The Limiting Oxygen Index is commonly referred to as the LOI. It is expressed in volume percent.
The Limiting Oxygen Index (LOI) is the minimum concentration of oxygen in a mixture of oxygen and nitrogen that is required to support the flaming combustion of a material.
It is expressed in volume percent (vol %). The index was first introduced in 1966 by Fenimore and Martin and is used to indicate the relative flammability of materials. It is standardised for composites in the US (ASTM D2863), France (NF T 51-071) and Internationally (ISO 4589). The method involves placing a sample vertically within a controlled atmosphere and its top inflamed with a burner.
The LOI test calculates the minimum concentration of oxygen in a mixture of oxygen and nitrogen that either maintains flame combustion of a material for 3 minutes or consumes a length of 5 centimeters of a sample.
Since atmospheric air contains an oxygen concentration of 20.95%, materials with LOI values less than 21% are classed as combustible while those with LOI values greater than 21% are classed as self-extinguishing, since their combustion cannot be sustained at ambient temperature without an external energy contribution. Materials with high LOI values generally exhibit better FR properties, that is, they prevent combustion and therefore propagation of fire to a wider environment.
LOI AT A GLANCE
The Limiting Oxygen Index of a material is the minimum concentration of oxygen, expressed as a percentage, that a material needs to support its combustion under certain conditions. The index is determined by exposing a sample of the material to a mixture of oxygen and nitrogen and measuring the concentration of oxygen required to sustain combustion.
Materials with an LOI > 21% are classed as non-flammable.