Chapter 15 -

Assist.  Prof.  Dr.  İlkay  KALAY  Materials  Science  and  Engineering  Department  

Çankaya  University    

MSE  226    ENGINEERING  MATERIALS  

(SECTIONS  01  &  02)  

Characteristics, Applications & Processing of Polymers

Chapter 15 - 2

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?

Chapter 15: Characteristics, Applications &

Processing of Polymers

• What are the primary polymer processing methods?

Chapter 15 - 3

Mechanical Properties •  i.e. stress-strain behavior of polymers

brittle polymer

plastic elastomer

σFS of polymer ca. 10% that of metals

Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less)

elastic modulus – less than metal

Adapted from Fig. 15.1, Callister 7e.

Chapter 15 - 4

Tensile Response: Brittle & Plastic

brittle failure

plastic failure

σ (MPa)

ε

x

x

crystalline regions

slide

fibrillar structure

near failure

crystalline regions align

onset of necking

Initial

Near Failure

semi- crystalline

case

aligned, cross- linked case

networked case

amorphous regions

elongate

unload/reload

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, Prentice-Hall, Inc., 1974, pp. 500-501.)

Chapter 15 - 5

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. • Compare to cold working in metals!

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.)

Chapter 15 - 6

• Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers)

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.)

Tensile Response: Elastomer Case σ (MPa)

ε

initial: amorphous chains are kinked, cross-linked.

x

final: chains are straight,

still cross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failure x

x

Chapter 15 - 7

• Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene

• Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin

Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.)

Thermoplastics vs. Thermosets

Callister, Fig. 16.9

T

Molecular weight

Tg

Tm mobile liquid

viscous liquid

rubber

tough plastic

partially crystalline solid crystalline

solid

Chapter 15 - 8

• Decreasing T... -- increases E -- increases TS -- decreases %EL • 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.)

T and Strain Rate: Thermoplastics

20

4 0

6 0

8 0

0 0 0.1 0.2 0.3

4°C

20°C

40°C

60°C to 1.3

σ (MPa)

ε

Data for the semicrystalline polymer: PMMA (Plexiglas)

Chapter 15 - 9

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

Adapted from Fig. 15.18, Callister 7e.

Chapter 15 - 10

• Stress relaxation test: -- strain to εο and hold. -- observe decrease in stress with time.

or

ttE

!

"=

)()(

• Relaxation modulus: • 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.

Time Dependent Deformation

time

strain tensile test

εo

σ(t)

• Data: Large drop in Er for T > Tg. (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.)

10 3

10 1

10 -1

10 -3

10 5

60 100 140 180

rigid solid (small relax)

transition region

T(°C) Tg

Er (10s) in MPa

viscous liquid (large relax)

Chapter 15 - 11

Polymer Fracture

fibrillar bridges microvoids crack

alligned chains

Adapted from Fig. 15.9, Callister 7e.

Crazing ≅ Griffith cracks in metals

– spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form

Chapter 15 - 12

Addition (Chain) Polymerization

–  Initiation

–  Propagation

–  Termination

Chapter 15 - 13

Condensation (Step) Polymerization

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

Chapter 15 -

POLYETHYLENE

HDPE pipe installation in storm drain project

in Mexico.

•  Most popular plastic in the world. 

•  The polymer you see most in daily life.

•  280 million metric tones of it was produced in 2011.

•  Largest use is as packaging materials, like films and foam; and for bottles and other containers that can be used in food, medical, and other consumer industries.

•  Grocery bags, shampoo bottles, children's toys, and even bullet proof vests

Chapter 15 -

POLYPROPYLENE

•  A thermoplastic polymer

•  In 2008, the global market for polypropylene had a volume of 45.1 million metric tons, which led to a turnover of about $65 billion (~ €47.4 billion)

•  Used in a wide variety of applications including packaging and labeling, textiles (e.g., ropes, thermal underwear and carpets), kitchenware, stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes.

Chapter 15 -

POLYPROPYLENE

Light Weight Polypropylene Automotive Parts

Chapter 15 -

POLYSTYRENE

•  A thermoplastic polymer •  Very inexpensive resin per unit weight •  One of the most widely used plastics •  Produced in several billion kilograms per year •  Polystyrene can be naturally transparent, but can be colored with colorants. •  Protective packaging (such as packing peanuts and CD and DVD cases), containers (such as "clamshells"), lids, bottles, trays, tumblers, and disposable cutlery

Chapter 15 -

POLYVINYL CHLORIDE (PVC) •  The third-most widely produced plastic, after polyethylene and polypropylene.

•  PVC is used in construction because it is more effective than traditional materials such as copper, iron or wood in pipe and profile applications.

•  Electric cables, clothing and furnature, flooring, healthcare

Chapter 15 -

PROPERTIES OF POLYMERS

Synthetic and natural polymers commonly consist of a collection of macromolecules of different molecular weights. • Polymers are fairly amorphous (noncrystalline). • Polymer chains tend to be flexible and easily entangled or folded. • They soften over a wide range of temperatures. • They may show some ordering. • The degree of crystallinity reflects the extent of the order.

Chapter 15 -

Properties of Polymers

Interactions between chains of a polymer lend elements of order to the structure of polymers.

Chapter 15 -

Properties of Polymers

Stretching the polymer chains as they form can increase the amount of order, leading to a degree of crystallinity of the polymer.

Chapter 15 -

Properties of Polymers

Such differences in crystallinity can lead to polymers of the same substance that have very different physical properties.

Chapter 15 -

Cross-Linking

Chemically bonding chains of polymers to each other can stiffen and strengthen the substance.

Chapter 15 -

Cross-Linking

Naturally occurring rubber is too soft and pliable for many applications.

Chapter 15 - 26

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

Chapter 15 - 27

•  Colorants –  Colorants impart a specific color to a polymer; they may be added in the form of dyes or pigments.

•  Flame Retardants –  Most polymers are flammable in their pure form; –  The flammability resistance of the remaining combustible polymers may be enhanced by additives called flame retardants.

POLYMER ADDITIVES

Chapter 15 - 28

•  Stabilizers –  Additives that counteract deteriorative processes are

called stabilizers –  Antioxidants –  UV protectants

•  Lubricants –  Added to allow easier processing –  “slides” through dies easier – ex: Na stearate

POLYMER ADDITIVES

Chapter 15 - 29

Processing Plastics - Molding

Molding is the most common method for forming plastic polymers.

The several molding techniques used include compression, transfer, blow, injection, and

extrusion molding.

For each, a finely pelletized or granulized plastic is forced, at an elevated temperature and by

pressure, to flow into, fill, and assume the shape of a mold cavity.

Chapter 15 - 30

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 - 31

Processing Plastics - Molding •  Injection molding •  The most widely used technique

–  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. )

Chapter 15 - 32

Processing Plastics – Extrusion

Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)

•  An extruder is a device that used a large screw to melt a polymer, compress it, & force it into a mold

•  Extremely commonly used •  Molding of a viscous thermoplastic under pressure through

an open-ended die

Chapter 15 - 33

Processing Plastics – Extrusion

Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)

•  A mechanical screw or auger propels through a chamber the pelletized material, which is successively compacted, melted, and formed into a continuous charge of viscous fluid

•  Extrusion takes place as this molten mass is forced through a die orifice.

Chapter 15 - 34

Polymer Types: Elastomers Elastomers – rubber •  Crosslinked materials

–  Natural rubber –  Synthetic rubber and thermoplastic elastomers

•  SBR- styrene-butadiene rubber styrene

– Silicone rubber

butadiene

Chapter 15 - 35

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

–  the fibers are drawn –  leads to highly aligned chains- fibrillar structure

Chapter 15 - 36

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

•  Films – blown film extrusion •  Foams – gas bubbles in plastic

Chapter 15 - 37

Blown-Film Extrusion

Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)

Fabrication of Fibers and Films

Continuous tubing is extruded through an annular die; then, by maintaining a carefully controlled positive gas pressure inside the tube and by drawing the film in the axial direction as it emerges from the die, the material expands around this trapped air bubble like a balloon

Chapter 15 - 38

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.

UHMWPE

Adapted from chapter-opening photograph, Chapter 22, Callister 7e.

Chapter 15 -

The inside of the golf ball is made out of rubber and the outside of the golf ball is made

out of plastic.

Chapter 15 - 40

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

Table 15.3 Callister 7e: Good overview of applications and trade names of polymers.

Summary

Chapter 15 - 41

Core Problems:

Self-help Problems:

ANNOUNCEMENTS Reading: