Reinforcements in multi-scale polymer composites: Processing, properties, and applications

Composites Part B 138 (2018) 122–139

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Reinforcements in multi-scale polymer composites: Processing, properties, and applications

T

Garima Mittala, Kyong Y. Rheea,∗, Vesna Mišković-Stankovića, David Huib a b

Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 446-701, South Korea Department of Mechanical Engineering, University of New Orleans, LA 70148, United States

A R T I C L E I N F O

A B S T R A C T

Keywords: Multiscale composites Nanomaterials Surface-modification Multifunctionality Fiber-reinforced polymer composites

Smart and novel materials require the replacement of conventional composites with superior ones, which requires an advanced class of composites with multi-functionality. Multi-scale composites are advanced composites that are reinforced with nanoscale materials along with macroscale fibers, and these materials have attracted the attention of researchers as well as various industries. Multi-scale composites have potential applications in almost every field due to their remarkable features like extraordinary mechanical, electrical, and optical properties; extremely high aspect ratios of the nanomaterial constituents; and the uniformity, flexibility, and stability of the fibers. To optimize the performance of these kinds of composites, it is crucial to understand the selection of appropriate reinforcements, processing, and utilization of these advanced materials. Most reviews in this area concentrate only on CNTs, while this review considers other nanomaterials too. Additionally, various methods to improve the dispersion of nanomaterials into the matrix are also discussed. Overall, this article focuses on the components of multi-scale composites, key challenges related to their processing, and the multi-functionality of designed multi-scale composites.

1. Introduction Researchers are constantly striving to make human life more convenient and to develop advanced technologies. Therefore, the demand for smart and novel materials has increased, and existing technologies are often replaced by more advanced options. Polymeric materials have already replaced conventional materials such as metals and ceramics due to their lightweight, easy processing, and low cost [1]. Moreover, polymers possess outstanding corrosion stability and good mechanical properties (ductility). However, there are some drawbacks associated with polymer components, such as low thermal stability, inferior chemical (e.g., acid) and environmental stability (UV), and low conductivity [2]. To avoid these kinds of issues, polymer composites were designed by reinforcing the matrices with a wide range of filler materials. Based on the desired performance of the final material, the polymer matrix can be reinforced with many kinds of fillers (particles, fibers, or platelets, synthetic or natural, organic or inorganic) at any scale (macro, micro, or nano) [3–6]. The performance of the final product synergistically depends upon the characteristics of the filler as well as those of the host material. Regardless, the performance of polymers decreases with time due to various causes such exposure to UV, high temperature, pH, and humidity [7]. Fiber-reinforced polymer

∗

Corresponding author. E-mail address: [email protected] (K.Y. Rhee).

https://doi.org/10.1016/j.compositesb.2017.11.028 Received 18 October 2017; Accepted 18 November 2017 Available online 26 November 2017 1359-8368/ © 2017 Elsevier Ltd. All rights reserved.

composites with remarkable properties were designed to overcome these problems and advance the overall performance of composites. In fiber-reinforced polymers (FRPs), fibers provide a unique set of properties such as good length to width ratio, environmental stability, uniformity, and flexibility, while the host matrix protects the fibers against unfavorable environmental conditions and maintains their position throughout the matrix [8]. Many studies show that FRPs can substitute for conventionally used materials. Currently, various kinds of synthetic or artificial fibers are used to reinforce the polymer matrix in order to improve the performance of the final materials [9]. These FRPs are often used in various industrial applications, including automobiles, military vehicles, sports equipment, and aerospace products, because of the their low weight, low cost, high durability, and high strength. However, there is always room for improvement, and researchers are constantly trying to advance the field of composites. To improve the out-of-plane performance of composites, more than one filler material is required [10]. Regardless, weight is a very crucial factor for many applications such as aircraft and automobiles, and large-scale fillers give rise to weight and voids, which subsequently narrow the scope of reinforcement in the matrices [11]. Hence, nano-fillers are broadly preferred as a reinforced material over macro-scale fillers. Over the past two decades, nanotechnology has been utilized in a

Composites Part B 138 (2018) 122–139

G. Mittal et al.

aggregating and settle to the bottom as soon as resin stirring stops [25]. Therefore, some researchers have decreased the curing time by using microwaves to accelerate the reaction [26]. Curing aids like microwaves, X-rays, and gamma-rays are more economically efficient and offer promising features like reduced curing time, energy savings, improved processing control, and higher heating efficiency compared to thermal curing [27].

wide range of applications. The extremely small dimensions of these materials give rise to their outstanding characteristics, such as exceptionally high aspect ratio and near zero weight (in comparison to the host material), that attract many researchers as well as industries. The desired composite material can be designed by exploiting the characteristics of reinforced nanomaterials that fluctuate with size, shape, type, dimensions, and production route [12]. Numerous studies have reported the enhanced mechanical, thermal, and electrical properties of nanoparticle-reinforced composites [13–15]. Composites constructed using reinforcing fibers (carbon, basalt, glass, aramid, natural fibers) together with nano-dimensional filler materials (graphene, carbon nanotube, nanoclay, metal nanoparticles) are known as multi-scale composites [16–18]. These kinds of composites are also known as multifunctional composites because they possess conventional load-sharing properties of the reinforced fibers as well as the additional functional properties (conductivity, sensing, thermal resistivity) associated with the particular nanomaterial. Moreover, the nanomaterials interact with the host resin to improve interactions and change texture and color. The field of nanotechnology has its own sets of challenges that hinder the development and commercialization of nanomaterial-reinforced composites. Processing issues, including homogenous dispersion and unsatisfactory interfacial interactions, are a major concern [19]. To obtain optimal composite performance, researchers have made efforts to alter the surface morphology or structure of the reinforced materials by incorporating chemical moieties that both improve the dispersion quality and the interfacial interactions between matrices and reinforcements [20–22]. This review focuses on the different components of the multi-scale polymer composites and different types of nano-reinforcements and their structures and properties. Further, critical issues regarding processing are discussed. Finally, the improved properties of multi-scale composites along with various applications are discussed.

3. Reinforced fibers Fibers play a crucial role in multiscale composites in determining the end performance of the synthesized material. It is well known that the defects present in a material decrease as the size decreases. Compared to the bulk material, a fiber possesses a larger surface area, a higher aspect ratio, and anisotropy that leads to the advanced performance of the material. Numerous studies report that the characteristics of the final composites are a function of fiber type, fiber length, volume fraction, fiber structure, sizing agent, morphology, and fiber orientation. Effect of fiber length: Zang et al. studied the effects of fiber length on glass fiber-reinforced poly(butylene terephthalate) composites and found that long fibers exhibit better mechanical properties compared to short fibers [28]. Although short fiber-reinforced composites are easy to process, provide versatility, and are cost-efficient, fiber degradation during the injection molding process results in some degradation in mechanical properties. In contrast, long fiber-reinforced composites are processed via pultrusion and exhibit excellent mechanical properties, fatigue resistance, durability, and impact resistance [28]. Effect of fiber loading: Generally, properties of the composite material increase by increasing the fiber loading in the matrix. However, after reaching a threshold limit, the properties of the composite material start to decrease because of poor mechanical interlocking, which degrades load transfer between the fibers and matrix [29,30]. Liu et al. studied the effects of fiber fraction on crack propagation rate and reported that the crack propagation rate was decreased by increasing the volume fraction of fibers, resulting in an improved crack growth threshold [31]. Effect of fiber orientation: Fiber loading and the method of processing significantly affect the orientation of the fibers in the matrix. Wang et al. studied the effects of fiber orientation on the Young's modulus of unidirectional GFRPs both theoretically and experimentally. They found that Young's modulus was lowest when the fibers were oriented at 45° and 60° for experimental and theoretical studies, respectively. Other factors affecting the properties of composite materials are responsible for this mismatch between theoretical and experimental outcomes [32]. Similarly, the shear strength of GFRPs oriented at 0° and 90° were compared with randomly oriented fibers [33]. The results showed that 0° composites possess higher interlaminar shear strength than that of 90° composites, while randomly oriented fibers showed higher in-plane shear strength. Based on the degree of fiber alignment relative to the loading direction, there are three type of fiber orientations, aligned, partially aligned, and randomly aligned. The strength and stiffness of the composite is highest when the fibers are parallel to the loading direction due to efficient stress transfer between fibers and the matrix [34]. Since the orientation of the fiber is influenced by the processing method, compression molded long GFRPs exhibit better strength than injection molded short GFRPs due to the better orientation in a preferred direction observed in compression molding [35]. Effects of fiber shape and type: Some studies showed that the shape of the reinforced fiber affects the performance of the composite [36]. To improve the compressive strength of composites, Bond et al. studied the effect of triangular-shaped glass-fiber reinforcement on composites and compared their properties with circular-shaped glass-fiber reinforced composites. They found that triangular fiber-based composites possess higher tensile and compression strength than circular fiber-based composites [37]. Similarly, Chandra et al. reported the effects of

2. Polymer matrix The matrix is a main component in a composite that protects fibers or fillers against abrasion and harsh environmental conditions. Moreover, it holds the fibers/fillers together and provides strength to the composite by absorbing energy via deformation during stress. When a load is applied to a composite material, the matrix transfers it to the reinforced material [23]. Features such as type, characteristics, and processing route need to be considered. Hence, it is essential to choose a matrix that can experience greater strain at break than the reinforced fibers. Thermosets and thermoplastics are two main types of polymer resins [23]. Thermosets can be cured at elevated temperature, and it is impossible to change them into their uncured form again. Thermosets possess strength and stability against adverse conditions because of complex cross-linking in the presence of catalyst. Conversely, thermoplastics are soft, do not cross-link with each other, and melt at higher temperatures. Thermoplastics have limited practical applications compared to thermosets because they have high viscosity and require high temperatures during processing. Commonly used thermosets are epoxies, vinyl esters, phenolic resins, polyesters, and urethanes. Epoxy resins are extensively used thermosets because of their outstanding thermo-mechanical properties, which improve the performance of the final material. Most epoxy resins have glycidyl ethers and amines and can be cross-linked in the presence of a hardener at room or higher temperatures. Many researchers have reported that the hardener improves the strength of the resin along with the glass transition temperature. Hence, the properties of an epoxy resin can be controlled by varying the type and chemical structure of the hardener, resin to hardener ratio, curing time, and curing temperature. For instance, Konuray et al. used a photobase generator as a curing agent and delayed the cross-linking of an epoxy resin to provide improved processing freedom [24]. However, some articles show that long curing times are unfavorable during nanocomposite processing as nanofillers start 123

Composites Part B 138 (2018) 122–139

G. Mittal et al.

elliptical fibers of different aspect ratios on macroscopic damping [38]. Agnese and Scarpa conducted a numerical and experimental study on biphasic composite structures using four-lobe star-shaped fibers and compared their properties with those of circular cylindrical fibers. They demonstrated that the star-shaped fiber produced improved energy dissipation that led to higher viscoelastic damping factors than those of cylindrical fibers [39]. Other factors like fiber density, sizing agents, and fiber diameter also affect the properties of reinforced composite materials. Generally, fibers are treated with different kinds of sizing agents such as starch, gelatin, oil, or wax to improve adhesion and handling. Overall, the appropriate selection of fibers with desired properties can help in the design of advanced composites with the desired performance. Different kinds of fibers are currently used to fabricate composites. Glass fibers, carbon fibers, basalt fibers, aramid fibers, and natural fibers are extensively used. In this review, we focus on glass fibers, carbon fibers, basalt fibers, and aramid fibers.

[40]. Glass fibers remain unchanged when exposed to high or low temperatures, and they show low moisture absorption. Hence, they are a good candidate for marine applications. Glass fibers are not flammable because they are an inorganic material. Although glass fibers are unaffected by most chemicals, they are highly sensitive to strong alkaline media and hot phosphoric acids. Glass fibers show high thermal conductivity and a low coefficient of thermal expansion due to air trapped within the glass. Glass fibers can be reinforced to make composites for printed circuit boards because they have insulating properties and a low dielectric constant [41]. Glass fiber-reinforced polymer (GFRP) composites are extensively used due to their cost-effectiveness, high quality, and environmental stability. The immense need for lightweight advanced materials for automobile or aircraft components suggest the importance of these materials in many industries. Moreover, GFRPs exhibit good mechanical properties like good impact resistance, better tensile strength than steel, less brittle behavior than carbon fiber, and good dimensional stability. Common examples of applications for reinforced glass fibers are fishing rods, boat hulls, wall paneling, heat insulation in refrigerators and stoves, garments, tires, and sporting goods. In comparison to other fibers, glass fibers are heavier, and this limits their applications as a reinforced material.

3.1. Glass fibers Glass fibers are lightweight, very strong, robust fibers that are formed by extrusion of thin strands of silica glass [40]. Glass fibers are preferred due to their easy processing, low cost, and higher strength in comparison to metals. Various forms of glass fibers are commercially available, such as woven fabrics, yarns, and textiles. The classification of various types of glass fibers is shown in Fig. 1 [41]. Glass fibers provide versatility and easy processing with low-cost. In addition, glass fibers possess high strength and good fire resistance

3.2. Carbon fibers Carbon fiber is one of the most desired forms of carbon because it possesses outstanding mechanical, thermal, and electrical properties. Carbon fiber has at least 92 wt% carbon, and these fibers can be short or

Fig. 1. Classification and physical properties of various glass fibers [41]. Reprinted with permission from SAGE publications.

124

Composites Part B 138 (2018) 122–139

G. Mittal et al.

continuous, single or woven. Carbon fibers can be crystalline, amorphous, or partially crystalline. Crystalline carbon fiber exhibits a typical graphitic structure with sp2 carbon atoms positioned in a two-dimensional honeycomb lattice. Two parallel layers of these honeycomb structures are linked through van der Waals forces and can slide on each other easily. Therefore, graphite is anisotropic in nature, and its in-plane and out-plane properties are different. Consequently, carbon fibers show a high elastic modulus parallel to the plane and a low elastic modulus perpendicular to the plane [42]. A similar trend is observed for electrical and thermal conductivities. Fibers with more than 99% carbon are known as graphite fibers. These kinds of fibers have remarkably high strength but low production yield. Carbon fiber-reinforced composites (CFRPs) have superior properties of low-density, light weight, high stiffness, a low coefficient of thermal expansion, excellent electrical conductivity, and chemical stability; therefore, they are used to design advanced components in various applications like aircraft, automobiles, sporting goods, and defense equipment [43]. The high modulus and light weight of carbon fiber allowed them to replace conventionally used aluminum and titanium alloys in aerospace applications. Mass production of carbon fiber-reinforced automobiles is still not mainstream due to the high production costs. However, high-end cars like Formula 1 cars are using carbon fiber-reinforced bodies because of the outstanding strength and for aesthetic reasons. Carbon fiber-reinforced composites are a good option for manufacturing high performance sports equipment like ice hockey sticks, crash helmets, tennis rackets, and golf clubs. In defense applications, decreasing the weight of a material can reduce the energy required to transport the equipment. Hence, carbon fibers can be an excellent choice for military applications. Due to their higher surface area, carbon fibers play an important role in chemical purification by absorbing toxic chemicals. In addition to these conventional applications, carbon fibers are used in the development of composites with advanced features. For instance, Pereira et al. developed a new flexible electronic device based on carbon fibers produced from PANI that can be used for applications like biosensors, biofuel production, and microfluidic devices [44]. Further, CRFPs are a very good candidate to replace conventional metal frameworks in the field of implant prosthodontics due to their remarkable stiffness, rigidity, and biocompatibility [45]. Similarly, Méjean and his colleagues conducted research in which they developed a novel and less hazardous electromagnetic-absorbing material using carbon fiber-reinforced epoxy foam. This new absorbing material can be a good substitute for conventionally used carbon particle-reinforced polyurethane foam [46]. Other forms of carbon fibers like helical carbon fibers, bamboo carbon fibers, activated carbon fibers, and recycled carbon fibers are used in applications like air filtering, fuel cells, lithium-ion batteries, hydrogen sorption, and catalytic support [47–50].

chemical stability. Although the composition of basalt fibers is similar to that of glass fibers, the properties of basalt fibers are far better than those of E-type glass fibers. Basalt fibers have a high breaking load and high Young's modulus along with excellent environmental stability [51]. Basalt fibers are resistant to UV and high-energy electromagnetic radiation. Basalt shows outstanding resistance to acidic, alkaline, and salt attack [52]. In comparison to other fibers, basalt fibers can be used in a broad temperature range (−269 °C–650 °C). Moreover, basalt fibers show excellent flame retardation, indicating that they can maintain their physical integrity during heat treatment even for prolonged periods. Basalt fibers are good reinforcements for concrete and marine industrial applications because of their high compression strength, chemical stability, and corrosion resistance. Additionally, basalt fibers have replaced asbestos in brake pads and other friction-based applications due to their excellent flame and abrasion resistance. Since the basalt fibers are more cost-effective than carbon fiber and possesses higher strength, they can be used for many industrial applications like making wind blades, tripods of cameras, and snowboards. The automotive industry has shown that basalt fiber-reinforced systems exhibit acoustic absorption, a high strength-to-weight ratio, and a low coefficient of thermal expansion. There are basalt fiber-reinforced tripods available in the market that are 20% lighter than aluminum.

3.4. Aramid fibers Aramid fibers were the first organic fiber synthesized with high strength and stiffness, and they are good candidates to replace asbestos [53]. Along with mechanical properties that are superior to steel or metal wires of equivalent weight, aramid fibers possess outstanding heat resistance. Therefore, these fibers are used to fabricate composites with advanced mechanical and flame resistant properties such as protective clothing, aircraft, automobiles, rope for off-shore oil rigs, industrial fillers, and bullet proof jackets. According to the U.S. Federal Trade Commission, an aramid fiber is any fiber in which the forming substance is a long-chain synthetic polyamide in which less than 85% of the amide linkages (-CO-NH-) are directly connected to two aliphatic groups. Kevlar, Technora, Heracron, Twaron, Nomex, and Teijinconex are the trade names for aramid fibers, among which the first four are examples of para-aramids, while the latter are meta-aramids [54]. The type of aramid (para- or meta-) depends on the position of the chemical bond. Para-aramids have bonds aligned in the long direction of the fiber, while meta-aramids have bonds in a zigzag pattern. Therefore, para-aramids exhibit better mechanical strength (3–7 times) than meta-aramids along with high flexibility and abrasion resistance. In contrast, meta-aramids exhibit superior heat, flame, chemical, and radiation resistance. Additionally, meta-aramids show good dissipation of energy perpendicular to the fiber and are consequently used in bullet-proof jackets and personal armor [55]. Aramid fibers can be used in load bearing applications because they have a higher tensile strength than steel and high stiffness [53]. Aramid fibers are low in density (lighter than glass fibers) and show a low creep rate. The energy absorption at failure (half of nylon) and shock resistance are attractive features of aramid fibers. Additionally, flame retardation and non-conductivity make aramid fibers suitable for safety clothes and insulator materials, respectively. The abrasion resistance of aramid fibers is better than that of glass and carbon fibers. However, there are some drawbacks of aramid fibers including their sensitivity to UV rays, poor compression strength in comparison to carbon fibers, and problems associated with cutting the fibers. Due to their properties, aramid fibers can be used in protective clothing and helmets, bulletproof jackets, body armor, drumheads, speaker woofers, ballistic applications, and flame-resistant suits and gloves.

3.3. Basalt fibers Basalt is a natural material that is readily available on earth. The origin of basalt is volcanic magma, which is dark in color and is produced by rapid cooling of molten lava. The cooling rate, source of lava, and geographical location of formation are the factors that affect the conditions and performance of basalt rock. Basalt fibers are a very thin fibrous form of basalt rock that have excellent mechanical and physical properties and good environmental stability [4]. Basalt fibers are considered a green industrial material due to their non-hazardous nature and efficient recyclability. Their easy processing and low cost make basalt fiber a good choice for the mass-production of advanced composites. Although carbon fibers have better mechanical properties than basalt fibers, the high production cost is a big obstacle to the mass production and application of carbon fiber-reinforced composites. Basalt fibers are 100% natural, and no additives are required during manufacturing. Basalt fiber exhibits high strength and modulus, extraordinary resistance to temperature and corrosion, and outstanding 125

Composites Part B 138 (2018) 122–139

G. Mittal et al.

4. Reinforcing nanomaterials

properties and remarkable mechanical properties [59,60]. Fire can be suppressed using CNT-reinforced materials [61]. Recently, Janas and his colleagues prepared a matrix-free CNT film and compared its flame-retardation performance with conventional materials. The outcomes revealed that the CNT films were durable, flexible, extremely lightweight, and can stop fire for twice as long as conventional flame-retardant materials [61]. CNTs can also provide super hydrophobicity to materials like cotton and steel, making them useful for industrial applications like waterproof textiles, self-cleaning materials, anti-fogging applications, lubrication, and low-friction coatings [62]. Moreover, CNTs have the potential to purify air and water. Researchers have found that hollow CNTs can be used as pores in filtration membranes as they provide easier transport to water molecules through smoother walls than other nanopores and reject salt ions [63]. The pores of CNTs can be customized for selective transport of the molecules. Further, CNT-based membranes can be used in air purification applications to protect the human body against deadly viral or bacterial infections [64]. CNT-based filters can remove a vast range of microbes by physical straining or puncture or depth filtration due to their high surface area and good adsorption capability. There are many reports suggesting that CNTs are potential candidates for next-generation sensing applications like gas sensing, humidity sensing, fluorescence sensing, electro-chemical sensing, and mechanical sensing because CNTs have high structural porosity and an extremely high surface to volume ratio [65]. The adsorbing capacity or sensitivity of the CNTs toward different molecules can be altered by attaching chemical groups on defect areas and end caps of the CNTs. The linear geometry, high surface area, and remarkable electrical conductivity favor the use of CNTs as electrodes in batteries and capacitors [66]. Due to their remarkable conductivity, CNTs are used as conductive fillers for various applications like electromagnetic shielding, conductive adhesives, and coaxial cables. Further, CNTs are building blocks for electronic circuits in molecular electronics. SWNTs are highly flexible and stretchable and have larger current densities (compared to copper or aluminum) that make them suitable replacements for metal electrodes for printable field-effect transistors (FETs) with high carrier mobilities [67]. CNTs also have very sharp tips that provide a more concentrated electric field, which consequently increases field emission, making them suitable for designing low-power electrical devices [68]. In addition, CNTs are used in advanced computing where good thermal conductivity is required.

4.1. Carbon nanotubes CNTs are one-dimensional cylindrical hollow structures made up of single pure graphite sheets, and the ends of the cylinders are capped with half of a fullerene structure, while the bends consist of pentagonal or heptagonal rings. The outstanding mechanical properties of CNTs are due to the strong bonding between the carbon atoms in carbon rings [3]. Additionally, free movement of electron charge in the nanotube is possible due to electron delocalization within the unique hexagonal pattern of the carbon rings. Similarly, CNTs possess remarkable thermal conductivity due to the vibration of carbon atoms in a way that produces heat conduction. The diameters of these tubes are on the nanometer scale, while the lengths are in microns. Consequently, they exhibit extremely high aspect ratios. The atoms of CNTs are arranged in three main categories, i.e., zigzag, armchair, and chiral, and the features of CNTs will change based on these configurations. For instance, tubes with armchair arrangements are always metallic, whereas metallic or semiconducting properties are observed for zigzag arranged tubes [56]. Similarly, the characteristics of CNTs depend upon the number of walls. Single-walled CNTs (SWNTs) exhibit superior electrical properties compared to multi-walled CNTs (MWNTs) due to the uniform distribution of current across the nanotube. CNTs are 100 times stronger than steel, yet one-sixth the density of steel. CNTs show very good plasticity and flexibility due to their hollow and closed structure [57]. Therefore, CNTs can bend to a small angle in the presence of an applied force and can fully regain their original shapes after removal of the applied stress. CNTs are a very good reinforcement candidate due to their extremely high aspect ratio (∼1.32 × 108) and near zero weight that helps in achieving advanced properties even with a small amount of reinforcements. CNTs show exceptional electrical properties that highly depend on the orientation of atoms and number of walls [56]. Because of the increased number of defects with increasing number of walls, MWNTs shows inferior electrical properties compared to SWNTs, while SWNTs show a uniform distribution of current throughout the tube and consequently possess excellent electrical properties. Likewise, the excellent thermal conductivity (for individual SWNT at 3500 Wm−1K−1) and a very low coefficient of thermal expansion are the attractive feature of SWNTS. Therefore, they can be used in molecular electronics. There are different synthesis routes for CNTs. Arc-discharge methods, laser-ablation methods, and chemical vapor deposition are extensively used. The method of synthesis is one of the factors that affects the performance of CNTs [58]. For instance, the CNTs grown using arc-discharge and laser-ablation are high quality and possess fewer voids and defects compared to those produced by chemical vapor deposition [3]. However, the production cost of CNTs grown using the arc-discharge or laser-ablation method is very high in comparison to that of chemical vapor deposition due to the high voltage and costly lasers. Therefore, the latter is suitable for mass-production and industrial applications of CNTs. Moreover, chemical vapor deposition is the method that supports the qualitative and quantitative production of CNTs by controlling their various growth parameters [3]. During synthesis, undesired particles or impurities like catalyst residue and amorphous residue are also formed and influence the performance of CNTs; this is why further processing like gel chromatography or separation through density gradient centrifugation paired with selective surfactant wrapping of CNTs is required [3]. Alternatively, CNT surfaces can be modified using chemicals or addition of surfactants on CNTs walls. However, these methods sometimes produce defects in CNT walls or break the CNTs into shorter lengths. Because of their high strength and low weight, CNTs are primarily used as reinforcing materials in all kinds of matrices to provide structural stability. Numerous studies indicate that CNTs are ideal candidates for bullet proof vests due to their high energy absorbing

4.2. Graphene Graphene is widely known as “wonder material” due to its extraordinary electrical, optical, mechanical, and chemical properties. It is an allotrope of carbon that is basically a building block for all graphitic applications. In graphene, sp2 carbon atoms are arranged in a hexagonal lattice to form a 2D layer. The unique properties of graphene include a large surface area (2630 m2/g), a high elastic modulus (1 TPa), outstanding conductivity (intrinsic carrier mobility of 2000000 cm2/V), high optical transparency, and high thermal conductivity (5000 W/mK). These properties make it a desirable nanofiller in a diverse range of applications [69]. A brief overview of the properties of graphene compared with different carbon nanomaterials is presented in Table 1 [13]. Nevertheless, the large-scale, cost-efficient, and high-quality production of graphene for real applications is still a major challenge. Since graphene is a high potential material, researchers are constantly trying to realize all of its advantages. Numerous preparation methods have been reported including mechanical and chemical exfoliation of graphite, chemical vapor deposition, unzipping of CNTs, solvothermal synthesis, and mechanical milling [3]. The choice of production method influences the characteristics and applications of the produced graphene. For instance, the exfoliation method is desirable due to its costefficiency, but the produced graphene is low quality. In contrast, 126

Composites Part B 138 (2018) 122–139

G. Mittal et al.

Table 1 The properties of graphene and other carbon allotropes [13]. Carbon allotropes

Graphite

Diamond

Fullerene (C60)

Carbon nanotube

Graphene

Hybridized form Crystal system Dimension Experimental specific surface area (m2 g−1) Density (g cm−3) Optical properties

sp2 Hexagonal Three ∼10–20

sp3 Octahedral Three 20–160

Mainly sp2 Tetragonal Zero 80–90

Mainly sp2 Icosahedral One ∼1300

sp2 Hexagonal Two ∼1500

2.09–2.23 Uniaxial

3.5–3.53 Isotropic

1500–2000a, 5–10b High Flexible non-elastic Electrical conductor

Electrical conductivity (S cm−1)

Anisotropic, 2––3 × 104a, 6b

900–2320 Ultrahigh – Insulator, semiconductor –

>1 Structure-dependent properties 3500 High Flexible elastic Metallic and semiconducting

>1 97.7% of optical transmittance

Thermal conductivity (W m−1 K−1) Hardness Tenacity Electronic properties

1.72 Non-linear optical response 0.4 High Elastic Insulator 10−10

Structure-dependent

a b

4840–5300 Highest (single layer) Flexible elastic Semimetal, zero-gap semiconductor 2000

Axial direction. Cross-axial direction.

flame retardant materials, and many industrial products are already made by adding various clay minerals into the polymer matrices. Additionally, clay minerals are extensively used as efficient absorbents to remove fluoride from water due to its low-cost, high absorption, and high surface area. Because of these remarkable properties, clay based composites are being used in various applications like the automobile industry, food packaging materials, coating and pigments, drug delivery, bio-sensors, and other medical devices [71].

chemical vapor deposition is used to produce customized (single layered or few-layered) and high quality graphene in bulk for industrial applications. Many reports related to tailoring the structure and properties of graphene as per the user requirements are available. Further, the use of graphene polymer matrices enhances the electrical, thermal, and mechanical properties of the composites. Another obstacle in employing graphene as a potential filler material is the non-uniform dispersion of graphene into polymer matrices [70].

4.4. Metal nanoparticles

4.3. Clays

Many inorganic nanoparticles are also used as nano-reinforcements, and some of them are mentioned in Table 2 [73].

The term “clay” indicates a group of layered silicates including metal oxides and cations that have attracted attention since prehistoric times because of their unique barrier and structural properties. Clays are one of the widely used filler materials with high surface area (750 m2/g) and low cost [71]. Clays are basically layered structures in which joined sheets of silica and alumina are arranged in a parallel manner, with around 1 nm interlayer spacing filled with exchangeable ions and OH groups. Generally, clays are crystalline, and the adjacent layers are attached through van der Waals forces. These layers possess a negative charge due to the isomorphic substitution within layers that is counterbalanced by Na+ or K+ ions. Clay minerals can be classified into 3 types based on silica and alumina sheet proportions of 1:1, 2:1, and 2:2 [72]. When clay is added the polymer matrix, its hydrophilic nature makes it difficult to disperse into a polymer matrix. This leads to poor interactions between the clay and polymer matrix. To enhance the compatibility between both materials, the clay particles are treated with organic modifiers that attach chemical moieties onto clay layers by an ion-exchange reaction. The interlayer spacing increases because of the grafting of organic cations. Consequently, organic polymer diffuses between the layers. Hence, the compatibility and the interfacial interactions between the polymer and clay particles increase [72]. Expansion after moisture absorption is a main attribute of the clay materials that varies with the type of clay mineral. For instance, kaolinite does not show any expansion due to very strong bonding between the layers. In contrast, MMT exhibits very weak bonding due to the cations, and the interlayer distance increases in the presence of water. Consequently, the surface area also increases. Similarly, a cation exchange capacity of 2:1 types is higher compared to others due to the isomorphic substitution and large number of exchange sites. Another interesting feature of clay minerals is flocculation and dispersion. Flocculation depends on the nature of ions present. For instance, Ca+ and H+ favor flocculation. Dispersion is affected by K+ and Na+, as Na+ possesses a thick electric double layer encompassing the ions that give rise to the suspension. Furthermore, clay minerals act as green

5. Processing of reinforcements for multi-scale composites 5.1. Processing of fiber reinforcements Some surface treatment methods have been reported to improve the mechanical and thermal properties of reinforced fibers [74]. These surface treatment methods both enhance the properties by altering the interfacial interactions between the filler and the matrix and protect the reinforced fibers against destruction. Sometimes these treatments are employed as pre-treatment methods that further support the attachment of various chemical moieties. These surface treatment methods can be divided into two major types: physical and chemical treatments. Electric discharge is an extensively practiced physical treatment method that alters the surface properties [75]. For instance, Su et al. employed oxygen-plasma treatment to modify the surface of Kevlar fibers, and this enhanced the interfacial interactions between fiber and matrix, resulting in improvements in the dielectric properties and water resistance [76]. Plasma treatment is highly desirable because it is very effective, does not produce any chemical waste, and is easily scalable. In addition, plasma treatment does not alter the bulk properties of the material, but affects the outer surface only. Plasma treatment participates in surface cleaning, ablation, introduction of free radicals, and alteration of surface energy. Another example of electric discharge treatment is corona discharge in which ionized air modifies the surface polarity due to the bombardment of ozone, oxygen, or free radicals [77]. However, the limitation of the corona treatment is that it is suitable only for 2D woven fabrics. For 3D structures, alternative methods like UV or ozone treatments are being used [78]. Another eco-friendly, cost-efficient, and effective physical method used to alter the fiber properties (especially natural fibers) is thermal treatment, which modifies the chemical structure of the fiber at higher temperature 127

Composites Part B 138 (2018) 122–139

Antibacterial and antiviral applications, biosensing, MRI, cancer diagnosis and photothermal cancer therapy Optical and photothermal properties due to surface plasmon resonance, thermal, electrical, antiangeogenic, catalytic, magnetic, and thermooptical properties

Optical properties due to surface plasmon resonance (SPR), antiangiogenic, structural, thermal, electrical and catalytic properties

(below the glass-transition temperature) by altering its properties including chemical-physical properties (like shrinkage reduction), decay resistance, crystallinity, weather resistance, and equilibrium moisture content [79]. Numerous chemical treatment methods have been reported that significantly improve the performance of the reinforced composites. Alkaline treatment (mercerization) is extensively used to enhance the interfacial properties between natural fibers and the matrix by introducing surface roughness and removing hydrogen bonds in network structure of fibers [80]. Another desirable chemical treatment is silane treatment, in which silane coupling agents are attached to the fiber surface. Silane coupling agents are multifunctional chemicals that interact with the fiber hydroxyl groups and alter the fiber surface properties like hydrophobicity. Moreover, the wettability, swelling, surface tension, interfacial interactions and compatibility with the matrix improve after silane treatment. For instance, Arun Prakash and Rajadurai treated and compared the interlaminar shear strength of glass fiber treated with acid, base, and silane coupling agents and found that the composite with silanized fiber exhibited superior properties [81]. After silanization, the interfacial adhesion between the matrix and the glass fiber increased, which restricted pull-out and mechanical failure. Another method is esterification or acetylation, which is a method used for plasticizing natural fibers in which acetyl groups react with hydroxyl groups of the fiber and alter the compatibility, introduce roughness, and improve interfacial adhesion between the fiber and matrix. Benzoylation, peroxide treatment, sodium chloride treatment, acrylation, steric acid treatment, triazine treatment, and permanganate treatment are other chemical methods reported in the literature. Fungal treatment is a bio-chemical treatment method that is an environmentally friendly and efficient way of modifying the surface of natural fibers. Fungal growth generates small holes on the fiber surface that improve adhesion by surface roughening. Additionally, fungi secrete enzymes that dissolve non-cellulosic components from the fiber surface. For instance, Pickering and his colleagues improved the strength of hemp fiber-reinforced composites by 22% after fungal treatment, which was 32% higher than the alkali-treated hemp fiberreinforced composites [82]. 5.2. Processing of nano-reinforcements

Magnetic

Ag

Au

5

6

7

Co-precipitation, microemulsions, sol–gel techniques, solvothermal, electrochemical, pulsed laser ablation and sonochemical method Microwave processing, ultrasonic spray pyrolysis,laser ablation, gamma irradiation, chemical reduction by inorganic and organic reducing agents, photochemical method, thermal decomposition of silver oxalate in water and in ethylene glycol and electrochemical synthesis Chemical reduction, physical reduction, photochemical reduction, solvent evaporation techniques, microwave irradiation

Physicochemical, optical, luminescent, thermal and mechanical properties Magnetic, caloric,physical and hydrodynamic properties SiO2 4

Sol–gel, flame synthesis, water-in-oil microemulsion processes

Optical, transport, mechanical and fracture properties Al2O3 3

Flame spray pyrolysis, reverse microemulsion, sol–gel, precipitation and freeze drying

ZnO 2

Sol–gel, homogeneous precipitation, mechanical milling, organometallic synthesis, microwave method, spray pyrolysis, thermal evaporation and mechanochemical synthesis

Optical properties, thermal conductivity, electrical, sensing, transport, magnetic and electronic properties

Photocatalysis, dye-sensitized solar cells, gas sensor, nanomedicine, skin care products, waste water treatment by removal of organic and inorganic pollutants and antimicrobial applications Electronic and optoelelectronic device applications, gas sensor, photocatalytic degradation of oraganic and inorganic pollutants for waste water treatment, cosmetics, medical filling materials, antimicrobial and anticancerous applications Waste water and soil treatment by removal of heavy metal ions and antimicrobial applications, ceramic ultrafilters and membranes to remove pathogenic microorganisms, for gas separation, in catalysis and absorption processes and drug delivery etc. Drug delivery, tissue engineering, carrier for antimicrobial applications, biosensing Biomedicine, cancer treatment, MRI, drug delivery, removal of toxic metal ions and antimicrobial applications Antibacterial and antifungal applications in water purification systems, paints and household products, antiviral applications against HIV-I and monkey pox virus, biosensing, Optical, electronic, spectral, structural, mechanical and anticorrosion properties TiO2 1

Hydrothermal, sonochemical, solvothermal, reverse micelles, sol gel, flame spray pyrolysis and nonhydrolytic approach

Applications Properties Synthesis Nanoparticle Sr. No.

Table 2 Synthesis, properties, and applications of selected inorganic nanoparticles [73].

G. Mittal et al.

Nanomaterials are potential reinforcements that can improve the performance of final composite materials even when used in very small amounts. However, there are different kinds of challenges associated with the use of nanomaterials, and achieving a homogenous dispersion is a main challenge. Nanomaterials have the tendency to agglomerate and form bundles due to high van der Waal interactions. Consequently, the surface area of the agglomerated nanomaterials decreases compared to the separated nanoparticles. Moreover, agglomeration hinders the uniform dispersion of nanomaterials throughout the matrix, which generates weak points and leads to the failure of the final material. To reach the full benefit of reinforced nanomaterials and advance the performance of the final materials, scientists have reported numerous methods such surface modification using physical or chemical methods, ultra-sonication, and grafting of nanomaterials onto reinforced fiber or the polymer. When it comes to preparing multi-scale composites, the first approach is to disperse nanofillers into the matrix along with fiber reinforcements using different mechanical, chemical, or physical methods. 5.2.1. Reinforcement in the matrix Mechanical methods: To homogenously disperse nanomaterials and improve filler-matrix interactions, it is essential to break the bundles into smaller sizes. After separation, it is necessary to stabilize the nanomaterials so that they do not form agglomerates again and instead remain properly dispersed. Hence, numerous methods have been reported to achieve the optimum dispersion of nanofillers in the host 128

Composites Part B 138 (2018) 122–139

G. Mittal et al.

other chemical groups. Nevertheless, sometimes acid treatment distorts the main structure of the material and affects the mechanical and electrical properties of the material. To avoid this problem, other physical methods like plasma treatment are used to functionalize the surface. Another approach to advance the functionality of nanomaterials and provide proper dispersion is grafting of synthetic polymers onto the nanomaterials [89]. Monomers can easily penetrate agglomerates and react with the nanoparticles. When polymerization takes place, the nanomaterials start separating. Moreover, the nanomaterials become hydrophobic after grafting, making them compatible with polymer matrices; hence, these grafted polymers are also known as compatibilizers, and their compatibilizing efficiency depends upon the structure of the polymer. For instance, Gong et al. grafted the surface of chemically reduced graphene oxide with PMMA using emulsion polymerization [90]. After grafting, the reduced graphene sheets were exfoliated, and the interactions between sheets and host matrix improved, leading to uniform dispersion of graphene sheets into the matrix. A composite with polymer-grafted graphene exhibited 42% and 15% increases in elastic modulus and tensile strength, respectively. Similarly, the antimicrobial properties of nano ZnO coatings were improved by grafting copolymers that reduced agglomeration of ZnO as well as enhanced the surface area [91]. Ligand exchange is also a widely used method for quantum dots that improve the uniform dispersion of nanomaterials by restricting selfaggregation [92]. Moreover, the properties of nanomaterials can be altered by ligand exchange methods [93].

matrix. Generally, mechanical methods are used to separate agglomerated nanomaterials using high shear forces. Most mechanical methods are single-step, cost-efficient, and eco-friendly. Ultra-sonication, mechanical mixing, ball-milling, and cryo-milling are examples of widely used mechanical methods to separate nanomaterials agglomerated in the bundles. Among these, ultra-sonication is the preferred technique in which the dispersion level can be controlled by controlling the frequency. Sumitomo et al. compared the dispersion efficiency of ultrasonic irradiation and mechanical stirring and found that the dispersion rate of ultrasonic irradiation is higher than that of mechanical stirring [83]. However, the high shear energy of ultra-sonication introduced defects or denatured the structure of the nanomaterials, decreasing the performance. Moreover, ultra-sonication is required for a prolonged time (at least 30 min). Otherwise, the nanomaterials start sinking to the bottom of the solution. Another widely practiced technique is ball-milling, which separates the agglomerates of nanomaterials using strong shear forces generated through high-frequency rotating metal balls. Many studies show that ball milling provides CNTs that are less entangled, have smaller lengths or diameters, and open ends. Sometimes the main properties decrease when using this technique. Moreover, heat is generated due to highspeed milling, which adversely affects the nanomaterials. To avoid issues related to the heat generation, cryo-milling was used where a closed milling chamber was jacketed with continuously flowing liquid nitrogen. The generated defects can be reduced and proper dispersion of nanofillers can be achieved because of the very low temperature and the reduced processing time. For instance, Mittal et al. studied the effects of adding cryo-milled CNTs into a PMMA matrix and found that the composites showed significant improvement in thermal, electrical, and shielding properties over the raw CNT-reinforced PMMA composites [84]. Chemical methods: Since the mechanical approaches use shear forces that distort the structure of nanomaterials and diminish their performance, chemical approaches are required. Numerous chemicals are used to modify or functionalize nanomaterial surfaces. These chemical methods are low-cost, large-scale, easy to conduct, and can inhibit the re-agglomeration of the dispersed nanomaterials. Under these methods, the dispersion of nanofillers into the matrix or filler-matrix interactions are improved either by altering the surface of the nanomaterial or by attaching chemical moieties onto the nanomaterial surface. Chemical methods provide a permanent solution to the issue of re-agglomeration by breaking bonds or interactions among agglomerated nanomaterials and generating steric repulsion by introducing chemical groups with larger molecules/long chains. Surface modification using silane coupling agents is extensively used, efficient, and is an easy way to advance the dispersion along with improved interfacial strength between the filler and matrix. Moreover, silane coupling agents improve the compatibility between the host and filler, which improves the performance of the final composite [85–87]. Numerous studies have been published regarding silane modification of nanomaterials along with layered structures. Hexagonal boron nitride (hBN), also known as white graphite, has parallel layers attached through van der Waals forces. However, it sediments at the bottom of the matrix due to poor interaction with the matrix. Mittal et al. reported that the van der Waals forces among layers are interrupted, and exfoliation occurred after silane modification. The thicknesses of the hBN stacks changed from a few hundred nanometers to a few nanometers because of exfoliation. Moreover, the chemical moieties of the silane coupling agent attached to the hBN surface, which interacted with the matrix and enhanced the properties of the composite [88]. Similarly, clay particles can be exfoliated and converted into nano-clay. Generally, the attachment of a coupling agent does not alter the mechanical or electrical properties of the materials. Further, many functionalization methods are used by researchers in which a nanomaterial is treated with concentrated acids that oxidize the surface of the material, which further reacts with the matrix or

5.2.2. Grafting of nanomaterials onto the fibers In this approach, nanofillers are grafted onto the traditional fibers using techniques like chemical vapor deposition, electrophoresis, or using chemical groups. These nanomaterial-coated fibers are also known as hierarchical fibers that exhibit advanced interfacial properties. Apart from multi-functionality, nanomaterials grafted onto fibers provide improved out-of-plane properties in the final material. The easiest and cheapest approach to coat nanomaterials onto the fibers is dip coating, in which the fiber is dipped into an aqueous solution of targeted nanomaterial and the coating is formed on both the outer and inner surfaces [94]. There are many examples showing that the properties of the materials were improved after coating the nanomaterials onto the fibers using dip-coating. For instance, Qi et al. coated MWNTs onto cellulose fibers using this technique. The MWCNTs were attached to the fibers due to the strong non-covalent interactions that both increased mechanical and electrical properties and advanced the sensing ability of the fibers towards external stimuli [95]. Similarly, Jamnani et al. improved the tensile and interlayer shear strength of glass fiber by attaching acid treated CNTs via dip-coating in the presence of a Nafion binding agent [96]. The coating thickness can be controlled by changing the number of coating cycles [97]. Another simple technique to coat nanomaterials onto the fibers is spray coating, which improves the out-of-plane properties and introduces multi-functionality by restricting the localization of nanofillers. Zhang et al. coated a connected network of CNTs onto carbon fiber prepreg and found that even a very small amount of CNTs (0.047 wt%) improved the Mode-I fracture toughness by 50% [98]. Additionally, the CNTs provided in situ damage sensing capability for the composite. Alternatively, the nanomaterials can be dispersed into a sizing agent that protects the fibers against damage or friction, improves the interfacial properties, and provides multi-functionality. Ashish and his colleagues adopted this method to coat glass fibers with a sizing agent mixed with CNTs; the resulting composite showed an improvement in glass transition temperature, coefficient of thermal expansion, and fracture toughness [99]. The improved interfacial interaction restricts the mobility of the chains and provides rigidity to the composite. Furthermore, Liu et al. provided multi-functionality to carbon fiber/epoxy 129

Composites Part B 138 (2018) 122–139

G. Mittal et al.

Fig. 2. Schematics of typical surface coating techniques of fibers and related fabrics: (a) CVD process, (b) electrophoresis process, (c) sizing process, and (d) spray coating process. Based on ref. [114 & 115]. Reprinted with permission from Elsevier.

composites by coating the carbon fiber with luminescent nanoparticledoped sizing agents [100]. The composite showed improved interfacial adhesion along with luminescent labeling properties. Furthermore, the nanomaterials can be grafted onto the fibers using binding or coupling agents. First, fibers are treated with chemicals so that reactive groups can be generated. Among all treatments, oxidative treatments are very common. Oxidative treatments generate –OH or –COOH groups on the fiber surface that easily react with other coupling or binding agents [101]. Tang et al. bound silver nanoparticles onto activated carbon fiber using chitosan as a binding agent and obtained excellent antibacterial properties [102]. Similarly, Hua and his colleagues simultaneously grafted CNTs and graphene oxide onto glass fiber, in which the glass fiber was first treated with silane coupling agent and immersed in an aqueous solution of CNTs and graphene oxide [103]. The synergistic effects of CNTs and graphene oxide improved the interfacial properties

and the wettability of the material. Chemical vapor deposition (CVD) is another method to craft nanomaterials (especially CNTs) onto fiber; in this method, CNTs are directly grown on the fiber surface at high temperature in the presence of metal catalyst [104]. Usually, hydrocarbon gases are used as a carbon source, and catalyst can be deposited onto the fiber surface by various methods like dipping, spraying, electrodeposition, sputtering, and thermal evaporation. Similar to other grafting methods, this also increases the surface area of the fiber and improves interfacial interlocking, which consequently leads to better stress transfer between the matrix and fibers. Most research has focused on carbon fibers infused with CNTs. For instance, Hansang Kim synthesized CNTs directly on woven carbon fabric before lay-up using a chemical vapor deposition method and fabricated a composite with enhanced crack detection sensitivity [105]. Since glass fiber has a low thermal resistance, it is difficult to perform 130

Composites Part B 138 (2018) 122–139

G. Mittal et al.

Fig. 3. (a) Schematic synthesis steps of functionalized clay and clay@CF, (b) SEM images of CF (c) and clay@CF, (d) TGA thermograms of CF and clay@CF, and (e) interfacial shear strength. Based on ref [122]. Reprinted with permission from Elsevier.

Fig. 2 depicts schematic diagrams of the grafting of CNTs onto fibers using different coating techniques [114,115].

CVD on a glass fiber in order to grow CNTs. Nevertheless, some scientists have tried to grow CNTs on a glass fiber surface using CVD. For instance, Rahman et al. uniformly coated CNTs onto woven glass. Iron acetate was used as a catalyst, and the growth temperate was kept below the glass transition temperature of the glass fibers. In-situ growth of CNTs onto the fiber improved stress transfer bridging between the fiber and matrix, and the resulting composites exhibited a ∼17% and ∼15% increase in flexural strength and flexural modulus compared to the neat composites, respectively [106]. Recently, He et al. enhanced the thermo-mechanical and electrical properties of glass fibers by growing CNTs directly on the surface using CVD [107]. They found that the properties of the fibers can be controlled by varying the CVD conditions. Moreover, they observed 12%, 21%, and 26% improvements in storage modulus, flexural modulus, and strength, respectively. Along with carbon and glass fibers, CVD is also applicable for inorganic fibers like SiC fabrics, aluminum silicate, alumina fiber, quartz fiber, and silica fiber. Similar to CVD, electrophoretic deposition (EPD) is a practical, scalable, and cost-efficient method used to graft nanomaterials onto fabrics [108]. In EPD, two parallel electrodes (i.e., the working and counter electrodes) are connected to the power supply, and positively and negatively charged particles are deposited onto the cathode and anode, respectively. For instance, carbonyl iron microparticles (CIP) were deposited onto carbon fibers using EPD in which CIPs were dispersed in acetone-ethanol solution along with stabilizer [109]. Similarly, negatively charged graphene oxide was also deposited onto glass fibers using EPD, and the thickness of the coating could be controlled by varying the voltage and time of EPD [110]. Likewise, acid-treated CNTs were deposited onto oxidized carbon fiber and showed a 60.2% enhancement in interlaminar shear strength [111]. In a typical EPD setup, the CNT concentration was 0.05 g/L, a voltage of 20 V was applied for 20 min, and the distance between the electrodes was 2 cm. To obtain a uniform coating of nanomaterials, the two electrodes must be in a parallel position. CNTs, graphene, and aramid nanofibers (ANF) were also coated onto the carbon fibers [112]. Lee et al. synthesized aramid nanofibers and suspended carbon fibers into the 3.5L ANF solution. During EPD, these ANF were coated onto carbon fibers and improved the surface energy and interfacial shear strength by 39.7% and 34.9%, respectively. It is reported that the coatings obtained using EPD are compact, and the coating thickness can be adjusted by controlling various parameters of EPD [113].

6. Properties of multiscale polymer composites Mechanical Properties: Multi-scale composites possess improved interfacial interactions and increased fiber surface area that helps the fibers to interact with a polymer matrix. The improved interlocking between polymer and fiber provides remarkable strength to the composite by advancing the stress transfer mechanism between fiber and matrix. Moreover, the reinforcing nanofillers reduce the number of voids formed during processing of a matrix and lead to a compact composite with high strength. Numerous reports show that the interlaminar shear strength (ILSS) is improved by incorporation of a nanophase in fiber-reinforced composites [116–119]. Srivastava et al. reported the effect of different nanofillers on mode I and mode II interlaminar fracture toughness of woven carbon fiber-reinforced epoxy composites [120]. They observed that a composite with graphene nanoplatelets showed 60.4% and 52.5% improvement in mode I and mode II toughness, respectively, compared to the neat composites. MWNTreinforced composites exhibited 44.2% and 29.4% increases in mode I and mode II toughness, respectively, in comparison to the neat composites. On the contrary, carbon black-reinforced composites showed mode I and mode II toughness even lower than that of the neat composites. Graphene provided better interactions with the matrix compared to the others because graphene possesses the highest surface area. Similar advancements in mechanical properties were observed by incorporating clay or inorganic nanomaterials into FRPs [121]. Furthermore, grafting of nanostructures on reinforced fibers further improved the mechanical property of the composites. The interfacial shear strength of carbon fiber-reinforced polymer composites increased by almost 33% after chemically grafting active clay due to efficient stress transfer bridging between the carbon fiber and matrix with the help of attached clay particles (Fig. 3) [122]. A 40–67% increase in interfacial shear strength of aramid fiber was observed after grafting TiO2 nanoparticles, which reduced the stress concentration between the fiber and the matrix by providing interfacial interlocking [123]. Du et al. reported 9.4% and 15.9% increases in flexural strength and flexural modulus, respectively, along with 10.2% and 25.4% increases in tensile strength and Young's modulus by coating graphene oxide on carbon fiber in carbon fiber-reinforced composites 131

Composites Part B 138 (2018) 122–139

G. Mittal et al.

this case, the LOI of the two composites increased from 19.0% to 23.2% and 23.7% after reinforcement with APP. Likewise, the flame retardant glass fiber-reinforced epoxy composites were prepared by adding a phenylphosphonate-based flame retardant that improved the LOI value of the composite up to 33% [135]. Furthermore, graphene, CNTs, and clay-reinforced flame retardant fiber composites were also synthesized. Hapuarachchi et al. reported that the heat release capacity of polylactide was reduced by 58% after reinforcement of CNTs and sepiolite nanoclays, and it was reduced by 45% after reinforcement of hemp fiber into polylactide [136]. Electrical properties: Since multiscale composites possess improved interfacial strength, the fillers make very good continuous networks that lead toward advanced electrical properties. For instance, Cortes et al. improved the transverse electrical conductivity of carbon fiber/ PEEK composites by at least 3 times by adding silver nanowires into the matrix [137]. Likewise, the electrical conductivity of carbon fiber/ epoxy composites was improved by dispersing CNTs via a freeze-drying method. The electrical conductivity along the longitudinal, transverse, and through thickness was improved by 49%, 189%, and 160%, respectively, indicating the favorable dispersion state of CNTs in that particular direction [138]. Beyond a particular loading of a conductive material, the electrical conductivity of the material suddenly increased. This point is known as the percolation threshold, which strongly depends upon the aspect ratio. This percolation threshold value is the concentration required to form a continuous conductive network. Generally, conductive fiber-reinforced composites show low throughthickness electrical conductivity, which limits their applications. This obstacle can be removed by grafting conductive fillers onto the fibers that enhance both in-plane and through-thickness electrical conductivity by creating an interconnected electrically conductive network. Kwon et al. coated graphene and CNTs onto carbon fiber using electrophoretic deposition and showed an improvement in the throughthickness electrical conductivity of a carbon fiber-reinforced composite of ∼1400% compared to the uncoated composites (Fig. 5) [139]. Along with electrical conductivity, electrical insulating properties can also be controlled using multiscale composites. Electrical insulation is required in some electronics applications. However, carbonaceous material possesses superior electrical conductivity that gives rise to electron leakage. To solve this issue, Zeng et al. fabricated a glass fiberreinforced bismaleimide–triazine composite reinforced with CNTs decorated with insulating SiO2 [140]. The formation of an insulating layer

compared to untreated composites [124]. Similarly, 1% reinforcement of CNTs into carbon fiber-reinforced epoxy composites enhanced the impact energy absorbance of the material [125]. Nevertheless, it was also reported that the properties start decreasing beyond the optimum loading of CNTs (or any other nanomaterials) due to the formation of aggregates of nanofillers that hindered the proper interaction of fillers with the matrix [126]. Thermal properties: A great number of studies showed that nanoparticle-reinforced fiber/polymer composites possess very good thermal properties due to the thermal stability of the fibers and improved interactions between the filler and matrix by nanofillers that provide stability and restrict the mobility of polymer chains during thermal treatment [127,128]. Kim et al. found a significant increase in glass transition temperature (from 57.2 °C to 62.4 °C) of fiber-reinforced epoxy composites after reinforcement of hybrid silica particles into the matrix [129]. Multiscale composites with advanced thermal properties were synthesized from graphene foam filled with carbon fiber-reinforced PDMS polymer (CF/GF/PDMS) by Zhao et al. [130]. CF/GF/ PDMS showed better thermal stability compared to the pure polymer and that without carbon fiber-reinforced graphene foam (i.e., GF/ PDMS) due to the crystallinity and rigidity of the carbon fiber. The temperature at which weight loss occurred shifted higher (from 450 °C to 550 °C) because of the improved interactions between the fiber and matrix. Moreover, due to the formation of a thermally conductive network of carbon fiber and graphene, CF/GF/PDMS showed 41% and 162% increases in thermal conductivity compared to GF/PDMS and pure PDMS, respectively. Li et al. lowered the coefficient of thermal expansion perpendicular to the carbon fiber by 30% by adding CNTs into 2D carbon fiber felt and pyrocarbon (PyC) composites via electrodeposition (Fig. 4) [131]. The lower coefficient was attributed to CNTs stiffening of the carbon matrix via improved interfacial bonding that restricted polymer mobility and decreased the contribution of carbon fibers in thermal expansion. Moreover, the flame-retardant properties of the material were improved by reinforcing materials applied at different scales [132]. Researchers have improved the flame retardance of natural fibers using flame retarding agents like ammonium polyphosphate (APP), magnesium hydroxide, and zinc borate [133,134]. Arjmandi and his colleagues improved the limiting oxygen index (LOI) of kenaf/polypropylene and rice husk/polypropylene composites by reinforcing them with APP [133]. Highly flammable materials possess low LOI, and vice versa. In

Fig. 4. SEM images of (a) CNTs introduced into felts by EPD, (b) the fracture surface of REF-C/C composites, (c) the fracture surface of CNT-C/C composites, and (d) CTE versus temperature of the composites in the Z direction. Based on ref [131]. Reprinted with permission from Elsevier.

132

Composites Part B 138 (2018) 122–139

G. Mittal et al.

Fig. 5. (a–f) SEM images of h-CFs at different EPD conditions and (g) in-plane and through-thickness electrical conductivities of h-CFRP and g-CFRP samples at different EPD conditions. Based on ref [139]. Reprinted with permission from Elsevier.

Fig. 6. (a) Schematic process of CFeMgO hybrid structure. SEM images of (b) pristine CF, (c) CFeCOOH, (d) CFeKH550, and (e) CFeMgO. (f) Thermal conductivity of pure CF/Nylon 6, CFeKH550/Nylon 6, and CFeMgO/Nylon 6 composites at various filler contents, and (g) surface resistivity of pure CF/Nylon 6, CFeKH550/Nylon 6, and CFeMgO/Nylon 6 composites at various filler contents. Based on ref [141]. Reprinted with permission from Elsevier.

composite with some unconventional properties. Self-healing is the process in which a material senses the damage and performs some actions or secretes a material to heal the damage. Selfhealing can be divided into two types: intrinsic and extrinsic. In intrinsic healing, the material reforms reversible covalent or non-covalent bonds to recover the damage, while in extrinsic healing, another phase or structure is added to the material. Micro-capsulation is a widely practiced approach for extrinsic healing in which the capsuled healing

on CNTs hinders the conductivity of CNTs and improves the electrical insulation of the composite. For similar reasons, Zhang et al. attached MgO nanoparticles onto carbon fibers via a coupling agent, followed by reinforcement in a nylon 6 matrix. The composite showed improved insulating properties due to the formation of an insulating layer of MgO that inhibits the electrical conductivity of carbon fibers (Fig. 6) [141]. Along with the abovementioned conventional properties, the introduction of multi-scale reinforcements into the matrix resulted in a 133

Composites Part B 138 (2018) 122–139

G. Mittal et al.

Fig. 7. Concept of the self-healing process using carbon nanotubes. Based on ref [145]. Reprinted with permission.

Fig. 8. Schematic representation of dimensional changes in polymeric solutions at surfaces and interfaces, in polymeric gels, and in polymer solids resulting from physical or chemical stimuli. Based on ref [146]. Reprinted with permission from Elsevier.

size and temperature, CNTs can break and release the filled healing agent to repair damage (Fig. 7) [145]. However, it is difficult to insert the healing agent into the tube and break the tube to release the healing fluid due to extremely small size and remarkable strength of CNTs. Therefore, CNTs are not yet used as containers to store healing agent. Most repair work is focused on self-healing composites; however, shape memory polymers (SMPs) are more desirable for realizing the full functionality of the material [146]. Multiscale composites are potential candidates for SMPs due to their excellent high elastic strain, scalable transition temperature, easy processing, light weight, and multi-functionality. Photoactive, magnetic-active, water-active, micro-wave heating, chemical, and electroactive effects are used as stimuli to induce shape memory effects in the material (Fig. 8) [146,147].

agent polymerizes to heal the damage or crack. Numerous reports based on self-healing fiber-reinforced composites have been published [142,143]. However, after healing of a crack by polymerization of resin, the damaged area becomes resin rich and can result in crack initiation due to the absence of reinforced fiber in that particular area. To avoid further crack initiation in that healed area, nanofillers are incorporated into the micro-capsules containing healing agent. Hence, the reinforced nanofillers increased the viscosity of the resin, leading to good self-healing properties [144]. The healing agent can be stored in many forms including nanotubes, microcapsules, and vessels. Since CNTs are extensively used in advanced composites, Lanzara et al. performed a molecular dynamic study to investigate the role of CNTs as a reservoir to contain healing agents and found that, depending on crack 134

Composites Part B 138 (2018) 122–139

G. Mittal et al.

Fig. 9. (a) Schematic illustration of a graphite stack oxidized to separate the individual layers of GO. (b) The role of GO in the interfacial bonding between carbon fiber and epoxy-based SMP matrix via van der Waals bonding and covalent crosslink, respectively. (c) Joule heating-induced shape recovery of SMP nanocomposite incorporated with 0.14 g carbon fiber and 0.03 g GO, and (d) snapshot of Joule heating-induced shape recovery in a SMP nanocomposite recorded by infrared video camera (VarioCAM HiRessl, JENOPTIK Infra Tec.). Based on ref [148]. Reprinted with permission from Elsevier.

Lu et al. prepared multi-scale graphene oxide/carbon fiber SMP composites that exhibited electrically driven shape recovery (Fig. 9) [148]. When the direct heating of SMP material is not achievable, electrically driven heating (or Joule heating) is very convenient. Proper dispersion and advanced interfacial interactions between reinforcements and the matrix in SMPs can be used in new applications. Practically, ‘one-way SMPs’ are being used for various applications in which external efforts are required to recover the original shape of the material due to the absence of full reversibility. This obstacle can be removed by ‘two-way SMPs’ that reform themselves to their original shape as soon as the stimulus is removed. Researchers are continuously making efforts to design ‘two-way SMPs’ [149]. Another remarkable property of multi-scale composites is sensing ability. Numerous reports are available on nanofibers decorated with different nanomaterials with great sensing ability. pH sensors with a fast response time (∼25 s for 4–11 pH) and high sensitivity (1.16 dBm/ pH) were fabricated by coating TiO2 on cone-shaped nanofibers [150]. Liang et al. fabricated a glucose sensor by coating graphene on silk fiber, followed by electrodeposition of spiked Pt nanospheres. Further, the enzyme was immobilized on the resulting film for use as a glucose biosensor electrode that showed very good sensitivity (150.8 μA mM−1 cm−2) toward glucose, with an ultralow detection limit [151]. Scientists are constantly making efforts to design smart sensors in which they can integrate different sensors. Calestani et al. developed a smart sensor by functionalizing commercially available carbon fiber with ZnO nanostructures. They found that the good conductivity of carbon fibers and good functional properties of ZnO gave rise to a piezoelectric sensor and a chemo-resistive gas sensor only at the intersection of two functionalized carbon fibers [152]. Fiber-reinforced composites are preferred to design sensors (especially mechanical sensors) due to their low-cost, flexibility, easy processing, and high sustainability [153]. Embedding is another way to include the sensing material into fiber-reinforced composites that both sense

damage or pressure and enhance sustainability [154]. 7. Applications of multiscale composites Multiscale composites can be employed in a large number of applications. Transportation applications (e.g., automobile industry, aeronautics, and defense) need advanced lightweight materials because the weight of the components is directly associated with fuel consumption. In the automobile industry, advanced multiscale composites are replacing heavy steel and injection molded automotive parts. Automotive components made of multiscale composites are lighter, safer, and more fuel-efficient. Carbon fiber-reinforced components provide excellent stiffness and strength along with good stability. Moreover, the strength of these composites can be tailored by incorporating nanofillers. Cars made of carbon fibers are popular due to their properties and aesthetics. However, these cars are still largely limited to racing cars due to their high production cost. However, BMW's i3 and Volkswagen's XL1 are examples of commercialized carbon fiber-based multiscale composites. Similar to automobiles, aircraft also require a lightweight body along with extraordinary strength in order to reduce fuel consumption. Around 50% of components in aircraft are made of composites that weigh up to 20–50% less than the original materials. Advanced composite technologies use mainly carbon fiber because it provides a high strength to weight ratio. However, the high cost of carbon fiber is still an issue. In addition, aramid fiber-reinforced composites are also used to design wing components with good stiffness and high impact resistance that protect the fuel-carrying engine pylons. Boeing and Airbus have designed aircraft in which around 50% of the components are made of multi-scale composites. Boeing has successfully replaced around 11000 metal components with 1500 composite components. These hybrid composites provide mechanical strength, damage 135

Composites Part B 138 (2018) 122–139

G. Mittal et al.

tolerance, thermal stability, and corrosion resistivity. Moreover, companies are adopting environmentally safe composites so that the aircraft can be recycled after retirement. Along with commercial aircrafts, these advanced composites are being used in space shuttles, fighter jets, hot air balloons, and gliders. The most common components made of multiscale composites are wing assemblies, helicopter rotor blades, fan blades, propellers, seats, and interiors. Marine applications also employ composites. In marine applications, corrosion is a key factor that affects the life span of the material. The use of multiscale polymer composites both protects against corrosion and provides the ease of manufacturing along with light weight. Glass fiber composites are highly desirable due to their low-cost and high corrosion resistance. Likewise, aramid fiber-based composites improve shock absorption properties and are being used to make racing powerboats in extreme sailing conditions. Carbon fiber-reinforced composites are being used to make hulls, keels, masts, and poles. Fothergill Composites Inc. has designed a cockpit using carbon and aramid fibers with aramid honeycomb core to protect drivers from accidents. Multiscale composites are used in the sporting goods industry to satisfy the demand for light weight, high strength, and low-cost sports materials. The fiber provides the required strength and flexibility to the material, and the reinforced nanophase eases the stress transfer by bridging the fiber and the matrix. Nanofillers can improve the properties of a material even when used in very small amounts, which make these materials cost-efficient. The most common examples of advanced sports equipment made of multiscale composites are tennis rackets, bicycles, golf clubs, skis, football helmets, kayaks, and hockey sticks. Likewise, in military-based applications, multiscale composites are highly desired due to their light weight, safety, durability, and multifunctionality for applications like drones, body armor, and vehicle parts (e.g., hoods, fenders, doors, window frames, load floors, ballistic panels, wheelhouse assemblies, engine covers, and battery covers). Another main application of multiscale composites is energy management. Components made of multiscale composites should exhibit good energy management so that energy can be stored, harvested, and distributed. Fuel cells, solar cells, marine turbines, power transmission, and supercapacitors are applications where multiscale composites are desired due to their multifunctionality. Further, multiscale composites are used in various medical applications like synthetic muscles, drug delivery, bio-sensors, dental implants, joint replacements and artificial organs. Other than these, multiscale composites can be used in bridges, tanks, pipes, and water purification systems.

compstruct.2015.08.116. [2] Azwa ZN, Yousif BF, Manalo AC, Karunasena W. A review on the degradability of polymeric composites based on natural fibres. Mater Des 2013;47:424–42. http:// dx.doi.org/10.1016/j.matdes.2012.11.025. [3] Mittal G, Dhand V, Rhee KY, Park S-J, Lee WR. A review on carbon nanotubes and graphene as fillers in reinforced polymer nanocomposites. J Ind Eng Chem 2015;21:11–25. http://dx.doi.org/10.1016/j.jiec.2014.03.022. [4] Dhand V, Mittal G, Rhee KY, Park S-J, Hui D. A short review on basalt fiber reinforced polymer composites. Compos Part B 2015;73:166–80. http://dx.doi.org/ 10.1016/j.compositesb.2014.12.011. [5] Al-Oqla FM, Sapuan SM, Anwer T, Jawaid M, Hoque ME. Natural fiber reinforced conductive polymer composites as functional materials: a review. Synth Met 2015;206:42–54. http://dx.doi.org/10.1016/j.synthmet.2015.04.014. [6] Hung P-Y, Lau K-T, Cheng L-K, Leng J, Hui D. Impact response of hybrid carbon/ glass fibre reinforced polymer composites designed for engineering applications. Compos Part B 2018;133:86–90. http://dx.doi.org/10.1016/j.compositesb.2017. 09.026. [7] Böer P, Holliday L, Kang THK. Interaction of environmental factors on fiber-reinforced polymer composites and their inspection and maintenance: a review. Constr Build Mater 2014;50:209–18. http://dx.doi.org/10.1016/j.conbuildmat. 2013.09.049. [8] Alkbir MFM, Sapuan SM, Nuraini AA, Ishak MR. Fibre properties and crashworthiness parameters of natural fibre-reinforced composite structure: a literature review. Compos Struct 2016;148:59–73. http://dx.doi.org/10.1016/j.compstruct. 2016.01.098. [9] Bakis CE, Bank LC, Brown VL, Cosenza E, Davalos JF, Lesko JJ, et al. Fiber-reinforced polymer composites for construction—state-of-the-art review. J Compos Constr 2002;6:73–87. http://dx.doi.org/10.1061/(ASCE)1090-0268(2002) 6:2(73). [10] Srinivasan M, Maettig P, Glitza KW, Sanny B, Schumacher A. Out of plane thermal conductivity of carbon fiber reinforced composite filled with diamond powder. Open J Compos Mater 2016;6:41–57. [11] Delucchi M, Ricotti R, Cerisola G. Influence of micro- and nano-fillers on chemicophysical properties of epoxy-based materials. Prog Org Coatings 2011;72:58–64. http://dx.doi.org/10.1016/j.porgcoat.2011.02.018. [12] Kenny JM, Torre L, Chiacchiarelli LM. The effect of processing routes on the thermal and mechanical properties of poly(urethane-isocyanurate) nanocomposites. J Appl Polym Sci 2015:132. http://dx.doi.org/10.1002/app.42750. [13] Wu Z-S, Zhou G, Yin L-C, Ren W, Li F, Cheng H-M. Graphene/metal oxide composite electrode materials for energy storage. Nano Energy 2012;1:107–31. http:// dx.doi.org/10.1016/j.nanoen.2011.11.001. [14] Michael FM, Khalid M, Walvekar R, Ratnam CT, Ramarad S, Siddiqui H, et al. Effect of nanofillers on the physico-mechanical properties of load bearing bone implants. Mater Sci Eng C 2016;67:792–806. http://dx.doi.org/10.1016/j.msec. 2016.05.037. [15] Lee S-Y, Shin HK, Park M, Rhee KY, Park S-J. Thermal characterization of erythritol/expanded graphite composites for high thermal storage capacity. Carbon N. Y 2014;68:67–72. http://dx.doi.org/10.1016/j.carbon.2013.09.053. [16] Vijayan PP, Puglia D, Al-Maadeed MASA, Kenny JM, Thomas S. Elastomer/thermoplastic modified epoxy nanocomposites: the hybrid effect of “micro” and “nano” scale. Mater Sci Eng R Rep 2017;116:1–29. http://dx.doi.org/10.1016/j. mser.2017.03.001. [17] Díez-Pascual AM, Naffakh M, Marco C, Gómez-Fatou MA, Ellis GJ. Multiscale fiberreinforced thermoplastic composites incorporating carbon nanotubes: a review. Curr Opin Solid State Mater Sci 2014;18:62–80. http://dx.doi.org/10.1016/j. cossms.2013.06.003. [18] Ladani RB, Ravindran AR, Wu S, Pingkarawat K, Kinloch AJ, Mouritz AP, et al. Multi-scale toughening of fibre composites using carbon nanofibres and z-pins. Compos Sci Technol 2016;131:98–109. http://dx.doi.org/10.1016/j.compscitech. 2016.06.005. [19] Korayem AH, Tourani N, Zakertabrizi M, Sabziparvar AM, Duan WH. A review of dispersion of nanoparticles in cementitious matrices: nanoparticle geometry perspective. Constr Build Mater 2017;153:346–57. http://dx.doi.org/10.1016/j. conbuildmat.2017.06.164. [20] Alam A, Wan C, McNally T. Surface amination of carbon nanoparticles for modification of epoxy resins: plasma-treatment vs. wet-chemistry approach. Eur Polym J 2017;87:422–48. http://dx.doi.org/10.1016/j.eurpolymj.2016.10.004. [21] Jalili MM, Davoudi K, Zafarmand Sedigh E, Farrokhpay S. Surface treatment of TiO2 nanoparticles to improve dispersion in non-polar solvents. Adv Powder Technol 2016;27:2168–74. http://dx.doi.org/10.1016/j.apt.2016.07.030. [22] Kumar A, Ghosh PK, Yadav KL, Kumar K. Thermo-mechanical and anti-corrosive properties of MWCNT/epoxy nanocomposite fabricated by innovative dispersion technique. Compos Part B Eng 2017;113:291–9. http://dx.doi.org/10.1016/j. compositesb.2017.01.046. [23] Sonnenfeld C, Mendil-Jakani H, Agogué R, Nunez P, Beauchêne P. Thermoplastic/ thermoset multilayer composites: a way to improve the impact damage tolerance of thermosetting resin matrix composites. Compos Struct 2017;171:298–305. http://dx.doi.org/10.1016/j.compstruct.2017.03.044. [24] Konuray AO, Fernández-Francos X, Ramis X. Latent curing of epoxy-thiol thermosets. Polym (United Kingdom) 2017;116:191–203. http://dx.doi.org/10.1016/ j.polymer.2017.03.064. [25] Chang J, Liang G, Gu A, Cai S, Yuan L. The production of carbon nanotube/epoxy composites with a very high dielectric constant and low dielectric loss by microwave curing. Carbon N. Y 2012;50:689–98. http://dx.doi.org/10.1016/j.carbon. 2011.09.029. [26] Odom MGB, Sweeney CB, Parviz D, Sill LP, Saed MA, Green MJ. Rapid curing and

7.1. Concluding remarks Based on the above discussion, we concluded that the performance of multiscale composites is affected by various factors such as type of matrix, type of fillers, interactions between fillers and host matrix, and size and shape of fillers. Since nano-reinforcements have the tendency to agglomerate, different processing methods are used for potential applications of composites with nano-reinforcements. Each processing method has pros and cons that determine the final physical, mechanical, thermal, and electrical properties of the composites. Moreover, these multi-dimensional fillers synergistically provide superior mechanical, thermal, and electrical properties along with other non-conventional properties like sensing capability, energy storage, and selfhealing. Composite materials with multiple fillers are also referred to as multi-functional materials and can significantly advance existing technologies. However, further research work is required to ensure that these technologies are cost effective. References [1] Liu Q, Paavola J. Lightweight design of composite laminated structures with frequency constraint. Compos Struct 2016;156:356–60. http://dx.doi.org/10.1016/j.

136

Composites Part B 138 (2018) 122–139

G. Mittal et al.

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

additive manufacturing of thermoset systems using scanning microwave heating of carbon nanotube/epoxy composites. Carbon N. Y 2017;120:447–53. http://dx.doi. org/10.1016/j.carbon.2017.05.063. Zhou J, Shi C, Mei B, Yuan R, Fu Z. Research on the technology and the mechanical properties of the microwave processing of polymer. J Mater Process Technol 2003;137:156–8. http://dx.doi.org/10.1016/S0924-0136(02)01082-8. Zhang D, He M, Qin S, Yu J. Effect of fiber length and dispersion on properties of long glass fiber reinforced thermoplastic composites based on poly(butylene terephthalate). RSC Adv 2017;7:15439–54. http://dx.doi.org/10.1039/ C7RA00686A. Rojo E, Alonso MV, Oliet M, Del Saz-Orozco B, Rodriguez F. Effect of fiber loading on the properties of treated cellulose fiber-reinforced phenolic composites. Compos Part B Eng 2015;68:185–92. http://dx.doi.org/10.1016/j.compositesb. 2014.08.047. Keck S, Fulland M. Effect of fibre volume fraction and fibre direction on crack paths in flax fibre-reinforced composites. Eng Fract Mech 2016;167:201–9. http:// dx.doi.org/10.1016/j.engfracmech.2016.03.037. Liu W, Luo L, Xu S, Zhao H. Effect of fiber volume fraction on crack propagation rate of ultra-high toughness cementitious composites. Eng Fract Mech 2014;124–125:52–63. http://dx.doi.org/10.1016/j.engfracmech.2014.03.007. Wang HW, Zhou HW, Gui LL, Ji HW, Zhang XC. Analysis of effect of fiber orientation on Young's modulus for unidirectional fiber reinforced composites. Compos Part B Eng 2014;56:733–9. http://dx.doi.org/10.1016/j.compositesb. 2013.09.020. Almeida JHS, Angrizani CC, Botelho EC, Amico SC. Effect of fiber orientation on the shear behavior of glass fiber/epoxy composites. Mater Des 2015;65:789–95. http://dx.doi.org/10.1016/j.matdes.2014.10.003. Efendy MGA, Pickering KL. Fibre orientation of novel dynamically sheet formed discontinuous natural fibre PLA composites. Compos Part A Appl Sci Manuf 2016;90:82–9. http://dx.doi.org/10.1016/j.compositesa.2016.07.003. Graupner N, Ziegmann G, Wilde F, Beckmann F, Müssig J. Procedural influences on compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites: influence of fibre loading, fibre length, fibre orientation and voids. Compos Part A Appl Sci Manuf 2016;81:158–71. http://dx.doi.org/10.1016/j. compositesa.2015.10.040. Pathan MV, Tagarielli VL, Patsias S. Effect of fibre shape and interphase on the anisotropic viscoelastic response of fibre composites. Compos Struct 2017;162:156–63. http://dx.doi.org/10.1016/j.compstruct.2016.11.046. Bond I, Hucker M, Weaver P, Bleay S, Haq S. Mechanical behaviour of circular and triangular glass fibres and their composites. Compos Sci Technol 2002;62:1051–61. http://dx.doi.org/10.1016/S0266-3538(02)00035-0. Chandra R, Singh SP, Gupta K. Micromechanical damping models for fiber-reinforced composites: a comparative study. Compos - Part A Appl Sci Manuf 2002;33:787–96. http://dx.doi.org/10.1016/S1359-835X(02)00019-2. Agnese F, Scarpa F. Macro-composites with star-shaped inclusions for vibration damping in wind turbine blades. Compos Struct 2014;108:978–86. http://dx.doi. org/10.1016/j.compstruct.2013.10.030. Stickel JM, Nagarajan M. Glass fiber-reinforced composites: from formulation to application. Int J Appl Glas Sci 2012;3:122–36. http://dx.doi.org/10.1111/j.20411294.2012.00090.x. Sathishkumar T, Satheeshkumar S, Naveen J. Glass fiber-reinforced polymer composites - a review. J Reinf Plast Compos 2014;33:1258–75. http://dx.doi.org/ 10.1177/0731684414530790. Newcomb BA. Processing, structure, and properties of carbon fibers. Compos Part A Appl Sci Manuf 2016;91:262–82. http://dx.doi.org/10.1016/j.compositesa. 2016.10.018. Shirvanimoghaddam K, Hamim SU, Karbalaei Akbari M, Fakhrhoseini SM, Khayyam H, Pakseresht AH, et al. Carbon fiber reinforced metal matrix composites: fabrication processes and properties. Compos Part A Appl Sci Manuf 2017;92:70–96. http://dx.doi.org/10.1016/j.compositesa.2016.10.032. Pereira AR, de Souza JCP, Iost RM, Sales FCPF, Crespilho FN. Application of carbon fibers to flexible enzyme electrodes. J Electroanal Chem 2016;780:396–406. http://dx.doi.org/10.1016/j.jelechem.2016.01.004. Menini M, Pesce P, Pera F, Barberis F, Lagazzo A, Bertola L, et al. Biological and mechanical characterization of carbon fiber frameworks for dental implant applications. Mater Sci Eng C 2017;70:646–55. http://dx.doi.org/10.1016/j.msec. 2016.09.047. Méjean C, Pometcu L, Benzerga R, Sharaiha A, Le Paven-Thivet C, Badard M, et al. Electromagnetic absorber composite made of carbon fibers loaded epoxy foam for anechoic chamber application. Mater Sci Eng B Solid-State Mater Adv Technol 2017;220:59–65. http://dx.doi.org/10.1016/j.mseb.2017.03.009. Raghubanshi H, Dikio ED, Naidoo EB. The properties and applications of helical carbon fibers and related materials: a review. J Ind Eng Chem 2016;44:23–42. http://dx.doi.org/10.1016/j.jiec.2016.08.023. Yang S, Zhu Z, Wei F, Yang X. Carbon nanotubes/activated carbon fiber based air filter media for simultaneous removal of particulate matter and ozone. Build Environ 2017;125:60–6. http://dx.doi.org/10.1016/j.buildenv.2017.08.040. Han Q, Yi Z, Wang F, Wu Y, Wang L. Preparation of bamboo carbon fiber and sandwich-like bamboo carbon fiber@SnO2@carbon composites and their potential application in structural lithium-ion battery anodes. J Alloys Compd 2017;709:227–33. http://dx.doi.org/10.1016/j.jallcom.2017.03.141. Moreno-Maroto JM, González-Corrochano B, Alonso-Azcárate J, Rodríguez L, Acosta A. Manufacturing of lightweight aggregates with carbon fiber and mineral wastes. Cem Concr Compos 2017;83:335–48. http://dx.doi.org/10.1016/j. cemconcomp.2017.08.001. Lee SO, Choi SH, Kwon SH, Rhee KY, Park SJ. Modification of surface functionality

[52]

[53] [54]

[55]

[56]

[57]

[58]

[59]

[60]

[61] [62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72]

[73]

[74]

[75]

[76]

[77]

137

of multi-walled carbon nanotubes on fracture toughness of basalt fiber-reinforced composites. Compos Part B Eng 2015;79:47–52. http://dx.doi.org/10.1016/j. compositesb.2015.03.077. Kim MT, Rhee KY, Jung I, Park SJ, Hui D. Influence of seawater absorption on the vibration damping characteristics and fracture behaviors of basalt/CNT/epoxy multiscale composites. Compos Part B Eng 2014;63:61–6. http://dx.doi.org/10. 1016/j.compositesb.2014.03.010. Jassal M, Ghosh S. Aramid fibres-An overview. Indian J Fibre Text Res 2002;27:290–306. Prashanth S, Subbaya KM, Nithin K, Sachhidananda S. Fiber reinforced composites - a review. J Mater Sci Eng 2017;6. http://dx.doi.org/10.4172/2169-0022. 1000341. Sahu NP, Khande DK, Patel GC, Sen PK, Bohidar SK. Study on aramid fibre and comparison with other composite materials, study on aramid fibre and comparison with other composite materials. Int J Innovat Res Sci Technol 2014;1:303–6. Dai H, Wong EW, Lieber CM. Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes. Science (80- ) 1996;272:523–6. http:// dx.doi.org/10.1126/science.272.5261.523. Pandya KS, Naik NK. Analytical and experimental studies on ballistic impact behavior of carbon nanotube dispersed resin. Int J Impact Eng 2015;76:49–59. http://dx.doi.org/10.1016/j.ijimpeng.2014.09.003. Mittal G, Dhand V, Rhee KY, Kim H-J, Jung DH. Carbon nanotubes synthesis using diffusion and premixed flame methods: a review. Carbon Lett 2015(16):1–10. http://dx.doi.org/10.5714/CL.2015.16.1.001. Haro EE, Odeshi AG, Szpunar JA. The energy absorption behavior of hybrid composite laminates containing nano-fillers under ballistic impact. Int J Impact Eng 2016;96:11–22. http://dx.doi.org/10.1016/j.ijimpeng.2016.05.012. Morka A, Jackowska B. Ballistic resistance of the carbon nanotube fibres reinforced composites - numerical study. Comput Mater. Sci 2011;50:1244–9. http://dx.doi.org/10.1016/j.commatsci.2010.03.046. Janas D, Rdest M, Koziol KKK. Flame-retardant carbon nanotube films. Appl Surf Sci 2017;411:177–81. http://dx.doi.org/10.1016/j.apsusc.2017.03.144. Nicola F De, Castrucci P, Scarselli M, Nanni F, Cacciotti I, Crescenzi M De. Superhydrophobic multi-walled carbon nanotube coatings for stainless steel. Nanotechnology 2015;26:145701. http://dx.doi.org/10.1088/0957-4484/26/14/ 145701. Ma L, Dong X, Chen M, Zhu L, Wang C, Yang F, et al. Fabrication and water treatment application of carbon nanotubes (CNTs)-Based composite membranes: a review. Membr (Basel) 2017;7:16. http://dx.doi.org/10.3390/ membranes7010016. Park KT, Hwang J. Filtration and inactivation of aerosolized bacteriophage MS2 by a CNT air filter fabricated using electro-aerodynamic deposition. Carbon N. Y 2014;75:401–10. http://dx.doi.org/10.1016/j.carbon.2014.04.019. Mittal M, Kumar A. Carbon nanotube (CNT) gas sensors for emissions from fossil fuel burning. Sensors Actuators, B Chem 2014;203:349–62. http://dx.doi.org/10. 1016/j.snb.2014.05.080. Zhang X, Cheng X, Zhang Q. Nanostructured energy materials for electrochemical energy conversion and storage: a review. J Energy Chem 2016;25:967–84. http:// dx.doi.org/10.1016/j.jechem.2016.11.003. Tang Y, Alva G, Huang X, Su D, Liu L, Fang G. Thermal properties and morphologies of MA–SA eutectics/CNTs as composite PCMs in thermal energy storage. Energy Build 2016;127:603–10. http://dx.doi.org/10.1016/j.enbuild.2016.06. 031. Lau PH, Takei K, Wang C, Ju Y, Kim J, Yu Z, et al. Fully printed, high performance carbon nanotube thin-film transistors on flexible substrates. Nano Lett 2013;13:3864–9. http://dx.doi.org/10.1021/nl401934a. Goenka S, Sant V, Sant S. Graphene-based nanomaterials for drug delivery and tissue engineering. J Control Release 2014;173:75–88. http://dx.doi.org/10.1016/ j.jconrel.2013.10.017. Mittal G, Rhee KY, Park SJ, Hui D. Generation of the pores on graphene surface and their reinforcement effects on the thermal and mechanical properties of chitosan-based composites. Compos Part B Eng 2017;114:348–55. http://dx.doi. org/10.1016/j.compositesb.2017.02.018. Zeng QH, Yu AB, Lu GQ (Max), Paul DR. Clay-based polymer nanocomposites: research and commercial development. J Nanosci Nanotechnol 2005;5:1574–92. http://dx.doi.org/10.1166/jnn.2005.411. Azeez AA, Rhee KY, Park SJ, Hui D. Epoxy clay nanocomposites - processing, properties and applications: a review. Compos Part B Eng 2013;45:308–20. http:// dx.doi.org/10.1016/j.compositesb.2012.04.012. Kango S, Kalia S, Celli A, Njuguna J, Habibi Y, Kumar R. Surface modification of inorganic nanoparticles for development of organic–inorganic nanocomposites—a review. Prog Polym Sci 2013;38:1232–61. http://dx.doi.org/10.1016/j. progpolymsci.2013.02.003. Xu F, Fan W, Zhang Y, Gao Y, Jia Z, Qiu Y, et al. Modification of tensile, wear and interfacial properties of Kevlar fibers under cryogenic treatment. Compos Part B Eng 2017;116:398–405. http://dx.doi.org/10.1016/j.compositesb.2016.10.082. Park S-J, Seo M-K, Rhee K-Y. Effect of Ar+ ion beam irradiation on the physicochemical characteristics of carbon fibers. Carbon N. Y 2003;41:592–4. http://dx. doi.org/10.1016/S0008-6223(02)00395-0. Su M, Gu A, Liang G, Yuan L. The effect of oxygen-plasma treatment on Kevlar fibers and the properties of Kevlar fibers/bismaleimide composites. Appl Surf Sci 2011;257:3158–67. http://dx.doi.org/10.1016/j.apsusc.2010.10.133. Hirvikorpi T, Vähä-Nissi M, Harlin A, Marles J, Miikkulainen V, Karppinen M. Effect of corona pre-treatment on the performance of gas barrier layers applied by atomic layer deposition onto polymer-coated paperboard. Appl Surf Sci 2010;257:736–40. http://dx.doi.org/10.1016/j.apsusc.2010.07.051.

Composites Part B 138 (2018) 122–139

G. Mittal et al.

[78] Gassan J, Gutowski VS. Effects of corona discharge and UV treatment on the properties of jute-fibre expoxy composites. Compos Sci Technol 2000;60:2857–63. http://dx.doi.org/10.1016/S0266-3538(00)00168-8. [79] Yuan J-M, Feng Y-R, He L-P. Effect of thermal treatment on properties of ramie fibers. Polym Degrad Stab 2016;133:303–11. http://dx.doi.org/10.1016/j. polymdegradstab.2016.09.012. [80] Fiore V, Di Bella G, Valenza A. The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites. Compos Part B Eng 2015;68:14–21. http://dx.doi.org/10.1016/j.compositesb.2014.08.025. [81] Arun prakash VR, Rajadurai A. Inter laminar shear strength behavior of acid, base and silane treated E-glass fibre epoxy resin composites on drilling process. Def Technol 2017;13:40–6. http://dx.doi.org/10.1016/j.dt.2016.11.004. [82] Pickering KL, Li Y, Farrell RL, Lay M. Interfacial modification of hemp fiber reinforced composites using fungal and alkali treatment. J Biobased Mater Bioenergy 2007;1:109–17. http://dx.doi.org/10.1166/jbmb.2007.012. [83] Sumitomo S, Koizumi H, Uddin MA, Kato Y. Comparison of dispersion behavior of agglomerated particles in liquid between ultrasonic irradiation and mechanical stirring. Ultrason Sonochem 2018;40:822–31. http://dx.doi.org/10.1016/j. ultsonch.2017.08.023. [84] Mittal G, Rhee K, Park S. The effects of cryomilling CNTs on the thermal and electrical properties of CNT/PMMA composites. Polym (Basel) 2016;8:169. http:// dx.doi.org/10.3390/polym8050169. [85] Mittal G, Dhand V, Rhee KY, Park SJ, Kim H-J, Jung DH. Investigation of seawater effects on the mechanical properties of untreated and treated MMT-based glass fiber/vinylester composites. Ocean Eng 2015;108:393–401. http://dx.doi.org/10. 1016/j.oceaneng.2015.08.019. [86] Pan C, Kou K, Jia Q, Zhang Y, Wu G, Ji T. Improved thermal conductivity and dielectric properties of hBN/PTFE composites via surface treatment by silane coupling agent. Compos Part B Eng 2017;111:83–90. http://dx.doi.org/10.1016/j. compositesb.2016.11.050. [87] Zhang Y, Rhee KY, Park S-J. Nanodiamond nanocluster-decorated graphene oxide/ epoxy nanocomposites with enhanced mechanical behavior and thermal stability. Compos Part B Eng 2017;114:111–20. http://dx.doi.org/10.1016/j.compositesb. 2017.01.051. [88] Mittal G, Rhee KY, Park SJ. Processing and characterization of PMMA/PI composites reinforced with surface functionalized hexagonal boron nitride. Appl Surf Sci 2017;415:49–54. http://dx.doi.org/10.1016/j.apsusc.2016.10.029. [89] Maurya NK, Kushwaha P, Mandal A. Studies on interfacial and rheological properties of water soluble polymer grafted nanoparticle for application in enhanced oil recovery. J Taiwan Inst Chem Eng 2017;70:319–30. http://dx.doi.org/10. 1016/j.jtice.2016.10.021. [90] Gong LX, Pei YB, Han QY, Zhao L, Wu L Bin, Jiang JX, et al. Polymer grafted reduced graphene oxide sheets for improving stress transfer in polymer composites. Compos Sci Technol 2016;134:144–52. http://dx.doi.org/10.1016/j. compscitech.2016.08.014. [91] Sontakke TK, Jagtap RN, Singh A, Kothari DC. Nano ZnO grafted on MAA/BA/ MMA copolymer: an additive for hygienic coating. Prog Org Coatings 2012;74:582–8. http://dx.doi.org/10.1016/j.porgcoat.2012.02.005. [92] Liu H, Li M, Shao G, Zhang W, Wang W, Song H, et al. Enhancement of hydrogen sulfide gas sensing of PbS colloidal quantum dots by remote doping through ligand exchange. Sensors Actuators. B Chem 2015;212:434–9. http://dx.doi.org/10. 1016/j.snb.2015.02.047. [93] Kim KM, Jeon JH, Kim YY, Lee HK, Park OO, Wang DH. Effects of ligand exchanged CdSe quantum dot interlayer for inverted organic solar cells. Org Electron Physics. Mater Appl 2015;25:44–9. http://dx.doi.org/10.1016/j.orgel.2015.05. 040. [94] Hung P-Y, Lau K-T, Fox B, Hameed N, Lee JH, Hui D. Surface modification of carbon fibre using graphene–related materials for multifunctional composites. Compos Part B Eng 2018;133:240–57. http://dx.doi.org/10.1016/j.compositesb. 2017.09.010. [95] Qi H, Liu J, Mäder E. Smart cellulose fibers coated with carbon nanotube networks. Fibers 2014;2:295–307. http://dx.doi.org/10.3390/fib2040295. [96] Dastorian Jamnani B, Hosseini S, Rahmanian S, Abdul Rashid S, Mustapha SB, Keshan Balavandy S. Grafting carbon nanotubes on glass fiber by dip coating technique to enhance tensile and interfacial shear strength. J Nanomater 2015;2015:1–7. http://dx.doi.org/10.1155/2015/149736. [97] Ghanbarian M, Taheri Nassaj E, Kariminejad A. Synthesis of nanostructural turbostratic and hexagonal boron nitride coatings on carbon fiber cloths by dipcoating. Surf Coatings Technol 2016;288:185–95. http://dx.doi.org/10.1016/j. surfcoat.2016.01.011. [98] Zhang H, Liu Y, Kuwata M, Bilotti E, Peijs T. Improved fracture toughness and integrated damage sensing capability by spray coated CNTs on carbon fibre prepreg. Compos Part A Appl Sci Manuf 2015;70:102–10. http://dx.doi.org/10.1016/ j.compositesa.2014.11.029. [99] Warrier A, Godara A, Rochez O, Mezzo L, Luizi F, Gorbatikh L, et al. The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix. Compos Part A Appl Sci Manuf 2010;41:532–8. http://dx.doi.org/10. 1016/j.compositesa.2010.01.001. [100] Liu D, Li B, Li G, Wang L, Yang X. Tagged and enhanced interface of carbon fiber/ epoxy by doping sizing agent with upconversion luminescent nanoparticles. Mater Lett 2017;196:37–41. http://dx.doi.org/10.1016/j.matlet.2017.02.132. [101] Wu G, Ma L, Liu L, Wang Y, Xie F, Zhong Z, et al. Interfacially reinforced methylphenylsilicone resin composites by chemically grafting multiwall carbon nanotubes onto carbon fibers. Compos Part B Eng 2015;82:50–8. http://dx.doi.org/ 10.1016/j.compositesb.2015.08.012. [102] Tang C, Hu D, Cao Q, Yan W, Xing B. Silver nanoparticles-loaded activated carbon

[103]

[104]

[105]

[106]

[107]

[108]

[109]

[110]

[111]

[112]

[113]

[114]

[115]

[116]

[117]

[118]

[119]

[120]

[121]

[122]

[123]

[124]

[125]

138

fibers using chitosan as binding agent: preparation, mechanism, and their antibacterial activity. Appl Surf Sci 2017;394:457–65. http://dx.doi.org/10.1016/j. apsusc.2016.10.095. Hua Y, Li F, Liu Y, Huang G-W, Xiao H-M, Li Y-Q, et al. Positive synergistic effect of graphene oxide/carbon nanotube hybrid coating on glass fiber/epoxy interfacial normal bond strength. Compos Sci Technol 2017;149:294–304. http://dx. doi.org/10.1016/j.compscitech.2017.06.024. Zhang L, De Greef N, Kalinka G, Van Bilzen B, Locquet JP, Verpoest I, et al. Carbon nanotube-grafted carbon fiber polymer composites: damage characterization on the micro-scale. Compos Part B Eng 2017;126:202–10. http://dx.doi.org/10. 1016/j.compositesb.2017.06.004. Kim H. Enhanced crack detection sensitivity of carbon fiber composites by carbon nanotubes directly grown on carbon fibers. Compos Part B Eng 2014;60:284–91. http://dx.doi.org/10.1016/j.compositesb.2013.12.063. Rahman AS, Rangari V, Jeelani S. Thermal and mechanical properties of woven glass fiber reinforced epoxy composites with carbon nanotubes grown in-situ. Int J Eng Sci 2015;4:54–61. He D, Fan B, Zhao H, Lu X, Yang M, Liu Y, et al. Design of electrically conductive structural composites by modulating aligned CVD-grown carbon nanotube length on glass fibers. ACS Appl Mater Interfaces 2017;9:2948–58. http://dx.doi.org/10. 1021/acsami.6b13397. Xiong C, Li T, Zhao T, Dang A, Li H, Ji X, et al. Reduced graphene oxide-carbon nanotube grown on carbon fiber as binder-free electrode for flexible high-performance fiber supercapacitors. Compos Part B Eng 2017;116:7–15. http://dx.doi. org/10.1016/j.compositesb.2017.02.028. Salimkhani H, Palmeh P, Khiabani AB, Hashemi E, Matinpour S, Salimkhani H, et al. Electrophoretic deposition of spherical carbonyl iron particles on carbon fibers as a microwave absorbent composite. Surfaces Interfaces 2016;5:1–7. http://dx.doi.org/10.1016/j.surfin.2016.09.004. Mahmood H, Tripathi M, Pugno N, Pegoretti A. Enhancement of interfacial adhesion in glass fiber/epoxy composites by electrophoretic deposition of graphene oxide on glass fibers. Compos Sci Technol 2016;126:149–57. http://dx.doi.org/10. 1016/j.compscitech.2016.02.016. Deng C, Jiang J, Liu F, Fang L, Wang J, Li D, et al. Influence of carbon nanotubes coatings onto carbon fiber by oxidative treatments combined with electrophoretic deposition on interfacial properties of carbon fiber composite. Appl Surf Sci 2015;357:1274–80. http://dx.doi.org/10.1016/j.apsusc.2015.09.178. Lee JU, Park B, Kim BS, Bae DR, Lee W. Electrophoretic deposition of aramid nanofibers on carbon fibers for highly enhanced interfacial adhesion at low content. Compos Part A Appl Sci Manuf 2016;84:482–9. http://dx.doi.org/10.1016/j. compositesa.2016.02.029. Li Z, Zhou Y, Yang Y, Dai H. Electrophoretic deposition of graphene-TiO2 hierarchical spheres onto Ti thread for flexible fiber-shaped dye-sensitized solar cells. Mater Des 2016;105:352–8. http://dx.doi.org/10.1016/j.matdes.2016.05.060. Lubineau G, Rahaman A. A review of strategies for improving the degradation properties of laminated continuous-fiber/epoxy composites with carbon-based nanoreinforcements. Carbon N. Y 2012;50:2377–95. http://dx.doi.org/10.1016/j. carbon.2012.01.059. Karger-Kocsis J, Mahmood H, Pegoretti A. Recent advances in fiber/matrix interphase engineering for polymer composites. Prog Mater Sci 2015;73:1–43. http://dx.doi.org/10.1016/j.pmatsci.2015.02.003. Shekar KC, Prasad BA, Prasad NE. Interlaminar shear strength of multi-walled carbon nanotube and carbon fiber reinforced, epoxy – matrix hybrid composite. Procedia Mater Sci 2014;6:1336–43. http://dx.doi.org/10.1016/j.mspro.2014.07. 112. Tüzemen MÇ, Salamcı E, Avcı A. Enhancing mechanical properties of bolted carbon/epoxy nanocomposites with carbon nanotube, nanoclay, and hybrid loading. Compos Part B Eng 2017;128:146–54. http://dx.doi.org/10.1016/j. compositesb.2017.07.001. Aluko O, Gowtham S, Odegard GM. Multiscale modeling and analysis of graphene nanoplatelet/carbon fiber/epoxy hybrid composite. Compos Part B Eng 2017;131:82–90. http://dx.doi.org/10.1016/j.compositesb.2017.07.075. Boroujeni AY, Tehrani M, Nelson AJ, Al-Haik M. Hybrid carbon nanotube-carbon fiber composites with improved in-plane mechanical properties. Compos Part B Eng 2014;66:475–83. http://dx.doi.org/10.1016/j.compositesb.2014.06.010. Srivastava VK, Gries T, Veit D, Quadflieg T, Mohr B, Kolloch M. Effect of nanomaterial on mode I and mode II interlaminar fracture toughness of woven carbon fabric reinforced polymer composites. Eng Fract Mech 2017;180:73–86. http://dx. doi.org/10.1016/j.engfracmech.2017.05.030. Simić DM, Stojanović DB, Brzić SJ, Totovski L, Uskoković PS, Aleksić RR. Aramid hybrid composite laminates reinforced with inorganic fullerene-like tungsten disulfide nanoparticles. Compos Part B Eng 2017;123:10–8. http://dx.doi.org/10. 1016/j.compositesb.2017.05.002. Zabihi O, Ahmadi M, Li Q, Shafei S, Huson MG, Naebe M. Carbon fibre surface modification using functionalized nanoclay: a hierarchical interphase for fibrereinforced polymer composites. Compos Sci Technol 2017;148:49–58. http://dx. doi.org/10.1016/j.compscitech.2017.05.013. Wang B, Duan Y, Zhang J. Titanium dioxide nanoparticles-coated aramid fiber showing enhanced interfacial strength and UV resistance properties. Mater Des 2016;103:330–8. http://dx.doi.org/10.1016/j.matdes.2016.04.085. Du SSen, Li F, Xiao HM, Li YQ, Hu N, Fu SY. Tensile and flexural properties of graphene oxide coated-short glass fiber reinforced polyethersulfone composites. Compos Part B Eng 2016;99:407–15. http://dx.doi.org/10.1016/j.compositesb. 2016.06.023. El Moumen A, Tarfaoui M, Lafdi K, Benyahia H. Dynamic properties of carbon nanotubes reinforced carbon fibers/epoxy textile composites under low velocity

Composites Part B 138 (2018) 122–139

G. Mittal et al.

[126]

[127]

[128]

[129]

[130]

[131]

[132]

[133]

[134]

[135]

[136]

[137]

[138]

[139]

compositesb.2017.04.005. [140] Zeng X, Yu S, Sun R, Xu J. Mechanical reinforcement while remaining electrical insulation of glass fibre/polymer composites using core–shell CNT@SiO2 hybrids as fillers. Compos Part A Appl Sci Manuf 2015;73:260–8. http://dx.doi.org/10. 1016/j.compositesa.2015.03.015. [141] Zhang J, Du Z, Zou W, Li H, Zhang C. MgO nanoparticles-decorated carbon fibers hybrid for improving thermal conductive and electrical insulating properties of Nylon 6 composite. Compos Sci Technol 2017;148:1–8. http://dx.doi.org/10. 1016/j.compscitech.2017.05.008. [142] Neisiany RE, Lee JKY, Khorasani SN, Ramakrishna S. Towards the development of self-healing carbon/epoxy composites with improved potential provided by efficient encapsulation of healing agents in core-shell nanofibers. Polym Test 2017;62:79–87. http://dx.doi.org/10.1016/j.polymertesting.2017.06.016. [143] Scheiner M, Dickens TJ, Okoli O. Progress towards self-healing polymers for composite structural applications. Polym (United Kingdom) 2016;83:260–82. http://dx.doi.org/10.1016/j.polymer.2015.11.008. [144] Bekas DG, Baltzis D, Paipetis AS. Nano-reinforced polymeric healing agents for vascular self-repairing composites. Mater Des 2017;116:538–44. http://dx.doi. org/10.1016/j.matdes.2016.12.049. [145] Lanzara G, Yoon Y, Liu H, Peng S, Lee W-I. Carbon nanotube reservoirs for selfhealing materials. Nanotechnology 2009;20:335704. http://dx.doi.org/10.1088/ 0957-4484/20/33/335704. [146] Liu F, Urban MW. Recent advances and challenges in designing stimuli-responsive polymers. Prog Polym Sci 2010;35:3–23. http://dx.doi.org/10.1016/j. progpolymsci.2009.10.002. [147] Liu T, Zhou T, Yao Y, Zhang F, Liu L, Liu Y, et al. Stimulus methods of multifunctional shape memory polymer nanocomposites: a review. Compos Part A Appl Sci Manuf 2017;100:20–30. http://dx.doi.org/10.1016/j.compositesa.2017.04. 022. [148] Lu H, Yao Y, Huang WM, Hui D. Noncovalently functionalized carbon fiber by grafted self-assembled graphene oxide and the synergistic effect on polymeric shape memory nanocomposites. Compos Part B Eng 2014;67:290–5. http://dx.doi. org/10.1016/j.compositesb.2014.07.022. [149] Ma L, Zhao J, Wang X, Chen M, Liang Y, Wang Z, et al. Effects of carbon black nanoparticles on two-way reversible shape memory in crosslinked polyethylene. Polym (United Kingdom) 2015;56:490–7. http://dx.doi.org/10.1016/j.polymer. 2014.11.036. [150] Pathak AK, Bhardwaj V, Gangwar RK, De M, Singh VK. Fabrication and characterization of TiO2 coated cone shaped nano-fiber pH sensor. Opt Commun 2017;386:43–8. http://dx.doi.org/10.1016/j.optcom.2016.11.021. [151] Liang B, Fang L, Hu Y, Yang G, Zhu Q, Ye X. Fabrication and application of flexible graphene silk composite film electrodes decorated with spiky Pt nanospheres. Nanoscale 2014;6:4264–74. http://dx.doi.org/10.1039/c3nr06057h. [152] Calestani D, Villani M, Culiolo M, Delmonte D, Coppedè N, Zappettini A. Smart composites materials: a new idea to add gas-sensing properties to commercial carbon-fibers by functionalization with ZnO nanowires. Sensors Actuators, B Chem 2017;245:166–70. http://dx.doi.org/10.1016/j.snb.2017.01.109. [153] Li W, He D, Dang Z, Bai J. In situ damage sensing in the glass fabric reinforced epoxy composites containing CNT-Al2O3 hybrids. Compos Sci Technol 2014;99:8–14. http://dx.doi.org/10.1016/j.compscitech.2014.05.005. [154] Nam IW, Park SM, Lee HK, Zheng L. Mechanical properties and piezoresistive sensing capabilities of FRP composites incorporating CNT fibers. Compos Struct 2017;178:1–8. http://dx.doi.org/10.1016/j.compstruct.2017.07.008.

impact. Compos Part B Eng 2017;125:1–8. http://dx.doi.org/10.1016/j. compositesb.2017.05.065. Chaudhry MS, Czekanski A, Zhu ZH. Characterization of carbon nanotube enhanced interlaminar fracture toughness of woven carbon fiber reinforced polymer composites. Int J Mech Sci 2017;131–132:480–9. http://dx.doi.org/10.1016/j. ijmecsci.2017.06.016. Matykiewicz D, Barczewski M, Knapski D, Skórczewska K. Hybrid effects of basalt fibers and basalt powder on thermomechanical properties of epoxy composites. Compos Part B Eng 2017;125:157–64. http://dx.doi.org/10.1016/j.compositesb. 2017.05.060. Sung DH, Kim M, Park Y-B. Prediction of thermal conductivities of carbon-containing fiber-reinforced and multiscale hybrid composites. Compos Part B Eng 2018;133:232–9. http://dx.doi.org/10.1016/j.compositesb.2017.09.032. Kim D, Chung I, Kim G. Study on mechanical and thermal properties of fiberreinforced Epoxy/Hybrid-silica composite. Fibers Polym 2013;14:2141–7. http:// dx.doi.org/10.1007/s12221-013-2141-9. Zhao Y-H, Zhang Y-F, Bai S-L, Yuan X-W. Carbon fibre/graphene foam/polymer composites with enhanced mechanical and thermal properties. Compos Part B Eng 2016;94:102–8. http://dx.doi.org/10.1016/j.compositesb.2016.03.056. Li K, Li L, Li H, Song Q, Lu J, Fu Q. Electrophoretic deposition of carbon nanotubes onto carbon fiber felt for production of carbon/carbon composites with improved mechanical and thermal properties. Vacuum 2014;104:105–10. http://dx.doi.org/ 10.1016/j.vacuum.2014.01.024. Liang S, Neisius NM, Gaan S. Recent developments in flame retardant polymeric coatings. Prog Org Coatings 2013;76:1642–65. http://dx.doi.org/10.1016/j. porgcoat.2013.07.014. Arjmandi R, Ismail A, Hassan A, Abu Bakar A. Effects of ammonium polyphosphate content on mechanical, thermal and flammability properties of kenaf/polypropylene and rice husk/polypropylene composites. Constr Build Mater 2017;152:484–93. http://dx.doi.org/10.1016/j.conbuildmat.2017.07.052. Jeencham R, Suppakarn N, Jarukumjorn K. Effect of flame retardants on flame retardant, mechanical, and thermal properties of sisal fiber/polypropylene composites. Compos Part B Eng 2014;56:249–53. http://dx.doi.org/10.1016/j. compositesb.2013.08.012. Zhao X, Yang L, Martin FH, Zhang X-Q, Wang R, Wang D-Y. Influence of phenylphosphonate based flame retardant on epoxy/glass fiber reinforced composites (GRE): flammability, mechanical and thermal stability properties. Compos Part B Eng 2017;110:511–9. http://dx.doi.org/10.1016/j.compositesb.2016.10.090. Hapuarachchi TD, Peijs T. Multiwalled carbon nanotubes and sepiolite nanoclays as flame retardants for polylactide and its natural fibre reinforced composites. Compos Part A Appl Sci Manuf 2010;41:954–63. http://dx.doi.org/10.1016/j. compositesa.2010.03.004. Cortes LQ, Racagel S, Lonjon A, Dantras E, Lacabanne C. Electrically conductive carbon fiber/PEKK/silver nanowires multifunctional composites. Compos Sci Technol 2016;137:159–66. http://dx.doi.org/10.1016/j.compscitech.2016.10. 029. Dong L, Li Y, Wang L, Hou F, Liu J. Spatial dispersion state of carbon nanotubes in a freeze-drying method prepared carbon fiber based preform and its effect on electrical conductivity of carbon fiber/epoxy composite. Mater Lett 2014;130:292–5. http://dx.doi.org/10.1016/j.matlet.2014.05.135. Kwon YJ, Kim Y, Jeon H, Cho S, Lee W, Lee JU. Graphene/carbon nanotube hybrid as a multi-functional interfacial reinforcement for carbon fiber-reinforced composites. Compos Part B Eng 2017;122:23–30. http://dx.doi.org/10.1016/j.

139