3. Plastics
- are a large and varied group of synthetic materials that are processes
by forming or molding into shape.
Elastomers or rubbers
- are a material that at room temperature stretches under a low stress
to at least twice its length and then quickly returns to almost its original
length upon removal of the stress.
3
4. Thermoplastics
- Linear or branched polymers which chains of molecules are NOT
INTERCONNECTED to one another.
- Low density, low tensile strength, high insulation, good corrosion
resistance.
- Are considered to fracture primarily in a brittle mode.
Thermosetting plastics
- Thermosetting or thermoset plastic are formed with a NETWORK
molecular structure of primary covalent bonds.
- High thermal and dimensional stability, rigidity, resistance to creep,
light weight.
- Are considered to fracture primarily by the brittle and ductile manner.
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5. Question 1
5
a.
Define and differentiate polymers, plastics and elastomers.
b.
Give 3 example of thermoplastic and thermosetting plastic.
c.
Give 2 example application of thermoplastic and thermosetting
plastic.
6. Advantages of Polymer
6
Wide range of properties.
Minimum finishing.
Minimum lubrication.
Good insulation.
Light weight.
Noise Reduction.
c)
Figure 10.1: Some application for engineering plastic
a) TV remote control casing
b) Semiconductor wafer wands
Nylon themoplastic reinforced with 30% glass fiber to replace aluminium in the manifold of the
turbodiesel engine
7. Polymerization
7
Polymerization:
- is the process by a small molecules consisting of one (monomer) or few
(oligomers) units are chemically joined to create a giant molecules.
Chain growth polymerization:
- Small molecules covalently bond to form long chains (monomers) which in
turn bond to form polymers.
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).
8. Chain Polymerization Steps
8
1.
Initiation:
A radical is needed.
Example: Ethylene
- One of free radicals react with ethylene molecule to form new longer
chain free radical.
2.
Propagation:
Process of extending polymer chain by addition of monomers.
Energy of system is lowered by polymerization.
3.
Termination: By addition of termination free radical.
Or by combining of two chains
Impurities.
9. Structural Feature of Polymers
9
The simple molecules that are covalently bonded into long chains are
called monomers.
The long chain molecule formed from the monomer units is called a
polymer.
The number of active bonds in a monomer has is called
functionality.
Homopolymers are polymeric materials that consist of polymer
chain made up of single repeating units.
Copolymers consist of polymer chains made up of two or more
chemically different repeating units that can be in different sequences.
11. Flexural and dynamic moduli
The flexural strength of a material is
defined as its ability to resist deformation
under load.
Flexural modulus is the ratio of stress to strain in flexural deformation.
Figure 10.43:
Tensile stress versus strain curves for PMMA at
various temperature. A britlle-ductile transition
occurs between 860C and 1040C.
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12. Viscoelestic deformation
Viscosity occur when temperature is above the glass transition
temperature.
Viscoelastic deformation of a material is the deformation by elastic
deformation and viscous flow of the material when stress is applied.
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13. Elastomeric deformation
The strength of thermoplastics cam be considerably increased by
addition of reinforcements.
Thermosetting plastic without reinforcements are strengthened by
the creation of a network of covalent bonding throughout the
structure of the material.
During the elastic deformation, covalent bond of the molecular
chains are stretch and distort, allowing the chain to elongate
elastically.
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14. Creep deformation
Polymeric materials subjected to a load may creep.
Creep is a time dependent permanent deformation with constant stress
or load.
Creep is low below Tg (above Tg, the behavior is viscoelastic). Glass fiber
reinforcements decreases creep.
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15. Stress relaxation
Stress relaxation is a reduction of the stress acting on a material over a
period of time at a constant strain due to viscoelastic deformation.
Stress relaxation is due to breaking and formation of secondary bonds.
Stress relaxation allow the material to attain a lower energy states
spontaneously if there is sufficient activation energy for the process to
occur.
15
16. t
0e
1
Ce
Q
RT
Where;
σ
σo
τ
T
R
C
= Stress after time t.
= Initial stress
= relaxation time.
= temperature
= molar gas constant.
= rate constant independent of temperature
16
17. Question 2
17
a.
b.
A stress of 7.6 MPa is applied to an elastomeric material at
constant strain. After 40 days at 200C, the stress decreases to
4.8 MPa.
i.
What is the relaxation time constant for this material?
ii.
What will be the stress after 60 days at 200C?
(Answer: 88.5 days, 3.6MPa)
The relaxation time for an elastomer at 250C is 40 days, while at
350C the relaxation time is 30 days. Calculate the activation
energy for this stress relaxation process. Given R = 8.314
(Answer : 22 kJ/mol)
18. Optical Properties of Polymers
18
Many plastics have excellent transparency.
If crystalline regions having high refractive index are larger than
wavelength of light, the light will be scattered.
Figure 15.7: Multiple internal reflections at the crystallineregion interfaces reduce the transparency of partly
crystalline thermoplastics.
19. Luminescence
19
Luminescence is the process by which substance absorbs energy and
spontaneously emits visible or near visible radiation.
Electrons are excited by input energy and drop to lower energy level.
Fluorescence: Emissions occur within 10-8 seconds after excitation.
Phosphorescence: Emissions occur 10-8 seconds after excitation.
Produced by material called phosphors.
Emission spectra can be controlled by adding activators.
20. Photoluminescence
20
Ultraviolet radiation from a mercury arc is converted into visible light
by using halophosphate phosphor.
In fluorescent lights, calcium halophosphate with 20% F - replaced
by Cl- is used.
Antimony ions (Sb3+) produce blue emission and manganese ions
(Mn2+) provide orange-red emission band).
21. Cathodoluminescence
21
Produced by energized cathode that generates
a beam of high energy bombarding electrons.
Examples:Electron microscope, CRO, TV Screen.
In TV screen, the signal is rapidly scanned
over the screen deposited with blue, green and
red emitting phosphors to produce images.
22. Intensity of luminescence:
I
I0
t
I0 = initial intensity
τ = relaxation time constant
I = fraction of luminescence after time t.
22
23. Question 3
23
a.
A colour TV phosphor has a relaxation time of 3.9 x 10-3 s. How long
will it take for the intensity of this phosphor material to decrease to
10% of its original intensity?
(Answer : 9 x 10-3s)
24. References
24
A.G. Guy (1972) Introduction to Material Science, McGraw Hill.
J.F. Shackelford (2000). Introduction to Material Science for
Engineers, (5th Edition), Prentice Hall.
W.F. Smith (1996). Principle to Material Science and Engineering, (3 rd
Edition), McGraw Hill.
W.D. Callister Jr. (1997) Material Science and Engineering: An
Introduction, (4th Edition) John Wiley.
