KINETICS OF FILM FORMATION IN ACRYLIC LATICES STUDIED WITH MULTIPLE-ANGLE-OF-INCIDENCE ELLIPSOMETRY AND ENVIRONMENTAL SEM

A combination of multiple-angle-of-incidence ellipsometry (MAIE) and environmental SEM (ESEM) was used to characterize the microstructure of acrylic latices during all four stages of film formation, starting from an aqueous colloidal dispersion (Stage I) and evolving to a continuous coating having no internal solid-solid interfaces (Stage IV). Stage II is usually defined as a close-packed array with water-filled interstices, and Stage III is defined as a densely packed array of deformed particles. This analysis identified an additional stage, II*, intermediate to the conventionally defined Stages II and III. The onset of this new stage, which coincides with the development of optical clarity, occurs at nearly the same time (normalized by the final film thickness), regardless of the glass transition temperature (T-g) of the latex polymer. The duration of Stage II* and the kinetics of particle coalescence in Stage III, on the other hand, are a function of T-g. A latex with a T-g well below the ambient temperature can deform rapidly to fill the space left by the evaporation of water. A latex with a higher T-g cannot, and so air voids and surface roughness develop and persist over measurable times.