Regarding combustion reactions and mechanisms, flame retardants generally operate through the following mechanisms:
I. Condensed-phase flame retardancy mechanism
At high temperatures, the flame retardant forms a condensed phase on the polymer surface. This layer isolates the material from air, inhibits heat transfer, and reduces the release of combustible gases, thereby achieving flame retardancy.
II. Free radical trapping mechanism
During polymer combustion, large quantities of HO· and H· radicals are generated, which promote gas-phase combustion reactions. If these radicals can be trapped and neutralized—thereby interrupting the free-radical chain reaction—combustion can be controlled and flame retardancy achieved.
III. Cooling mechanism
The flame retardant undergoes endothermic processes such as dehydration, phase changes, decomposition, or other heat-absorbing reactions. This lowers the temperature of the polymer surface and the combustion zone, preventing thermal degradation and reducing the volatilization of combustible gases. Ultimately, this disrupts the conditions necessary to sustain combustion, achieving flame retardancy.
IV. Non-combustible gas mechanism
Upon heating, these flame retardants decompose to release large amounts of non-combustible gases (such as CO2, N2, NH3, H2O, HCl, and HBr). These gases dilute the combustible gases and lower the oxygen concentration in the combustion zone, thereby inhibiting combustion.
V. Synergistic mechanism
Flame retardancy is a highly complex process; in practice, many flame-retardant systems rely on the simultaneous action of multiple mechanisms. Combining different flame retardants to leverage their interactions and enhance overall flame-retardant performance is known as a synergistic system.
Flame retardancy is a highly complex process; in reality, many flame-retardant systems involve the simultaneous operation of multiple flame-retardant mechanisms.
Ammonium polyphosphate: In a phosphorus-nitrogen system (intumescent flame retardant), the phosphorus and nitrogen compounds form an intumescent char layer on the surface at high temperatures. This layer acts as a protective barrier that insulates against heat and blocks oxygen, while the nitrogen-containing compounds function as blowing agents and char reinforcers. The phosphorus provides an acid source (dehydrating agent) that reacts with the resin to promote char formation. The nitrogen provides a gas source (blowing agent) that causes the system to expand and foam—further facilitating the formation of the char layer—resulting in a porous, foamed char structure.
Post time: Oct-13-2026
