HIGH-INTENSITY INFRARED DRYING STUDY .2. CASE OF THIN COATED FILMS

This study describes the combined drying (convective and infrared) of aqueous coated films under high infrared densities (i.e. 5-18 kW m-2). This shrinking, highly hygroscopic and transparent material exhibited specific characteristics which are very different to those observed in a pure convective drying process. Thus, the strong influence of the radiative properties of the support (absorptivity, reflectivity) on the drying kinetics and on the film temperatures of the material was observed. Modelling of the drying process, based on a diffusional model written using a mobile frame of coordinates (Lagrangian coordinates) and associated with an overall heat balance in the support + coated film system, has proved capable of expressing the drying curves and temperature evolution very precisely. Before simulation, two main coating properties were deduced from drying experiments: (i) the mean absorptivity as a function of the mean moisture content, and (ii) the apparent diffusivity as a function of the temperature and of the local moisture content. So, this simulation lets us approach quantitatively the process control and the risk of bubbling (or swelling) during the final drying period.