Assessing Thermal Stability - the Challenge of Powders

Assessing the thermal stability of process materials and mixtures is an essential step in understanding the process safety profile of a process. Many laboratories use Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA) techniques to affect this analysis and, for liquid systems, decomposition reactions can be readily assessed and quantified. More sensitive thermal techniques are available to supplement the DSC and DTA “screening” techniques including adiabatic calorimetric methods such as Accelerating Rate Calorimetry (ARC), Advanced Reactive System Screening Tool (ARSST), Vent Sizing Package (VSP II), Adiabatic Dewar Calorimeter (ADC II) and Phi Tec II. In short, the assessment of thermal stability of liquid systems is well understood and robust.

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 also increases and this can result in dangerous self-heating – smoldering / glowing possibly leading to flaming combustion.  

This paper examines the challenges in assessing the thermal stability of powders and demonstrates how traditional thermal techniques such as DSC and 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 appraised and summarized, and a selection of thermal stability data on solids with different behaviors are presented to highlight the dangers of pursuing a one-dimensional DSC or DTA program for the thermal stability assessment of solids.

The aim of the paper is to highlight a robust strategy for thermal stability assessment, specifically in relation to powders, and to exemplify the shortcomings of some traditional thermal analysis techniques through the presentation of test data from a range of techniques.