Interfacial improvement of carbon fiber reinforced epoxy composites by tuning the content of curing agent in sizing agent

This paper presents an approach to increase the interfacial shear strength (IFSS) of carbon fiber/epoxy composites by regulating the quantity of chemical bonds at both interfaces of fiber/sizing and sizing/matrix through adding different contents of curing agent (respectively 0, 15%, 30% and 60% of full stoichiometry in epoxy sizing agent) and 150 °C pretreatment on sized fibers. Fibers with 15% stoichiometric curing agent have the maximum IFSS of 122.4 MPa. The reinforcing mechanisms, explored by analyzing the surface physicochemical properties of fibers, the interphase modulus, the wettability and chemical bonding between fibers and matrix, were ascribed to the formation of a gradient interphase in cross-linking density, as well as the maximal polar component of surface energy and the minimal contact angle. After 150 °C pretreatment, fibers with 30% stoichiometric curing agent could form balanced chemical bonds at both fiber/sizing and sizing/matrix interfaces, and thus improve IFSS. Interestingly, variation of IFSSs for untreated sized fibers shows a positive correlation with the ratio of polar to dispersion components, while opposite relationship is observed for 150 °C treated sized fibers.