CHARACTERISTICS, APPLICATIONS AND

PROCESSING OF POLYMERS

Materials of Engineering

ENGR 151

2

POLYMER FORMATION

Synthesis of large (polymer) molecules is called

polymerization.

There are two types of polymerization

Addition (or chain) polymerization

Monomer units are attached one at a time to form a linear

macromolecule

Composition of resultant product molecule is exact multiple of

original reactant monomer

Condensation (step) polymerization

Formation of polymers by stepwise intermolecular chemical

reactions that may involve more than one monomer species

Usually small molecular weight by-product, e.g. water

3

ADDITION (CHAIN) POLYMERIZATION

Three distinct steps:

Initiation, propagation, termination

Initiation: An active center capable of propagation is

formed by a reaction between an initiator (or catalyst)

species and the monomer unit

E.g. Polyethylene

4

ADDITION (CHAIN) POLYMERIZATION

Three distinct steps:

Initiation, propagation, termination

Propagation: Linear growth of polymer chain by sequential

addition of monomer units to active growing chain molecule

5

ADDITION (CHAIN) POLYMERIZATION

Three distinct steps:

Initiation, propagation, termination

Termination: May occur in different ways

Active ends of two propagating chains may link together to form one

molecule (combination)

6

ADDITION (CHAIN) POLYMERIZATION

Three distinct steps:

Initiation, propagation, termination

Termination: May occur in different ways

Two growing molecules react to form “dead chains”

(disproportionation)

7

CONDENSATION (STEP) POLYMERIZATION

Condensation (step) polymerization

Formation of polymers by stepwise intermolecular

chemical reactions that may involve more than

one monomer species

Usually small molecular weight by-product, e.g.

water

No reactant species has the has the chemical

formula of the repeat unit

Reaction times are generally slower than those for

addition polymerization

8

CONDENSATION (STEP) POLYMERIZATION

E.g. Polyester

9

CONDENSATION (STEP) POLYMERIZATION

10

POLYMER ADDITIVES

Improve mechanical properties, tensile and compressive strengths, abrasion resistance, toughness, dimensional and thermal stability, 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.

11

POLYMER ADDITIVES

• Plasticizers

• Improve the flexibility, ductility and toughness of polymers

• Added to reduce the glass transition temperature Tg below room temperature

• Presence of plasticizer transforms brittle polymer to a ductile one

• Commonly added to PVC - otherwise it is brittle

Stabilizers

Antioxidants

UV protectants

12

POLYMER ADDITIVES (CONT.)

• Lubricants

– Added to allow easier processing

– polymer “slides” through dies easier

– ex: sodium stearate

• Colorants

– Dyes and pigments

• Flame Retardants (e.g. textiles and children’s toys)

– Substances containing chlorine, fluorine, and boron

13

PROCESSING OF PLASTICS

Thermoplastic

can be reversibly cooled & reheated, i.e. recycled

heat until soft, shape as desired, then cool

ex: polyethylene, polypropylene, polystyrene.

• Thermoset

– when heated forms a molecular network (chemical reaction)

– degrades (doesn’t melt) when heated

– a prepolymer molded into desired shape, then

chemical reaction occurs

– ex: urethane, epoxy

14

PROCESSING PLASTICS – COMPRESSION

MOLDING

Thermoplastics and thermosets polymer and additives placed in mold cavity

mold heated and pressure applied

fluid polymer assumes shape of mold

Fig. 15.23, Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of

Polymer Science, 3rd edition. Copyright ©

1984 by John Wiley & Sons, New York.

Reprinted by permission of John Wiley &

Sons, Inc.)

15

PROCESSING PLASTICS – INJECTION MOLDING

Fig. 15.24, Callister & Rethwisch 9e. (From F. W. Billmeyer, Jr., Textbook of

Polymer Science, 3rd edition. Copyright ©

1984 by John Wiley & Sons, New York.

Reprinted by permission of John Wiley &

Sons, Inc.)

Thermoplastics and some thermosets • when ram retracts, plastic pellets drop from hopper into barrel

• ram forces plastic into the heating chamber (around the spreader) where the plastic melts as it moves forward

• molten plastic is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold

Barrel

16

PROCESSING PLASTICS – EXTRUSION

Fig. 15.25, Callister & Rethwisch 9e. (Reprinted with permission from Encyclopædia

Britannica, © 1997 by Encyclopædia Britannica, Inc.)

Thermoplastics • plastic pellets drop from hopper onto the turning screw

• plastic pellets melt as the turning screw pushes them forward by the heaters

• molten polymer is forced under pressure through the shaping die to form the final product (extrudate)

17

PROCESSING PLASTICS – BLOWN-FILM

EXTRUSION

Fig. 15.26, Callister & Rethwisch 9e. (Reprinted with permission from Encyclopædia

Britannica, © 1997 by Encyclopædia Britannica, Inc.)

18

POLYMER TYPES – FIBERS

Fibers - length/diameter >100 Primary use is in textiles. Fiber characteristics:

high tensile strengths high degrees of crystallinity structures containing polar groups

• Formed by spinning

– extrude polymer through a spinneret (a die containing many small orifices)

– the spun fibers are drawn under tension

– leads to highly aligned chains - fibrillar structure

19

POLYMER TYPES – MISCELLANEOUS

Coatings – thin polymer films applied to surfaces – i.e., paints, varnishes

protects from corrosion/degradation

decorative – improves appearance

can provide electrical insulation

• Adhesives – bonds two solid materials (adherands)

– bonding types:

1. Secondary – van der Waals forces

2. Mechanical – penetration into pores/crevices

• Films – produced by blown film extrusion

• Foams – gas bubbles incorporated into plastic

20

ADVANCED POLYMERS

Molecular weight ca. 4 x 106 g/mol

Outstanding properties

high impact strength

resistance to wear/abrasion

low coefficient of friction

self-lubricating surface

Important applications

bullet-proof vests

golf ball covers

hip implants (acetabular cup)

UHMWPE

Adapted from chapter-

opening photograph,

Chapter 22, Callister 7e.

Ultrahigh Molecular Weight Polyethylene (UHMWPE)

21

ADVANCED POLYMERS

styrene

butadiene

Thermoplastic Elastomers

Styrene-butadiene block copolymer

hard

component

domain

soft

component

domain

Fig. 15.22, Callister & Rethwisch 9e. Fig. 15.21(a), Callister & Rethwisch 9e.

22

ISSUES TO ADDRESS...

• How does corrosion occur?

• Which metals are most likely to corrode?

• What environmental parameters affect

corrosion rate?

• How do we prevent or control corrosion?

CHAPTER 17:

CORROSION AND DEGRADATION OF

MATERIALS

23

• Corrosion: -- the destructive

electrochemical

attack of a material.

-- Ex: Rusting of

automobiles and

other equipment

• Cost: -- 4 to 5% of the Gross National Product (GNP)*

-- in the U.S. this amounts to just over $400 billion/yr**

* H.H. Uhlig and W.R. Revie, Corrosion and Corrosion Control: An Introduction

to Corrosion Science and Engineering, 3rd ed., John Wiley and Sons, Inc.,

1985.

**Economic Report of the President (1998).

THE COST OF CORROSION

© E

HS

tock/iS

tockphoto

24

ELECTROCHEMICAL CORROSION

For metallic materials, corrosion process is normally electrochemical Transfer of electrons from one species to the other

Metals give up electrons – Oxidation reaction

Electrons may be taken up by hydrogen to form hydrogen gas – Reduction reaction

25

• Two reactions are necessary: -- oxidation reaction:

-- reduction reaction:

• Other reduction reactions in solutions with dissolved oxygen:

-- acidic solution -- neutral or basic solution

Adapted from Fig. 17.1,

Callister & Rethwisch 9e. (From M. G. Fontana, Corrosion

Engineering, 3rd edition. Copyright

© 1986 by McGraw-Hill Book

Company. Reproduced with

permission.)

ELECTROCHEMICAL CORROSION

Zinc

Oxidation reaction Zn Zn 2+

2e - Acid solution

reduction reaction

H + H +

H 2 (gas)

H +

H +

H +

H +

H +

flow of e- in the metal

Ex: consider the corrosion of zinc in an acid solution

26

ELECTRODE POTENTIALS

Not all metals oxidize to form ions with the same degree of ease, e.g. electrochemical cell

Reduction (deposition) occurs for copper at the expense of oxidation (corrosion or dissolving) of iron

27

ELECTRODE POTENTIALS

Electron motion results in electric current

28

ELECTRODE POTENTIALS

If copper electrode is replaced by zinc electrode, reaction is reversed Zinc corrodes while iron deposits

29

ELECTRODE POTENTIALS

Behavior of electrochemical cell and associated corrosion/deposition reactions are dependent on nature of metals Relative properties of metals used for electrodes

determines direction of flow of electrons (electric current)

Concept of standard electrode potential

Measured w.r.t. an inert reference metal (Pt) electrode

30

STANDARD HYDROGEN ELECTRODE

• Two outcomes:

0o

metal <V (relative to Pt)

Standard Electrode Potential Adapted from Fig. 17.2,

Callister & Rethwisch 9e.

-- Corrosion

-- Metal is the anode (-)

Pla

tinum

meta

l, M

M n+ ions

ne - H2(gas)

25°C 1M M n+ sol’n 1M H + sol’n

2e -

e - e -

H +

H +

-- Electrodeposition

-- Metal is the cathode (+)

M n+ ions

ne -

e - e -

25°C 1M M n+ sol’n 1M H + sol’n

Pla

tinum

meta

l, M

H +

H + 2e -

0o

metal >V (relative to Pt)

H2(gas)

31

STANDARD EMF SERIES

metal o

• Metal with smaller

V corrodes. • EMF series

Au

Cu

Pb

Sn

Ni

Co

Cd

Fe

Cr

Zn

Al

Mg

Na

K

+1.420 V

+0.340

- 0.126

- 0.136

- 0.250

- 0.277

- 0.403

- 0.440

- 0.744

- 0.763

- 1.662

- 2.363

- 2.714

- 2.924

metal V metal o

Data based on Table 17.1,

Callister 9e.

mo

re a

nodic

m

ore

cath

odic

DV =

0.153V

o

Adapted from Fig. 17.2,

Callister & Rethwisch 9e.

-

1.0 M

Ni 2+ solution

1.0 M

Cd 2 + solution

+

25°C Ni Cd

Cd o

Ni o

• Ex: Cd-Ni cell

V < V Cd corrodes

32

STANDARD EMF SERIES

33

GENERALIZED REACTIONS

-- Potential Difference:

34

CORROSION IN A GRAPEFRUIT

Zn 2+

2e - oxidation reaction

Acid

H + H +

H +

H +

H +

H +

H + - +

Zn (anode) Cu (cathode)

reduction reactions

35

EFFECT OF SOLUTION CONCENTRATION AND

TEMPERATURE

• Ex: Cd-Ni cell with

standard 1 M solutions

-

Ni

1.0 M

Ni 2+ solution

1.0 M

Cd 2 + solution

+

Cd 25°C

• Ex: Cd-Ni cell with

non-standard solutions

n = #e- per unit

oxid/red

reaction

(= 2 here) F =

Faraday's

constant

= 96,500

C/mol. • Reduce VNi - VCd by

-- increasing X

-- decreasing Y

-- increasing T

- +

Ni

Y M

Ni 2+ solution

X M

Cd 2 + solution

Cd T

36

EFFECT OF SOLUTION CONCENTRATION AND

TEMPERATURE

• R: Gas constant

• T: absolute temperature

• n: number of electrons participating in

either of the half cell reactions

• F: Faraday’s constant

37

EFFECT OF SOLUTION CONCENTRATION AND

TEMPERATURE

• R: Gas constant

• T: absolute temperature

• n: number of electrons participating in

either of the half cell reactions

• F: Faraday’s constant

38

EXAMPLE

39

EXAMPLE