Further uses of the heat of oxidation in chemical hazard assessment



Flammability: The “net heat of oxidation” technique described in an earlier publication is extended to predicting the lower flammable limits, lower limit flame temperatures, and limiting oxygen concentrations of chlorinated organic fuels having H:Cl ratios greater than unity. A new Rule is derived for predicting the effect of initial temperature on the lower flammable limits and limiting oxygen concentrations of organic fuels. It is suggested that this Rule be used in preference to the modified “Burgess-Wheeler” Rule. The effect of initial pressure is discussed.

Instability: Net heats of oxidation (kcal/mol oxygen) for a series of disparate fuel groups are compared with “ΔHD,” the maximum heat of decomposition (cal/g) calculated using CHETAH methodology. Given the reasonable assumption that CHETAH's “maximum heat of decomposition” cannot exceed the net heat of combustion “ΔHC,” examination is made as to whether the ratio of these parameters (each expressed in units of kcal/mol), coined the “Reaction Heat Ratio” (RH), provides a useful new indicator for instability assessment.

Of these parameters, the net heat of oxidation (ΔHC/S) is the best indicator to help assign NFPA Instability Ratings. However, ΔHC/S cannot generally be used to assign ratings for organo-peroxides. Also, its performance as an indicator for hazardous polymerization depends on the ΔHC/S difference between the reacting monomer and the polymer product, so it should become increasingly unreliable as the monomer ΔHC/S approaches -100 kcal/mol oxygen. The ranking method tacitly assumes organic polymers to have a constant heat of oxidation of about -100 kcal/mol oxygen. Errors in this assumption must invalidate the ranking approach where ΔHC/S differences are small. Finally, separate “cut-offs” must be used at each NFPA Instability Rating for organo-nitrates versus other organics containing combinations of CHON atoms. Additional materials need to be examined to extend this preliminary analysis. The net heat of oxidation would be a useful additional output parameter of the CHETAH program, if only for its application in flammability assessment. No conclusions are drawn regarding the usefulness of net heat of oxidation or RH in conducting CHETAH hazard assessments, since this procedure requires consideration of several variables. However, the analysis may be helpful to the ASTM E 27.07 subcommittee responsible for developing the program. For example, the -ΔHD ⩾ 700 cal/g cut-off used to assign a “high” CHETAH hazard rating typically corresponds to organic materials rated NFPA 1, the second to lowest hazard rating.