Steel's widespread use in engineering applications is hampered by its susceptibility to corrosion. Traditional anti-corrosion coatings, while offering some protection, fail upon damage, necessitating timely manual repair, which is often impractical. Biologically inspired self-healing materials offer a potential solution, repairing damage autonomously. Waterborne polyurethane (WPU) is an attractive base material for coatings due to its environmentally friendly nature and good adhesion, but its inherent corrosion resistance is limited. Graphene and graphene oxide (GO) nanosheets offer exceptional barrier properties and mechanical strength. The challenge lies in creating a coating with uniformly dispersed GO to maximize these benefits and achieve self-healing capabilities. This research explores a strategy to overcome these limitations by combining a self-healing WPU elastomer with GO, leveraging the hydrogen bonding capabilities of the polymer and the photothermal properties of GO to achieve a highly oriented, self-healing, and anti-corrosion coating. The researchers utilized a biomimetic approach, inspired by the layered structure of nacre and the healing mechanisms of biological systems, to achieve a highly oriented GO-WPU nanocomposite with superior protective capabilities.

The introduction reviewed existing anti-corrosion coating technologies, including metal coatings, polymer coatings, and organic-inorganic hybrid coatings. It highlighted the limitations of current passive coatings, emphasizing the need for self-healing capabilities to extend coating lifespan and ensure safety. Existing research on self-healing polymers and their application in anti-corrosion coatings was discussed. The use of graphene and GO in enhancing the properties of polymer composites was reviewed, noting the importance of uniform dispersion and the benefits of highly oriented structures. Previous studies showed the potential of graphene-based coatings, but concerns about defects affecting performance were acknowledged. The biomimetic approach, inspired by natural materials and biological systems such as the pearl layer structure and tumor vascular lipids, provided valuable insights for designing advanced coatings. Challenges associated with existing high-orientation barrier structure fabrication methods were mentioned, highlighting the need for a simpler, scalable process.

The study involved synthesizing a self-healing WPU elastomer using a one-pot polycondensation reaction of polycaprolactone diol (PCL), hexamethylene diisocyanate (HMDI), 2,2-bis(hydroxymethyl)butyric acid (DMBA), and 4,6-diaminopyrimidine. The formation of multiple hydrogen bonds in the WPU was confirmed using in situ temperature-dependent FT-IR spectroscopy and two-dimensional infrared correlation spectroscopy (2D-COS). The mechanical properties of the WPU elastomer were evaluated through tensile testing, showing high tensile strength, toughness, and fracture energy. Damage resistance was assessed using a needle-impact test and notched tensile testing. Self-healing capabilities were evaluated by observing the recovery of mechanical properties after inducing damage and subjecting the material to heating at 50°C. Graphene oxide (GO) was prepared via a modified Hummers' method. The morphology and thickness of GO nanosheets were characterized using atomic force microscopy (AFM). UV-Vis spectroscopy was used to confirm the structure of GO. WPU-GO composite coatings were prepared using a roller coating method, promoting the alignment of GO nanosheets within the WPU matrix to form a highly oriented lamellar structure. Electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET) were used to assess the anti-corrosion performance of the coatings in 3.5 wt% NaCl solution. Large-scale immersion tests in 5.0 wt% NaCl solution were conducted to evaluate the long-term anti-corrosion performance. The near-infrared (NIR) triggered self-healing capabilities of the WPU-GOLM coating were investigated using NIR laser irradiation. The surface morphology, and roughness of the coatings, and the corroded substrates were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Finally, the mechanism of the anti-corrosion performance was explored.

The synthesized WPU elastomer displayed exceptional mechanical properties: a tensile strength of 39.89 MPa, toughness of 300.3 MJ m⁻³, and fracture energy of 146.57 kJ m⁻². The material exhibited remarkable damage resistance and self-healing capabilities, fully recovering its mechanical properties after 12 h at 50 °C. The incorporation of GO significantly enhanced the anti-corrosion performance of the WPU coating. EIS analysis showed that the impedance modulus of the WPU-GOLM coating was an order of magnitude higher than that of the blank WPU coating, indicating superior corrosion resistance. SVET measurements confirmed the effective self-healing and protection of the WPU-GOLM coating in 3.5 wt% NaCl solution. Large-scale immersion tests showed that the WPU-GOLM coating maintained its integrity and prevented corrosion for 246 h in 5.0 wt% NaCl solution, significantly outperforming the WPU coating. The WPU-GOLM coating also exhibited NIR-triggered self-healing, fully repairing damage within 30 s of irradiation. The superior performance of the WPU-GOLM coating was attributed to the highly oriented lamellar structure of the GO nanosheets, which created a robust physical barrier against corrosive media and provided a tortuous pathway for ion diffusion. The hydrogen bonding between GO and the WPU matrix further enhanced the coating's strength and self-healing capacity. Zeta potential measurements confirmed the stable dispersion of GO within the WPU matrix.

The results demonstrate the successful synthesis of a bio-inspired self-healing and anti-corrosion coating with superior performance compared to existing solutions. The unique combination of a self-healing WPU elastomer with highly oriented GO nanosheets resulted in a coating that addresses the limitations of traditional anti-corrosion coatings. The high toughness and self-healing ability of the WPU, along with the excellent barrier properties of GO, contribute to the exceptional protection of the underlying substrate. The use of a simple and scalable roller coating method for coating preparation makes this approach suitable for industrial applications. The findings highlight the potential of this biomimetic strategy for designing multifunctional coatings for various applications requiring enhanced durability and self-healing capabilities in demanding environments. Future research could focus on exploring other types of nanomaterials to further enhance the coating's performance and exploring the coating's efficacy in different corrosive environments.

This research successfully demonstrated the creation of a bio-inspired self-healing and anti-corrosion coating by combining a self-healing WPU elastomer with highly oriented GO nanosheets. The resulting WPU-GOLM coating exhibited superior mechanical properties, damage resistance, self-healing capability, and anti-corrosion performance compared to WPU alone. The findings offer a promising approach for developing durable and self-repairing protective coatings for various applications. Further investigation into the long-term durability and performance under diverse environmental conditions is recommended, along with exploring other filler materials and polymer systems.

While the study demonstrates the efficacy of the WPU-GOLM coating in laboratory settings, further research is needed to assess its long-term durability and performance under real-world conditions. The scalability and cost-effectiveness of the fabrication process for large-scale industrial applications require further investigation. The current study focused on carbon steel substrates; examining the compatibility and performance of the coating on other metallic substrates would broaden its applicability. The influence of different environmental factors, such as temperature fluctuations and UV exposure, on the long-term performance and self-healing efficiency of the coating warrants further investigation.