A recent article published in Scientific Reports demonstrated the efficacy of ZnO and CuO nanoparticles (NPs) as heat and corrosion-resistant pigments in paint formulations. The NPs were synthesized using environmentally friendly plant extracts, and their performance was assessed based on properties such as oil absorption, hydrogen ion concentration, fineness of grinding, moisture content, bleeding, and loss on ignition.
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Background
Protecting outdoor carbon steel equipment, such as exhausters, furnaces, chimneys, grills, and ovens, is challenging, especially when applying decorative coatings. These coatings must enhance aesthetics while also providing resistance to various weather conditions, exhibiting high heat and corrosion resistance. Coatings used on cooking tools must also adhere to food safety regulations.
Silicon-based coating materials are promising for high-temperature applications due to their thermal stability. For example, polysiloxane is commonly used as a binder in such coatings. Zn-rich inorganic coatings utilizing ethyl silicate as a binder are specifically designed to prevent steel corrosion in harsh environments.
While many coatings have been developed for heat or corrosion resistance, few address both criteria simultaneously. Thus, this study investigated the heat and corrosion resistance of metal oxide NP-based pigments in paint formulations.
Methods
ZnO and CuO NPs were synthesized using okra plant extract in combination with zinc acetate and copper nitrate solutions, respectively. Characterization was performed using Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Particle size distribution and zeta potential were measured using a Zeta-sizer.
Pigments comprising ZnO and CuO were evaluated for oil absorption, bleeding, moisture content, hydrogen ion concentration (pH value), fineness of dispersion, and loss on ignition. These pigments were then combined with silicon resins in a 2:1 ratio to prepare two paint formulations using a ball mill.
The prepared paints were coated on a steel surface using a film applicator with a thickness of 80 ± 5 μm. Several mechanical and physical assessments of the paint films were conducted. These included thickness quantification, specular gloss measurements, film hardness, flexibility, adhesion, and resistance to rapid deformation.
Heat resistance was assessed using Method A for interior service coatings and Method B for exterior service coatings. Corrosion resistance was evaluated through salt spray exposure to 5 % NaCl for 500 hours, with corrosion quantified in terms of visible rusting (spot, general, pinpoint, or hybrid rust), blistering, photographic inspection, and scribe failure.
Results and Discussion
The pH values of the synthesized NP pigments were 11 (ZnO) and 8 (CuO), indicating strong and moderate alkalinity, respectively. ZnO NPs showed a higher oil absorption value than CuO NPs (115 vs. 75), suggesting that more binder is required when using ZnO in paints. However, the fineness of grind values indicated better dispersion of ZnO NPs in pigment-vehicle systems.
Both ZnO and CuO NPs exhibited low moisture content, and no bleeding was observed for either type in various solvents, demonstrating their stability in coatings. The pigments maintained their form and color when exposed to heat and light, indicating excellent resistance to environmental factors. Furthermore, no weight loss on ignition was noted for either ZnO or CuO NPs, suggesting strong thermal stability.
SEM images of the NP-based paint formulations revealed no morphological irregularities, indicating good dispersion of the NPs in the pigment vehicle. The dry paint films maintained their integrity during testing, showing resistance to harsh chemicals such as Na2CO3, H2O, and H2SO4 even after 48 hours of exposure.
No damage or cracking was observed in the ZnO and CuO NP-based paint films upon exposure to elevated temperatures up to 500 °C, both retaining uniformity and color. ZnO NP-based paint demonstrated greater thermal stability compared to CuO NP-based paint. Additionally, the gradual incorporation of ZnO and CuO NPs reduced blister density and spot rusting in the paint films, enhancing overall corrosion resistance.
Conclusion
The researchers successfully synthesized ZnO and CuO NPs using environmentally friendly plant extract methods and analyzed their effectiveness as pigments.
Incorporating these metal oxide NPs into paint formulations with silicon resins resulted in films with significant physicomechanical properties, including heat, chemical, and corrosion resistance. In a 500-hour salt spray test, ZnO NP-based coatings outperformed CuO NP-based coatings.
The findings of this study indicate the potential of metal oxide NPs for applications in heat- and corrosion-resistant pigments.
Journal Reference
El-Wahab, HA., et al. (2024). Efficacy of zinc and copper oxide nanoparticles as heat and corrosion-resistant pigments in paint formulations. Scientific Reports. DOI: 10.1038/s41598-024-74345-0, https://www.nature.com/articles/s41598-024-74345-0
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