Highly curable self-healing vitrimer-like cellulose-modified halloysite nanotube/epoxy nanocomposite coatings

Self-healing and self-repairing coatings are stimuli-responsive materials, which are of crucial importance in developing advanced smart systems. However, a few has been attempted to unveil the association between the microstructure and properties of self-healing organic coatings. Herein, silane coupled cellulose-functionalized halloysite nanotubes (HNT-C)/epoxy nanocomposite coatings showing vitrimer-like behavior were developed and their crosslinking, thermal, mechanical, and self-healing behaviors were discussed. Nanocomposites with variable pristine HNT and HNT-C contents (0.1, 0.3, and 0.5 wt%) were prepared, where epoxy/HNT-C (0.3 wt %) took Good cure label in terms of Cure Index, surprisingly with a cure enthalpy three times higher than that of blank epoxy. Moreover, the tensile strength and elongation at break of the assigned nanocomposite coating were enhanced by 16% and 56%, respectively. Evidence of cellulose functionalization was in SEM image provided from the fracture surface suggesting smooth breakup mechanism for epoxy and epoxy/HNT, in contrast to a roughly-broken fracture behavior for epoxy/HNT-C. Surface functionalization of nanotubes with cellulose decreased the relaxation time by five times in some cases, which was ascribed to the esterification reaction in the system. The self-healing behavior of the coatings through transesterification of beta-hydroxyl ester at higher temperature was theoretically interpreted in terms of relaxation time of the epoxy chains. The vitrimers rapidly responded to the intra/inter molecular forces because of the transesterification reaction so that the ester groups were exchanged with hydroxyl groups that assisted autocatalytic reactions. The epoxy/HNT-C nanocomposites developed in this work are fast cure coatings with acceptable toughness and self-healing behavior.