Adhesion of pressure-sensitive adhesives to automotive coatings: influence of topcoat composition
Résumé
This study characterizes the influence of automotive coating composition on the adhesion of pressure-sensitive adhesives (PSA). Automotive topcoats are generally constituted of a basecoat/clearcoat system, but a layer of lacquer is used for white colour coating. X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) are used to study the topcoat surface composition. Siloxane additives are detected on the clearcoat surface but not on the lacquer surface. Peel tests show that the adhesion of PSA to the lacquer is higher than to the clearcoat. Moreover, study of clearcoat formulated without additives confirms that the siloxane additives are unfavourable to the adhesion of PSA. When the PSA is peeled from a clearcoat, analyses of failure surfaces indicate that the failure seems to propagate within the PSA near the interface. On the contrary, the failure seems to propagate within the bulk of the lacquer during peeling. This difference is explained by XPS analysis of lacquer: the outermost surface is constituted of a layer of oxygenated unidentified compounds, which are detected on the failure surface of PSA. Chemical bonds between these compounds and the lacquer network seem to be weaker than the bonds between these compounds and the PSA. Analyses point out that the ethanol cleaning can remove the additives from the topcoat surface and improve considerably the adhesion to the clearcoat. If the topcoat is cleaned before applying the PSA, the failure propagates deeper within the PSA than if the topcoat is not cleaned.