Using Beer’s Law to Model Percent Transmittance of Multilayer Composite Coatings

By Don Campbell, Antonette St. Pierre

Transmission of light through automotive topcoat and primer layers can lead to degradation of the underlying electrocoat layer and to topcoat delamination. To protect against this, it is critical that transmission of both ultraviolet wavelengths and certain visible wavelengths be effectively blocked by the topcoat and primer layers. The clearcoat, basecoat, and primer each have their own role and combine to protect against light transmission.

The transmittance of these combined layers is typically measured by the Integrating Sphere UV-Visible Spectrophotometer. It would both simplify measurement of the top-coat systems and allow better system modeling if these layers could be measured separately and combined using Beer’s Law to obtain the transmittance of the total topcoat system.

Although some automotive pigments function by reflecting or scattering, we demonstrate here that log-linear models similar to Beer’s Law for absorption can still be used to combine these layers and interpolate the effect of individual layer thickness on the total topcoat transmittance.