Evolution of surface structure and chemistry of pigmented coatings during drying

Pigmented coatings made of styrene-butadiene (SB) latex and kaolin clay with a wide range of pigment volume concentration (PVC) were quenched and freeze-dried at different stages of film formation. The surface was characterized by XPS, SEM, gloss and surface energy measurements. The SB/clay ratio increased at the surface with drying time; The latex-pigment interaction at the surface, at and below the critical pigment volume concentration (CPVC), is different from that above CPVC. While at and below CPVC latex coalescence is almost complete at the second critical concentration (SCC), where air enters into the coating, above CPVC latex continues coalescing after SCC. This behavior of latex coalescence at the surface above CPVC is different from that found in the bulk. Even at PVC as low as 40%, XPS results indicate that the surface is still heterogeneous and not covered totally with latex. Surface energy and its polar component decreased during film formation. (C) 2000 Elsevier Science S.A. All rights reserved.