Flame Retardants
There are five main ways in which these act::
- Coating the exposed area with, for example, an intumescent coating
- Decomposing to produce large volumes of incombustible gases which dilute the oxygen supply
- Promoting endothermic reactions which reduce the temperature to below that needed to sustain combustion or permit ignition
- Influencing the decomposition reaction in the solid phase so that less flammable products are produced
- Inhibiting the free radical oxidation process in the flame and so quench the flame
Intumescent coatings contain a number on ingredients. One of these produces large amounts of incombustible gas to foam the coating. Another must provide a source of carbon, which, when acted upon by a third, a dehydrating agent, will provide the foamed char.
A coating may be formed by melting an incombustible additive such as certain borates.
Materials like alumina trihydrate act in two ways: the decomposition reaction is endothermic, at the same time liberating molecules of water to help quench the flame.
Phosphorous compounds in cellulosics influence the decomposition of the cellulose to produce more char and less flammable vapours.
Halogen compounds decompose in the flame so dissipating the energy. Some help is needed to transport the halogen to the flame zone and for this synergests are used, the most important of which being antimony oxide.
Within each sub group there is a number of materals, and the choice is usually decided by the absence of undesirable side effects.
Thus the final choice may be that which:
- Causes least reduction in impact strength
- Is most compatible and so dies not bloom out from the plastics
- Has least effect on the cure rate of a thermosetting compound
- Produces least smoke when it does burn
