Mechanism of Color Variation in Spraying of Metallic Waterborne Basecoat

Mechanism of color variation in spraying of a metallic automotive waterborne basecoat has been studied by measuring variations of aluminum flake orientation in wet films, film thickness and nonvolatile concentration with time after spraying, and by evaluating the flake content in baked films. Though the Mechanism is similar to that for solventborne basecoat which is provided in our previous papers, the flakes are found to be slightly oriented parallel to the substrate surface by the flow of the droplet containing the flakes resulting from impingement of the droplet on the substrate during spraying and to be remarkably oriented by the shrinkage of the film thickness during drying (flash, heated flash and baking). These results are attributed to larger thickness of the film which plays as a buffer layer for impinging droplets during spraying and lower nonvolatile concentration immediately after spraying which increases the shrinkage of the film thickness throughout drying. These result from only a little water evaporation during spraying. The degree of the parallel orientation by the film shrinkage decreases, as the film thickness immediately after spraying increases. This is due to the increase in the random orientation of the flakes by the effects such as gravity and evaporative convection since the increase in the film viscosity caused by the water evaporation is delayed by the increase in the film thickness, when the film viscosity immediately after spraying is not high enough to prohibit the random orientation.