DSC Measurements of CIIR Elastomers
Purpose
Elastomers often show relatively weak thermal effects when measured by DSC. To illustrate this, DSC measurements are performed on CIIR as a typical example of an elastomer based on one type of polymer.
Sample
Vulcanized CIIR elastomer (chlorinated isoprene isobutene rubber) with the following composition:
CIIR: 51.3%
Vulcanizing agent: 4.4%
Fillers: 41.4%
Plasticizers: 2.0%
Other additives: 1.0%
Conditions
Measuring cell: DSC822e with liquid nitrogen cooling option
Pan: Aluminum 40 µl, pierced lid.
Sample preparation: Cube of 21.718 mg cut from the starting material
DSC measurement: Heating from –140 °C to 120 °C at 10 K/min (1st run). Afterward cooling to –140 °C at 5 K/min and then repeating the heating program (2nd run).
Atmosphere: Nitrogen, 50 ml/min
Interpretation
The glass transition can be seen in the DSC curves at about –60 °C. The low step height is due to the relatively high filler content of about 50%. The step height for the pure polymer, , can be calculated from the Δcpvalue using the equation:
where wp is the proportion of polymer in the sample. For pure CIIR, this gives a value of 0.386 J/gK. A value of Δcp,100% less than 0.4 J/gK is typical for halogenated polymers.
Weak endothermic events can be seen at about 20 °C and 90 °C in both the first and the second measurements (marked with arrows). In both cases, the peak area is about 0.8 J/g. The peaks could be due to the melting of about 2% polyethylene wax, which is present as additive.
In the first measurement curve, an endothermic peak is observed at about 100 °C. The area is 1.9 J/g. This peak could originate from the release of moisture. If one assumes the enthalpy of vaporization of water to be 2400 J/g, then the peak corresponds to the loss of 0.017 mg water. The sample crucible was reweighed after the DSC measurement. The measured weight loss was 0.020 mg. This peak can therefore be assigned to the loss of adsorbed water. Such water peaks are often observed in the first heating run of DSC curves of elastomers.
Conclusion
DSC curves often exhibit other effects apart from the glass transition. These provide information about the material concerned. The effects, however, are often very weak. This means that large sample weights have to be used.
DSC Measurements of CIIR Elastomers | Thermal analysis Handbook No.HB463 | Application published in METTLER TOLEDO TA Application Handbook, Elastomers Volume 2.