Industrial waste lignosulphonate to functionalized azo pigments: an application to epoxy-polyamine composite coating

Lignosulphonate-based azo pigments are synthesized by functionalizing industrial waste lignosulphonate material through a diazo coupling reaction. It has dispersion and metal surface protection function by coating. Aniline, 4,4′-bis(4-aminophenoxy) biphenyl (BAP-Biphenyl), and o-dianisidine were used to synthesize three lignosulphonate-based azo pigments. ¹H-NMR, FT-IR, HRMS, and UV spectroscopic methods confirm the pigment formation through the diazo coupling reaction of lignosulphonate and products. By using TGA and DSC analysis, the thermal behaviour of azo pigments was examined. Successive increase in mass also suggests successful diazo coupling reaction. The light scattering technique gives particle size of azo pigments for aniline-azo-LS, BAP-Biphenyl-azo-LS and o-dianisidine-azo-LS were found to be 1578, 2387, and 1917 nm, respectively. Additionally, the negative zeta potential was noticed indicating the pigments’ dispersion ability. This dispersible ability was utilized for paint formulation, and chemical resistance of metal coating was observed. The formulation has good mechanical stability, and TGA suggests the thermal stability over 200 °C. This lignin-based functional pigment will find new forms of an environmentally friendly and sustainable method for a cleaner utilization of lignin in metal coatings.