FLUID MECHANICS AND TRANSPORT PHENOMENA Drying and Film Formation of Industrial Waterborne Latices I. Ludwig, W. Schabel and M. Kind Universita ¨t Karlsruhe (TH), 76128 Karlsruhe, Germany J.-C. Castaing and P. Ferlin Rhodia Recherches et Technologies, Aubervilliers, France DOI 10.1002/aic.11098 Published online January 19, 2006 in Wiley InterScience (www.interscience.wiley.com). Experimental evidence is given for the mechanism of film formation from industrial waterborne latices using Inverse-Micro-Raman-Spectroscopy (IMRS). In the vertical direction of the film drying is gas-side controlled, indicated by uniform water concen- tration profiles. In the horizontal direction inhomogeneous drying resulting from a hor- izontal mass flux toward the edge of the film and the formation of a drying front are observed. The completeness of film formation is tested by so-called IMRS redispersion experiments. For hard latices (T experiment ^ T mff ) particle deformation is incomplete and the final coating—although transparent and optically clear—is a porous structure with a network of surfactant material located at the particle interfaces. The use of a film-forming aid lowers the polymer’s minimum film formation temperature (T mff ) and facilitates particle deformation and polymer interdiffusion. The result is a nonporous film structure where individual particles and a network of surfactant material are no longer observed. IMRS redispersion experiments are compared with pictures of the final coating surface obtained from atomic force microscopy (AFM). Ó 2007 American Institute of Chemical Engineers AIChE J, 53: 549–560, 2007 Keywords: water-based coatings, latex, drying, film formation, Raman spectroscopy Introduction As a result of stricter environmental regulations, aqueous latex dispersions, among others such as aqueous polyurethane dispersions or alkyd emulsions, are considered to constitute one alternative for future coatings and paints compared to solvent-based formulations. Industry invests significant time and effort into the development of water-based systems that show a drying behavior comparable to that of the established solvent-based formulations. Further improvements in the application properties of water-based formulations require a fundamental understanding of the drying and film formation mechanism from colloidal dispersions. During the last decades many researchers have dealt with the question of latex film formation, 1–3 revealing that film formation from aqueous latex dispersions is very complex. Experimental studies on a broad variety of latex dispersions prepared and dried under many different conditions and con- ducted by a large number of different methods have led to a flood of information containing valuable details of latex film formation but that are often not transferable even to closely related systems. A large number of experimental techniques have been used to investigate different aspects of spacing and defor- mation of the latex particles: Structural changes in films cast from soft particles protected by hydrophilic mem- branes have been extensively studied by small-angle neu- tron scattering (SANS). 4–7 The same technique has also been applied to study particle coalescence and surfactant ID: 3b2server Date: 13/2/07 Time: 07:55 Path: J:/Wiley_Sgml/JOHNWILEY/3B2-Final-Pdfs/Electronic/WEB_PDF/Process/C2AIC#060082 Ó 2007 American Institute of Chemical Engineers AIChE Journal March 2007 Vol. 53, No. 3 549