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Multiwalled Carbon Nanotube Reinforced Epoxy Nanocomposites for Vibration Damping Anand Joy, Susy Varughese, Sankaran Shanmugam, and Prathap Haridoss ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01865 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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ACS Applied Nano Materials
Multiwalled Carbon Nanotube Reinforced Epoxy Nanocomposites for Vibration Damping Anand Joy1, Susy Varughese2, Sankaran Shanmugam1, and Prathap Haridoss1* 1Department
of Metallurgical and Materials Engineering, IIT Madras, Chennai 600036, India
2Department
of Chemical Engineering, IIT Madras, Chennai 600036, India Abstract
Multiwalled carbon nanotubes (CNT) are used as reinforcement in epoxy modified with adduct, i.e. block
co-polymer
acrylonitrile
of
carboxyl-terminated
(CTBN)
and
diglycidyl
butadiene ether
of
bisphenol A (DGEBA). Microstructure analysis reveals uniform
dispersion
of
a
phase
with
spherical
morphology in a continuous phase. The mean diameter of the spherical particles increased with CNT and adduct content. At 30 wt% adduct, adding CNT results in exceptional growth of spherical particles. Loss factors in the first two modes were determined by using
free
vibration
test
in
cantilever
mode.
Significant increase in loss factor is obtained for the epoxy nanocomposites modified with 30 wt% adduct and at all wt.% CNTs (i.e. 0.5, 1 and 2.5). Vibration damping mechanism in this nanocomposite is likely due 1
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to relative sliding of spherical particles during dynamic loads, which dissipates energy. The reduction in the distance between spherical particles as well as the reduction in the elastic modulus of continuous phase facilitate relative sliding.
Keywords:
Carbon
nanotube,
Epoxy,
Damping,
Nanocomposite, CTBN, Phase separation, Polymer blend, Vibration isolation
*Corresponding author: Dr. Prathap Haridoss; Email:
[email protected] 1
Introduction Vibration damping is a highly demanding and challenging
problem
encountered
by
design
engineers
and
material
scientists. Structures exposed to vibration leads to fatigue loading which causes unanticipated failure [1]. In passenger carriers,
such
as
automobiles,
aircraft
etc.,
vibration
causes noise and unfavourable motions to the passenger [2, 3]. In a material system, vibration damping occurs via energy dissipating events known as vibration damping mechanisms [4]. These mechanisms are essentially restoration of structures ranging from atomic or molecular level to macro level [5]. Dominance of a mechanism over the other possible mechanisms depends
on
many
internal
and
2
external
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factors.
Internal
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factors
include
presence
of
point
defects,
dislocations,
grain boundaries, reinforcement material etc. [5]. External factors
include
temperature,
frequency,
input
stress
or
strain and preload [6]. Damping can be achieved by active or passive methods [3, 7, 8]. Inherent material properties are utilized to achieve passive damping [3]. Surface damping treatment is a passive damping treatment which is easy to apply, less complex, compact and relatively cheap [3]. It is broadly classified into free layer damping and constrained layer damping [9]. In free layer damping, a damping material is applied on to either one or both the sides of the structure. Under dynamic loads the damping material undergo cyclic tension and compression deformation. In constrained layer damping, a metal plate known as “constraining layer” is applied to the damping material. Whenever
dynamic
loads
are
applied,
the
damping
layer
undergoes shear deformation due to the constraining of metal plate. In both of these treatments, the damping material is either spray coated or adhesive bonded to the base structure. Performance of surface damping treatment depends on thickness and stiffness of the adhesive layer too [5, 10]. Which can be avoided if the damping material itself has adhesive property. Epoxy is a thermosetting polymer well known for its adhesive properties [11, 12]. However, they exhibit poor vibration damping characteristics [13]. One of the initial studies to improve
the
vibration
damping 3
of
epoxy
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was
done
by
the
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incorporation of continuous glass fibre and graphite fibre [14]. This study revealed the dependence of loss factor on type and orientation of fibre. Shape memory alloys such as TiNi embedded epoxy composites exhibited loss factor around 0.2 above austenite finish temperature in the free vibration analysis [15]. Blending of epoxy with carboxyl- terminated butadiene-acrylonitrile amine-terminated
(CTBN)
[16–20]
liquid
butadiene-acrylonitrile
rubber
and
[21,
22]
(ATBN)
rubber improves vibration damping. A loss factor of 0.084 compared to 0.037 of neat epoxy was obtained for 25 wt% CTBN modified epoxy composites in free vibration analysis [17]. Liquid rubber modified epoxy systems are characterized with two phase morphology. In this, the spherical rubber particles are dispersed in the continuous epoxy matrix [23]. Since the deformation of rubber particles under vibratory loads is restrained
by
the
rigid
epoxy
matrix,
a
significant
improvement in vibration damping is not observed. However, vibration
damping
in
these
materials
can
be
improved
significantly by reducing the gap between the rubber particles in epoxy matrix. The inter connected phase structure exhibited higher
damping
structure
[21].
compared
to
distributed
Increasing
the
liquid
spherical rubber
domain content
increases the size of distributed spherical domains [24]. As a result the distance between spherical domains decreases. However, this is still not sufficient to get significant improvement in vibration damping.
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In this work, we report a high damping composite based on epoxy reinforced with multiwalled carbon nanotube (CNT) and modified with an adduct of epoxy and CTBN. Adduct is a copolymer of epoxy resin and CTBN [25]. Significant improvement in vibration damping is observed beyond a certain level of adduct content. Mechanism of vibration damping is predicted by microstructure analysis. A unique microstructure formation due
to
the
reaction
epoxy/adduct/CNT behaviour,
induced
composite
which
is
phase
results
explained
separation in
based
a on
peak
in
the
damping
microstructure
analysis of the composite.
2 2.1
Experimental Details Materials
Araldite GY 257 is a low viscose (viscosity at 25 oC is 450 650 mPa·s) epoxy resin based on bisphenol–A modified with an aromatic
glycidyl
ether
(DGEBA).
Aradur
polyamidoamine (PAMAM) hardener (viscosity at 25
140 oC
is
a
is 10000
- 25000 mPa·s). Both were purchased from Huntsman Company, Mumbai, India. CTBN liquid rubber with acrylonitrile content of 26 % (Hypro 1300×13) and number average molecular weight of 3150 was purchased from CVC thermoset specialties, NJ, USA. Mixing ratio of resin to hardener is 2:1 by weight. CNT was synthesized in house using arc discharge technique as
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reported earlier [26, 27]. The average diameter of CNT was 15.2 ± 5.5 nm and length was around 1 µm.
2.2
Nanocomposite fabrication
Adduct of DGEBA and CTBN was prepared by stirring 40 phr CTBN in DGEBA at 120
oC
for 4 h under slow Nitrogen purge. The
reaction is shown in Figure 1. Carboxyl group of CTBN reacts with epoxide group of DGEBA to form DGEBA – CTBN – DGEBA adduct and the chain extends further as reaction proceeds (Figure S1 in supporting information) [20, 28].
Figure 1: Reaction between DGEBA and CTBN to form adduct
Appropriate amount of adduct was added to DGEBA and stirred using a magnetic stirrer for 30 min. CNTs were dispersed in epoxy by solution mixing technique [29]. Based on the cost, availability and also on the results from previous studies [30,
31]
ethanol
was
selected
as
solvent.
CNT
was
ultrasonicated in ethanol using a bath type ultrasonicator 6
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(CREST ultrasonics, NJ, USA) at 25 kHz and 132 kHz operated in
parallel
for
10
min
to
break
the
agglomerates.
This
dispersion was added to the blend of adduct and DGEBA and stirred for 30 min using a magnetic stirrer. Increase in the level (by volume) after the addition of CNT/ethanol dispersion was noted. After this, the dispersion was placed on a hot plate and heated at 85
◦C
till the volume level reduces to
the initial level. PAMAM hardener was then added with 2:1 resin to hardener ratio by weight. It was mixed using a magnetic stirrer for 15 min. After degassing, this mixture was injected to a Teflon mould lined with silicone rubber using a hand operated plunger. This enabled proper handling of the injection pressure to address the higher viscosity of the mixture. Size of the mould was 200 mm in length 10 mm in width and 4 mm in thickness. Details about the injection process
is
given
in
section
S.2.
information. Curing was done at 100
oC
in
the
supporting
for 1 h and then at
room temperature for 7 days. Nanocomposite with 10, 20, 30 wt% of adduct and 0.5, 1, 2.5 wt% CNT were fabricated. Samples were coded as EAxCy where E stands for cross linked epoxy matrix, A is for the adduct, x is the wt% of adduct, C is for the CNT and y is wt% of CNT.
2.3
Characterization and testing Test for vibration damping was conducted according to
ASTM E756-05 [10]. Nanocomposites in Oberst beam configuration 7
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(damped on one side) were tested to find the damping ratios at different modes of vibration. Aluminum of thickness 2 mm was chosen as the base material and 2 mm thick nanocomposite layer was cast on to this. The beam dimension was 200 mm × 10 mm (length × width) (Figure S3). The free length was 180 mm. An initial tip displacement was given to the beam using an impact hammer (Model 5800B4, DynapulseTM, Dytran Instruments) and
the
tip
displacement
was
recorded
using
a
Dytran
accelerometer having sensitivity 100 mV/g. The frequency range of
the
accelerometer
was
5
to
10000
Hz.
The
weight
of
accelerometer was 2 g and the weight of the beams was around 20 g. The weight of the accelerometer is relatively small. Hence, the effect of mass of accelerometer on the response of the beams is negligible. Further, no bending was observed due to the attachment of accelerometer to the beams. NI 9234 data acquisition system of ’National Instruments’ was used to acquire signals through m+p SO analyser application. The results were obtained as amplitude vs. frequency, called as frequency response function (FRF). Minimum five samples were tested in each configuration. In the given length two modes were excited. Loss factors in this two modes were obtained by half power bandwidth method. From this the loss factors of the nanocomposites
were
calculated
by
using
Ross–Kerwin–Ungar
(RKU) equations [32] given in the ASTM standard specified above. Average value of loss factor with standard deviation was plotted against adduct and CNT content.
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The density of the nanocomposites were found out using fluid displacement technique. Weight in air and weight in water of the samples for each composition were determined using Sartorius CP124S weighing balance. At least five samples were tested and the average value with standard deviation was plotted against CNT content. The evolution of two phase morphology was analysed by placing
a
small
drop
of
the
sample
on
a
glass
slide
immediately after adding PAMAM hardener. Stirring for 1 min was
carried
out
to
ensure
proper
mixing.
This
was
characterized by phase contrast microscopy (PCM) in Olympus IX71 inverted optical microscope. Images were taken at regular intervals
of
fractured
by
time.
The
bending
nanocomposite
for
samples
microstructure
were
analysis.
cryo The
fractured surfaces were sputter coated with gold and viewed under FEI Quanta 400 FEG scanning electron microscope (SEM). The
microstructure
developed
in
the
nanocomposites
were
analysed. The average diameter with standard deviation of the spherical domains was measure using ImageJ software. Elastic moduli of each phase in the nanocomposite was determined using Park NX10 atomic force microscope (AFM) in contact mode.
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3 3.1
Results and Discussion Free Vibration damping
The FRFs of composite beams in Oberst condition are shown in Figure 2. Damped natural frequency (ωd) in the first and second modes are varying with respect to adduct and CNT content. This variation is significant in the second mode and is shown in Table 1. Damped natural frequencies in mode 2 (ωd2) of the nanocomposite beams with adduct modifier are slightly lower than that of neat epoxy beam. The beams with
Figure 2: Frequency response function of neat epoxy and nanocomposites. Two modes were excited for 180 mm free length
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30 wt % adduct at all wt % of CNT have the lowest ωd2. The variation in ωd2 is attributed to the effects of damping. Damped natural frequency varies with damping ratio (ζ) as given by; d n 1 2 where ωn is the undamped (ζ = 0) natural frequency. As the damping increases the natural frequency of the damped system decreases. However ωd2 of nanocomposites without adduct modifier are higher than that of neat epoxy. This is attributed to the increase in flexural rigidity of these
beams.
Natural
frequency
rigidity (EI) as, n n L
2
varies
with
the
flexural
EI l L4 , given by Euler - Bernoulli
beam theory. Here L is the beam length, E is elastic modulus, I is the second moment of inertia of cross section with respect to bending axis and ρl is the mass per unit length. For cantilever beam β1L = 1.875, β2L = 4.694 gives the first two natural frequencies.
Table 1: Variation of mode 2 natural frequency with adduct and CNT content
CNT content (wt %) 0 0.5 1 2.5
Damped natural frequency in mode 2 (Hz) Adduct content (wt %) 0 10 20 30 257 ± 4
245 ± 5.3
242 ± 5
264 ± 10 268 ± 4.5 263.5 ± 4.7
244 ± 2 245 ± 4.6 248 ± 5
252 ± 4.7 255 ± 4 252 ± 4
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240 ± 5.4 230 ± 4 226 ± 5 231 ± 6
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Loss
factors
in
mode
1
(η1)
and
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mode
2
(η2)
of
the
nanocomposites are shown in Figure 3(a) and 3(b). Addition of adduct to epoxy improved the loss factor in both the modes. Two distinct trends in the variation of loss factor with the addition of CNT have observed. In nanocomposites with 10 and 20 wt% adduct, incorporation of CNT has no effect or slightly reduced the loss factor in both the modes. Similar results were
reported
in
previous
studies
[24,
33].
Whereas
in
nanocomposites with 30 wt% adduct, the incorporation of CNT increased the loss factor significantly. In mode 1, EA30C1 exhibited highest loss factor, with an increase of 119 % from that of the neat epoxy. In mode 2, EA30C0.5 exhibited highest loss factor, with an increase of 98 % from that of the neat epoxy. Earlier studies reported 13 % increase in loss factor for epoxy reinforced with 0.5 wt % CNT and 32 % increase for 2.5 wt 3: % CNT 34].inCompared to mode these, a significant Figure Loss [8, factor mode 1 and 2 are calculated improvement in according vibrationto damping is obtained for the from the FRFs ASTM E756–05 [10] and shown in nanocomposites discussed in this study. (a) and (b) respectively Loss factor calculation using RKU equations depends on the density of the damping material. Hence, the effect of addition of adduct and CNT to epoxy on the density of the final nanocomposite
was
tested
and
is
shown
in
Figure
4.
The
variation in the density of nanocomposite with 10, 20 and 30 wt% adduct is well within the limits of standard deviation of that of neat epoxy, which has a density of 1118.2 ± 9.3 kg/m3.
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In nanocomposites with 10 and 20 wt% adduct, with the incorporation of CNT, negligible variation in density is observed. However, the addition of CNT to the nanocomposites with 30 wt% adduct resulted in the lowest density among all the nanocomposites. EA30C0.5 has the lowest density which is 2.5 % lower than that of neat epoxy. This reduction will not affect significantly in the calculation using RKU equations.
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Significant improvement in vibration damping was obtained especially when CNT is incorporated to epoxy modified with 30 wt% adduct. Vibration damping mechanisms need to be examined to
substantiate
energy
this
dissipating
improvement. events,
such
Damping as
occurs
various
through
relaxation
processes of polymer chains [35, 36], interfacial movements
Figure 4: Variation of density of the nanocomposite [6,
8,
37,
adduct and CNT content. 38],with inter-tube sliding in CNT
[39–43]
etc.
Detailed microstructure analysis of the nanocomposites using SEM and PCM was done to formulate the mechanism of vibration damping.
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3.2 3.2.1
Microstructure analysis Reaction induced phase separation
One of the factors that contribute to vibration damping in material systems is the microstructure developed during the processing. PCM images in Figure 5 were taken 1 min after adding the cross linking agent. The presence of spherical domains structures were characterized in the initial stages of cross linking (Figure 5(a)). More number of spherical domains
appeared
as
time
progressed
due
to
the
phase
separation (Figure 5(b)). These domains grow in size (Figure 5(c) and 5(d)). However, the growth is hindered as viscosity increases due to cross linking. Previous studies suggest that the phase separation in the blend
of
epoxy
and
adduct
can
occur
either
by
spinodal
decomposition [20, 21] or by nucleation and growth [44–46]. Phase separation by spinodal decomposition results in cocontinuous
and
interconnected
phase
structure
[21,
47].
Whereas, the morphology as a result of nucleation and growth, is spherical domains distributed in continuous phase [48, 49]. Similar morphology is obtained in the nanocomposites of this study (Figure 5(a) – 5(d)). Hence, it is believed that the phase separation occurs by nucleation and growth mechanism.
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3.2.2
Particle size and distribution of CNT
SEM
images
showing
variation
in
microstructure
of
nanocomposites with respect to adduct and CNT content are shown in Figure 6. A continuous phase and a dispersed phase with spherical morphology are observed. Average diameter of the dispersed phase with standard deviation corresponding to each adduct and CNT content are shown in Figure 6. It is found that
the
average
diameter
increased
with
adduct
and
CNT
content. A significant increase is observed with 0.5, 1 and 2.5 wt% CNT and 30 wt% adduct. The observed diameter of adduct in EA30C0 is around 0.99 ± 0.16 µm, which has increased to 65 ± 25.8 µm in EA30C0.5 due to the presence of 0.5 wt% CNT.
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Higher rate of curing is expected at lower adduct and CNT contents. This results in higher viscosity which restricts
Figure 5: Phase contrast optical microscope images of cross linking of EA30C0, 1 min after adding PAMAM (a), images (b), (c) and (d) were taken successively. Time stamps are provided with each image the coalescence of adduct particles [16]. Cross linking rate decreases as the adduct and CNT content increases. This allows the growth as well as coalescence of adduct particles for an extended period of time [44]. Hence, the diameter of spherical adduct particles increases with adduct and CNT content. Such a trend has been reported earlier [20, 50–52]. Substantial
increase
in
the
diameter
of
spherical
domains was observed when CNT is added to epoxy with 30 wt% adduct (Figure 6). Morphology evolution of EA10C0.5 and EA30C0.5 nanocomposites are shown in Figure 7. These images were taken 17
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20, 30 and 40 min after adding PAMAM cross linking agent. The
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black dots marked in Figure 7 are CNT aggregates. In EA10C0.5
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these dots are well distributed in the matrix. Whereas in
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EA30C0.5 most of these dots are situated on the surface of the
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spherical domains. This is again substantiated with the SEM
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images shown in Figure 8. In EA20C1, the CNTs are found in the
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continuous epoxy phase (Figure 8 (a) – 8(d)). In EA30C2.5 the
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irregular areas of spherical adduct phase (Figure 8(e)) were
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magnified (Figure 8 (f)). CNTs are found to be protruding out
Figure
7:
Morphology
evolution
of
EA10C0.5
and
EA30C0.5
nanocomposites after 20, 30 and 40 min of adding cross linking agent
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Figure 8: SEM images of EA20C1 and EA30C2.5 nanocomposite. CNTs are found in the continuous phase (a and c). Images (b) and (d) are magnified view of CNTs in (a) and (c) respectively. In image (e), the spherical domains have few irregular Figure 6: SEM micrographs of fracture surfaces. Average areas. One such area is magnified (b). On further analysis diameter with standard deviation of the second phase we could see CNTs protruding out from the spheres (h) particles is given the box from the surface of these domains (Figure 8 (h)). Growth of nuclei
occurs
through
two
mechanisms;
diffusion
of
the
monomers (here adduct and DGEBA molecules) from the bulk, and surface reactions [53]. It is believed that the presence of CNTs on the surface of the growing particles enhances the diffusion of monomers from the bulk by acting as defects on the surface of the growing spherical adduct particles. This also results in the reduction of available number of DGEBA molecules for cross linking. Thus, the nuclei experience less resistance towards growth. This might be the reason for the substantial
increase
of
spherical
domain
size
in
CNT
reinforced epoxy nanocomposites modified with 30 wt% adduct. The DGEBA molecules left outside form the continuous phase.
27
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The amount of continuous phase in 30 wt% adduct modified nanocomposites have reduced significantly (Figure 6). This also reveals that the number of DGEBA molecules available for cross linking was quite less as a result of its diffusion to the spherical particles.
3.3 Vibration damping mechanism AFM scanning of the samples was conducted and the elastic modulus of points 0.05 µm apart along a line was calculated using Hertzian model. Figure 9 shows the variation of elastic modulus of four compositions with distance along the line of scan. An abrupt increase in elastic modulus indicates the presence of continuous phase.
28
Figure
9:
Variation
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of
elastic
distance along the sample surface
modulus
with
scanning
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Which is marked with dashed circles. The continuous phase in EA30C0.5 has very low elastic modulus compared to the other three nanocomposites shown in Figure 9. This is due to the diffusion
of
DGEBA
molecules
into
the
spherical
adduct
particles. Which resulted in the reduction of number DGEBA molecules available for cross linking with PAMAM. Hence, it is determined that the continuous phase in EA30C0.5, EA30C1 and EA30C2.5 is highly flexible. As a result, energy is dissipated as heat during frictional sliding of spherical particle with respect to the matrix as well as between themselves. A reduction in loss factor was obtained for 10 and 20 wt% adduct
modified
nanocomposites.
Whereas
a
significant
improvement is obtained for nanocomposites with 30 wt% adduct. The gap between distributed spherical adduct particles in 30 wt% adduct modified nanocomposites is lesser than that in 10 and 20 wt% adduct modified nanocomposites (Figure 6). AFM analysis also revealed that the continuous phase in 30 wt% adduct
modified
compared
to
nanocomposites
that
in
10
and
has 20
lower wt%
elastic adduct
modulus modified
nanocomposites. This implies that compared to 10 and 20 wt% adduct
modified
nanocomposites,
30
wt%
adduct
modified
nanocomposites facilitate relative sliding. This dissipate more energy and results in higher loss factor. The loss factor in mode 2 is found to be higher than that in mode 1 (Figure 3). At a particular excitation frequency 29
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some mechanisms dominate and the damping is proportional to the quantity of energy dissipated by these mechanisms. Hence, higher modes can give higher damping ratio if the mechanism active at the higher modes are capable of dissipating more amount of energy. Similar results were reported in previous studies [8, 34]. Mode 2 occurred at higher frequency (250 Hz) compared to mode 1 (30 Hz). More relative sliding might occur between
spherical
particles
and
matrix
as
well
as
with
themselves. This could dissipate more energy. This led to higher loss factor in mode 2 compared to that in mode 1. A prominent decrease in loss factor in both the modes is observed for EA30C2.5 nanocomposite. Agglomeration of CNTs can occur
since
its
amount
is
more
in
this
nanocomposite.
Diffusion into spherical adduct particles would be difficult for the agglomerated CNTs. Hence, a fraction of CNTs are retained in the continuous phase. This reduces the flexibility of the continuous phase. As a result loss factor decreases.
4
Conclusions Nanocomposites of CNT reinforced epoxy, modified with
adduct of epoxy and CTBN, were fabricated and tested for vibration damping properties. In the first mode, epoxy with 1 wt% CNT and 30 wt% adduct exhibited highest loss factor. An increase of 119 % compared to that of neat epoxy is obtained. In the second mode, epoxy with 0.5 wt% CNT and 30 wt% adduct 30
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exhibited highest loss factor. An increase of 98 % compared to that of neat epoxy is obtained. Significant improvement in loss factor is obtained for 30 wt% adduct modified epoxy nanocomposites reinforced with CNT. Which is as a result of the modification of microstructure of the nanocomposites when CNTs were incorporated to it. Larger spherical adduct particles were formed as a result of phase separation. Diffusion of DGEBA molecules to spherical adduct particles reduced the elastic modulus of continuous phase. Under the application of dynamic loads these particles slide relative to each other. This is facilitated by the increased flexibility of continuous phase. Energy is dissipated as heat during the frictional sliding of spherical particle with respect to the matrix as well as between themselves. CNTs played
major
role
in
developing
a
microstructure
which
enhanced the vibration damping.
5
Acknowledgement
The authors gratefully acknowledge Prof. S. Raja, Department of
Chemical
Sciences,
IISER,
Kolkata
for
the
technical
discussions and the Naval Research Board (NRB) of Government of
India
for
the
funding
support
DNRD/05/4003/NRB/333).
Supporting Information 31
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(Project
number:
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Synthesis of adduct, Fabrication of nanocomposite
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