Investigating the effect of structural changes of alkali lignins caused by reduction and methylation reaction on the optical characteristics of lignin-containing papers

In this study, changes in optical behavior of poplar Kraft lignin and bagasse Soda lignin, before and after reducing with sodium borohydride and sodium hydrosulfite (dithionite) and also after methylation using dimethyl sulfate, was monitored by measuring optical indices (brightness, k/s, whiteness and Post-Color numbers) of impregnated papers following accelerated thermal aging. The structural changes in lignins caused by reductive reactions, were mostly verified by using absorbance changes (UV-Vis spectrometry) and FI-IR spectroscopy. It was observed that, regarding elimination of chromophoric carbonyl and quinone groups, sodium borohydride showed better and more sustainable performance than sodium hydrosulfite. Therefore, it could be inferred that to improve the brightness of lignin-containing papers and persistance of this brightness, sodium borohydride could exhibit more efficiency and higher applicability.