Heat Loss In Accelerating Rate Calorimetry Analysis And Thermal Lag For High Self-Heat Rates

Accelerating rate calorimetry (ARC) is a common tool used in thermal stability evaluation of hazardous materials to provide self-heat rate and pressure data that are used to model the kinetics of a reaction and even calculate relief vent sizing. The ARC accomplishes a conservative approach by maintaining adiabatic conditions. However, an issue with ARC instrument heating occurs when a reaction with a high self-heat rate is analyzed that results in nonadiabatic conditions and a "thermal lag". Testing was conducted to determine the self-heat rate threshold for the ARC instrument and to better understand telltale features that occur during analysis of samples with high self-heat rates. The results indicate that while the oven cannot maintain adiabatic conditions, the convective heat loss from the sample container to the ARC oven does not have a significant effect on either the measured self-heat rate or the calculated overall heat of reaction. However, the conductive heat loss from the sample container to the connection fitting does, causing the total heat measured in ARC analysis to be lower by 16% for an adiabatic ARC test, 33% for a nonadiabatic ARC test, and similar to 20% for a typical ARC test in comparison with DSC measurements. A corrected phi estimation is proposed with a correction factor to take into consideration the conductive heat loss to address this issue. In addition, the thermal lag caused by the ARC temperature measurement does create a significant difference between the measured peak pressure rate and peak self-heat rate occurrences. Analysis of the results showed that a simple linear correction can be applied to high self-heat rate results for liquid samples, while the pressure rate is still increasing, to correct for these peak rate differences. The pressure rate after correction matches the self-heat rate and better agrees with the prediction from kinetic modeling. Such corrections can be applied to solid samples, where there is an additional thermal lag due to resistance to heat transfer in the solid sample itself. Accounting for the thermal lag in reactions with high self-heat rates is important when modeling of the data is used for kinetic parameters and especially for relief vent sizing.