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.
