Transcript EBB 220/3 POLYMER RHEOLOGY - USM :: Universiti Sains Malaysia

EBB 220/3
POLYMER RHEOLOGY
Introduction

Flow process in manufacturing polymer products
can be represented as follows:
Raw Materials
Activities:
Molecular and compositional
modification & enhancement
Processing
Activities:
Rheology & equipment
design studies
Final Products
Activities:
Product design & End
properties investigation
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.

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

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
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

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.
e.
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
a.
b.
c.
d.
Molecular structure,
morphology,
Polymer melt,
Blends and polymer modification
 Can be studies by relationship between the
rheological properties and materials structure.
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
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
Example of flow
Plate of area A
Force =F
Velocity =
V/U
Direction
of flow
Fluids
Stationary Plate
Starting position of the
fluid particles
Force = F
Velocity = V
Direction
of flow
Velocity
Profile
Stationary plate
Viscosity
S
A
F
θ
D
1. Consider 2 plates (A= area of the plate),
separated by distance, D
1. The space between them is occupied by
the liquid
3. One plate moves relatively to the other
with velocity U
4. The movement is resisted by the viscous
reaction in the fluid
5. Since the movement is in shear, the
Reaction is the shear viscosity
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
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
Shear rate is a measure of the rate
of shear deformation
Rheogram for Newtonian liquids.
A - high viscosity, B - low viscosity.
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
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)???
Newtonian and non-Newtonian bahavior -Dilatant behavior can cause processing
difficulties
Variation of apparent viscosity with shear rate
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.
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:
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 viscosity
May be defined in two ways;
1. Calculating apparent viscosity, ηa
2. Calculating consistency viscosity, ηc
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 – the slope of the line at the
chosen value of Rate of shear
ηo
ηa
The ηa is greater than ηc
Non- newtonian flow

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
Behaviour of viscous material
•
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
de
 
dt
= viscosity
de/dt = strain rate
Non time dependence flow

Shear rate for non time dependence flow can
represents mathematically the shear stress as:
g  f (t )

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
Shear rate Vs flow for non time
dependence flow
Bingham Body
Pseudoplastic fluid
S
h
e
a
r
S
t
r
e
s
s
Newtonian fluid
Dilatant fluid
Shear Rate
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:
t  g  t y

where  plastic viscosity that reach a infinity when shear
rate almost zero (g 0) and reach a value  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.
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
Pseudoplastic

Pseudoplastic, or shear-thinning fluids
have a lower apparent viscosity at
higher shear rates.
Pseudo-plastic substance.
Pseudo-plastic substance
with yield value
Newtonian & Pseudoplastic Flow
Newtonian
Viscosity
Shear Thinning
Shear Rate
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.
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
Time dependence flow

Flow properties that dependence
with time are dependence on:
Types of shear flow,
2. Flow history
3. Moulding time.
1.

This types of flow showed a
reversible conditions
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)
Viscoelastic behaviour
Polymer is called viscoelastic because:
•
•
Showing both behaviour
behaviour
•
Instantaneously elastic strain followed by
viscous time dependent strain
elastic & viscous
Influenced of temperature on viscosity

Understanding the influenced of temperature
with the melt viscosity is importance in:


Polymer processing
To estimate the thermal resistance
particular materials
of

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.

The Andrade or Arrhenius equations can relate the
activation energy during chain mobility as
A
Where
 Ea 


 RT 
= viscosity of polymer melt
AEa = activation energy
R = Universal gas constant
T = Temperature (°K)
A = Arrhenius constant

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:
 Ea  1 
Log   
 
 RT  T 

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.
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
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
Piston
Polymer
melt
Pressure
Transducer
Barrel
Constant shear
rate Rheometer
Atmosphere
pressure
Extrudate
Example of flow
Flow phenomena:
Rod climbing & extrudate swell
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
Students Activity

Discuss with the person next to you what
you understand on the importance of
rheology in polymer processing