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.