VISCOSITY -

Viscosity is a measure of how free flowing a liquid is. The more syrupy and thick the liquid, the higher the viscosity.

In simple terms, viscosity can be referred to as the 'thickness' of a liquid - or its resistance to flow. In the coatings industry, this behavior is one of the key parameters.
The viscometer instruments use a variety of methods to determine the viscosity of a fluid. Viscometers (also called viscosimeter) only measure under one flow condition.

In paint industries, viscosity is commonly measured with a Zahn cup, in which the efflux time is determined and given to customers. The efflux time can also be converted to kinematic viscosities (cst) through the conversion equations.

Zahn Cup Viscometers -

• Quickly measures the viscosity of liquids such as paint, lacquer, varnish, syrup and oil.

• Measures viscosity in the 18 to 1725 centistokes range in 90 seconds or less.

Also used in paint, a Stormer viscometer uses load-based rotation in
order to determine viscosity. The viscosity is reported in Krebs units
(KU), which are unique to Stormer viscometers.

The "one-point" tests are inherently limited, in that they cannot describe the material behavior over a range of shear rates that may be essential for proper product performance. Many coatings, for example, are sprayed onto substrates - requiring very low viscosity at very high shear rates - but then must stay on the substrates without further dripping - requiring fast structural recovery and, therefore, high viscosity at essentially zero shear rate. Thus, an automated, multi-speed, rheological or viscometric test is appropriate.