EBB 220/3POLYMER RHEOLOGY
Flow process in manufacturing polymer products can be represented as follows:
IntroductionIntroduction
ProcessingActivities:
Rheology & equipment design studies
Final ProductsActivities:
Product design & End properties investigation
Raw MaterialsActivities:
Molecular and compositional modification & enhancement
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.
IntroductionIntroduction
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
IntroductionIntroduction
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
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.
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
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
Importance of rheologyImportance of rheology
The importance of rheological studies are:
a. Can identify the behaviour of flow during flowing together with factors that influenced the flow of polymers.
b. Can predict the real complex processing condition through easier component and predict the final properties of polymer
c. Can relate the qualitative and quantitative parameters such as output and used of materials properties
d. 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
e. In some cases, factors asa. Molecular structure, b. morphology,c. Polymer melt, d. Blends and polymer modification
Can be studies by relationship between the rheological properties and materials structure.
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
FlowFlow
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
FlowFlow
Example of flowExample of flow
Plate of area A
Fluids
Direction of flow
Force =F Velocity = V/U
Stationary Plate
Starting position of the fluid particles
Force = F
Velocity = V
Direction of flow Velocity
Profile
Stationary plate
Viscosity
1. Consider 2 plates (A= area of the plate), separated by distance, D1. The space between them is occupied by the liquid3. One plate moves relatively to the other with velocity U4. The movement is resisted by the viscous reaction in the fluid5. Since the movement is in shear, the Reaction is the shear viscosity
FAS
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
θ
Viscosity, η = Shear stress/Rate of shear strain = ζ / (d γ/dt) = ζ / γ
Viscosity
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
Rheogram for Newtonian liquids. A - high viscosity, B - low viscosity.
Shear rate is a measure of the rate of shear deformation
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
Newtonian Liquid
Viscosity
Variation of apparent viscosity with shear rate
Newtonian and non-Newtonian bahavior
- 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
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
Viscosity
Thixotropic substance at different shear rates.
When the relationship of shear stress versus shear rate is non-linear two types of viscosity at any value of shear rate can be obtained:
1. 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
2. Constant viscosity from slope taken from a line at
particular value of shear rate for materials that showed non newtonian behaviour
Viscosity
When the curve is nonlinear, the viscosityMay be defined in two ways;1. Calculating apparent viscosity, ηa
2. Calculating consistency viscosity, ηc
Viscosity
ηa – is the slope of the secant line
from the origin to the shear stress
at the given value of shear rate
ηc – the slope of the line at the chosen value of Rate of shear
The ηa is greater than ηc ηa
ηc
ηo
ηo – viscosity at a very low shear Rate, which behave like Newtonian behavior
Most of the polymer systems not follow Newtonian law.
Non Newtonian flow can be classified into 3 parts as:
1. Non time dependence flow,
1. Time dependence flow
1. Viscoelastic flow
Non- newtonian flowNon- newtonian flow
• Materials will demonstrate behaviour:
1. At low strain rate – behave according to the Newtonian relationship
2. Totally dependent with time.
3. Stress being function of strain rate
4. Stress independent of strain
dt
de
= viscosity
de/dt = strain rate
Behaviour of viscous materialBehaviour of viscous material
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
1. Bingham body flow,
2. Pseudoplastic flow,
3. Newtonian flow
4. Dilatant flow
)( f
Non time dependence flowNon time dependence flow
Shear rate Vs flow for non time dependence flow
Shear rate Vs flow for non time dependence flow
Bingham Body
Pseudoplastic fluid
Newtonian fluid
Dilatant fluid
Shear Rate
Shear Stress
Body Bingham is elastic solid ideal materials that their structure will collapse when the stress applied greater than their yield stress y,
Shear stress for body Bingham are proportional with shear rate given as:
where plastic viscosity that reach a infinity when shear rate almost zero ( 0) and reach a value when shear rate approach infinity value ( no limits).
Materials that represents model Bingham including emulsion and suspension with high concentration such as paint, printing ink, clay slurry and plastic emulsion.
y
Body Bingham flowBody Bingham 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
Pseudoplastic flowPseudoplastic flow
Pseudoplastic Pseudoplastic, or shear-thinning fluids
have a lower apparent viscosity at higher shear rates.
Pseudo-plastic substance.Pseudo-plastic substancewith yield value
Newtonian & Pseudoplastic FlowNewtonian & Pseudoplastic Flow
Viscosity
Newtonian
Shear Thinning
Shear Rate
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.
Dilatant FlowDilatant Flow
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
Flow properties that dependence with time are dependence on:
1. Types of shear flow,
2. Flow history
3. Moulding time.
This types of flow showed a reversible conditions
Time dependence flowTime dependence 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)
Viscoelastic FlowViscoelastic Flow
• Polymer is called viscoelastic because:
• Showing both behaviour elastic & viscous behaviour
• Instantaneously elastic strain followed by viscous time dependent strain
Viscoelastic behaviourViscoelastic behaviour
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.
Influenced of temperature on viscosity
Influenced of temperature on viscosity
The Andrade or Arrhenius equations can relate the activation energy during chain mobility as
Where = viscosity of polymer melt AEa = activation energy R = Universal gas constant T = Temperature (°K) A = Arrhenius constant
RT
Ea
A
When taking the logarithm plot from log 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.
TRT
ELog a 1
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
Instruments for rheology measurements
Instruments for rheology measurements
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 rotational viscometer Schematic diagram of a cone and plate viscometer.
Instruments for viscosity measurements
Atmosphere pressure
Pressure Transducer
Polymer melt Barrel
Piston
Extrudate
Constant shear rate Rheometer
Example of flow
Flow phenomena: Rod climbing & extrudate swell
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
Example of exams questionExample of exams question
Discuss with the person next to you what you understand on the importance of rheology in polymer processing
Students ActivityStudents Activity