The heat produces combustible gases from the pyrolysis of the polymer. The proper ratio between these gases and oxygen then causes the polymer to catch fire. Combustion results in the production of heat, which is dissipated and fed back. This thermal feedback pyrolyzes the polymer and keeps the combustion going.
To limit the establishment of this combustion cycle, one (or several) ingredient must be removed. Flame retardants can hinder or even inhibit the combustion process through several actions.
Additives can degrade endothermically, thereby cooling the substrate below the temperature required to sustain the combustion process. For example, borax with crystal water releases crystal water by endotherm, thereby suppressing the temperature rise of the substrate and exerting a flame retardant effect. Different metal hydroxides also follow this principle and their efficiency depends on the amount incorporated in the polymer. In addition, the molten droplets often generated during the cracking of some thermoplastic polymers can play a certain flame retardant effect because they can leave the combustion zone and take away the reaction heat.
Additives can form stable barriers at higher temperatures or decompose into foam-like substances, which have low thermal conductivity. In this way, the heat transfer from the heat source to the material can be reduced, the escape of combustible gas can be suppressed, and the contact between the air and the material can be isolated. For example, phosphate ester compounds and fire-resistant foam coatings can work according to this mechanism.
Flame retardants with this function can generate a large amount of non-flammable gases when thermally decomposed, such as CO₂, NH₃, HCI and H₂O. These gases dilute the oxygen in the air and the flammable gases from the decomposition of the substrate so that the lower ignition limit of the gas mixture cannot reached. For example, ammonium phosphate, ammonium chloride, ammonium carbonate, etc. can generate these incombustible gases when heated.
Its function is to change the thermal decomposition mode of the polymer material, thereby inhibiting the generation of flammable gases. For example, cellulose is decomposed into carbon and water by dehydration reaction with acid or alkali. Because it does not produce flammable gas, it cannot catch fire. Ammonium chloride, ammonium phosphate, phosphate ester, etc. can decompose to produce such substances, which can catalyze the carbonization of condensed rings of materials to achieve the purpose of flame retardancy.
Inhibitory Effect (Capture Free Radicals)
The combustion of polymer is essentially a free radical chain reaction of thermal decomposition products. Some substances can capture the active intermediates (HO·, H·, O·, HOO·, etc.) of the combustion reaction, which can effectively inhibit the free radical chain reaction and make the combustion process reduce speed until flame goes out. Commonly used bromine, chlorine and other organohalogen compounds act as this inhibitory effect.
Enhancement Effect (Synergistic Effect)
Some additives do not have flame retardant effect or have little flame retardant effect when used alone, but can significantly improve their flame retardant effect when used in combination with other flame retardants. This effect is called enhancement or synergistic effect. The combination of antimony trioxide and halogen compounds is the most typical example. As a result, not only can the flame retardant efficiency be improved, but also the amount of the flame retardant used can be reduced.