1. Introduction
A composite material is a material which is produced from two or more constituent materials [1, 2]. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements [3, 4]. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions. There are various reasons where new material can be favored. Typical examples include materials which are less expensive, lighter, stronger or more durable when compared with common materials, as well as composite materials inspired from animals and natural sources with low carbon footprint [5, 6]. Composite materials are generally used for buildings, bridges, and structures [7, 8]. They are also being increasingly used in general automotive applications. The most advanced examples perform routinely on spacecraft and aircraft in demanding environments.
Fiber-reinforced polymers include carbon fiber reinforced polymer and glass-reinforced plastic. If classified by matrix then there are thermoplastic composites, short fiber thermoplastics, long fiber thermoplastics or long fiber-reinforced thermoplastics [9, 10]. There are numerous thermoset composites [11, 12]. Many advanced thermoset polymer matrix systems usually incorporate aramid fiber and carbon fiber in an epoxy resin matrix [13, 14]. Composite materials are created from individual materials. These individual materials are known as constituent materials, and there are two main categories of it. One is the matrix and the other reinforcement [15, 16]. A portion of each kind is needed at least. The reinforcement receives support from the matrix as the matrix surrounds the reinforcement and maintains its relative positions. The properties of the matrix are improved as the reinforcements impart their exceptional physical and mechanical properties [17, 18]. The mechanical properties become unavailable from the individual constituent materials by synergism [19, 20]. At the same time, the designer of the product or structure receives options to choose an optimum combination from the variety of matrix and strengthening materials.
To shape the engineered composites, it must be formed [21, 22]. The reinforcement is placed onto the mold surface or into the mold cavity. Before or after this, the matrix can be introduced to the reinforcement. The matrix undergoes a melding event which sets the part shape necessarily. This melding event can happen in several ways, depending upon the matrix nature, such as solidification from the melted state for a thermoplastic polymer matrix composite or chemical polymerization for a thermoset polymer matrix [23, 24]. Although the two phases are chemically equivalent, semi-crystalline polymers can be described both quantitatively and qualitatively as composite materials. The crystalline portion has a higher elastic modulus and provides reinforcement for the less stiff, amorphous phase. Different processing techniques can be employed to vary the percent crystallinity in these polymer matrix composite materials and thus the mechanical properties of these materials [25, 26]. In many cases these materials act like particle composites with randomly dispersed crystals known as spherulites. However, they can also be engineered to be anisotropic and act more like fiber reinforced composites [27, 28]. In the case of spider silk, the properties of the material can even be dependent on the size of the crystals, independent of the volume fraction. Ironically, single component polymeric materials are some of the most easily tunable composite materials known.
Usually, the composite’s physical properties are not isotropic in nature. But they are typically anisotropic. For instance, the composite panel’s stiffness will usually depend upon the orientation of the applied forces and moments [29, 30]. In general, particle reinforcement is strengthening the composites less than fiber reinforcement [31, 32]. It is used to enhance the stiffness of the composites while increasing the strength and the toughness [33, 34]. Because of their mechanical properties, they are used in applications in which wear resistance is required. For example, hardness of cement can be increased by reinforcing gravel particles, drastically [35, 36]. Particle reinforcement a highly advantageous method of tuning mechanical properties of materials since it is very easy implement while being low cost [37, 38]. In general, continuous fiber reinforcement is implemented by incorporating a fiber as the strong phase into a weak phase, matrix. The reason for the popularity of fiber usage is materials with extraordinary strength can be obtained in their fiber form [39, 40]. Non-metallic fibers are usually indicating a very high strength to density ratio compared to metal fibers because of the covalent nature of their bonds.
Fiber-reinforced plastic is a composite material made of a polymer matrix reinforced with fibers [41, 42]. The fibers are usually glass, carbon, aramid, or basalt. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use [43, 44]. Carbon fiber reinforced polymer, or carbon fiber reinforced plastic, or carbon fiber reinforced thermoplastic, is an extremely strong and light fiber-reinforced plastic which contains carbon fibers. Carbon fiber reinforced polymers can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness are required, such as aerospace, superstructures of ships, automotive, civil engineering, and an increasing number of consumer and technical applications [45, 46]. The binding polymer is often a thermoset resin such as epoxy, but other thermoset or thermoplastic polymers, such as polyester, vinyl ester, or nylon, are sometimes used. The properties of the final carbon fiber reinforced polymer product can be affected by the type of additives introduced to the binding matrix [47, 48]. The most common additive is silica, but other additives such as rubber and carbon nanotubes can be used.
Carbon nanotubes are generally elongated hollow, tubular bodies with a linear graphene structure. They are typically only a few atoms in circumference and may be single-walled or multi-walled. Carbon nanotubes are recognized as possessing excellent mechanical, chemical, electrical, and thermal properties and have potential uses in a diverse number of applications [49, 50]. One use of carbon nanotubes has been to add them to polymer matrices as separate fillers or as reinforcing agents [51, 52]. However, the more recent development of attaching polymers to carbon nanotubes to form polymer-carbon nanotube composites offers exciting new potential uses [53, 54]. By chemically or physically linking the carbon nanotubes to the polymer chains, the resultant polymer-carbon nanotube composites benefit from the mechanical, thermal, and electrical properties of the carbon nanotubes to provide multifunctional new lightweight materials [55, 56]. Attempts to make such polymer-carbon nanotube composites include chemically modifying the ends or the side walls of carbon nanotubes with functional groups, which then react to form, or to link with, polymer chains [57, 58]. The process involves the functionalization of the sidewalls and the ends of carbon nanotubes with diazonium species using an electrochemical process [59, 60]. The functional group is then actively involved in a polymerization process which results in a polymer-carbon nanotube composite material in which the carbon nanotubes are chemically involved.
Carbon nanotubes have very anisotropic structures, and may be formed in various shapes such as single-walled, multi-walled, and rope shapes. The carbon nanotubes may have semiconducting or conducting characteristics depending on how they are coiled, different energy gaps depending on their chirality and diameters, and particular quantum effects due to quasi-one-dimensional structures. The present study is focused primarily upon the electrical and thermal properties of epoxy matrix composite materials reinforced with multi-walled carbon nanotubes under different weight fraction conditions. Stable suspensions of carbon nanotubes are achieved in water with the use of surfactants, and non-covalent and covalent attachment of polymers. Scanning electron microscopy characterization is performed and electrical resistance is measured. Mechanical properties are studied and the loading rate is continuously adjusted to keep a constant representative strain rate. The Oliver-Pharr method is used to analyze partial load-unload data in order to calculate the indentation elastic modulus as a function of the indenter penetration. The present study aims to provide an improved method for the preparation of epoxy matrix composite materials reinforced with multi-walled carbon nanotubes with reduced volume resistivity and enhanced thermal conductivity. Particular emphasis is placed upon the effect of carbon nanotube weight fraction on the volume resistivity and thermal conductivity of the epoxy matrix composite materials reinforced with multi-walled carbon nanotubes.