1. EBB 220/3 POLYMER RHEOLOGY

2. Introduction
Flow process in manufacturing polymer products can be represented as follows:

3. Rheology = Science of deformation and flow of matter
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.

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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

10. To produce a product with optimum processing properties.
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.

11. 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

12. 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

13. Example of flow Plate of area A Force =F Velocity = V/U
Fluids
Direction of flow
Force =F Velocity = V/U
Stationary Plate

14. Starting position of the fluid particles
Force = F
Velocity = V
Direction of flow
Velocity Profile
Stationary plate

15. 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 S A θ D
Shear stress, ζ = Shear force/Area of the shear face
= F/A Nm-2
Shear strain,γ = Amount of shear displacement, S/Distance between shearing surfaces (D)
= Tan θ
Viscosity, η = Shear stress/Rate of shear strain
= ζ / (d γ/dt) = ζ / γ

16. Viscosity η rapidly decreases as temperature increases.
The unit of viscositiy was poise, P, or centipoise, cP.
1 mPa·s = 1 cP.
η 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.

17. 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

18. 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

19. 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.

20. 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

21. Viscosity When the curve is nonlinear, the viscosity
May be defined in two ways;
Calculating apparent viscosity, ηa
Calculating consistency viscosity, ηc
ηo – viscosity at a very low shear
Rate, which behave like
Newtonian behavior
ηa – is the slope of the secant line
from the origin to the shear stress
at the given value of shear rate ηc
ηc – the slope of the line at the
chosen value of Rate of shear ηo ηa
The ηa is greater than ηc

22. 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

23. 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

24. 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

25. Shear rate Vs flow for non time dependence flow

26. Bingham Body
Pseudoplastic fluid
Newtonian fluid
Dilatant fluid
Shear Rate
Shear Stress

27. 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.

28. 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

29. Pseudoplastic
Pseudoplastic, or shear-thinning fluids have a lower apparent viscosity at higher shear rates.
Pseudo-plastic substance
with yield value
Pseudo-plastic substance.

30. Newtonian & Pseudoplastic Flow
Viscosity
Newtonian
Shear Thinning
Shear Rate

31. 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.

32. 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

33. Flow properties that dependence with time are dependence on:
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

34. 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)

35. Viscoelastic behaviour
Polymer is called viscoelastic because:
Showing both behaviour elastic & viscous behaviour
Instantaneously elastic strain followed by viscous time dependent strain

36. 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.

37. 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

38. 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.

39. 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

40. 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

41. Instruments for viscosity measurements
Piston
Polymer melt
Barrel
Constant shear rate Rheometer
Pressure Transducer
Atmosphere pressure
Extrudate

42. Example of flow

43. Flow phenomena: Rod climbing & extrudate swell

44. 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

45. Students Activity
Discuss with the person next to you what you understand on the importance of rheology in polymer processing