The role of pigments in self-stratifying epoxy/silicone coatings

The integration of pigments in self-stratifying coatings represents a critical step towards the industrial realization of this technology. However, the role of pigments in the self-stratification process and the parameters determining their positioning in the coating remain to be fully elucidated. This work systematically studies eight different pigments in an epoxy/silicone-based coating composition for their effects on stratification and final distribution across coating and interfaces. The key processing parameters investigated included: the binder phase in which the pigment was initially dispersed (epoxy or silicone phase), pigment concentration, binder phase mixing ratio, and applied film thickness. Rheological measurements of the binder phases upon pigmentation, cross-sectional coating analyses, particle characterization and surface energy studies of the pigments and binder phases were conducted.

Pigments, when originally dispersed into the epoxy phase, were always retained in this phase. Self-stratification was strongly determined by pigment-induced rheological changes of the epoxy phase, which varied with pigment type and concentration. Moreover, the addition of wetting and dispersing additives or organic solvents exhibited a positive effect on film formation. Conversely, self-stratification was worse overall when pigments were initially incorporated into the silicone phase. Depending on the pigment type, this was due to migration of pigments to the epoxy phase or their adsorption at the interface of the binder phases. These unexpected dependent phenomena are concluded to be an interplay between interfacial interactions between the binder phases and the pigment particles, which lead to preferential wetting behavior, and the kinetics of the binder phase displacement at the pigment surface.