The chemical functionality of the carbon fiber surface is key to the properties of carbon fiber reinforced composites (CRFP), especially their mechanical performance.
In commercial virgin carbon fiber (vCF) production, anodic oxidation is the accepted method for surface activation. For recycled carbon fibers (rCF), alternative fiber surface treatments are necessary, which are applicable to short fibers and recycled carbon fiber patches. For this purpose, plasma treatment is a promising procedure to modify selectively carbon fiber surface functionalities by the composition of the process gas and by optimized process parameters. In this study, recycled carbon fibers from a thermal recycling process are plasma treated by a plasma-jet, which enables a potential-free indirect surface treatment. This technique is significantly less destructive compared to classic plasma treatments and allows a better control of the surface functionalization. The effect of three different process gases (air, nitrogen and dinitrogen monoxide) and the influence of the distance between the plasma-jet and the fiber surface are studied with the aim to increase the oxygen and nitrogen content on the recycled carbon fiber (rCF) surfaces. Higher surface coverage of oxygen- and nitrogen-containing functional groups is supposed to lead to a better adhesion between the carbon fiber and the matrix, in our case an epoxy resin. The elemental compositions and functional groups of the treated carbon fiber surfaces are studied by x-ray photoelectron spectroscopy. Further fiber properties like tensile strength, tensile modulus and surface roughness are investigated. Finally, the fiber-matrix-adhesion of functionalized rCF within an epoxy resin re-infiltrated CFRP sample is investigated by single fiber push-out tests.