ZINC COATED STEEL EPOXY ADHESIVE SYSTEMS - INVESTIGATION OF THE INTERFACIAL ZONE BY FTIR SPECTROSCOPY

The present study takes advantage of the ability of Fourier Transform Infrared Spectroscopy (FTIR) for the analysis of ultrathin organic fil ms on metals. FTIR in the reflection mode (IRRAS) is used in order to study the interaction of ultrathin films of dicyandiamide (hardener of most one-pack epoxy resins) with various substrates, model ones such as gold or zinc and industrial ones such as steel and zinc-coated stee ls. Pure zinc surfaces and, to a lesser extent, zinc-coated steels are shown to react with dicyandiamide after heating at 180 degrees C, as evidenced by the frequency shift of the absorption band (at about 2200 cm(-1)) characteristic for nitrile groups. As real systems consist of thick layers of a fully formulated adhesive cured onto a metallic sub strate, the direct investigation of such a buried interphase is no lon ger possible by FTIR and by most of the known spectroscopies. Some mec hanically tested specimens are then analysed, after failure, by FTIR m icrospectrometry. The spectra obtained, corresponding to the fracture initiation zone which is about 100 mu m in diameter, advocate for the presence of an ultrathin layer of modified polymer still covering the substrate.