DSC Measurements of a Mineral Oil

Purpose

Mineral oils are often used as plasticizers or processing agents in elastomers. An example of a DSC measurement is discussed.

Sample

Mineral oil

Conditions 

Measuring cell: DSC822e with IntraCooler cooling option

Sample preparation: A drop of 17.68 mg. The oil was stirred well beforehand.

DSC measurement: Heating from –60 °C to 100 °C at 10 K/min. The sample was inserted in the measuring cell that had been cooled to –60 °C.

Atmosphere: Nitrogen, 50 ml/min

Interpretation

The DSC curve exhibits a glass transition at about –40 °C followed by an exothermic crystallization peak at –20 °C. The two small peaks at 30 °C and 40 °C are due to melting processes. 

Evaluation

At first sight, the melting peaks appear to be smaller than the crystallization peak. A peak evaluation gives values of 4 J/g for the crystallization peak and 0.8 J/g for die melting peaks. In such an evaluation, the large difference indicates that part of the melting behavior has not been measured - there are either additional melting peaks at higher temperatures or the baseline for the peak evaluation has not been correctly drawn. In this case, the baseline must set differently because no further melting processes are observed at higher temperatures. With these types of material, it can be fairly safely assumed that the enthalpy of fusion is equal to the enthalpy of crystallization, i.e. the sum of the area over both peaks should be zero. A corresponding baseline for peak evaluation is drawn in the diagram. The baseline is in fact the extrapolation of the measurement curve above 50 °C to lower temperature. The interpretation of the evaluation is that crystals melt in a broad melting range between –10 °C and 20 °C immediately after crystallization. To evaluate the glass transition, the second tangent was drawn so that it corresponds to the extrapolation of the baseline for the peak evaluation. This gave a glass transition temperature of about –39 °C and a step height, Δcp, of 0/45 J/g.

Conclusions

The DSC curves of mineral oils exhibit not only glass transitions but also crystallization and melting effects. In the evaluation of the measured curves, the total energy balance must be taken into account. The DSC curves of elastomers will also show similar peaks arising from the crystallization and melting of oils.

^{DSC Measurements of a Mineral Oil | Thermal Analysis Application No. HB 487 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2}