A model for the dehydration of waterborne basecoat
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In vehicle manufacturing, a common topcoat process sequence is the application of a waterborne basecoat, partial dehydration, application of a solventborne clearcoat, then curing at approximately 150°C. Anecdotal evidence suggests that under “harsh” (high temperature and velocity) dehydration conditions, a skin can be formed, which can trap excess water in the basecoat. Upon curing, the excess water forms vapor that can burst through the topcoat, leading to surface imperfections, termed pinholes or pops.
A forward-time center-distance finite difference model, incorporating only diffusion and evaporation and experimentally determined coefficients, was used to simulate the water concentration in the layers of a paint film. Simulations were conducted at constant temperatures (27°C and 82°C) as well as by using temperature traces from a full-size dehydration oven under harsh (82°C) and gentle (54°C) conditions. The model accurately predicted the overall solids content after dehydration. However, in both temperature scenarios, the surface layer water concentration was less at the low temperature for the same overall solids content. This counter-intuitive finding is attributed to the increase in diffusivity with temperature, which allows water to be supplied to the surface faster in the high temperature run in each scenario. Therefore, this simple diffusion-evaporation model is not suitable for simulating water entrapment leading to popping.
KeywordsDrying waterborne defects automotive-OEM process modeling simulation
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