1. Chapter 7: Polymers Part 2
DMT125 Materials Science
Chapter 7: Polymers
Part 2
25 January 2013
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2. Thermoplastics General properties Relatively low densities
Relatively low tensile strength
Good electrical insulative materials
Dielectric strength
voltage gradient that produces electrical breakdown through the material
Unit: volts/mm
Relatively low maximum use temperature
Range from 54 deg C to 149 deg C
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3. Thermoplastics Polyethylene (PE)
Clear to whitish, translucent thermoplastic material
Often fabricated into clear thin films
2 types
Low density (LDPE) – branched chain structure
High density (HDPE) – straight chain structure
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4. Thermoplastics
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5. Thermoplastics
LDPE lowers its degree of crystallinity & its density due to branched chain structure
Also lowers strength because it reduces intermolecular bonding forces
HDPE can pack more closely together to increase crystallinity & strength due to having very little branching on the main chains
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6. Thermoplastics Most extensively used plastic material Low cost
Has many industrially important properties
Toughness at room temperature
Low temperature with sufficient strength
Good flexibility over a wide range of temperatures
Excellent corrosion resistance
Excellent insulating properties
Odorlessness & tastelessness
Low water-vapor transmission
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7. Thermoplastics Applications Containers Electrical insulations
Chemical tubing
House wares
Packaging & materials handling
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8. Thermoplastics Polyvinyl chloride (PVC)
Presence of large chlorine atom that is essentially amorphous & does not crystallize
Strong cohesive forces between polymer chains due mainly to strong dipole moments caused by chlorine atoms
Large negative chlorine atoms cause steric hindrance & electrostatic repulsion, reducing flexibility of polymer chains
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9. Thermoplastics PVC homopolymer
Relatively high strength (51 to 63 Mpa) plus brittleness
Medium heat-deflection temperature ( 57 deg C to 82 deg C at 0.5 Mpa)
Good electrical properties (16,745 to 51,220 V/mm)
High solvent resistance
Flame & chemical resistance produced by the presence of high chlorine content in PVC
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10. Thermoplastics
PVC can only be used for a few applications without addition of a number of compounds to the basic material so that it can be processed & converted into a finished product
Compounds added are
Plasticizers: impart flexibility to polymeric materials
Heat stabilizers: prevent thermal degradation during processing & extending life of finished product
Lubricants: aid the melt flow of PVC compounds during processing & prevent adhesion to metal surfaces
Fillers: lower the cost of PVC
Pigments: used to give color, opacity & weatherability
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11. Thermoplastics Rigid PVC
Addition of rubbery resins can improve melt flow during processing
Absorb & disperse impact energy so that the impact resistance of the material is increased
Used for pipe, siding, window frame & etc
Plasticized PVC
Produce softness, flexibility & extensibility
Adjusting these properties via plasticizer-polymer ratio
Outperforms rubber, textiles & paper
Used for furniture, shoes, wire insulation & etc
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12. PVC pipe & valve
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13. Thermoplastics Polypropylene (PP) Presence of methyl group
Stronger but less flexible material
Increase glass transition temperature
Higher melting & heat deflection temperatures
Temperature max: 120 deg C without deformation
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14. Thermoplastics Good chemical, moisture & heat resistance Low density
Good surface hardness
Dimensional stability
Applications
House wares
Packaging
Appliance parts
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15. Thermoplastics Polytetrafluorothylene (PTFE)
Fluorinated polymer formed by the free radical chain polymerization of tetrafluroethylene gas to produce linear chain polymers of ---CF2--- units
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16. Thermoplastics
Crystalline polymer with crystalline melting point of 327 deg C
Highly dense crystalline polymeric material
Density is high (2.13 to 2.19 g/cm3)
Exceptional resistance to chemicals
Insoluble in all organics with exception of a few fluorinated solvents
Impact strength is high
Tensile strength, wear & creep resistance are low
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17. Thermoplastics
Adding fillers such as glass fibres can increase strength
Slippery & waxy & has low coefficient of friction
Applications
Chemically resistant pipe & pump parts, high temperature cable insulation, O rings & etc
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18. P
PTFE flexible exhaust
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19. PTFE flexible exhaust inside shot
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20. Thermosetting plastics (thermosets)
Formed with a network molecular structure of primary covalent bonds
Some are cross-linked by a chemical reaction that occurs at room temperature (cold setting thermosets)
Although cured parts can be softened by heat, their covalent bonding cross links prevent from being restored to the flowable state that existed before plastic resin was cured
Cannot be reheated & remelted as thermoplastics
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21. Advantages High thermal stability High rigidity
High dimensional stability
Resistance to creep & deformation under load
Light weight
High electrical & thermal insulating properties
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22. General properties Density slightly high
Tensile strength relatively low (28 to 103 Mpa)
With addition of glass filling, tensile strength can be increase up to 207 Mpa
High impact strength
Good dielectric strength (0.5 to 2.6 x 104 V/mm)
Maximum use temperature ranges from 77 deg C to 288 deg C
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23. Phenolics Low cost Good electrical & heat insulating properties
Good mechanical properties
Limited in color (usually black or brown)
High hardness
High rigidity
High strength
Applications
Wiring devices
Electrical switchgear & etc
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24. Epoxy resins Low cure shrinkage Good adhesion to other materials
Good chemical & environmental resistance
Good mechanical properties
Good electrical insulating properties
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25. Allows the liquid epoxy resin to quickly & thoroughly wet surfaces
Low molecular weight gives the material exceptionally high molecular mobility during processing
Allows the liquid epoxy resin to quickly & thoroughly wet surfaces
High reactivity of epoxides group with curing agents like amines provides high degree of cross-linking & good hardness, strength & chemical resistance
Applications
High voltage insulators
Encapsulation of transistors & etc
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26. Deformation & strengthening
Can be primarily
Elastic
Plastic (permanent)
Combination of both
Below Tg, thermoplastics deform by elastic (-40 deg & 68 deg C)
Above Tg, deform by plastic (122 deg C & 140 deg C)
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27. 25 January 2013
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28. As temperature increases
thermoplastics gradually soften
Secondary bonding forces between molecular chains become weaker
Strength decreases
When heated through its Tg
Strength decreases due to a pronounced decrease in the secondary bonding forces
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29. Creep & fracture
Creep – deformation under a constant applied load at a constant temperature continues to increase with time
Temperature at which creep of a polymeric material takes place is also important factor to determine creep rate
Creep rate is relatively low due to restricted molecular chain mobility
Above Tg
deforms easily by a combination of elastic & plastic deformation – viscoelastic behavior
Molecular chains slide past each other more easily – viscous flow
Reinforce with glass fibres greatly increase the creep moduli & reduce creep rate
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30. Creep & fracture Fracture Can be considered either
Brittle
Ductile
Intermediate between two extremes
Unreinforced thermosets fracture primarily in brittle mode
Thermoplastics fracture in either both
Above Tg, it will fracture in ductile mode
Below Tg, it will fracture in brittle mode
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31. Creep & fracture
Temperature greatly affect the fracture mode of thermoplastics
Thermosets heated above room temperature become weaker & fracture at lower stress level but still fracture primarily in brittle mode
Due to covalent bonding is retained at elevated temperature
Strain rate is also an important factor
Slower strain rate favoring ductile fracture due to molecular chain realignment
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32. Creep & fracture Brittle fracture Crazes – distorted localized regions
Formed in a highly stressed region of the material & consists of an alignment of molecular chains combined with a high density of interdispersed voids
Ductile fracture
Exhibit plastic yielding before fracture
Molecular linear chains uncoil & slip past each other & gradually align closer together in the direction of the applied stress
Eventually, the stress on the chains becomes too high, the covalent bonds of main chains break & fracture
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34. Copyright © Mr.Mohd. Azarulsani b. Md. Azidin
END
Copyright © Mr.Mohd. Azarulsani b. Md. Azidin
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