polymers-part 2 properties and production
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POLYMERS-Part 2
Properties and Production
MSE 226-Engineering Materials
Lecture-12
Molecular Structure
Direction of increasing strength
B ranched Cross-Linked Network Linear
secondary bonding
The physical properties of polymers are affected from the structure of
polymer chains
Polyethylene,
PVC, nylon Rubber Epoxies,
polyurethane
Branches lower polymer density
i.e. High density polyethylene (HDPE): linear polymer
Low density polyethylene (LDPE) : brached polymer
Wan der walls
or hydrogen
Adapted from Fig. 14.13,
Callister & Rethwisch 8e.
Polymer Crystallinity
Many bulk polymers that are crystallized from melt are semicrystalline
and forma spherulite structure. (composed of lamellar crystal and
amorhous material)
(Polyethylene, PVC, PP, nylon)
Mechanical Properties
Metals vs. Polymers
Polymers have very low elastic modulus and tensile strength
FS (fracture strength) of polymer 10% that of metals
Polymers elongate more than 1000% possible, but metals rarely
plastically elongate more about 100%.
Polymers become more softer and ductile as the rate of deformation
increases.
Mechanical properties of polymers are much sensetive to temperature
changes near room temperature.
Stress-strain behavior of polymers
brittle polymer
plastic
Elastomer (totally elastic)
Adapted from Fig. 15.1,
Callister 7e.
Mechanical Properties
Yield
point
Tensile strength
corresponds to stress
at fracture
brittle failure
plastic failure
(MPa)
e
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
Tensile Responce: Brittle&Plastic
Deformation of semicrystalline polymer (spherulitic structure)
Epoxies,
polyurethane
(Polyethylene, PVC, PP, nylon)
Results of drawing:
• increases the elastic modulus (E) in the stretching direction
• increases the tensile strength (TS) in the stretching direction
• decreases ductility (%EL)
Predeformation by Drawing
In plastic deformation region, crystalline blocks and tie chain
align in the direction of tensile axis. This process is called
DRAWING. Used for improving the mechanical properties
of polymer fibers and films. i.e. Monofilament fishline
Annealing after drawing...
• Specimen is heated to high temp., near its melting point
• The material will recrystallize to form spherulitic structure
• Reverses effects of drawing.
• Comparable to cold working in metals!
Factors Effecting the Mechanical Properties
of Semi-crystalline Polymers
1- Temperature and Strain rate
2- Molecular weight
3- Degree of crystallinity
4- Predeformation by drawing
5- Heat treating
• Decreasing temperature
-- increases E
-- increases TS
-- decreases %EL
20
4 0
6 0
8 0
0 0
0.1 0.2 0.3
4°C
20°C
40°C
60°C
(MPa)
e
Data for the semicrystalline polymer: PMMA (Plexiglas)
Temperature and Strain Rate:
Factors Effecting the Mechanical Properties
of Semi-crystalline Polymers
• Increasing strain rate...
-- same effects as decreasing T.
Molecular weight and Degree of Crystallinity
Factors Effecting the Mechanical Properties
of Semi-crystalline Polymers
TS increases with increasing molecular weight
Crystallinity affect the extent of the intermolecular secondary bonding.
Increasing crystallinity: Strength is enhanced but polymer become
brittle
For example: for polyethylene modulus increases an order of magnitude
as crystallinity is raised from 0.3 to 0.6
Predeformation by Drawing
Strength and modulus can be increased by deformation in tension
(drawing)
Molecular chains slip past one another and become highly oriented
Employed in the production of FIBERS and FILMS
Factors Effecting the Mechanical Properties
of Semi-crystalline Polymers
Annealing
Causes increase in the %crystallinity, crystallite size
For undrawn polymer annealing heat treatment causes;
1-an increase in tensile modulus and yield strength
2-reduction in ductility
SHRINK-WRAP POLYMER FILMS
PVC, PE or polyolefin plastically deformed
about 20-300% to provide aligned film. Then
film is wrapped around object to be packaged.
When heated to about 100-150oC, this
prestrecthed film shrinks to recover 80-90% of
its initial deformation, which gives tightly
strecthed film.
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.)
(MPa)
e
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
Tensile Response: Elastomer Case
Elastomer: Rubber like elasticity
Ability to deform to quite large deformations and elastically spring back to their
original shape
Cross-linked
polymer
Unstrained Case: Elastomer is amorphous and
composed of crosslinked molecular
chains (twisted, linked and coiled)
Elastic Deformation: Chains elongate in the stress
direction and uncoiling,
untwisting occurs
Release of load: Chains spring back
Tensile Response: Elastomer Case
For a polymer to be elastomeric it mustn’t easily crystallize (they are
amorphous)
Elastomer behavior seen in cross-linked polymers. Cross-linking occurs
during vulcanization process.
Vulcanization process: Sulphur atoms are added to heated polymer
Chains of sulphur atoms bond with adjacent polymer backbone chains and
crosslink them.
ε
unvulcanized
vulcanized Elastic modulus, tensile strength
and oxidation resistance
increase by vulcanization
Polymer Fracture
Thermosetting Polymers (heavily cross-linked network):
The mode of fracture is brittle
Thermoplastic Polymers
Capable of experiencing ductile to brittle transition
Both ductile and brittle modes are possible
Thermoplastics are brittle when
- temp.< Tg (glass transition temp)
- strain rate increased
- there is sharp notch
- Specimen thickness is increased
Ductile in the vicinity of Tg
fibrillar bridges microvoids crack
alligned chains
Adapted from Fig. 15.9,
Callister 7e.
Crazing Griffith cracks in metals
Crazing of Thermoplastic Polymers
- Crazes are regions of very localized plastic deformation, which lead to the
formation of small and interconnected microvoids
– spherulites plastically deform to fibrillar structure
– microvoids and fibrillar bridges form
Polymer Fracture
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 (tacticity) – affects Tm only
Adapted from Fig. 15.18,
Callister 7e.
Melting and Glass Transition Temp.
Melting and glass transition temperature determine the upper and
lower temperature limits for numerous applications, especially for
semicrystalline polymers
• 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.)
Callister, Fig. 16.9
T
Molecular weight
Tg
Tm mobile liquid
viscous liquid
rubber
tough plastic
partially crystalline solid crystalline
solid
Melting and Glass Transition Temp.
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
Polymer Additives
• Stabilizers
– Antioxidants
– UV protectants
• Lubricants
– Added to allow easier processing
– “slides” through dies easier – ex: Na stearate
• Colorants
– Dyes or pigments
• Flame Retardants
– Cl/F & B
Polymer Additives
Polymer Types
Depending on the end use polymers are groupped as;
- Plastics
- Elastomers (or rubbers)
- Fibers
- Coatings
- Adhesives
- Foams
- Films
• 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. )
Processing of Plastics: Molding
Plastics:
- Have some structural rigidity
- Usually have high degree of crystallinity
- If amorphous: use below Tg; if semicrystalline: use below Tm
- Thermoplastics or thermosets (PE, PP, PVC, epoxies, phenolics)
• Injection molding
– 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. )
Processing of Plastics: Molding
Adapted from Fig. 15.25,
Callister 7e. (Fig. 15.25 is from
Encyclopædia Britannica, 1997.)
Processing of Plastics: Extrusion
Elastomers – rubber
• Crosslinked materials
– Natural rubber
– Synthetic rubber and thermoplastic elastomers
• SBR- styrene-butadiene rubber styrene
– Silicone rubber
butadiene
Polymer Types (acc. to end use)
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
Polymer Types: Fibers
• 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
Polymer Types
Blown-Film Extrusion
Adapted from Fig. 15.26, Callister 7e.
(Fig. 15.26 is from Encyclopædia
Britannica, 1997.)
Blown Film- Extrusion
• 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.
Advanced Polymers
The Stem, femoral head, and the AC socket are made from Cobalt-chrome metal alloy or ceramic, AC
cup made from polyethylene
• General drawbacks to polymers: -- E, y, 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, y, Tapplication -- Larger Kc
-- Easier to form and recycle
• Elastomers (rubber):
-- Large reversible strains!
• Thermosets (epoxies, polyesters):
-- Larger E, y, Tapplication
-- Smaller Kc
Summary
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