If an oxidising agent is present during a measurement, oxidation of the sample can occur [2. Chap. 2]. In this process, electrons are transferred from the substance under consideration to the oxygen present an an oxide is formed as a reaction product. Oxidation is an exothermic process, the onset of which is represented in a DSC signal is rarely recorded. In order to test the preload and stability of substances, it is possible to make statements about the oxidative behavoiur of a sample with the help of the DSC measurement. To ensure a comparable analysis, there are various standards for oxidation reactions. THese specify uniform measurement condtions for similar results. The first of these oxidation testts is the oxidtion onset temperature (OOT) measurement. Here a static atmophere is used in which the oxidant oxygen is already present. The test specimen is then heated at a defined heating rate. The temperature from which the DSC signal shows the signigicant bend of the oxidation is the OOT. The evaluation is carried out by two tangents which are applid to the DSC curve before and after this kink. The corresponding intersection point represents the desired onset temperature. THe method is illusrated in Fig. 1.


Figure 1: Illustrated of an oxidation onset temperature measurement [3, p. 8]

The second oxidation stability test is the oxidation induction time (OIT), here after referred to as OIT) and is illustrated in Fig. 2. In this process, the sample is brought to a defined temperature below the OOT under inert, i.e. reaction agent-free, conditions and held there. At a certain point in time, the oxidant is then introduced into the sample chamber. The time from he introduction of the oxidant to the start of oxidation on the sample, also marked by the bend in the DSC signal, is the OIT. The evaluation is analogous to the OOT but as a function of time.

Figure 2: Representation  of an oxidation-induction-time measurement [3, p. 7]

In order to guarantee economic and reproducible measurements for very ocidation-stable compounds, high-pressure DSCs are used in practice. The OIT is shortened depending on the pressure. Due to the higher oxygen concentration as a result of better heat and oxidant exchange, the result is more pronounced with a steeper change in slope.

Decomposition during oxidation

Decomposition refers to the breakdown of a compound into smaller molecules or elements [1, section 3.4.4]. If no oxidant such as atmospheric oxygen is present during a measuement, pyrolysis starts at a substance-dependent temperature. The substance system is split by the input of heat and decomposes. In a DSC, this can be implemented by using an inert sample gas such as nitrogen, as otherwise oxidation can occur, which affects this process. Decomposition is an endothermic process.

[1] B. Wunderlich, Thermal Analysis of Polymeric Materials. Berlin, Heidelberg: Springer-Verlag Berlin Heidelberg, 2005.
[2] G. W. Ehrenstein, G. Riedel und P. Trawiel, Thermal analysis of plastics: Theory and practice. Munich: Hanser, 2004.
[3] Plastics – Differential Scanning Calorimetry (DSC), ISO 11357-6, 2008.