Separation of Melting and Decomposition using High Heating Rates

Introduction

A lot of information about organic substances can be derived from the melting peak in a DSC measurement. This includes the degree of purity, composition, polymorphic effects, the enthalpy of melting and the melting point itself. The information can only be obtained with adequate accuracy if the sample is chemically stable during the melting process.

Many organic materials, however, start to decompose as soon as melting begins. As a result, the DSC curve exhibits two overlapping thermal events that cannot easily be separated. Furthermore, decomposition leads to contamination of the sample and in turn to a change in the measured melting point [1].

One possibility to separate melting and decomposition is to measure the sample at a high heating rate. This approach takes advantage of the effect that the temperature of the melting process is not (or is only slightly) dependent on the heating rate whereas a chemical reaction is shifted to higher temperatures at higher heating rates.

If the heating rate is sufficiently high, it should therefore be possible to shift a decomposition reaction to a temperature high enough to separate the processes of melting and decomposition. Measurements using conventional DSC, however, show that the technical limit of the DSC is frequently reached before the two processes can in fact be separated.

Figure 1 shows an example of an organic pigment that was measured at heating rates of 100 and 150 K/min. In both curves, it is apparent that melting and decomposition overlap; the thermal events cannot be correctly evaluated in either of the curves. Much higher heating rates are therefore necessary to separate the two effects. Such rates can now be achieved using the METTLER TOLEDO Flash DSC 1 [2–4], which can reach heating rates of up to 2.4 million K/min or 40,000 K/s.

In this article, Flash DSC 1 measurements are shown in which the melting and decomposition effects of an organic substance are separated and individually analyzed.

Experimental details

Measurements in the DSC 1 were performed using standard 40-μL aluminum crucibles at heating rates of 100 and 150 K/min. The sample mass was approximately 4 mg. The Flash DSC 1 measurements were performed at heating rates of 50 to 5000 K/s (3000 to 300,000 K/min). Silicone oil was applied to the UFS 1 chip sensor of the Flash DSC 1 on the sample side and on the reference side in order to ensure good contact between the sample and the sensor [3]. Samples with a mass of about 3 ng were positioned on the sensor using a hair.

Results and discussion

Flash DSC 1 measurements

Figure 2 displays the results obtained from Flash DSC 1 measurements at a heating rate of 100 K/s (6000 K/min). The curves indicate that this heating rate is still too low to separate the two effects.

The two curves however show that the melting peak is shifted to higher temperature. The reason for this is that the decomposition reaction is shifted to higher temperature. At 100 K/s, the peak maximum is at about 400 °C while at 2.5 K/s (150 K/min) it is about 380 °C. The degree of decomposition in the melting range is therefore lower so the sample is less contaminated and melts at a higher temperature.

To separate the processes, measurements were then performed at a heating rate of 5000 K/s (300,000 K/min) as shown in Figure 3. The black curve in Figure 3 displays the measurement at a heating rate of 5000 K/s (300,000 K/min). At this heating rate, the melting process is completely separated from the decomposition process.

Conclusions

The Flash DSC 1 allows physical effects to be separated if the maximum possible heating rate of 2.4 million K/min is sufficient to shift a heating rate-dependent effect to such an extent that it no longer overlaps the second effect. Both effects can then be correctly and quantitatively evaluated.

_{Separation of Melting and Decomposition using High Heating Rates | Thermal Analysis Application No. UC 415 | Application published in METTLER TOLEDO Thermal Analysis UserCom 41}