What is the interaction between flow and viscosity?

The driving force for the leveling process is surface tension. The forces resisting leveling are viscosity and, to a lesser extent, the elasticity of the coating. Therefore, for easy leveling, a low viscosity coating is recommended. However, low viscosity coatings cannot always be used. If the viscosity is low, it will be difficult to deposit heavier coatings. If the coating is applied to vertical surfaces, high viscosity is required to prevent sagging.

Figure 3.2 Thixotropic circuit

Waterborne paints are often pseudoplastic: they exhibit a rate-dependent viscosity. They may have low viscosity (20 to 30 mPa sec) at high shear rates (104 sec–1), such as experienced in roll coating operations, and higher viscosities at low shear rates (1 to 30 mPa sec). 3pa sec) (0.01 to 10sec–1), prevalent during the leveling process. Such coatings do not level well due to high viscosity at low shear rates.

Rheological index is sometimes used as an indicator of the leveling ability of a coating. Rheological index is defined as the ratio of high shear rate viscosity to low shear rate viscosity. If the rheological index is 1, the coating is Newtonian; if it is greater than 1, the coating is dilatant; if it is less than 1, the coating is pseudoplastic. A large rheological index favors good leveling; it should exceed 0.25 for an aqueous system with acceptable leveling properties.