Introduction to polymer science &

engineering(IPSE) Lab

Presented to: Sir Sajid Naseem

Rheology& Rheometers

Scientists are still confused that how to define rheology

Rheology

Rheology=study of deformation and flow

What is rheology to scientists?

Rheology is

Yield stressesViscoelastic effectsMemory effectsShear thickening and shear thinning

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.

Rheological measurements

Flows are of following main types

Steady simple shear flow

Unsteady simple shear flow

Extensional flow

Flow classification

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.

Steady 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

Unsteady simple shear 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

Extensional Flow

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

Flow behaviour

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

Models for the Shear viscosity

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

Polymer applications

Rheology of paints

Capillary rheometer

Calculation of shear stress

Calculation of shear rate

Calculation of power law parameters

Calculation of viscosity

Correction of results or baggley correction.

Calculations

Dilute polymer solution

Concentrated polymer solution

Polymer solution

Viscosity of dilute polymer solution

Glass capillary tube viscometer

Viscotek relative viscometer

Not a viscosity

Unit:dl/g

Inverse of molecular density

Intrinsic viscosity

Applications:

Fiber spinning

Film casting

Polymer manufacturing process

Methods:

Capillary viscometer

Extrusion rheometer

Rotational viscometer

Viscosity of concentrated polymer solution

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

Choice of a ViscometerThere 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

Capillary ViscometerIt 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

Design of Capillary viscometerThree 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

Cannon–Fenske viscometerIt 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

Orifice viscometersIt 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

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

Rotational viscometer

•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

CONTENTS

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 ”

construction

Rotational viscometers consist of two basic parts separated by the fluid being tested These parts may be

concentric cylinders (cup and bob)

two parallel plates

a low angle cone and a plate

WORKING PRINCIPLE

ROTATION Torque

Shearing action

ANGULAR VELOCITY

Viscosity

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

Types

Concentric cylinder viscometerPlate and cone viscometerParallel plate viscometer

Concentric cylinder viscometer

It consists of two cylinders, one within the other (cup and bob), keeping the specimen between them

Outer cylinder (cup) Inner cylinder(bob)

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= torqueΩ= relative angular velocityH= length of inner cylinderRi= radius of inner cylinderRo= radius of outer cylinderError in calculations can be reduced by reducing the ratio of inner to outer radius that ratio should be equal to 1

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.

Likewise we can find out shear rate and torque:

Cone-plate viscometerIn 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

Parallel plate viscometerIn 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 plateH= distance between two plates

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.

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

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

Rod viscometerThe 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

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.

RHEOMETER• 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

ROTATIONAL VISCOMETERSFor 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.

1. CONE-N-PLATE VISCOMETERThe 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.

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.

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.

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

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.

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.

CONCLUSION:

• 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.

• 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.