From black liquor to tinted sunscreens: Washing out Kraft lignin unpleasant odor and improving its properties by lignin nanoparticle preparation

Abstract

Kraft lignin is a major byproduct of pulp and paper processing, presenting outstanding properties, including antioxidant action and UV and visible light absorption. Despite being a promising low-cost and renewable ingredient for dermo-cosmetic formulations, lignin use is restricted primarily because of its unpleasant odor and poor dispersibility in water. This study presents an integrated and environmentally friendly method to eliminate lignin odor, followed by improving its dispersibility in water through lignin nanoparticle (LNP) production, and, finally, evaluating their application in cosmetic formulations to act as antioxidant and UV-blocker. Starting with black liquor, Kraft lignin was precipitated and washed with water at 70°C, effectively removing lignin odor by a straightforward method, reducing its sulfur content from 5.6 to 2.9 wt%, and the ashes to negligible values without compromising the lignin structure. LNPs were successfully produced by the self-assembly method, yielding spherical and colloidally stable particles (zeta potential ca. −30 mV) with diameters ranging between 25 and 200 nm, outperforming particles prepared by sonication and bulk lignin itself. LNPs prepared by self-assembly method showed exceptional antioxidant activity, eliminating up to 79.2 % of DPPH• radicals (52 % more than bulk lignin) and showing in vitro SPF of ca. 15 with only 0.5 wt% of particle concentration, in addition to the improved dispersibility in cosmetic formulation.