composite materials module

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MODULE 6 Introduction to New materials 6.1 Motivation: Composites: Many of modern technologies require materials with unusual properties, which can’t be met by conventional materials like metals, polymers and ceramics. This led to the development of composite materials. Composites consist of more than one material type. Fiberglass, a combination of glass and a polymer, is an example. Concrete and plywood are other familiar composites. Many new combinations include ceramic fibers in metal or polymer matrix. Wood is best example for natural composite. Ploymers: Because of their immense diversity, we are on the verge of a major expansion in what polymers can do and how we use them. Polymers can conduct electricity and emit light, and be used to produce transistors. They are the basis of the nanotechnology revolution through their use as photoresists. They are used in medical devices and are vital elements in the coming revolution in nano-bio-technology. In the future, polymers can be expected to play a role in almost all technological advances, since we can now control their molecular structure, atom by atom, to form synthetic polymers with uses limited only by our imagination. Nano Materials: A motivation in nanoscience is to try to understand how materials behave when sample sizes are close to atomic dimensions. Size and shape effects leads to unique properties and the opportunity to use such nanostructured materials in novel applications and devices. Phenomena occurring on this length scale are of interest to physicists, chemists, biologists, electrical and mechanical engineers, and computer scientists, making research in nanotechnology a frontier activity in materials science.

Transcript of composite materials module

MODULE 6

Introduction to New materials

6.1 Motivation:

Composites: Many of modern technologies require materials with unusual properties, which can’t be met by conventional materials like metals, polymers and ceramics. This led to the development of composite materials. Composites consist of more than one material type. Fiberglass, a combination of glass and a polymer, is an example. Concrete and plywood are other familiar composites. Many new combinations include ceramic fibers in metal or polymer matrix. Wood is best example for natural composite.

Ploymers: Because of their immense diversity, we are on the verge of a major expansion in what polymers can do and how we use them. Polymers can conduct electricity and emit light, and be used to produce transistors. They are the basis of the nanotechnology revolution through their use as photoresists. They are used in medical devices and are vital elements in the coming revolution in nano-bio-technology. In the future, polymers can be expected to play a role in almost all technological advances, since we can now control their molecular structure, atom by atom, to form synthetic polymers with uses limited only by our imagination.

Nano Materials:A motivation in nanoscience is to try to understand how materials behave when sample sizes are close to atomic dimensions. Size and shape effects leads to unique properties and the opportunity to use such nanostructured materials in novel applications and devices. Phenomena occurring on this length scale are of interest to physicists, chemists, biologists, electrical and mechanical engineers, and computer scientists, making research in nanotechnology a frontier activity in materials science.

6.2 Syllabus:

Module Details Hrs6 Introduction to New materials:

Composites: Basic concepts of composites, Processing of composites, advantages over metallic materials, various types of composites and their applications.

Nano Materials: Introduction, Concepts, synthesis of nano materials, examples, applications and nano composites.

Polymers: Basic concepts, Processing methods, advantages and disadvantages over metallic materials, examples and applications.

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6.3 Books Recommended:

1. Materials Science and Engineering by William D. Callister, Jr. – Adapted by R. Balasubramaniam. Wiley India (P) Ltd. 2. The Structure and Properties of Materials Vol I: M. G. Moffet, G. T. W. Pearsall & J. Wulff. 3. Material Science and Metallurgy, By V.D. Kodgire. 4. Metallurgy for Engineer- E.C. Rollason - ELBS SOC. And Edward Arnold, London. 5. Mechanical Behaviour of Materials- Courtney- McGraw Hill International New Delhi. 6. Introduction of Engineering Materials, By B.K. Agrawal, McGraw Hill Pub. Co. ltd

6.4 Weightage in University Examination: Approx 8-10 Marks.

6.5 Objective :

After studying this chapter you should be able to do the following:

Composites:

To know the different class of materials called composites To study different composites and their strengthening mechanisms with examples. To understand the importance and application of composites.

Ploymers:

To understand the characteristics, applications, and processing of polymeric materials, their mechanical behavior and different methods to improve their performance.

Nano Materials:

Define key terms in nanotechnology Explain some of the ways nanomaterial properties differ from molecules and microscale

particles Describe some of the physical and chemical characteristics that can change at the

nanoscale Describe some of the major classes of nanomaterials produced today and their properties

and potential benefits

6.6 Composite

6.6.1Concept of Composite:

Fibers or particles embedded in matrix of another material are the best example of modern-day composite materials, which are mostly structural.

Laminates are composite material where different layers of materials give them the specific character of a composite material having a specific function to perform. Fabrics have no matrix to fall back on, but in them, fibers of different compositions combine to give them a specific character. Reinforcing materials generally withstand maximum load and serve the desirable properties.

Further, though composite types are often distinguishable from one another, no clear determination can be really made. To facilitate definition, the accent is often shifted to the levels at which differentiation take place viz., microscopic or macroscopic.

In matrix-based structural composites, the matrix serves two paramount purposes viz., binding the reinforcement phases in place and deforming to distribute the stresses among the constituent reinforcement materials under an applied force.

The demands on matrices are many. They may need to temperature variations, be conductors or resistors of electricity, have moisture sensitivity etc. This may offer weight advantages, ease of handling and other merits which may also become applicable depending on the purpose for which matrices are chosen.

Solids that accommodate stress to incorporate other constituents provide strong bonds for the reinforcing phase are potential matrix materials. A few inorganic materials, polymers and metals have found applications as matrix materials in the designing of structural composites, with commendable success. These materials remain elastic till failure occurs and show decreased failure strain, when loaded in tension and compression.

Figure 6.1 : Classification of Matrix Materials

Composites cannot be made from constituents with divergent linear expansion characteristics. The interface is the area of contact between the reinforcement and the matrix materials. In some cases, the region is a distinct added phase. Whenever there is interphase, there has to be two interphases between each side of the interphase and its adjoint constituent. Some composites provide interphases when surfaces dissimilar constituents interact with each other. Choice of fabrication method depends on matrix properties and the effect of matrix on properties of reinforcements. One of the prime considerations in the selection and fabrication of composites is that the constituents should be chemically inert non-reactive. Figure M1.1.1 (f) helps to classify matrices.

6.6.2 Advantages and Limitations of Composites Materials

Advantages of Composites

Summary of the advantages exhibited by composite materials, which are of significant use in aerospace industry are as follows:

1. High resistance to fatigue and corrosion degradation. 2. High ‘strength or stiffness to weight’ ratio. As enumerated above, weight savings are

significant ranging from 25-45% of the weight of conventional metallic designs. 3. Due to greater reliability, there are fewer inspections and structural repairs. 4. Directional tailoring capabilities to meet the design requirements. The fibre pattern can

be laid in a manner that will tailor the structure to efficiently sustain the applied loads. 5. Fibre to fibre redundant load path. 6. Improved dent resistance is normally achieved. Composite panels do not sustain damage

as easily as thin gage sheet metals. 7. It is easier to achieve smooth aerodynamic profiles for drag reduction. Complex double-

curvature parts with a smooth surface finish can be made in one manufacturing operation. 8. Composites offer improved torsional stiffness. This implies high whirling speeds,

reduced number of intermediate bearings and supporting structural elements. The overall part count and manufacturing & assembly costs are thus reduced.

9. High resistance to impact damage. 10. Thermoplastics have rapid process cycles, making them attractive for high volume

commercial applications that traditionally have been the domain of sheet metals. Moreover, thermoplastics can also be reformed.

11. Like metals, thermoplastics have indefinite shelf life. 12. Composites are dimensionally stable i.e. they have low thermal conductivity and low

coefficient of thermal expansion. Composite materials can be tailored to comply with a broad range of thermal expansion design requirements and to minimise thermal stresses.

13. Manufacture and assembly are simplified because of part integration (joint/fastener reduction) thereby reducing cost.

14. The improved weatherability of composites in a marine environment as well as their corrosion resistance and durability reduce the down time for maintenance.

15. Close tolerances can be achieved without machining. 16. Material is reduced because composite parts and structures are frequently built to shape

rather than machined to the required configuration, as is common with metals. 17. Excellent heat sink properties of composites, especially Carbon-Carbon, combined with

their lightweight have extended their use for aircraft brakes. 18. Improved friction and wear properties. 19. The ability to tailor the basic material properties of a Laminate has allowed new

approaches to the design of aeroelastic flight structures.

The above advantages translate not only into airplane, but also into common implements and equipment such as a graphite racquet that has inherent damping, and causes less fatigue and pain to the user.

Limitations of Composites

Some of the associated disadvantages of advanced composites are as follows:

1. High cost of raw materials and fabrication. 2. Composites are more brittle than wrought metals and thus are more easily damaged. 3. Transverse properties may be weak. 4. Matrix is weak, therefore, low toughness. 5. Reuse and disposal may be difficult. 6. Difficult to attach.

Repair introduces new problems, for the following reasons: 1. Materials require refrigerated transport and storage and have limited shelf life. 2. Hot curing is necessary in many cases requiring special tooling. 3. Hot or cold curing takes time. 4. Analysis is difficult. 5. Matrix is subject to environmental degradation

6.6.3 Comparison with Metals

Requirements governing the choice of materials apply to both metals and reinforced plastics. It is, therefore, imperative to briefly compare main characteristics of the two.

1. Unidirectional fibre composites have specific tensile strength (ratio of material strength to density) about 4 to 6 times greater than that of steel and aluminium.

2. Unidirectional composites have specific -modulus (ratio of the material stiffness to density) about 3 to 5 times greater than that of steel and aluminium.

3. Fatigue endurance limit of composites may approach 60% of their ultimate tensile strength. For steel and aluminium, this value is considerably lower.

4. Fibre composites are more versatile than metals, and can be tailored to meet performance needs and complex design requirements such as aero-elastic loading on the wings and the vertical & the horizontal stabilisers of aircraft.

5. Fibre reinforced composites can be designed with excellent structural damping features. As such, they are less noisy and provide lower vibration transmission than metals.

6. High corrosion resistance of fibre composites contributes to reduce life- cycle cost. 7. Composites offer lower manufacturing cost principally by reducing significantly the

number of detailed parts and expensive technical joints required to form large metal structural components. In other words, composite parts can eliminate joints/fasteners thereby providing parts simplification and integrated design.

8. Long term service experience of composite material environment and durability behaviour is limited in comparison with metals.

6.6.4 Particle-reinforced composites

This class of composites is most widely used composites mainly because they are widely available and cheap. They are again two kinds: dispersion-strengthened and particulate- reinforced composites. These two classes are distinguishable based upon strengthening mechanism – dispersion-strengthened composites and particulate composites.

In dispersion-strengthened composites, particles are comparatively smaller, and are of 0.01-0.1μm in size. Here the strengthening occurs at atomic/molecular level i.e. mechanism of strengthening is similar to that for precipitation hardening in metals where matrix bears the major portion of an applied load, while dispersoids hinder/impede the motion of dislocations. Examples: thoria (ThO2) dispersed Ni-alloys (TD Ni-alloys) with high-temperature strength; SAP (sintered aluminium powder) – where aluminium matrix is dispersed with extremely small flakes of alumina (Al2O3).

Figure 6.2. Particulate reinforced composite

Figure 6.3. Particulate reinforced composite

Particulate composites are other class of particle-reinforced composites. These contain large amounts of comparatively coarse particles. These composites are designed to produce unusual combinations of properties rather than to improve the strength.

Particulate composites are used with all three material types – metals, polymers and ceramics. Cermets contain hard ceramic particles dispersed in a metallic matrix. Eg.: tungsten carbide (WC) or titanium carbide (TiC) embedded cobalt or nickel used to make cutting tools. Polymers are frequently reinforced with various particulate materials such as carbon black. When added to vulcanized rubber, carbon black enhances toughness and abrasion resistance of the rubber. Aluminium alloy castings containing dispersed SiC particles are widely used for automotive applications including pistons and brake applications.

Concrete is most commonly used particulate composite. It consists of cement as binding medium and finely dispersed particulates of gravel in addition to fine aggregate (sand) and water. It is also known as Portland cement concrete. Its strength can be increased by additional reinforcement such as steel rods/mesh.

6.6.5 Fiber-reinforced composites

Most fiber-reinforced composites provide improved strength and other mechanical properties and strength-to-weight ratio by incorporating strong, stiff but brittle fibers into a softer, more ductile matrix. The matrix material acts as a medium to transfer the load to the fibers, which carry most off the applied load. The matrix also provides protection to fibers from external loads and atmosphere.

These composites are classified as either continuous or discontinuous. Generally, the highest strength and stiffness are obtained with continuous reinforcement. Discontinuous fibers are used only when manufacturing economics dictate the use of a process where the fibers must be in this form.

The mechanical properties of fiber-reinforced composites depend not only on the properties of the fiber but also on the degree of which an applied load is transmitted to the fibers by the matrix phase. Length of fibers, their orientation and volume fraction in addition to direction of external load application affects the mechanical properties of these composites.

6.6.6 Structural composites

These are special class of composites, usually consists of both homogeneous and composite materials. Properties of these composites depend not only on the properties of the constituents but also on geometrical design of various structural elements. Two classes of these composites widely used are: laminar composites and sandwich structures.

Laminar composites: there are composed of two-dimensional sheets/layers that have a preferred strength direction. These layers are stacked and cemented together according to the requirement. Materials used in their fabrication include: metal sheets, cotton, paper, woven glass fibers embedded in plastic matrix, etc. Examples: thin coatings, thicker protective coatings, claddings, bimetallics, laminates. Many laminar composites are designed to increase corrosion resistance while retaining low cost, high strength or light weight.

Sandwich structures: these consist of thin layers of a facing material joined to a light weight filler material. Neither the filler material nor the facing material is strong or rigid, but the composite possesses both properties. Example: corrugated cardboard. The faces bear most of the in-plane loading and also any transverse bending stresses. Typical face materials include Al-alloys, fiber-reinforced plastics, titanium, steel and plywood. The core serves two functions – it separates the faces and resists deformations perpendicular to the face plane; provides a certain degree of shear rigidity along planes that are perpendicular to the faces. Typical materials for core are: foamed polymers, synthetic rubbers, inorganic cements, balsa wood. Sandwich structures are found in many applications like roofs, floors, walls of buildings, and in aircraft for wings, fuselage and tailplane skins.

6.7 Nano Materials

6.7.1. IntroductionNanomaterials are cornerstones of nanoscience and nanotechnology. Nanostructure science and technology is a broad and interdisciplinary area of research and development activity that has been growing explosively worldwide in the past few years. It has the potential for revolutionizing the ways in which materials and products are created and the range and nature of functionalities that can be accessed. It is already having a significant commercial impact, which will assuredly increase in the future.

6.7.2 What are nanomaterials?Nanoscale materials are defined as a set of substances where at least one dimension is less than approximately 100 nanometers. A nanometer is one millionth of a millimeter - approximately 100,000 times smaller than the diameter of a human hair. Nanomaterials are of interest because at this scale unique optical, magnetic, electrical, and other properties emerge. These emergent properties have the potential for great impacts in electronics, medicine, and other fields.

6.7.3. Classification of NanomaterialsNanomaterials have extremely small size which having at least one dimension 100 nm or less. Nanomaterials can be nanoscale in one dimension (eg. surface films), two dimensions (eg. strands or fibres), or three dimensions (eg. particles). They can exist in single, fused, aggregated or agglomerated forms with spherical, tubular, and irregular shapes. Common types of nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes. Nanomaterials have applications in the field of nano technology, and displays different physical chemical characteristics from normal chemicals (i.e., silver nano, carbon nanotube, fullerene, photocatalyst, carbon nano, silica). According to Siegel, Nanostructured materials are classified as Zero dimensional, one dimensional, two dimensional, three dimensional nanostructures. Nanomaterials are materials which are characterized by an ultra fine grain size (< 50 nm) or by a dimensionality limited to 50 nm. Nanomaterials can be created with various modulation dimensionalities as defined by Richard W. Siegel: zero (atomic clusters, filaments and cluster assemblies), one (multilayers), two (ultrafine-grained overlayers or buried layers), and three (nanophase materials consisting of equiaxed nanometer sized grains).

6.7.4. Methods for creating nanostructuresThere are many different ways of creating nanostructures: of course, macromolecules or nanoparticles or buckyballs or nanotubes and so on can be synthesized artificially for certain specific materials. They can also be arranged by methods based on equilibrium or near-equilibrium thermodynamics such as methods of self-organization and self-assembly (sometimes also called bio-mimetic processes). Using these methods, synthesized materials can be arranged into useful shapes so that finally the material can be applied to a certain application.

a)Mechanical grindingMechanical attrition is a typical example of ‘top down’ method of synthesis of nanomaterials, where the material is prepared not by cluster assembly but by the structural decomposition of coarser-grained structures as the result of severe plastic deformation. This has become a popular method to make nanocrystalline materials because of its simplicity, the relatively inexpensive equipment needed, and the applicability to essentially the synthesis of all classes of materials. The major advantage often quoted is the possibility for easily scaling up to tonnage quantities of material for various applications. Similarly, the serious problems that are usually cited are;1. contamination from milling media and/or atmosphere, and2. to consolidate the powder product without coarsening the nanocrystalline microstructure.In fact, the contamination problem is often given as a reason to dismiss the method, at least for some materials. Here we will review the mechanisms presently believed responsible for formation of nanocrystalline structures by mechanical attrition of single phase powders, mechanical alloying of dissimilar powders, and mechanical crystallisation of amorphous materials. The two important problems of contamination and powder consolidation will be briefly considered.

Fig. 6.4 Schematic representation of the principle of mechanical millingMechanical milling is typically achieved using high energy shaker, planetary ball, or tumbler mills. The energy transferred to the powder from refractory or steel balls depends on the rotational (vibrational) speed, size and number of the balls, ratio of the ball to powder mass, the time of milling and the milling atmosphere. Nanoparticles are produced by the shear action during grinding.Milling in cryogenic liquids can greatly increase the brittleness of the powders influencing the fracture process. As with any process that produces fine particles, an adequate step to prevent oxidation is necessary. Hence this process is very restrictive for the production of non-oxide materials since then it requires that the milling take place in an inert atmosphere and that the powder particles be handled in an appropriate vacuum system or glove box. This method of synthesis is suitable for producing amorphous or nanocrystalline alloy particles, elemental or compound powders. If the mechanical milling imparts sufficient energy to the constituent powders a homogeneous alloy can be formed. Based on the energy of the milling process and thermodynamic properties of the constituents the alloy can be rendered amorphous by this processing.

6.8Polymers

6.8.1Introduction: Polymers play a very important role in human life. In fact, our body is made of lot of polymers, e.g. Proteins, enzymes, etc. Other naturally occurring polymers like wood, rubber, leather and silk are serving the humankind for many centuries now. Modern scientific tools revolutionized the processing of polymers thus available synthetic polymers like useful plastics, rubbers and fiber materials. As with other engineering materials (metals and ceramics), the properties of polymers are related their constituent structural elements and their arrangement. The suffix in polymer ‘mer’ is originated from Greek word meros – which means part. The word polymer is

thus coined to mean material consisting of many parts/mers. Most of the polymers are basically organic compounds, however they can be inorganic (e.g. silicones based on Si-O network). This chapter introduces classification of polymers, processing and synthesis of polymers, followed by mechanism of deformation and mechanical behavior of polymers.

6.8 .2 Polymer types and Polymer synthesis & processing

Polymers are classified in several ways – by how the molecules are synthesized, by their molecular structure, or by their chemical family. For example, linear polymers consist of long molecular chains, while the branched polymers consist of primary long chains and secondary chains that stem from these main chains. However, linear does not mean straight lines. The better way to classify polymers is according to their mechanical and thermal behavior. Industrially polymers are classified into two main classes – plastics and elastomers .

Plastics are moldable organic resins. These are either natural or synthetic, and are processed by forming or molding into shapes. Plastics are important engineering materials for many reasons. They have a wide range of properties, some of which are unattainable from any other materials, and in most cases they are relatively low in cost. Following is the brief list of properties of plastics: light weight, wide range of colors, low thermal and electrical conductivity, less brittle, good toughness, good resistance to acids, bases and moisture, high dielectric strength (use in electrical insulation), etc.

Plastics are again classified in two groups depending on their mechanical and thermal behavior as thermoplasts (thermoplastic polymers) and thermosets (thermosetting polymers).

Thermoplasts: These plastics soften when heated and harden when cooled – processes that are totally reversible and may be repeated. These materials are normally fabricated by the simultaneous application of heat and pressure. They are linear polymers without any cross-linking in structure where long molecular chains are bonded to each other by secondary bonds and/or inter-wined. They have the property of increasing plasticity with increasing temperature which breaks the secondary bonds between individual chains. Common thermoplasts are: acrylics, PVC, nylons, polypropylene, polystyrene, polymethyl methacrylate (plastic lenses or perspex), etc.

Thermosets: These plastics require heat and pressure to mold them into shape. They are formed into a permanent shape and cured or ‘set’ by chemical reactions such as extensive cross-linking. They cannot be re-melted or reformed into another shape but decompose upon being heated to too high a temperature. Thus thermosets cannot be recycled, whereas thermoplasts can be recycled. The term thermoset implies that heat is required to permanently set the plastic. Most thermosets composed of long chains that are strongly cross-linked (and/or covalently bonded) to one another to form 3-D network structures to form a rigid solid. Thermosets are generally stronger, but more brittle than thermoplasts. Advantages of thermosets for engineering design applications include one or more of the following: high thermal stability, high dimensional

stability, high rigidity, light weight, high electrical and thermal insulating properties and resistance to creep and deformation under load. There are two methods whereby cross-linking reaction can be initiated – cross-linking can be accomplished by heating the resin in a suitable mold (e.g. bakelite), or resins such as epoxies (araldite) are cured at low temperature by the addition of a suitable cross-linking agent, an amine. Epoxies, vulcanized rubbers, phenolics, unsaturated polyester resins, and amino resins (ureas and melamines) are examples of thermosets.

Elastomers: Also known as rubbers, these are polymers which can undergo large elongations under load, at room temperature, and return to their original shape when the load is released. There are number of man-made elastomers in addition to natural rubber. These consist of coil-like polymer chains those can reversibly stretch by applying a force.

Processing of polymers mainly involves preparing a particular polymer by synthesis of available raw materials, followed by forming into various shapes. Raw materials for polymerization are usually derived from coal and petroleum products. The large molecules of many commercially useful polymers must be synthesized from substances having smaller molecules. The synthesis of the large molecule polymers is known as polymerization in which monomer units are joined over and over to become a large molecule. More upon, properties of a polymer can be enhanced or modified with the addition of special materials. This is followed by forming operation. Addition polymerization and condensation polymerization are the two main ways of polymerization.

Addition polymerization, also known as chain reaction polymerization, is a process in which multi-functional monomer units are attached one at a time in chainlike fashion to form linear/3-D macro-molecules. The composition of the macro-molecule is an exact multiple of for that of the original reactant monomer. This kind of polymerization involves three distinct stages – initiation, propagation and termination. To initiate the process, an initiator is added to the monomer. This forms free radicals with a reactive site that attracts one of the carbon atoms of the monomer. When this occurs, the reactive site is transferred to the other carbon atom in the monomer and a chain begins to form in propagation stage. A common initiator is benzoyl peroxide. When polymerization is nearly complete, remaining monomers must diffuse a long distance to reach reactive site, thus the growth rate decreases.

The process for polyethylene is as follows

Here R. represents the active initiator. Propagation involves the linear growth of the molecule as monomer units become attached to one another in succession to produce the chain molecule, which is represented, again for polyethylene, as follows

As we need polymers with controlled molecular weight, polymerization needs to be terminated at some stage. Propagation may end or terminate in different ways. First, the active ends of two propagating chains may react or link together to form a non-reactive molecule, as follows:

thus terminating the growth of each chain or an active chain end may react with an initiator or other chemical species having a single active bond, as follows:

with the resultant cessation of chain growth. Polyethylene, polypropylene, PVC, and polystyrene are synthesized using addition polymerization.

Condensation polymerization, also known as step growth polymerization, involves more than one monomer species; and there is usually a small molecular weight by-product such as water, which is eliminated. The repeat unit here forms from original monomers, and no product has the chemical formula of mere one mer repeat unit. The polymerization of dimethyl terephthalate and

ethylene glycol to produce polyester is an important example. The by-product, methyl alcohol, is condensed off and the two monomers combine to produce a larger molecule (mer repeat unit). Another example, consider the formation of a polyester from the reaction between ethylene glycol and adipic acid; the intermolecular reaction is as follows:

This stepwise process is successively repeated, producing, in this case, a linear molecule. The intermolecular reaction occurs every time a mer repeat unit is formed. Reaction times for condensation are generally longer than for addition polymerization. Polyesters, phenol-formaldehyde, nylons, polycarbonates etc are produced by condensation polymerization. Condensation polymerization reactions also occur in sol-gel processing of ceramic materials. Some polymers such as nylon may be polymerized by either technique.

Polymers, unlike organic/inorganic compounds, do not have a fixed molecular weight. It is specified in terms of degree of polymerization – number of repeat units in the chain or ration of average molecular weight of polymer to molecular weight of repeat unit. Average molecular weight is however defined in two ways. Weight average molecular weight is obtained by dividing the chains into size ranges and determining the fraction of chains having molecular weights within that range. Number average molecular weight is based on the number fraction, rather than the weight fraction, of the chains within each size range. It is always smaller than the weight average molecular weight.

Most of polymer properties are intrinsic i.e. characteristic of a specific polymer. Foreign substances called additives are intentionally introduced to enhance or modify these properties. These include – fillers, plasticizers, stabilizers, colorants, and flame retardants. Fillers are used to improve tensile and compressive strength, abrasion resistance, dimensional stability etc. wood flour, sand, clay, talc etc are example for fillers. Plasticizers aid in improving flexibility, ductility and toughness of polymers by lowering glass transition temperature of a polymer. These are generally liquids of low molecular weight. Stabilizers are additives which counteract

deteriorative processes such as oxidation, radiation, and environmental deterioration. Colorants impart a specific color to a polymer, added in form of either dyes (dissolves) or pigments (remains as a separate phase). Flame retardants are used to enhance flammability resistance of combustible polymers. They serve the purpose by interfering with the combustion through the gas phase or chemical reaction.

Polymeric materials are formed by quite many different techniques depending on (a) whether the material is thermoplast or thermoset, (b) melting/degradation temperature, (c) atmospheric stability, and (d) shape and intricacy of the product. Polymers are often formed at elevated temperatures under pressure. Thermoplasts are formed above their glass transition temperatures while applied pressure ensures that the product retain its shape. Thermosets are formed in two stages – making liquid polymer, then molding it.

Different molding techniques are employed in fabrication of polymers. Compression molding involves placing appropriate amount of polymer with additives between heated male and female mold parts. After pouring polymer, mold is closed, and heat and pressure are applied, causing viscous plastic to attain the mold shape. Figure-6.5 shows a typical mould employed for compression molding.

Figure 6.5: Schematic diagram of a mould employed for compression molding

Transfer molding differs from compression molding in how the materials is introduced into the mold cavities. In transfer molding the plastic resin is not fed directly into the mold cavity but into a chamber outside the mold cavities. When the mold is closed, a plunger forces the plastic resin into the mold cavities, where and molded material cures. In injection molding, palletized materials is fed with use of hopper into a cylinder where charge is pushed towards heating chamber where plastic material melts, and then molten plastic is impelled through nozzle into the enclosed mold cavity where product attains its shape. Most outstanding characteristic of this

process is the cycle time which is very short. The schematic diagram of injection-molding machine is shown in figure-6.6

Figure 6.6: Schematic diagram of injection-molding machine

Extrusion is another kind of injection molding, in which a thermoplastic material is forced through a die orifice, similar to the extrusion of metals. This technique is especially adapted to produce continuous lengths with constant cross-section. The schematic diagram of a simple extrusion machine is shown in figure- 6.7

Figure 6.7: Schematic diagram of a simple extrusion machine

Blow molding of plastics is similar to blowing of glass bottles. Polymeric materials may be cast similar to metals and ceramics.

6.8.3 Characteristics and typical applications of few plastic materials.

a) Thermo plastics

1. Acrylonitrile-butadiene-styrene (ABS):

Characteristics: Outstanding strength and toughness, resistance to heat distortion; good electrical properties; flammable and soluble in some organic solvents.

Application: Refrigerator lining, lawn and garden equipment, toys, highway safety devices.

2. Acrylics (poly-methyl-methacrylate)

Characteristics: Outstanding light transmission and resistance to weathering; only fair mechanical properties.

Application: Lenses, transparent aircraft enclosures, drafting equipment, outdoor signs

3. Fluorocarbons (PTFE or TFE)

Characteristics: Chemically inert in almost all environments, excellent electrical properties; low coefficient of friction; may be used to 260o C; relatively weak and poor cold-flow properties.

Application: Anticorrosive seals, chemical pipes and valves, bearings, anti adhesive coatings, high temperature electronic parts.

4. Polyamides (nylons)

Characteristics: Good mechanical strength, abrasion resistance, and toughness; low coefficient of friction; absorbs water and some other liquids.

Application: Bearings, gears, cams, bushings, handles, and jacketing for wires and cables

5. Polycarbonates

Characteristics: Dimensionally stable: low water absorption; transparent; very good impact resistance and ductility.

Application: Safety helmets, lenses light globes, base for photographic film

6. Polyethylene

Characteristics: Chemically resistant and electrically insulating; tough and relatively low coefficient of friction; low strength and poor resistance to weathering.

Application: Flexible bottles, toys, tumblers, battery parts, ice trays, film wrapping materials.

7. Polypropylene

Characteristics: Resistant to heat distortion; excellent electrical properties and fatigue strength; chemically inert; relatively inexpensive; poor resistance to UV light.

Application: Sterilizable bottles, packaging film, TV cabinets, luggage

8. Polystyrene

Characteristics: Excellent electrical properties and optical clarity; good thermal and dimensional stability; relatively inexpensive

Application: Wall tile, battery cases, toys, indoor lighting panels, appliance housings.

9. Polyester (PET or PETE)

Characteristics: One of the toughest of plastic films; excellent fatigue and tear strength, and resistance to humidity acids, greases, oils and solvents

Application: Magnetic recording tapes, clothing, automotive tire cords, beverage containers.

b) Thermo setting polymers

1. Epoxies

Characteristics: Excellent combination of mechanical properties and corrosion resistance; dimensionally stable; good adhesion; relatively inexpensive; good electrical properties.

Application: Electrical moldings, sinks, adhesives, protective coatings, used with fiberglass laminates.

2. Phenolics

Characteristics: Excellent thermal stability to over 150o C; may be compounded with a large number of resins, fillers, etc.; inexpensive.

Application: Motor housing, telephones, auto distributors, electrical fixtures.

6.9 Multiple Choice Questions:

1. Composite materials are classified based on:

(a) Type of matrix (b) Size-and-shape of reinforcement

(c) Both (d) None

2. Major load carrier in dispersion-strengthened composites

(a) Matrix (b) Fiber (c) Both (d) Can’t define

3. Usually softer constituent of a composite is

(a) Matrix (b) Reinforcement

(c) Both are of equal strength (d) Can’t define

4. Usually stronger constituent of a composite is

(a) Matrix (b) Reinforcement

(c) Both are of equal strength (d) Can’t define

5. Last constituent to fail in fiber reinforced composites

(a) Matrix (b) Fiber

(c) Both fails at same time (d) Can’t define

6. Size range of dispersoids used in dispersion strengthened composites

(a) 0.01-0.1 μm (b) 0.01-0.1 nm (c) 0.01-0.1 mm (d) None

7. Rule-of-mixture provides _________ bounds for mechanical properties of particulate composites.

(a) Lower (b) Upper (c) Both (d) None

8. Al-alloys for engine/automobile parts are reinforced to increase their

(a) Strength (b) Wear resistance (c) Elastic modulus (d) Density

9. Mechanical properties of fiber-reinforced composites depend on

(a) Properties of constituents (b) Interface strength

(c) Fiber length, orientation, and volume fraction (d) All the above

10. Longitudinal strength of fiber reinforced composite is mainly influenced by

(a) Fiber strength (b) Fiber orientation

(c) Fiber volume fraction (d) Fiber length

11. The following material can be used for filling in sandwich structures

(a) Polymers (b) Cement (c) Wood (d) All

12. Not an example for laminar composite

(a) Wood (b) Bimetallic (c) Coatings/Paints (d) Claddings

13. The word ‘polymer’ meant for material made from ______________.

(a) Single entity (b) Two entities (c) Multiple entities (d) Any entity

14. One of characteristic properties of polymer material __________ .

(a) High temperature stability (b) High mechanical strength

(c) High elongation (d) Low hardness

15. Polymers are ___________ in nature.

(a) Organic (b) Inorganic (c) Both (a) and (b) (d) None

16. These polymers can not be recycled:

(a) Thermoplasts (b) Thermosets

(c) Elastomers (d) All polymers

17. In general, strongest polymer group is __________ .

(a) Thermoplasts (b) Thermosets (c) Elastomers (d) All polymers

18. These polymers consist of coil-like polymer chains:

(a) Thermoplasts (b) Thermosets (c) Elastomers (d) All polymers

19. Strong covalent bonds exists between polymer chains in __________ .

(a) Thermoplasts (b) Thermosets (c) Elastomers (d) All polymers

20. Following is the unique to polymeric materials:

(a) Elasticity (b) Viscoelasticity (c) Plasticity (d) None

21. Elastic deformation in polymers is due to _____________ .

(a) Slight adjust of molecular chains (b) Slippage of molecular chains

(c) Straightening of molecular chains (d) Severe of Covalent bonds

22. Kevlar is commercial name for ___________ .

(a) Glass fibers (b) Carbon fibers

(c) Aramid fibers (d) Cermets

Answers:

1. c 2. a 3. a 4. b 5. a 6. a 7. c

8. b 9. d 10. a 11. d 12. a 13. c 14. c

15. c 16. b 17. b 18. c 19. b 20. b 21. a

22. c

6.10 Theory Questions

1. What are composites? 2. Why do we need composites? 3. How the composites are made? 4. How the presence of dispersed phase improves the properties? 5. What are structural composites?6. Cite the differences in behavior and molecular structure for thermoplastic and thermosetting

polymers.7. Briefly describe the crystalline state in polymeric materials.8. Describe a typical polymer molecule in terms of its chain structure and, in addition, how the

molecule may be generated from repeat units.