Research on Epoxy Based Composites

#### **Chapter 2**

## A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay

*Shanti Kiran Zhade, Syam Kumar Chokka, V. Suresh Babu and K.V. Sai Srinadh*

#### **Abstract**

Polymer nanocomposites are currently one of the most rapidly growing families of materials, and they are finding use in a wide range of industrial applications, including aerospace and defense. The broad usage of composites is because of their consolidated mechanical properties. Glass fiber reinforced epoxy composites are available for the last few decades. The idea of adding nano clay into it has emerged in the late first decade of this century. This study is aimed at reporting the effects of the addition of nano clay into GFRP on its mechanical properties. The new composite formed is epoxy-glass composites reinforced with nano clay (EGCN). Nano clay has a crystal structure that facilitates the formation of intercalated and exfoliated mixture with liquid epoxy during mixing which results in good dispersion of Nano clay thereby resulting in improved mechanical properties compared to GFRP. The work done by several researchers in this area and the results obtained are reported in this article. The improved mechanisms of failures were discussed with the addition of nano clays.

**Keywords:** nano clay particles, GFRP composites, mechanical properties, interlaminar shear strength

#### **1. Introduction**

In the current era, much effort is invested in developing new composite materials which are superior to existing materials in terms of their mechanical and physical properties. Quite a large number of studies have been published in the area of behavior, characterization, and modeling of composite materials, from metal matrix composites [1–3] to polymer matrix composites [4–7]. There are several definitions for composite materials, but the common feature of each definition is the presence of two or more constituents with an interface between them. Traditional metal matrixbased composites are made of heavy materials. In the aerospace and automobile sector, the fuel consumption is proportional to the weight of the body of the vehicle. A study by A. K. Dhingra [8] has shown that a minimum of 20% of the cost is saved if polymer composites (PMCs) replace the metal structures and the operating and

maintenance costs are also very low. Chakraverty AP [9] stated that Polymer composites are easy to repair, have good durability, and maintenance is simple. There is a consistent requirement for composites in the industries with the invention of new applications. Glass fiber-epoxy composites are widely used in the making of aircraft and automobile body parts and are not only limited to these fields but also used in ship building, structural applications in civil engineering, pipes for the transport of liquids, electrical insulators in reactors, etc. GFRPs have been in use since 1936. As the requirements for weight reduction continuously increase the mechanical properties of GFRPs can be tailored by adding micro fillers and it further evolved by adding nanofillers. The epoxy resin in GFRP firmly holds the fibers together and helps in uniform load distribution throughout the composite.

Nanocomposites are materials that are created by introducing nanoparticles into a matrix. There is a drastic improvement in mechanical properties with the addition of nanomaterials into various matrix materials. In general, the content of nanoparticles that can be added to the composite ranges between 0.5% and 5%. It is because of the high surface area of nanomaterials at a given weight content compared to the micronsized powder of the same material. Plenty of research is in progress to develop nanocomposites with multiple functionalities. The term "polymer nanocomposite" broadly describes any number of multicomponent systems where the primary component is the polymer matrix and the filler material has at least one dimension below 100 nm [10]. Polymer nanocomposites are generally lightweight, require low filler loading, are often easy to process, and provide property enhancements extending orders of magnitude beyond those realized with traditional composites.

Filler is a term that encompasses a vast number of materials and plays a significant role in improving composite properties. Fillers help minimize cost, enhance properties, and improve the composites. Fillers also increase the mechanical properties and reduce shrinkage of the composites during curing. Proper selection of matrix and filler combination will lead to the creation of composites with high mechanical and thermo-mechanical properties which are comparable to metals.

Montmorillonite is natural clay with a high charge density. Charge density is the total number of cations in between the silicate layers of montmorillonite which can be substituted with organic cations. Montmorillonite nanoparticles are naturally hydrophilic but if treated with alkylammonium ions, the particles become organophilic. The organically treated montmorillonite when dispersed in liquids like epoxy forms gels [11, 12]. The length of the ammonium ions has a strong impact on the resulting structure of nanocomposites. Lan et al. [13] showed that alkylammonium ions with chain lengths larger than eight carbon atoms favor the synthesis of exfoliated nanocomposites, whereas alkylammonium ions with shorter chains led to the formation of intercalated nanocomposites. Alkylammonium ions based on secondary amines have also been successfully used [14]. A schematic diagram showing the substitution of alkylammonium ions in place of interlayer cations is shown in **Figure 1**. The structure of the organic cations between silicate layers depends on the charge density of clay [15]. In **Figure 1**. alkylammonium ions adopt a paraffin type of structure due to which the spacing between the clay layers increased by about 10 A°. Alkylammonium ions permit lowering the surface energy of clay so that organic species with different polarities can get intercalated between the clay layers.

Based on the above discussion, it is observed that out of the three types of surface modifiers alkyl ammonium ions are the most popular because they have a higher affinity with silicate layers compared to amino acids and silanes. Depending on the layer charge density of the clay, alkyl ammonium ions may adopt different structures *A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

**Figure 1.**

*The cation-exchange process between alkylammonium ions and cations initially intercalated between the clay layers.*

between the clay layers. Alkyl ammonium ions reduce the electrostatic interactions between silicate layers thus facilitating the diffusion of a polymer molecule between clay platelets or galleries [16].

#### **2. Mechanical properties**

#### **2.1 Tensile properties**

EGCN exhibited 54% improvement in modulus at 10 wt% addition of octadecyl ammonium treated fluorohectorite (ME-ODA) but there was a 36% decrease in strength while ductility was also reduced (shown **Figure 2a**). The stress–strain curve of GRE exhibited ductile behavior, and the EGCN exhibited brittle behavior [17]. Bozkurt et al. [18] reported that when MMT is added to epoxy-noncrimp glass fabric composite, up to 6 wt% there was no improvement in strength and stiffness while both decreased beyond 6 wt%. This unchanging behavior is attributed to the dominant effect of noncrimp glass fibers over the nanoclay effect (shown **Figure 2b**). Shi et al. [19] reported the effect of "Magnetic stirring" and the high shear mixing technique (HSMT) on the tensile behavior of EGCN. When the magnetic stirring method was used, the increment in modulus of EGCN was about 19.4%, 22.2%, and 27.7% at 1, 2, and 3 wt% of nanoclay respectively. The increased modulus is credited to the good dispersion of clay layers. At 1 wt% nanoclay, the tensile properties were compared between composites; one composite consists of an epoxy-clay mixture processed by magnetic stirring, and the other by mechanical stirring. There was about a 7.9% and 5.7% increase in σ**UTS** and modulus for the EGCN with matrix processed by mechanical stirring (shown **Figure 2c**). The epoxy molecular chains were prevented from moving when the load was applied. The clay layers hindered the molecular chains because of strong adhesion and chemical bond between organoions and epoxy, thereby enhancing the stiffness of the laminate. This mechanism of clay particles hindering epoxy molecular movement was also described by other researchers [20–22]. The formation of clay aggregates at low clay contents, i.e., at 2 and 3 wt% clay addition was also reported in some literature [23–25]. The increased tensile properties with the addition of various surface-modified nanoclays under different mixing conditions and composite making methods are given in **Table 1**.

Voids are formed while mixing nanoclay and hardener, and increase with clay content; due to an increase in the viscosity of the mixture, the removal of these gas bubbles becomes difficult when kept in degassing chamber. In addition to aggregates and voids, there is a possibility of a decrease in strength by other means, that is,

#### **Figure 2.**

*Changes in tensile properties of EGCN'S at various conditions.*

through interruption of crosslinking of chains by silicate layers as a result of the reaction of epoxy molecules with organoions, which breaks the continuity of the crosslinks, this claim is not yet fully established though [35, 36]. The laminate fabricated from the matrix which was prepared by HSMT provided the enhancement in strength and modulus by 7.9% and 5.7% as compared to the laminates made by using the matrix prepared by direct mixing technique (DMT) [19].

Gurusideswar and Velmurugan [29] carried out tension tests on laminates with the addition of Garamite-1958 (alkyl ammonium treated clay) at crosshead speeds of 0.5, 5, 50, and 500 mm/min. The stress–strain plot for EGCN at 1.5 wt% of nanoclay and a testing speed of 5 mm/min was linear elastic with 9.9% elongation and failed suddenly. At 500 mm/min there was a rise in strength, modulus, and ductility by about 17%, 10.7%, and 33.3% compared to the values at quasi-static loading, i.e., at 5 mm/min (shown **Figure 2d**). σ**UTS** is more sensitive to strain rate compared to the modulus which is due to the dominant behavior of fibers in strength, whereas the modulus is influenced by clay. The same behavior was exhibited by glass/epoxy composite (GRE). The increase in clay content by up to 5 wt% did not change the elongation (i.e., 9.9%) at quasi-static loading, but at high strain rates.


*A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*



*A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

#### **Table 1.**

*Effect of Nano clay on various properties.*

the elongation has reduced (i.e. 9.6% at 500 mm/min). This is attributed to the high brittleness induced at high clay addition. At 1.5 wt% of nanoclay addition, the inversely proportional behavior between elongation and strain rate was not observed, unlike the case of 5 wt% clay added composite. The increase in strength is mainly attributed to the presence of fibers and the increase in modulus is mainly attributed to the silicate platelets which restrict the movement of epoxy molecules [37–41].

The optimum value of clay content is 1.5 wt%, whereas all tensile characteristics were improved at a high strain rate, the slight decrease in properties above 1.5 wt% nanoclay is attributed to agglomeration and a weak interfacial bond between epoxy and clay. An increase in strain rate in the range of 0.0006 s−1-0.6 s−1 increased the strength and elongation of GRE. Okoli and Smith [42] reported that there was a decrease in percentage elongation when GRE specimens were tested at various strain rates. Okoli and Smith [42] added that the decrease in elongation at high strain rates is explained with the help of Eyring theory of viscosity; while formulating this theory, an assumption has been made which states that the molecules of polymer need to cross the potential energy barriers to deform when a load is applied. Based on this assumption a linear model is developed which states that the plot between yield stress and a logarithm of strain rate is linear. This increase in yield stress with the logarithm of strain rate implies decreased plastic deformation of the matrix due to decreased movement of crosslinked epoxy molecules at high strain rates. The constrained movement of molecules is ascribed to the lack of time available for the molecules to relax at high strain rates [43–45]. But according to Gurusideswar and velmurugan [29], this effect was absent at 500 mm/min as there was an increased elongation for GRE at 500 mm/min compared to the elongation at quasi-static loading rate, which implies that 500 mm/min is not high enough to restrict the molecules' relaxation. Hussain F [45] stated that the high modulus of clay is also one of the attributes for an increase in tensile properties and improved deformation mechanisms. Li X et al. [46] Reported that the exfoliated structures have a high surface area of contact between silicate platelets and resin; therefore the transfer of load to clay platelets also will be more compared to the load transferred in intercalated structures. Withers et al. [47] reported an 11.7%, 10.6%, and 10.5% increase in strength, modulus, and elongation with 2 wt% of Cloisite 30B loading into glass-epoxy due to exfoliated morphology (shown **Figure 2e**).

Gurusideswar and Velmurugan [31] reported the behavior of EGCN with the addition of GARMITE-1958 and testing speeds varying between quasi-static rate of 0.00167 s−1 to very high strain rates of 315 s−1, 385 s−1, 445 s−1 which are far higher compared to the strain rates in the range of 0.0001-0.1 s−1. GRE exhibited about 106% and 67% improvement in modulus and strength at 445 s−1 compared to quasi-static conditions. EGCN exhibited about a 150% rise in modulus and 84% rise in strength at 1.5 wt% clay addition and 445 s−1 strain rate. This substantial rise in modulus and strength of EGCN is attributed to viscoelastic nature, damage accumulation behavior of epoxy which was also reported by Brown et al. [48] for GRE, restriction of polymers chain mobility in the matrix and at the fiber-matrix interface due to good adhesion between clay platelets and epoxy allowed better stress transfer to all the fibers. Similar findings were reported by many authors [49, 50]. Jeyakumar et al. [33] reported the mechanical properties of EGCNs with the addition of Cloisite 93A into epoxy-glass. Nanoclay was mixed into acetone using a mechanical stirrer for 30 min. Epoxy resin of the required weight was added to the acetone-clay mixture heated to 80°C and mixed for 1 hr. During this process, acetone gets evaporated and the epoxy clay mixture remains. The remaining mixture is ultrasonicated for uniform mixing.

#### *A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

The testing results of the prepared samples showed that the σ**UTS** improved by 6.6%, 16.6%, and 23.58% at 1, 3, and 5 wt% of nanoclay, whereas tensile modulus improved by 8.4%, 14%, and 23.66%. With the further addition of nanoclay, the decreasing trend started (shown in **Figure 2f**).

Achutha et al. [34] attempted to optimize the parameters such as nanoclay wt% and glass fiber content in EGCN. In addition, the samples were also subjected to hydrothermal conditions. A set of samples were soaked in cold water for 70 days and dried and another set of samples was boiled in hot water for 2 hrs and dried. It was reported from these studies that hydrothermal aging conditions showed 42.69% contribution to tensile properties whereas nanoclay content showed 24.57% contribution and fiber content showed 30.23% contribution. Achutha et al. [34] reported that nanoclay does not act as a load-bearing instrument but it warrants load transfer to fibers as the interface between matrix and fiber becomes strong, which also hinders crack propagation. The samples treated with cold water exhibited lower σ**UTS** and those treated with hot water exhibited the lowest strength due to the moisture absorbed at the interface which weakens the interface strength. Moisture absorption increases with temperature.

Prabhakar et al. [11] studied the effect of Nanomer I.28E on the mechanical properties of EGCN. In addition, the glass fiber was treated with silane and acid to check for the effects of both treatments. The results indicated that a combination of silanetreated glass fibers and Nanomer I.28E in the composite exhibited the highest σ**UTS** which was 130% compared to EGCN with untreated glass fiber and unmodified MMT particles. Prabhakar et al. [11] showed that any increase in an interfacial bond due to the addition of nanoclay led to increases in both σ**UTS** and hardness of the composite. The treatment of fibers and organic modifiers on MMT formed a strong interface.

#### **2.2 Flexural properties**

Haque et al. [26] reported 24% and 17% enhancement in flexural strength (σ**f**) and modulus at 1 wt% addition of Nanomer 1.28E in EGCN (shown **Figure 3a**). Kornmann et al. [17] reported 6% and 27% improvement in flexural modulus and σ**f** of EGCN at 10 wt% addition of ME-ODA (shown **Figure 3b**). The increase in σ**f** is linked to the existence of nano-silicate layers at the interface of the fiber, which might have improved interfacial properties. Another possible illustration is the fact that the compressive strength of epoxy is enhanced by the presence of the silicate layers so that it in turn enhances the bending strength of the laminate. Bozkurt et al. [18] reported 16% and 13% improvement in σ**f** and modulus at 6 wt% addition of OMMT (shown **Figure 3c**). It was observed from the fracture surface that fracture occurred along with the fiber-matrix interface and the fracture surface seems to have roughness indicating a strong interface. The laminate without clay showed a smooth fracture surface, which means the interface was weak. The increased flexural properties with the addition of various surface-modified nanoclays under different mixing conditions and making methods are given in **Table 1**.

Manfredi et al. [12] stated that the addition of Cloisite 10A in EGCN laminates caused the flexural modulus and strength to rise by 20% and 29%. The addition of Cloisite 30B did not cause any increment in the modulus of epoxy. It could be because of the collapse of clay particles, i.e. the particles were aggregated and the layers were not separated in the matrix. The modulus of clay nanoparticles is about 170 GPa (shown in **Figure 3d**). Therefore, when a strong bond is formed between matrix and clay it will result in an increased modulus of the laminate [51]. The increase in

bending strength is attributed to the presence of silicate layers upon the glass fiber surface which improves the adhesion between the interface of matrix and glass fibers. The other possible reason for the improvement in the bending strength of laminate could be the increase in compressive strength of the epoxy. Shi and Kanny [19] reported that EGCN showed about 23% and 14% enhancement in modulus and strength at 3 wt% of Cloisite 30B. This enhancement is ascribed to the presence of intercalated silicate platelets of clay which interrupted the molecular motion of epoxy [52, 53]. The composites consisting of matrix processed by HSMT have shown 9.7% and 8.5% improvement in strength and modulus at 1 wt% (shown **Figure 3e**).

Sharma et al. [30] observed improvement in σf up to 5 wt% addition of nanoclay (shown **Figure 3f**). This increment is attributed to the presence of layered silicates on the glass fiber surface enhancing the adhesive bond between the epoxy matrix and glass fibers. In the range of 6 – 8 wt% of OMMT σf is reduced which was attributed to the agglomeration of OMMT in the EGCN. The uniform distribution and dispersion of silicate layers in epoxy resin are limited by the weight content of OMMT, when this content exceeds its percolation threshold (the ability of the liquid resin to pass through clay particles so that all the particles get wetted) there is a tendency to form particle aggregates [27]. The increased viscosity hinders the dispersion and favors the formation of agglomerates [54]. The fracture surface of GRE has shown that the fibers pulled out from the matrix had a smooth surface texture, whereas EGCN showed less fiber pullout with rough surfaces of fiber and matrix indicating the strong bond between fiber and epoxy and improved stress transfer between fiber and matrix. At 8 wt% clay addition, there were agglomerates formed fully in the EGCN [55].

At 40% and 60% volume of glass fiber reinforcement into epoxy-clay matrix, there was about 20% and 8% improvement in σ**f** at 3 wt% of Nanomer I.30E (shown **Figure 3g**) [32]. This increment is attributed to the ability of the matrix to transfer the load to all the fibers. When nanoclay is not present in the matrix, it cannot transfer the load to all fibers, and thus crack propagates along with the matrix, and there will be low resistance to crack propagation. At low fiber volumes, i.e., at 40%, GRE exhibited interlaminar fracture as the crack propagated through the matrix between fiber layers and confined itself to layers near the top of the composite where the loading point is located so that the load was not transferred to all the layers, whereas EGCN exhibited translaminar fracture as the fiber layers break vertically at the loading point which requires more energy because the load is transferred to all the fiber layers [56].

With the increase in Vf of fiber to 60%, there was a reduction in the effect of nanoclay and both GRE and EGCN have failed predominantly in translaminar fracture mode which should occur only for EGCNs. This is because, at higher Vf of fibers, the fabric layers are well compacted to fit in the same volume of the composite, thereby the crimp zones present in the fabric will get interlocked with adjacent fabric layers, thus strengthening the interlaminar regions. Hence the crack propagation is resisted along interlaminar regions by the interlocked crimp zones and fracture occurs by rupture of glass fibers along the translaminar direction. These interlocks could resist interface shearing; thus, at higher Vf, crack propagation proceeds with the rupture of fiber fabric layers [32]. Srikanth I et al. [23] stated that At further higher fiber volumes, i.e., at >60%, fiber wetting became difficult, so there is a chance of failure by both mechanisms, i.e., interlaminar and translaminar crack propagation, thereby decreasing strength.

Jeyakumar et al. [33] stated that with the addition of Cloisite 93A into EGCN there was a significant improvement in flexural properties. With the addition of 1, 3, 5 wt% of Cloisite 93A, there was about 10.4%, 41.2%, and 52.3% increase in σ**f** and also

*A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

#### **Figure 3.**

*Changes in flexural properties of EGCN'S at various conditions.*

18.75%, 62.5%, and 118.75% improvement in flexural modulus. Beyond the 5 wt% addition of nanoclay, there was a decreasing trend (shown in **Figure 3h**). Najafi et al. [20] conducted experiments on EGCNs by adding pristine MMT and subjected some samples to hygrothermal conditions which consists of immersing the specimens in distilled water at 80°C for 10 weeks. The flexural curves for both neat GRE and EGCN exhibited linear behavior, EGCN subjected to hygrothermal conditions exhibited a gradual decrease in slope. At 3 and 5 wt% addition of MMT, there was about 8% and

12% improvement in flexural modulus, and 10.7% and 6.3% improvement in σ**f** was observed. At 3 wt% of MMT addition, the properties were optimum. The samples treated by hygrothermal conditioning exhibited very poor flexural properties due to decreased interface bond strength caused by water absorption. Prabhakar et al. [11] stated that the addition of silane treated glass fiber in epoxy has resulted in improved flexural properties due to enhanced interface bonding between fiber and matrix compared to the composite reinforced with untreated fiber. The addition of Pristine MMT and Nanomer I.28E has not shown any considerable improvement but rather reduced the σ**f**. There was about a 29% increase in σ**f** of epoxy-silane treated fiber composite compared to epoxy-untreated fiber composite.

#### **2.3 Fracture toughness**

At 1 and 2 wt% addition of Nanomer I.28E, there was about 28% and 32% improvement in fracture toughness of clay-epoxy nanocomposite compared to NE, whereas EGCN exhibited about 20 and 23% improvement in fracture toughness for the same clay contents compared to GRE. Above 5 wt%, there was a decreasing tendency (shown **Figure 4a**) [26]. In the single edge notch bending test conducted by Bozkurt et al. [18], at 10 wt% addition of OMMT, the KIC of EGCN improved by 5% but MMT did not show significant improvement (shown **Figure 4b**). The load applied is in the in-plane of the specimen. Therefore the fracture mechanism consisted of fiber-matrix debonding, fiber pullout, and fracture. The increased fracture toughness of the composites with the addition of various surface modified nanoclays under different mixing conditions and various making methods are given in **Table 1**.

Zulfli and Chow [27] stated that with the addition of nanoclay, KIC improved. This improvement was ascribed to the strengthening of the interface between fiber and matrix by the presence of OMMT at the interface and increased resistance to crack propagation because of OMMT [55]. Swaminathan and Shivakumar [21] stated

**Figure 4.**

*Changes in fracture toughness of EGCN'S under various conditions.*

#### *A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

that the major mechanism for increased toughness in composites was because of the deflection of the crack around clay tactoids. OMMT resists the crack from propagating because of which bowing and pinning of the crack take place [21]. The toughening effect of OMMT is limited by agglomeration. Tsai and Wu [22] reported a continuous decrease in Mode-I fracture toughness with the addition of nanoclay due to the brittleness induced in the composite which caused the crack to propagate at a faster rate, whereas pristine GRE composite exhibited ductile nature compared to EGCN with high clay content, so the crack propagation was slow and needed more energy for failure.

Jeyakumar et al. [33] reported that with the addition of Cloisite 93A into glassepoxy, there was a conspicuous increase in fracture toughness of EGCN. For neat epoxy it was 0.9 MPa-m1/2, for glass-epoxy it was 1.1 MPa-m1/2. At 1, 3, and 5 wt% addition of nanoclay, the increase in fracture toughness of EGCN was about 36%, 63%, and 86% respectively compared to GRE. Beyond 5 wt% addition, there was a decreasing tendency (shown in **Figure 4c**). Therefore, it was concluded that the saturation limit is 5 wt% of nanoclay for the experimental conditions adopted by Jeyakumar et al. [33]. Senthil Kumar et al. [24] reported that with the addition of Cloisite 25A in EGCN, there was a considerable improvement in Mode-I fracture toughness of EGCN. At 2, 4, 6, and 8 wt% addition of nanoclay, there was about 118.85%, 9%, 56.55%, and 38.5% improvement in fracture toughness. Beyond 8 wt% addition, there was a decreasing trend (shown in **Figure 4d**). The increase in fracture toughness is attributed to the fiber bridging effect. At 10 wt% addition of nanoclay there was a decrease in the property, which is ascribed to the poor distribution of matrix between the fiber laminas.

#### **2.4 Interlaminar shear strength (ILSS)**

ILSS is a matrix dependent property, which means the strengthening of the matrix improves ILSS because the interface between the epoxy-clay matrix and the glass fiber becomes strong [57]. Therefore if the ILSS of the matrix is enhanced, then the ILSS of the composite also will get enhanced. The increase in ILSS of the composite is owing to the enhanced interfacial area between matrix and clay, the enhanced bond between resin and fiber, and the improved morphology of the matrix. The failure in ILSS mode is acknowledged as a critical mode of failure in FRP laminates. Thus there is a necessity to study the ILSS characteristics of the nanocomposites. It is proved that the shear strength of FRPs is remarkably enhanced with the incorporation of nanoclays [36]. EGCN with 1 and 2 wt% added Nanomer I.28E had shown 44% and 20% improvement in ILSS compared to GRE. The rough interface between the epoxy-fiber in fracture surface indicates a strong bond, whereas GRE and NE have shown a smooth interface which implies a weaker interface bond (shown **Figure 5a**) [26]. The increased ILSS of the composites with the addition of various surface modified nanoclays under different mixing conditions and making methods are given in **Table 1**. Bozkurt et al. [18] reported a decrease in ILSS of EGCN with the addition of MMT and OMMT. The ILSS of GRE is noted to be 32.7 MPa. But when the clay is added, it is observed that the laminate with the addition of clay reports a small decrease than when MMT is added; the decrease is high when OMMT is added. This decreasing trend is attributed to the creation of air voids in the interlaminar region while making the composite. The susceptibility to form voids in the interlaminar region is observed to be more when OMMT was added and further study is required to establish this phenomenon.

**Figure 5.** *Changes in ILSS of EGCN'S at various conditions.*

The ILSS characteristics of GRE and EGCN with the addition of Cloisite 10A and Cloisite 30B were evaluated by Manfredi et al. [12]. There was a small increase of 7.5% in ILSS of EGCN with the addition of Cloisite 10A, but Cloisite 30B had no influence (shown **Figure 5b**). The trend of improvement with the addition of Cloisite 10A and decrease with the addition of Cloisite 30B was reported in the flexural properties section also. Laminates with Cloisite 10A have shown high flexural modulus and high σ**f**. The morphologies of the composites indicated that the addition of Cloisite 30B had not provided strong adherence between matrix and fiber, but Cloisite 10A provided strong bonding between matrix and fiber. There is also a high attraction between Cloisite 10A and glass fiber surfaces since both are ceramic materials. The matrix without clay has shown a smooth and brittle surface at failure, whereas the matrix with nanoclay addition has shown a rough surface at failure which is also in line with the impact characteristics [12]. EGCN showed an 18.5% improvement in ILSS with the addition of 1 wt% of nanoclay by the magnetic stirring method. Above 1 wt%, there was a decreasing trend which is attributed to the aggregates of silicate tactoids and voids, whereas EGCN consists of a matrix processed by HSMT exhibited a 24% increase in ILSS, which might be attributed to the high shear force, which resulted in good dispersion of nanoclay platelets (shown **Figure 5c**) [19].

Jeyakumar et al. [33] reported that with the addition of Cloisite 93A, the ILSS of EGCN improved notably. At 1, 3, and 5 wt% addition of nanoclay in EGCN, there was about an 8%, 16%, and 38% increase in ILSS (shown **Figure 5d**). The presence of nanoclay brought about strong adhesion amongst nanoclay and epoxy matrix and in this manner enhanced the shear properties of the composites. Beyond 5 wt% the

*A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

ILSS started decreasing which might be due to the non-uniform scattering of nanoclay. Anni et al. [57] stated that with the addition of organically modified nanoclay into woven flax fiber reinforced epoxy, there was a rise in ILSS. Before reinforcing the fibers, some flax fibers were washed in distilled water, some treated with alkali solution, some with saline solution, and some others treated with nanoclay dispersed solution, to graft the nanoclay particles onto the flax fibers. The improvement in ILSS with the addition of these four kinds of treated fibers in ILSS was observed to be 8%, 10%, 17.9% compared to the composite reinforced only with water treated fibers.

Senthil Kumar et al. [24] reported that with the addition of Cloisite 25A into EGCN there was a significant increment in the ILSS property of EGCN. There was an increasing trend in the property up to 2 wt% addition of nanoclay, after that, it started decreasing. At 2 wt% of nanoclay addition, there was about a 70% increase in ILSS of EGCN (shown **Figure 5e**). ILSS mainly depends on matrix behavior if the matrix is tough, the ILSS is increased. The addition of nanoclay makes the matrix tough because the crack propagation is hindered by the clay platelets and the stress distributed to the fibers will be uniform as the interface becomes stronger. At 10 wt% addition of nanoclay, the ILSS decreased by 3% compared to GRE. Lim et al. [58] showed that the geometry of the interface between epoxy-nanoclay platelets may also influence ILSS.

#### **2.5 Impact strength**

The impact strength of the composite depends mainly on the strength of the matrix and the ability of the fiber matrix to withstand the impact loads. At 5 wt% addition of Cloisite 10A, the EGCN has exhibited a 23% improvement in impact strength; this improvement is attributed to the creation of a complex path for the fracture propagation, as the layered silicate platelets hinder the extension of microcracks created in the matrix (shown **Figure 6a**) [59]. The increase in the strength of the fiber-matrix interface has decreased the resistance to impact force. Manfredi et al. [12] stated that the failure strength of EGCN depends on two factors, one being the tortuous path formed by clay platelets, and another being the fiber-matrix interface strength. The well-dispersed nanoclay platelets hinder crack propagation by diverting the crack to a longer path or splitting it into sub cracks that require more energy, whereas a strong fiber-matrix interface reduces the impact resistance. The laminates were made with low fiber content hence the properties of the laminate are mainly dependent on the matrix behavior. An improvement in the impact characteristics of the nanocomposite with no glass fiber reinforcement was observed. The enhancement in the impact characteristics was observed for laminates with nanocomposite matrix irrespective of the clay type [12].

Shi and Kanny [19] carried out an Izod impact test at a high strain rate to study the impact characteristics of EGCN. When the matrix incorporated into the laminate was processed by magnetic stirring, the impact strength of the laminate was noticed to be decreasing with the addition of Cloisite 30B. A sudden decrease in impact strength of 27% is observed for the laminate at 1 wt% clay; further addition of clay did not affect impact strength (shown **Figure 6b**). The sudden decrease at 1 wt% clay is attributed to the agglomeration and air voids in the matrix, Siddiqui et al. [60] addressed the same finding, whereas 44.9% improvement in the impact strength.

At 1 wt% nanoclay was observed when the laminate prepared was incorporated with a matrix processed by HSMT [19]. The changes in the impact strength of the composites with the addition of various surface modified nanoclays under different mixing conditions and making methods are given in **Table 1**.

#### **Figure 6.**

*Changes in impact strength of EGCN'S under various conditions.*

Zulfli and Chow [27] reported that the impact strength of the laminates with Nanomer 1.28E incorporated in the matrix exhibited a higher value compared to GRE. This improvement in the impact characteristics was ascribed to strong adhesion between Nanomer 1.28E and epoxy which implies that the resin has wetted all layers of the nanoclay particles. This, therefore, enhances the energy required to debond the fiber and matrix due to the strong bond. Yasmin et al. [49] stated that the enhanced impact strength of the laminate is because of the complex path for cracks to propagate through the matrix. The OMMT and glass fiber provides a synergistic increment to the impact characteristics. OMMT at the fiber matrix interface acts as an interfacial modifier while the stress transfer from the matrix to fiber gets enhanced through clay particles; thus as the clay content at the fiber matrix interface increases, higher stress levels can be taken by the composite because of which better characteristics were attained [40]. But the content of clay that can be added to the epoxy is limited by the agglomeration and air voids that are formed while mixing the clay into the resin. Rafiq Ahmad et al. [41] added Nanomer I.30E into EGCN to evaluate its effect on the impact strength of EGCN. The laminates were stroked with low-speed impact forces ranging between 10 and 50 J. The optimum property was obtained at 1.5 wt% of nanoclay addition with 23% improvement in the maximum load required to damage the specimen and 11% improvement in stiffness. Also, a notable decrease in physical damage was observed for EGCN compared to GRE (shown **Figure 6c**).

Najafi et al. [20] studied the effect of the addition of pristine MMT into EGCN on impact strength. To study the effect of hygrothermal aging, some EGCN specimens were immersed in distilled water at 80°C for 10 weeks. At 3 wt% nanoclay addition there was about a 7% increase in impact strength for EGCN. At 5 wt%, the impact strength reduced nearly by 5% compared to the value obtained at 3 wt%, and this decrease was attributed to agglomerates. Also, the brittleness of EGCN increased with the addition of nanoclay, causing the energy absorption to decrease [59]. The 3 wt% and 5 wt% nanoclay added EGCN subjected to hygrothermal conditioning exhibited a 7.23% and 10.47% decrease in impact strength compared to the control specimen which *A Review on Mechanical Properties of Epoxy-Glass Composites Reinforced with Nanoclay DOI: http://dx.doi.org/10.5772/intechopen.102159*

was dry GRE. The conditioned GRE exhibited about 13% decrease compared to dry GRE, whereas for 3 and 5 wt% added, conditioned EGCN exhibited about 14% and 8% increase compared to conditioned GRE (shown **Figure 6d**). In both dry and conditioned states, the 3 wt% added EGCN's exhibited good impact strength compared to the control specimen. Prabhakar et al. [11] stated that EGCN reinforced with acid treated glass fiber and MMT exhibited the highest impact strength out of all the composites made using neat glass fiber, silane treated glass fiber, and acid treated glass fiber, MMT, and Nanomer I.28E. Neat GRE exhibited the second highest impact strength value. The next highest impact strength was exhibited by EGCN with silane treated fiber and Nanomer I.28E. Compared to neat GRE the former one was 2% superior in property and the latter one is 2% inferior in the property. Prabhakar et al. [11] stated that a decrease in impact strength was compensated by an improvement in hardness of composites added with Nanomer I.28E and silane treated fiber, because the increase in hardness increases the brittleness, thereby reducing the energy absorption capability.

#### **3. Conclusion**

After reviewing the existing literature available on EGCNs reinforced with surface modified nanoclays, it is clear that the interfacial bond between reinforced fibers and the matrix is enhanced which resulted in enhancement in the mechanical properties of the composite. The enhanced fiber-matrix interface strength is due to good adhesion between clay platelets and epoxy allowing better stress transfer to all the fibers.

#### **Conflict of interest**

The authors have declared no conflict of interest.

#### **Author details**

Shanti Kiran Zhade1 , Syam Kumar Chokka<sup>2</sup> \*, V. Suresh Babu3 and K.V. Sai Srinadh3

1 RGUKT, Basar, India

2 Ellenki College of Engineering and Technology, India

3 NIT, Warangal, India

\*Address all correspondence to: chokka.syamkumar@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 3**

## Fiber Inclusions-Based Epoxy Composites and Their Applications

*Nassima Radouane and Abdelkrim Maaroufi*

#### **Abstract**

Because of their low cost, lightweight, easy production methods, and design flexibility, polymer-based composites are widely employed in a wide range of applications. Because of its high specific strength, superior mechanical characteristics, super adhesiveness, heat and solvent resistance, and so on, epoxy polymer or polyepoxide represent a significant majority of matrix composites. As a result, fiber fillers-reinforced epoxy resin composites have been investigated for a variety of applications, including high-tech in the ballistic, aircraft, automobile, construction, and sports sectors. In this chapter, the manufacturing procedures of fiber-reinforced epoxy composites have been described. Different categories of fiber are used as fillers in an epoxy matrix and their morphology is discussed as a function of the obtained properties.

**Keywords:** carbon fiber, glass fiber, plant fiber, epoxy composites, applications

#### **1. Introduction**

In recent decades, our societies have been confronted with climatic disturbances and resource use, leading to the degradation of ecosystems. In order to combat these threatening changes, the international community is committed to finding new ways of producing and creating value, including light-weighting structures and the valorization of lignocellulosic biomass as possible solutions towards sustainable innovation [1–3]. Indeed, light-weighting implies a reduction in production energy, raw materials produced and materials to be managed at the end of life [4, 5]. The reduction in mass also leads to a reduction in the energy consumption of means of transport and their emissions of pollutants. For these reasons, sandwich structures are increasingly used instead of monolithic structures in various applications, thanks to their lightness, their mechanical performance in bending and their thermal, vibratory, and acoustic features.

Because of their good features, they may be found in vital industries such as aeronautics, automotive, sports, marine, and construction. These properties include high mechanical strength and stiffness, high-impact resistance, low weight, corrosion resistance, and low maintenance costs [6]. Traditionally, composite materials are reinforced with synthetic fibers such as glass, carbon, aramid, or ceramic fibers. These fibers are used because of their strength, stiffness, low moisture absorption,

and good compatibility with polymer resins. Glass fibers are the most commonly used because of their low cost, ease of production, and specific mechanical characteristics.

The epoxy matrix combined with rigid fiber allows for the creation of building materials with high stiffness and strength. Given the variety of technical and material options, developing a composite material necessitates taking into account the chemical and physical interactions between all components [7]. As a result, the effects of production processes, fiber reinforcement type, and reactive or nonreactive modifiers on the characteristics of epoxy composites remain intriguing study issues. Many different types of synthetic and natural fibers are used to strengthen the epoxy matrix, including glass, carbon, basalt, aramid, ramie, hemp, jute, and flax [8].

In this chapter, a detailed description of epoxy polymer was represented. Moreover, various fiber types such as glass, carbon, and plant materials. In addition, some fabrication procedures of epoxy reinforced fiber composite are reported. Furthermore, a representation of some applications was described as well as the coming challenges.

#### **2. Thermosetting organic matrix: Epoxy resin**

Epoxy resin is a thermosetting polymer. It comprises two parts: an epoxy base catalyst and an amine-containing hardener (-NH2 or -NH). During cross-linking, each hydrogen atom in the amine group opens the epoxy ring and produces a polymer chain (**Figure 1**) [9]. The glass transition temperature denoted T, increases with the rate of crosslinking. Thanks to its 3D polymeric structure and high phase change temperature, epoxy phase change temperature, epoxy achieves good mechanical and thermal properties [10].

**Table 1** shows the advantages and disadvantages of epoxy thermoset resin. Compared to thermoplastic resins, epoxy resins are more brittle on impact due to their susceptibility to cracking. According to Vieille et al. [12], the impact response of thermoset matrix composites has some weak points:

#### **Figure 1.**

*Main chemical reactions taking place during the curing of an epoxy resin.*


#### **Table 1.**

*Advantages and disadvantages of epoxy resin [11].*


Moreover, a high brittleness of this family of resins is also the cause of the pseudoplastic behavior of the composite. Upon impact, the opening of intralaminar and interlaminar cracks is triggered. At the same time, epoxy debris forms and blocks the closure of the cracks after impact, which is impact, which is unfavorable to the impact resistance of the composite [13].

#### **3. Fiber materials and types**

Composite materials are categorized based on their content, which includes the base material (matrix) and the filler material. A matrix or binder material is the basic material that binds or retains the filler material in structures, whereas filler material is present in the form of sheets, pieces, particles, fibers, or whiskers of natural or synthetic material. Composite based fibers are categorized into three major groups based on their structure, as shown in **Figure 2**.

#### **3.1 Epoxy resins reinforced with glass fibers**

Fiber or particulate inclusions in epoxy matrix with different types and shapes are studied by many researchers to characterize their mechanical, electrical, thermal

#### **Figure 2.** *Composites structure types.*

and so on properties [14–17]. Glass fibers are the most often used synthetic fibers because of their high strength and durability, thermal stability, impact resistance, chemical, friction, and wear qualities. However, machining glass fiber-reinforced polymers (GFRPs) using traditional machining techniques is generally slow, difficult, and results in lower tool life [18]. They are easily made from raw material, which is readily available in an almost limitless supply. There are numerous types of GFs that are often utilized in GFRP composites, depending on the raw materials used and their quantities in fabrication (see **Figure 3**). GFs also have the disadvantage of being disposed of at the end of their useful life [19]. Glass fiber reinforced polymer

**Figure 3.** *Glass classification.*

*Fiber Inclusions-Based Epoxy Composites and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.104118*


**Table 2.**

*Example of composites-based epoxy reinforced glass fibers.*

composites were created using various production technologies and are widely employed in a variety of applications [20]. Because of their superior mechanical qualities, glass fiber reinforced composites have received more attention in recent years. Glass fibers have excellent features such as high strength, flexibility, stiffness, durability, and so on. The characteristics of GFRP composites improved when the amount of glass fiber was increased. The mechanical and thermal properties of different polymer composites reinforced with glass fiber when exposed to mechanical stress are been listed in the following **Table 2**.

#### **3.2 Epoxy resins reinforced with carbon fibers**

Carbon fibers were first used in 1880 by T. Edison as a filament in lamps. From 1960 onwards, research was directed towards the development of high modulus and high strength carbon fibers. The carbon fibers are more required in applications that need more stiffness. Carbon fiber-reinforced polymer (CFRP) composites have extensive uses in aircraft, automotive, sports, and a variety of other sectors. In the literature, many other fillers type such as particulate and fiber fillers [27, 28]. In general, carbon fibers can be categorized by their mechanical properties, manufacturing methods, application field, precursor, fiber materials, final heat treatment temperature, and their function.

#### **3.3 Epoxy resins reinforced with plant fibers**

Nowadays, industrial businesses are concentrating on providing environmentally friendly products, and the globe is moving towards sustainable development. Because of their biodegradability, natural fibers are employed in the production of such eco-friendly products. The key causes influencing the rising use of natural fiberreinforced composites are increased awareness of concerns such as pollution, waste of raw materials and energy, and depletion of petroleum reserves (FRC). Due to their lightness, mechanical performance, ability to integrate functions, physical–chemical resistance, and ease of processing, composite materials have made considerable progress in terms of volume and have dominated practically all sectors. Such as wood fiber which are transformed using the steam explosion process and are treated at various steam pressures. Because of the increased explosion pressure, the fiber's affinity for water, mechanical characteristics, and dissolving ability in caustic solution diminish after the steam explosion [29].

#### **4. Fabrication methods**

The preferred procedure is determined mostly by the resin used, the length of the fibers, the required qualities of the composite material, and the production run and rate (**Figure 4**) [30].


**Figure 4.** *Different procedures of fabrication for polymer composites-based fibers.*

*Fiber Inclusions-Based Epoxy Composites and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.104118*


According to studies, the constraints of each technique and the production parameters employed during composite processing might induce undesired internal flaws into the material, such as bubbles or cavities, poor or rich areas, delamination, shrinkage, and so on [30]. As a result, these flaws can compromise the mechanical characteristics of composite materials. Several investigations on carbon and glass reinforced composites have been conducted. Liu et al. investigated the influence of autoclave pressure cycling on the porosity of a [0/90]3 s carbon/epoxy cross-linked composite. In comparison to tensile strength and modulus, they demonstrated a considerable sensitivity to porosity in the interlaminar shear strength and flexural parameters of the composites. They discovered that when the porosity inside the composites is less than 4%, the interlaminar shear strength reduces by roughly 8% for every 1% increase in porosity [31]. Gu et al. investigated void formation by transforming hygroscopic water absorbed by glass and carbon fibers, as well as trapped air, into vapor bubbles as a result of the temperature rise during the thermocompression process [6]. Compared to synthetic fibers, the problems associated with the processing of plant fiber composites are more complicated due to the particular characteristics of this type of fiber. The use of plant fibers for resin reinforcement necessitates careful consideration of the production conditions. The essential criteria for regulating the thermal deterioration of the fibers are the process temperature and time. To reduce viscosity, the hot-molding temperature must be higher than the melting point of the resin, and the time must be long enough to allow the molten resin to permeate the fibers, assuring good adhesion between the reinforcement and the matrix. In conflict with these needs, the melting temperature and time should be as low as possible to slow down the thermal deformations that occur and cause fissures and permanent damage to the fibers, as well as the pectin breakdown, which begins at 180°C [32, 33].

#### **5. Applications**

E-glass fibers are commonly used as reinforcements in shipbuilding, while carbon fibers saturated with epoxy resin are commonly used in aeronautics. The use of synthetic fibers in composites is supported by their high chemical resistance, compatibility with most impregnation resins, and mechanical and thermal performance. However, the usage of this sort of reinforcement is no longer adequate: On the one hand, their comparatively large density penalizes them; on the other hand, they endanger the health and the environment. Since the 1980s, these environmental problems have become a major concern for our society and the media. And since then, the industrial optimization of eco-composites is booming thanks to their high specific mechanical properties. The limits of applications are constantly being pushed back through the development of fiber preforms and the adaptation of processing methods.

#### **5.1 Automobile**

Automobile body sections, such as engine hoods, dashboards, and storage tanks, are made using natural fiber reinforcements such as flax, hemp, jute, sisal, and ramie. The VARTM manufacturing technology was used for these composite constructions, and its liability was tested through structural testing and impact stress analysis. As a consequence, the material's weight was reduced while its stability and strength were improved. The increase in safety characteristics was tested using the head impact criterion (HIC), and it was discovered that composite constructions with natural fiber reinforcements are appropriate for automotive body sections [34–36]. **Figure 5** depicts the external body elements of a Volkswagen x11 crazy carbon fiber replica.

The automobile sector, in particular, has shown a genuine commitment to economic and environmental concerns by using natural fibers in different non-structural components (dashboards, door panels, spare wheel covers, etc.) with the goals of lowering mass, fuel consumption, and emissions (**Figure 5**).

#### **5.2 Aerospace**

Fiber-reinforced epoxy composites manifest the properties required for aircraft interior panels, such as resistance to heat and flame and disposal of materials. Fiberreinforced epoxy composite shows a variety of applications in the aerospace industry

#### **Figure 5.**

*Volkswagen xl1 carbon fiber body pieces, adapted from [36] under a creative commons license. (a) the 45 kg of natural fibers in a Mercedes S-class. (b) Car door panel made of natural fibers. (c) Spare wheel cover made of natural fibers.*

#### *Fiber Inclusions-Based Epoxy Composites and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.104118*

due to its superior mechanical properties and lightweight structure. Conductive fibers in the layer of fiber composite structure eliminate the requirement of separate wires for transceivers of communication devices. High stiffness with a lower coefficient of thermal expansion is achieved when P100 graphite fibers diffused in 6061 aluminum matrix composite material are employed to the high gain antenna of the Hubble space telescope [37].

For example, the wing of the plane is a composite material, the fiber is carbon fiber and the resin is epoxy. The manufacturing technique involves resin infusion: all the reinforcing fibers are dried, shaped and then the resin is infused into the reinforcement. The choice of polymer matrix must both ensure good performance for the finished wing after curing and also maintain a well-tuned reactivity, not too high to allow the wing to be infused, which can take several hours, but enough to allow the reactions to take place effectively.

#### **5.3 Marine**

Components and structures functioning in the marine environment are subjected to significant stresses caused by wind, waves, and tides. Furthermore, they must endure hostile and harsh environmental conditions throughout their lives, including being placed in the splash zone if not submerged in seawater. The use of polymer composites in maritime systems has been the subject of much research in recent decades, showing the potential benefits of replacing various components such as ship hulls, propeller blades, wind, and tidal turbine blades, to name a few [38]. For example, in the offshore construction (seawater piping, stairways and walkways, firewater piping, grating, fire and blast walls, cables and ropes, storage vessels, and so on), valves and strainers, fans and blowers, propeller vanes, gear cases, condenser shells, and so on.

And more other applications, which we will not be able to represent all of them such as:


#### **6. Challenges**

Understanding the significant material characteristics of fiber/epoxy constituents, as well as the fundamental structures and availability of production technologies, is required for the use of fiber/epoxy composites in a range of applications.

Furthermore, the manufacturing technique used has an effect on the ultimate qualities of the material. The cost of materials is influenced by production volume the bigger the volume of production, the lower the cost of materials. In the instance of the car industry, increasing production volume increases the risk of investing in raw materials while building manufacturing set-up based on production rate and cycle time. In addition, the product's design complexity increases the cycle time, decreasing the manufacturing pace.

### **7. Conclusions**

On fiber-reinforced composite manufacturing, current progress, novel advancements, and future research prospects are summarized and presented. However, the ongoing demand for composite constructions necessitates a large consumption of environmentally hazardous components. Certain fibers (for example, carbon fibers) utilized to improve qualities in numerous sectors are a significant hindrance to recycling at the end of the composites' life. As a result, the current environmental crisis, which has reached a tipping point, necessitates immediate and objective action to cut greenhouse gas emissions. As a result, obtaining advanced composites from renewable energy resources would be the best ecological answer. Furthermore, future research areas might focus on recycling current composites into high-value alternative goods. Furthermore, new innovative methods for post-consumer waste treatment must be developed. Additionally, new sophisticated technologies for post-consumer waste treatment must be developed, or existing FRP composite production technologies must be improved.

### **Conflict of interest**

The authors declare no conflict of interest.

*Fiber Inclusions-Based Epoxy Composites and Their Applications DOI: http://dx.doi.org/10.5772/intechopen.104118*

#### **Author details**

Nassima Radouane1,2\* and Abdelkrim Maaroufi2

1 UDSMM (EA 4476), MREI-1, Université du Littoral Côte d'Opale, Dunkerque, France

2 Laboratory of Composite Materials, Polymers and Environment, Department of Chemistry, Faculty of Sciences, University of Mohammed V, Rabat, Morocco

\*Address all correspondence to: nassima.radouane@univ-littoral.fr

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 4**

## Synthesis and Properties of Epoxy-Based Composites

*Srikanta Moharana and Bibhuti B. Sahu*

#### **Abstract**

Epoxy-based composites are of great interest among academic and industrial researchers owing to their low cost, superior mechanical properties, large specific strength, super adhesiveness with good thermal and solvent resistance in recent times. However, the effect of carbon-based nanofiller reinforced epoxy composites is of prime focus due to their significant mechanical, dielectric and electrical performances for technological applications in broad fields of nanoscience and technology. There is a greater influence on the properties of the nanofiller reinforced epoxy matrix composites depending on the concentration of various types of nanofillers. The processing techniques play a crucial role in the prediction of attractive and suitable properties of the various nanofiller reinforced epoxy composites. There are several processing methods that have been employed to accomplish a superior degree of dispersion of nanofillers in the epoxy matrix. This current chapter portrays the simultaneous focus on their preparation techniques and effect of the dielectric, electrical and mechanical properties of various carbon nanofillers (such as fullerene, carbon nanotubes (CNTs), carbon nanofibers (CNFs) & graphene) filled epoxy resin composites for a broad spectrum of technological applications. We hope this chapter will facilitate the concrete in-depth ideas to the readers on the progress of various synthesis techniques and properties of different nanofiller reinforced epoxy composite systems.

**Keywords:** epoxy, nanofiller, carbon nanotubes, graphene, composites, mechanical, dielectric, electrical, properties

#### **1. Introduction**

The emergence of new technological fields is associated with the development of new hybrid polymeric composite materials with high-performance practical applications. These composite materials have several interesting multifunctional properties including superior strength, high stiffness or modulus of elasticity, durability, corrosion resistance, better thermal stability, enhanced electrical and electronic properties, lightweight with highest specific stiffness and strength along the direction of the reinforcing fiber, dimensional stability, good temperature, chemical resistance, flex performance and ease of processing with cost-effectiveness in contrast to other types of material [1–3]. However, there are different composite systems, which are used in the field of manufacturing technology. Among, epoxides, phenolics, polyurethane

and polyimides are commonly used as the matrix of the materials for the progress of advanced hybrid composite materials. The epoxy resin is one of the significant polymeric materials among academic and industrial researchers due to its remarkable versatile properties like thermal, electrical, mechanical performances, superior thermal and chemical resistance [4–7]. The thermoset resin such as epoxy is currently in wide use for the composite industry. This is because of their significant chemical, corrosion resistance, excellent adhesion performance, low shrinkage and lesser price with their challenging applications. Generally, the cured epoxy gives rise to large modulus, strength, good resistance to creep and high performance at elevated temperature due to its extreme cross-linked microstructure. The improved toughness of cured epoxy resin is commonly advantageous owing to its poor ductility. The betterment of toughness is a proficient approach for incorporating rigid or reactive rubbery particles into the epoxy network. It is the consequence at the expense of glass transition temperature and strength of this polymeric matrix [8–10]. Epoxy is intensely associated with daily life in the form of packaging, coating, adhesive and electrical insulating materials as well as applications in electrical appliances, semi-conductors, etc. [10, 11].

In the descended decade's, aluminum is one of the common metals around the globe, which is about 8% of abundance on earth's crust and seen in the form of oxides including karolinite, bauxite, nepheline and alunite. Sir Humprey Davy in the year 1808 revealed the existence of aluminum and further Oersted in 1825 formed its tiny pellets. However, Wohler a German scientist (1845) verified the specific gravity and aluminum lightness. They also discovered certain performances such as ease of deformation, air stability and its melting with a blow torch. The metals are characterized by high corrosion resistance, superb machining performances, superior thermal and electrical properties, large ductility, low strength, hardness and wear resistance [12, 13]. Due to high corrosion resistance and its lightness, aluminum and its alloy based materials played a vital role for the production of equipment (panels, roofs and frames) of packaging materials in the area of food and transportation (vehicle and aircraft parts) [13, 14]. Due to the broad spectrum of attractive properties and potential applicability of aluminum academic and industrial researchers have converged their focus on increasing the strength of aluminum and its hardness through solid solution and hardening. Also, they have progressed the aluminum-based metal matrix composite materials by the reinforcement of various fillers in the matrix of aluminum [12–14]. Recently, various researchers are putting their efforts to attempt for developing appropriate materials using aluminum-based alloys for end used applicability in the field of aerospace application, cast aluminum engine is used on flier, the manufacture of wind ribs (aluminum 2050). The high static strength of aluminum 7079 and 7075 are used to give sufficient toughness and corrosion resistances [3, 15, 16]. The composite of aluminum-graphite shows superior thermal conductivity as a result of the appreciable contribution coming out of the metal matrix. The aluminum-based metal matrix composite systems preserve the advantageous properties of both the reinforcement and matrix by associating the vital strength of the reinforcement with the ductility of the aluminum matrix [17, 18]. Many researchers have reported about the synthesis of carbon black powders by using agricultural byproducts including coconut shell, apricot stones, sugarcane, bagasse, nutshells, tobacco stems and forest residues. It is observed that the coconut shell shows costly disposal and cause environmental problem. Thus, they adopted an appropriate technique for the synthesis of carbon black using coconut shell owing to their superior natural structure and small ash contents *via* pyrolysis route (carbonization of coconut shell) with the application of temperature ranging from 550°C to 900°C. [3, 19–22]. However, activated carbon

#### *Synthesis and Properties of Epoxy-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.104119*

developed through the conversion of coconut shell can be used as filler in processing the composites, which have potential utility in significant adsorbent for purification of water or industrial treatment and municipal effluents. The addition of these filler may also diminish the cost for waste disposal with a cheap alternative as compared to the commercial carbons [3, 21–24]. The byproducts (barley husk and coconut shell) reinforced thermoplastic is a better alternative for wood fibre-based hybrid composite materials. The experimental results revealed that barley husk and coconut shell are thermally stable at high temperature than that of soft wood fiber with different percentages of cellulose content (50% barley husk and 34% coconut shell) [25, 26]. Both coconut shell and barley husk are of large carbon-rich layers on their surface as compared to soft wood fiber. The superior tensile strength is observed in the barley husk fiber reinforced composite than that of the soft wood fibre-based composite. Moreover, the coconut shell and barley husk reinforced composite exhibited 80% and 40% improved elongation at break, 20 and 35% superior impact strength as compared to soft wood fiber composite systems [22, 27, 28].

The synthesis of composites is the combination of two materials such as matrix and reinforcement to form a hybrid composite material with excellent electrical and mechanical performances. The matrix is in the bulk form, which employs reinforcement with a strong bond [29–31]. However, the reinforcement is normally embedding additives for enhancing the properties of the material. The thermoplastic and thermosetting polymers are usually used as a matrix in the polymer composites. On the other hand, thermosetting polymer matrix exhibits better stiffness and superior strength than that of the thermoplastic polymeric materials [32]. Epoxy resin is one of the most commonly used thermosetting polymer matrix with better mechanical performances and good adhesive property with the incorporation of reinforcement particles [33, 34]. Thus, epoxy resin reinforced filler-based composites have immense interest among scientist, researchers for the development of hybrid composites for their use in the field of composite manufacturing industries, automobiles, paints and coating industries [35, 36]. The epoxy-based materials have common shortcomings including low impact strength and weak wear resistance, which can be overcome by selecting proper reinforcement of filler particles in the matrix; this will be useful in the field of tribological application. In epoxy-based composites, the epoxy reinforced nanoparticles show the large surface area with substantial interaction between matrix and filler particles [35–39]. However, nanocomposites are generally lightweight than that of the micro composites due to the relatively high density of the micro-additive fillers [40]. The enhancement of wear resistance and mechanical performances is due to the incorporation of hard oxide and carbide-based nanoparticles (including silicon oxide, alumina and tungsten) into the matrix [41–43]. A significant technique for the preparation of nanocomposites is needed for the homogeneous dispersion of the nanoparticles reinforced epoxy matrix *via* sonication technique [44]. The most crucial factors which influence the properties of the composites are curing conditions, molecular bonding between reinforcement and epoxy matrix with ratio of curing agent [45, 46]. There are various researchers have made to synthesize epoxy resin reinforced nano-filler-based hybrid composites for improving the mechanical and electrical performance of the composite systems. The different nanofillers (such as alumina, fullerene, graphene, carbon nanotubes (CNTs), carbon fibers (CFs), etc.) reinforced epoxy matrix composites result superior thermal conductivity, large thermal stability and better wearable resistance [44–47].

This chapter is organized on the basis of various filler-based nanomaterials used in epoxy resin composites, which includes carbon-based nanomaterials, fullerene,

graphene, CNTs, CFs, nanoclay reinforced epoxy resin composites have been presented in minute details in Section I. This section comprises of a brief introduction of the specific filler materials and its effect on epoxy resin followed by detailed discussion on the filler reinforced epoxy composite systems. Section II portrays the vivid insight into the synthesis techniques of carbon-based nano-filler reinforced epoxy composites. In Section III, we have especially emphasized on dielectric and mechanical properties of different filler reinforced epoxy resin composite systems.

### **2. Different carbon based nanofillers reinforced epoxy composites**

Carbon-based epoxy composites are considered to be one of the most promising groups among the advanced materials of current times due to their distinctive physical and chemical properties. These materials exhibit significant properties as a result of the introduction of nanofillers into the matrix materials gives rise to unexpected properties, which make them distinguished due to unparallel design possibilities. In this context, Kroto et al. [48] in 1985 have discovered fullerene, which is the allotrope of carbon, where in fullerene the molecules of carbon atoms are well connected by single and double bonds. The family of fullerene is then extended with other forms of synthesis of carbon-based nanomaterials like single-walled carbon nanotubes (SWCNTS) (1991), multiwalled carbon nanotubes (MWCNTs) (1993) and graphene

*Carbon-based nanoparticles (a) graphene, (b) fullerene, (c) SWCNTs and (d) MWCNTs. Reprinted with permission from Ref. [49].*

(2004) (as shown in **Figure 1**) have gained immense attention to the scientists for further research [49–55].

The carbon-based graphene nanofillers [49, 53–55] play an important role for the synthesis of composites in the field of nanoscience and technology. A single layer carbon sheet of graphene with sp2 hybridization (two dimensional) is arranged in a hexagonally packed lattice structure analogous to a honey comb. They have unique performances including high charge mobility at room temperature, high surface area, good optical transparency, large young's modulus and superior conductivity. However, it is observed that fullerene may be considered as fascinating reinforcements in comparison to CNT or graphene due to their zero-dimensional structure of carbon molecules, which exist in the form of spherical, tube shape and ellipsoid. The fullerene related to C60 is called Bucky ball or Buckminster fullerene spherical in shape, which corresponds to a soccer ball [49, 54–56]. The carbon nanotubes (CNTs) [57–61] a new allotrope of carbon is normally thin hollow cylindrical fullerene structure in the diameter of nano-scale range and little micron length with significant properties. Recently, CNTs are considered to be the most talented candidates as reinforcement for polymer composites due to their high aspect ratio, high young's modulus, tensile strength, large thermal conductivity and approximately 1000 times larger than that of the electrical capacity than copper and thermally stable in vacuum at 2800°C [60, 61].

According to the number of concentric layers of carbon atoms, CNT is available at single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), which shows multiple SWCNTs arranged in a concentric and coaxial manner as well as more flexible in nature than that of the MWCNTs. Moreover, CNTs appeared as outstanding materials and can be used as conducting nanofillers in the polymer matrix to give high-performance composites. Similarly, one dimensional carbon nanofibers (CNFs) [8] show a hollow cylindrical structure with lower cost and ease of processing than that of the pure CNTs. However, the carbon-based materials with a diameter less than 500 nm, do not exhibit structure of CNT falls into the class of CNFs and CNT, which exhibits clear structure as compared to CNFs. In this section, there are various studies have been made to achieve significant properties in epoxy-based carbon nanofillers composites [8, 57–61]. For instance, Kim et al. [62] have reported surface modified epoxy-based CNTs composites with superior homogeneity of CNTs in the epoxy matrix. They also studied their effect on the rheological and mechanical performances of the resultant composites. The multiwalled carbon nanotubes (MWCNTs) reinforced epoxy resin composites are of uniform dispersion in the matrix through the ultrasonication technique. The synthesized MWCNTs reinforced epoxy composites have superior Young's modulus and strength with optimized parameters were reported by Montazeri and his co-workers [63]. Allaoui et al. [64] have fabricated MWCNTs based rubber epoxy matrix composites with different concentrations of MWCNTs contents. It is observed that the resultant composite system has enhanced electrical performance with the increase of filler concentration in the epoxy matrix. However, the lower weight percentage (0.5 wt%) of MWCNTs reinforced epoxy composites exhibits a considerable increase of tensile strength and Young's modulus were reported by Montazeri and his group [65]. The incorporation of MWCNTs into the epoxy composites showed significant improvement of the thermal and mechanical performances *via* oxidation, acylation and amidation. It is also noticed that the enhancement of the electrical conductivity of the silane modified MWCNT-epoxy composites as compared to unmodified MWCNT-epoxy composites has been reported by Shen et al. [66]. Similarly, Choi et al. [67] have studied

the effect of silver-plated MWCNTs into the epoxy matrix. It is also noticed that the synthesized composites are of improved thermal conductivity with the increase of filler contents and time duration.

#### **2.1 Fullerene reinforced epoxy composites**

The contemporary demand for the development of lightweight hybrid composite materials with high specific strength, stiffness and improved tribological performance for end used applicability in the areas of aerospace and automotive industries [49, 56, 68]. During the past few times, fullerene and fullerene-based composites have been synthesized and used in various applications particularly in the thin films, organic polymers and hybrid organic-inorganic composites in the field of microelectronics [68]. There are various established techniques for the preparation of fullerene-based hybrid composites including solid-state reaction technique, liquid state techniques, deposition and spraying technique [56]. Rafiee et al. [69] have fabricated fullerene reinforced epoxy composites with various concentrations of filler content. It is revealed that the resultant composites with Young's modulus, fracture toughness, ultimate tensile strength were significantly improved in the epoxy matrix. It is reported that the incorporation of fullerene into the epoxy matrix shows superior properties as compared to neat nanosilica, nano-alumina and nano-titania filled epoxy composites; this may be due to the hollow structure, which results in increasing surface area and reduced weight simultaneously. However, the dispersion of fullerene into polymer matrix is easier than that of the other carbon based nanofillers which includes CNTs and CNFs, etc. The 1D and 2D filler-based materials are more prone to entanglement to each other than spherical fullerene particles. There are various research works related to fullerene modified polymer composites were reported by Ayesha et al. [70, 71]. However, the reports have been made based on fullerene modified epoxy-fiber composites. For example, fullerene modified epoxy carbon fiber reinforced polymer composites were fabricated and studied their effect of various mechanical performance of fullerene concentrations in the matrix. It is observed that 0.5% of epoxy matrix composites have improved interlaminar fracture toughness (60%) and also enhanced tension and compression up to 12% were reported by Ogasawara et al. [72]. Similarly, Jiang et al. [73] have reported about fullerene modified epoxy (1–3% concentration) reinforced fiber composites show improvement in the bonding strength between unidirectional carbon fiber and matrix. This improvement may be due to the suppression of fracture at interfacial layers of fibers, which is attributed to the presence of fullerene nanoparticles in the polymer matrix.

#### **2.2 Carbon nanotube and carbon nanofiber reinforced epoxy composites**

Several techniques including chemical vapor deposition (CVD), arc discharge and laser ablation techniques have been used to synthesize carbon nanotubes and CNT reinforced polymer composites in the past few decades [74–84]. The improvement of physical properties of the epoxy composites by the incorporation of different carbon-based nanofillers was reported by Liu et al. [85]. One of the key problems for the preparation of carbon nanotubes-epoxy composite system is the agglomeration or aggregation in the matrix. Thus, various techniques and studies related to CNT-based epoxy composites have been adopted to enhance the better dispersion and reduction of agglomeration or aggregation in the matrix [86, 87]. Moreover, the direct use of CNT in the epoxy matrix without surface treatment may give a

#### *Synthesis and Properties of Epoxy-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.104119*

marginal improvement in the composites. Besides, the unmodified CNT embedded into the epoxy resin with one dimensional structure forms poor bonding, which is the high tendency to give entanglement with each other results in some problems. The structural performance is enhanced with the application of strong acids to convert the C**–**C bonds into various functional groups of amine, amide, etc. on the side surface of the CNT *via* plasma treatment or UV/ozone treatments [88]. In addition, the attachment of CNTs into the metal particles with enhanced homogeneity and interfacial adhesion results for improving the structural properties as compared to neat CNT-epoxy composites [89]. Similarly, carbon nanofibers are also creating some difficulties in uniform dispersion of these particles in the epoxy matrix owing to their structure. There are various physical (sonication, mechanical stirring, plasma treatment, high-temperature heat treatment) and chemical (surface functionalization through modifying agents, surfactants, etc.) methods are employed to synthesize homogeneous dispersion of CNF in the epoxy matrix. It is also observed that, using these techniques for preparation may affect the properties of the bulk composites. For example, the high-temperature heating technique for preparation of composites may reduce the interfacial strength, this lead to decrease the structural properties [90–93]. The high concentration of filler in the polymeric matrix may affect the final properties of the composites. To overcome this shortcoming, the incorporation of lower filler loading into the matrix gives better homogeneity as well as enhance structural properties of the composite systems. Moreover, for the utilities of CNTs various optimized parameters are essential to achieve better homogeneity, desired orientation and functionalization to extract the utmost benefits of this marvel material.

#### **2.3 Graphene reinforced epoxy composites**

The two-dimensional macromolecule graphene and its derivative have extensively been explored due to their application as nanofillers in graphene reinforced composites. The production of nanofillers must be attainable at a large scale with low cost for the progress of graphitic fillers from natural graphite, which make them suitable nano-filler-based materials for reinforcement in the field of composites *via* simple processing technique. These graphene-based composites are of immense interest among academic and industrial researchers owing to the excellent electrical, thermal and mechanical performance of graphene [49, 54, 55, 94–97]. However, there are various nanofillers including graphene nano platelets or sheets (GNPs or GNs) and graphene oxide (GO) shows additional flexibility at the nano-scale due to the stability of macromolecule and multitude of alternatives for more functionalization for the composite systems. An enormous modified forms of carbon fiber and polymer phase of carbon fiber reinforced composites (CFRC) are found in various literatures. The ultrafine GNP has been utilized to exfoliate graphite flake producing graphite nanoplatelets (GNP), on the other hand graphene oxide (GO) *via* modified hummer's technique exhibits oxidation followed by exfoliation of bulk graphite [96, 97]. Graphite is of strong planar structure with each carbon atom forming three covalent bonds with adjacent atoms. Whereas graphene oxide (GO) contains different functional groups (hydroxyl, epoxy and carboxylic acid groups) joined through sp3 hybridized carbon atom but it may partially retain a number of sp2 hybridized carbon depending on their reaction conditions. Moreover, it is also observed that the presence of polar functional group present in the surface of the graphene oxide, which results in bonding with polymer matrix to form strong interfaces. It is reported that the significant properties of graphene play an important role in the scientific research communities to develop

graphene-based epoxy matrix for the application in structural composites. Besides, these fillers are applied for additional needs to develop composites with attractive properties like thermal, electrical conductivity and mechanical strength [98–101]. Alexopoulo et al. [102] reported on the fabrication of epoxy-based GNP composite systems and studied their various properties on the effect of size on the GNP particle embedded into the epoxy matrix. It has also been noticed that the synthesized composites show the formation of agglomeration at higher concentrations (>5 wt%) within the matrix. Further, at lower concentration (0.25 wt%) of GNs, it is revealed that there is considerable improvement in properties (toughness, flexural strength and flexural modulus) of the composites.

The epoxy-based composites using reduced graphene oxide act as fillers, which give rise to significant improvement in properties (tensile, impact and flexural) of the resultant composite systems. However, it is interesting to observe that the functionalised graphene nano-sheet (GNS) reinforced epoxy composite at lower filler concentration shows superior fracture toughness, fracture energy, stiffness, strength, and fatigue resistance than that of the neat CNT based epoxy composites [103–106]. The advantages of GNS as compared to CNTs raise the structural properties of epoxy, which may be attributed to the larger surface area with improved adhesion of fillermatrix owing to the wrinkled surface and the two-dimensional geometry of GNS [105, 106]. Graphite continues to attract considerable attention among researchers due to its excellent mechanical, electrical properties, low density with ease of processing and low cost. However, graphite commonly exists as a layered material and these layers are closely packed through Van der Waals force. For the efficient use of graphite as filler in polymer composites, its layer must be separated partly to achieve expanded graphite (EG). It is difficult to intercalate monomers into the interlayer of graphite to produce composites if the raw graphite is used as reinforcement; also, it is not possible to disperse graphite layers in the epoxy matrix. To overcome this adversity, the preparation of expanded graphite (EG) from raw graphite is exposed to a strong oxidizer (e.g., HNO3, H2SO4, KMNO4). The EG and GO both are the derivatives of graphite and are considered to be ultimate nano-filler materials for epoxy matrix. It is due to the presence of covalent bonds having hydroxyl, phenolics and epoxide functional groups on their basal planes and also it is located on the carboxyl and carbonyl group at the sheet edges. The presence of these functional groups enables them to strongly hydrophilic in nature. The EG is readily dispersed in water and reinforced into matrix with the help of these functional groups for the synthesis of composites [107–109]. However, two-dimensional graphene-based nanomaterials show little agglomeration because of their high aspect ratio, which affects the mechanical properties of the resultant composites. Therefore, suitable dispersion and exfoliation techniques are essential to facilitate better structural properties.

#### **2.4 Nanoclay reinforced composites**

The nano-clay reinforced polymer-based composites have earned much attention among both academic as well as industrial sectors due to incorporation in a small amount of nanoclay considerably improves the mechanical performance of the neat polymers. The nanoclay based two dimensional nano-material are naturally occurring in the form of platelets, which include a few to 1000 sheets. These are mainly silicate and comparatively inexpensive than that of the other nanomaterials. The other types of nanomaterial possess a larger surface area with a high aspect ratio (>50) and are thermally stable. Usually, the use of nanoclay in the matrix improves their properties

#### *Synthesis and Properties of Epoxy-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.104119*

with good optical transparency for a suitable selection of nano-filler reinforced in the polymer composites [49, 110].

Montmorillonite (MMT) is an aluminosilicate [111], which is extensively used in the clay based nanofillers (**Figure 2**). The thickness and lateral dimensions are observed in the montmorillonite around 5 nm and 500 nm, respectively. The single sheet of montmorillonite has a stiffness of about 250 GPa. Moreover, the large stiffness was observed in the clay minerals, which makes them appropriate for improving the structural performance of the polymer-based composites [111–113]. In recent few times, it has been reported about the epoxy-based glass and carbon composites, which may be embedded in the glass reinforced polymer composites. In addition, a montmorillonite (Nanomer I.30E) based clay mineral was used in this research [49]. They have reported each platelet are approximately 1 nm thickness and internal dimension around 300–600 nm with a high aspect ratio. Moreover, the compression strength is improved about 15–20% for particularly 5 wt% of nanoclay reinforced epoxy matrix composite systems. It is also observed that the incorporation of a small amount of nanoclay into the epoxy composites enhanced the mechanical performance using reinforcement of clay particles in the polymer matrix, which results the increase in impact and interfacial properties of the composite systems. Especially, the current research is based on nanoclay composites, which consist of the polymer as matrix material and nanoclay as the reinforcement particle (act as nanofiller) in the composite systems. Several properties (including mechanical, thermal and electrical) and structural aspects of the polymer matrix are enhanced due to the incorporation of nanoclay [114, 115]. These nanoclay based materials have been used as cost effective substituents with significant strength characteristics and considerable enhancement of properties on the nanoclay reinforced polymer-based hybrid composite systems. Several studies have been carried out on nanoclay reinforced polymer composites [113–119]. For instance; Hussain et al. [120] have reported that the natural fiber composite is robustly dependent on the optimum fiber length and weight percentage

#### **Figure 2.**

*Schematic illustration of the montmorillonite clay (MMT) structure. Side view of tetrahedron units of MMT assembled through weak van der Waals and electrostatic forces to form the primary particles and top view of MMT shows hexagonal structure of oxygen and hydroxyl ligands of the octahedral layer. Reprinted with permission from Ref. [111].*

of fiber. It is also reported that the mechanical properties of the polymer composite system are improved by the incorporation of a little amount of nanoclay in to the matrix [121].

#### **3. Synthesis of carbon-based nano-filler reinforced epoxy composites**

Several methods have been employed for the synthesis of various nano-filler reinforced epoxy resin composites in recent few times. In this section, we have emphasized mainly on three methods (including in-situ intercalative polymerization, solution mixing and melt blending) for preparation of composites, which are extensively discussed in details.

#### **3.1** *In-situ* **intercalative polymerization method**

*In situ* polymerization is a very effective method that allows carbon-based filler particles to be uniformly dispersed in the matrix and therefore gives strong interaction between polymeric matrix and reinforcing filler in the composite systems. This technique generally involves the polymerization of monomers in the presence of dispersed filler particles. The polymerization is initiated through the incorporation of filler and suitable initiator with the application of heat or radiation after diffusion during the synthesis of the polymeric materials [122, 123]. Several researchers have been reported about the preparation of composites by using *in situ* polymerization methods and achieving superior mechanical properties with the low value of percolation threshold as compared to the other techniques (like solution compounding or melt blending method) [124–127]. However, in-situ polymerization is also been used to give non-covalent composites based on different polymeric matrices including polyethylene (PE) [128] and PMMA [129], etc. Ray and Okamoto [130] reported that the *in-situ* polymerization method referred as intercalation polymerization method is applied for the preparation of GDs based nanocomposites. Zhang et al. [124] have fabricated graphene sheet reinforced epoxy resin composites by in-situ polymerization method. It is observed that the prepared composites have significant improvement in the Young's modulus and thermal stability at 0.7 wt% of GNs content in the matrix. It is also noticed that there is an extensive enhancement in the thermal conductivity of the resultant graphene sheet-epoxy composite systems using in-situ polymerization method [131]. Ying and his co-workers [132, 133] have fabricated surfactant incorporated CNT into the epoxy composites *via* in-situ polymerization technique. It is observed that the tensile strength and ultimate strain are enhanced with an increase in the modified filler contents in the CNT reinforced epoxy composite systems.

Kotsilkova et al. [134] have reported on amine and epoxy grafted MWCNTs based composites. They have successfully synthesized and measured various performances (including rheological, dc conductivity, radio frequency and microwave properties) of the composites. The enhancement of the thermal and mechanical properties of the MWCNTs reinforced epoxy composites were utilized by in-situ polymerization method, which is reported by Theodore et al. [135]. The preparation of the epoxybased nanocomposites using functionalized vapor grown carbon nanofiber (VGCNF) by in-situ polymerization technique. It is revealed that the functionalized VGCNFepoxy composite systems have superior tensile modulus and strength than that of the neat VGCNF and epoxy matrix with enhanced thermal stability [136]. The carbon

#### *Synthesis and Properties of Epoxy-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.104119*

nanofiber (CNF) based epoxy composites with various weight percentages of silanemodified CNF and unmodified CNF contents *via* in-situ polymerization technique were prepared. It is revealed that the synthesized composites with different properties (thermal, mechanical and electrical) were analyzed as a function of different weight percentages of modified and unmodified CNF contents [66, 137]. The key features of this technique offer covalent bonding between surface-functionalized CNTs and polymeric matrix, which result enhancement of mechanical performance of the composites with strong interfacial bond [86, 138]. On the other hand, the shortcomings of this method are that the dispersion of filler particles in the polymeric matrix occurred during the preparation of nanocomposites, so the plenty of energy is necessary, which might affect the large production of polymer-based nanocomposites. Similarly, Ramezanzadeh et al. [139] have prepared conducting polypyrrole (PPy) and zinc doped polypyrrole functionalized graphene oxide (GO) nano-sheets for the development of high-performance epoxy-based composite systems with enhanced thermal and mechanical performance. The results showed that the nanocomposites are enhanced about 54% for elongation at break and 115% for the energy at break, and there is an improvement of tensile strength and energy (21% and 32.44%) at break for the zinc doped GO-PPy in the epoxy composites.

#### **3.2 Solution mixing/solution blending/solvent casting**

Solution-induced intercalation technique is a simple, ease of processing and efficient approach to produce polymer-based nanocomposites. This technique is more suitable for both small- and large-scale preparation of the nanocomposites. In this technique, initially an appropriate proportion of polymeric mixtures were dissolved in a proper solvent. Then the different types of filler particles (such as graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), etc.) at various weight percentages are reinforced into the polymeric solution with constant stirring either by magnetic stirrer or mechanically or by using ultrasonication technique for dissolution of the particles in the matrix. Once the complete mixing of solution is over, then the mixtures were dried to eliminate the solvent and ultimately resultant composite materials are molded with suitable mold to give necessary shape and making suitable for characterization and further properties measurement. Similarly, the solvent casting method has been used to synthesize composites in the form of thin films. This method deals with the uniform dissolution of ingredients in a suitable solvent and then evaporated through a drying device. However, the solvent casting technique shows higher mixing quality, thinner film, high purity and better clarity than that of the melt mixing technique [140]. Besides, the solution or polymer film is exposed to moderately low thermal or mechanical stress during the preparation technique, which results insignificant degradation or side reactions.

There are various literatures available with epoxy-based composites by using this technique and then studied several properties of those composites as a function of different weight percentage of filler contents. Similar to epoxy resin, other various polymers (such as PS, PMMA, PVDF and PI) and its co-polymers [P(VDF-HFP), P(VDF-TrFE), etc.] are used to fabricate nanocomposites with the addition of several fillers into matrix using this technique [141–146]. For instance; Lv et al. [147], Prolongo et al. [148] and Allaoui et al*.* [64] have fabricated carbon nano-fibers reinforced epoxy-based composites with different weight percentages of CNF contents and studied their properties. Similarly, Choi et al. [149] have synthesized polycarbonate-based carbon nanofiber composites. They have also examined mechanical

properties and electrical resistivity with the incorporation of various percentages of CNTs contents. Moreover, the limitation of this technique is the requirement of the high amount of solvent in the preparation of nanocomposites for industrial point of view, which may not be environmentally friendly and cost-effective.

#### **3.3 Melt blending method**

Melt blending is the facile, cost-effective, eco-friendly and conventional technique for the synthesis of various thermoplastic polymer-based composites. In this process, there is no such solvent used for preparation. Also, it is one of the most efficient approaches for industries in large scale production of the composite materials. During the last few times, most of the industrial researchers have preferred this synthesis technique for preparing nanocomposites. In this technique, initially, polymeric materials and filler particles are normally mixed systematically and then the homogeneous mixture is subjected to annealing greater than the melting point of the polymeric material. Besides, the polymeric material is melted and combined with apposite amount of the filler particles using an extruder. The process of melt blending is carried out in the presence of an inert gas including argon, nitrogen or neon. However, this melt blending technique has enormous benefits over insitu intercalative polymerization or polymer solution intercalation method. This technique is compatible with present industrial processes including extrusion and injection molding techniques and thus it is a most popular method. Various academic and industrial researchers have fabricated different polymer-based nanocomposites (PP-CNT, nylon 6-MWCNT, PC-MWCNT and PS-MWCNTs, etc.) using this technique [150–153]. For instance; Jin and his co-workers [154] have fabricated PMMA-MWCNTs based composites by using this technique. In the synthesized composite system CNTs shows no obvious damage or breakage as well as uniformly dispersed within the polymeric matrix. The preparation of polylactide (PLA) exfoliated graphite (EG) based nanocomposites was synthesized using this technique. It is seen that the composite exhibits a considerable increase in the thermal degradation temperature with the increase in the EG contents. The mechanical and electrical performances are also improved with the continuous increase of graphite contents. A similar work on carbon-based material (such as graphite nano-sheet and MWCNTs, etc.) reinforced epoxy composites is fabricated by Kim et al. and his group [155, 156] using this technique and also studied their mechanical and rheological properties of the resultant composites.

#### **4. Different properties of filler reinforced epoxy composites**

Recently, epoxy-based composites are of great importance for their significant dielectric, electrical, thermal and mechanical performances. In this section, we have especially emphasized on properties (dielectric, electrical and mechanical) of the various filler reinforced epoxy-based composites.

#### **4.1 Dielectric and electrical properties of epoxy-based polymer composites**

Varma et al. [157] have fabricated calcium copper titanate (CaCu3Ti4O12; CCTO) and metallic aluminum (Al) powder-filled epoxy-based tri-phase composites with various weight percentages of filler contents. It is observed that the dielectric constant

#### *Synthesis and Properties of Epoxy-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.104119*

of the composites is greatly improved near the percolation threshold. The maximum dielectric constant was achieved (≈700) of the three-phase epoxy composites, which is much larger than that of the two-phase epoxy-CCTO (≈70) and neat epoxy matrix. These flexible three-phase composites are potential candidates for practical application in the field of energy storage devices [158]. The perovskite-type ceramic (BaTiO3) based epoxy composites with a different weight percentage of filler contents have been reported by Kuo et al. [159]. They studied the dielectric properties of these composites and obtained a high dielectric constant (≈44) and negligible dielectric loss (<1) at 40 wt% of filler content in the epoxy matrix. The synthesized threephase composites have been uniformly dispersed in the polymeric matrix. However, the dielectric constant of these composites is proportional to the volume ratio of the ceramic contents and remained constant with the application of temperature and frequencies. The fabricated composites have larger than that of the commercial ceramic-filled polymer composites. Bhattacharya and Tummala [160] have fabricated PMN-PT filled epoxy composites with various weight percentages of filler contents. It is revealed that the high dielectric constant (≈29) was achieved in the 40 wt% of filler-filled epoxy composites with superior homogeneity within the epoxy matrix. The dielectric performance of Ni particle doped epoxy composites at 40 and 55 wt% of filler contents have been analyzed in the various frequency regions from 1 to 107 Hz and temperature range of −20 to 200°C. The dielectric constant of these Ni particle filled epoxy composites is improved with the increase of Ni particle content and simultaneously reduces the frequency at room temperature. The value of the dielectric constant is increased with the increase of filler contents due to the improved dipole and interfacial polarization effect. On the other hand, the dielectric loss value was reduced with the higher concentration of filler contents has reported by Chen and his co-workers [161]. The multifunctional polymer-based composites comprising of Fe3O4 fillers and epoxy resin as the matrix was analyzed in different concentration of magnetite Fe3O4 contents. It is observed that the composites with larger dielectric constant and suppressed dielectric loss at low frequencies. However, the synthesized composites with a larger volume fraction of filler contents appeared at the percolation threshold with an increase in the value of conductivity [162]. Xie et al. [163] have fabricated the modified hexagonal boron nitride reinforced epoxy composites with enhanced dielectric properties *via* simple free radical polymerization technique. It is observed that PGMA grafted h-BN reinforced epoxy matrix composite has improved thermal conductivity with increase of filler contents. The modified composite shows larger dielectric constant than that of the unmodified one and pristine epoxy matrix. Similarly, the dielectric loss of the resultant composites was achieved negligible at suitable frequencies. Zhang et al. [164] have fabricated core-shell satellite structured BaTiO3 nanoparticles with polydopamine (PDA) layers and silver (Ag) nanoparticles incorporated into the epoxy matrix. It is observed that the epoxy nanocomposites with BT-PDA and BT-PDA-Ag fillers showed enhanced dielectric constant (≈9) and negligible dielectric loss (0.024) at microwave frequencies (10 GHz) for 20 vol% of filler contents. However, it is also noticed that the composite shows homogeneous dispersion with uniform particle size, due to strong interfacial interaction between modified particles and polymer matrix. Meng [165] and his co-workers have developed thermally stable honokiol derived epoxy resin nanocomposites with excellent thermal and dielectric properties. The results showed that the fabricated composites have excellent dielectric constant (9.74) and minimized dielectric loss values (0.026) at 1 KHz. Moreover, it is also confirmed the better thermal stability, thermal conductivity and high specific heat in the composites.

#### **4.2 Mechanical properties of epoxy-based composites**

Zhao et al. [166] have synthesized hyper branched graphene oxide structured based epoxy nanocomposites. The results showed excellent engineering application performances of the composite systems. The synthesized nanocomposites have uniform dispersion in the epoxy resin matrix and are combined with the matrix through chemical bonds, which shows strong interfacial active force and enhancing the load transfer efficiency of the matrix to hyper-branched polymer-graphene composites. However, these composites exhibited excellent mechanical properties [impact strength (58.53%), tensile strength (83.29%), and compression strength (57%)] with considerable increase of 0.2 wt% for HPB-GO contents than that of the neat epoxy matrix. Also, it is noticed that there is 80% increase in thermal conductivity (0.32 W m−1 K−1) of the synthesized nanocomposites. The incorporation of rGO in the epoxy matrix in the composites improved the strength and Young's modulus is about 500 and 70%. It is observed that rGO plays a significant role in strengthening the epoxy than that of the GO. However, rGO efficiency in the improvement of modulus and strength is about 10–35% than those of GO. The significant results for r-GO composite are due to the efficiency of interfacial adhesion between r-GO sheet and epoxy molecules [167]. The enhancement of strength can be ascribed to the outstanding load bearing capacity of reduced GO sheets as well as excellent load transfer from matrix to reinforcements. They also suggest an increase in the value of modulus and strength in the composites for 1 wt% rGO reinforced polymeric matrix. The fabrication of graphene- nano-alumina based epoxy composites reported by Osman and his co-workers [168]. It is observed that both tensile strength and storage modulus are improved by 22.56% and 4.6%, which is much larger in contrast to the neat epoxy matrix. The thermal conductivity of the resultant composites is also improved by 23.4% with increase in the filler contents. The incorporation of alumina particles on the surface of the graphene not only reduces the electron transfer but also eliminates the agglomeration of graphene. Khan [169] and his co-workers have reported improvement of thermo-mechanical performance of carbon fiber and glass fiber reinforced epoxy composites. These composites were characterized by using universal testing machine (UTM) with tensile strength and Young's modulus. The tensile strength of carbon fiber reinforced epoxy composites is enhanced to 844.44%, 951.11% and 1122.22% with selected 40, 50 and 60 wt% of carbon fiber contents. On the other hand, the tensile strength of glass fiber reinforced epoxy composites is also enhanced about 156.66%, 171.10% and 197.77% for 40, 50 and 60 wt% of glass fiber contents. Karle et al. [170] have systematically studied CaSiO3 particulate fillers reinforced epoxy-based composites and analyzed their hardness, flexural strength and impact resistance through mechanical performance. It is found that the addition of CaSiO3 particles (1–2 wt%) in the matrix results effective enhancement of mechanical properties than that of the neat epoxy. Park [171] and his co-workers have investigated the reinforcement effect of molybdenum sulfide (MoS2) nano-sheets on the mechanical performance of the epoxy-based composites. The fabricated high performance epoxy composites are extremely enhanced the fracture toughness (55–81%), flexural strength (25–66%), modulus (0.7–6%), impact strength (31–118%) and strong interfacial interaction (1–21% surface free energy) than that of the pristine epoxy matrix. The thermal and mechanical performance of the epoxy reinforced modified iron oxide nanoparticles reported by Baghdadi et al. [172]. It is revealed that the PDA modified Fe3O4 based epoxy composites normally improved as compared to neat epoxy matrix and unmodified one. The maximum enhancement in

tensile strength (34%) and fracture toughness (13%) is observed in the epoxy-based composites. The graphene oxide-epoxy composites with improved failure strength (48.3%) and toughness (1185.2%) for 0.0375 wt% of GO within the epoxy matrix. The fabricated composites with enhanced properties may be due to the uniform dispersion of the GO in the epoxy matrix through two-phase extraction technique using an aqueous dispersion of the GO were reported by Yang and his co-workers [173]. Bortz et al. [174] have reported helical carbon nanofibers to achieve graphene nanoribbon and then oxidized to get GO. This composite exhibit improved tensile strength (7.57 MPa) and modulus (3.32 GPa) at 0.5 and 0.1 wt% of GO loading into the polymer matrix. Moreover, the flexural strength was also improved by 12% and 23% for the addition of fillers into the matrix.

Munoz et al. [175] have synthesized GO based epoxy composites and studied their mechanical properties for various wt% of GO contents. It is revealed that the composites with enhanced elastic modulus and flexural modulus at 0.3 wt% of GO contents in the epoxy matrix. Fang et al. [176] have reported methylene dianiline (MDA) modified rGO-epoxy composites with different weight percentages of filler contents. In these composite systems, the fracture toughness and flexural strength is enhanced by 94% and 92% for 0.6 wt% filler loading. Seong et al. [177] have fabricated MDA modified GNP-epoxy composites and investigated their mechanical properties. The 1.5 phr MDA modified GNP content shows 120% and 63% enhancement in impact toughness and storage modulus in the epoxy-based composites. Naebe [178] and his co-workers have reported the effect of thermally reduced graphene oxide (TRG) with bingel reaction (FG) based epoxy composites. It is observed that 0.1 wt% of filler content in the FG-epoxy composite shows larger flexural strength (15% and 22%) and higher storage modulus (6% and 16%) than that of the pristine epoxy matrix. Similarly, Guo et al. [179] have synthesized GO modified triazine derivatives (GO-TCT-DETA) show homogeneous dispersion in the epoxy matrix. It is also exhibited higher flexural strength (49%) and modulus (15%) for 0.1 wt% of GO-TCT-DETA-based epoxy composites than that of the pristine epoxy matrix.

#### **5. Summary**

The epoxy-based composites have become potential candidates for the application in different technological fields owing to their excellent physical, chemical and electrical performances**.** The present chapter reviews on the research work related to carbon based nanofillers (e.g., fullerene, CNTs, CNFs, graphene & nanoclay) reinforced epoxy-based composites. However, several researchers have reported the variety of reinforcement in order to explore mechanical, dielectric & electrical performances to predict their behavior suitable for technological viability of the epoxy-based composites. According to the literatures, it has been found that the use of nanofillers into the epoxy matrix, which help to enhance the mechanical and electrical performance of the composite systems than that of the micron sized filler particles. Moreover, with the increase of concentration of nano-filler content results in the enhancement of certain properties of the epoxy-based composites up to the threshold value, in most of the cases after attaining the threshold value the properties again initiate to fall, this may be ascribed to the presence of large clusters into the epoxy resin. The incorporation of minimum amount of nano-filler contents is indispensible into the matrix to avoid agglomeration in the composite systems. Therefore, we have especially emphasized on most suitable synthesis techniques for the processing of nano-filler

reinforced epoxy-based composites. Further, our efforts have been made for providing higher insight on the properties (like dielectric, electrical and mechanical) of various nanofillers reinforced epoxy composites. The unique properties of nanofiller reinforced epoxy composites also reflected as potential applicability in the field of electronics, energy storage, gas sensors and aerospace.

#### **Acknowledgements**

The authors gratefully acknowledge the support provided by Centurion University of Technology and Management, Odisha, India for carrying out the present research work.

### **Conflicts of interest**

The authors declare no conflict of interest.

### **Author details**

Srikanta Moharana1 \* and Bibhuti B. Sahu<sup>2</sup> \*

1 School of Applied Sciences, Centurion University of Technology and Management, Odisha, India

2 Department of Physics, Veer Surendra Sai University of Technology, Odisha, India

\*Address all correspondence to: srikantanit@gmail.com; bibhubhusan78@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Synthesis and Properties of Epoxy-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.104119*

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