Fabrication of Light-Colored Lignin Microspheres for Developing Natural Sunscreens with Favorable UV Absorbability and Staining Resistance

As a natural macromolecular sun blocker, industrial lignin has potential in preparing value-added body sunscreens, but the dark color of lignin causes serious staining, which limits the practicability of lignin sunscreens. In this study, a light-colored acetic acid bamboo lignin microsphere was fabricated to address the staining issue and facilitate the marketing promotion of lignin sunscreens. The bamboo lignin microsphere was obtained by self-assembly using acetic acid/water as solvent and was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, two-dimensional hetero-nuclear single quantum correlation (2D HSQC), and particle diameter to illustrate the decoloration mechanism. The result revealed that the bulk density of the lignin microsphere was decreased due to the smaller particle size and larger space between lignin particles, which diluted the concentration of chromophores macroscopically and consequently resulted in the light color of lignin. Besides the morphology influence, the acetic acid bamboo lignin shows less unsaturated structure and fewer methoxyl groups in the chemical structural aspect, which jointly make it the lightest-colored lignin microsphere ever reported. Afterward, light-colored lignin sunscreens were prepared by blending color-reduced lignin microsphere and achieved favorable UV absorbability. The SPF value is as high as 87.2 with an addition amount of 8% of acetic acid lignin microsphere. Below an SPF of 56.8, the lignin-based sunscreens do not cause skin staining, which well satisfies the daily needs of consumers.