Nylon, a synthetic polymer also known as polyamide, is prized across industries for its strength and durability. Standard, untreated nylon is not inherently flame resistant. While nylon is combustible, its unique thermal properties—specifically its thermoplastic nature—dictate how it reacts in a fire scenario.
How Nylon Reacts to Heat and Flame
The immediate reaction of nylon to a heat source is to soften and shrink away from the flame, a characteristic typical of thermoplastic polymers. This softening occurs at distinct temperature points, such as approximately 220°C for Nylon 6 and 260°C for Nylon 6,6. As heat persists, the material melts and then burns slowly, releasing flammable hydrocarbon gases.
A hazard of burning nylon is the production of molten drips, known as “melt drip.” These superheated liquid droplets adhere to the skin or other surfaces, causing severe, deep contact burns. While the melting action may cause the material to self-extinguish once the heat source is removed, continuous exposure sustains combustion.
During thermal decomposition, the material releases dense smoke (white or blackish, depending on oxygen availability). The combustion produces volatile compounds, including carbon monoxide and hydrogen cyanide. Burning nylon emits a distinctive odor, often described as smelling like celery or plastic, resulting from the chemical breakdown of polyamide linkages. The residue is typically a hard, black, irregular bead.
Distinguishing Flammability from Flame Resistance
To assess a material’s safety profile, it is necessary to distinguish between flammability and true flame resistance. A material is considered flame resistant if it has an intrinsic ability to self-extinguish when the ignition source is removed and does not melt or drip. Flammability describes how easily a substance ignites and sustains a burn.
The Limiting Oxygen Index (LOI) is a scientific measure that quantifies the minimum percentage of oxygen required in the surrounding atmosphere for a material to sustain combustion. Normal air contains approximately 21% oxygen. Untreated nylon has a relatively low LOI, typically ranging between 20% and 26%.
Since the LOI of standard nylon is close to or slightly above the oxygen concentration in ambient air, it is considered a combustible material that burns readily once ignited. This contrasts with truly flame-resistant materials, such as aramid fibers, which have a significantly higher LOI and prevent melt-dripping. The thermoplastic nature of nylon and the resulting melt-drip hazard limit its classification as truly flame resistant in its unmodified state.
Specialized Uses and Fire Retardant Additives
Because of its excellent mechanical properties, nylon is widely used in applications where fire safety is a concern, such as electrical components, automotive parts, and industrial carpets. To meet stringent fire safety standards, nylon must be compounded with specialized fire retardant (FR) additives. These additives disrupt the combustion cycle by forming a protective char layer or diluting flammable gases.
Common FR additives incorporated into the polymer matrix include phosphorus-based compounds, often favored for their effectiveness and reduced environmental impact. Nitrogen-based systems are also employed, sometimes combined with phosphorus to create a synergistic effect that increases fire safety performance.
The incorporation of these compounds significantly modifies nylon’s thermal behavior, increasing its LOI and reducing the melt-dripping effect. Treated Nylon 6,6, for instance, can achieve an LOI of 32% and meet the strict UL94 V0 flammability rating required for many electronic applications. While these treatments improve safety, the resulting material does not possess the same intrinsic flame resistance as naturally non-flammable fibers like Nomex.