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Polymer Composites; Properties, Performance and Applications

Hüsnügül Yılmaz Atay

Department of Materials Science and Engineering, İzmir Katip Celebi University, Çiğli, İzmir, 35620, Turkey

Polymer composites are one of the most important advanced materials created through the development of specialized process and synthesis technology. They are one of the most dynamic sectors of polymer research and industry, representing one of the fastest growing areas in polymer science. It has been provided the up-to-date account of the situation of Polymer Composite Materials in this chapter from the basic science to the latest innovations. Starting with the fundamental definitions, variety of properties and manufacturing methods, a facile way to understand the new techniques in polymer composite materials has been provided to the reader. Also it has been mentioned more recent processes and applications, briefly.

Keywords: Advanced materials; Polymer composites; Properties; Performance; Applications

1. Introduction

Advanced materials are novel materials which have engineered properties created through the development of specialized process and synthesis technology. They hold the key to our future by improving the properties over their replacement weight, strength, formability, conductivity etc. Composites, which can be defined as one of the most important of these advanced materials, are the materials consisting of two or more chemically and physically different phases (matrix phase and dispersed phase) separated by a distinct interface. Matrix phase is the primary phase having a continuous character. The matrix is usually more ductile and less hard phase, and it holds the dispersed phase and shares a load with it. Dispersed (reinforcing) phase is embedded in the matrix in a discontinuous form usually stronger than the matrix, therefore, it is sometimes called reinforcing phase. In the polymer composites, generally it is said to be polymer matrix composites, strong fibers embedded in a resilient plastic that holds them in place. Thanks to this technology, lighter and stronger polymer composite materials are being designed to be used at high temperatures and resist corrosion better than conventional metals or plastics in various commercial and military applications [1,2,3]. Materials such as glass, boron and aramid have extremely high tensile and compressive strength. However, when stressed, random surface flaws will cause each material to crack and fail well below its theoretical breaking point because their properties are not readily apparent in solid form. To make a success of this, the material is produced in fibre form, thereby they will be restricted to a small number of fibres with the remainder exhibiting the material's theoretical strength. In this context, exceptional properties can be achieved when the resin systems are reinforced with such fibres [1,2,3]. As a general view, it seems that large part of the produced polymer composites are used in the building sectors, however, they are also used in transportation (moulded parts, fuel and gas tanks), aerospace (satellites and aircraft structures), marine, biomedical (dental fixtures, prosthetic devices), electronics and in recreation industries. Moreover, it continues to be researched their performance and applications [1,2,3]. The material evolution which occurred millions of years ago has led to the development of composites in the 20th century. Simultaneously with composite usage as fuselage materials, aircraft engines have also used this technology. Advanced Composite Materials now forms about 80% of structural weight in fighter aircraft, adding to their speed & maneuverability. A helicopter rotor blade was made from composites in 1990. An all composite aircraft ‘Avtek 400’ has been made by using Kevlar in the fuselage because graphite alone is too brittle (Figure 1) [1].

Fig. 1 Avtek 400 aircraft.

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

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Composites are produced when two or more materials or phases are used together to give a combination of properties that cannot be attained otherwise. They can be selected to give unusual combinations of stiffness, strength, weight, high-temperature performance, corrosion resistance, hardness, or conductivity. Actually, composites can highlight how different materials can work in synergy. Abalone shell, wood, bone, and teeth are examples of naturally occurring composites. People have been making composites for many thousands of years. One early example is mud bricks. Mud can be dried out into a brick shape to give a building material. It is strong if you try to squash it, but it breaks quite easily if you try to bend it. So, it has good compressive strength, but poor tensile strength. Straw seems very strong if you try to stretch it, but you can crumple it up easily. By mixing mud and straw together it is possible to make bricks that are resistant to both squeezing and tearing and make excellent building blocks. Another ancient composite is concrete. Concrete is a mix of aggregate, cement and sand. It has good compressive strength, it resists squashing. In more recent times it has been found that adding metal rods or wires to the concrete can increase its tensile strength. Concrete containing such rods or wires is called reinforced concrete. Microscale composites include such materials as carbon or glass fiber reinforced plastics. These composites offer significant gains in specific strengths and are finding increasing usage in airplanes, electronic components, automotives, and sporting equipment [4,5]. On the basis of matrix phase, composites can be classified into metal matrix composites (MMCs), ceramic matrix composites (CMCs), and polymer matrix composites (PMCs) [3]. Polymer blends have invaded the ever-growing market of automobile and electronic industries and much of the equipment that is in day-to-day use in our homes. In addition, they are very useful for designing new materials that potentially fulfill the green requirements. In addition, polymer composites provide the ability to lower costs without sacrificing those properties that are most desired. They provide the ability to control or tailor properties without the invention or creation of completely new polymers, extending engineering resin performance by diluting it with a low cost polymer adjusting the composition of the blend to customer specifications. It can be formed a high performance blend from synergistically interacting polymers. It can be provided understanding of the underlying fundamentals that control miscibility. Finally, recycling industrial and/or municipal scrap can be possible [1]. The first modern polymeric composite material was fibreglass. Thousands of years natural polymers have been blended with naturally occurring fillers, fibers and many other substances. In this century, the development of synthetic polymers has led to the development of high performance polymer composites. It is still widely used today for boat hulls, sports equipment, building panels and many car bodies. The matrix is a plastic and the reinforcement is glass that has been made into fine threads and often woven into a sort of cloth. On its own, the glass is very strong but brittle and it will break if bent sharply. The plastic matrix holds the glass fibres together and also protects them from damage by sharing out the forces acting on them [5]. Use of pure polymers as structure materials is limited by low level of their mechanical properties, namely strength, modulus, and impact resistance. Reinforcement of polymers by strong fibrous network permits fabrication of polymer matrix composites, which is characterized by high specific strength, high specific stiffness, high fracture resistance, good abrasion resistance, good impact resistance, good corrosion resistance, good fatigue resistance and low cost. Their main disadvantages are low thermal resistance and high coefficient of thermal expansion. The biggest advantage of polymer composite materials is that they are light as well as strong. By choosing an appropriate combination of matrix and reinforcement material, a new material can be made that exactly meets the requirements of a particular application. Polymer composites also provide design flexibility because many of them can be moulded into complex shapes.

2. Structure

The term polymer-matrix composite is applied to a number of plastic-based materials in which several phases are present. It is often used to describe systems in which a continuous phase (the matrix) is polymeric and another phase (the reinforcement) has at least one long dimension. Polymer matrix material often called a resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, polypropylene, polyether ether ketone, and others. The reinforcement materials are often fibers but can also be common ground minerals (Figure 2) [6,7].

Fig. 2 Polymer composites structures.


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2.1 Matrix

Various polymeric materials have been used as matrix for the polymer composites. Varieties of polymers for composites are thermoplastic polymers, thermosetting polymers, elastomers, and their blends. Thermoplastics consist of linear or branched chain molecules having strong intramolecular bonds but weak intermolecular bonds. They can be reshaped by application of heat and pressure and are either semicrystalline or amorphous in structure. Examples include polyethylene, polypropylene, polystyrene, nylons, polycarbonate, polyacetals, polyamide-imides, polyether ether ketone, polysulfone, polyphenylene sulfide, polyether imide, and so on. Thermosets have cross-linked or network structures with covalent bonds with all molecules. They do not soften but decompose on heating. Once solidified by cross-linking process they cannot be reshaped. Common examples are epoxies, polyesters, phenolics, ureas, melamine, silicone, and polyimides. An elastomer is a polymer with the property of viscoelasticity, generally having notably low Young’s modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred when referring to vulcanizates. Each of the monomers that link to form the polymer is usually made of carbon, hydrogen, oxygen, and silicon. Elastomers are amorphous polymers existing above their glass transition temperature, so that considerable segmental motion is possible. Natural rubber, synthetic polyisoprene, polybutadiene, chloroprene rubber, butyl rubber, ethylene propylene rubber, epichlorohydrin rubber, silicone rubber, fluoroelastomers, thermoplastic elastomers, polysulfide rubber, and so on are some of the examples of elastomers [6].

2.2 Fibre Reinforcements and Particulate Fillers

Reinforcement usually adds rigidity and greatly impedes crack propagation. Thin fibers can have very high strength, and provided they are mechanically well attached to the matrix they can greatly improve the composite's overall properties. Fiber-reinforced composite materials can be divided into two main categories normally referred to as short fiber-reinforced materials and continuous fiber-reinforced materials. Continuous reinforced materials will often constitute a layered or laminated structure. The woven and continuous fibre styles are typically available in a variety of forms, being pre-impregnated with the given matrix (resin), dry, uni-directional tapes of various widths, plain weave, harness satins, braided, and stitched. The short and long fibers are typically employed in compression moulding and sheet moulding operations. These come in the form of flakes, chips, and random mate which can also be made from a continuous fibre laid in random fashion until the desired thickness of the ply / laminate is achieved. Common fibers used for reinforcement include glass fibers, carbon fibers, cellulose (wood/paper fiber and straw) and high strength polymers for example aramid. Silicon carbide fibers are used for some high temperature applications. Many engineering polymers that contain fillers and extenders are particulate composites. A classic example is carbon black in vulcanized rubber. Carbon black consists of tiny carbon spheroids only 5 to 500 nm in diameter. The carbon black improves the strength, stiffness, hardness, wear resistance, resistance to degradation due to ultraviolet rays, and heat resistance of the rubber. Nanoparticles of silica are added to rubber tires to enhance their stiffness. Extenders, such as calcium carbonate (CaCO), solid glass spheres, and various clays, are added so that a smaller amount of the more expensive polymer is required. The extenders may stiffen the polymer, increase the hardness and wear resistance, increase thermal conductivity, or improve resistance to creep; however, strength and ductility normally decrease. Introducing hollow glass spheres may impart the same changes in properties while significantly reducing the weight of the composite. Other special properties can be obtained. Elastomer particles are introduced into polymers to improve toughness. Polyethylene may contain metallic powders, such as lead, to improve the absorption of fission products in nuclear applications [4]. Important changes in the properties of plastics resulting from the incorporation of special additives permit their use in applications where the polymer alone would have had small chance to meet certain performance specifications. Fillers and reinforcements are solid additives that differ from the plastic matrices with respect to their composition and structure. The basic role of a filler is to increase the bulk at low cost, thereby improving economics while, by definition, the main function of a reinforcing filler is to improve the physical and mechanical properties of the basic polymer. Out of these properties, stiffness and strength are the most important among ‘short-term’ properties in engineering applications. Resistance to creep and fatigue failure are principal long-term properties. The effect of heat on both these groups of properties is essential. The heat deflection temperature effects dimensional stability. All these properties can be upgraded by reinforcing fillers. Non-mechanical properties of the basic polymer, e.g., electrical properties, abrasion resistance, flammability, may also be strongly modified by the presence of reinforcing fillers. With fibrous fillers, the influence of the fibre aspect ratio and the anisotropic effect of fibre orientation can further magnify the improvements. That is why the most effective reinforcing fillers are fibres of high modulus and strength.


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Modern fillers can take on many of the functions of reinforcements. Usually, fibres and lamina structure are counted as reinforcements, while the ball type additives are counted as fillers. Inert fillers or extender fillers increase the bulk, solve some processing problems, and lower the price; no improvement is seen in the mechanical or physical properties compared with the unfilled polymer, although by a higher thermal conductivity, they improve the production rates. Active fillers, enhancers, and reinforcements produce specific improvements of certain mechanical or physical properties, including modulus, tensile and impact strength, dimensional stability, heat resistance, and electrical properties. Most particulate fillers have a higher specific gravity than polymers, but some, such as hollow ceramic or glass spheres, expandable beads, have been developed to reduce the weight of the compound. Liquid extenders are used in elastomers. Particulate mineral additives are classified as two- or three-dimensional. The two-dimensional silicates in layers (such as talc and mica) essentially induce rigidity and thermal stability, but do not attain the stiffening effect of fibre-type reinforcements. Average particle diameter, specific surface and surface energy are decisive in the reinforcing effect of fillers in elastomers. Enhancers and reinforcing fillers induce the some improvements in thermoplastics. For instance, increasing in tensile strength and tensile stress at break, and in compressive and shear strength, increasing of the modulus of elasticity, stiffness and hardness of the composite material, increasing of heat deflection temperature and decreasing of the temperature dependence on the mechanical values, improving creep behaviour and bend-creep modulus, as well as partial impact strength and reducing the viscoelastic yield under load and lower shrinkage, giving good dimensional stability [8]. The action of active fillers can be attributed to three causes. First of all, chemical bond formation between filler and the material need to be reinforced. Immobilisation of polymer segments attached to the filler surface by secondary or primary valence bonds, an interfacial layer with characteristic properties thus appearing. When the polymer molecules are subjected to stress with energy absorption, they can slide off the filler surface; the impact energy is thus uniformly distributed and the impact strength increases. Detailed theories on these aspects are important. For their use in plastics the most important characteristics of fillers are: chemical composition, particle shape, average diameter, grain distribution, specific surface and value of surface energy, thermo-oxidative and UV-stability for outdoor applications and moisture and water-soluble compounds content [1,9]. For a controlled modification of the various properties of the composite materials, certain characteristics of the filler/reinforcement are necessary. The main requirements for the properties of fillers and reinforcements necessary to obtain of composite materials that can be further used in specific applications are [8]: • The chemical purity of fillers is very important. • Low moisture absorption and high bulk density; they should preserve their properties during storage prior to compounding. • Optimum compounding is achieved with fillers and reinforcements of a certain particle size, intimate wettability through the polymer matrix, which does not present static charge, no shortening of the reinforcing fibres taking place, which means a good dispersion behaviour. • Filler particles should be as round as possible with a small specific surface, low surface energy, and absorptivity, thus assuring a low viscosity during compounding. A high compounding rate is obtained with fillers having low specific heat and high thermal conductivity. A composite with high tensile strength and high elongation is obtained when using a filler/reinforcement having high strength in comparison with the matrix, high length/diameter ratio, and good fibre/matrix adhesion, as well as a good distribution in the matrix while, for high flexural strength, it is very important to obtain, additionally a smooth surface in the finished article [1]. Fillers with low compressibility and small round particles are suitable for obtaining composites with high compressive strength from crystalline polymers. Fibrous or lamellar reinforcements with a high length/diameter ratio, high modulus of elasticity in comparison with the matrix, high orientation in the direction of the force profile and good adhesion are used for composites requiring high stiffness and high modulus of elasticity. Good long-term behaviour and fatigue and weathering resistance of the composite materials are achieved with filler/reinforcement with permanent polymer/matrix bonds, good resistance to heat, light, water, chemicals, etc. Also, a low cost results when low cost filler is used with low processing cost and maximum possible degree of filling; the other requirements regarding the properties being, of course, fulfilled [1]. On the other hand, it is well known that mineral fillers have a catalytic effect on resin cure and that the effect is specific to each resin and each curing system. Thus, polyester resins cured with a benzoyl peroxide initiator are less sensitive to the mineral surface than the same resin cured with a cobalt-promoted ketone peroxide initiator. Glass fibres treated with a chrome finish, retarded gelation of a polyester more than those treated with a silane finish. Barium sulphate, calcium carbonate and zircon promoted a much faster cure than an unfilled resin. With some materials such as clay, silica and talc, the inhibition of cure is severe enough to limit their usefulness in highly filled systems. Most polymers undergo shrinkage during cure. Therefore, many composites are translucent when removed hot from the press, but they become opaque due to crazing as the composite cools. Mixing finely divided particulate fillers with resin produces total expansion, yet merely transfers interfacial stresses from a macro to a microscale. The result is that


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particulate filled plastics generally have lower mechanical properties than the milled filled plastics [1]. The value of filler depends on the difference between the coefficients of thermal expansion that must have minimum values.

3. Factors Affecting Properties of Polymer Composites

The macroscopic behaviour of particulate composites is affected by the size, shape, and the distribution of the inclusions. The interfacial adhesion between the matrix and inclusion is important. Addition of rigid particles to polymers or other matrices can produce a number of desirable effects; for example an increase in stiffness, a reduction in the coefficient of thermal expansion and an improvement in creep resistance and fracture toughness [4]. The modulus of a filled resin results from a complex inter-play between the properties of the individual constituent phases; the resin, the filler and the interfacial region. In the case of non-spherical particles, the degree of orientation with respect to the applied stress is also important [11]. Therefore, many factors must be considered when designing a fiber-reinforced composite, including the length, diameter, orientation, amount, and properties of the fibers, the properties of the matrix, and the bonding between the fibers and the matrix.

3.1 Interfacial Adhesion

The behavior of a composite material belongs to the basis of the combined behavior of the reinforcing element, polymer matrix, and the fiber/matrix interface. To attain superior mechanical properties the interfacial adhesion should be strong. Matrix molecules can be anchored to the fiber surface by chemical reaction or adsorption, which determine the extent of interfacial adhesion. The developments in atomic force microscopy and nano indentation devices have facilitated the investigation of the interface. The interface is also known as the mesophase [6]. The interface between the filler or reinforcing material and the polymeric matrix is essential in polymeric composites [12]. The interface adjusts the composite’s mechanical resistance. Concentration of mechanical stress occurs, as a result of the differences between the matrix’s coefficient of thermal expansion and the coefficient of the filler or reinforcing agent, due to contraction at crosslinking and to crystallisation. At the same time, it acts as a site for the production of chemical reactions. The force acting on a polymeric composite’s matrix is transmitted to the filler or reinforcing material through the interface. Its resistance and durability are determined by several factors which govern the matrix’s adhesion to the surface of the included material. With a view to increasing the interfacial resistance between the included material and the matrix, the surface of the filling or reinforcing material is usually treated by special techniques, so that to create the conditions in which the interface should assure shifting – without any discontinuities – from the properties of the matrix to those of the inclusion material. The adhesion force between the solid polymers and other substances, along with other parameters, such as the contact surface and the diffusion distance, are strongly influenced by the type and magnitude of the intermolecular forces manifested between adhesion parameters [13]. The adhesion forces may be characterised by the application of different types of methods, such as direct measurement of the force, spectral determination of the chemical composition of the solid surfaces and physico-chemical determination of the energetic interactions [1].

3.2 Shape and Orientation of Dispersed Phase Inclusions

Particles have no preferred directions and are mainly used to improve properties or lower the cost of isotropic materials [14]. The shape of the reinforcing particles can be spherical, cubic, platelet, or regular or irregular geometry. Particulate reinforcements have dimensions that are approximately equal in all directions. Large particle and dispersion-strengthened composites are the two subclasses of particle-reinforced composites. A laminar composite is composed of two dimensional sheets or panels, which have a preferred high strength direction as found in wood. The layers are stacked and subsequently cemented together so that the orientation of the high strength direction varies with each successive layer (Figure 3) [15].

Fig. 3 Fillers in material improve strength.


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3.3 Properties of the Matrix Properties

The chief advantages of polymers as matrix are low cost, easy processability, good chemical resistance, and low specific gravity. On the other hand, low strength, low modulus, and low operating temperatures limit their use [16]. The matrix supports the fibers and keeps them in the proper position, transfers the load to the strong fibers, protects the fibers from damage during manufacture and use of the composite, and prevents cracks in the fiber from propagating throughout the entire composite. The matrix usually provides the major control over electrical properties, chemical behavior, and elevated temperature use of the composite. Polymer matrices are particularly common. Thermoplastics and thermosets polymer materials are available in short glass fiber-reinforced grades. These composites are formed into useful shapes by different processes. Sheetmolding compounds and bulk-molding compounds are typical of this type of composite. Thermosetting aromatic polyimides are used for somewhat higher temperature applications [4]

4. Bonding and Failure

Particularly in polymer and metal-matrix composites, good bonding must be obtained between the various constituents. The fibers must be firmly bonded to the matrix material if the load is to be properly transmitted from the matrix to the fibers. In addition, the fibers may pull out of the matrix during loading, reducing the strength and fracture resistance of the composite if bonding is poor. In some cases, special coatings may be used to improve bonding. Glass fibers are coated with a silane coupling to improve bonding and moisture resistance in fiberglass composites. Carbon fibers similarly are coated with an organic material to improve bonding. Boron fibers can be coated with silicon carbide or boron nitride to improve bonding with an aluminum matrix; in fact, these fibers are called Borsic fibers to reflect the presence of the silicon carbide (SiC) coating. Another property that must be considered when combining fibers into a matrix is the similarity between the coefficients of thermal expansion for the two materials. If the fiber expands and contracts at a rate much different from that of the matrix, fibers may break or bonding can be disrupted, causing premature failure [4].

5. Production methods

Various conventional methods of composite fabrication are as old as 40 years and have their origins in the World War II. During this period, technology in all spheres of life has jumped by leaps and bounds which has had its impact on the way these methods are used today. It is now easy and clear to demarcate these methods between conventional and new. What is new today, e.g., robotic moulding, has its roots in the old (hand-lay up, lamination, autoclave, filament winding, pultrusion, etc.) and the boundary between them is fuzzy and not sharp [1]. Many advances in composite technology have taken place during the intervening era. For instance, development of computerised controls development of advanced thermoset and thermoplastic resins to withstand high temperatures for aerospace, widening the scope of composites to include underwater applications, such as offshore platforms, development of aramid fibres and new grades of carbon fibres, and sensor and artificial neural network technologies etc. The tremendous versatility can be achieved by combining two or more materials on a sub-microscopic, microscopic, or even macroscopic scale to obtain properties which are not previously available for a much greater variety of applications.

5.1 Resin Selection

Generally following five resins currently used for the production of composites, other than for research and development purposes are epoxides, polyesters, phenolics, silicones and polyimides. Epoxides have excellent mechanical properties, dimensional stability, chemical resistance, low water absorption, self-extinguishing, low shrinkage, good anrasion resistance, excellent adhesion properties. They can be produced by compression moulding, flament winding, hand lay-up, mat moulding and pressure bag moulding. They require heat curing for maximum performance. Polyesters are the simplest, most versatile, economical and most widely used family of resins. They have chemical resistance, especially to acids. They can be produced similar to epoxides, besides pultrision, encapsulation and centrifugal casting. The limitation of polyesters is degradation by strong oxidisers, aromatic solvents, concentrated caustics. Phenolic resins have good acid resistance, good electrical properties and high heat resistance. They dissolve in caustic environments unless specially treated. Silicones have highest heat resistance, low water absorption, excellent dielectric properties, high arc resistance.


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5.2 Fabrication of Composites

The fabrication and shaping of composites into finished products often combines the formation of the material itself during the fabrication process [17]. The important processing methods are hand lay-up, bag molding process, filament winding, pultrusion, bulk molding, sheet molding, resin transfer molding, injection molding, and so on [6].

5.2.1 Hand Lay-Up

The oldest, simplest and most commonly used method for the manufacture of both small and large reinforced products is the hand lay-up technique. A flat surface, a cavity or a positive-shaped mold, made from wood, metal, plastic, or a combination of these materials may be used for the hand lay-up method.

5.2.2 Bag Molding Process

It is one of the most versatile processes used in manufacturing composite parts. In bag molding process, the lamina is laid up in a mold and resin is spread or coated, covered with a flexible diaphragm or bag, and cured with heat and pressure. After the required curing cycle, the materials become an integrated molded part shaped to the desired configuration [14]. Three basic molding methods involved are pressure bag, vacuum bag, and autoclave.

5.2.3 Pultrusion

It is an automated process for manufacturing composite materials into continuous, constant cross-section profiles. In this technique, the product is pulled from the die rather than forced out by pressure. A large number of profiles such as rods, tubes, and various structural shapes can be produced using appropriate dies.

5.2.4 Filament

Winding filament winding is a technique used for the manufacture of surfaces of revolution such as pipes, tubes, cylinders, and spheres and is frequently used for the construction of large tanks and pipe work for the chemical industry. High-speed precise lay down of continuous reinforcement in predescribed patterns is the basis of the filament winding method.

5.2.5 Preformed Molding Compounds

A large number of reinforced thermosetting resin products are made by matched die molding processes such as hot press compression molding, injection molding, and transfer molding. Matched die molding can be a wet process but it is most convenient to use a preformed molding compound or premix to which all necessary ingredients are added [13]. This enables the attainment of faster production rate. Molding compounds can be divided into three broad categories: dough molding, sheet molding, and prepregs.

5.2.6 Resin Transfer Molding

Resin transfer molding has the potential of becoming a dominant low-cost process for the fabrication of large, integrated, high performance products. In this process, a dry reinforced material that has been cut and shaped into a preformed piece is placed in a prepared mold cavity. The resin is often injected at the lowest point and fills the mold upward to reduce the entrapping of air. When the resin starts to leak into the resin trap, the tube is clamped to minimize resin loss. When excess resin begins to flow from the vent areas of the mold, the resin flow is stopped and the mold component begins to cure. Once the composite develops sufficient green strength it can be removed from the tool and postcured.

5.2.7 Injection Molding

Injection molding is a manufacturing process for both thermoplastic and thermosetting plastic materials. Composites is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity (Figure 4). Injection molding is used to create many things such as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, and most other plastic products available today. It is ideal for producing high volumes of the same object [17]. Some advantages of injection molding are high production rates, repeatable high tolerances, and the ability to use a wide range of materials, low labor cost, minimal scrap losses, and little need to finish parts after molding. Some disadvantages of this process are expensive equipment investment, potentially high running costs, and the need to design moldable parts.


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Fig. 4 Injection Moulding of thermoplastics.

5.2.8 Reaction Injection Molding

Reaction injection molding is similar to injection molding except that thermosetting polymers are used, which requires a curing reaction to occur within the mold. Common items include automotive bumpers, air spoilers, and fenders. First, the two parts of the polymer are mixed together. The mixture is then injected into the mold under high pressure using an impinging mixer. The most common processable material is polyurethane, but others include polyureas, polyisocyanurates, polyesters, polyepoxides, and nylon 6. For polyurethane, one component of the mixture is polyisocyanate and the other component is a blend of polyol, surfactant, catalyst, and blowing agent. Automotive applications comprise the largest area of use for produced products. Polymers have been developed specifically for exterior body panels for the automotive industry. Non-E-coat polymers offer an excellent combination of stiffness, impact resistance, and thermal resistance for body panel applications. These provide excellent paintability and solvent resistance with the ability to achieve high distinction of image when painted.

5.2.9 Reinforced Reaction Injection Molding

If reinforcing agents are added to the mixture of reaction injection molding setting then the process is known as reinforced reaction injection molding. Common reinforcing agents include glass fibers and mica. This process is usually used to produce rigid foam automotive panels. A subset of reinforced reaction injection molding is structural reaction injection molding, which uses fiber meshes for the reinforcing agent. The fiber mesh is first arranged in the mold and then the polymer mixture is injection molded over it.

5.2.10 Spray-Up

In spray-up process, liquid resin matrix and chopped reinforcing fibers are sprayed by two separate sprays onto the mold surface. The fibers are chopped into fibers of 25–50 mm length and then sprayed by an air jet simultaneously with a resin spray at a predetermined ratio between the reinforcing and matrix phase. The spray-up method permits rapid formation of uniform composite coating, however, the mechanical properties of the material are moderate since the method is unable to use continuous reinforcing fibers.

6. Applications

Fueled by the need to surpass the limitations of conventional materials, recent years have seen a large increase in engineering applications of advanced fiber reinforced polymer composite materials in many major industries, such as aerospace and defense, automotive, construction, marine, and oil and gas industries. Fiber reinforced composites are very attractive for use in engineering applications due to their highly favorable material properties, including high strength-to-weight and stiffness-to-weight ratios, corrosion resistance, and light weight. Studies conducted to date have demonstrated numerous advantages offered by fiber reinforced composites in various engineering applications. In the broadest sense, an extreme environment can be any application for polymer composites where the ambient conditions compromise a material's durability. The most demanding extreme conditions may be those encountered in the air, on the land, under the sea, or in space, each with its unique set of requirements. One of the application area of polymer composites is aerospace structures. The military aircraft industry has mainly led the use of polymer composites. In commercial airlines, the use of composites is gradually increasing. Space shuttle and satellite systems use graphite/epoxy for many structural parts (Figure 5) [6,18].


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Fig. 5 Schematic section of a helicopter rodor blade. In marine, they can be used in the boat bodies, canoes, kayaks, and so on. In automotive industry they are used in ody panels, leaf springs, drive shaft, bumpers, doors, racing car bodies, and so on. As sports goods; golf clubs, skis, fishing rods, tennis rackets, and so on. Bulletproof vests and other armor parts. Chemical storage tanks, pressure vessels, piping, pump body, valves, and so on. For biomedical applications, they can be used in medical implants, orthopedic devices, X-ray tables. In addition, bridges made of polymer composite materials are gaining wide acceptance due to their lower weight, corrosion resistance, longer life cycle, and limited earthquake damage. Besides, they are used in electrical applications; panels, housing, switchgear, insulators, and connectors etc [19]. Consequently, polymer composites can be used at high and low temperatures, corrosive environments, including salt water and salt spray, or changing aqueous environments, high stresses, fatigue, or multiaxial loading etc.

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