Development of Robust Electrically Insulated Carbon Nanotube Yarns for Sensing in Conductive Composites

Abstract

Carbon nanotubes have novel mechanical, thermal and electrical properties that alongside their tailorable aspect ratio can be integrated and widely distributed in composite structures to detect damage and monitor strain in real time. With the growing demand for conductive and functional composites, there is an increasing need to incorporate CNT yarns in them due to their inherent multifunctional properties. However, CNT fibers are conductive and need to be coated. Insulation of CNT yarns in conductive reinforcements like carbon fiber-reinforced composites can be challenging due to their highly porous structure, small size and non-uniform construction. This work implements a facile approach for coating of CNT yarns through a simple dip-coating of a measured length of CNT fiber in an adhesive. The adhesive serves to electrically insulate the yarn against short-circuiting in conductive medium and to preserve it against deterioration in harsh environment. It was observed that the molecular size, viscosity and adherence properties of the coating material plays a key role in achieving a uniform and durable coating. These factors alongside weight and the coefficient of thermal expansion of the coating materials was considered in selecting low density polyethylene as the coating agents for this study. Among the polymers tested, Dow Corning 732 provided the best coating for a highly piezoresistive CNT yarn while coating with Permatex-80050 increased the strain-tofailure of the CNT yarn.