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Halogen flame retardants include single halogen flame retardants and halogen/antimony synergistic flame retardant systems.
Halogen flame retardants mainly exert their effects in the gas phase.
When materials undergo thermal cracking, they can react with oxygen in the atmosphere to form the H2-O2 system: H ·+HX · → H2+X ·, which is a reversible reaction. When the temperature increases, the equilibrium constant decreases, so the flame retardant efficiency of halogen derivatives will decrease in high fire, and temperature affects the flame retardant efficiency of halogen containing compounds. The equilibrium constant K, the larger K, the greater the degree of reaction.
1200-1300 ℃ and higher temperatures - Bromine has oxidizing properties and can catalyze chain reactions in the gas phase, and there is no flame retardant effect at this time; At 230-1230 ℃, the equilibrium constant KHBr>>KHCl. Bromides and chlorides have higher flame retardancy at the ignition temperature of polymers.
The flame retardant efficiency of fluoride is low - the activation energy of the reaction between HF and H · is too high to occur.
The flame retardant efficiency of iodide is also low - the interaction between RH and I · does not easily generate HI.
The flame retardant efficiency of chloride is lower than that of bromide, which may be due to the fact that the reaction between HCl and H · is close to thermal neutrality, and this reaction is highly likely to proceed in the opposite direction, regenerating H ·.
· The flame retardant mechanism of halogenated flame retardants is based on the breaking of C-X bonds.
Thermal stability ranking: C-F>C-Cl>C-Br>C-I
· The flame retardant efficiency of halogenated flame retardants is related to the strength of C-X bonds.
a) Iodide: The strength of the C-I bond is too low, and iodide is unstable and cannot be used as a flame retardant.
b) Fluoride: very stable, not conducive to quenching free radicals in flames, cannot be used as a flame retardant; But some are applicable to X/Sb synergistic systems.
c) Aliphatic halogen derivatives and aromatic bromides:
The bond strength and stability of aliphatic halogen derivatives are low, and they are prone to decomposition; HX molecules can be generated at low temperatures, and their flame retardant effect is higher than that of aromatic derivatives.
The thermal stability of aromatic bromides is higher than that of aliphatic compounds, but their photostability is not as good as that of aliphatic compounds.
d) Bromine based flame retardants and chlorine based flame retardants
Bromine based flame retardant efficiency is higher than chlorine based: C-Br bond energy is lower, and bromine free radicals and HBr are generated more timely during combustion; Although HCl can be generated over a larger range, its concentration is lower at the flame front.
· The gas-phase flame retardant effect of halides is mainly due to their ability to capture free radicals that propagate thermal oxidation in flames.
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