Observations of sagging in architectural paints

Sagging of an alkyd and a latex paint is presented. Two forms are studied; firstly, when the paint film sags smoothly and, secondly, when the sag progresses as a ‘tear-shaped’ drip or ‘curtain’ due to uneven application. In the case of smooth sagging, the actual sag flow is measured by the distance that a white-blue boundary moves as the film dries. Employing two colours of paint also allows the flow within the coating to be determined as a function of depth, as the coating is ablated in an oxygen plasma. This experiment shows that most of the flow takes place near the substrate, as expected. A simple model is constructed for the smooth sagging of the alkyd paint showing that the major influences on the sagging performance, besides the non-linear time-dependent rheology, are the increase in viscosity of the continuous phase and the thinning that occurs as the coating flows vertically. A third colour, green, is used for drops applied on top of an otherwise smooth coating in order to observe the flow after uneven application. This experiment shows that the sag proceeds in a wave motion and that the material in the ‘drip’, when dry, is not the material forming the original uneveness. Sagging of unevenly applied coatings proves to be intractable to simple analysis.