Improving the Appearance of a 3-Coat-1-Bake Coating Film by Minimizing the Shrinkage Difference between the Three Layers

Abstract

The 3-coat-1-bake coating system has been widely employed in automobile body painting. This study examined whether the appearance (waviness) of the coating film can be improved by reducing the difference in the shrinkage percentage (by weight) between the primer surfacer and the basecoat and the clearcoat after flow/leveling stops in the clearcoat while baking. To delay the time of flow/leveling stops in the clearcoat (tC) and to reduce the difference in the shrinkage percentage between the basecoat and clearcoat, solventborne clearcoats were prepared by blocking all or part of the isocyanate group with 3,5-dimethylpyrazole (DMP). tC was measured using the electric-field tweezers system while baking at 140 °C. The respective shrinkage percentages of the primer surfacer, basecoat, and clearcoat (ωS, ωB, and ωC, respectively) were measured after tC. tC increased as the DMP content of the solventborne clearcoat increased. The ωC value is lower than the ωS and ωB values when the DMP content is zero; however, the ωC value increased when tC increased, and the ωS and ωB values decreased as tC increased. Wavescan Wa, Wb, Wc, and Wd decreased (i.e., improved the appearance) as |ωS − ωB| + |ωB − ωC| decreased. We confirmed that reducing the difference in the shrinkage percentage between the primer surfacer, basecoat, and clearcoat after tC is an effective way to improve the appearance.