Flame retardants refer to a variety of substances that are added to combustible materials to prevent fires from starting or to slow the spread of fire and provide additional escape time. Polymers have a wide range of mechanical and chemical properties and differ in combustion behavior. Therefore, they need to be matched to the appropriate flame retardants in order to retain key material functionalities.
Fig 1. The hazard of fire to polymers
Mechanism of Flame Retardant
The most effective chemical action may take place by the following ways.
Reaction in the gas phase: The radical gas phase combustion process is interrupted by the flame retardant, resulting in cooling of the system, reducing and eventually suppressing the supply of flammable gases.
Reaction in the solid phase: The flame retardant builds up a char layer and shields the material against oxygen and provides a barrier against the heat source (flame).
The less effective physical action may take place by the following ways.
Cooling: Energy absorbing (Endothermic) processes triggered by additives and/or the chemical release of water cool the substrate to a temperature below that required for sustaining the combustion process.
Formation of a protective layer (coating): The material is shielded with a solid or gaseous protective layer and protected from heat and oxygen necessary for the combustion process.
Dilution: Inert substances (fillers) and additives evolving non-combustible gases dilute the fuel in the solid and gaseous phases.
Categories of Flame Retardants
Brominated Flame Retardants (BFRs)
BFRs are commonly used to prevent fires in electronics and electrical equipment. This area accounts for more than 50% of their applications - for example in the outer housings of TV sets and computer monitors.
Flame Retardants Based on Phosphorus Compounds (PFRs)
The class of Phosphorus-containing flame retardants covers a wide range of inorganic and organic compounds and include both reactive (chemically bound into the material) and additive (integrated into the material by physical misering only) compounds. They have a broad application field, and a good fire safety performance.
Mineral Flame Retardants
Fine precipitated ATH and MDH (< 2 µm) are used in melt compounding and extrusion of thermoplastics like cable PVC or polyolefins for cables. For use in cable, ATH and more often MDH are coated with organic materials to improve their compatibility with the polymer. Coarser ground and air separated grades can be used in liquid resin compounding of thermosets for electrical applications, seats, panels and vehicle parts.
Fig 2. Mineral flame retardants used in cables
Nitrogen-containing Flame Retardants
Melamine is mainly used in polyurethane foams, whereas melamine yanurate is used in nylons or in olypropylene intumescent formulations in conjunction with ammonium polyphosphate. The phosphate, oly- and pyrophosphates of melamine contain both nitrogen and hosphorus and are used in nylons. Some specific formulations, triazines, isocyanurates, urea, guanidine and cyanuric acid derivatives re used as reactive compounds.
Other Flame Retardants
Several other substances show flame retarding effects and are used in commercial applications like borates, antimony trioxide, zinc compound, Nanocomposites…