Adhesion of polyurethanes on native metal surfaces – stability and the role of urea-like species

Water is present in most technical applications of polyurethane (PU) adhesives, both from atmosphere and as adsorbate on the substrate. Hence, studies on the fundamental adhesion mechanisms have not only to cover the PU monomers but possible secondary reaction products like amines and ureas on the given substrate as well.In this work, diluted solutions of the monomer 4,4′-methylene diphenyl diisocyanate (4,4′–MDI) in tetrahydrofuran (THF) are exposed to the native surfaces of Au, Al and Cu in dried air. The resulting monomer adsorbate layers on the metals are stored in dried air for several hours. Therefore, any water essentially originates from the native metal surfaces. The chemical structure and the orientation of the adsorbed monomer molecules are evaluated in situ as a function of storage time by infrared external reflection absorption spectroscopy (IR-ERAS). A sequence of rinsing cycles with pure THF helps to discriminate between physisorbed and chemisorbed molecules.The changes of the spectra with storage time are significant and specific for each of the metal substrates. They reveal the formation of urea-like species which are easily removed by THF. Hence, they do not contribute to adhesion. The remaining isocyanate molecules adhere strongly and specifically to the substrates. The corresponding strong spectral modifications point to a chemical adhesion mechanism.