Limiting Oxygen Index (LOI)
 

Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index) (ASTM D2863-00)

 The Limiting Oxygen Index (LOI) is the minimum percentage of oxygen it takes in an air-like gas mixture to support flaming combustion.  In the unmodified state, i.e. no flame retardants, polymers vary greatly in their ability to support combustion at normal atmospheric conditions.  The relative resistance to burning is based on the chemical composition of each polymer.  The LOI is a method of ranking the polymers based on combustibility. This property is very important for plastics used in aviation.  As the normal amount of oxygen in air is approximately 21%, polymers with indexes below 21 will burn continuously at normal conditions.  The LOI is measured by placing the samples in a flow of oxygen/nitrogen gas and increasing the concentration of oxygen until the sample will support combustion.  The next lowest oxygen concentration is the value used as the index.