For a thermoset, the ion viscosity is the frequency independent resistivity (ρDC). In most cases, ion viscosity varies in proportion to mechanical viscosity prior to gelation, and to modulus after. This makes ion viscosity a useful measure of material state through the entire cure.
Dielectric Cure Curve
Figure 1 shows the typical ion viscosity behavior of a thermoset with one temperature ramp step and one temperature hold step.
Figure 1. Typical ion
viscosity behavior of a curing thermoset
The dielectric cure
curve is characterized by four Critical Points (CP):- CP(1)—A user defined level of ion viscosity, generally applied to determine the onset of material flow.
- CP(2)— Ion viscosity minimum, which closely corresponds to the mechanical viscosity minimum.
- CP(3)—Inflection point, when the reaction rate begins to slow, can be related to gelation but does not indicate gelation.
- CP(4)—A user defined slope to define the end of cure.
Dielectric/Conductivity Sensors
Dielectric instrumentation quantifies the resistance (R) and capacitance (C) between two electrodes at a specific frequency. It is possible to model the Material Under Test (MUT) between the two electrodes as a resistance in parallel with a capacitance (Figure 2).
Figure 2. Electrical
model of dielectric Material Under Test
Figure
3 shows simple parallel plate electrodes, which are capable of measuring the dielectric
properties of material between them. The A/D ratio is a figure of
merit, where ‘A’ is the electrode area and ‘D’ is the distance between them. A
larger A/D ratio represents greater sensor sensitivity.
Figure 3. Comparison of
parallel plate and interdigitated electrodes
The
A/D ratio is also the scaling factor used to calculate permittivity
from capacitance, and resistivity from resistance. However, the value D can vary
with pressure or with material expansion and contraction, leading to erroneous
results.Interdigitated electrodes, shown in Figure 3, are the alternative solution, where the electrodes are supported by a rigid substrate, and therefore the planar structure remains unchanged with pressure or with MUT expansion and contraction. A bulk measurement is made by the parallel plate sensor, whereas a surface measurement is made by an interdigitated sensor.
As a rule of thumb, interdigitated electrodes with the same width and separation “see” into material to a depth roughly equal to the width of the electrode. The A/D ratio can also be applied to interdigitated electrodes as a figure of merit, and is the scaling factor to calculate resistivity and permittivity. A typical disposable dielectric/conductivity sensor is shown in Figure 4, with interdigitated electrodes of 100 µm width.
Figure 4. Disposable
dielectric/conductivity sensor on polyimide flex circuit
This
sensor is built as a Kapton® flex circuit, and is thin enough to be
introduced between the plys of a laminate, and may be disposed of after use. A
large A/D ratio of 160, with correspondingly high sensitivity, will be the
result of the narrow electrodes, which are too small to be identified in the
figure. The trade-off is measuring dielectric
properties only within 100 µm of the surface.A reusable dielectric/conductivity sensor embedded in a platen for a small press is shown in Figure 5. This sensor is built with interdigitated electrodes embedded in ceramic. It has an A/D ratio of 10. When installed as depicted, the sample can be placed, heated and compressed in the press, and Reusable sensors are ideal for QA/QC applications, where repetitive testing is common. Figure 5 shows the wider electrodes. This sensor can measure more deeply into the material, but has lower sensitivity due to the smaller A/D ratio.
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