Fire retardants are chemical substances added to materials to slow down or prevent ignition and the spread of fire. These compounds are mixed into plastics, textiles, and building materials to increase the time available for escape during a fire. They work by interfering with the chemical process of combustion, which requires fuel, heat, and an oxidizing agent like oxygen. Fire retardants manipulate how heat and gas interact to suppress the flame.
The Chemistry of Fire Suppression
Fire retardant chemicals function through two main modes of action: the condensed phase (the solid or liquid material) and the gas phase (the flame above the material). Condensed phase action involves creating a protective char layer on the surface of the burning material. This dense, carbonaceous barrier insulates the underlying material from heat and limits the escape of flammable gases, starving the fire of new fuel.
Gas phase action works directly on the flame by disrupting the chemical chain reactions that sustain combustion. Fire is maintained by highly reactive molecules called free radicals, such as hydrogen and hydroxyl radicals. Gas phase retardants release molecules that quickly scavenge and neutralize these radicals, breaking the continuous reaction cycle and extinguishing the flame.
Halogen-Based Compounds
Halogen-based fire retardants, historically the most common type, contain elements like bromine or chlorine. These compounds, such as certain brominated flame retardants (BFRs), operate primarily through the gas phase mechanism. When exposed to heat, the halogenated chemicals decompose to release hydrogen halide gases, like hydrogen bromide (HBr).
The released halogen species are highly effective at reacting with the free radicals that propagate the flame, such as H• and •OH, terminating the combustion chain reaction. This radical-scavenging action is efficient, meaning only a small amount of the chemical is needed for significant fire protection. These compounds were widely used in electronics, plastics, and furniture foams due to their low cost and high performance.
However, the widespread use of halogenated compounds, particularly polybrominated diphenyl ethers (PBDEs), has been restricted or phased out globally. Concerns center on their chemical persistence and tendency to bioaccumulate in the environment and living organisms. When they burn, they can release corrosive and toxic gases, including hydrogen halides and dioxins, posing a health risk. This shift has driven the industry toward developing safer, non-halogenated alternatives.
Phosphorus and Nitrogen-Based Retardants
Phosphorus and nitrogen-based retardants have largely replaced halogenated compounds in many modern applications, often working synergistically. These chemicals, including organophosphates and melamine derivatives, primarily function in the condensed phase to promote char formation. When heated, phosphorus compounds break down to form polyphosphoric acids, which act as strong dehydrating agents on the material’s surface.
This dehydration encourages the creation of a stable, dense layer of carbon char, preventing the underlying material from reaching its ignition temperature. The char layer acts as a physical barrier, insulating the bulk material from heat and blocking the diffusion of oxygen and flammable breakdown products. In intumescent systems, the char layer expands into a protective foam, further enhancing the insulating effect.
Nitrogen components, such as those in melamine-based chemicals, contribute to fire suppression in multiple ways. They enhance the performance of phosphorus compounds by improving the stability and integrity of the carbon char. Nitrogen-containing compounds also release non-combustible gases like nitrogen and ammonia, which dilute the concentration of flammable gases and oxygen in the fire zone. This dual action makes the phosphorus and nitrogen combination an effective modern flame retardant technology.
Mineral and Inorganic Flame Retardants
Mineral and inorganic compounds represent an important class of fire retardants, valued for their non-toxicity and environmentally friendly profile. The most common examples are aluminum trihydroxide (ATH), \(\text{Al(OH)}_3\), and magnesium hydroxide, \(\text{Mg(OH)}_2\). These minerals are often added in high concentrations to materials like cable insulation and roofing membranes.
The primary mechanism for these inorganic retardants is cooling and dilution, occurring through an endothermic decomposition reaction. When ATH is exposed to temperatures around \(200^\circ\text{C}\), it absorbs heat and decomposes into aluminum oxide and water vapor. Magnesium hydroxide requires a higher temperature, decomposing around \(340^\circ\text{C}\), making it suitable for polymers processed at higher temperatures.
The release of water vapor has two main effects: it absorbs heat, cooling the burning surface, and the steam dilutes the concentration of flammable gases and oxygen in the flame area. The residual metal oxide, such as aluminum oxide, can also form a protective, non-flammable layer on the surface. Although these mineral-based systems require higher loading levels, they are a clean way to achieve fire safety.