Curing study and optimization of a polyurethane-based model paint coated on sheet molding compound - Part I: Polymerization and drying modelling

This study deals with the drying I curing process of a model car painting a pplied on SMC type support and more precisely with the bubbling phenomenon (craters, chips, bubbles, etc.) which appears during the industrial drying of these materials. Firstly, the polymerization and the drying kinetics were experimentally det ermined for many operating conditions. Concerning the polymerization kineti cs determination, different classical analytical methods were used as the d ifferential scanning calorimetry (DSC) and the microdielectrometry, this la st one allowing the determination of the polymerization degree in situ and in real time. All these techniques lead to coherent data which were interpr eted by a third order autocatalytic model. Furthermore, we observed a uniqu e characteristic curve interpreted by the Di Benedetto's model relating the glass transition temperature of the paint film and the reaction conversion degree whatever the aerothermal conditions were. Then, a laboratory dryer equipped with convective, infrared or combined inf rared + convective heating was set up to measure the influence of the opera ting parameters on the classical drying curves. So, we studied the influenc e of the various aerothermic parameters (air temperature and velocity, infr ared flux density, etc.) on the drying kinetics and on the temperature prof ile curves. Two types of substrate - iron and SMC - and different paint for mulations (additives, pigments, etc.) in convective, infrared or combined ( infrared + convective) drying mode were successively investigated. A Fickian diffusion model, taking into account the film shrinkage (lagrangi an coordinates) associated with an overall heat balance of the support + co ated film system, was able to predict the drying curves and the temperature profiles accurately.