Title ~ Thermal Analysis
Author ~ R. P. W. Scott
Section ~ Dielectric Thermal Analysis.
Dielectric Thermal Analysis
Many substances, for example polymers, can exhibit both dielectric and conductive characteristics. Consequently, if such a sample is situated between two metal plates that form a type of capacitor, and a sinusoidal alternating current applied across the plates, a current will flow through the capacitor partly as a capacity current and partly as a resistive current. The resistive portion of the current that flows through the capacitor will be in phase with applied alternating voltage but the capacity current will be 90o out of phase with the applied alternating voltage. Measurement of the amplitude and individual phase differences of the voltage and current through the capacitor provides dielectric data for the polymer and the extent of ionic conductance. In turn, these values will also provide information regarding the chemical and physical structure of the polymer. The phase difference between alternating voltage and current across a pure resistor will be 0o and the phase difference across a pure capacitor will be 90o. If the polymer provides a combination of resistance and capacity then this will result in a phase angle having a value lying between 0o and 90o.
In general, it can be deduced that the magnitude of the dielectric constant (or the permittivity) of the material examined will indicate the degree of dipole alignment existing in the sample. Conversely the dipole loss factor (another electrical property that can be calculated from the current and voltage amplitude and phase difference) gives a measure of the ionic conductance that occurs through the sample. It is clear that such measurements can be used both for research and the quality control of products indicating the degree and extent of the polymerization process. In addition the polymerization process can be examined as the polymerization proceeds. As polymers are extensively employed in the manufacture of electrical products such information can be essential. A photograph of a Triton dielectric thermal analysis sample device is shown in figure 12.
Figure 12, A Sample Holder
for Dielectric Thermal Analysis
The sample, usually in the form of thin sheets is held between two metal disks across which the alternating current potential is applied. The whole is situated in an oven the temperature of which can be held to given value or programmed to a suitable temperature-time profile. Care must be taken to ensure close contact between the metal plates and the sample and the samples should be relatively thin (less than 2 mm if possible) as this increases the capacity and indirectly increases the signal –to-noise of the system.
The results from the examination of an epoxy resin during the curing procedure are shown in figure 13.
Figure 13 A Curve Relating
the Loss Factor with Time for a Sample of Epoxy Resin During Curing at 100oC.
The loss factor represents the presence of ionic conduction in the curing resin and so the curves indicate that during the curing process ions are produced. However, as the curing proceeds, the ions are removed and ionic conductance eventually disappears.