Examining conductivity, current density, and sizings applied to carbon fibers during manufacture and their effect on fiber-to-matrix adhesion in epoxy polymers

Abstract

This study provides a comprehensive study of carbon fiber surface treatment conditions for epoxy resins. A set of 27 fibers, derived from DowAska precursor, with alterations to manufacturing conditions only occurring at the electrochemical oxidation and sizing bath. Electrolyte (NH4HCO3) conductivity was varied between 8, 16 and 24 μS/cm while oxidative current density was varied between 0.5, 1.0 and 1.5 A/m2. Subsequent to oxidation, fibers were coated with either epoxy, polyamide or polyurethane compatible sizing. This study is the first to consider electrolyte conductivity, oxidative current density and sizing variables simultaneously and links them to interfacial shear strength (IFSS). Extensive chemical, physical and mechanical characterization was conducted to quantify the changes to fiber properties as a result of treatment variation. Results reveal that varying combinations of electrolyte and applied current density not only vary the degree of oxidation on the fiber, but the types of surface chemistry installed (e.g. ratio of COH, CO, and COOH). Somewhat counter-intuitively, at high conductivities and current densities the degree of oxidized carbon species on the fiber surface is decreased. Indeed, this study shows combinations of surface treatment variables which could be used to promote the formation of surface bound functional groups in preference (e.g. phenolic groups in preference to COOH) to enhance interfacial bonding for a given resin. Determination of fiber roughness for all samples showed no statistical difference between samples, suggesting that mechanical interlocking effects do not play a role in the variations in interfacial adhesion observed herein.