Glass Transition and Melting of EPDM - METTLER TOLEDO

Glass Transition and Melting of EPDM

Purpose

To show how the melting peak and glass transition can be separated and evaluated using EPDM as an example

 

Sample

Unvulcanized EPDM (EPG 6170) 

 

Conditions

Measuring cell: DSC822e with liquid nitrogen cooling option

Pan: Aluminum 40 µl, with pierced lid

Sample preparation: Cube of approx. 5 mg cut from the starting material. Before the measurement, the sample was heated to 100°C and then cooled to the starting temperature at 5 K/min

DSC measurement: Heating from 120 °C to 100 °C at 10 K/min  

Atmosphere: Nitrogen, 50 ml/min 

 

Evaluations

With such broad melting peaks, it is not always so easy to determine the beginning of the peak with good reproducibility. To illustrate the problem, the same measurement curve was evaluated in three different ways. In evaluation (1), the entire endothermic deviation from the baseline was taken as the melting peak. Since the slopes of the curves before and after this peak are different, the baseline type “spline” was used. 

In evaluation (2), the step in the curve was interpreted as the glass transition. The peak that followed was evaluated as the melting peak. In evaluation (3), the extrapolation of the DSC curve to lower temperature from the melt (above 70 °C) was used as the baseline for the melting peak. This curve also corresponds to the tangent for the glass transition evaluation. The following table summarizes the evaluation results. 

Interpretation

The different evaluation methods yield peak areas that differ by about r25%. This raises the question as to which evaluation has the lowest systematic errors and the best reproducibility. It is in fact evaluation (3). The other two evaluations, and in particular evaluation (1), are based on incorrect interpretations of the measurement curve.

In the example considered, linear extrapolation of the heat flow curve from the melt corresponds to a good approximation to the baseline of the melting peak determined by the heat capacity of the melt (without melting peak). This line also corresponds to the tangent used for the evaluation of the glass transition. 

 

Conclusions

With broad melting peaks, such as for example often occur with EPDM, the straight line extrapolated to lower temperature from temperatures above the melting range should be used as baseline for the peak evaluation. This also allows the glass transition and the melting peak to be separated. With EPDM, melting begins immediately after the glass transition. This has to do with the relatively broad size distribution of the crystallites.

 

Glass Transition and Melting of EPDM | Thermal Analysis Handbook No.HB406 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 1