JEC COMPOSITES MAGAZINE - Issue #144 - January/February 2022 - 49

AC and DC cure monitoring of composites
(MUT) in a parallel plate cell consists of a resistance, RMUT
allel with a capacitance, CMUT
, is calculated from bulk resistance, RMUT
, in par,
as shown in Figure 1.a. and Figure
1.b. These models include boundary layer capacitances, which can
form on electrodes and affect low-frequency AC measurements.
For both AC and DC measurements, the material property of resistivity,
ρMUT
, scaled by a
factor that depends on the sensor. AC resistivity has both frequency-independent
and frequency-dependent components. In an
oscillating electric field, frequency-independent resistivity arises
from the flow of mobile ions while frequency-dependent resistivity
arises from the rotation of stationary dipoles. For resins, frequency-independent
resistivity correlates with material state and is
the property most used in studying thermoset and composite cure.
DC resistivity (or resistance) in thermosets has been used interchangeably
with frequency-independent resistivity (or resistance),
but in fact they are different because of electrochemical effects. To
avoid confusion, they are defined more accurately below:
- DC resistivity (ρDC
Hz with a DC bias
- Frequency-independent resistivity (ρFI
) - thermoset resistivity measured at f = 0
) - thermoset resistivity
that is constant or nearly constant across a range of AC frequencies
and measured without a DC bias
Frequency-independent resistivity has particular importance for
cure monitoring. Before gelation, the amount of polymerization
affects both mechanical viscosity and the movement of ions, and
therefore influences ρFI
. For many materials, the change in frequency-independent
resistivity is proportional to the change in
mechanical viscosity [1,2,3]. To emphasize this relationship, the
term ion viscosity (IV), or AC ion viscosity (IVAC
as a synonym for frequency-independent resistivity and is defined
as IV = ρFI
or IVAC
= ρFI
.
Comparison: AC and DC cure monitoring
PR 520N [4] is a pre-mixed epoxy system engineered for resin transfer
moulding (RTM). At room temperature, PR 520N is a thick offwhite
paste and the manufacturer specifies full cure after two hours
at 179°C. To compare AC and DC measurements, PR 520N was applied
to Mini-Varicon [5] dielectric/conductivity sensors and heated
to 180°C for two hours. An LT 451 dielectric cure monitor [6] measured
AC dielectric properties at multiple excitation frequencies and
an LT 440 Dielectric Channel [7] measured DC resistance, which
was converted to DC ion viscosity (DC resistivity).
Fig. 3: Frequency-independent ion viscosity, with composite of frequencies,
following entire isothermal cure of PR 520N epoxy at 180°C
Fig. 2: AC resistivity of PR 520N during isothermal cure at 180 °C, VAC = 2.0 V
Ideally, AC ion viscosity should be the same as DC ion viscosity
(DC resistivity). The few previously published comparisons
show good correlation between the two during mid- to end of
cure [10,11,12]. During early to mid-cure, however, resins act as
electrolytes and are affected by electrochemical phenomena. As
a result, AC and DC results can be considerably different in the
early stages of composite processing.
The LT-440 circuit used for DC measurements has an internal
N°144 January - February 2022 / jec composites magazine
49
), has been used
Figure 2 is a plot of AC resistivity during isothermal cure at
180°C. Although AC resistivity has both frequency-independent
and frequency-dependent components, for convenience all data
are plotted against an axis labelled Ion viscosity.
Three features are apparent in the plot of Figure 2:
- Distortion of 1 Hz curve during early cure, from 0 to 15 minutes,
due to electrode polarization and the resulting boundary
layer capacitance [8]
- Curves that overlap or nearly overlap, indicating the dominance
of frequency-independent resistivity (ρFI
)
- Caused by flow of mobile ions - true ion viscosity
- Before gelation, change of ρFI
of viscosity
- After gelation, change of ρFI
modulus
is often proportional to change
is often proportional to change of
- Curves that diverge, indicating the dominance of frequency-dependent
resistivity (ρAC
)
- Caused by rotation of dipoles
- For certain frequencies, does not correlate well with cure state
After CureView [9] data analysis software removed frequency-dependent
resistivity, the remaining frequency-independent resistivity
is shown in Figure 3. These data can now properly be called
ion viscosity and accurately follow cure state for the entire test.
At about 120 minutes, the ion viscosity curve approaches a horizontal
slope to indicate the reaction is slowing and cure is ending.
This behaviour agrees with the manufacturer's specifications of
full cure after two hours at 179°C.
The ion viscosity curve of Figure 3 is actually a composition of
frequencies from 1 Hz to 10 kHz. In the general case, depending
on the material and circumstances, the data from only one frequency
is often sufficient and may be used instead of a composition
of frequencies.
JEC COMPOSITES MAGAZINE - Issue #144 - January/February 2022

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