Strategies for the attainment of dismantleable adhesive systems

Dismantleable structural adhesion is required to facilitate the recycling and re-use of critical materials and to enable flexible manufacturing. There are a number of reported ways to achieve dismantleable adhesion. These include: the incorporation of functional fillers such as thermally expandable microspheres (TEMs) and chemical foaming agents (CFAs) in the adhesive phase; and designing joints which on the application of a potential di?erence lead to disbonding. These approaches have shown > 90 % reduction in initial adhesion levels but have limitations in terms of their applicability in industrially-useful bonding systems. The current Project will introduce a number of novel methods for achieving dismantleable adhesion with aluminium-to-aluminium and carbon fibre reinforced polymer (CFRP)-to-CFRP joints. These methods are based upon: microwave triggerable reactive fillers within the adhesive phase; ohmic heating of a semiconductive adhesive; and thermally decomposable interphases. Expandable graphite (EG) exfoliates upon heating, which can be triggered when exposed to microwaves. When the EG is added to the adhesive phase and microwaved, a solid piece of adhesive forms a flaky powder. When exposed to microwaves, CFRP-to-CFRP joints bonded using an adhesive containing EG failed within 40 s. Semi-conductive adhesives were produced which underwent ohmic heating when a constant current was applied to them. Aluminium-to-aluminium joints were manufactured which were subsequently bonded using an adhesive which had functionalised carbon fibre added. Static load tests were carried out with a 196 N load; when 50 A was passed through a joint containing 30 wt% carbon fibre in the adhesive, the sample failed in 3 s. Thermally decomposable interphases were developed for dismantling aluminiumto-aluminium bonded joints. The aluminium substrate was anodised and then subsequently sealed in boiling water to form a hydrated layer on the surface. A solgel layer was then applied to the hydrated oxide and joints were manufactured. On heating, the hydrated layer released water which initiated the reverse condensation reaction of the sol-gel, causing a failure at the interphase. In all the above cases, the novel approaches have proven to be extremely effective with initial adhesion levels of 15 MPa and above. They either reduce to zero joint strength after triggering, or, fail in static load tests within a few seconds. This Thesis will provide a scientific understanding of the novel methods used and for the observed reduction in joint strengths.