Fundamentally all dielectric measurements are made by measuring the voltage and current between a pair of electrodes in order to determine the conductance and capacitance between those electrodes. Conductance is a measure of the material's dissipation of energy, while capacitance is a measure of the material's storage of energy. Both conductance and capacitance depend on the geometry of the electrodes- that is, their shape, width and separation--and the material between the electrodes. In dielectric cure monitoring, the conductance of the material is of greatest interest.
Almost all materials contain ions, which are electrons, charged atoms or charged molecular complexes. The application of a voltage between a pair of electrodes will create an electric field which forces those ions to move from one electrode to the other. Ions encounter viscous drag as they flow through a medium filled with molecules, and their mobility through this medium determines the conductivity. At this point keep in mind that resistivity is the inverse of conductivity and we can see how resistivity is directly related to viscosity. Ions flowing through a very fluid, watery material have a high mobility and conductivity--resulting in low resistivity which correlates with low viscosity. Conversely ions flowing though a very stiff material have a low mobility and conductivity--yielding a high resistivity corresponding to the high viscosity. It is important to note that past some point in the cure the physical viscosity will climb so high that it is no longer measurable, even though the cross- linking reaction has not reached completion. Because the increasing polymerization continues to affect ionic motion, dielectric measurements retain sensitivity past the time when ion and physical viscosity deviate. Consequently, with proper interpretation, dielectric measurements are useful throughout the entire cure for determining changes in viscosity and rigidity, and are extremely sensitive in determining the end of cure.
For an example of dielectric cure monitoring data on the cure of a polyester bulk molding compound, click here.
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