Flame retardants are materials that inhibit or resist the spread of fire. Naturally occurring substances such as asbestos as well as synthetic materials, usually halocarbons such as polybrominated diphenyl ether (PBDEs), polychlorinated biphenyls (PCBs) and chlorendic acid derivates, most often dibutyl chlorendate and dimethyl chlorendate, have been used in this capacity. Generally, these classes of flame retardant compounds are the most common: aluminium hydroxide, magnesium hydroxide, and various hydrates; organobromines and organochlorines; phosphorus, in the form of organophosphates, halogenated phosphorus compounds, and red phosphorus; antimony trioxide; and boron compounds, mostly borates.
Tetrakis(hydroxymethyl) phosphonium salts, made by passing phosphine gas through a solution of formaldehyde and a mineral acid such as hydrochloric acid, are used as flame retardants for textiles.
Other flame retardants include chlorinated paraffins, polybrominated biphenyls (PBB), pentabromodiphenyl ether (pentaBDE), octabromodiphenyl ether (octaBDE), decabromodiphenyl ether (decaBDE), hexabromocyclododecane (HBCD), tri-o-cresyl phosphate, tris(2,3-dibromopropyl) phosphate (TRIS), bis(2,3-dibromopropyl) phosphate, tris(1-aziridinyl)-phosphine oxide (TEPA), and others.
Mechanisms of function
Endothermic degradation
Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various hydrates. The reaction removes heat from the surrounding, thus cooling the material. The use of hydroxides and hydrates is limited by their relatively low decomposition temperature, which limits the maximum processing temperature of the polymers.
Dilution of fuel
Inert fillers, eg. talc or calcium carbonate, act as diluents, lowering the combustible portion of the material, thus lowering the amount of heat per volume of material it can produce while burning.
Thermal shielding
A way to stop spreading of the flame over the material is to create a thermal insulation barrier between the burning and unburned parts. Intumescent additives are often employed; their role is to turn the polymer into a carbonized foam, which separates the flame from the material and slows the heat transfer to the unburned fuel.
Dilution of gas phase
Inert gases (most often carbon dioxide and water) produced by thermal degradation of some materials act as diluents of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, and slowing the reaction rate.
Gas phase radical quenching
Chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide. These react with the highly reactive H* and OH* radicals in the flame, resulting in an inactive molecule and a Cl* or Br* radical. The halogen radical has much lower energy than H* or OH*, and therefore has much lower potential to propagate the radical oxidation reaction the burning is. Antimony compounds tend to act in synergy with halogenated flame retardants. The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects (especially fine electronics) subjected to the released smoke.
