Mode of action
Flame retardants may slow down or even interrupt the combustion process by physical or chemical action in the solid, liquid or gas phase. They interfere during heating, decomposition, ignition or flame spread with the course of the fire. The most important processes are:
- Physical action by cooling (endothermic process of FR decomposition) or diluting the substrate in the gas phase (i.e. formation of water) and the solid phase (alumina trihydrate and magnesium hydroxide), or by coating the substrate (shielding it against the attack of oxygen and heat) with phosphorous and nitrogen compounds.
- Chemical action in the gas phase interferes with the combustion processes by eliminating the high energy H and OH radicals by halogen halides from halogenated flame retardants, metal halogen compounds from antimony trioxide, and phosphorous-containing fragments from phosphorous flame retardants (“flame poisoning”).
In the solid phase, the flame retardant forms a carbonaceous layer on the surface of the polymer by dehydration, formation of double bonds, thus initiating cyclization and cross-linking (phosphorous, nitrogen compounds, intumescent systems).