Indigo Dyeing of Mongolian Cashmere Fiber

Mongolian cashmere sliver, yarn, and fabric were dyed and bleached with a solution of ascorbic acid and iron sulfate at $70^{\circ}C$, and then dyed using natural indigo powder at the dyeing temperature of $25^{\circ}C$ to $90^{\circ}C$ for 15-90 minutes using the IR dyeing machine. K/S values of bleached samples decreased significantly when dyed above $70^{\circ}C$ dyeing temperature for a longer dyeing time. Bleached cashmere fabric showed a greater loss of tensile strength than unbleached cashmere fabric, even when the samples were dyed at $40^{\circ}C$. With a higher dyeing temperature, yarns lost fullness, became thinner, and the pores between the yarns were enlarged. The x-ray diffraction pattern exhibited a prominent increase in crystallinity and the protein assay indicated a loss of protein in the bleached sample dyed at $90^{\circ}C$. Thinning of scales, fractured or raised tip of scales, and roughness on the entire surface of the fiber were also observed. The results indicate that bleaching and high temperature dyeing cause a serious damage to cashmere fibers. In addition, bleaching and high temperature dyeing cause significant fiber damage. Natural indigo dyeing using low temperature dyeing is recommended to produce blue color cashmere.