Mechanisms of Flame Retardants
2015-05-16 08:33 | writer: admin
Flame retardants can act physically or chemically and sometimes both by physically and chemically interfering at particular stages of burning. The different mechanisms are:
Endothermic Degradation : Certain compounds break down endothermically when they are subjected to high temperatures. Magnesium and aluminium hydroxides are such examples. Various hydrates also act similarly. The reaction takes off heat from the surroundings, thus cooling the material.
Dilution of Fuel: Substances, which evolve inert gases on decomposition, dilute the fuel in the solid and gaseous phases. 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 that it can produce while burning. Thus the concentrations of combustible gases fall under the ignition limit.
Thermal Shielding : A thermal insulation barrier is created between the burning and the yet-to-burn parts. Intumescent additives are sometimes applied that turn the polymer into a carbonized foam, resultantly separating the flame from the material and slowing down the heat transfer to the unburned fuel.
Dilution of Gas Phase : Inert gases, mostly carbon dioxide and water, act as diluent of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, thus slowing the reaction rate. These gases are produced by thermal degradation of some materials.
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 thus has much lower potential to propagate the radical oxidation reactions of combustion. 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 subjected to the released smoke.