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The Structure of Metals Mechanical Behavior, Testing and Manufacturing Properties of Materials Physical Properties of Materials Metal Alloys: Structure and Strengthening by Heat Treatment Ferrous Metals and Alloys: Production, General Properties and Applications.
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The Structure of Metals • Mechanical Behavior, Testing and Manufacturing Properties of Materials • Physical Properties of Materials • Metal Alloys: Structure and Strengthening by Heat Treatment • Ferrous Metals and Alloys: Production, General Properties and Applications
6. Nonferrous Metals and Alloys: Production, General Properties and Applications 7. Polymers: Structure, General Properties and Applications 8. Ceramics, Graphite and Diamond: Structure, General Properties and Applications 9. Composite Materials: Structure, General Properties, and Applications
Chapter Objectives • Structure of polymers, polymerization processes, crystallinity, and glass-transition temperature. • How temperature and deformation rate affect the properties of thermoplastics. • Differences between thermoplastics and thermosets. • Properties and applications of polymers, their advantages and limitations.
Chapter Outline • Introduction • The Structure of Polymers • Thermoplastics • Thermosetting Plastics • Additives in Plastics • General Properties and Applications of Thermoplastics • General Properties and Applications of Thermosetting Plastics • Biodegradable Plastics • Elastomers (Rubbers)
7.1 Introduction • Plastics are one of numerous polymeric materials and have extremely large molecules (macromolecules or giant molecules). • The advantages of polymers in terms of the following characteristics: • Corrosion resistance and resistance to chemicals • Low electrical and thermal conductivity • Low density • High strength-to-weight ratio (particularly when reinforced) • Noise reduction
7.1 Introduction • Noise reduction • Ease of manufacturing and complexity of design possibilities • Relatively low cost • Other characteristics that may or may not be desirable (depending on the application), such as low strength and stiffness (Table 7.1), high coefficient of thermal expansion, low useful-temperature range—up to about 350°C, and lower dimensional stability in service over a period of time.
7.1 Introduction • Plastics can be formed, machined, cast, and joined into various shapes with relative ease. • Plastics are available commercially as film, sheet, plate, rods, and tubing of various cross-sections. • An outline of the basic process of making various synthetic polymers is given in Fig. 7.1.
7.2 The Structure of Polymers • The properties of polymers depend largely on the structures of individual polymer molecules, molecule shape and size, and how molecules are arranged to form a polymer structure. • Polymers are long-chain molecules that are formed by polymerization (that is, by the linking and cross-linking of different monomers). • A monomer is the basic building block of a polymer. • The word mer (from the Greek meros, meaning part) indicates the smallest repetitive unit; its use is similar to that of the term unit cell in crystal structures of metals.
7.2 The Structure of Polymers • The term polymer means many mers (or units), generally repeated hundreds or thousands of times in a chainlike structure. • Fig 7.2 shows the molecular structure of various polymers. These are examples of the basic building blocks for plastics.
7.2 The Structure of Polymers • In condensation polymerization polymers are produced by the formation of bonds between two types of reacting mers. • Fig 7.3 shows the examples of polymerization. (a) Condensation polymerization of nylon 6,6 and (b) addition polymerization of polyethylene molecules from ethylene mers.
7.2 The Structure of Polymers • This process is also known as step-growth or step-reaction polymerization, because the polymer molecule grows step-by-step until all of one reactant is consumed. • In addition polymerization (also called chain-growth or chain-reaction polymerization), bonding takes place without reaction by-products,
7.2 The Structure of Polymers Molecular Weight • The sum of the molecular weights of the mers in a representative chain is known as the molecular weight of the polymer. • The spread of the molecular weights in a chain is referred to as the molecular weight distribution (MWD). • A polymer’s molecular weight and its MWD have a strong influence on its properties.
7.2 The Structure of Polymers Molecular Weight • Fig 7.4 shows the effect of molecular weight and degree of polymerization on the strength and viscosity of polymers.
7.2 The Structure of Polymers Degree of Polymerization • It is convenient to express the size of a polymer chain in terms of the degree of polymerization (DP), which is defined as the ratio of the molecular weight of the polymer to the molecular weight of the repeating unit. • The higher the DP, the higher is the polymer’s viscosity or its resistance to flow. • High viscosity adversely affects the ease of shaping and, thus, raises the overall cost of processing.
7.2 The Structure of Polymers Bonding • During polymerization, the monomers are linked together by covalent bonds, forming a polymer chain. • Because of their strength, covalent bonds also are called primary bonds. • The polymer chains are, in turn, held together by secondary bonds, such as van der Waals bonds, hydrogen bonds, and ionic bonds. • Secondary bonds are weaker than primary bonds by one to two orders of magnitude.
7.2 The Structure of Polymers Linear Polymers • The chain-like polymers shown in Fig. 7.2 are called linear polymers because of their sequential structure. • Fig 7.5 shows the schematic illustration of polymer chains. (a) Linear structure— thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as in polyethylene. (c) Cross-linked structure—many rubbers or elastomers have this structure, and the vulcanization of rubber produces this structure. (d) Network structure, which is basically highly cross-linked—examples are thermosetting plastics, such as epoxies and phenolics.
7.2 The Structure of Polymers Branched Polymers • The properties of a polymer depend not only on the type of monomers but also on their arrangement in the molecular structure. • In branched polymers (Fig. 7.5b), side-branch chains are attached to the main chain during the synthesis of the polymer.
7.2 The Structure of Polymers Cross-linked Polymers • Generally three-dimensional in structure, cross-linked polymers have adjacent chains linked by covalent bonds (Fig. 7.5c). • Polymers with a cross-linked chain structure are called thermosets, or thermosetting plastics. • Fig 7.6 shows the behavior of polymers as a function of temperature and (a) degree of crystallinity and (b) cross-linking. The combined elastic and viscous behavior of polymers is known as viscoelasticity.
7.2 The Structure of Polymers Network polymers • These polymers consist of spatial (three-dimensional) networks of three or more active covalent bonds (Fig. 7.5d). • A highly cross-linked polymer also is considered a network polymer.
7.2 The Structure of Polymers Copolymers and terpolymers • If the repeating units in a polymer chain are all of the same type, the molecule is called a homopolymer. • Copolymers contain two types of polymers (for example, styrene-butadiene, which is used widely for automobile tires). • Terpolymers contain three types (for example, ABS (acrylonitrilebutadiene-styrene), which is used for helmets, telephones, and refrigerator liners).
Example 7.1 Dental and medical bone cement Polymethylmethacrylate (PMMA) is an acrylic polymer commonly used in dental and medical applications as an adhesive and commonly is referred to as bone cement. There are a number of forms of PMMA, but this example describes one common form involving an addition-polymerization reaction. PMMA is delivered in two parts: a powder and a liquid, which are mixed by hand. The liquid wets and partially dissolves the powder, resulting in a liquid with viscosity on the order of similar to that of vegetable oil. The viscosity increases markedly until a “dough” state is reached in about five minutes and fully hardens from the dough state in an additional five minutes.
Example 7.1 Dental and medical bone cement The powder consists of high molecular weight poly[(methylmethacrylate)- costyrene] particles of about 50 micro-meter in diameter, containing a small volume fraction of benzoyl peroxide. The liquid consists of a methyl methacrylate (MMA) monomer, with a small amount of dissolved n,n dimethyl-p-toluidine (DMPT). When the liquid and powder are mixed, the MMA wets the particles (dissolving a surface layer of the PMMA particles) and the DMPT cleaves the benzoyl peroxide molecule into two parts to form a catalyst with a free electron (sometimes referred to as a free radical). The resulting catalyst causes rapid growth of PMMA from the MMA mers, so that the final material is a composite of high molecular weight PMMA particles interconnected by PMMA chains. A schematic diagram of fully set bone cement is shown in Fig. 7.7.
7.2.2 Crystallinity • Polymers such as polymethylmethacrylate, polycarbonate, and polystyrene are generally amorphous; that is, the polymer chains exist without long-range order. • The crystalline regions in polymers are called crystallites. • Fig 7.8 shows the amorphous and crystalline regions in a polymer. • The crystalline region (crystallite) has an orderly arrangement of molecules. The higher the crystallinity, the harder, stiffer, and less ductile the polymer.
7.2.2 Crystallinity • These crystals are formed when the long molecules arrange themselves in an orderly manner, similar to the folding of a fire hose in a cabinet or of facial tissues in a box. • A partially crystalline (semi-crystalline) polymer can be regarded as a two-phase material, one phase being crystalline and the other amorphous. • By controlling the rate of solidification during cooling and the chain structure, it is possible to impart different degrees of crystallinity to polymers, although never 100%.
7.2.2 Crystallinity Effects of crystallinity • The mechanical and physical properties of polymers are greatly influenced by the degree of crystallinity: as crystallinity increases, polymers become stiffer, harder, less ductile, more dense, less rubbery, and more resistant to solvents and heat (Fig. 7.6). • The increase in density with increasing crystallinity is called crystallization shrinkage and is caused by a more efficient packing of the molecules in the crystal lattice.
7.2.3 Glass-transition temperature • The temperature at which a transition occurs is called the glass-transition temperature, Tg, also called the glass point or glass temperature. • To determine Tg, the specific volume of the polymer is determined and plotted against temperature, and marked by a sharp change in the slope of the curve. • Fig 7.9 shows the Specific volume of polymers as a function of temperature. Amorphous polymers, such as acrylic and polycarbonate, have a glass-transition temperature, but do not have a specific melting point, Partly crystalline polymers, such as polyethylene and nylons, contract sharply while passing through their melting temperatures during cooling.
7.2.3 Glass-transition temperature • The glass-transition temperature varies with different polymers (Table 7.2).
7.2.4 Polymer Blends • The brittle behavior of amorphous polymers below their glass-transition temperature can be reduced by blending them—usually with small quantities of an elastomer. • These polymer blends are known as rubber modified polymers. • Advances in blending involve several components, creating polyblends that utilize the favorable properties of different polymers. • Miscible blends (mixing without separation of two phases) are created by a process similar to the alloying of metals that enables polymer blends to become more ductile.
