chapter7 - CHAPTER 7 POLYMERS STRUCTURE GENERAL PROPERTIES...

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Unformatted text preview: CHAPTER 7 POLYMERS: STRUCTURE, GENERAL PROPERTIES, AND APPLICATIONS 7-1 Widely Used Because: Resistance to corrosion and chemicals Low electrical and thermal conductivity Low density High strength-to-weight ratio Noise reduction Wide choice of colors and transparencies Ease of manufacture Relatively low cost 7-2 Fillers Plasticizers ‘, Stabilizers L. _ .1 Colorants ‘ Flame retardants Lubricants Thermoplastics: Acryiics. ABS. nyions. k . E pub-carbonates. polyethylenes. Mer Heat. pressure. ‘ ; polp'x'lnyi chloride. etc. or ' l : Polymer mers cam yst I 1 . -——.—x l '—I__ Thermosets: Epoxtes. pnenohcs. l 3 polyxmldes. etc. ' l Polvmenzation: ‘ condensation. Amorphous. Elastomers: Natural and synthetic addition Partliprystallme l rubbers. sxhcones [near Cross-Linking lvuremanes BLE Branched " p0 ‘ ' ' Homopolymer Copolymer - Terpolymer :IGURE 7.1 Outline of topics described in Chapter 7. __.__.._~._______________H_________—_ TABLE 7,1 # RANGE OF MECHANICAL PROPERTIES FOB VARIOUS ENQINEEERVINGHPLASTICS AT ROOM TEMPERATURE ‘ ______________________.___.__-——-——————-~— UTS E ELONGATION -' POISSON'S RATIO MATERIAL (M Pa) (G Pa) (96) (v) ABS 28-55 1.4—2.8 75-5 — ABS. reinforced 100 7.5 — 0.35 Acetal 55—70 1 .4—3.5 75—25 —— Acetal, reinforced 1 35 1 0 — . _ 0.35—0.40 Acrylic . 40—75 1 4—3.5 50—5 ’ — Cellulosic 10—48 . 0.4-4 .4 100-5 ‘ - _ _ -—,— Epoxy - . 35—140 " 3.5-1? ' 10—1 ’ ‘ ' L- Epoxy, reinforced 70—1400 ‘ 217-52 4—2 — fluorocarbon ' ‘ 7—48 0.1-2 300—100 0.46—0.48 Nylon 55—83 1 4—2.8 200-60 0.32—0.40 Nylon, reinforced 70-210 2-10 10—1 —— Phenolic 28-70 28—21 2-0 —— Polycarbonate 55—70 2.5-3 125—1 0 0.38 Polycarbonate, reinforced 1 1 O 6 6-4 — Polyester 55 2 300—5 0.38 Polyester, reinforced 1 10-160 8.3—1 2 3—1 ' — Polyethylene 7-40 0.1 —1 .4 1 000—1 5 0.46 Polypropylene 20—35 - 0.7—1 .2 ' 500-10 — Polypropylene, reinforced 40—100 3.5-6 4—2 I —- Polystyrene ' 14—83 -- 1.4-4 ‘ 60—1 ---- ~ - - 0.35 Polyvinyl chloride ‘ 7—55 0.014—4 450—40 . — ________________________._——————-—-—-—-* | Commercial L polymers l -.I' Tensile and ’ impact strength] l Pl‘t‘ men)! ‘I Viscosity/I l FIGURE 7.3 Effect of molecular weight and 10-000 ill-000300 degree of polymerization Molecular weight. on the strength and dcgrcc 0f Polymerization viscosity of polymers. Glass-Transition Temperature (Tg) Below Tg amorphous polymers do not have a specific melting point. At low temperatures they are hard, rigid, brittle, and glassy. At higher temperatures they are rubbery or leathery. (polycarbonate is an exception) 7-6 Figure 7.7 Specific vqu-me‘ specific volume of polymers as a function of temperature. Arncrphous polymers, such as acrylic and polycarbonate. have a glass-transition temperature. T9, but do not have a loacific melting point, Tm. Partly crystalline polymers. Inch as polyethylene and nylons, contract sharply at their maltlng points during cooling. ' Temperature ¥ Thermoplastics Reversible heat cool cycle Common thermoplastics include acrylics, cellulosics, nylons, polyethylenes, and polyvinyl chloride Becomes a viscous fluid when heated above Tg 7-8 Figure 7.9 ‘27: £315.? = ‘- " -’ ' 10 70 ' m-..“ A. 60 ‘. .- 8 5" 50 c: H . 35 6 40 83 g ‘ 2 E - 4 30 1'3 A to _ ' ' 20 . , 2 , . 10 . _ 0 , 0 ; 0 5 A 10 15 20 25 ‘ 30 7'; “gm". A. . ., ,_ . - . Strain (%)- A Effect of temperature on. the stresststrain curve for cellu- loset’acetate, "a ‘ thermoplastic. Note“'the~ large drop in Strength and increase in‘ ductility with a relatively small increase in temperature-Source: After ,T.’ S. Carswell and :‘f H. K- Nasonc v ’3: .~‘- LIT; . 7 . _. , . I 7‘ ‘ r ‘l 7 7-9 Thermosets Long-chain molecules cross-linked in a 3-dimensional arrangement During polymerization the network is completed and the shape is set (non-reversible) Common thermosets include epoxy, polyester, urethane, melamine, phenolic Better mechanical properties than thermoplastics 7-10 Rigid and brittle (melamine. "phenolic! Tough and ductile 7 (ABS, nylon) Stress Soft and flexible (polyethylene, PTFE) Strain \.. ___. a? General terminology describing the behavior of th‘ree Wpes of plastics. ‘PT FE (polytetrafluoroethylene) is Teflon, a trade naméfSaurce': R. L. E. Brown. ' 7-11 Figure 7.10 . . ,. 1. -... Jr. -. - 0 h“, r. .c u. -. _ ' an . - _. . - ‘3'?f“€j,.~-,.LM':: an ._ l C l .- _ ., t. '.-.a. ' I. a -a'. Kfi'- :18 . o .. 32 Low-deceity' ' _ - polyethylene High-impact polypropylene ' I" Poiwanyréwe Impact strength Polymethylmethacrylate M 0 32 90 Temperature PF) Effect of temperature on the impact strength of various plastics. Small changes in temperature can have a signifi- cant effect on impact stength. Source: P. C. Powell. _____________________.____——-———- Additives Used to modify (improve) selected characteristics, e.g., stiffness, strength, color, weatherability, flammability, arc resistance, and ease of processing Fillers wood flour,silica, clay, glass Plasticisers improve flexibility, EX. PVC Carbon black (sunlight) Lubricants 7-13 TABLE 73 M GENERAL RECOMMENDATIONS FOR PLASTIC PRODUCTS I _________________.__—————————~—-—-—- DESIGN REQUIREMENT APPLICATIONS PLASTICS E. -. Mechanical strength Gears, cams, rollers, valves, fan Acetal, nylon, phenolic, ‘ ‘ blades, impellers, pistons polycarbonate \ Functional and decorative Handles, knobs, camera and ABS, acrylic, cellulosic, , battery cases, trim moldings, phenolic, polyethylene, pipe fittings polypropylene, polystyrene, polyvinyl chloride Housings and hollow shapes Power tools, pumps, housings, ABS, cellulosic, phenolic, sport helmets, telephone cases polycarbonate, polyethylene, polypropylene, polystyrene Functional and transparent Lenses, goggles, safety glazing, Acrylic, polycarbonate, signs, food»processing polystyrene, polysulfone equipment, laboratory hardware Wear resistance Gears, wear strips and liners, Acetal, nylon, phenolic, bearings, bushings, roller-skate polyimide, polyurethane. wheels . ultrahigh molecular weight polyethylene M— 7-14 Elastomers (Rubbers) Recover from large deformations quickly Hardness measured by durometer . Natural rubber (latex) Synthetic rubbers EX. neoprene, silicone, and polyurethane ...
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