Cotton, a natural textile fiber, is primarily composed of cellulose, a polymer also found in wood and paper. Understanding the thermal properties of this material is important due to its widespread use in clothing, home furnishings, and industrial applications. While untreated cotton is highly flammable, the specific temperature at which it burns is not fixed but is influenced by the method of heating.
The Critical Ignition Temperature
The autoignition temperature is the point at which cotton ignites without an external flame. For pure cellulose fiber, this temperature generally falls between \(360^{\circ}\)C and \(400^{\circ}\)C (\(680^{\circ}\)F to \(752^{\circ}\)F). This is distinct from the flash point, which is the lower temperature where the material releases enough flammable vapors to ignite when an external ignition source is introduced.
The flash point for cotton can be as low as \(150^{\circ}\)C (\(302^{\circ}\)F) when an external flame is applied. However, the fabric must first reach about \(250^{\circ}\)C (\(482^{\circ}\)F) to begin thermal decomposition. This process releases the volatile, flammable gases necessary to sustain a flame. Real-world scenarios are complicated by factors like air flow, density, and contaminants.
The Chemical Process of Burning
The combustion of cotton is a complex oxidation process that begins with heating. Before visible flames appear, the cotton undergoes thermal decomposition, or pyrolysis, starting as low as \(250^{\circ}\)C to \(300^{\circ}\)C. During pyrolysis, the long cellulose polymer chains break down into smaller molecules, which are released as gases and tars.
These decomposition products include flammable gases like methane and carbon monoxide, as well as non-flammable gases like carbon dioxide and water vapor. A solid, carbon-rich residue called char is also left behind. When these flammable gases mix with oxygen and reach a sufficiently high temperature, they ignite, resulting in a visible flame. The heat generated by this combustion accelerates further pyrolysis, creating a self-sustaining cycle of burning.
How Treatments Change Flammability
Textile finishes and physical properties can alter the burning characteristics of cotton. Applying a flame retardant finish modifies the thermal degradation pathway. These chemical treatments often work by promoting the formation of a thicker char layer at lower temperatures, which acts as an insulating barrier to prevent heat from reaching the underlying cotton.
Other flame retardants operate in the gas phase by releasing non-combustible gases, such as water vapor or carbon dioxide. These gases dilute the concentration of flammable gases and oxygen near the material. The presence of moisture also affects ignition, as the heat must first be used to evaporate any water content before the fiber’s temperature can increase enough for pyrolysis to begin. This requires more energy to achieve ignition.
The physical structure of the cotton material also plays a significant role; a lightweight, loosely woven fabric will ignite and spread flame much faster than a heavy, tightly woven material. Furthermore, blending cotton with synthetic fibers, such as polyester, creates a complicated flammability profile. While untreated cotton burns and leaves a soft ash, synthetic blends combine the rapid flame spread of the natural fiber with the melting and dripping behavior of the synthetic component, which can cause severe burns.