Mechanism of Flame Retardancy

Flame retardant systems for synthetic or natural polymers can act physically and/or chemically by interfering at particular stages of burning

By cooling Endothermic processes triggered by the flame retardants cool the substrate.

By forming a protective layer: The heat transfer is impeded, fewer pyrolysis gases are evolved, and the oxygen is excluded.

By dilution.: Substances, which evolve inert gases on decomposition, dilute the fuel in the solid and gaseous phases. The concentrations of combustible gases fall under the ignition limit.

Reaction in the gas phase: The free radical mechanism of combustion processes which takes place in the gas phase could be interrupted by flame retardants.

Reaction in the solid phase: One mechanism is the accelerated breakdown of polymers.

Types of Flame Retardants:

Brominated flame retardants

Chlorinated flame retardants

Phosphorous-containing flame retardants {Phosphate ester such as Tri phenyl phosphate

Nitrogen-containing flame retardants (i.e. Melamines)

Inorganic flame retardants.

These can be further classified as:

Inorganic, Organo Phosphorous, Halogenated organic and Nitrogen based compounds.

2: Halogenated organic flame retardants are further classified as containing either Chlorine or Bromine {Brominates Flame Retardants BFR}

There are three types BFRs currently produced. These are Poly Brominated DiPhenyl Ethers {PBDE}, Tetra Bromo Bisphenol A {TBBPA} and Hexa Bromo Cyclodecane {HBCD} The PBDEs that are commonly used in products are Deca, Octa, and Penta BDE .The concentration of BFRs in products ranges from 5 to 30 % .Compounds containing Iodine are known, but of limited utility as flame retardants, due to their poor thermal stability and dark colour of iodine. Compounds containing Fluorine generally exist as functional polymers rather than materials to be added to other polymeric systems to provide flame retardancy. These polymers are oxidatively stable and only decompose at very high temperature.

Antimony oxide is another important component flame retardant composition, containing halogen, particularly Chlorine and Bromine. It is totally ineffective if used with out halogen. The Tri oxide is the common material used although the Pentoxide can also use. The pentoxide has a much finer particle size and is more effective per unit weight added than the trioxide. Polyesters are very sensitive to residual acidity in all forms of antimony oxide. Alkaline salts of antimony oxides are used in these critical cases. Antimony oxide acts as synergists with chlorine and bromine.

Antimony tri bromide is a dense white product and is one of the main components of the typical white smoke that is seen from burning polymers containing halogen and antimony oxide. High levels of water from normal combustion cause reversion of SbBr3 to HBR and Sb203.The remaining antimony oxide is then available to react with fresh HBR from decomposing brominated compound. Typically compounds used in flame retardant application contain either 40 to 70 % Chlorine or 45 to 80% Bromine, depending on the flame retardant requirements from 20 to 40 parts of Brominated compound would be used per 100 parts of polymer. Antimony oxide used is typically 1/4^th to that of the halogenated material.