Dependence of Wetting Behavior on the Thickness of Highly Viscoelastic Films

Both static and dynamic contact angle measurements were used to examine the impact of thickness on the wetting of highly viscoelastic films. The films are composed of a thermally stripped, solvent-borne acrylic polymer. Results show that wetting behavior is strongly influenced for polymer coatings < 10 mu m thick and plateaus above this. This dependency is attributed to the extent to which surface tension forces can induce deformation. For thinner coatings, surface deformation is limited, which allows propagation of the three-phase line, resulting in low contact angles for sessile drop measurements and little or no discernible pinning in dynamic wetting measurements. With increasing film thickness, residual deformations due to the pinning of the wetting line start to appear and become quite distinct at thicknesses corresponding to the plateau wetting behavior. The onset of pinning is accompanied by nonwetting contact angles for sessile drops and stick-and-break propagations in dynamic measurements. The observed relationship between wetting behavior and decreased coating weight is similar to that reported previously between (static and dynamic) contact angle and increased wetting rate. It appears both relationships result from an increase in the elastic response of the film. The findings reviewed demonstrate that both film thickness and wetting rate can be used to control drop movement on viscoelastic surfaces.