Dyeing of Silk through Iron Ion-Induced Formation of Hydrophobic Lignin Coatings

There is a continuous demand for a specific type of Chinese silk fabric, which is one of China's national intangible cultural heritage items called Xiang-Yun-Sha. Xiang-Yun-Sha is silk fabric dyed using the vegetable dyes from a yam and the mud from the Pearl Delta River. The ingenious and sophisticated dyeing process inspires us to explore new dyes for silk. Through learning from the traditional mud dyeing process of Xiang-Yun-Sha, especially the contents of the mud, lignin and iron species are recognized as key factors. Yet little attention has been dedicated to lignin as a dye despite its wide applications. In this study, the feasibility of using lignin to dye silk is investigated using sodium lignosulfonate and alkaline lignin as model systems. Thanks to their advantageous polyphenolic features, such as stacking tendency and ability to form stable complexes with iron ions, lignin-related compounds show excellent performance in serving as dyes by themselves, as well as protective coatings for other dyes; the dyed silk fabric could inherit anti-ultraviolet property from lignin. Considering that iron is inexpensive and that leftover lignin is a vastly underutilized biomass resource, this study offers a promising strategy to exploit the enormous potential of these naturally occurring materials by developing high value-added products.