Assessing Thermal Stability: The Challenge of Powders
When it comes to solid materials, thermal instability can result not only from self-reaction or molecular fragmentation (decomposition), but also from oxidation – reaction of the solid with atmospheric oxygen. As the median particle size of a product decreases, the surface area to volume ratio increases and this can accelerate the self heating process – smoldering / glowing possibly leading to flaming combustion – from modest temperatures. This article examines the challenges in assessing the thermal stability of powders and demonstrates how traditional thermal techniques such as Differential Scanning Calorimeter (DSC) and Differential Thermal Analysis (DTA) can fail to diagnose self-heating phenomena. Specific techniques for powder thermal stability assessment (such as the Diffusion Cell, Aerated Cell and Basket Tests) are summarized, and a selection of thermal stability data on solids with different behaviors are presented to highlight the shortcomings of pursuing a one-dimensional DSC or DTA program for the thermal stability assessment of solids.