RHEOLOGY METHODS

By

Asabuwa N. F.

Presentation Outline:Introduction

What is rheology?Importance of rheology StudiesFunctioning principle of rheology Important variables in rheology methods

RheometryMeasurement of rheological properties

Melt Index instrumentRotational rheometersCapillary rheometers

Conclusions

IntroductionThe term 'Rheology' was invented by Professor Bingham Lafayette College in 1920s,and was inspired by a Greek quotation “panta rei” meaning “everything flows”.

This method of material analysis was first used by Professor Bingham and Reiner on the 29th of April 1929 when the American Society of Rheology was founded in Columbus, Ohio State, USA.

Rheology mostly analyses mechanical properties which include physical properties of liquids and solids by describing the strain and flow behavior.

What is Rheology?

Rheology is the study of flow and deformation of materials and how the flow is affected by stresses, strains and time.

It applies to substances which have a complex structure such as polymers, cheese, emulsion and other biological materials.

Rheology (particularly in the study of the viscous flow of polymers) is very important because it gives a relationship between the properties, structure and processing of the materials.

Rheology helps describe the mechanical behavior of materials as a function of stresses (shear rate), strain , temperature and pressure in order to develop materials with the correct processing behavior based on their viscosity, elasticity and time. These parameters can be calculated using a dynamic mechanical testing method at melt temperatures.

Importance of Rheology Studies

Functioning Principle of Rheological analyses

This method is mostly based on the analyses of the viscous(Newtonian Fluids) and viscoelastic (Non Newtonian Fluids) behavior of material at their melt temperature.This method is based on the flow and deformation of matter in all the three states (solid, liquid and gas).

Important variables in Rheology methods

In order to measure a material’s rheology, five criteria must be met: geometric boundary conditions, stress, strain, strain rate and mode of deformation (shear rate) .

In regards to this we have some important variables during analysis;

Rheology (Viscosity models)

- ideal fluid

the viscosity is zero, this is used for the analysis of potential flow

problems such as found in aerodynamics

- Newtonian fluid

a good viscosity model for gases and simple liquids such as water

- Non Newtonian fluid

the viscosity of the fluid changes with shear rate. For some fluids, it

increases with shear rate (dilatant fluids) while for others it decreases

with shear rate (pseudoplastic behavior). This pseudoplastic (or shear thinning) behavior is what we observe for polymers.

Factors affecting viscosity- pressure - molecular weight- temperature - shear rate - Time

Rheometry

This is the use of rheometers or viscometers to determine the rheological properties of materials with the measured values based on force, length and time.

Rheometers are devices use to determine both the viscous and viscoelastic properties of materials depending on the force exerted in both the rotational and oscillatory test.

While on the other hand viscometers are devices use to determine the viscosity depending on the rotation, temperature and time.

Measurement of Rheological properties

Rheological properties can be measured using either 1. Melt Flow Indexer:

Actually this instrument is mostly use for quality control. It does not compute a material property. Its mainly to compare the relative flow behavior ofdifferent materials based on the viscosity.

2. Rotational Rheometers

In a rotational rheometer the sample is sheared between

two plates or a cone and plate geometry. The viscosity is calculated as the ratio of the applied stress and the applied deformation rate (rotation speed). This instrument in the case of polymers is use to extract information about the average molecular weight and molecular weight distribution via measurement of the viscoelastic properties and thus determine how this affects the processing characteristics of the material.

Modern rotational rheometers are capable of many tests for a full characterization of the material which involves;

• Flow Rates

• Creep Tests

• Stress Relaxation

• Small amplitude sinusoidal Oscillatory Testing

Types of Rotational rheometers

(a) concentric cylinder rheometer consist of an inner rotating cylinder with a stationary outer cylinder

(b) cone and plate rheometer consist of a flat plate and a low angle cone rotating against the flat plate.

(c) parallel plate rheometers consist of two disk with an adjustable gap for holding the sample.

(d) Dual slit plate rheometer consist of two plate with one sliding over the other

(e) Van Rheometers

Provides a useful method for analyzing rheologically complex materials. It is mainly use to measure the yield stress and examine the structural characteristics of a dispersion or suspension.

Advantages:

1. The wall slip is avoided

2. There is less disturbance of the sample structure compared to the other rheometers

3. Can be also use for larger particle size samples.

3. Capillary Rheometers

Capillary rheometers are mostly used to examine processing behavior, rather than just determining the rheological parameters. In a capillary rheometer the material is forced through a slit or round die. From the pressure drop and the volume flow rate, a

steady state viscosity as a function of the applied rate (piston speed) or stress (applied pressure) is measured.

Advantages:

(1) Can operate at high shear rates

(2) May be closer to real processing situation than a rotational rheometer

Disadvantages:

(1) Shear rate is not uniform

(2) Wall slip

(3) Melt fracture

(4) Difficult to clean

In addition we also have gravity driven capillary rheometers also called viscometers but mostly function for viscous Newtonian fluids.

Conclusion

Rheology is very sensitive to small changes of the material’s structure therefore its an ideal for characterization with the rheology structure relationship being the key to the development of new materials. Thus rheology is one of the most suitable tool for process and quality control.

Notwithstanding rheology is a standard in most material industries (e.g polymer Industry) to characterize materials with the goal to develop

new and better materials with the desired processing and end-use properties.

Teşekkürler