Structural and Conductive Adhesives Enabled by Single-Walled Carbon Nanotubes

Adhesives are increasingly being employed as alternatives to mechanical fasteners in aerospace and other engineering applications. While structural adhesives are commonly employed on aircraft, electrical bonding continues to be achieved with rivets as the existing electrically conductive adhesives suffer from low strength due to the high loading (typically 25-30 vol.%) of conductive filler particles. Carbon nanotubes are one of the more attractive candidates for development of multifunctional adhesives for both structural and conductive bonding because of their high conductivity and high aspect ratio. The latter enables creation of conductive pathways at much lower loading (< 1 vol.%) and, therefore, conductivity can be achieved without degrading mechanical performance. Single-walled carbon nanotubes (SWCNTs) offer the highest intrinsic conductivity and aspect ratio as well as the lowest percolation threshold, which is associated with higher conductivity at a fixed loading. In this work, SWCNTs were incorporated at low loading (0.5 - 3 wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability. Mechanical properties of composite-to-composite joints were evaluated using ASTM-based lap shear and peel tests. Bulk electrical conductivities over 1 S/m were achieved without degrading the joint structural performance in the selected test methods. The mechanical and electrical performance of a SWCNT-modified epoxy adhesive was also studied for aluminum-to-aluminum bonding. It was found that the integration of 1 wt% SWCNTs can considerably improve joint Mode I fracture toughness by similar to 35% due to mechanisms such as crack bridging. The electrical resistance of the bondline was also consistent with the electrical conductivity of SWCNT-modified adhesive films (similar to 10(-3) S/m), but slightly lower than the bulk electrical conductivity measured on thicker samples.