Standard Procedure Of Oxygen Index Test Methods

Oxygen index tester test methods, which describes the tendency of a material to sustain a flame, are widely used as a tool to investigate the flammability of polymers. They provide a convenient, reproducible, means of determining a numerical measure of flammability. A further attraction is that the test method uses inexpensive equipment and only requires a small sample size. These methods have been used to systematical investigate the relative flammabilities of fire-retarded materials, frequently comparing the effectiveness of fire-retardants and fire-retardancy mechanisms.

The quintessential feature of oxygen-index methods is that the sample is burnt within a controlled atmosphere. The standard procedure is to ignite the top of the sample, using a gas flame which is withdrawn once ignition has occurred, and to find the lowest oxygen concentration in an upward flowing mixture of nitrogen and oxygen which just supports sustained burning. The criticality criterion typically takes the form of a minimum burning length: either specifying that the sample must burn for a certain length of time or that a specified length of material be consumed. The effectiveness of fire retardants is measured by the change in the critical oxygen concentration that they induce as a function of their concentration.


Marginally-stable materials form a natural set for a quantification of the efficiency of fire-retardant mechanisms. We achieve this by finding the value of the relevant continuation parameter to increase the LOI of these materials to 28.0, the transition between slow-burning and self-extinguishing polymers, and to 100, the threshold for intrinsically non-flammable materials.

It should be realised that our classification of materials (flammable, slow-burning, self-extinguishing, intrinsically non-flammable) is specific to the limiting oxygen index tester, i.e. a material that is self-extinguishing here is not necessarily self-extinguishing in another test method. The tenet in the limiting oxygen index is that the higher the value of the LOI the `safer' the material. However, we stress that results from one test method do not necessarily agree with another (Emmons 1974). The reasons for this were alluded to in the opening paragraph. Thus throughout this paper an assignment of a material as being self-extinguishing is short-hand for ``self-extinguishing in the limited oxygen index test''.