The characterisation of the interfacial chemistry of adhesion of rigid polyurethane foam to aluminium

The interfacial interactions between a rigid polyurethane foam (RPUF) and aluminium have been studied to understand the mechanisms of adhesion. Three different blowing systems are used in the production of the foam: chemical blowing, physical blowing and a mix of chemical and physical blowing systems. In addition an unfoamed system has been examined for comparison of the catalysts behaviour with and without blowing agents and the surfactant. Peeled failure surfaces have been examined by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF–SIMS). To examine the intact interfacial regions of the RPUFs cured against aluminium, samples have been sectioned by microtomy. The failure surfaces of the aluminium sides exhibit relatively clean aluminium surfaces with RPUF residues observed for all three foamed systems; such thin RPUF layers (ca. 1 nm) indicate good adhesion (and a cohesive failure) between foam and substrate and that the interfacial adhesion is higher than the cohesive strength of the foam. The unfoamed system behaves in a similar manner but has a higher peel strength. A fragment indicative of covalent bond formation between isocyanate and aluminium (nominal mass at 102 u: AlCHNO₃ ⁻) is observed on the failure surface of aluminium side, where RPUF/aluminium interface region is present, for all foams. The catalyst used in these formulations, pentamethyldiethylenetriamine (PMDETA), is concentrated at the interface area. Whilst examination of the sectioned specimens shows that the silicone surfactant is concentrated within the cell area fulfilling its role on cell formation and stabilisation, and is not segregated at the RPUF/aluminium interface.