Rheology in Coatings, Principles and Methods
Published Online: 15 SEP 2006
Copyright © 2000 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Analytical Chemistry
How to Cite
Eley, R. R. 2006. Rheology in Coatings, Principles and Methods . Encyclopedia of Analytical Chemistry. .
- Published Online: 15 SEP 2006
Rheology by definition is the study of the deformation and flow behavior of materials. An operational definition would be “the study of the response of certain materials to the stresses imposed on them”. Rheology seeks to understand the relationship between applied force, or stress, and the resulting deformation, particularly for materials showing nonsimple responses. Such a relationship expressed mathematically is termed a constitutive equation. Perhaps the most familiar such equation is Hooke's law for elastic solids, which states the linear relationship between the stress σ and the deformation or strain γ [Equation 1]:
where the modulus G is the constant of proportionality. However, rheology is primarily concerned not with the solid state but with the study of fluid behavior, and the simplest constitutive relation for a liquid is Newton's law [Equation 2]:
where the coefficient of viscosity or, more commonly, the viscosity η expresses the proportionality between stress and, now, not strain but the strain rate, .
For the ideal, simple materials described above, G and η are called material constants because they are dependent only on the thermodynamic variables of temperature, pressure, and concentration, similar to a material property such as the density. However, rheology is not concerned with either Hookean solids or Newtonian liquids. It is rather concerned with materials whose constitutive relationships involve complex behavior. For such materials, G and η are no longer material constants but material functions. Their values will now depend on variables such as the magnitude of applied stress, strain, strain rate, and even the deformation history. In fact, for rheologically interesting materials, the behavior is somewhere between that of a Newtonian liquid and an elastic solid. Such materials are termed non-Newtonian, and may display time-dependent, plastic and viscoelastic behaviors.
Paints and industrial coatings, creams and lotions, inks, adhesives, ceramic slips, solder pastes, foods, medicines, etc. are representative of the range of materials whose commercial viability depends on having the correct rheology for the application. In turn, the required rheological properties must be defined with reference to the specific process at hand. This article introduces the science of rheology, with description of basic terms, models, and methods used. The mechanisms responsible for complex rheological behavior and the use of rheological measurements to understand and control the flow of materials are described. The emphasis is on coatings and coatings processes as points of reference, but the principles discussed apply to a wide range of materials and industrial products.