Self-Healing of Microcracks and Scratches in a Carbon-Fiber Reinforced Epoxy Vitrimer by Conventional or Remote Heating

This study addresses the extension of the service life of carbon-fiber reinforced epoxies by inducing thermal healing of microcracks through the use of a vitrimer as a polymeric matrix. Our aim was to explore the feasibility of using a blend of selected carboxylic acids (citric, glutaric, and sebacic acids) and commercial monomers to design a matrix specifically developed for technological implementation in composites with the ability of intrinsic repair of microcracks under moderate (even remote) heating treatments. The selection of the formulation (the acid blend, catalysts, and monomers) was the result of an exhaustive prescreening analysis of processing requisites and final properties. The glass transition temperature of the cured vitrimer composite measured by differential scanning calorimetry (DSC) is 94 °C, a value lying in the range required for several technological applications, whereas stress relaxation to (1/e) of the initial value took ∼4.7 h at 180 °C and only 1.1 h at 200 °C. Composites containing 50 vol % of carbon fibers could be successfully prepared by compression molding. Acoustic emission tests proved the formation and partial healing of microcracks during tensile tests performed until 350 MPa. Surface scratches could also be healed by remote activation using near-infrared irradiation (NIR). These first results under nonoptimized thermal cycles are a proof of concept that microcrack and scratch healing can be produced in high glass-transition temperature epoxy-based carbon-reinforced composites.