Adhesion and crosslinking in epoxy resin steel assemblies

A novel technique has been employed to investigate the development of interfacial bonding between steel and epoxy resins. Whereas such systems are usually rigid, precluding use of the very informative peel test, we have used spring steel as a flexible adherend and peeled this from the (relatively) rigid crosslinked polymers. Peel energy has been assessed for 180 degrees and 90 degrees tests, using a cylindrical former to limit irreversible deformation of the steel. Cure cycles for the resins DGEBA/DDA and DGEBA/DDS have been studied using DSC, and results exploited in such a way that peel tests could be effected on assemblies for which the total degree of polymer crosslinking was standardised, yet polymer/steel contact time during crosslinking was varied. The degree of potential reactivity of the polymers with respect to the steel was thus controlled. It was found, for both polymers, that measured adhesion energy was an approximately linear function of the fraction of crosslinking agent that reacted whilst epoxy/steel contact was maintained. Master curves for the two systems have been plotted, irrespective of cure conditions, the DGEBA/DDS system presenting better adhesion. Although no direct evidence of type and/or number of interfacial bonds is presently available, a simple argument suggests that chemical reactions occurring at ca. 1% of available surface sites may markedly improve adhesion.