“Creep” is the technical expression used to describe deformation or flow of a material under a constant force. When paint is applied to a horizontal surface with a brush, lines created by the bristles are visible immediately afterwards. Within a short amount of time, the lines may disappear and leave a perfectly smooth surface. This flow behavior is referred to as “leveling”. When paint is applied on a wall with a brush, paint droplets may suddenly coalesce and appear to slide down the wall. This flow behavior is referred to as “sag”. Leveling is desirable, whereas sag is not. Paint manufacturers must evaluate new formulations for these characteristics to confirm that flow performance will satisfy customer expectations.
Gravity is the force in play that affects the flow behavior of paint in the above examples. It is a relatively small force compared to the brushing action that applies the paint to the horizontal and vertical surfaces. Flow behavior of paint during sag and leveling phenomena takes a longer time to occur compared to quick brushing action when painting. Rheological tests that properly characterize sag and leveling must mimic these conditions – namely a low force level applied to a paint sample over an extended time interval – to confirm that performance of the paint formulation is acceptable.