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

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