Flame retardants are added to or are part of flammable materials, mainly plastics and textiles.
The term “flame retardant” refers to a function, not a family of chemicals. A variety of different chemicals, with different properties and structures, act as flame retardants and these chemicals are often combined for effectiveness. Several type of flame retardants can me mixed to each other to increase efficiency via synergetic effects.
The efficacy of flame retardants appears in their potential to prevent ignition, to delay the time to ignition and flame spread, to reduce heat release and eventually in delaying or even preventing flashover. Flame retardants allow materials and products to meet high fire safety performance requirements specified in regulations and fire tests. This is also shown in full-scale fire test studies on flame retarded technical and consumer products. Flame retardants help to save lives by extending escape time and to reduce property losses.
Mode of action
A flame retardant is a substance that may inhibit or even extinguish a fire by physical or chemical action during heating, decomposition, ignition and flame spread of the combustible material. In many cases, flame retardants act both physically and chemically.
They may work to:
- Disrupt combustion by stopping the chemical chain reaction by eliminating the H and OH radicals
- Limit the process of decomposition by physically insulating the available fuel sources from the material source with a fire-resisting “char” layer
- Cool and dilute the flammable gases and oxygen concentrations in the flame formation zone by emitting water, nitrogen or other inert gases.
Physical action
Flame retardants may induce physical action by cooling the substrate (endothermic process of flame retardant decomposition), diluting the substrate and the gas phase (i.e. formation of water) with flame retardants like alumina trihydrate and magnesium hydroxide, or by charring and coating the substrate (shielding it against the attack of oxygen and heat) with phosphorus- and nitrogen-containing flame retardants.
Chemical action
Chemical action initiated by flame retardants interferes with the combustion process by eliminating the high energy radicals in the gas phase (i.e. extinguishing the flame) with halogenated or phosphorus-containing flame retardants, or form a carbonaceous layer by cyclization and cross-linking with phosphorus- and nitrogen-containing flame retardants, thus coating the substrate.
Additive flame retardants
Additive flame retardants are incorporated in the plastic either prior to, during, or, more frequently, following polymerization. They are generally used in thermoplastics, elastomers and thermosets. If they are compatible with the plastic they act as plasticizers, otherwise they are considered as fillers. Polymeric flame retardants are increasingly used as such or as blends in other polymers.
Reactive flame retardants
Reactive flame retardants are reactive components chemically built into the polymer molecule, together with the other starting components. This prevents them from bleeding out of the polymer and volatilizing. In addition, they have no plasticizing effect and do not affect the thermal stability of the polymer. They are mainly used in thermosets (polyesters, epoxy resins, polyurethanes), in which they can easily be incorporated.
Synergists
Combinations of additive or reactive flame retardants with other substances can produce an additive, synergistic or antagonistic effect. While the additive effect is the sum of the individual actions, the effects of synergism and antagonism are higher and lower, respectively. When used alone, synergists show no or only negligible effectiveness. The synergistic effect occurs when synergists are used together with specific flame retardants.
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