Innovative Routes for the Fabrication of Advanced Multifunctional Protective Coatings with Combined Improved Barrier, Smart Active Feedback, Self-Repairing, and Antimicrobial Properties for Materials Protection
Authors: Njoku Demian Ifeanyi , Cui Miaomiao , Shang Baihua , Li Ying 
Published: February 2020

Due to the massive cost involved in combating corrosion related problems in varied industries, research into the development of new materials that can minimize this loss is a priority area. Although many protective methods have been proposed in recent times for the protection of metallic materials, organic protective coatings are inevitable because of their high adhesion strengths, good barrier properties, cost efficiency and ability to accommodate nanomaterials and active species in their matrix. However, since the strengths of organic coatings are not comparable to those of metals, coating layers can be damaged or scratched during handling and / or when deployed in service due to several environmental factors thereby losing their protective properties. In the present report, active inhibiting agents (benzotriazole and 2-mercaptobenzothiazole) were successfully loaded into clay nanoparticles and embedded into epoxy polymeric coating matrix to (a) isolate the active species from the coating matrix, (b) delay the release of the active agents, and (c) avoid the deactivation of the active agents. This approach yielded good results in delaying the time needed for the exhaustive release of the encapsulated active species following the Peppas and Weibull models, but a continuous outward diffusion of the encapsulated active species was unavoidable even after the corrosion actions have been mitigated in the damaged sites. To overcome this setback and enhance the active protective efficacy of the anticorrosive protective coatings, single/double bilayers of pH-sensitive polyelectrolytes gels based on chitosan cross-linked with epichlorohydrin (CTS-ECH) and glutaraldehyde (CTS-GTA), and complexes of chitosan with polymethacrylic acid (CTS-PMA) and polyacrylic acid (CTS-PAA) were deposited on the loaded nanotubes by layer-by-layer (LBL) methods. The permeability properties of the polyelectrolytes films deposited on the loaded nanotubes were designed to vary with the pH of the solution thereby introducing a smart/intelligent release of the active species. In this way, an innovative route for the fabrication of advanced multifunctional protective coatings possessing multiple functions such as intelligent active feedback, self-healing, antimicrobial (antifouling) and improved barrier properties was proposed. The suitability of the as-received clay nanotubes to encapsulate the active agents, the success of the loading process, and the formations of tube end capping and polyelectrolytes gel multilayers on the loaded nanotubes were confirmed by different spectroscopic measurements (SEM, TEM, FTIR, XRD and TGA/DTG). The improved passive barrier performances of the various innovative coatings were revealed by long-term in-situ impedance spectroscopic (EIS), while their active feedback and self-healing abilities were determined by advanced electrochemical, optical, and spectroscopic techniques. The optical and spectroscopic techniques revealed (a) the degree of pit formation on the steel surface (after the top coat is removed), (b) iron rust formation around the artificially marked defects, and (c) the ability of the marked defects to self-heal over lengthened exposure times. The advanced electrochemical techniques revealed the mitigation of the corrosion process at the defective zones by the released inhibitor molecules that saturate the defective zones and the self-healing of the defective zones by the polyelectrolytes hydrogels used for the nanotube’s interfacial treatments. Furthermore, sophisticated optical / microscopic techniques were adopted to reveal the antimicrobial effects of the innovative coatings. The antimicrobial effects of the innovative coatings were related to the biocidal effects of the polyelectrolyte gel constituents. Various physical and mechanical tests were adopted to compare the physical/mechanical properties of the various protective coatings. The results from the various investigations afforded insights into the effects of exposure time on the protective and failure behaviours of both the reference coatings and the modified coatings. And In overall, an innovative route for the fabrication of protective organic coatings with advanced multifunctional properties for the protection of materials has been proposed.