Like certain pop stars, plastics are generally flammable. Therefore it is often necessary to add flame retardant plastic additives to meet governmental and industry regulatory standards. Flame retardant plastic additives are compounds added to plastics and other materials to inhibit, suppress or delay combustion. These compounds are useful in impending burning in the ignition phase of the fire. They do not prevent charring or melting nor do they increase the heat resistance of a material. Flame retardants are also not effective when a fire is fully engaged. There is no universal additive for all plastics, rather each flame retardant is specific to a particular polymer and a specific test of flammability.
To understand how flame retardants work, it is necessary to know how materials burn. When we see the flame of a burning object, we are seeing the combustion of flammable gases given off during its thermal decomposition. This process is call pyrolysis.
Pyrolysis is the process in which the long-chain molecules of a polymer chain degrade into smaller hydrocarbon molecules and flammable gases. These gases mix with oxygen and exothermic chemical reactions occur producing free radicals (H and OH). In complete combustion, H2O and Co2 are produced as well as other material specific gases. The way that flame retardants interfere in the pyrolysis of a polymer depends on the flame retardant(s) used and the plastic burning. Flame retardants can be either active or reactive. Active means blended into a polymer and reactive means inserted in the polymer molecule. Both types can suppress ignition in the vapor (gaseous) state of a fire and in the condensed (solid) state. Three common processes include endothermic degradation, gas phase radical quenching, and thermal shielding.
1. Endothermic Degradation
Mineral compounds such as aluminum and magnesium hydroxides are well known as antacids but they can also be used as flame retardants. These plastic additives break down endothermically when subjected to high heat. This process removes heat from the plastic and cools the material. While high processing temperatures for plastics and relatively low decomposition temperatures for hydroxides and hydrates can limit their use, aluminum hydroxide did account for over 70% of total market demand in 2012. Mineral flame retardants are usually additive and include boron compounds, antimony oxides, huntite, hydromagnesite and zinc oxides
2. Gas Phase Radical Quenching
The most common flame retardant plastic additives are brominated flame retardants (BRFs). Brominated flame retardants are organohalogen compounds. Chlorinated and brominated compounds undergo thermal degradation releasing hydrogen chloride and hydrogen bromide. These react with H and OH radicals in the flame producing chlorine and bromine radicals. As the halogen radicals are less reactive than H or OH radicals, they retard the oxidation reactions of the flame. Halogenated flame retardants are low in cost and work on a wide variety of polymers. It should be noted that halogenated retardants are considered toxic to humans and animals and many have been banned.
3. Thermal Shielding
Phosphorus flame retardants, which include phosphate-ester compounds, are non-halogenated compounds that act on the solid state of combustible materials. Phosphorus forms phosphoric acid when heated which chars the solid forming a thick glassy layer of carbon. This charring prohibits pyrolysis and thus less fuel is available for the fire. Phosphorous flame retardant plastic additives can be additive or reactive. Additionally, one can purchase halogenated organic phosphorus compounds that include a halogen and phosphorus. The halogen acts on the gas phase of a fire and the phosphorus inhibits fire in the sold state.
Most flame retardant plastic additives are combined to improve their overall efficacy. For example, brominated compounds are often used in synergy with antimony trioxide. This combination acts as a catalyst to speed the release of bromine and chlorine radicals in gas phase radical quenching.
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