The use of expandable graphite as a disbonding agent in structural adhesive joints

Expandable graphite (EG) offers the potential to be used as a controllable adhesive disbonding mechanism in an adhesively bonded joint. The graphite particles can be incorporated, in small quantities, into the adhesive layer. Heat can be used to trigger the expansion and thereby facilitate disassembly of structurally bonded assemblies. An experimental programme was developed to investigate the engineering capabilities of different types of bonded joints, made with an epoxy and a polyurethane (PU) adhesive, containing different amounts of EG. The short- and long-term mechanical properties of lap shear and wedge cleavage joints were evaluated. In contrast to the incorporation of more conventional physical and chemical foaming agents into bondlines, typically in excess of a 10% addition level, it was found that the addition of just a few per cent of graphite was sufficient to provide a reliable disbonding mechanism. Moreover, the usual adverse side effects on joint performance, associated with incorporating functional additives, were far more limited.