C. Zou et al.: The
ffect of Water Absorption on the Dielectric Properties of Epoxy Nanocomposites
1070-9878/08/$25.00 © 2008 IEEE
ffect of Water Absorption on the Dielectric Properties
of Epoxy Nanocomposites
Chen Zou, J. C. Fothergill
Department of Engineering,
University of Leicester,
Leicester, LE1 7RH, UK
S. W. Rowe
Schneider Electric, Centre de Recherche A2,
Quai Paul-Louis Merlin,
38050- Grenoble-Cedex 9, Grenoble, France
In this research, the influence of water absorption on the dielectric properties of epoxy
resin and epoxy micro-composites and nano-composites filled with silica has been
studied. Nanocomposites were found to absorb significantly more water than unfilled
However, the microcomposite absorbed less water than unfilled epoxy:
corresponding to the reduced proportion of the epoxy in this composite. The glass
transition temperatures (T
of all the samples were measured by both differential
scanning calorimetry and dielectric spectroscopy. The T
decreased as the water
absorption increased and, in all cases, corresponded to a drop of approximately 20 K as
the humidity was increased from 0% to 100%. This implied that for all the samples, the
amount of water in the resin component of the composites was almost identical. It was
concluded that the extra water found in the nanocomposites was located around the
surface of the nanoparticles.
This was confirmed by measuring the water uptake, and
the swelling and density change, as a function of humidity as water was absorbed.
water shell model, originally proposed by Lewis and developed by Tanaka, has been
further developed to explain low frequency dielectric spectroscopy results in which
percolation of charge carriers through overlapping water shells was shown to occur.
This has been discussed in terms of a percolation model.
At 100% relative humidity,
water is believed to surround the nanoparticles with a thickness of approximately 5
A second layer of water is proposed that is dispersed but sufficiently
concentrated to be conductive; this may extend for approximately 25 nm.
If all the
water had existed in a single layer surrounding a nanoparticle, this layer would have
been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water
surrounding a given size of nanoparticle appeared to be independent of the
concentration of nanoparticles but approximately proportional to water uptake.
particles that have surfaces that are functionalized to be hydrophobic considerably
reduce the amount of water absorbed in nanocomposites under the same conditions of
humidity. Comments are made on the possible effect on electrical aging.
water absorption, epoxy, nanocomposites, glass-transition
temperature, differential scanning calorimetry, dielectric spectroscopy, swelling,
percolation, electrical aging.