Development of coal tar-free coatings: acetylated lignin as a bio-additive for anticorrosive and UV-blocking epoxy resins

Abstract

Coal tar epoxy (CTE) coatings promote effective protection of steel structures under critical conditions of atmosphere and immersion. However, according to the international guideline, coal tar has been reported as a mutagenic and carcinogenic compound and has been prohibited for use in countries such as the USA and Japan. Aiming to replace the use of CTE coatings, this paper proposes the use of Kraft lignin, a bio-polyphenol considered safe and obtained as waste from the pulp and paper industry, to develop a coal tar free epoxy coating. In order to promote better compatibilization with bisphenol A diglycidyl ether (DGEBA), lignin was subjected to the acetylation process. The epoxy-lignin resin was obtained by incorporating acetylated lignin into DGEBA. Three types of epoxy-lignin resin were prepared, namely: DGEBA/7.5% lignin, DGEBA/15% lignin and DGEBA/30% lignin. The chemical structure of the resin was evaluated by Fourier transform infrared spectroscopy (FTIR). For the coating preparation, the resin was cured using isophorone diamine (IPDA). The resultant thermosetting were analyzed regarding their chemical, thermal, anticorrosive and UV-blocking properties using the DGEBA and CTE coatings as reference. The anticorrosive properties of the coatings were evaluated by electrochemical impedance spectroscopy (EIS). The adhesion of the coatings was evaluated by the pull-off method (ASTM D4541). A complete absorption of UV light in the UV-A and UV-B regions was observed in the coatings add with acetylated lignin, a property very valuable in order to prevent the UV damage and the durability reduction of epoxy coatings. EIS results showed that the epoxy-lignin coatings (with 7.5 and 15% lignin content) presented modulus of impedance values comparable to the commercial coating and superior to the CTE coating, indicating the potential use of this coating as a corrosion protector.