The concept that flammability is fundamentally related to the potential thermal energy available per unit of volume of material emerged from attempts to correlate the effect of composition variables on the flammability of neoprene vulcanizates as measured by the oxygen index (O.I.) test. The origins of this test clearly show that it is a highly specific measure of flammability—the tendency of a composition to continue to burn once ignited—and that it is thermodynamically related to the heat of combustion of materials. This relationship is developed to a linear correlation which includes a wide variety of synthetic and natural materials and permits reasonable prediction of O.I. values from elemental analysis. Polymeric materials containing carbon and oxygen in atom ratios of less than 6 to 1 are more flammable than predicted. The effect of atmospheric temperature on O.I. can be predicted in relation to the O.I. value at normal temperature. This effect is shown to be independent of the composition of the material being tested. These two correlations permit the construction of a simple general map of flammability against which experimental data can be compared and judgments made with respect to the significant variables involved. There appears to be a significant relation between O.I. data, as viewed from these correlations, and the data of other flammability tests.
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Paul Johnson (1974) studied this question.