Simulation of latex drying: from the fundamentals to complex formulation – LatexDry

While it is well known that the drying of latex films play a critical role in paints, cosmetics and inks, a realistic description of the drying process is in fact not yet available. The final properties of the film, such as its adhesion, its optical and mechanical properties depend not only on the formulation of the suspension, but also on the drying process itself and on the way that its components behave during drying. Films not only consist of solvent and colloids, but also of various additives such as surfactants, mineral charges, pigments... which are added in order to obtain the final desired properties. Drying essentially consists in the evaporation of the solvent that involves several complex physical phenomena. The process of drying creates fluxes of matter, both along the plane of the film, as well as in the direction perpendicular to its thickness. These fluxes are greatly modified when the volume fraction of the solid charges reach the random close packing, and they finally stop, leading to the final local composition of the film. Two approaches have been followed in order to describe these phenomena: (i) an empirical one, based on trial and error in order to optimize the film formulation and process leading to the desired properties and (ii) an analytical one, based on a simplification of the system and of the phenomena at play during drying, that allow a precise understanding of the phenomena at play, but that cannot describe the drying of real films.The goal of our project is to develop a numerical simulation tool of the mechanisms at work during drying and to predict the final properties of the dried film and in particular its local final composition and its adhesive strength on its support. To this end, we will use a cellular automata approach that will allow the description of the local equation of transport of each component of the suspension. Having begun to implement this approach for the simplest situations, we will now use its versatility in order to implement progressively the different components and the different flows of matter at work during drying.To this aim, we will also perform experimental measurements of two kinds, the first ones being used as basic ingredients for the development of the numerical model and the second ones serving to validate the results of the model:(i) Precise optical profilometry measurements will be performed, with the main aim of determining the curvature of the surface close to the front between the region of the film where the volume fraction of the suspension exceeds close packing (the gel phase), and the less concentrated, liquid phase region. This determination will be used as an ingredient of the numerical model as it will allow the assessment of the capillary pressures responsible for flows inside the film.(ii) The numerical results will be compared with the experimental measurements of the particle distribution across the film, as measured by X-ray tomography and to the adhesion forces exerted by the film during and after drying onto the substrate, as measured by Traction Force Microscopy.This project will provide the scientific and industrial community with a realistic model of film drying, taking into account the full composition and all the transport phenomena at work during drying. Moreover, the precise measurement of the profile with time will lead to fundamental knowledge about the origins of the flows during drying. By detailing the rational mechanism linking the initial formulation to the final properties of the film, this project will provide guidance towards improving the formulation of latices with a reduced content in volatile organic compounds.