EBB 427

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EBB 427

• Technology and Application of Engineering Polymers

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EBB 427

• Course Synopsis
• This course covers topics on technology and
  applications of various polymers in engineering
  applications.
• The course covers the properties and the
  processing techniques for three types of
  polymeric materials such as thermoset,
  thermoplastics and elastomer.
• It also covers the examples of new polymeric
  materials and commercially available polymeric
  materials, for instance thermoplastic and
  thermoset for general and engineering
  applications

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EBB 427
Contribution of Assessment
Final Examination 70 Coursework 30
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Teaching Plans / Syllabus
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Teaching Plans / Syllabus
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Teaching Plans / Syllabus
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Teaching Plans / Syllabus
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References

• R J Young and P A Lovell, Introduction to
  Polymers, Chapman Hall, 1992.
• R J Crawford, Plastics Engineering, Pergamon
  Press, 1990.
• D H Morton-Jones, Polymer Processing, Chapman
  Hall, 1989.
• N G McCrum, C P Buckley, C B Bucknall, Principles
  of Polymer Engineering, Oxford/ University Press,
  1988.
• An Introduction to Rubber Technology, Andrew
  Ciesielski, Rapra Technology Ltd,1999.
• Rubber Technology Handbook, Werner Hofmann,
  Hanser Publisher, 1989.

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Revision
What is the difference between polymers,
plastics and resins???
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Revision

• Molecular Arrangement of Polymers
• Think of how spaghetti noodles look on a plate -
  Amorphous organization.
• An Amorphous polymers are generally transparent.
• This is an important characteristic for many
  applications such as food wrap, plastic windows,
  headlights and contact lenses.

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Revision

• Molecular Arrangement of Polymers
• The translucent and opaque polymers - crystalline
  arrangement.
• By definition a crystalline arrangement has
  atoms, ions, or in this case, molecules in a
  distinct pattern.
• You generally think of crystalline structures in
  salt and gemstones, but not in plastics.
• Just as quenching can produce amorphous
  arrangements, processing can control the degree
  of crystallinity. The higher the degree of
  crystallinity, the less light can pass through
  the polymer.
• Therefore, the degree of translucence or
  opaqueness of the polymer is directly affected by
  its crystallinity.

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• Classification of Polymer
• Linear chain molecules - Thermoplastics
• Branched chain molecules - Thermoplastics
• Weakly cross-linked chain molecules - Elastomers
• Highly cross-linked molecules - Thermosets
• Thermoplastic - meaning that once the polymer is
  formed it can be heated and reformed over and
  over again (facilitates recycling)
• . Thermosets Elastomers - can not be remelted.

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Revision
Characteristics of plastics when compared to
Metals and Ceramics

• Applications of Plastics
• Packaging
• Medical
• Recreational
• Textiles
• Furniture Housewares
• Transportation
• Construction, etc.

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Some Important Terminology

• Polymer
• Plastics
• Resin
• Synthetic polymer
• Natural polymer

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• Polymer- long molecules made up of smaller
  molecules-joined together by chemical bonds
• Plastics- Large molecules (synthetically made or
  naturally occuring), are highly modified
• Resin- Polymer that has not been formed into its
  final useful shape
• Synthetic polymer- polymer that do not occur
  naturally, they are manufactured
• Natural polymer- polymer that occur in nature

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Some Important Terminology

• Backbone atom
• Branching
• Copolymer
• Homopolymer
• Monomer

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• Backbone atom- atom along the main chain of a
  polymer
• Branching- side chain of a polymer main chain
• Copolymer- a polymer formed from more than the
  minimum number of monomer, i.e. ABS
• Homopolymer- polymer made from the minimum number
  of monomer type
• Monomer- a single unit that can be combined with
  others to form a polymer

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Some Important Terminology

• Crosslink
• Curing
• Thermoplastic
• Thermoset
• Catalyst

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• Crosslink- covalent bond between polymer chain
• Curing- process of hardening a polymer by the
  formation of crosslink
• Thermoplastic- a polymer solid at room temp, that
  can be melted and cooled to solidify in the
  desired shape
• Thermoset- a polymer that may be either liquid or
  solid at room temp., when heated it will harden
  and cure
• Catalyst- a molecule or material that facilitates
  a chemical reaction, but does not become part of
  the reaction

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Some Important Terminology

• Amorphous
• Crystallinity
• Degree of crystallinity
• Melting point (Tm)
• Steric effect

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• Amorphous- no regular structural pattern occur in
  a area of polymer
• Crystallinity- area within a polymer in which a
  polymer molecules fold into a tight, regular
  structure
• Degree of crystallinity- the amount of structure
  that is crystalline as opposed to the amount that
  is amorphous
• Melting point (Tm)- the temperature at which
  material changes from solid to liquid (vice
  versa)
• Steric effect- The influence of molecule shapes
  on the properties of a material

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Some Important Terminology

• Glass transition temperature (Tg)
• Virgin material
• Aging
• Degradation
• decomposition

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• Glass transition temperature (Tg)-
• Virgin material- Resin that has not been
  previously processed
• Aging- long term, low temperature degradation
• Degradation- the decomposition of a material
• Decomposition- the breaking of primary bond in a
  molecule

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Flow properties of polymer melts

• Rheology- science of the deformation and flow of
  bodies
• Rheometry- the technology of measuring the flow
  behavior
• In plastic processing, the materials to be
  processed must be in flowable condition- through
  increase in temperature caused melting,
  dissolving the materials in solvent, etc.
• During such processing, the viscosity is
  important

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• Traditional materials 2 categories
• Elastic solid (iron, concrete, copper, etc)-
  materials completely recover their shape
  restore energy
• Viscous fluid (water, oil, etc)- materials that
  flow when exposed to an imposed shear force, do
  not return to original shape
• Polymer- do not follow the trend of traditional
  materials.Why???

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Viscosity

• Viscosity is a measure of the resistance of a
  fluid to deform under shear stress
• Viscosity describes a fluid's internal resistance
  to flow and may be thought of as a measure of
  fluid friction (water is "thin", having a lower
  viscosity, while vegetable oil is "thick" having
  a higher viscosity)
• During flow process in plastic processing
  machinery, the melt is subjected to shear
• This can be illustrated by 2 plate model (next
  slide)

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Viscosity

• Consider 2 plates (A area of the plate),
• separated by distance, D
• The space between them is occupied by
• the liquid
• One plate moves relatively to the other
• with velocity U
• The movement is resisted by the viscous
• reaction in the fluid
• Since the movement is in shear, the
• Reaction is the shear viscosity

F
A
S
?
D
Shear stress, ? Shear force/Area of the shear
face F/A Nm-2
Shear strain,? Amount of shear displacement,
S/Distance between shearing surfaces (D) Tan ?
Viscosity, ? Shear stress/Rate of shear strain
? / (d ?/dt)
? / ?
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Viscosity

• The unit of viscositiy was poise, P, or
  centipoise, cP.
• 1 mPas 1 cP.
• ? rapidly decreases as temperature increases.
• Ideal fluids are called Newtonian. The viscosity
  is independent of the rate of shear

Shear rate is a measure of the rate of shear
deformation
Rheogram for Newtonian liquids. A - high
viscosity, B - low viscosity.
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Newtonian Liquid

• Newtonian liquid, where shear stress is
  proportional to shear rate, with the
  proportionality constant being the viscosity
• A Newtonian fluid (named for Isaac Newton) is a
  fluid that flows like water
• For example, water is Newtonian, because it
  continues to exemplify fluid properties no matter
  how fast it is stirred or mixed.
• If the liquid is not Newtonian, a plot of shear
  vs. the rate of shear is not a straight line but
  a curve

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Dilatant

• A dilatant material is one in which viscosity
  increases with the rate of shear (also termed
  shear thickening).
• The dilatant effect can be seen more readily with
  a mixture of corn starch and water

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Pseudoplastic

• Pseudoplastic, or shear-thinning fluids have a
  lower apparent viscosity at higher shear rates.

Pseudo-plastic substance with yield value
Pseudo-plastic substance.
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Viscosity

• - Most polymer melts rubber compound
• behave in pseudoplastic.
• How can we relate the pseudoplastic
• behavior to the morphology of the polymer
• (long chain coiled in complex structure)???
• Dilatant behavior can cause processing
• difficulties

Newtonian and non-Newtonian bahavior
Variation of apparent viscosity with shear rate
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Viscosity

• Thixotropy
• Thixotropy is the property of some non-newtonian
  pseudoplastic fluids to show a time-dependent
  change in viscosity .
• Viscosity decreases as the material is stirred
  until some minimum value is reached. It increases
  again when the substance is no longer agitated.
• Many gels and colloids are thixotropic materials,
  exhibiting a stable form at rest but becoming
  fluid when agitated

Thixotropic substance at different shear rates.
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Viscosity

• When the curve is nonlinear, the viscosity
• May be defined in two ways
• Calculating apparent viscosity, ?a
• Calculating consistency viscosity, ?c

?o viscosity at a very low shear Rate, which
behave like Newtonian behavior
?a is the slope of the secant line from the
origin to the shear stress at the given value of
shear rate
?c
?c the slope of the line at the chosen value
of Rate of shear
?o
?a
The ?a is greater than ?c
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Viscometers

• are employed to measure viscosity.
• Capillary viscometer
• Rotational rheometer
• Simple shear viscometer
• Cone plate rheometer
• Parallel plate viscometer
• Tensile extensional viscometer

Schematic diagram of a cone and plate viscometer.
Schematic diagram of a rotational viscometer
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Melt Flow Index (MFI)

• The Melt Flow Index is a measure of the ease of
  flow of the melt of a thermoplastic polymer or a
  measure of the ability of the material's melt to
  flow under pressure.
• It is defined as the weight of polymer in grams
  flowing in 10 minutes through a capillary of
  specific diameter and length by a pressure
  applied via prescribed alternative gravimetric
  weights for alternative prescribed temperatures.
• The melt flow rate is an indirect measure of
  molecular weight, high melt flow rate
  corresponding to low molecular weight
• The melt flow rate is inversely proportional to
  the viscosity of the melt at the conditions of
  the test

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MFI Apparatus

• Comprises a cylinder containing polymer melt
  which loaded from above by a piston carrying a
  weight.
• There is a capillary die at the bottom of the
  cylinder
• The procedure is to measure the output by cutting
  off sections of extrudate at known time intervals
  and weighing them

How to relate MFI with molecular weight???
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Melting of Thermoplastic

• Originally solid, must be heated to above its
  melting or softening point
• The heat comes from 2 sources
• 1. The external heat supplied-i.e. by heater on
  the barrel of extruder, etc
• 2. Heat generated when a highly viscous fluid
  being sheared at high shear rate

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Latent heat

• the amount of energy in the form of heat that is
  required for a material to undergo a change of
  phase (also known as "change of state").
• Two latent heats are typically described. One is
  the latent heat of fusion (melting), and the
  other is the latent heat of vaporization
  (evaporation).
• They are so named as to describe the direction of
  heat flow from one phase to the next
• solid ? liquid ? gas.
• The energy change is endothermic when going from
  solid to liquid to gas, but exothermic when going
  in the opposite direction.

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Specific heat capacity

• The typical unit for specific heat capacity is
  the kilojoule per kilogram kelvin, kJkg-1C-1
• the amount of energy required to raise the
  temperature of one kilogram of the substance by
  one Celcius. Heat capacity can be measured by
  using calorimetry. The SI unit would be joule per
  kilogram celsius

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Freezing of Melts

• The reverse of the melting process
• The molding must be removed from a mould without
  danger of its distortion.
• To estimate cooling rate, need to find thermal
  diffusivity,