1. Introduction

As an important chemical additive, flame retardants are widely used in various materials to improve their fire resistance. The mechanism of action of flame retardants is complex and diverse, but can be summarized into four major principles: heat absorption, covering, chain reaction inhibition and non-flammable gas asphyxiation. This article will explain these four principles in detail to better understand the working mechanism of flame retardants.

2. Endothermic effect

Heat absorption is an important mechanism of flame retardants. During the combustion process, the fire source will release a large amount of heat in a short period of time, causing the surrounding temperature to rise rapidly. Flame retardants undergo a strong endothermic reaction under high temperature conditions, absorbing part of the heat released by combustion, thereby reducing the temperature of the combustible surface. This process not only reduces the heat radiated to the combustion surface, but also reduces the heat required for the vaporized combustible molecules to break down into free radicals, thereby inhibiting the combustion reaction. Taking Al(OH)3 flame retardant as an example, it improves its flame retardant properties by increasing the heat capacity of the polymer so that it absorbs more heat before reaching the thermal decomposition temperature.

3. Covering effect

Covering is another important mechanism of flame retardants. After adding flame retardants to combustible materials, they can form a glassy or stable foam covering layer at high temperatures. This covering layer isolates oxygen and has the functions of heat insulation, oxygen isolation, and preventing the escape of combustible gases. In this way, flame retardants can prevent the spread of flames, thereby achieving the purpose of flame retardancy. For example, organophosphorus flame retardants can produce cross-linked solid substances or carbonized layers with more stable structures when heated. These substances can prevent further pyrolysis of polymers and prevent the thermal decomposition products inside them from entering the gas phase to participate in the combustion process.

4. Inhibit chain reaction

Inhibiting chain reactions is one of the key mechanisms of action of flame retardants. According to the chain reaction theory of combustion, free radicals are required to maintain combustion. Flame retardants can act on the gas phase combustion zone, capture free radicals in the combustion reaction, and thus prevent the spread of flames. In this process, flame retardants can reduce the flame density in the combustion zone, and ultimately reduce the combustion reaction rate until it stops. Halogen-containing flame retardants are a typical example, whose evaporation temperature is the same or similar to the polymer decomposition temperature. When the polymer is decomposed by heat, the flame retardant also evaporates at the same time. At this time, the halogen can capture free radicals in the combustion reaction, prevent the spread of flames and reduce the combustion rate.

5. Asphyxiation by non-flammable gases

The asphyxiation of non-combustible gases is another important mechanism of flame retardants. When heated, flame retardants decompose into non-combustible gases, which can dilute the concentration of combustible gases decomposed from combustibles and reduce it to below the lower limit of combustion. These non-combustible gases also dilute the oxygen concentration in the combustion zone, further preventing the combustion from proceeding. In this way, flame retardants can effectively inhibit the spread of flames and the continuation of combustion.

VI. Conclusion

The four principles of flame retardants - heat absorption, covering, chain reaction inhibition and non-flammable gas asphyxiation - together constitute the core mechanism of their fire protection performance. In practical applications, different types of flame retardants may exert multiple mechanisms of action at the same time to achieve the best fire protection effect. By deeply understanding and mastering these mechanisms of action, we can better use flame retardants to protect people's lives and property.