Limiting Oxygen

Another property, used to compare the flammabiUty of textile fibers, is the limiting oxygen index (LOI). This measured quantity describes the minimum oxygen content (%) in nitrogen necessary to sustain candle-like burning. Values of LOI, considered a measure of the intrinsic flammabiUty of a fiber, are Hsted in Table 2 in order of decreasing flammabiUty. 

Table 2. Limiting Oxygen Index of Textile Fibers...

Fig. 4. Effect on limiting oxygen index (LOI) of (---) antimony trioxide (---) Flamtard S, and Flamtard H added to (a) the basic unfilled PVC...

ASTM D2863-87 limiting oxygen index ease of extinction... 

Limiting Oxygen Index. The minimum concentration of oxygen in an O2/N2 mixture that supports combustion of a vertically mounted test specimen is called the limiting oxygen index (3,4). Test specimens are 0.65 x 0.3 cm x 12.5 cm. The principal advantage of this test is its reproducibiUty which makes it useful for quaUty control. The main disadvantage is that the results rarely correlate with the results of other fire tests. 

The limiting oxygen index of Tefzel as measured by the candle test (ASTM D2863) is 30%. Tefzel is rated 94 V-0 by Underwriters Laboratories, Inc., in their burning test classification for polymeric materials. As a fuel, it has a comparatively low rating. Its heat of combustion is 13.7 MJ/kg (32,500 kcal/kg) compared to 14.9 MJ /kg (35,000 kcal/kg) for poly(vinyHdene fluoride) and 46.5 MJ /kg (110,000 kcal/kg) for polyethylene. 

When exposed to fire. Teflon PEA contributes Httle in fuel value and is self-extinguishing when the flame is removed. The fuel value is approximately 5.4 MJ/kg (2324 Btu/lb). It passes the UL 83 vertical-flame test and is classified as 94VE-0 according to UL 94. The limiting oxygen index (LOI) by ASTM D2863 is above 95%. 

The self-ignition temperature of PVF film is 390°C. The limiting oxygen iadex (LOI) for PVF is 22.6% (98), which can be raised to 30% ia antimony oxide-modified film (99). Hydrogen fluoride and a mixture of aromatic and aUphatic hydrocarbons (100) are generated from the thermal degradation of PVF. Toxicity studies, ie, survival and time to iacapacitation, of polymers, ceUulosics (101,102), and airplane iaterior materials (103) expose... 

Elame-spread and smoke-density values, and the less often reported fuel-contributed semiquantitive results of the ASTM E84 test and the limited oxygen index (LOI) laboratory test, are more often used to compare fire performance of ceUular plastics. AH building codes requite that ceUular plastics be protected by inner or outer sheathings or be housed in systems aH with a specified minimum total fire resistance. Absolute incombustibHity cannot be attained in practice and often is not requited. The system approach to protecting the more combustible materials affords adequate safety in the buildings by aHowing the occupant sufficient time to evacuate before combustion of the protected ceUular plastic. 

Properties and Applieations. Aryloxyphosphazene elastomers using phenoxy and J-ethylphenoxy substituents have found interest in a number of appHcations involving fire safety. This elastomer has a limiting oxygen index of 28 and contains essentially no halogens. It may be cured using either peroxide or sulfur. Peroxide cures do not require the allyhc cute monomer. Gum physical properties are as follows (17) ... 

Alternatives to Arresters Alternatives to the use of flame arresters include fast-ac ting isolation valves, vapor suppression systems, velocity-type devices in which gas velocity is designed to exceed flashback velocity, and control of the flammable mixture (NFPA 69 standard, Explosion Prevention Systems ). The latter alternative frequently involves reduction of oxygen concentration to less than the limiting oxygen concentration (LOC) of the gas stream. 

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