EBB 220/3 POLYMER RHEOLOGY

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EBB 220/3POLYMER RHEOLOGY
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Introduction

• Flow process in manufacturing polymer products
  can be represented as follows

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Introduction

• Rheology Science of deformation and flow of
  matter
• A very high performance polymer granules or
  pellets (raw materials) is useless if it cannot
  be transformed into a practically useable
  products
• Transformation means deformation and flow of
  polymer raw materials into a specified and
  required shapes
• The rheology of polymer powder or pallet is
  importance in first section melts or liquids.

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Introduction

• In melt processing of thermoplastics polymers ?
  rheological studies give initial information on
  how these polymer behave during actual polymer
  processing.
• e.g effect of temperature, pressure geometry
  on polymer flow behaviour in processes such as
  extrusion injection moulding
• Flow behaviour is important in injection molding,
  compression moulding, blow moulding, calendering
  cold forming and spinning of fibres
• It is also importance in the formulation of
  polymeric materials in preparing for fabrication
  process especially extrusion and mill rolling

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• For many simple fluids the study of rheology
  involves the measurement of viscosity ? the
  viscosity depends primarily on temperature and
  hydrostatic pressure
• However the rheology of polymers is much more
  complex because the fluid shows non ideal
  behaviour

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• All these rheological properties depend upon the
  rate of shear, the molecular weight, structure of
  polymers the concentration of additives and
  temperature.
• In most cases, flow is involved in the processing
  and fabrication of the plastics.

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• The degree of orientations is determined by
  rheological behaviour of the polymer and nature
  of the flow in fabrication process
• Molecular orientation hence influence the
  mechanical properties of moulded object films and
  fibres

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Importance of rheology

• Mechanical properties that shown by any polymer
  products is the most importance factors
  considered by manufactured and user.
• In actual conditions ? the optimum mechanical
  properties is not importance if the product could
  not be process as faster, simple or easier and
  relatively low cost
• Flow involved is rheological studies that also
  involved
• types and degree of orientation
• Flow properties in actual processing

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• The importance of rheological studies are
• Can identify the behaviour of flow during flowing
  together with factors that influenced the flow of
  polymers.
• Can predict the real complex processing condition
  ? through easier component and predict the final
  properties of polymer
• Can relate the qualitative and quantitative
  parameters such as output and used of materials
  properties

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• Can choose the suitable polymer for specific
  processing conditions and services
• To produce a product with optimum processing
  properties.
• ? importance in real processing to produce
  maximum output with minimum input
• In some cases, factors as
• Molecular structure,
• morphology,
• Polymer melt,
• Blends and polymer modification
• ? Can be studies by relationship between the
  rheological properties and materials structure.

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Flow

• Flow is the continuous deformation under an
  influenced of constant force
• ? any particle of materials will not back to the
  original positions after the force of deformation
  been released
• All the body in the nature will flow if given a
  period of time and appropriate temperature even
  with very low applied force

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Flow

• Ability to flow for a molten materials depends on
  the molecular chain mobility that hold molecule
  together.
• Low mobility with high degree of chain
  entanglement ? will influenced the ability to
  flow and the process ability of polymeric
  materials

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Example of flow
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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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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 relationship of shear stress t versus
  shear rate g is non-linear ? two types of
  viscosity at any value of shear rate can be
  obtained
• Apparent viscosity ? from slope taken from a line
  that connect the value of shear stress with shear
  rate at any point of shear rate from the origin
• Constant viscosity ? from slope taken from a line
  at particular value of shear rate for materials
  that showed non newtonian behaviour

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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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Non- newtonian flow

• Most of the polymer systems not follow Newtonian
  law.
• Non Newtonian flow can be classified into 3 parts
  as
• Non time dependence flow,
• Time dependence flow
• Viscoelastic flow

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Behaviour of viscous material

• Materials will demonstrate behaviour
• At low strain rate behave according to the
  Newtonian relationship
• Totally dependent with time.
• Stress being function of strain rate
• Stress independent of strain

h viscosity de/dt strain rate
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Non time dependence flow

• Shear rate for non time dependence flow can
  represents mathematically the shear stress as
• In rheological studies there are 4 types of flow
  that not dependence with time
• Bingham body flow,
• Pseudoplastic flow,
• Newtonian flow
• Dilatant flow

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Shear rate Vs flow for non time dependence flow
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Body Bingham flow

• Body Bingham is elastic solid ? ideal materials
  that their structure will collapse when the
  stress applied greater than their yield stress
  ty,
• Shear stress for body Bingham are proportional
  with shear rate given as
• where h plastic viscosity that reach a infinity
  when shear rate almost zero (g? 0) and reach a
  value h when shear rate approach infinity value
  (g ? no limits).
• Materials that represents model Bingham ?
  including emulsion and suspension with high
  concentration such as paint, printing ink, clay
  slurry and plastic emulsion.

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Pseudoplastic flow

• Viscosity of pseudoplastic flow decreased with
  the increased in shear rate ? it showed the shear
  thinning behaviour
• During real processing that involved a higher
  range of shear rate ? no problems of flowing for
  pseudoplastic materials
• At suppressed condition ? molecule has higher
  entanglement and will have random conformation or
  orientation
• Under the applications of shear force ? uncoiled
  of molecule chain occur and the orientation of
  molecule increased even though the occurrence of
  Brownian movement will try to gives the original
  conformation (the condition where no force
  occurred)
• At very high shear rate ? the almost Newtonian
  behaviour was observed for materials with
  pseudoplastic flows

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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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Newtonian Pseudoplastic Flow
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Dilatant Flow

• Viscosity value for Dilatant flow increased with
  increasing shear rate
• ? its enable the polymer to be process at high
  shear rate due to the ability to flow polymer is
  low.
• Dilatant behaviour normally shown by polymer with
  high suspension such as PVC and materials with
  non uniform particles shape ? ? materials that
  difficult to be compressed under high shear rate.
  
• Dilatant behaviour is hardly shown for molten
  polymer except under a special condition ? where
  the melt crystallization occurred during flow.

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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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Time dependence flow

• Flow properties that dependence with time are
  dependence on
• Types of shear flow,
• Flow history
• Moulding time.
• This types of flow showed a reversible conditions

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Viscoelastic Flow

• This flow are shown by materials that has the
  dominant viscous behaviour but has the elastic
  recovery after the deformation.
• Viscoelastic flow has a properties in between the
  solid and liquid behaviour.
• Please refer the viscoelastic behaviour
  (viscoelasticity)

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Viscoelastic behaviour

• Polymer is called viscoelastic because
• Showing both behaviour elastic viscous
  behaviour
• Instantaneously elastic strain followed by
  viscous time dependent strain

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Influenced of temperature on viscosity

• Understanding the influenced of temperature with
  the melt viscosity is importance in
• Polymer processing
• To estimate the thermal resistance of particular
  materials
• Big variation in viscosity with range of
  temperature ? represent the materials need a
  higher activation energy
• polymer molten viscosity that dependence on
  temperature have a higher temperature from glass
  transition temperature Tg or their melting
  temperature Tm.

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• The Andrade or Arrhenius equations can relate the
  activation energy during chain mobility as
• Where h viscosity of polymer melt
• AEa activation energy
• R Universal gas constant
• T Temperature (K)
• A Arrhenius constant

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• When taking the logarithm plot from log h against
  log (1/T) ?will given one straight line where
  the slope is the same? activation energy
  according to this equations
• If viscosity at various temperature taken at
  constant shear stress ? activation energy is
  supposed to be constant and not dependence on
  shear stress where it been taken.
• If the viscosity at constant temperature at
  various shear rate ? activation energy dependence
  on shear rate
• ? example activation energy decreased with
  increasing shear rate
• However the flow according to Arrhenius
  equations? activation energy almost not
  dependence on temperature.

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Instruments for rheology measurements

• A very popular types of instruments to measure
  viscosity is capillary rheometer or viscometer
• It function in conditions of load and forced is
  constant or at constant volume rate
• In conditions of constant shear stress ?
  measurement of flow rate was taken based on the
  speed of piston
• Pressure at the outer layer of die ? is measured
  using the pressure transducer

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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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Instruments for viscosity measurements
Piston
Polymer melt
Barrel
Constant shear rate Rheometer
Pressure Transducer
Atmosphere pressure
Extrudate
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Example of flow
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Flow phenomena Rod climbing extrudate swell
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Example of exams question

• What are the importance of rheological studies in
  polymer processing.
• Discuss the non-newtonian behaviour of polymeric
  materials.
• What are the influenced of pseudoplastic flow
  towards polymer processing?
• Most polymers melt exhibit pseudoplastic
  characteristics under shear conditions. How these
  differ from those of Newtonian fluids

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Students Activity

• Discuss with the person next to you what you
  understand on the importance of rheology in
  polymer processing

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