Alternatives to coal tar coatings

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 many countries. Aiming to replace the use of CTE coatings, a new 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 compatibilisation 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. For the coating preparation, the resin was cured using isophorone diamine (IPDA).

Absorption of UV light

The resultant thermosetting were analysed 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.

The study has been published in Progress in Organic Coatings, Volume 161, December 2021.