JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition - 77

resin. To this end, the uncured resin must
be in contact with a dielectric sensor
consisting of two electrodes. A sinusoidal
electrical voltage (excitation) is applied
to the electrodes and the resulting
current and corresponding phase shift
are measured (see Fig. 2). The response
signal correlates with the ion mobility
(also known as ion conductivity) of
the resin and the alignment of dipoles.
As the curing reaction progresses, the
sample material becomes increasingly
viscous. As a consequence, the mobility
of the charge carriers decreases, followed
by a corresponding attenuation of the
amplitude and an increased phase shift.
The dielectric coefficient εr describes
the resulting signal, which is comprised
of a real part εr' (dielectric permittivity)
and an imaginary part εr'' (dielectric loss
factor). In addition, εr'' is proportional
to the ion conductivity σ, which is the
reciprocal value of the ion viscosity ν [2].
Since ion viscosity, just like shear viscosity, changes as a function of the degree of
curing, DEA is the ideal tool for in-situ
monitoring of the cross-linking progress
of any thermosetting resin.
A typical cure monitoring measurement curve at 1 kHz for an epoxy resin
is displayed in Fig. 3, where the ion
viscosity and sample temperature are
shown over time. The logarithmic ion
viscosity exhibits a significant decrease
from 107.8 Ω*cm to 106.9 Ω*cm as the
sample temperature increases from room
temperature to 150°C, which enhances
the mobility of ions in the uncured resin.
The sharp increase in ion viscosity after
8.7 minutes indicates the beginning of
network formation by cross-linking of
the epoxy, which hinders the ion mobi-

lity. This dielectric behaviour
correlates with the
rheological viscosity, which can be
analyzed by means
of a plate-plate
rheometer.
Main features of
DEA:
- Describes the
entire sequence of
curing behaviour
from 0% to 100% in Fig. 3: Typical measurement curve for an epoxy resin sample as a function of tempeone measurement. rature and time, recorded with an IDEX sensor inside NETZSCH's DEA lab furnace
In contrast, rheometry can only
be applied before
vitrification of the
resin and not to
the final end of
curing.
- Does not require
a defined sample
geometry; only
requires a thin
coating on the
sensor.
- Can be applied
in the laboratory
Fig. 4: Typical applications for DEA technology in the composite industry, from labas well as during
scale resin development in a temperature-controlled furnace, over coupon testing in
part manufacturing. a lab-scale press up to an industrial-scale compression moulding process
- Yields spatially
nique across a wide range of application
dispersed information about the surface
areas in the composite industry. It helps
or the bulk of the resin (Z-information),
monitor and understand curing behaas DEA sensors can be placed at different
viour during the research and developthicknesses across a moulded part.
ment of resin formulations as well as pro- Monitors the resin flow front (X-Y
cess development in lab-scale processes.
information) during infusion processes
Furthermore, DEA technology can be
when multiple sensors are used.
used for on-line quality control in the
manufacturing of composite parts with
Dielectric analysis is an essential techthermosetting matrices (Fig. 4).

DEA solutions

Fig. 2: Dipoles and ions in the resin are stimulated by an electrical field generated by a sinusoidal voltage; the
response signal is a phase-shifted sinusoidal current

Netzsch Analyzing & Testing offers a
versatile portfolio of tailored DEA solutions for every step along the composite
processing chain. The state-of-the-art
DEA 288 Epsilon instrument, with its
wide frequency range (up to 1 MHz)
and fast data acquisition (< 5 ms), can

No88 April 2014

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JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition
