Facile and Green Preparation of High UV-Blocking Lignin/Titanium Dioxide Nanocomposites for Developing Natural Sunscreens

Lignosulfonate (LS), one type of industrial lignin and natural macromolecular sun blocker, was applied to modify TiO2 via one-pot hydrothermal esterification. The reaction mechanism and composite structure of LS@TiO2 were investigated by DLS, TEM, AFM, TG, UV, FTIR, XPS, XRD, ESR, and contact angle measurements. The results show that the esterification modification occurred between the carboxyl groups of lignin and the hydroxyl groups on the surface of TiO2. Lignosulfonate of 3.4 wt % with a thickness of 13 nm effectively coated the normal TiO2, and the LS@TiO2 composites were nanoscale with an average size of 119 nm. Hydroxylated TiO2 particles were more reactive, and the contents of LS on TiO2 were processed by 0.1 and 1 M HCl and increased to 5.0 wt % (LS@TiO2-0.1M) and 6.0 wt % (LS@TiO2-1M), respectively. Chemically coated LS not only improved the dispersibility of TiO2 in the substrates, but also significantly boosted its UV-blocking ability. Therefore, TiO2@LS nanocomposites were applied as the sole active in the pure cream and the sunscreen performance was compared with that of TiO2. The sun protection factor (SPF) values of creams containing 5, 10, and 20 wt % LS@TiO2 were 16, 26, and 48, respectively. They were 30–60% higher than those of the creams containing the same amount of TiO2. The sunscreen performance of LS@TiO2 improved with LS content. The SPF value of pure cream containing 10 wt % LS@TiO2-1M reached 50+. The possible UV-blocking enhancement mechanism of LS@TiO2 was also revealed. Preparation of LS@TiO2 nanocomposites provides a facile and green way for value-added utilization of lignin biomass and highly efficient UV-blocking of nano-TiO2 materials.