Synergy of modified lignin and p-coumaric acid for improving the function of sun-protection in sunscreen

Lignin is a potential natural sunlight protector because its benzene derivative structure can absorb UV rays. Pure lignin is not good enough for sunblock because lignin is not fully dispersed in cream, and the range of absorbed UV rays is not broad. In this work, lignin was modified with polybutyl acrylate (PBA) and self-assembled into microspheres (L-PBA-NPs) to enhance its compatibility in cream, and it was blended with p-coumaric acid (CA) to extend its light absorbance. The hydrophobicity of grafted PBA on lignin and the microsphere of the modified lignin provided an ability to disperse in oily pure cream. To improve the sun protection factor (SPF) of the blended lignin-based sunscreen, CA with different UV absorbing ability was mixed inside the modified lignin to form CA@L-PBA-NPs. When CA and L-PBA-NPs (1:1) were added in cream with 5%, the SPF of the blended lignin sunscreen reached 18.8. After irradiation of the CA@L-PBA-NPs sunscreen under UV for 3 h, the SPF of the blended lignin-based sunscreen increased slightly because of more conjugated structures produced in lignin. The CA@L-PBA-NPs in sunscreens have great potential as a green ingredient to substitute for the small-molecule sunscreen active ingredients in commercial sunscreen products.