Demethylation and Acetylation Modification of Alkali Lignin and Their Potential Applications in Sunscreen

In order to improve the utilization of alkali lignin (AL) as an effective component for ultraviolet (UV) shielding, demethylation and acetylation modification were carried out to improve the UV absorption performance of lignin. Then, lignin-based sunscreens were successfully prepared by mixing the modified lignin and commercial cream without UV shielding ingredients. The modified alkali lignin was comprehensively characterized in terms of its molecular weight, functional groups and structural properties by GPC, UV spectroscopy and 31 P NMR. The results showed that the Mw of all three lignin feedstocks (AL, AL MeOH and AL Acetone ) was decreased with prolonged demethylation time. Compared to the original feedstock, demethylated AL had a darker color and improved UV absorption performance due to the increased phenolic hydroxyl content (approximately 4.35 mmol/g). 31 P-NMR spectra showed that the guaiacyl phenolic hydroxyl content decreased rapidly after acetylation, causing the sample color to become lighter. Among all lignin-based sunscreens, DAL Acetone achieved the highest SPF value of 11.23, a 69.4% increase over its pre-reaction level and a 7.58-fold enhancement compared to the original lignin. In summary, this study opens a promising avenue for repurposing industrial lignin as a sustainable biomaterial in high-value sectors like UV-blocking agents and cosmetic formulations.