CHAPTER

10

Polymeric Materials

10-1

Introduction to Polymers

• Polymers many parts

Polymers

Plastics

Thermoplastics Thermosetting Plastics

Elastomers

Can be reheated and

formedinto new materials

Cannot be reformed by reheating.

Set by chemical reaction.

10-2

Plastics - Advantages

• Wide range of properties.

• Minimum finishing.

• Minimum lubrication.

• Good insulation.

• Light weight.

• Noise Reduction.

RemoteControl

Wafer bandsAir intake manifold

Figure 7.1

Royalty-Free/CORBIS. (b) Charles O’Rear/ CORBIS. (c) Tom pantages 10-3

Polymerization

• Chain growth polymerization: Small molecules covalently bond to form long chains (monomers) which in turn bond to form polymers.

• Example: Ethylene

• Functionality: Number of active bonds in a monomer.

H H

C C

H H

n

H H

C C

H H

Heat

PressureCatalyst

n

n=degree of Polymerization (DP).(range: 3500-25000

DP =Molecular mass of polymer(g/mol)

Mass of a mer (g/mer)

10-4

Chain Polymerization - Steps

• Initiation: A Radical is needed. Example H2O2

• One of free radicals react with ethylene molecule to form new longer chain free radical.

In General

10-5

Chain Polymerization – Steps (cont..)

• Propagation: Process of extending polymer chain by addition of monomers.

• Energy of system is lowered by polymerization.

• Termination:- By addition of termination free radical. Combining of two chains Impurities.

R CH2 CH2 + CH2 CH2 R CH2 CH2 CH2 CH2

R(CH2 CH2)m + R’(CH2 CH2)n R(CH2 CH2)m R (CH2 CH2)n R’

Coupling of two chains

10-6

Average Molecular Weight

• Average molecular weight determined by special physical and chemical techniques.

• Example:

i

iim f

MfM

mM = average molecular weight of

thermoplastics.Mi = Mean molecular weight of each molecular range selected. fi = Weight fraction of the material havingMolecular weights of a selected molecularWeight range.

mM= 19,550

1

= 19,550 g/mol

10-7

Structure of Noncrystalline Linear Polymers

• Zig-Zag configuration in ethylene due to 109 degree angle between carbon covalent bonds.

• Chains are randomly entangled.

• Entanglement increases tensile strength.

• Branching decreases tensile strength.

Figure 7.4

Figure 7.5

After W. G. Moffatt, G. W. Pearsall, and J. Wulff, “The structure and Properties of Materials,” vol I: “Structure,” Wiley, 1965, p.65,10410-8

Vinyl and Vinylidene Polymers

• Vinyl polymers: One of the hydrogen atom is replaced by another atom or group of atoms.

• Vinylidene Polymers: Both hydrogen of carbon are replaced by another atom or group of atoms.

Figure 7.6

Figure 7.7

10-9

Homopolymer and Copolymers

• Homopolymers: Polymer chain is made up of single repeating units.

Example: AAAAAAAA

• Copolymers: Polymer chains made up of two or more repeating units. Random copolymers: Different monomers randomly

arranged in chains. Eg:- ABBABABBAAAAABA Alternating copolymers: Definite ordered alterations of

monomers. Eg:- ABABABABABAB Block copolymers: Different monomers arranged in long

blocks. Eg:- AAAAA…….BBBBBBBB…… Graft copolymers: One type of monomer grafted to long chain

of another. Eg: AAAAAAAAAAAAAAAAAAABBB

BBB

10-10

Other Methods of Polymerization

• Stepwise Polymerization:

Monomers chemically

react with each other to

produce linear polymers

and a small molecule of

byproduct.

• Network polymerization:

Chemical reaction takes

place in more than two

reaction sites

(3D network).

Figure 7.10

Figure 7.11

10-11

Industrial Polymerization

Raw Materials:Natural gas, Petroleum and coal

Granules, pellets, powders or liquids.Polymerizatio

n•Bulk polymerization :

Monomer and activator

mixed in a reactor and

heated and cooled as desired

• Solution polymerization: Monomer

dissolved in non-reactive solvent

and catalyst.

• Suspension polymerization: monomer

and catalyst suspended in water.

• Emulsion polymerization: Monomer

and catalyst suspended in water along with emulsifier.

After W. E. Driver, “Plastics Chemistry and Technology,” Van Nostrand Reinhold, 1979, p.19

Figure 7.12

10-12

Solidification of Thermoplastics.

• There is no sudden change in specific volume on cooling in noncrystalline thermoplastics.

• In crystalline thermoplastics, sudden decrease in specific volume occurs due to more efficient packing of polymer chains.

Tg = glass transition temperature.

Tg

abovebelowGlassbrittle

Rubbery

Tg for polyethylene is –1100CFor PVC it is 820C

Figure 7.14

10-13

Structure of Partly Crystalline Thermoplastics

• Longest dimension of crystalline region is 5-50 nm.

• Fringed micelle model: Long polymer chains of 5000 nm wandering successively through a series of disordered and ordered region.

• Folded chain model: sections of molecular chains folding on themselves.

Figure 7.16 Figure 7.17

Polyethylene-folded chain

After F. Rodriguez, “priciples of Polymer Systems,” 2nd ed., McGraw-Hill, 1982,p.42After R. L. Boysen, Olefin Polymers, in “Encyclopedia of Chemical Technology,” vol. 16, Wiley, 1981, p.405.

10-14

Stereoisomerism in Thermoplastics

• Stereoisomer:- Same chemical composition but different structural arrangements. Atactic stereoisomer:- Pendent methyl

group of polypropylene is randomly

arranged on either side of main carbon

chain. Isotactic stereoisomer:- The pendent

methyl group is always on same side

of the carbon chain. Syndiotactic stereoisomer:- The

pendant group regularly alternates

from one side of the chain to the

other side.

Figure 7.19

After G. Crepsi and L. Luciani, in “Encyclopedia of Chemical Technology,” vol. 16, Wiley, 1982, p.454.10-15

Processing of Plastic Materials

• Injection Molding: uses

reciprocating screw

mechanism.

• More uniform delivery

of melt for injection.

• High quality, low labor

cost, but high initial cost.

Figure 7.21

Figure 7.22

After J. Brown, “ Injection Molding of Plastic Components,” McGraw-Hill, 1979, p.28.10-16

Extrusion, Blow molding and Thermo Forming

• Extrusion: Melted plastic forced by a rotating screw through a opening and used to produce pipes, rods etc.

• Blow molding: Compressed air is blown into heated cylinder or pipe of plastic to press it against the wall of mold.

• Thermoforming: heated plastic sheet is forced into contours of a mold by pressure.

Figure 7.23

After H. S. Kauffman and J. J. Falcetta(eds.), “Introductin to Polymer Science and Technology” Wiley, 1977, p.462.10-17

Processes for Thermosetting

• Compression molding: Pressure is applied on heated plastic by upper mold and the molten plastic fills the cavities. Low initial cost, simple. Less wear and abrasion

of molds. Difficult to mold complex

parts Creates flash (spills).

Figure 7.25

After B. B. Seymour, Plastics Technology, in “ Encyclopedia of Chemical Technology,” vol. 15, Wiley, 1968, p.802.10-18

Transfer Molding

• A plunger forces plastic resin, placed outside mold, into mold cavities through runners and gate.

No flash formed. Multiple parts at a

time. Can be used for small

and intricate parts.

• Injection molding is also used to process thermosetting plastics.

• Special heating-cooling jackets are added to standard injection molding machine.

Figure 7.26

Courtesy of Plastics Engineering Co., Sheboygan, Wisc.10-19

General Purpose Thermoplastics

• Polyethylene, polyvinyl chloride (PVC) polypropylene and polyesters account for most plastic materials sold.

Materials Engineering, May 1972

Table 7.210-20

Polyethylene• Clear to whitish translucent thermoplastic. • Types

Low density

High Density

Linear low density

• Applications: containers, insulation, chemical tubing, bottles, water pond liners etc.

Figure 7.28Table 7.3

10-21

Polyvinyl Chloride and Copolymers

• PVC is amorphous, does not recrystallize.

• Chlorine atoms produce large dipole moments and also hinder electrostatic repulsion.

• PVC homopolymer has high strength (7.5 to 9 KSI) and is brittle.

• Compounding of PVC: Modifies and improves properties. Plasticizers: Impart flexibility. Eg – Phthalate. Heat Stabilizers: Prevent thermal degradation. Eg – lead and

tin compounds. Lubricants: Aid in melt flow of PVC. Eg – Waxes and fatty

esters. Fillers: Lower the cost. Eg – Calcium Carbonate. Pigments : Give color.

10-22

Polypropylene

• Low density, good chemical resistance, moisture resistance and heat resistance.

• Good surface hardness and dimensional stability.

• Applications: Housewares, appliances, packaging,

laboratory ware, bottles, etc.

H H

C C

H CH3n

• Methyl group substitute every other carbon atom in carbon polymer chain.• High melting (165-1770Cand heat deflection temperature.

10-23

Polystyrene

• Applications: Automobile interior parts, dials and knobs of appliances and housewares.

H H

C C

H n

• Phenyl ring present on every other carbon atom.• Very inflexible, rigid, clear and brittle.• Low processing cost and good dimensional stability.• Poor weatherability and easily attacked by chemicals.

10-24

Polyacrylonitrile and Styrene-Acrylonitrile (SAN)

Polyacrylonitrile

• High strength.

• Good resistance to

moisture and solvents.

• Applications: sweaters and blankets. Commoner for SAN and ABS resins.

SAN• Random amorphous

copolymer of styrene and acrylonitrile.

• Better chemical resistance, high heat deflection temperature, toughness and load bearing characteristics than polyester alone.

• Applications: Automotive instrument lenses, dash components, knobs, blender and mixer bowls.

H H

C C

H C N n

Does not Melt.

10-25

ABS

• ABS = Acrylonitrile + Butadiene + Styrene (Three monomers).

• Applications: Pipe and fittings, automotive parts, computer and telephone housings etc.

Figure 7.31

Table 7.4

After G. E Teer, ABS and Related Multipolymers, in Modern Plastics Encyclopedia,” McGraw-Hill, 1981- 1982.10-26

Polymethyl Methacrylate (PMMA)

• An acrylic commonly known as Plexiglas.

• Applications: Glazing of aircraft, boats, skylights, advertising signs etc.

H CH3

C C

H CCH3

O

n

• Rigid and relatively strong.• Completely amorphous and very transparent.

10-27

Fluoroplastics

• Monomers have one or more atoms of fluorine.

• Polytetrafluoroethylene(PTFE):

• Applications: Chemically resistant pipe, parts, molded electrical components, nonstick coating etc.

F F

C C

F F n

Melting Point1700C

• Exceptionally resistant to chemicals.• Useful mechanical properties at a wide temperature range.• High impact strength but low tensile strength.• Good wear and creep resistance.

10-28

Polychlorotrifluroethylene (PCTFE)

Applications: Gaskets, chemical processing equipments, seals and electric components.

F F

C C

F Cl n

MeltingPoint2180C

• Chlorine atom substitutes for every fourth fluorine atom.•Can be extruded and mold easily.

10-29

Engineering Thermoplastics

• Low density, low tensile strength.

• High insulation, good corrosion resistance.

Table 7.5

10-30

Polyamides (Nylons)

• Main chain structure incorporates repeating amide group.

• Processed by injection molding.

• Examples:

O H

C N

Amide linkage

10-31

Properties of Nylon

• High strength due to hydrogen bonding between molecular chain.

• Flexibility of carbon chain contributes to molecular flexibility, low melt viscosity and high lubricity.

• Applications: Electrical equipments, gears, auto parts, packaging etc.

Figure 7.35

After M. I. Kohan(ed.), “Nylon Plastics,” Wiley, 1983, p.274 10-32

Polycarbonate

• Applications: Precision parts, cams, gears, helmets, power tool housings and computer terminals.

• High strength, toughness and dimensional stability. • Very high impact strength.• high heat deflection temperature.• Resistance to corrosion.

10-33

Phenyl Oxide Based Resins

• Produced by oxidative coupling of phenolic monomers.

• Applications: Electric connectors, TV tuners, small

machine housing, dashboards and grills.

• High rigidity, strength, chemical resistance, dimensional stability and heat deflection temperature.

• Wide temperature range, low creep

• High modulus.

10-34

Acetals

• Strongest (68.9 Mpa) and stiffest (2820 Mpa) thermoplastics.

• Homopolymer is harder and rigid than copolymer.

• Low wear and friction but flammable.

• Applications: Fuel systems, seat belts, window handles of automobiles, couplings, impellers, gears and housing.

H

C O

H n

Polyoxymethylenemp: 1750C

2 Types• Homopolymers• Copolymers

• Excellent long term load carrying capacity and dimensional stability.

10-35

Thermoplastic Polyesters

Good insulator: independent of temperature and humidity.

• Applications: Switches, relays, TV tuner components, circuit boards, impellers, housing and handles.

• Phenylene ring provides rigidity.

• Good strength and resistant to most chemicals.

10-36

Polysulfone and Polyphenylene Sulfide.• Polysulfone: Phenylene ring provides high strength

and rigidity.

• Applications: Electrical connectors, cores, circuit boards, pollution control equipments.

• Polyphenylene Sulfide:- • Rigid and strong.• Highly crystalline.

• No chemical can dissolve it below 2000C.• Applications: Chemical process equipment, emission

control equipment, electrical connectors.

• Can be used for long time at high temperature.

S

n

Mp: 2880C

10-37

Polyetherimide and Polymer Alloys

• Polyetherimide:

• High heat and creep resistance and rigidity.• Good electric insulation. • Applications: High voltage circuit breaker housing, coils etc.• Polymer alloys: Mixture of structurally different

homopolymers or copolymers optimizes properties.• Some degree of compatibility needed.• Example:- Bayblend MC2500 (ABS/Polycarbonate)

10-38

Thermosetting Plastics

• High thermal and dimensional stability, rigidity, resistance to creep, light weight.

Table 7.7

Source: Materials Engineering, May 1972. 10-39

Phenolics

• Low cost, good insulating and mechanical properties.

• Produced by polymerization of phenol and formaldehyde.

• General purpose compounds: Usually wood flour filled to increase impact resistance.

• High impact strength compounds: Filled with cellulose and glass fibers.

• High electrical insulating compounds: Mineral (Mica) filled.

• Heat resistant compounds: Mineral filled.

• Applications: Wiring devices, auto transmission parts, plywood lamination, adhesives, shell molding.

10-40

Epoxy Resins

• Good adhesion, chemical resistance and mechanical properties.

• High molecular mobility, low shrinkage during hardening.

• Applications: Protective and decorative coating, drum lining, high voltage insulators and laminates.

OCH2 C

H

Epoxidegroup

10-41

Unsaturated Polyesters

• Low viscosity and can be reinforced with low viscosity materials.

• Open mold lay up or spray up techniques are used to process many small parts.

• Compression molding is used for big parts. • Applications: Automobile panels and body parts, boat

hulls, pipes, tanks etc.

• Have reactive double Carbon-Carbon covalent bonds.

10-42

Amino Resins (Ureas and Melamines)

• Formed by reaction of formaldehydes with compounds having –NH2 group.

• Combined with cellulose fillers to produce low cost products with good mechanical properties.

• Applications: Electrical wall plates, molded dinnerware, buttons, control buttons, knobs, flooring etc.

10-43

Elastomers (Rubbers)

• Natural rubber: Produced from latex of Havea Brasiliensis tree.

• Vulcanization: Heating rubber with sulfur and lead carbonate.

• Increases tensile strength.

• Restricts molecular movement

by crosslinking of molecules.

H CH3 H H C C C C H H

n

Figure 7.41

10-44

Natural Rubber - Properties

Table 7.8

Figure 7.43

After M. Eisenstadt, “Introduction to Mechanical properties of Materials,” Macmillan, 1971, p.89.10-45

Synthetic Rubbers

• Styrene-Butadiene rubber (SBR): Most widely used.• Greater elasticity than natural rubbers.• Tougher and stronger, war resistant.• Absorbs organic solvents and swell.• Nitrile Rubbers: 55-82% Butadiene and 45-18%

acrylonitrile.• Resistance to solvents and wear. Less flexible.• Polychloroprene: Increased resistance to oxygen, ozone, heat and weather.• Low temperature flexibility, high cost.

H Cl H H

C C C C

H H n

Figure 7.44

10-46

Vulcanization of Polychloroprene Elastomers

• Silicone Rubbers:

• Wide temperature

range.

• Used in gaskets,

electric insulation etc.

2ZnCl2 + MgO OH2Zn + MgCl Cl

H2O

X

Si O

X

CH3

Si O

CH3n n

Example

10-47

Deformation of Thermoplastics

• Below Tg Elastic deformation. Above Tg Plastic deformation.

Elastic deformation

Elastic or plastic deformation

Plastic deformation

Figure 7.45

Figure 7.46

After T. Alfrey, “mechanical Behavior of Polymers,” Wiley-Interscience, 1967.After M. Eisenstadt, “Introduction to Mechanical properties of Materials,” Macmillan, 1971,p.264.

10-48

Strengthening of Thermoplastics

• Increasing average molecular mass increases strength upto a certain critical mass.

• Degree of crystallinity increases strength, modulus of elasticity and density.

• Chain slippage during permanent deformation can be hindered by introduction of pendant atomic groups to main carbon chain.

• Strength can be increased by bonding highly polar atoms on the main carbon chain.

10-49

Strengthening of Thermoplastics (Cont..)

• Strength can be increased by introduction of oxygen and nitrogen atoms into main carbon chain.

• Introduction of phenylene

ring into main polymer

chain with other elements

increases strength.

• Adding plastic fibers

increases the strength.

• Thermosetting plastics can be strengthened by reinforcements and creation of covalent bonds by chemical reaction during setting.

Figure 7.49

After J. A. Sauer and K. D. Pae, in “Introductin to Polymer Science and Technology,” Wiley, 1977, p.331.10-50

Effects of Temperature on Strength

• Thermoplastics soften as temperature increases.

• Strength dramatically decreases after Tg.

• Thermosets also become weaker but not viscous.

• Thermosets are more stable at high temperature than thermoplastics.

Figure 7.50

After H. E Barker and A. E.Javitz, Plastic Modeling Materials for structural and Mechanical Applications, Electr. Amnuf., May 1960.10-51

Creep and Stress Relaxation of Polymers

• Creep increases with increased tensile stress and temperature.

• Creep is low below Tg. Above Tg, the behavior is viscoelastic.

• Glass fiber reinforcements decreases creep.

• Stress relaxation: Decrease in stress at constant strain.

• Due to breaking and formation of secondary bonds.

RT

Q

t

Ce

e

1

0

σ = Stress after time t. = Initial stress

τ = relaxation time.T= temperature, R= molar gas constant.

0

10-52

Fracture of Polymers

• Thermosetting plastics Primarily brittle mode.

• Thermoplastics ductile or brittle depending on the temperature.

Figure 7.52

Figure 7.55

Figure 7.53

After P. beahan, M bevis, and D. Hull, J. mater. Sci., 8: 162(1972).10-53

Biopolymers

• Polymers are used in biomedical applications Cardiovascular, Opthalmic and Orthopaedic

implants Dental implants, dental cements and denture bases

• Low density, easily formed

and can be made biocompatible.

• Recent development – biodegradable polymers.

Cardiovascular Applications

• Heart valves can be stenotic or incompetent

• Polymers are used to make artificial heart valves

• Leaflets are made from biometals

• Sewing ring made from PTFE or

PETConnected to heart tissue

• Blood clogging is side effect

• PTFE is used as vascular graft to bypass clogged arteries.

• Blood oxygenators : Hydrophobic polymer membranes used to oxygenate blood during bypass surgery

Air flows on one side and blood on the other side and oxygen diffuses into blood.

Opthalmic Applications

• Eye glasses, contact lenses and Intraocular implants are made of polymers.

• Hydrogel is used to make soft contact lenses Absorbs water and allows snug fit Oxygen permeable Made of poly-HEMA

• Hard lenses made from PMMA Not oxygen permeable Mixed with Siloxanylalkyl Metacrylate and metacrylic acid to make permeable and hydrophilic.

• Intraocular implants are made of PMMA

Orthopaedic Applications

• Bone cement: Fills space between implant and bone – PMMA

Centrifuging and vacuum techniques minimize porosity

• Used in joint prosthesis (Knee and Hip replacements)

• Other applications: Drug delivery systems: Polymer matrix with

drug implanted inside the body Struture materials: High tensile and knot pull

strength. Non-absorbable: Polypropylene, Nylon Absorbable : Polyglycolic acid.

Future – Tissue Engineering

• Polymers can be synthesized and blend to suite the applications

• Biodegradable polymers are used as scaffolding for generation of new tissues

• In future, tissues can be generated in vivo or in vitro for repair or replacement.