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Chapter 15: Polymers - Characteristics, Applications, and Processing

Chapter 15: Polymers - Characteristics, Applications, and Processing. 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.

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Chapter 15: Polymers - Characteristics, Applications, and Processing

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  1. Chapter 15:Polymers - Characteristics, Applications, and Processing 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?

  2. Mechanical Properties • i.e. stress-strain behavior of polymers brittle polymer FS of polymer ca. 10% that of metals plastic elastomer elastic modulus – less than metal Strains – deformations > 1000% possible (for metals, maximum strainca. 10% or less)

  3. Temperature and Strain Rate s (MPa) 8 0 Data for the 4°C semicrystalline 6 0 polymer: PMMA 20°C (Plexiglas) 4 0 40°C 20 to 1.3 60°C 0 0 0.1 0.2 0.3 e • Decreasing T... -- increases E -- increases TS -- decreases %EL • Increasing strain rate... -- same effects as decreasing T.

  4. fibrillar structure s (MPa) Near Failure brittle failure x near onset of failure necking plastic failure x Initial unload/reload e aligned, networked crystalline cross- case regions linked slide case semi- amorphous crystalline crystalline regions case regions align elongate Tensile Response: Brittle & Plastic

  5. Predeformation by Drawing • Drawing -- 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. • Compare to cold working in metals! Is it like Strain Hardening ?

  6. Tensile Response: Elastomer s (MPa) brittle failure x plastic failure x x elastomer final: chains e are straight, still cross-linked Deformation initial: amorphous chains are is reversible! kinked, cross-linked. • Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)

  7. alligned chains microvoids fibrillar bridges crack Polymer Fracture – spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form

  8. Melting vs. Glass Transition Temp. What factors affect Tm and Tg? • Both Tm and Tg increase with increasing chain stiffness • Chain stiffness increased by • Bulky sidegroups • Polar groups or sidegroups • Double bonds or aromatic chain groups • Tg is about 0.5–0.8Tm in K

  9. T Callister, rubber viscous Fig. 16.9 mobile Tm liquid tough liquid plastic Tg partially crystalline crystalline solid solid Molecular weight Dependence of polymer properties on Molecular weight and Temperature

  10. Strain-time response of materials Completely elastic Pure viscous Viscoelastic

  11. Time Dependent Deformation • Data: Large drop in Er for T > Tg. (amorphous polystyrene) 5 10 rigid solid Er(10s) (small relax) 3 10 in MPa transition tensile test 1 10 region eo strain -1 10 viscous liquid -3 s(t) (large relax) 10 60 100 140 180 T(°C) time Tg • Sample Tg(C) values: • Relaxation modulus: PE (low density) PE (high density) PVC PS PC -110 - 90 + 87 +100 +150 • Stress relaxation test: -- strain to eo and hold. -- observe decrease in stress with time.

  12. Propagation • Termination Addition (Chain) Polymerization • Initiation

  13. Condensation (Step) Polymerization

  14. Polymer Additives Improve mechanical properties, processability, durability, etc. • Fillers • Added to improve tensile strength & abrasion resistance, toughness, thermal and dimension stability & decrease cost • ex: carbon black, silica gel, wood flour, glass, limestone, etc. • Plasticizers • Improve flexibility, ductility and toughness • Added to reduce the glass transition temperature Tg • commonly added to PVC - otherwise reduction of hardness and stiffness

  15. Polymer Additives • Stabilizers • Antioxidants • UV protectants • Lubricants • Added to allow easier processing • “slides” through dies easier – ex: Na • Colorants • Dyes or pigments • Flame Retardants • Cl/F & B

  16. Thermoplastics vs. Thermosets • Thermoplastics: -- little crosslinking -- ductile -- soften with heating -- polyethylene, polypropylene, polycarbonate, polystyrene • Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften with heating -- vulcanized rubber, epoxies,polyester resin, phenolic resin

  17. 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

  18. Processing Plastics - Molding • Compression and transfer molding • thermoplastic or thermoset

  19. Processing Plastics - Molding • Injection molding • thermoplastic & some thermosets

  20. Processing Plastics – Extrusion

  21. Blown-Film Extrusion

  22. Processing Plastics • Blow Molding • Casting

  23. Polymer Types: Elastomers Elastomers – rubber • Crosslinked materials • Natural rubber • Synthetic rubber and thermoplastic elastomers • SBR- styrene-butadiene rubber styrene butadiene – Silicone rubber

  24. Polymer Types: Fibers Fibers - length/diameter >100 • Textiles are majority 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 • the fibers are drawn • leads to highly aligned chains- fibrillar structure

  25. 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: • Secondary bonds • Mechanical bonding • Films – blown film extrusion • Foams – gas bubbles in plastic

  26. 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. • Liquid Crystal Polymers • LCD displays UHMWPE

  27. Summary • General drawbacks to polymers: -- E, sy, 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, sy, Tapplication -- Larger Kc -- Easier to form and recycle • Elastomers (rubber): -- Large reversible strains! • Thermosets (epoxies, polyesters): -- Larger E, sy, Tapplication -- Smaller Kc

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