continuous, non-retracting filament from the droplet, stop timing and record the time taken; this is the gelation time of the sample. Step 4: Repeat the measurement three times for each sample, calculate and record the average value.

[0064] The results of observations at room temperature and testing at high temperature gel time are recorded in Table 1.

[0065] Table 1. Results of observation at room temperature and gelation time at high temperature:

[0066] Test results show that the sample of Application Example A remained in a liquid state after being stored at 25±2℃ for 72 hours. The sample of Application Comparative Example E, stored under the same conditions, changed from a high-viscosity liquid to a gel within 48 hours. Under heating conditions of 120℃, the gelation time of the sample of Application Example A was 273 seconds, while the gelation time of the freshly prepared sample of Application Comparative Example E was 51 seconds.

[0067] The above phenomenon is attributed to the form in which the curing agent component exists in the system. In application example A, the latent curing agent-1 used is prepared via a pre-reaction step. This process causes a proton transfer between the primary amine group of the basic amine and the thiol group of the acidic thiol, forming a quaternary ammonium salt-thiol salt ion pair with ionic bonding characteristics. This ion pair structure is stable at room temperature, limiting the direct reactivity of the amine and thiol groups as curing agents, thus the system exhibits long-term liquid storage capacity.

[0068] In contrast, in Comparative Example E, the amine and thiol components physically coexist in the system as free functional groups. These two functional groups can directly undergo ring-opening reactions with epoxy groups at room temperature, leading to a continuous increase in system viscosity and eventual gelation. Under heating conditions, the ion pair structure in Application Example A dissociates due to thermal input, releasing highly reactive amine and thiol anions, initiating a controlled curing reaction. This process explains the system's ability to achieve effective curing at high temperatures, while its potential is determined by the structural stability of the ion pair at room temperature.

[0069] Test Example 3: Storage Stability Assessment The purpose of this test is to quantitatively evaluate the storage stability of different single-component systems by periodically measuring the viscosity of samples under specified accelerated aging conditions.

[0070] Test sample: Sample A: The single-component epoxy curing system prepared in Example A; Sample D: The single-component epoxy curing system prepared in Comparative Example D; Sample E: A one-component epoxy curing system prepared in Comparative Example E; Sample F: A single-component epoxy curing system prepared using Comparative Example F.

[0071] The testing method is as follows: Step 1: Using a rotational viscometer, test the initial viscosity of each sample when it is freshly prepared under constant temperature water bath conditions of 25.0±0.1℃. Step 2: Take 50g of each sample and place it in a sealed container, then put it in a constant temperature oven at 40±1℃ for accelerated aging. Step 3: On the 7th, 14th and 30th days of aging, the samples were removed from the oven and allowed to cool to 25.0±0.1℃. The viscosity was then measured again using the same rotational viscometer and test parameters. Step 4: If the sample gels during the test period, record it as "gel" and stop subsequent viscosity tests.

[0072] The viscosity changes of each sample during the accelerated aging process are recorded in Table 2.

[0073] Table 2. Viscosity changes during accelerated aging:

[0074] Test results showed that under accelerated aging conditions at 40℃, the viscosity of each sample changed to varying degrees over time. The sample in Application Example A had a viscosity of 4250 mPa·s after 30 days of storage. The sample in Comparative Example D showed a viscosity increase to 8900 mPa·s after 30 days. The sample in Comparative Example E showed a significant viscosity increase after 7 days and gelled before the 14-day test node. The sample in Comparative Example F showed a viscosity increase from 12550 mPa·s to 17900 mPa·s.

[0075] The viscosity stability of Application Example A is attributed to its composition and phase structure. The latent curing agent-1 in the system exists in ion-pair form, and its spontaneous dissociation and reaction rate at 40°C are effectively suppressed. Simultaneously, the introduction of the dielectric constant jump additive (PC) forms a physically dispersed microphase within the epoxy matrix, encapsulating or isolating most of the ion-pair curing agent within this phase, further reducing its contact frequency with the epoxy matrix on a macroscopic scale, thereby limiting viscosity growth.

[0076] In Comparative Example D, lacking a dielectric constant-changing additive, the curing agent ion pairs are directly dispersed in the epoxy resin. At aging temperatures, the random dissociation and reaction of some ion pairs lead to a continuous increase in system viscosity. The rapid gelation phenomenon in Comparative Example E is due to the amine and thiol components existing as highly reactive free functional groups, failing to form stable ion pair structures, thus reacting rapidly with the epoxy resin at aging temperatures. The viscosity increase in Comparative Example F reflects the characteristics of the solid dispersion system; its stability depends on the solubility of the curing agent particles (DICY) in the epoxy resin, and its viscosity change is related to the slight dissolution of particles or changes in interparticle interactions.

[0077] Test Example 4: Curing Behavior Test (DSC Analysis) The purpose of this test is to obtain the heat flow changes of each sample during the heating process using differential scanning calorimetry (DSC) to determine its initial curing temperature (T_onset), peak curing temperature (T_peak), and total curing reaction enthalpy (ΔH), thereby characterizing its thermosetting behavior.

[0078] Test sample: Sample A: The single-component epoxy curing system prepared in Example A; Sample B: The one-component epoxy curing system prepared in Example B; Sample C: The one-component epoxy curing system prepared in Example C; Sample D: The single-component epoxy curing system prepared in Comparative Example D; Sample E: A one-component epoxy curing system prepared in Comparative Example E; Sample F: A single-component epoxy curing system prepared using Comparative Example F.

[0079] The testing method is as follows: Step 1: Using a precision electronic balance, accurately weigh 10-15 mg of the liquid sample to be tested, place it in an aluminum DSC crucible, and seal it using a capping machine; Step 2: Place the sealed crucible into the sample chamber of the differential scanning calorimeter, using an empty sealed crucible as a reference. Step 3: Under the protection of nitrogen atmosphere (flow rate 50 mL / min), heat the sample from 25℃ to 250℃ at a constant heating rate of 10℃ / min and record the heat flow curve; Step 4: Process the obtained heat flow curve using analysis software to determine the onset temperature (T_onset), peak temperature (T_peak) of the exothermic peak, and the total curing reaction enthalpy (ΔH) obtained by integrating the exothermic peak.

[0080] The DSC test data for each sample are recorded in Table 3.

[0081] Table 3. DSC curing behavior data for each system:

[0082] Test data show that the peak curing temperatures (T_peak) of application examples A, B, and C are concentrated in the range of 128.7℃ to 138.9℃. Comparative example E has the lowest T_peak at 91.2℃, and its initial curing temperature is also significantly lower than the other samples. Comparative examples D and F have significantly higher T_peaks, at 164.8℃ and 176.3℃, respectively. Regarding the enthalpy of curing reaction (ΔH), application examples A, B, and C, as well as comparative examples E and F, all exhibit relatively high values, while comparative example D has the lowest enthalpy value at 291.3 J / g.

[0083] The curing behavior exhibited in Application Example A stems from the specific combination of a latent curing agent and a dielectric constant-jumping additive within its system. The ion-pair structure in the latent curing agent provides stability at room temperature. When the system is heated, the compatibility of the dielectric constant-jumping additive (PC) with the epoxy resin matrix increases, leading to a change in the local polar environment of the system. This enhanced polarity promotes the dissociation of ion pairs, releasing highly reactive amine and thiol anions, thereby initiating a concentrated and complete curing reaction at a moderate temperature of 134.8°C, characterized by a single, sharp exothermic peak and a high enthalpy of reaction.

[0084] In contrast, Comparative Example D, lacking a dielectric constant abrupt change additive, relies solely on thermal energy input for ion pair dissociation, requiring a higher temperature (T_peak = 164.8℃). Furthermore, the dissociation process is not concentrated, leading to incomplete curing and a lower enthalpy. Comparative Example E, lacking a stable ion pair structure, exhibits high reactivity of its free amine and thiol functional groups at a lower temperature (T_onset = 64.7℃). The curing temperature of Application Example A is significantly lower than that of the traditional DICY curing agent in Comparative Example F. Data also shows that Application Examples B and C exhibit similar curing behavior to A, demonstrating that this technology has a consistent mechanism of action regardless of the raw material selection.

[0085] Test Example 5: Thermal Properties Testing of Cured Products (TGA, DMA) The purpose of this test is to determine the 5% thermogravimetric temperature (Td5) and glass transition temperature (Tg) of the cured products prepared from each system by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA), respectively, in order to evaluate their final thermal stability and heat resistance rating.

[0086] Test sample: Sample A - Cured product: Prepared by curing the system of Application Example A; Sample B - Cured product: Prepared by curing the system of Application Example B; Sample C - Cured product: Obtained by curing the system of Application Example C; Sample D - Cured product: prepared by curing the system of Comparative Example D; Sample E - Cured product: prepared by curing the system of Comparative Example E; Sample F - Cured product: prepared by curing the system of comparative example F.

[0087] The testing method is as follows: 1) Preparation of cured samples: Step 1: Pour the liquid systems prepared in each application example and application comparative example into steel molds preheated to 80°C, and remove air bubbles under vacuum. Step 2: Based on the peak curing temperature (T_peak) of each sample in Table 3, set the corresponding curing program: Samples A, B, and C are cured at 140℃ for 2 hours; Sample D is cured at 165℃ for 2 hours; Sample E is cured at 95℃ for 2 hours; Sample F is cured at 180℃ for 2 hours. Step 3: After curing, allow the mold to cool naturally at room temperature, then demold to obtain a standard cured sample for subsequent testing.

[0088] 2) Thermogravimetric analysis (TGA): Step 1: Cut 10-15 mg block samples from each cured sample plate; Step 2: Place the sample in the platinum crucible of the thermogravimetric analyzer and heat it from 50°C to 600°C at a heating rate of 10°C / min under a nitrogen atmosphere (flow rate 100 mL / min). Step 3: Record the curve of sample weight change with temperature, and record the temperature at which the sample loses 5% of its initial weight as Td5.

[0089] 3) Dynamic Mechanics Analysis (DMA): Step 1: Process the cured sample into a standard strip with dimensions of 35mm × 12mm × 3mm; Step 2: Install the spline on the three-point bending fixture of the dynamic mechanical analyzer; Step 3: At a vibration frequency of 1 Hz and a heating rate of 3℃ / min, the sample was heated from 30℃ to 200℃, and the changes in storage modulus (E') and loss factor (tanδ) with temperature were recorded. Step 4: Define the temperature corresponding to the peak value of the loss factor (tanδ) curve as the glass transition temperature (Tg) of the cured material.

[0090] The thermal performance test data of each cured product are recorded in Table 4.

[0091] Table 4. Thermal property test data for each cured product:

[0092] Test results show that the cured products prepared from the systems of Application Examples A, B, and C have glass transition temperatures (Tg) ranging from 126.3℃ to 134.2℃ and 5% thermogravimetric temperatures (Td5) ranging from 358.1℃ to 363.5℃. The cured product of Comparative Example D has the lowest Tg (109.5℃) and Td5 (340.8℃). The cured product of Comparative Example E has a Tg of 87.6℃. The cured product of Comparative Example F has the highest Tg (147.9℃) and Td5 (372.6℃).

[0093] The thermal properties exhibited by the cured products of Application Examples A, B, and C are a result of their curing process. The concentrated and complete exothermic behavior observed in DSC testing corresponds to the formation of a uniform and highly crosslinked three-dimensional network structure. This network structure macroscopically manifests as a high glass transition temperature and thermal decomposition temperature. This result depends on the promoting effect of dielectric constant-jumping additives on ion pair dissociation at specific temperatures, enabling the curing reaction to proceed efficiently within a controllable, moderate temperature range.

[0094] The low Tg and low Td5 of Comparative Example D directly reflect defects in its crosslinking network. Due to the lack of dielectric constant jump additives, its curing reaction was incomplete (as shown by the lower ΔH in Table 3), resulting in insufficient crosslinking network density and a decrease in its heat resistance and thermal stability. The low Tg value of Comparative Example E is attributed to its rapid but disordered curing process at low temperatures, which easily leads to the formation of a non-uniform network structure. The high Tg and Td5 of Comparative Example F are as expected, resulting from its full curing at high temperatures. However, Application Examples A, B, and C achieved similar thermal stability (Td5) and reached the heat resistance level (Tg) required for specific applications at significantly lower curing temperatures than Comparative Example F.

[0095] Test Example 6: Mechanical Properties Testing of Cured Materials (Tensile Properties) The purpose of this test is to evaluate the basic mechanical properties of cured products prepared from each system by measuring their tensile strength and elongation at break.

[0096] Test sample: Sample A - Cured product: Prepared by curing the system of Application Example A; Sample B - Cured product: Prepared by curing the system of Application Example B; Sample C - Cured product: Obtained by curing the system of Application Example C; Sample D - Cured product: prepared by curing the system of Comparative Example D; Sample E - Cured product: prepared by curing the system of Comparative Example E; Sample F - Cured product: prepared by curing the system of comparative example F.

[0097] The testing method is as follows: Step 1: The cured sample prepared in Test Example 5 is processed into a standard dumbbell-shaped tensile test strip using a stamping machine or CNC milling machine in accordance with GB / T 2567-2008 standard; Step 2: Place the prepared sample strips in an environment of 23±2℃ and 50±5% relative humidity for more than 24 hours; Step 3: Perform a tensile test on the specimen on a universal testing machine, with the tensile rate set to 2 mm / min; Step 4: Record the maximum tensile stress (i.e., tensile strength) and the corresponding gauge length elongation (i.e., elongation at break) of the specimen at fracture. Collect at least 5 valid data points for each sample group, calculate and record their average value.

[0098] The tensile property test data of each cured product are recorded in Table 5.

[0099] Table 5. Test data of mechanical properties of various cured products:

[0100] Test results show that the cured products prepared from application examples A, B, and C have tensile strengths ranging from 68.7 MPa to 72.1 MPa and elongation at break ranging from 4.15% to 5.12%. The tensile strength of the cured product of comparative example D is 55.4 MPa. The tensile strength and elongation at break of the cured product of comparative example E are the lowest, at 41.6 MPa and 1.87%, respectively. The cured product of comparative example F has the highest tensile strength (78.2 MPa), but its elongation at break (3.09%) is lower than that of the cured products of application examples A, B, and C.

[0101] The mechanical properties exhibited by the cured products in Application Examples A, B, and C are a direct reflection of their internal cross-linked network structure. This system, through a combination of a latent curing agent and a dielectric constant-jumping additive, achieves a concentrated and complete curing reaction at a set curing temperature (as shown in Test Example 4), forming a homogeneous network with high cross-linking density. This network structure can effectively withstand and transfer external loads, thus exhibiting high tensile strength. Simultaneously, the uniformity of the network reduces stress concentration points, allowing it to undergo a certain degree of deformation before fracture, resulting in a moderate elongation at break.

[0102] In contrast, the low tensile strength of Comparative Example D's cured material is a result of incomplete curing (as shown by the lower Tg in Test Example 5). Its internal cross-linking density is insufficient, and the network structure contains a large number of unreacted chain segments, failing to form an effective load-bearing skeleton. The low strength and low elongation of Comparative Example E's cured material reflect its brittle characteristics, attributed to the rapid and disordered curing process at low temperatures, which resulted in a non-uniform network with numerous defects and high internal stress. The high strength and low elongation of Comparative Example F's cured material are typical characteristics of its rigid network structure formed during high-temperature curing. This technical solution, through a combination of specific components, achieves a combination of mechanical properties that combines high tensile strength with a certain degree of flexibility (elongation at break) at a lower curing temperature.

[0103] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A temperature-latent curing agent, characterized in that, It is prepared by reacting raw materials containing component A and component B under preset conditions; Wherein, component A is a polyamine compound containing at least one primary or secondary amine group, and component B is a polythiol compound; The molar ratio of the active amine hydrogen in component A to the thiol group in component B is 1:(0.9-1.1).

2. The temperature-latent curing agent according to claim 1, characterized in that, The polyamine compound is isophorone diamine, m-phenylenediamine, or polyetheramine D-230.

3. The temperature-latent curing agent according to claim 1, characterized in that, The polythiol compound is pentaerythritol tetra(3-mercaptopropionate) or trimethylolpropane tri(3-mercaptopropionate).

4. The temperature-latent curing agent according to claim 1, characterized in that, The molar ratio of the active amine hydrogen to the thiol group is 1:

1.

5. The temperature-latent curing agent according to claim 1, characterized in that, The curing agent is an ion-pair compound formed by a proton transfer reaction between the amine group of the polyamine compound and the thiol group of the polythiol compound.

6. A method for preparing a temperature-latent curing agent according to any one of claims 1-5, characterized in that, Includes the following steps: The polythiol compound was added dropwise to the polyamine compound at a temperature of 15–30°C, and the reaction was continued for 60–120 minutes after the addition was completed to obtain the temperature-latent curing agent.

7. The preparation method according to claim 6, characterized in that, The temperature conditions are 20–25°C.

8. The preparation method according to claim 6, characterized in that, The preparation method is carried out under solvent-free conditions.

9. A one-component epoxy curing system, characterized in that, It contains the following components in parts by weight: Epoxy resin: 100 parts; The temperature-latent curing agent according to any one of claims 1-5: 40-50 parts; Dielectric constant jump additive: 10-30 parts.

10. The single-component epoxy curing system according to claim 9, characterized in that, The dielectric constant jump additive is selected from propylene carbonate or sulfolane.

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