Limiting oxygen index

The oxygen index ( OI abbreviated, or LOI Limiting Oxygen Index ) is a parameter for describing the fire behavior of plastics. It is the minimum oxygen concentration of an oxygen - nitrogen mixture at which the combustion of a vertically arranged test piece holding under the test conditions. The test was proposed by Fenimore and Martin 1977 and initially set in the American ASTM standard D 2863.

Background

Each combustion requires oxygen. Under standard conditions, the oxygen concentration of the air is 21 %. A plastic, which requires more than the oxygen contained in the air for combustion should be self-extinguishing.

Measurement methods

The sample, the oxygen index is to be determined is in an upright glass pipe which is flowed through by an oxygen -nitrogen mixture ignited from the top. After removal of the igniting flame the fire behavior is observed. The flame burns longer than 180 seconds and reaches a located 50 mm below the upper edge of the measuring mark, the oxygen concentration is reduced in the following test, increases in the other case. This is continued for a time, to burn at a certain concentration 50 % of the specimens.

Requirements

From the International Union of Railways (UIC ), the following requirements have been established with respect to the oxygen index:

Criticism

The oxygen index is certainly a good basis for the development of materials, eg the investigation of the effect of flame retardants. However, the test conditions, such as burn-up of the top and increased oxygen concentration, impractical. In many cases, the burn takes place from below. Thus the material is preheated in the upper part, allowing it to burn in air with an oxygen content below the oxygen index.

Standardize

  • DIN EN ISO 4589 Plastics - Determination of burning behavior by oxygen index
  • ASTM D 2863-77 Measuring the Minimum Oxygen Concentration to Support Candle -like Combustion of Plastics ( Oxygen Index)
  • NT Fire 013 (Norway )
  • AS 2122.2-1978 Combustion propagation Characteristics of plastics. Part 2 - Determination of Minimum Oxygen Concentration for Flame Propagation Following Top Surface Ignition of Vertically Oriented Specimens. (Australia)
  • BS 2782: Part 1: Methods 141 A to 141 D: 1978; Plastics Part 1: Thermal Properties. Method 141A: Oxygen index of combustion of a rigid bar of 10 mm × 4 mm nominal cross section. ( UK )
  • NF T 51-071 May 1977 Matières plastiques, réaction au feu de l' indice d' oxygène determination. (France)
  • JIS K 7201-1972 (Japan)
  • GOST 12.1.044 (Russia)
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