Hygrothermal aging of a filled epoxy resin - Schneider Electric

Schneider Electric 2007 - Conferences publications
mass of 6% measured for the unfilled samples, this
value leads to a filler content of 58% which corresponds
well with the “rated” filler content of 60%. To simplify
the comparisons, the variations of the residual masses
were neutralized in the results displayed in Figure 3.
This shows the relative mass losses between the initial
mass values of the samples and the residual masses.
A mass loss between 50°C and 200°C can be observed,
as in unfilled samples. In these cases, the variations
correspond well with the water uptake measured before.
The resin decomposition can be observed in the same
temperature range as above. As shown in the zoom in
Figure 3, decomposition started at lower temperatures
for longer conditioning periods. This is clear if we
compare the curves for 50 and 70 days, which fit the
curve of 14 days until about 200°C and decrease faster
between 200°C and 300°C. This indicates an evolution
of the material produced by the hygrothermal
conditioning after about 50 days. Since this effect was
not encountered in samples without fillers, the
epoxy/silica interface regions are necessarily involved.
We have already mentioned that the mass measurements
suggest an accumulation of matter close to these
interfaces. We may consider that this evolution is due to
an hydrolysis of the filler/matrix interface regions which
creates a degraded inter-phase region. Since the filler
content is large, this degraded region may also fill the
whole gap between neighbouring fillers and constitute
“weak” path between them. This hydrolysis occuring
after about 50 days is probably correlated with the
increase of the mass uptake observed at the same time.
The possible explanation of this is as follows: free space
is created by the hydrolytic reactions consisting mainly
in the attack of the ester linkages and creation of acid
groups [4]. This allows extra water molecules to
accumulate between the filler and the degraded region.
However, the extra quantity of water involved in this
mechanism would be low in comparison with the total
quantity of absorbed water, which was almost entirely
lost between 50°C and 150°C. Further experiments are
under way to confirm this hypothesis.
DMA results
The dynamic mechanical analysis was carried out on
four filled and four unfilled samples after 0, 5, 14 and
50 days of conditioning. The evolution of the elastic
moduli E’ is displayed in Figure 4a. The magnitudes for
the filled samples were about three times larger in the
glassy state and five times larger in the rubbery state.
The elastic modulus decreased during conditioning,
especially in the glassy state. In practice, water
molecules break the hydrogen bonds established inside
the network. The bound water increases also the
mobility of the polymer chains and causes a decrease of
the elastic modulus. This is the well-known waterinduced
plasticization effect.
In the rubbery state, we would expect the effect to be
less marked since the chains are already mobile. Thus in
this case, the modulus E’ depends theoretically on the
density of cross-links in the epoxy network [14].
Nevertheless, the conditioning induced a decrease of the
elastic moduli E’ for the filled samples. For unfilled
samples, the measurements were conducted at the lower
limit of sensitivity of the DMA instrument. Such a
decrease of the rubbery modulus has already been
observed in [15]. It is not due to the plasticization but
may be attributed to a degradation of the polymer,
which probably results from hydrolysis of the resin.
(a)
M(Minitial-Mfinal)%
100
90
80
70
60
50
40
30
20
10
0
REF
5 days
14 days
50 days
70 days
REF
5 days
14 days
50 days
70 days
0 100 200 300 400 500 600
T(°C)
Figure 3: Weight loss during a TGA dynamic test for
filled samples before (•) and after conditioning at 80°C
and 80%HR during 5 (+), 14 (), 50 () and 70 () days
with a zoom in the temperature range 0-350°C between
92 and 100%.
(b)
Figure 4: Elastic modulus E’ (a) and loss modulus E’’
(b) vs temperature of unfilled (---) samples and filled
( ___ ) samples before (•) and after conditioning at 80°C,
80%HR during 5(+), 14() and 50 () days.
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