A model for adhesion-producing interactions of zinc oxide surfaces with alcohols,amines, and alkenes

The interactions between paint/adhesive polymers and metal surfaces that are critical for adhesion have been studied theoretically. This study used zinc oxide as a model of a galvanized steel surface, and ammonia, water, and ethylene as models for amino, hydroxy, and unsaturated functionalities in paint/adhesive polymers. Ab initio molecular orbital calculations were carried out on zinc oxide and zinc oxide dimer. Geometries were optimized at the HF/3-21G level and relative energies were calculated by CASSCF/3-21G and by MP2 with the DZP basis set of Wachters and Hay. Ethylene forms a stable complex with zinc oxide dimer that has a stabilization energy of 24.9 kcal/mol. Insertion of ethylene into zinc oxide dimer to form a stable six-membered ring adduct occurs with a surprisingly low activation energy of 8.8 kcal/mol. The binding energy of ammonia with zinc oxide dimer is 38.5 kcal/mol and the activation energy for insertion of ammonia forming covalent Zn-NH₂ and O-H bonds is calculated to be 9.6 kcal/mol. Aminolysis of zinc oxide dimer with two ammonia molecules has a predicted barrier height of 6.7 kcal/mol. The transition structure for Zn-O bond rupture with one NH₃ and one H₂O molecule is only 1.5 kcal/mol higher in energy than the reactant cluster. The calculations suggest that alkenes, amines, and alcohols could readily form covalent bonds with the ZnO surface, thereby facilitating adhesion of the polymer containing these functional groups to a galvanized surface.