Tuning of renewable biomass lignin into high value-added product: Development of light resistant azo-lignin colorant for coating application

Abstract The valorization of lignin by virtue of functionalization of its aromatic backbone with different moieties through diazo coupling reaction and its utilization as a leather chemical has been attempted. The 4-aminoazobenzene functionalized lignin based azo polymer (4-AAB-IL) was characterized using Nuclear Magnetic Resonance Spectroscopy (NMR), Heteronuclear Single Quantum Correlation (HSQC), Fourier - Transform Infrared Spectroscopy (FT-IR), UV–visible absorption, and dynamic light scattering techniques. The CHNS analysis reveals the increase in the percentage of nitrogen after diazotization which could have originated from the diazo functional groups. UV–visible absorption spectra of azo-modified lignin exhibit the characteristic features of azobenzene. The photoisomerization of lignin bearing azobenzene from trans→cis isomer was understood from the characteristic UV–visible spectral changes monitored during the course of the reaction. Notably the photostationary state (PSS), an indicator for the extent of trans isomer converted into cis isomer, PSStrans/cis is found to be 92:8 for 4-AAB-IL whereas that of the parent compound 4-aminoazobenzene (4-AAB) is 32:68. This indicates the higher photostability of 4-AAB-IL which would have originated from the restrictions in the structural motions around the diazo moiety imposed by the lignin skeleton, which is essential for the isomerization reaction. Since the azo compounds are evolved as dyes in leather industries, the applicability of synthesized 4-AAB-IL on leather was tested and the results showed excellent light fastening properties even after prolonged irradiation. In addition, the 4-AAB-IL provides darker shade to the crust leather when compared to the 4-AAB, thus these results suggest that functionalization of lignin could pave the way to alter the color of the lignin in a desired way by employing suitable substituents. This functional lignin based colorant would find a new green and sustainable pathway for cleaner application of lignin in coating.