Exploration of the Flame Retardant Mechanisms and Synergistic Effects of Different Types of Flame Retardants in the Glue of Automatic Fire Suppression Material

In modern materials science, glue, as a widely used adhesive, its safety is of crucial importance. Especially in Automatic Fire Suppression Material, the flame retardant performance of the glue is directly related to the fire prevention effect of the material and the safety of people and property in case of a fire. The addition of flame retardants is a key means to improve the flame retardant performance of the glue. Different types of flame retardants, such as phosphorus-based, halogen-based, and nitrogen-based ones, play their flame retardant roles in unique ways. Meanwhile, the synergistic effect among them provides more possibilities for optimizing the flame retardant performance of the glue. In-depth research on the flame retardant mechanisms, differences, and synergistic effects of these flame retardants in the glue is of great theoretical and practical significance for the development of high-performance fireproof glue.

Basic Overview of Flame Retardants

Flame retardants are functional additives that can endow flammable polymers with flame retardancy. According to their composition and structure, they are mainly divided into inorganic flame retardants and organic flame retardants. Among them, organic flame retardants include halogen-based, phosphorus-based, nitrogen-based, etc. In the glue of Automatic Fire Suppression Material, these flame retardants inhibit or slow down the combustion process of the glue through different physical and chemical actions.

Flame Retardant Mechanisms of Different Types of Flame Retardants

Phosphorus-based Flame Retardants

1. Condensed Phase Flame Retardant Mechanism: Phosphorus-containing additives mainly function in the condensed phase. When the glue is heated, the phosphorus-based flame retardants first decompose to form phosphoric acid, which further dehydrates to form metaphosphoric acid, and finally polymerizes into polyphosphoric acid. Polyphosphoric acid has strong dehydrating properties, which can promote the dehydration and charring of the base material of the glue, forming a dense char layer on the surface of the glue. This char layer has the functions of heat insulation and oxygen isolation, reducing the heat conduction from the flame to the condensed phase, preventing the diffusion of oxygen into the interior of the glue, and also inhibiting the escape of combustible gases from the glue, thus effectively preventing the spread of combustion. For example, in some studies on the application of organic phosphorus-based flame retardants in epoxy resin glue, it was found that the polyphosphoric acid generated by the decomposition of the flame retardant rapidly carbonizes the epoxy resin matrix, and the increased thickness of the formed char layer significantly improves the flame retardant performance of the glue.
2. Gas Phase Flame Retardant Mechanism: During the combustion process, the phosphorus-based flame retardants will decompose to generate PO· free radicals when heated. These free radicals can capture the active free radicals such as H· and HO· in the combustion reaction, thus interrupting the chain reaction of combustion. PO· + H· = HPO. By reducing the concentration of free radicals in the flame, the rate of the combustion reaction is decreased, achieving the flame retardant effect. In addition, some volatile phosphorus compounds generated during the decomposition of phosphorus-based flame retardants can also dilute the oxygen and combustible gases around the flame, further inhibiting the combustion.

Halogen-based Flame Retardants

3. Gas Phase Flame Retardant Mechanism: Halogen-based flame retardants decompose to generate hydrogen halide (HX) at high temperatures. Hydrogen halide can react with the active substances HO· in the chain reaction of the flame. HX + HO·→X· + H2O, reducing the concentration of free radicals, thus slowing down or terminating the chain reaction of combustion and achieving the flame retardant purpose. Taking bromine-based flame retardants as an example, the HBr generated by their decomposition can effectively capture the HO· free radicals in the flame and inhibit the combustion reaction. At the same time, the relatively high density of hydrogen halide will cover the surface of the glue, blocking the contact between oxygen and the glue and playing the role of isolating the air.
4. Condensed Phase Flame Retardant Mechanism: The residues after the decomposition of halogen-based flame retardants to produce HX under combustion conditions can promote the dehydration and charring of polymer materials, forming a flame-retardant charred layer. For example, in the research on the application of halogen-based flame retardants in polyolefin glue, the hydrogen halide generated by the decomposition of the flame retardant causes the cross-linking and charring of the polyolefin matrix, and the formed charred layer can prevent the transfer of heat and oxygen, improving the flame retardant performance of the glue. However, when halogen-based flame retardants burn, they will produce a large amount of smoke and toxic and corrosive gases, which limits their application in some fields with high environmental and safety requirements.

Nitrogen-based Flame Retardants

5. Gas Phase Flame Retardant Mechanism: Nitrogen-based flame retardants decompose to form non-combustible gases such as NH3 and N2 during combustion. These gases can dilute the flammable gases and reduce the oxygen concentration, thus inhibiting the combustion. For example, melamine and its derivatives, as common nitrogen-based flame retardants, during the combustion of the glue, the ammonia and nitrogen gases generated by decomposition can effectively dilute the combustible gases around the flame, making it difficult for the combustion to continue. At the same time, these non-combustible gases can also take away part of the heat, reducing the temperature of the glue and further playing a flame retardant role.
6. Condensed Phase Flame Retardant Mechanism: Some nitrogen-based flame retardants can chemically react with the glue matrix to form an expanded char layer structure. This char layer will expand when heated, forming a porous and dense protective layer to isolate the transfer of oxygen and heat, thus achieving the flame retardant purpose. For example, in some nitrogen-phosphorus-based flame retardants in the glue, the nitrogen element and the phosphorus element work synergistically to form an expanded char layer in the condensed phase, improving the flame retardant performance of the glue.

Differences in the Action Mechanisms of Different Types of Flame Retardants

7. Different Action Stages: The action of phosphorus-based flame retardants in the condensed phase runs through the entire combustion process, from promoting char formation to forming a stable char layer protection; halogen-based flame retardants mainly capture free radicals rapidly in the gas phase and have a significant effect on inhibiting the chain reaction at the initial stage of combustion; nitrogen-based flame retardants, during the combustion process, dilute the combustible gases in the gas phase by decomposing to produce non-combustible gases, and it is also common to form an expanded char layer in the condensed phase. For example, at the initial stage of a fire, the halogen-based flame retardants quickly decompose to produce hydrogen halide to capture free radicals, while the phosphorus-based flame retardants gradually begin to decompose to promote char formation, and the nitrogen-based flame retardants also start to release non-combustible gases.
8. Different Action Modes: Phosphorus-based flame retardants mainly act through chemical reactions, such as dehydration to form char and capture free radicals; halogen-based flame retardants mainly rely on the chemical reaction between the decomposition products and free radicals and the physical covering and blocking effect; nitrogen-based flame retardants mainly act through the physical dilution of non-combustible gases produced by decomposition and, in some cases, the physical blocking effect of forming an expanded char layer. For example, the formation of a char layer by phosphorus-based flame retardants is a chemical change, the decomposition of halogen-based flame retardants to produce hydrogen halide is a chemical change, but its covering and blocking is a physical change, and both the release of non-combustible gases and the formation of an expanded char layer by nitrogen-based flame retardants mainly involve physical changes.
9. Different Product Effects: Halogen-based flame retardants produce toxic, corrosive gases and a large amount of smoke when burning, which pose great harm to the environment and human body; the combustion products of phosphorus-based flame retardants are relatively more environmentally friendly, but they may have a certain impact on some physical properties of the glue; the combustion products of nitrogen-based flame retardants are usually non-combustible gases, which are more environmentally friendly, but their flame retardant efficiency is relatively low when used alone. For example, the hydrogen halide gas produced by the combustion of halogen-based flame retardants is corrosive and can damage equipment and the human respiratory tract. Phosphorus-based flame retardants may slightly reduce the flexibility of the glue, and nitrogen-based flame retardants need to be used in combination with other flame retardants to better exert their flame retardant effect.

Synergistic Effects of Different Types of Flame Retardants

10. Phosphorus-Halogen Synergistic Effect: When phosphorus-based flame retardants and halogen-based flame retardants are used together, there is a synergistic effect. The phosphorus element can promote the formation of the char layer, while the halogen element captures free radicals in the gas phase. The two cooperate with each other to improve the flame retardant effect. For example, in some studies, it was found that when an organic phosphorus-based flame retardant and a bromine-based flame retardant were simultaneously added to polycarbonate glue, compared with using a single flame retardant, the flame retardant performance of the glue was significantly improved, the oxygen index was obviously increased, and the combustion time was shortened. This is because the char layer formed by the phosphorus-based flame retardant can provide an attachment point for the hydrogen halide generated by the decomposition of the halogen-based flame retardant, making it more effective in capturing free radicals. At the same time, the halogen-based flame retardant inhibits the oxidation of the char layer and enhances the stability of the char layer.
11. Phosphorus-Nitrogen Synergistic Effect: When phosphorus-based flame retardants and nitrogen-based flame retardants act synergistically, they can also enhance the flame retardant effect. The phosphorus element promotes char formation, and the nitrogen element decomposes to produce non-combustible gases, and the two complement each other. When phosphorus-nitrogen-based flame retardants are added to some epoxy resin glues, the results show that a more dense and expanded char layer is formed when the glue burns, and the generated non-combustible gases effectively dilute the combustible gases, greatly improving the flame retardant performance of the glue. This is because the ammonia gas and other gases generated by the decomposition of the nitrogen-based flame retardant can promote the decomposition of the phosphorus-based flame retardant and accelerate the char formation process. At the same time, the char layer formed by the phosphorus-based flame retardant can fix the gases generated by the decomposition of the nitrogen-based flame retardant, enhancing the heat insulation and oxygen isolation effects.
12. Halogen-Nitrogen Synergistic Effect: When halogen-based flame retardants and nitrogen-based flame retardants are used synergistically, the flame retardant effect of halogen-based flame retardants in the gas phase and the dilution effect of non-combustible gases generated by the decomposition of nitrogen-based flame retardants are combined, which can also improve the flame retardant performance of the glue. For example, in some rubber glues, when a bromine-based flame retardant and a melamine-based nitrogen-based flame retardant are added at the same time, it is found that the flame retardant performance of the glue is improved. This is because the hydrogen halide generated by the decomposition of the halogen-based flame retardant reacts with the ammonia gas and other gases generated by the decomposition of the nitrogen-based flame retardant to form a compound with a synergistic flame retardant effect. At the same time, the ammonia gas and other gases also dilute the combustible gases around the flame, improving the flame retardant effect.

Conclusion

Different types of flame retardants play their flame retardant roles in the glue of Automatic Fire Suppression Material through their unique flame retardant mechanisms. Phosphorus-based flame retardants focus on char formation and free radical capture in the condensed phase, halogen-based flame retardants mainly inhibit the chain reaction and block oxygen in the gas phase, and nitrogen-based flame retardants act in both the gas phase and the condensed phase by decomposing to produce non-combustible gases. There are obvious differences in their action mechanisms, but in practical applications, through reasonable compounding and synergistic effects, the flame retardant performance of the glue can be significantly improved. In the future, with the continuous improvement of environmental protection and safety requirements, the research and development of more efficient, environmentally friendly, and low-toxic flame retardants and in-depth exploration of the synergistic action mechanisms among them will be an important development direction in the field of glue flame retardancy. This will not only help improve the application safety of glue in fields such as automatic fire suppression materials but also promote the technological progress of the entire flame retardant materials industry.

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