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Dielectric thermal analysis (DETA) is a materials science technique in which an oscillating electric field is used to analyze changes in the physical properties of a number of polar materials. This thermal analysis technique can be used with materials in a range of forms, from thin films and sheet materials to powders or liquids. Another similar technique is dynamic mechanical analysis (DMA), in which a mechanical force is used instead of an electrical field.
How does Dielectric Thermal Analysis Work?
In dielectric thermal analysis, an oscillating AC electrical field is applied to a sample of material, and measurements of the material’s capacitance and conductance are taken as a function of time, temperature and frequency. Some of the electrical charge will be stored in the material sample (capacitance) and the rest of the electrical charge will be dissipated through the material sample (conductance). The relaxation properties of the material sample will determine the sample’s rates of capacitance and conductance.
In practice, a material sample will be placed in contact with two parallel plate electrodes, and a sinusoidal voltage is applied to one of the electrodes. The response is then measured at the second electrode, with the sinusoidal current being weakened and shifted in accordance with ion mobility and dipole alignment. The dipoles will attempt to align with both the ions and the electrical field, and move toward the electrode of opposite polarity. The oscillating voltage signal is applied at frequencies from 0.001 to 100,000 Hz.
Applications of Dielectric Thermal Analysis
Dielectric thermal analysis can be used in investigations into the curing behavior of thermosetting resin systems, adhesives, paints, and composite materials. Dielectric thermal analysis can also be used to characterize polar materials including:
- PVC – polyvinyl chloride
- PVDF – polyvinylidene fluoride
- PMMA – poly(methyl methacrylate)
- PVA – poly(vinyl acetate)
Polymer properties examined by DETA are permittivity (ε’), the measure of the degree of alignment of the molecular dipoles to the electrical field, and the loss factor (ε”), which represents the energy required for the reorientation of the dipoles and ions. The dissipation factor (tan δ = ε”/ε’) and the conductivity (σ-1) are also examined.
With DETA, the dielectric constant and polarizability of polymers are easily detected during phase transitions such as the glass transition (Tg), a unique characteristic of polymers in which a material will transition from a hard state (glassy) to a viscous state as the temperature of the polymer is increased. Other transitions include melting and crystallization and secondary transitions.
Other applications of dielectric thermal analysis are monitoring curing kinetics of epoxy and urethane systems.
Benefits of Dielectric Thermal Analysis
The applications for dielectric thermal analysis are very similar to that of dynamic mechanical analysis and differential scanning calorimetry (DSC). DSC is the most widely used thermal analysis technique and has been used to analyze food, pharmaceuticals, polymers, plastics, glasses, ceramics, and proteins. However, dielectric thermal analysis has a number of advantages over both DMA and DSC.
For instance, the data gathered through DETA are not affected by curing reactions, the breakdown of additives like blowing agents and degradation reactions, and data can be gathered from samples in a wide range of forms, from solids and liquid, to powders or pellets. DETA is also more sensitive than DSC, allowing it to obtain better data on samples in which there is a small polymer fraction, and, while dynamic mechanical analysis is constrained by its 100 Hz limit, dielectric thermal analysis is able to offer frequencies beyond 100 Hz. Additionally, DETA allows data to be gathered from probe samplers placed a long way away from the instrument itself. Lastly, dielectric thermal analysis equipment can be used at a laboratory scale or even in industry scale for in-process monitoring, for example in curing or polymerization reactions.
Sources and Further Reading
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