Defined heating and cooling rates are important parameters for DSC measurements. International standards recommend a heating rate of 10 K/min or 20 K/min (ISO 11357, DIN 53765, ASTM E 793, ASTM E 794) when striving for thermodynamic equilibrium. In contrast, the objective of quality control and assurance in polymer processing is to obtain meaningful measurement results faster by means of higher heating rates (e.g. 40 K/min). The primary aim is to compare a current measurement on a rejected part sample to a reference sample. The operator dutifully carries out temperature calibration at higher heating rates and records a shift of the melting peak temperature to higher values, but is then often surprised that the DSC measurement on the real polymer sample does not deliver the desired result. The high heating rate causes thermal effects to be displaced; individual peaks or melting phases can no longer be reliably separated.
In figure 1, the relatively high heating rate of 40 K/min for semi-crystalline polybutylene terephthalate (PBT) no longer shows the typical beta melting phase seen in smaller crystallites, but rather only the main melting peak (here at 228°C). If attempting material identificiation, it could be incorrectly assumed here that the material in question is polyamide 6 (PA 6). The lower heating rate of 10 K/min already shows the beta phase clearly separated from the main peak at 217°C; this is typcial for PBT and does not occur for PA6.
Controlled cooling from the melt carried out with an intracooler or liquid nitrogen yields the crystallization behavior of PBT (figure 2). As the cooling rate increases, both the beginning of solidification (extrapolated onset temperature) and the crystallization temperature shift to lower values (figure 3). As the cooling rate increases, the crystallization peak not only gets larger but also extends across a broader temperature range. Although considerably higher cooling rates are employed in injection molding, DSC yields important information as to when or at which temperature the part can be demolded from the tool safely and without danger of distortion.