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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.

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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].

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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.

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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

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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.

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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

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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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