Designing Carbon Fiber Composite Interfaces: Reactive Sizing Derived from Terpenes

Terpenes, typically obtained from conifer plants, were chemically modified to include functional groups that could react with both the surface of carbon fiber and the surrounding epoxy resin. This strategy was coupled with the installation of reactive amines (NH2) and thiols (SH) on the fiber surface to specifically react with these sizings. Increases in interfacial shear strength (IFSS) up to 195% were observed for fibers coated in these reactive sizings in combination with the tailored surface chemistry. Improvements in IFSS were also observed for unmodified fibers, suggesting passive functionalization. The presence of persistent carbon-centered radicals was proposed as a way that the fiber surface could react with the sizing agents, stabilized by the inherent high degree of conjugation present within the fiber structure. Interrogating commercially available carbon fiber via electron paramagnetic resonance (EPR) spectroscopy confirmed the presence of radicals within or on the carbon fiber surface (g-value = 1.9984).