Flammability Studies of Isomeric Structures of Ethane Derivatives and Percolation Theory

In order to analyze how isomeric structures will affect the combustion hazard, the lower flammability limits (LFL) of 1,1,1-trifluoromethane (R-143a) and 1,1,2- trifluoroethane (R-143) have been measured using a modified ASTM E681 method. The modification is a spark ignition source in which the current is set at 10mA and the voltage is adjusted (7–12 kV) so that dielectric breakdown just begins to occur rather than 30mA at 15kV specified by the ASTM E681-04 method. An earlier study on the LFL of difluoromethane indicated that flammability limits by the modified ASTM E681 method are affected by vessel size. In order to investigate the vessel volume effect on the measured LFL, experimental measurements of the LFL of R-143 and R-143a are made at 21.5◦C using 5-, 12-, and 20-L vessels. For vessel volumes of 5-L and larger, the LFL systematically shift to higher concentrations of R-143 and R-143a with increasing vessel sizewhich is consistent with a percolationmodel. Extrapolation of themeasured LFL data to infinite vessel size using a percolationmodel yielded 3.57 and 2.96 mol · m−3 for R-143a and R-143, respectively.