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 c ases, application of adhesives is effected without prior degreasing of the steel sheet. As a consequence, oil originally at the steel/adhesive interfa ce may potentially modify both initial and long-term behaviour. Direct stud y of the evolution in situ of such steel/oil/ adhesive systems would seem e xceedingly difficult, if not impossible, with techniques presently availabl e. Notwithstanding, we have attempted to model the situation, albeit by rep lacing 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 displacem ent 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 spec troscopy and, in particular, developed a heated ATR cell. The technique all ows evolution of chemical composition to be followed as a function of time during the curing process of the adhesive (appearance or disappearance of c hemical 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. O ur results point out the importance of chronology in the phenomena of oil-e limination and curing within the interphase (this chronology has a great in fluence on mechanical properties of real structural joints). The influence of oil formulation has been considered, as have the cure conditions. Finall y, the potential perturbations of oil on the kinetics and mechanisms of int erphase formation have been estimated and are discussed.