Effects of Different Curing Methods on Flame Retardancy and Bonding Strength of Fireproof Adhesives and Optimization of Curing Process Parameters

Fireproof adhesives are widely used in construction, electronics, aerospace, and other fields. Their flame retardancy and bonding strength are key indicators for measuring product quality. As a critical step in the forming process of fireproof adhesives, the curing method significantly affects these two properties. Selecting appropriate curing methods and optimizing their process parameters can effectively enhance the comprehensive performance of fireproof adhesives to meet the requirements of different application scenarios.

Effects of Different Curing Methods on Flame Retardancy of Fireproof Adhesives

1. Room Temperature Curing

Room temperature curing fireproof adhesives achieve curing mainly through slow chemical reactions between moisture or oxygen in the air and active groups in the adhesive. In terms of flame retardancy, due to the slow curing speed, the distribution of flame retardants within the fireproof adhesive is relatively uniform, but the cross – linked network formed during curing is not dense enough. This makes it easier for flame retardants to volatilize and migrate at high temperatures, reducing the sustained effect of flame retardants. Meanwhile, the slow curing process may cause some flame retardants to fail to fully participate in the reaction, unable to fully exert their flame – retarding efficiency. Therefore, the flame retardancy of room temperature curing fireproof adhesives is relatively limited, and their fireproof capability will decline under long – term high temperatures or strong fire sources.

2. Heat Curing

Heat curing accelerates the chemical reaction rate of each component in the fireproof adhesive by raising the temperature, promoting the cross – linking reaction. Under high – temperature action, the fireproof adhesive can quickly form a dense cross – linked network structure, which can effectively restrict the migration and volatilization of flame retardants, enabling flame retardants to continuously function during combustion. For example, some phosphorus – and nitrogen – containing flame retardants can chemically react with the matrix of the fireproof adhesive during heat curing to form an intumescent char layer, which has good heat and oxygen insulation properties, thereby significantly improving the flame retardancy of the fireproof adhesive. However, if the heating temperature is too high or the time is too long, it may cause the decomposition and failure of flame retardants, instead reducing the flame – retarding effect.

3. Light Curing

In light – curing fireproof adhesives, under the irradiation of specific wavelength light, photoinitiators absorb light energy to generate free radicals, initiating monomer polymerization and cross – linking reactions. Light curing is extremely fast and can form a cured layer in a short time. For flame retardancy, rapid curing can quickly fix flame retardants in place, preventing their migration during the curing process. However, due to the limited light penetration ability, for thicker fireproof adhesive layers, the surface may be well – cured while the interior is incompletely cured, affecting the overall flame retardancy. In addition, internal stress may be generated during light curing, causing micro – cracks in the adhesive layer. These cracks may become channels for heat and oxygen transfer during combustion, weakening the flame – retarding effect.

Effects of Different Curing Methods on Bonding Strength of Fireproof Adhesives

1. Room Temperature Curing

The room temperature curing process is mild, causing little thermal impact on the bonded materials and being less likely to cause material deformation or damage, making it suitable for bonding temperature – sensitive materials. However, due to the slow curing speed, the molecular diffusion and penetration process between the adhesive and the surface of the bonded material are also relatively slow, making it difficult to form strong chemical bonding and physical adsorption. Moreover, during the long curing process, external environmental factors (such as humidity, dust, etc.) easily interfere with the adhesive layer, leading to defects at the bonding interface, which limits the improvement of the bonding strength of room temperature curing fireproof adhesives.

2. Heat Curing

Heating can accelerate the diffusion and interaction between adhesive molecules and the surface molecules of the bonded material, promoting the formation of chemical bonds and the enhancement of physical adsorption, which is conducive to improving the bonding strength. At the same time, the rapidly formed dense cross – linked network structure also provides a more stable mechanical support for bonding. However, excessively high heating temperatures may cause thermal deformation, aging, and other problems of the bonded materials, reducing the compatibility between the material surface and the adhesive, and instead leading to a decrease in bonding strength. In addition, the thermal stress generated during heat curing may also form defects at the interface between the adhesive layer and the bonded material, affecting the bonding performance.

3. Light Curing

The cured layer formed instantly by light curing can quickly bond the bonded materials together, which has advantages for some bonding scenarios requiring rapid positioning and fixing. Due to the rapid curing process, the adhesive may cure before fully penetrating into the micro – structures of the bonded material surface, resulting in insufficient physical meshing between the adhesive layer and the bonded material. Moreover, if the internal stress generated by light curing cannot be effectively released, stress concentration will occur at the bonding interface, reducing the bonding strength. Especially under external force, the adhesive layer is prone to separate from the bonded material.

Optimization Methods for Curing Process Parameters

1. Room Temperature Curing

• Temperature and Humidity Control: Appropriately increasing the temperature of the room temperature curing environment (generally controlled at 20 – 30℃) can accelerate the curing speed to a certain extent and promote the reaction and diffusion between molecules. At the same time, controlling the environmental humidity within a suitable range (40% – 60%) can avoid the formation of a water film on the surface of the adhesive layer, which affects the curing quality and bonding strength.
• Addition of Promoters: Adding an appropriate amount of curing promoters, such as organic amine promoters, to the fireproof adhesive formula can accelerate the curing reaction process, shorten the curing time, and increase the cross – linking density, thereby improving flame retardancy and bonding strength. However, the amount of promoters added needs to be noted, as excessive addition may affect the storage stability of the fireproof adhesive.

2. Heat Curing

• Optimization of Heating Temperature and Time: Determine the optimal curing temperature and time curve for the fireproof adhesive through experiments. Generally, thermal analysis experiments such as differential scanning calorimetry (DSC) are first carried out to understand the exothermic peak and reaction process of the fireproof adhesive’s curing reaction, and based on this, a reasonable heating temperature range (usually slightly lower than the exothermic peak temperature) and curing time are set. For example, for a certain type of fireproof adhesive, heating at 120 – 150℃ for 2 – 3 hours can obtain better curing effects, which can not only ensure sufficient cross – linking but also avoid the decomposition of flame retardants and damage to the bonded materials.
• Stage Heating: Adopt a staged temperature – rising method for curing. First, cure the fireproof adhesive at a lower temperature to reduce the generation of internal stress, and then gradually increase the temperature to complete the full curing. This method can effectively improve the problem of internal stress caused by rapid heating and improve the stability of bonding strength and flame retardancy.

3. Light Curing

• Selection of Suitable Photoinitiators and Light Sources: Select photoinitiators whose absorption wavelength matches the emission wavelength of the light source according to the formula and curing requirements of the fireproof adhesive to improve the photoinitiation efficiency. At the same time, reasonably select the type (such as UV LED lights, mercury lamps, etc.), power, and irradiation distance of the light source. Generally, the higher the light source power and the closer the irradiation distance, the greater the light intensity and the faster the curing speed. However, too high light intensity may cause excessive surface curing while insufficient internal curing, so the optimal parameters need to be determined through experiments.
• Multiple Light Curing: For thicker fireproof adhesive layers, use multiple light curing methods. After each light irradiation, allow the adhesive layer to have a certain time for internal reactions and stress relaxation, and then carry out the next light irradiation. This can effectively improve the overall curing degree, reduce internal stress and defects, and improve flame retardancy and bonding strength.

Conclusion

Different curing methods have complex effects on the flame retardancy and bonding strength of fireproof adhesives. Room temperature curing is mild but has limited performance improvement; heat curing can significantly improve performance but requires attention to temperature and time control; light curing is fast but has problems of uneven curing and internal stress. By optimizing their respective curing process parameters, such as controlling environmental conditions, adjusting temperature – time curves, and selecting appropriate initiation systems, the shortcomings of each curing method can be made up to a certain extent, achieving the coordinated improvement of flame retardancy and bonding strength of fireproof adhesives to meet the application requirements of fireproof adhesives in different fields. In the future, with the continuous development of materials science and curing technology, it is believed that more efficient and environmentally friendly curing methods and optimization methods will be applied to the production and application of fireproof adhesives.

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