Migration of reactable UVAs and HALS in automotive plastic coatings

Loss of light stabilizers from automotive finishes can significantly reduce the durability of coatings on plastic substrates. Minimizing movement by retaining the stabilizers in the targeted layer can greatly improve coating longevity. This article examines the migratory patterns of a series of traditional UVA and HALS additives in fully formulated 1K silane crosslinked coatings applied over both plastic and steel substrates, comparing their migratory patterns to that of reactable light stabilizers containing hydroxyl functionality. Analysis of the coating layers with the various UVAs and HALS revealed that functionalization of the light stabilizer with reactable hydroxy groups can prevent migration into the plastic; whereas nonreactable light stabilizers migrate into lower coating system layers and even into the plastic substrate. The greatest extent of migration does not occur during initial cure of the wet coating, but during exposure. Significant depletion of the nonreactable light stabilizers from the topcoat occurs as early as 1500 hours of Xenon boro/boro exposure. This paper was awarded first place in the Shelby F. Thames Best Paper Award, at the 30th International Waterborne, High-Solids and Powder Coatings Symposium, February 26–28, 2003, in New Orleans, LA.