Shear viscosity is defined as the ratio of shear to shear strain rate at the strain rate of interest.
Although viscosity is often quoted as a number without reference to strain rate, it is actually a function of strain rate. The strain rate dependence and, in some cases, time dependence of viscosity need to be determined if meaningful correlations with coating phenomena are to be established. In the case of coatings, shear strain rates of interest range from about a few thousand reciprocal seconds (such as during spraying) to reciprocal hundredths of a second (after application).
There are a variety of techniques that can be used to measure the viscosity of paint formulations. Instruments with single or undefined strain rates should be avoided in coating rheology studies. Viscosity needs to be measured over a wide range of strain rates if meaningful correlations with coating phenomena are to be established.
The most accepted technique to determine the strain rate dependence of viscosity is to use constant strain rate experiments in torsion. This can be done in cone-plate (for low velocities) or concentric cylinder geometry (for higher velocities). However, oscillatory or dynamic measurements are also often used for the same purpose. Assuming that the shear strain rate and frequency are equivalent, the complex viscosity is equal to the steady state constant velocity viscosity (ie the Cox-Merz rule is valid). However, the applicability of the Cox-Merz rule is by no means universal, and its validity needs to be demonstrated before dynamic measurements can replace steady-state measurements. The capillary technique employed in several commercial instruments is generally not suitable for coating studies as it is better suited for measuring viscosity at higher strain rates.
