Hygrothermal aging of a filled epoxy resin - Schneider Electric

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Hygrothermal aging of a filled epoxy resin - Schneider Electric

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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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