4. Scientists are still confused
that how to define rheology
Rheology=study of
deformation and flow
5. What is rheology to scientists?
Rheology is
Yield stresses
Viscoelastic effects
Memory effects
Shear thickening and shear thinning
6. Rheological measurements
Deformation is the relative displacement of points of a
body. It can be divided into two types: flow and elasticity.
Flow is irreversible deformation; when the stress is
removed, the material does not revert to its original form.
This means that work is converted to heat.
Elasticity is reversible deformation; the deformed body
recovers its original shape, and the applied work is largely
recoverable. Viscoelastic materials show both flow and
elasticity.
7. Flow classification
Flows are of following main types
Steady simple shear flow
Unsteady simple shear flow
Extensional flow
8. Steady simple shear flow
Shear flow is a flow which occurs when the fluid is placed
between the two plates and the two plates move at different
velocities
The viscosity function η,the primary and secondary normal
stress coefficients ψ 1 and
ψ 2 respectively are the three viscometric functions
which completely determine the state of stress in any
rheologically steady shear flow.
9. Unsteady simple shear flow
Unsteady simple shear flow occurs when the stresses are
time dependent
Small amplitude oscillatory flow, stress growth, stress
relaxation, creep and constrained flow are some examples
of such flows
10. Extensional Flow
There is no shear flow in this type.
The volume of the fluid remains constant in this type of
flow
It occurs when the material is longitudinally stretched as
for example in fiber spinning
11. Flow behaviour
The viscoelastic nature of the polymers whether filled or
unfilled bring them under the category called the non
newtonian fluids
Newtonian fluids as we know are the fluids which obey the
newton’s law of viscosity
The simplest example of newtonian fluid is water
12.
13.
14.
15.
16. Models for the Shear viscosity
At low shear rate range the unfilled polymer behave as
Newtonian fluids
As shear rate increases the viscosity begins to decrease
showing the pseudo plastic behavior.Under normal
conditions the high shear rate regions are neglected and the
curve of unfilled polymers become similar to the filled
polymers
17. Polymer applications
Polymer Processing
Polymers are used in the construction and a large number
of other applications so these are now indispensable
In most of the cases the melt processing is been carried out
but there are many examples in which processing of
solution is also taking place such as in the formation of
films and fiber of heat sensitive polymers
20. Calculations
Calculation of shear stress
Calculation of shear rate
Calculation of power law parameters
Calculation of viscosity
Correction of results or baggley correction.
27. Rheological Measurements
Viscometers are used to measure rheological properties
Viscometer is defined as instruments used to measure viscosity
They differ on the basis of geometry and shear rates
There are three main types
Capillary Viscometer
Rotational Viscometer
Moving Body Viscometer
28. Choice of a Viscometer
There are number of criteria to be kept in mind;
Nature of material to be tested
Material’s viscosity
Materials elasticity
The dependence of viscosity on temperature
The degree of accuracy required
29. Capillary Viscometer
It is the most oldest and popular way
follows Hagen_poiseulle equation i.e. η= πr^4pt/8VL
If we are assuming laminar flow and pressure is constant then equation becomes
ν = η/ρ
They are useful for measuring precise viscosities of dilute polymer solutions
They can’t measure absolute viscosities
Always measure viscosity relative to a reference liquid
30. Design of Capillary viscometer
Three main design of capillary viscometer
Ostwald glass capillary viscometer,
Cannon–Fenske viscometer,
Ubbelohde viscometer
Ostwald glass capillary viscometer,
It is a u shaped tube with to bulb reservoirs
The time of flow of liquid between to etched marks is taken as function of viscosity
31. Cannon–Fenske viscometer
It is excellent for general use
It consist of long capillary tube
Both reservoirs are present on the same vertical axis
Ubbelohde viscometer.
It is particularly useful for measurement at different concentration
It is the modified form of Ostwald viscometer
32. Orifice viscometers
It is also known as cup viscometer
It is typically a cup with a hole in the bottom
The time required for the liquid to flow out is measured to determine viscosity
Uses
It is used to measure control flow properties in the manufacturing, processing and
applications of inks, dyes, paints and lubricating oils
33. Limitations
It should not be used for setting product specifications
It is only designed for Newtonian fluids
It should not be used for thixotropic materials
34.
35. CONTENTS
•What is rotational viscometer
•Construction
•Working principle
•Determination of viscosity
•Types and their brief introduction and working
•Moving body viscometer
•Types and their brief introduction
36. Rotational viscometer
What is rotational viscometer?
rotational viscometer is an instrument that is
“
used to find out the viscosity of a fluid by using
action of rotation ”
39. DETERMINATION OF VISCOSITY
Viscosity can be calculated by this formula
η=K(stress term/shear rate term)
K =is constant
Stress term=may torque load and deflection
Shear rate term=rpm (revolution per min)
ASTM followed =D2196
41. Concentric cylinder viscometer
It consists of two cylinders, one within the other (cup
and bob), keeping the specimen between them
Inner cylinder(bob) Outer cylinder (cup)
42. Concentric cylinder
The relationship between viscosity, angular
velocity, and torque for a Newtonian fluid in a
concentric cylinder viscometer is given by the
Margules equation
M= torque
Ω= relative angular velocity
H= length of inner cylinder
Ri= radius of inner cylinder
Ro= radius of outer cylinder
Error in calculations can be reduced by reducing the ratio of
inner to outer radius that ratio should be equal to 1
43. Error correction
In case of Newtonian fluids:
Reduce Ro/Ri
In case of non Newtonian fulids:
The correction appears as an addition, ho
The data are plotted as M/Ω vs h and extrapolation is made to
a value of ho at M/Ω = 0.
The quantity (h + ho) is substituted for h in the various
equations.
45. Cone-plate viscometer
In a cone–plate viscometer (Fig. 25), a low angle (≤3 )
cone rotates against a flat plate with the fluid sample
between them.
With careful calibration and good temperature
control it can be a very effective research and
Viscosity can be measured through this formula
46. Parallel plate viscometer
In parallel plate viscometers the gap width is usually larger and
can be varied freely
The wide gap means that there is less sensitivity to temperature
changes
with the plate–plate instrument, the velocity, and therefore the
shear rate, varies with the distance from the center of the plate.
This makes viscosity data more difficult to evaluate.
Rp= radius of plate
H= distance
between two
plates
47. CHARACTERISTICS
•more efficient than capillary viscometer
•They can be used with a wide range of materials because
opacity, settling, and non-Newtonian behavior do not cause
difficulties.
•shear rates as a function of time can be measured.
Therefore, they are useful Viscosities over a range of for
characterizing shear thinning and time-dependent behavior .
48. Moving body viscometer
In moving body viscometers, the motion of a
ball, bubble, plate, needle, or rod through a material is
monitored.
The Stokes’ equation relating viscosity to the fall of a solid
body through a liquid may be written as equation 34,
where r is the radius of the sphere;
ds and dl are the density of the sphere
and the liquid, respectively;
g is the gravitational force;
and v is the. velocity of the sphere
49. Ball viscometer
Ball is fall in the fluid
Travel through the fluid
Speed of ball in fluid determines the viscosity of fluid
Used for suspension and polymer melts
ASTM D3121
50. Rod viscometer
The falling rod viscometer, sis based on the
movement of a rod rather than a plate through
the fluid.
In the 1990s, the Laray falling rod viscometer
became a standard test instrument in the ink
industry (ASTM D4040),
and more recent versions of the falling rod
viscometers are capable of precise
measurements of polymer melts and solutions
51. Needle viscometer
In the falling needle viscometer (ASTM D5478), the
moving body is a glass or stainless steel needle that
falls vertically through the fluid. The viscous
properties and density of the fluid are derived from
the velocity of the needle.
technique is useful for the characterization of polymer
melts and concentrated solutions.
52. • A rheometer is an instrument for measuring the
rheological properties:
1. It can apply a deformation mode to the material and
measure the subsequent force generated.
2. It can apply a force mode to a material and measure
the subsequent deformation.
• Rheometers used for determining the material
functions of thermoplastic melts can be divided into
two broad categories: 1. rotational type and
2. capillary type
53. ROTATIONAL VISCOMETERS
For thermoplastic melt studies, rotational viscometers with
either the cone-n plate or
parallel-disk configuration are used.
B: cone-and-plate viscometer. C: parallel disk viscometer.
The basic limitation in rotational viscometers is that they are
restricted in their use only to low shear rates for unidirectional
shear and low-frequency oscillations during oscillatory shear.
54. 1. CONE-N-PLATE VISCOMETER
The sample, is trapped between the circular conical disk at the bottom
and the circular horizontal plate at the top. The cone is connected to
the drive motor which rotates the disk at various constant
speeds, whereas the plate is connected to the torque-measuring device
in order to evaluate the resistance of the sample to the motion.
It can be used to measure shear rate, shear stress, normal stress
difference, oscillatory shear .
55. 2. PARALLEL-DISK VISCOMETER
The parallel-disk viscometer used for measuring the shear
stress and normal stress difference of molten
thermoplastics is similar in principle to the cone-n plate
viscometer except that the lower cone is replaced by a
smooth circular disk.
This type of viscometer was initially developed for
measuring the rheological properties of rubber. It can be
used for polymer melts of extremely high viscosity and
elasticity.
56. CAPILLARY RHEOMETERS
They are used for determining the rheological properties of polymer
melts.
1. Constant Plunger Speed Circular Orifice Capillary
Rheometer:
It extrudes the polymer melt through a capillary with a circular
orifice using a plunger at constant speeds.
The major advantage of this type of capillary rheometer is that
higher-shear rate levels than those attainable in rotational
viscometers can be achieved.
57. 2. Constant Plunger Speed Slit Orifice Capillary
Rheometer:
This rheometer has a slit orifice cross section rather than a
circular one.
It extrudes the polymer melt through a capillary with a slit
orifice using a plunger at constant speeds
58. 3. Constant Speed Screw-Extrusion-type Capillary
Rheometers:
These type of capillary rheometers are capable of
generating rheological data from medium-to-high
shear rates. These rheometers have been used for
rheological studies of polymer melts but have not
become as popular as the plunger type capillary
rheometers because they need a much larger quantity
of polymer feed.
59. 4. Constant Pressure Circular Orifice Capillary
Rheometer (Melt Flow Indexer):
This rheometer is also similar to Constant Plunger Speed
Circular Orifice Capillary Rheometer except for two
differences.
First, the capillary used is of very short length, and
second, the polymer melt is extruded by the use of dead
weights (i.e., constant pressure) rather than constant
plunger speed.
60. • Rheological measurements are often used as an
effective tool for
1. Quality control of raw materials, manufacturing
process/final product
2. Predicting material performance
•Melt rheology is concerned with the description of
the deformation of the material under the influence of
stresses. Deformation and flow naturally exist when
the thermoplastics are melted and then reformed into
solid products of various shapes.
61. • All polymer melts are viscoelastic materials; that is, their
response to external load lies in varying extent between
that of a viscous liquid and an elastic solid.
• A polymer melt represents a cluster of
entangled, flexible strings of varying lengths. It is these
entanglements that provide the resistance to deformation
and, therefore, with increasing molecular weight, the melt
viscosity goes up, processibility worsens although, of
course, mechanical properties improve.
Notas do Editor
Rheology is concerned with relating the response of a material to the forces that act upon it. As you have seen from the virtual field trips, the ways in which rocks can respond to forces are highly variable. Not only do different rock types respond in different ways but also the same rock type can show various responses depending on the conditions under which the force is applied. As a general principal however, take it that rocks deform in the easiest way open to them. By that we mean that if it takes less force to break a rock than it does to distort it, the rock will break.
