Improved Mechanical Performance of Epoxy Systems with Low Loadings of Milled Carbon Fiber via Chemical Surface Modification

The chemical surface modification of carbon fibers has been a prolific area of literature in recent years, but little has been done to translate these advances to short discontinuous carbon fibers. Routinely used as fillers in composite materials, the prospect of using chemical surface modification to improve the performance of milled carbon fibers carries appeal. This work describes the adaptation of conventional electrochemical reduction of aryl diazonium salts to recycled milled carbon fibers and explores the improvements in mechanical performance achieved therein in a simple epoxy resin system. Increasing fiber mass loadings are examined using conventional unmodified milled carbon fiber, and two species of chemically modified fibers, one featuring benzyl alcohol and the other nitrophenyl groups. At low loadings, milled carbon fibers with a surface featuring aryl alcohol functionality significantly improve the mechanical properties of epoxy specimens, increasing flexural strength by an average of 35% and tensile strength by 31.5%. Significant improvements are also seen at low loadings (<1% w/w) using aryl nitro-modified fibers. It is proposed that fibers modified with these functionalities promote hydrogen-bonding at the fiber–polymer interface and result in greater mechanical performance, and that this improvement is pronounced at ≤1 wt%.