Chapter 15: Processing of Polymers

Transcription

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 mechanical response of polymers compare to ceramics and metals? What are the primary polymer processing methods? Chapter 15-1

2 Mechanical Properties i.e. stress-strain behavior of polymers brittle polymer TS of polymer ca. 10% that of metals elastic modulus less than metal plastic elastomer Strains deformations > 1000% possible (for metals, maximum strain ca. 10% or less) Adapted from Fig. 15.1, Callister 7e. Chapter 15-2

3 Temperature influence Strain rate influence: slow => High T Chapter 15 -

4 Elastic behavior ( σ = Eε ) Viscoelastic behavior asing T Increa Viscous behavior f15 05 pg528 Chapter 15 -

5 Viscoelastic relaxation modulus E r ( t) = constant σ (t) t ε 0 strain ε 0 Creep modulus E c ( t) = σ 0 ε ( t ) constant stress σ 0 f15_06_pg529 Chapter 15 -

6 A: highly crystallized B: Rubber or elastomeric C: amorphous Chapter 15 -

7 Polymer Fracture Crazing spherulites plastically deform to fibrillar structure microvoids and fibrillar bridges form alligned chains fibrillar bridges microvoids crack Adapted from Fig. 15.9, Callister 7e. Chapter 15-7

8 Tensile Response: Brittle & Plastic Near Failure Initial σ(mpa) x brittle failure onset of necking x unload/reload l d plastic failure ε fibrillar structure near failure aligned, crosslinked case networked case semicrystalline case amorphous regions elongate crystalline regions align crystalline regions slide Chapter 15-8

9 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 (~ 3 times) -- increases the tensile strength (TS) in the stretching direction (~2-5 times) -- decreases ductility (%EL) Annealing after drawing decreases alignment -- reverses effects of drawing. Compare to cold working in metals! Adapted from Fig , 13 Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp ) Chapter 15-9

10 Other factors that influence the mechanical properties p of semicrystalline polymers Molecular Weight A TS = TS M n TS tensile strength with infinite molecular weight A a constant; M n number - average molecular weight Degree of Crystallinity closely l packed din an ordered dand parallel larrangement... Increase tensile molulus, strength, brittleness Heat treating for undrawn polymers increase in the percent crystallinity, crystal size and perfection increase in tensile modulus, yield strength and breittleness Chapter 15-10

11 Tensile Response: Elastomer Case σ(mpa) initial: amorphous chains are kinked, cross-linked. x brittle failure plastic failure x elastomer Deformation is reversible! ε x final: chains are straight, still cross-linked Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) Amorphous, not easily crystallize Chain bonds easily rotate in responding to applied force Delayed plastic deformation Chapter 15-11

12 Crystallization, melting and glass transition in polymers Crystallization: -- Nucleation and growth -- follow Avrami Equation y=1-exp(-kt n ) -- polyethylene polypropylene polycarbonate polystyrene Melting: -- melting over a range of T -- T m depends on history of the polymer, crystallization P/T -- T m depends on heating rate, higher rate => higher T m Glass transition: -- transition from rubbery to rigid state -- reversible at T g Chapter 15-12

13 Thermoplastics vs. Thermosets Thermoplastics: little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: large crosslinking 1984.) (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- rubber, epoxies, polyester resin, phenolic resin T mobile liquid highly crystalline solid viscous liquidid rubber tough plastic partially crystalline solid Molecular weight T m T g Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., Chapter 15-13

14 Melting vs. Glass Transition Temp. What other factors affect T m and T g? g Both T m and T g increase with increasing chain stiffness Chain stiffness increased by 1. Bulky sidegroups 2. Polar groups (e.g.cl) 3. Double bonds chain groups Normally 05T 0.5T m <T T g <0.8T m Adapted from Fig , Callister 7e. Chapter 15-14

15 t15_02_pg547 Chapter 15 -

16 Polymer types Plastics Elastomers Fibers Relative Rigid Rubbery High TS Any degree of crystallinity All molecular structures and configurations Work below T m or T g, crosslinked e.g. polyamide (nylon), polystyren amorphous Highly twisted kinked k and coiled crosslinked molecular chains hi e.g. Natural polyisoprene p (rubber), chloroprene Highly crystallinity and high molecular weight e.g. polyamide (nylon), polyester Chapter 15-16

17 Addition (Chain) Polymerization Initiation Propagation Termination Chapter 15-17

18 Condensation (Step) Polymerization Chapter 15-18

19 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 T g commonly added d to PVC -otherwise it is brittle Stabilizers Colorants Antioxidants Dyes or pigments pg UV protectants Flame Retardants Cl/F & B Chapter 15-19

20 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 d (not melts) when heated mold the prepolymer (linear polymer) then allow further reaction (curing with heating or catalysts) to form crosslinked or network structures. ex: urethane, epoxy Chapter 15-20

21 Processing Plastics - Molding Compression and transfer molding thermoplastic or thermoset Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley & Sons, ) Chapter 15-21

22 Processing Plastics - Molding Injection molding thermoplastic & some thermosets Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 2nd edition, John Wiley & Sons, ) Chapter 15-22

23 Processing Plastics Extrusion Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.) Chapter 15-23

24 Fabrication of fibers (spinning) Melt spinning: molten Dry spinning: dissolved in volatile solvent which will be evaporated Wet spinning: i precipitate solvent with second solvent Chapter 15-24

25 Blown-Film Extrusion Adapted from Fig , Callister 7e. (Fig is from Encyclopædia Britannica, 1997.) Chapter 15-25

26 Summary General drawbacks to polymers: -- E, σy, Tapplication are generally small. -- Deformation is often T and dtime dependent. d Thermoplastics (PE, PS, PP, PC): -- Smaller E, σy, Tapplication -- Larger Kc (fracture strength) -- Easier to form and recycle Elastomers (rubber): -- Large reversible strains! Thermosets (epoxies, polyesters): -- Larger E, σy, Tapplication f l -- Smaller Kc Table 15.3 Callister 7e: Good overview of applications and trade names of polymers. Chapter 15-26

27 Homework Reading: Core Problems: Self-help Problems: Chapter 15-27