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Benard cells and other convection effects.

Benard cells are defects that look as if someone has imprinted a series of hexagonal shapes on the surface of the paint film. They also may be four or five sided or even roughly circular. Often there is a mixture of forms as shown in the figure. Benard cells have very noticeable centers and are produced by convection circulation patterns induced by solvent evaporation in paint films. They can occur during air drying as well as baking. The whole cell is in motion with currents streaming up the center and flowing down at the cell edges. These flow patterns lead to segregation and deposition of the different coating components in different parts of the cell. The cells do not occur singly, but in networks that cause color irregularities in coatings with multiple pigments and can produce surface effects such as roughness, low gloss and waviness in virtually any coating, including clears. It is rare for clears to show the hexagonal surface pattern, but I am convinced that some of the waviness in clearcoats and the occasional waves induced in automotive basecoats by clearcoats both are due to convection flow.

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Benard cells may be quite large (several cm across) or so small that a microscope is needed to see them. In the latter case, the cells may give a dull or matte appearance to the paint film. I recall a self-flatting white coil coating that depended on accidentally formed Benard cells to give it the correct low gloss. In colored coatings, the circulation within the cells can cause pigment separation leading to color changes. For example, blue pigment may float to the surface and be trapped at the edges of the cells giving them dark blue borders. A related defect, floating, may occur in the form of streaks or mottling rather than obvious cells, but is the consequence of convection flow. Pigment flocculation contributes to cell formation and floating because it encourages separation of pigments that are not flocculated.

Convection flow can be prevented by lowering the film thickness, increasing the viscosity of the coating, minimizing surface tension gradients across the film surface and preventing or reducing flocculation. Thin multiple coats are less likely to suffer convection flow than a single thick paint film. Raising the viscosity will slow down the circulation pattern. Introducing a surfactant or a low energy polymer or oligomer that comes to the coating surface prevents surface tension gradients that arise from solvent evaporation and uneven heating. Reducing the rate of evaporation by reformulating with slower solvents has helped in some cases, but can make things worse if the coating does not dry rapidly enough. Flocculation can be prevented or reduced through better dispersion practices and the use of wetting agents.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email publications@coatingstech.org.
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Title Annotation:Coatings Clinic
Author:Schoff, Clifford K.
Publication:JCT CoatingsTech
Date:Feb 1, 2005
Words:514
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