In 1991, the discovery of carbon nanotubes (CNTs) by Sumio Iijima created a global scientific phenomenon in the field of nanotechnology [1]. A CNT is defined as a one-atom thick sheet of graphite rolled into a tube with a diameter of one nanometer, which is classified as a single-wall carbon nanotube (SWCNT); if there are additional or multiple graphene tubes around the core of an SWCNT, this is known as a multiwalled carbon nanotube (MWCNT). Diameters are fractions of nanometers and tens of nanometers. Lengths can be up to a number of centimeters, with both their ends normally capped by fullerene-like structures [2]. It is believed that the unique properties of CNTs have opened new era in the material world, especially in the field of conductive polymer and CNT-based nanocomposites. Since then, different kinds of techniques of have been developed for CNT-incorporated polymer matrices with the aim to fabricate new advanced materials with multifunctional properties. Some of these properties were designed to transfer the unique electrical properties associated with CNTs to insulating polymer matrices with the aim of obtaining better conducting polymer composites.

Theoretical and experimental results on CNTs have showed a high modulus of elasticity: greater than 1 TPa (the elastic modulus of diamond is 1.2 TPa). In addition, CNTs also possess a strength that is 10–100 times higher than the resilient steel at a fraction of the weight [3]. Additionally, CNTs have an excellent thermal stability of up to 2800 °C in vacuum and an electrical conductivity in the vicinity of 10³ S/cm, with an electric-current-carrying capacity that is 1000 times higher and thermal conductivity of about 1900 W m⁻¹ K⁻¹ (which is about twice as high as diamond) [4,5]. SWCNTs in a hexagonal honeycomb structure consist of sp² hybridized carbon in a that is rolled into a hollow tube morphology, while MWCNTs consist of multiple concentric tubes encircling one another [6]. To date, CNTs have shown increasing interest as potential conductive fillers and reinforcements for polymeric composites. Apart from their high electrical conductivity, CNTs have unique electronic and optical properties for the development of organo-electronic devices [7]. High conductivity can be achieved at a very low concentration of CNTs of between 0.0025 and 4 wt.%, owing to their high aspect ratio (L/D, where L is length of a CNT and D is diameter of a CNT) from hundreds to 1000. All these superiorities allow CNTs to have tremendous potential for nanotechnology fields, especially for use as composite fillers and reinforcements in order to enhance the mechanical, electrical, and thermal properties of resulting composite systems. Many potential applications for CNTs, including microwave absorption [8,9], corrosion protection [10,11], reinforced materials in natural fiber composites [12,13], electromagnetic interference shielding (EMI) [14,15], batteries [16,17], solar cells [18,19,20,21], chemical sensors [22,23,24], hydrogen storage [25,26], field-emission materials [27,28], and adsorbents [29,30], have been reported.

Besides the aforementioned characteristic of CNTs, the rigidity, chemical inertness, and strong π–π interactions of pristine CNTs cannot be synthesized and fabricated due to the difficulties of dissolving or dispersing them in common volatile organic solvents or polymeric matrices. Such actions rely on the agglomeration properties of nanotubes while considering the electrostatic interaction and Van der Walls forces of CNTs that impart the low dispersion properties [31]. Furthermore, the physical nature of the nanosized CNTs plays an important role in dispersing them into a polymer matrix, as well as for a polymer to encapsulate onto a CNT surface. It has been proven that these bundles and agglomerates led to the deterioration of the mechanical and electrical properties of composites compared to the theoretical predictions for individual CNTs [32]. In other words, the dispersion of CNTs does not merely depend on the geometrical problem that related to the length and size of the CNTs alone; instead, it involves a technique that separates individual CNTs from highly entangled and agglomerated CNTs and then stabilize the CNTs in a polymer matrix in order to avoid further agglomeration [33]. Thus, the chemical modification of the side walls of the surfaces of CNTs is needed to improve their dispersion or solubility in solvents or polymers, as well as to improve their interaction and reactivity with polymers by hydrogen bonding interaction [34].

2. Applications and Potential Use of Carbon Nanotube-Reinforced Polymer Composites

Polymeric composites are one of the most well-known materials that have lightweight properties and high durability for various functions [232,233,234,235,236,237,238,239]. Polymer composites exploit a wide range of applications due to their all-around excellent performance in mechanical, thermal, and electrical properties [240,241,242,243,244,245]. The inclusion of nanofillers inside polymer resins could provide promising properties for materials in almost every sector [246,247,248]. Due to high cost of carbon fibers, CNTs can be added in small quantities in polymeric composites but exhibit while exhibiting strong mechanical properties [249]. For instance, the application of CNTs as reinforcements in polymeric composites could establish significantly high mechanical strength and elastic modulus values in comparison other high performance fibers such as Kevlar and carbon [143,250,251]. To be specific, their tensile strength and elastic modulus have been recorded at 150 GPa and 1 TPa, respectively, which marks them as tremendously stronger and stiffer, as well as three-to-five times lighter, than steel. These properties can be characterized by using a proper testing facilities [252,253] to ensure the qualities are on par with current conventional materials. Since these materials have shown significant enhancement in term of their material properties, Table 11 summarizes recent research on CNT–polymer composites conducted in various sectors.

Table 11. Recent progress of CNT–polymer composites.

Applications	Types of CNT	Polymers	References
Biomedical goods, space vehicles, and stations	SWCNT	Poly (4-methyl-1-pentene)	[254]
Biocatalytic films	SWCNT	PMMA	[255]
Actuators and sensors for biomedical Applications	MWCNT	Poly (vinyl alcohol) and poly(2-acrylamido-2-methyl-1-propane sulfonic acid)	[256]
Supercapacitor electrode materials	MWCNT	PPy, Poly-(3,4-ethylenedioxythiophene) and PANI	[257]
External body components of automotive, yarn fiber, conductive plastic, and hot melt adhesives	MWCNT	PE	[258]
Electronics, electrostatic discharge, and automotive and industrial goods	SWCNT and MWCNT	Polyamide	[6]
Wind turbine blade and flame retardant	SWCNT and MWCNT	PU	[259]

CNTs are emerging advanced materials with outstanding mechanical, electrical, and thermal properties and highly interfacial contact areas. in comparison to other polymers, cnt–polymer composites have received more attention among material scientists due to the good compatibility between cnt and polymers. Figure 11 shows the potential and current applications of CNT–polymer composites including electronics, automobiles, textiles, aerospace, sport equipment, sensors, energy storage devices, and filters [260,261,262].

Figure 11. Application of CNT–polymer composites.

3.1. CNTs Reinforced Polymer Applications

3.1. Electronic Application

The advanced applications of CNT–polymer composites are rising quickly in the electronic field, especially in the development in electronic devices. The growing demand for advanced materials with customize electrical properties makes CNTs the most attractive nanomaterials for electrical and electronic devices. The enhancement of field emission properties can result in the improvement of the efficiency of electronic devices.

Additionally, CNT–polymer composites are useful when preparing solar cells. According to Sibinski et al. (2011) [266], elastic CNT–polymer composites have a high potential to produce new photovoltaics through a screen-printing technique. They discovered that the CNT–polymer composite had a high optical transmittance with less costly manufacturing process. The nanocomposite had a better elastic behavior and significantly strong optical and electrical parameters, which gives them potential use as coatings in solar cells.

3.2. Aerospace Application

The aerospace industry requires very high strength and durable materials to be embedded as components in astronautic equipment. Since CNTs has various characteristics, the materials have been widely studied to evaluate their potential to act as constituents of composite materials. The CNT–polymer composites are highly suitable for the aerospace and aeronautical fields. For the aerospace field, the CNT–polymer composites have been actively studied by researchers in order to enhance the electrical performance of composites with epoxy resin. Thus, CNT–polymer composites are essential in the aerospace field due to their structural properties that could be applied to such areas as in anti-radar protectors, antistatic materials, and spacecraft [267]. CNT-reinforced epoxy polymer composites have been commonly utilized in air/spacecraft developments since 2006. CNTs are emerging advanced materials that allow a structure to be lightweight, have elevated temperature resistance, and have high strength-to-weight ratio. In general, the 1 wt.% CNT composites doubled the Young’s modulus and yield strength compared a pure epoxy laminate, as shown in Table 12.

Table 12. Yield strength and Young’s modulus at different strain levels and CNT loading.

CNTs (wt. %)	σ10% (MPa)	Young’s Modulus (MPa)	Yield Strength (MPa)
0	4	EO = 118	1
1	8	236 (2 × EO)	3
4	10	456 (3.9 × EO)	6

CNT–polymer composites can exhibit significant changes in their resistivity value, which is important for high-fidelity circuits in aerospace application. Additionally, the technology of electromagnetic interference (EMI) shielding was developed with CNT–PP composites by Al-Saleh and Sundararaj (2009) [270]. They indicated that the shielding from CNT–polymer composites provides absorption (major shielding mechanism) and reflection (secondary shielding mechanism). Moreover, the EMI shielding effectiveness of CNT–PP composites was elevated with increases in CNT content and shielding plate thickness, which showed the efficiency of the CNT nanocomposites. Next, CNT reinforced polymer composites could act as heat absorbing media that are useful in aerospace industries, e.g., as electromagnetic wave absorption materials [271].

3.3. Automotive Application

In the automotive sector, nanocomposite materials—especially CNT–polymer composites—could be beneficial in many ways, including the improvement of existing technologies. CNT reinforced polymer composites could be applied to automobile parts including exhaust systems, catalytic converters, suspension and breaking systems, electronic equipment, engines, power strain materials, and body parts [272,273]. Previously, traditional fillers such as mica, calcium carbonate, and talc were widely applied in automotive parts in order to offer higher melt viscosity, optical clarity, and better stiffness properties. For instance, glass fiber was introduced due to its high in stiffness, but it is difficult to fabricate and thus incurs high production costs. Additionally, traditional fillers and glass fibers have to be implemented with high loading to improve dimensional stability, increase the mechanical modulus, and increase surface quality. Thus, the introduction of CNT–polymer composites in this industry could aid the aforementioned issues of traditional fillers.

CNT fillers are effective at lower concentrations (0.2 wt.%) in polymeric composites because they can significantly enhanced dimensional and thermal stability, as well as reduce weight [259,274]. CNT–polymer composites also play significant roles in automobile engineering. CNT–polymer composites have been found to possess a high strength-to-weight ratio because a lightweight vehicle could allow for a vehicle to have a lower fuel consumption. This would result in the reduction of carbon dioxide emissions by the vehicle, which could help to reduce global warming. Yang et al. (2012) [275] discovered that a 25% reduction in vehicle weight would reduce crude barrel consumption by up to 250 million barrels per year. Many car manufacturing companies have employed nanocomposites in trunk lids, car seats, dashboard coverings, and roofs [276].

Furthermore, the addition of MWCNTs in epoxy composites would increase the adhesion strength of the matrix, which would subsequently contribute to lower water intake, hydrophobicity, and corrosion resistance [277]. Another study conducted by Lee et al. (2010) [278] found that the inclusion of CNTs and montmorillonite in epoxy resin would permit good anti-oxidation and flame retardant properties. These findings showed that CNT–reinforced rubber composites have a high potential to produce high-performance vehicle tires. According to Jia and Wei (2017) [279], the application of CNT rubber composites in tires would induce a high thermal conductivity and a low hysteresis. This would cause the tread base and shoulder parts to reduce the heat accumulation, which would subsequently prolong tire durability.

3.4. Sensors

CNT reinforced polymer composites have the significant ability to detect chemicals in the air for various purposes. Because they allow for the good ability to sense gas molecules, they would benefit space exploration; environmental monitoring; and medical, industrial and agricultural applications. For instance, the detection of carbon monoxide, nitrogen oxide, and ammonium is required to monitor environmental pollution in the industrial and medical environments. According to Kong et al. (2000) [280], individual SWCNT composites have been established in chemical sensor applications. It was discovered that the exposure to chemical molecules such as nitrogen oxide and ammonium of semi-conducting CNTs would provide changes in electrical resistance. Currently, electrical sensors implement carbon black polymer composites. This shows that the CNT–polymer composites could provide better and faster responses than current materials. Another study carried out by Sattari et al. (2014) [281] discovered that methane gas was efficiently sensed by CNT/polyaniline composites at room temperature. Likewise, Rajabi et al. (2013) [282] applied CNT/polyvinyl chloride (PVC) mixed matrix membranes for gas separation applications. Khan et al. (2014) [283] also established that similar nanocomposites that can function as indicator electrodes for titration of the potentiometric materials.

3.5. Sporting Goods

The promising values of CNT–polymer composites in this modern era have led many material scientists and engineers to conduct various studies in many study areas. One of the most stimulating characteristics of such composites are their light weight, high strength property, and strong stiffness property, which render them superlative fillers [284]. Due to the fact that CNT–polymer composites have high stiffness and strength values, the nanocomposites have been turned into structural products and applications such as civil engineering structures and sporting goods [285]. For superior composite sporting goods such as badminton rackets and golf sets, epoxy has been used to reinforce the CNT fillers. As such resins of this class have excellent specific strength, stiffness, chemical resistance, and dimensional stability [232,235,237]. However, there are still many challenges for CNT-reinforced thermosetting polymer composites. These issues include the development of material features of nanocomposites when transferring the mechanical, thermal, and electrical properties of CNTs to epoxy composites [286].

3.6. Wind Turbine Blades

The renewable energy sector is currently growing rapidly to replace conventional energy such as coal and petroleum. One of fastest growing energy production sectors is wind energy. According to the US Department of Energy, the country aims to generate “green” energy as at least 20% of its total energy needs with wind-generated electricity by 2030. Thus, this industry intends to produce a high efficiency and optimum production of energy by generating large blades that are lighter in weight. This is due to the fact that the production of wind energy increases with the square-area of rotor radius [287,288]. The current goal in the field is to produce larger wind blades with good mechanical properties, light weight, and long fatigue life. However, this goal is huge challenge for many researchers as most lightweight, high strength, and high stiffness materials would have high costs in raw material and production.

In order to overcome this issue, a CNT-reinforced polymer matrix is potential material to be implemented in the production of wind blades. Based on previous studies, the application of CNT fillers as strengthening agents has highlighted the influence of CNTs on the stiffness and strength of composites. More recent research found that the inclusion of CNT fillers in composites could enhance fatigue resistance and subsequently prolong fatigue life [289]. Thus, many researchers are recently working on CNT-reinforced thermoset polymer composites in order to enhance tensile and fatigue properties for wind blade applications. In general, epoxy polymers are not suitable for the large scale production of wind blades because they have a shorter fatigue life and poorer fracture toughness. This, in turn, limits the operating life and reliability of wind blades in long term use. Thus, studies have to focus on the long-term prospective of CNT–polymer composites under cyclic loading to be utilized for structural applications that require an increased fatigue life.

According to Böger et al. (2010) [290], the inclusion of 0.2 wt.% in epoxy polymer could enhance the fatigue resistance of composites. This could be done by dispersing the CNTs throughout epoxy resin with help from copolymers via sonication. Moreover, those tensile and dynamic mechanical properties were evaluated for pure epoxy and CNT–epoxy composites with five different load levels (25, 30, 40, 45, and 50 MPa). At the end of the experiment, it was established that CNT/epoxy composites exhibited a long fatigue life and significant improvements of fatigue properties in high cycles. Moreover, the improvement in fatigue life occurred due to the pull-out of the CNTs and crack bridging at the crack interface, thus showing that CNT-reinforced polymer composites would be the most promising candidates with high fatigue lives to be used in major structural and dynamic applications.

3.7. Environmental Remediation

Globally, the increase in the pollution rate due to urbanization and industrialization has caused tremendous negative effects on environmental ecosystems [291,292]. Flora and fauna can be adversely affected by various types of contaminants such as chemical, physical, radiological, and biological contaminants [293]. Water contamination has become a worldwide problem over past few decades because of the disposal of contaminated waste in water systems. Preventative measures have to be implemented in order to reduce catastrophic effects on the environment.

The application of CNT–composites is one way to remedy excessive environmental pollution. CNTs have special adsorption capacities for different types of environmental pollutants by a large accessible external surface area, a high aspect ratio of fibrous shapes, and strong electrostatic interactions with charge pollutants in water [294]. In detail, CNTs can absorb pollutant particles on their external surfaces, open-ended portions, groves at the line boundary of carbon nanotube bundles, and the interstitial pores among the tube bundles [295]. It can be seen that CNTs have a good membrane separation ability that is especially useful for water treatment processes. CNT surface structures also have cytotoxic effects that inhibit the growth of microbes. Nanofillers have also been shown to contribute self-cleaning properties to CNT filters [296]. CNTs also considered to be good catalyzers for immobilized enzymes. In this case, immobilized enzymes on CNTs have shown more stability, broad pH ranges, more storage stability, better capacitive deionization, and more reusability. Yan et al. (2011) [297] successfully removed aniline aromatic compounds from water molecules by implementing immobilized enzymes of Delftia sp. XYJ6 on CNTs. Zhai et al. (2013) [298] showed that CNT–horseradish peroxidase enzyme could remove phenolic compounds from polluted water. Additionally, CNT-reinforced polymer membranes have a good ability for diffusivity, which makes them highly significant for water purification and adsorption systems for heavy metals ions, small molecules, organic chemicals, and radionuclides [299].