In our August issue, we learned that Lindsay Doyle formulated an industrial primer using an alkyd-amino system pigmented with TiO2 and carbon black. Her testing indicated that the formulation met all the customer specifications. However, when tested by Saul Vance, the gloss reading decreased due to the occurrence of a hexagonal honeycomb pattern on the surface. Saul had only modified the formulation with the addition of some ketone solvent. He also eliminated the flash time before baking the panel in the oven. What was the cause of the honeycomb pattern and how was Saul going to solve the problem?
Saul Vance recognized the honeycomb pattern on the surface of the coating as Bénard cells. Bénard cells form in solvent-based coating films when there is a temperature gradient between the substrate (high temperature) and the surface (lower temperature). This temperature gradient causes convection currents in the coating. When the temperature gradient reaches a critical value, the flow pattern of the system suddenly changes. Solvent rises through the film to the surface, where evaporation occurs, resulting in cooling, viscosity build, and increasing surface tension. As the viscosity rises and the density increases, the material flows back into the film, down to the hot substrate surface, where the cycle repeats. The surface tension change also impacts the movement of the currents in the cells. Material flows from a region of high surface tension to one of low surface tension as it moves back down into the film. This dynamic process of changing density, surface tension, and temperature results in the flow forming hexagonal cells as the solvent moves up from the middle and down at the edges.