Normally, when wool and cotton are blended together, two separate dye baths are required.
Wool takes up most of the dye, while the cotton is left mostly undyed.
ARS' simple approach is to reverse the chemical charge of cotton from negative to positive before dyeing so that both fabrics are positively charged. Wool is already positive. The scientists use positively charged compounds called cationic fixatives, which are typically applied after cotton is dyed to help it retain color. Since the dye is negatively charged and opposites attract, the cotton and wool dye to a uniform shade because the dye is attracted equally to both fibers. Experimental textile treatments like this one may broaden the market for cool-weather garments made of wool and cotton blends.