A Study on Water Absorption and its Effects on Strength of Nano Organoclay-epoxy Composites
Dinh Van Chau
Nanoclay-epoxy composites have been applied widely since they
have excellent properties which resulted from the incorporation of the nanoclay
material in epoxy matrix. However, the improved properties can degrade by presence
of water in the epoxy matrix, leading to limitation of service life of the material.
Diffusion of water having 40 and 60°C in the nanoclay-epoxy composites made
by direct mixing process was investigated through immersion test. The strength
of the materials was also measured by using the three point bending test. The
results showed that the two-stage diffusion process occurred when the composites
immersed in water at 60°C whereas the Fickian diffusion mode was observed
in case of water at 40°C. The saturation and the diffusion rate are influenced
by organoclay loading and by temperature. Increase of clay loading resulted
in an increase of the water saturation and reduction of diffusion rate of water.
The results also showed the significant influence of temperature on water absorption
of the composites. Water saturation, increased about 20% and water diffusion
rate was 5 time-higher when temperature of water shifted from 40 to 60°C.
Investigation of flexural strength was carried out by use of the three point
bending test. The results showed decrease of flexural strength with an increase
of clay loading for the composites. However, there was no significant change
of flexural strength during the immersion. Results for the neat resin showed
to the contrary.
Received: June 13, 2012;
Accepted: August 08, 2012;
Published: September 08, 2012
Nanocomposites are a new class of materials in which the ultrafine phase dimensions
ranges from 1 to 100 nm (Komarneni, 1992). Much investigation
has proven that these materials have new and improved properties in comparison
to their micro- and macro-composite counterparts. In general, polymer nanocomposites,
a branch of the nanocomposite material show an improvement in terms of properties
to conventional filed polymer. The polymer-layered silicate nanocomposites is
a special example for the improvement, in which markedly improved properties
such as modulus and strength (Kojima et al., 1993a),
permeability (Yano et al., 1993; Kojima
et al., 1993b), shrinkage (Kelly et al.,
1994; Haque and Armeniades, 1986), heat resistance
and flammability (Giannelis, 1996; Lee
et al., 1997) is attributed nanometer-size dispersion in the base
Epoxies are considered as the best polymeric materials for many applications
and widely used in industry as matrix materials of fiber-reinforced composites
due to their superior characteristics such as good mechanical properties and
good resistance to chemicals. Thus, this resin is significantly utilized in
industry as a matrix material, especially in the aerospace and automotive industry
(Kshirsagar et al., 2000; Becker
et al., 2003). However, the service life of epoxy based composites
materials can be decreased due to the degradation of epoxy matrix caused by
working environment such as temperature, moisture and corrosive elements (Kshirsagar
et al., 2000; Kim et al., 2005). With
dispersion of nanoclay in the epoxy matrix to perform a nanocomposite, the material
is expected to not only overcome the disadvantages of the base polymer and also
to obtain new characteristics.
The clay-epoxy nanocomposite materials can be produced by two common methods:
the direct mixing and the solution mixing (Jiankun et
al., 2001; Zerda and Lesser, 2001; LeBaron
et al., 1999; Kornmann et al., 2001).
These techniques produce intercalated or intercalated/exfoliated composites
rather than fully exfoliated composites (Zerda and Lesser,
2001). It is generally believed that the diffusion performance of clay nanocomposites
normally influenced by the clay content, the aspect ratio of the barrier (ratio
of width over length of the barrier) and the dispersion of the silicate layers
in which the reduction of water permeability was attributed to the high aspect
ratio of the clay platelets and the high degree of exfoliation (Ke
and Yongping, 2005; Gain et al., 2005; Picard
et al., 2007; Gusev and Lusti, 2001; Fredrickson
and Bicerano, 1999; Nielsen, 1967; Bharadwaj,
2001). This reduction is one of the most excellent properties that made
the material to be applied widely in industry recently.
Degradation process, such as chemical degradation, reduction of mechanical
properties etc., occur when the composite materials contacted to the environment.
The environment penetration into a polymer composite is influenced by the composites
characteristics such as density and the order of pore, defects or contamination
and the polymer-environment affinity (Wong and Broutman,
1985; Diamant et al., 1981; Adamson,
1980; Moy and Karasz, 1980). In the studies of liquid
molecular transport into a polymer membrane, diffusion has been classified as
Case 1 (Fickian type), Case 2 (relaxation-controlled) and non-Fickian (anomalous)
(Thomas and Windle, 1980; Frisch,
1980; Peterlin, 1980; Hansen,
1980; Astaluta and Sarti, 1978). Penetrant molecules
diffuse into the membrane until the concentration is equal over the whole of
the membrane (saturation state). When Fickian diffusion is assumed, the time-dependent
relative concentration of the liquids into the polymer membrane can be expressed
by the following equation (Vergnaud, 1991):
in which C(t,x) and C∞ are the concentrations of
the liquid inside the membrane at time t, position x and saturate state, D is
diffusivity which depends on the nature of liquid-polymer interactions and h
is the thickness of the membrane. The boundary conditions for solving Eq.
1 are: C = 0 when t = 0, 0≤x≤h; C = C∞ when t>0,
x = 0, x = h and ∂C/∂x = 0 when x = 0, t>0. The diffusion coefficient
has been calculated by Crank (1975):
where, Mt and M∞ are the mass uptake at time t
and saturation state, respectively. The diffusivity can be calculated from the
initial slope of the environmental uptake Mt/M∞
versus time (t1/2/h) as:
The water content may predict by using the equation as follow (Shen
and Springer, 1976):
The high order absorption of water is a major disadvantage of epoxy resins.
Furthermore, the absorbed water is considered as a main factor resulted in degradation
of functional, structural and mechanical properties of the composites (Lee
and Neville, 1957; Lu et al., 2001; Nunez
et al., 1999; Yano et al., 1993).
Therefore, understanding of diffusion behavior of water in particular clay epoxy
nanocomposite system is need for application of the material.
In this study, the water absorption behavior of nano organoclay epoxy composites
was investigated. Effects of clay loading, environment temperature on the absorption
water barrier characteristic are discussed. Mechanical properties such as flexural
strength, flexural modulus during environmental exposure, were also studied
and discussed in the study.
MATERIALS AND METHODS
Materials and sample preparation: Three components are need to form
the clay-epoxy nanocomposites using for the investigation: the Bisphenol A Epoxy
Epomik R140 made by Mitsui Chemical Co., Ltd used as the matrix component of
the composites; the diamine having commercial name as Jeffamine D230 made by
Huntsman Co. used as curing agent and the organoclays received from Nanocor
Inc. named as Nanomer I.28E.
The organoclay was swelled with the curing agent and mixed with a mechanical
stirrer at 2500 rpm for 1.5 h at 60°C, followed by ultra-sonication for
an additional 1 h. When the swelling process had been finished, the epoxy resin
was added. It should notice that before the curing process started, degassing
process under vacuum was applied. The curing was done in two stages: the first
stage was carried out at 70°C for 6 h while the second was at 110°C
for 6 h. The load of the clay varied from 0 to 6 phr (parts per hundred resin).
Table 1 shows code and formula of the prepared samples.
The test pieces having dimension of 60x25x1 mm were cut out from the prepared
sample sheets. The pieces were then dried up at 50°C for 72 h to ensure
that the remaining moisture/gas was removed before the immersion test was carried
|| Sample used for investigation
|| Schematic of the immersion test apparatus
Immersion test: Immersion experiment was carried out by simply immersing
the test pieces in de-ionized water. The test pieces were fixed in a polytetrafluoroethylene
holder to avoid contact surface each other and immersed in separable flash bottles
which filled with the water. Temperature of the water was constantly set at
40 and 60°C. Figure 1 shows a schematic of the immersion
At interval time, the specimens were taken out, wiped by filter paper to remove
attached water on their surfaces and weighed by a balance with 0.1 mg accuracy.
A change in mass was measured related to initial mass.
Flexural test: Specimens, taken out from the immersion chamber at interval
time of immersion, were measured the flexural modulus and strength by the three
point bending test. The test was done regarding to ASTM D790 (American Society
for Testing and Materials ASTM, 2005) with Shimadzu Autograph
AGS-1KNJ machine. Speed of the crosshead was set at 15 mm min-1 constantly.
RESULTS AND DISCUSSION
The water uptake during immersion was measured by comparing the test specimens
weight to its initial weight. The time-dependent uptake amount for the different
organoclay loadings is shown in Fig. 2 to 5.
It can be seen that the uptake increased linearly with increasing time at first,
then leveled off.
||Weight change vs. time of CE0 sample under exposure of de-ionized
||Weight change vs. time of CE2 sample under exposure of de-ionized
||Weight change vs. time of CE4 sample under exposure of de-ionized
The leveling off is considered as saturation state of the absorption process.
The prediction curves based on Eq. 3 and 4
are also superimposed in Fig. 2-5. The water
diffusion characteristic into the samples was shown in Table 2.
||Weight change vs. time of CE6 sample under exposure of de-ionized
|| Water diffusion characteristics of samples
Effect of clay loading: The results showing in Fig. 2
to 5 as well as in Table 2 confirmed the
influence of the clay loading on water diffusion behavior into the materials.
The saturation degree and saturation time increased with an increase of the
clay content, whereas the diffusivity decreased with an increase of the clay
loading. For the neat epoxy exposed to the 40°C water (Fig.
2, Table 2), the saturation obtained as 2.091% after about
2078 h of the immersion and the diffusivity was as 2.96x10-4 mm2
h-1. It is also seen that the water diffusion behavior was fitted
well with the Fickian model. Exposure to the same water environment, the nanocomposites
of 2, 4 and 6 phr clay loading became saturated at 2.27, 2.407 and 2.496% within
3364, 3969 and 4356 h and the diffusivity decreased to 2.70x10-4,
2.47x10-4 and 2.27x10-4 mm2 h-1,
respectively (Fig. 3-5). The similar results
were also obtained for the samples exposed to the 60°C water environment.
As seen in these figures, saturation value was lowest as about 2.499% for the
neat epoxy, higher as 2.699 and 2.899% for the 2 and 4 phr clay nanocomposites
then highest as 3.001% for the 6 phr clay nanocomposite, whereas, the diffusion
rate was the highest for the neat epoxy then the lower for the 2 and 4 phr clay
nanocomposites and the lowest for the 6 phr clay nanocomposite. These results
are similar to that reported by Abacha et al. (2009),
Okada et al.(1990), Lan and
Pinnavaia (1994) and Messersmith and Giannelis (1995).
Thus, presence of incorporated nanoclay particles in the epoxy matrix resulted
in the increase of saturation degree and the decrease of water diffusion rate.
As discussed by Popineau et al. (2005), the water
molecules after diffused in the composites can only stay in micro-voids and
form clusters. If all micro-voids/free volumes were filled by diffused water
molecules, the saturation is attained. The number of micro-voids/free volume
may increase when more organoclay was added and that is considered as a reason
for the increase of saturation order with an increase of clay loading. The decrease
of the diffusion rate with an increase of clay loading is attributed by dispersion
of clay particles within the epoxy matrix and by exfoliation order of the composites
(Okada et al., 1990; Lan
and Pinnavaia, 1994; Messersmith and Giannelis, 1995).
Under the same preparation conditions, the higher clay loading composite may
result in higher order of the exfoliation and that contribute to the better
barrier characteristic of the composite.
Effect of temperature: Temperature affected significantly the diffusion
behavior of water into the materials. As seen in Fig. 2-5,
the saturation value and the diffusion rate increased when the temperature of
water environment shifted from 40 to 60°C. For instance, for the neat epoxy
resin (Fig. 2), the water saturation was higher as about 12.5%
at 60°C than those at 40°C. For the 2, 4 and 6 phr clay nanocomposite
(Fig. 3-5) this difference is about 18.89,
20.44 and 20.23%, respectively. Remarkable 5 time increase of the diffusivity
was observed when water temperature shifted to 60 from 40°C. Because the
difference between saturation values as well as diffusion rates at 40 and 60°C
is the same among the composite systems then it may conclude that the effect
of temperature was mainly acted on the epoxy matrix. An explanation for the
effect of temperature would be an expansion/creation of micro-voids/free volume
inside the material by thermal relaxation of polymer chain of the epoxy matrix.
An interesting was found when observation effect of temperature on the water
diffusion behavior of the material. The two stage diffusion (non-Fickian mode)
occurred when the samples immersed in water at 60°C. This phenomenon is
attributed to chemical interaction of diffused water molecules to the functional
groups of the epoxy matrix and the thermal relaxation effect (Nguyen
and Martin, 1996).
Mechanical properties: The result of three-point bending test under
wet condition is shown in Fig. 6 and 7.
|| Flexural strength of samples
|| Flexural modulus of samples
Figure 6, the initial flexural strength decreased with the
increase of the nanoclay loading. It may result from agglomeration of the nanoclay
in epoxy matrix. The adhesion strength and interfacial stiffness of interface
between the nanoclay particles and the epoxy matrix are the important factors
which contribute flexural strength of the composites due to they are significant
to stress transfer and elastic deformation from the epoxy matrix to the fillers.
The result of flexural modulus showed in Fig. 7 is accordance
with the manner showed in Fig. 6.
Retention of properties as same as at initial, during service time, is an important
criteria to evaluate application potential of any materials. In terms of mechanical
properties, the flexural strength of the test pieces was measured during the
immersion test and its result is shown in Fig. 8. An evaluable
information, as seen in this figure, is that there are not much change of the
strength of nanoclay-epoxy composites samples while the strength of the neat
epoxy reduced about 20% after immersed 100 h, then maintaining as long as the
immersion time. It can be explained by distortion produced around the amine
salt resulted the decrease of the C-N bond of the epoxy matrix.
||Normalized flexural strength (wet condition) vs. Immersion
time in the 40°C de-ionized water
When the organoclay is added, this distortion may be hindered, therefore,
the strength is retention.
This study concluded that diffusion behavior of water into the organoclay-epoxy
composites is influenced by the clay loading and temperature of environment.
The water situation increased with an increase of the clay content whereas the
diffusion rate decreased with an increase of the clay loading. The uniform dispersion
of clay particles in the epoxy matrix and higher order of exfoliation in the
composites are considered as a main reason resulted in the effect. Temperature
effect on diffusion behavior of water into the materials is significant. The
saturation value increased about 20% when temperature shifted from 40 to 60°C.
The diffusion rate increased about 5 times when temperature changed in these
values. The effect of the temperature is attributed only by the thermal effect
of epoxy matrix. Two-stage diffusion occurred for the polymers immersed in water
Flexural strength of the samples decreased with increasing the organoclay loading
for the composite samples. However, the retention of this property during immersion
test is the same as those at initial which did not obtain for the neat resin
The author are grateful to the University of Engineering and Technology, Vietnam
National University, Hanoi (Vietnam) for financial support, Grant No. CN.11.05.
This work was also partial financial support from the National Foundation for
Science and Technology Development (Vietnam), Grant No. 103. 06-2011.64.
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