Evaluation of Norbornene-Based Adhesives to Amine-Cured Epoxy for Self-Healing Applications

Abstract

High adhesive strengths are essential in self-healing polymers. In these novel materials, healing is triggered by crack propagation through embedded microcapsules in an epoxy matrix, which then release the liquid healing agent into the crack plane. Subsequent exposure of the healing agent to an embedded chemical initiator triggers ring-opening metathesis polymerization (ROMP), bonding the crack faces closed. In order to improve self-healing efficiencies in these systems, it is necessary to improve the adhesion of the polymerized healing agent with the epoxy matrix. In this study, the adhesive shear strength between different norbornene-based healing agents and an amine-cured epoxy resin was evaluated using single lap shear specimens. The healing agents tested include endo-dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) and DCPD/ENB blends. 5-Norbornene-2-methanol (NBM) and 5-norbornene-2-exo, 3-exo-dimethanol (NBDM) were used as adhesion promoters because they contain hydroxyl groups which can form hydrogen bonds with the amine-cured epoxy adherend. A custom synthesized norbornene-based crosslinking agent was also added to improve the adhesion of the polymerized ENB by increasing its crosslink density after ROMP. The effects of catalyst loading, polymerization time and cure temperature on the adhesive bond strength are studied in detail.