(497e) Multiscale Modeling and Optimizationof Wet-On-Wet Topcoat Curing

Curing of polymeric coating is a critical step in automotive coating manufacturing.  In the curing process, solvent evaporation, film thickness reduction and cross-linking reactions take place in a multistage operational environment.  The reactive drying operation consumes a large amount of energy, which provides radiation heat and convection air flows.  Therefore, saving energy without compromising the coating quality becomes the top priority for manufacturing process design.  On the other hand, the management of VOC emission is also critical.  Comparing with the traditional wet-on-dry technique, i.e., applying a wet clearcoat film on a dried basecoat layer, the wet-on-wet coating development which omits basecoat drying process is becoming a widely adopted baking and curing technique in automotive industries.  Solvent evaporation, film thickness reduction and cross-linking reactions occur within the very thin layers of basecoat and clearcoat simultaneously.  The solvent residual must be controlled under a certain level in order to prevent coating defects.  Due to the operational complexity and measurement difficulty, the process dynamics is not fully understandable, and process and product quality optimization is the area that is yet to be explored. 

In this paper, the mechanisms of solvent evaporation and film formation in wet-on-wet thermoset coating’s baking and curing process are studied.  We introduce a multiscale modeling and simulation method to characterize the wet-on-wet coating curing process.  The model allows to quantify macroscale panel heating, solvent removal, film thickness change, and surface topology progression, as well as mesoscale cross-linking reaction and elastically effective crosslink density.  The solvent mass loss rate is controlled by the solvent evaporation rate initially and then by the diffusion of solvent through the paint film.  A unique solvent concentration profile through both basecoat and clearcoat can be developed by simulating the internal solvent diffusion and solvent evaporation.  Model-based simulation also provides the opportunity for optimizing the reactive drying process.  A pre-heating process is introduced to reduce solvent residual to the desired level prior to occurrence of cross-linking reaction.  The case study on optimal topcoating will demonstrate methodological efficacy, where industrial data are used for solution validation.