Study on surface and mechanical fiber characteristics and their effect on the adhesion properties to a polycarbonate matrix tuned by anodic carbon fiber oxidation

Polyacrylonitrile (PAN) based high strength carbon fibers were anodically o xidized using the galvanostatic mode in alkaline electrolyte solutions to i nfluence the chemical surface composition. The change of chemical and physi cal properties was investigated using scanning electron microscopy (SEM), p hotoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDX) and contact angle as well as zeta (zeta)-potential measurements. An initially improved wettability for polar liquids, particularly water, wa s observed for oxidized carbon fibers. This result was confirmed by zeta-po tential measurements. The chemical state of the oxygen containing surface g roups changes during anodic oxidation in K2CO3/KOH due to further oxidation of C-OH and C=O groups to COOH groups. Therefore the surface acidity incre ases, which leads to a shift of the isoelectric point to lower pH values an d increases the negative zeta(plateau) value. The zeta-pH as well as the ze ta-concentration dependence show the same tendency. During anodic oxidation of carbon fibers in KNO3/KOH electrolyte solution beside 'normal' (like C- OH, C=O and COOH) surface oxides also carboxylate groups (COO-K+) were form ed at the fiber surface in contrast to an oxidation in K2CO3/KOH which intr oduces 'normal' surface oxides. No influence could be observed of such an a nodic oxidation on the single fiber tensile strength. Contact angle measure ments of polycarbonate melt droplets onto single carbon fibers show no depe ndence of the surface composition. The interfacial shear strength, measured using the microdroplet pull-off test were compared with the thermodynamic work of adhesion. The calculated as well as the measured adhesion show the same absence of any influence of fiber treatment. (C) 1999 Elsevier Science Ltd. All rights reserved.