Comparison of surface-chemical methods for characterizing carbon fiber-epoxy resin composites

The surface characteristics of differently treated carbon fibers have been studied by wetting, inverse gas chromatographic (IGC), streaming potential, and X-ray photoelectron spectroscopic (XPS) measurements. The results of these surface-chemical methods were related to the adhesion properties (measured as debonding shear strength) of these carbon fibers in model epoxy resin composites. Despite their incomplete theoretical background, IGC, contact angle, and electrokinetic measurements provide valuable information about the dispersive and the Lewis or Brønsted acid-base interaction forces proceeding from differently treated carbon fiber surfaces. A direct correlation was found between the electron acceptor parameters of the carbon fiber surfaces, determined by contact angle and IGC, and the debonding shear strength of carbon fiber-epoxy resin model composites. By means of XPS it was shown that the electron acceptor centres at the carbon fiber surfaces are due to C-OH functional groups. These experimental results lead to the assumption that an increased number of functional alcoholic surface groups can contribute to an improved adhesion in carbon fiber-epoxy resin composites.