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Chapter 5/1/2008 15: Characteristics, Applications & Processing of Polymers ISSUES TO ADDRESS... What are the tensile properties of polymers and how are they affected by basic microstructural features? Hardening, anisotropy, and annealing in polymers. How does the elevated temperature mechanical response of polymers compare to ceramics and metals? What are the primary polymer processing methods? Mechanical Properties i.e. stress-strain behavior of polymers brittle polymer of polymer ca. 10% that of metals plastic elastomer elastic modulus less than metal FS Callister 7e. Strains deformations > 1000% possible (for metals, maximum strain ca. 10% or less) Chapter 15 - 1 Adapted from Fig. 15.1, Chapter 15 - 2 Tensile Response: Brittle & Plastic Near Failure Predeformation by Drawing Drawing (ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) Annealing after drawing... -- decreases alignment -- reverses effects of drawing. (MPa) x brittle failure onset of necking fibrillar structure near failure plastic failure x Initial unload/reload aligned, networked crosscase linked case semicrystalline case amorphous regions elongate crystalline regions align crystalline regions slide Adapted from Fig. 15.13, Callister 7e. (Fig. 15.13 is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp. 500-501.) Compare to cold working in metals! Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, PrenticeHall, Inc., 1974, pp. 500-501.) Chapter 15 - 3 Chapter 15 - 4 Tensile Response: Elastomer Case (MPa) x brittle failure Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig. 15.15, Callister 7e. (Fig. 15.15 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.) Thermoplastics vs. Thermosets Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene T mobile liquid viscous liquid Callister, rubber Fig. 16.9 tough plastic partially crystalline solid Tm Tg x plastic failure elastomer x final: chains are straight, still cross-linked crystalline solid Thermosets: Molecular weight Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer, initial: amorphous chains are kinked, cross-linked. Deformation is reversible! Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) Chapter 15 - 5 -- large crosslinking Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin Chapter 15 - 6 1 5/1/2008 T and Strain Rate: Thermoplastics Decreasing T... -- increases E -- increases TS -- decreases %EL Melting vs. Glass Transition Temp. What factors affect Tm and Tg? Both Tm and Tg increase with increasing chain stiffness Chain stiffness increased by 1. Bulky sidegroups 2. Polar groups or sidegroups 3. Double bonds or aromatic chain groups Regularity effects Tm only (MPa) 80 4 C 60 40 20 0 60 C 0 0.1 0.2 20 C 40 C to 1.3 0.3 Data for the semicrystalline polymer: PMMA (Plexiglas) Increasing strain rate... -- same effects as decreasing T. Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.) Adapted from Fig. 15.18, Callister 7e. Chapter 15 - 7 Chapter 15 - 8 Time Dependent Deformation Stress relaxation test: -- strain to and hold. -- observe decrease in stress with time. tensile test o Polymer Fracture Crazing Griffith cracks in metals spherulites plastically deform to fibrillar structure microvoids and fibrillar bridges form alligned chains Data: Large drop in Er for T > Tg. 105 Er (10s) 3 in MPa 10 101 rigid solid (small relax) region transition (amorphous polystyrene) Adapted from Fig. 15.7, Callister 7e. (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) strain (t) time 10 -1 viscous liquid 10-3 (large relax) 60 100 140 180 T( C) Tg Relaxation modulus: E r (t ) (t ) o Sample Tg( C) values: PE (low density) PE (high density) PVC PS PC - 110 - 90 + 87 +100 +150 Selected values from Table 15.2, Callister 7e. fibrillar bridges microvoids crack Adapted from Fig. 15.9, Callister 7e. Chapter 15 - 9 Chapter 15 - 10 Addition (Chain) Polymerization Initiation Propagation Condensation (Step) Polymerization Termination Chapter 15 - 11 Chapter 15 - 12 2 5/1/2008 Polymer Additives Improve mechanical properties, processability, durability, etc. Fillers Added to improve tensile strength & abrasion resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. Plasticizers Added to reduce the glass transition temperature Tg commonly added to PVC - otherwise it is brittle Polymer Additives Stabilizers Antioxidants UV protectants Lubricants Added to allow easier processing slides through dies easier ex: Na stearate Colorants Dyes or pigments Flame Retardants Cl/F & B Chapter 15 - 13 Chapter 15 - 14 Processing of Plastics Thermoplastic can be reversibly cooled & reheated, i.e. recycled heat till soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene, etc. Thermoset when heated forms a network degrades (not melts) when heated mold the prepolymer then allow further reaction ex: urethane, epoxy Processing Plastics - Molding Compression and transfer molding thermoplastic or thermoset Adapted from Fig. 15.23, Callister 7e. (Fig. 15.23 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, 1984. ) Chapter 15 - 15 Chapter 15 - 16 Processing Plastics - Molding Injection molding thermoplastic & some thermosets Adapted from Fig. 15.24, Callister 7e. (Fig. 15.24 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, 1971. ) Processing Plastics Extrusion Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclop dia Britannica, 1997.) Chapter 15 - 17 Chapter 15 - 18 3 5/1/2008 Polymer Types: Elastomers Elastomers rubber Crosslinked materials Natural rubber Synthetic rubber and thermoplastic elastomers SBR- styrene-butadiene rubber styrene Polymer Types: Fibers Fibers - length/diameter >100 Textiles are main use Must have high tensile strength Usually highly crystalline & highly polar Formed by spinning ex: extrude polymer through a spinnerette Pt plate with 1000 s of holes for nylon ex: rayon dissolved in solvent then pumped through die head to make fibers butadiene Silicone rubber the fibers are drawn leads to highly aligned chains- fibrillar structure Chapter 15 - 19 Chapter 15 - 20 Polymer Types Coatings thin film on surface i.e. paint, varnish To protect item Improve appearance Electrical insulation Adhesives produce bond between two adherands Usually bonded by: 1. Secondary bonds 2. Mechanical bonding Blown-Film Extrusion Films blown film extrusion Foams gas bubbles in plastic Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclop dia Britannica, 1997.) Chapter 15 - 21 Chapter 15 - 22 Advanced Polymers Ultrahigh molecular weight polyethylene (UHMWPE) Molecular weight ca. 4 x 106 g/mol Excellent properties for variety of applications bullet-proof vest, golf ball covers, hip joints, etc. Adapted from chapteropening photograph, Chapter 22, Callister 7e. Summary General drawbacks to polymers: -- E, y, Kc, Tapplication are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement. Thermoplastics (PE, PS, PP, PC): -- Smaller E, y, Tapplication -- Larger Kc Table 15.3 Callister 7e: -- Easier to form and recycle Good overview Elastomers (rubber): of applications -- Large reversible strains! and trade names Thermosets (epoxies, polyesters): of polymers. -- Larger E, y, Tapplication -- Smaller Kc UHMWPE Chapter 15 - 23 Chapter 15 - 24 4 5/1/2008 ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 15 - 25 5

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