Leveling Additives Affect Film Formation of Powder Coatings
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).