Method and experimental characterization of the surface free energy of carbon fibers used in composite systems

Abstract

AbstractSurface treatments are known to affect the interfacial properties in composite systems, including some mechanical properties, as well as the ease of handling. In this study, a simple surface free energy (SFE) characterization technique is proposed to assess the effect of fiber surface treatments on the wetting behavior and SFE of carbon fibers. Several fiber surface treatments were applied. Commercially available T300 carbon fibers from a unidirectional tape were desized and then subjected to nitric acid oxidation as well as air plasma oxidation. Functionalization of the fiber surface was confirmed via X‐ray photoelectron spectroscopy. Oxidation led to enhanced surface roughness as confirmed by measurements of the fibers' surface area using the Brunauer, Emmett, and Teller method. However, the wetting behavior of the fibers does not follow either the Wenzel or the Cassie–Baxter equations, possibly due to the dominant effect of the chemical interactions and the chemical heterogeneity of the fiber surface. It is shown that the polar component of the treated fibers' SFE increases while the dispersive component remains unaffected. The increase of the polar component is due to fiber surface functionalization. Both plasma and wet oxidation treatments enhanced the interfacial shear strength of carbon/epoxy specimens. Optimum conditions may result with treatment duration, as shown for a 2‐min air plasma treatment. The suggested technique to measure contact angles may prove beneficial for fast and accurate research work and also for quality control of surface treatments or coating processes.