Effect of natural melanin nanoparticles on a self-healing cross-linked polyurethane

Natural melanin nanoparticles (MNs) stripped from cuttlefishes were purified and characterized for the first time. The reactive groups (–OH or –NH2) of the MNs at a concentration of 6.0 mmol/g react with isocyanate groups in pre-polyurethane (pre-PU) to form cross-links, further strengthening the self-healing of PU based on the reversible acylhydrazone bond that was synthesized for the first time in this study. Then, the performances of different mass fractions of self-healing PU modified with MNs (0.5, 1, 1.5, 2, 2.5 wt%) were characterized. We found that PU modified with 1.5 wt% of MNs showed better performance due to the adjustment of the micro-phase separation in PU, with the σ and ε values improved from 0.81 MPa and 405.44% to 2.75 MPa and 483.17%, respectively. Surprisingly, the self-healing efficiency, defined by the recovery of ε, was improved from 91.73% to 99.58% due to the strong interactions between the MNs and PU. The aim of this study was to perfectly combine the organic active fillers—melanin nanoparticles—with self-healing materials to enhance their performance.