(224d) Particle Segregation in Drying Coatings

Paints and coatings are composite materials made from multiple components, each imparting various properties on the final dried application. In many cases the arrangement of different components is desired to be non-uniform. Examples include anti-fouling or self cleaning agents at the film/air interface, or an increased amount of adhering particles at the substrate.

The mechanism by which particles in a film come into close packing during solvent evaporation has an important role to play in the final film morphology. As the solvent evaporates from the top surface, the particles can develop a non-uniform concentration across the vertical height of the film. This process is described by the Peclet number, which determines the relative importance of the evaporative induced convection and the particle diffusion. By tuning the different diffusion rates of a particle mixture we can achieve a vertical segregation in a film, driven by evaporation.

By applying classical diffusion to a two component hard sphere system, a theory for this method of segregation has been derived. The diffusion behaviour of the particles predicted by this theory has been modelled numerically as a coupled PDE system and has shown a significant segregation between two types of particle under varying drying conditions.

This system is also investigated experimentally using controlled evaporation of water from drying latex films containing two components, each with a different particle radius. Volume fractions of particles of each component were evaluated both on the surface, using Atomic Force Microscopy, and through the cross-section of the films, using Freeze Fracture SEM and Nuclear Magnetic Resonance techniques.