Drying inhomogeneity of waterborne latex particles

Waterborne latex (polymer particles suspended in water) forms a polymeric coating after being dried. Contrary to ideally uniform drying, particles in real latexes distribute inhomogeneously in space during drying. This can undermine performances of coatings, such as drying front, cracking, delamination and skin layer formation. To understand drying inhomogeneity, we develop “OCT-Gravimetry- Video” method by combining optical coherence tomography (OCT), analytical balance and video camera together to monitor the drying process. Microscopic images by OCT give the spatial distribution of particles and the internal structure of coating. Water evaporation rates measured by gravimetry and film’s appearance by video are correlated with OCT results. We use this method to study the mechanism of skin layer formation, during which the film dries early on top and forms an impermeable skin that extends the film drying time considerably. Water-soluble additives, that can delay coalescence, were found able to reduce the drying time dramatically.