Leveling is a critical step in obtaining a smooth and uniform coating. During the application of paint, imperfections such as ripples or grooves often appear on the surface. For the coating to be acceptable, these defects need to be eliminated before the wet coating (fluid) cures.
It is generally accepted that surface tension is the main driver of flow out in a paint, while resistance to flow is the viscosity of the paint. The result of leveling is to reduce the surface tension of the film. Figure 2.7 illustrates the levelness of the newly formed sinusoidal surface of a continuous fused film. For films with a concentric sinusoidal surface, as shown in Figure 2.7, Rhodes and Orchard gave equations relating the leveling velocity tv to viscosity and surface tension.
where t and a are the final and initial amplitudes, γ is the wavelength, and h is the average thickness of the film. This formula is only valid when γ is greater than h. From the equation it is evident that leveling is due to larger film thickness, smaller wavelength, higher surface tension and lower melt viscosity.
However, the issue of the relative viscosity used in Equation 2.11 has not been fully resolved. Lin 18 suggests calculating the stress due to surface tension using one of several available methods.
Then, the viscosity at this shear stress is obtained from a predetermined flow curve. This may require measuring viscosity at very low strain rates. On the other hand, it is recommended to use
Figure 2.7 Desirable sinusoidal surface
The viscosity in Equation 2.11 is the zero shear value. Both methods yield similar results except when the material is highly sensitive to strain rate (n < 1).
When a material has a yield stress, surface tension needs to overcome the yield stress to initiate flow or leveling. Therefore, we replace λ in Equation 2.11 with λ':
This equation means that a coating fluid with a low yield stress should level off quickly. The requirement for leveling conflicts with the requirement for low sag or slump (high yield stress). Wu 35 claims that a shear-thinning fluid with an index of 0.6 exhibits the lowest sagging provided that the viscosity is 50 poise in reciprocal seconds.
Since such a fluid has no yield stress, it should flatten out nicely. This rheological behavior can be obtained in oligomeric powder coatings at temperatures close to their melting point or in solution coatings with high solids content. It is difficult to see how this behavior can be achieved in all cases, especially for latex dispersions with yield stress.
