Grafting of Curcumin onto Caffeic Acid for Biomass Derivative toward Enhanced Photostability and Color Uniformity in Dyed Silk

Curcumin (CCM) is a high-performance natural yellow pigment with excellent chromaticity and versatile bioactivities, holding great potential for eco-friendly textile dyeing. Nevertheless, its industrial utilization is severely restricted by inherent photolability and uneven color distribution. Conventional modifications, including metal mordanting, structural derivatization, and synthetic antioxidant blending, improve the UV stability of CCM, but introduce inevitable drawbacks such as chromatic deviation, toxic residues, and competitive dye–fiber binding interference. Herein, a fully biobased, water-dispersible, and UV-resistant CCM derivative (CCMMod) was fabricated via CDI-mediated covalent condensation between CCM and plant-derived caffeic acid (CA) with intrinsic UV-shielding capability. FTIR, UV–vis spectroscopy, and HPLC–MS results confirm successful covalent grafting, whereby CCMMod achieves significantly enhanced hydrophilicity and photoretention capacity compared with pristine CCM. Dye adsorption on silk follows a multistage behavior involving rapid interfacial binding and gradual intrafibrillar diffusion toward equilibrium, and the kinetic process is well fitted by the pseudo-second-order model based on noncovalent interactions between CCMMod and silk protein groups. Relative to neat CCM-dyed silk, CCMMod-treated fabrics exhibit superior washing and rubbing fastness, with the light fastness rating improved from 1.5 to 3.5. After 20 h of standard UV irradiation, CCMMod-dyed silk retains 59.9% of its original K/S value, while pure CCM-dyed silk only maintains 10.8%. Notably, silk dyed with 5% owf CCMMod acquires a UPF value above 40, benefited from the synergistic UV attenuation of curcuminoid and caffeoyl moieties. This eco-friendly CDI-based conjugation strategy effectively overcomes CCM’s intrinsic photolability, providing a scalable and universal platform for renewable dye modification and sustainable low-carbon textile dyeing.