Evaluation of the self-healing capabilities and resistance to thermal cycling of a shape-memory epoxy coating containing polycaprolactone microspheres

Abstract

The aim of this work was to prepare a shape-memory epoxy coating containing polycaprolactone microspheres (SMEP-PCL) and study its resistance to thermal cycling, which is yet to be evaluated in coatings prepared with similar approaches, as well as further characterize its self-healing capabilities. Firstly, the prepared microspheres were characterized by scanning electron microscopy (SEM) and laser diffraction. Afterwards, the dispersion of the PCL microspheres in the coating was examined and experiments were conducted with differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) to determine the glass transition temperature and investigate the curing reaction of the prepared coating, respectively. Then, the healing promoted by the reversible plasticity and the melt of the polycaprolactone microspheres mechanisms were studied by SEM, Raman Imaging, electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). Finally, thermal cycling tests were conducted and the samples were later visually evaluated for thermal stress damage and by EIS and pull-off adhesion strength tests. The SMEP-PCL of the present work was able to reproduce the self-healing results obtained from previous reports, even improving upon the self-healing efficiency obtained in other studies, in the SVET experiment. The results of the EIS measurements after the thermal cycles, pointed out to a minor reduction on the barrier properties of the SMEP and SMEP-PCL, which should be further investigated. Nonetheless, the adherence of both coatings was unhindered by the thermal cycles, which is a highly positive result for the future application of coatings based on this approach. Also, the SMEP-PCL exhibited substantially higher average values of adhesion strength compared to the SMEP, showing that the addition of PCL microspheres significantly improved the adherence of the coating and signaling great compatibility between them. This work also underlines how the healing efficiency of coatings with this approach are highly dependent on the type and extension of damage, as the healing observed in the two types of damage studied were exceedingly different.