Autoignition temperature

Substances which spontaneously ignite in a normal atmosphere at naturally ambient temperatures are termed pyrophoric.

Autoignition temperatures of liquid chemicals are typically measured using a 500-millilitre (18 imp fl oz; 17 US fl oz) flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659.

When measured for plastics, autoignition temperature can also be measured under elevated pressure and at 100% oxygen concentration. The resulting value is used as a predictor of viability for high-oxygen service. The main testing standard for this is ASTM G72.

Autoignition time equation

The time ${\displaystyle t_{\text{ig}}}$ it takes for a material to reach its autoignition temperature ${\displaystyle T_{\text{ig}}}$ when exposed to a heat flux ${\displaystyle q^{″}}$ is given by the following equation:

${\displaystyle t_{\text{ig}}=\frac{\pi }{4}k\rho c{\left[\frac{T_{\text{ig}}-T_0}{q^{″}}\right]}^2,}$

where k = thermal conductivity, ρ = density, and c = specific heat capacity of the material of interest, ${\displaystyle T_0}$ is the initial temperature of the material (or the temperature of the bulk material).