Engineering Materials vol 2 Part 10 pot

216 Engineering Materials 2 Polymers 217 C. Polymers and composites 218 Engineering Materials 2 Polymers 219 Chapter 21 Polymers Introduction Where people have, since the industrial revolution, used metals, nature uses polymers. Almost all biological systems are built of polymers which not only perform mechan- ical functions (like wood, bone, cartilage, leather) but also contain and regulate chem- ical reactions (leaf, veins, cells). People use these natural polymers, of course, and have done so for thousands of years. But it is only in this century that they have learned how to make polymers of their own. Early efforts (bakelite, celluloid, formaldehyde plastics) were floppy and not very strong; it is still a characteristic of most simple synthetic polymers that their stiffness (for a given section) is much less than that of metal or, indeed, of wood or bone. That is because wood and bone are composites: they are really made up of stiff fibres or particles, embedded in a matrix of simple polymer. People have learned how to make composites too: the industries which make high-performance glass, carbon, or Kevlar-fibre reinforced polymers (GFRP, CFRP, KFRP) enjoy a faster growth rate (over 10% per year) than almost any other branch of materials production. These new materials are stiff, strong and light. Though expens- ive, they are finding increasing use in aerospace, transport and sporting goods. And there are many opportunities for their wider application in other fields like hiking equipment, medical goods and even apparently insignificant things like spectacle frames: world-wide, at least 1,000,000,000 people wear spectacles. And the new polymers are as exciting as the new composites. By crystallising, or by cross-linking, or by orienting the chains, new polymers are being made which are as stiff as aluminium; they will quickly find their way into production. The new process- ing methods can impart resistance to heat as well as to mechanical deformation, open- ing up new ranges of application for polymers which have already penetrated heavily into a market which used to be dominated by metals. No designer can afford to neglect the opportunities now offered by polymers and composites. But it is a mistake to imagine that metal components can simply be replaced by components of these newer materials without rethinking the design. Polymers are less stiff, less strong and less tough than most metals, so the new component requires careful redesign. Composites, it is true, are stiff and strong. But they are often very anisotropic, and because they are bound by polymers, their properties can change radically with a small change in temperature. Proper design with polymers requires a good understanding of their properties and where they come from. That is the func- tion of the next four chapters. 220 Engineering Materials 2 In this chapter we introduce the main engineering polymers. They form the basis of a number of major industries, among them paints, rubbers, plastics, synthetic fibres and paper. As with metals and ceramics, there is a bewilderingly large number of polymers and the number increases every year. So we shall select a number of “generic” polymers which typify their class; others can be understood in terms of these. The classes of interest to us here are: (a) Thermoplastics such as polyethylene, which soften on heating. (b) Thermosets or resins such as epoxy which harden when two components (a resin and a hardener) are heated together. (c) Elastomers or rubbers. (d) Natural polymers such as cellulose, lignin and protein, which provide the mechan- ical basis of most plant and animal life. Although their properties differ widely, all polymers are made up of long molecules with a covalently bonded backbone of carbon atoms. These long molecules are bonded together by weak Van der Waals and hydrogen (“secondary”) bonds, or by these plus covalent cross-links. The melting point of the weak bonds is low, not far from room temperature. So we use these materials at a high fraction of the melting point of the weak bonds (though not of the much stronger covalent backbone). Not surprisingly, they show some of the features of a material near its melting point: they creep, and the elastic deflection which appears on loading increases with time. This is just one import- ant way in which polymers differ from metals and ceramics, and it necessitates a different design approach (Chapter 27). Most polymers are made from oil; the technology needed to make them from coal is still poorly developed. But one should not assume that dependence on oil makes the polymer industry specially vulnerable to oil price or availability. The value-added when polymers are made from crude oil is large. At 1998 prices, one tonne of oil is about $150; 1 tonne of polyethylene is about $800. So doubling the price of oil does not double the price of the polymer. And the energy content of metals is large too: that of aluminium is nearly twice as great as that of most polymers. So polymers are no more sensitive to energy prices than are most other commodities, and they are likely to be with us for a very long time to come. The generic polymers Thermoplastics Polyethylene is the commonest of the thermoplastics. They are often described as linear polymers, that is the chains are not cross-linked (though they may branch occa- sionally). That is why they soften if the polymer is heated: the secondary bonds which bind the molecules to each other melt so that it flows like a viscous liquid, allowing it to be formed. The molecules in linear polymers have a range of molecular weights, and they pack together in a variety of configurations. Some, like polystyrene, are amorphous; others, like polyethylene, are partly crystalline. This range of molecular weights and packing geometries means that thermoplastics do not have a sharp melting Polymers 221 point. Instead, their viscosity falls over a range of temperature, like that of an inor- ganic glass. Thermoplastics are made by adding together (“polymerising”) sub-units (“monomers”) to form long chains. Many of them are made of the unit H C H H C R repeated many times. The radical R may simply be hydrogen (as in polyethylene), or —CH 3 (polypropylene) or —Cl (polyvinylchloride). A few, like nylon, are more com- plicated. The generic thermoplastics are listed in Table 21.1. The fibre and film-forming polymers polyacrylonitrile (ACN) and polyethylene teraphthalate (PET, Terylene, Dacron, Mylar) are also thermoplastics. Thermosets or resins Epoxy, familiar as an adhesive and as the matrix of fibre-glass, is a thermoset (Table 21.2). Thermosets are made by mixing two components (a resin and a hardener) which react and harden, either at room temperature or on heating. The resulting polymer is usually heavily cross-linked, so thermosets are sometimes described as network polymers. The cross-links form during the polymerisation of the liquid resin and hardener, so the structure is almost always amorphous. On reheating, the addi- tional secondary bonds melt, and the modulus of the polymer drops; but the cross- links prevent true melting or viscous flow so the polymer cannot be hot-worked (it turns into a rubber). Further heating just causes it to decompose. The generic thermosets are the epoxies and the polyesters (both widely used as matrix materials for fibre-reinforced polymers) and the formaldehyde-based plastics (widely used for moulding and hard surfacing). Other formaldehyde plastics, which now replace bakelite, are ureaformaldehyde (used for electrical fittings) and melamine- formaldehyde (used for tableware). Elastomers Elastomers or rubbers are almost-linear polymers with occasional cross-links in which, at room temperature, the secondary bonds have already melted. The cross-links pro- vide the “memory” of the material so that it returns to its original shape on unloading. The common rubbers are all based on the single structure C H C R A B C D E F H C H H C H n with the position R occupied by H, CH 3 or Cl. They are listed in Table 21.3. 222 Engineering Materials 2 Natural polymers The rubber polyisoprene is a natural polymer. So, too, are cellulose and lignin, the main components of wood and straw, and so are proteins like wool or silk. We use cellulose in vast quantities as paper and (by treating it with nitric acid) we make celluloid and cellophane out of it. But the vast surplus of lignin left from wood process- ing, or available in straw, cannot be processed to give a useful polymer. If it could, it COOCH 3 Thérmoplastic Composition Uses Polyethylene, PE Tubing, film, bottles, cups, electrical insulation, packaging. Table 21.1 Generic thermoplastics A B C D E F H C H n Partly crystalline. Polypropylene, PP Same uses as PE, but lighter, stiffer, more resistant to sunlight. A B C D E F H C H n Partly crystalline. Polytetrafluoroethylene, PTFE Teflon. Good, high-temperature polymer with very low friction and adhesion characteristics. Non-stick saucepans, bearings, seals. A B C D E F F C F n Partly crystalline. Polystyrene, PS Cheap moulded objects. Toughened with butadiene to make high-impact polystyrene (HIPS). Foamed with CO 2 to make common packaging. A B C D E F H C H n Amorphous. Polyvinylchloride, PVC Architectural uses (window frames, etc.). Plasticised to make artificial leather, hoses, clothing. A B C D E F H C H n Amorphous. Polymethylmethacrylate, PMMA Perspex, lucite. Transparent sheet and mouldings. Aircraft windows, laminated windscreens. A B C D E F H C H n Amorphous. Nylon 66 Textiles, rope, mouldings. Partly crystalline when drawn. H C CH 3 H C C 6 H 5 H C Cl CH 3 C C 6 H 11 NO() n Polymers 223 Elastomer Composition Uses Polyisoprene Natural rubber. Table 21.3 Generic elastomers (rubbers) Amorphous except at high strains. Polybutadiene Synthetic rubber, car tyres. Amorphous except at high strains. Polychloroprene Neoprene. An oil-resistant rubber used for seals. Amorphous except at high strains. A B C D E F H C H n C C C CH 3 H H H A B C D E F H C H n C C C H H H H A B C D E F H C H n C C C Cl H H H Thermoset Composition Uses Epoxy Fibreglass, adhesives. Expensive. Table 21.2 Generic thermosets or resins A B C D E F CH 3 C CH 3 n C 6 H 4 O C 6 H 4 O CH 2 CH OH CH 2 Amorphous. Polyester Fibreglass, laminates. Cheaper than epoxy. A B C D E F Amorphous. Phenol-formaldehyde Bakelite, Tufnol, Formica. Rather brittle. C 6 H 2 A B C D E F OH CH 2 n CH 2 Amorphous. CH 2 OH C CH 2 OH n (CH 2 ) m C O O O C 224 Engineering Materials 2 ) n Natural polymer Composition Uses Cellulose Framework of all plant life, as the main structural component in cell walls. Table 21.4 Generic natural polymers Amorphous.Lignin The other main component in cell walls of all plant life. Protein Crystalline ( C 6 H 9 O 6 Gelatin, wool, silk. A B C D E F n NH C C H O R R is a radical. Partly crystalline. Table 21.5 Properties of polymers Polymer Cost (UK£ Density Young’s Tensile ($US) tonne − 1 ) (Mg m − 3 ) modulus strength (20°C 100 s) (MPa) (GPa) Thermoplastics Polyethylene, PE (low density) 560 (780) 0.91–0.94 0.15–0.24 7–17 Polyethylene, PE (high density) 510 (700) 0.95–0.98 0.55–1.0 20–37 Polypropylene, PP 675 (950) 0.91 1.2–1.7 50–70 Polytetrafluoroethylene, PTFE – 2.2 0.35 17–28 Polystyrene, PS 650 (910) 1.1 3.0–3.3 35–68 Polyvinyl chloride, PVC (unplasticised) 425 (595) 1.4 2.4–3.0 40–60 Polymethylmethacrylate, PMMA 1070 (1550) 1.2 3.3 80–90 Nylons 2350 (3300) 1.15 2–3.5 60–110 Resins or thermosets Epoxies 1150 (1600) 1.2–1.4 2.1–5.5 40–85 Polyesters 930 (1300) 1.1–1.4 1.3–4.5 45–85 Phenolformaldehyde 750 (1050) 1.27 8 35–55 Elastomers (rubbers) Polyisoprene 610 (850) 0.91 0.002–0.1 ≈10 Polybutadiene 610 (850) 1.5 0.004–0.1 Polychloroprene 1460 (2050) 0.94 ≈0.01 Natural polymers Cellulose fibres 1.5 25–40 ≈1000 Lignin 1.4 2.0 – Protein 1.2–1.4 –– Polymers 225 would form the base for a vast new industry. The natural polymers are not as complic- ated as you might expect. They are listed in Table 21.4. Material data Data for the properties of the generic polymers are shown in Table 21.5. But you have to be particularly careful in selecting and using data for the properties of polymers. Specifications for metals and alloys are defined fairly tightly; two pieces of Type 316L stainless steel from two different manufacturers will not differ much. Not so with polymers: polyethylene made by one manufacturer may be very different from polyethylene made by another. It is partly because all polymers contain a spectrum of molecular lengths; slight changes in processing change this spectrum. But it is also because details of the polymerisation change the extent of molecular branching and the degree of crystallinity in the final product; and the properties can be further changed by mechanical processing (which can, in varying degrees, align the molecules) and by proprietary additives. For all these reasons, data from compilations (like Table 21.5), or data books, are at best approximate. For accurate data you must use the manufacturers’ data sheets, or conduct your own tests. Fracture Glass Softening Specific heat Thermal Thermal toughness temperature expansion (J kg − 1 K − 1 ) conductivity coefficient (20°C) T g (K) temperature (W m − 1 K − 1 )(MK − 1 ) (MPa m 1/2 ) T s (K) 1–2 270 355 2250 0.35 160–190 2–5 300 390 2100 0.52 150–300 3.5 253 310 1900 0.2 100–300 –– 395 1050 0.25 70–100 2 370 370 1350–1500 0.1–0.15 70–100 2.4 350 370 – 0.15 50–70 1.6 378 400 1500 0.2 54–72 3–5 340 350–420 1900 0.2–0.25 80–95 0.6–1.0 380 400– 440 1700–2000 0.2–0.5 55–90 0.5 340 420– 440 1200–2400 0.2–0.24 50–100 –– 370–550 1500–1700 0.12–0.24 26–60 – 220 ≈350 ≈2500 ≈0.15 ≈600 – 171 ≈350 ≈2500 ≈0.15 ≈600 – 200 ≈350 ≈2500 ≈0.15 ≈600 –– – – – – –– – – – – –– – – – – [...]... influence the way in which these materials are used? 22 8 Engineering Materials 2 Chapter 22 The structure of polymers Introduction If the architecture of metal crystals is thought of as classical, then that of polymers is baroque The metal crystal is infused with order, as regular and symmetrical as the Parthenon; polymer structures are as exotic and convoluted as an Austrian altarpiece Some polymers,... range 103 to 105 The molecular weight of a polymer is simply the DP times the molecular weight of the monomer Ethylene, C2H4, for example, has a molecular weight of 28 If the DP for a batch of polyethylene is 104 , then the molecules have an average molecular weight of 28 0,000 The word “average” is significant In all commercial polymers there is a range of DP, and thus of molecular lengths (Fig 22 .2a)... crystal is 1.014 Mg m–3 at 20 °C The density of amorphous polyethylene at 20 °C is 0.84 Mg m–3 Estimate the percentage crystallinity in: (a) a low-density polyethylene with a density of 0. 92 Mg m–3 at 20 °C; (b) a high-density polyethylene with a density of 0.97 Mg m–3 at 20 °C Answers: (a) 46%, (b) 75% 23 8 Engineering Materials 2 Chapter 23 Mechanical behaviour of polymers Introduction All polymers have a... carpet Nonetheless, the crystallinity is good enough for the polymer to diffract X-rays like a Fig 22 .5 A chain-folded polymer crystal The structure is like that of a badly woven carpet The unit cell, shown below, is relatively simple and is much smaller than the polymer chain 23 4 Engineering Materials 2 Fig 22 .6 A schematic drawing of a largely crystalline polymer like high-density polyethylene At the... but so does the viscosity; it is hard to mould polyethylene if 23 0 Engineering Materials 2 Fig 22 .2 (a) Linear polymers are made of chains with a spectrum of lengths, or DPs The probability of a given DP is P (DP); (b) and (c) the strength, the softening temperature and many other properties depend on the average DP the DP is much above 103 The important point is that a material like polyethylene does... composite theory (Chapter 25 ) A stress σ produces a strain which is the weighted sum of the strains in each sort of bond ε = f σ σ f (1 − f ) + (1 − f ) = σ +  E1 E2 E2   E1 (23 .2) Here f is the fraction of stiff, covalent bonds (modulus E1) and 1 − f is the fraction of weak, secondary bonds (modulus E2 ) The polymer modulus is −1 E= (1 − f ) σ f = +  E1 E2  ε  (23 .3) If the polymer is completely... thermosetting polymers, or resins The simplest linear-chain polymer is polyethylene (Fig 22 .3a) By replacing one H atom of the monomer by a side-group or radical R (sausages on Fig 22 .3b, c, d) we obtain the vinyl group of polymers: R = Cl gives polyvinyl chloride; R = CH3 gives The structure of polymers 23 1 Fig 22 .3 (a) Linear polyethylene; (b) an isotactic linear polymer: the side-groups are all on... understanding the properties that polymers exhibit Molecular length and degree of polymerisation Ethylene, C2H4 , is a molecule We can represent it as shown in Fig 22 .1(a), where the square box is a carbon atom, and the small circles are hydrogen Polymerisation breaks The structure of polymers 22 9 Fig 22 .1 (a) The ethylene molecule or monomer; (b) the monomer in the activated state, ready to polymerise with... Engineering Design Guide No 17: The Engineering Properties of Plastics, Oxford University Press, 1977 Problems 21 .1 What are the four main generic classes of polymers? For each generic class: (a) give one example of a specific component made from that class; (b) indicate why that class was selected for the component 21 .2 How do the unique characteristics of polymers influence the way in which these materials. .. complicated than that The growing ends of a small bundle of crystallites (Fig 22 .7a) trap amorphous materials between them, wedging them apart More crystallites nucleate on the bundle, and they, too, splay out as they grow The splaying continues until the crystallites bend back on themselves and touch; then it can go no further (Fig 22 .7b) The spherulite then grows as a sphere until it impinges on others, . coefficient (20 °C) T g (K) temperature (W m − 1 K − 1 )(MK − 1 ) (MPa m 1 /2 ) T s (K) 1 2 270 355 22 50 0.35 160–190 2 5 300 390 21 00 0. 52 150–300 3.5 25 3 310 1900 0 .2 100 –300 –– 395 105 0 0 .25 70 100 2. 0.1–0.15 70 100 2. 4 350 370 – 0.15 50–70 1.6 378 400 1500 0 .2 54– 72 3–5 340 350– 420 1900 0 .2 0 .25 80–95 0.6–1.0 380 400– 440 1700 20 00 0 .2 0.5 55–90 0.5 340 420 – 440 120 0 24 00 0 .2 0 .24 50 100 –– 370–550. 370–550 1500–1700 0. 12 0 .24 26 –60 – 22 0 ≈350 25 00 ≈0.15 ≈600 – 171 ≈350 25 00 ≈0.15 ≈600 – 20 0 ≈350 25 00 ≈0.15 ≈600 –– – – – – –– – – – – –– – – – – 22 6 Engineering Materials 2 There are other