Migration of cross-linking agents to the surface during ageing of a structural epoxy adhesive

During the thermal degradation of a modified structural epoxy adhesive, based on TGDDM and cured with DDA and DDS, there is migration of nitrogen- and sulphur-containing molecules from the bulk towards the surface, as demonstrated by X-ray analysis. In this paper, it is shown that the thermal behaviour of these molecules is similar to that of the cross-linking agents themselves. The hypothesis of a surface layer consisting of DDA and DDS only in a 1/1 molar ratio is consistent with the observed concentrations. Results strongly suggest that the molecules which migrate at the surface are indeed the cross-linking agents.