Lecture - 16 Applied Rheology in Polymer Processing

8/10/2019 Lecture - 16 Applied Rheology in Polymer Processing

1/12

Applied Rheology in Polymer

Processing


2/12

Introduction

The polymer components may

be manufactured either fromi) low viscosity liquid polymers or

their solutions and emulsions like

monomer solutions in the reaction

injection moulding or rubber latexes

for making latex goods

ii) Polymer or rubber melts to

manufacture various extruded or

moulded components

The processing stages and

equipments used in the

manufacture of latex and

solution based components are

not highly energy intensive due

to the low viscosity of the

starting fluids.

On the other hand solid

polymers (both rubbers and

plastics) are used for the

corresponding component

manufacture and the energy

required for their processing isvery high because of their high

melt viscosity.

The elastic nature of rubbers

presents additional processing

problems.


3/12

Introduction...

The important equipments used in the

polymer processing are mixers,

calenders, extruders, injection and

other moulding systems.

During processing the polymer melts

are subjected to different stresses like

shear, compression, tensile and are

deformed accordingly under high

temperature, pressure and shear rateconditions.

The compounds flow through varying cross

sections while passing through different

equipment thereby requiring high-energy

consumption. The viscosity and therefore the rheological

behaviour of these compounds plays a very

important role in optimizing the energy

consumption in these high energy intensiveprocesses.

Shear: Strain produced bypressure in the structure ofsubstance, so that each layerslides over the next.


4/12

Flow situation in different processing equipment

In processing equipments the melt flow involves the high degree of shearing

deformation of polymer matrix achieved by making it pass through small cross-

section area like

i) nip region in a two-roll mill associated with manual cutting and foldingoperations

ii) rotor tip and the chamber wall area in a Banbur, mixer associated with cross

flows due to design features of the rotors

iii) Screw tip and the barrel wall in the intermix along with the cross flows

obtained due to special screw and barrel cross section design.

Banbury mixer Barrier screw design for

continuous mixer

Schematic view of

transfer mix (shear mix)


5/12

Flow situation in Extruder

The polymer melt flow through the screw

channel and the gap between the screw

flight top and barrel surface in a screw

extruder .

The flow of melt through screw extruder is

complex in nature due to screw design.

The melt is subjected to high pressure and

shear stress in a continuously varying crosssection area.

Because of high pressure there may result

of back flow of the melt and also

considerableheat generation.

At the end of extruder the melt enters thedie region and also the die cross section is

much smaller as compared to barrel the

melt is subjected to the flow contraction

and circulation.

Therefore the knowledge of

rheological behaviour of the melt

is very essential for the analysis

of such flow problems.

Die entry flow pattern


6/12

Why Rheology ?

The manufacture of different polymer componentsrequires the polymers to be processed in various process

equipment.

The polymer melt and solutions are likely to be

subjected to shear and extensional deformations at different

deformation rates under various applied stresses duringflow through different flow channels under high pressure

and temperature.

The volumetric flow rate pressure drop relationship,

which depends on the rheological behaviour of the

polymers, is of considerable importance from the point ofview of energy consumption in processing.

The study of rheological behaviour of polymers therefore forms an essential part of

polymer processing.


7/12

Rheological principles

Hooke in 1676 developed the first

scientific relationship between the extent

of deformation and applied force based on

the experimental observations of changein length of an elastic metallic rod

subjected to the elongational force in the

direction of length.

The relationship is known asHookeslawand states that in the linear range, the

deformation is directly proportional to the

applied force. The materials which follow

this relationship, are known as the

Hookean solids.

The science of rheology deals with the study of deformation of materials

under applied stresses. It describes the interrelation between the extent and

rate of deformation with magnitude of stress.


8/12

Rheological principles...

Newton in 1686 studied thedeformation of simple liquids

and established that the rate of

deformation of liquids is

directly proportional to the

applied shear stress.

The relationship is known as

Newtonslaw of viscosityand

all fluids which follow this

relationship, are known aspurely viscous Newtonian

fluids.


9/12


Thepurely elastic materials are capable

of storing all the energy used in

deformation and releasing it as useful

work as soon as the applied force isremoved.

Thepurely viscous fluids on the other

hand dissipate all the energyas heat due

to viscous resistance. These materials

start flowing as soon as subjected to theshear stress and do not go back to their

original state i.e., they flow irreversibly.

Purely elastic and purely viscous nature

form two extreme cases of

deformational behaviours.

In 1867 Maxwell suggested that

all substances must possess boththese characteristics to a varying

degree.

Polymer melts and solutions show

both characteristics when

deformed implying there by part

of deformational energy will be

stored as elastic energy and the

remaining will be dissipated as

heat due to the viscous drag. Such

systems are known asviscoelastic

materials.


10/12


The stored elastic energy manifest itself

in different forms depending on the

application for example, in melt

extrusion it shows up as die swellwhere the extrudate diameter is larger

than die diameter or the rod climbing

effectwhere stirred solution climbs the

stirrer rod as against the newtonian

fluids which show a downward

parabolic profile.


11/12


The polymers contain long

chain molecules in coiled

spatial structure, which duringflow or rotation tend to uncoil

and orient in the direction of

application of the force

resulting in stressing of the

chains and storing of elastic

energy.

This stored elastic energy is the cause of

developing normal stresses giving rise to die

swell or the rod climbing effect. This is

known as the stress relaxation process in

which the long chain molecules tend toacquire their original spatial configurations

showing some sort of memory effect and

therefore these fluids are also known asmemory fluids.


12/12

Documents

Lecture - 16 Applied Rheology in Polymer Processing