Bio-based anticorrosive coatings for the heavy-duty industry

High performance anticorrosive coatings are essential for protecting large steel structures in harsh environments. However, to promote sustainability, there is a growing demand to replace fossil-fuel derived components in anticorrosive coatings with bio-based alternatives. Among renewable materials, lignin is considered a promising candidate for polymeric resins. However, its high molar mass and heterogeneity limit its direct incorporation into coating formulations. As a consequence, lignin is often fractionated and/or selectively modified to reduce the heterogeneity and enhance its reactivity and solubility in organic solvents, thereby improving its compatibility with resin systems.
In this PhD project, we integrated lignin into epoxy-based anticorrosive coatings as a bio-based alternative to traditional fossil-derived ingredients. The research focussed on both unmodified and chemically modified (epoxidized) lignin, exploring their potential to enhance coating performance for anticorrosive applications.

Initially, to evaluate its role as a structure-reinforcing component and as a replacement for traditional pigments and fillers, unmodified Kraft lignin was incorporated into epoxy novolac coatings. The findings revealed that size-fractionated (i.e., sieved) fine lignin particles significantly improved the mechanical properties and corrosion resistance of the coatings, achieving performance comparable to commercial coatings. However, unmodified lignin is limited to replacing pigments and fillers, not the resin component. This limitation prompted further exploration of chemical modification techniques to enable lignin’s use as a resin component.

Subsequently, in a close collaboration with Mats Johansson’s group at KTH, particularly with PhD student Alessio Truncali, we developed epoxidized Kraft lignin (EKL) as a particulate resin and incorporated it into epoxy novolac (EN) and diglycidyl ether bisphenol F (DGEBF) coatings. The research highlighted the superior performance of coatings containing size-fractionated epoxidized lignin (SF-EKL), particularly in terms of adhesion strength, impact resistance, and rust creep resistance, compared to non-size-fractionated lignin and commercial coatings.

This thesis also examined the performance of lignin-based coatings under high pressure and high temperature conditions, simulating the harsh environments encountered in the oil and gas industry. The results showed that lignin-based coatings can withstand significant pressure and temperature, with optimal performance observed up to 120 °C and 100 bar. However, limitations were noted at higher temperatures, particularly in the presence of CO₂.

Finally, we investigated the solubility of lignin in various solvents, identifying benzyl alcohol as an effective solvent for lignin. Benzyl alcohol not only achieved complete solubility of lignin, but also selectively solubilized lignin fractions with desirable molecular and functional properties, offering a promising alternative to conventional solvents, such as ethanol, methanol, and acetone, in lignin processing.

Overall, this thesis demonstrates that lignin, both unmodified and epoxidized, holds significant potential as a sustainable material for anticorrosive coatings. The research provides a foundation for the development of bio-based coatings that can replace or complement fossil-based systems, contributing to the advancement of bio-based industrial products. Future work should focus on scaling up these processes and further exploring the long-term performance and environmental impact of lignin-based coatings in real-world applications.