Fall 2000 Vol. 2, No. 3

Adhesion quality and optical appearance of coatings are determined by film formation. Several factors, including application parameters, rheological behavior and interfacial properties, interact in a complicated manner (1). Simply put, surface tension can be regarded as the driving force and viscosity as the resistance of wetting and leveling (2-5). This article discusses the interfacial properties. The rheological behavior of powder coating melts, as well as the influence of different application parameters (e.g. particle-size distribution, film thickness, etc.) on film formation are not discussed.

Wetting of a substrate and leveling of a fluid film strongly depend on the surface tension of the coating. However, both processes have opposing requirements in terms of surface tension. If the surface tension is too high, poor wetting occurs, which leads to defects such as craters. On the other hand, if the surface tension is too low, the leveling is adversely influenced, which leads to wavy surfaces known as orange peel (6). Moreover, surface flow can cause surface defects due to local surface-tension differences, known as the Marangoni Effect (7,8). These differences can arise from temperature gradients and local inhomgenieties (e.g. contaminations). Therefore, the surface tension of a coating must be controlled and adjusted (9,10).