Mechanism for environmental etch of acrylic melamine-based automotive clearcoats: identification of degradation products

This paper focuses on defining the reaction mechanism involved in the envir onmental etch of acrylic melamine-based automotive clearcoats via an identi fication of reaction products. This has been accomplished through a compara tive study of products formed on acid treatment of neat crosslinkers, and t hose formed following laboratory acid treatment and field exposure of acryl ic melamine clearcoat systems. Bulk elemental, X-ray photoelectron spectros copy (XPS), and infrared (IR) analyses of sulfuric acid-treated melamine cr osslinkers show that acid hydrolysis results primarily in the formation of melamine sulfate. Melamine sulfate formation was also observed following la boratory and field exposure of acrylic melamine clearcoat systems. These re sults confirm that the primary mode of melamine crosslink decomposition is through hydrolysis of acetal linkages and subsequent formation of melamine sulfates. However, data show that hydrolysis of pendent amino groups on the triazine ring also occurs.