Silk and wool are more easily dyed with azo dyes than other fabrics like nylon or cotton because they are polypeptides.
Polypeptides or polymers made of amino acids are what makeup wool and silk. Natural protein fibers like wool and silk, synthetic polyamide fibers like nylon, and, to a lesser extent, acrylic fibers and blends of these fibers, are typically dyed with acid dyes. They are applied to these fibers from a dyebath under acidic or neutral conditions, thus their name. The azo group is the most widely used chromophore in acid dyes. When an ionic bond is formed between the protonated amine group of the fiber and the acid group of the dye, acid dyes that are anionic and soluble in water are applied to wool, silk, and nylon
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