Formation, structure and morphology of polyurethane-metal interphases

Thin films (few 10 nm to some μm) of different two-part polyurethane adhesive systems (polyether resins and aromatic isocyanate hardener) are characterised on the native metal surfaces of gold, aluminium, copper and steel, thus acting as a model for the interphase in technically relevant material compounds. It is investigated in how far the curing processes, the resulting chemical structure and the morphology of the polymer films differ from the bulk. The comparison of films on the different metals and with the bulk provides conclusions on the properties of the interphase at polymer-metal contacts. The variation of the film thickness gives access to microstructure gradients. The chemical structure of the films is characterised by IR spectroscopy during curing at room temperature (RT kinetics) and in the cured state. Furthermore, microscopic (e.g. SFM) and additional spectroscopic techniques (e.g. XPS) characterise the films and their surface with respect to morphology, composition, homogeneity and topography. The results reveal specific features in the interphase caused by adhesive interactions, catalysis, phase separation and transport phenomena on the different metal surfaces. Remarkable quantitative differences can be seen in the chemical structure formation of thin PU films with respect to reaction rate and final degree of cure.