Performance of Lignin as a Sustainable Anticorrosion Coating

Synthetic polymers play a pivotal role in many industrial applications that includes their utilization as barrier coatings for corrosion protection of metal surfaces. However, use of such non-renewable coatings results in environmental pollution both during production and use. As such, there is a global effort to find/produce more sustainable metal coatings from renewable resources including biomass-based polymers. Consequently, this thesis investigates the performance of technical lignin—a primary waste from biomass processing industries—as an organic binder in anticorrosion coatings, with a focus on the industrial applicability of these materials and associated deposition techniques. Electrochemical properties of stainless steel spin-coated with two different organosolv lignin (dissolved in 1,4-Dioxane) were investigated. Results showed that the coatings enhanced the resistance of surfaces against corrosion with a lignin source-dependent variation of the barrier properties. In order to address the limited lignin solubility in many organic solvents, the screening of a series of industrially-applicable organic solvents was undertaken. Findings indicated that two solvents—diethylene glycol monobutyl ether (DEGBE) and propylene glycol monomethyl ether (PGME)—act as strong solvents for a kraft and an organosolv lignin, and that DEGBE also has a plasticizing effect on lignin. However, electrochemical analysis of lignin-coated steel prepared from PGME following prolonged immersion (24 hours) in 5 wt.% NaCl, showed that these coatings offer limited protection. Furthermore, cracking of lignin-PGME coatings was observed, which was found to be mitigated by addition of triethyl citrate (TEC) as a plasticizer. An alternative and more environmentally benign route for the preparation of lignin-based coatings was further achieved by the preparation of aqueous dispersions of colloidal lignin particles (CLPs) using DEGBE as the starting solvent in a solvent-exchange procedure. Consequently, it was possible to prepare combined lignin-cellulose composite coatings using electrophoretic deposition (EPD) from aqueous dispersions at low deposition potentials, and resulted in coatings with enhanced durability during long term immersion (15 days) in 3.5 wt.% NaCl electrolyte. An important outcome of this process was the coalescense of CLPs during drying—as a result of the DEGBE—that enabled the formation of compact coatings. Such techniques and coalescing characteristics could be exploited in the preparation of water-borne lignin layers with enhanced corrosion protection capabilities as part of a future fully sustainable coating formulation.