Chestnut shell dyes extracted by deep eutectic solvent in green dyeing of silk fibers for enhanced washable, antibacterial and UV protection properties

As an important biomass resource, chestnut shells are often discarded and wasted. In this study, we report an innovative, low-cost and environmentally friendly approach of dyeing silk fibers with natural pigments extracted from chestnut shells using deep eutectic solvent (DES) and imbuing them with antibacterial and ultraviolet protection properties. To improve the extraction ratio of chestnut shell pigment, DES of choline chloride and urea (ChCl/Urea) with high solubility performance was adopted to extract pigment from chestnut shells and then directly used for silk dyeing. The color spectral curve, washing durability, antibacterial and ultraviolet resistance of dyed silk were tested and compared with the silk dyed by metal salt mordant. The result showed that compared with ethanol extraction, DES extraction method had a higher extraction rate and better stability of the extraction solution. Owing to the swelling, surface modification and high permeability of ChCl/Urea DES to silk fibers, the affinity between the pigment solution and silk fibers was enhanced, and the pigment molecules entered the amorphous area inside the silk fibers, forming a tight bond with the fiber matrix during dyeing. Therefore, the dyed silk fabric has excellent washing durability and soft luster, and the color strength (K/S) remained unchanged after three washings. Meanwhile, the antibacterial activity and ultraviolet protection coefficient of silk dyed with chestnut shell pigment reached above 97% and 42.26, respectively, which exhibits excellent antibacterial and ultraviolet protection properties.