Most potentially flammable materials in the modern world can be treated with a variety of flame retardants.
It is important to understand that materials to be rendered fire safe will have different compositions. They are, therefore, appropriately matched with a specific fire retardant that provides effective protection alone, or with the addition of a synergist, further enhancing the technology of flame retardancy where necessary.
Flame retardants interact with the fire cycle (Figure b.) in order to prevent, delay or stop it. They act at different stages, depending on their chemical basis.
Firstly, understanding how a fire develops
The Fire Triangle (according to Emmons)
A fire can basically be split into three phases, the initiating fire, the fully developed fire and the decreasing fire. The fire starts with an ignition source (for example a match) setting combustible material (for example an upholstered armchair) on fire. The fire spreads, heats up the surroundings and once the materials in the room have formed enough flammable gases and are sufficiently hot, flashover takes place and the whole room is engulfed in the fire.
This is the start of the fully developed fire where temperatures up to 1200 °C can be reached. The fire will later decrease as the available fire load is consumed by the fire or if the fire occurs in a totally closed room the fire can die because of oxygen deficiency.
The fundamental parameters governing a fire are:
Combustibility: Will a material burn?
Ignitability: If it is combustible, how and when will it ignite?
Spread of flame: Once ignited how quickly will the flames spread?
Heat release: What will be the rate and total amount of heat released?
To understand how flame retardants work, it is necessary to see
How materials burn? (Figures a and b)
Solid materials do not burn directly: they must first be decomposed by heat (pyrolysis) to release flammable gases.
Visible flames appear when these flammable gases burn with the oxygen (O2) in the air.
If solid materials do not break down into gases, then they will only smoulder slowly and often self extinguish, particularly if they “char” and form a stable carbonaceous barrier which prevents access of the flame to the underlying material.
However, as we all know, even materials such as wood do in fact burn vigorously, because once ignited the heat generated breaks down long-chain solid molecules into smaller molecules which transpire as gases.
The gas flame itself is maintained by the action of high energy radicals (that is H. and OH. in the gas phase) which decompose molecules to give free carbon which can react with oxygen in air to burn to CO2, generating heat energy.
By their chemical and/or physical action, flame retardants prevent or even suppress the process of combustion during a particular phase of the fire cycle. This can be either during heating, ignition, flame spread or decomposition (pyrolysis).
Most effective chemical action of flame retardants
The reaction in the gas phase :
…… where the flame retardant interrupts the radical gas phase combustion process resulting in a cooling of the system, a reduction and suppression of the supply of flammable gases.
The reaction in the condensed phase :
…… where the flame retardant builds up a char layer, smothering the material and inhibiting the oxygen supply, thereby providing a barrier against the heat source or already ignited flame from another source.
Less effective physical action of Flame retardants
Can take place by :
Cooling : where the additive or chemically–induced release of water, cools the underlying substance to a temperature that is unable to sustain the burning process
Coating : where the substance is shielded with either a solid or gaseous layer, protecting it against the heat and oxygen required for combustion to take place
Dilution : Chemically inactive substances and additives turn into non-combustible gases which dilute the fuel in the solid and gaseous phases of the fire cycle (see below figure b).
The Fire Cycle
Any energy source (heat, incandescent material or a small flame) can be the initial ignition source (1).
- Energy transmitted by the ignition source to the polymer creates a degradation where pyrolysis takes place (2).
- Pyrolysis is a process that degrades the polymer’s long-chain molecules into smaller hydrocarbon molecules, the flammable gases (4)which are emitted to the gas phase. In the condensed phase, the result is an inert carbonised material, called char (3).
- In the gas phase, flammable gases are mixed with oxygen from the air. The proper mix of oxygen and fuel is reached in the combustion zone (5), where hundreds of exothermic chemical reactions take place involving high-energy free radicals (e.g. H. and OH.), fuel and oxygen.
- A perfect combustion would theoretically produce H2O and CO2. In real life, incomplete combustion products are also emitted during a fire (CO, PAHs, HCN, etc) (6).
- Energy (7) emitted during exothermic reactions is transmitted to the polymer and reinforces pyrolysis.
This allowing the reaction to sustain itself.