The role of flame retardants
1, endothermic effect
The heat released by any combustion in a short period of 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 lowered, radiated to the combustion surface and acted on the gasification. The heat of flammable molecules cleavage into free radicals is reduced, and the combustion reaction is suppressed to some extent. Under high temperature conditions, the flame retardant undergoes a strong endothermic reaction, absorbs part of the heat released by combustion, reduces the temperature of the surface of the combustible material, effectively suppresses the formation of flammable gases, and prevents the spread of combustion. The flame retardant mechanism of Al(OH)3 flame retardant is to increase the heat capacity of the polymer to absorb more heat before reaching the thermal decomposition temperature, thereby improving its flame retardant performance. Such flame retardants fully exert their characteristics of inhaling heat when combined with water vapor, and improve their own flame retardant ability.
After the flame retardant is added to the combustible material, the flame retardant can form a glass-like or stable foam coating layer at high temperature to isolate oxygen, and has the functions of heat insulation, oxygen barrier, and preventing the combustible gas from escaping outward, thereby achieving flame retardant. purpose. For example, an organophosphorus flame retardant can produce a crosslinked solid material or a carbonized layer which is more stable in structure when heated. The formation of the carbonized layer on the one hand prevents the polymer from further pyrolysis, and on the other hand prevents the internal thermal decomposition products from entering the gas phase to participate in the combustion process.
3. Suppressing chain reaction
According to the chain reaction theory of combustion, it is the free radicals required to sustain combustion. The flame retardant acts on the gas phase combustion zone to capture free radicals in the combustion reaction, thereby preventing the flame from propagating, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction rate until termination. For example, a halogen-containing flame retardant whose evaporation temperature and polymer decomposition temperature are the same or similar, when the polymer is thermally decomposed, the flame retardant also volatilizes. At this time, the halogen-containing flame retardant and the thermal decomposition product are simultaneously in the gas phase combustion zone, and the halogen can capture the free radicals in the combustion reaction and interfere with the combustion chain reaction.
4, non-combustible gas suffocation
When the flame retardant is heated, the non-combustible gas is decomposed, and the concentration of the combustible gas which decomposes the combustible material is diluted to below the lower limit of combustion. At the same time, it also has a diluting effect on the oxygen concentration in the combustion zone, preventing the combustion from proceeding and achieving the flame retarding effect.