Graphite Fiber Surface Analysis by X-Ray Photoelectron Spectroscopy and Polar/Dispersive Free Energy Analysis

Graphite fibers of less than 10 microns in diameter are used as the reinforcing agents in structural epoxy matrix composite materials. Their efficiency in reinforcing is determined to a large degree by their degree of bonding to the epoxy. Commercial surface treatments maximize this adhesion without quantifying the interaction. The purpose of this work has been to relate the surface composition of commercial treated and untreated fibers to the 'real' fiber surface incorporated in composites. Hercules A and HM filters, treated and untreated, have been studied using X-ray photoelectron spectroscopy (XPS) and contact angle measurements before and after a 300 C vacuum heat treatment. XPS spectra indicated multiple chemical states of carbon and oxygen and a reduction in the high energy binding state of carbon on the treated A fiber after vacuum heat treatment. Elemental surface concentrations have been calculated using corrected XPS peak areas. Contact angles were measured on these same fibers using a micro-Wilhelmy technique which incorporated eight liquids having a wide range of polar to dispersive free energy ratios. Determination of the fiber surface free energies and the proportions due to polar and dispersive components was made. Good Correlation was found between the oxygen concentration as measured by XPS and the change in polar/dispersive ratios for the fibers with surface treatments. (Author)