No abstract

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.

One of the challenges of coating plastic substrates for exterior automotive applications involves the tendency for light stabilizers, which inhibit UV degradation of the coating, to migrate out of the topcoat and into the underlying plastic substrate. The consequent depletion of the stabilizer from the topcoat could account for significantly poorer durability for coatings applied on plastic than on a nonpermeable substrate such as steel.These studies investigate the migration of both nonreactable and reactable UV absorbers (UVAs) and hindered amine light stabilizers (HALS) in fully formulated 2K urethane coatings applied on thermoplastic olefin (TPO) and steel substrates. Extensive migration of the nonreactable stabilizers occurs during accelerated exposure of the cured coating, and this migration correlates with poorer performance on TPO. Functionalization of the stabilizer with a group which covalently reacts into the coating will prevent much of this migration, and the improved degree of retention of the reactable light stabilizers in the topcoat correlates with improved performance on TPO substrates. In addition, stabilizer variables, such as the type and extent of functionalization as well as their chemical structure, have an effect on both the migration and performance of the stabilizers. T he use of plastics for automotive structural components has increased dramatically in recent years. Most of these plastic parts require coatings that are not only matched with the steel body in color and appearance, but that also meet the same durability criteria. Historically, coatings on plastic parts have not been able to match steel coatings in weatherability. Problems in durability may result from the somewhat different composition of the flexible plastic coating versus rigid coatings. They may also be caused by the migration of low molecular weight components from the plastic into the coating 1 as well as the migration of light stabilizers from the coating into the plastic. [2][3][4] In this article, we demonstrate that the latter mechanism-migration of conventional stabilizers out of the topcoat and into the plastic substrates-can account for a significant portion of the poorer performance observed in coatings applied on plastic versus on steel. Figure 1 illustrates a typical coating system construction for coatings on steel and on TPO. Although some limited migration of light stabilizers from the topcoat (basecoat + clearcoat) into the primer layers on the steel can occur, this is largely prevented by prebaking the primers before applying the basecoat and clearcoat layers, as well as by their highly crosslinked nature. In any case, any migration would be limited to the coating layers, since no migration into the steel substrate could take place. In contrast, all the paint layers on the TPO were applied wet-on-wet-on-wet. In addition, the TPO, which is much thicker than the two primer layers on steel, and is also noncrosslinked, can act as a virtually unlimited "sink" for the migration of stabilizers, resulting in...

No abstract

No abstract