Improvement of the bonding between carbon fibers and an epoxy matrix using a simple sizing process with a novolac resin

Novolac resin (NR) sizing modification of the carbon fiber/epoxy composites bonding was investigated by sizing NR on the carbon fibers surface using a conventional sizing process, i.e., immersing fibers tows in a NR solution. Thus, three different types of carbon fibers/epoxy composites were prepared, which NR is introduced in three ways: (1) pure matrix for comparison, (2) adding NR in the matrix, (3) adding NR in the sizing. Interlaminar shear strength (ILSS) using short beam shear test and impact strength were investigated. The results showed that both interfacial and impact properties of composites were maximized using the NR sizing on fibers. ILSS and impact strength were improved by 22.5% and 56.8%, respectively, compared with the composite without NR. This way introduces no damage to the fibers and has larger advantage than the matrix modification by dispersing NR in epoxy resins. The fracture and surface morphologies of the composite specimens were characterized by scanning electron microscopy (SEM). SEM clearly highlighted a change in the fracture mechanism from a clearly adhesive failure to a cohesive failure.