A ToF-SIMS Investigation of the Thermodynamics and Bonding of Polymeric Methylene Diphenyl Diisocyanate on Oxidised Aluminium and Iron Surfaces

Polymeric methylene diphenyl diisocyanate (PMDI) is the major component of polyurethane formulations and as a result the adhesion, or indeed abhesion, of polyurethanes, in a variety of forms (foams, coatings and adhesives), to metal substrates will be a function of the interactions between PMDI and metal surfaces. In this paper the adsorption of PMDI on oxidised metal (aluminium and iron) substrates has been investigated. The thermodynamics of adsorption has been examined by the construction of adsorption isotherms derived from ToF-SIMS data. At low solution concentration, the adsorption isotherns of PMDI are not of the Langmuir type, but are shown to conform to Langmuir adsorption at higher solution concentrations (> 1 g L-1). The interaction between the PMDI and iron is probably an acid-base interaction, and thus the adsorption of small PMDI molecules is displaced by larger PMDI molecules on the iron surface above a critical solution concentration. By contrast, such displacement is small on the aluminium surface as a result of dominance of covalent bond formation between PMDI and the metal substrate