(7cw) Skin Layer Formation during Drying of Latex Films

During drying of a waterborne latex coating film with a dry thickness in the millimeter range, the top surface of the film can get dried early and form a polymer skin while beneath it is still wet. Skin layer impedes water evaporation, which determines the total film drying time to a large extent. This study focuses on the kinetic process of skin layer formation and investigates how water-soluble additives affect the skin and drying process. Besides the global gravimetric analysis, we monitor the drying evolution of film structure at a local position by innovatively using optical coherence tomography (OCT). During the whole film drying process, skin appeared on the top of the film when the water evaporation rate became negligible. The thickness of skin, determined by OCT, was not fixed but increased slowly with the square root of time. Some additives (surfactants, dispersants and other water-soluble polymers), mixed with a latex, were found to promote the thickening rate of the skin and shorten the total film drying time. We propose that those additive molecules, residing between boundaries of deformed particles within the skin, retard particle coalescence and constitute continuous pathways for water evaporation. This study is expected to provide a guide of both theoretical and experimental significance in solving the drying problems caused by the skin layer formation.

Research Interests: colloids, nanoparticle, waterborne latex coating, optical characterizations

Teaching Interests: colloids, nanoparticle, waterborne latex coating, optical characterizations