The determination of critical pigment volume concentration (CPVC) in organic coatings with fluorescence microscopy

The critical pigment volume concentration (CPVC) in pigmented organic coatings is one of the most important parameters in coating formulation (Asbeck and Van Loo [1]). It is usually determined experimentally by tracing the change of those void formation-related properties with PVC or detecting the appearance of voids in micrograph. However, these methods often underestimate the CPVC value because of the non-uniform dispersion of pigment. As discussed by Fishman et al. (1992, 1993 [2,3]), locally poor dispersion can cause local volumes in a coating film to have a local pigment volume concentration (PVC – local volume fraction) that exceeds the CPVC, even if the global or bulk value is not yet exceeded. PVCs at or above the global CPVC develop many voids, and so a possible measure of the CPVC is a rapid increase in local voids. In this work, we report a novel method for experimental determination of CPVC. In this method, fluorescent dye is used to mark the voids in coating layer and the fraction of fluorescent area is obtained from fluorescence image. The area fraction, which is a measure of voids volume fraction, is then plotted with PVC to obtain CPVC value. This method eliminates the influence of non-uniform dispersion of pigment. To illustrate the utility of this method, the CPVC values of various Mg-rich primers were determined with this method. The results showed that a reliable and accurate CPVC value can be obtained by this technique.