Bénard cells are hexagonal cells formed on the coating film due to the swirling flow resulting from solvent evaporation in thin films. The cell is subject to a movement in which the flows generate hills on the centers and holes at the cell boundaries. These flows result in the decomposition of different coating components thus accumulating at various sections of the cell leading to the formation of color and surface differences. These cells become more evident as the coating dries or hardens. They may not be observed if they are spread out during hardening/stoving. However, the cell appearance is generally fixed on the hardening film and is observed as a defect. (Figure.1: A typical Benard cell pattern on an industrial coating) 

Fig 1.

Bénard cells can be large enough to be spotted by the naked eye or small enough to be observed only after magnifying 50 times. Small cells may cause the coated surface to appear matt or dull to the naked eye. When viewed with a sufficiently magnifying lens, a weblike appearance is observed comprised mostly of hexagons but ranging from pentagons to heptagons. The rapid mobility inside the cell in pigment systems may result in color changes due to pigment decomposition. This phenomenon is known as floating, because one of the pigments rises up towards the film surface. The presence of Bénard cell pattern can be seen when the surface with color difference is viewed more closely. Bénard cell patterns can be spotted easily in metallic coatings since they result in an unacceptable surface appearance. 

In films with a thickness of several millimeters or more, Bénard cells develop due to convection caused by gravity. Whereas in films with thickness ranging between 10-100m that is typical for coatings, Bénard cells generate as a result of the surface tension gradients formed due to the changes in temperature and concentration inside the coating. Both gravity an surface tension gradients act as driving forces for Bénard cell formation in films with thickness values between these two groups. (Figure.2: A schematic drawing of Bénard cells with a cellular convection pattern caused by surface tension based movements) 

Fig 2.

Experimental studies have illustrated that the formation of Bénard cells may be suppressed by reducing the thickness of the applied film, increasing the viscosity of the coating and minimizing surface tension gradients that may develop across the film surface. It is less likely for multiple thin layer application to generate Bénard cells compared with a single thick layer application. The increase of the coating viscosity through using a resin with high molecular weight or a thickener additive will reduce Bénard cell mobility or even stop it completely. Silicon oil migrating to the coating surface which can suppress the surface tension differences due both to solvent evaporation or nonhomogeneous heating or the inclusion of additives such as low energy polymer or low energy oligomers may effectively eliminate this problem. 

Coating chemists are generally disappointed when they try to eliminate Bénard cells by way of a single measure. As an example, a thickening additive generally tends to prevent Bénard cells, however it may have an adverse impact on the levelling of the coating thus resulting in fixed roller traces. Silicon oil may generally eliminate Bénard cells, however it may also result in intercoat adhesion failure. It takes significant effort to eliminate Bénard cells using a single material without causing other adverse side effects. 

Fig 3.

On the other hand, using two materials simultaneously may generally be very effective. Each material contributes to the coating at a level that will not be sufficient to eliminate Bénard cells by itself but in the meantime not causing other adverse side effects. The use of a surface tension reducing additive or polymer together with a thixotrope or a thickener is among the examples of effective combinations that can be used for suppressing the formation of Bénard cells. (Figure.3: Schematic cross-section of a Bénard cell illustrating the upwards flow of the fluid at the center towards the edges of the cell followed by a downwards flow from the cell boundaries)