Uploaded by Farzana Ramzol

Properties Processing and Applications of Polymers

advertisement
BMFG 1213
ENGINEERING MATERIALS
Sem/Session : 1/2016-2017
Week 10
Title : Characteristics, Processing
and Application of Polymer
Chapter 14 -
Outlines
• 10.1 Types of Polymers
• 10.2 Crystallization, melting and glass
transition phenomena in polymers
• 10.3 Polymer processing
• 10.4 Applications of polymers
Chapter 14 - 2
Learning Objectives
• Describe a typical polymer molecule in terms of its
chain structure and repeat units.
• Calculate number-average and weight-average
molecular weights and degree of polymerization for a
specified polymer.
• Cite the differences in physical and mechanical
characteristics of thermoplastic, thermosetting
polymers, and elastomers.
• Name and briefly describe the polymer processing
methods and its applications.
Chapter 14 - 3
ISSUES TO ADDRESS...
• What are the general structural and chemical
characteristics of polymer molecules?
• What are some of the common polymeric
materials, and how do they differ chemically?
• How is the crystalline state in polymers different
from that in metals and ceramics ?
• What are the tensile properties of polymers and how
are they affected by basic microstructural features?
• What are the primary polymer processing methods?
Original source: CALLISTER, W. D. & RETHWISCH, D. G. 2009. Materials Science and Engineering: An Introduction, 8th Edition, Wiley.
Chapter 14 - 4
What is a Polymer?
Poly
many
mer
repeat unit
repeat
unit
repeat
unit
repeat
unit
H H H H H H
C C C C C C
H H H H H H
H H H H H H
C C C C C C
H Cl H Cl H Cl
Polyethylene (PE)
Poly(vinyl chloride) (PVC)
H
C
H
H H
C C
CH3 H
H H
C C
CH3 H
H
C
CH3
Polypropylene (PP)
Adapted from Fig. 14.2, Callister & Rethwisch 8e.
Chapter 14 - 5
Ancient Polymers
• Originally natural polymers were used
– Wood
– Rubber
– Cotton
– Wool
– Leather
– Silk
• Oldest known uses
– Rubber balls used by Incas
– Noah used pitch (a natural polymer)
for the ark
Chapter 14 - 6
Molecular Structures for Polymers
secondary
bonding
Linear
Branched
Cross-Linked
Network
Adapted from Fig. 14.7, Callister & Rethwisch 8e.
Chapter 14 - 7
Classification of Polymers
• Thermoplastic
– composed of long chains produced by joining together
monomers; they typically behave in a ductile manner. The
chains may or may not have branches.
• Thermoset
– composed of long chains (linear or branched) of molecules
that are strongly cross-linked to one another to form threedimensional network structures.
• Elastomer
– known as rubbers. They have an elastic deformation of more
than 200%. These may be thermoplastics or lightly crosslinked thermosets.
Chapter 14 - 8
Polymer Composition
Most polymers are hydrocarbons
– i.e., made up of H and C
• Saturated hydrocarbons
– Each carbon singly bonded to four other atoms
– Example:
• Ethane, C2H6
H
H
C
H
H
C
H
H
Chapter 14 - 9
Chapter 14 -10
Unsaturated Hydrocarbons
• Double & triple bonds somewhat unstable –
can form new bonds
– Double bond found in ethylene or ethene - C2H4
H
H
C C
H
H
– Triple bond found in acetylene or ethyne - C2H2
H C C H
Chapter 14 - 11
Chemistry and Structure of
Polyethylene
Adapted from Fig.
14.1, Callister &
Rethwisch 8e.
Note: polyethylene is a long-chain hydrocarbon
- paraffin wax for candles is short polyethylene
Chapter 14 -12
Bulk or Commodity Polymers
Chapter 14 -13
Bulk or Commodity Polymers (cont)
Chapter 14 -14
Bulk or Commodity Polymers (cont)
Chapter 14 -15
Synthesis of Polymers
• There are two types of polymerization
– Addition (or chain) polymerization
• (sometimes called chain reaction polymerization) is a
process by which monomer units are attached one at a
time in chainlike fashion to form a linear macromolecule.
– Condensation (step) polymerization
• formation of polymers by stepwise intermolecular
chemical reactions that may involve more than one
monomer species. There is usually a small molecular
weight by-product such as water that is eliminated (or
condensed)
Chapter 14 -16
Addition (Chain) Polymerization
– Initiation
– Propagation
– Termination
Chapter 14 -17
17
Condensation (Step)
Polymerization
Chapter 14 -18
18
MOLECULAR WEIGHT
• Molecular weight, M: Mass of a mole of chains.
Low M
high M
Not all chains in a polymer are of the same length
— i.e., there is a distribution of molecular weights
Chapter 14 -19
MOLECULAR WEIGHT DISTRIBUTION
Adapted from Fig. 14.4, Callister & Rethwisch 8e.
total wt of polymer
Mn 
total # of molecules
M n  xi Mi
M w  wi Mi
Mi = mean (middle) molecular weight of size range i
xi = number fraction of chains in size range i
wi = weight fraction of chains in size range i
Chapter 14 -20
Molecular Weight Calculation
Example: average mass of a class
Student
Weight
mass (lb)
1
104
2
116
3
140
4
143
5
180
6
182
7
191
8
220
9
225
10
380
What is the average
weight of the students in
this class:
a) Based on the number
fraction of students in
each mass range?
b) Based on the weight
fraction of students in
each mass range?
Chapter 14 -21
Molecular Weight Calculation (cont.)
Solution: The first step is to sort the students into weight ranges.
Using 40 lb ranges gives the following table:
weight
range
number of
students
Ni
mass (lb)
81-120
121-160
161-200
201-240
241-280
281-320
321-360
361-400
total
number
2
2
3
2
0
0
0
1
Ni
10
mean
weight
number
Calculateweight
the number and weight
fraction
fraction
fraction of
students in each weight
Wi
xirange as follows:
wi
NiWi
Ni
mass (lb)
wi 
xi 
110
0.2
Ni
 NiWi
0.117
142
0.2
0.150
184
0.3
0.294
For example:
for the 81-120 lb range
223
0.2
0.237
 2
 0
x811200.000

 0.2
10
0
0.000
0
0.0002 x 110

 0.117
380
0.1 w 81120
0.202
1881
NiWi 
1881
total
weight
Chapter 14 -22
Molecular Weight Calculation (cont.)
weight
range
mean
weight
mass (lb)
Wi
mass (lb)
81-120
121-160
161-200
201-240
241-280
281-320
321-360
361-400
110
142
184
223
380
number
fraction
xi
weight
fraction
wi
0.2
0.2
0.3
0.2
0
0
0
0.1
0.117
0.150
0.294
0.237
0.000
0.000
0.000
0.202
Mn   xi Mi  (0.2 x 110 0.2 x 142 + 0.3 x 184 + 0.2 x 223 + 0.1 x 380) =188 lb
Mw   wi Mi  (0.117 x 110 0.150 x 142 + 0.294 x 184
Mw   wi Mi  218 lb
+ 0.237 x 223 + 0.202 x 380) = 218 lb
Chapter 14 -23
Degree of Polymerization, DP
DP = average number of repeat units per chain
H H H H H H H H H H H H
H C C (C C ) C C C C C C C C H
DP = 6
H H H H H H H H H H H H
Mn
DP 
m
where m  average molecular weight of repeat unit
for copolymers this is calculated as follows:
m  fi mi
Chain fraction
mol. wt of repeat unit i
Chapter 14 -24
Copolymers
two or more monomers
polymerized together
• random – A and B randomly
positioned along chain
• alternating – A and B
alternate in polymer chain
• block – large blocks of A
units alternate with large
blocks of B units
• graft – chains of B units
grafted onto A backbone
A–
B–
Adapted from Fig.
14.9, Callister &
Rethwisch 8e.
random
alternating
block
graft
Chapter 14 -25
Crystallinity in Polymers
Adapted from Fig.
14.10, Callister &
Rethwisch 8e.
• Ordered atomic
arrangements involving
molecular chains
• Crystal structures in terms
of unit cells
• Example shown
– polyethylene unit cell
Chapter 14 -26
Polymer Crystallinity
• Crystalline regions
– thin platelets with chain folds at faces
– Chain folded structure
Adapted from Fig.
14.12, Callister &
Rethwisch 8e.
10 nm
Chapter 14 -27
Polymer Crystallinity (cont.)
Polymers rarely 100% crystalline
• Difficult for all regions of all chains to
become aligned
crystalline
region
• Degree of crystallinity
expressed as % crystallinity.
-- Some physical properties
depend on % crystallinity.
-- Heat treating causes
crystalline regions to grow
and % crystallinity to
increase.
amorphous
region
Adapted from Fig. 14.11, Callister 6e.
(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,
and J. Wulff, The Structure and Properties of
Materials, Vol. III, Mechanical Behavior, John Wiley
and Sons, Inc., 1965.)
Chapter 14 -28
Melting & Glass Transition Temps.
What factors affect Tm and Tg?
•
•
•
Both Tm and Tg increase with
increasing chain stiffness
Chain stiffness increased by
presence of
1. Bulky sidegroups
2. Polar groups or sidegroups
3. Chain double bonds and
aromatic chain groups
Regularity of repeat unit
arrangements – affects Tm only
Adapted from Fig. 15.18,
Callister & Rethwisch 8e.
Chapter 14 -29
29
Mechanical Properties of Polymers –
Stress-Strain Behavior
brittle polymer
plastic
elastomer
elastic moduli
– less than for metals
Adapted from Fig. 15.1,
Callister & Rethwisch 8e.
• Fracture strengths of polymers ~ 10% of those for metals
• Deformation strains for polymers > 1000%
– for most metals, deformation strains < 10%
Chapter 14 -30
30
Mechanisms of Deformation —
Semicrystalline (Plastic) Polymers
s(MPa)
Stress-strain curves adapted
from Fig. 15.1, Callister &
Rethwisch 8e. Inset figures
along plastic response curve
adapted from Figs. 15.12 &
15.13, Callister & Rethwisch
8e. (15.12 & 15.13 are from
J.M. Schultz, Polymer
Materials Science, PrenticeHall, Inc., 1974, pp. 500-501.)
fibrillar
structure
x brittle failure
onset of
necking
plastic failure
near
failure
x
unload/reload
e
crystalline
block segments
separate
undeformed
structure
amorphous
regions
elongate
crystalline
regions align
Chapter 14 -31
31
Mechanisms of Deformation—Brittle
Crosslinked and Network Polymers
Initial
Near
Failure
s(MPa)
Initial
x brittle failure
Near
Failure
x plastic failure
aligned, crosslinked
polymer
e
network polymer
Stress-strain curves adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Chapter 14 -32
32
Mechanisms of Deformation—
Elastomers
s(MPa)
x brittle failure
x
plastic failure
elastomer
x
e
initial: amorphous chains are
kinked, cross-linked.
final: chains
are straighter,
still
cross-linked
Stress-strain curves
adapted from Fig. 15.1,
Callister & Rethwisch 8e.
Inset figures along
elastomer curve (green)
adapted from Fig. 15.15,
Callister & Rethwisch 8e.
(Fig. 15.15 is from Z.D.
Jastrzebski, The Nature
and Properties of
Engineering Materials,
3rd ed., John Wiley and
Sons, 1987.)
deformation
is reversible (elastic)!
• Compare elastic behavior of elastomers with the:
-- brittle behavior (of aligned, crosslinked & network polymers), and
-- plastic behavior (of semicrystalline polymers)
(as shown on previous slides)
Chapter 14 -33
33
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
Chapter 14 -34
34
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 8e. (Fig. 15.23 is
from F.W. Billmeyer, Jr.,
Textbook of Polymer Science,
3rd ed., John Wiley & Sons,
1984.)
Chapter 14 -35
35
Processing Plastics – Injection Molding
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
Fig. 15.24, Callister & Rethwisch
8e. (Fig. 15.24 is from F.W.
Billmeyer, Jr., Textbook of
Polymer Science, 2nd edition,
John Wiley & Sons, 1971.)
Barrel
Chapter 14 -36
36
Processing Plastics – Extrusion
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)
Fig. 15.25, Callister & Rethwisch 8e.
(Fig. 15.25 is from Encyclopædia
Britannica, 1997.)
Chapter 14 -37
37
Processing Plastics – Blown-Film
Extrusion
Fig. 15.26, Callister & Rethwisch 8e.
(Fig. 15.26 is from Encyclopædia
Britannica, 1997.)
Chapter 14 -38
38
Polymer Applications
• Plastics
– The largest number of different polymeric materials comes
under the plastic classification. Plastics are polymer
materials that have some structural rigidity under load and
are used in general-purpose applications. Plastic materials
may be either thermoplastic or thermosetting polymers.
• Elastomers
– As described earlier
Chapter 14 -39
39
Trade names, characteristics, and typical applications for a number of plastics
Chapter 14 -40
40
Characteristics and typical applications for five commercial elastomers
Chapter 14 -41
41
Polymer Additives
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 below room temperature
– Presence of plasticizer transforms brittle polymer to a
ductile one
– Commonly added to PVC - otherwise it is brittle
Chapter 14 -42
42
Polymer Additives (cont.)
• Stabilizers
– Antioxidants
– UV protectants
• Lubricants
– Added to allow easier processing
– polymer “slides” through dies easier
– ex: sodium stearate
• Colorants
– Dyes and pigments
• Flame Retardants
– Substances containing chlorine, fluorine, and boron
Chapter 14 -43
43
Advanced Polymers
Ultrahigh Molecular Weight Polyethylene (UHMWPE)
• Molecular weight ca. 4 x 106 g/mol
• Outstanding properties
–
–
–
–
high impact strength
resistance to wear/abrasion
low coefficient of friction
self-lubricating surface
UHMWPE
• Important applications
– bullet-proof vests
– golf ball covers
– hip implants (acetabular cup)
Adapted from chapteropening photograph,
Chapter 22, Callister 7e.
Chapter 14 -44
44
Summary
•
•
•
•
•
•
•
•
Polymer molecular structure
Classification of polymers
Synthesis of polymers
Molecular weight and degree of polymerization
Polymer crystallinity
Polymer melting and glass transition temperatures
Mechanical properties of polymers
Polymer Processing
-- extrusion, compression molding, injection molding,
blow molding
• Polymer applications
Chapter 14 -45