Crack-Healing Behavior of Epoxy-Amine Thermosets

This work offers detailed experimental evidence for crack healing in fully formed epoxy-amine polymer networks. Compact tension specimens of diglycidyl ether of bisphenol A (EPON-828) and 4,4'-methylene biscyclohexanamine were synthesized at stoichiometry and with an excess or paucity of an amine curing agent. Healing efficiencies were measured on the basis of the regain in the fracture load after a healing protocol was applied. For all systems investigated, the average healing efficiency for first fracture was greater than 50% when healing was conducted at 185 degrees C for 1 h. The crack-healing behavior was highly repeatable at all stoichiometries and healing was found to occur for repeated fracture-heal cycles of the same specimen. On the basis of results from size exclusion chromatography for the extractable phase, infrared spectroscopy, and scanning electron microscopy, it is postulated that healing is primarily due to mechanical interlocking of the nodular topology of a fractured crack interface that occurs in the rubbery state and is set in place by vitrification upon cooling. A 1/2 power dependence of the healing efficiency on the healing time was observed, and this suggests that the interlocking of topographical features is governed by diffusive processes. For networks cured with a large excess of epoxide groups, the recovered fracture load was higher than that of virgin specimens. In this case, polyetherification or homopolymerization of previously unreacted epoxy groups increased the healing efficiency significantly, and this suggests that low degrees of covalent bonding can significantly enhance healing behavior in these systems. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2191-2201, 2009