Effect of the interplay between fiber surface chemistry and sizing reactivity on fiber matrix interaction in carbon fiber reinforced epoxy resin

A comprehensive picture of the interaction between fibers and matrix in carbon fiber–reinforced epoxy resin is given. We present new data on fiber wetting by a resin, interphase formation, and fiber matrix adhesion and develop a model for fiber matrix interaction, considering earlier studies on fiber surface chemistry and interaction with sizings. The effect of surface chemistry is investigated by oxygen functionalization of carbon fibers by anodic oxidation utilizing two electrolytes. To address the influence of sizing reactivity, two epoxy-based sizings are applied with different number of epoxide groups per molecule. The fiber matrix adhesion is investigated by single fiber push-out tests. The resulting interfacial fracture toughness of functionalized fibers in epoxy resins depends on the fiber surface chemistry. Based on quantitative measurements, an influence of surface topography on the fiber matrix adhesion can be ruled out. Application of a standard sizing does not change the composite's interfacial fracture toughness compared to the unsized case. In contrast, use of a reactive sizing generally increases fiber matrix adhesion. The systematic variation of interfacial fracture toughness is discussed based on the number of bonds formed between fiber and resin. Bonding between hydroxyl surface functionalities and epoxy groups of resin or sizing is assumed to dominate.