The mechanism of action of 4,4′-diaminodiphenylmethane as an epoxy resin curing agent and its formulation optimization

Overview of 4,4'-diaminodimethane (MDA) as an epoxy resin curing agent

4,4'-diaminodiphenylmethane (4,4'-Diaminodiphenylmethane, referred to as MDA) is an important organic compound and is widely used in high-performance composite materials, electronic packaging, aerospace and other fields. As a curing agent for epoxy resin, it has excellent mechanical properties, heat resistance and chemical stability. The MDA molecular structure contains two active amino groups, which can cross-link with the epoxy groups in the epoxy resin to form a three-dimensional network structure, thus imparting excellent mechanical properties and durability to the cured product.

The chemical formula of MDA is C13H12N2 and the molecular weight is 196.25 g/mol. Its appearance is white or light yellow crystalline powder, with a melting point of about 87-90°C and a density of 1.17 g/cm³. MDA has good solubility and can be soluble in common organic solvents such as, etc., but is insoluble in water. These physical properties make MDA highly operable and applicable in industrial applications.

In epoxy resin systems, MDA functions not only as a curing agent, it can also provide additional functions during the curing process. For example, MDA can increase the glass transition temperature (Tg) of the cured product, enhance the heat resistance and dimensional stability of the material. In addition, MDA can improve the toughness of epoxy resin, reduce the risk of brittle fracture, and make it perform better when withstand shock or vibration. Therefore, MDA plays an indispensable role in high-performance epoxy resin composites.

Reaction mechanism of MDA and epoxy resin

MDA, as a curing agent for epoxy resin, has a reaction mechanism mainly based on the chemical reaction between amino groups and epoxy groups. To better understand this process, we first need to understand the basic structure of MDA and epoxy resins and their reactive sites.

Structure and Reactive Activity of MDA

The molecular structure of MDA is connected by two rings through a methylene group (-CH2-), each with an amino group (-NH2) on each ring. These two amino groups are the main reactive sites of MDA, and they are able to open rings with the epoxy group (-O-CH2-CH2-O-) in the epoxy resin to form stable covalent bonds. Specifically, nitrogen atoms in the amino group carry lone pair of electrons, which can attack carbon atoms in the epoxy group, causing the epoxy ring to open and form new chemical bonds. This process not only consumes epoxy groups, but also forms hydroxyl groups (-OH) and imine groups (-NH-), further promoting the progress of the cross-linking reaction.

Structure and reactivity of epoxy resin

Epoxy resin is a type of polymer containing epoxy groups. The common types are bisphenol A (Bisphenol A) and epoxy chloride (Epichloro)Epoxy Resin (DGEBA) is a bisphenol A type epoxy resin (Epoxy Resin, DGEBA) made by polycondensation of ohydrin. The molecular chain of this epoxy resin contains multiple epoxy groups, which are the main reactive sites of the epoxy resin. When the epoxy resin is mixed with MDA, the epoxy group will quickly react with the amino group of MDA to form a crosslinking network.

Reaction steps and kinetics

The curing reaction between MDA and epoxy resin is usually divided into the following steps:

1. Initial contact stage: The amino group of MDA contacts the epoxy group in the epoxy resin for the first time, and a local crosslinking structure begins to form. At this time, the reaction rate is slow, mainly because the concentration of the reactants is low and the diffusion rate between the reactants is limited.

2. Fast reaction stage: As the reaction progresses, more epoxy groups are consumed and the crosslinking network gradually expands. At this time, the reaction rate is significantly accelerated because the newly formed hydroxyl and imine groups further promote the ring-opening reaction of the epoxy group. This stage is a critical period in the entire curing process, which determines the performance of the final cured product.

3. Crosslinking network formation stage: When most of the epoxy groups are consumed, the crosslinking network is basically formed. At this time, the reaction rate gradually slows down, and the remaining small amount of epoxy groups continues to react with the amino groups of MDA, further improving the crosslinking structure. Finally, the cured product exhibits a highly crosslinked three-dimensional network structure, which imparts excellent mechanical properties and heat resistance to the material.

Factors that affect reaction rate

The reaction rate of MDA and epoxy resin is affected by a variety of factors, mainly including the following points:

• Temperature: Temperature is one of the key factors affecting the reaction rate. Generally speaking, the higher the temperature, the faster the reaction rate. However, excessively high temperatures may lead to side reactions, affecting the quality of the cured product. Therefore, in practical applications, an appropriate curing temperature is usually selected to equilibrium the reaction rate and product quality.

• Catalytics: Appropriate catalysts can significantly increase the reaction rate and shorten the curing time. Commonly used catalysts include tertiary amine compounds, imidazole compounds, etc. These catalysts can promote the ring opening reaction of epoxy groups and accelerate the formation of cross-linking networks.

• Reactant ratio: The ratio of MDA to epoxy resin will also affect the reaction rate. Generally, the more MDA is used, the faster the reaction rate, but excessive MDA may lead to increased brittleness of the cured product. Therefore, reasonable control of MThe ratio of DA to epoxy is the key to optimizing the formulation.

• Ambient Humidity: Although MDA and epoxy resins themselves are not affected by humidity, in humid environments, moisture may react with epoxy groups to produce by-products, thereby reducing curing efficiency . Therefore, during the curing process, we should try to maintain a dry environment to avoid moisture interference.

Advantages and limitations of MDA as an epoxy resin curing agent

MDA, as an efficient epoxy resin curing agent, has many unique advantages, but also some limitations. Below we analyze the advantages and disadvantages of MDA from different perspectives and discuss how to overcome its limitations through formula optimization.

Advantages of MDA

1. Excellent mechanical properties
   The crosslinking network formed by the reaction of MDA with epoxy resin is very dense, giving the cured product extremely high strength and rigidity. Research has shown that epoxy resin composites cured with MDA have excellent tensile strength, compression strength and bending strength. For example, the tensile strength of MDA-cured epoxy resin can reach more than 100 MPa at room temperature, which is much higher than other types of curing agents. In addition, MDA can improve the impact resistance of the material, reduce the risk of brittle fracture, and make it perform better when withstand shock or vibration.

2. High heat resistance
   MDA-cured epoxy resins have high glass transition temperatures (Tg), usually between 150-200°C. This means that the material can still maintain good mechanical properties and dimensional stability in high-temperature environments, and is suitable for high-temperature applications such as aerospace and electronic packaging. Compared with other curing agents, MDA can significantly improve the heat resistance of epoxy resins and extend the service life of the material.

3. Good chemical stability
   MDA-cured epoxy resin has strong resistance to chemical substances such as acids, alkalis, and salts, and is not easily corroded or degraded. This makes the materials perform well in harsh chemical environments and are suitable for chemical equipment, anticorrosion coatings and other fields. In addition, MDA cured products also have excellent weather resistance and can be used outdoors for a long time without being affected by factors such as ultraviolet rays and moisture.

4. Low volatile and toxicity
   MDA has low volatility and hardly produces harmful gases during curing, reducing the harm to the environment and operators. Compared with some traditional curing agents (such as isocyanates), MDA is more safe and meets modern environmental protection requirements. In addition, MDA is low in toxicity and has long-term contactTouch has a small impact on human health and is suitable for use in areas such as food packaging and medical devices that require high safety requirements.

Lights of MDA

Although MDA has many advantages, it also has some limitations, which are mainly reflected in the following aspects:

1. Long curing time
   The reaction rate of MDA with epoxy resin is relatively slow, especially at low temperatures, and the curing time can be as long as hours or even days. This is an obvious disadvantage for some application scenarios that require rapid curing (such as on-site construction, rapid molding). To solve this problem, the reaction process can be accelerated by adding a catalyst or increasing the curing temperature, but this may increase costs or affect material performance.

2. More brittle
   Although MDA can improve the strength and rigidity of epoxy resins, it can also lead to increased brittleness of the material, especially in low temperature environments. This is because the MDA-cured crosslinking network is too dense, limiting the movement of the molecular chain, making the material prone to brittle fracture when it is subjected to external forces. To solve this problem, toughening agents (such as rubber, nanofillers) can be added to the formula to improve the toughness of the material while maintaining its high strength.

3. Rare price
   MDA is relatively high in production, resulting in its relatively expensive market price. This makes MDA less competitive in some cost-sensitive application areas (such as construction, furniture manufacturing). To solve this problem, cost can be reduced by optimizing the formulation, reducing the amount of MDA or finding alternative curing agents, while ensuring that the performance of the material is not affected.

4. Poor storage stability
   MDA is prone to moisture absorption at room temperature, especially in humid environments, which may cause it to deteriorate or fail. Therefore, the storage conditions of MDA are relatively strict and usually need to be stored in sealed and stored in a dry environment. This increases the difficulty of production and use, especially in large-scale industrial applications, which can cause inconvenience. To solve this problem, it is possible to consider developing new moisture-proof packaging materials or modified MDA to improve its storage stability.

Recipe Optimization Strategy

To give full play to the advantages of MDA as an epoxy resin curing agent while overcoming its limitations, formulation optimization is crucial. Through reasonable formulation design, the performance of cured products can be effectively improved, production costs can be reduced, and the needs of different application scenarios can be met. Here are several common recipe optimization strategies:

1. Add toughener

Although MDA-cured epoxy resin has excellent strength and rigidity, it is highly brittle, especially in low-temperature environments, it is prone to brittle fracture. To solve this problem, an appropriate amount of toughening agent can be added to the formula to improve the toughness of the material. Common toughening agents include:

• Rubber toughening agents: such as carboxy-butylnitrile rubber (CTBN), terminal carboxy-polybutadiene (PTC), etc. These rubber tougheners can form an interpenetrating network structure (IPN) with epoxy resin during the curing process, effectively dispersing stress and preventing crack propagation. Studies have shown that adding an appropriate amount of rubber toughener can increase the impact strength of the cured product by 2-3 times while maintaining its high strength.

• Thermoplastic toughening agents: such as polyether sulfone (PES), polycarbonate (PC), etc. These thermoplastic tougheners can form a blend system with epoxy resin during the curing process, significantly improving the toughness and impact resistance of the material. In addition, thermoplastic toughener also has good processing properties, which facilitates subsequent molding and processing.

• Nanofillers: such as nanosilica (SiO2), nanoclay, etc. These nanofillers can enhance the toughness of the material at the microscopic scale while improving its mechanical properties and heat resistance. Studies have shown that adding an appropriate amount of nanofiller can increase the tensile strength and modulus of the cured product by 10%-20%, respectively, and significantly improve its fatigue resistance.

2. Use catalyst

The reaction rate of MDA with epoxy resin is relatively slow, especially at low temperatures, and the curing time may last for several hours or even days. To solve this problem, an appropriate amount of catalyst can be added to the formula to accelerate the reaction process. Commonly used catalysts include:

• Term amine catalysts: such as triethylamine (TEA), benzyl di(BDMA), etc. These catalysts can promote the ring opening reaction of epoxy groups and significantly increase the reaction rate. Studies have shown that adding an appropriate amount of tertiary amine catalyst can shorten the curing time to 1-2 hours without affecting the performance of the cured product.

• imidazole catalysts: such as 2-methylimidazole (2MI), 2-ylimidazole (2PI), etc. These catalysts have high catalytic efficiency and can accelerate the reaction process at lower temperatures. In addition, imidazole catalysts also have good heat resistance and stability, and are suitable for high-temperature curing applications.

• Metal complex catalysts: such as tetrabutyl titanate (TBOT), triisopropyl aluminate (TAA), etc. These metal complex catalysts can promote the ring opening reaction of epoxy groups through coordination, significantly increasing the reaction rate. Studies have shown that adding an appropriate amount of metal complex catalyst can shorten the curing time to less than 30 minutes, while improving the heat resistance and chemical stability of the cured product.

3. Control the ratio of reactants

The ratio of MDA to epoxy resin has an important influence on the performance of the cured product. Generally speaking, the more MDA is used, the greater the cross-linking density of the cured product, the higher the strength and rigidity, but the brittleness will also increase accordingly. Therefore, rationally controlling the ratio of MDA to epoxy resin is the key to optimizing the formulation. Generally, the molar ratio of MDA to epoxy resin is about 1:1, but in actual applications, it can be adjusted appropriately according to specific needs. For example:

• Increase the dosage of MDA: If you need to obtain higher strength and rigidity, you can appropriately increase the dosage of MDA. Studies have shown that when the molar ratio of MDA to epoxy resin is increased to 1.2:1, the tensile strength and modulus of the cured product are increased by 15%-20%, respectively, but the brittleness also increases accordingly. To solve this problem, a proper amount of toughener can be added to the formula to balance strength and toughness.

• Reduce the dosage of MDA: If you need to obtain better toughness and processing performance, you can appropriately reduce the dosage of MDA. Studies have shown that when the molar ratio of MDA to epoxy resin is reduced to 0.8:1, the impact strength of the cured product is significantly improved while maintaining a high tensile strength and modulus. In addition, reducing the amount of MDA can also reduce costs and improve economic benefits.

4. Introduce functional additives

In order to impart more functionality to the cured product, some functional additives can be introduced into the formulation. For example:

• Conductive fillers: such as graphene, carbon nanotubes, silver powder, etc. These conductive fillers can form a conductive network in the cured product, imparting excellent electrical conductivity to the material. Research shows that adding an appropriate amount of conductive filler can reduce the resistivity of the cured product to below 10^-3 Ω·cm, and is suitable for electromagnetic shielding, conductive coatings and other fields.

• Flame retardants: such as aluminum hydroxide (ATH), magnesium hydroxide (MDH), phosphorus-based flame retardants, etc. These flame retardants can form a thermal insulation layer in the cured product, preventing flame spread and improving the fire resistance of the material. Studies have shown that adding an appropriate amount of flame retardant can increase the limit oxygen index (LOI) of the cured product to more than 30%, reaching the UL94 V-0 flame retardant standard.

• Light stabilizers: such as ultraviolet absorbers (UVAs), light stabilizers (HALS), etc. These light stabilizers can absorb or reflect ultraviolet rays, preventing the material from degrading under long-term light and prolonging its service life. Studies have shown that adding an appropriate amount of light stabilizer can significantly improve the weather resistance of the cured products and are suitable for long-term outdoor use.

5. Optimize the curing process

In addition to formula optimization, the selection of curing process also has an important impact on the performance of cured products. In order to obtain an excellent curing effect, appropriate curing process parameters such as temperature, pressure, time, etc. can be selected. For example:

• Increase the curing temperature: Within a certain range, increasing the curing temperature can significantly speed up the reaction rate and shorten the curing time. Studies have shown that when the curing temperature is increased from 80°C to 120°C, the curing time can be shortened from 6 hours to 2 hours, while the mechanical properties and heat resistance of the cured products are improved.

• Use segmented curing: For complex products or thick-walled parts, segmented curing can be used, that is, initial curing is performed first at a lower temperature and then at a higher temperature Secondary curing. This can prevent excessive internal stresses generated during one curing process, resulting in deformation or cracking of the product. Research shows that using the segmented curing process can obtain a more uniform crosslinked structure, which improves the dimensional stability and mechanical properties of the cured products.

• Apply pressure: Applying a certain pressure during the curing process can promote the diffusion of reactants, increase cross-linking density, and reduce the formation of bubbles and pores. Studies have shown that applying a pressure of 0.1-0.5 MPa can increase the density of cured products by 5%-10%, while improving their surface quality and mechanical properties.

Progress in domestic and foreign research and future prospects

In recent years, domestic and foreign scholars have made significant progress in the research of MDA as an epoxy resin curing agent, especially in the fields of formulation optimization, reaction mechanism and application. The following is a review of relevant research progress and a prospect for future development directions.

Progress in domestic and foreign research

1. In-depth study of reaction mechanism
   Early research mainly focused on the reaction mechanism between MDA and epoxy resin, revealing the ring-opening reaction process between amino groups and epoxy groups. In recent years, with the advancement of experimental techniques and theoretical simulation methods, researchers have gained a deeper understanding of reaction kinetics, crosslink network structures, and side reaction mechanisms. For example, Li et al.[1] via In-situ InfraredSpectroscopy (FTIR) and nuclear magnetic resonance (NMR) technology monitored the reaction process between MDA and epoxy resin in real time. It was found that the initial stage of the reaction was mainly monosubstituted products, and then the multisubstituted products and crosslinked structures gradually formed. In addition, Wang et al. [2] used molecular dynamics simulation (MD) to study the reaction path between MDA and epoxy resin, revealing the interaction and energy change laws between reactant molecules, providing a theoretical basis for optimizing reaction conditions.

2. Research on formula optimization
   To improve the performance of MDA cured epoxy resin, the researchers carried out a lot of formulation optimization work. For example, Zhang et al. [3] successfully prepared high-strength and high-toughness epoxy resin composite materials by introducing nano-silicon dioxide (SiO2) as a toughening agent. Studies have shown that the addition of nano SiO2 not only improves the tensile strength and modulus of the cured product, but also significantly improves its impact resistance. In addition, Chen et al. [4] developed a new type of imidazole catalyst that can quickly cure the MDA/epoxy resin system at low temperatures, shortening the curing time and reducing energy consumption. The catalyst also has good heat resistance and stability, and is suitable for high-temperature curing applications.

3. Expansion of application fields
   With the continuous improvement of MDA cured epoxy resin performance, its application areas are also expanding. For example, in the field of aerospace, MDA cured epoxy resin is widely used in key parts such as aircraft structural parts and engine parts due to its excellent heat resistance and dimensional stability. Studies have shown that the glass transition temperature (Tg) of MDA cured epoxy resin can reach above 200°C, and can maintain good mechanical properties under high temperature environments. In addition, in the field of electronic packaging, MDA cured epoxy resin is widely used in high-end electronic products such as integrated circuits and semiconductor devices due to its excellent electrical insulation properties and chemical corrosion resistance. Research shows that the dielectric constant of MDA cured epoxy resin is as low as below 3.0, which can effectively reduce signal transmission losses and improve the performance of electronic products.

Future Outlook

Although MDA has achieved remarkable research results as an epoxy resin curing agent, there are still many challenges to be solved. Future research can be carried out from the following aspects:

1. Develop new curing agents
   In order to further improve the performance of cured products, researchers can explore and develop new types of curing agents, such as sulfur and phosphorus-containing functional curing agents. These curing agents can not only react with epoxy groups, but also impart more functions to the material, such as flame retardant, conductivity, self-healing, etc. In addition, curing agents with special structures and properties can also be developed through molecular design and synthesis technology to fully realize the development of curing agents with special structures and properties.Suitable for the needs of different application scenarios.

2. Green and sustainable development
   With the continuous improvement of environmental awareness, the development of green and sustainable curing agents has become an important development direction in the future. For example, researchers can explore the use of renewable resources such as natural vegetable oils and biomass as raw materials to develop green and environmentally friendly curing agents. These curing agents not only have excellent performance, but also reduce dependence on fossil resources and reduce environmental pollution. In addition, biodegradable curing agents can be developed through biodegradable technology to realize the recycling of materials and promote the development of green chemistry.

3. Research and Development of Smart Materials
   Smart materials refer to materials that can sense changes in the external environment and respond to them. Future research can develop smart materials with functions such as self-healing, shape memory, and sensing based on the characteristics of MDA cured epoxy resin. For example, by introducing self-healing agents or shape memory polymers, the cured product can be given the self-healing ability and shape memory function, so that it can be automatically repaired after being damaged and restored to its original performance. In addition, it is also possible to develop intelligent materials with sensing functions by introducing conductive fillers or piezoelectric materials to achieve real-time monitoring and feedback.

4. Scale of industrial applications
   Although MDA cured epoxy resins exhibit excellent performance in laboratories, their scale production in industrial applications still faces many challenges. Future research can focus on issues such as how to reduce production costs, improve production efficiency, and optimize production processes. For example, by developing high-efficiency catalysts, improving curing processes, optimizing formulation design, etc., the production efficiency of MDA cured epoxy resin can be significantly improved, production costs can be reduced, and its wide application in more fields can be promoted.

Summary

4,4'-diaminodimethane (MDA) is a curing agent for epoxy resin. With its excellent mechanical properties, high heat resistance and good chemical stability, it is used in high-performance composite materials, electronic packaging, Aerospace and other fields have been widely used. Through in-depth research on the reaction mechanism between MDA and epoxy resin, we learned that the amino groups of MDA can open rings with epoxy groups, forming a dense crosslinking network structure, giving excellent performance to the cured product. However, MDA also has limitations such as long curing time, high brittleness and high price. Through reasonable formulation optimization strategies, such as adding toughener, using catalysts, controlling the proportion of reactants, introducing functional additives and optimizing the curing process, these limitations can be effectively overcome, further improving the performance of cured products, and meeting the needs of different application scenarios. .

In the future, with the continuous deepening of research and continuous innovation of technology, MDA will be solidifiedEpoxy resins are expected to be widely used in more fields. Especially in the development of new curing agents, green and sustainable development, smart material research and development, and scale of industrial applications, MDA cured epoxy resin will usher in broader development prospects.

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