Hygrothermal aging of a filled epoxy resin - Schneider Electric

Technical collection
Hygrothermal aging of a filled
epoxy resin
2007 - Conferences publications
E. Brun
P. Rain
G. Teissédre
C. Guillermin
S. Rowe

Schneider Electric 2007 - Conferences publications
2007 InternationalConferenceon Solid Dielectrics, Winchester, UK, July 8-13, 2007
Hygrothermal aging of a filled epoxy resin.
E. Brun 2 , P. Rain 1* , G. Teissèdre 1 , C. Guillermin 2 , S. Rowe 2
1 Grenoble Electrical Engineering Lab (G2Elab), CNRS- Université de Grenoble, Grenoble - France
2 Schneider Electric, Grenoble, France
* E-mail : pascal.rain@grenoble.cnrs.fr
Abstract: The hygrothermal conditioning of an epoxy
resin at 80°C under 80% RH has been followed by
weight measurements, thermogravimetric analysis
(TGA) and dynamical mechanical analysis (DMA). The
samples are either filled with 60% by weight of silica
flour or are not filled. Above an apparent saturation
value of about 1.5% reached within a few days, a slight
but significant mass uptake was observed in the filled
resin, especially after 50 days. The TGA showed an
evolution of the filled samples with conditioning after
50 days as well, which was not observed on unfilled
samples. For the filled samples, the elastic modulus in
the rubbery state decreased with conditioning. These
evolutions have been attributed to the formation of a
degraded inter-phase region due to hydrolysis occurring
after the debonding of the filler-matrix interface caused
by the absorbed water.
INTRODUCTION
Filled epoxy resin has been used for many years in
electrical engineering. The material is submitted to
thermal and electrical stress and is in contact with the
environment. The mechanisms leading to the occurrence
of electrical breakdown are still not properly
understood. A great variety of electrical, thermal,
mechanical, chemical phenomena may be involved in
the ageing of these polymeric insulations. Since the
material may be exposed to a humid environment, the
present work focuses on the specific influence of water
on the material and its effects on the electrical rigidity.
From an electrical point of view, the impact of water on
the dielectric behaviour of filled epoxy resins has been
extensively described. As concerns the electrical
rigidity, the breakdown voltages of wet materials may
fall by a factor of 5 to 10 in comparison with a dry
material [1-3]. In composites, the interfaces between the
matrix and the mineral fillers are known to be zones of
weakness [4-6]. The shape of the fillers may also
influence the breakdown voltage [7]. Furthermore, the
deleterious influence of water on epoxy resin is well
known [8]. The main mechanisms leading to physical
and chemical degradation of epoxy resin have been
illustrated or at least foreseen [9]. With the use of a
FTIR spectrometer, water layers of a few hundred nm
have been measured at an epoxy/glass interface [10].
This order of magnitude is in accordance with the MEB
observations reported in [3]. Water may accumulate at
the interfaces and lead to a filler/matrix debonding,
which may be followed by mechanical cracks
propagating in the bulk material [11].
For a better understanding of the overall mechanisms,
the impact of the hygrothermal conditioning on the
physical properties have thus been carried out first.
Electrical characterisations will ensue. In the following,
mass uptakes, thermogravimetric analyses and dynamic
mechanical analyses are reported.
EXPERIMENTAL PART
Materials, Sampling and Conditioning
The material used is a DGEBA based epoxy resin filled
or not with silica flour and cured with an anhydrid acid.
The filler content is 60% by weight. The fillers’ sizes
range between a few 0.1 μm and 200 μm.
The components were mixed, cast in a mould then
maintained at 100°C during one hour. After crosslinking,
samples were demoulded and then cured at
130°C during 16 hours. The sheets are 0.5 mm thick.
Glass transition temperatures of the filled and unfilled
samples are 72°C and 70°C respectively.
Samples are cleaned with alcohol and dried in an oven
at 50°C for 24 hours. After this preparation, their initial
mass was measured. The samples were then conditioned
in a climatic chamber at 80°C under 80%RH.
Characterisations
Moisture uptake measurements: Samples were
periodically withdrawn from the climatic chamber.
Before mass measurements were made, the temperature
and hygrometry of the samples were stabilized. For this
purpose, they were laid in a small quantity of water
initially at 80°C. After 15 minutes, both water and
sample were at ambient temperature. Samples were then
dried with a suitable paper. The mass uptake was
measured with an electronic Ohauss balance Explorer.
Thermogravimetric analysis (TGA): The thermal
stability of the materials has been evaluated throughout
conditioning by TGA with a TA instruments® 2050.
The mass loss of the samples was measured during a
temperature rise of 3°C/min between -40°C and 850°C.
Experiments were carried under nitrogen to avoid added
oxidation. A gas flow of 45 mL/min inside the oven
allowed the extraction of the thermolysis by-products.
The relative mass loss of samples with initial mass of 10
1-4244-0750-8/07/$20.00©2007 IEEE.
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Schneider Electric 2007 - Conferences publications
to 20 mg is reported hereafter.
Dynamic Mechanical Analysis (DMA): The evolution
of mechanical properties and thermal transitions during
conditioning were measured by DMA with a TA
instruments® 2980 using the 3-point bending mode.
Taking into account the elastic domain of the material,
the measurement conditions were the following: a
dynamic magnitude of 50µm and a static force of 140%
of the dynamic force. The samples were rectangular in
shape 40×10×0.5mm 3 . The frequency was 1Hz and the
temperature rise was 3°C/min from -40°C to 140°C.
RESULTS AND DISCUSSION
Moisture uptake results
Three different samples, either filled or not filled, were
measured for each conditioning duration. Mean values
and standard deviations are reported in Figure 1. For a
better comparison of the two materials, mass variations
of the filled samples were calculated taking into account
the initial mass of unfilled resin, evaluated by the mean
filling content of 60%.
Water saturation of not filled samples is observed after
about 5 days. The water uptake was then of 1.5%. The
mass uptake of filled samples is lower at the beginning
than the one of unfilled ones: the kinetics of water
absorption is slower. The water diffusion is impeded by
the silica grains, which slow down its propagation
throughout the whole material. After 5 days
conditioning, the mass variation was close to the one of
unfilled samples but measurements tend to indicate that
the mass uptake continues to progress after the quasi–
saturation has been reached. This specific behaviour of
filled samples has already been reported in [12].
Furthermore, a slope increasing after about 50 days has
been observed in a similar way to that shown in [13].
These measurements will be completed to confirm this
tendency. This difference observed between the two
materials can be attributed to phenomena taking place at
the epoxy/silica interfaces. The following mechanism
can be proposed. Firstly, water molecules break the
physical bounds between the silica grains and the resin
and form H-bounds with the polymer. Then, some water
mass uptake (%)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Unfilled samples
Filled samples
0 20 40 60 80
Conditioning time (days)
Figure 1: Weight gain in unfilled () and filled ()
samples as a function of conditioning time.
accumulates between the two phases, part of which
possibly reacts with the resin.
TGA results
Thermogravimetric analysis was carried out for filled
and not filled samples after until 70 days of
conditioning. The thermal behaviour of unaged sample
are mentioned as “reference” in the figures. The dots on
the graphics are not measurements points but are added
to improve visualisation of the data.
The thermograms are displayed in Figure 2 for unfilled
samples. The derivatives of the mass loss curves are
also displayed to highlight the differences between the
samples. A detail of the curves between 50°C and
200°C is enlarged. This shows a significant decreasing
of the mass between 50°C and 120°C in aged samples,
which is not observed in reference samples. This mass
loss is slightly lower than the water uptake measured by
gravimetry. It corresponds to the evaporation of
absorbed water. Xu et al [13] observed an initial, weak,
peak of the derivative weight in the same temperature
range that they attributed to the evaporation of water
contained only in the free volumes. They considered
that the bound water was released at 200-300°C where
they observe a slight difference between as-cured and
aged samples. Our results do not confirm this point. The
main mass loss occurred between 300 and 450°C. For
all aged unfilled samples investigated so far, the
derivative weights display a maximum at about 398°C
and a shoulder (maximum of the second derivative) at
364°C. This corresponds to resin decomposition. At
800°C, a residual mass of about 6% was measured
which was not observed in similar experiment under
oxygen flow. MEB analysis showed the presence of
carbon with low quantities of oxygen. MEB photos
showed blackish sheets. This residue was then mainly
composed of carbon graphite.
For filled samples, residual masses were scattered and
not correlated with the duration of conditionning. The
mean value was 64%. Taking into account the residual
Figure 2: Weight loss during a TGA dynamic test for
unfilled samples before (•) and after conditioning at
80°C and 80%HR during 5 (+), 14 () and 50 () days.
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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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Schneider Electric 2007 - Conferences publications
The glass transition temperatures have been taken at the
maxima of the curves of loss moduli E’’ (Figure 4b).
For the reference samples, the Tg were 70°C and 72°C
respectively for the unfilled and filled samples. These
maxima shifted towards the low temperatures during
conditioning, thus highlighting water-induced
plasticization. This decrease occurred mainly within the
first days of conditioning and is more important in the
unfilled samples. After a 50 days conditioning, the shift
of the Tg values were of 14K and 11K for the unfilled
and filled samples respectively. Similar evolutions have
been already mentioned [16]. A secondary peak, which
would have indicated the heterogeneity of the material
[6, 12, 17], was not visible here.
CONCLUSION
Evidence of aging of filled epoxy resin conditionned at
80°C and 80%RH were provided by weight
measurements, thermogravimetric analysis (TGA) and
dynamical mechanical analysis (DMA).
Above an apparent saturation value of about 1.5%
reached within a few days, a slight but significant mass
uptake was observed in the filled resin. An
accumulation of water at the epoxy/silica interface can
be inferred, part of which may have chemically reacted
with the polymer.
Thermogravimetric analysis showed a degradation of
filled samples after a conditioning period longer than 50
days, which does not occur in unfilled samples.
The decrease of the elastic modulus E’ and of the glass
transition temperature Tg underlined the plasticization
of the samples during conditioning. For the filled
samples, the elastic modulus in the rubbery state
decreased with conditioning.
These results suggest the creation of a degraded interphase
region between the silica and the epoxy matrix
due to the hydrolysis of the resin.
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