STUDY AND MODELING OF COATED CAR PAINTING FILM BY INFRARED OR CONVECTIVE DRYING

In this study, we described and modelled some coupled material transfo rmations with heat and mass transfer phenomena which occur during the convective or the infrared drying of coated films of model car paintin gs. For these very complex reactive systems -highly shrinking, hygrosc opic, semi-transparent (infrared)-the vaporization process is coupled with the polymerization reaction itself which determines significantly the material properties of the dry paint coating. Three model systems (paint + support) were successively defined and investigated by assoc iation of five supports of different radiative or adhesive properties -polish aluminium, blackened aluminium, glass, galvanized iron and com posite plastic (SMC)-with three model paintings : epoxy-amine system, polyurethane system and polyvinylalcohol (PVA). Two laboratory combine d dryers (convective or infrared) were set up in order to control and regulate precisely the main process parameters : aerothermic condition s (temperature and velocity), spectrum (NIR or MIR) and flux density o f infrared radiation. Firstly, the kinetics of the polymerization were followed all along the process by three different techniques : DSC (D ifferential Scanning Calorimetry), SEC (Size Exclusion Chromatography) , FT-IRS (FOURIER Transform Infrared Spectroscopy). The glass transiti on temperature of the material during the drying process was deduced f ront DSC data with the fractional conversion of the monomer. These two parameters were well correlated by the DI BENEDETTO's relationship, t hus defining some characteristic curve of the painting system. Drying curves -temperature profiles and drying rates-were determined for PVA systems for many operating conditions, principally infrared flux densi ty, spectra type or air velocity. The two main coating properties nece ssary for the modelling were deduced from drying experiments 1: the me an radiative absorptivity as a function of the mean moisture content, 2: the water apparent mass diffusivity as a function of the temperatur e and of the local moisture content. A diffusive model written with a mobile frame of coordinates (lagrangian coordinates)-associated with a n overall heat balance of the support + coating system, has proved cap able of predecting very well the drying curves and the temperatures pr ofiles. So, this whole set of results -associated with other material properties (rheological, adhesive, color, etc...)-will be useful to ra tionally optimize the industrial dryers operating in the car manufacto ries.