Melting and Crystallization Behavior of Vulcanized and Unvulcanized Silicone Elastomers
Purpose
To determine the influence of vulcanization on crystallization and melting behavior using silicone rubber as an example.
Sample
Silicone elastomers, vulcanized and unvulcanized.
Conditions
Measuring cell: DSC822e with liquid nitrogen cooling option
Pan: Aluminum 40 µl, pierced lid.
Sample preparation: Cubes of approx. 20 mg cut from the starting material
DSC measurement: The sample was placed in the measuring cell at 25 °C and cooled at the maximum cooling rate to the starting temperature. It was then heated from –140 °C to 100 °C at 10 K/min (1st run). Afterward it was cooled at 5 K/min to –140 °C and the first heating program repeated (2nd run). The measuring cell was purged with nitrogen.
Atmosphere: Nitrogen, 50 ml/min
The first heating run of the unvulcanized material exhibits a glass transition at about –120 °C followed by cold crystallization at about –90 °C. The crystallites then melt in the range –60 °C to –30 °C.
In the cooling run, the sample crystallizes. The maximum of the crystallization peak is at about –80 °C. In the second heating run, only a very broad and flat glass transition can be seen. Cold crystallization does not occur and the sample begins to melt from about – 80 °C onward. The behavior of the vulcanized sample is similar to that of the unvulcanized material. The most obvious differences are that the crystallization peak in the cooling run is broader and shifted to lower temperature. In the second heating run, a very small exothermic peak can be seen in the region of the cold crystallization.
Evaluation
In the first heating runs, cold crystallization occurs after the glass transition. Melting begins immediately afterward. If the peaks are evaluated separately, it is often difficult to choose the right the peak limits. This can be lead to misinterpretation and poorly reproducible evaluations. For better reproducibility of the evaluation the baseline for integration should be drawn both peaks (as shown in the diagram of the unvulcanized sample). The result yields a value for the enthalpy of fusion that characterizes the sample as it was before the measurement. It is therefore a measure of the crystallinity of the sample before the measurement. To determine the enthalpy of crystallization separately for the cold crystallization or the enthalpy of fusion, partial areas can be determined with the same baseline. The upper limit of the crystallization peak is the point of intersection of the DSC curve with the baseline. This temperature then also corresponds to the lower limit of the melting peak. In the cooling and second heating runs, only one peak occurs. In this case, the evaluation is much simpler. For the sake of clarity, only the evaluations of the unvulcanized sample are shown in the diagram.
The following table summarizes the evaluation results, where Δh is the peak area (enthalpies of fusion or crystallization) and T is the maximum temperature of the peak. Negative signs for Δh indicate exothermic processes.
Interpretation
In the first heating runs, the total peak areas are about 9 J/g and 6 J/g. This means that, on heating, more material melted than crystallized before in cold crystallization. Prior to the first measurement, the samples were therefore semicrystalline. The samples crystallized partially during cooling from room temperature to the starting temperature of –140 °C. The vulcanized sample attained a lower degree of crystallization. This indicates that the cross-linking hinders crystallization.
This effect can be clearly seen in the cooling curves. The unvulcanized material crystallizes earlier and in a narrower temperature range. Besides this, the enthalpy of crystallization and therefore the degree of crystallization after cooling is greater than for the vulcanized sample.
The small exothermic peak at –93.9 °C in the second heating run also shows that, with the vulcanized sample, a maximum degree of crystallinity is not attained at a cooling rate of 5 K/min. The enthalpies of crystallization and fusion from the cooling runs and second heating runs agree well. From the measurement results, the following conclusions can be drawn about the difference in crystallization behavior of vulcanized and unvulcanized silicone elastomers:
Conclusions
Although all the crystallites in silicone elastomers have melted at the temperatures at which they are normally used, an analysis of the crystallization and melting behavior allows information on the nature of the material to be obtained. A direct comparison of the different measurement curves can also be used for quality control purposes. The crystallization process is influenced by vulcanization.
Melting and Crystallization Behavior of Vulcanized and Unvulcanized Silicone Elastomers | Thermal Analysis Application No. HB449 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2