Shelf stability of reactive adhesive formulations: A case study for dicyandiamide-cured epoxy systems

Positron annihilation lifetime spectroscopy was used to study the curing behavior of epoxy resins of DGEBA with a hardener of dicyandiamide, from the initial viscous state of the formulation to the cured polymer. Isothermal measurements were done in the range from 50 to 80 °C. Gelation resulted in an increase of the intensity of ortho-positronium (o-Ps) and a sharp decrease in the lifetime. Change of o-Ps lifetime was used as a measure for the degree of curing. Applying the Johnson–Mehl–Avrami–Kolmogorov approach, these changes could be characterized and growth exponent ‘n’ and reaction constant ‘k’ were determined. Hence, we are able to project the shelf life of this one-component epoxy at room temperature by extrapolation.