Statistical optimization of treatment conditions for the electrochemical oxidation of PAN-based carbon fiber by response surface methodology: Application to carbon fiber/epoxy composite

In the current study, the effect of three independent variables; electrochemical oxidation time along with concentration and temperature of an acidic electrolyte solution on the oxygen content of surface groups (O/C ratio) and tensile strength of PAN-based carbon fiber have been investigated. The electrochemical oxidation process of carbon fibers is carried out in nitric acid solution (as the electrolyte) with a constant potential oxidation, obtained from a cyclovoltammetric method. Box-Behnken design, which is subset of the response surface methodology, has been employed to present mathematical models as the function of physical factors for prediction of O/C ratio and tensile strength behavior of the electrochemically oxidized carbon fibers and also optimizing mentioned surface chemistry and mechanical properties of carbon fiber. Overally, 17 experiments with 5 replicates at the center point were designed. The minimum and maximum values of O/C ratio were 0.0107 wt% and 0.7339 wt% which occurred in design runs 11 and 13, respectively. A also, the maximum and minimum values for tensile strength were 3087 MPa and 2221 MPa which occurred in design runs 17 and 8, respectively. The optimization results, obtained using the optimization part of design expert software, showed that the most optimal O/C ratio and tensile strength were to be 0.5045 wt% and 2574 MPa, respectively, and achieved at 15 wt% of the electrolyte concentration, oxidation time of 10 min and temperature oxidation of 20 °C. However, preparing and testing five replicate samples and taking the average gave 0.4912 wt% and 2565 MPa for O/C ratio and tensile strength, respectively. In addition, electrochemical oxidation caused a roughening of the fibers surface and decrease of their tensile strength. Change in adhesion between the epoxy matrix and the carbon fiber was measured by ILSS (interlaminar shear strength).