Time Dependency
Fluid behavior characterized by a decrease in viscosity under constant shear stress followed by a slow recovery when the stress is removed describes the structural recovery of a coating color. The thixotropic index is the ratio of viscosity measurements taken at two different rotational speeds, or the area of the hysteresis loop between ascending and descending shear rate curves. This index determines the rate of structural breakdown and subsequent rebuilding of a paperboard coating during and after its application.
Coating Application
High-speed coating applications require that the formulation flows easily under the doctor blade but recovers its structure quickly once applied to the paperboard. If the recovery is too slow, the coating may run, resulting in uneven coverage, or penetrate too deeply into the porous paperboard web. Conversely, if the structure recovers too rapidly, the coating will not level properly, leading to surface roughness and poor printability.
The index helps formulations strike the correct recovery time to achieve a smooth, uniform surface finish.
Structure Recovery
Adjusting the dosage of synthetic rheology modifiers or natural starches allows the chemist to control this recovery behavior. This adjustment ensures that the coating sets at the optimal point in the drying process.