The mechanism of flame retardants is complex and not clear yet. It is generally believed that halogen compounds decompose when heated by fire, and the decomposed halogen ions react with polymer compounds to produce hydrogen halide. The latter reacts with the active hydroxyl radicals (Ho •) that proliferate massively during the combustion of polymer compounds, reducing their concentration and slowing down the combustion speed until the flame is extinguished. Among halogens, bromine has greater flame retardancy than chlorine. The role of phosphorus containing flame retardants is that when they burn, they form metaphosphoric acid, which polymerizes into a very stable poly state, becoming a protective layer of plastics and isolating oxygen. [1]
Flame retardant exerts its flame retardant effect through several mechanisms, such as endothermic effect, covering effect, chain reaction inhibition, asphyxiating effect of incombustible gas, etc. Most flame retardants achieve the purpose of flame retardancy through several mechanisms.
1. Endothermic action
The heat released by any combustion in a short time is limited. If a part of the heat released by the fire source can be absorbed in a short time, the flame temperature will be reduced, the heat radiated to the combustion surface and acting on cracking the vaporized combustible molecules into free radicals will be reduced, and the combustion reaction will be suppressed to a certain extent. Under high temperature conditions, the flame retardant has a strong endothermic reaction, absorbs part of the heat released by combustion, reduces the temperature of the surface of combustibles, effectively suppresses the generation of combustible gases, and prevents the spread of combustion. The flame-retardant mechanism of Al (OH) 3 flame retardant is to increase the heat capacity of polymer, so that it can absorb more heat before reaching the thermal decomposition temperature, so as to improve its flame-retardant performance. This kind of flame retardant gives full play to its large amount of heat absorption when combined with water vapor, and improves its own flame retardancy.
2. Covering effect
After the flame retardant is added to the combustible materials, the flame retardant can form a glass like or stable foam covering layer under high temperature, isolate oxygen, and have the functions of heat insulation, oxygen isolation, and preventing combustible gases from escaping outward, so as to achieve the purpose of flame retardancy. For example, organophosphorus flame retardants can produce crosslinked solid substances or carbonized layers with more stable structure when heated. On the one hand, the formation of the carbonized layer can prevent the further pyrolysis of the polymer, and on the other hand, it can prevent the internal thermal decomposition products from entering the gas phase to participate in the combustion process.
3. Inhibition chain reaction
According to the chain reaction theory of combustion, what is needed to maintain combustion is free radicals. The flame retardant can act on the gas phase combustion zone, capture the free radicals in the combustion reaction, so as to prevent the propagation of the flame, reduce the flame density in the combustion zone, and finally reduce the combustion reaction speed until it ends. For example, the evaporation temperature of halogen-containing flame retardant is the same or similar to the decomposition temperature of polymer. When the polymer is decomposed by heat, the flame retardant will also volatilize. At this time, the halogen-containing flame retardant and the thermal decomposition product are in the gas-phase combustion zone at the same time, so the halogen can capture the free radicals in the combustion reaction and interfere with the chain reaction of combustion.
4. Asphyxiating effect of incombustible gas
When the flame retardant is heated, it decomposes incombustible gas and dilutes the concentration of the combustible gas decomposed from the combustible to below the lower limit of combustion. At the same time, it also dilutes the oxygen concentration in the combustion zone, prevents the combustion from continuing, and achieves the flame retardant effect.
Post time: Sep-13-2022