Foundations of Materials Science and Engineering Third Edition
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Transcript Foundations of Materials Science and Engineering Third Edition
CHAPTER
10
Polymeric Materials
10-1
Introduction to Polymers
•
Polymers
many parts
Polymers
Plastics
Thermoplastics
Can be
reheated and
formed
into new
materials
10-2
Elastomers
Thermosetting Plastics
Cannot be reformed
by reheating.
Set by chemical reaction.
Plastics - Advantages
•
Wide range of
properties.
• Minimum finishing.
• Minimum lubrication.
Remote
Control
• Good insulation.
• Light weight.
• Noise Reduction.
Wafer bands
Air intake manifold
Figure 7.1
10-3
Polymerization
•
Chain growth polymerization: Small molecules
covalently bond to form long chains (monomers) which
in turn bond to form polymers.
• Example: Ethylene
H H
H H
n
Heat
C
C
H H
•
10-4
n=degree of
Polymerization (DP).
(range: 3500-25000
Pressure
Catalyst
C
C
H H n
Molecular mass of polymer(g/mol)
Mass of a mer (g/mer)
DP =
Functionality: Number of active bonds in a monomer.
Chain Polymerization - Steps
•
Initiation:
A Radical is needed.
Example H2O2
In General
• One of free radicals react with ethylene molecule to
form new longer chain free radical.
10-5
Chain Polymerization – Steps (cont..)
•
Propagation: Process of extending polymer chain by
addition of monomers.
R
•
•
CH2 CH2 + CH2 CH2
R CH2 CH2 CH2 CH2
Energy of system is lowered by polymerization.
Termination: By addition of termination free radical.
Combining of two chains
Impurities.
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.
Mm
•
fi M i
fi
Example:
M m = average molecular weight of
thermoplastics.
Mi = Mean molecular weight of each
molecular range selected.
fi = Weight fraction of the material having
Molecular weights of a selected molecular
Weight 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.
Figure 7.4
• Entanglement increases tensile strength.
• Branching decreases tensile strength.
10-8
Figure 7.5
Vinyl and Vinylidene Polymers
•
Vinyl polymers: One of the hydrogen atom is replaced
by another atom or group of atoms.
Figure 7.6
• Vinylidene Polymers: Both hydrogen of carbon are
replaced by another atom or group of atoms.
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.
10-10
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: AAAAAAAAAAAAAAAAAAA
B
B
B
B
B
B
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).
10-11
Figure 7.10
Figure 7.11
Industrial Polymerization
Raw Materials:
Natural gas, Petroleum
and coal
Granules, pellets,
Polymerization powders or liquids.
•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.
10-12
Figure 7.12
After W. E. Driver, “Plastics Chemistry and Technology,” Van Nostrand Reinhold, 1979, p.19
Solidification of Thermoplastics.
•
There is no sudden change in specific volume on
cooling in noncrystalline thermoplastics.
Tg = glass transition temperature.
above
Glass below
Rubbery
Tg
brittle
Tg for polyethylene is –1100C
For PVC it is 820C
Figure 7.14
•
10-13
In crystalline thermoplastics, sudden decrease in
specific volume occurs due to more efficient packing of
polymer chains.
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.
Polyethylene-folded chain
Figure 7.16
10-14
Figure 7.17
After F. Rodriguez, “priciples of Polymer Systems,” 2nd ed., McGraw-Hill, 1982,p.42
After R. L. Boysen, Olefin Polymers, in “Encyclopedia of Chemical Technology,” vol. 16, Wiley, 1981, p.405.
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
10-15 After G. Crepsi and L. Luciani, in “Encyclopedia of Chemical Technology,” vol. 16, Wiley, 1982, p.454.
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 7.22
Figure 7.21
10-16
After J. Brown, “ Injection Molding of Plastic Components,” McGraw-Hill, 1979, p.28.
Extrusion, Blow molding and Thermo Forming
•
Extrusion: Melted plastic forced by a rotating screw
through a opening and used to produce pipes, rods etc.
Figure 7.23
•
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.
10-17 After H. S. Kauffman and J. J. Falcetta(eds.), “Introductin to Polymer Science and Technology”
Wiley, 1977, p.462.
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
10-18
After B. B. Seymour, Plastics Technology, in “ Encyclopedia of Chemical Technology,” vol. 15, Wiley, 1968, p.802.
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.
Figure 7.26
•
Injection molding is also used to process thermosetting
plastics.
• Special heating-cooling jackets are added to standard
injection molding machine.
10-19 Courtesy of Plastics Engineering Co., Sheboygan, Wisc.
General Purpose Thermoplastics
•
Polyethylene, polyvinyl chloride (PVC) polypropylene
and polyesters account for most plastic materials sold.
Table 7.2
10-20
Materials Engineering, May 1972
Polyethylene
•
•
Clear to whitish translucent thermoplastic.
Types
Low density
High Density
Linear low density
Table 7.3
Figure 7.28
• Applications: containers, insulation, chemical tubing,
bottles, water pond liners etc.
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
H H
C
C
• Methyl group substitute every other carbon
atom in carbon polymer chain.
• High melting (165-1770Cand heat deflection
temperature.
H CH3 n
•
Low density, good chemical resistance, moisture
resistance and heat resistance.
• Good surface hardness and dimensional stability.
• Applications: Housewares, appliances, packaging,
laboratory ware, bottles, etc.
10-23
Polystyrene
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.
• Applications: Automobile interior parts, dials and
knobs of appliances and housewares.
10-24
Polyacrylonitrile and Styrene-Acrylonitrile (SAN)
Polyacrylonitrile
•
H H
C
C
Does not
Melt.
H C N n
• High strength.
• Good resistance to
moisture and solvents.
• Applications: sweaters
and blankets.
Commoner for SAN and
ABS resins.
10-25
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.
ABS
• ABS = Acrylonitrile + Butadiene + Styrene (Three
monomers).
Table 7.4
• Applications: Pipe and fittings, automotive parts,
computer and telephone housings etc.
Figure 7.31
10-26After G. E Teer, ABS and Related Multipolymers, in Modern Plastics Encyclopedia,” McGraw-Hill, 1981- 1982.
Polymethyl Methacrylate (PMMA)
• An acrylic commonly known as Plexiglas.
H CH3
C
C
O
H C
• Rigid and relatively strong.
• Completely amorphous and
very transparent.
CH3 n
• Applications: Glazing of aircraft, boats, skylights,
advertising signs etc.
10-27
Fluoroplastics
•
•
Monomers have one or more atoms of fluorine.
Polytetrafluoroethylene(PTFE):
F F
• Exceptionally resistant to
Melting
chemicals.
Point
C C
• Useful mechanical properties
0
170 C
at a wide temperature range.
F F n
• High impact strength but low
tensile strength.
• Good wear and creep resistance.
• Applications: Chemically resistant pipe, parts, molded
electrical components, nonstick coating etc.
10-28
Polychlorotrifluroethylene (PCTFE)
F
F
C
C
F
Cl n
Melting
Point
2180C
• Chlorine atom substitutes
for every fourth fluorine atom.
•Can be extruded and mold
easily.
Applications: Gaskets, chemical processing equipments,
seals and electric components.
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
O H
group.
Amide linkage
C
N
• Processed by injection molding.
• Examples:
10-31
Properties of Nylon
•
High strength due to hydrogen bonding between
molecular chain.
Figure 7.35
•
Flexibility of carbon chain contributes to molecular
flexibility, low melt viscosity and high lubricity.
• Applications: Electrical equipments, gears, auto parts,
packaging etc.
10-32
Polycarbonate
• High strength, toughness and
dimensional stability.
• Very high impact strength.
• high heat deflection
temperature.
• Resistance to corrosion.
• Applications: Precision parts, cams, gears, helmets,
power tool housings and computer terminals.
10-33
Phenyl Oxide Based Resins
•
Produced by oxidative coupling of phenolic monomers.
• High rigidity, strength, chemical resistance, dimensional
stability and heat deflection temperature.
• Wide temperature range, low creep
• High modulus.
• Applications: Electric connectors, TV tuners, small
machine housing, dashboards and grills.
10-34
Acetals
•
Strongest (68.9 Mpa) and stiffest (2820 Mpa)
thermoplastics.
2 Types
H
• Homopolymers
Polyoxymethylene
• Copolymers
mp: 1750C
C O
• Excellent long term load carrying capacity
n
and dimensional stability.
• 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
10-35
Thermoplastic Polyesters
• Phenylene ring provides rigidity.
• Good strength and resistant to most chemicals.
Good insulator: independent of temperature and
humidity.
• Applications: Switches, relays, TV tuner components,
circuit boards, impellers, housing and handles.
10-36
Polysulfone and Polyphenylene Sulfide.
•
Polysulfone: Phenylene ring provides high strength
and rigidity.
• Can be used for long time
at high temperature.
• Applications: Electrical connectors, cores, circuit
boards, pollution control equipments.
• Polyphenylene Sulfide:Mp: 2880C
• Rigid and strong.
S
• Highly crystalline.
n
• No chemical can dissolve it below 2000C.
• Applications: Chemical process equipment, emission
control equipment, electrical connectors.
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
10-39
Phenolics
•
•
•
•
•
•
•
10-40
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.
Epoxy Resins
•
Good adhesion, chemical resistance and mechanical
properties.
O
Epoxide
CH2
C group
H
•
High molecular mobility, low shrinkage during
hardening.
• Applications: Protective and decorative coating, drum
lining, high voltage insulators and laminates.
10-41
Unsaturated Polyesters
• Have reactive double
Carbon-Carbon covalent
bonds.
•
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.
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.
H
C
H
CH3
C
H
C
H
C
H
n
• Vulcanization: Heating rubber with sulfur and lead
carbonate.
• Increases tensile strength.
• Restricts molecular movement
by crosslinking of molecules.
Figure 7.41
10-44
Natural Rubber - Properties
Table 7.8
Figure 7.43
10-45
After M. Eisenstadt, “Introduction to Mechanical properties of Materials,” Macmillan, 1971, p.89.
Synthetic Rubbers
•
•
•
•
•
•
•
•
10-46
Styrene-Butadiene rubber (SBR): Most widely used.
Greater elasticity than natural
rubbers.
Tougher and stronger, war
Figure 7.44
resistant.
Absorbs organic solvents and swell.
Nitrile Rubbers: 55-82% Butadiene and 45-18%
acrylonitrile.
Resistance to solvents
H Cl H H
and wear. Less flexible.
Polychloroprene: Increased resistance C C C C
to oxygen, ozone, heat and weather.
Low temperature flexibility, high cost. H
Hn
Vulcanization of Polychloroprene Elastomers
2ZnCl2 + MgO
OH
H2O
2Zn
+ MgCl
Cl
•
•
X
Silicone Rubbers:
Wide temperature
Si
range.
• Used in gaskets,
X
electric insulation etc.
10-47
CH3
Example
O
Si
n
CH3
O
n
Deformation of Thermoplastics
•
Below Tg
Elastic deformation. Above Tg
Plastic deformation.
Elastic deformation
Elastic or plastic deformation
Plastic deformation
Figure 7.46
Figure 7.45
10-48
After T. Alfrey, “mechanical Behavior of Polymers,” Wiley-Interscience, 1967.
After M. Eisenstadt, “Introduction to Mechanical properties of Materials,” Macmillan, 1971,p.264.
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.
Figure 7.49
• Thermosetting plastics can be strengthened by
reinforcements and creation of covalent bonds by
chemical reaction during setting.
10-50After J. A. Sauer and K. D. Pae, in “Introductin to Polymer Science and Technology,” Wiley, 1977, p.331.
Effects of Temperature on Strength
•
•
Thermoplastics soften as temperature increases.
Strength dramatically decreases after Tg.
Figure 7.50
• Thermosets also become weaker but not viscous.
• Thermosets are more stable at high temperature than
thermoplastics.
10-51
After H. E Barker and A. E.Javitz, Plastic Modeling Materials for structural and Mechanical Applications, Electr. Amnuf., May 1960.
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.
t
σ = Stress after time t.
0 = Initial stress
e
0
1
10-52
Ce
Q
RT
τ = relaxation time.
T= temperature, R= molar gas constant.
Fracture of Polymers
•
Thermosetting plastics
Primarily brittle mode.
• Thermoplastics
ductile or brittle depending on
the temperature.
Figure 7.52
Figure 7.53
Figure 7.55
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
PET
Connected 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.