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.