Presentation on theme: "Chapter 15: Characteristics, Applications & Processing of Polymers"— Presentation transcript:
1 Chapter 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 mechanicalresponse of polymers compare to ceramics and metals?• What are the primary polymer processing methods?
2 Mechanical Properties i.e. stress-strain behavior of polymersbrittle polymerplasticelastomerelastic modulus – less than metalAdapted from Fig. 15.1, Callister 7e.Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less)
3 Tensile Response: Brittle & Plastic (MPa)fibrillar structurenearfailureInitialNear Failurexbrittle failureonset ofcrystallineregions alignneckingplastic failurexcrystallineregionsslideamorphousregionselongatealigned,cross-linkedcasenetworkedunload/reloadesemi-crystallinecaseStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
4 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 thestretching direction-- increases the tensile strength (TS) in the-- decreases ductility (%EL)• Annealing after drawing...-- decreases alignment-- reverses effects of drawing.• Compare to cold working in metals!Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )
5 Tensile Response: Elastomer Case (MPa)final: chainsare straight,stillcross-linkedxbrittle failureStress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)plastic failurexxelastomerinitial: amorphous chains arekinked, cross-linked.Deformationis reversible!e• Compare to responses of other polymers:-- brittle response (aligned, crosslinked & networked polymer)-- plastic response (semi-crystalline polymers)
6 Thermoplastics vs. Thermosets Callister,Fig. 16.9TMolecular weightTgTmmobileliquidviscousrubbertoughplasticpartiallycrystallinesolid• Thermoplastics:-- little crosslinking-- ductile-- soften w/heating-- polyethylenepolypropylenepolycarbonatepolystyrene• Thermosets:-- large crosslinking(10 to 50% of mers)-- hard and brittle-- do NOT soften w/heating-- vulcanized rubber, epoxies,polyester resin, phenolic resinAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)
7 T and Strain Rate: Thermoplastics (MPa)• Decreasing T...-- increases E-- increases TS-- decreases %EL• Increasingstrain rate...-- same effectsas decreasing T.20468Data for the4°Csemicrystallinepolymer: PMMA20°C(Plexiglas)40°Cto 1.360°Ce0.10.20.3Adapted from Fig. 15.3, Callister 7e. (Fig 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.)
8 Melting vs. Glass Transition Temp. What factors affect Tm and Tg?Both Tm and Tg increase with increasing chain stiffnessChain stiffness increased byBulky sidegroupsPolar groups or sidegroupsDouble bonds or aromatic chain groupsAdapted from Fig , Callister 7e.
9 Time Dependent Deformation • Stress relaxation test:• Data: Large drop in Erfor T > Tg.(amorphouspolystyrene)Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)1031-1-3560100140180rigid solid(small relax)transitionregionT(°C)TgEr (10s)in MPaviscous liquid(large relax)-- strain to eo and hold.-- observe decrease instress with time.timestraintensile testeos(t)• Relaxation modulus:• Sample Tg(C) values:PE (low density)PE (high density)PVCPSPC- 110- 90+ 87+100+150Selected values from Table 15.2, Callister 7e.
10 Polymer Fracture Crazing Griffith cracks in metals – spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges formfibrillar bridgesmicrovoidscrackalligned chainsNote: crack spreads by breaking C-C bondsDuctile plastics have large craze zones that absorb large amounts of energy as it spreadsAdapted from Fig. 15.9, Callister 7e.
13 Polymer AdditivesImprove mechanical properties, processability, durability, etc.FillersAdded to improve tensile strength & abrasion resistance, toughness & decrease costex: carbon black, silica gel, wood flour, glass, limestone, talc, etc.PlasticizersAdded to reduce the glass transitiontemperature Tgcommonly added to PVC - otherwise it is brittlePolymers are almost never used as a pure materialMigration of plasticizers can be a problem
14 Polymer Additives Stabilizers Antioxidants UV protectants Lubricants Added to allow easier processing“slides” through dies easier – ex: Na stearateColorantsDyes or pigmentsFlame RetardantsCl/F & B
15 Processing of Plastics Thermoplastic –can be reversibly cooled & reheated, i.e. recycledheat till soft, shape as desired, then coolex: polyethylene, polypropylene, polystyrene, etc.Thermosetwhen heated forms a networkdegrades (not melts) when heatedmold the prepolymer then allow further reactionex: urethane, epoxyCan be brittle or flexible & linear, branching, etc.
16 Processing Plastics - Molding Compression and transfer moldingthermoplastic or thermosetMale & female parts to moldAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed.,John Wiley & Sons, )
17 Processing Plastics - Molding Injection moldingthermoplastic & some thermosetsAdapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition,John Wiley & Sons, )Polymer is melted in chamber & then the molten polymer is forced under pressure into the die
18 Processing Plastics – Extrusion An extruder is a device that used a large screw to melt a polymer, compress it, & force it into a moldExtremely commonly usedAdapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)
20 Polymer Types: Fibers Fibers - length/diameter >100 Textiles are main useMust have high tensile strengthUsually highly crystalline & highly polarFormed by spinningex: extrude polymer through a spinnerettePt plate with 1000’s of holes for nylonex: rayon – dissolved in solvent then pumped through die head to make fibersthe fibers are drawnleads to highly aligned chains- fibrillar structure
21 Polymer Types Coatings – thin film on surface – i.e. paint, varnish To protect itemImprove appearanceElectrical insulationAdhesives – produce bond between two adherandsUsually bonded by:Secondary bondsMechanical bondingFilms – blown film extrusionFoams – gas bubbles in plastic
22 Blown-Film ExtrusionAdapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.)
23 Advanced Polymers Ultrahigh molecular weight polyethylene (UHMWPE) Molecular weight ca. 4 x 106 g/molExcellent properties for variety of applicationsbullet-proof vest, golf ball covers, hip joints, etc.UHMWPEAdapted from chapter-opening photograph, Chapter 22, Callister 7e.
24 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-- Easier to form and recycle• Elastomers (rubber):-- Large reversible strains!• Thermosets (epoxies, polyesters):-- Larger E, sy, TapplicationTable 15.3 Callister 7e:Good overviewof applicationsand trade namesof polymers.