gt;gt; Chapter One gt; gt;Advances in Polymer Composites: Macro- and Microcomposites – State of the Art, New Challenges, and Opportunitiesgt; gt; gt;Josmin P. Jose, Sant Kumar Malhotra, Sabu Thomas, Kuruvilla Joseph, Koichi Goda, and Meyyarappallil Sadasivan Sreekalagt; gt; gt; gt;1.1 Introductiongt; gt; Composites can be defined as materials that consist of two or more chemically and physically different phases separated by a distinct interface. The different systems are combined judiciously to achieve a system with more useful structural or functional properties nonattainable by any of the constituent alone. Composites, the wonder materials are becoming an essential part of today's materials due to the advantages such as low weight, corrosion resistance, high fatigue strength, and faster assembly. They are extensively used as materials in making aircraft structures, electronic packaging to medical equipment, and space vehicle to home building. The basic difference between blends and composites is that the two main constituents in the composites remain recognizable while these may not be recognizable in blends. The predominant useful materials used in our day-to-day life are wood, concrete, ceramics, and so on. Surprisingly, the most important polymeric composites are found in nature and these are known as natural composites. The connective tissues in mammals belong to the most advanced polymer composites known to mankind where the fibrous protein, collagen is the reinforcement. It functions both as soft and hard connective tissue. gt; Composites are combinations of materials differing in composition, where the individual constituents retain their separate identities. These separate constituents act together to give the necessary mechanical strength or stiffness to the composite part. Composite material is a material composed of two or more distinct phases (matrix phase and dispersed phase) and having bulk properties significantly different from those of any of the constituents. Matrix phase is the primary phase having a continuous character. Matrix is usually more ductile and less hard phase. It holds the dispersed phase and shares a load with it. Dispersed (reinforcing) phase is embedded in the matrix in a discontinuous form. This secondary phase is called the dispersed phase. Dispersed phase is usually stronger than the matrix, therefore, it is sometimes called reinforcing phase. gt; Composites in structural applications have the following characteristics: gt; • They generally consist of two or more physically distinct and mechanically separable materials. gt; • They are made by mixing the separate materials in such a way as to achieve controlled and uniform dispersion of the constituents. gt; • They have superior mechanical properties and in some cases uniquely different from the properties of their constituents. gt; Wood is a natural composite of cellulose fibers in a matrix of lignin. Most primitive man-made composite materials were straw and mud combined to form bricks for building construction. Most visible applications pave our roadways in the form of either steel andaggregate reinforced Portland cement orasphalt concrete. Reinforced concrete is another example ofcomposite material. The steel and concrete retain their individual identities in the finished structure. However, because they work together, the steel carries the tension loads and concrete carries the compression loads. gt; Most advanced examples perform routinely on spacecraft in demanding environments. Advanced composites have high-performance fiber reinforcements in a polymer matrix material such as epoxy. Examples are graphite/epoxy, Kevlar/epoxy, and boron/epoxy composites. Advanced composites are traditionally used in the aerospace industries, but these materials have now found applications in commercial industries as well. gt; gt; gt;1.2 Classification of Compositesgt; gt; On the basis of matrix phase, composites can be classified into metal matrix composites (MMCs), ceramic matrix composites (CMCs), and polymer matrix composites (PMCs) (Figure 1.1). The classifications according to types of reinforcement are particulate composites (composed of particles), fibrous composites (composed of fibers), and laminate composites (composed of laminates). Fibrous composites can be further subdivided on the basis of natural/biofiber or synthetic fiber. Biofiber encompassing composites are referred to as biofiber composites. They can be again divided on the basis of matrix, that is, nonbiodegradable matrix and biodegradable matrix. Bio-based composites made from natural/biofiber and biodegradable polymers are referred to as green composites. These can be further subdivided as hybrid composites and textile composites. Hybrid composites comprise of a combination of two or more types of fibers. gt; gt; 1.2.1 gt;Polymer Matrix Compositesgt; gt; Most commercially produced composites use apolymer 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 (PEEK), and others. The reinforcement materials are often fibers but can also be common ground minerals [6]. The various methods described below have been developed to reduce the resin content of the final product. As a rule of thumb, hand lay up results in a product containing 60% resin and 40% fiber, whereas vacuum infusion gives a final product with 40% resin and 60% fiber content. The strength of the product is greatly dependent on this ratio. gt; PMCs are very popular due to their low cost and simple fabrication methods. Use of nonreinforced 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 PMCs, which is characterized by the following: gt; a) High specific strength gt; b) High specific stiffness gt; c) High fracture resistance gt; d) Good abrasion resistance gt; e) Good impact resistance gt; f) Good corrosion resistance gt; g) Good fatigue resistance gt; h) Low cost gt; gt; The main disadvantages of PMCs are gt; a) low thermal resistance and gt; b) high coefficient of thermal expansion. gt; gt; gt;Factors Affecting Properties of PMCsgt; gt; gt;Interfacial Adhesiongt; The behavior of a composite material is explained on the basis of the combined behavior of the reinforcing element, polymer matrix, and the fiber/matrix interface (Figure 1.2). 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 (AFM) and nano indentation devices have facilitated the investigation of the interface. The interface is also known as the mesophase. gt; gt;Shape and Orientation of Dispersed Phase Inclusions (Particles, Flakes, Fibers, and Laminates)gt; Particles have no preferred directions and are mainly used to improve properties or lower the cost of isotropic materials. 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 1.2 Schematic model of interphase. gt; gt;Properties of the Matrixgt; Properties of different polymers will determine the application to which it is appropriate. 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. Varieties of polymers for composites are thermoplastic polymers, thermosetting polymers, elastomers, and their blends. gt; gt;Thermoplastic polymers:gt; Thermoplastics consists 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. gt; gt;Thermosetting polymers:gt; Thermosetts 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. gt; gt;Elastomers:gt; An elastomer is a polymer with the property of viscoelasticity, generally having notably low Youngs 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. At ambient temperatures, rubbers are relatively soft (gt;Egt; ~ 3 MPa) and deformable; their primary uses are for seals, adhesives, and molded flexible parts. 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. gt; gt; gt;Fabrication of Compositesgt; gt; The fabrication and shaping of composites into finished products often combines the formation of the material itself during the fabrication process. 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. gt; gt;Hand Lay-Upgt; The oldest, simplest, and the 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. gt; gt;Bag Molding Processgt; 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. Three basic molding methods involved are pressure bag, vacuum bag, and autoclave. gt; gt;Pultrusiongt; 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. gt; gt;Filament Windinggt; 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. gt; gt;Preformed Molding Compoundsgt; 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. This enables the attainment of faster production rate. Molding compounds can be divided into three broad categories: dough molding, sheet molding, and prepregs. gt; gt;Resin Transfer Moldinggt; Resin transfer molding (RTM) has the potential of becoming a dominant low-cost process for the fabrication of large, integrated, high performance products. In this process, adry reinforced material that has been cut and shaped into a preformed piece, generally called a perform, 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 (Figure 1.3). gt; gt;Injection Moldinggt; 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. 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. 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. gt; gt;Reaction Injection Molding (RIM)gt; RIM is similar to injection molding except that thermosetting polymers are used, which requires a curing reaction to occur within the mold. Common items made via RIM 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 RIM processable material is polyurethane (generally known as PU-RIM), 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 RIM-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 (DOI) when painted. gt; gt;Reinforced Reaction Injection Moldinggt; If reinforcing agents are added to the mixture of RIM setting then the process is known as reinforced reaction injection molding (RRIM). Common reinforcing agents include glass fibers and mica. This process is usually used to produce rigid foam automotive panels. A subset of RRIM is structural reaction injection molding (SRIM), 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. gt; gt;Spray-Upgt; 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 1–2" (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. gt; gt;(Continues...)gt; gt; gt; gt;gt; gt;gt;gt; Excerpted from gt;Polymer Composites, Macro- and Microcompositesgt; Copyright © 2012 by John Wiley & Sons, Ltd. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the;Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.