Composite material is a material, composed or made of two or more distinct phases (matrix phase and dispersed phase), having significantly different bulk properties from those of any of the constituents (Campo 2008; Alok et al. 2011; Katz and Milewski 1978; Rosato 2004; Biron 2007).

Matrix Phase

Matrix phase is the primary phase, having a continuous character, usually a softer and ductile phase. It helps to hold the dispersed phase. A key role of the matrix is to serve as a binder of the fibers with desired shape and protect them from mechanical or chemical damages. Composite materials can be categorized based on the matrix material (metal, ceramic, and polymer) and the material structure.
Based on the nature of the matrix material of the composite it can be classified into metal matrix composites (MMC), ceramic matrix composites (CMC), and polymer matrix composites (PMC). This book chapter is about discussion on polymer matrix composites (PMC).
Unsaturated polyester (UP) and epoxiy (EP) are examples of thermoset whereas polycarbonate (PC), polyvinylchloride, nylon, and polysterene are thermoplastic.

Dispersed (Reinforcing) Phase

Dispersed phase is the secondary phase of composite, embedded or dispersed in the matrix in a discontinuous manner. In the composite, the mainly dispersed phase is a greater load carrier than the matrix; therefore it is also known as the reinforcing phase. When small additives like metal alloys, doped ceramics or polymers mix in dispersed phases they are not considered as composite materials since their bulk properties are similar to those of their base constituents. (Physical properties of steel and pure iron are almost the same.)
According to the classification of composites, PMC is the material consisting of polymer matrix and reinforcing dispersed phase (glass, carbon, steel or Kevlar fibers). PMCs are widely used due to their simple fabrication methods and low cost.
Reinforcement of polymers with a strong fibrous network permits fabrication of PMC which provides advantage over non-reinforced polymers in terms of mechanical properties. Reinforced PMCs provide better tensile strength, high stiffness, high fracture toughness, good abrasion resistance, good puncture resistance, and good corrosion resistance. The main disadvantages of PMCs are associated with low thermal resistance and high coefficient of thermal expansion.
Properties of PMCs are determined by properties of the fibers, orientation of the fibers, concentration of the fibers, and properties of the matrix.
Two types of polymers are used as matrix materials for fabrication composites, one is thermosets which are generally epoxies or phenolics and the other is thermoplastics (LDPE, HDPE, polypropylene (PP), nylon, and acrylics, etc.).
Fiberglass, carbon fiber, and Kevlar (aramid) fibers are widely used to make PMCs.

Fiberglass: Glass Fiber Reinforced Polymers

PMC is reinforced by glass fibers (fiberglass is a common name). Use of glass as reinforcing fibers in PMSc shows better corrosion resistance and high tensile strength, which may go up to 590 psi. The making of glass fiber reinforced polymer composite is simple and needs only low-cost technology.
Glass fibers are made of molten silica-based or other formulation glass, from which glass extruded and then gathered to strands. The strands are used for preparation of yarns, rovings, woven fabrics, and mat glass fiber products (Fig. 21).

Different kinds of glass fibers are used for making PMCs depending on the end requirement. For excellent electrical insulator PMCs, the most popular and inexpensive E-glass fibers are used. The designation letter “E” means “electrical” (E-glass is insulator). The composition of E-glass which is an excellent insulator ranges from 50–57 % SiO2, 10–17 % A1203, 17–26 % CaO, and 9–14 % B203. For high strength PMCs, S-Glass is used. It has applications in military and aerospace areas. S-glass is generally made of silica (SiO2), magnesia (MgO), and alumina (Al2O3).
Besides S-glass and E-glass, there are S + R-glass and C-glass. S + R glass is strongest and most expensive and has a diameter half of that of E-glass. C-glass is used for preparation of corrosion and chemical resistant PMCs, widely used for manufacturing storage tanks, pipes, and other chemical resistant equipment.
The widely used matrix materials for manufacturing fiberglass-PMCs are unsaturated polyesters (UP), epoxies (EP), nylon (polyamide), polycarbonate (PC), polystyrene (PS), and polyvinylchloride (PVC).
Orientations in the reinforcing glass fibers in fiberglass layers of fiberglass-PMCs are very important as they greatly affect the anisotropy behavior of the final materials. Fiberglass normally contains between 42 % and 72 % glass fibers by concentration.
Glass fiber reinforced polymer matrix composites are manufactured by open mold processes, closed mold processes, and the pultrusion method.
Fiberglass-PMCs show excellent features in terms of high strength-to-weight ratio, high modulus of elasticity-to-weight ratio, corrosion resistance, insulating properties, thermal resistance (with respect to polymer matrix).
Fiberglass materials are used for manufacturing, surfboards, gliders, kit cars, sports cars, microcars, karts, bodyshells, boats, kayaks, flat roofs, lorries, K21 infantry fighting motor vehicles, minesweeper hulls, pods, domes and architectural features where a light weight is necessary, high end bicycles, body-parts for automobiles, such as the Anadol (Anadol was Turkey’s first passenger vehicle), Reliant (Reliant was a British car manufacturer), Airbus A320 (A320 is a short- to medium-range, narrow-body, commercial passenger jet airliners manufactured by Airbus), and radome ((combination of word of radar and dome) is a weatherproof enclosure that protects a microwave antenna). Fiberglass reinforced plastics (FRP), also known as glass reinforced plastics (GRP) are a modern composite material, used in chemical plant equipment manufacturing like tanks and vessels by hand lay-up and filament winding processes using BS4994-British Standard. (BS4994 is a British standard related to this application which still remains a key standard for the specification design and construction of vessels and storage tanks using reinforced plastics.)
Besides the above applications, fiberglass materials are also used in UHF-broadcasting antennas, large commercial wind turbine blades, velomobiles (a bicycle car), and printed circuit boards used in electronics that consist of alternating layers of copper and fiberglass, which is technically known as FR-4. FR-4 is a grade designation given to glass-reinforced epoxy laminate sheets, tubes, rods and printed circuit boards (PCB). Glass fiber composite is also used in preparation of RF coils used in MRI scanners.

Carbon Fiber Reinforced Polymer Composites

Carbon fiber-PMCs are similar to fiberglass-PMCs, in which carbon fibers are used instead of fiberglass as reinforced materials in polymer matrix. The reinforcing dispersed phase may be in the form of carbon fibers, commonly woven into a cloth.
Carbon fibers are used in continuous or discontinuous form during PMCs manufacturing. It is expensive compared to glass fiber but has high specific mechanical properties to weight, with a very high modulus of elasticity, which can match that of steel in terms of tensile strength which may go up to more than 1,000 ksi (7 GPa). In addition it possesses very low density of 114 lb/ft3 (1,800 kg/m3) and high chemical inertness. These properties make carbon fibers one for potential reinforcement. The disadvantage of carbon (graphite) fibers is being brittle, accountable for a catastrophic mode of failure.
Some of the various types of carbon fibers available are “ultra-high modulus” (UHM), “high modulus” (HM), “intermediate modulus” (IM), “high tensile” (HT), and super high tensile (SHT).
“Ultra-high modulus” (UHM) carbon fibers has modulus of elasticity of about 65,400 ksi (450 GPa) whereas “high modulus” (HM) has modulus of elasticity is in the range 51,000–65,400 ksi (350–450 GPa). For “intermediate modulus” (IM) has modulus of elasticity in the range 29,000–51,000 ksi (200–350 GPa). For “high tensile” (HT) carbon fibers generally have tensile strength of 436 ksi (3 GPa) and modulus of elasticity of 14,500 ksi (100 GPa) whereas for super high tensile (SHT) carbon fiber, tensile strength is about 650 ksi (4.5 GPa).
Carbon fibers are manufactured by PAN-based carbon fibers, the most well-liked type of carbon fibers. Polyacrylonitrile (PAN) is used as a precursor for preparing PAN-based carbon fibers. In this method, the polyacrylonitrile precursor goes through several steps to become a carbon fiber (thermal oxidation at 200 C, carbonization in nitrogen atmosphere at 1,200 C for several hours, and graphitization at 2,500 C). Coal tar or petroleum asphalt is used as the precursor in pitchbased carbon fibers.
Epoxy, polyester, and nylon are among some of the polymers used as a matrix for preparation of carbon fiber based PMCs. This composite is generally prepared by open mold, closed mold, and the pultrusion method. Carbon fiber reinforced-PMCs are light in weight, show high strength, high modulus elasticity, high fatigue, good electrical conductivity, corrosion resistance, good thermal-stability, and low impact resistance.
Carbon fiber reinforced-PMCs are widely used in automotive, marine and aerospace applications, golf clubs, skis, tennis racquets, fishing rods, light weight bicycle frames, artificial light weight legs, etc.

Kevlar (Aramid) Fiber Reinforced Polymers

Kevlar fibers were originally developed as a replacement for steel in automotive tires, because of its high tensile strength-to-weight ratio, by this measure it is five times stronger than steel on an equal weight basis. Kevlar is a trade name, registered by DuPont Co. in 1965. It is an aramid fiber and its chemical name is polypara-phenylene terephthalamide. It is synthesized from 1,4-phenylene-diamine (para-phenylenediamine) and terephthaloyl chloride through a condensation reaction in solution from.
Apart from high tensile strength, it has very high modulus of elasticity, very low elongation breaking point, very low coefficient of thermal expansion, high chemical inertness, high fracture and high cut resistance, textile processability, excellent flame resistance, and toughness. It also shows high impact resistance and low density.
The disadvantages of Kevlar are its ability to absorb moisture, it is difficult to cut, and has low compressive strength.
There are several grades of Kevlar which are developed for various applications. Kevlar 29 – high strength, (~3,700 MPa) used for bullet-proof vests, composite armor reinforcement, helmets, cars, etc. Kevlar 49 has a high modulus about 132 GPa, high strength (~3,810 MPa), low density, and is used in aerospace, automotive, and marine applications. Kevlar 149 which has an ultra-high modulus (about 187 GPa), high strength (about 3,600 MPa), low density highly crystalline fibers, and is used for composite aircraft components.
The name of some other modified Kevlar are Kevlar K100 (colored version of Kevlar), Kevlar K119, Kevlar K129, Kevlar AP (has 15 % higher tensile strength than K-29), Kevlar XP (lighter weight resin). Kevlar KM2 is used as enhanced ballistic resistance for armor applications. Most of those Kevlar fibers are used in aerospace armor areas where mechanical, chemical properties, and weight play an important role.
UV degradation is the main drawback of Kevlar fiber and the ultraviolet present in sunlight degrades and decomposes Kevlar, so it needs protection during outdoors application.
However a combination of Kevlar and carbon fibers, a hybrid fabric, further improves their properties and give very high tensile strength, high impact, and abrasion resistance.
Epoxies (EP), vinylester, and phenolics (PF) are the most widely used matrix materials for manufacturing Kevlar (aramid) fiber reinforced-PMCs. Kevlar (aramid) fiber reinforced-PMCs are manufactured by open mold processes, closed mold processes, and the pultrusion method.

Effect of Length and Orientation of Reinforcing Material on PMCs

For fibrous composites, dispersed phase in form of fibers improves strength, stiffness and fracture toughness of the material, there is hindered crack growth in the directions normal to the fiber, and strength increases significantly when the fibers are arranged in a particular direction (preferred orientation) and a stress is applied along the same direction. In general PMCs strength is higher in long-fiber compared to that of short-fiber.
Short-fiber reinforced composites, consisting of dispersed phase in the form of discontinuous fibers, has a limited ability to distribute the load but is able to share the load. In addition orientation of the fibers in composite also decides the end properties of PMCs.
Short-fiber can exist in random preferred orientation in composites, whereas long-fiber reinforced composites consist of a reinforced matrix that can exist in the form of continuous fibers with unidirectional or bidirectional orientation.

Particulate Composites

Besides reinforcing fibers, a different kind of particle is used to make polymer composite. Choice of particle as a reinforcing agent depends on the end use of composites. Particulate composites consist of a matrix reinforced with a dispersed phase in the form of particles. Effect of the dispersed particles on the composite properties depends on the particles size. Very small particles, (less than 0.25 μm in diameter) finely distributed in the matrix, prevent the deformation of the material by restricting the dislocations movement. This strengthening effect is similar to the metal alloy’s “age hardening”. It is clearly found that, for a given particle amount, the mechanical strength of composite increases with decreasing particle size. As for example, mechanical properties or strength of kaolin filled nylon 6,6 composites increase with decreasing mean particle size (Bradley 1999). There is a large improvement in tensile strength with decreasing particle size. This indicates that the strength increases with increasing surface area of the filled particles through a more efficient stress transfer mechanism (Fu et al. 2008).
However, it is noted that for particles with size larger than 100 nm, the composite strength is reduced with increasing particle loading whereas for nanoparticle particles, with size 10 nm or lower, the strength of particle composites trend is reversed with loading. To conclude, particle size and amount of loading clearly has a significant effect on the strength of particulate-filled polymer composites.
Interface adhesion quality, between particle (reinforcing) and polymer matrix on fiber-reinforced composites is very important, control the strength and toughness of PMCs. The adhesion strength at the interface decides the load transfer between the components. However the Young’s modulus is not affected by this interfacial adhesion quality because, for small loads or displacements, debonding is not yet reported. Evaluation of adhesion between two different materials can be done by comparing surface properties of the particle (reinforcing) with respect to the polymer matrix. The basic mechanisms related to polymer surface are responsible for adhesion at the molecular level. The strength of micro-particle-filled composites either decreases or increases with particle content. This can be explained by interfacial adhesion, between particle and matrix, which significantly affects the strength of particulate composites. Effective stress transfer is the key factor, contributes to the strength of two-phase composite materials. The stress transfer at the particle-polymer interface is inefficient for weakly bonded particles which leads to discontinuity in the form of de-bonding. As a result composite strength decreases with increasing particle loading. However, for well-bonded or compatible particles addition into a polymer matrix will lead to an increase in strength especially for nanoparticles with high surface areas.
For example, it is reported that the interface bonding strength between alumina nanoparticles and vinyl ester resin shows decreased strength due to particle agglomeration, but when functionalization alumina nanoparticles are used this leads to a strong interfacial bonding between particle and matrix. This significantly increases both the modulus and strength of the composite.
In general, therefore, quality of interfacial adhesion between particles and matrix has a very significant effect on composite fracture toughness. Strong adhesion leads to high toughness in thermoplastic matrices but not necessarily in thermosetting matrices due to different failure mechanisms. To summarize, the strength of particulate composites is determined not only by particle size and quality of interfacial adhesion between particle and matrix but also by the amount of particle loading. Various trends in composite strength have been observed due to the interplay between these three factors (particle size, amount of loading, and interfacial adhesion), which cannot always be separated.
Use of hard particles such as ceramic particles prevent wear and abrasion of particulate composites and allow materials designed to work in high temperature applications, whereas copper and silver particles provide composites with high electrical conductivity matrices; for refractory use tungsten and molybdenum are used as dispersed phase to work in high temperature electrical applications.

Laminate Composites

Laminate composites are made when a fiber reinforced composite consists of several layers with different fiber orientations, it is also called multilayer (angleply) composite.
These layers are arranged in different anisotropic orientations as a matrix reinforced with a dispersed phase in the form of sheets. It directs the increased mechanical strength where mechanical properties of the material are low. Scheme 1 shows the various techniques for the preparation of polymer matrix composites.