Approaching Typical Metallic Conductivities in Polymer Nanocomposites for Lightning Strike Protection

ABSTRACT Among all performance targets a new aircraft design seeks to achieve, reduction in weight is of primary importance since it has a direct influence on the energy costs. In pursuing this target, an increasing proportion of composite materials has been introduced into modern aircrafts' structures. Perhaps the most important structural component, the fuselage has not been spared by change. Every change requires adjustment, and thus the problem of lightning strikes to aircraft has resurfaced as the conductive aluminum skin was replaced by a lower conductivity carbon fiber/polymer composite counterpart. Bonding a continuous metallic mesh to the surface of the skin has solved the problem, only to see the quest for mass minimization resume as this additional metal along with its adhesive and matrix contributes to the weight without benefits on the structural front. A better technology is sought. Upon gradually incorporating conductive fillers in a non-conductive matrix, the material undergoes a sudden change from an insulator to a conductor at a concentration called the percolation threshold which differs for every matrix-filler combination and filler dispersion state. Research on carbon nanotubes composites has highlighted the ability of high aspect ratio fillers to reach percolation at a very low concentration. However, issues with contact resistance between individual particles inhibit the maximum performance such composites can historically reach. If a polymer composite is to fulfill the requirements of lightning strike protection, the filler used should display simultaneously a high conductivity, a low contact resistance and a propensity to form connected networks through the matrix. One strategy to obtain such a combination of properties is to force the percolation of metallic fillers through self-organization upon forming the composite. We explored such an avenue by combining an aqueous colloidal epoxy dispersion with a water-based silver precursor ink. Upon solvent evaporation, elemental silver precipitates preferentially in the continuous aqueous phase, effectively segregating the conductive phase in a connected topology and thus promoting the conductivity of the composite. A very low percolation threshold was achieved, at only 0.27 vol.%, whereas typical silver fillers percolation occurs around 20 vol. %.

Résumé

as this additional metal along with its adhesive and matrix contributes to the weight without benefits on the structural front. A better technology is sought. Upon gradually incorporating conductive fillers in a non-conductive matrix, the material undergoes a sudden change from an insulator to a conductor at a concentration called the percolation threshold which differs for every matrix-filler combination and filler dispersion state. Research on carbon nanotubes composites has highlighted the ability of high aspect ratio fillers to reach percolation at a very low concentration. However, issues with contact resistance between individual particles inhibit the maximum performance such composites can historically reach. If a polymer composite is to fulfill the requirements of lightning strike protection, the filler used should display simultaneously a high conductivity, a low contact resistance and a propensity to form connected networks through the matrix. One strategy to obtain such a combination of properties is to force the percolation of metallic fillers through self-organization upon forming the composite. We explored such an avenue by combining an aqueous colloidal epoxy dispersion with a water-based silver precursor ink. Upon solvent evaporation, elemental silver precipitates preferentially in the continuous aqueous phase, effectively segregating the conductive phase in a connected topology and thus promoting the conductivity of the composite. A very low percolation threshold was achieved, at only 0.27 vol.%, whereas typical silver fillers percolation occurs around 20 vol. %.