In many cases, the materials are required to have flame retardancy, the commonly used occasions are electronic appliances, and the automobile industry also has the flame retardant requirements, but generally low. Flame retardants can be achieved by adding flame retardants. Most plastics have flammability. With the wide application of plastics in building, furniture, transportation, aviation, aerospace and electrical appliances, improving the flame retardancy of plastics has become an urgent task.
Flame retardant also called fire-resistant or fire retardant, flame retardant polymer with flammable functional additives in nature; they are mostly in the periodic table of elements of VA (P), VIIA (bromine and chlorine) and ⅢA (antimony, aluminum) compound elements.
Having smoke suppression effects of molybdenum compounds, tin compounds and iron compounds are flame retardant category, mainly used in flame retardant plastics demand, delay or prevent plastic especially polymer plastics combustion, the ignition time of growth, light self extinguishing, it is hard to ignite.
Principle of flame retardancy
1) endothermic action
Any burning out in a relatively short period of time the heat is limited, if can absorb part of the heat released by the fire in a relatively short period of time, then the flame temperature will reduce the radiation to the molecular surface and combustible pyrolysis and combustion will have effect on gasification into free radicals will reduce the heat, combustion reaction will be suppressed to a certain extent.
Under high temperature, the flame retardant has a strong endothermic reaction, absorbs part of the heat released from combustion, reduces the surface temperature of combustibles, effectively inhibits the formation of flammable gases and prevents the spread of combustion. The flame retardancy mechanism of Al (OH) 3 flame retardants is to increase the heat capacity of the polymer and make it absorb more heat before reaching the thermal decomposition temperature, so as to improve its flame retardancy. This kind of flame retardant gives full play to the characteristics of large amount of heat absorption when it combines with water vapor to improve its own flame retardancy.
After adding flame retardants in combustible materials, flame retardant can form glass or stable foam overlay at high temperature, and isolate oxygen. It has the functions of heat insulation, oxygen isolation and preventing flammable gas from escaping outward, so as to achieve the purpose of flame retardancy.
If the organophosphorus flame retardants are heated, they can produce cross-linked solid material or carbonized layer which is more stable in structure.
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 thermal decomposition of the organism into the gas phase and participate in the combustion process.
3) inhibition of chain reaction
According to the chain reaction theory of combustion, the free radical is needed to maintain the combustion. The flame retardant can act on the gaseous combustion area, catch the free radicals in the combustion reaction, thus prevent the flame from spreading, and decrease the flame density of the combustion zone, and ultimately reduce the burning reaction rate until the termination.
If a halogen flame retardant is used, the evaporation temperature is the same or similar as the decomposition temperature of the polymer. When the polymer is heated and decomposed, the flame retardant is also volatilized simultaneously. At the same time, halogen containing flame retardants and thermal decomposition products are in the gas-phase combustion zone at the same time. Halogen can capture free radicals in the combustion reaction, thereby preventing the flame from spreading, reducing the flame density of the combustion zone, and ultimately decreasing the burning reaction rate until the termination.
4) the effect of unflammable gas asphyxiation
When the flame retardant is heated, the unflammable gas is decomposed, and the concentration of the combustible gas, which is decomposed from the combustibles, is diluted to the lower limit of the combustion limit. At the same time, it also has a dilution effect on the oxygen concentration in the combustion zone, which prevents the continuous combustion of the combustion and reaches the effect of flame retardancy.
PVC plastic is a flame retardant plastic and extinguish being deviated from fire,but it is not nonflammable. When fire is severe, PVC will burn and release toxic gases. Therefore, special requirements for the environment need to use PVC flame retardant formulations.