Rheology in general
Rheology is the science of deformation and flow of materials. Every material is influenced by external forces. Rheology deals with the relationship between force, deformation and time.
A subset of Rheometry is Viscometry, which is the relationship between force and deformation rate, which is used to classify materials as Newtonian(viscosity independent of shear rate), or non-Newtonian. Viscosity is a measure of a material's resistance to flow.
Most materials show both viscous and elastic behavior and are therefore called viscoelastic materials. These properties can be determined by making dynamic measurements in oscillatory shear mode.
The aim of Rheometry
Using rheological studies, one can mimic different processes and predict the influence on product properties or changes in formulations. The measurements can be used to study material structure, for raw-material or process control and for application properties, such as consistency, shear- or storage stability against sedimentation.
Every application requires its own practical tests, but very often it is difficult to optimize and to see small differences in performance using only these tests. Rheology measurements can reveal these small variations.
These measurements can also be of help in the development of new products, giving better understanding of the processes governing the final properties.
For more information about basic rheology see these links:
 Basic Rheology (Powerpoint presentation)
 Classification of liquids (Powerpoint presentation)
 Dynamic properties (Powerpoint presentation)
Rheology in our applications:
Rheology in Coatings
Rheology in Building applications
The forces that have an influence on the rheology of coatings can range from gravitational that have an influence on phenomena as leveling, sagging and sedimentation up to high shear forces that are acting upon the paint when it is brushed or sprayed.
The prehistory of the paint is crucial. A rheological measurement of for instance leveling has to be done on paint where all structure is broken to mimic the practical test.
The curve below shows a typical viscosity curve of a water based paint. The shear rate area in which the physical properties is taking place is noted below.
The shear rate area for the most commonly used viscometers is also noted.
Examples of tests for different properties
|
Practical test |
Rheology property |
Rheology measurement |
Physical properties |
|
Leveling acc. to Leneta ASTM D 4062 |
Low shear viscosity
Thixotropy
Viscoelasticity |
Viscosity curve
Dynamic structural build up
Creep/recovery |
The leveling of paint after brushing and rolling |
|
Sagging ASTM D 4400 |
Low shear viscosity
Thixotropy
Viscoelasticity |
Viscosity curve
Dynamic structural build up
Creep/recovery |
The sagging of a vertical paint film after application |
|
Roller spatter |
Viscoelasticity
High shear
viscosity |
Oscillation frequency sweep
Viscosity curve |
Spatter when rolling a paint |
|
Hiding power |
High shear viscosity |
Viscosity curve |
High paint thickness build |
|
Storage stability |
Linear viscoelastic region
Critical stress/strain value
Zero shear viscosity |
Oscillation Stress/strain sweep
Creep |
Stability against sedimentation |
Some of the practical tests in building applications are also very often depending on “the feeling” and the experience by the user.
Rheological studies can help us to determine the consistency of e.g. plasters, joint fillers or tile adhesives, how easy/nice they are to apply, consistency variations, setting properties, anti-sagging properties, etc.
The shear thinning property is for example very important when the plaster or mortar is going to be applied by trowelling or spraying.
The shear thinning properties for some Bermocoll additives in cement mortar
Example of tests for different properties
|
Practical tests |
Rheology
property |
Rheology measurements |
Physical Properties |
|
Ring test
(AN method CCD 1097) |
Magnitude of Elastic modulus, G’
Critical stress value
Viscosity at low shear stress |
Oscillation stress / strain sweep
Creep
Viscosity curve |
Consistency
Flow
Structure build up |
|
Flow table
(ASTM method C 230-68 ) |
Shear thinning
|
Oscillation stress/strain sweep at a high frequency
|
Flow
|
|
Sliding test
(EN 1308; DIN 18156) |
Viscoelasticity
Thixotropy |
Oscillation time sweep
Creep |
Consistency
Slip resistance
Structure build up under stress |
|
Manual application |
Medium to high viscosity |
Viscosity curve |
Flow behaviour
Workability |
It can be difficult to translate Rheology data from a water solution into a ready formulation in an application.
It is always important to make the measurements with the exact formulation, under the right conditions, as all components influence the consistency and property by interactions with each other.
|