Exploring the Barrier Property of High-Performance Polyurethane-Silica-Lignin Coatings on Carbon Steel

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The use of lignin in anticorrosive coatings has provided efficient corrosion protection to metallic alloys. 1,2 We have combined lignin-based polyurethane (PU) with silica to produce eco-friendly anticorrosive coatings with high anticorrosive performance, mechanical strength and bactericidal action. 3 The PU was synthesized from methyl diphenyl diisocyanate (MDI), glycerol and lignin; the PU pre-polymer was functionalized with APTES to later form a hybrid with the polysilanol network from TEOS. The reference coating 4 was prepared using 100% glycerol (PU-Si-0Lig) as the polyol source, while the other two specimens contain glycerol and 0.2 g (Pu-Si-0.2Lig) or 0.4 g (Pu-Si-0.4Lig) of lignin, keeping the predetermined proportion NCO:OH = 1.16. The incorporation of lignin into the polymer matrix led to an increase in the thermal stability (295 to 304 °C), smoothness of the coating surface (4.3 to 1.7 nm) and improvement of the coating mechanical properties, such as coating hardness (362 to 376 MPa). In addition, the lignin-containing coatings present bactericidal activity against Escherichia coli. The corrosion protection of the PU-Si-Lignin hybrid coatings deposited on carbon steel was analyzed by electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl at room temperature. The tests were conducted in a three-electrode cell, consisting of the coated carbon steel substrate as the working electrode, a platinum mesh as a counter electrode, and an Ag|AgCl|KCl sat as the reference electrode. PU-Si-0Lig and PU-Si-0.2Lig present fair corrosion protection with low-frequency impedance |Z lf | values around 10 6 Ω.cm 2 after immersion for up to 14 days. Conversely, PU-Si-0.4Lig presents |Z lf | ~ 10 10 Ω.cm 2 , phase angle near -90 ° over seven frequency decades, and a lifespan of 100 days immersed in seawater. The equivalent electric circuit (EEC) used to fit the PU-Si-0.4Lig revealed that the EIS data of the initial 14 days can be represented by a model containing only one time constant. This means the coating has a highly capacitive behavior with insignificant water uptake in the first two weeks. Altogether, these results demonstrated the potential of PU-Si-Lig coating to be applied as an efficient anticorrosive barrier with enhanced mechanical and antibacterial properties. Acknowledgements: This work was supported by the Brazilian funding agencies CNPq, FAPESP, CAPES and PROPe UNESP. Keywords: Renewable feedstock, green coating, PU-silica, lignin, corrosion protection. (1) Harb, S. V.; Cerrutti, B. M.; Pulcinelli, S. H.; Santilli, C. V.; Hammer, P. Siloxane–PMMA Hybrid Anti-Corrosion Coatings Reinforced by Lignin. Surf Coat Technol 2015 , 275 , 9–16. https://doi.org/http://dx.doi.org/10.1016/j.surfcoat.2015.05.002. (2) Cao, Y.; Liu, Z.; Zheng, B.; Ou, R.; Fan, Q.; Li, L.; Guo, C.; Liu, T.; Wang, Q. Synthesis of Lignin-Based Polyols via Thiol-Ene Chemistry for High-Performance Polyurethane Anticorrosive Coating. Compos B Eng 2020 , 200 , 108295. https://doi.org/https://doi.org/10.1016/j.compositesb.2020.108295. (3) Alinejad, M.; Henry, C.; Nikafshar, S.; Gondaliya, A.; Bagheri, S.; Chen, N.; Singh, S. K.; Hodge, D. B.; Nejad, M. Lignin-Based Polyurethanes: Opportunities for Bio-Based Foams, Elastomers, Coatings and Adhesives. Polymers (Basel) 2019 , 11 (7). https://doi.org/10.3390/polym11071202. (4) Braz, Á. G.; Pochapski, D. J.; Pulcinelli, S. H.; Santilli, C. V. Effects of Curing Temperature and Silica Content on the Structure and Properties of a Sol–Gel PU-Silica Hybrid Coating for Corrosion Protection. ACS Applied Engineering Materials 2023 , 1 (7), 1739–1751. https://doi.org/10.1021/acsaenm.3c00148.