Valorization of Sulfonated Kraft Lignin as a Natural Dye for the Sustainable Dyeing of Wool Fabrics: Effect of Peroxide Oxidation

Lignin is an abundant and complex biopolymer with inherent color but has only a few applications. It has the potential to replace synthetic acid dyes for the coloration of wool fibers in brown shades, as traditional dyeing with acid dyes produces toxic effluent needing costly treatment. In this work, the valorization of sulfonated lignin was explored by using it as a natural dye for the coloration of wool fabrics alone and with hydrogen peroxide (H2O2). The dyed fabrics were characterized by reflectance and diffuse reflectance spectroscopies to assess the color yield and ultraviolet (UV) protection performance, respectively. The dyed fabrics exhibited reasonably good color yield and satisfactory colorfastness to washing (Grade 3/4), which improved to Grade 4 by the oxidation treatment with H2O2. The scanning electron microscopy (SEM) images show that when fabrics are dyed alone with lignin, deposition of lignin particles is visible on the fiber surface, but with hydrogen peroxide, no deposition of lignin is visible on the fiber surface, and the color strength of the fabric became almost double. The UV light transmission through the fabric decreased from 6.73 and 13.23% at 311 and 365 nm to 2.19 and 5.23%, respectively. The attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of the dyed fabrics showed increased absorption of lignin sulfonate by the fabric, suggesting depolymerization of lignin by H2O2 to smaller macromolecules easing absorption into the wool fiber. Lignin sulfonate could be a cheap and sustainable alternative to harmful synthetic dyes for producing brown shades on wool fabric.