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CHAPTER II
LITERATURE REVIEW
Combination of a polymer matrix and additives that have at least one dimension in the
nanometer range constitute polymer nanocomposites. The additives can be uni-dimensional
(examples include nanotubes and fibres), two-dimensional (which include layered minerals
like clay), or three-dimensional (including spherical particles). Over the past decade, polymer
nanocomposites have attracted considerable interests in both academia and industry, owing to
their outstanding mechanical properties like elastic stiffness and strength with only a small
amount of the nanoadditives. This is caused by the large surface area to volume ratio of
nanoadditives when compared to the micro and macro additives. Other superior properties of
polymer nano-composites include barrier resistance, flame retardancy, scratch/wear
resistance, as well as optical, magnetic and electrical properties.
A great deal of literature has been published, particularly over the last decade,
highlighting the properties of polymers and their composites with nanomaterials. The
inclusion of nanoparticles over the matrices has proven to exhibit a high potential for
significantly improving mechanical properties of the polymers. Nevertheless, most of the
available data refer to static properties while the behavior under thermal and mechanical
loading is rarely investigated. A brief overview on some of the most important work is
presented below.
2.1. Epoxy-thermoset
Epoxy resins are widely applied as a composite matrix mainly due to their thermal,
mechanical, chemical, and corrosion resistance. A favorable property of epoxy resins is the
low viscosity in the uncured state that enables the resin to be processed without the use of
high pressure equipments. One application of great interest is an epoxy matrix for fiber-
21
reinforced composites. Several composite manufacturing methods rely on the low viscosity of
epoxy resins such as resin transfer molding (RTM), vacuum assisted resin transfer molding
(VARTM), hand lay-up, and filament winding [1]. However epoxy systems have their
inherent brittleness and poor resistance to crack propagation. Recently it has been reported
that the toughening of the cross-linked epoxy resins can be achieved by the incorporation of
high-performance thermoplastics (thermoplastics having high glass transition temperature)
[2-4] without lowering the Tg of the system.
2.2. Toughening of polymers
Thermoset epoxy composites are most often used in high-performance applications on
account of their unique performance-to-cost ratio compared to polyester based composites.
They generally possess excellent properties and are suitable for a large number of processing
techniques. This results from the different chemistries, blending components and pre-
polymerisation stages that are used. Despite their versatility, the applications they are
intended for, demand an increasingly high performance. In particular, resin-related properties
such as the glass transition temperature (Tg), toughness, and dimensional stability are of
prime interest. In general, as the glass transition temperature of a resin system is raised, a
decrease in toughness and dimensional stability is observed. This depends on the fact that all
these properties are related to the cross-link density of the cured resins, which rigidifies the
molecular network, decreases its deformability, and increases the process-induced shrinkage
[5-7]. As a consequence, when formulating a new resin, all the above mentioned property
requirements, as well as the resin processability must be taken into account. The difficulty
lies in optimizing the processability of the resin without decreasing the thermomechanical
properties. This becomes crucial when using modifiers, which are widely used for toughening
brittle matrix systems in epoxy composites to improve crack resistance and inhibit
22
interlaminar failure. Commercial tougheners such as rubber, thermoplastic or glass particles
can affect their Tg by varying degrees, but they always limit the processability of the resin
systems [8-13]. Furthermore, these modifiers can be filtered out during impregnation of the
composite fibre structure. In processing techniques such as resin transfer moulding (RTM)
where long flow distances and high shear forces are present during fibre bed impregnation,
viscosity and the filtration of modifiers are critical issues. At present, no efficient tougheners
seemed to be applicable to such resins and, more generally speaking, no modifiers could
fulfill their task of property improvement without affecting the general performance of the
resin system.
2.3. Toughening agents
Among rubber modifiers, the most used are reactive liquid butadiene-acrylonitrile
rubbers [14] or preformed rubber particles (for example core shell particles [15]). One major
concern with butadiene-acrylonitrile rubbers is the high level of unsaturation in their
structure, which provides sites for degradation reactions in oxidative and high temperature
environments [16]. It is also possible that traces of free acrylonitrile, a carcinogen, may be
present and this is a strong limitation in the use of these materials [17]. The addition of
reactive liquid rubbers or preformed rubber particles results in high increase in the viscosity
of uncured blends [18, 19], that consequently cannot be readily used as matrices for
composites manufactured with composite manufacturing techniques due to dispersion
problems.
Use of high performance engineering thermoplastics, such as poly(ether imide)s [20,
21], polycarbonate [22-24], poly (phenylene oxide) [25], and poly(ether sulfone) [26] and
interesting data on carboxyl terminated polyethylene glycol adipate [27] have been reported.
Engineering thermoplastics are favored over rubbers because of their high glass transition
23
temperature, high elastic modulus, better solvent resistance, and toughness. Their thermal
properties are advantageous in avoiding significant decreases in the elastic modulus and in
glass transition temperature, which are usually observed in rubber-modified epoxy blends.
However, the use of thermoplastics results in high increases in the blend’s viscosity, even if
low molecular mass polymers are used [28, 29]. Moreover, the cost of high performance
engineering thermoplastics has restricted their use as modifiers to the formulation of special
applications such as advanced fiber- reinforced composites for aerospace and Formula One
race cars. The miscibility and curing process of the system epoxy + thermoplastic was
carried out, specifically diglycidyl ether of bisphenol A (DGEBA)+ PVAc, where PVAc is a
ductile polymer of moderate Tg, and could be seen as a potential epoxy modifier to improve
toughness and cure shrinkage [30].
2.4. Unsaturated polyester as toughening agent
In order to improve the toughness and flexural strength and other thermo-mechanical
properties of epoxy resin, it is proposed to use unsaturated polyester as a toughening agent.
Unsaturated polyester (UP) resins are one of the most widely used resins for the fabrication
of polymer composites because of their competitive cost and ease of processing [31].
Unsaturated polyester is expected to function as the best toughening material for epoxy
resins, because of its versatile behavior like flexibility, high thermal stability, heat resistance,
low water absorption and chemical resistance. It is observed that the introduction of
unsaturated polyester into epoxy resin improves the impact strength and thermal stability, but
reduces the stress-strain properties and glass transition temperature.
24
2.5. Interpenetrating polymer networks (IPNs)
Interpenetrating polymer networks (IPNs) usually exhibit the properties of partial
compatibility, and broad distribution of the molecular relaxation [32]. A broader temperature
range of thermal transition is shown to be suitable for the application as damping materials.
Excellent damping properties have been demonstrated from IPNs [33-36]. As part of
endeavor to pursue IPNs with excellent damping properties, a series of unsaturated
polyester/epoxy IPNs have been developed [37]. Moreover, attempts at imparting flame
retardancy to organic polymers via addition of flame retardant additives have been of strong
interest for the past few years [38-41]. However, the addition of some flame retardants may
significantly decrease the mechanical properties of polymers [42, 43]. It is preferred that the
mechanical properties of the polymers would not be affected drastically with the addition of
flame retardants. It is even more desirable if the mechanical properties can be improved by
the addition of flame retardants. Saravanos and Chamis [44, 45] demonstrated that the
addition of beam-plate or shell-shaped structures of flame retardants can enhance the
damping properties of materials.
Reactive blending with thermoset resins can lead to deactivating the end groups of
UPR chains. In recent years, chemical modification by reactive blending of UPR and other
thermosets via semi interpenetrating polymer networks (IPNs) and hybrid polymer networks
(HPNs) has been reported. Blending of epoxy resin and polyesters resulting in IPNs [46-49]
has been extensively studied. Dinakaran et al. [50] have developed an intercrosslinked
network of unsaturated polyester-bismaleimide-modified epoxy matrix system.
Interpenetrating networks of varying percentages of bismaleimide in vinyl ester oligomer-
modified unsaturated polyester matrices have also been reported [51]. Hybrid polymer
networks of polyurethane prepolymers and unsaturated polyester have been developed with
increase in mechanical properties of the resin and laminates [52-57]. Similarly, chemical
25
bonding between elastomer and UPR using methacrylate end-capped nitrile rubber or epoxy-
terminated nitrile rubber (ETBN) or isocyanate end-capped polybutadiene are attractive
routes [58]. The mechanical properties of resins and laminates are improved by this
technique.
Recently, hyperbranched polymers have substituted the traditional modifiers in order
to overcome the limitations of the latter [59]. Epoxy-functionalized hyperbranched polymers
have proven to be feasible as modifiers of formulations that are processed by RTM
techniques [60]. The effects of hydroxyl hyperbranched polymers on blend viscosity
compared with linear thermoplastics, have been reported previously [61], confirming the
advantages of hyperbranched polymers over poly(ether sulfone)s. Furthermore, partial
substitution with epoxy groups of the hydroxyl groups on the shell of the hyperbranched
polymers has been shown to be more effective in reducing blend viscosity [62]. The
hyperbranched polymers used as epoxy modifiers have been shown to increase the viscosity
of the blend up to values that allow for their use in processes like resin film infusion (RFI),
but precludes their use for resin transform mechanism (RTM) or vacuum assisted resin
transform mechanism (VARTM).
2.6. Polyester-Epoxy hybrid formulations
Polyester-epoxy hybrid powder coatings contain both epoxy resins and carboxyl-
terminated polyester resins. Polyester producers market “high reactivity”, “active” or “low
temperature curing” polyester resins which have been admixed with catalysts during
production. These catalysts are intended to speed up the curing rate or to lower the cure
temperature of high-gloss hybrid coatings [63].
26
2.7. Unsaturated Polyester-toughened Epoxy systems
The unsaturated polyester-toughened epoxy is designed to withstand high impact
loads especially at location like corners where fibers are few. Park et al. [64] and Harani et al.
[65] used unsaturated polyester for toughening the epoxy resin. Though bamboo is
extensively useful as a valuable forest material from time immemorial (because of its high
strength-to-weight ratio), the studies on these fibers are meagre. Bamboo fibers coated with
the blend of epoxy/unsaturated polyester (UP), are chemically resistant and tensile properties
are to be studied to ascertain whether epoxy/UP bamboo system can be effectively used for
making the composites.
2.8. Fillers
Fillers play important roles in modifying the polymer characteristics and reducing the
cost of their composites. In conventional polymer composites, many inorganic fillers with
dimensions in the micrometer range, e.g. calcium carbonate, glass beads and talc have been
used extensively to enhance the mechanical properties of polymers. Such properties can
indeed be tailored by changing the volume fraction, shape, and size of the filler particles. A
further improvement of the mechanical properties can be achieved by using filler materials
with a larger aspect ratio such as short glass fibers. It is logical to anticipate that the
dispersion of fillers with dimensions in the nanometer level having very large aspect ratio and
stiffness in a polymer matrix, could lead to even higher mechanical performances. These
fillers include layered silicates and carbon nanotubes. Carbon nanotubes (CNTs) have a
substantially larger aspect ratio (~1000) in comparison with layered silicates (~200). Rigid
inorganic nanoparticles with a smaller aspect ratio are also good reinforcing and/or
toughening materials for the polymers, but the dispersion of these nanofillers in the polymers
is rather poor due to their incompatibility with polymers and large surface-to-volume ratio.
27
Therefore, organic surfactant and compatibilizer additions are needed in order to improve the
dispersion of these nanofillers in polymeric matrices as absorbed in the layered silicate
surfaces which are hydrophilic and require proper modification of the clay surfaces through
the use of organic surfactants. The product obtained known as ‘organoclay’, can be readily
delaminated into nanoscale platelets by the polymer molecules, leading to the formation of
polymer–clay nanocomposites. These nanocomposites belong to an emerging class of
organo-inorganic hybrid materials that exhibit improved mechanical properties at very low
loading levels compared to conventional microcomposites. CNTs are recognized to
agglomerate and entangle easily during processing of the nanocomposites, leading to poor
mechanical properties. Several techniques such as ultrasonic activation, in situ
polymerization and surfactant addition are commonly used to disperse CNTs in polymer
matrices.
2.9. Epoxy nanocomposites
At least three classes of nanocomposites can be distinguished, depending on the
morphology of the filler: layered nanocomposites, whisker (or nanotubes) based
nanocomposites and isodimensional nanocomposites. In recent years, the attention has been
mainly focused on the first class of nanocomposites, especially those obtained from layered
silicates [66, 67] in thermoplastic or thermosetting matrices [68] since they often
demonstrated a remarkable improvement in thermal and mechanical properties compared to
traditional microcomposites. Other fillers, such as carbon black or fumed silica, have been
largely used as additives to improve the properties of polymers, such as imparting uv-
resistance, to control rheological properties, and quite recently, as nanometric fillers with
potentially interesting reinforcing capabilities.
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2.10. Epoxy-organoclay nanocomposites
Polymer nanocomposites reinforced with nanoclay particles have attracted greater
attention because of their unique mechanical, thermal and physical properties along with
excellent transport characteristics that are offered by the layered structure of clay particles
with extremely high aspect ratios. Remarkable ten-fold increase in tensile strength and
modulus is reported with exfoliated organoclays in rubbery epoxy [69]. These property gains
were at the expense of ductility, which decreased with increasing clay content [70]. In
contrast, the reinforcement efficiency offered by the similar organoclay in glassy epoxy
matrices is not as remarkable as in the rubbery epoxy, and the tensile strength often showed
lower values than that of the neat epoxy [71].
The effect of organoclay nanoparticles on the rheology and development of the
morphology and properties for epoxy/organoclay nanocomposites has been studied by Dean
et al. [72]. While intercalated structures were obtained in all cases, the interlayer spacing
increased with the curing temperature and rheological studies suggest that intergallery
diffusion and catalyzation of the curing process are essential for exfoliation of the silicate
layers. Lee has reported that Tensile strength and Young’s modulus increase with the
treatment of organo-surfactants and with the increasing clay loading content in epoxy resin
[73]. Partially intercalated nanocomposites were synthesized by Isik et al. and absorbed the
basal spacing of organically modified montmorillonite powder increases from 1.83 to 3.82
nm upon mixing into the DGEBA-hardener system. Tensile strength and strain at break show
a maximum at 1 wt% montmorillonite content in epoxy-montmorillonite binary systems [74].
For the first time, confocal microscopy was used by Langat et al. [75] to obtain
quantitative information on mesoscale dispersion of Nile Blue-tagged MMT platelets in cured
epoxy matrix. There is evidence of aggregated clay platelet regions in the hand mixed
sample. Sonication of the sample improved the separation of clay platelets and the combined
29
effect of sonication as well as use of reactive clay gave well dispersed clay platelet
distribution in epoxy nanocomposites. Visual inspection and image analysis of the confocal
images confirmed the superior quality of the clay dispersion in sonicated samples as opposed
to hand mixed nanocomposite samples.
Lai [76] studied the effect of epoxy treatment on clay surface resulting in a
remarkable increase in decomposition temperature as measured from TGA, which reflects the
ameliorating effect on thermal stability of the composites. The epoxy treatment also resulted
in a significant improvement in melt flow index of the Poly(ethylene terephthalate-co-
ethylene naphthalate)/Cloisite/epoxy system even at a high temperature, giving rise to a much
wider process window (245–265oC from 245–250
oC) for the Poly(ethylene terephthalate-co-
ethylene naphthalate)/Cloisite. The organoclay reinforcement gave rise to a higher tensile
strength and modulus than the neat Poly(ethylene terephthalate-co-ethylene naphthalate), at
the expense of much reduced ductility. Epoxy treatment of organoclay resulted in further
improvements of all three tensile properties, partly confirming the improved interactions
between functional groups in epoxy, Poly(ethylene terephthalate-co-ethylene naphthalate)
matrix and organoclay surface.
The nanocomposites made with the high pressure mixing method (HPMM) showed
dramatic improvement in fracture toughness at very low clay loading over the pristine resin
properties as reported by Liu et al. [77].
Kim et al. [78] have observed that the presence of organoclay in epoxy matrix
increased the glass transition temperature (Tg) significantly whether the nanocomposites
were in a dry or wet condition. The Tg for both the nanocomposite and neat epoxy displayed
a linear decrease with increasing moisture content. Akbari et al. [79] reported that the
Compressive and flexural strength of epoxy decreases with increasing the organoclay content.
30
From Dynamic mechanical analysis (DMA), maximum 25% improvement of storage
modulus in nanocomposites was achieved by Hussain and et al. [80]. In nanocomposites
significant modulus improvement has been achieved in rubbery state, which showed the
reinforcing effect of nanoclay. An increase in tensile modulus upto 47% has been achieved
for clay concentration monotonically, however opposite trend has been observed for tensile
strength. The observed lower tensile strength is attributed to the aggregation of the nanoclay
in epoxy systems, as confirmed by SEM and TEM micrographs. Wide angle X-ray diffraction
(WAXD) results also confirmed intercalated morphology, but no significant improvement in
Tg has been achieved in nanocomposites.
2.11. Epoxy-silanated clay nanocomposites
The possibility of grafting some silane molecules on the surface of the clay mineral
layers [81, 82] has been attempted recently and concluded that the grafted minerals act as
better reinforcing materials by physical or reactive blending in polymer matrix. In fact,
reactive OH groups are present in the clay minerals at the edges and at the structural defects
which can be easily functionalized and get compatible with polymers [83-85]. Therefore, the
functionalization of the clay mineral can take place at three possible sites: at the interlayer
space, at the external surface [86] and at the edges [82]. The silanization at the interlayer and
at the edges can increase the distance between the layers [87-88], and the silane treated clay-
epoxy system exhibited improved quasi-static fracture toughness [89]. In some cases, the
silanization reaction has also been conducted into organophilic clay minerals to enhance their
compatibility with the polymer: these twice-functionalized organo-clay minerals are
described by Chen [90].
The intercalation or eventually the exfoliation of clay mineral layers after
modification can be achieved by different methods. Some of them request the melting of the
31
polymer and can affect its stability; others are solvent-based, thus releasing large amounts of
volatile compounds [91]. In situ intercalative polymerization is also proposed: in this process,
which is often solvent based, the kind of guest species to be intercalated are the reactive
monomers. In situ crosslinking photopolymerisation of solvent free curable systems is
interesting: the procedure, which involves the use of UV light and is called UV-curing,
guarantees the building up of polymeric thermoset matrices via a fast and environmental
friendly process with low energy consumption and no emission of volatile organic
compounds [92]. There are recent reports on clay mineral-based hybrids, obtained by UV-
curing oligomers in the presence of organophilic clay minerals [93, 94].
2.12. Epoxy-silica nanoparticles
Sun and co-workers [95] have compared the effects of nano-sized and micro-sized
fillers in epoxy composites by means of differential scanning calorimetry, thermo-mechanical
and dielectric relaxation measurements and inferred that the glass transition temperature was
reduced with increase of nanofillers, whereas it did not change in the corresponding
microcomposites. Contrasting effects for different fillers in various matrices, both increase
[96] and decrease [97] of the thermo-mechanical properties and even more complicated non-
monotonic trends were observed with the best performance at filler level as low as 5% by
weight [98]. The need to improve the mechanical properties of epoxy matrices is encouraging
the use of silica nanoparticles as reinforcing agent and the best performance was obtained
with surface-functionalized silica nanoparticles [99]. On the other hand, untreated silica
nanoparticles appeared to deteriorate thermal properties in epoxy matrices mainly because of
the presence of residual moisture and organics. The thermo-mechanical properties of the
silica-filled epoxy nanocomposites were found to decrease for 10 and 20 phr filled samples,
while a trend inversion was observed for 30 phr filled samples. Retardation of thermo-
32
mechanical properties, such as glass transition temperatures, dynamic storage modulus and
tensile modulus, was explained by postulating the presence of polymer–filler interactions
limiting the cross-linking degree attained by the polymer matrix during composites curing
and the presence of residual moisture and organics [95].
Adebahr et al. [100] proposed a novel route to prepare nanocomposites consisting of
monodispersed SiO2 nanoparticles and reactive resin. The addition of 23 wt.% of particles
subjected to thermal anhydride curing induced a 66% increase in the stress intensity factor
[KIC], while UV curing led to an improvement of 82% at 50 wt.%. Ragosta et al. [101]
improved the mechanical properties of epoxy resin adding 10 wt.% of silica particles with a
diameter of 10-15 nm. The normalized elastic modulus reached the value of 1.5, while the
normalized yield strength increased up to 1.3. The addition of silica raised the fracture energy
of the epoxy matrix by a factor of ~4, whereas the increase of stress intensity factor (KIc) was
twofold.
Zheng et al. [102] found that the addition of 3 wt.% of silica nanoparticles within
epoxy matrix lead to an increase in tensile strength of 115%, while the impact strength
increased by 56%. The strain to failure was generally improved through the nanomodification
with enhancements up to 20%, whereas the ultimate tensile strength was not significantly
affected by the fumed silica particles. The most important improvements were achieved in
fracture toughness, up to an increase of 54% at 0.5 vol.% of AMEO-fumed silica, thus
indicating a strong effect on crack propagation resistance already at the low filler contents. In
the fatigue test, some enhancements were visible only at 0.5 vol.%, but not so significant. The
indifferent behavior in static and dynamic loading seemed to suggest that different
toughening mechanisms could act in the two cases.
33
The influence of nanoparticle SiO2 content on fracture behavior of nanocomposites
showed that the nanocomposite with 3wt% nanoparticle had higher fracture toughness and
larger deformation resisting capability than other nanocomposites.
2.13. Epoxy-CNT nanocomposites
Carbon nanotubes (CNT) have attracted the attention of researchers worldwide,
because they show superior physical and electrical potentials, which allow them to be applied
to the whole gamut of technologies ranging from microelectronics to aerospace [103]. The
unique mechanical properties of carbon nanotubes, their high strength and stiffness and the
enormous aspect ratio make them a potential structural element for the improvement of
mechanical properties [104].
Meng-Kao Yeh et al. [105] have observed that the addition of MWNTs in the epoxy
matrix increases the Young’s modulus of MWNTs/epoxy nanocomposites by 51.8% for the
nanocomposites with 5 wt.% of MWNTs when compared with the epoxy specimen. The
tensile strength of the MWNTs/epoxy nanocomposites also increases by 17.5% for 3 wt.% of
MWNTs additive. ZhengYaping et al. [106] showed that when the content of the MWNTs-
NH2 on epoxy resin increases to 0.6%, the bending strength and flexural modulus can be
increased by 100% and 58% respectively. Yijun Li et al. [107] fabricated the DWNT/epoxy
composite fibers by immersing well aligned DWNT strands into epoxy solutions, and
observed that in low concentrations of soaking solutions, their tensile strength and young’s
modulus are found to increase by factors of 25% and 75% compared with those of the
original strands, respectively. Tsu-Wei Chou et al. [108] examined the nanoscale structure
evolution and demonstrated that the shear mixing induced by the calendaring approach results
in a high degree of nanotube dispersion in epoxy composite. The processed nanocomposites
exhibited significantly enhanced fracture toughness at low nanotube concentrations. Mingxin
34
Ye et al. [109] established that different amino groups on the surface of the MWCNTs have a
significant effect on the thermal and mechanical properties of the epoxy composites.
Florian H. Gojny et al. [110] demonstrated the applicability of nanotube/epoxy-
systems as matrix for FRPs and the capability of RTM-technique to manufacture these
nanoreinforced composites. They found that manufacturing of resins with nanotube contents
of more than 0.5 wt% is still a challenge, due to the enormous surface area of CNTs and the
resulting increase in viscosity. Brinson et al. [111] developed different amino-finctionalized
carbon nanotubes to bind polymers for various biological applications. Huaihe Song et al.
[112] observed increase in mechanical properties of epoxy composites with addition of
MWCNTs, and the combination of chemical functionalization of MWCNTs and high energy
sonication improved the interfacial adhesion of CNTs to the epoxy matrix. Daniel Wagner et
al. [113] compared the stiffness and strength of pristine carbon nanotube-based composites
with 1wt% functionalized MWCNTs spread in a rubbery matrix and observed a significant
increase in stiffness and strength due to good nanotube dispersion and strong interfacial
bonding. Kin- tak Lau [114] demonstrated the solvent effect on SWNT’s dispersion in
acetone, ethanol, and DMF, and the consequence of thermal and mechanical properties of
SWNT bundle/epoxy composites. The thermo-mechanical properties and matrix-cracking
behaviors of CSCNT-dispersed carbon fiber reinforced plastics (CFRP) were investigated by
Tomohiro Yokozeki et al. showed that the dispersion of CSCNT resulted in the enhancement
of stiffness and strength and the decrease of residual thermal strain in composite laminates
[115]. The mechanical, thermo-mechanical and electrical properties of a brittle epoxy resin
reinforced with 1% of MWCNTs were investigated by Avile et al. [116]. The nanocomposite
containing the amine-functionalized MWNTs exhibit higher thermal stability and mechanical
properties because the surface treatments provide relatively more homogeneous dispersion of
35
CNTs and stronger interaction between the CNTs and the polymer matrix, which implies the
existence of an unique interphase region [117].
However, the realization of nanotube-reinforced epoxy resin can be achieved only by
solving certain critical problems encountered viz. the lack of interfacial adhesion, which is
vital for load transfer in composites, because of the atomically smooth surface of nanotubes
and the poor dispersion of nanotubes in the epoxy matrix, as the high surface energy and
intrinsic Van der Waals forces make CNTs to aggregate and entangle together spontaneously
2.14. Filled polymer nanocomposites containing other functionalized nanoparticles
2.14.1. Epoxy-Titanium dioxide composites
Many studies have demonstrated that titanium oxide (TiO2) can be used as a suitable
filler for epoxy based matrices. Ragosta et al. [118] found that adding 10wt% of TiO2
nanoparticles within epoxy resin increased considerable percentage of fracture toughness.
Evora and Shukla [119] studied the dynamic behaviors of high strain rate of polyester/TiO2
nanocomposite using a split Hopkinson pressure bar apparatus, which showed a moderate
stiffening effect with increasing particle volume fraction, but the ultimate strength has no
markable change. Siegel et al. [120] obtained an increase of 15% of the strain to failure
filling an epoxy resin with 10 wt.% of nanometric TiO2 particles. Lin et al. [121] reported that
tensile and impact strength of titanium dioxide and montmorillonite filled epoxy resin
reached a maximum for a filler content of 5-8 vol.% and decreases at higher filler contents,
sometimes even below that of the neat resin. In the literature, the toughening effect due to the
addition of particles to polymers has been studied for a long time [122-124]. Yang et al. [125]
investigated the fracture behavior of polyamide 66 filled with TiO2 nanoparticles. With the
increase of the TiO2 content from 1 to 3 vol.%, the plastic zone around the crack tip
decreased and the density of dimples near the pre-notched area increased. Thus, the energy
36
absorbed during crack propagation should be higher for the nano-reinforced matrix than for
the pure polyamide.
2.14.2. Epoxy-alumina composites
The potential of alumina particles as suitable filler in epoxy based systems are also
reported. Zhang et al. [126] established that, the Al2O3 (micro-nano) particles enhance the
fracture toughness with polyester matrix, strongly influenced by the particle matrix adhesion.
More significant enhancements in fracture toughness (almost 100% at 4.5 vol.% of Al2O3
nanoparticles in unsaturated polyester) were achieved improving the particle-matrix adhesion
through a silane surface. Wetzel et al. [127] studied the effects of nano (alumina) and micro-
spherical (calcium silicate) particle addition to epoxy resin and found increases in flexural
modulus (upto 35%), strength (upto 20%) and Charpy impact energy (up to 35%). In a
following, interesting work [128], neat epoxy reinforced with Al2O3 nanoparticles at different
volume contents was investigated. The 10 vol.% epoxy/Al2O3nanocomposite exhibited
significant improvements in flexural modulus (around 40%), strength (15%) and fracture
toughness (120%). Furthermore, the crack propagation threshold and resistance turned out to
be improved dramatically, with the crack propagation rates for nanocomposites being orders
of magnitude slower than neat resin for the same range of SIF.
2.14.3 Composites with other fillers
The addition of nanoparticles has exhibited high potential for proven mechanical
properties of polymers. Battistella et al. [129] obtained an increase of 54% of fracture
toughness by filling an epoxy resin with 0.5 vol% of fuming silica modified with 3-
aminopropyltrimethoxy silane. ZhanhuGuo et al. [130] studied the effect of functionalized
ZnO nanoparticle on the optical and mechanical properties of vinyl resin.
37
F. Hussain et al. [131] obtained an increase of 47% of the tensile modulus in filling an
epoxy with 5wt% of clay. Y.K. Choi et al. [132] obtained a maximum tensile strength and
young’s modulus resulted at 5wt% of cup-stacked carbon nanotubes filling. N. Chisholm et
al. [133] observed an average of 20-30% increase in mechanical properties with 1.5wt%
loading of carbon/SiC on epoxy resin. A.V. Rajula et al. [134] have used epoxy and polyester
as coating materials and observed that alkali treated epoxy and polyester coated fibers have
shown an increase in tensile strength of 55% and 88% respectively over the uncoated fibers.
Recent fracture studies of nanocomposites are concentrated on static/dynamic fracture
toughness and microscopic fracture behaviors. Sue et al. [135] studied the fracture process
and the fracture mechanisms of α-zirconium phosphate based epoxy nanocomposites with
and without core-shell rubber toughening. Ratna et al. [136] studied the improvement of
impact properties in epoxy/clay nanocomposites, and the fracture surface analysis are
performed by scanning electron microscope (SEM). Park et al. [137] measured the interfacial
properties of epoxy/red mud nanocomposites in the context of critical stress intensity factor
and critical strain energy release rate. Park et al. [138] detected the fracture of carbon fiber in
carbon nanotube (CNT)/epoxy composites by nondestructive acoustic emission. Kornmann et
al. [139] showed that epoxy-layered silicate nanocomposite formation could simultaneously
improve fracture toughness and Young's modulus, without adversely affecting tensile
strength. Bernd et al. [140] introduced various amounts of micro- and nano-scale particles
into an epoxy polymer matrix for its reinforcement, and the energy dissipating fracture
mechanisms are explained, and the influence of these particles on the impact energy, flexural
strength, dynamic mechanical/thermal properties and block-on-ring wear behavior were
investigated. Gam et al. [141] studied the morphology and fracture mechanisms of two
nanoclay-filled epoxy systems using both microscopy and spectroscopy tools. Becker et al.
[142] studied an increasing toughness of the nanocomposites with increasing clay content in
38
the intercalated epoxy-layered silicate nanocomposites. Zerda and Lesser [143] prepared
intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy, and the
fracture toughness improvements were demonstrated and the fracture-surface topology was
examined using scanning electron and tapping-mode atomic force microscopes. Frohlich et
al. [144] prepared high-performance epoxy hybrid nanocomposites, and the fracture surfaces
revealed extensive matrix shear yielding for the neat resin and the predominant fracture mode
like crack bifurcation and branching. Haque et al. [145] determined mechanical properties
such as interlaminar shear strength, flexural properties and fracture toughness for both
conventional S2-glass/epoxy composites and S2-glass fiber reinforced nanocomposites. Li
XD et al. [146] prepared the nanoclay-reinforced agarose nanocomposite films with varying
weight concentration ranging from 0 to 80% of nanoclay and measured the structural
characterization by transmission electron microscopy (TEM), scanning electron microscopy
(SEM) and atomic force microscope.
2.15. Challenges on nanocomposite fabrication
Different toughening mechanisms have been mentioned in polymer technology, such
as the localized inelastic matrix deformation and void nucleation, particle debonding, crack
deflection, crack pinning, crack tip blunting, particle deformation or breaking at the crack tip.
However, it is still an open question which is the effective mechanisms responsible for
toughening on nanocomposites [147]. Furthermore, experimental techniques and descriptive
models are based on macro-mechanical concepts. Thus, their application to nanocomposites
is not straightforward and indeed questionable. Particle-matrix debonding and localized
deformations in the process zone ahead of the crack tip are probably responsible for the
considerable toughening effect brought about by nano modification. Recent experimental
investigations by Johnsen et al. on silica nanoparticles-reinforced epoxy polymers confirm
39
these assumptions [148]. Because of the very high specific surface area, even very low filler
contents can significantly contribute to matrix reinforcement. Especially interface related
effects, such as debonding mechanisms and void nucleation could play a significant role even
at low volume contents. Although classical mechanical theories concerning particle
toughening sometimes even predict a decrease of toughening contribution with decreasing
particle size, the increasing amount of interfacial area and absolute number of particles in the
process zone can be reasons for the experimentally observed increases in fracture toughness
[149]. Xie et al. [150] reported the improvement of the mechanical properties of PVC with
the addition of CaCO3. At 5 vol.%, optimal performances were achieved in Young’s
modulus, tensile yield strength, strain to failure and Charpy impact energy. The filler enabled
ductile fracture caused by elevated triaxial stresses at the neck region and consequently
debonding at the particle-matrix interface. Increasing the load, the ligaments between the
voids were stretched increasing the energy consumption. Lazzeri et al. [151] showed that the
addition of 10 vol.% of uncoated CaCO3 led to an increase in Young’s modulus and yield
stress and to a decrease in impact strength. On the other hand, if the particles were covered
with stearic acid, the tensile properties slightly dropped and the impact strength linearly
increased with the stearic acid surface concentration. The fracture surface analysis showed
cavities and voids due to debonding and deformation bands in the stress whitened areas. The
void formation allows for a plastic deformation of the interparticle ligaments, which is
assumed to be the main absorbing energy mechanism [152]. Yang et al. [153] investigated
the fracture behavior of polyamide 66 filled with TiO2 nanoparticles. With the increase of the
TiO2 content from 1 to 3 vol.%, the plastic zone around the crack tip decreased and the
density of dimples near the pre-notched area increased. Thus, the energy absorbed during
crack propagation should be higher for the nanoreinforced matrix than for the pure
polyamide. In the last decades the greatest part of the researches carried out on
40
nanomodification was oriented to thermoplastic matrix nanocomposites, however the
attention of the scientific community has recently moved to the nanomodification of
thermosetting resins, in view of their possible application as matrix for ternary and fibre
reinforced laminates. Because of their size in the nanometer region, nanoparticles are smaller
enough to penetrate into the fibrerovings and to act as matrix reinforcement in FRP
laminates. Chisholm et al. [154] investigated the influence of a nanocomposite matrix on a
laminate composite. The presence of 1.5 wt.% of SiC nanoparticles within the epoxy resin
increased the tensile modulus of the nano-modified matrix of 44% and the tensile strength of
15%. The tensile modulus of the corresponding laminate increased to 23.5% and the tensile
strength 11%. Also in the flexural test the laminate containing 1.5 wt.% of SiC in the matrix
showed improvements (39% in strength and 12% in modulus). However, when the nano-
reinforcement was increased from 1.5% to 3%, a worsening of both tensile and flexural
properties of the composite was observed. Kinloch et al. [155] investigated the fracture
behavior of GFRP laminates with a nano-modified epoxy matrix and found considerable
increase in fracture toughness when using silica nanoparticles alone and in combination with
a CTBN toughening.
The recent works on the generation of polymer nanocomposites and their
character studies have formed the basis of the present work with certain objectives.
41
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