1. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
EFFECT OF THE ENVIRONMENTAL DEGRADATION ON THE
VISCOELASTIC RESPONSE OF NANO MODIFIED EPOXIES AND
CFRPs
N. M. Barkoula*, G. Gkikas, A. Makri, T.E. Matikas, A. Paipetis
Dept. of Materials Science & Engineering, University of Ioannina,
P.O. Box 1186, GR-45110 Ioannina, GREECE
*nbarkoul@cc.uoi.gr
Abstract
Carbon Nanotubes (CNTs) addition in the matrix of Carbon Fibre Reinforced Plastics
(CFRPs) results to significant improvement of the damage tolerance of the CFRPs. This has
been attributed to the increased interfacial area, between the CNTs and the matrix. Although
this might be beneficial for the damage tolerance of the modified composites, it could lead to
deterioration of the in service durability of such systems. It is well known that Epoxies (EPs)
are susceptible to heat and moisture particularly when they operate in varying environments.
The properties dominated by the matrix or the fibre-matrix interface are degraded by
moisture absorption, whereas the properties that are dominated by the fibres are less
influenced. No experimental results are available so far on the effect of environmental
degradation on the viscoelastic response of these materials. Therefore, the purpose of this
study is to evaluate the viscoelastic performance of the aforementioned materials in extreme
environments. For this reason, multi wall CNTs were incorporated in an epoxy system, which
was subsequently used for the manufacturing of the modified CFRPs. All systems were
subjected to hydrothermal loading. The conditioned modified EPs and CFRPs were tested in
dynamic three-point bending using Dynamic Mechanical Analysis (DMA).
KEYWORDS: CNTs, Epoxies, Carbon Fibre Composites, Viscoelastic Response,
hygrothermal loading
1 Introduction
The introduction of nanoscaled reinforcement in otherwise conventional fibre reinforced
composites has opened an exciting new area in composites research. The unique properties of
these materials combined with the design versatility of fibrous composites may offer both
enhanced mechanical properties and multiple functionalities.
One of the most attractive nanofillers for the matrix modification of fibre composites are
Carbon Nanotubes (CNTs). CNTs possess unique properties such as elastic modulus at the
terra scale, huge aspect ratio, as well as extremely large specific surface area (or interfacial
area, when incorporated in the matrix). Moreover, the conductivity of such systems has been
reported to directly correlate to (i) the macroscopic strain applied on the material and (ii) the
internal damage that accumulates within the structural component throughout its service life.
[1]. Recent research has proven that when CNTs are used as an additive in the matrix of
Carbon Fibre Reinforced Plastics (CFRPs) at weight fractions as low as 0.5% result to the
1

2. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
spectacular improvement of the fracture toughness of the material [2], as well as the
substantial prolongation of its fatigue life. According to these studies, the improvement of the
damage tolerance of these systems is attributed to the incorporation of an additional interfacial
area, i.e. that between the CNTs and the matrix. This interfacial area is expected to activate
additional energy dissipation mechanisms related to interfacial sliding, fibre pull out and
bridging as well as crack bifurcation and arrest at the nanoscale. Furthermore, five different
blending techniques of incorporating MWCNTs into an epoxy matrix were examined recently
[3]. In general the use of solvents together with sonication produced enhanced mechanical
properties e.g. Young’s Modulus, Bending Modulus and dynamic properties such as Tg and
Storage Modulus. In a recent communication [4] it was emphasized that that specific surface
area of nanosized particles is huge, indicating that a large proportion of the surrounding
matrix will be in contact with the interface or even a separate phase – the interphase- will be
developed with properties different to those of the bulk matrix.
However, one challenge for the applicability of CFRPs is related to their environmental
durability. It is well known that epoxy matrix composites are susceptible to heat and moisture
particularly when they operate in varying environments. The amount of moisture absorbed by
the matrix is significantly different to that absorbed by the reinforcing phase. The presence of
moisture and stresses associated with moisture-induced expansion may deteriorate the matrix
related properties of the composite and as a result, have an adverse effect on the damage
tolerance and structural stability. The higher the temperature the higher the moisture uptake
rate of the composites and the delamination nucleation [5-18]. Furthermore the interfacial
adhesion degradation is dependent on the conditioning temperature and exposure time.
Among the properties of polymer matrix composites that are negatively affected by moisture
uptake is the stiffness [5, 6], the interfacial strength [7], the interlaminar interface [8-16], the
damping ratio [17,18] etc. Some of the mechanisms occurring during moisture absorption
include weakening of the fibre-matrix interface [8,11,15,16], plasticization and swelling of the
matrix and in some cases even softening of the matrix [11]. From the above one can conclude
that in the case of CFRPs, the properties dominated by the matrix or the fibre-matrix interface
are degraded by moisture absorption, whereas the properties that are dominated by the fibres
are less influenced. One important point is that in crosslinking resins, the ability of CNTs to
absorb or donate electrons may well affect the crosslinking densilty. As discussed above,
matrix, and reinforcement-matrix interface are more prone to absorb water and alter their
properties. The fact that CNT-reinforced composites possess increased interfacial area may be
beneficial for their fracture toughness; however, this could prove to be their weakest point in
terms of in service durability.
Although CFRPs have received a lot of focus, very few papers have been published on the
environmental degradation of CNTs-reinforced composites [4, 19-21]. The work by Zhang et.
al. [19,20] is an analytical approach on the hygrothermal effects on the pull-out force and on
the interfacial stress transfer of CNTs-reinforced composites that takes into consideration the
mismatch of the thermal and moisture expansion coefficients of CNTs and polymer. The work
by Barkoula et al. [22] is the first experimental study on the effect of water absorption on the
interlaminar shear strength (ILSS) of CNT modified CFRPs and the effect of hydrothermal
loading on the material properties using non destructive means. The ultimate goal of this
publication was to relate the hydrothermally induced changes of the material to electrical
resistivity changes.
Based on the above it is expected that the hygrothermal conditioning of the aforementioned
systems will alter their viscoelastic response and more specifically their damping properties,
glass transition temperature and dynamic modulus. Therefore, the purpose of this study is to
evaluate the viscoelastic performance of the aforementioned materials in extreme
2

3. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
environments. To this end, multi wall CNTs were incorporated in a commercial epoxy system
via high shear mechanical mixing which was subsequently used for the manufacturing of
quasi isotropic laminates CFRPs, using the wet lay up method. Modified matrices with CNT
content varying from 0.1% to 1% were manufactured. All modified resins were used to
manufacture un-reinforced rectangular cast specimens. The resin with the 0.5% CNT content
was subsequently used for the manufacturing of the modified CFRPs. All systems were
subjected to hydrothermal loading. During the environmental conditioning, the composites
were weighted in specified intervals and the water absorption vs. time was recorded for both
the modified and a reference system.
The conditioned composite systems were subsequently tested in dynamic three-point bending
in order to study their viscoelastic behaviour. The properties of the modified systems were
compared to the properties of unmodified composites that were subjected to identical
conditioning.
2. Experimental
2.1. Materials
MWCNTs produced by catalysed CVD, were supplied by ARKEMA, France. The MWCNTs
had diameters of 10-15 nm and length higher than 500 nm, resulting in aspect ratios
(length/diameter) varying between 30 and 50. The nanotubes were dried in an oven overnight
prior to use. The epoxy system used for the fabrication of CNT doped resins and CFRPs was
the Araldite LY564/ Aradur HY2954 from Huntsman Advanced Materials, Switzerland.
2.2. Specimen preparation
The dispersion of the MWCNTs in the epoxy took place in a Torus Mill device (VMA
Getzmann GmbH). The torus mill introduces high shear forces by a high-speed rotating disc
and reduces the nanoparticle agglomerates due to the milling effect generated by zirconium
dioxide beads. The beads have a diameter of 1.2 to 1.7 mm and cause strong shear action and
collision effects. The dissolver-disc provides additional shear forces and maintains the vortex
flow. The compound is stirred in a vacuum container to avoid air inclusion. The vortex flow
achieved by the geometry of the disc leads to continuous mixing of the compound. The
mixing speed was at 2000 rpm for 3 h. Modified matrices with CNT content varying from
0.1% to 1% were manufactured. All modified resins were used to manufacture un-reinforced
rectangular cast specimens. The resin with the 0.5% CNT content was subsequently used for
the manufacturing of the modified CFRPs.
16 plies of quasi-isotropic CF laminas [(0/+45/-45/90)2]s, supplied by Hexcel, Germany with
specific 160 gr/m2
weight, were used for the manufacturing of CFRPs. Each panel was hand
laid-up and then processed in an autoclave, using the vacuum bag technique. A reference
panel was also manufactured with neat resin for direct comparison. Two laminates of CFRP
materials were tested in total; one having a modified matrix with the addition of CNT, and the
other having an unmodified (neat) matrix. DMA specimens were cut (50x10xt mm3
) from the
aforementioned laminates using a diamond saw.
2.3. Environmental conditioning
A water bath with temperature control was used for the hydrothermal conditioning of the
specimens. Before placing the test specimens into the water bath, they were put in an oven of
50 °C to dry until their weight was stabilised. Their weight was measure using an analytical
balance with accuracy of ±0.1 mg. The specimens were afterwards placed in distilled water at
80 °C, controlled to ±1 °C and were exposed up to about 1200 h. The specimens used for
3

4. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
moisture uptake measurement were removed at specified time intervals (see table 1), wiped,
air dried for 5 min and, then weighted. The moisture uptake kinetics was measured at different
intervals of the conditioning time. The weight gain was calculated according to the following
equation:
( ) )1(100%)( ∗
−
=
d
dw
m
mm
tM
where md is dry weight and mw is wet weight of the specimen.
2.4. Dynamic Mechanical Analysis
DMA measurements prior and after exposure were performed on a DMTA 983 (DuPont)
device in flexular configuration. Thermal scans from 35 to 200 °C were conducted at a
heating rate of 1 °C/min at 1 Hz and constant amplitude (0.1 mm for the CFRPs and 0.5 mm
for the EPs).
3. Results and Discussion
Figure 1 presents the weight gain of the neat and CNT-modified epoxy matrices and the neat
and 0.5% CNT modified CFRP laminates. All systems reached saturation within the time
frame of the hydrothermal exposure. The neat epoxy exhibited the least weight gain at
saturation compared to all modified systems. The doped epoxies exhibited increased water
uptake ranging without clear trend between the CNT content and the relative weight gain for
the modified matrix systems. The pronounced difference in the water uptake between the
matrices and the laminates was due to the presence of the carbon fibres, which did not exhibit
any water absorption. There were no visible differences between the two laminates. It can
therefore been concluded that the presence of the carbon fibre reinforcement was masking any
increase in the water uptake created by the CNTs.
Figure 1. Weight gain versus square root of time for the neat and modified epoxy matrices and the 0.5% CNT
modified CFRP specimens [22]
4
0 5 10 15 20 25
0,0
0,5
1,0
1,5
2,0
∆W/W[%]
Time
1/2
[h
1/2
]
Neat Resin
Doped Resin 0.3%
Doped Resin 0.5%
Doped Resin 1%
0 5 10 15 20 25
0,0
0,5
1,0
1,5
2,0
Doped CFRP
Neat CFRP
Doped Resin 0.5%
Neat Resin
∆W/W[%]
Time
1/2
[h
1/2
]

5. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
Figure 2. Storage Modulus and loss factor of the neat and CNT-modified EPs as a function of temperature
before and after hydrothermal exposure
5

6. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
Figure 2 presents the DMA results of the neat and CNT-modified epoxies as a function of the
% CNT content, before and after exposure. In these graphs the variation of the Storage
Modulus and the loss factor (tanδ) as a function of temperature is presented. In all cases it can
be observed that the water absorption led into degradation of the Storage Modulus, which
shifted to lower values. In the case of the loss factor, it can be seen that the water exposure
introduced a broadening of the peak around the glass transition temperature (Tg) which is
slightly more pronounced at increased CNT contents. Next to that no considerable trend of the
Tg can be observed due to the water exposure. The variation of the Tg was small and non-
monotonic both as a function of CNT content, as well as a function of the water exposure.
Figure 3 presents the DMA results of the neat and CNT-modified CFRPs before and after
exposure. In these graphs the variation of the Storage Modulus and the loss factor (tanδ) as a
function of temperature. In all cases it can be observed that there was no influence of the
exposure on the stiffness and the glass transition temperature (Tg) of the neat and doped
composites. This can be attributed to the water absorption results presented in Figure 1, where
no significant difference was observed in the water uptake of the CFRP specimens, due to the
masking effect provided by the presence of the carbon fibres as explained above.
Doped (D) and Neat (N) CFRPs
1
10
100
25 50 75 100 125 150 175 200
Temperature [degC]
StorageModulusE'[GPa]
CFRP-N before
CFRP-N after
CFRP-D before
CFRP-D after
Doped (D) and Neat (N) CFRPs
0,01
0,1
1
25 50 75 100 125 150 175 200
Temperature [degC]
tanδ[1]
CFRP-N before
CFRP-N after
CFRP-D before
CFRP-D after
Figure 3. Storage Modulus and loss factor of the neat and CNT-modified CFRPs as a function of temperature
before and after hydrothermal exposure
Figure 4 summarizes the results presented in Figure 2 for the neat and CNT-modified EPs,
before and after exposure. The data for the Storage Modulus for all specimens are taken from
the glassy region (50 °C). From these data it can be seen that Storage Modulus increases with
the increase of the CNT content up to 0.5% and afterwards decreases. This holds for both
exposed and non-exposed specimens. The incorporation of low weight fractions of CNTs (up
to 0.3%) into the EP matrix caused small changes into the glass transition also (shift to higher
values). Above a certain fraction the opposite effect can be observed. This behavior can be
explained in terms of the interaction of the CNTs with the epoxy at the CNT/EP interface.
Due to the higher surface area and the interfacial interactions, a reduced mobility of the EP is
obtained, which leads to increased stiffness and increased thermal stability. The dispersed
nanotubes dissipate energy due to resistance against viscoelastic deformation of the
surrounding EP matrix. Above a certain CNT, which here was estimated at about 0.5%, the
nanotubes tend to agglomerate, leading to less energy dissipating in the system under visco-
elastic deformation. The decrease of the Storage Modulus due to the water absorption can be
explained due to increase mobility around the CNT/EP interface due to the presence of water
and lowering of the stress transfer efficiency of the modified systems. It is interesting to note
6

7. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
there was not monotonic change in the Tg before and after hydrothermal exposure. From
Figure 4 it can be seen that the Tg increased after exposure in most cases compared to the un-
exposed condition. In the past a decrease of the Tg has been observed due to water absorption,
in EP based systems. This has been attributed mainly to plasticization of the EP by moisture.
Another explanation could be less crosslinking of the interface due to the presence of water.
0
0,5
1
1,5
2
2,5
3
3,5
0 0,1 0,3 0,4 0,5 0,6 0,8 1
% CNT content
StorageModulus[GPa]
BEFORE
AFTER
0
20
40
60
80
100
120
140
160
180
200
0 0,1 0,3 0,4 0,5 0,6 0,8 1
% CNT contentTg[degC]
BEFORE
AFTER
Figure 4. Storage Modulus (at 50°C) and loss factor of the neat and CNT- modified EPs as a function of CNT
content before and after hydrothermal exposure
Though in most of the published literature [23-27] an increase in the peak of tanδ and a shift
to lower temperatures of the glass transition region with an increase of the water content is
observed, in our case, the opposite is observed as in case of Zhou and Lucas [28-29] and
Papanicolaou et al [30]. This kind of behavior can only be explained on the basis of respective
recent findings reported [28-30]. According to their findings, water molecules bind with
epoxy resins through hydrogen bonding. Two types of bound water were found in epoxy
resins. The binding types are classified as Type I or Type II bonding, depending on
differences in the bond complex and activation energy. They revealed that the change of Tg
does not depend solely on the water content absorbed in epoxy resins, that the Tg depends on
the hygrothermal history of the materials. They also proposed that for a given epoxy system,
higher values of Tg resulted for longer immersion time and higher exposure temperature and
the water/resin interaction characteristics (Type I and Type II bound water) have quite
different influence on Tg variation. Type I bound water disrupts the initial interchain Van der
Waals force and hydrogen bonds, resulting in increased chain segment mobility acting as a
plasticizer and decreasing Tg. In contrast, Type II bound water contributes, comparatively, to
an increase in Tg in water saturated epoxy resin by forming a secondary crosslink network.
4. Conclusions
In this paper the viscoelastic performance of CNT modified EPs and CFRPs after exposure in
extreme environments was studied. Multi wall CNTs were incorporated in an epoxy system
which was subsequently used for the manufacturing of quasi isotropic laminates CFRPs.
Modified matrices with CNT content varying from 0.1% to 1% were manufactured. All
modified resins were used to manufacture un-reinforced rectangular cast specimens. The resin
with the 0.5% CNT content was subsequently used for the manufacturing of the modified
CFRPs. All systems were subjected to hydrothermal loading. The conditioned composite
systems were subsequently tested in dynamic three-point bending in order to study their
viscoelastic behaviour. The properties of the modified systems were compared to the
properties of unmodified composites that were subjected to identical conditioning. Based on
the obtained results, it can be concluded:
7

8. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
1) There were very little differences between the neat and the modified epoxy resins in term
of weight gain. The neat epoxy system exhibited slightly less water uptake than the modified
systems. In the case of the composite laminates, there was practically no observed difference
in terms of weight gain versus time.
2) The exposure of the CNT-modified EPs into hydrothermal loading influenced slightly the
viscoelastic properties. In all cases it was observed that the water absorption led into
degradation of the Storage Modulus, which shifted to lower values. In the case of the loss
factor, it was seen that the water exposure introduced a broadening of the peak around the
glass transition temperature (Tg) which was slightly more pronounced at increased CNT
contents. Next to that no considerable trend of the Tg was observed due to the water exposure.
The variation of the Tg was small and non-monotonic both as a function of CNT content, as
well as a function of the water exposure. The CNT-modified CFRPs did not show any
deterioration due to exposure into hydrothermal loading, which was explained due to the lack
of any kind of difference in the water up-take curves.
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9. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
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9

10. 14TH
EUROPEAN CONFERENCE ON COMPOSITE MATERIALS
7-10 June 2010, Budapest, Hungary
Paper ID: 045-ECCM14
11. Selzer R., Friedrich K., “Influence of Water Up-take on Interlaminar Fracture
Properties of Carbon Fibre-Reinforced Polymer Composites”, Journal of Materials
Science, 30, 334-338, 1995.
12. Asp L. E., “The Effects of Moisture and Temperature on the Interlaminar
Delamination Toughness of a Carbon/Epoxy Composite”, Composites Science and
Technology, 58(6), 967-977, 1998.
13. Wang S., Kowalik D. P., Chung D. D. L., “Effects of the Temperature, Humidity, and
Stress on the Interlaminar Interface of Carbon Fiber Polymer-Matrix Composites,
Studied by Contact Electrical Resistivity Measurements”, The Journal of Adhesion,
78, 189-200, 2002.
14. Wang S., Chung D. D. L., “Effect of Moisture on the Interlaminar Interface of a
Carbon Fiber Polymer-Matrix Composites, Studied by Contact Electrical Resistivity
Measurement”, Composite Interface, 9(5), 453-458, 2002.
15. Ray B. C., “Temperature Effect During Humid Ageing on Interfaces of Glass and
Carbon Fibers Reinforced Epoxy Composites, Journal of Colloid and Interface
Science, 298, 111-117, 2006.
16. Botelho E. C., Pardini L. C., Rezende M. C., “Hygrothermal Effects on the Shear
Properties of Carbon Fiber/Epoxy Composites” Journal of Materials Science, 41,
7111-7118, 2006.
17. Lai J. Y., Young K. F., “Dynamics of Graphite/Epoxy Composite Under Delamination
Fracture and Environmental Effects”, Composite Structures, 30, 25-32, 1995.
18. Botelho E. C., Pardini L. C., Rezende M. C., “Damping Behavior of Hygrothermally
Conditioned Carbon Fiber/Epoxy Laminates” Journal of Applied Polymer Science,
106, 3143-3148, 2006.
19. Zhang Y. C., Wang X., “Hygrothermal Effects on Initial Frictional Pull-Out Force of
CNTs-Reinforced Composites”, Journal of Thermal Stresses, 29, 67-92, 2006.
20. Zhang Y. C., Wang X., “Hygrothermal Effects on Interfacial Stress Transfer
Characteristics of Carbon Nanotubes-reinforced Composites System”, Journal of
Reinforced Plastics and Composites, 25(1), 71-88, 2006.
21. Yip M. C., Wu H. Y., “Fatigue and Electrical Properties of CNT/Phenolic Composites
under Moisture-Temperature Effects”, Key Engineering Materials, 334-335, 769-772,
2007.
22. Barkoula, N. M., Paipetis, A., Matikas, T., Vavouliotis, A., Karapappas, P.,
Kostopoulos.: Environmental degradation of carbon nanotube modified carbon fibre
reinforced laminates: an electrical resistivity study. Mechanics of Composite Materials
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