Oil-covered substrates: A model study of the evolution in the interphase during cure of an epoxy adhesive

The car industry uses adhesive bonding for bodywork assembly and, in many cases, application of adhesives is effected without prior degreasing of the steel sheet. As a consequence, oil originally at the steel/adhesive interface may potentially modify both initial and long-term behaviour. Direct study of the evolution in situ of such steel/oil/ adhesive systems would seem exceedingly difficult, if not impossible, with techniques presently available. Notwithstanding, we have attempted to model the situation, albeit by replacing the steel substrate by a material more conducive to a systematic and intimate study of the zone of contact, with an aim to follow any displacement of the oil in the interphase solid/oil/adhesive during the formation of the assemblage and its influence on behaviour. We have used Infra-Red spectroscopy and, in particular, developed a heated ATR cell. The technique allows evolution of chemical composition to be followed as a function of time during the curing process of the adhesive (appearance or disappearance of chemical species, crosslinking, etc...) and over a depth of several microns. As a consequence, a better understanding of behaviour and localisation of oil within a (model) structural joint has been obtained and is discussed. Our results point out the importance of chronology in the phenomena of oil-elimination and curing within the interphase (this chronology has a great influence on mechanical properties of real structural joints). The influence of oil formulation has been considered, as have the cure conditions. Finally, the potential perturbations of oil on the kinetics and mechanisms of interphase formation have been estimated and are discussed.