Decomposition or thermal degradation
What is thermal degradation?
Thermal decomposition (or thermolysis) corresponds to the breaking of molecules under the effect of heat. It is to be differentiated from combustion which is an exothermic oxidation-reduction reaction. Thermal degradation is often an endothermic reaction, but in some cases it can be athermal or exothermic.
In industry, the stability conditions of chemical compounds must be known. They correspond to conditions for which no thermal decomposition occurs, even for long periods of time. However, real aging experiments are sometimes too long to implement. This is why accelerated aging experiments should be performed.
How does DSC allow accelerated aging studies to be carried out?
Differential Scanning Calorimetry (DSC) allows a sample to be placed under controlled conditions of temperature, pressure, hygrometry and gas atmosphere. The heat flux emitted or absorbed by the sample is measured continuously, and makes it possible to detect degradation phenomena which are not athermal.
The sample undergoes a temperature ramp up to temperatures which can reach more than 1000°C in certain cases. As long as no thermal phenomenon takes place, the thermal signal corresponds only to the heat necessary for the heating of the sample to follow the temperature ramp of the calorimeter (heat capacity effect). The presence of other thermal phenomena on the thermogram indicates that changes in the sample are probably taking place. Depending on the sample analyzed, this may correspond to phase changes, reactions catalyzed by temperature or thermal degradation.
Several techniques make it possible to identify the causes of the thermal phenomena observed. A first approach consists in carrying out 2 consecutive heating cycles of the sample, to see if the observed phenomenon is reversible or not. Thermal degradation is in fact an irreversible phenomenon which does not recur during a second temperature ramp. At the same time, it is possible to stop the heating ramp at any time and analyze the sample using conventional analytical techniques, in order to check its integrity: HPLC, GC, MS, NMR, etc.
The identification by DSC of thermal degradation phenomena makes it possible to know the conditions for which this degradation occurs rapidly. To estimate the conditions at which a sample will degrade more slowly, extrapolations must be made.
The deliverable of this type of analysis is a report containing the DSC results (thermograms) and their analysis, as well as the results of additional analyzes if necessary.