# Chitosan Nanocomposite Coatings for Food, Paints, and Water Treatment Applications
## 2 Chitosan and its Properties
### Self-Healing Properties  
The ability of a material to heal or repair damages automatically or with some external stimulation independently is called self-healing. Numerous polymeric materials with self-healing capabilities have been fabricated in recent years [62; 63; 64; 65]. Self-healing materials, being capable of forming reversible bonds or reactions in the networks, enhance the durability of the materials. An example of chitosan-based self-healing materials that have attracted attention are anticorrosion self-healing paints [66; 67; 68; 69]. Despite significant achievements in the development of chitosan-based self-healing materials, several challenges still need to be addressed for wider applications. An appropriate balance between mechanical strength, self-healing capacities, and mechanical robustness is required for the fabrication of newly developed, chitosan-based, self-healing materials.  
Figure 4: Transmission Electron Micrographs (TEM) of _E. coli_ (**a**) treated with buffer and (**b–d**) chitosan of 50 kDa molecular weight, (Reproduced with permission from [61], Copyright © 2010, Elsevier).  
Applications of self-healing coatings include automotive refinish on the backside of smartphones to stop the development of corrosion in scratches [70], etc. Due to excellent film-forming properties, superior adhesion to metallic surfaces, and self-healing abilities, chitosan-based self-healing coatings have been effective in protecting metal surfaces and metallic pieces [71; 72]. Two main approaches have been pursued for corrosion protection in self-healing coatings: (i) the fixing of defects by adding polymerizing agents in polymeric coating matrix and (ii) by using corrosion inhibitors that can protect corroding areas [73]. In anti-corrosion paints, self-healing refers to both dynamic care of the substrate and structural repair of the coatings, offering superior protecting ability and increased longevity of the coating compared to other protective coatings [71]. Chitosan- and cerium (Ce)-based self-healing coatings have been reported to protect aluminium alloy 2024 from corrosion [68; 74]. 2-Mercaptobenzothiazzo (MBT) has been used as an effective corrosion inhibitor in chitosan-based coating for aluminium alloys 2024. The study revealed that MBT has strong inhibiting ability and even after one week in a full immersion condition no corrosion attack was reported [71]. In the study, the surface properties of the chitosan coatings were also improved by chemical grafting using poly (ethylene-alt-maleic anhydride) (PEMA) and poly (maleic anhydride-alt-1-octadecene) (PMAO) to increase its hydrophobicity, which is important for corrosion protection in atmospheric conditions. Due to good wettability and adhesion properties it was argued that chitosan provides corrosion protection, simultaneously working as a reservoir for the corrosion inhibitor which prevents the formation of pittings on aluminium alloy. The grafting of chitosan at the coating/solution interface with PEMA and PMAO provided an adequate hydrophobic effect, especially in the case of chitosan loaded with MBT, leading to the delay of ingress of electrolyte towards the metal interface. The combination of active corrosion protection due to MBT and the surface hydrophobicity conferred by grafting could be the reason behind the efficient protection from corrosion to aluminium alloy 2024 [71].