Cotton is a plant-based, organic fiber. As a cellulosic fiber, cotton is inherently combustible, meaning it provides its own fuel source for a fire. Understanding cotton’s flammability requires examining its physical behavior when exposed to a flame and the underlying chemistry that drives combustion.
How Cotton Burns
Cotton fabric ignites almost immediately when exposed to an open flame. It burns rapidly and continuously with a soft, yellowish flame. This quick ignition is due to cotton’s relatively low flashpoint, with an ignition temperature for solid fabric typically falling between 210°C and 235°C (410°F and 455°F).
Unlike synthetic fibers such as polyester or nylon, cotton does not melt or shrink away from the heat source. The combustion process leaves behind a very fine, light gray or white ash that easily crumbles to the touch. This non-melting behavior is a safety distinction, as synthetic materials can melt and adhere to the skin, causing severe burns. The characteristic smell is similar to burning paper or wood, reflecting its natural origin.
The Chemical Structure of Flammability
The high flammability of cotton is rooted in its chemical composition, which is over 90% cellulose. Cellulose is a highly crystalline polymer composed of repeating glucose units. These tightly packed chains serve as a dense source of carbon and hydrogen, which are the primary fuel components for fire.
When heated, cotton undergoes thermal decomposition called pyrolysis, which occurs before visible flames appear. During pyrolysis, the chemical bonds in the cellulose chains break down, largely driven by heat absorption. This degradation releases a mixture of highly volatile, flammable gases and tars into the air.
The most abundant of these volatile products is a substance called levoglucosan, which is a six-carbon oxygenate molecule. Levoglucosan and other flammable gases mix with oxygen in the surrounding air, creating the fuel-air mixture that ignites and sustains the visible flame. Because cotton fibers are woven into fabric, the material has a high surface area that allows rapid oxygen exposure, which further accelerates the burning process.
Fire Retardants and Treated Cotton
The flammability of cotton can be significantly reduced through the application of fire retardant treatments, which modify the combustion chemistry. These treatments primarily work by altering the thermal decomposition pathway of the cellulose fiber. Instead of producing highly flammable levoglucosan, the treated cotton is chemically directed to break down into more non-volatile carbon-rich char and water vapor.
This process is known as a condensed phase mechanism, where the char layer forms a glassy, insulating barrier on the fiber’s surface. The char physically blocks heat transfer to the underlying material and limits the escape of any remaining volatile gases, effectively starving the fire. Many effective retardants use phosphorus or nitrogen compounds, as these elements are highly efficient at promoting the formation of this stable carbonaceous char.
A secondary mechanism involves the gas phase, where some retardants release non-combustible gases like carbon dioxide and water vapor directly into the flame zone. These non-flammable gases dilute the concentration of oxygen and the flammable pyrolysis products, which slows the rate of combustion. These chemical modifications are employed to meet strict safety standards for materials like children’s sleepwear and industrial protective uniforms.