Oxygen index

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The oxygen index (abbreviated OI, or LOI = Limiting Oxygen Index) is a parameter used to describe the fire behavior, especially of plastics. It is the minimum oxygen concentration of an oxygen - nitrogen mixture below which combustion will continue under the test conditions.

The testing of a vertically arranged test piece was proposed by Fenimore and Martin and initially specified in 1977 in the American ASTM standard D 2863.

The lower its oxygen index, the better a substance burns. If oxygen is chemically bound in a substance, it can be released in the event of a fire and react with the carbon it contains. Wood, for example, contains around 42% oxygen. In maple, birch or pine as a solid therefore the oxygen index is lower than that of nylon , PVC PVC hard, soft, polyester or wool and can therefore also known as green wood a fire load form, which fires in chips camps clearly.

background

Every combustion requires oxygen. Under standard conditions , the oxygen concentration in the air is approx. 21%. A plastic that requires a higher concentration of oxygen to burn should be self-extinguishing.

Measurement method

Oxygen index measuring device

Upper burn

The sample, the oxygen index of which is to be determined, is ignited from above in a standing glass tube through which an oxygen-nitrogen mixture flows. After removing the pilot flame, the fire behavior is observed. If the flame burns for longer than 180 seconds or if it reaches a measuring mark 50 mm below the upper edge, the oxygen concentration is reduced in the following experiment, in the other case it is increased. This is carried out until 50% of the test specimens burn at a certain concentration.

Other methods

Among other things, in the standard ECSS-Q-70-21A of the ECSS of the ESA , in addition to a burn test of a vertical material edge in a closed chamber, a determination of the oxygen index in the case of burn-up after ignition by a gas flame arranged below the sample is standardized for materials used in space travel .

conditions

The International Union of Railways (UIC) established the following requirements for the oxygen index:

scope of application Requirement for OI
Curtain fabrics ≥ 28
Laminated panels ≥ 35
Glass fiber reinforced unsaturated polyester resin molded materials GF-UP ≥ 26

criticism

The oxygen index determination with upper burn-up is certainly a good basis for material development, e.g. B. the investigation of the effect of flame retardants . However, the test conditions (burn-up from above and increased oxygen concentration) are impractical because in many cases the burn-up takes place from below. As a result, the material is preheated in the upper part, which means that it can burn in air with an oxygen content below the oxygen index.

Norms

  • DIN EN ISO 4589 Plastics - Determination of fire behavior using the 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 x 4 mm nominal cross section. (Great Britain)
  • NF T 51-071, May 1977: Matières plastiques, réaction au feu, détermination de l'indice d'oxygène. (France)
  • JIS K 7201-1972 (Japan)
  • GOST 12.1.044 (Russia)

Individual evidence

  1. ↑ CP Fenimore, FJ Martin "Candle-type Test for Flammability of Polymers" in Modern Plastics 43 (1966), p. 141.
  2. ↑ Wood structure: fire and explosion protection - fire theory, Kohlhammer-Verlag, Stuttgart, 1998, quoted by: Werner Bauer (fire expert): Risk of fire in wood processing companies , at schadenprisma.de, (PDF file)
  3. ↑ Werner Bauer (fire expert): Risk of fire in wood processing companies , at schadenprisma.de, (PDF file)
  4. ↑ https://www.dlr.de/qp/Portaldata/44/Resources/dokumente/einrichtung/ecss-q-70-21a_dlr_final_version.pdf ECSS-Q-70-21A of October 4, 1999
  5. ^ Jürgen Troitzsch "Fire behavior of plastics", Hanser Verlag 1982.